Surface-Enhanced Infrared Spectra of Nicotinic Acid and Pyridoxine on Copper Substrates: What Is the Effect of Temperature and Deposition Conditions?

Nicotinic acid and pyridoxine, adsorbed on electrochemically prepared copper substrates, were studied by Surface-Enhanced Infrared Absorption (SEIRA) spectroscopy. Assignment of the bands in SEIRA spectra to appropriate vibrational modes was based on results of DFT calculations. The role of dissolved gas in deposition solution was elucidated for adsorption of nicotinic acid. Temperature dependence of SEIRA spectral features of both nicotinic acid and pyridoxine was studied in the temperature range 15–50 °C. In order to evaluate the temperature-induced changes of SEIRA spectra of both two adsorbates, the bands were fitted with Voigt profiles and their heights and widths were compared. Principal Component Analysis (PCA) and Partial Least Squares (PLS) regression were applied. It was found out that heating even to 50 °C does not induce any observable changes in molecular structure of adsorbates. However, noticeable changes of band intensities and widths were detected and assigned to the partial reorientation of adsorbed molecules, to the changes of intermolecular interactions within the adsorbed layers, and to a minor extent, to the other changes of metal–adsorbate systems properties, for example, to the changes of surface moisture. The changes of metal reflectivity are considered to be the minor source of spectral variation.