NAD(P)H SENSORS BASED ON ENZYME ENTRAPMENT IN FERROCENE-CONTAINING POLYACRYLAMIDE-BASED REDOX GELS

NADH and NADPH sensors were developed by entrapping lipoamide dehydrog enase (LD) and glutathione reductase (GR), respectively, in a redox ge l formed by the copolymerization of vinylferrocene with acrylamide and N,N'-methylenebisacrylamide. Addition of LD or GR to the gel polymeri zation mixture resulted in a significant acceleration of free-radical copolymerization, The redox gels were secured on the surface of a carb on paste electrode with a dialysis membrane, and the resultant enzyme electrodes showed linear amperometric response to their substrates (NA DH for LD and NADPH for GR) up to 3 mM when the immobilized ferrocene (Fc) and entrapped enzyme concentrations were similar to 0.90 mM and 1 .0 mg/mL, respectively. The substrate concentration over which the cat alytic current was found to be linear depended on the concentrations o f both the Fc mediator and the enzyme in the redox gels. The observed linearity indicates that the enzyme electrodes can be used as sensors to quantitate both NADH and NADPH in aqueous solutions. The pH-activit y profiles of the enzyme electrodes as well as their storage and opera tional stabilities were examined, It was unexpectedly observed that bo th entrapped LD and GR exerted different effects on the electrochemica l properties of the immobilized Fc/Fc(+) redox couple, although they h ave similar structural and catalytic properties. In LD-containing gels , the redox couple is electrochemically irreversible (Delta E-p = 285 mV), while in GR-containing gels, the mediator exhibits the same quasi -reversible electrochemical behavior as in the absence of protein (Del ta E-p = similar to 95 mV), The electrocatalytic currents (i(c)) of th e enzyme electrodes vs enzyme loading were investigated, and it nas fo und that high LD loading (greater than or equal to 1.5 mg/mL) reduced i(c) but high GR loading did not It is concluded that the GR-containin g redox gel electrode is highly suitable for use with dehydrogenases t hat require NADP(+) as a cofactor.