XPS analysis of glassy carbon electrodes chemically modified with 8-hydroxyquinoline-5-sulphonic acid

Glassy carbon (GC) modified electrodes were obtained by cycling the potential in an 8-hydroxyquinoline-5-sulphonic acid (HQSA) solution. These electrodes were successfully tested as sensors of some species of alimentary and pharmaceutical interest, showing improved performances with respect to those of unmodified GC electrodes and of GC electrodes cycled under the same experimental conditions but in the absence of HQSA. As a matter of fact, in the wide potential range explored for modifying the electrodes, even in the absence of HQSA, complex redox processes leading to the production of several functional groups take place at the surface of glassy carbon itself. An XPS investigation was consequently performed to better understand the effective nature of active species present on the surface of HQSA modified electrodes. The spectroscopic experiments involved acquiring survey and detailed scans of an HQSA powder standard sample and of GC electrodes cycled both in the presence and in the absence of HQSA. The experimental value of the binding energy of the S2p3/2 level of HQSA-modified electrodes was found equal to that of the HQSA standard powder, thus confirming that HQSA molecules are adsorbed on the surface of the GC/HQSA electrodes and that they maintain their chemical structure and properties.