Labelfree chemical analysis at nanoscale spatial resolution

Abstract

Vibrational spectroscopy (mid-infrared and Raman spectroscopy) is without a question a powerful tool for molecular analysis and has therefore found wide acceptance for label free chemical imaging of organic materials and plant and animal tissues. However, as with any optical imaging technique, the spatial resolution of infrared and Raman spectroscopy are limited by diffraction to roughly around the employed wavelength. We can circumvent the diffraction limit by building a nearfield imaging system, i.e. by moving the detector and/or the light source as close to our specimen as possible. One approach to move the “detector” closer that has found wide acceptance for mid-IR spectroscopy is to use photothermal expansion induced by a tuneable pulsed laser for detection of local absorption. This AFM-IR (atomic force microscopy induced resonance) technique reads the local thermal expansion using an AFM tip to enable nanometre scale spatial resolution chemical imaging. AFM-IR can be used to collect mid-IR absorption spectra from nanoscale samples that resemble conventional bulk spectra [1]. An exciting aspect of AFM-IR is that it can be used to apply multivariate chemical imaging techniques that are well established for IR microscopy at the nanoscale. These enable to combine information from multiple spectra or multiple single wavelength images into actual maps of chemical composition, which can reveal inclusion bodies in cells. We can also leverage full spectra to create images showing us – secondary structure specific - intracellular protein distribution at the nanoscale [2].

References

[1] A. Dazzi, C. B. Prater, Q. Hu, D. B. Chase, J. F. Rabolt, and C. Marcott, “AFM–IR: Combining Atomic Force Microscopy and Infrared Spectroscopy for Nanoscale Chemical Characterization,” Appl. Spectrosc., vol. 66, no. 12, pp. 1366–1384, 2012, doi: 10.1366/12-06804. [2] A. C. V. D. dos Santos et al., “Nanoscale Infrared Spectroscopy and Chemometrics Enable Detection of Intracellular Protein Distribution,” Anal. Chem., Dec. 2020, doi: 10.1021/acs.analchem.0c02228.