Highly sensitive vibrational imaging by fluorescence detection of mid-infrared photothermal effect

Mid-infrared photothermal (MIP) imaging is an emerging technique where a visible beam is deployed to sense the photothermal lensing effect induced by infrared absorption of molecules. This technology provides sub-micron spatial resolution defined by the visible probe beam. Yet, the photothermal lensing effect is a weak effect for most materials. For instance, the diffraction coefficient of poly (methyl methacrylate) changes around 0.1% per Kelvin. Thus, highly sensitive probes are desired to improve the detection sensitivity. Here, we report a fluorescence-enhanced mid-infrared photothermal (FE-MIP) microscope with high sensitivity. MIP microscopy uses a pump-probe strategy in which a mid-infrared light vibrationally excites a sample and a visible light probes the thermal effect. Instead of measuring the scattering modulated by mid-infrared absorption, we deploy thermal sensitive fluorescent dyes as the probe, and measure the modulated fluorescence intensity in both confocal mode and wide field mode. We demonstrate high imaging sensitivity and component specificity through fluorescence labeling.