Ontogenetic colour change of a sexual ornament in males of a damselfly: female mimicry, crypsis or both?

Female mimicry by males is a widespread phenomenon in several taxa and may be involved in aggression avoidance or facilitated access to resources. In early developmental stages, female mimicry may be a mechanism involved in signalling sexual immaturity or, when coupled with strategies related to visual camouflage, may be involved in the avoidance of male-male agonistic interactions. Here, we addressed whether the delayed colour maturation of a sexual ornament in males of Mnesarete pudica damselflies might be a case of crypsis, female mimicry or both. We analysed how conspecifics and predators perceive the pigmented wings of juvenile males by contrasting the wing spectra against a savannah background and the wings of both juvenile and sexually mature males and females. Our results based on the modelled visual system of conspecifics and predators suggest that the colour maturation of juvenile males may function as both crypsis and female mimicry. We discuss whether these results related to age- and sexual-dichromatism might be a mechanism to avoid unwanted intraspecific interactions or to avoid territorial and aggressive males. We conclude that the female mimicry and crypsis in juvenile males of M. pudica are mechanisms involved in avoidance of predators and unwanted intraspecific interactions, and the signalling of sexual maturity.

Decoupling Temperature-Volume Effects on Poly[2-Methoxy-5-(2'-Ethylhexyloxy)-1,4-Phenylene-Vinylene] Films at the ß-Relaxation Temperature.

In this paper, we report a simple procedure to decouple effects from temperature and volume on the emission properties of thin films of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV). This procedure consists of applying a positive pressure close to the ß-relaxation temperature, Tß (∼220 K), of MEH-PPV, which controls the molecular movement to retain a disordered state for the polymer chains even after the sample is cooled from room temperature. Such decoupling could be confirmed by calculating the photoluminescence (PL) spectra line shape using a semi-empirical model based on molecular exciton and Franck-Condon transitions, and with electron-vibrational modes coupling being parameterized with the Huang-Rhys factor. We also show that the decoupling between temperature and volume effects does not occur if the molecular movement is restricted either by thermal annealing or by depositing the MEH-PPV film on a rigid substrate. This latter finding may be exploited in designing thermally stable devices.

Temperature effect on the electron-vibrational mode coupling of a fully conjugated polyfluorene derivative.

Photoluminescence (PL) and electroluminescence (EL) spectra were used to probe the thermal relaxation processes of the poly(9,9'-n-dihexyl-2,7-fluorenediiylvinylene-alt-1,4-phenylenevinylene) (LaPPS16) electroluminescent polymer. A theoretical model of molecular excitons and Franck-Condon transitions were used to analyze the line shape of the radiative transitions. It was possible to correlate directly the electron-vibrational mode coupling, i.e., the Huang-Rhys parameter, and the polymer relaxation processes due to the effects of molecular dynamics on the electronic states. The results showed different dependences of the thermal relaxation process on PL and on EL due to the molecular dynamics restraints of LaPPS16. This could explain the efficiency variation in organic light emitting diodes where the external electric field would decrease the degrees of freedom of the polymer and activate specific non-radiative channels. Molecular relaxation temperatures of the LaPPS16 polymer are proposed.

Photoluminescence Study of Ammonium-Free Chemical Bath Deposition of CdS Nanoparticles on Polyester Substrate.

Self-assembled cadmium sulfide (CdS) thin solid films were synthesized by chemical bath deposition (CBD) technique for different deposition times (20 min.­24 h) on transparent and flexible polyester substrates using sodium acetate complex agent. CBD-CdS films were also deposited on glass (BK7) and quartz substrates, as reference. CBD-CdS films on polyester substrate showed a homogeneous deposition, reduction of chemical or structural defects (green emission), and large photoluminescence efficiency in comparison of CBD-CdS films deposited on Si-based substrates. Simulation of PL temperature dependence for polyester CdS films using the rate equation for the carrier population showed the presence of two decay pathways due to cadmium and sulphide defects at crystallite.

Conjugation Length Distribution in Poly(p-phenylenevinylene) (PPV) Films.

We studied the absorption line-shape of poly(p-phenylenevinylene) (PPV) films deposited via spin coating and Langmuir-Blodgett techniques with the intent of identifying the conjugation length distribution in these two types of films, a key morphological aspect of conjugated polymer films. We treated the excitons in the polymer as independent oligomer excitons and modeled the absorption spectra of the individual oligomers using simple expressions for the oligomer size dependence of the gap energy, the line-broadening factor, the transition dipole moment and the Huang-Rhys parameter. We validated these expressions by independent measurements on phenyl-based oligomers and Density Functional Theory calculations. Our results show clear evidence that, for both types of PPV films, the conjugation length distribution depends exponentially on the segment size. Our results also set a lower limit, of about ten repeat units, for the maximum exciton length of three different phenyl-based oligomers.

Kinetics of Thermal Conversion of Conjugated Polymers Investigated from Their Optical Absorption Spectra.

We report on a phenomenological, theoretical model to calculate the rate constants and activation energies for the thermal conversion reactions from poly(xylylidene tetrahydrothiophenium chloride) (PTHT) into poly(p-phenylenevinylene) (PPV) using the optical absorption spectra of spin-coated films. The probabilities of electron transitions were calculated considering Franck-Condon states with a Gaussian distribution of conjugated segments and molecular excitons. The dependence on the conjugation degree (n) for the energy gap, transition dipole moment, and electron-phonon coupling were obtained semiempirically using published data for PPV. Fitting was performed for the C-C stretching of the aromatic ring 1550 cm(-1), for it is considered the most optically active. The isotherms for consumption and formation of PPV segments were fitted using a first-order and consecutive reactions, respectively. With this modeling we could identify the most probable reactions, where the formation of longer PPV segments n ≥ 3 occurs only via reactions from smaller conjugated segments (i.e., n = 1 and 2). The activation energies tend to decrease with longer conjugation lengths. Significantly, the modeling allows us to predict the conversion temperature and chemical composition yielding a predefined distribution of conjugated segments, which can be applied to any polymer undergoing thermal conversion, decomposition, or photo-oxidation.

Emission ellipsometry used to probe aggregation of the luminescent 2,1,3-benzothiadiazole dyes and ordering in an E7 liquid crystal matrix.

The incorporation of dyes in liquid crystal matrices has been exploited to produce enhanced displays, but it can also be used to probe ordering in liquid crystals and to assess intermolecular interactions and dye aggregation. In this study, we investigated polarized absorption and emission of the luminescent dyes 4,7-bis(2-(4-(decyloxy)phenyl)ethynyl)-[2,1,3]-benzothiadiazole (1A) and 4,7-bis{2-[4-(4-decylpiperazin-1-yl)phenyl]ethynyl}-[2,1,3]-benzothiadiazole (5A) in the E7® liquid crystal. The electronic structures of both 1A and 5A dyes were affected by the matrix and by the analysis of the line shape of emission we could determine that the dyes form J aggregates. This achievement is significant because obtaining this type of information for small molecules requires ordered matrices, which is difficult to obtain for these dyes. Using emission ellipsometry we were able to determine the ordering of the E7 molecules, but this was possible only with the larger 5A dye. The smaller 1A was not entirely ordered in the E7 matrix and this calls for caution in other types of work where dopants are used as probes to infer the properties of the matrix. The emission ellipsometry data for the dyes allowed us to detect the enhanced birefringence in the matrix, thus confirming the theoretical prediction.

Revisiting the conformational transition model for the pH dependence of BSA structure using photoluminescence, circular dichroism, and ellipsometric Raman spectroscopy.

Changes in pH affect metabolic pathways, primarily by modulating enzyme conformations, which is why a detailed analysis of pH-driven conformational transitions is required to understand the underlying biochemistry of diseases and biological organisms. In this work, we examined the pH-driven conformational dynamics of Bovine Serum Albumin (BSA), within the framework of the Foster Model. Circular Dichroism and Raman Optical Activity showed the conversion of helical into ß-rich structures in the acid and basic regions, while an opening of BSA tertiary structure was shown by the upsurging of accessibility of ANS-BSA binding sites and the increasing of random contributions at regions F and B. We could then revisit the Foster Model by introducing two additional intermediate conformational states and structural reorganization at extreme pH values. This expanded model opens up new possibilities concerning protein-molecule interactions, promising far-reaching implications for fields such as drug design and biomaterials.

Photoluminescent ellipsometric circular dichroism.

A novel spectroscopic technique, photoluminescent ellipsometric circular dichroism (PECD), which distinguishes all radiative electronic transitions related to molecular chiral centers. Additionally, it is proposed as complementary to the ellipsometric Raman spectroscopy (ERS) technique, thus establishing a relationship between vibrational modes and electronic transitions, associated with molecular chiral centers. In this way, PECD turns into a powerful technique for chiral material characterization. The PECD technique was performed on a chiral oligomer (1R,2R)-diiminocyclohexane, and its derivative polymer. A complete photophysical characterization in solution was performed to corroborate the new PECD technique.

Correlation of electronic and vibrational properties with the chiro-optical activity of polyfluorene copolymers.

The rationale of this paper is to shed some light on the origin of the optical response of two similar chiral fluorene copolymers in correlation with their vibrational modes, to understand how a chiral center placed in a ramification affects the optical properties of the main chain. Various spectroscopic ellipsometric techniques, in the scope of the Stokes theory were used to characterize the optical-vibrational behavior of the polyfluorenes: ellipsometry in emission (EE), transmission (TE), and Raman (ERS). The results showed that the optical activity and the emission of the circularly polarized light depends substantially on the interaction of the chiral carbon in the ramification and the main chain through specific optically active vibrational modes, for each sample. One interesting achievement was to find the absolute dextrorotatory configuration of the studied molecules, that could induce a helicoidal structure to the entire material.

Optical spectroscopy study of the interaction between curcumin and acrylic polymers.

This paper presents the results of a study conducted on the interaction between curcumin, a compound with several biomedical applications in traditional and modern medicine, and the acrylic polymers poly(methyl methacrylate), poly(ethyl methacrylate), and poly(n-butyl methacrylate), through photophysical experiments in curcumin/acrylic polymers casting films. Optical absorption intensity at ~340 nm increases relatively to its maximum at ~417 nm when the amount of curcumin in the polymeric film decreases, due to a significant change in the concentration of the isomers cis- or trans-form of curcumin, regardless of the acrylic polymer. Fluorescence (FL) spectra of the films depend on the curcumin concentration in the matrix with well-resolved line shape. They show two distinct bands, one at ~525 nm, for higher curcumin concentration (5.00 mmol.L-1), related to the aggregated curcumin species, and another at ~465 nm, for lower concentration of curcumin (0.10 mmol.L-1), related to the effects of the solvent on the conformational structure of the curcumin molecule and the presence of the trans-form of curcumin. The parameter Kagg, related to the contribution of the aggregated curcumin, shows the influence of the polymeric lateral chain length of the matrix in the de-aggregation of the curcumin. The Huang-Rhys factor indicates that curcumin aggregated species are conformationally more stable, and that the isolate species depends on the chemical environment and the matrix/curcumin interaction, decreasing its conformational degrees of freedom. Arrhenius plots, obtained via FL experiment in function of the sample temperature, show that, for higher curcumin concentration, the value for the relaxation energy process is not well defined, due the decrease in the interaction between the matrix and the curcumin molecules. With these results, it is possible to infer that the interaction matrix/curcumin must occur via lateral chemical alkyl groups.

Bacterial Photoinactivation Using PLGA Electrospun Scaffolds.

The use of ultraviolet (UV) and blue irradiation to sterilize surfaces is well established, but commercial applications would be enhanced if the light source is replaced with ambient light. In this paper, it is shown that nanofibers can be explored as an alternative methodology to UV and blue irradiation for bacterial inactivation. It is demonstrated that this is indeed possible using spun nanofibers of poly[lactic-co-(glycolic acid)] (PLGA). This work shows that PLGA spun scaffolds can promote photoinactivation of Staphylococcus aureus and Escherichia coli bacteria with ambient light or with laser irradiation at 630 nm. With the optimized scaffold composition of PLGA85:15 nanofibers, the minimum intensity required to kill the bacteria is much lower than in antimicrobial blue light applications. The enhanced effect introduced by PLGA scaffolds is due to their nanofiber structures since PLGA spun nanofibers were able to inactivate both S. aureus and E. coli bacteria, but cast films had no effect. These findings pave the way for an entirely different method to sterilize surfaces, which is less costly and environmentally friendly than current procedures. In addition, the scaffolds could also be used in cancer treatment with fewer side effects since photosensitizers are not required.

Energy transfer mechanisms in pH-tuned aggregates of curcumin.

Curcumin ((1,7-bis-(4-hidroxi-3-metoxifenil)-1,6-heptadieno-3,5-diona)), is a natural yellow-orange polyphenol dye, obtained from Curcuma longa rhizomes, with vast potential in the biotechnology field, such as anti - inflammatory and cancer preventive properties, and a very environmental-sensible structure, mostly resulting in aggregate formations, with low solubility in aqueous solvents and remarkable pH dependence. Hence, it is essential to comprehend its aggregation mechanisms, optical properties and conformational equilibrium to develop curcumin-based drugs and biomaterials for several applications. With the purpose of understand such concepts and its properties, the results presented in this work, obtained through spectroscopic measurements and the Hill's modeling, show a conformational equilibrium, driven by pH, between the enol-enolate and aggregated forms in curcumin, and the respective energy transfer processes between these species. Additionally, the photophysical mechanisms for the energy transfer between aggregated forms, observed with the ellipsometry measurements, showed an "all-or-nothing" character, pointing to a very complex conformation for curcumin aggregates.

Induced secondary structure in nanostructured films of poly(p-phenylene vinylene).

The control of emission properties in luminescent polymers such as poly(p-phenylene vinylene) (PPV) is important for various applications, and may be achieved with suitable molecular architectures in nanostructured films. This paper reports on optical properties of PPV films, using ellipsometry measurements for emitted light in the scope of the Stokes' theory. Organized PPV films obtained with the Langmuir-Blodgett (LB) method exhibited high degree of polarization for the emitted light, while cast films emitted mainly non-polarized light. From ellipsometry data, a secondary structure was inferred for poly(xylylidene tetrahydrothiophenium) chloride (PTHT), a PPV precursor, in solution, which is retained only to a small extent in the PPV cast film as thermal conversion was performed close to the glass transition temperature of PPV. On the other hand, a higher intensity of emitted light with circular polarization was observed for the LB film, which is attributed to PPV molecular secondary structure that was enhanced during the LB film deposition. Circular dichroism experiments were performed to corroborate this hypothesis. It is suggested that such a secondary structure has not been predicted in theoretical models for PPV because possible conformational changes induced in the processing steps are not taken into account.

On the role of epigallocatechin-3-gallate in protecting phospholipid molecules against UV irradiation.

Catechin molecules such as epigallocatechin-3-gallate (EGCG) are capable of attenuating the biomolecular damage induced by UV radiation, possibly through molecular mechanisms involving the cell membranes. In this study, we confirmed the protective role of EGCG against UV of 1,2-dipalmitoyl-sn-glycero-3-[phospho-rac-(1-glycerol) (sodium salt) (DPPG) in liposomes and cast films. The incorporation of EGCG increased the stability of DPPG liposomes as indicated by UV-vis absorption spectra. Using 2D correlation spectroscopy to analyse the spectra, we found that DPPG and EGCG are co-helpers and complement each other against degradation induced by UV. At the molecular level, UV irradiation affects the phosphate and carbonyl groups of DPPG, in addition to triggering the oxidation and opening of the pyrogallol ring of EGCG. Since EGCG can be incorporated into liposomes and is a strong shield against UV radiation, one may envisage its use in anti-ageing and sunscreen creams, and in dermal drug delivery.

Effect of blue light irradiation on the stability of phospholipid molecules in the presence of epigallocatechin-3-gallate.

In this paper, we report on the effects from epigallocatechin-3-gallate (EGCG), a phytochemical flavonoid present in green tea, on Langmuir monolayers of 1,2-dipalmitoyl-sn-glycero-3-[phospho-rac-(1-glycerol) (sodium salt) (DPPG), including experiments with blue light irradiation. EGCG was found to interact with both the DPPG headgroups and hydrophobic tails, thus affecting the lipid packing according to surface pressure and surface potential isotherms and polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS) data. Blue light irradiation caused considerable changes in the surface pressure isotherms and PM-IRRAS spectra of DPPG monolayers, but the effects were considerably less when EGCG was present. For the surface pressure isotherms, for instance, no irradiation effect could be measured for mixed EGCG-DPPG monolayers. It is concluded that EGCG protected the DPPG molecules from degrading upon blue light irradiation, which means that EGCG may be a preventive and therapeutic agent to decrease photosensitivity of phospholipids to blue light oxidative damage, a pathogenic mechanism in skin disorders.

Polycaprolactone/Gelatin Nanofiber Membranes Containing EGCG-Loaded Liposomes and Their Potential Use for Skin Regeneration.

Polymeric scaffolds incorporating plant-derived compounds, produced by electrospinning, have attracted attention in the field of skin tissue engineering. This study evaluates the sustained antioxidant activity of polycaprolactone (PCL)/gelatin nanofibers prepared by electrospinning and incorporating loaded liposomes of epigallocatechin-3-gallate (EGCG), a strong antibacterial and antioxidant molecule found in green tea, that significantly accelerates the wound-healing process. The morphology and the structural properties of the membranes were characterized by scanning electron microscopy (SEM) and FTIR spectroscopy. Results revealed that the EGCG released from PCL+gelatin nanofibers scavenges the toxic ROS species generated by exposure to either H2O2 or UV radiation and slows down the oxidation events associated with damage. This study provides the basis for development of promising nanofiber formulations containing EGCG that might enhance repair/regeneration of skin tissue.

Emission ellipsometry as a tool for luminescent liquid crystal phase transition identification.

In the present work, we have studied the liquid crystalline properties of a luminescent molecule, which presents smectic mesophases, using the emission ellipsometry technique. A methodology with photoselection of the excitation light to improve the experiment is presented. An empirical model is proposed to explain the experimental results. The Stokes parameters of the emitted light allowed the correlation of changes in the polarization degree, ellipticity angle, and rotation angle with phase transitions.

Tuning Structural Changes in Glucose Oxidase for Enzyme Fuel Cell Applications.

Stabilization and electrical contacting of redox enzymes with electrodes are fundamental requirements for bioelectronics devices, including biosensors and enzyme fuel cells (EFCs). In this study, we show increased glucose oxidase (GOx) stability by immobilization with Nafion. The immobilization process affected GOx conformation but was not detrimental to its activity, which was maintained for more than 120 days. The GOx/Nafion system was interfaced to a carbon cloth electrode and assembled in a prototypal EFC fed with glucose. Polarization and power density curves demonstrated that GOx/Nafion system was able to generate power, exploiting a Nafion-assisted electron transfer process to the electrode. Our findings are consistent with the onset of pH-dependent conformational equilibrium for the enzyme secondary structure and its active site. Significantly, the protective effect exerted by Nafion on the enzyme structure may be tuned by varying parameters such as the pH to fabricate durable EFCs with good electrocatalytic performance.

Nanomolding the Surface of Polymer Films.

Slight changes in the experimental procedures of the micro contact printing (ACP) technique are introduced here, which allow for using polymers soluble in distinct solvents to fabricate submicrometric 2D periodic structures. Highly reproducible secondary and tertiary poly(dimethylsiloxane) (PDMS) molds could be produced, as demonstrated in atomic force microscopy images and light diffraction experiments. The replication of tertiary molds with no residues of PDMS demonstrates the feasibility of large-scale production with distinct polymers. The plane wave propagation along the tertiary poly(3,4-ethylenedioxythiophene) with poly(hydrogen 4-styrene sulfonate) molds was simulated with a finite-difference time-domain algorithm. A strong wave propagation was observed in the region containing the structures acting as a wave guide, in agreement with the results from the experimental absorption measurements. Furthermore, we show that the optical properties of the molds and their roughness can be tuned by choosing the polymers (including biopolymers) for printing pillars and tracks, thus bringing new possibilities for nanomolding of polymer surfaces for photonics, organic electronics and bioelectronics.