Optical spectroscopy microscopy Publications

Tefillin are Jewish ritual artifacts consisting of leather cases, containing inscribed slips, which are affixed with leather straps to the body of the tefillin practitioner. According to current Jewish ritual law, the tefillin cases and straps are to be colored black. The present study examines seventeen ancient tefillin cases discovered among the Dead Sea Scrolls in caves in the Judean Desert. All seventeen cases display grain surfaces with a very dark, nearly black appearance. We start with a hypothesis that the cases were intentionally colored black in antiquity using either a carbon-based or iron-gall-based paint or dye. The aim of this study is to test this hypothesis by subjecting these tefillin cases to a battery of examinations to assess the presence of carbon and iron used as pigments, and of organic materials which may have been used as binding agents in a paint. The tests deployed are: (1) macroscopic and microscopic analyses; (2) multispectral imaging using infrared wavelengths; (3) Raman spectroscopy; (4) Fourier transform infrared spectroscopy (FTIR); and (5) scanning electron microscope (SEM) and energy dispersive X-ray (EDX) spectroscopy. The results of these tests found no traces of carbon-based or iron-gall-based pigments, nor of organic compounds which may have served as binders in a paint. These results suggest that our posited hypothesis is unlikely. Instead, results of the SEM examination suggest it more likely that the black color on the surfaces of the tefillin cases is the result of natural degradation of the leather through gelatinization. The Judean Desert tefillin likely represent tefillin practices prior to when the rabbinic prescription on blackening tefillin was widely practiced. Our study suggests that the kind of non-blackened tefillin which the later rabbis rejected in their own times may well have been quite common in earlier times.

Many animals exhibit remarkable colors that are produced by the constructive interference of light reflected from arrays of intracellular guanine crystals. These animals can fine-tune their crystal-based structural colors to communicate with each other, regulate body temperature, and create camouflage. While it is known that these changes in color are caused by changes in the angle of the crystal arrays relative to incident light, the cellular machinery that drives color change is not understood. Here, using a combination of 3D focused ion beam scanning electron microscopy (FIB-SEM), micro-focused X-ray diffraction, superresolution fluorescence light microscopy, and pharmacological perturbations, we characterized the dynamics and 3D cellular reorganization of crystal arrays within zebrafish iridophores during norepinephrine (NE)-induced color change. We found that color change results from a coordinated 20° tilting of the intracellular crystals, which alters both crystal packing and the angle at which impinging light hits the crystals. Importantly, addition of the dynein inhibitor dynapyrazole-a completely blocked this NE-induced red shift by hindering crystal dynamics upon NE addition. FIB-SEM and microtubule organizing center (MTOC) mapping showed that microtubules arise from two MTOCs located near the poles of the iridophore and run parallel to, and in between, individual crystals. This suggests that dynein drives crystal angle change in response to NE by binding to the limiting membrane surrounding individual crystals and walking toward microtubule minus ends. Finally, we found that intracellular cAMP regulates the color change process. Together, our results provide mechanistic insight into the cellular machinery that drives structural color change.

Pharmaceutical waste and contaminants pose a significant global concern for water and food safety. The detection of piperidine, a common residue in drug and supplement synthesis, is critical due to its toxic nature to both humans and animals. In this study, we develop a plasmonic-based detector for surface enhanced Raman scattering (SERS) measurements. The plasmonic device is composed of triangular cavities, milled in silver thin film, and protected by a 5 nm SiO₂ layer. Due to the confined and enhanced electromagnetic field, remarkable sensitivity to piperidine with a concentration of 10⁻⁸ M in water is achieved. Despite the relatively small polarizability of piperidine, high sensitivity is observed even when using a low numerical aperture of 0.3, attributed to the directional scattering from our plasmonic device. Thus, it offers a cost-effective alternative to traditional high numerical aperture used in SERS, and the ability to use a portable Raman device for a cheaper and faster analysis.

Spherical particles with diameters within the wavelength of visible light, known as spherulites, manipulate light uniquely due to their spatial organization and their structural birefringence. Most of the known crystalline spherulites are branched, and composed of metals, alloys, and semi-crystalline polymers. Recently, a different spherulite architecture is discovered in the vision systems of decapod crustaceans - core-shell spherulites composed of highly birefringent ((Formula presented.)) organic single-crystal platelets, with exceptional optical properties. These metastructures, which efficiently scatter light even in dense aqueous environments, have no synthetic equivalence and serve as a natural proof-of-concept as well as synthetic inspiration for thin scattering media. Here, the synthesis of core-shell spherulites composed of guanine crystal platelets (((Formula presented.)) is presented in a two-step emulsification process in which a water/oil/water emulsion and induced pH changes are used to promote interfacial crystallization. Carboxylic acids neutralize the dissolved guanine salts to form spherulites composed of single, radially stacked, β-guanine platelets, which are oriented tangentially to the spherulite surface. Using Mie theory calculations and forward scattering measurements from single spherulites, it is found that due to the single-crystal properties and orientation, the synthetic spherulites possess a high tangential refractive index, similarly to biogenic particles.

Nanocomposite materials, integrating nanoscale additives into a polymer matrix, hold immense promise for their exceptional property amalgamation. This study delves into the fabrication and characterization of polyetherimide (PEI) nanocomposite strings fortified with multiwall WS₂ nanotubes. The manufacturing process capitalizes on the preferential alignment of WS₂ nanotubes along the string axis, corroborated by scanning electron microscopy (SEM). Mechanical measurements unveil a remarkable acceleration of strain hardening in the nanocomposite strings, chiefly attributed to the WS₂ nanotubes. Structural analyses via X-ray diffraction (XRD) and wide-angle X-ray scattering (WAXS) reveal intriguing structural alterations during tensile deformation. Notably a semi-crystalline framework ∼100 nm in diameter surrounding the WS₂ nanotubes emerges, which is stabilized by the π-π interactions between the PEI chains. The amorphous majority phase (97% by volume) undergoes also major structural changes upon strain becoming more compact and closing-up of the distance beweeetn the PEI chains. Dynamic mechanical analysis (DMA) demonstrates improved thermal stability of the evolved semi-crystalline π-π oriented PEI molecules, characterized by delayed thermal “structural melting”, underscoring the pivotal role of the WS₂ nanotubes in reinforcing the nanocomposite. The insight gained in this study of WS₂ nanotube-reinforced PEI nanocomposite strings, could offer diverse applications for such tailor-made polymeric materials.

The sixth-generation wireless communication (6G) extends the electromagnetic pollution up to the terahertz band. The two-dimensional titanium carbide of intercalated structure Ti₃C₂T_x MXene attracts much attention because it exhibits high terahertz electromagnetic interference (EMI) shielding effectiveness (SE) due to the large intrinsic electrical conductivity as well as few-atom monosheet thickness. Scientists worldwide are making continuous efforts to optimize the MXene structures for EMI shielding application. Innovatively, we demonstrate a chemical method to bend the nanoflake by grafting two types of alkane, octane (C₈H₁₈) and dodecane (C₁₂H₂₆), onto the surface terminals. The chain length of alkane exceeds the bond length of surface functionalities T_x (=O, −OH, −F) to introduce intrananoflake and internanoflake strains into Ti₃C₂T_x MXene. The nanoflake’s deformation leads to Raman peak redshift and broader line width. The element distribution shows that the alkane increases the oxygen-fluorine ratio of Ti₃C₂T_x MXene from 3:2 to 3:1 (Ti₃C₂T_x-C₈H₁₈) and even up to 4:1 (Ti₃C₂T_x-C₁₂H₂₆). Electronic microscopy (SEM/TEM) shows obvious edge-fold and tensile/compressive deformation of the nanoflake. The EMI SE of Ti₃C₂T_x-C₁₂H₂₆ achieves 35 dB, which is higher than those of Ti₃C₂T_x-C₈H₁₈ (26 dB) and Ti₃C₂T_x MXene (22 dB). The alkane grafting increases the absorption coefficient of the MXene thin film by more than 50% but has negligible contribution to the refractive index. Meanwhile, the conductivity of Ti₃C₂T_x-C₈H₁₈ MXene is over twice higher than that of Ti₃C₂T_x MXene, whereas the conductivity of Ti₃C₂T_x-C₁₂H₂₆ is three times higher than that of Ti₃C₂T_x MXene. The nanoflake curvature of alkane grafted Ti₃C₂T_x MXene enlarges the specific surface area and causes topological defects, which increase the absorption as well as the conductivity so that the terahertz EMI SE is enhanced correspondingly. The realization of Ti₃C₂T_x MXene of curved nanoflake for the enhancement of terahertz EMI SE is valuable for 6G electromagnetic protection.

Our work describes the nanofibrous materials of tungsten disulfide, which can be tuned by the precursor's crystallinity degree. The carefully formulated nanofibers create the morphological crossroad between fullerenes (0D), nanotubes (1D), plates (2D), and a nonwoven web of nanofibers (3D), containing all the advantageous properties of the presented material categories. Our synthetic methodology (electrospinning, reductive sulfidation) allows scale-up to industrial production. In addition, we studied the optical properties of the WS₂ nanofibers using extinction and absolute absorption measurements. The results of the optical analysis further indicate the higher crystallinity of the closed stacked fullerene-based structure. By comparing the extinction with the absorbance, we find that all the examined nanostructures display typical polaritonic spectra. However, the open plate structure exhibits a stronger scattering and thus better pronounced polaritonic features. Moreover, the ability to control the morphology allows for variating polaritonic features of the final nanofibrous material, which can directly impact the potential optoelectronic and photocatalytic applications.

While individual single-wall carbon nanotubes (SWCNTs) have remarkable strength and electrical conductivity, SWCNT networks fabricated from dispersions have inferior properties due to nanotube bundling, limiting the potential applications of SWCNT materials. Herein, a common dye molecule (purpurin) is used to exfoliate SWCNTs via noncovalent functionalization and to fabricate SWCNT materials by a simple solution-based process. The advantageous noncovalent interactions result in efficient exfoliation and metallic SWCNT enrichment, affording SWCNT materials with high mechanical robustness and electrical conductivity. This method is used to prepare mechanically robust SWCNT films and flexible transparent conductive electrodes.Purpurin, a common dye molecule, is utilized to efficiently exfoliate single-wall carbon nanotubes (SWCNTs) in an aqueous solution. The advantageous noncovalent interactions result in metallic SWCNT enrichment, affording the fabrication of flexible, robust, and highly electrical conductive SWCNT networks. Transparent conductive electrodes based on those networks show excellent optoelectronic performance suitable for application in touch screens and LEDs.image

Radiation curing (photocuring) of thermosetting polymers, such as acrylate resins, is a common technology with diverse applications, such as in adhesives, coatings, advanced manufacturing, medical-related technologies, and more. Photocuring of acrylate thermosets is initiated via a radical-induced cleavage of a photocuring agent, like bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (BAPO), which activates the vinyl C═C double bond of the acrylate moiety. Here, we show that addition of semiconducting WS₂ nanoparticles (NPs) accelerates the photocuring process. Using electron paramagnetic resonance (EPR) of ethanol-water solutions, the mechanism of the radical reactions of BAPO and WS₂ NPs is investigated. It is found that the two photocuring agents operate according to entirely different mechanisms, which has been discussed in great detail. In contrast to BAPO, which is photocleaved during the curing process, the WS₂ NPs remain unchanged, leading to major mechanical reinforcements of the cured acrylate film.

We examined lithic artifacts from the late Neanderthal site Sesselfelsgrotte (Bavaria, Germany) in order to evaluate the possibility of fire use and intentional flint heat treatment performed by late Neanderthals. We analyzed 1113 flint pieces from the G-layer complex (~60 to 45 kya; Micoquian) and 946 from the lower-layer complex (~115 to 70 kya; Mousterian). Based on macroscopic traits associated with the exposure of flint to heat and fire, we assigned artifacts to one of three groups: burnt, unburnt, and possibly intentionally heated. Our results show that while both complexes demonstrate the clear presence of fire, fire is more common in the younger G-layer complex. Moreover, possibly intentionally heated pieces are significantly more frequent in the G-layer complex, especially among the tools and specifically among side scrapers, suggesting a link between heat treatment and the production of these tools, most probably due to their functional and cultural significance. We therefore suggest that the flint in the G-layer complex of Sesselfelsgrotte underwent intentional heat treatment. The proportions of burnt flint artifacts in both complexes suggest an intensification in fire use at the site over time, while the appearance of possibly intentionally heated artifacts in the G-layer complex suggests the development of this advanced pyrotechnology by Neanderthals sometime between these two timeframes. Our results are supported by sedimentological and faunal data. We view these results as further indication of the advanced cognitive and technological capabilities of Neanderthals, which did not fall short of those of early modern humans.

The synthesis of complex new nanostructures is challenging but also bears the potential for observing new physiochemical properties and offers unique applications in the long run. High-temperature synthesis of ternary WSe2xS2(1-x) (denoted as WSSe) nanotubes in a pure phase and in substantial quantities is particularly challenging, requiring a unique reactor design and control over several parameters, simultaneously. Here, the growth of WSSe nanotubes with the composition 0 ≤ x < 1 from W18O49 nanowhiskers in an atmospheric chemical vapor deposition (CVD) flow reactor is investigated. The oxide precursor powder is found to be heavily agglomerated, with long nanowhiskers decorating the outer surface of the agglomerates and their core being enriched with oxide microcrystallites. The reaction kinetics with respect to the chalcogen vapors varies substantially between the two kinds of oxide morphologies. Insights into the chemical reactivity and diffusion kinetics of S and Se within W18O49 nanowhishkers and the micro-oxide crystallites were gained through detailed microscopic, spectroscopic analysis of the reaction products and also through density functional theory (DFT) calculations. For safety reasons, the reaction duration was limited to half an hour each. Under these circumstances, the reaction was completed for some 50% of the nanotubes and the other half remained with thick oxide core producing new WOx@WSSe core-shell nanotubes. Furthermore, the selenium reacted rather slowly with the WOx nanowhiskers, whereas the more ionic and smaller sulfur atoms were shown to diffuse and react faster. The yield of the combined hollow and core-shell nanotubes on the periphery of the agglomerated oxide was very high, approaching 100% in parts of the reactor boat. The nanotubes were found to be very thin (∼80% with a diameter

WS₂ nanotubes present many new technologies under development, including reinforced biocompatible polymers, membranes, photovoltaic-based memories, ferroelectric devices, etc. These technologies depend on the aspect ratio (length/diameter) of the nanotubes, which was limited to 100 or so. A new synthetic technique is presented, resulting in WS₂ nanotubes a few hundred micrometers long and diameters below 50 nm (aspect ratios of 2000-5000) in high yields. Preliminary investigation into the mechanistic aspects of the two-step synthesis reveals that W₅O₁₄ nanowhisker intermediates are formed in the first step of the reaction instead of the ubiquitous W₁₈O₄₉ nanowhiskers used in the previous syntheses. The electrical and photoluminescence properties of the long nanotubes were studied. WS₂ nanotube-based paper-like material was prepared via a wet-laying process, which could not be realized with the 10 μm long WS₂ nanotubes. Ultrafiltration of gold nanoparticles using the nanotube-paper membrane was demonstrated.

Our ability to synthesize life-inspired systems and materials is intimately connected to our understanding of the interplay between reactions, diffusion, phase separation, and self-assembly. The interaction between non-linear reactions (autocatalysis) and liquid-liquid phase separation is particularly interesting because of the emergence of functional properties and structures through instabilities, which are hard to predict theoretically without the help of experimental model systems. In this work, we studied systems where chemical autocatalysis is coupled to complex coacervation and the formation of oil-in-water droplets. The autocatalysis is driven by a nucleophilic chain reaction and is coupled to complex coacervation through the formation of tri- and tetracationic species. Interestingly, we observed the formation of hierarchical colloids when we used reactants that can form oil droplets in addition to coacervate droplets. This work illustrates a mechanism for the formation of complex, hierarchical microstructures by kinetically controlled self-assembly regulated by non-linear chemical reaction networks.

Noble metal nanoparticles (NPs) and particularly gold (Au) have become emerging materials in recent decades due to their exceptional optical properties, such as localized surface plasmons. Although multiple and relatively simple protocols have been developed for AuNP synthesis, the functionalization of solid surfaces composed of soft matter with AuNPs often requires complex and multistep processes. Here we developed a facile approach for functionalizing soft adhesive flexible films with plasmonic AuNPs. The synthetic route is based on preparing Au nanoislands (AuNI) (ca. 2-300 nm) on a glass substrate followed by hydrophobization of the functionalized surface, which in turn, allows efficient transfer of AuNIs to flexible adhesive films via soft-printing tape lithography. Here we show that the AuNI structure remained intact after the hydrophobization and soft-printing procedures. The AuNI-functionalized flexible films were characterized by various techniques, revealing unique characteristics such as tunable localized plasmon resonance and Raman enhancement factors beneficial for chemical and biological sensing applications.

Dioxobimanes, colloquially known as bimanes, are a well-established family of N-heterobicyclic compounds that share a characteristic core structure, 1,5-diazabicyclo[3.3.0]octadienedione, bearing two endocyclic carbonyl groups. By sequentially thionating these carbonyls in the syn and anti isomers of the known (Me,Me)dioxobimane, we were able to synthesize a series of thioxobimanes, representing the first heavy-chalcogenide bimane variants. These new compounds were extensively characterized spectroscopically and crystallographically, and their aromaticity was probed computationally. Their potential role as ligands for transition metals was demonstrated by synthesizing a representative gold(I)-thioxobimane complex.

Anthropogenic calcite is a form of calcium carbonate produced through pyrotechnological activities, and it is the main component of materials such as lime binders and wood ash. This type of calcite is characterized by a significantly lower degree of crystallinity compared with its geogenic counterparts, as a result of different formation processes. The crystallinity of calcite can be determined using infrared spectroscopy in transmission mode, which allows decoupling particle size effect from atomic order and thus effectively distinguish anthropogenic and geogenic calcites. On the contrary, Raman micro-spectroscopy is still in the process of developing a reference framework for the assessment of crystallinity in calcite. Band broadening has been identified as one of the proxies for crystallinity in the Raman spectra of geogenic and anthropogenic calcites. Here we analyze the full width at half maximum of calcite bands in various geogenic and anthropogenic materials, backed against an independent crystallinity reference based on infrared spectroscopy. Results are then used to assess the crystallinity of anthropogenic calcite in archaeological lime binders characterized by different states of preservation, including samples affected by the formation of secondary calcite, and tested on micromorphology thin sections in which lime binders are embedded in sediments.

Scleractinia coral skeleton formation occurs by a heterogeneous process of nucleation and growth of aragonite in which intraskeletal soluble organic matrix molecules, usually referred to as SOM, play a key role. Several studies have demonstrated that they influence the shape and polymorphic precipitation of calcium carbonate. However, the structural aspects that occur during the growth of aragonite have received less attention. In this research, we study the deposition of calcium carbonate on a model substrate, silicon, in the presence of SOM extracted from the skeleton of two coral species representative of different living habitats and colonization strategies, which we previously characterized. The study is performed mainly by grazing incidence X-ray diffraction with the support of Raman spectroscopy and electron and optical microscopies. The results show that SOM macromolecules once adsorbed on the substrate self-assembled in a layered structure and induced the oriented growth of calcite, inhibiting the formation of vaterite. Differently, when SOM macromolecules were dispersed in solution, they induced the deposition of amorphous calcium carbonate (ACC), still preserving a layered structure. The entity of these effects was species-dependent, in agreement with previous studies. In conclusion, we observed that in the setup required by the experimental procedure, the SOM from corals appears to present a 2D lamellar structure. This structure is preserved when the SOM interacts with ACC but is lost when the interaction occurs with calcite. This knowledge not only is completely new for coral biomineralization but also has strong relevance in the study of biomineralization on other organisms.

Many animals undergo changes in functional colors during development, requiring the replacement of integument or pigment cells. A classic example of defensive color switching is found in hatchling lizards, which use conspicuous tail colors to deflect predator attacks away from vital organs. These tail colors usually fade to concealing colors during ontogeny. Here, we show that the ontogenetic blue-to-brown tail color change in Acanthodactylus beershebensis lizards results from the changing optical properties of single types of developing chromatophore cells. The blue tail colors of hatchlings are produced by incoherent scattering from premature guanine crystals in underdeveloped iridophore cells. Cryptic tail colors emerge during chromatophore maturation upon reorganization of the guanine crystals into a multilayer reflector concomitantly with pigment deposition in the xanthophores. Ontogenetic changes in adaptive colors can thus arise not via the exchange of different optical systems, but by harnessing the timing of natural chromatophore development. The incoherent scattering blue color here differs from the multilayer interference mechanism used in other blue-tailed lizards, indicating that a similar trait can be generated in at least two ways. This supports a phylogenetic analysis showing that conspicuous tail colors are prevalent in lizards and that they evolved convergently. Our results provide an explanation for why certain lizards lose their defensive colors during ontogeny and yield a hypothesis for the evolution of transiently functional adaptive colors.

Stony corals (order: Scleractinia) differ in growth form and structure. While stony corals have gained the ability to form their aragonite skeleton once in their evolution, the suite of proteins involved in skeletogenesis is different for different coral species. This led to the conclusion that the organic portion of their skeleton can undergo rapid evolutionary changes by independently evolving new biomineralization-related proteins. Here, we used liquid chromatography-tandem mass spectrometry to sequence skeletogenic proteins extracted from the encrusting temperate coral Oculina patagonica. We compare it to the previously published skeletal proteome of the branching subtropical corals Stylophora pistillata as both are regarded as highly resilient to environmental changes. We further characterized the skeletal organic matrix (OM) composition of both taxa and tested their effects on the mineral formation using a series of overgrowth experiments on calcite seeds. We found that each species utilizes a different set of proteins containing different amino acid compositions and achieve a different morphology modification capacity on calcite overgrowth. Our results further support the hypothesis that the different coral taxa utilize a species-specific protein set comprised of independent gene co-option to construct their own unique organic matrix framework. While the protein set differs between species, the specific predicted roles of the whole set appear to underline similar functional roles. They include assisting in forming the extracellular matrix, nucleation of the mineral and cell signaling. Nevertheless, the different composition might be the reason for the varying organization of the mineral growth in the presence of a particular skeletal OM, ultimately forming their distinct morphologies.

The Scleractinia coral biomineralization process is a representative example of a heterogeneous process of nucleation and growth of biogenic CaCO3 over a mineral phase. Indeed, even if the biomineralization process starts before settlement, the bulk formation of the skeleton takes place only when the larvae attach to a solid substrate, which can be Mg-calcite from coralline algae, and the following growth proceeds on the Mg-calcite surface of the formed baseplate of the planula. Despite this peculiarity and central role of the Mg-calcite substrate, the in vitro overgrowth of CaCO3 on single crystals of Mg-calcite, or calcite, in the presence of magnesium ions and the soluble organic matrix (SOM) extracted from coral skeletons has not been performed until now. In this study, the SOMs from Stylophora pistillata and Oculina patagonica skeletons were used in a set of overgrowth experiments. The overgrown CaCO3 was characterized by microscopic, diffractometric, and spectroscopic techniques. Our results showed that CaCO3 overgrowth in the presence of S. pistillata or O. patagonica SOM produces different effects. However, there appears to be a minor distinction between samples when magnesium ions are present in solution. Moreover, the Mg-calcite substrate appears to be a favorable substrate for the overgrowth of aragonite, differently from calcite. These observations fit with the observed settling of coral larvae on Mg-calcite-based substrates and with the in vivo observation that in the planula aragonite forms on first-formed Mg-calcite crystals. The overall results of this study highlight the importance of magnesium ions, either in the solution or in the substrate, in defining the shape, morphology, and polymorphism of biodeposited CaCO3. They also suggest a magnesium-dependent biological control on the deposition of coral skeletons.

We demonstrate the formation of highly interpenetrated frameworks. An interesting observation is the presence of very large adamantane-shaped cages in a single network, making these crystals new entries in the collection of diamondoid-type metal-organic frameworks (MOFs). The frameworks were constructed by assembling tetrahedral pyridine ligands and copper dichloride. Currently, the networks’ degree of interpenetration is among the highest reported and increases when the size of the ligand is increased. Highly interpenetrated frameworks typically have low surface contact areas. In contrast, in our systems, the voids take up to 63% of the unit cell volume. The frameworks are chiral but formed from achiral components. The chirality is manifested by the coordination chemistry frameworks around the metal center, the structure of the helicoidal channels and the motifs of the individual networks. Channels of both handedness are present within the unit cells. This phenomenon shapes the walls of the channels, which are composed of 10, 16, or 32 chains correlated to the degree of interpenetration 10-, 16- and 32-fold. By changing the distance between the center of the ligand and the coordination moieties, we succeeded in tuning the diameter of the channels. Relatively large channels were formed, having diameters up to 31.0 Å × 14.8 Å.

Biomineralization processes exert varying levels of control over crystallization, ranging from poorly ordered polycrystalline arrays to intricately shaped single crystals. Coccoliths, calcified scales formed by unicellular algae, are a model for a highly controlled crystallization process. The coccolith crystals nucleate next to an organic oval structure that was termed the base plate, leading to the assumption that the base plate is responsible for the oriented nucleation of the crystals via stereochemical interactions. In recent years, several works focusing on a well-characterized model species demonstrated a fundamental role for indirect interactions that facilitate coccolith crystallization. Here, we develop the tools to extract the base plates from five different species, giving the opportunity to systematically explore the relations between base plate and coccolith properties. We used multiple imaging techniques to evaluate the structural and chemical features of the base plates under native hydrated conditions. The results show a wide range of properties, overlaid on a common rudimentary scaffold that lacks any detectable structural or chemical motifs that can explain direct nucleation control. This work emphasizes that it is the combination between the base plate and the chemical environment inside the cell that cooperatively facilitate the exquisite control over the crystallization process.

Nanotubes of transition metal dichalcogenides such as WS2 and MoS2 offer unique quasi-1D properties and numerous potential applications. Replacing sulfur by selenium would yield ternary WS2(1–x)Se2x (0 ≤ x ≤ 1; WSSe) nanotubes, which are expected to reveal strong modulation in their absorption edge as a function of selenium content, x Se. Solid WO2.72 oxide nanowhiskers were employed as a sacrificial template to gain a high yield of the nanotubes with a rather uniform size distribution. Though sulfur and selenium belong to the same period, their chemical reactivity with oxide nanowhiskers differed appreciably. Here, the closed ampoule technique was utilized to achieve the completion of the solid–vapor reaction in short time scales instead of the conventional flow reactor method. The structure and chemical composition of the nanotubes were analyzed in detail. X-ray and electron diffractions indicated a systematic modulation of the WSSe lattice upon increasing the selenium content. Detailed chemical mapping showed that the sulfur and selenium atoms are distributed in random positions on the anion lattice site of the nanotubes. The optical excitonic features and absorption edges of the WSSe nanotubes do not vary linearly with the composition x Se, which was further confirmed by density functional theory calculations. The WSSe nanotubes were shown to exhibit strong light–matter interactions forming exciton–polariton quasiparticles, which was corroborated by finite-difference time-domain simulations. Transient absorption analysis permitted following the excited state dynamics and elucidating the mechanism of the strong coupling. Thus, nanotubes of the ternary WSSe alloys offer strong band gap tunability, which would be useful for multispectral vision devices and other optoelectronic applications.

Biogenic purine crystals function in vision as mirrors, multilayer reflectors and light scatterers. We investigated a light sensory organ in a primarily wingless insect, the jumping bristletail Lepismachilis rozsypali (Archaeognatha), an ancestral group. The visual system of this animal comprises two compound eyes, two lateral ocelli, and a median ocellus, which is located on the front of the head, pointing downwards to the ground surface. We determined that the median ocellus contains crystals of xanthine, and we obtained insights into their function. To date, xanthine biocrystals have only been found in the Archaeognatha. We performed a structural analysis, using reflection light microscopy, cryo-FIB-SEM, microCT and cryo-SEM. The xanthine crystals cover the bottom of a bowl-shaped volume in the median ocellus, in analogy to a tapetum, and reflect photons to light-sensitive receptors that are spread in the volume without apparent order or preferential orientation. We infer that the median ocellus operates as an irregular multifocal reflector, which is not capable of forming images. A possible function of this organ is to improve photon capture, and by so doing assess distances from the ground surface when jumping by determining changes in the intensity and contrast of the incident light.

Soft corals (Cnidaria, Anthozoa, Octocorallia, Alcyonacea) produce internal sclerites of calcium carbonate previously shown to be composed of calcite, the most stable calcium carbonate polymorph. Here we apply multiple imaging and physical chemistry analyses to extracted and in-vivo sclerites of the abundant Red Sea soft coral, Ovabunda macrospiculata, to detail their mineralogy. We show that this species’ sclerites are comprised predominantly of the less stable calcium carbonate polymorph vaterite (> 95%), with much smaller components of aragonite and calcite. Use of this mineral, which is typically considered to be metastable, by these soft corals has implications for how it is formed as well as how it will persist during the anticipated anthropogenic climate change in the coming decades. This first documentation of vaterite dominating the mineral composition of O. macrospiculata sclerites is likely just the beginning of establishing its presence in other soft corals. Statement of significance: Vaterite is typically considered to be a metastable polymorph of calcium carbonate. While calcium carbonate structures formed within the tissues of octocorals (phylum Cnidaria), have previously been reported to be composed of the more stable polymorphs aragonite and calcite, we observed that vaterite dominates the mineralogy of sclerites of Ovabunda macrospiculata from the Red Sea. Based on electron microscopy, Raman spectroscopy, and X-ray diffraction analysis, vaterite appears to be the dominant polymorph in sclerites both in the tissue and after extraction and preservation. Although this is the first documentation of vaterite in soft coral sclerites, it likely will be found in sclerites of other related taxa as well.

Poly(L-lactic acid) (PLLA) is a biocompatible, biodegradable, and semi-crystalline polymer with numerous applications including food packaging, medical implants, stents, tissue engineering scaffolds, etc. Hydroxyapatite (HA) is the major component of natural bone. Conceptually, combining PLLA and HA could produce a bioceramic suitable for implants and bone repair. However, this nanocomposite suffers from poor mechanical behavior under tensile strain. In this study, films of PLLA and HA were prepared with small amounts of nontoxic WS₂ nanotubes (INT-WS₂ ). The structural aspects of the films were investigated via electron microscopy, X-ray diffraction, Raman microscopy, and infrared absorption spectroscopy. The mechanical properties were evaluated via tensile measurements, micro-hardness tests, and nanoindentation. The thermal properties were investigated via differential scanning calorimetry. The composite films exhibited improved mechanical and thermal properties compared to the films prepared from the PLLA and HA alone, which is advantageous for medical applications.

Fully cured epoxy resins are typically brittle materials but according to recent research, cured epoxy fibers exhibit a singular mechanical behavior, including yielding followed by large deformation, and very high strength, toughness, and modulus. These properties appear to intensify as the fiber diameter decreases. The microstructural origin of this unusual behavior has not been fully determined. Here we use confocal polarized Raman spectroscopy to monitor the apparent molecular reorientation induced by plastic deformation of epoxy fibers, both qualitatively and quantitatively. Based on these and previous X-ray diffraction measurements, a likely molecular explanation for the extreme mechanical behavior of micro-sized epoxy fibers is proposed.

We demonstrate the solvent-free amorphous-to-cocrystalline transformations of nanoscale molecular films. Exposing amorphous films to vapors of a haloarene results in the formation of a cocrystalline coating. This transformation proceeds by gradual strengthening of halogen-bonding interactions as a result of the crystallization process. The gas–solid diffusion mechanism involves formation of an amorphous metastable phase prior to crystallization of the films. In situ optical microscopy shows mass transport during this process, which is confirmed by cross-section analysis of the final structures using focused ion beam milling combined with scanning electron microscopy. Nanomechanical measurements show that the rigidity of the amorphous films influences the crystallization process. This surface transformation results in molecular arrangements that are not readily obtained through other means. Cocrystals grown in solution crystallize in a monoclinic centrosymmetric space group, whereas the on-surface halogen-bonded assembly crystallizes into a noncentrosymmetric material with a bulk second-order nonlinear optical response.

The most abundant mineral produced in the wood and leaves of trees is calcium oxalate monohydrate (whewellite), and after burning the wood the ash obtained is calcite. In the case of the Tamarix sp. tree, the freshly prepared ash is calcium sulfate (anhydrite). The aim of this study is to determine the calcium sulfate mineral phase in the fresh wood of Tamarix aphylla prior to burning. SEM images of the crystals show that they express smooth faces, are about 5–15 microns in their longest dimensions and are located in the ray cells. Fourier transform infrared spectroscopy (FTIR) and Raman microspectroscopy of the crystals in the wood and after extraction, both showed that the crystals are composed of calcium sulfate hemihydrate (bassanite). As elemental analyses of the crystals showed that in addition to calcium and sulfur, around 20 atom percent of the cations are sodium and potassium, we also obtained an X-ray powder diffraction pattern. This pattern excluded the possibility that the crystals are composed of another related mineral, and confirmed that, indeed, the crystals in the T. aphylla wood are composed of calcium sulfate hemihydrate (bassanite).

The synthesis of high-quality WS2 and more so of MoS2 (multiwall) nanotubes in substantial amounts from oxide precursors is a very challenging and important undertaking. While progress has been offered by a recent report, the present work presents another step forward in the synthesis of MoS2 nanotubes with a narrow size distribution and better crystallinity than before. Williamson–Hall analysis of the X-ray diffraction data is used to analyze the crystallinity and strain in the nanotubes. This analysis shows that the crystallinity and average diameter of the WS2 and MoS2 nanotubes reported here (type II) are better than those obtained according to the previous methods (type I). Size selection by centrifugation reported by others is used here to prepare several fractions of WS2 and MoS2 nanotubes according to their average diameter. The high refractive index of WS2 and more so MoS2 enables the nanotubes to trap light by total internal reflection, turning them into nanocylindrical resonators and thereby supporting cavity-mode resonances. The extinction, net absorption, and transient absorption of suspensions of MoS2 and WS2 nanotubes of different (average) diameters were investigated. A strong coupling effect between optical cavity modes and the A and B excitons was observed for the WS2 and MoS2 nanotubes with diameters above 80 and 60 nm, respectively. These conclusions are also supported by transient absorption measurements. Finite-difference time-domain (FDTD) calculations support the experimental findings, confirming the strong coupling effect in the WS2 and MoS2 nanotubes. These results are important not only for their own sake but also because they may bear on the new photocatalytic applications of such nanotubes.

The facile fabrication of free-floating organic nanocrystals (ONCs) was achieved via the kinetically controlled self-assembly of simple perylene diimide building blocks in aqueous medium. The ONCs have a thin rectangular shape, with an aspect ratio that is controlled by the content of the organic cosolvent (THF). The nanocrystals were characterized in solution by cryogenic transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering. The ONCs retain their structure upon drying, as was evidenced by TEM and atom force microscopy. Photophysical studies, including femtosecond transient absorption spectroscopy, revealed a distinct influence of the ONC morphology on their photonic properties (excitation energy transfer was observed only in the high-aspect ONCs). Convenient control over the structure and function of organic nanocrystals can enhance their utility in new and developed technologies.

Anthropogenic pollution from marine microplastic particles is a growing concern, both as a source of toxic compounds, and because they can transport pathogens and other pollutants. Airborne microplastic particles were previously observed over terrestrial and coastal locations, but not in the remote ocean. Here, we collected ambient aerosol samples in the North Atlantic Ocean, including the remote marine atmosphere, during the Tara Pacific expedition in May-June 2016, and chemically characterized them using micro-Raman spectroscopy. We detected a range of airborne microplastics, including polystyrene, polyethylene, polypropylene, and poly-silicone compounds. Polyethylene and polypropylene were also found in seawater, suggesting local production of airborne microplastic particles. Terminal velocity estimations and back trajectory analysis support this conclusion. For technical reasons, only particles larger than 5 µm, at the upper end of a typical marine atmospheric size distribution, were analyzed, suggesting that our analyses underestimate the presence of airborne microplastic particles in the remote marine atmosphere.

Reflective assemblies of high refractive index organic crystals are used to produce striking optical phenomena in organisms based on light reflection and scattering. In aquatic animals, organic crystal-based reflectors are used both for image-formation and to increase photon capture. Here we report the characterization of a poorly-documented reflector in the eye of the shrimp L. vannamei lying 150 μm below the retina, which we term the proximal reflective layer (PR-layer). The PR-layer is made from a dense but disordered array of polycrystalline isoxanthopterin nanoparticles, similar to those recently reported in the tapetum of the same animal. Each spherical nanoparticle is composed of numerous isoxanthopterin single crystal plates arranged in concentric lamellae around an aqueous core. The highly reflective plate faces of the crystals are all aligned tangentially to the particle surface with the optical axes projecting radially outwards, forming a birefringent spherulite which efficiently scatters light. The nanoparticle assemblies form a broadband reflective sheath around the screening pigments of the eye, resulting in pronounced eye-shine when the animal is viewed from a dorsal-posterior direction, rendering the eye pigments inconspicuous. We assess possible functions of the PR-layer and conclude that it likely functions as a camouflage device to conceal the dark eye pigments in an otherwise largely transparent animal.

Purpose: To evaluate the riboflavin (RF) concentration and distribution in the corneal stroma and the risk for endothelial photodamage during corneal crosslinking (CXL) following 10-and 30-minute impregnation. Methods: De-epithelialized rabbit corneas were subjected to impregnation for 10 and 30 minutes with different RF formulations. Human corneal endothelial cells (HCECs) were subjected to different RF concentrations and ultraviolet A (UVA) dosages. Assays included fluorescence imaging, absorption spectroscopy of corneal buttons and anterior chamber humor, and cell viability staining. Results: After 10 and 30 minutes of impregnation, respectively, anterior chamber fluid showed an RF concentration of (1.6 ± 0.21)•10⁻⁴ % and (5.4 ± 0.21)•10⁻⁴ %, and trans-corneal absorption reported an average corneal RF concentration of 0.0266% and 0.0345%. This results in a decrease in endothelial RF concentration from 0.019% to 0.0056%, whereas endothelial UVA irradiance increases by 1.3-fold when changing from 30 to 10 minutes of impregnation. HCEC viability in cultures exposed to UVA illumina-tion and RF concentrations as concluded for the endothelium after 10-and 30-minute impregnation was nonstatistically different at 51.0% ± 3.9 and 41.3 ± 5.0%, respectively. Conclusions: The risk for endothelial damage in CXL by RF/UVA treatment does not increase by shortened impregnation because the 30% increase in light intensity is accompanied by a 3.4-fold decrease of the RF concentration in the posterior stroma. This is substantiated by similar endothelial cell toxicity seen in vitro, which in fact appears to favor 10-minute impregnation. Translational Relevance: This study offers compelling arguments for (safely) shorten-ing RF impregnation duration, reducing patients’ burden and costly operation room time.

Ion diffusion affects the optoelectronic properties of halide-perovskites (HaPs). Until now, the fastest diffusion has been attributed to the movement of the halides, largely neglecting the contribution of protons, on the basis of computed density estimates. Here, the process of proton diffusion inside HaPs, following deuterium–hydrogen exchange and migration in MAPbI₃, MAPbBr₃, and FAPbBr₃ single crystals, is proven through D/H NMR quantification, Raman spectroscopy, and elastic recoil detection analysis, challenging the original assumption of halide-dominated diffusion. The results are confirmed by impedance spectroscopy, where MAPbBr₃- and CsPbBr₃-based solar cells respond at very different frequencies. Water plays a key role in allowing the migration of protons as deuteration is not detected in its absence. The water contribution is modeled to explain and forecast its effect as a function of its concentration in the perovskite structure. These findings are of great importance as they evidence how unexpected, water-dependent proton diffusion can be at the basis of the ≈7 orders of magnitude spread of diffusion (attributed to I⁻ and Br⁻) coefficient values, reported in the literature. The reported enhancement of the optoelectronic properties of HaP when exposed to small amounts of water may be related to the finding.

Production of stone artefacts using pyro-technology is known from the Middle and Upper Palaeolithic of Europe and the Levant, and the Middle Stone Age in Africa. However, determination of temperatures to which flint artefacts were exposed is impeded by the chemical and structural variability of flint. Here we combine Raman spectroscopy and machine learning to build temperature-estimation models to infer the degree of pyro-technological control effected by inhabitants of the late Lower Palaeolithic (Acheulo-Yabrudian) site of Qesem Cave, Israel. Temperature estimation shows that blades were heated at lower median temperatures (259 °C) compared to flakes (413 °C), whereas heat-induced structural flint damage (for example, pot-lids and microcracks) appears at 447 °C. These results are consistent with a differential behaviour for selective tool production that can be viewed as part of a plethora of innovative and adaptive behaviours of Levantine hominins >300,000 years ago.

Paramagnetic relaxation enhancement (PRE) is the current strategy of choice for enhancing magnetic resonance imaging (MRI) contrast and for accelerating MRI acquisition schemes. Yet, debates regarding lanthanides’ biocompatibility and PRE-effect on MRI signal quantification have raised the need for alternative strategies for relaxation enhancement. Herein, we show an approach for shortening the spin-lattice relaxation time (T1) of fluoride-based nanocrystals (NCs) that are used for in-vivo 19F-MRI, by inducing crystal defects in their solid-crystal core. By utilizing a phosphate-based rather than a carboxylate-based capping ligand for the synthesis of CaF2 NCs, we were able to induce grain boundary defects in the NC lattice. The obtained defects led to a ten-fold shorter T1 of the NCs’ fluorides. Such paramagnetic-free relaxation enhancement of CaF2 NCs, gained without affecting neither their size nor their colloidal characteristics, improved 4-fold the obtained 19F-MRI signal-to-noise ratio, allowing their use, in-vivo, with enhanced hot-spot MRI sensitivity.

The non-stoichiometric misfit layered compounds (MLC) of the general formula ((MX)(1+y))(m)(TX2)(n) (abbreviated herein as MX-TX2) have been investigated quite extensively over the last 30 years. Here MX is a atomic slab of a material with distorted rocksalt structure and TX2 is a layered compound with hexagonal (octahedral) coordination between the metal T atom and the chalcogen X atom. Recognizing the mismatch between the two (MX and TX2) sublattices, nanotubes from the MLC of different compositions were described in the past. In particular, semimetallic nanotubes belonging to the family LnX-TaX2 with Ln = rare earth atom and X = S, Se, Te have been studied in the past. While some of them, like LaS-TaS2 were obtained with moderately high yields, others like YbS-TaS2 were scarce. In the present study, a new strategy for promoting the yield of such MLC nanotubes by alloying the LaS sublattice with another Ln atom is proposed. Detailed transmission electron microscopy investigation of the (mixed) Ln(x)La((1-x))S-TaS2 (Ln = Pr, Sm, Ho, Yb) nanotubes show clearly that the substituting Ln atom resides in the rocksalt LaS sublattice of the nanotubes. Raman measurements show distinct differences between mixed tubes with open-shell (Pr, Sm, Ho) and close-shell (La, Yb) rare-earth atoms. Density functional calculations show that the interplay between two important factors determine the enhanced stability of the mixed nanotubes- the size and electronic structure of the substituting rare-earth atom. The smaller is the substituting rare-earth atom (larger Z number), the more dissimilar it is to the original La atom. This dissimilarity enhances the incommensurability between the Ln(x)La((1-x))5 and the TS2 subunits, promoting thereby the stability of the mixed MLC. However, the electronic structure of the Ln atom was found to play a more significant role. The MLC lattice of the LaS-TaS2 is electron-rich and consequently the 4d(z)(2) level of Ta is full. The unoccupied 4f levels of the substituent open-shell atoms (Pr, Sm, Ho), which are positioned below the Fermi level, serve as electron acceptors. Consequently, the Ln substitution is found to enhance the stability of the mixed lattice and nanotubes thereof. This strategy can be employed for enhancing the yield of these and other misfit nanotubes using different substituents of the right size and energy profile.

We present the analysis of a family of nanotubes (NTs) based on the quaternary misfit layered compound (MLC) YxLa1-xS-TaS2. The NTs were successfully synthesized within the whole range of possible compositions via the chemical vapor transport technique. In-depth analysis of the NTs using electron microscopy and spectroscopy proves the in-phase (partial) substitution of La by Y in the (La,Y)S subsystem and reveals structural changes compared to the previously reported LaS-TaS2 MLC-NTs. The observed structure can be linked to the slightly different lattice parameters of LaS and YS. Raman spectroscopy and infrared transmission measurements reveal the tunability of the plasmonic and vibrational properties. Density-functional theory calculations showed that the YxLa1-xS-TaS2 MLCs are stable in all compositions. Moreover, the calculations indicated that substitution of La by Sc atoms is electronically not favorable, which explains our failed attempt to synthesize these MLC and NTs thereof.

In the last decade, hybrid organic-inorganic halide perovskites have emerged as a new type of semiconductor for photovoltaics and other optoelectronic applications. Unlike standard, tetrahedrally bonded semiconductors (e.g., Si and GaAs), the ionic thermal fluctuations in the halide perovskites (i.e., structural dynamics) are strongly coupled to the electronic dynamics. Therefore, it is crucial to obtain accurate and detailed knowledge about the nature of the atomic motions within the crystal. This has proved to be challenging due to low thermal stability and the complex, temperature-dependent structural phase sequence of the halide perovskites. Here, these challenges are overcome and a detailed analysis of the low-frequency lattice mode symmetries is provided in the low-temperature orthorhombic phase of methylammonium-lead iodide. Raman measurements using linearly and circularly polarized light at 1.16 eV excitation are combined with density functional perturbation theory (DFPT). By performing an iterative analysis of Raman polarization-orientation dependence and DFPT mode analysis, the crystal orientation is determined. Subsequently, accounting for birefringence effects detected using circularly polarized light excitation, the symmetries of all of the observed Raman-active modes at 10 K are assigned.

Little is known about shell formation of calcareous dinoflagellates, despite the fact that they are one of the major calcifying organisms of the phytoplankton. Here, calcitic cyst formation in two representative members of calcareous dinoflagellates is investigated using cryo-electron microscopy (cryo-SEM and cryo-FIB-SEM) in combination with micro-Raman and infrared spectroscopy. Only calcein-AM and not calcein enters these cells, indicating active uptake of calcium and other divalent cations. Multifunctional vacuoles containing crystalline inclusions are observed, and the crystals are identified as anhydrous guanine in the beta-form. The same vacuolar enclosures contain dense magnesium-, calcium-, and phosphorous-rich mineral bodies. These bodies are presumably secreted into the outer matrix where calcite forms. Calcite formation occurs via multiple independent nucleation events, and the different crystals grow with preferred orientation into a dense reticular network that forms the mature calcitic shell. We suggest a biomineralization pathway for calcareous dinoflagellates that includes (1) active uptake of calcium through the membranes, (2) deposition of Mg2+- and Ca2+-ions inside disordered MgCaP-rich mineral bodies, (3) secretion of these bodies to the inter-membrane space, and (4) Formation and growth of calcite into a dense reticulate network. This study provides new insights into calcium uptake, storage and transport in calcifying dinoflagellates.Statement of significanceLittle is known about the shell formation of calcareous dinoflagellates, despite the fact that they are one of the major calcifying organisms of the phytoplankton. We used state-of-the-art cryo-electron microscopy (cryo-SEM and cryo-FIB-SEM) in combination with micro-Raman spectroscopy to provide new insights into mineral formation in calcifying dinoflagellates.To date, intracellular crystalline calcite was assumed to be involved in calcite shell formation. Surprisingly, we identify these crystalline inclusions as anhydrous guanine suggesting that they are not involved in biomineralization. Instead, a key finding is that MgCaP-rich bodies are probably secreted into the outer matrix where the calcite shell is formed. We suggest that these bodies are an essential part of Ca-uptake, -storage and -transport and propose a new biomineralization model. 

Multiwall WS2 nanotubes (and fullerene-like nanoparticles thereof) are currently synthesized in large amounts, reproducibly. Other than showing interesting mechanical and tribological properties, which offer them a myriad of applications, they are recently shown to exhibit remarkable optical and electrical properties, including quasi-1D superconductivity, electroluminescence, and a strong bulk photovoltaic effect. Here, it is shown that, using a simple dispersion-fractionation technique, one can control the diameter of the nanotubes and move from pure excitonic to polaritonic features. While nanotubes of an average diameter >80 nm can support cavity modes and scatter light effectively via a strong coupling mechanism, the extinction of nanotubes with smaller diameter consists of pure absorption. The experimental work is complemented by finite-difference time-domain simulations, which shed new light on the cavity mode-exciton interaction in 2D materials. Furthermore, transient absorption experiments of the size-fractionated nanotubes fully confirm the steady-state observations. Moreover, it is shown that the tools developed here are useful for size control of the nanotubes, e.g., in manufacturing environment. The tunability of the light-matter interaction of such nanotubes offers them intriguing applications such as polaritonic devices, in photocatalysis, and for multispectral sensors.

Formation of a p-n junction-like with a large built-in field is demonstrated at the nanoscale, using two types of semiconducting nanoparticles, CsPbBr₃ nanocrystals and CdSe nanoplatelets, capped with molecular linkers. By exploiting chemical recognition of the capping molecules, the two types of nanoparticles are brought into mutual contact, thus initiating spontaneous charge transfer and the formation of a strong junction field. Depending on the choice of capping molecules, the magnitude of the latter field is shown to vary in a broad range, corresponding to an interface potential step as large as ∼1 eV. The band diagram of the system as well as the emergence of photoinduced charge transfer processes across the interface is studied here by means of optical and photoelectron based spectroscopies. Our results propose an interesting template for generating and harnessing internal built-in fields in heterogeneous nanocrystal solids.

The eyes of many fish contain a reflecting layer of organic crystals partially surrounding the photoreceptors of the retina, which are commonly believed to be composed of guanine. Here we study an unusual fish eye from Stizostedion lucioperca that contains two layers of organic crystals. The crystals in the outer layer are thin plates, whereas the crystals in the inner tapetum layer are block-shaped. We show that the outer layer indeed contains guanine crystals. Analyses of solutions of crystals from the inner layer indicated that the block-shaped crystals are composed of xanthopterin. A model of the structure of the block-shaped crystals was produced using symmetry arguments based on electron diffraction data followed by dispersion-augmented DFT calculations. The resulting crystal structure of xanthopterin included, however, a problematic repulsive interaction between C=O and N of two adjacent molecules. Knowing that dissolved 7,8-dihydroxanthopterin can oxidize to xanthopterin, we replaced xanthopterin with 7,8-dihydroxanthopterin in the model. An excellent fit was obtained with the powder X-ray diffraction pattern of the biogenic crystals. We then analyzed the biogenic block-shaped crystals in their solid state, using MALDI-TOF and Raman spectroscopy. All three methods unequivocally prove that the block-shaped crystals in the eye of S. lucioperca are crystals of 7,8-dihydroxanthopterin. On the basis of the eye anatomy, we deduce that the guanine crystals form a reflective layer producing the silvery color present on part of the eye surface, whereas the block-shaped crystals backscatter light into the retina in order to increase the light sensitivity of the eye.

The Eastern Mediterranean Sea scleractinian Oculina patagonica, demonstrates high resilience to repeated seasonal bleaching events, a trait potentially allowing the species to survive through a radically changing climate. However, the physiological and morphological contributors that make this plasticity of O. patagonica possible are poorly understood. Here we use a long-term in vitro induced bleaching experiment where colonies were reared in a dark environment to examine how O. patagonica colonies can survive without endosymbionts. We assessed the physiological, morphological and genetic adaptations that accompany our controlled bleaching. Measurements reveal changes to respiration and calcification rates both at 3 and 12 months following the initiation of the darkness experiment, coupled with corresponding macromorphology traits. Upon placing in the dark environment, O. patagonica begins the bleaching process while demonstrating acclimation in which the coral appears to divert its energy to survival resulting in the expulsion of the Symbiodiniaceae population. In addition, the coenosarc exhibits degradation where the coral transforms from a colonial living to a solitary one. Once bleached, we observe adaptation by the solitary polyps characterized by a lower respiration rate yet, regaining their calcification activity and are continuing gametogenesis. However, under bleaching conditions, the newly formed skeletons differ substantially from non-bleached colonies, clearly suggesting an environmental influence on the skeleton morphology. Overall, our study reveals that O. patagonica shows phenotypic plasticity allowing the species to withstand losing their beneficial endosymbionts so as to prosper as a solitary coral. The mechanisms used by this highly resilient coral may provide clues to what corals may require to be able to adapt to life without photosynthetic symbionts.

In reef-building corals, larval settlement and its rapid calcification provides a unique opportunity to study the bio-calcium carbonate formation mechanism involving skeleton morphological changes. Here we investigate the mineral formation of primary polyps, just after settlement, in two species of the pocilloporoid corals: Stylophora pistillata (Esper, 1797) and Pocillopora acuta (Lamarck, 1816). We show that the initial mineral phase is nascent Mg-Calcite, with rod-like morphology in P. acuta, and dumbbell morphology in S. pistillata. These structures constitute the first layer of the basal plate which is comparable to Rapid Accretion Deposits (Centers of Calcification, CoC) in adult coral skeleton. We found also that the rod-like/dumbbell Mg-Calcite structures in subsequent growth step will merge into larger aggregates by deposition of aragonite needles. Our results suggest that a biologically controlled mineralization of initial skeletal deposits occurs in three steps: first, vesicles filled with divalent ions are formed intracellularly. These vesicles are then transferred to the calcification site, forming nascent Mg-Calcite rod/pristine dumbbell structures. During the third step, aragonite crystals develop between these structures forming spherulite-like aggregates.Statement of SignificanceCoral settlement and recruitment periods are highly sensitive to environmental conditions. Successful mineralization during these periods is vital and influences the coral's chances of survival. Therefore, understanding the exact mechanism underlying carbonate precipitation is highly important. Here, we used in vivo microscopy, spectroscopy and molecular methods to provide new insights into mineral development. We show that the primary polyp's mineral arsenal consists of two types of minerals: Mg-Calcite and aragonite. In addition, we provide new insights into the ion pathway by showing that divalent ions are concentrated in intracellular vesicles and are eventually deposited at the calcification site. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd.

Guanine crystals are used by certain animals, including vertebrates, to produce structural colors or to enhance vision, because of their distinctive reflective properties. Here we use cryo-SEM, cryo-FIB SEM and Raman spectroscopic imaging to characterize crystalline inclusions in a single celled photosynthesizing marine dinoflagellate species. We demonstrate spectroscopically that these inclusions are blocky crystals of anhydrous guanine in the beta-polymorph. Two-dimensional cryo-SEM and three-dimensional cryo-FIB-SEM serial block face imaging show that the deposits of anhydrous guanine crystals are closely associated with the chloroplasts. We suggest that the crystalline deposits scatter light either to enhance light exploitation by the chloroplasts, or possibly for protection from UV radiation. This is consistent with the crystal locations within the cell, their shapes and their sizes. As the dinoflagellates are extremely abundant in the oceans and are a major group of photosynthesizing marine organisms, the presence of guanine crystals in this marine organism may have broad significance.

Misfit-layered compounds (MLCs) are formed by the combination of different lattices and exhibit intriguing structural and morphological characteristics. MLC SrxLa1-xS-TaS2 nanotubes with varying Sr composition (10, 20, 40, and 60 Sr atom %, corresponding to x = 0.1, 0.2, 0.4 and 0.6, respectively) were prepared in the present study and systematically investigated using a combination of high-resolution electron microscopy and spectroscopy. These studies enable detailed insight into the structural aspects of these phases to be gained at the atomic scale. The addition of Sr had a significant impact on the formation of the nanotubes with higher Sr content, leading to a decrease in the yield of the nanotubes. This trend can be attributed to the reduced charge transfer between the rare earth/S unit (LaxSr1-xS) and the TaS2 layer in the MLC which destabilizes the MLC lattice. The influence of varying the Sr content in the nanotubes was systematically studied using Raman spectroscopy. Density functional theory calculations were carried out to support the experimental observations.

Shelled pteropods are holoplanktonic mollusks that build lightweight shells containing aragonite crystals. The complex shell architecture is composed of well-aligned, curved aragonitic fibers. Each curved fiber is continuously crystalline. We used in vivo micro-Raman spectroscopy to study the mineral composition of shells of living Creseis acicula pteropods at the larval (veliger) and adult stages. The spectra obtained from the growing edge have weak and broad peaks indicative of a highly disordered nascent aragonite phase. The disordered precursor phase is detected in the newly formed regions both at the shell edge, and during thickening in the internal part of the shell. As the shell grows and thickens throughout the life of the animal, the mineral matures from a disordered transient precursor phase to crystalline aragonite. We conclude that the shell of C. acicula is formed via a disordered nascent form of aragonite, which, being isotropic, facilitates the formation of the convoluted morphology in the continuously crystalline fibers of aragonite.

Nanomaterials that possess the ability to upconvert two low-energy photons into a single high-energy photon are of great potential to be useful in a variety of applications. Recent attempts to realize upconversion (UC) in semiconducting quantum dot (QD) systems focused mainly on fabrication of heterostructured colloidal double QDs, or by using colloidal QDs as sensitizers for triplet-triplet annihilation in organic molecules. Here we propose a simplified approach, in which colloidal QDs are coupled to organic thiol ligands and UC is achieved via a charge-transfer state at the molecule-dot interface. We synthesized core/shell CdSe/CdS QDs and replaced their native ligands with thiophenol molecules. The alignment of the molecular HOMO with respect to the QD conduction band resulted in the formation of a new charge-transfer transition from which UC can be promoted. We performed a series of pump-probe experiments and proved the non-linear emission exhibited by these QDs is the result of UC by sequential photon absorption, and further characterized the QD-ligand energy landscape by transient absorption. Finally, we demonstrate that this scheme can also be applied in a QD solid.

The challenge of nanowire assembly is still one of the major obstacles toward their efficient integration into functional systems. One strategy to overcome this obstacle is the guided growth approach, in which the growth of in-plane nanowires is guided by epitaxial and graphoepitaxial relations with the substrate to yield dense arrays of aligned nanowires. This method relies on crystalline substrates which are generally expensive and incompatible with silicon-based technologies. In this work, we expand the guided growth approach into noncrystalline substrates and demonstrate the guided growth of horizontal nanowires along straight and arbitrarily shaped amorphous nanolithographic open guides on silicon wafers. Nanoimprint lithography is used as a high-throughput method for the fabrication of the high-resolution guiding features. We first grow five different semiconductor materials (GaN, ZnSe, CdS, ZnTe, and ZnO) along straight ridges and trenches, demonstrating the generality of this method. Through crystallographic analysis we find that despite the absence of any epitaxial relations with the substrate, the nanowires grow as single crystals in preferred crystallographic orientations. To further expand the guided growth approach beyond straight nanowires, GaN and ZnSe were grown also along curved and kinked configurations to form different shapes, including sinusoidal and zigzag-shaped nanowires. Photoluminescence and cathodoluminescence were used as noninvasive tools to characterize the sine wave-shaped nanowires. We discuss the similarities and differences between in-plane nanowires grown by epitaxy/graphoepitaxy and artificial epitaxy in terms of generality, morphology, crystallinity, and optical properties.

Composites of poly(l-lactic acid) (PLLA) reinforced by adding inorganic nanotubes of tungsten disulfide (INT-WS2) were prepared by solvent casting. In addition to the pristine nanotubes, PLLA nanocomposites containing surface modified nanotubes were studied as well. Several surface-active agents, including polyethylene imine (PEI), were studied in this context. In addition, other biocompatible polymers, like poly d,l-lactic acid (PDLLA) and others were considered in combination with the INT-WS2. The nanotubes were added to the polymer in different proportions up to 3 wt %. The dispersion of the nanotubes in the nanocomposites were analyzed by several techniques, including X-ray tomography microscopy (Micro-XCT). Moreover, high-temperature rheological measurements of the molten polymer were conducted. In contrast to other nanoparticles, which lead to a considerable increase of the viscosity of the molten polymer, the WS2 nanotubes did not affect the viscosity significantly. They did not affect the complex viscosity of the molten PLLA phase, either. The mechanical and tribological properties of the nanocomposites were found to improve considerably by adding the nanotubes. A direct correlation was observed between the dispersion of the nanotubes in the polymer matrix and its mechanical properties.

We report the synthesis, characterization, and photo-physical properties of two new ruthenium(II)-phenol-imidazole complexes. These bio-mimetic complexes have potential as photocatalysts for water splitting. Owing to their multiple phenol-imidazole groups, they have a higher probability of light-induced radical formation than existing complexes. The newly synthesized complexes show improved overlap with the solar spectrum compared to other ruthenium(II)-phenol-imidazole complexes, and their measured lifetimes are suitable for light-induced radical formation. In addition, we conducted solvatochromic absorption measurements, which elegantly follow Marcus theory, and demonstrate the symmetry differences between the two complexes. The solvatochromic measurements further imply electron localization onto one of the ligands. The new complexes may find applications in photocatalysis, dye-sensitized solar cells, biomedicine, and sensing. Moreover, their multiple chelating units make them promising candidates for light-activated metal organic radical frameworks, i.e. metal-organic frameworks that contain organic radicals activated by light.

Misfit layered compounds (MLC) with the composition (LaS)(1.15)TaS2 (for simplicity denoted as LaS-TaS2) and LaS-NbS2 were prepared and studied in the past. Nanotubes of LaS-TaS2 could be easily synthesized, while tubular structure of the LaS-NbS2 were found to be rather rare in the product. To understand this riddle, quaternary alloys of LaS-NbxTa(1-x)S2 with ascending Nb concentration were prepared herein in the form of nanotubes (and platelets). Not surprisingly, the concentration of these quaternary nanotubes shrank (and the relative density of platelets increased) with increasing Nb content in the precursor. The structure and chemical composition of such nanotubes was elucidated by electron microscopy. Conceivably, the TaS2 in the MLC compounds LnS-TaS2 (Ln = lanthanide atom) crystallizes in the 2H polytype. High resolution transmission electron microscopy showed however that, invariably, MLC nanotubes prepared from 80 at% Nb content in the precursor belonged to the 1T polytype. Raman spectroscopy of individual tubes revealed that up to 60 at% Nb, they obey the standard model of MLC, while higher Nb lead to large deviations, which are discussed in brief. The analysis indicated also that such nanotubes do not exhibit the pattern assigned to charge density wave transition so typical for binary 1T-TaS2. The prospect for revealing interesting quasi-1D behavior of such quaternary nanotubes is also briefly discussed.

Quantifying ion concentrations and mapping their intracellular distributions at high resolution can provide much insight into the formation of biomaterials. The key to achieving this goal is cryo-fixation, where the biological materials, tissues and associated solutions are rapidly frozen and preserved in a vitreous state. We developed a correlative cryo-Scanning Electron Microscopy (SEM)/Energy Dispersive Spectroscopy (EDS) protocol that provides quantitative elemental analysis correlated with spatial imaging of cryo-immobilized specimens. We report the accuracy and sensitivity of the cryo-EDS method, as well as insights we derive on biomineralization pathways in a foraminifer. Foraminifera are marine protozoans that produce Mg-containing calcitic shells and are major calcifying organisms in the oceans. We use the cryo-SEM/EDS correlative method to characterize unusual Mg and Ca-rich particles in the cytoplasm of a benthic foraminifer. The Mg/Ca ratio of these particles is consistently lower than that of seawater, the source solution for these ions. We infer that these particles are involved in Ca ion supply to the shell. We document the internal structure of the MgCa particles, which in some cases include a separate Si rich core phase. This approach to mapping ion distribution in cryo-preserved specimens may have broad applications to other mineralized biomaterials.Statement of significanceIons are an integral part of life, and some ions play fundamental roles in cell metabolism. Determining the concentrations of ions in cells and between cells, as well as their distributions at high resolution can provide valuable insights into ion uptake, storage, functions and the formation of biomaterials. Here we present a new cryo-SEM/EDS protocol that allows the mapping of different ion distributions in solutions and biological samples that have been cryo-preserved. We demonstrate the value of this novel approach by characterizing a novel biogenic mineral phase rich in Mg found in foraminifera, single celled marine organisms. This method has wide applicability in biology, and especially in understanding the formation and function of mineral-containing hard tissues. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The synthesis and characterization of nanotubes from misfit layered compounds (MLCs) of the type (LnS)(1+y)TaS2 (denoted here as LnS-TaS2; Ln=Pr, Sm, Gd, and Yb), not reported before, are described (the bulk compound YbS-LaS2 was not previously documented). Transmission electron microscopy and selected area electron diffraction showed that the interlayer spacing along the c axis decreased with an increase in the atomic number of the lanthanide atom, which suggested tighter interaction between the LnS layer and TaS2 for the late lanthanides. The Raman spectra of the tubules were studied and compared to those of the bulk MLC compounds. Similar to the bulk MLCs, the Raman spectra could be divided into the low-frequency modes (110-150cm(-1)) of the LnS lattice and the high-frequency modes (250-400cm(-1)) of the TaS2 lattice. The Raman spectra indicated that the vibrational lattice modes of the strained layers in the tubes were stiffer than those in the bulk compounds. Furthermore, the modes of the late lanthanides were higher in energy than those of the earlier lanthanides, which suggested larger charge transfer between the LnS and TaS2 layers for the late lanthanides. Polarized Raman measurements showed the expected binodal intensity profile (antenna effect). The intensity ratio of the Raman signal showed that the E-2g mode of TaS2 was more sensitive to the light-polarization effect than its A(1g) mode. These nanotubes are expected to reveal interesting low-temperature quasi-1D transport behavior.

The formation of atherosclerotic plaques in the blood vessel walls is the result of LDL particle uptake, and consequently of cholesterol accumulation in macrophage cells. Excess cholesterol accumulation eventually results in cholesterol crystal deposition, the hallmark of mature atheromas. We followed the formation of cholesterol crystals in J774A.1 macrophage cells with time, during accumulation of LDL particles, using a previously developed correlative cryosoft X-ray tomography (cryo-SXT) and stochastic optical reconstruction microscopy (STORM) technique. We show, in the initial accumulation stages, formation of small quadrilateral crystal plates associated with the cell plasma membrane, which may subsequently assemble into large aggregates. These plates match crystals of the commonly observed cholesterol monohydrate triclinic structure. Large rod-like cholesterol crystals form at a later stage in intracellular locations. Using cryotransmission electron microscopy (cryo-TEM) and cryoelectron diffraction (cryo-ED), we show that the structure of the large elongated rods corresponds to that of monoclinic cholesterol monohydrate, a recently determined polymorph of the triclinic crystal structure. These monoclinic crystals form with an unusual hollow cylinder or helical architecture, which is preserved in the mature rodlike crystals. The rod-like morphology is akin to that observed in crystals isolated from atheromas. We suggest that the crystals in the atherosclerotic plaques preserve in their morphology the memory of the structure in which they were formed. The identification of the polymorph structure, besides explaining the different crystal morphologies, may serve to elucidate mechanisms of cholesterol segregation and precipitation in atherosclerotic plaques.

Films combining hydroxyapatite (HA) with minute amounts (ca. 1 weight %) of (rhenium doped) fullerene-like MoS2 (IF) nanoparticles were deposited onto porous titanium substrate through electrophoretic process (EPD). The films were analyzed by scanning electron microscopy (SEM), X-ray diffraction and Raman spectroscopy. The SEM analysis showed relatively uniform coatings of the HA + IF on the titanium substrate. Chemical composition analysis using energy dispersive X-ray spectroscopy (EDS) of the coatings revealed the presence of calcium phosphate minerals like hydroxyapatite, as a majority phase. Tribological tests were undertaken showing that the IF nanoparticles endow the HA film very low friction and wear characteristics. Such films could be of interest for various medical technologies. Means for improving the adhesion of the film to the underlying substrate and its fracture toughness, without compromising its biocompatibility are discussed at the end.

Aragonite skeletons in corals are key contributors to the storage of atmospheric CO2 worldwide. Hence, understanding coral biomineralization/calcification processes is crucial for evaluating and predicting the effect of environmental factors on this process. While coral biomineralization studies have focused on adult corals, the exact stage at which corals initiate mineralization remains enigmatic. Here, we show that minerals are first precipitated as amorphous calcium carbonate and small aragonite crystallites, in the pre-settled larva, which then evolve into the more mature aragonitic fibers characteristic of the stony coral skeleton. The process is accompanied by modulation of proteins and ions within these minerals. These findings may indicate an underlying bimodal regulation tactic adopted by the animal, with important ramification to its resilience or vulnerability toward a changing environment.

The eyes of some aquatic animals form images through reflective optics. Shrimp, lobsters, crayfish, and prawns possess reflecting superposition compound eyes, composed of thousands of square-faceted eye units (ommatidia). Mirrors in the upper part of the eye (the distal mirror) reflect light collected from many ommatidia onto the photosensitive elements of the retina, the rhabdoms. A second reflector, the tapetum, underlying the retina, back-scatters dispersed light onto the rhabdoms. Using microCT and cryo-SEM imaging accompanied by in situ micro-X-ray diffraction and micro-Raman spectroscopy, we investigated the hierarchical organization and materials properties of the reflective systems at high resolution and under close-to-physiological conditions. We show that the distal mirror consists of three or four layers of plate-like nanocrystals. The tapetum is a diffuse reflector composed of hollow nanoparticles constructed from concentric lamellae of crystals. Isoxanthopterin, a pteridine analog of guanine, forms both the reflectors in the distal mirror and in the tapetum. The crystal structure of isoxanthopterin was determined from crystal-structure prediction calculations and verified by comparison with experimental X-ray diffraction. The extended hydrogen-bonded layers of the molecules result in an extremely high calculated refractive index in the H-bonded plane, n = 1.96, which makes isoxanthopterin crystals an ideal reflecting material. The crystal structure of isoxanthopterin, together with a detailed knowledge of the reflector superstructures, provide a rationalization of the reflective optics of the crustacean eye.

We report here a new methodology for the formation of freestanding nanotubes composed of individual gold nanoparticles (NPs) cross-linked by coordination complexes or porphyrin molecules using WS ₂ nanotubes (INT-WS ₂) as a template. Our method consists of three steps: (i) coverage of these robust inorganic materials with monodispersed and dense monolayers of gold NPs, (ii) formation of a molecular AuNP network by exposing these decorated tubes to solutions containing a ruthenium polypyridyl complex or meso-tetra(4-pyridyl)porphyrin, and (iii) removal of the INT-WS ₂ template with a hydrogen peroxide solution. Nanoindentation of the template-free AuNP tubes with atomic force microscopy indicates a radial elastic modulus of 4 GPa. The template-free molecular AuNP tubes are characterized using scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, and micro-Raman spectroscopy. The methodology provides a convenient and scalable strategy for the realization of molecular AuNP tubes with a defined length and diameter, depending on the dimensions of the template.

Designing supramolecular nanotubes (SNTs) with distinct dimensions and properties is highly desirable, yet challenging, since structural control strategies are lacking. Furthermore, relatively complex building blocks are often employed in SNT self-assembly. Here, we demonstrate that symmetric bolaamphiphiles having a hydrophobic core comprised of two perylene diimide moieties connected via a bipyridine linker and bearing polyethylene glycol (PEG) side chains can self-assemble into diverse molecular nanotubes. The structure of the nanotubes can be controlled by assembly conditions (solvent composition and temperature) and a PEG chain length. The resulting nanotubes differ both in diameter and cross section geometry, having widths of 3 nm (triangular-like cross-section), 4 nm (rectangular), and 5 nm (hexagonal). Molecular dynamics simulations provide insights into the stability of the tubular superstructures and their initial stages of self-assembly, revealing a key role of oligomerization via side-by-side aromatic interactions between bis-aromatic cores. Probing electronic and photonic properties of the nanotubes revealed extended electron delocalization and photoinduced charge separation that proceeds via symmetry breaking, a photofunction distinctly different from that of the fibers assembled from the same molecules. A high degree of structural control and insights into SNT self-assembly advance design approaches toward functional organic nanomaterials.

Active control over the shape, composition, and crystalline habit of nanocrystals has long been a goal. Various methods have been shown to enable postsynthesis modification of nanoparticles, including the use of the Kirkendall effect, galvanic replacement, and cation or anion exchange, all taking advantage of enhanced solid-state diffusion on the nanoscale. In all these processes, however, alteration of the nanoparticles requires introduction of new precursor materials. Here we show that for cesium lead halide perovskite nanoparticles, a reversible structural and compositional change can be induced at room temperature solely by modification of the ligand shell composition in solution. The reversible transformation of cubic CsPbX3 nanocrystals to rhombohedral Cs4PbX6 nanocrystals is achieved by controlling the ratio of oleylamine to oleic acid capping molecules. High-resolution transmission electron microscopy investigation of Cs4PbX6 reveals the growth habit of the rhombohedral crystal structure is composed of a zero-dimensional layered network of isolated PbX6 octahedra separated by Cs cation planes. The reversible transformation between the two phases involves an exfoliation and recrystalliztion process. This scheme enables fabrication of high-purity monodispersed Cs4PbX6 nanoparticles with controlled sizes. Also, depending on the final size of the Cs4PbX6 nanoparticles as tuned by the reaction time, the back reaction yields CsPbX3 nanoplatelets with a controlled thickness. In addition, detailed surface analysis provides insight into the impact of the ligand composition on surface stabilization that, consecutively, acts as the driving force in phase and shape transformations in cesium lead halide perovskites.

Organic crystalline materials are used as dyes/pigments, pharmaceuticals, and active components of photonic and electronic devices. There is great interest in integrating organic crystals with inorganic and carbon nanomaterials to create nanocomposites with enhanced properties. Such efforts are hampered by the difficulties in interfacing organic crystals with dissimilar materials. Here, an approach that employs organic nanocrystallization is presented to fabricate solution-processed organic nanocrystal/carbon nanotube (ONC/CNT) hybrid materials based on readily available organic dyes (perylene diimides (PDIs)) and carbon nanotubes. The hybrids are prepared by self-assembly in aqueous media to afford free-standing films with tunable CNT content. These exhibit excellent conductivities (as high as 5.78 ± 0.56 S m⁻¹), and high thermal stability that are superior to common polymer/CNT hybrids. The color of the hybrids can be tuned by adding various PDI derivatives. ONC/CNT hybrids represent a novel class of nanocomposites, applicable as optoelectronic and conductive colorant materials.

Detecting prostate specific antigen (PSA) in a questioned body fluid stain or other sexual assault casework items strongly indicates the presence of semen and is extremely useful when no sperm cells are observed. However, it has several false-positives which may prevent confirmative semen classification. Namely, the presence of PSA and nucleated cells in male urine could lead a forensic examiner to an incorrect body fluid conclusion. Micro Raman spectroscopy is a molecular spectroscopy based on the inelastic scattering of monochromatic light and its potential to detect a wide range of body fluids has been demonstrated in recent years. In the present study, we show how the combination of PSA tests and micro Raman spectroscopy offers a simple, non-destructive and quick method for confirmatory semen detection. After positive PSA tests, micro Raman spectroscopy can easily corroborate semen presence and exclude the possibility of urine false positive detection. The sensitivity of this practice was demonstrated by measuring micro Raman spectra of diluted urine and semen (up to 1:100), as well as their spectra after extraction from cloth and swabs. These results strongly show the advantages of combining micro Raman spectroscopy and PSA tests when examining sexual assault casework items.

Mixed alloy Mo_xW_(1 − x)S₂ (0 ≤ x ≤ 1) bulk samples are characterized by low wavenumber Raman spectroscopy. The results provide a convenient and reliable means for systematically determining the sample Mo/W composition. The absence of disorder effects in its interlayer motion (unlike for the intralayer motion) and the lack of Mo/W composition dependence of the shear mode force constants enable the facile employment of a reduced monoatomic linear chain model by treating the bulk alloy as a series of ‘single balls’ of mass (xm_Mo + (1 − x)m_W + 2m_S) attached by interlayer springs with a composition-independent interlayer force constant of 2.52N/m. Shear mode Raman scattering analysis may consequently be complementary to the electronic spectroscopy tools (X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS)), commonly used for mixed alloy bulk composition characterization.

Keywords: Ophthalmology

Both in vivo and ex vivo observations support the hypothesis that bone mineral formation proceeds via disordered precursor phases. The characteristics of the precursor phases are not well defined, but octacalcium phosphate-like, amorphous calcium phosphate-like, and HPO₄^2--enriched phases were detected. Here we use in vivo Raman spectroscopy and high-resolution wide-angle X-ray diffraction (WAXD) to characterize and map at 2 μm resolution the mineral phases in the rapidly forming tail fin bones of living zebrafish larvae and zebrafish larvae immediately after sacrifice, respectively. Raman spectroscopy shows the presence of an acidic disordered calcium phosphate phase with additional characteristic features of HPO₄^2- at the bone-cell interface. The complexity in the position and shape of the ν₁ PO₄ peak viewed by in vivo Raman spectroscopy emphasizes the heterogeneity of the mineral during bone formation. WAXD detects an additional isolated peak, appearing alone or together with the characteristic diffraction pattern of carbonated hydroxyapatite. This unidentified phase is located at the interface between the mature bone and the surrounding tissue, similar to the location at which the disordered phase was observed by Raman spectroscopy. The variable peak positions and profiles support the notion that this is an unstable disordered precursor phase, which conceivably crystallized during the X-ray diffraction measurement. Interestingly, this precursor phase is co-aligned with the c-axes of the mature bone crystals and thus is in intimate relation with the surrounding collagen matrix. We conclude that a major disordered precursor mineral phase containing HPO₄^2- is part of the deposition pathway of the rapidly forming tail fin bones of the zebrafish.

TiO₂/CdSe/CuSCN extremely thin absorber (ETA) solar cells are found to give relatively high values of open-circuit voltage (>0.8 V) but low currents upon annealing the cadmium selenide (CdSe) in air (500 °C). Annealing in N₂ produces much lower photovoltages and slightly lower photocurrents. Band structure measurements show differences between the two annealing regimes that, however, appear to favor the N₂-annealed CdSe. On the other hand, chemically resolved electrical measurements (CREM) of the cells reveal marked differences in photo-induced charge trapping, in particular at absorber grain boundaries of the air versus N₂-annealed systems, correlated with the formation of Cd-O species at the CdSe surface. Using transient absorption and photovoltage decay, pronounced lifetime differences are also observed, in agreement with the strong suppression of charge recombination. The results point to a multiple role of grain surface-oxidation, which both impedes electron injection from the CdSe to the TiO_2, but, much more significantly, enhances hole injection to the CuSCN via passivation of hole traps that act as efficient recombination centers. Upon annealing in air, extremely thin absorber solar cells based on CdSe-sensitized titania show relatively high values of open-circuit voltage but low currents. This stems from the fact that while oxidation impedes electron injection from the CdSe to the TiO_2, the balance between hole extraction and recombination is improved in favor of the former.

Optical gain from colloidal quantum dots has been desired for several decades since their discovery. While gain from multiexcitations is by now well-established, nonradiative Auger recombination limits the lifetime of such population inversion in quantum dots. CdSe cores isovalently doped by one to few Te atoms capped with rod-shaped CdS are examined as a candidate system for enhanced stimulated emission properties. Emission depletion spectroscopy shows a behavior characteristic of 3-level gain systems in these quantum dots. This implies complete removal of the 2-fold degeneracy of the lowest energy electronic excitation due to the large repulsive excitonexciton interaction in the doubly excited state. Using emission depletion measurements of the trap-associated emission from poorly passivated CdS quantum dots, we show that 3-level characteristics are typical of emission resulting from a band edge to trap state transition, but reveal subtle differences between the two systems. These results allow for unprecedented observation of long-lived population inversion from singly excited quantum dots.

We used a helical polymer backbone (polyacrylamide) as a scaffold to organize perylene diimide chromophores into well-defined foldamers, which further undergo self-assembly into supramolecular tube-like arrays in aqueous media, as revealed by cryo-TEM imaging. The arrays are supramolecular polymers, whose structure is templated by folded primary building blocks, representing a useful tool for directing self-assembly. Exciton migration in the supramolecular arrays was studied by transient absorption and revealed a moderate exciton diffusion propensity.

In covalent polymerization, a single monomer can result in different polymer structures due to positional, geometric, or stereoisomerism. We demonstrate that strong hydrophobic interactions result in stable noncovalent polymer isomers that are based on the same covalent unit (amphiphilic perylene diimide). These isomers have different structures and electronic/photonic properties, and are stable in water, even upon prolonged heating at 100 °C. Such combination of covalent-like stability together with structural/functional variation is unique for noncovalent polymers, substantially advancing their potential as functional materials. A strong hold: Strong hydrophobic interactions result in stable noncovalent polymer isomers derived from a single covalent unit. These isomers have different electronic and photonic properties and are stable in water, even upon prolonged heating to 100 °C.

The formation of donor/acceptor junctions in hybrid nanomaterials is predicted to enhance photocatalytic activity as compared to single-component semiconductor systems. Specifically, nanomaterials containing a junction of n-type cadmium sulfide (CdS) and p-type copper sulfide (Cu₂S) formed via cation exchange have been proposed as potential photocatalysts for reactions such as water splitting. Herein, we study the elemental distribution of Cu within these nanostructures using analytical transmission electron microscopy techniques. The resulting effects of this elemental distribution on photocatalytic activity and charge dynamics were further studied using a model photoreduction reaction and transient absorption spectroscopy. We find that copper diffusion in the hybrid nanostructure quenches the exciton lifetime and results in low photocatalytic activity; however, this effect can be partially mitigated via selective extraction. These results provide a deeper understanding of the physical processes within these hybrid nanostructures and will lead to more rational design of photocatalyst materials.

A new technique of gold nanoparticle (AuNP) growth on the sidewalls of WS₂ inorganic nanotubes (INT-WS₂) and the surface of MoS₂ fullerene-like nanoparticles (IF-MoS₂) is developed to produce metal-semiconductor nanocomposites. The coverage density and mean size of the nanoparticles are dependent on the HAuCl₄/MS₂ (M = W, Mo) molar ratio. AuNPs formation mechanism seems to involve spatially divided reactions of AuCl₄^- reduction and WS ₂/MoS₂ oxidation taking place on the surface defects of the disulfide nanostructures rather than directly at the AuNP-INT/IF interface. A strong epitaxial matching between the lattices of the gold nanoparticles and the INT-WS₂ or IF-MoS₂ seems to suppress plasmon resonance in the nanocomposites with small (

A methodology leading to facile self-assembly of crystalline aromatic arrays in dilute aqueous solutions would enable efficient fabrication and processing of organic photonic and electronic materials in water. In particular, soluble 2D crystalline nanosheets may mimic the properties of photoactive thin films and self-assembled monolayers, covering large areas with ordered nanometer-thick material. We designed such solution-phase arrays using hierarchical self-assembly of amphiphilic perylene diimides in aqueous media. The assemblies were characterized by cryogenic transmission electron microscopy (cryo-TEM), revealing crystalline order and 2D morphology (confirmed by AFM studies). The order and morphology are preserved upon drying as evidenced by TEM and AFM. The 2D crystalline-like structures exhibit broadening and red-shifted absorption bands in UV-vis spectra, typical for PDI crystals and liquid crystals. Photophysical studies including femtosecond transient absorption spectroscopy reveal that two of the assemblies are superior light-harvesters due to excellent solar spectrum coverage and fast exciton transfer, in one case showing exciton diffusion comparable to solid-state crystalline systems based on perylene tetracarboxylic dianhidride (PTCDA).

Purpose. We evaluated the efficacy and safety of photochemical corneal stiffening by palladium bacteriochlorin 13′-(2-sulfoethyl)amide dipotassium salt (WST11) and near infrared (NIR) illumination, using ex vivo and in vivo rabbit eye models. Methods. Corneas of post mortem rabbits and living rabbits were pretreated topically with 2.5 mg/mL WST11 in saline or in 20% dextran T-500 (WST-D), washed and illuminated with an NIR diode laser (755 nm, 10 mW/cm2. Studies with corneas of untreated fellow eyes served as controls. Tensile strength measurements, histopathology, electron spin resonance, and optical spectroscopy and fluorescence microscopy were used to assess treatment effects. Comparative studies were performed with standard riboflavin/ultraviolet-A light (UVA) treatment. Results. WST11/NIR treatment significantly increased corneal stiffness following ex vivo or in vivo treatment, compared to untreated contralateral eyes. The incremental ultimate stress and Young's modulus of treated corneas increased by 45, 113, 115%, and 10, 79, and 174% following 10, 20, and 30 minutes of incubation with WST11, respectively. WST-D/NIR had a similar stiffening effect, but markedly reduced post-treatment edema and shorter time of epithelial healing. WST11/NIR and WST-D/NIR generate hydroxyl and superoxide radicals, but no singlet oxygen in the cornea. Histology demonstrated a reduction in the keratocyte population in the anterior half of the corneal stroma, without damage to the endothelium. Conclusions. Treatment of rabbit corneas, with either WST11/NIR or WST-D/NIR, increases their biomechanical strength through a mechanism that does not involve singlet oxygen. The WST-D/NIR treatment showed less adverse effects, demonstrating a new potential for clinical use in keratoconus and corneal ectasia after refractive surgery.

The majority of protein spectroelectrochemical methods utilize a diffusing, chemical mediator to exchange electrons between the electrode and the protein. In such methods, electrochemical potential control is limited by mediator choice and its ability to interact with the protein of interest. We report an approach for unmediated, protein spectroelectrochemistry that overcomes this limitation by adsorbing protein directly to thiol self-assembled monolayer (SAM) modified, thin (10 nm), semitransparent gold. The viability of the method is demonstrated with two diverse and important redox proteins: cytochrome c and azurin. Fast, reversible electrochemical signals comparable to those previously reported for these proteins on ordinary disk gold electrodes were observed. Although the quantity of protein in a submonolayer adsorbed at an electrode is expected to be insufficient for detection of UV-vis absorption bands based on bulk extinction coefficients, excellent spectra were detected for each of the proteins in the adsorbed state. Furthermore, AFM imaging confirmed that only a single layer of protein was adsorbed to the electrode. We hypothesize that interaction of the relatively broad gold surface plasmon with the proteins' electronic transitions results in surface signal enhancement of the molecular transitions of between 8 and 112 times, allowing detection of the proteins at much lower than expected concentrations. Since many other proteins are known to interact with gold SAMs and the technical requirements for implementation of these experiments are simple, this approach is expected to be very generally applicable to exploring mechanisms of redox proteins and enzymes as well as development of sensors and other redox protein based applications.

Intraband hole relaxation of colloidal Te-doped CdSe quantum dots is studied using state-selective transient absorption spectroscopy. The dots are excited at the band edge, and the defect band bleach caused by state filling of the hole is probed. Close to the defect energy, the hole relaxation is substantially slowed down, indicating a gap separating the defect state from the CdSe band edge. A clear dependence of the relaxation time with the QDs size is presented, implying that the hole relaxation is mediated by longitudinal optical (LO) phonon modes of the CdSe host. In addition, we find that overcoating the quantum dots by two monolayers of a ZnS shell extends the hole relaxation time by a factor of 2, suggesting a combined effect of LO phonons and surface effects governing intraband hole relaxation.

We present a self-assembly method to construct CdSe/ZnS quantum dot-gold nanoparticle complexes. This method allows us to form complexes with relatively good control of the composition and structure that can be used for detailed study of the exciton-plasmon interactions. We determine the contribution of the polarization-dependent near-field enhancement, which may enhance the absorption by nearly two orders of magnitude and that of the exciton coupling to plasmon modes, which modifies the exciton decay rate.

Background: Major circulation pathologies are initiated by oxidative insult expansion from a few injured endothelial cells to distal sites; this possibly involves mechanisms that are important to understanding circulation physiology and designing therapeutic management of myocardial pathologies. We tested the hypothesis that a localized oxidative insult of endothelial cells (ECs) propagates through gap junction inter-cellular communication (GJIC). Methodology/Principal Findings: Cultures comprising the bEnd.3 cell line, that have been established and recognized as suitable for examining communication among ECs, were used to study the propagation of a localized oxidative insult to remote cells. Spatially confined near infrared illumination of parental or genetically modified bEnd.3 cultures, pretreated with the photosensitizer WST11, generated O₂•^- and •OH radicals in the illuminated cells. Time-lapse fluorescence microscopy, utilizing various markers, and other methods, were used to monitor the response of non-illuminated bystander and remote cells. Functional GJIC among ECs was shown to be mandatory for oxidative insult propagation, comprising de-novo generation of reactive oxygen and nitrogen species (ROS and RNS, respectively), activation and nuclear translocation of c-Jun N-terminal kinase, followed by massive apoptosis in all bystander cells adjacent to the primarily injured ECs. The oxidative insult propagated through GJIC for many hours, over hundreds of microns from the primary photogeneration site. This wave is shown to be limited by intracellular ROS scavenging, chemical GJIC inhibition or genetic manipulation of connexin 43 (a key component of GJIC). Conclusion/Significance: Localized oxidative insults propagate through GJIC between ECs, while stimulating de-novo generation of ROS and RNS in bystander cells, thereby driving the insult's expansion.

Self-assembly in aqueous medium is of primary importance and widely employs hydrophobic interactions. Yet, unlike directional hydrogen bonds, hydrophobic interactions lack directionality, making difficult rational self-assembly design. Directional hydrophobic motif would significantly enhance rational design in aqueous self-assembly, yet general approaches to such interactions are currently lacking. Here, we show that pairwise directional hydrophobic/π- stacking interactions can be designed using well-defined sterics and supramolecular multivalency. Our system utilizes a hexasubstituted benzene scaffold decorated with 3 (compound 1) or 6 (compound 2) amphiphilc perylene diimides. It imposes a pairwise self-assembly mode, leading to well-defined supramolecular polymers in aqueous medium. the assemblies were characterized using cryogenic electron microscopy, small-angle X-ray scattering, optical spectroscopy, and EPR. Supramolecular polymerization studies in the case of 2 revealed association constants in 10 ⁸ M ^-1 range, and significant enthalpic contribution to the polymerization free energy. The pairwise PDI motif enables exciton confinement and localized emission in the polymers based on 1 and 2's unique photonic behavior, untypical of the extended π-stacked systems. Directional pairwise hydrophobic interactions introduce a novel strategy for rational design of noncovalent assemblies in aqueous medium, and bring about a unique photofunction.

The energetics and dynamics of multiply excited states in single material colloidal quantum dots have already been shown to exhibit universal trends. Here we attempt to identify similar trends in exciton-exciton interactions within compound colloidal quantum dots. For this end, we thoroughly review previously available data and also present experimental data on several newly synthesized systems, focusing on core/shell nanocrystals with a type-II band alignment. A universal condition for the transition from binding to repulsion of the biexciton (type-I-type-II transition) is established in terms of the change in the exciton radiative lifetime. A scaling rule is also presented for the magnitude of exciton-exciton repulsion. In contrast, we do not identify a clear universal scaling of the non-radiative Auger recombination lifetime of the biexciton state. Finally, a perspective on future applications of engineered multiexcitonic states is presented.

Photosynthetic organisms utilize interacting pairs of chlorophylls and bacteriochlorophylls as excitation energy donors and acceptors in light harvesting complexes, as photosensitizers of charge separation in reaction centers, and maybe as photoprotective quenching centers that dissipate excess excitation energy under high light intensities. To better understand how the pigments local environment and spatial organization within the protein tune its ground- and excited-state properties to perform different functions, we prepared and characterized the simplest possible system of interacting bacteriochlorophylls within a protein scaffold. Using HP7, a high-affinity heme-binding protein of the HP class of de novo designed four-helix bundles, we incorporated 13²-OH-zinc-bacteriochlorophyllide-a (ZnBChlide), a water-soluble bacteriochlorophyll derivative, into specific binding sites within the four-helix bundle protein core. We capitalized on the rich and informative optical spectrum of ZnBChlide to rigorously characterize its complexes with HP7 and two variants, in which a single heme-binding site is eliminated by replacing histidine residues at positions 7 or 42 by phenylalanine. Surprisingly, we found the ZnBChlide binding capacity of HP7 and its variants to be higher than for heme: up to three ZnBChlide pigments bind per HP7, or two per each single histidine variant. The formation of dimers within HP7 results in dramatic quenching of ZnBChlide fluorescence, reducing its quantum yield by about 80%, and the singlet excited-state lifetime by 2 orders of magnitudes compared to the monomer. Thus, HP7 and its variants are the first examples of a simple protein environment that can isolate a self-quenching pair of photosynthetic pigments in pure form. Unlike its complicated natural analogues, this system can be constructed from the ground up, starting with the simplest functional element, increasing the complexity as needed.

We experimentally investigate carrier multiplication (CM) in type II CdTe/CdSe quantum dot (QD) heterostructures by the means of a simple and robust subnanosecond transient photoluminescence spectroscopy setup. Experimental conditions were set to minimize the blurring of the CM signature by extraneous effects. The extracted photon energy threshold for CM is consistent with previous studies in CdSe and CdTe QDs (around 2.65 times the type II energy band gap) and we can infer an upper bound to CM yield. This study indicates that, while CM is probably present in type II QD heterostructures below the CM threshold for each constituent separately, it exhibits only a modest yield.

We report on the synthesis of organic dye-metal nanoparticle hybrids from two thiol-derivatized perylenediimide (PDI) ligands and 1.5 nm gold nanoparticles. The hybrids form spherical nanostructures when cast from 40% methanol/chloroform solution and toluene. The spherical aggregates are in the size range 50-230 nm in 40% MeOH/CHCl₃ mixture and 100-400 nm in toluene solution, as evidenced by transmission electron microscopy (TEM). Scanning electron microscopy (SEM) measurements show that these spherical aggregates are vesicles with a hollow interior. The π-π interactions of the perylenediimides are the predominant driving force leading to the aggregation of the hybrids, whereby the sizes of the nanospheres can be regulated via the PDI linker moiety and solvent choice. Femtosecond transient absorption studies of the hybrids reveal complex photophysical behavior involving electron transfer from the gold nanoparticles to the PDI moieties. This study shows that the formation of well-defined hybrid nanostructures as well as tuning their sizes can be achieved through employing a combination of the capping ligand choice and regulating the solvophobic interactions between the ligands.

(Chemical Equation Presented) Four from one: Nanoscale ribbons, tubes, vesicles, and platelets can be formed from the self-assembly of a single covalent unit, which is based on an amphiphilic perylene diimide functionalized with a terpyridine ligand (see picture). The assembly diversity arises from the encoding of multiple inputs through hydrophobic interactions and metal coordination.

Design of an extensive supramolecular three-dimensional network that is both robust and adaptive represents a significant challenge. The molecular system PP2b based on a perylene diimide chromophore (PDI) decorated with polyethylene glycol groups self-assembles in aqueous media into extended supramolecular fibers that form a robust three-dimensional network resulting in gelation. The self-assembled systems were characterized by cryo-TEM, cryo-SEM, and rheological measurements. The gel possesses exceptional robustness and multiple stimuli-responsiveness. Reversible charging of PP2b allows for switching between the gel state and fluid solution that is accompanied by switching on and off the material's birefringence. Temperature triggered deswelling of the gel leads to the (reversible) expulsion of a large fraction of the aqueous solvent. The dual sensibility toward chemical reduction and temperature with a distinct and interrelated response to each of these stimuli is pertinent to applications in the area of adaptive functional materials. The gel also shows strong absorption of visible light and good exciton mobility (elucidated using femtosecond transient absorption), representing an advantageous light harvesting system.

Light-induced radical generation is the hallmark of fundamental processes and many applications including photosynthesis and photodynamic therapy (PDT). In this manuscript, we present two novel observations made upon monitoring light-induced generation of reactive oxygen species (ROS) in aqueous solutions by WST11, a water-soluble derivative of the photosynthetic pigment Bacteriochlorophyll a (Bchl). Using a host of complementary experimental techniques including time-resolved spectroscopy at the subpicosecond to the millisecond range, ESR spectroscopy, electrochemistry, spectroelectrochemistry, oximetry, and protein mass spectroscopy, we first show that in aqueous solutions WST11 generates only superoxide (O(2)(-center dot)) and hydroxyl (OH(center dot)) radicals with no detectable traces of singlet oxygen. Second, we show that WST11 makes a noncovalent complex with human serum albumin (HSA) and that this complex functions as a photocatalytic oxidoreductase at biologically relevant concentrations enabling approximately 15 cycles of electron transfer from the associated HSA protein to molecular oxygen in the solution. These findings rule out the paradigm that porphyrin and chlorophyll based PDT is mainly mediated by formation of singlet oxygen, particularly in vascular targeted photodynamic therapy (VTP) with sensitizers that undergo photoactivation during circulation in the plasma, like [Pd]-Bacteriopheophorbide (WST09, Tookad). At the same time, our findings open the way for new design paradigms of novel sensitizers, since O2(-center dot) and OH(center dot) radicals are well-recognized precursors of important pathophysiological processes that can be activated for achieving tumor eradication. Moreover, the finding that promiscuous protein scaffolds become sinks for holes and electrons when holding light-activated pigments provides a new insight to the evolution and action mechanism of natural light activated oxidoreductases (such as photosynthetic reaction cente

Self-assembling systems, whose structure and function can be reversibly controlled in situ are of primary importance for creating multifunctional supramolecular arrays and mimicking the complexity of natural systems. Herein we report on photofunctional fibers self-assembled from perylene diimide cromophores, in which interactions between aromatic monomers can be attenuated through their reduction to anionic species that causes fiber fission. Oxidation with air restores the fibers. The sequence represents reversible supramolecular depolymerization-polymerization in situ and is accompanied by a reversible switching of photofunction.

In this study, we show how light can be absorbed by the body of a living rat due to an injected pigment circulating in the blood stream. This process is then physiologically translated in the tissue into a chemical signature that can be perceived as an image by magnetic resonance imaging (MRI). We previously reported that illumination of an injected photosynthetic bacteriochlorophyll-derived pigment leads to a generation of reactive oxygen species, upon oxygen consumption in the blood stream. Consequently, paramagnetic deoxyhemoglobin accumulating in the illuminated area induces changes in image contrast, detectable by a Blood Oxygen Level Dependent (BOLD)-MRI protocol, termed photosensitized (ps)MRI. Here, we show that laser beam pulses synchronously trigger BOLD-contrast transients in the tissue, allowing representation of the luminous spatiotemporal profile, as a contrast map, on the MR monitor. Regions with enhanced BOLD-contrast (7-61 fold) were deduced as illuminated, and were found to overlap with the anatomical location of the incident light. Thus, we conclude that luminous information can be captured and translated by typical oxygen exchange processes in the blood of ordinary tissues, and made visible by psMRI (Fig. 1). This process represents a new channel for communicating environmental light into the body in certain analogy to light absorption by visual pigments in the retina where image perception takes place in the central nervous system. Potential applications of this finding may include: non-invasive intra-operative light guidance and follow-up of photodynamic interventions, determination of light diffusion in opaque tissues for optical imaging and possible assistance to the blind.