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 (1O2) 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 1O2 to kill cancer cell as demonstrated in this study.