Correlation of surface-enhanced Raman scattering (SERS) with the surface density of gold nanoparticles: evaluation of the critical number of SERS tags for a detectable signal.

Vincenzo Amendola

doi:10.3762/bjnano.10.102

Abstract

The use of plasmonic nanotags based on the surface-enhanced Raman scattering (SERS) effect is highly promising for several applications in analytical chemistry, biotechnological assays and nanomedicine. To this end, a crucial parameter is the minimum number of SERS tags that allows for the collection of intense Raman signals under real operating conditions. Here, SERS Au nanotags (AuNTs) based on clustered gold nanoparticles are deposited on a substrate and analyzed in the same region using Raman spectroscopy and transmission electron microscopy. In this way, the Raman spectra and the surface density of the SERS tags are correlated directly, showing that 1 tag/µm2 is enough to generate an intense signal above the noise level at 633 nm with an excitation power of only 0.65 mW and an acquisition time of just 1 s with a 50× objective. The AuNT density can be even lower than 1 tag/µm2 when the acquisition time is extended to 10 s, but must be increased to 3 tags/µm2 when a 20× objective is employed under the same excitation conditions. In addition, in order to observe a linear response, it was found that 10 SERS AuNTs inside the probed area are required. These findings indicate that a better signal-to-noise ratio requires high-magnification optics, while linearity versus tag number can be improved by using low-magnification optics or a high tag density. In general the suitability of plasmonic SERS labels for ultrasensitive analytical and biomedical applications is evident.

Highlights

In surface-enhanced Raman scattering (SERS), the Raman scattering cross-section of molecules adsorbed on the surface of plasmonic nanostructures is enormously increased compared to the same isolated molecules [1,2,3,4,5]
This work reports on the study of SERS tags obtained by laser ablation synthesis in liquid solution (LASiS) of gold (Au) nanoparticles, their coating with three different Raman reporters that are resonant at 633 nm, and their stabilization with a biocompatible and hydrophilic coating. Their performance was tested by correlating the Raman signal to the density of Au nanotags (AuNTs) per unit area, as obtained by transmission electron microscopy (TEM) analysis of the same area probed by Raman spectroscopy
AuNTs containing MG, MGITC and HITC, examples of which are shown in Figure 1B, have a hierarchical structure with one or more large Au nanoparticles surrounded by smaller particles, all grouped in a nanoaggregate with size of the order of tens of nanometers

Summary

Introduction

In surface-enhanced Raman scattering (SERS), the Raman scattering cross-section of molecules adsorbed on the surface of plasmonic nanostructures is enormously increased compared to the same isolated molecules [1,2,3,4,5]. When the analytes can be concentrated on a surface or in a well-defined volume, SERS tags allow for the collection of an intense Raman signal revealing the presence of the targeted molecule.

Results

Conclusion