Molecular Materials through Microdroplets: Synthesis of Protein-Protected Luminescent Clusters of Noble Metals

Atomically precise noble metal nanoclusters protected with proteins have emerged as a new research frontier in nanoscience due to their unique optical and chemical properties as well as promising applications. In the present work, we have employed an ambient electrospray technique to synthesize protein-protected luminescent clusters of gold and silver within the time scale of a few microseconds, which typically takes hours. In the absence of an electric field, the spray results in nanoparticles and no cluster formation was noticed. Synthesis of these clusters in microdroplets leads to severalfold enhancement in the rate of cluster formation. Spectroscopic investigations such as optical absorption, transmission electron microscopy, and matrix-assisted laser desorption ionization mass spectrometry confirm the molecular nature of the particles formed. Luminescence of electrospray-synthesized clusters shows multifold enhancement as compared to the clusters synthesized in the solution phase. Luminescence of the clusters synthesized in microdroplets increases with the distance traveled by the spray. The formation of clusters via electrospray affects the secondary structure of the protein, and its conformation is different from that of the parent protein. The Au@BSA cluster is utilized for in vitro imaging of retinoblastoma NCC-RbC-51 cells demonstrating a biological application of the resultant material. The absence of solvents and additional reagents enhances the sustainability of the method.