Robust, reproducible, recyclable SERS substrates: monolayers of gold nanostars grafted on glass and coated with a thin silica layer.

We prepared and characterized recyclable surface enhanced Raman spectroscopy (SERS) active glass chips. Gold nanostars were grafted on properly functionalized glasses by means of electrostatic interactions and then they were coated with a silica layer of controllable thickness in the nanometer range. The SERS activity of the obtained substrates were tested in terms of reproducibility and homogeneity intra-samples and inter-samples from different batches using the Raman reporter as the model compound rhodamine 6G. The uncoated substrates were used as reference to evaluate the effect of silica spacers on SERS enhancement factors (EFs). The chemical route to obtain silica-coated SERS chips is described in detail, and the morphology and the optical response of substrates have been characterized. We demonstrate that SERS substrates coated with 1 nm silica conserve a good EF, and that the coating confers to the SERS platform an extreme robustness leading to reusability of the substrates.

Tailored coating of gold nanostars: rational approach to prototype of theranostic device based on SERS and photothermal effects at ultralow irradiance.

The last decade has come across an increasing demand for theranostic biocompatible nanodevices possessing the double ability of diagnosis and therapy. In this work, we report the design, synthesis and step-by-step characterization of rationally coated gold nanostars (GNSs) for the SERS imaging and photothermal therapy of HeLa cancer cells. The nanodevices were realized by synthesizing GNSs with a seed growth approach, coating them with a controlled mixture of thiols composed of a Raman reporter and a polyethylene glycol with a terminal amino group, and then reacting these amino groups with folic acid (FA), in order to impart selectivity towards cancer cells which overexpress folate receptors on their membranes. After a complete characterization, we demonstrate that these FA-functionalized GNSs (FA-GNSs) are able to bind selectively to the membranes of HeLa cells, acting as SERS tags and allowing SERS imaging. Moreover, we demonstrate that once bound to HeLa cell membranes, FA-GNSs exhibit photothermal effect which can be exploited to kill the same cells in vitro using laser irradiation in the NIR at a very low and safe irradiance. We thus demonstrate that the FA-GNSs designed following the described approach are an efficient prototype of theranostic nanodevices.

Tunable coating of gold nanostars: tailoring robust SERS labels for cell imaging.

Surface modification of noble metal nanoparticles with mixed molecular monolayers is one of the most powerful tools in nanotechnology, and is used to impart and tune new complex surface properties. In imaging techniques based on surface enhanced Raman spectroscopy (SERS), precise and controllable surface modifications are needed to carefully design reproducible, robust and adjustable SERS nanoprobes. We report here the attainment of SERS labels based on gold nanostars (GNSs) coated with a mixed monolayer composed of a poly ethylene glycol (PEG) thiol (neutral or negatively charged) that ensure stability in biological environments, and of a signalling unit 7-Mercapto-4-methylcoumarin as a Raman reporter molecule. The composition of the coating mixture is precisely controlled using an original method, allowing the modulation of the SERS intensity and ensuring overall nanoprobe stability. The further addition of a positively charged layer of poly (allylamine hydrocloride) on the surface of negatively charged SERS labels does not change the SERS response, but it promotes the penetration of GNSs in SH-SY5Y neuroblastoma cells. As an example of an application of such an approach, we demonstrate here the internalization of these new labels by means of visualization of cell morphology obtained with SERS mapping.

Labeling interacting configurations through an analysis of excitation dynamics in a resonant photoemission experiment: the case of rutile TiO2.

A detailed study of resonant photoemission at Ti L(2,3) edges of insulating rutile TiO(2-x) thin film is presented. Pure TiO(2) resonating structures, defect-related resonances, resonant Raman-Auger and normal LVV Auger emissions are tracked, including an unpredicted two-hole correlated satellite below the non-bonding part of the valence band. The analysis of excitation dynamics unambiguously addresses the origin of these features and, in particular, the extent of charge transfer effects on the Ti-O bonding in the valence band of rutile, disclosing further applications to the more general case of, formally, d(0) oxides.

Photoinduced π-π* band gap renormalization in graphite.

As is well-known, the character of the π orbitals is of paramount importance for the chemical properties of the carbon allotropes and their derived compounds. While at equilibrium the nature of these orbitals is well understood, their photoinduced nonequilibrium behavior is under investigation. Here, we demonstrate that when a UV-laser pulse excites a carrier density larger than 10% of the π* density of state in graphite, a renormalization of the π-π* band gap takes place. This result has been achieved by detecting the transient reflectivity and the associated decay time of an infrared probe following the excitation of a UV pump pulse tuned across the π-π* absorption resonance. The pump photon energy at which both the transient reflectivity and the decay time are maximum is downshifted by 500 meV with respect to the relative absorption maximum at equilibrium. This finding is interpreted as a transient π-π* band gap shrinking of similar magnitude, near the M point of the Brillouin zone.

Electronic excitations in synthetic eumelanin aggregates probed by soft X-ray spectroscopies.

Electronic excitations of condensed phase eumelanin aggregates are investigated with soft X-ray spectroscopies. Resonant photoemission data indicate that mechanisms of charge delocalization may occur when electrons are excited about 3 eV above the first unoccupied electronic level. An average, lower limit value of 1.6 fs was estimated for the lifetime of the excited C 1s-pi* states.

Structural and spectroscopic properties of pure and doped Ba6Ti2Nb8O30 tungsten bronze.

Pure and doped Ba(6)Ti(2)Nb(8)O(30) (BTN), obtained by substituting M = Cr, Mn, or Fe on the Ti site (Ba(6)Ti(2-x) M(x)Nb(8)O(30), x = 0.06 and 0.18) and Y and Fe on the Ba and Ti sites, respectively (Ba(6-x)Y(x)Ti(2-x)Fe(x)Nb(8)O(30), x= 0.18), are synthesized. The influence of cation doping on the local structure, the cation oxidation state, and the possible defect formation able to maintain the charge neutrality are investigated by spectroscopic (electron paramagnetic resonance (EPR) and micro-Raman), structural (X-ray powder diffraction) and transport (impedance spectroscopy, thermoelectric power) measurements, in the temperature range of 300-1200 K in air and N(2) flow. Starting from the valence state of the doping ions (Fe(3+), Cr(3+), and Mn(2+)), determined by EPR, and from thermoelectric power measurements, evidencing a negative charge transport, different charge-compensating defect equilibria, based on the creation of positive electron holes or oxygen vacancies and electrons, are discussed to interpret the conductivity results.

Cation distribution in LiMgVO4 and LiZnVO4: structural and spectroscopic study.

The room temperature cation occupancy in LiMgVO(4) and LiZnVO(4) crystallographic sites is obtained by means of the combined use of X-ray powder diffraction (XRPD), (7)Li and (51)V magic angle spinning nuclear magnetic resonance (MAS NMR), and micro-Raman measurements. In the LiMgVO(4) Cmcm orthorhombic structure, the 4c (C(2)(v) symmetry) tetrahedral vanadium site is fully ordered; on the contrary, the Li 4c tetrahedral site and the 4b (C(2)(h) symmetry) Mg octahedral site display about 22% of reciprocal cationic exchange. Higher cationic disorder is observed in LiZnVO(4): the three cations can distribute on the three tetrahedral and distinct sites of the R-3 structure. XRPD and MAS NMR analysis results highly agree for what concerns vanadium ion distribution on the three cationic sites (about 25, 26, and 47%). From the full profile fitting of XRPD patterns with the Rietveld method, it is also obtained that Li(+) displays a slightly preferred occupation of the T1 position (approximately 55%) and Zn(2+) of the T2 position (approximately 46%). The vibrational spectra of the two compounds are characterized by different peak positions and broadening of the Raman modes, reflecting the cation distribution and the local vibrational unit distortion. A comparison is also made with recent Raman results on Li(3)VO(4). High temperature XRPD measurements rule out possible structural transitions up to 673 K for both compounds.

Ferromagnetism on a paramagnetic host background: the case of rutile TM:TiO(2) single crystals (TM = Cr, Mn, Fe, Co, Ni, Cu).

Single crystals of TiO(2) rutile doped with Cr, Mn, Fe, Co, Ni, and Cu were grown with the flux method in a Na(2)B(4)O(7) melt. The samples, checked in their structural and phase homogeneity by x-ray diffraction and micro-Raman spectroscopy, were single-phase needle-shaped crystals several millimetres long. Paramagnetic and ferromagnetic behaviours at room temperature were observed and they are discussed also in connection with the magnetic properties of undoped TiO(2) crystals.

Transport and structural properties of pure and Cr doped Li3VO4.

This study deals with the effects of 5 and 10% chromium additions on the transport and structural properties of Li3VO4. The Cr substitution is easily obtained without impurity phases and does not affect the room- and high-temperature host crystal structure, as evidenced by X-ray powder diffraction and micro-Raman analysis. The EPR signals are interpreted in terms of quantified amounts of Cr ions in 5+ and 3+ valence states. Suitable 7Li and 51V MAS NMR spectra simulations agree with the EPR results about the relative amount of Cr5+ and Cr3+ ions substituted in V5+ and Li+ sites, respectively. The Cr3+ presence on Li site, also suggested by Raman results and Rietveld refinements, requires Li vacancies to maintain the charge neutrality. The p-type conductivity, suggested by the positive thermoelectric power coefficients, significantly increases by the cation doping up to an order of magnitude.