Tailoring NIR-II photoluminescence of single thiolated Au25 nanoclusters by selective binding to proteins

Abstract Atomically precise gold nanoclusters (Au NCs) are a fascinating class of nanomaterials that exhibit molecule-like properties and have outstanding photoluminescence (PL), which is highly dependent on their structure and chemical environment. Their ultrasmall size, molecular chemistry, and biocompatibility make them extremely appealing for selective biomolecule labeling in investigations of biological mechanisms at the cellular and anatomical levels. In this work, we report a simple route to incorporating a preformed Au25 nanocluster into a model bovine serum albumin (BSA) protein. A new approach combining small-angle X-ray scattering and molecular modeling provides a clear localization of a single Au25 within the protein to a cysteine residue on the gold nanocluster surface. Attaching Au25 to BSA strikingly modifies the PL properties with enhancement and a redshift in the second near-infrared window (NIR-II). An extensive study based on a bottom-up approach that uses mixed-ligand nanoclusters Au25pMBA(18−x)Cysx with x=2, 5, 18 supported by experimental data (steady state, time-resolved spectroscopy) and theoretical calculations (DFT) provides new hints at the origin of NIR-II emission in such nanoclusters and their subsequent enhancement when selectively binding to a cysteine-rich protein. This study paves the way to controlling the design of selectively sensitive probes in biomolecules through a ligand-based strategy to enable the optical detection of biomolecules in a cellular environment by live imaging.