Modification of surfaces with thin organic films by reaction with aryldiazonium salts

Abstract

In this work, the modification of conducting substrates with thin (nanometer thick) aryl films via reaction with aryldiazonium salts was investigated. Two methods were used: modification by electro-reduction of the aryldiazonium salts and modification by spontaneous reaction of aryldiazonium the salts with the surface at open circuit potential. The majority of the studies were undertaken using p-nitrobenenze diazonium salt, which gives electro-active nitrophenyl (NP) films at the surface that can be detected and characterized by cyclic voltammetry.

Films prepared spontaneously on carbon and gold electrodes at open circuit potential were characterized by electrochemistry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and water contact angle measurements. At both carbon and gold, spontaneous modification proceeds via electron transfer from the surface to the diazonium salt.Furthermore, on both types of surface, spontaneously prepared NP films were found to be loosely packed multilayers of less than 5 nm in thickness.

The spontaneous reaction was utilized for the patterning of carbon, gold, silicon and copper surfaces by microcontact printing (μCP) with diazonium salts. The presence of spontaneously formed films upon printing was confirmed by cyclic voltammetry and AFM. The films were demonstrated to be useful for the tethering of further molecules to the surface. Patterns prepared by μCP were imaged using scanning electron microscopy (SEM) and condensation figures. The preparation of two-component systems, with different chemical functionalities attached to different, well-defined, regions of the surface, was demonstrated.

The optimization of the long term activity of glucose oxidase hydrogels by covalent attachment of the hydrogels to modified carbon electrodes was investigated. Covalent attachment was demonstrated, but the resulting electrode-hydrogel surfaces did not show long-term activities superior to those for physisorbed hydrogels. It is suggested that the limiting factor for long-term hydrogel activity is not adhesion of the hydrogel to the surface, but degradation of enzymatic activity by H2O2.