Gold nanoclusters possess outstanding physical and chemical attributes that make them excellent scaffolds for construction of chemical and biological sensors. The paper reported synthesis of double gold nanoclusters/graphene oxide (D-GNCs/GO) and its application as a new fluorescence probe for Hg²⁺ detection. In the study, the amine group was introduced into GO sheets through the EDC/NHS mediated reaction to form positively charged GO sheets (GO-NH₃⁺). After GNC@Lys was mixed with GNC@BSA to form negatively charged D-GNCs, the D-GNCs was assembled on the surface of GO-NH₃⁺ with electrostatic interaction. The study demonstrated that the interaction between GNC@Lys and GNC@BSA increases fluorescence intensity of the GNC@BSA and leads to more sensitive fluorescence response towards Hg²⁺, for which the sensitivity is more than 3-fold that of single GNC@BSA. The interaction between GO and GNCs accelerates the reaction of D-GNCs/GO with Hg²⁺, for which the reaction rate is more than 3-fold that of single D-GNCs. Owing to prominent synergistic effects between GNC@Lys, GNC@BSA and GO, the nanosensor based on the D-GNCs/GO displays a surprisingly enhanced sensitivity and rapidity for Hg²⁺ detection. The fluorescence peak intensity linearly decreases with increasing Hg²⁺ concentration in the range of 1.0 × 10⁻⁵ to 5.0 × 10⁻¹³ M with a detection limit of 1.8 × 10⁻¹³ M (S/N = 3). The analytical method presents an obvious advantage of sensitivity, rapidity and repeatability when compared with present Hg²⁺ optical sensors. It has been successfully applied to detection of Hg²⁺ in water samples. The study also opens a new avenue for fabrication of fluorescent hybrids, which holds great potential applications in sensing, spectral encoding, bioimaging and catalysis.