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.

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.

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.

Real time monitoring of water level and temperature in storage fuel pools through optical fibre sensors.

We present an innovative architecture of a Rayleigh-based optical fibre sensor for the monitoring of water level and temperature inside storage nuclear fuel pools. This sensor, able to withstand the harsh constraints encountered under accidental conditions such as those pointed-out during the Fukushima-Daiichi event (temperature up to 100 °C and radiation dose level up to ~20 kGy), exploits the Optical Frequency Domain Reflectometry technique to remotely monitor a radiation resistant silica-based optical fibre i.e. its sensing probe. We validate the efficiency and the robustness of water level measurements, which are extrapolated from the temperature profile along the fibre length, in a dedicated test bench allowing the simulation of the environmental operating and accidental conditions. The conceived prototype ensures an easy, practical and no invasive integration into existing nuclear facilities. The obtained results represent a significant breakthrough and comfort the ability of the developed system to overcome both operating and accidental constraints providing the distributed profiles of the water level (0-to-5 m) and temperature (20-to-100 °C) with a resolution that in accidental condition is better than 3 cm and of ~0.5 °C respectively. These new sensors will be able, as safeguards, to contribute and reinforce the safety in existing and future nuclear power plants.

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.

The interaction of photoexcited carbon nanodots with metal ions disclosed down to the femtosecond scale.

Fluorescent carbon nanodots are a novel family of carbon-based nanoscale materials endowed with an outstanding combination of properties that make them very appealing for applications in nanosensing, photonics, solar energy harvesting and photocatalysis. One of the remarkable properties of carbon dots is their strong sensitivity to the local environment, especially to metal ions in solution. These interactions provide a testing ground for their marked photochemical properties, highlighted by many studies, and frequently driven by charge transfer events. Here we combine several optical techniques, down to femtosecond time resolution, to understand the interplay between carbon nanodots and aqueous metal ions such as Cu2+ and Zn2+. We find that copper inhibits the fluorescence of carbon dots through static and diffusional quenching mechanisms, and our measurements allow discriminating between the two. Ultrafast optical methods are then used to address the dynamics of copper-dot complexes, wherein static quenching takes place, and unveil the underlying complexity of their photocycle. We propose an initial increase of electronic charge on the surface of the dot, upon photo-excitation, followed by a partial electron transfer to the nearby ion, with 0.2 ps and 1.9 ps time constants, and finally a very fast (≪1 ps) non-radiative electron-hole recombination which brings the system back to the ground state. Notably, we find that the electron transfer stage is governed by an ultrafast water rearrangement around photo-excited dots, pointing out the key role of solvent interactions in the photo-physics of these systems.

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.

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.

Luminescence from nearly isolated surface defects in silica nanoparticles.

A structured emission/excitation pattern, proper of isolated defects, arises in a vacuum from silica nanoparticles. The luminescence, centered around 3.0-3.5 eV, is characterised by a vibronic progression due to the phonon coupling with two localised modes of frequency ∼1370 cm(-1) and ∼360 cm(-1), and decays in about 300 ns at 10 K. On increasing the temperature, the intensity and the lifetime decrease due to the activation of a non-radiative rate from the excited state. Concurrently, the temperature dependence of the lineshape evidences the low coupling with non-localised modes of the matrix (Huang-Rhys factor S ~ 0.2) and the poor influence of the inhomogeneous broadening. These findings outline an uncommon behaviour in the field of the optical properties of defects in amorphous solids, evidencing that the silica surface can allocate luminescent defects almost disentangled from the basal network.

Poly-L-lactide acid-modified scaffolds for osteoinduction and osteoconduction.

Poly-L-lactide acid (PLLA) scaffold has been modified to enhance its osteoconductive and osteoinductive properties in view of a bone tissue engineering application. Two approaches have been followed: (i) coating with laminin or fibronectin and (ii) grafting with arginine-glycine-aspatic acid (RGD) or SIKVAV peptides. Moreover we have added a bioactive molecule 1,25-(OH)2 D3 into the scaffold that shows better cellular interaction to implement osteoinduction and osteogenesis. The two coatings promoted only cell adhesion in the very short term while even if grafted scaffolds had cell seeding efficiency similar to ungrafted PLLA, the grafted ones supported better the proliferation of seeded human osteoblast (hOB) and human mesenchymal stem cells (hMSCs) over 1 week of culture. Our data showed that in view of bone integration and bone regeneration, PLLA grafting with RGD can be considered a good substrate to induce hOB adhesion and proliferation but having no significant effect on the osteogenic induction, the scaffold has to be reinforced with osteoinductive molecules. It can be concluded from reverse transcriptase polymerase chain reaction results, alkaline phosphatase activity and mineralization assays that 1,25(OH)2 D3 reinforced RGD-PLLA keeps increased cell proliferation supported by an upregulation of the studied osteogenic markers and induced hMSCs differentiation into osteoblasts demonstrating osteoinductivity and osteoconductivity of the new formulated scaffold. These results can lead to a future application of RGD-D3-PLLA as an osteogenic material for bone replacement..

Cardiovascular biomaterials: when the inflammatory response helps to efficiently restore tissue functionality?

The evaluation of biological host response to implanted materials permits the determination of the safety and biocompatibility of biomedical devices, prostheses and biomaterials. Once a biomaterial is introduced into the body to a corresponding implant site, a sequence of events occurs promoting the activation of inflammatory mediators such as leukocytes and the release of signaling molecules such as cytokines and growth factors, evoking an inflammatory and wound healing process. This review examines the cellular and molecular mechanisms involved in the foreign body reaction, especially how cytokines impact the overall inflammatory response to devices. It also reviews how these events can be modulated by the physical and chemical properties of the biomaterials such as wettability, chemistry and geometry of surface. Particular attention is dedicated to the cardiovascular field, where the use of synthetic polymers has several limitations such as thrombogenicity and risk of infection. New materials and strategies to improve biomaterial characteristics are discussed.

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.

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.

Biological evaluation of materials for cardiovascular application: the role of the short-term inflammatory response in endothelial regeneration.

Because of their suitable bio-mechanical properties, polymeric materials, such as Poly(L-lactic acid) (PLLA), and poly (lactic-co-glycolic acid) (PLGA), are often used in the biomedical field, in particular for cardiovascular applications. Implanted materials induce several events related to the inflammatory reaction, such as macrophage adhesion and activation with following cytokine release. This work considered the effect of macrophage adhesion and related cytokine release on endothelial cells (PAOEC) proliferation and migration. Slight differences have been shown by the macrophages reaction when in contact with PLLA, PLGA, or PLLA/PLGA blend. However, these differences showed to differently enhance endothelial cells behavior in terms of wound healing. These data suggest the inflammatory reaction as a useful way to consider concerning materials biocompatibility, in order to optimize the endothelial regeneration following vascular prosthetic implants.

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.

PCBs contamination does not alter aerobic metabolism and tolerance to hypoxia of juvenile sole (Solea solea L. 1758).

Coastal habitats play a major role as nurseries for many fish species; however, they are also submitted to pollutants and oxygen fluctuations. Fry's concept of metabolic scope for activity was used to evaluate the effect of polychlorinated biphenyls (PCBs) on the aerobic metabolism in juvenile common sole (0-1 year old). Aerobic metabolic scope (AMS) in control and PCB-contaminated fish via food pathway was determined using respirometry techniques. Furthermore, the hypoxia tolerance in control and PCB-contaminated fish was evaluated by assessing their critical oxygen concentration (O(2crit)). Our results showed that while PCB-contaminated fish were able to maintain a constant AMS and O(2crit), PCBs tend to affect their aerobic metabolism by acting on maximal oxygen consumption (MO(2max)) in hypoxia and standard metabolic rate, but only at the highest PCB concentration between 30 and 60 days of exposure. In conclusion, we can hypothetise that the tested PCB-exposures may not impair the tolerance to hypoxia and the survival of common sole in their natural environment.

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.

The biological response of poly(L-lactide) films modified by different biomolecules: role of the coating strategy.

The interactions between the surface of synthetic scaffolds and cells play an important role in tissue engineering applications. To improve these interactions, two strategies are generally followed: surface coating with large proteins and surface grafting with small peptides. The proteins and peptides more often used and derived from the extracellular matrix, are fibronectin, laminin, and their active peptides, RGD and SIKVAV, respectively. The aim of this work was to compare the effects of coating and grafting of poly(L-lactide) (PLLA) films on MRC5 fibroblast cells. Grafting reactions were verified by X-ray photoelectron spectroscopy. Cell adhesion and proliferation on coated and grafted PLLA surfaces were measured by cell counting. Vinculin localization and distribution were performed on cell cultured on PLLA samples using a fluorescence microscopy technique. Finally, western blot was performed to compare signals of cell adhesion proteins, such as vinculin, Rac1, and RhoA, as well as cell proliferation, such as PCNA. These tests showed similar results for fibronectin and laminin coated PLLA, while RGD grafting is more effective compared with SIKVAV grafting. Considering the overall view of these results, although coating and grafting can both be regarded as effective methods for surface modification to enhance cell adhesion and proliferation on a biomaterial, RGD grafted PLLA show better cell adhesion and proliferation than coated PLLA, while SIKVAV grafted PLLA show similar adhesion but worse proliferation. These data verified different biological effects depending on the surface modification method used.