Angela Zoppi

Characterisation of painting materials from Eritrea rock art sites with non-destructive spectroscopic techniques

Rock painting samples from Eritrean archaeological sites were studied by means of micro-Raman spectroscopy and proton-induced X-ray emission technique (PIXE). Hematite and manganese oxides/hydroxides were determined in red and black paints, respectively. Since colours do not contain carbon, the paintings cannot be dated with 14 C. Moderate amounts of calcium carbonate or sulphate were also observed in most red drawings, while traces of phosphorus were found by PIXE only in a few red and black samples.

A study on the anisole–water complex by molecular beam–electronic spectroscopy and molecular mechanics calculations

An experimental and theoretical study is made on the anisole-water complex. It is the first van der Waals complex studied by high resolution electronic spectroscopy in which the water is seen acting as an acid. Vibronically and rotationally resolved electronic spectroscopy experiments and molecular mechanics calculations are used to elucidate the structure of the complex in the ground and first electronic excited state. Some internal dynamics in the system is revealed by high resolution spectroscopy.

Shift-excitation Raman difference spectroscopy-difference deconvolution method for the luminescence background rejection from Raman spectra of solid samples.

The feasibility of the shift-excitation Raman difference spectroscopy–difference deconvolution (SERDS-DDM) method for fluorescence suppression from Raman spectra of solid samples is discussed. For SERDS measurements a tunable diode laser source with an emission band centered at 684 nm is coupled to a conventional micro-Raman apparatus and a monochromator device is used for checking the excitation frequency stability. The shifted Raman spectra are then mathematically treated and a deconvolution procedure is used to reconstruct the Raman spectrum devoid of fluorescence. Two different cases are presented. In the first one, fluorescence is intrinsic to the sample and the Raman spectrum of cinnabar pigment is finally reconstructed. In the second, the presence of an external luminescence background in the spectrum of a pure sulfur crystal is considered. The SERDS-DDM reconstructed spectra are compared with spectra obtained via multi-point baseline subtraction and a significant improvement in the detection of weak bands is demonstrated. Practical insights for the application of this method are presented as well.

A new compact instrument for Raman, laser-induced breakdown, and laser-induced fluorescence spectroscopy of works of art and their constituent materials.

A small, potentially transportable prototype instrument capable of carrying out Raman, laser-induced breakdown (LIB), and laser-induced fluorescence (LIF) spectroscopy using a single pulsed laser source was developed for the analysis of cultural heritage objects. The purpose of this instrumentation is to perform fast and reliable analysis of surfaces with minimum damage to an object. For this purpose, a compact (51 x 203 x 76 mm) nanosecond Q-switched neodymium doped yttrium aluminum garnet laser (8 ns, 20 Hz, 0.01-115 mJ/pulse) was used as an irradiation source. The use of a nanosecond-gated detector sensitive between 180 and 900 nm allows the acquisition of elemental emissions in LIB spectroscopy and can also be employed for both LIF and time-resolved Raman spectroscopy. In this work, attention is focused on the description of the instrument and its optical components, and two examples of applications for the analysis of pigments and binding media used in works of art are presented.

Functionalized Au/Ag nanocages as a novel fluorescence and SERS dual probe for sensing.

We obtained chitosan-protected Au/Ag nanocages (NCs), i.e., hollow and porous metallic nanoparticles, by galvanic replacement reaction. Subsequently, we functionalized the NCs with a fluorescent derivative of 4-methoxy-1,8-naphtalimide (NAFTA6). The plasmonic properties of these structures, which exhibit an extinction maximum in the 700-800 nm range, allowed their use as SERS active substrates for excitation at 785 nm and an efficient identification of the vibrational bands of NAFTA6, in spite of the low ligand concentration (<10(-5) M). Furthermore, NAFTA6 could also be identified from its fluorescence emission. The proposed functionalization with fluorescent compounds opens the way to the application of metal NCs using double-wavelength detection. Namely, Raman spectroscopy in the near infrared and fluorescence emission in the visible region, with considerable potential especially for in vivo medical applications, as the plasmonic band is centered in the visible light region where biological fluids and tissues are transparent.

Influence of cobalt doping on the structural, optical and luminescence properties of sol-gel derived TiO2 nanoparticles

Abstract We have investigated the structural and optical properties of sol–gel derived cobalt-doped Titanium dioxide (TiO2) nanoparticles. X-ray diffraction patterns exhibit that Co-doped TiO2 forms in anatase phase without any formation of secondary phases. SEM images reveal that the as-grown nanoparticles are almost spherical in size with average particle size in the range of 35–50 nm. Raman spectra recorded at room temperature show that Co doping cancels the rutile phase that was present in the pure TiO2 sample. The energy band gap calculated using UV/Vis spectra and Tauc’s relation is found to be of direct bandgap nature and its value decreases with the doping concentration of cobalt. Our photoluminescence results indicate that a photoluminescence emission is found to be centred around 380 nm in the ultraviolet and around 680 nm in the visible range of spectrum.

Gold nanostars as SERS-active substrates for FT-Raman spectroscopy

Gold nanostars having extinction maxima in the near infrared spectral region were synthesized according to surfactant-free or CTAB-based procedures. The SERS activity of these colloidal suspensions by 1064 nm excitation was tested with a thiolic derivative of naphtalimide as a ligand molecule. The shielding effect of the colloid on the observed Raman response was taken into account as well as the influence of nanoparticle concentration and the addition of chitosan, as stabilizer. A nanomolar limit of detection and a Raman enhancement factor of more than 105 were found. In particular, an important finding is that CTAB-free nanostars, even in the more biocompatible chitosan-stabilized form, still behaved as good SERS substrates in the near infrared where the occurrence of the “optical tissue window” points to their use in biological systems with minimum fluorescence background.

Metal-clad optical waveguide fluorescence device for the detection of heavy metal ions

Abstract. We developed Hg2+-sensing chips by decorating the external surface of metal-clad optical waveguides with a monolayer of Hg2+-sensitive fluorescent molecular probes. The emission properties of the original water-soluble form of the molecule were previously found to be selectively quenched in the presence of Hg2+ ions. The fabricated samples were tested with optical waveguide fluorescence spectroscopy by putting them in contact with a 5-μM water solution of Hg2+ ions and recording the emission spectra versus incubation time. The estimate of the limit of detection was 150 nM. A preliminary evaluation of the selectivity of the structure was also performed by using Cd2+ as possible interfering analytes.

Does the bazhenovite structure really contain a thiosulfate group? A structural and spectroscopic study of a sample from the type locality

Abstract Bazhenovite was originally reported as a rare, thiosulfate-containing mineral, with the chemical formula CaS5⋅CaS2O3⋅6Ca(OH)2⋅20H2O. The structure was predicted to be layered, but no structural details were given. A crystal of “bazhenovite” from the pyritized siderite fragments occurring in the melt products of burning dumps at the type locality in the Chelyabinsk coal basin, South Urals, Russia has been examined by single-crystal X-ray diffraction and vibrational spectroscopy (FTIR and Raman). The structure was solved in space group P21/c and refined assuming twinning on {100} to a final Robs = 5.03% (731 reflections) and Rall = 6.98% (966). Although the unit-cell dimensions of the examined crystal [a = 8.391(2), b = 17.346(6), and c = 8.221(4) Å, β = 119.33(5)°, V = 1043.2(8) Å3 ] match those of the original bazhenovite description [a = 8.45(1), b = 17.47(1), and c = 8.24(1) Å, β = 119.5°, V = 1059 Å3 ], the thiosulfate group was not detected by either structural analysis or spectroscopic investigations. The structure is an alternating sequence of two kinds of layers, labeled A and B respectively, stacked along [010]. The A layer is the ordered part of the structure and consists of a linkage of Ca(OH)2(H2O)6 antiprisms and Ca(OH)4(H2O)2 octahedra. Taking into account both structural and spectroscopic results, the B layer is inferred to consist of a disordered assemblage of S32-, and to a lesser extent, S42- groups, with the possible presence of additional H2O and H2S. The possibility that bazhenovite and the mineral here examined could represent two distinct phases differing slightly from each other with respect to the thiosulfate content is discussed.

Structural Determinations and Dynamics on Floppy Molecular Systems

We discuss on the central role of very high resolution spectroscopy for the study of molecular systems weakly bonded or flexible. It will appear evident how the lack of high resolution results can lead to wrong conclusions. The paper will focalize the attention in particular on two different cases: one involving the hydrogen bonded complex anisole‐water, the other involving the very floppy 1,3‐benzodioxole (BDO) molecule. In the first case the issue is the determination of the structure of the complex that cannot be correctly inferred from resonance enhanced multi photon ionization (REMPI) data and ab initio calculations. In the second case the non‐rigidity of the molecule and the possibility of the interaction of two low frequency modes (ring‐puckering and ring‐butterfly) have lead to a wrong assignment of the laser induced fluorescence (LIF) vibronic spectrum.