A horseradish peroxidase (HRP)-Au nanoclusters (AuNCs) bionanocomposite was prepared simply by mixing HRP and NaAuCl4 in an alkaline aqueous solution (pH 12) at 37 degrees C for 12 h under vigorous stirring. Cast-coating the HRP-AuNCs bionanocomposite and then Nafion on a glassy carbon electrode (GCE) yielded a Nafion/HRP-AuNCs/GCE. This electrode exhibited a pair of well-defined cyclic voltammetric peaks of the immobilized HRP with a formal potential of -0389 V vs SCE and an electron transfer rate constant of 3.19 s(-1) in pH 7.0 phosphate buffer. The HRP also exhibits a high bioactivity. This electrode showed high electrocatalytic activity for reduction of both O-2 and H2O2, as a result of the synergistic effects of HRP and nearby AuNCs. This electrode gave a linear amperometric response to H2O2 from 0.01 to 3.27 mmol L-1 with a sensitivity of 0.86 mu A mM(-1) cm(-2) and a limit of detection (LOD, S/N = 3) of 0.99 mu M. (C) 2017 Elsevier B.V. All rights reserved.