Nanoscale phenomena occurring at liquid-solid interfaces govern many chemical and biological processes. However, imaging such 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 through a ZnSe prism, which provides spatially confined excitation for minimized IR water absorption, reduced far-field background, and enhanced directional signal emission and sensitivity. Emission of the tip-excited sample dipoles peaks at angles close to the critical angle for the ZnSe/H2O interface and can exceed emission values of a dipole in a uniform medium by over an order of magnitude due to near-field coupling of high wavevector components of the dipole field into free-space forbidden photon modes in ZnSe. 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. In combination with local field enhancement of the infrared excitation using Au nanotriangle substrates we achieve monolayer sensitivity with chemical specificity at the few zeptomole level. This technique thus extends related in-liquid label-free nanospectroscopy efforts including PTIR and TERS to a wide range of systems including thin structures and biological systems with typically weak Raman cross-sections. 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.

In-situ chemical and biological nano-imaging using in liquid infrared scattering scanning near-field optical microscopy

ACS Spring 2022

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