AFM–IR: Combining Atomic Force Microscopy and Infrared Spectroscopy for Nanoscale Chemical Characterization

TLDR

AFM‑IR produces IR spectra that contain the same molecular‑structure information as conventional IR spectroscopy, enabling researchers to apply existing IR expertise to nanoscale measurements. This review surveys polymer and life‑science applications of AFM‑IR for obtaining nanoscale IR spectra and images. AFM‑IR acquires IR absorption spectra and images with 50–100 nm spatial resolution, providing label‑free mapping of IR‑absorbing species in biological materials and polymers, including polymer blends, multilayer films, thin films for devices, microdomains, pharmaceutical blends, and electrospun nanofibers with controllable polarization. Experiments demonstrate sub‑cellular chemical spectroscopy, spatially resolved IR spectra and high‑resolution chemical images at specific wavenumbers, stiffness mapping via cantilever resonance, and molecular‑orientation information from polarized IR excitation.

Abstract

Polymer and life science applications of a technique that combines atomic force microscopy (AFM) and infrared (IR) spectroscopy to obtain nanoscale IR spectra and images are reviewed. The AFM-IR spectra generated from this technique contain the same information with respect to molecular structure as conventional IR spectroscopy measurements, allowing significant leverage of existing expertise in IR spectroscopy. The AFM-IR technique can be used to acquire IR absorption spectra and absorption images with spatial resolution on the 50 to 100 nm scale, versus the scale of many micrometers or more for conventional IR spectroscopy. In the life sciences, experiments have demonstrated the capacity to perform chemical spectroscopy at the sub-cellular level. Specifically, the AFM-IR technique provides a label-free method for mapping IR-absorbing species in biological materials. On the polymer side, AFM-IR was used to map the IR absorption properties of polymer blends, multilayer films, thin films for active devices such as organic photovoltaics, microdomains in a semicrystalline polyhydroxyalkanoate copolymer, as well as model pharmaceutical blend systems. The ability to obtain spatially resolved IR spectra as well as high-resolution chemical images collected at specific IR wavenumbers was demonstrated. Complementary measurements mapping variations in sample stiffness were also obtained by tracking changes in the cantilever contact resonance frequency. Finally, it was shown that by taking advantage of the ability to arbitrarily control the polarization direction of the IR excitation laser, it is possible to obtain important information regarding molecular orientation in electrospun nanofibers.