High-sensitivity infrared vibrational nanospectroscopy in water

Mid-infrared vibrational spectroscopy is a universal label-freetool for identifying molecular compounds in chemical and bio-logical samples on the basis of their‘fingerprint’vibrationalabsorption lines. Vibrational spectroscopy with nanometer spa-tial resolution can reveal the chemical composition of samples atthe nanoscale, and several scanning-probe techniques have beendeveloped to address this need1–22. It is also highly desirable tostudy biological and chemical samples in their native aqueousenvironments rather than in air. In aqueous environments, how-ever, the sensitivity of the current vibrational nanospectroscopytechniques deteriorates dramatically7,23. Here, we report thefirstmid-infrared nanospectroscopy technique that retains nanoscalesensitivity and spatial resolution when the sample and the scan-ning probe are completely immersed in water. This methodovercomes challenges including water absorption and scatteringand the mechanical damping of cantilever vibrations. We fur-ther demonstrate spectroscopy and imaging of 20- to 50-nm-thick polymer samples with a 25-nm spatial resolution in thebiologically relevant Amide I and II spectral regions.