Chemical Analysis Below the Diffraction Limit using Infrared-Coupled Atomic Force Microscopy (AFM-IR)

Abstract

High resolution infrared spectroscopy is a popular technique for investigating biological structures. It is relatively simple to use, and in some cases considered to be a non-destructive technique. By combining atomic force microscopy and infrared spectroscopy (AFM-IR) into a single bench-top instrument, it is possible to resolve chemical differences on the scale of ca. 100 to 200 nm, which often reveals information that could not have been obtained with conventional infrared microspectroscopy. The AFM-IR technique is based on observing the rapid thermal expansion and contraction of material due to the absorption of nanoseconds-long IR radiation pulses, which is collectively known as the photothermal induced resonance (PTIR) phenomenon. This rapid movement is captured by an AFM cantilever equipped with a sharp tip that is in direct contact with the sample material. The resulting amplitude of the ringdown response is directly related to the absorption characteristics of the material across a given range of wavenumbers. Therefore, AFM-IR spectral band shapes are similar to the bulk IR measurement and the spectra are searchable against existing databases. By further modulating the pulse frequency of the infrared laser radiation to coincide with the contact resonance of the AFM cantilever, sensitivity is enhanced, enabling the detection of ∼ 20 nm-thick organic materials. In this presentation, we will examine several biological systems using this AFM-IR technique. Spectral changes in the IR spectra can be seen through the whole or cross-sections of proteinaceous materials. Functional group IR images acquired using the AFM-IR technique also reveal the spatial distribution of chemical species in the form of absorption characteristics can be achieved at below the diffraction limit.