Light-Harvesting Photothermal Hotspots Enabled by NIR Scattering-Absorption Coupling.

Efficient near-infrared (NIR) photothermal conversion is constrained by photon loss from scattering and attenuation in complex media. We present a scattering-absorption coupling strategy that pairs broadband photon scattering with plasmonic absorption to deliver supra-additive heating. Nanodiamonds (ND) multiply scatter and trap NIR photons, elevating local photon density, while neighboring gold nanostars (AuNS) convert the enriched flux into heat with high efficiency. A multi-scale framework links mechanism to outcome: finite-difference time-domain calculations quantify AuNS absorbed-power gains in the proximity of ND, and Monte Carlo photon transport yields a depth-dependent photon-density enhancement that together predicts the measured temperature rises. In agarose phantoms and bacterial plates, ND increases NIR absorption and, when combined with AuNS, produces greater-than-additive heating. In cells, endogenous endocytosis brings ND and AuNS into lysosomal microenvironments that act as confined "hotspots" enabling low-power ablation; the power density required for 50% cell kill (LD50) is reduced by a factor of 4.2 relative to AuNS alone. In in vivo experiments, AuNS and ND also exhibited a more potent photothermal therapeutic effect on mouse tumors. This principle provides a design rule for coupling scatterers and absorbers to overcome photon scarcity, offering a biocompatible route to efficient photothermal action with potential across oncology, anti-infective therapy, and other light-managed applications.