The electrochemical behavior of methylene green (MG) adsorbed on a silica surface modified with niobium oxide (SN) was investigated, using modified carbon paste electrodes. It was also used in an electrocatalytic study of NADH oxidation. The electrode showed a high stability attributed to the presence of SN, which avoids the leaching of the mediator from the electrode surface. The formal potential (E-0') of the adsorbed MG was -35 mV vs SCE, showing a shift of 30 mV toward more positive potential values, compared to the MG dissolved in aqueous solution. This shift was assigned to the interaction between the basic nitrogen of MG and the acid sites of SN. The variation of the solution pH between 4 and 8 did not affect the stability nor the formal potential. However, for solution pH lower than 4 the formal potential was affected by the acidity of the medium. The electrocatalytic oxidation of NADH at the electrode was investigated. In the solution pH between 5 and 8 the electrocatalytic activity remained almost constant, giving a response signal of 13.3 nA L mumol(-1) cm(-2) and a K-Mapp of 1.4 x 10(-5) mol L-1. The electrode gave a linear response range between 5.0 x 10(-4) and 4.0 x 10(-3) mol L-1 NADH concentration at pH 7.0 at an applied potential of 50 mV vs SCE. Applying a flow injection analysis system, the electrode showed a better analytical performance for NADH detection, presenting a linear response range between 6.0 x 10(-5) and 1.0 x 10(-3) mol L-1, with an analytical frequency of 30 determinations/h, a detection limit of 8.2 x 10(-6) mol L-1, and a precision for 25 replicates of 1% expressed as a relative standard deviation. (C) 2002 Elsevier Science (USA).2541113119