  Clinical identification and fundamental study of viruses rely on the
detection of viral proteins or viral nucleic acids. Yet, amplification-based
and antigen-based methods are not able to provide precise compositional
information of individual virions due to small particle size and low-abundance
chemical contents (e.g., ~ 5000 proteins in a vesicular stomatitis virus).
Here, we report a widefield interferometric defocus-enhanced mid-infrared
photothermal (WIDE-MIP) microscope for high-throughput fingerprinting of single
viruses. With the identification of feature absorption peaks, WIDE-MIP reveals
the contents of viral proteins and nucleic acids in single DNA vaccinia viruses
and RNA vesicular stomatitis viruses. Different nucleic acids signatures of
thymine and uracil residue vibrations are obtained to differentiate DNA and RNA
viruses. WIDE-MIP imaging further reveals an enriched \b{eta} sheet components
in DNA varicella-zoster virus proteins. Together, these advances open a new
avenue for compositional analysis of viral vectors and elucidating protein
function in an assembled virion.
