Monolayer-sensitive infrared imaging of DNA stripes using apertureless near-field microscopy

Monolayer-sensitive chemical imaging with a lateral spatial resolution of approximately 200 nm is demonstrated. Apertureless near-field scanning infrared microscopy was employed to study samples patterned with regions of DNA and hexadecanethiol. The scattering of the infrared radiation was modulated by an oscillating metallic probe, and the scattered signal was demodulated at twice the probe's oscillation frequency. The maps of the near-field signal reveal patterns that are not readily observable in the topographic images, indicating the absence of topography-related artifacts. The decrease in the scattering from areas coated with 24 base pair long DNA molecules is attributed to a phosphate stretching absorption band. The near-field absorption is observed to be larger than the far-field absorption.