The synthesis of a novel bidentate ligand-protected copper nanocluster via a solid-state strategy is reported. Single-crystal X-ray diffraction analysis result reveals that the copper nanocluster features an octahedral core (Cu) coordinated by six ligands. Noncovalent interactions (C-Hπ and ππ) exist between the copper nanoclusters. The copper nanocluster displays luminescence even at 250 °C. The luminescence intensity is linearly correlated with temperature changes. The copper nanocluster can assemble into luminescent nanosheets whose emission is quenched by 4-nitrophenol. Spectroscopic analysis and theoretical calculations results demonstrate that the inner filter effect and electron transfer cause the above quenching effect. A probe based on luminescent nanosheets was constructed for β-galactosidase activity determination. The linearity range is 3.3-91.8 U·L, and the limit of detection is 0.45 U·L. This probe was also evaluated for determination of the β-galactosidase activity in human serum via spiking experiments. The recoveries ranged from 96.2% to 101.8%.