Enhancement of the super-resolved optical photothermal infrared signal increase at lower temperatures

Optical photothermal infrared (O-PTIR) microspectroscopy is an emerging label-free IR super-resolution technique with a growing base of applications, primarily in soft materials. The photothermal effect has traditionally been studied at room temperature, but apart from expanding the scope of samples, there was an open question whether the signal itself would benefit from a lower temperature, as the photoinduced heating could be more localized, thus increasing the signal. To enable such measurements, a temperature- controlled stage was fitted into the O-PTIR microscope to evaluate the O-PTIR signal of tristearin, biological tissue and polymers during temperature lowering from room temperature down to -100 °C. Therefore, we report the first successful temperature-controlled spectroscopic measurements and imaging of samples at subzero temperatures using O-PTIR. Moreover, for the first time, we observed an up to 3 times increase in O-PTIR signal with decreasing temperature, with a maximum in the -20 to -40 °C range. This also propagates to an increase in the limit of detection of a thin protein film sample from a thickness of ca. 400 nm to ca. 300 nm. Finally, we discuss which physical factors contribute to the observed enhancement.