Integrated wide-field photothermal reflectance and transmittance microscopy for the detection of individual nanoparticles

Nanoparticles are extensively utilized in biomedicine and plasmonic sensing; therefore, precise measurement techniques for individual NPs are required. Photothermal microscopy offers high sensitivity, label-free specificity, and rapid imaging for nanoparticle detection. Among photothermal microscopy modalities, wide-field photothermal microscopy facilitates high-throughput analysis with simplified alignment but suffers from reduced signal-to-noise ratio due to increased shot noise. This study presents a compact wide-field photothermal microscopy system capable of performing both photothermal reflectance and transmittance imaging. The system integrates two CCD cameras, two probe beams, and a pump laser into a conventional microscope setup, allowing the alignment-free detection of individual nanoparticles. Photothermal transmittance imaging improves the photothermal contrast through enhanced forward scattering and interference. Comparative experiments demonstrated that photothermal transmittance achieves a significantly higher signal-to-noise ratio than photothermal reflectance, particularly when the surrounding medium is optically transparent. Optical simulations using the finite-difference time-domain method support the observed enhancement, which is attributed to strong modulation of the transmitted probe field. The developed platform enables robust, high-throughput characterization of individual nanoparticles and provides practical advantages for nanoparticle analysis under varying sample conditions.