Carbon paste electrode incorporating calcium-exchanged zeolite modified with Methylene Blue for amperometric detection of NADH

A new composite electrode material consisting of Methylene Blue (as redox mediator), a calcium-exchanged synthetic zeolite (as immobilization support) and carbon paste (as conductive material) (MB- 13XCa-CPEs) was obtained and characterized. Electrocatalytic oxidation of NADH was studied at this modified electrode and a better electrocatalytic activity in comparison with the electrode incorporating the same zeolite without calcium (MB-13X-CPEs) was put on evidence. The electrochemical properties (formal standard potential, peak currents and peak-to-peak separation) were different in the case of the calcium-exchanged zeolite, probably owing to the formation of a complex between mediator and Ca2+. Cyclic voltammetry, rotating disk electrode and amperometry were used to study NADH electrocatalytic oxidation. The kinetic parameters of this electrocatalytic process were evaluated in the presence or absence of Ca2+ in zeolite, assuming the formation of an intermediate charge-transfer complex between mediator and NADH. At low overpotentials (0 mV vs. Ag vertical bar AgCl/KClsat) a higher linear NADH concentration range and a lower detection limit is observed for MB-13XCa -CPEs (10(-7) -3 center dot 10(-5) M; 0.8 mu M) than for MB-13X- CPEs (10(-6) center dot 10(-4) M; 4.3 mu M).