ABSTRACT
Various biocatalyzed transformations induce the polymerization of a thin film [37], or the precipitation of an insoluble product [38, 39] on the transducer, leading to the electrode support insulation (increase of electrode resistance and decrease of interfacial electrontransfer) or to an increase in the mass associated with a
piezoelectric crystal. Figure 8 exemplifies several biocatalyzed transformations that lead to the precipitation of an insoluble product on the transducer. Peroxidase-mediated oxidation of 4-chloro-1-naphthol, (7), or 3,3′,5,5′-tetramethylbenzidine, TMB, (9) by
H2O2 to form the insoluble products, (8) and (10), respectively, or the alkaline phosphatase oxidative hydrolysis of 5-bromo-4-chloro-3-indoylphosphate, (11), that forms the indigo derivative, (12), represent biocatalytic transformations that precipitate an insoluble product on the surface [38, 39]. Similarly, horseradish peroxidase, HRP, mediated polymerization of phenol yields a polymer
film on surfaces [37]. The insoluble product generated on the electrode insulates the conductive support and introduces a barrier for interfacial electron transfer. Faradaic impedance spectroscopy was found to be a sensitive transduction method to probe the formation of the insoluble product on the conductive support [34]. Similarly, microgravimetric, quartz-crystal-microbalance measurements were used to probe the formation of the precipitate on an Au-quartz piezoelectric crystal [34]. These amplification routes and the respective electronic transduction methods were used to develop enzyme-sensing electrodes [34a, b], and immunosensors for the amplified detection of an antibody [34c].
