Origin of Red Emission in Protein-Stabilized Copper Nanoparticles: Evidence for Cu I -Metallothionein-Like Cluster Formation

Nanoparticles ( NP ) produced in or added to biological milieu will spontaneously form a shell composed of biomolecules, most commonly proteins, around the NP core. The explicit interaction of the protein shell with the NP core remains poorly resolved, particularly for NP based on metals essential to life. Red emissive copper nanoclusters ( RCuNC ) serve as a synthetic model for the Cu 0 NP -protein interface, and have been developed as biocompatible sensors, though the mechanism underlying their red emission is still unclear. Herein, we identify that the red emission from RCuNC does not originate from the Cu 0 NP but instead from previously unidentified Cu I -metallothionine ( MT )-like clusters. Emission decay measurements, Cu I -quantification assays, native polyacrylamide gel electrophoresis imaging experiments, and direct protein metalation with Cu I identify at least two distinct populations of Cu that form during the reduction of Cu II in the presence of proteins. Our findings reveal that approximately 47% of the total Cu in the as-prepared bovine serum albumin-stabilized RCuNC is present as Cu I . Our results underscore the need for the scrutiny of the assignment of emitting species in copper-treated protein samples prepared under reducing conditions, while revealing the opportunity for the development of protein-based sensors with red-emitting embedded Cu I - MT -like clusters.