Ultrasensitive Tip- and Antenna-Enhanced Infrared Nanoscopy of Protein Complexes
Surface-enhanced infrared absorption (SEIRA) using resonant plasmonic nanoantennas enables zeptomolar detection sensitivity of (bio)analytes, although with diffraction limited spatial 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 complexes 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 line shapes, which can be modeled by using coupled harmonic oscillators. The role of the antenna resonance frequency on the coupling 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 identification and imaging through a combination of tip- and antenna-enhanced IR nanoscopy.
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The Journal of Physical Chemistry C
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