Single Molecules and Single Gold Nanoparticles in the Spotlight

Michel ORRIT^1*

¹Huygens-Kamerlingh Onnes Lab, Leiden University, Leiden, Netherlands

* Presenter:Michel ORRIT, email:orrit@physics.leidenuniv.nl

I shall present several optical methods giving access to signals from single molecules and single gold nanoparticles. The latter objects are interesting because of their strong interaction with light mediated by their plasmon resonance, and because they are chemically and photochemically very stable. The experiments are based on fluorescence or photoluminescence, scattering, absorption detected by photothermal contrast, purely refractive effects leading to shifts of the plasmon resonance, or plasmon-enhanced fluorescence of weak emitters.
The optical isolation of single gold nanoparticles leads to their spectroscopic study on a single-particle basis and, through their plasmonic properties, to the study of their direct surroundings. I shall illustrate this general idea with some recent experiments from our laboratory.
Besides fluorescence, other optical signals can provide enough contrast to detect single molecules. The sensitivity of photothermal contrast can be pushed to the detection of individual organic absorbers with negligible fluorescence yields [1].
The strong optical near field close to the tips of gold nanorods enhances coupling to optical waves, both for excitation and for emission [2], and make the surface plasmon resonance sensitive to refractive index changes in the environment. This effect allowed us to detect individual non-absorbing proteins [3]. These biomolecules were bound to the functionalized metal surface with preferential attachment to the tips of the nanorod. The binding and unbinding of single protein molecules from the solution give rise to sudden absorption steps, and could be of use in micro-analytical applications and in-situ sensing.
By photothermal methods, we have monitored the nanosecond dynamics of steam nanobubbles formed in a liquid around a nanoparticle upon plasmonic heating [4]. We show that steam nanobubbles are extremely nonlinear systems that present dynamics in the nanosecond time scale and are very sensitive to perturbations.
Photothermal contrasts can be enhanced in near-critical fluids. We have recently used liquid xenon to detect single molecules of the conjugated polymer MEH-PPV. These molecules are too photosensitive for photothermal detection in regular molecular liquids such as glycerol.
[1] A. Gaiduk et al., Science 330 (2010) 353.
[2] H. Yuan et al. Angew. Chem. 52 (2013) 1217.
[3] P. Zijlstra et al., Nat. Nanotech. 7 (2012) 379.
[4] L. Hou et al., New J. Phys. 17 (2015) 013050.
[5] L. Hou et al., Nano Lett. 17 (2017) 1575.

Keywords: single molecules, gold nanoparticles, photoluminescence, photothermal microscopy, interferometric scattering