PNNL

Abstract

Surface enhanced infrared absorption (SEIRA) us-ing resonant plasmonic nanoantennas enables zeptomolar detection sensitivity of (bio)analytes, although with diffraction limited spa-tial resolution. In contrast, infrared scattering-scanning near-field optical microscopy (IR s-SNOM) allows simultaneous imaging and spectroscopy with nanometer spatial resolution through vibrational coupling to the antenna mode of a probe tip. Herein, we combine these two approaches to image distributions of ferritin protein com-plexes adsorbed onto IR-resonant Au nanoantennas. The joint tip- and antenna-enhancement yields single protein complex sensitivity due to coupling with the vibrational modes of the bioanalytes. The coupling is revealed through IR s-SNOM spectra in the form of Fano lineshapes, which can be modelled using coupled harmonic oscillators. The role of the antenna resonance frequency on the cou-pling strength and the sensitivity is explored. Inhomogeneities in protein density within the film can be detected at single protein complex level. This work paves the way for single protein identi-fication and imaging through a combination of tip- and antenna-enhanced IR nanoscopy.