Chemical modification of the surface of elemental carbon comprises a first stage of attaching a compound with an azo group to the elemental carbon and then a second stage of decomposing the azo group in the presence of one or more compounds with an olefinic group so that decomposition of the azo group forms radicals attached to the carbon surface and a said radical forms a covalent bond to a said olefinic group. The second stage may proceed as a polymerization of a vinyl monomer with a redox active group such as ferrocene, anthracene or anthraquinone. The resulting polymer-modified carbon may be used in an electrochemical sensor.