Fingerprinting a living cell by raman integrated mid-infrared photothermal microscopy

Vibrational spectroscopic imaging techniques, based on infrared absorption or Raman scattering, allow for noninvasive chemically specific visualization of biological systems. The infrared and Raman modalities with different selection rules provide complementary information. Specifically, infrared microscopy provides strong signals in the fingerprint region, but suffers from low spatial resolution. Raman microscopy provides submicrometer resolution, but requires a long acquisition time. We developed a system that combines the strengths of both techniques by integrating confocal Raman microspectroscopy to the recently developed mid-infrared photothermal microscopy. This hybrid system is capable of fast infrared photothermal imaging of living cells with submicrometer resolution to identify points of interest, followed by a full-spectrum Raman analysis of the identified objects. In addition, a fingerprint photothermal spectrum can be acquired by scanning the wavelengths of the infrared laser. Comprehensive vibrational fingerprint mapping of live cells, demonstrated in adipocytes and single bacteria, promises broad applications of this technology in biology and material science.