A Combination Study of Electrochemical Surface-Enhanced Raman Spectroscopy and Density Functional Theory for Redox Reaction Processes of Thionine Adsorbed on a Nanostructured Gold Electrode

Abstract

Electrochemical Surface-Enhanced Raman Spectroscopy (EC-SERS) enables the detection of in situ electrochemical redox processes at the molecular level and, in conjunction with Density Functional Theory (DFT) calculations, elucidates the chemical enhancement and adsorption configurations of molecules. This study investigates the EC-SERS of thionine on gold nanoparticles (AuNPs), revealing that the resonance Raman scattering in solid thionine originates from intramolecular electronic transitions. DFT calculations confirm that thionine preferentially adsorbs on AuNPs via the N atom of 1,4-thiazine ring. The SERS signal of thionine arises from both electromagnetic enhancement and chemical enhancement of photo-induced charge transfer between gold and the molecule. The study also provides a quantitative relationship between the Raman peak intensity of thionine and the applied potential during in-situ electrochemical reduction, as well as the adsorption kinetics of thionine onto AuNPs, which follows the Langmuir model and forms a stable self-assembled monolayer at concentrations > 10 μM. The average adsorption energy (ΔG) value for thionine adsorbed on AuNPs was found to be -5.96 kcal/mol. These findings provide molecular-scale insights into the electronic and vibrational behavior of thionine on AuNPs, offering valuable guidance for the development of advanced SERS substrates and electrochemical sensors.