Radiative engineering of nanoantenna arrays for ultrasensitive vibrational spectroscopy of proteins

Infrared absorption spectroscopy offers direct access to the vibrational signatures of molecular structure. Although absorption cross sections are nearly 10 orders of magnitude larger than the Raman cross sections, they are small in comparison with those of fluorescent labels. Sensitivity improvements are therefore required in order for the method to be applicable to single molecule/monolayer studies. In this work, we demonstrate a plasmon enhanced vibrational spectroscopy technique which allow for the measurement of molecule specific signatures at the monolayer level. Specifically, we show 4-5 order of magnitude enhancement of the amide-I and II backbone signature of protein monolayers, the signal resulting from only zeptomole quantities of molecules.