Directional Nanoplasmonic Antennas for Self-Referenced Refractometric Molecular Analysis

Martin Wersäll Department of Applied Physics, Chalmers University of Technology, Göteborg, Sweden Ruggero Verre Department of Applied Physics, Chalmers University of Technology, Göteborg, Sweden Mikael Svedendahl Department of Applied Physics, Chalmers University of Technology, Göteborg, Sweden Peter Johansson Department of Applied Physics, Chalmers University of Technology, Göteborg, Sweden Mikael Käll Department of Applied Physics, Chalmers University of Technology, Göteborg, Sweden Timur Shegai Department of Applied Physics, Chalmers University of Technology, Göteborg, Sweden

Plasmonic metal nanoparticles are regarded as important ingredients in a wide range of promising sensing schemes due to their unique capability to sense minute changes in the ambient dielectric environment. Several optical sensing methods, including tracking of the localized surface-plasmon resonance (LSPR) peak through optical extinction or scattering spectroscopy, have been demonstrated with competitive performance. LSPR refractometric detection limits down to the order of Δn = 10^-6 RIU (refractive index units), and even single molecules in the case of single nanoparticle measurements, have been reported, evincing the future potential of plasmonic nanoparticles for a variety of molecular analysis applications.

However, the readout in LSPR molecular analysis is traditionally performed by following resonance peak shifts, which often requires rather complex and expensive instrumentation such as a spectrometer, spectrally stable light source, and accurate referencing to achieve reasonable signal-to-noise levels.

Motivated by the possibility to reduce both complexity and expense associated with LSPR-based sensors, we have previously demonstrated that Pd−Au heterodimers can be used for hydrogen sensing using a self-referenced optical readout based on directional scattering arising from the built-in material asymmetry of the nanostructures [1]. In this study we instead exploit the directional angular scattering from nanodimers comprised by gold nanodisks with different aspect ratios [2] to perform single-wavelength self-referenced spectrometer-free measurements based on tracing the ratio of intensities scattered to the left- and right along the dimer axis. Besides refractometric bulk sensing, a biotin-neutravidin recognition model was performed utilizing the new sensing scheme. Overall we demonstrate that LSPR molecular analysis can be performed in a simple and cost-effective manner by building an optical setup consisting of cheap optical components, which in turn results in an approach that may prove useful for future LSPR-based label-free biosensing.

[1] T. Shegai, et al. “Directional Scattering and Hydrogen Sensing by Bimetallic Pd−Au Nanoantennas” Nano Lett. 12, 2464−2469 (2012).

[2] M. Wersäll, et al. “Directional Nanoplasmonic Antennas for Self-Referenced Refractometric Molecular Analysis” J. Phys. Chem. C. 118, 21075-21080 (2014).

wersall@chalmers.se