Plasmonic Effects of Dual-Metal Nanoparticle Layers for High-Performance Quantum Dot Solar Cells

To improve quantum dot solar cell performance, it is crucial to make efficient use of the available incident sunlight to ensure that the absorption is maximized. The ability of metal nanoparticles to concentrate incident sunlight via plasmon resonance can enhance the overall absorption of photovoltaic cells due to the strong confinement that results from near-field coupling or far-field scattering plasmonic effects. Therefore, to simultaneously and synergistically utilize both plasmonic effects, the placement of different plasmonic nanostructures at the appropriate locations in the device structure is also critical. Here, we introduce two different plasmonic nanoparticles, Au and Ag, to a colloidal PbS quantum dot heterojunction at the top and bottom interface of the electrodes for further improvement of the absorption in the visible and near-infrared spectral regions. The Ag nanoparticles exhibit strong scattering whereas the Au nanoparticles exhibit an intense optical effect in the wavelength region where the absorption of light of the PbS quantum dot is strongest. It is found that these dual-plasmon layers provide significantly improved short-circuit current and power conversion efficiency without any form of trade-off in terms of the fill factor and open-circuit voltage, which may result from the indirect contact between the plasmonic nanoparticles and colloidal quantum dot films.

     

Characterization of lignin in situ by photoacoustic spectroscopy

Photoacoustic spectroscopy is a recently developed nondestructive analytical technique that provides ultraviolet, visible, and infrared absorption spectra from intensely light scattering, solid, and/or optically opaque materials not suitable for conventional spectrophotometric analysis. In wood and other lignocellulosics, the principal ultraviolet absorption bands, in the absence of photosynthetic pigments, arise from the aromatic lignin component of the cell walls. Photoacoustic spectra of extracted lignin fragments (milled wood lignin) and synthetic lignin-like polymers contain a single major absorption band at 280 nanometers with an absorption tail extending beyond 400 nanometers. Photoacoustic spectra of pine, maple, and oak lignin in situ contain a broad primary absorption band at 300 nanometers and a longer wavelength shoulder around 370 nanometers. Wheat lignin in situ, on the other hand, exhibits two principle absorption peaks, at 280 nanometers and 320 nanometers. The presence of absorption bands at wavelengths greater than 300 nanometers in intact lignin could result from (a) interacting, nonconjugated chromophores, or (b) the presence of more highly conjugated structural components formed as the result of oxidation of the polymer. Evidence for the latter comes from the observation that, on the outer surface of senescent, field-dried wheat culms (stems), new absorption bands in the 350 to 400 nanometer region predominate. These new bands are less apparent on the outer surface of presenescent wheat culms and are virtually absent on the inner surface of either senescent or presenescent culms, suggesting that the appearance of longer wavelength absorption bands in senescent wheat is the result of accumulated photochemical modifications of the ligin polymer. These studies also demonstrate photoacoustic spectroscopy to be an important new tool for the investigation of insoluble plant components.     

Comparative Theoretical Study of the Optical Properties of Silicon/Gold,Silica/Gold Core/Shell and Gold Spherical Nanoparticles

The scattering and absorption efficiencies of light by individual silicon/gold core/shell spherical nanoparticles in air are analysed theoretically in the framework of Lorenz-Mie formalism. We have addressed the influence of particle-diameter and gold-shell thickness on the scattering and absorption efficiencies of such nano-heterostructures. For comparison, we also considered the famous silica/gold core/shell nanoparticle and pure gold nanoparticle. Our simulation clearly shows that the optical response of the illuminated Si/Au core/shell nanoparticle differs markedly from that of the famous SiO₂/Au heterostructure which in turn does not show a significant difference with that of the pure gold nanoparticle. This difference is clearly evident for shell thickness to outer particle radius ratio of less than 0.5. It manifests itself essentially by the occurrence of a strong and sharp absorption resonance beyond the wavelength of 600 nm where the silica/gold and the pure gold nanoparticles never absorb. The characteristics of this resonance are found to be sensitive to the particle diameter and the shell thickness. In particular, its spectral position can be adjusted over a wide spectral range from the visible to the mid-IR by varying the particle diameter and/or the shell thickness.     

On phytochrome absorption and the phytochrome photoequilibrium in a green leaf: environmental sensitivity and photoequilibrium time

The average, corrected attenuance spectra for both spectral forms of phytochrome in a mature leaf were calculated. Optical masking by chlorophyll together with the detour effect (optical path lengthening effect) due to multiple light scattering led to large changes in both the Qy band shape and wavelength position and the effective intensity of the weak vibrational bands increases. The Pfr/Pr oscillator-strength-ratio between 400-750 nm (0.93 in vitro), becomes 1.63 in a leaf. Thus the dominant absorption form is Pfr. These two values permit calculation of the phytochrome photoequilibrium under conditions of "daylight" illumination both in vitro and in folia. These values are 0.6 and 0.38 respectively. Previous literature estimates for the situation in vitro, based on the 660/730 nm absorption ratio, yielded values close to 0.6. It is demonstrated that this large decrease in the phytochrome photoequilibrium in a leaf has the effect of translating this parameter to a position on the dose (red/far-red light ratio)-response (Pfr/Ptot) plot towards greater sensitivity to changes in the environmental red/far-red ratio. The increased sensitivity factor is almost five-fold for the "daylight" environment and is even greater for the various "shade-light" environments. The approximate time taken to attain photoequilibrium (1/e lifetime) has also been calculated for phytochrome in a leaf in different light environments. For the "daylight" environment the photoequilibration time is approximately 5 s, which increases into the 20-80 s interval under different degrees of "shade light". Thus, despite the strong optical masking by chlorophyll in a mature leaf, the phytochrome photoequilibrium is attained quite rapidly on a physiological time scale.     

High-resolution optical absorption-difference spectra of the triplet state of the primary donor in isolated reaction centers of the photosynthetic bacteria Rhodopseudomonas sphaeroides R-26 and Rhodopseudomonas viridis measured with optically detected magnetic resonance at 1.2 K

We have recorded triplet optical absorption-difference spectra of the reaction center triplet state of isolated reaction centers from Rhodopseudomonas sphaeroides R-26 and Rps. viridis with optical absorption-detected electron spin resonance in zero magnetic field (ADMR) at 1.2 K. This technique is one to two orders of magnitude more sensitive than conventional flash absorption spectroscopy, and consequently allows a much higher spectral resolution. Besides the relatively broad bleachings and appearances found previously (see, e.g., Shuvalov V.A. and Parson W.W. (1981) Biochim. Biophys. Acta 638, 50–59) we have found strong, sharp oscillations in the wavelength regions 790–830 nm (Rps. sphaeroides) and 810–890 nm (Rps. viridis). For Rps. viridis these features are resolved into two band shifts (a blue shift at about 830 nm and a red shift at about 855 nm) and a strong, narrow absorption band at 838 nm. For Rps. sphaeroides R-26 the features are resolved into a red shift at about 810 nm and a strong absorption band at 807 nm. We conclude that the appearance of the absorption bands at 807 and 838 nm, respectively, is due to monomeric bacteriochlorophyll. Apparently, the exciton interaction between the pigments constituting the primary donor is much weaker in the triplet state than in the singlet state, and at low temperature the triplet is localized on one of the bacteriochlorophylls on an optical time scale. The fact that for Rps. sphaeroides the strong band shift and the monomeric band found at 1.2 K are absent at 293 K and very weak at 77 K indicates that these features are strongly temperature dependent. It seems, therefore, premature to ascribe the temperature dependence between 293 and 77 K of the intensity of the triplet absorption-difference spectrum at 810 nm (solely) to a delocalization of the triplet state on one of the accessory bacteriochlorophyll pigments.     

Spectral manifestations of different types of acriflavine binding to DNA in ultraviolet and visible region]

Difference absorption spectra (complex-sum of the initial reagents) are obtained in the visible and longwave UV region for the system of actiflavine and DNA in a number of cases differing in initial and final degrees of DNA filling by the dye, in particular separately for two types of dye binding to DNA. For these binding types conventional absorption spectra are calculated. In the visible region for the first binding type ("strong" binding) red shift of the absorption band is observed; for the second type ("weak" binding) we observed splitting of the band, short wavelength component being highly prevailing, and hypochromism. In the UV region for both binding types the spectra changed in approximately similar way; a slight blue shift and a rather remarkable hypochromism are observed. It is shown that the dye brings the main contribution into the spectral changes in the UV region. If to take into account the spectral properties of molecular aggregates the data obtained are compatible with the intercalation model for "strong" binding and dye stacking on DNA for "weak" binding.     

Circular dichroism and magnetic circular dichroism spectra of chlorophylls a and b in nematic liquid crystals. II. Magnetic circular dichroism spectra

Absorption and magnetic circular dichroism (MCD) spectra are reported for chlorophyll (Chl) a and Chl b dissolved in nematic liquid crystal solvents. The spectra were measured with the dye molecules oriented uniaxially along the direction of. the magnetic field and measuring light beam. It is significant that under such conditions the MCD spectra recorded in the wavelength region of the Q and Soret bands of the chlorophyll are essentially unchanged with respect to rotation of the sample cell around this axis, even though there is almost complete orientation of the chlorophyll molecules by the liquid crystals. The MCD spectra of Chl a and b in the nematic liquid crystal solvents used in this study are surprisingly similar to the spectra obtained under isotropic conditions. These results illustrate an important technique with which to examine the optical spectra of dyes oriented in liquid crystal matrices in which the anisotropic effects can be reduced the negligible proportions by the application of a strong magnetic field parallel to the direction of the measuring light beam. The first deconvolution calculations are reported that describe the deconvolution of pairs of absorption and MCD spectra, in the Q and B band regions, for both Chl a and b. The spectral analysis to obtain quantitative estimates of transition energies was accomplished by carrying out detailed deconvolution calculations in which the both the absorption and MCD spectral envelopes were fitted with the same number of components; each pair of components had the same hand centres and bandwidth values. This procedure resulted in an assignment of each of the main transitions in the absorption spectra of both Chl a and b. Chl a is clearly monomeric, with Qy, Qx, By and Bx located at 671, 582, 439 and 431 nm, respectively. Analysis of the spectral data for Chl b located Qy, By and Bx, at 662, 476 and 464 nm, respectively.     

Peptide-conjugated gold nanorods for nuclear targeting

Resonant electron oscillations on the surface of noble metal nanoparticles (Au, Ag, Cu) create the surface plasmon resonance (SPR) that greatly enhances the absorption and Rayleigh (Mie) scattering of light by these particles. By adjusting the size and shape of the particles from spheres to rods, the SPR absorption and scattering can be tuned from the visible to the near-infrared region (NIR) where biologic tissues are relatively transparent. Further, gold nanorods greatly enhance surface Raman scattering of adsorbed molecules. These unique properties make gold nanorods especially attractive as optical sensors for biological and medical applications. In the present work, gold nanorods are covalently conjugated with a nuclear localization signal peptide through a thioalkyl-triazole linker and incubated with an immortalized benign epithelial cell line and an oral cancer cell line. Dark field light SPR scattering images demonstrate that nanorods are located in both the cytoplasm and nucleus of both cell lines. Single cell micro-Raman spectra reveal enhanced Raman bands of the peptide as well as molecules in the cytoplasm and the nucleus. Further, the Raman spectra reveal a difference between benign and cancer cell lines. This work represents an important step toward both imaging and Raman-based intracellular biosensing with covalently linked ligand-nanorod probes.     

Circular dichroism of soybean leghemoglobin.

Circular dichroic (CD) spectra of soybean leghemoglobin, and some of its liganded derivatives were measured over the wavelength range of 650 to 200 nm. The heme-related circular dichroic bands in the visible, Soret and ultraviolet wavelength regions exhibit Cotton effects characteristic of each of the compounds examined. The positions of the dichroic bands vary with ligand substitutions and the oxidation state of the iron. All leghemoglobin derivatives, except the apoprotein, exhibit negative circular dichroic bands in the region of Soret absorption. In this region the optical activity of compounds with high-spin moments is greater than that of compounds with low or intermediate spin moments. The ellipticity of the heme band at about 260 nm is also altered by ligand binding and spin state. The dichroic spectra in the far-ultraviolet region indicated a high extent of alpha-helical structure (about 70%) in the native leghemoglobin and its liganded derivatives. The helicality of the apoprotein seems to diminish suggesting a decrease caused by the removal of the heme.     

Electronic and resonance Raman spectra of iron(III) complexes of enterobactin, catechol, and N-methyl-2,3-dihydroxybenzamide.

Resonance Raman electronic absorption and circular dichroism spectra and pH titration curves are reported for the trianionic ferric complexes of enterobactin, catechol, and N-methyl-2,3-dihydroxybenzamide (MDHB). The spectral signatures of the enterobactin and MDHB complexes are virtually identical and differ from those of the catechol complex in ways that reflect the influence of the amide group on the electronic structure. Excitation in either the visible charge-transfer bands or the near-ultraviolet pi-pi* bands enhances Raman bands associated with benzene ring modes, although the relative enhancements differ markedly in the two regions. The data stronly support a structural model in which iron is bound exclusively to the phenolate oxygen atoms in all three complexes.     

Effect of anthocyanins, carotenoids, and flavonols on chlorophyll fluorescence excitation spectra in apple fruit: signature analysis, assessment, modelling, and relevance to photoprotection

Whole apple fruit (Malus domestica Borkh.) widely differing in pigment content and composition has been examined by recording its chlorophyll fluorescence excitation and diffuse reflection spectra in the visible and near UV regions. Spectral bands sensitive to the pigment concentration have been identified, and linear models for non-destructive assessment of anthocyanins, carotenoids, and flavonols via chlorophyll fluorescence measurements are put forward. The adaptation of apple fruit to high light stress involves accumulation of these protective pigments, which absorb solar radiation in broad spectral ranges extending from UV to the green and, in anthocyanin-containing cultivars, to the red regions of the spectrum. In ripening apples the protective effect in the blue region could be attributed to extrathylakoid carotenoids. A simple model, which allows the simulation of chlorophyll fluorescence excitation spectra in the visible range and a quantitative evaluation of competitive absorption by anthocyanins, carotenoids, and flavonols, is described. Evidence is presented to support the view that anthocyanins, carotenoids, and flavonols play, in fruit with low-to-moderate pigment content, the role of internal traps (insofar as they compete with chlorophylls for the absorption of incident light in specific spectral bands), affecting thereby the shape of the chlorophyll fluorescence excitation spectrum.     

Action Spectra of Photosynthetic Activities of Chlorella ellipsoidea

• 1) Suspensions of Chlorella show an even stronger light scattering than suspensions of chloroplasts of spinach. The bands of absorption are thus broadened and, at higher concentrations, moved to lower wave-lengths. The intensity of the photosynthesis closely follows the curves of light scattering, a fact partly explaining the high efficiency of green light. Calculated per unit thermoelectrically measured incident energy the action spectrum shows bands at 660–670 nm and c. 500 nm and a comparatively high level of the whole region 500–560 nm.
• 2) Flash experiments show the existence of a steady state carotene/xanthophyll that is moved to reduction (c/x > 1) in blue and green light and to oxidation (c/x < 1) in red light. All experiments point to the existence of two light reactions, the first one involving excitation of carotenoids, with ferredoxin-TPN as acceptor, the second one involving excitation of chlorophyll, with the cytochrome system of the chloroplasts acting as donors of electrons and thus completing an energy converting circulation between pigments and enzyme systems.
• 3) The operation of combined light reactions appears also from experiments with simultaneous or succedaneous illumination with monochromatic light of different wave-lengths. Some effects may be explained from separate excitations of carotenoids and chlorophylls, others may depend on still unknown photic reactions.
• 4) The action spectrum in ultrared shows a positive band at c. 900 nm but no or only very small effects in the region 950–1400 nm. Ultrared radiation has on the other hand an enhancing effect on the light excitation in the visible spectrum. A combination of infrared and visible radiation shows a roughly linear relation between incident energy and photosynthetic effect.
• 5) All experiments were performed in the region of linear relation between intensity of incident light and O₂-production. Induced effects of combined monochromatic regions show a very rapid initial change in the steady states that in one or two minutes simmers down to a balanced state of continued photosynthesis. No change was observed in the total quantity of the pigments.

     

R-phycoerythrins having two conformations for the same aggregate

The visible circular dichroism (CD) spectrum of an R-phycoerythrin (Porphyra tenera) is composed of several positive bands. The protein in aqueous buffer very slowly exhibits changes in the CD spectrum of its chromophores, a band at 489 nm undergoes an increase in intensity and a red shift. When the band reached a 493 nm maximum, the spectrum became very stable. The aggregation state of the protein did not change during this spectral conversion. The chromophore CD spectrum was also obtained in the presence of a low concentration of urea or sodium thiocyanate, and the identical change in the CD was noted, but the change was much faster. The visible absorption and CD in the far UV spectra were unaffected by urea. Unchanged visible absorption and protein secondary structure (61% alpha helix) contradicted by comparatively salient alterations in the visible CD spectra suggested very subtle structural changes are influencing some of the chromophores. For a second R-phycoerythrin (Gastroclonium coulteri), the CD of the chromophores had a negative band on the blue edge of the spectrum. This is the first negative CD band observed for any R-phycoerythrin. Treatment of this protein with low concentrations of urea produced a change in the visible CD with the negative band being completely converted to a positive band. Fluorescence studies showed that the treatment by urea did not affect energy migration. Deconvolution of the CD spectra were used to monitor the chromophores. The results demonstrated that the same aggregate of each R-phycoerythrin could exist in two conformations, and this is a novel finding for any red algal or cyanobacterial biliprotein. The two forms of each protein would differ in tertiary structure, but retain the same secondary structures.     

THE PIGMENT-PROTEIN COMPOUND IN PHOTOSYNTHETIC BACTERIA : II. THE ABSORPTION CURVES OF PHOTOSYNTHIN FROM SEVERAL SPECIES OF BACTERIA

Absorption curves have been obtained in the spectral region of 450 to 900 mµ for the water soluble cell juice of four species of photosynthetic bacteria, Spirillum rubrum (strain S1), Rhodovibrio sp. (strain Gaffron), Phaeomonas sp. (strain Delft), and Streptococcus varians (strains C11 and orig.). These curves all show maxima at 790 and 590 mµ due to bacteriochlorophyll, whose highest band, however, occurs at 875, 855, or 840 mµ depending on the species. The bacteria that appear red rather than brown have a band at 550 mµ due to a carotinoid pigment. An absolute absorption curve of bacteriophaeophytin has maxima at 530 and 750 mµ. The extraction of cell juice by supersonic vibration does not change the position of the absorption bands or of the light absorbing capacity of the pigment.     

Temperature regime of biological tissue under photodynamic therapy

An analytical model is proposed to calculate heating of human skin cover under laser light action of photodynamic therapy. A photosensitizer of "Fotolon" is taken as an example. Temperatures of skin surface and of deep dermis regions are studied as a function of time under pulsed and stationary irradiation of skin surface at the wavelength of 665 nm corresponding to the maximum of the photosensitizer absorption band. It is shown that, under the action of a short light pulse, the photosensitizer can lead to an essential temperature rise of dermis due to a considerable increase in its absorption coefficient. However, this rise does not destruct tissue cells because of the short action. Under stationary irradiation, the photosensitizer concentration has a low effect on the temperature regime of tissue. This is related with the specific features in heating of the medium by red light, where the main thermal process in skin is heat transfer over tissue volume from epidermis having a substantially larger absorption coefficient than that of dermis in the said spectral range. The role of blood perfusion in dermis and its effect on the temperature regime of tissue are evaluated.     

Synthesis,luminescence, and excited-state absorption properties of disubstituted perylene diimide derivatives modified at bay region

Three A–π–A or D–π–D perylene diimide ( PDI ) derivatives with varied groups on π-conjugate were synthesized and characterized. The photophysical properties of these compounds were systematically studied by spectral experiments and density functional theory calculations. All compounds displayed intense absorption bands at 300–800 nm wavelengths. However, diverse groups on the π-conjugate influenced the UV–vis absorption. Electron-withdrawing groups on PDI-2 caused a slight red shift at the 350–400 nm wavelength and a blue shift after 400 nm wavelength. At the same time, the electron-donating substituents on PDI-3 caused an obvious red shift of this band. These PDI derivatives exhibited emission in solution at room temperature (λ_em = 500–850 nm). The quantum yield of PDI-3 decreased, while the electron-donating substituents were introduced to the π-conjugated motifs. However, the quantum yield of PDI-2 increased when electron-withdrawing substituents were introduced to the π-conjugated motifs. In addition, PDI-1 and PDI-2 exhibited broad triplet transient absorption in the visible region. These photophysical properties could help us to understand the relationship between structure and photophysical properties of perylene diimide derivatives and exploit more original perylene diimide-based optical functional materials.     

Temperature regime of biological tissue under photodynamic therapy

An analytical model is proposed to calculate heating of human skin cover under laser light action of photodynamic therapy. A photosensitizer of «Fotolon» is taken as an example. Temperatures of skin surface and of deep dermis regions are studied as a function of time under pulsed and stationary irradiation of skin surface at the wavelength of 665 nm corresponding to the maximum of the photosensitizer absorption band. It is shown that, under the action of a short light pulse, the photosensitizer can lead to an essential temperature rise of dermis due to a considerable increase in its absorption coefficient. However, this rise does not destruct tissue cells because of the short action. Under stationary irradiation, the photosensitizer concentration has a low effect on the temperature regime of tissue. This is related with the specific features in heating of the medium by red light, where the main thermal process in skin is heat transfer over tissue volume from epidermis having a substantially larger absorption coefficient than that of dermis in the said spectral range. The role of blood perfusion in dermis and its effect on the temperature regime of tissue are evaluated.     

Variation in ultraviolet reflectance by carotenoid-bearing feathers of tanagers (Thraupini: Emberizinae: Passeriformes)

Avian visual sensitivity encompasses both the human visible range (400–700 nm) and also near‐ultraviolet (UV) wavelengths (320–400 nm) invisible to normal humans. I used reflectance spectrophotometry to assess variation in UV reflectance for yellow, orange and red plumage in 67 species of tanager (Passeriformes). Previous chemical studies, and my analysis of reflectance minima, suggest that carotenoids are the dominant pigments in yellow, orange and red tanager plumage. Spectra recorded over the range of wavelengths to which birds are sensitive (320–700 nm) were invariably bimodal, with both a plateau of high reflectance at longer (> 500 nm) wavelengths and a distinct secondary peak at UV (< 400 nm) wavelengths. Within this overall framework, variation in UV reflectance was expressed within well‐defined quantitative limits: (1) peak reflectance was always lower than the corresponding plateau of reflectance at longer visible wavelengths; (2) the intensity of peak reflectance declined steadily below 350 nm; (3) wavelengths of peak reflectance clustered between 350 and 370 nm. Significant correlations were detected between various measures of total reflectance in the UV and visible wavebands, but not between various measures of spectral location of UV and visible reflectance. I propose that the strong absorption band at short visible wavelengths (～ 380–550 nm) responsible for bimodal spectra of carotenoids in vitro is also responsible for bimodal reflectance by carotenoid‐based plumage colours. The construction of the UV and visible reflectance bands from different sides of this same absorbance band provides a mechanism for the observed covariation between UV and visible wavelengths. Lack of an association between the spectral locations of the UV and visible reflectance bands may result from the limited variation in spectral location of the UV band. These patterns suggest that plumage colours are subject to constraints, just as are more traditional morphological characters. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 84 , 243–257.     

不同水氮条件下水稻冠层反射光谱与植株含水率的定量关系

研究了不同土壤水氮条件下水稻 (Oryzasativa) 冠层光谱反射特征和植株水分状况的量化关系。结果表明, 水稻冠层近红外光谱反射率随土壤含水量的降低而降低, 短波红外光谱反射率随土壤含水量的降低而升高。相同土壤水分条件下, 高氮水稻的冠层含水率高于低氮水稻的冠层含水率 ;同一水分条件下, 高氮处理的可见光区和短波红外波段光谱反射率低于低氮处理, 近红外波段光谱反射率高于低氮处理。发现拔节后比值植被指数 (R810 /R460 ) 与水稻叶片含水率和植株含水率呈极显著的线性相关, 模型的检验误差 (RootmeansquareError, RMSE) 分别为 0.93和 1.5 0。表明比值植被指数R810 /R460 可以较好地监测不同生育期水稻叶片和植株含水率。     

Quantitative scattering of melanin solutions

The optical scattering coefficient of a dilute, well-solubilized eumelanin solution has been accurately measured as a function of incident wavelength, and found to contribute <6% of the total optical attenuation between 210 and 325 nm. At longer wavelengths (325-800 nm), the scattering was less than the minimum sensitivity of our instrument. This indicates that ultraviolet and visible optical density spectra can be interpreted as true absorption with a high degree of confidence. The scattering coefficient versus wavelength was found to be consistent with Rayleigh theory for a particle radius of 38 +/- 1 nm. Our results shed important light on the role of melanins as photoprotectants.