How to record single molecules on a loop.

In our study, we demonstrate that opto-plasmonic resonators can respond in unexpected ways to reversible surface reactions. Optical whispering-gallery modes coupled to localised surface plasmons may yet offer specific signatures from biological analytes.
How to record single molecules on a loop.

Molecular dynamics is an aspect of nature easier to discuss than it is to observe. The diffraction limit restricts us to visualise features down to about half the wavelength of light, which we have historically sidestepped in a number of ways. We’ve looked at static images of DNA, for instance, for many decades now through X-ray crystallography. In real time, we can also tag a substance like a protein with a fluorescent label to convey its structural information. Tagging a molecule, however, changes the way it behaves and so characterising it becomes entangled with the way we probe it. Optical cavities are suited for these sorts of applications since light can be forced to repeatedly encounter a molecule. With optical cavities we can inspect each molecule and its motion at the nanoscale, i.e. below the diffraction limit, without the need for labeling.

In the Vollmer group at the Living Systems Institute of the University of Exeter, we exploit optical whispering-gallery modes coupled to plasmonic antennae to study single molecules. A micron-sized glass sphere, about which light circulates, is loaded with gold nanoparticles that further enhance and shrink the light. The hybrid opto-plasmonic resonance that emerges is used to monitor the surface reactions ongoing on the nanoparticles themselves. In a way, we are listening in on molecular chatter and trying to piece together the conversation.

One of the existing challenges with the above method is specificity. Although we can detect molecules as small as atomic ions, we cannot necessarily tell where they are or what they are made up of – we can only discern certain aspects about their dynamics. Properties such as a chirality are buried underneath factors such as the location of the sensing site and fluctuations in the nanoparticle. In our work, we took steps to address this by considering regulated reaction pathways through disulfide bonds. These reaction pathways end on single thiol sensing sites on the nanoparticle, providing a highly repeatable means to measure molecular characteristics. Such an approach could hold the key to decoding the charge and chirality state of individual molecules.

The main advantage of thiol/disulfide exchange is its reversibility. By strategically placing linker molecules on our gold nanoantennae, we can pick out interactions with thiolated species based on restricting their charge and cycling. The sensor signal follows distinct patterns based on the presence or absence of reducing agent, oscillating in time in a controlled or random fashion, respectively. In this way the monomer or dimer state of the leaving group can be identified per reaction. The opto-plasmonic resonance responds both in frequency and linewidth to single-molecule activity, hinting to more sophisticated mechanisms at play. Our results suggest potential plasmon-vibrational mode coupling that may allow for molecular fingerprinting. The vision of a molecular scanner feature, such as in Star Trek’s tricorder, may be closer to reality than you may think.

For more details, you can read our full paper "Optoplasmonic characterisation of reversible disulfide interactions at single thiol sites in the attomolar regime" published in Nature Communications.

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