Covalent Grafting of Glassy Carbon Electrodes with Diaryliodonium Salts:  New Aspects

The applicability and versatility of the recently communicated procedure for the grafting of conducting carbon substrates by diaryliodonium salts is expanded. We have found that several types of organic arylic layers can be formed on the carbon surface and that the chemical functionalities of the thus formed layers can be varied extensively over electron withdrawing (for example, −NO2) to electron donating (for example, −OMe) groups. A comparative study involving the grafting of aryldiazonium salts reveals that, despite the two approaches being similar, iodonium salts exhibit spontaneous grafting to a significantly lower extent. Nevertheless, the grafted layer becomes less accessible to proton transport as visualized from a greater reluctance toward the reduction of surface-confined nitro groups to amino groups in acidic medium. Employment of unsymmetrical iodonium salts opens up the interesting possibility of forming organic films consisting of a mixture of two different aryl groups. Alternatively, such composite layers may be prepared by selecting iodonium and diazonium salts with comparable reduction properties. Analysis of the surfaces is carried out by means of cyclic voltammetry, X-ray photoelectron spectroscopy, and ToF-SIMS (time-of-flight secondary-ion mass spectrometry). The ToF-SIMS analysis primarily serves to provide unambiguous evidence for the covalent attachment of the organic layers to the surface.