Resonance Raman Imaging Research Articles

Label-free tracking of cytochrome C oxidation state in live cells by resonance Raman imaging.

Free interconversion of cytochrome C (CytC) between native ferrous (Cyt-FeII) and oxidized ferric (CytC-FeIII) states is necessary to maintain the respiratory function of mitochondria. Disturbances in CytC-FeIII to total CytC ratio may indicate mitochondrial dysfunction and apoptosis. Thus, tracking CytC oxidation state delivers important information about cellular physiology. In this work, we propose a novel methodology based on resonance Raman (rR) imaging optimized uniquely to track and qualitatively analyze the transition of Cyt-FeII to CytC-FeIII within live cells without affecting their morphology. None of the commonly used excitation lines allows such clear-cut differentiation, contrary to the 405 nm applied in this work. The presented methodology provides a novel pathway in the label-free detection of ferrous and ferric heme proteins.

Establishment of a Method for the Introduction of Poorly Water-Soluble Drugs in Cells and Evaluation of Intracellular Concentration Distribution Using Resonance Raman Imaging.

Label-free measurement is essential to understand the metabolism of drug molecules introduced into cells. Raman imaging is a powerful method to investigate intracellular drug molecules because it provides in situ label-free observation of introduced molecules. In this study, we propose that Raman imaging can be used not only to observe the intracellular distribution of drug molecules but also to quantitatively visualize the concentration distribution reflecting each organelle in a single living cell using the Raman band of extracellular water as an intensity standard. We dissolved poorly water-soluble all-trans-retinoic acid (ATRA) in water using a cytocompatible amphiphilic phospholipid polymer, poly[2-methacryloyloxyethyl phosphorylcholine-co-n-butyl methacrylate] (PMB) as a solubilizing reagent, introduced it into cells, and obtained the intracellular concentration distribution of ATRA. ATRA was concentrated in the cells and mainly localized to mitochondria and lipid droplets, interacting strongly with mitochondria and weakly with lipid droplets. Poorly water-soluble β-carotene was also introduced into cells using PMB but was not concentrated intracellularly, indicating that β-carotene does not interact specifically with intracellular molecules. We established a protocol for the solubilization and intracellular uptake of poorly water-soluble molecules using PMB and obtaining their concentration distribution using Raman microscopy.

Fluorescence-free bis(dithiolene)nickel dyes for surface-enhanced resonance Raman imaging in the second near-infrared window

Second near-infrared window (NIR-II, 1000–1700 nm) imaging is one of the foremost optical imaging techniques. However, surface-enhanced Raman scattering (SERS)-based research in this optical region remains in its infancy, mainly because of a lack of suitable NIR-II Raman reporters. Herein, we report the first example of a nickel dithiolene complex as a NIR-II resonance Raman reporter with intense long wavelength absorption (ε = 9.58 × 104 m−1 cm−1 at 1007 nm), fluorescence-free features and ultrahigh affinity to noble metal surfaces with its eight sulfur atoms. Surface-enhanced resonance Raman scattering nanoprobes constructed with such reporters enable high contrast and highly photostable lymph node imaging far superior to that possible with existing NIR-I and NIR-II SERS nanoprobes. The developed NIR-II nanoprobes allow deep optical penetration (8 mm) as well as in vivo SERS detection of deep-seated microtumors in mice.

Redox Imbalance and Biochemical Changes in Cancer by Probing Redox-Sensitive Mitochondrial Cytochromes in Label-Free Visible Resonance Raman Imaging.

Simple SummaryGliomas comprise around 30% of human brain tumors, while invasive ductal carcinoma (IDC) comprises around 80% of human breast cancers. The aim of our study was to show that cancerogenesis affects the redox status of mitochondrial cytochromes, which can be tracked by using Raman spectroscopy and imaging. Our results confirmed that human breast cancer and brain tumor demonstrate a redox imbalance compared to normal tissues. We have shown the correlation between the intensity of cytochromes Raman bands at 750, 1126, 1337 and 1584 cm−1 and malignancy grade for brain and breast cancers.To monitor redox state changes and biological mechanisms occurring in mitochondrial cytochromes in cancers improving methods are required. We used Raman spectroscopy and Raman imaging to monitor changes in the redox state of the mitochondrial cytochromes in ex vivo human brain and breast tissues at 532 nm, 633 nm, 785 nm. We identified the oncogenic processes that characterize human infiltrating ductal carcinoma (IDC) and human brain tumors: gliomas; astrocytoma and medulloblastoma based on the quantification of cytochrome redox status by exploiting the resonance-enhancement effect of Raman scattering. We visualized localization of cytochromes by Raman imaging in the breast and brain tissues and analyzed cytochrome c vibrations at 750, 1126, 1337 and 1584 cm−1 as a function of malignancy grade. We found that the concentration of reduced cytochrome c becomes abnormally high in human brain tumors and breast cancers and correlates with the grade of cancer. We showed that Raman imaging provides additional insight into the biology of astrocytomas and breast ductal invasive cancer, which can be used for noninvasive grading, differential diagnosis.

Single-Molecule Resonance Raman Spectroscopy

Near-field Raman spectroscopy has attracted significant attention as a tool for performing chemical analysis with submolecular spatial resolution. The mechanism in atomic and molecular scale Raman spectroscopy, however, is not well understood due to the strong perturbation of the molecule during measurements in previous studies. In particular, the resonance effect that greatly enhances Raman scattering has not been clarified on a single molecule scale. Here we demonstrate single-molecule resonance Raman imaging with a scanning tunneling microscope (STM) at submolecular resolution of copper naphthalocyanine molecules. In our experiment, an ultra-thin insulating NaCl film is used as the substrate to keep the molecule in an undisturbed condition for performing a precise analysis. We have succeeded in resonance Raman spectroscopy by tuning the excitation wavelength to the intrinsic electronic transitions of the molecule. This makes it possible to acquire accurate Raman spectra with a short measurement time at single molecule sensitivities. The resonance Raman maps show three different spatial distribution patterns depending on the symmetry of the vibration modes. These results are explained by considering the interaction between the electromagnetic field of the plasmon in the tip-substrate gap and the intrinsic molecular symmetry, thereby a selection rule for the plasmon-enhanced resonant Raman effect is established.

Imaging lutein and zeaxanthin in the human retina with confocal resonance Raman microscopy

Lutein and zeaxanthin are xanthophyll carotenoids that are highly concentrated in the human macula, where they protect the eye from oxidative damage and improve visual performance. Distinguishing lutein from zeaxanthin in images of the human retina in vivo or in donor eye tissues has been challenging because no available technology has been able to reliably differentiate between these two carotenoids, which differ only in the position of one C = C bond. Here, we report the differential distributions of lutein and zeaxanthin in human donor retinas mapped with confocal resonance Raman microscopy. Zeaxanthin is highly concentrated in the fovea, extending from the inner to the outer limiting membranes, with especially high concentrations in the outer plexiform layer, while lutein is much more diffuse at relatively lower concentration. Our results imply that zeaxanthin may play a more important role than lutein in human macular health and disease.

Microsphere packages of carotenoids: intact sea urchin eggs tracked by Raman spectroscopy tools.

Although natural exposure to ambient UV radiation in oligotrophic seawater at small depths can reach the levels responsible for cellular damage, the sea urchin Paracentrotus lividus is frequently in such sites, particularly on the southern Adriatic Sea shore. Spawning their eggs and spending their early life stage in rocky shores at depths of 0.5-2 m are the results of their successful adaptation strategies, although adults may dwell at greater depths. Surprisingly, there is a paucity of reports regarding the carotenoid content in sea urchin eggs. Beyond their important role in photoprotection against high UV exposure, cell division and early development, the content and distribution of carotenoids contribute to the successful survival of sea urchins and also determine the color of their gonads (roe), which is of commercial importance as a delicacy. Herein, for the first time, we have described the carotenoid content and distribution in intact, freshly released eggs of P. lividus species, non-destructively employing resonance Raman spectroscopy and imaging; near-infrared Raman spectroscopy revealed additional molecular carotenoid content. Echinenone and β-carotene resonance Raman signals were the most intense, and they were identified as the principal carotenoids that are preferentially accumulated in eggs rather than in gonads. Raman imaging in confocal mode revealed the uniform distribution of the carotenoid signal over the whole eggs, while the distribution of proteins appeared spotted. Egg carotenoids generally maintained their identity after 2 months of dry storage, with slight signs of C[double bond, length as m-dash]C bond oxidation. The potential utilization of P. lividus sea urchin eggs as valuable microsphere packages of native carotenoids is discussed.

Resonance Raman Spectroscopy and Imaging of Franck-Condon Vibrational Activity and Morphology in Conjugated Polymers for Solar Cells.

Vibrational reorganization influences photophysical outcomes in conjugated polymers used as active materials for optoelectronic devices. Excited state geometric rearrangements typically involve many displaced vibrations, yet most materials design schemes rely solely on pure electronic models with limited predictive capability. Although the coupling of vibrational motions to electronic processes occurs over a broad range of time scales, resolving structural displacements immediately following photon absorption can be particularly insightful for understanding the intrinsic stabilities of excited states. These Franck-Condon vibrational relaxation processes occur on time scales of <1 ps in polymers and mainly involve high-frequency skeletal motions. Establishing correlations between Franck-Condon vibrational reorganization and steady-state material properties could generate new avenues for informing materials design, which is especially important in the fast-paced field of organic photovoltaics (OPV) where seemingly elegant strategies often fail but molecular-level insights are usually lacking. The goal of this Account is to highlight relationships between molecular structure, packing, and vibrational reorganization in OPV systems, such as blends of conjugated polymers with fullerenes. Resonance Raman spectroscopy (RRS) is a sensitive probe of Franck-Condon activity in OPV materials, and signals are bolstered by large resonance enhancements and low backgrounds from quantitative fluorescence quenching. Our group has undertaken extensive RRS investigations of heterogeneous OPV materials in functioning device environments to uncover new insights of the multidimensional excited state potential energy landscape and fluctuations with local morphology. Time-dependent quantum mechanical approaches facilitate this effort by providing an intuitive theoretical framework to access dynamical perspectives of Raman transitions. Moreover, dynamics regimes of Franck-Condon excited state structural evolution can be selected simply by tuning excitation energies. This excitation detuning approach also reveals structurally and electronically distinct conformers with unique Franck-Condon signatures typically concealed under inhomogeneously broadened absorption line shapes. Interestingly, long and rich progressions of overtone and combination transitions-rare for large molecules with multiple displaced modes-are frequently resolved that exhibit strong sensitivity to the local chromophore environment. These harmonic features encode useful dynamics information by serving as internal "clocks" of Franck-Condon vibrational activity in addition to enabling quantitative estimates of mode-specific displacements. RRS attributes may be further exploited to perform noninvasive imaging of functioning OPV devices in concert with variable frequency electrical imaging probes. This approach generates direct spatial correlations between morphology-dependent Franck-Condon vibrational activity and material performance metrics (e.g., photocurrent generation) on submicrometer size scales.

Fluorescence imaging of stained red blood cells with simultaneous resonance Raman photostability analysis.

Optical spectroscopic imaging of biological systems has important applications in medical diagnosis, biochemistry, and image-guided surgery. Vibrational spectroscopy, such as Raman scattering, provides high chemical selectivity but is limited by weak signals and a large fluorescence background. Fluorescence imaging is often used by introducing specific dyes in biological systems to label different system parts and to increase the image contrast. However, the extrinsic fluorescence of the staining molecules often masks the intrinsic vibrational signals of biomolecules, which could also be simultaneously detected using the same excitation laser source. Therefore, fluorescence staining is often accompanied by the loss of other important complimentary information. For example, the high laser power often used for the rapid, high-quality imaging could lead to photo-induced suppression or bleaching of the fluorescence and Raman signals resulting in sample photodamage. Therefore, simultaneous imaging and photodamage analysis need to be performed in a controlled bioimaging experiment. Here we perform simultaneous spectroscopic bioimaging and photostability analysis of rhodamine 6G (R6G) stained red blood cells (RBCs) using both fluorescence and resonance Raman imaging in a single 532 nm laser excitation experiment. We develop a corresponding data processing algorithm which allows separation of the two spectroscopic signals. We control the relative intensity of the R6G and RBC signals by varying the excitation laser power and simultaneously monitor the photostability of RBCs. We observe no significant photodamage of RBCs through the absence of changes in the relative Raman peak intensities. Conversely, the R6G molecules show bleaching with the suppression of both the fluorescence and resonance Raman signals. Our approach may be generalized to other types of stained cells with the appropriate selection of fluorescent dyes and excitation sources.

Impact of Citric Acid on the Impregnation of CoMoP/γ-Al2O3 Catalysts: Time and Spatially Resolved MRI and Raman Imaging Study

An in situ characterization methodology based on the magnetic resonance imaging (MRI) and Raman imaging is applied to investigate the effect of citric acid in the impregnation step of molybdenum catalysts promoted with cobalt in the presence of phosphorus and supported on γ-alumina. MRI provides temporal and spatial information of the transport of species of the impregnation solutions within the porosity with a spatial resolution of 39 × 39 µm. Raman imaging gives information about the chemical nature of the species deposited on the support with a spatial resolution of 16.2 × 16.2 µm. The effect of citric acid strongly depends on the ratio between the additive and molybdenum used in the impregnation solution. For a ratio of 0.2, at the end of impregnation, molybdenum ions are in an egg-yolk distribution either in polymeric or monomeric form. For a ratio of 0.7, cobalt ions can be either in the form of aqua complexes in an egg-yolk distribution or in the form of H2PMo11CoO405− heteropolyanion, which can improve the proximity between molybdenum and cobalt ions. This work gives new insights concerning the role of citric acid on the metal distribution profiles obtained at the end of impregnation, which can be used to control the final active phase distribution.

Highly Selective Mitochondrial Targeting by a Ruthenium(II) Peptide Conjugate: Imaging and Photoinduced Damage of Mitochondrial DNA

AbstractMitochondrial DNA (mtDNA) plays a crucial but incompletely understood role in cellular biochemistry and etiology of numerous disease states. Thus, there is an urgent need for targeted probes that can dynamically respond to changes to mtDNA such as copy number in live cells, but it is difficult to permeate the mitochondrial membrane of the living cell. Now, a ruthenium(II) light‐switching probe targeted by peptide vectorization selectively to mitochondrial nucleoids is presented. Evidence for DNA binding by the probe in live cells is derived from confocal fluorescence microscopy, resonance Raman, and luminescence lifetime imaging. While viable under imaging conditions, specific staining of mitochondrial DNA permitted efficient and selective photoinduced toxicity on a cell‐by‐cell basis under higher excitation intensities. This powerful combination of imaging and photocytotoxicity is an important step towards realizing phototheranostic application of such RuII probes.

Highly Selective Mitochondrial Targeting by a Ruthenium(II) Peptide Conjugate: Imaging and Photoinduced Damage of Mitochondrial DNA.

Mitochondrial DNA (mtDNA) plays a crucial but incompletely understood role in cellular biochemistry and etiology of numerous disease states. Thus, there is an urgent need for targeted probes that can dynamically respond to changes to mtDNA such as copy number in live cells, but it is difficult to permeate the mitochondrial membrane of the living cell. Now, a ruthenium(II) light-switching probe targeted by peptide vectorization selectively to mitochondrial nucleoids is presented. Evidence for DNA binding by the probe in live cells is derived from confocal fluorescence microscopy, resonance Raman, and luminescence lifetime imaging. While viable under imaging conditions, specific staining of mitochondrial DNA permitted efficient and selective photoinduced toxicity on a cell-by-cell basis under higher excitation intensities. This powerful combination of imaging and photocytotoxicity is an important step towards realizing phototheranostic application of such RuII probes.

Targeting Photoinduced DNA Destruction by Ru(II) Tetraazaphenanthrene in Live Cells by Signal Peptide.

Exploiting NF-κB transcription factor peptide conjugation, a Ru(II)-bis-tap complex (tap = 1,4,5,8-tetraazaphenanthrene) was targeted specifically to the nuclei of live HeLa and CHO cells for the first time. DNA binding of the complex within the nucleus of live cells was evident from gradual extinction of the metal complex luminescence after it had crossed the nuclear envelope, attributed to guanine quenching of the ruthenium emission via photoinduced electron transfer. Resonance Raman imaging confirmed that the complex remained in the nucleus after emission is extinguished. In the dark and under imaging conditions the cells remain viable, but efficient cellular destruction was induced with precise spatiotemporal control by applying higher irradiation intensities to selected cells. Solution studies indicate that the peptide conjugated complex associates strongly with calf thymus DNA ex-cellulo and gel electrophoresis confirmed that the peptide conjugate is capable of singlet oxygen independent photodamage to plasmid DNA. This indicates that the observed efficient cellular destruction likely operates via direct DNA oxidation by photoinduced electron transfer between guanine and the precision targeted Ru(II)-tap probe. The discrete targeting of polyazaaromatic complexes to the cell nucleus and confirmation that they are photocytotoxic after nuclear delivery is an important step toward their application in cellular phototherapy.

Discovery of carotenoid red-shift in endolithic cyanobacteria from the Atacama Desert

The biochemical responses of rock-inhabiting cyanobacteria towards native environmental stresses were observed in vivo in one of the Earth’s most challenging extreme climatic environments. The cryptoendolithic cyanobacterial colonization, dominated by Chroococcidiopsis sp., was studied in an ignimbrite at a high altitude volcanic area in the Atacama Desert, Chile. Change in the carotenoid composition (red-shift) within a transect through the cyanobacteria dominant microbial community (average thickness ~1 mm) was unambiguously revealed in their natural endolithic microhabitat. The amount of red shifted carotenoid, observed for the first time in a natural microbial ecosystem, is depth dependent, and increased with increasing proximity to the rock surface, as proven by resonance Raman imaging and point resonance Raman profiling. It is attributed to a light-dependent change in carotenoid conjugation, associated with the light-adaptation strategy of cyanobacteria. A hypothesis is proposed for the possible role of an orange carotenoid protein (OCP) mediated non-photochemical quenching (NPQ) mechanism that influences the observed spectral behavior. Simultaneously, information about the distribution of scytonemin and phycobiliproteins was obtained. Scytonemin was detected in the uppermost cyanobacteria aggregates. A reverse signal intensity gradient of phycobiliproteins was registered, increasing with deeper positions as a response of the cyanobacterial light harvesting complex to low-light conditions.

Application of Magnetic Resonance Imaging and Raman Imaging to study the impact of phosphorus in impregnation of hydrotreatment catalysts

Magnetic Resonance Imaging is applied, for the first time, to monitor in-situ the impregnation step of hydrotreatment catalysts with an aqueous solution composed simultaneously of molybdenum, cobalt and phosphorus. This technique provides information about spatial distribution of all metal precursors inside the catalyst pellet with a significantly improved spatial resolution of 39μm×39μm compared to the literature. Streamline Raman Imaging yields complementary information about molybdenum speciation at equilibrium. For a molybdenum based catalyst, a slow transport of molybdenum complexes through γ-alumina is observed by Magnetic Resonance Imaging, which suggests a strong interaction between metal precursor and the support. Presence of polymeric molybdenum ions is found near the edges as well as the formation of an Anderson-type heteropolyanion. In the presence of phosphorus, interaction between molybdenum and γ-alumina is weaken since a preferential adsorption to phopshorus is observed. This phenomenon results in an increase in the transport rate of molybdenum complexes. When phosphorus is added to a molybdenum based catalysts promoted by cobalt, preferential interactions between metal promotor and the support are observed rather than with molybdenum species. This innovative Magnetic Resonance Imaging −Streamline Raman Imaging methodology allows a better control of the deposition process during impregnation and to identify the descriptors that most influence this preparation step. Finally, this methodology can be applied to other type of supported catalysts.

Unveiling the Aggregation of Lycopene in Vitro and in Vivo: UV-Vis, Resonance Raman, and Raman Imaging Studies.

The present study investigates the structure of lycopene aggregates both in vitro and in vivo using ultraviolet-visible (UV-vis) and Raman spectroscopies. The electronic absorption bands of the J- and H-aggregates in vitro shift to lower and higher energies, respectively, compared to that of the lycopene monomer. Along with these results, the frequencies of the ν1 Raman bands were shifted to lower and higher frequencies, respectively. By plotting the frequencies of the ν1 Raman band against the S0 → S2 transition energy, a linear relationship between the data set with different aggregation conformations can be obtained. Therefore, the band positions depending on the different conformations can be explained based on the idea that the effective conjugated C═C chain lengths within lycopene molecules are different due to the environmental effect (site-shift effect) caused by the aggregation conformation. Applying this knowledge to the in vivo measurement of a tomato fruit sample, the relationship between the aggregation conformation of lycopene and the spectral patterns observed in the UV-vis as well as Raman spectra in different parts of tomato fruits was discussed in detail. The results showed that the concentration of lycopene (particularly that of the J-aggregate) specifically increased, whereas that of chlorophyll decreased, with ripening. Furthermore, Raman imaging indicated that lycopene with different aggregate conformations was distributed inhomogeneously, even within one sample. The layer formation in tomato tissues with high concentrations of J- and H-aggregates was successfully visualized. In this manner, the presence of lycopene distributions with different aggregate conformations was unveiled in vivo.

Ionizing radiation exposure reveals instability of purified domains in polymer/fullerene solar cells

Solar cells based on conjugated polymer and fullerene blends exhibit morphology-dependent stability towards ionizing radiation exposure. Blend thin film solar cells of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C60-butyric acid methyl ester (PCBM) were irradiated up to 1Mrad with X-rays (10–60KeV) and investigated with resonance Raman and photocurrent/photovoltage modulation spectroscopy and imaging. Bulk blend morphological characteristics were pre-selected by controlling film processing conditions (i.e., solvent and thermal annealing). As-cast P3HT/PCBM devices with a large fraction of a well-mixed phase display improvements in performance upon application of post-irradiation annealing treatments. On the other hand, devices annealed prior to irradiation showed irreversible performance degradation that could not be recovered following annealing treatments after exposure. Resonance Raman spectroscopy and imaging following irradiation up to 1Mrad reveals virtually no changes in the P3HT structural and packing integrity for both types of devices. Intensity modulated photocurrent/photovoltage spectroscopy measurements indicate that radiation-induced performance degradation is concentrated in the vicinity of device contacts, evident from large alterations in charge extraction kinetics. We propose that charge transfer from the aluminum cathode leads to formation of a space charge zone which becomes more pronounced in devices with greater phase purification (i.e., larger PCBM crystallites in the vicinity of the cathode). Importantly, well-mixed P3HT/PCBM regions, which are more plentiful in as-cast devices, appear to mitigate radiation-induced performance degradation since application of post-irradiation annealing treatments restores device performances to pristine levels. Our results suggest that greater morphological stability, such as polymer/fullerene systems forming co-crystals, is key for suppressing performance losses from ionizing radiation exposure.

Detection of herbicide effects on pigment composition and PSII photochemistry in Helianthus annuus by Raman spectroscopy and chlorophyll a fluorescence

The effects of herbicides from three mode-of-action groups – inhibitors of protoporphyrinogen oxidase (carfentrazone-ethyl), inhibitors of carotenoid biosynthesis (mesotrione, clomazone, and diflufenican), and inhibitors of acetolactate synthase (amidosulfuron) – were studied in sunflower plants (Helianthus annuus). Raman spectroscopy, chlorophyll fluorescence (ChlF) imaging, and UV screening of ChlF were combined to evaluate changes in pigment composition, photosystem II (PSII) photochemistry, and non-photochemical quenching in plant leaves 6d after herbicide application. The Raman signals of phenolic compounds, carotenoids, and chlorophyll were evaluated and differences in their intensity ratios were observed. Strongly augmented relative content of phenolic compounds was observed in the case of amidosulfuron-treated plants, with a simultaneous decrease in the chlorophyll/carotenoid intensity ratio. The results were confirmed by in vivo measurement of flavonols using UV screening of ChlF. Herbicides from the group of carotenoid biosynthesis inhibitors significantly decreased both the maximum quantum efficiency of PSII and non-photochemical quenching as determined by ChlF. Resonance Raman imaging (mapping) data with high resolution (150,000–200,000 spectra) are presented, showing the distribution of carotenoids in H. annuus leaves treated by two of the herbicides acting as inhibitors of carotenoid biosynthesis (clomazone or diflufenican). Clear signs were observed that the treatment induced carotenoid depletion within sunflower leaves. The depletion spatial pattern registered differed depending on the type of herbicide applied.

Mapping of the electronic band gap along the axis of a single InAs/InSbxAs1-x heterostructured nanowire.

Herein, we report the graded electronic band gap along the axis of individual heterostructured WZ-ZB InAs/InSb0.12As0.88 nanowires. Resonance Raman imaging has been exploited to map the axial variation in the second excitation gap energy (E1) at the high symmetry point (L point) of the Brillouin zone. We relate the origin of the observed evolution of the gap energy to the fine tuning of the alloy composition from the tip towards the interface of the nanowire. The electronic band structures of InAs, InSb and InSbxAs1-x alloy systems at x = 0.125, 0.25, 0.50, 0.75 and 0.875, using the all electron density functional theory code Wien2k, are reported. The measured band gap along the axis of the InAs/InSb0.12As0.88 nanowire is correlated with the calculated gap energy at the A point and the L point of the Brillouin zone for InAs and InSb0.125As0.875, respectively. We draw a one-to-one correspondence between the variation of the E1 gap and the fundamental E0 gap in the calculated electronic band structure and propose the graded fundamental gap energy across the axis of the nanowire.

Resonance Raman imaging as a tool to assess the atmospheric pollution level: carotenoids in Lecanoraceae lichens as bioindicators.

Raman spectroscopy differentiation of carotenoids has traditionally been based on the ν 1 position (C = C stretching vibrations in the polyene chain) in the 1500-1600 cm(-1) range, using a 785 nm excitation laser. However, when the number of conjugated double bonds is similar, as in the cases of zeaxanthin and β-carotene, this distinction is still ambiguous due to the closeness of the Raman bands. This work shows the Raman results, obtained in resonance conditions using a 514 mm laser, on Lecanora campestris and Lecanora atra species, which can be used to differentiate and consequently characterize carotenoids. The presence of the carotenoid found in Lecanoraceae lichens has been demonstrated to depend on the atmospheric pollution level of the environment they inhabit. Astaxanthin, a superb antioxidant, appears as the principal xanthophyll in highly polluted sites, usually together with the UV screening pigment scytonemin; zeaxanthin is the major carotenoid in medium polluted environments, while β-carotene is the major carotenoid in cleaner environments. Based on these observations, an indirect classification of the stress suffered in a given environment can be assessed by simply analysing the carotenoid content in the Lecanoraceae lichens by using resonance Raman imaging.