A method is described for the preparation of copper(II)-modified keratin-capped gold nanoclusters (AuNCs) with adjustable Au/Cu molar ratio through a two-step synthetic route. The introduction of Cu(II) is known to cause quenching of the fluorescence of such AuNCs. It is found, however, that the Cu(II) loaded AuNC (AuNC-Cu) display strongly enhanced peroxidase-like activity and improved chemical stability. This is assumed to be due to the synergistic effect of the gold and copper atoms and in contrast to the single components (pure AuNCs and copper ions). The kinetic parameters of the new peroxidase mimic show a higher K value (12.1 × 10 s) and a lower K value (53 μM) for HO (compared to those of conventional AuNCs). The catalytic activity is stable and remains essentially unchanged after two months. The interactions of AuNCs with Cu(II) were characterized by fluorescence spectroscopy, UV-vis spectroscopy and X-ray photoelectron spectroscopy. Based on these findings, a glucose colorimetric assay at 452 nm was developed that has a detection range from 1.6 to 800 μM and a 0.26 μM detection limit. Graphical abstract Copper ion-modified keratin-capped gold nanoclusters (AuNC-Cu) exhibit enhanced peroxidase-like activity owing to the synergistic effect of the gold and copper atoms which is in contrast to pure AuNCs.