Revue Roumaine de Chimie, 2007, 52(1–2), 67–74 Dedicated to Professor Ionel Haiduc on the occasion of his 70th anniversary NADH ELECTROCATALYTIC OXIDATION AT GLASSY CARBON PASTE ELECTRODES MODIFIED WITH MELDOLA BLUE ADSORBED ON ACIDIC α-ZIRCONIUM PHOSPHATE Carmen Ioana LADIU, a José R. GARCÍA, b Ionel Catalin POPESCU *a and Lo GORTON c a Department of Physical Chemistry, ”Babes-Bolyai” University, 400028 Cluj-Napoca, Roumania b Department of Organic and Inorganic Chemistry, University of Oviedo, 33006 Oviedo, Spain c Department of Analytical Chemistry, Lund University, P.O. Box 124, SE-2100 Lund, Sweden Received February 6, 2007 The electrochemical behavior and the electrocatalytic activity for the NADH oxidation at glassy carbon paste electrodes (GCPEs) modified with Meldola Blue (MB) adsorbed onto acidic α-zirconium phosphate (ZP * ) have been investigated. Cyclic voltammetry and rotating disc electrode measurements, performed in Tris buffer solution (pH 7) in absence and in presence of different NADH concentrations, showed that: (i) the formal standard potential (E°’) of MB was found pH dependent, according to the equation ) 10 1 log( 029 . 0 059 . 0 © a pK pH pH E E − + + − ° = , where the best fitting value for pK a was estimated to be 4.3 ± 0.4; (ii) the best electrocatalytic response was obtained for MB-ZP * - modified GCPEs having a ratio between the glassy carbon powder (Sigradur K) and MB-ZP * of 19:1; (iii) the presence of polyethyleneimine in the paste has a significant beneficial effect on the electrocatalytic activity of the investigated modified electrodes. As a general conclusion, it was stated that the particular features of the MB-ZP*-GCPEs are related to the ZP* crystallinity. INTRODUCTION ∗ The preparation of modified electrodes for NADH oxidation is of constant interest in developing new biosensors with higher sensitivity, selectivity and stability as well as efficient biofuel cells 1-6 based on cofactors regeneration 7 . For this reason, different mediators (quinines 8, 9 , flavins 10 , or other quinoic type compounds such as phenoxazines, phenothiazines, and phenazines 11-14 ) were immobilised by simple adsorption on various support materials such as: zirconia, titania 15, 16 , zirconium or titanium phosphates 17-21 , silica gel 12, 22-24 , and zeolites 25, 26 . The use of these support materials, onto or into which the mediators are strongly bound avoiding their leaching under experimental conditions, is also important for electrocatalytic purposes in order to: (i) improve the mediator electrochemical activity and stability 10, 12, 17, 27 , (ii) shift the mediator potential into the optimal potential range 28 , (iii) improve the ∗ Corresponding author: cpopescu@chem.ubbcluj.ro rate of the electrocatalytic reaction 17, 18 , and (iv) to decrease the effects of adsorption of products and other side reactions. Meldola Blue (MB), a phenoxazine compound, is a two electron-proton acceptor type mediator, which has a high oxidation reaction rate with NADH 29-32 . When immobilised onto an electronically conducting materials (such as graphite 31, 32 ) or onto amorphous homemade zirconium phosphate (ZP) 11, 17 MB can be used to obtain highly efficient modified electrodes for electrocatalytic oxidation of NADH. In the last case, it was noticed that the formal potential (E°’) of MB is apparently pH independent and has a value in the optimal potential region for amperometric detection 20 . Crystalline α-zirconium phosphate (α- Zr(HPO4)2·H2O; α-ZP) was used in many research areas as a catalyst, ion exchanger, molecular sieve, electrode material, and chemical modified matrix 33 . Taking into account that crystalline α-ZP is a good ion conductor, a good ion exchanger, and its structure (layered material)