Cristiana Lofrumento

A remote scanning Raman spectrometer for in situ measurements of works of art

In conservation science, one of the main concerns is to extract information from an artistic surface without damaging it. Raman spectroscopy has emerged in recent years as a reliable tool for the non-destructive analysis of a wide range of inorganic and organic materials in works of art and archaeological objects. Nevertheless, the technique is still mainly limited to the analysis of micro-samples taken from artistic surfaces. The development of an instrument able to perform non-contact analysis of an area of a few square centimeters aims to further increase the employment of this technique. This paper describes the development of a prototype Raman scanning spectrometer based on a diode laser, a 2D scanning mirror stage and a custom optical system, which can map a surface of 6 cm in diameter at a working distance of 20 cm. The device exhibits collecting optics with a depth of field close to 6 cm, which makes the Raman system suitable for the analysis of non-flat surfaces and three-dimensional objects. In addition, the overall dimensions and weight of the instrument have been limited in order to make the device transportable and, in principle, usable for in situ measurements. Details on the design of the device, with particular emphasis on the collecting optical system, and on results of the characterization tests carried out to assess its performances are reported. Finally, an example of an application involving the identification of pigments from a model painting is presented.

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.

Alternative SERRS probes for the immunochemical localization of ovalbumin in paintings: an advanced mapping detection approach.

In the field of analytical chemistry, many scientific efforts have been devoted to develop experimental procedures for the characterization of organic substances present in heterogeneous artwork samples, due to their challenging identification. In particular, performances of immunochemical techniques have been recently investigated, optimizing ad hoc systems for the identification of proteins. Among all the different immunochemical approaches, the use of metal nanoparticles - for surface enhanced Raman scattering (SERS) detection - remains one of the most powerful methods that has still not been explored enough for the analysis of artistic artefacts. For this reason, the present research work was aimed at proposing a new optimized and highly efficient indirect immunoassay for the detection of ovalbumin. In particular, the study proposed a new SERRS probe composed of gold nanoparticles (AuNPs) functionalised with Nile Blue A and produced with an excellent green and cheap alternative approach to the traditional chemical nanoparticles synthesis: the laser ablation synthesis in solution (LASiS). This procedure allows us to obtain stable nanoparticles which can be easily functionalized without any ligand exchange reaction or extensive purification procedures. Moreover, the present research work also focused on the development of a comprehensive analytical approach, based on the combination of potentialities of immunochemical methods and Raman analysis, for the simultaneous identification of the target protein and the different organic and inorganic substances present in the paint matrix. An advanced mapping detection system was proposed to achieve the exact spatial location of all the components through the creation of false colour chemical maps.

Safranin-O dye in the ground state. A study by density functional theory, Raman, SERS and infrared spectroscopy

The analysis of ground state structural and vibrational properties of Safranin-O is presented. The experimental results, obtained by FTIR, Raman and SERS spectroscopy, are discussed in comparison to the results of DFT calculations carried out at the B3LYP/6-311+G(d,p) level of theory. The calculated spectra reproduce quite satisfactorily the experimental data. The calculated Safranin-O equilibrium structure and the assignment of the vibrational spectra are reported as well. From the changes between Raman and SERS spectra a model is presented for the interaction of Safranin-O with silver nanoparticles.

Multivariate Analysis of Combined Fourier Transform Near-Infrared Spectrometry (FT-NIR) and Raman Datasets for Improved Discrimination of Drying Oils

This work explores the application of chemometric techniques to the analysis of lipidic paint binders (i.e., drying oils) by means of Raman and near-infrared spectroscopy. These binders have been widely used by artists throughout history, both individually and in mixtures. We prepared various model samples of the pure binders (linseed, poppy seed, and walnut oils) obtained from different manufacturers. These model samples were left to dry and then characterized by Raman and reflectance near-infrared spectroscopy. Multivariate analysis was performed by applying principal component analysis (PCA) on the first derivative of the corresponding Raman spectra (1800-750 cm−1), near-infrared spectra (6000–3900 cm−1), and their combination to test whether spectral differences could enable samples to be distinguished on the basis of their composition. The vibrational bands we found most useful to discriminate between the different products we studied are the fundamental v(C C) stretching and methylenic stretching and bending combination bands. The results of the multivariate analysis demonstrated the potential of chemometric approaches for characterizing and identifying drying oils, and also for gaining a deeper insight into the aging process. Comparison with high-performance liquid chromatography data was conducted to check the PCA results.

Resonance Raman Spectra of o-Safranin Dye, Free and Adsorbed on Silver Nanoparticles: Experiment and Density Functional Theory Calculation

The properties of o-Safranin (SO) dye in the first electronic excited state were studied with combined experimental and theoretical methods. The electronic absorption spectra of SO molecules are measured in water solution and in the presence of silver nanoparticles. The normal Raman (NRS) and resonance Raman (RR) spectra of solid SO and the surface enhanced Raman (SERS) and surface enhanced resonance Raman (SE[R]RS) spectra of SO adsorbed on silver nanoparticles are measured at different excitation energies. The enhancement factors for selected vibrational bands of the RR, SERS, and SE[R]RS spectra of SO have been obtained with respect to the NRS spectra of the solid after a careful evaluation of the experimental conditions. The data furnished useful information on the excited electronic states and the interactions of SO with silver nanoparticles. The experimental results are discussed on the basis of DFT and TD-DFT calculations (B3LYP/6-311+G(d,p)) on the isolated SO molecule.

Non-destructive and non-invasive analyses shed light on the realization technique of ancient polychrome prints.

Five polychrome prints representing famous painters, such as Albrecht Dürer, were analyzed using a non-destructive and non-invasive methodology as required by the artwork typology. The diagnostic strategy includes X-ray fluorescence (XRF), reflectance micro-infrared (microFTIR) and micro-Raman (microRaman) spectroscopy. These prints were realized with a la poupée method that involves application of the polychrome inks on a single copper plate, before the printing process. A broad range of compounds (i.e., cinnabar, red lead, white lead, umber earth, hydrated calcium sulfate, calcium carbonate, amorphous carbon, and Prussian blue) was employed as chalcographic inks, using linseed oil as a binding medium. Gamboge was identified in the delicate finishing brush touches realized in watercolor.

Ceramic findings from the archaeological site at Aiano-Torraccia di Chiusi (Siena, Italy): a multi-analytical approach

In 2005, the remains of a Roman villa, dating from the early fourth to the sixth centuries ad, were discovered at the archaeological site of Aiano-Torraccia di Chiusi (Siena, Italy). After being abandoned in the sixth century ad, the complex was occupied by a group of Ostrogothic or Lombardic artisans in the period between the sixth and the seventh centuries ad. Many ceramic remains (coarse pottery and red slip ceramics) from the first to the seventh centuries ad have been discovered on this archaeological site. These findings have been analysed using different analytical techniques (optical microscopy (OM), X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscope-energy dispersive spectroscopy (SEM-EDS), attenuated total reflection Fourier transform infrared (ATR-FTIR), and micro-Raman in order to characterize the ceramic body, the coating, the temper, and to investigate the compositional relationship between the different kinds of ceramics. The use of different techniques on the same samples yielded information at different scales. OM and SEM-EDS yielded interesting information on the coarse pottery: the analyses performed on some minerals and rock fragments suggest that stone tesserae from the Roman villa (in the form of numerous marble fragments) were used in the production of this pottery. Bulk analyses (XRD and XRF) and subsequent micro-analyses (SEM-EDS, ATR-FTIR, and micro-Raman) of the red slip pottery revealed clear chemical, mineralogical and textural differences: some ceramics (the TCC sample group) typically have a Fe-enriched coating while others (the INGR sample group) present a clear difference in grain size but no chemical or mineralogical differences between the ceramic body and the coating.

Microanalysis of Organic Pigments in Ancient Textiles by Surface-Enhanced Raman Scattering on Agar Gel Matrices

We review some new methods based on surface-enhanced Raman scattering (SERS) for the nondestructive/minimally invasive identification of organic colorants in objects whose value or function precludes sampling, such as historic and archeological textiles, paintings, and drawing. We discuss in detail the methodology we developed for the selective extraction and identification of anthraquinones and indigoids in the typical concentration used in textiles by means of an ecocompatible homogeneous nanostructured agar matrix. The extraction system was modulated according to the chemical properties of the target analyte by choosing appropriate reagents for the extraction and optimizing the extraction time. The system has been found to be extremely stable, easy to use and produce, easy to store, and at the same time able to be analyzed even after long time intervals, maintaining its enhancement properties unaltered, without the detriment of the extracted compound. Highly structured SERS band intensities have been obtained from the extracted dyes adopting laser light excitations at 514.5 and 785 nm of a micro-Raman setup. This analytical method has been found to be extremely safe for the analyzed substrates, thus being a promising procedure for the selective analysis and detection of molecules at low concentration in the field of artworks conservation.

The Raman and SERS spectra of indigo and indigo-Ag2 complex: DFT calculation and comparison with experiment

Using time-dependent density functional theory in conjunction with B3LYP functional and LANL2DZ/6-31+g(d,p) basis sets, static and pre-resonance Raman spectra of the indigo-Ag2 complex have been calculated. Structure optimization, excitation energies and pre-resonance Raman spectra of the indigo molecule have been obtained at the same level of theory. The available experimental Raman spectra at 1064, 785 and 514nm and the SERS spectra at 785 and 514nm have been well reproduced by the calculation. Experimental SERS spectra are confronted with the calculated pre-resonance Raman spectra obtained for the indigo-Ag2 complex. The Raman activities calculated under the infinite lifetime approximation show a strong dependence upon the proximity to the energy and the oscillator strength of the excitation electronic transition. The comparison of the integrated EFs for indigo and indigo-Ag2 calculated Raman spectra, gave some hints as to the enhancement mechanisms acting for the different excitation wavelengths. Whereas for excitation at a wavelength corresponding to 785nm, the enhancement mechanism for the Raman spectrum of the metal complex seems the chemical one, the strong increment (ten times) of the integrated EF of the Raman spectra of the complex in the case of 514nm excitation, suggests the onset of other enhancement mechanisms. Assuming that intra-cluster transitions with high oscillator strength can be thought of as to mimic surface plasmons excitations, we suggest the onset of the electromagnetic mechanisms (EM) as the origin of the Raman spectrum enhancement. Nevertheless, other enhancement effects cannot be ruled out, as a new molecular transition gains strength in the proximity of the excitation wavelength, as a consequence of the symmetry lowering of the molecule in the complex. A large variation across vibrational modes, by a factor of at least 104, was found for the EFs. This large variation in the EFs can indicate that B-term Herzberg-Teller scattering, due to metal and/or charge transfer states, can feed intensity to the inactive (in the molecule) and/or non totally symmetric modes.