Nanoscale Infrared Spectroscopy of Biological Systems

Abstract

Infrared microspectroscopy is a popular technique for investigating biological structures. It is relatively simple to use, and considered to be a non-destructive technique. By combining atomic force microscopy and infrared spectroscopy (AFM-IR), it is possible to resolve chemical differences on the scale of ∼100 to 200 nm, which often reveals information that could not have been obtained with conventional infrared microspectroscopy. Currently, AFM-IR spectroscopy has the ability to collect IR spectroscopic information below the diffraction limit with lateral resolution of ∼ 100 nm. However, there are still some limitations that prevent its use on many important nanoscale systems. One of the main limitations is the thickness of the sample required for examination (> 100 nm). Overcoming these limitations has a dramatic impact by enabling widespread use of nanoscale IR spectroscopy for spatially resolved chemical characterization. The use of a quantum cascade laser (QCL) as the IR source significantly increases the sensitivity of AFM-IR. The QCL has repetition rates 1000 times higher than previous lasers used for AFM-IR. This allows the ability to pulse the laser at the resonant frequency of the AFM cantilever giving rise to a high IR sensitivity mode referred to as resonance enhanced infrared nanospectroscopy (REINS). Additional enhancement of the AFM-IR signal results when a gold-coated AFM tip is used, producing a “lightning-rod” effect which enhances the intensity of the exciting electric field at the tip. Furthermore, if the sample is deposited onto a gold substrate, the local electric field is further enhanced allowing for chemical identification of samples as thin as 25 nm. In this presentation we will demonstrate the effectiveness of the AFM-IR technique on several biological systems, including; IR spectroscopy of a monolayer film of Halobacterium salinarium on a gold substrate and IR chemical imaging of Streptomyces bacteria.