Azathioprine Electrochemical Adsorption onto the Reduced Graphene Oxide Sheets in Absence and in the Presence of Polyaniline

The azathioprine (AZA) electrochemical adsorption onto the screen-printed carbon electrodes (SPCE) modified with the reduced graphene oxide (RGO) sheets in the absence and in the presence of polyaniline–emeraldine salt (PANI-ES) is reported in this work. Using cyclic voltammetry (CV), in the case of the SPCE modified with the RGO sheets non-functionalized and functionalized with PANI-ES, respectively, an irreversible process at the electrode/electrolyte interface is highlighted to take place. In the case of the SPCE modified with the non-functionalized RGO sheets (SPCERGO), the oxidation-reduction processes induce an up-shift of the AZA Raman lines from 856 and 1011 cm-1 to 863 and 1020 cm-1, respectively. These variations indicate an AZA adsorption onto the surface of the SPCE modified with the RGO sheets that takes place throught the imidazole and pyrimidine cycles of mercaptopurine, when the generation of the π–π* bonds between the mercaptopurine structure and hexagonal carbon cycles of RGO occurs. The electrochemical functionalization of the RGO sheets with PANI-ES is proved by the appearance of the Raman lines at 1165, 1332-1371, 1496 and 1585 and 1616 cm-1. The oxidation-reduction processes induced at the interface of the SPCE modified with PANI-ES functionalized RGO sheets and the electrolyte consisting into a phosphate buffer (PB) and AZA lead to the generation of new positive charges onto the PANI macromolecular chain and the adsorption of the drug on the working electrode surface that takes place via the π–π* bonds established between the benzene/quinoide rings of PANI and the imidazole/ purine cycles of AZA. These results indicate that the SPCE modified with the PANI-ES functionalized RGO sheets shows potential applications in the field of sensors for AZA detection.

Effect of Surface Roughness on Electrochemical Adsorption/Desorption of Dopamine by Carbonaceous Electrodes

Electrochemical adsorption/desorption of dopamine by carbonaceous electrodes upon voltage variation is the key process of neurotransmitter detection through fast scan cyclic voltammetry. In the present study, <i>ab initio</i> molecular dynamics simulation empowered by image-charge method was applied to calculate the adsorption/desorption free energy profile of dopamine and dopamine o-quinone at fixed electrode potentials using our newly developed open-source CP2K simulation package. It was found that the activation barriers for both adsorption and desorption were substantially reduced with increasing surface roughness of the carbonaceous electrodes. For example, on the flat graphene electrode, the activation barrier for dopamine adsorption at V₀=−0.4V is 1.34 kcal/mol, while its counterpart on the curved nanotube electrode drops to 0.82 kcal/mol. Moreover, the diffusion coefficient of dopamine decreases by approximately 60% when it is moving close to the graphene electrode, while its diffusion is accelerated by up to 100% when the nanotube electrode is adopted. The faster diffusion alongside the reduced activation barrier greatly facilitates the electrochemically driven adsorption/desorption of dopamine by nanotube electrodes, in consistent with experimental findings that a rougher carbonaceous surface is critical for fast scan cyclic voltammetry.

Effect of Surface Roughness on Electrochemical Adsorption/Desorption of Dopamine by Carbonaceous Electrodes

Electrochemical adsorption/desorption of dopamine by carbonaceous electrodes upon voltage variation is the key process of neurotransmitter detection through fast scan cyclic voltammetry. In the present study, <i>ab initio</i> molecular dynamics simulation empowered by image-charge method was applied to calculate the adsorption/desorption free energy profile of dopamine and dopamine o-quinone at fixed electrode potentials using our newly developed open-source CP2K simulation package. It was found that the activation barriers for both adsorption and desorption were substantially reduced with increasing surface roughness of the carbonaceous electrodes. For example, on the flat graphene electrode, the activation barrier for dopamine adsorption at V₀=−0.4V is 1.34 kcal/mol, while its counterpart on the curved nanotube electrode drops to 0.82 kcal/mol. Moreover, the diffusion coefficient of dopamine decreases by approximately 60% when it is moving close to the graphene electrode, while its diffusion is accelerated by up to 100% when the nanotube electrode is adopted. The faster diffusion alongside the reduced activation barrier greatly facilitates the electrochemically driven adsorption/desorption of dopamine by nanotube electrodes, in consistent with experimental findings that a rougher carbonaceous surface is critical for fast scan cyclic voltammetry.