A biosensor for detecting the aromatic substance 4-nitrophenol based on Agrocybe aegerita peroxygenase (AaP) immobilized with chitosan-stabilized gold nanoparticles is presented here This biosensor measures the enzymatic product of 4-nitrophenol peroxygenation 4-nitrocatechol which is electrochemically detected in the presence of hydrogen peroxide Cyclic voltammetry and amperometry were used to characterize the proposed biosensor The linear range of the AaP biosensor for the detection of 4-nitrophenol was between 10 and 30 mu M with a detection limit of 0 2 mu M (based on the S/N =3) The catalytic property of AaP to oxidize 4-nitrophenol was compared with two other heme proteins a camphor-hydroxylating cytochrome P450 monooxygenase (P450(cam) CYP101) and horseradish peroxidase (HRP) The results revealed that only AaP is capable of catalyzing the hydroxylation of 4-nitrophenol into 4-nitrocatechol Consequently AaP could be a particularly potent biocatalyst that may fill the gap between cytochrome P450s and common heme peroxidases (C) 2010 Elsevier Ltd All rights reserved