Optical observation of single atomic ions interacting with plasmonic nanorods in aqueous solution

This is the author accepted manuscript. The final version is available from the publisher via the DOI in this recordPlasmonic nanoparticles provide the basis for a multitude of applications in chemistry, health care and optics because of their unique properties. Nanoparticle-based techniques have evolved into powerful tools for studying molecular interactions with single-molecule resolution. Here we show that this sensing capability can be used to detect single atomic ions in aqueous medium. We monitored interactions of single zinc and mercury ions with plasmonic gold nanorods (NRs) resonantly coupled to our whispering gallery mode sensor. Our system's ability to discern permanent binding and transient interaction allows us to study the different interaction kinetics of both ion species. The detection of transient interactions enables us to confirm statistically that the sensor signals originate from single ions. Furthermore, we reveal how the ion–NR interactions evolve with respect to the medium's ionic strength as mercury ions amalgamate with gold and zinc ions eventually turn into probes of highly localized surface potentials.Max Planck Societ

In situ observation of single-molecule surface reactions from low to high affinities

This is the author accepted manuscript. The final version is available from the publisher via the DOI in this recordUnderstanding reactions occurring between ligand molecules and nanomaterial surfaces is essential in the field of nanoscience. The conventional methods for characterizing such surface-based reactions allow only for the analysis of the end product of a reaction, although the reaction path proceeds through the transient interaction of reactants and with kinetics dependent on environmental parameters. Here we study single molecule reaction kinetics associated with gold nanoparticle surfaces in an aqueous medium by utilizing whispering-gallery-mode microcavity sensors. Our approach resolves transient as well as permanent interaction kinetics of ligand molecules at the nanoparticle interface in situ, over a broad range of affinities and even under conditions where no net product is formed. This enables us to monitor and characterize reactions during the entire procedure of a bottom-up surface modification, ranging from the deposition of ligands to the confirmation of their functionality. We demonstrate this prospect by studying surface reaction kinetics with respect to the species of ligand head groups, tethered molecules, and inhibitors in addition to subsequent bio-specific reactions between tethered molecules and analytes.Max Planck Societ