Mid-infrared photothermal imaging: from cellular volumes to nanostructured interfaces
Embargo Date
2028-01-29
OA Version
Citation
Abstract
Vibrational spectroscopic imaging is a powerful tool in life science and material science to distinguish molecular conformations and study chemical dynamics. Molecular vibration signatures can be revealed through Raman scattering and infrared absorption. Mid-infrared (mid-IR) spectroscopy contains rich bond-selective information of key metabolites including proteins, lipids, nuclei acids and glucose. However, the mid-IR wavelengths that provide strong vibrational contrasts also limit the spatial resolution. Overcome this limitation, mid-infrared photothermal (MIP) microscopy has been developed through probing thermo-optic effect or thermal expansion induced by infrared absorption, enabling subcellular chemical imaging beyond video rates. However, three-dimensional (3D) chemical imaging speed is not sufficient for live cell studies. This dissertation developed two methods for volumetric chemical imaging at video rates. Firstly, by exploiting thermo-sensitive fluorescence, we demonstrate fluorescence-detected mid-infrared photothermal Fourier light field (FMIP-FLF) microscopy, achieving 8 volumes per second with 0.5–0.9 µm lateral and 0.8–1.1 µm axial resolution across a 60×60×5.6 µm3 volume. FMIP-FLF visualized protein in bacteria and lipid distributions in living cells and revealed altered lipid metabolism in drug-resistant pancreatic cancer cells. Secondly, we present photothermal relaxation intensity diffraction tomography (PRIDT) that encodes IR-induced refractive index changes through photothermal relaxation and reconstructs them via intensity diffraction tomography. PRIDT achieves 15 Hz volumetric chemical imaging with 264 nm lateral and 1.12 µm axial resolution over a 50×50×10 µm3 volume, enabling label-free, video-rate visualization of lipid and protein metabolism in live cells.
Beyond imaging speed, MIP sensitivity is limited by the weak interaction between IR photons and molecules. To address this, two ultrasensitive infrared photothermal spectroscopy methods were developed. We first report mid-infrared encoded plasmonic scattering (MIREPS) spectroscopy, which leverages a nanoparticle-on-film cavity with strong plasmonic field enhancement and extremely high sensitivity to the spacing defined by the analyte molecules inside the nanogap to detect nitrile or nitro groups in ~130 molecules. Next, we introduce mid-infrared photothermal microscopy to map the hot spot distribution and thermal response of a mid-IR-resonant plasmonic metasurface. Metasurface-enhanced infrared photothermal (MEIP) microscope achieves a detection limit as low as 0.24 monolayer surface coverage of bovine serum albumin.
Together, FMIP-FLF, PRIDT, MIREPS, and MEIP establish new strategies for high-speed, high-throughput, high-sensitivity and 3D-resolved infrared photothermal imaging, opening pathways for quantitative chemical analysis of low-abundance molecules in living systems.
Description
2026
License
Attribution 4.0 International