We report a confocal interferometric mid-infrared photothermal (MIP) microscope and its application to label-free detection of biological nanoparticles down to single virus level. We apply the interferometric scattering principle to detect the weak photothermal effect induced by infrared absorption in a viral particle. We validated this method with two kinds of viruses, namely the vesicular stomatitis virus (VSV) and poxvirus. The single virus spectra generated by MIP microscopy show high consistency in the same group, with dominant peaks contributed by the amide I and amide II vibrations. The ratio of these two peaks are significantly different between VSV and poxvirus, highlighting the potential of using MIP microscopy for label-free differentiation of viral particles. This all-optical chemical imaging method opens a new way for spectroscopic detection of biological nanoparticles in a label-free manner and may facilitate in predicting and controlling outbreaks of emerging virus strains.
Vibrational Spectroscopic Detection of Single Virus by a Confocal Interferometric Mid-Infrared Photothermal Microscope