Strategies for modifying electrode surfaces to control electrochemical reactivity via immobilized molecules have become highly developed, indicating great promise in the area of electronic and optical devices, chemical sensors, and models for the study of interfacial reactivity and fundamental charge transfer processes. Ferrocenyl thiols adsorbed onto gold electrodes have been widely used in many electrochemical applications in mixed or one-components SAMs, in some cases in very adverse environments (e.g., in concentrated sulfuric acid; Issa, T.B. et al. ACS Sym. Ser., 1998, 690, 257). In the now classic demonstration (Hickman, J.J. et al. Science, 1991, 252, 688) of mixed-SAM pH sensors, ferrocenyl and quinone thiols (cf. description of product cat. no TE 004) were used to form a mixed monolayer on Au microelectrodes to give a two-terminal, voltammetric microsensor with reference and sensor functions on the same electrode. The detection was based on measurement of the potential difference of current peaks for oxidation and reduction of the reference (ferrocene) and indicator (quinone). Since the quinone has a half-wave potential that is pH-sensitive it was possible to use it as a pH indicator; the ferrocene was a pH-insensitive reference.