Optimizing TEMPO-Functionalized Surfaces: Unraveling the Impact of Diazotization Conditions on Immobilization Efficiency

In this study, glassy carbon surfaces were modified via the electrochemical reduction of diazonium salts bearing a TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) moiety. This work investigates the impact of diazotization conditions on the redox state of TEMPO and its subsequent immobilization efficiency on electrodes. Two diazotization methods were employed: one using nitrosonium tetrafluoroborate (NOBF₄) and the other using tert-butyl nitrite (tBuONO) in combination with boron trifluoride acetonitrile complex (BF₃-CH3CN). Cyclic voltammetry and electrochemical simulations revealed that the redox state of TEMPO is significantly influenced by the diazotization agents. NOBF₄ primarily oxidizes TEMPO to its oxoammonium form (TEMPO⁺), while the tBuONO/BF₃-CH3CN system leads to the formation of non-redox species alongside TEMPO⁺. The electrochemical grafting of these TEMPO-derivatized diazonium salts onto glassy carbon electrodes showed that the presence of TEMPO radicals during the electroreduction process hinders immobilization efficiency. This effect is attributed to the spontaneous reduction of diazonium salts by TEMPO radicals, which decreases the availability of diazonium species for surface grafting. The formation of non-redox species, which are regenerated after immobilization on the surface, appears to be a promising approach for achieving reproducible layers with higher surface concentrations.