First, fluorescent HRP-AuNCs was synthesized. The blackish-green solution of the as-prepared HRP-AuNCs emits intense red fluorescence (λem max = 650 nm) along with a weak peak at 450 nm under UV light excitation (365 nm). In contrast, the control HRP solution was reddish-brown and emitted no fluorescence (Fig. 1). The fluorescence of the HRP-AuNCs in the presence of H2O2 (50 mM) was measured. Notably, the fluorescence at 450 nm significantly increased, while the peak at 650 nm remarkably reduced in the presence of H2O2. The change of the microenvironment around the AuNCs may be one of the reasons for fluorescence quenching. The results indicate that HRP after synthetic reaction remains active and enables the catalytic reaction to H2O2.