Abstract
Shell-isolated nanoparticles (SHINs) have attracted increasing interest for non-interfering plasmonic enhanced sensing in fields such as materials science, biosensing, and in various electrochemical systems. The metallic core of these nanoparticles is isolated from the surrounding environment preventing direct contact or chemical interaction with the metal surface, while still being close enough to enable localized surface plasmon enhancement of the Raman scattering signal from the analyte. This concept forms the basis of the shell isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) technique. To date, the vast majority of SHIN designs have focused on SiO2 shells around spherical nanoparticle cores and there has been very limited published research considering alternatives. In this article, we introduce a new polymer-based approach which provides excellent control over the layer thickness and can be applied to plasmonic metal nanoparticles of various shapes and sizes without compromising the overall nanoparticle morphology. The SHIN layers are shown to exhibit excellent passivation properties and robustness in the case of gold nanosphere (AuNP) and anisotropic gold nanostar (AuNS) core shapes. In addition, in situ SHINERS spectro-electrochemistry measurements performed on both SHIN and bare Au nanoparticles demonstrate the utility of the SHIN coatings. Correlated confocal Raman and SEM mapping was achieved to clearly establish single nanoparticle SERS sensitivity. Finally, confocal in situ SERS mapping enabled visualisation of the redox related molecular structure changes occurring on an electrode surface in the vicinity of individual SHIN-coated nanoparticles.
Highlights
The vast majority of shell isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) studies reported in the literature to date have focused on a thin silica (SiO2) shell surrounding a spherical gold or silver plasmonic nanoparticle core
Mercaptoundecanoic acid (MUA) was selected as the base layer due to the molecule's tendency to form a densely-packed self-assembled monolayer (SAM) layer on gold,[47] which has the ability to completely passivate an electrode surface preventing any electrochemical reactions.[48]
Characterisation of the new SHINs indicated that the AuNP and AuNS cores could be synthesised with little polydispersity and that the prepared SHIN layers physically isolated the cores from exterior redox interactions
Summary
The vast majority of SHINERS studies reported in the literature to date have focused on a thin silica (SiO2) shell surrounding a spherical gold or silver plasmonic nanoparticle core. Using methylene blue (MB); which is a popular biological stain and redox indicator,[44] as well as being an important benchmark compound to determine catalytic activity;[45] the SHINERS performance of both the AuNP and AuNS SHINs was demonstrated using in situ confocal SERS mapping of an electrode surface under an applied potential. These experiments highlight the utility of our approach for the non-interfering analysis of electrochemical processes at spatial resolutions approaching the single nanoparticle level
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