  Interferometric scattering microscopy has been a very promising technology
for highly sensitive label-free imaging of a broad spectrum of biological
nanoparticles from proteins to viruses in a high-throughput manner. Although it
can reveal the specimen's size and shape information, the chemical composition
is inaccessible in interferometric measurements. Infrared spectroscopic imaging
provides chemical specificity based on inherent chemical bond vibrations of
specimens but lacks the ability to image and resolve individual nanoparticles
due to long infrared wavelengths. Here, we describe a bond-selective
interferometric scattering microscope where the mid-infrared induced
photothermal signal is detected by a visible beam in a wide-field common-path
interferometry configuration. A thin film layered substrate is utilized to
reduce the reflected light and provide a reference field for the
interferometric detection of the weakly scattered field. A pulsed mid-IR laser
is employed to modulate the interferometric signal. Subsequent demodulation via
a virtual lock-in camera offers simultaneous chemical information about tens of
micro- or nano-particles. The chemical contrast arises from a minute change in
the particle's scattered field in consequence of the vibrational absorption at
the target molecule. We characterize the system with sub-wavelength polymer
beads and highlight biological applications by chemically imaging several
microorganisms including Staphylococcus aureus, Escherichia coli, and Candida
albicans. A theoretical framework is established to extend bond-selective
interferometric scattering microscopy to a broad range of biological micro- and
nano-particles.
