Ultrasensitive Mid-Infrared Detection through Ligand-Driven Local Heating in Lanthanide-doped Nanoparticles

Mid-infrared (MIR) upconversion spectroscopy, leveraging well-established visible/near-infrared (vis/NIR) detection techniques, holds significant potentials for a wide range of scientific and industrial applications. The recent finding that lanthanide upconversion nanoparticles (UCNPs) can exhibit MIR-induced luminescence presents a compelling technique to conventional upconversion techniques, opening new possibilities for compact, room-temperature MIR sensors. However, the intrinsically low absorption cross-section of lanthanide ions limits their MIR sensitivity. Here, we overcome this challenge by employing ligand-capped UCNPs, where organic molecules with strong and broadband MIR absorption serve as localized heating sources under MIR irradiation. The increased temperature modulates lanthanide ions' photoluminescence (PL) via thermally sensitive energy transfer processes. This thermally mediated mechanism enables broadband (5–10 μm) MIR detection at room temperature, achieving a detectivity of 4.8 E8 Jones at 6.3 μm, outperforming commercial pyroelectric detectors. The system exhibits a response time of approximately 2 ms and demonstrates spectral fidelity and gas sensing performance competitive with state-of-the-art Fourier-transform infrared (FTIR) systems. Our results establish a new design paradigm for MIR photodetection that bridges molecular photonics and upconversion nanotechnology, paving the way for next-generation optical sensing platforms.