Bovine Serum Albulmin Protein‐Templated Silver Nanocluster (BSA‐Ag13): An Effective Singlet Oxygen Generator for Photodynamic Cancer Therapy

This paper reports a novel synthesis approach of bovine serum albumin (BSA) protein-templated ultrasmall (<2 nm) Ag nanocluster (NC) with strong singlet oxygen generation capacity for photodynamic therapy (PDT). An atomically precise BSA-Ag13 NC (i.e., 13 Ag atoms per cluster) is successfully synthesized for the first time by using NaOH-dissolved NaBH4 solution as the controlling reducing agent. The ubiquitous size of BSA-Ag13 NC results in unique behaviors of its photoexcited states as characterized by the ultrafast laser spectroscopy using time-correlated single photon counting and transient absorption techniques. In particular, triply excited states can be largely present in the excited BSA-Ag13 NC and readily sensitized molecular oxygen to produce singlet oxygen (1 O2 ) with a high quantum efficiency (≈1.26 using Rose Bengal as a standard). This value is much higher than its Au analogue (i.e., ≈0.07 for BSA-Au25 NC) and the commonly available photosensitizers. Due to the good cellular uptake and inherent biocompatibility imparted by the surface protein, BSA-Ag13 NC can be applied as an effective PDT agent in generating 1 O2 to kill cancer cell as demonstrated in this study.