Electrodes modified with nitrofluorenone derivatives as a basis for new biosensors

We report about the electrocatalytic properties of electrodes modified by a dsorption of nitro-fluorenone derivatives. The stable, adherent monolayer o f these catalyst precursors can be transformed electrochemically into the c orresponding hydroxylamine compounds (R-NO2 + 4e + 4H(+) double right arrow R-NHOH + H2O). The completely reversible two electron oxidation of the hyd roxylamine leads to the nitroso compounds (R-NHOH double right arrow R-NO 2e + 2H(+)) that exhibit high catalytic activity in the electrooxidation o f NADH at low overpotentials (-30 mV vs. Ag/AgCl) and therefore constitute a new family of efficient redox mediators for biosensor applications. A sig nificant increase in catalytic activity (up to 500%) is observed after addi tion of calcium ions to the electrolyte. This is explained by a specific an d bridging complexation between the coenzyme's phosphate groups and a carbo xyl group present in the catalyst molecule. The interaction favours the con tact between NADH and the surface confined catalyst, leading to a higher el ectron transfer efficiency. This interaction can be used in an approach of molecular level design for controlled monolayer deposition of catalyst, Ca2 +, NAD(+) and enzyme. A very simple and inexpensive modification scheme, es sentially based on electrostatic attraction, leads to electrodes that can b e employed as reagentless biosensors for the electrochemical detection of c ommon and commercially interesting analytes like glucose. (C) 2001 Elsevier Science B.V. All rights reserved.