A. Bertoluzza

Raman and infrared spectra on silica gel evolving toward glass

Abstract Infrared and Raman spectra of gels obtained from specially prepared solutions of Si(OC2H5)4 (TEOS) which have been thermally treated in the 40–800°C temperature range, are reported and discussed with reference to the spectra of fused quartz. The results show that the gel to glass transformation is an hydrolytic polycondensation process, which takes plase gradually and is practically completed in the samples treated at 800°C. Particularly revealing is the behaviour of the bands due to OH (associated and/or unassociated H-bonded) stretching modes, including those of water, which are weakened by increasing temperature; the behaviour of the band due to the SiOH stretching mode; the behaviour of the network bands, some of which become more intense with increasing temperature, due to asymmetric and symmetric stretching and the bending SiOSi modes. In the Raman spectra of gels heated at different temperatures two peaks also appear at ≈600 and ≈490 cm−1, which are also present in the Raman spectrum of fused quartz, they are related to network defects of the glass structure, for which no definite interpretation has yet been given in the literature.

Raman spectroscopic studies of silk fibroin from Bombyx mori

This study was focused on the conformational characterization of Bombyx mori silk fibroin in film, fiber and powder form by means of Fourier transform Raman spectroscopy. Native and regenerated silk fibroin films prepared by casting dilute silk fibroin solutions (<1%, w/v) display characteristic conformationally sensitive bands at 1660 cm-1 (amide I), in the range 1276–1244 cm-1 (a complex amide III region with multiple detectable maxima) and at 1107 and 938 cm-1. This spectral pattern can be related to a prevalently random coil conformation, with traces of α-helix. Liquid silk, prepared by casting the silk gland content (fibroin concentration 20–25%, w/v), shows almost the same wavenumbers in the amide I and III ranges, while differences appear below 1000 cm-1, where three bands at 952, 930 and 867 cm-1 increase in intensity. The spectral differences between films and liquid silk are discussed with a view to identifying possible markers for silk I structure, a crystalline modification of silk fibroin. The treatment of both native and regenerated films with 50% (v/v) methanol solution induces the conformational transition to a β-sheet structure, as demonstrated by the shift of amide I to 1665 cm-1 and the appearance of new maxima at 1262 and 1236 cm-1 (amide III) and at 1084 cm-1. When liquid silk is cast at above 50°C, the prevailing conformation taken by silk fibroin is β-sheet, whatever the rate of drying. By comparing the Raman spectra of silk fibroin fiber and powder, both having a β-sheet structure, a difference in the tyrosine doublet bands and in the amide I band can be observed. The value of the I853/I830 (Rtyr) intensity ratio increases in the powder while amide I shifts to lower wavenumbers, suggesting that the hydrogen bonds involving the tyrosil residues are weaker in the powder than in the fiber.

Further investigations on Raman spectra of silica gel evolving toward glass

Abstract In a previous investigation a continuous evolution of silica gel structure toward that of v-SiO 2 was verified by Raman spectroscopy. Modifications on the intensity of some Raman bands usually associated in the literature to “defects” of the structure were observed. In the present work the study has been extended to the transformation with temperature of silica gels obtained from tetraethoxysilane (TEOS) with different H 2 O/TEOS molar ratios. Raman spectra as well as thermogravimetric data are discussed in terms of the effect of this ratio on the local structure and the sintering process of silica gel.

Raman and solid state13C-NMR investigation of the structure of the 1 : 1 amorphous piroxicam : ?-cyclodextrin inclusion compound

The results of a Raman and solid state 13C-NMR spectroscopic investigation aimed at studying the conformation of piroxicam (P) and its interaction with beta-cyclodextrin (betaCD) in 1 : 1 amorphous PbetaCD inclusion compound are reported. The 1700-1200 cm(-1) FT-Raman and the 13C CP/MAS NMR spectra of 1 : 1 PbetaCD inclusion compound are discussed and assigned in comparison with those of the three main modifications of piroxicam (alpha, beta, and monohydrate). The FT-Raman and 13C-NMR results show that in 1 : 1 PbetaCD inclusion compound piroxicam mainly assumes the zwitterionic structure typical of monohydrate, even if the presence of a different structure, that is, beta form, is not excluded. Piroxicam monohydrate, differently from alpha and beta forms, is characterized by a zwitterionic structure with an internal proton transfer and an increased charge delocalization, as shown by our spectroscopic results. The charge delocalization characteristic of this zwitterionic structure gives rise to the interaction with betaCD via electrostatic and hydrogen bonds. The possibility of a host-guest interaction between piroxicam and betaCD is not excluded; the guest molecule can be accommodated in betaCD cavity by interaction via hydrophobic bonds.

Micro-Raman spectroscopy for the crystallinity characterization of UHMWPE hip cups run on joint simulators☆

Abstract In this work Raman microspectrometry was used to evaluate the crystallinity changes of hip cups made of ultra-high molecular weight polyethylene (UHMWPE). In vitro experiments were carried out using hip joint movement-wear simulators, run for five million cycles in water or bovine calf serum. The hip cups were subjected to mechanical friction with ceramic femoral heads (alumina–zirconia blend). The crystallinity of the polymer hip cups was studied as a function of inner surface position and thickness, from the stressed surface to the unstressed outer one. The Partial Least Square (PLS-l) calibration was used to correlate the Raman spectra with the crystallinity of the polymer measured with DSC calorimetry.

Raman and infrared spectra of As2Sx chalcogenide glasses with x ⩽ 3

Abstract Raman laser and far infrared spectra of As2Sx glasses with x ⩽ 3 are given and discussed. The purpose of the work is to bring a vibrational spectroscopic contribution to the study of these glasses and to the hypothesis, still under discussion in the literature, that they might be constituted either by a homogeneous vitreous phase or by a mixture of As2S3 and As4S4. Our results confirm a phase separation, with formation of s-As4S4, below a certain value of x, which depends not only on the preparation method of the samples but also on other factors such as melting time. Laser irradiation of s-As4S4 modifies its Raman spectrum. Such a phenomenon is attributable to two principal factors, either a partial polymerization or formation of a species richer in arsenic. A structural and formation model of the As2Sx glasses is given, starting from a more generalized structural model of vitreous As2S3 which is an accord with the vibrational results and those by the diffraction method in the literature.

Raman defect peaks in the spectra of Na2O silica gels evolving toward glass

Abstract In previous works the continuous thermal evolution toward v-SiO2 of gels obtained from tetraethoxysilane (TEOS) and with different H2O/TEOS molar ratios was studied by Raman spectroscopy; Raman defect peaks at ∼ 600 and ∼ 490 cm−1 were observed both in gels and in the v-SiO2. In this work the structural effects of small amounts of Na2O on silica gels evolving toward glass are studied in order to give a further experimental contribution to the problem of “Raman defect peaks”. Some considerations on the nature of these defect peaks are now presented considering all the results so far obtained.

NEAR INFRARED SURFACE-ENHANCED RAMAN SPECTROSCOPIC STUDY OF ANTIRETROVIRALY DRUGS HYPERICIN AND EMODIN IN AQUEOUS SILVER COLLOIDS

Abstract The near infrared surface-enhanced Raman spectra (NIR SERS) of antiretroviraly drugs hypericin and its analogs emodin and bianthrone were studied at different drug concentration, dimethylsulfoxide concentration, pH and time. The differences observed in the SERS spectra when varying some of these parameters are attributed to changes in the adsorbate coverage and orientation of these molecules on the silver colloids, and can be related to the monomeric drug concentration existing in the solution. Two different drug orientations on the metal surface can be deduced: perpendicular and planar, which can be characterized by two different SERS profiles. The drug reorientation on the surface is indicative of a change in the oligomer-monomer equilibrium in the solution, what implies that the SERS technique can be used as an indirect method to monitor the intermolecular interactions of these molecules in water. In addition the UV-visible absorption spectra of these drugs also reveals the existence of intermolecular interactions in water and the temperature dependence of these interactions.

Applications of Raman spectroscopy to the ophthalmological field : Raman spectra of soft contact lenses made of poly-2-hydroxyethylmethacrylate (PHEMA).

Abstract Raman and i.r. spectra of soft contact lenses made of PHEMA are discussed in relation with their structural properties and clinical applications. Complementary TG,DSC and TMA measurements are also presented. The results of this study show that such lenses present the requested characteristics of biocompatibility.

Hydrogen bonds in calcium acid phosphates by infrared and Raman spectra

Abstract Infrared and Raman spectra of Ca(H2PO4)2, Ca(H2PO4)2·H2O and deuterated analogues are presented and discussed. The behaviour of the asymmetric stretching vibration νOH and of the in-plane δOH and out-of-plane γOH deformations shows that hydrogen bonds of different strengths are present in both compounds, in agreement with X-ray diffraction data. The perturbation brought about by the water molecule on the strength of the hydrogen bond is also considered.