Under-Etched Plasmonic Disks on Indium Tin Oxide for Enhanced Refractive Index Sensing on a Combined Electrochemical and Optical Platform.

Hans Dyrnesli,Duncan S Sutherland,Gunnar Klös

doi:10.3390/ma13040853

Abstract

A simple approach to enhance the refractive index sensitivity of gold nanodisks immobilized on electrically conducting indium tin oxide (ITO) substrates has been demonstrated. A two-fold increase in sensitivity to bulk refractive index change was achieved by substrate under-etching of gold nanodisks on ITO in 50 mM sulfuric acid. The influence of an intermediate titanium adhesion layer was investigated and was found to markedly influence the etching pattern and time. Etching with an adhesion layer resulted in enhanced refractive index sensitivity on disk-on-pin like structures after long etching times, whereas etching of disks deposited directly on ITO resulted in a disk-on-pincushion like configuration and similarly enhanced sensitivity already at shorter times. The gold disks remained electrically connected to the ITO substrate throughout etching and allowed site-specific electrodeposition of poly(3-aminophenol) at the nanodisks, showing enhanced thin-film refractive index sensitivity. This work demonstrates a simple method for enhancing refractive index sensitivity of nanostructures on ITO substrates for combined electrochemical and optical platforms, and subsequently a method to modify the surface of the electrically connected nanostructures, which has potential application in biosensing.

Highlights

Plasmonic nanostructures have been the subject of intense research interest due to their potential application in biological/chemical sensing [1], and as nanoantennas and nanolenses for manipulating light below the diffraction limit [2]
20 nm tall gold nanodisks with a diameter of 104 ± 5 nm and with a 2 nm titanium adhesion layer were fabricated on indium tin oxide (ITO) substrates
We have studied approaches to increase the refractive index sensitivity of plasmonic nanostructures immobilized on substrates by removing parts of the substrate

Summary

Introduction

Plasmonic nanostructures have been the subject of intense research interest due to their potential application in biological/chemical sensing [1], and as nanoantennas and nanolenses for manipulating light below the diffraction limit [2]. Interactions of light with sub-wavelength nanoparticles of specific metals (e.g. Ag, Au, and Al), are dominated, in particular spectral regions in the UV, visible and NIR, by incident field-driven oscillations of conduction electrons spatially confined to the nanoparticles. The sensitivity of the plasmon resonance spectral position to surrounding dielectric properties allows the use of LSPRs for refractive index sensing with a wavelength-based sensitivity that is increased for nanostructures with longer wavelength resonances [5,6]. Immobilization allows control over particle spacing and orientation, which is of fundamental importance in, for example, nanolensing [2], LSPR-based directional scattering [7]

Methods

Results

Conclusion