Superradiant Thomson scattering from graphite in the extreme ultraviolet.

We study the Thomson scattering from highly oriented pyrolitic graphite excited by the extreme ultraviolet, coherent pulses of FERMI free electron laser (FEL). An apparent nonlinear behavior is observed and fully described in terms of the coherent nature of both exciting FEL beam and scattered radiation, producing an intensity-dependent enhancement of the Thomson scattering cross-section. The process resembles Dicke's superradiant phenomenon and is thus interpreted as the observation of superradiant Thomson scattering. The process also triggers the creation of coherent, low-q ([Formula: see text] 0.3 Å[Formula: see text]), low energy phonons. The experimental data and analysis provide quantitative information on the sample characteristics, absorption, scattering factor, and coherent phonon energies and populations and open the route for the investigation of the deep nature of complex materials.

Infrared Resonance Raman of Bilayer Graphene: Signatures of Massive Fermions and Band Structure on the 2D Peak.

Few-layer graphene possesses low-energy carriers that behave as massive Fermions, exhibiting intriguing properties in both transport and light scattering experiments. Lowering the excitation energy of resonance Raman spectroscopy down to 1.17 eV, we target these massive quasiparticles in the split bands close to the K point. The low excitation energy weakens some of the Raman processes that are resonant in the visible, and induces a clearer frequency-separation of the substructures of the resonance 2D peak in bi- and trilayer samples. We follow the excitation-energy dependence of the intensity of each substructure, and comparing experimental measurements on bilayer graphene with ab initio theoretical calculations, we trace back such modifications on the joint effects of probing the electronic dispersion close to the band splitting and enhancement of electron-phonon matrix elements.

Physical and chemical properties and degradation of MAPbBr3 films on transparent substrates.

To date, the potential exploitation of hybrid organic-inorganic perovskites (HOIPs) in photovoltaic technologies has been significantly hampered by their poor environmental stability. HOIP degradation can be triggered by conventional operational environments, with excessive heating and exposure to oxygen and moisture significantly reducing the performances of HOIP-based solar cells. An imperative need emerges for a thorough investigation on the impact of these factors on the HOIP stability. In this work, the degradation of methylammonium lead bromide (CH3NH3PbBr3) thin films, deposited via spin-coating on indium tin oxide (ITO) and strontium titanate (STO) substrates, was investigated by combining Raman and ultraviolet-visible (UV-Vis) absorption spectroscopy, as well as optical and fluorescence microscopy. We assessed the physical and chemical degradation of the films occurring under diverse preservation conditions, shedding light on the byproducts emerging from different degradation pathways and on the optimal HOIP preservation conditions.

Probing Enhanced Electron-Phonon Coupling in Graphene by Infrared Resonance Raman Spectroscopy.

We report on resonance Raman spectroscopy measurements with excitation photon energy down to 1.16 eV on graphene, to study how low-energy carriers interact with lattice vibrations. Thanks to the excitation energy close to the Dirac point at K, we unveil a giant increase of the intensity ratio between the double-resonant 2D and 2D^{'} peaks with respect to that measured in graphite. Comparing with fully ab initio theoretical calculations, we conclude that the observation is explained by an enhanced, momentum-dependent coupling between electrons and Brillouin zone-boundary optical phonons. This finding applies to two-dimensional Dirac systems and has important consequences for the modeling of transport in graphene devices operating at room temperature.

Advanced Raman Spectroscopy Detection of Oxidative Damage in Nucleic Acid Bases: Probing Chemical Changes and Intermolecular Interactions in Guanosine at Ultralow Concentration.

DNA/RNA synthesis precursors are especially vulnerable to damage induced by reactive oxygen species occurring following oxidative stress. Guanosine triphosphates are the prevalent oxidized nucleotides, which can be misincorporated during replication, leading to mutations and cell death. Here, we present a novel method based on micro-Raman spectroscopy, combined with ab initio calculations, for the identification, detection, and quantification of oxidized nucleotides at low concentration. We also show that the Raman signature in the terahertz spectral range (<100 cm-1) contains information on the intermolecular assembly of guanine in tetrads, which allows us to further boost the oxidative damage detection limit. Eventually, we provide evidence that similar analyses can be carried out on samples in very small volumes at very low concentrations by exploiting the high sensitivity of surface-enhanced Raman scattering combined with properly designed superhydrophobic substrates. These results pave the way for employing such advanced spectroscopic methods for quantitatively sensing the oxidative damage of nucleotides in the cell.

Phonon Engineering in Twinning Superlattice Nanowires.

One of the current challenges in nanoscience is tailoring the phononic properties of a material. This has long been a rather elusive task because several phonons have wavelengths in the nanometer range. Thus, high quality nanostructuring at that length-scale, unavailable until recently, is necessary for engineering the phonon spectrum. Here we report on the continuous tuning of the phononic properties of a twinning superlattice GaP nanowire by controlling its periodicity. Our experimental results, based on Raman spectroscopy and rationalized by means of ab initio theoretical calculations, give insight into the relation between local crystal structure, overall lattice symmetry, and vibrational properties, demonstrating how material engineering at the nanoscale can be successfully employed in the rational design of the phonon spectrum of a material.

Dyes from the Ashes: Discovering and Characterizing Natural Dyes from Mineralized Textiles.

Vesuvius eruption that destroyed Pompeii in AD 79 represents one of the most important events in history. The cataclysm left behind an abundance of archeological evidence representing a fundamental source of the knowledge we have about ancient Roman material culture and technology. A great number of textiles have been preserved, rarely maintaining traces of their original color, since they are mainly in the mineralized and carbonized state. However, one outstanding textile sample displays a brilliant purple color and traces of gold strips. Since the purple was one of the most exclusive dyes in antiquity, its presence in an important commercial site like Pompeii induces us to deepen the knowledge of such artifacts and provide further information on their history. For this reason, the characterization of the purple color was the main scope of this research, and to deepen the knowledge of such artifacts, the SERS (Surface Enhanced Raman Scattering) in solution approach was applied. Then, these data were enriched by HPLC-HRMS analyses, which confirmed SERS-based hypotheses and also allowed to hypothesize the species of the origin mollusk. In this context, a step-by-step integrated approach resulted fundamental to maximize the information content and to provide new data on textile manufacturing and trade in antiquity.

Crystalline, Phononic, and Electronic Properties of Heterostructured Polytypic Ge Nanowires by Raman Spectroscopy.

Semiconducting nanowires (NWs) offer the unprecedented opportunity to host different crystal phases in a nanostructure, which enables the formation of polytypic heterostructures where the material composition is unchanged. This characteristic boosts the potential of polytypic heterostructured NWs for optoelectronic and phononic applications. In this work, we investigate cubic Ge NWs where small (∼20 nm) hexagonal domains are formed due to a strain-induced phase transformation. By combining a nondestructive optical technique (Raman spectroscopy) with density-functional theory (DFT) calculations, we assess the phonon properties of hexagonal Ge, determine the crystal phase variations along the NW axis, and, quite remarkably, reconstruct the relative orientation of the two polytypes. Moreover, we provide information on the electronic band alignment of the heterostructure at points of the Brillouin zone different from the one (Γ) where the direct band gap recombination in hexagonal Ge takes place. We demonstrate the versatility of Raman spectroscopy and show that it can be used to determine the main crystalline, phononic, and electronic properties of the most challenging type of heterostructure (a polytypic, nanoscale heterostructure with constant material composition). The general procedure that we establish can be applied to several types of heterostructures.

Hexagonal Silicon Realized.

Silicon, arguably the most important technological semiconductor, is predicted to exhibit a range of new and interesting properties when grown in the hexagonal crystal structure. To obtain pure hexagonal silicon is a great challenge because it naturally crystallizes in the cubic structure. Here, we demonstrate the fabrication of pure and stable hexagonal silicon evidenced by structural characterization. In our approach, we transfer the hexagonal crystal structure from a template hexagonal gallium phosphide nanowire to an epitaxially grown silicon shell, such that hexagonal silicon is formed. The typical ABABAB... stacking of the hexagonal structure is shown by aberration-corrected imaging in transmission electron microscopy. In addition, X-ray diffraction measurements show the high crystalline purity of the material. We show that this material is stable up to 9 GPa pressure. With this development, we open the way for exploring its optical, electrical, superconducting, and mechanical properties.

Molecular mechanisms behind BRACO19 and human telomeric G-quadruplex interaction.

Human telomeres (HTs) can form DNA G-quadruplex (G4), an attractive target for anticancer and antiviral drugs. HT-G4s exhibit inherent structural polymorphism, posing challenges for understanding their specific recognition by ligands. Here, we aim to explore the impact of different topologies within a small segment of the HT (Tel22) on its interaction with BRACO19, a rationally designed G4 ligand with high quadruplex affinity, already employed in in-vivo treatments. Our multi-technique approach is based on the combined use of a set of contactless spectroscopic tools. Circular dichroism and UV resonance Raman spectroscopy probe ligand-induced conformational changes in the G4 sequence, while UV-visible absorption, coupled with steady-state fluorescence spectroscopy, provides further insights into the electronic features of the complex, exploiting the photoresponsive properties of BRACO19. Overall, we find that modifying the topology of the unbound Tel22 through cations (K+ or Na+), serves as a critical determinant for ligand interactions and binding modes, thus influencing the HT-G4's assembly capabilities. Furthermore, we show how fluorescence serves as a valuable probe for recognizing cation-driven multimeric structures, which may be present in living organisms, giving rise to pathological forms.

Signatures of Polaron Dynamics in Photoexcited MAPbBr3 by Infrared Spectroscopy.

Hybrid organic-inorganic perovskites (HOIPs) have attracted considerable attention in the past years as photoactive materials for low-cost, high-performance photovoltaics. Polaron formation through electron-phonon coupling has been recognized as the leading mechanism governing charge carrier transport and recombination in HOIPs. In this work, two types of MAPbBr3 film samples deposited on different substrates (transparent insulating SrTiO3 and a heterostructure mimicking a functioning photovoltaic cell) were photoexcited with above-bandgap radiation at 450 nm, and the effects of illumination on the sample were analyzed in the infrared region. The infrared absorbance detected at different powers of the photoexciting laser allowed us to obtain an estimate of the characteristic decay time of photoexcited polaron population of the order of 100-1000 ns. When focusing on the absorption features of the MA molecular cation in the region of the NH stretching modes, we observed the influence of hydrogen bonding and the effect of the polaron dynamics on the cation reorientation.

Nanoscale Surface-Enhanced Raman Spectroscopy Investigation of a Polyphenol-Based Plasmonic Nanovector.

The demand for next-generation multifunctional nanovectors, combining therapeutic effects with specific cellular targeting, has significantly grown during the last few years, pursuing less invasive therapy strategies. Polyphenol-conjugated silver nanoparticles (AgNPs) appear as potential multifunctional nanovectors, integrating the biorecognition capability and the antioxidant power of polyphenols, the antimicrobial activity of silver, and the drug delivery capability of NPs. We present a spectroscopic and microscopic investigation on polyphenol-synthesized AgNPs, selecting caffeic acid (CA) and catechol (CT) as model polyphenols and using them as reducing agents for the AgNP green synthesis, both in the presence and in the absence of a capping agent. We exploit the plasmonic properties of AgNPs to collect Surface-Enhanced Raman Scattering (SERS) spectra from the nanosized region next to the Ag surface and to characterize the molecular environment in the proximity of the NP, assessing the orientation and tunable deprotonation level of CA, depending on the synthesis conditions. Our results suggest that the SERS investigation of such nanovectors can provide crucial information for their perspective biomedical application.

Sensing tissue engineered cartilage quality with Raman spectroscopy and statistical learning for the development of advanced characterization assays.

Nasal chondrocyte-derived engineered cartilage has been demonstrated to be safe and feasible for the treatment of focal cartilage lesions with promising preliminary evidences of efficacy. To ensure the quality of the products and processes, and to meet regulatory requirements, quality controls for identity, purity, and potency need to be developed. We investigated the use of Raman spectroscopy, a nondestructive analytical method that measures the chemical composition of samples, and statistical learning methods for the development of quality controls to quantitatively characterize the starting biopsy and final grafts. We provide a proof-of-concept to show how Raman spectroscopy can be used to identify the types of tissues found in a nasal septal biopsy, i.e., hyaline cartilage and perichondrium, for a novel tissue identity assay. The tissues could be classified with a sensitivity of 89% and specificity of 77%. We also show how clinically relevant and mature nasal chondrocyte-derived engineered cartilage can be assessed with Raman spectroscopy for the development of potency assays. The maturity of engineered grafts, based on the quantified ratio of glycosaminoglycans to DNA and histological score, could be accurately assessed (R2 = 0.78 and 0.89, respectively, between predicted and measured values). Our results demonstrate the potential of Raman spectroscopy for the development of characterization assays for regenerative therapies that could be integrated into a good manufacturing practice-compliant process.

Exploring the Potentiality of a SERS-Active pH Nano-Biosensor.

The merging of the molecular specificity of Raman spectroscopy with the extraordinary optical properties of metallic nanoarchitectures is at the heart of Surface Enhanced Raman Spectroscopy (SERS), which in the last few decades proved its worth as powerful analytical tool with detection limits pushed to the single molecule recognition. Within this frame, SERS-based nanosensors for localized pH measurements have been developed and employed for a wide range of applications. Nevertheless, to improve the performances of such nanosensors, many key issues concerning their assembling, calibration and stability, that could significantly impact on the outcome of the pH measurements, need to be clarified. Here, we report on the detailed characterization of a case study SERS-active pH nanosensor, based on the conjugation of gold nanoparticles with the pH-sensitive molecular probe 4-mercaptobenzoic acid (4MBA). We analyzed and optimized all the aspects of the synthesis procedure and of the operating conditions to preserve the sensor stability and provide the highest responsiveness to pH. Exploiting the dependence of the SERS spectrum on the protonation degree of the carboxylic group at the edge of the 4MBA molecules, we derived a calibration curve for the nanosensor. The extrapolated working point, i.e., the pH value corresponding to the highest sensitivity, falls at pH 5.6, which corresponds to the pKa value of the molecule confined at the nanoparticle surface. A shift of the pKa of 4MBA, observed on the molecules confined at the nanostructured interface respect to the bulk counterpart, unveils the opportunity to assembly a SERS-based pH nanosensor with the ability to select its working point in the sensitivity region of interest, by acting on the nanostructured surface on which the molecular probe is confined. As a proof-of-concept, the nanosensor was successfully employed to measure the extracellular pH of normal and cancer cells, demonstrating the capability to discriminate between them.

Antifolate SERS-active nanovectors: quantitative drug nanostructuring and selective cell targeting for effective theranostics.

One of the frontiers of nanomedicine is the rational design of theranostic nanovectors. These are nanosized materials combining diagnostic and therapeutic capabilities, i.e. capable of tracking cancer cells and tissues in complex environments, and of selectively acting against them. We herein report on the preparation and application of antifolate plasmonic nanovectors, made of functionalized gold nanoparticles conjugated with the folic acid competitors aminopterin and methotrexate. Due to the overexpression of folate binding proteins on many types of cancer cells, these nanosystems can be exploited for selective cancer cell targeting. The strong surface enhanced Raman scattering (SERS) signature of these nanovectors acts as a diagnostic tool, not only for tracing their presence in biological samples, but also, through a careful spectral analysis, to precisely quantify the amount of drug loaded on a single nanoparticle, and therefore delivered to the cells. Meanwhile, the therapeutic action is implemented based on the strong toxicity of antifolate drugs. Remarkably, supplying the drug in the nanostructured form, rather than as a free molecule, enhances its specific toxicity. The selectivity of the antifolate nanovectors can be optimized by the design of a hybrid folate/antifolate coloaded nanovector for the specific targeting of folate receptor α, which is overexpressed on numerous cancer cell types.

A new multi analytical approach for the identification of synthetic and natural dyes mixtures. The case of orcein-mauveine mixture in a historical dress of a Sicilian noblewoman of nineteenth century.

In this paper, the application of a multi-analytical approach for the characterisation of synthetic and natural dyes in a historical textile is presented. The work is focused on a historical dress of a Sicilian noblewoman, dating from about 1865-1870. Firstly, SERS on fibre was performed, in order to individuate the classes of dyes employed. The SERS spectra suggested the presence of two main dyes: mauveine and orcein. In order to confirm these preliminary results, two different extraction protocols were applied. The extracts obtained were analysed by ESI-MS, MALDI-ToF and UHPCL-MS analyses, confirming the SERS results. In particular, the application of the ammonia mild extraction technique allowed to selectively extract the phenoxazonic dyes, separating them already in the extraction step from the synthetic ones. Thanks to this multi-analytical approach, this dress could be considered as one of the first examples of employment of synthetic dyes in association with natural ones.