Papain-directed synthesis of luminescent gold nanoclusters and the sensitive detection of Cu2+

► A simple and “green” approach has been developed to synthesize the papain-Au nanoclusters. ► Papain is a commercially available inexpensive protein and could be obtained in large quantities. ► The nanoclusters show intense red emission in both solution and powder forms. ► The red emission remained unchanged after ∼3 months and at a buffer pH range of 6-12. ► The use of these Au clusters for the simple, sensitive, selective, and label-free cupric ion detection has been demonstrated.