Photophysics-Guided Upconversion Nanosystems for Sensing

applications. To address this challenge, advances in materials design, most notably core-shell architectures that regulate energy migration and suppress surface quenching, have substantially boosted upconversion efficiency and spectral tunability. Complementary surface engineering via chemical modifications has further enhanced colloid stability, biocompatibility, and targeting specificity. In parallel, optical field engineering strategies, including superlensing effects and plasmonic coupling, have expanded the functional scope of upconversion platforms beyond conventional luminescence. Together, these developments have established upconversion nanoparticles as a robust physical interface between optical excitation and biological response.In this Account, we focus on recent progress in integrating upconversion nanoparticles with diverse physical modalities for biosensing and biointerfacing. We first outline the photophysical principles underlying photon upconversion and summarize key strategies for enhancing efficiency and signal fidelity. We then survey upconversion nanoparticle-based platforms that couple optical emission with electrical, mechanical, and thermal readouts. In optical microscopy, upconversion nanoparticles enable long-term single-particle tracking of neuronal transport and support super-resolution imaging through nonlinear emission processes and surface-migration depletion. When interfaced with electrophysiological measurements, these nanoparticles allow real-time monitoring of transmembrane water transport including flux through ion channels. Upconversion-assisted optogenetics further enables noninvasive neuromodulation without implanting optical fibers. Besides optical and electrical modalities, upconversion nanoparticles have been applied to force sensing over a wide dynamic range and to subcellular thermometry with high spatial precision. Incorporation of upconversion nanoparticles into device architectures extends these capabilities to stochastic photoluminescence encoding, infrared vision through retinal nanoantennas, and biocompatible contact lenses for near-infrared color perception.Looking ahead, the main challenges include further improving quantum yield under biologically permissible excitation, reducing excitation power requirements while maintaining high brightness, establishing long-term biosafety, and advancing toward clinical translation. Emerging directions such as data-driven materials design, artificial intelligence-guided optimization, and integration with regenerative medicine and microrobotics may help overcome these hurdles. By uniting advances in photophysical control with biological sensing and actuation, upconversion nanoplatforms hold strong potential to transform imaging, diagnostics, and therapeutic intervention.