Charge-Transfer Complex Combining Reduced Cluster with Enhanced Stability for Combined Near-Infrared II Photothermal Therapy.

In the search for materials with enhanced near-infrared (NIR) photothermal properties and capability of providing environment-sensitive therapy, a method that combines isolated components into one nanocomposite is developed. The technique simultaneously involves redox, charge-transfer formation, and ionic complexation. During the polyoxophosphomolybdate (PMo) cluster mixing with biosafe chromogen 3,3',5,5'-tetramethylbenzidine (TMB), the reduced state (rPMo) and the oxidized TMB in the state of charge-transfer complex (cTMB) emerge spontaneously. The two reduced and oxidized components with charges form a stable ionic complex that resists physiology, saline, broad pH, and elevated temperature. Both the rPMo and cTMB contribute to the total sustainable photothermal conversion efficiency of 48.4% in the NIR-II region. The ionic complex exhibits biocompatibility in in vitro cell viability evaluation and is demonstrated to enter tumor cells with sustained photothermal property and complexation stability. Due to the local acidity that triggers further interaction among rPMo clusters, a distinct accumulation of the ionic complex at the tumor position is observed after caudal vein injection. Moreover, a remarkable local NIR-II photothermal image appears. The diminishment of tumor in mice with maintained body weight demonstrates the comprehensive effect of this NIR-II photothermal therapeutic material.