Atomically precise nanoclusters of coinage metals for photothermal energy conversion: recent progress and biomedical...

Atomically precise nanoclusters (APNCs) of metals have emerged as a unique class of nanomaterials that bridge the gap between molecular complexes and metallic nanoparticles. Their precisely defined atomic structures, tunable electronic states, ultrasmall dimensions (<3 nm), and strong optical absorption spanning the visible to near-infrared (NIR) regions (NIR-I: 700-1000 nm, and NIR-II: 1000-2500 nm) have attracted significant attention. Owing to their discrete electronic structures, APNCs exhibit highly tunable excited-state dynamics and luminescence ranging from visible to NIR-II. Apart from photoluminescence, their photothermal properties, namely the ability to convert absorbed light into heat, have recently become an area of growing interest. Emerging studies demonstrate that APNCs can achieve remarkably high photothermal conversion efficiencies (PCE), in some cases surpassing plasmonic nanoparticles, carbon-based materials, and semiconductor nanomaterials. Their strong NIR absorption is particularly advantageous for biomedical applications because NIR light offers deeper tissue penetration and minimal photodamage, enabling photothermal therapy, photoacoustic imaging, and related theranostic technologies. Furthermore, water-soluble APNCs, especially Au APNCs, combine multiple advantages such as low cytotoxicity, ultrasmall size, efficient tumor accumulation, and rapid renal clearance, making them promising candidates for cancer treatment. APNCs have also demonstrated rapid and efficient heat generation in the solid state. Notably, several Au APNCs exhibit broad absorption profiles that closely overlap with the solar irradiance spectrum, particularly in the NIR region, creating opportunities for solar-driven water evaporation, thermal energy harvesting, and thermoelectric applications. This review summarizes recent progress in the photothermal properties of coinage-metal APNCs, discusses the fundamental mechanisms governing light-to-heat conversion, and highlights emerging applications, particularly for Au APNCs in photothermal cancer therapy and photoacoustic imaging. We also highlight critical research gaps and discuss future directions for advancing APNC-based photothermal energy conversion.