Single nanoparticle plasmonic sensors

Abstract

The adoption of plasmonic nanomaterials in optical sensors, coupled with the advances in detection techniques, has opened the way for biosensing with single plasmonic particles. Single nanoparticle sensors offer the potential to analyse biochemical interactions at a single-molecule level, thereby allowing us to capture even more information than ensemble measurements. We introduce the concepts behind single nanoparticle sensing and how the localised surface plasmon resonances of these nanoparticles are dependent upon their materials, shape and size. Then we outline the different synthetic approaches, like citrate reduction, seed-mediated and seedless growth, that enable the synthesis of gold and silver nanospheres, nanorods, nanostars, nanoprisms and other nanostructures with tunable sizes. Further, we go into the aspects related to purification and functionalisation of nanoparticles, prior to the fabrication of sensing surfaces. Finally, the recent developments in single nanoparticle detection, spectroscopy and sensing applications are discussed.

Keywords: metal nanoparticles; optical sensors; sensors; single molecule detection; surface plasmons.

Figure 1

General rules of shape-control in silver-assisted, seed-mediated gold nanoparticles synthesis, where the shape of nanoparticles can be controlled simply by altering the reaction rate or altering the silver concentration. Scale bars are 200 nm. Adapted with permission from reference [12]. Copyright 2012 American Chemical Society.

Figure 2

The four major surface modification approaches for functionalisation of nanoparticles, given a peptide as an example of a biomolecule and PEG as the antifouling material. (1) Covalent attachment: covalent reactions such as EDC/NHS coupling occur between the antifouling layer and biomolecule end groups; (2) Electrostatic: use cationic/anionic antifouling materials and biomolecules to allow charge-charge conjugation; (3) Direct thiol reaction: both anti-fouling material and biomolecules loaded through thiol binding to the surface; (4) Secondary interaction: ligands are loaded onto the surface followed by the biomolecules containing a specific receptor to allow ligand-receptor specific conjugation.

Figure 3

Schematic diagram of the optical setup for the detection of the gold nanoparticles in a nanoarray, where the scattered light is directed to a colour CCD, spectrometer CCD and the eyepiece. The nanoarray is spectroscopically analysed by scanning along the region-of-interest in sequential steps. The single-particle spectral information is then recreated into a pseudo-image for further analysis. Adapted with permission from [101]. Copyright 2011 American Chemical Society.

Figure 4

(a) The optical setup for the total internal reflection and illumination of a single gold nanorod (magnified) for the detection of single protein binding. Inset at the top left is the theoretical LSPR shift that is expected in response to a large increase in local refractive index, such as the binding of proteins. Top panel in (b) shows the spectrum of a single Au nanorod, before and after the binding of single proteins. The bottom panel in (b) shows the difference in intensity between pre and post-binding of the protein; (c) The resonance wavelength of the single Au nanorod, monitored as a function of time, shows multiple single protein binding events, while the resonance wavelength remains flat for the protein-free solution. The peaks in the histogram (d) correspond to single protein-binding events. Reprinted with permission from [70]. Copyright 2012 American Chemical Society.

Figure 5

(a) Schematic diagram of the setup used to illuminate Au nanorods with a superluminescent diode through a glass prism. (b) Image of the modified Au nanorods on the surface obtained with the CCD and (c) illustration of the red-shift in LSPR scattering during the binding of proteins to the Au nanorod (inset). Reprinted with permission from [114]. Copyright 2015 American Chemical Society.