Light‐Harvesting Photothermal Hotspots Enabled by NIR Scattering‐Absorption Coupling

ABSTRACT 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 (LD ₅₀ ) 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.