In liquid infrared scattering scanning near-field optical microscopy for chemical and biological nano-imaging
Imaging heterogeneous biological systems with simultaneous intrinsic chemical specificity and nanometer spatial resolution in their typical native liquid environment has remained a long-standing challenge. Here we demonstrate a general approach of chemical nano-imaging in liquid based on infrared scattering scanning near-field optical microscopy (IR s-SNOM). It is enabled by combining AFM operation in a fluid cell with evanescent IR illumination via total internal reflection, which provides spatially confined excitation for minimized IR water absorption, reduced far-field background, and enhanced directional signal emission and sensitivity. We demonstrate in-liquid IR s-SNOM vibrational nano-imaging and conformational identification of catalase nano-crystals and spatio-spectral analysis of biomimetic peptoid sheets with monolayer sensitivity and chemical specificity at the few zeptomole level. This work establishes the principles of in-liquid and in-situ IR s-SNOM spectroscopic chemical nano-imaging and its general applicability to bio-molecular, cellular, catalytic, electrochemical, or other interfaces and nano-systems in liquids or solutions.
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