Photoinduced charge separation in functional carbon-silver nanohybrids.

In recent times, nanoscience is devoting growing interest to the easy assembly of well-established nanomaterials into hybrid nanostructures displaying new emerging features. Here, we study the photophysicochemical response of binary nanohybrids obtained by the spontaneous coupling of luminescent carbon dots to silver nanoparticles with controlled surface charge. Evidence of the successful coupling is obtained by steady-state and time resolved optical measurements and further confirmed by direct imaging. We demonstrate strong interactions within nanohybrids, which can be modelled in terms of a sub-picosecond electron transfer from photoexcited carbon dots to silver nanoparticles. Accordingly, newly designed nanohybrids display significant photocatalytic performance demonstrated by the photodegradation of methylene blue under ultraviolet-visible light. Our results provide an exhaustive picture of the optical response of these self-assembled carbon-silver nanohybrids and show their promise as a new class of eco-friendly materials for light-driven catalytic applications.

UV photobleaching of carbon nanodots investigated by in situ optical methods.

Carbon dots are a family of optically-active nanoparticles displaying a combination of useful properties that make them attractive for many applications in photonics and photochemistry. Despite the initial claims of high photostability of carbon dots even under prolonged illuminations, several recent studies have evidenced their photobleaching (PB) under UV light, detrimental for some applications. A study of the mechanism and dynamics of carbon dot PB can be considered a useful route to gather relevant information on the underlying photophysics of these nanoparticles, which is still widely debated. Here we report a study of the PB of carbon dots under UV light, conducted through optical experiments under well-controlled illumination conditions. In particular, the use of a laser as an irradiation source allows a precise control of the irradiated volume, and provides accurate estimates and control of the administered energy. Besides, our setup allows spectroscopic measurements to be carried out in situ at the irradiated site, thus allowing us to investigate in real time the progress of photobleaching effects through a time-resolved approach. Therefore, our experiments allow the precise kinetics of the undergoing PB process to be captured which is found to be significantly affected by disorder and photoselection effects. Furthermore, our study discloses several pieces of information on the nature of the main blue chromophore absorbing at 340 nm and emitting at 430 nm, and on its PB mechanism. We propose that the emissive units consist in small molecular-like chromophores adsorbed on carbon dot surfaces and are in a dynamical equilibrium with free diffusing molecules in solution. Their photobleaching proceeds in two distinct steps: in the first phase, linear absorption of UV photons rapidly converts the molecular surface chromophores into a non-emissive form, likely through an isomerization, causing the disappearance of the fluorescence properties but almost no changes in the optical absorption spectra. At higher fluences, a complete destruction of the optically-active centers is observed, which completely wipes out all the absorption features of surface chromophores and only leaves a fully carbonized, yet non-fluorescent, dot core.

Different natures of surface electronic transitions of carbon nanoparticles.

The photoluminescence behaviour of carbon-based nanodots is still debated. Both core and surface structures are involved in the emission mechanism, and the electronic transitions can be modified by external agents such as metal ions or pH, but the general relation between the structure and the optical function is poorly understood. Here, we report a comparative study on the effects of these variables, changing the core structure from crystalline to amorphous, and modifying the surface structure by different passivation procedures. Our results highlight that the emission mechanism of the tunable visible fluorescence is identical for crystalline and amorphous samples, indicating the independence of the emission from the core structure. Furthermore, surface functionalization weakly influences the emission peak position, but has large consequences on their interaction with different metal ions. This suggests the involvement of quasi-degenerate electronic states originating from the high density of different interacting groups on the surface. Finally, we report the presence of an unusual ultraviolet emission band for the amorphous sample, likely involving localized molecular-type chromophores with carboxyl ends. Our findings provide new information on the emission mechanisms of CDs and can be used to engineer sub-types of CDs displaying very similar emission features, but specifically tailored for different sensing applications.

Vulnerability of OFDR-based distributed sensors to high γ-ray doses.

Vulnerability of Optical Frequency Domain Reflectometry (OFDR) based sensors to high γ-ray doses (up to 10 MGy) is evaluated with a specific issue of a radiation-hardened temperature and strain monitoring system for nuclear industry. For this, we characterize the main radiation effects that are expected to degrade the sensor performances in such applicative domain: the radiation-induced attenuation (RIA), the possible evolution with the dose of the Rayleigh scattering phenomenon as well as its dependence on temperature and strain. This preliminary investigation is done after the irradiation and for five different optical fiber types covering the range from radiation-hardened fibers to highly radiation sensitive ones. Our results show that at these high dose levels the scattering mechanism at the basis of the used technique for the monitoring is unaffected (changes below 5%), authorizing acceptable precision on the temperature or strain measurements. RIA has to be considered as it limits the sensing range. From our vulnerability study, the OFDR sensors appear as promising candidates for nuclear industry even at doses as high as 10 MGy.

Radiation effects on optical frequency domain reflectometry fiber-based sensor.

We investigate the radiation effects on germanosilicate optical fiber acting as the sensing element of optical frequency domain reflectometry devices. Thanks to a new setup permitting to control temperature during irradiation, we evaluate the changes induced by 10 keV x rays on their Rayleigh response up to 1 MGy in a temperature range from -40°C up to 75°C. Irradiation at fixed temperature points out that its measure is reliable during both irradiation and the recovery process. Mixed temperature and radiation measurements show that changing irradiation temperature leads to an error in distributed measurements that depends on the calibration procedure. These results demonstrate that Rayleigh-based optical fiber sensors are very promising for integration in harsh environments.

Radiation tolerant fiber Bragg gratings for high temperature monitoring at MGy dose levels.

We report a method for fabricating fiber Bragg gratings (FBG) resistant to very severe environments mixing high radiation doses (up to 3 MGy) and high temperatures (up to 230°C). Such FBGs have been written in two types of radiation resistant optical fibers (pure-silica and fluorine-doped cores) by exposures to a 800 nm femtosecond IR laser at power exceeding 500 mW and then subjected to a thermal annealing treatment of 15 min at 750°C. Under radiation, our study reveals that the radiation induced Bragg wavelength shift (BWS) at a 3 MGy dose is strongly reduced compared to responses of FBGs written with nonoptimized conditions. The BWS remains lower than 10 pm for temperatures of irradiation ranging from 25°C to 230°C without noticeable decrease of the FBG peak amplitude. For an applicative point of view, this radiation induced BWS corresponds to an additional error on the temperature measurements lower than 1.5°C, opening the way to the development of radiation-tolerant multi-point temperature sensors for nuclear industry.

Stimuli-responsive photoluminescent and structural properties of MIL-53(Al) MOF for sensing applications.

Metal-organic frameworks (MOFs) are an intriguing group of porous materials due to their potential influence on the development of indispensable technologies like luminescent sensors and solid-state light devices, luminescent multifunctional nanomaterials. In this research work we explored MIL-53(Al), an exceptional class of MOF that, along with guest adsorption, undergoes structural transitions exhibiting breathing behavior between narrow pore and large pore under temperature and mechanical stress. Therefore, we opted for the time resolved luminescence and FT-Raman spectroscopy to investigate the mechanochromic and thermochromic response of this material under external stimuli. Intriguingly, when subjected to temperature changes, MIL-53(Al) exhibited a ratiometric fluorescence behavior related to the reversible relationship of photoluminescence emission intensity with respect to temperature. Moreover, under higher mechanical stress MIL-53(Al) displayed turn-on behavior in emission intensity, hence offering a thrilling avenue for the application in mechanically deformed-based luminescent sensors and ratiometric fluorescence temperature sensors.

Importance of spin-orbit interaction for the electron spin relaxation in organic semiconductors.

Despite the great interest organic spintronics has recently attracted, there is only a partial understanding of the fundamental physics behind electron spin relaxation in organic semiconductors. Mechanisms based on hyperfine interaction have been demonstrated, but the role of the spin-orbit interaction remains elusive. Here, we report muon spin spectroscopy and time-resolved photoluminescence measurements on two series of molecular semiconductors in which the strength of the spin-orbit interaction has been systematically modified with a targeted chemical substitution of different atoms at a particular molecular site. We find that the spin-orbit interaction is a significant source of electron spin relaxation in these materials.

Non pigmented melanocytic nevus of the oral cavity: a case report with emphasis on the surgical excision procedures.

We report a case of a 37-year-old caucasian woman presenting a 1 cm pinkish nodular asymptomatic lesion of the hard palate, slowly growing in the last years. The lesion underwent to biopsy. Histological analysis showed the nevus tissue layered under a continuous squamous epithelium. The stroma contained nests of medium-sized round cells, with regular monomorphous nuclei. The nevus cells were immunohistochemically positive for S-100 protein, while melanin, visualized by Masson-Fontana silver staining, was absent. Therefore a diagnosis of non pigmented melanocytic nevus was formulated. Because of its rarity and to avoid any risk of malignant transformation, a surgical treatment with wide excision was chosen; the surgical wound was previously covered with a membrane of fibrin and autologous platelets, and subsequently sutured, resulting in a total heal. This procedure seems to be the most reliable to approach melanocytic lesions of the oral cavity. Clinical diagnosis of non-pigmented nevi, either flat or protruding, is difficult, because the nevus shows a pinkish colour that is indistinguishable from that of the surrounding mucosa. Moreover, attention is required when similar clinical evidence occurs, because the localization inside the oral cavity may offer several problems of differential diagnosis.

The effect of hypoxia on ventilation frequency in startled common sole Solea solea.

Ventilation frequency (F(V) ) in motionless common sole Solea solea was measured before and after a startling stimulus in normoxia and in hypoxia (15% air saturation). Startling reduced F(V) in normoxia (from mean ±s.e. 41 ± 3·3 beats min⁻¹ to near zero, i.e. 2·0 ± 1·8 beats min⁻¹) and in hypoxia (from mean ±s.e. 80 ± 4·4 to 58·8 ± 12·9 beats min⁻¹). It is suggested that the maintenance of high F(V) in hypoxia may increase the probability of detection by predators compared to normoxia.

Photocycle of point defects in highly- and weakly-germanium doped silica revealed by transient absorption measurements with femtosecond tunable pump.

We report pump-probe transient absorption measurements addressing the photocycle of the Germanium lone pair center (GLPC) point defect with an unprecedented time resolution. The GLPC is a model point defect with a simple and well-understood electronic structure, highly relevant for several applications. Therefore, a full explanation of its photocycle is fundamental to understand the relaxation mechanisms of such molecular-like systems in solid state. The experiment, carried out exciting the sample resonantly with the ultraviolet (UV) GLPC absorption band peaked at 5.1 eV, gave us the possibility to follow the defect excitation-relaxation dynamics from the femto-picosecond to the nanosecond timescale in the UV-visible range. Moreover, the transient absorption signal was studied as a function of the excitation photon energy and comparative experiments were conducted on highly- and weakly-germanium doped silica glasses. The results offer a comprehensive picture of the relaxation dynamics of GLPC and allow observing the interplay between electronic transitions localized on the defect and those related to bandgap transitions, providing a clear evidence that the role of dopant high concentration is not negligible in the earliest dynamics.

The role of mechanical stretching in the activation and localization of adhesion proteins and related intracellular molecules.

The molecular complexity of the processes which lead to cell adhesion includes membrane and cytoskeletal proteins, involved in the focal adhesion formation, as well as signaling molecules tightly associated with the main intracellular regulatory cascades (Akt/PKB and MAPK/Erk). Dynamic environments, which create substrate deformations at determined frequencies and timing, have significant influences on adhesion mechanisms and in general in cellular behavior. In this work, we investigated the role of mechanical stretching (10% substrate deformation, 1 Hz frequency applied up to 60 min) on adhesion proteins (vinculin and focal adhesion kinase-FAK), related RhoGTPases (Rac1 and RhoA), and intracellular pathways (Akt/PKB and MAPK/Erk) in terms of activation and membrane recruitment in relation with cytoskeletal changes observed (membrane ruffling and filopodia formation). These changes are due to intracellular molecular rearrangements, acting with sequential concerted dynamics, able to modify the cytoskeletal conformation. The observed cellular response adds some important issues for better understanding the cellular behavior in environment which mimic as close as possible the physiological conditions.

Evolution of photo-induced defects in Ge-doped fiber/preform: influence of the drawing.

We have studied the generation mechanisms of two different radiation-induced point defects, the Ge(1) and Ge(2) centers, in a germanosilicate fiber and in its original preform. The samples have been investigated before and after X-ray irradiation using the confocal microscopy luminescence and the electron paramagnetic resonance techniques. Our experimental results show the higher radiation sensitivity of the fiber as compared to the perform and suggest a relation between Ge(1) and Ge(2) generation. To explain our data we have used different models, finding that the destruction probability of the Ge(1) and Ge(2) defects is larger in fiber than in preform, whereas the generation one is similar. Finally we found that the higher radiation sensitivity of the fiber at low doses is essentially related to the presence of germanium lone pair center generated by the drawing.

Overstressed mechanical stretching activates survival and apoptotic signals in fibroblasts.

The interest of scientists in the effects of mechanical stresses on cells is growing, in order to reproduce and understand cell behaviour in an environment closely reproducing physiological conditions. There have been many studies showing that mechanical stimulations are involved in regulating the proliferation, apoptosis and synthesis of proteins and cell morphology. In this study, we have considered the effects of a 20% stretching mechanical stress on MRC5 lung fibroblast cells in order to verify the role of survival/apoptotic pathways. As a survival pathway, the activation of Akt has been studied in association with pro-apoptotic or anti-apoptotic signals such as the Bax/Bcl-2 ratio and cleavage of caspases 3 and 9. Findings have shown the effects of overstressed cellular stretching to be a balance of a cause-and-effect reaction between survival and apoptosis.

Serum anti-p53 antibodies as a diagnostic tumor marker: observations in patients with malignant and premalignant oral cavity lesions.

AIM: To determine whether serum anti-p53 antibody (p53-Abs) positivity in patients with oral carcinoma corresponds with tumor localization, histological grade, stage, and recurrence. METHODS: The study population was divided into three groups: controls; patients with a premalignant lesion; and patients with oral squamous cell carcinoma (SCC). The third group was composed of patients attending outpatient services for pathological diagnosis or for follow-up monitoring only. The cancer patients had undergone resective surgery in local anesthesia. Serum p53-Abs levels were measured using a commercial enzyme-linked immunosorbent assay (ELISA) and monitored over a 3-year follow-up period. RESULTS: Controls and patients with premalignant lesions did not test positive for p53-Abs at ELISA testing. Patients with a malignant lesion tested positive at initial diagnosis when a high histopathological grade lesion was present or localized to the posterior region of the oral cavity. Postoperative serum p53-Abs levels gradually declined until complete seronegativity. Patients with a recurrent tumor tested positive for p53-Abs. CONCLUSION: Seropositivity for p53-Abs may be associated with histopathological tumor grade, localization, and recurrence. The findings suggest that serum p53-Abs analysis is a useful diagnostic marker for oral SCC.

Room temperature instability of E'gamma centers induced by gamma irradiation in amorphous SiO2.

We study by optical absorption measurements the stability of E'(gamma) centers induced in amorphous silica at room temperature by gamma irradiation up to 79 kGy. A significant portion of the defects spontaneously decay after the end of irradiation, thus allowing the partial recovery of the transparency loss initially induced by irradiation. The decay kinetics observed after gamma irradiation with a 0.6 kGy dose closely resembles that measured after exposure to 2000 pulses of pulsed ultraviolet (4.7 eV) laser light of 40 mJ/cm(2) energy density per pulse. In this regime, annealing is ascribed to the reaction of the induced E'(gamma) centers with diffusing H(2) of radiolytic origin. At higher gamma doses, the decay kinetics becomes unexpectedly slower notwithstanding the progressive growth of the concentration of induced defects. In particular, the annealing kinetics of E'(gamma) centers after 79 kGy irradiation is inconsistent with the reaction parameters between the defect and H(2). To explain this result, on the basis of the quantitative analysis of the kinetics, we propose water-related species to be responsible for the slow room temperature annealing of E'(gamma) after irradiation with such a dose. This model is qualitatively supported by results obtained by IR absorption measurements, which show an increase of the absorption in the spectral region of Si-OH groups.

GABAA receptors expression pattern in rat brain following low pressure distension of the stomach.

It is known that gastric mechanoreceptor stimuli are widely integrated into neuronal circuits that involve visceral nuclei of hindbrain as well as several central brain areas. GABAergic neurons are widely represented in hindbrain nuclei controlling gastric motor functions, but limited information is available specifically about GABA(A)-responding neurons in brain visceral areas. The present investigation was designed to determine the central sensory neuronal pathways and their GABA(A)-alpha1 and -alpha3 receptor presenting neurons that respond to gastric mechanoreceptor stimulation within the entire rat brain. Low pressure gastric distension was used to deliver physiological mechanical stimuli in anesthetized rats, and different protocols of gastric distension were performed to mimic different stimulation patterns with and without sectioning vagal and/or splanchnic afferent nerves. Mapping of activated neurons was investigated using double colorimetric immunohistochemistry for GABA(A)-alpha1 or -alpha3 subunits and c-Fos. Following stomach distension, neurons expressing GABA(A) receptors with alpha1 or alpha3 subunits were detected. Low frequency gastric distension induced c-Fos expression in nucleus tractus solitarii (NTS) only, whereas in the high frequency gastric distension c-Fos positive nuclei were found in lateral reticular nucleus and in NTS in addition to some forebrain areas. In contrast, during the tonic-rapid gastric distension the neuronal activation was found in hindbrain, midbrain and forebrain areas. Moreover different protocols of gastric stimulation activated diverse patterns of neurons presenting GABA(A)-alpha1 or -alpha3 receptors within responding brain nuclei, which may indicate a probable functional significance of differential expression of GABA(A)-responding neurons. The same protocol of gastric distension performed in vagotomized rats has confirmed the primary role of the vagus in the response of activation of gastric brain areas, whereas neuronal input of splanchnic origins was shown to play an important role in modulating the mechanogastric response of brain areas.

Functionalization of a poly(D,L)lactic acid surface with galactose to improve human keratinocyte behavior for artificial epidermis.

The production of artificial epidermis using reabsorbable polymeric matrices is one of possible goals; one of most used strategies in this field is the polymer substrate functionalitation using specific growth factors, in order to accelerate and improve keratinocyte adhesion and proliferation. In this study films of poly(D,L)lactide (P(D,L)LA), have been functionalized with various concentrations of galactose (GAL, 1-5-10%, w/v) conjugated with poly-L-lysine (PLL) using 1-etil-3-(3-diaminopropil) carbodiimide (EDC) as a coupling agent. GAL is a disaccharide present in the extracellular matrix (ECM) and it is bind by Galectines, a family of cell receptors whose activation regulate the cell-matrix interaction and cell growth and apoptosis. One of these receptors, Galectin-7 (Gal-7), is selectively expressed by human keratinocytes. Spontaneously immortalized human keratinocytes (HaCaT) that express high level of Gal-7 were allowed to adhere for 4 h in serum free condition on control P(D,L)LA (PLA), and on PLA-GAL and cell proliferation; the production of matrix metalloproteinases (MMP-2, MMP-9, and MMP-28), involved in cellular migration and tissue homeostasis have been analyzed after 24 h. The presence of GAL onto the polymer surface increased both cell adhesion, spreading and proliferation along with MMP-9 and MMP-28 production, suggesting that polymer functionalization using GAL could be an useful tool for the production of an artificial epidermis.

Generation of defects in amorphous SiO(2) assisted by two-step absorption on impurity sites.

Generation of the Si dangling bond defect in amorphous SiO(2) (E' centre) induced by tunable pulsed UV laser radiation was investigated by in situ optical absorption measurements. The defect generation efficiency peaks when the photon energy equals ∼5.1 eV, it depends quadratically on laser intensity and is correlated with the native linear absorption due to Ge impurities. We propose a model in which the generation of E' is assisted by a two-step absorption process occurring on Ge impurity sites.