pH-sensitive phosphorescence in penicillamine-coated Au22 nanoclusters: Theoretical and experimental insights

•Water-soluble and bio-compatible D-Penicillamine-wrapped Au22(D-pen)18 nanoclusters with red-NIR phosphorescence quantum yield of 7.39% were synthesized.•Highly sensitive pH-conditioned phosphorescence intensity and lifetime near neutral condition provides high contrast for tumor imaging.•Mechanism underlying pH-sensitivity were unravelled through DFT calculations.•Alkaline solution attenuates the spin-orbital coupling and destabilizes structures, thus quenching phosphorescence yield.•In vivo and in vitro imaging of tumor tissues and cells confirm broad bands for excitation and luminescent throughout biological windows. Penicillamine-coated Au22 nanoclusters have an intensified yet pH-adjustable red to near infrared-I (NIR-I) phosphorescence due to their unique electronic structure, which endues them a high potential for tumor imaging and detecting in nanotheranostics. This study reports a sort of engineered Au22(D-pen)18 nanoclusters with a high quantum yield 7.39% as well as pH-conditioned phosphorescence intensity and lifetime that were addressed experimentally and theoretically. Through spin-orbital coupling calculation based on time-dependent density functional theory (TD-DFT), this study ascertains that the deprotonation of carboxylic groups on the D-penicillamine gives rise to a series of changes in the electronic structure of Au22(D-pen)18 nanocluster, leading to an ensuing pH-conditioned phosphorescence from pH-dependent spin–orbit coupling matrix element during inter-system crossing. The deprotonation also alters the bonding and interaction between Au-S core of the Au22(D-pen)18 nanocluster, which compromises the structural stability and potentially generates non-radiative decay with a quenched phosphorescence. The unique in-vitro and in vivo tumor-imagingadvantage of Au22(D-pen)18 nanocluster qualifies it as an efficient tumor-targeting phosphorescence sensor with pH-sensitivity capability for bright contrast to distinguish cancer cells from normal ones. This study provides a comprehensive view on the preparation and characterization of Au22(D-pen)18 nanoclusters, as well as the underlying mechanism of pH-conditioned phosphorescence and application prospects beside uniqueness in methodology, shedding light on further investigation for rational design and engineering of surface of ligand-coated nanoclusters.