Thermo-optoplasmonic single-molecule sensing on optical microcavities
Whispering-gallery-mode (WGM) resonators are powerful instruments for single-molecule sensing in biological and biochemical investigations. WGM sensors leveraged by plasmonic nanostructures, known as optoplasmonic sensors, provide unprecedented sensitivity down to single atomic ions. In this article, we describe that the response of optoplasmonic sensors upon the attachment of single protein molecules strongly depends on the intensity of WGM. At low intensity, protein binding causes red shifts of WGM resonance wavelengths, known as the reactive sensing mechanism. By contrast, blue shifts are obtained at high intensities, which we explain as thermo-optoplasmonic (TOP) sensing, where molecules transform absorbed WGM radiation into heat. To support our conclusions, we experimentally investigated seven molecules and complexes; we observed blue shifts for dye molecules, amino acids and anomalous absorption of enzymes in the near-infrared spectral region. As an example of application, we propose a physical model of TOP sensing that can be used for the development of single-molecule absorption spectrometers. Significance StatementA notable contribution to the optical detection of single molecules has been brought about by using optical microcavities, specifically whispering-gallery-mode resonators that combine optical and plasmon resonances for the most sensitive single-molecule and ion detection. Adding absorption measurements to a single-molecule technique that operates in an aqueous solution would provide powerful new detection capabilities for sensing the properties of single molecules. We demonstrate here, for the first time, the detection of absorption cross-sections of protein molecules on optoplasmonic microcavities and validate the technique with seven different molecules and complexes. A proof-of-concept for single-molecule optoplasmonic absorption spectrometers is presented, based on a thermo-optoplasmonic sensing mechanism.