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Highly homogeneous and robust SERS conductive surfaces with large gold coverages and small inter-particle distances for a combined electrochemical-SERS investigation to monitor switchable redox molecules and electroactivated interfacial reactions.

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.

DNA/RNA synthesis precursors are especially vulnerable to damage induced by reactive oxygen species occurring following oxidative stress. Guanosine triphosphates are the prevalent oxidized nucleosites, 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 low concentrations of oxidized nucleotides. 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 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., 11 pages, 7 figures

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., This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement 756365) and from the Swiss National Science Foundation research Grant (Project Grant 200021_165784). R.R. acknowledges financial support by the Ministerio de Economi ́ a, Industria y Competitividad (MINECO) under Grant FEDER-MAT2017- 90024-P and the Severo Ochoa Centres of Excellence Program under Grant SEV-2015-0496 and by the Generalitat de Catalunya under Grant 2014 SGR 301. X.C. acknowledges financial support by the Ministerio de Economi ́ a, Industria y Competitividad under Grant TEC2015-67462-C2-1-R (MINECO/FEDER). The calculations have been performed at the Red Española de Supercomputacion (RES) within the ́ project “Phonons dispersion relation in GaP crystal phase superlattices” (FI-2017-1-0026). Solliance and the Dutch province of Noord-Brabant are acknowledged for funding the TEM facility. M.D.L. acknowledges support from the Swiss National Science Foundation Ambizione Grant (Grant PZ00P2_179801). C.F. acknowledges financial support from Sapienza University scholarship “Borsa di Perfezionamento all’Estero 2017−2018”. The authors thank A. Campo and M. Carballido for nanowire manipulation and Z. Azdad for preliminary work.

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.

The results of Fourier Transform Infrared absorption and Raman spectroscopy studies are reported for solid and liquid neat butane-1,4-diamine (DAB) and for the molecule in different solvents. Ab initio molecular dynamics and static calculations are reported in the work as an interpretative tool for the measurements and provide an analysis of the relative population of DAB conformers, both in vacuo and in the liquid phase. X-ray diffraction data for liquid DAB are presented to further consolidate the interpretation of the conformation and structure of the biomolecule in the liquid phase. The study of DAB in polar solutions suggests a high level of structuring of the molecules in the liquid, as a consequence of the effect of solvent on the molecular charge distribution.

Raman scattering is a powerful inelastic light scattering technique able to probe the vibrational properties of materials. This technique has been successfully employed in semiconductor nanowires to provide information on their fundamental properties, such as the phononic properties, the crystal composition, and the electronic band structure. When performed in a polarization-resolved manner on a single nanowire, Raman spectroscopy can even allow addressing the nanowire’s crystal structure. This is a fact of pivotal importance, as crystal phase is emerging as a novel degree of freedom in the bandgap engineering and phonon engineering of materials, and the control of the crystal phase is a possibility uniquely offered by nanowires. Indeed, recent advances in the synthetic growth of nanowires have given access to crystal phases (e.g., hexagonal phase in Si and Ge) that in the bulk can only be obtained under extreme pressure conditions, and it is possible to controllably switch between different crystal phases during the growth of nanowires. The realization and, even more, the interpretation of polarized Raman experiments on nanowires can be non-trivial, as several issues have to be considered. Therefore, in this chapter, we provide the basic theoretical background necessary to calculate Raman selection rules and interpret polarization-resolved Raman spectra of semiconductor nanowires. We also discuss the main ingredients of a Raman setup, with a focus on the scattering geometries typically used for nanowires. We highlight the main differences in the Raman spectra of nanowires with cubic and hexagonal crystal symmetries, and we treat also the case of the most challenging type of heterostructure: a nanoscale crystal-phase homostructure. Finally, we discuss resonant Raman experiments that allow the determination of the energy of some electronic transitions in nanowires. We focus mostly on a very new material system, namely Ge nanowires with controlled crystal phase, but the general procedure that we establish can be applied to several types of nanostructures.

Exploiting the theory of solitons in a nonlinear elastic medium we predict a novel phenomenon called a phononic tsunami, which is characterized by the dramatic increase of the local amplitude of phonon modes. To elucidate the possible experimental detection of this phenomenon we propose to use a wedge-shaped sample in which a sharp edge serves for the emulation of the shoaling effect and such a local enhancement can be observed. Together with eigenfrequencies of transverse and longitudinal phonon modes of a system we find the characteristic dispersion relations that can be considered as a hallmark of a phononic tsunami. We justify our predictions by means of analytical calculations and numerical simulations showing a possible realization of this nonlinear effect in such a geometry. Our results provide the framework for the implementation of new kind experiments aimed at realizing and investigating a phononic tsunami phenomenon in relevant materials., Comment: 14 pages, 8 figures

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.

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.

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., I.Z. acknowledges financial support from the Swiss National Science Foundation research grant (Project Grant No. 200021_165784). C.F. acknowledges financial support from Sapienza University scholarship “Borsa di Perfezionamento all’Estero 2017–2018”. R.R. acknowledges financial support by the Ministerio de Economia, Industria y Competitividad (MINECO) under grant FEDER-MAT2017-90024-P and the Severo Ochoa Centres of Excellence Program under Grant SEV-2015-0496 and by the Generalitat de Catalunya under grants no. 2017 SGR 1506. L.V. acknowledges financial support from the IDEX Paris-Saclay (ANR-11-IDEX-0003-02). This work was partly supported by the ANR HEXSIGE project (ANR-17-CE030-0014-01) of the French Agence Nationale de la Recherche. R.R. and M.A. thank Giacomo Giorgi and Thanayut Kaewmaraya for useful discussions.

The programmable assembly of DNA strands is a promising tool for building tailored bottom-up nanostructures. Here, we present a plasmonic nanosystem obtained by the base-pairing mediated aggregation of gold nanoparticles (NPs) which are separately functionalized with two different single-stranded DNA chains. Their controlled assembly is mediated by a complementary DNA “bridge” sequence. We monitor the formation of DNA assembled NP aggregates in solution, and we study their Surface Enhanced Raman Scattering (SERS) response by comparison with the single NP constituents. We interpret the revealed SERS signatures in terms of the molecular and NP organization at the nanoscale, demonstrating that the action of the DNA bridge molecule yields regular NP aggregates with controlled interparticle distance and reproducible SERS response. In perspective, this demonstrates the potential of the present system as a stable, biocompatible, and recyclable SERS sensor.

We present an application of Raman spectroscopy for understanding the chemisorption catalytic processes occurred in impregnated activated carbon used in hydrogen sulfide (H2S) adsorption from biogas. Downstream of an H2S adsorption capacity study, conducted employing commercial carbon at different operating conditions as sorbent in a fixed bed flow reactor, spent carbon samples who returned suitable H2S removal performances were studied by means of Raman spectroscopy. Raman analysis showed that among all the H2S metal impregnates, copper was the chemical responsible for H2S adsorption, since observed Raman spectra exhibited a perfect overlapping with copper sulfide (CuS) signature. A further confirmation was given by a thermal decomposition test conducted through the Raman optical apparatus over an H2S-exposed carbon sample, by estimating anti-Stokes and Stokes Raman processes relative efficiency. Raman results turned out to be consistent with thermogravimetry tests results executed over the same spent carbon samples. The experiment witnesses the capability of Raman spectroscopy to determinate the chemical nature of compounds resulting from H2S adsorption, overcoming the lack in chemical information encountered by elemental techniques such as energy-dispersive X-ray. We thus propose Raman spectroscopy as a way to integrate traditional chemical investigation procedures.

The temperature dependence of the surface plasmon resonance in small metal spheres is calculated using an electron gas model within the Random Phase Approximation. The calculation is mainly devoted to the study of spheres with diameters up to at least 10 nm, where quantum effects can still be relevant and simple plasmon pole approximation for the dielectric function is no more appropriate. We find a possible blue shift of the plasmon resonance position when the temperature is increased while keeping the size of the sphere fixed. The blue shift is appreciable only when the temperature is a large fraction of the Fermi energy. These results provide a guide for pump and probe experiments with a high time resolution, and tailored to study the excited electron system before thermalisation with the lattice takes place., 5 pages, 3 figures

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.

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.

Recent progresses in nanotechnology and their application to biomedical settings have generated great advantages in dealing with early cancer diagnosis. The identification of specific properties of cancer cells, such as the expression of particular plasma membrane molecular receptors, has become crucial in revealing the presence and in assessing the stage of development of the disease. Here we report on a single cell screening approach based on Surface Enhanced Raman Scattering (SERS) microimaging. We fabricated a SERS-labelled nanovector based on the biofunctionalization of gold nanoparticles with folic acid. After treating the cells with the nanovector, we were able to distinguish three different cell populations from different cell lines (cancer HeLa and PC-3, and normal HaCaT lines), properly chosen for their different expression of folate binding proteins. The nanovector, indeed, binds much more efficiently on cancer cell lines than on normal ones, resulting in a higher SERS signal measured on cancer cells. These results pave the way for applications in single cell diagnostics and, potentially, in theranostics.

The identification of dyes from ancient textiles represents a challenging issue requiring high proficiency and versatility, that must be guided by the principles of non-destructiveness. The complexity of the dye/textile system influences both the choice of sampling and extraction procedures and the analytical methodology. For instance, in order to preserve both the fibre and the original chromophores on it, the extraction procedures commonly used for natural dyes have evolved in last few decades towards increasingly mild conditions. Along with this approach, the use of solid substrates for sampling has become prominent in several fields of Cultural Heritage diagnostics.With reference to these two aspects, the aim of this work is to combine gel substrates and a mild basic solution for the extraction of anthraquinone dyes (madder and cochineal) from wool textiles, in order to evaluate the versatility of the extraction procedure for different analytical techniques. Because of their wide use in the field of Conservation, agar and nanorestore gel substrates soaked in an ammonia-EDTA solution were tested. The dye-extracting substrates were then analysed with Raman and SERS spectroscopy, whilst the fraction re-extracted from them was examined with HPLC-HRMS. Finally, scanning electron microscopy was used to monitor the integrity of the wool after the extraction treatment with the ammonia-EDTA solution.HPLC-HRMS results show that the basic solution in both gels is effective for the dye extraction, and no alteration of wool fibres was evidenced by SEM-EDS. The Raman spectra give no information about the dyes, but only substrate signals are obtained. Conversely, SERS spectroscopy makes it possible to distinguish the two dyes, especially using the 633 nm laser on the agar substrate.

As already discussed in the introductory chapters, the wide interest and research focused on plasmonic nanostructures for near field coupling and enhanced field confinement has paved the way for the development of numerous specific applications in diverse fields, from sensors technology to medical diagnostics. Among these applications, plasmonic substrates for SERS spectroscopy, sensing and SERS-based chemical analysis have attracted much interest. In this chapter, we will present a study on the SERS efficiency of self-assembled mesoscopic nanoparticle aggregates by means of spectroscopy, atomic force microscopy and electromagnetic simulations. We will also discuss the preparation of EBL-template guided, self-assembled nanoparticle cluster arrays on solid substrates and their performances as SERS substrates for biosensing.

In this chapter, we will report on the application of folate-based SERS active nanovectors to single cell screening for cancer diagnostics. Owing to the overexpression of folate receptor on cancer cells, their selective targeting with the SERS-nanovector can be achieved. A single cell screening procedure based on Raman microimaging can be used to discriminate normal and cancer cells and the level of folate receptor expression can even be inferred by SERS measurements over statistically relevant cell populations. Therapeutic features of the nanovectors can be implemented by substituting folate molecule with toxic antifolate drugs. As the characterization of folate/antifolate nanovectors was discussed in Chap. 3, here we will mainly focus on the biophysical results of the experiment.

The essential building blocks of SERS-active systems we have studied are noble metal nanoparticles (Nps). They constitute the metallic substrate crucially contributing to SERS amplification thanks to the enhancement of the local electromagnetic field. Nowadays, Nps of various shape, size and material can be fabricated with a high degree of precision and their chemical properties can be tuned by employing different ligands. In this chapter, we will present the functionalization of noble metal Nps with a Raman reporter and their further conjugation with a biomolecule (folic acid). The characterization of the system with multiple techniques will be presented, and its SERS signature will be thoroughly discussed.

An interesting approach to the study of matter is represented by spectroscopy, that infers information about materials and biosystems by driving their interaction with controlled probes, as beams of light, electrons and neutrons. In this chapter, the basic physics of Raman and Surface Enhanced Raman Scattering (SERS) will be discussed. Furthermore, we will present a review of the scientific literature concerning the themes that are relevant for the Thesis, and in particular the fabrication of nanocolloid-based SERS substrates for sensing and biosensing and the application of SERS to biomedical problems.

The equilibrium optical phonons of graphene are well characterized in terms of anharmonicity and electron–phonon interactions; however, their non-equilibrium properties in the presence of hot charge carriers are still not fully explored. Here we study the Raman spectrum of graphene under ultrafast laser excitation with 3 ps pulses, which trade off between impulsive stimulation and spectral resolution. We localize energy into hot carriers, generating non-equilibrium temperatures in the ~1700–3100 K range, far exceeding that of the phonon bath, while simultaneously detecting the Raman response. The linewidths of both G and 2D peaks show an increase as function of the electronic temperature. We explain this as a result of the Dirac cones’ broadening and electron–phonon scattering in the highly excited transient regime, important for the emerging field of graphene-based photonics and optoelectronics., Non-equilibrium ultrafast processes in graphene entail relaxation pathways involving electron–electron and electron–phonon scattering events. Here, the authors probe graphene optical phonons at high electronic temperatures by means of Raman spectroscopy under pulsed excitation

The first page of this article is displayed as the abstract., Rohit Chikkaraddy opened the discussion of the Introductory Lecture: Regarding quantifying the chemical enhancement, you showed a systematic change in the SERS enhancement for halide substituted molecules due to charge transfer from the metal. Is the extra enhancement due to an inherent increase in the Raman cross-section of the molecule? How do you go about referencing, as the charge transfer changes the vibrational frequency? Richard Van Duyne answered: The extra enhancement is not due to an increase in the Raman cross section, as that is ratioed out in the calculation of the enhancement factor. The charge transfer (CT) process does not transfer a complete electron, it is a fractional degree of CT. Thus the change in vibrational frequency is small. DFT calculations that provide eigenvectors allow one to reference the vibrational modes of the free molecule with those of the adsorbed molecule. Sylwester Gawinkowski asked: You have shown that the enhancement factor curve is redshifted relative to the plasmon resonance band and has a maximum at about 800 nm. This means that the SERS signal should be strongest for excitations in the near infrared spectral region. Why do most SERS reports, particularly related to single molecule SERS, have the excitation in the green or red spectral range and not in the near infrared?

Jurgen Popp opened a general discussion of the paper by Roy Goodacre: What is the advantage of using SERS instead of ordinary Raman spectroscopy?

The electronic and molecular structure, as well as the chemical nature of noble metal (silver, gold) nanoparticles (AgNPs, AuNPs) stabilized by the negatively charged hydrophilic organic thiol Sodium 3-mercapto-1-propanesulfonate (3-MPS) has been probed combining Synchrotron Radiation-induced X-ray Photoelectron Spectroscopy (SR-XPS) and Surface Enhanced Raman Spectroscopy (SERS). Complementary information about the electronic, chemical and molecular structure is obtained combining SERS and XPS data, allowing to evidence structural similarities and differences related to the noble metal choice. These results coherently suggest the effective 3-MPS chemical absorption onto the nanoparticle surface, which allows to obtain a stable and biocompatible system, suitable for ad hoc further functionalization as required for several applications in biotechnology. It was observed that the NPs molecular overlayer is made up by covalent bonds between thiols and metallic surface atoms, without degradation of the molecule. The metals SR-XPS spectra analysis collected on the silver and gold nanoparticles points out a small surface to volume atomic percent ratio, as a consequence of the NPs size (≥5 nm). Surface-enhanced Raman Scattering (SERS) spectroscopy was employed for the investigation of the NP molecular conjugation. SERS spectra acquired on the capped Ag and AuNPs are in very good agreement and suggest that the same organic layer is covering the NPs. The conjugation with 3-MPS is confirmed by comparing SERS data with the conventional Raman signature of the ligand. The reproducibility of SERS measurements overall points out the high purity and homogeneity of the system and the good integrity of the organic capping layer.

We realized and tested a strategy for developing reproducible and stable two-dimensional gold nanoparticle cluster arrays arranged on silicon substrates, to be used for surface-enhanced Raman spectroscopy. We combined electron beam lithography and molecular functionalization to finely control the shape of the nanoparticle assemblies. Atomic force microscopy analysis allowed to optimize the procedure of nanoparticle cluster array fabrication in terms of electron beam dose and chemical protocol, enabling to achieve a good regularity in spacing and size (i.e. area and layer number) of the clusters. MicroRaman space resolved measurements were undertaken on large 2D arrays (100 × 100 μm2) made up of regularly microsized clusters. The high surface enhanced Raman signal measured on the structures highlights the full correspondence of the spatial and optical periodicity achieved. This standardized procedure represent the basis to realize versatile platforms for nano-optical investigation and towards an efficient high-sensitive multiplex sensing.

A recent work reported on different solid phases obtained on the protic ionic liquid propyl-ammonium nitrate (PAN) under a pressure up to 2 GPa. However, the experimental parameters and measurement protocols driving the solidification process were not clarified. Here, we report on and discuss three different Raman measurements on PAN carried out over different pressure ranges (up to 4 GPa). Exploiting different pressure increasing rates for each measurement we obtained a variety of solid phases. The differences between these phases are highlighted by visual inspection, by different solidification pressure values and by the Raman spectroscopic features relative to the anion stretching mode and to the cation alkyl chain vibration. Our results indicate different local structures surrounding the ionic couple in the solid phase and the capability of pressure to lock the cation alkyl chain in conformations different from those of the liquid state. These evidences also suggest the use of systematic and careful measurement protocols in order to reproduce a particular solid phase of PAN under pressure.

The coherent oscillations of the surface electron gas, known as surface plasmons, in metal nanostructures can give rise to the localization of intense electromagnetic fields at the metal-dielectric interface. These strong fields are exploited in surface enhanced spectroscopies, such as Surface Enhanced Raman Scattering (SERS), for the detection and characterization of molecules at very low concentration. Still, the implementation of SERS-based biosensors requires a high level of reproducibility, combined with cheap and simple fabrication methods. For this purpose, SERS substrates based on self-assembled aggregates of commercial metallic nanoparticles (Nps) can meet all the above requests. Following this line, we report on a combined micro-Raman and Atomic Force Microscopy (AFM) analysis of the SERS efficiency of micrometric silver Np aggregates (enhancement factors up to 109) obtained by self-assembly. Despite the intrinsic disordered nature of these Np clusters, we were able to sort out some general rules relating the specific aggregate morphology to its plasmonic response. We found strong evidences of cooperative effects among the NPs within the cluster and namely a clear dependence of the SERS-efficiency on both the cluster area (basically linear) and the number of stacked NPs layers. A cooperative action among the superimposed layers has been proved also by electromagnetic simulations performed on simplified nanostructures consisting of stacking planes of ordered Nps. Being clear the potentialities of these disordered self-assembled clusters, in terms of both easy fabrication and signal enhancement, we developed a specific nanofabrication protocol, based on electron beam lithography and molecular functionalization, that allowed for a fine control of the Np assemblies into designed shapes fixing their area and height. In particular, we fabricated 2D ordered arrays of disordered clusters choosing gold Nps owing to their high stability. AFM measurements confirmed the regularity in spacing and size (i.e. area and layer number) of the aggregates. Preliminary SERS measurements confirm the high signal enhancement and demonstrate a quite good reproducibility over large number of aggregates within 100×100 µm2 2D super-structure. The availability of such a multisensor could allow a careful statistical analysis of the SERS response, thus leading to a reliable quantitative estimate of the presence of relevant molecular species even at ultra-low concentration.

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.

A study of the Surface Enhanced Raman Scattering (SERS) from micrometric metallic nanoparticle aggregates is presented. The sample is obtained from the self-assembly on glass slides of micro-clusters of silver nanoparticles (60 and 100 nm diameter), functionalized with the organic molecule 4-aminothiophenol in water solution. For nanoparticle clusters at the micron scale, a maximum enhancement factor of 10(9) is estimated from the SERS over the Raman intensity ratio normalized to the single molecule contribution. Atomic force microscopy, correlated to spatially resolved Raman measurements, allows highlighting the connection between morphology and efficiency of the plasmonic system. The correlation between geometric features and SERS response of the metallic structures reveals a linear trend of the cluster maximum scattered intensity as a function of the surface area of the aggregate. On given clusters, the intensity turns out to be also influenced by the number of stacking planes of the aggregate, thus suggesting a plasmonic waveguide effect. The linear dependence results weakened for the largest area clusters, suggesting 30 mu m(2) as the upper limit for exploiting the coherence over large scale of the plasmonic response. (C) 2014 AIP Publishing LLC.

Thin films of copper oxide were grown by radio frequency-magnetron sputtering in an oxygen-argon environment onto silicon substrates at two different oxygen partial pressures (15% and 23%). Post deposition annealing in vacuum environment was conducted on the films at different temperatures (between 250 °C and 550 °C). We investigated the thin films by Scanning Electron Microscopy, Energy Dispersive X-ray, X-ray diffraction, Raman spectroscopy, Atomic Force Microscopy and Kelvin Probe Force Microscopy. These studies show that post-growth annealing in vacuum results in thin films with different morphological and stoichiometric properties. Furthermore, the oxygen partial pressure conditions during deposition have an impact over the obtained oxide phases: high oxygen partial pressure leads to the formation of two different oxide phases, i.e. CuO and Cu2O, while low oxygen partial pressure leads to the formation of Cu2O thin films. Notably, we also uncovered a parasitic crystallite growth as a result of aging on the pristine and low-temperature annealed samples, and we found out that high temperature annealing prevents this kind of aging.