pH dependent raman spectra of nicotinic acid – spectra of aqueous solutions and surface enhanced raman spectra on silver electrodes

Abstract

Raman spectra of nicotinic acid have been measured in aqueous solutions in the pH‐range between 1.0 and 12.5. Raman band assignments between 500 cm−1 and 1750 cm−1 are carried out by referring to the compound's solid state Raman spectrum and to spectra of related molecules. Changes of the Raman spectra with solution pH are correlated to the structure change of the nicotinic acid molecule. The surface enhanced Raman spectra (SERS) of nicotinic acid (NA) on a polycrystalline silver electrode are reported in the range 500–1750 cm−1 as a function of pH and applied potential. The results are presented in order to determine the type of adsorbed species (anion, cation, zwitterion). Therefore the SER spectra are compared with the pH dependent solution spectra of NA. These investigations show – in the presence of a non specifically adsorbing electrolyte (0.1 mol l−1 Na2SO4) – the nicotinate anion as the only surface active species detectable within the pH range of 1.4 to 12.5. A detailed analysis of the SER spectra reveals, that the anion exhibits two different molecular orientations towards the metal electrode, a presumably planar alignment dominating at low and medium pH values and a perpendicular one at low proton concentrations. In the intermediate range both orientations exist side by side. The ratio of planar to perpendicular oriented molecules depends on applied potential and increases with growing cathodic polarisation.

Monitoring of Silver Electrodeposition onto HOPG Electrodes in the Presence of Picolinic Acid by in Situ Surface-Enhanced Raman Spectra Measurements

“…19 Similar results concerning pyridinecarboxylic acids adsorption on electrochemically activated silver electrodes have been published. 12,20,21,37 Thus, ex situ SERS spectra show that PA molecules undergo deprotonation upon adsorption, the Pic − anion being the stable adsorbate species, even for the solution at pH 3.0 where the HPic zwitterion predominates (Table 2). Moreover, the intense signal at around 240 cm −1 attributed to both Ag−N and Ag−O stretching contributions supports the validity of a chemisorption process for the Pic − anion on silver, via both the ring nitrogen and carboxylate groups as well as the feasible oxidation of crystallites.…”

“…The influence of picolinic acid (PA) molecules on the nucleation and growth mechanism has been recently studied for copper and silver deposition. − The reported results have shown that PA species can be strongly adsorbed on the growing nuclei surface, thereby inhibiting the surface reactivity, and as a consequence, the crystallites grow as dendritic microstructures. Many SERS studies of adsorbed pyridine carboxylic acids on metal substrates have been mainly focused on the effect produced by changing the solution pH ,− and the applied potential, , whereas a few publications have been on in situ studies of the electrodeposition process, i.e., formation of a new phase with structures (SERS active sites) that induce preferential adsorption of the additive. ,− The aim of this study is the molecular tracking of the picolinic acid molecules during the silver particles electroformation onto HOPG by in situ Raman spectroscopy, under different experimental conditions. Spectroscopic analysis of the adsorbed species from solutions at different pH has allowed the chemical composition in the electrochemical interface to be correlated to the deposited silver structures.…”

Identification and characterization of pharmaceuticals using Raman and surface‐enhanced Raman scattering

“…lactate), and malate dehydrogenases. Nicotinic acid 61 62,63 were characterized using Raman and SERS techniques. For example, in the case of nicotinamide, the pH-dependent SERS spectra (Fig.…”