  Mid-infrared photothermal microscopy is a new chemical imaging technology in
which a visible beam senses the photothermal effect induced by a pulsed
infrared laser. This technology provides infrared spectroscopic information at
sub-micron spatial resolution and enables infrared spectroscopy and imaging of
living cells and organisms. Yet, current mid-infrared photothermal imaging
sensitivity suffers from a weak dependance of scattering on temperature and the
image quality is vulnerable to the speckles caused by scattering. Here, we
present a novel version of mid-infrared photothermal microscopy in which
thermo-sensitive fluorescent probes are harnessed to sense the mid-infrared
photothermal effect. The fluorescence intensity can be modulated at the level
of 1% per Kelvin, which is 100 times larger than the modulation of scattering
intensity. In addition, fluorescence emission is free of speckles, thus much
improving the image quality. Moreover, fluorophores can target specific
organelles or biomolecules, thus augmenting the specificity of photothermal
imaging. Spectral fidelity is confirmed through fingerprinting a single
bacterium. Finally, the photobleaching issue is successfully addressed through
the development of a wide-field fluorescence-enhanced mid-infrared photothermal
microscope which allows video rate bond-selective imaging of biological
specimens.
