Thickness control of the silica shell: a way to tune the plasmonic properties of isolated and assembled gold nanorods

Angela Candreva,Massimo La Deda,Wiktor Lewandowski

doi:10.1007/s11051-022-05402-w

Abstract

By combining photophysical measurements with transmission electron microscopy, we proved that the thickness of the silica shell around gold nanorods determines the position of the longitudinal plasmonic band when they are isolated in solution or assembled in solid. The silica thickness has been tuned by modulating the reaction time and the ratio between CTAB-coated gold nanorods and TEOS concentration, obtaining gold nanorods covered by a silica shell with a thickness varying from 3.5 to 24 nm. Considering this shell as a spacer between the gold cores, it is possible to modulate the coupling of the localized surface plasmon resonance (LSPR) of neighboring nanorods. Moreover, the comparison between the extinction spectra in solution and in solid, recorded from nanorods covered by silica shell with different thickness, can be used to estimate the inter-nanoparticles distance required for plasmon interaction. We found that LSPR coupling is effective when the distance between the gold cores is no more than 10 nm. When the distance is greater, the nanorods do not interact with each other.

Highlights

Gold nanoparticles (AuNPs) with various shapes, size, and organization show unique scattering spectra depending on their geometries (Hao et al 2004; Xia et al 2003; Creighton and Eadon 1991; Candreva et al 2020)
Silica shell modifies the spectral position of the AuNRs longitudinal localized surface plasmon resonance (LSPR) peak (Fig. 1b): the peak at 836 nm of AuNR@SiO2 solution is redshifted compared to that of the AuNR@CTAB solution, positioned at 783 nm
Various parameters control the growth and thickness of silica shell of gold nanorods, but the obtained results show that the impact of the parameters is different, and small variations of someone can produce very important effects

Summary

Introduction

Gold nanoparticles (AuNPs) with various shapes, size, and organization show unique scattering spectra depending on their geometries (Hao et al 2004; Xia et al 2003; Creighton and Eadon 1991; Candreva et al 2020). To spatially organize anisotropic gold nanoparticles, two strategies should in principle be followed: one based on the interparticle forces due to a proper functionalization of the AuNPs (direct assembly) (Ni et al 2010; Kumar et al 2013; Kawamura et al 2007; Ahijado-Guzmán et al 2016); the other based on the presence of an orienting medium (indirect assembly) (Coursault et al 2012; Rožič et al 2017). Plasmonic coupling was found to be highly dependent on interparticle spacing and the effectiveness of the NP coating in modulating the plasmonic field. The effectiveness of plasmonic coupling increases as the distance between the nanorods decreases and the number of interacting NPs increases. (Kumar et al 2013; Sahu and Raj n.d.)

Methods

Results

Conclusion