Session: Current Topics in Biological Chemistry:

Biological systems are inherently heterogeneous over a hierarchy of length scales from molecular to macroscopic dimensions. However, imaging biological structures 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 water IR 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 photons 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.

Brian O'Callahan

Pacific Northwest National Laboratory

Appearances