Self-assembled monolayers with biospecific affinity for lactate dehydrogenase for the electroenzymatic oxidation of lactate

Abstract

Surface modified gold electrodes with high biospecific affinity for NAD(H)-dependent lactate dehydrogenase have been prepared by covalent attachment of several traizine dyes to stepwise functionalized mixed alkanethiol self-assembled monolayers. The biospecific affinity of such ligand-anchored monolayers to bind submonolayer amounts of enzyme was demonstrated from the course of the protein adsorption events monitored by surface plasmon resonance. Electroenzymatic activity measurements of lactate dehydrogenase modified surfaces for the reaction of lactate oxidation, carried out ‘ex situ’ at different stages of protein layer growth, allowed the optimization of the preparative procedure to yield reproducible enzymatic electrodes with a low amount of unspecifically bound protein. A short adsorption time, as well as a high concentration of enzyme in the solution used for protein layer growth, led to lactate dehydrogenase-modified gold electrode surfaces with a high electroenzymatic activity arising mainly from biospecifically bound species. The lowest amount of unspecifically adsorbed protein was found for ligand-anchored monolayers prepared from mixed alkanethiol underlayers with an excess of positively charged groups. The lack of electroenzymatic activity shown by lactate dehydrogenase modified electrodes in the absence of soluble coenzyme (NAD +) indicates that none of the investigated ligand-anchored monolayers could provide an efficient electronic pathway from the metal to the active site of the enzyme. Therefore, the monolayers acted just as an anchoring system for lactate dehydrogenase.