Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to study the effect of anthraquinone (AQ) films on the charge transfer rate of beta-nicotinamide adenine dinucleotide (NAD(+)), dopamine (DA), and ferricyanide on glassy carbon (GC) electrodes in solutions of different pH. Maximum blocking action on the Fe(CN)(6)(3-) redox probe was observed at pH 7 and open-circuit potential (OCP). However, maximum electron hopping effect was observed at pH 9 at both -0.58 V and -0.85 V for Fe(CN)(6)(3-), pH 7 at -0.58 V for NAD(+), and pH 9 at -0.58 V for DA, suggesting that electron hopping in AQ films on a GC surface is dependent on both pH and electrode potential. These findings lend support for the application of these films in the detection of soluble redox probes such as NAD(+) and DA at biological pH values (from 7 to 9). (C) 2011 Elsevier Ltd. All rights reserved.