The basic electrochemistry of carbon paste electrodes modified with two new redox mediators, nitro-fluorenone derivatives, one neutral and one negatively charged, adsorbed on zirconium phosphate has been studied as well as their ability to oxidize NADH electrocatalytically. Cyclic voltammetry of these chemically modified electrodes performed in different aqueous buffer solutions at different pHs revealed that the immobilized redox species present very stable redox properties and the formal potential is virtually independent of the pH and/or the constituents of the contacting solution. The second order rate constants for the electrocatalytic oxidation of NADH were evaluated from rotating disk electrode experiments, and found to be of the order of between 10(6) and 10(7) M-1 s(-1), among the highest reported so far compared with literature values. The catalytic efficiency can be enhanced by addition of Ca2+ ions to the electrolyte solution and it is shown that Ca2+ affects both the neutral and negatively charged mediator. (C) 2003 Elsevier B.V. All rights reserved.