Computational Optimization of the Size of Gold Nanorods for Single-Molecule Plasmonic Biosensors Operating in Scattering and Absorption Modes.

Teresa Staniszewska,Maciej Szkulmowski,Seweryn Morawiec

doi:10.1021/acs.jpcc.1c02510

Teresa Staniszewska, Maciej Szkulmowski + Show 1 more

Open Access

https://doi.org/10.1021/acs.jpcc.1c02510

Copy DOI

Abstract

We present a comprehensive computational study on the optimization of the size of gold nanorods for single-molecule plasmonic sensing in terms of optical refractive index sensitivity. We construct an experimentally relevant model of single-molecule–single-nanoparticle sensor based on spherically capped gold nanorods, tip-specific functionalization and passivation layers, and biotin-streptavidin affinity system. We introduce a universal figure of merit for the sensitivity, termed contrast-to-noise ratio (CNR), which relates the change of measurable signal caused by the discrete molecule binding events to the inherent measurement noise. We investigate three distinct sensing modalities relying on direct spectral measurements, monitoring of scattering intensity at fixed wavelength and photothermal effect. By considering a shot-noise-limited performance of an experimental setup, we demonstrate the existence of an optimum nanorod size providing the highest sensitivity for each sensing technique. The optimization at constant illumination intensity (i.e., low-power applications) yields similar values of approximately 20 × 80 nm2 for each considered sensing technique. Second, we investigate the impact of geometrical and material parameters of the molecule and the functionalization layer on the sensitivity. Finally, we discuss the variable illumination intensity for each nanorod size with the steady-state temperature increase as its limiting factor (i.e., high-power applications).

Highlights

Noble-metal nanoparticles sustaining plasmonic resonances are perfectly suited to detect and study single organic molecules with no need for fluorescent labeling.[1−3] The detection of biomolecules is facilitated by pronounced spectral shifts of the resonance induced by the binding of an analyte molecule to the receptor molecule stabilized on the nanoparticle’s surface.The resonant optical response of metallic nanoparticles, i.e., localized surface plasmon resonance (LSPR), arises from the coupling of light to the collective oscillations of free electrons confined within a nanoparticle
We create an experimentally relevant model of a single-particle biosensor consisting of a spherically capped gold nanorod placed at the water/glass interface with tip-specific functionalization and passivation layers and biotin-streptavidin affinity system
We introduced a universal figure of merit, termed contrast-to-noise ratio (CNR), which relates the change of observable signal caused by discrete molecule binding events to the detection noise

Summary

Introduction

Noble-metal nanoparticles sustaining plasmonic resonances are perfectly suited to detect and study single organic molecules with no need for fluorescent labeling.[1−3] The detection of biomolecules is facilitated by pronounced spectral shifts of the resonance induced by the binding of an analyte molecule to the receptor molecule stabilized on the nanoparticle’s surface.The resonant optical response of metallic nanoparticles, i.e., localized surface plasmon resonance (LSPR), arises from the coupling of light to the collective oscillations of free electrons confined within a nanoparticle. Among various geometries of metallic nanoparticles suitable for biosensing applications,[9] gold nanorods (GNRs) are commonly employed[10−13] as their longitudinal LSPR can be tuned across the visible and near-infrared wavelength range by varying their aspect ratio Their elongated shape red-shifts the resonance away from the interband transition reducing plasmon damping and increasing the near-field enhancements.[14] GNRs can be synthesized by wet chemistry methods providing high-quality single-crystal nanoparticles and good control over the size and shape monodispersity.[15] Gold is the usual material of choice due to its chemical stability and better control of synthesis compared with silver, despite the latter being able to provide stronger plasmonic response.[16] Importantly, the existing chemical protocols allow for selective functionalization of highly curved surfaces of nanoparticles, i.e., the tips of the nanorod and passivation of remaining surfaces

Methods

Results

Discussion

Conclusion