Origin of enhanced signal from TERS measurements on functionalized nanoparticles.

Recent work by Nanonics Users Z.D. Schultz and H. Wang (Analyst, 2013, 138, 3150-3157) on understanding the origin of TERS signals from functionalized nanoparticles was featured on the front cover of Analyst and identified as a “hot article” by its editors.  In this work , the authors used a Nanonics MV4000 customized with a homebuilt Raman microscope to measure the TERS signal of gold nanoparticles functionalized with proteins and compare that with SERS measurements of clustered gold nanoparticles to understand the source of enhancement of the TERS signal and its relation to location of the enhanced molecule with respect to the gap.  The ability to understand the source of the enhancement of the TERS signal provides flexibility in preparing complicated biological samples such as receptors for TERS imaging beyond the traditional method of affixing the target to a metallic surface, which cannot be applied to many materials (e.g. biological receptors).

Background:

Surface enhanced Raman spectroscopy (SERS) is a surface sensitive technique where Raman scattering is enhanced by molecules adsorbed on a rough or nanostructured metal surface and is used for ultrasensitive detection resulting in single molecule sensitivity in a variety of fields including materials and biology.  TERS, or tip enhance Raman spectroscopy, also results in enhancement of the Raman signal and employs scanning a very sharp metallic or metalized tip to create a “nano-antenna” resulting in high resolution and enhanced Raman signal, and has been used to investigate carbon nanotubes, semiconductors, and biologically relevant molecules.  In TERS, substantial enhancement has been observed from gap-modes that arise from image dipoles as the tip approaches a metal surface. However, a limitation of gap-mode TERS is the requirement of a metallic or conducting substrate.  To address this limitation, nanoparticles on dielectric substrates interacting with the TERS tip have been shown to generate enhancement comparable to gap-modes and thus broadens the applicability of TERS with a more flexible sample preparation.

Goal and findings

The goal of this study is elucidate the location of the enhanced molecule within the gap that leads to the TERS signal, and to understand whether nanoparticles bound to proteins can provide additional TERS enhancement without the analyte of interest residing directly in the gap junction.  The authors focus on the biotin-streptavidin system, a popular ligand-receptor binding system.

This work suggests that TERS can distinguish between a protein located within the gap junction and a protein bound to the nanoparticle outside the gap (see schematics enclosed in blue and green rectangles in figure below, respectively); these results are compared with the SERS signal from aggregated gold nanoparticles (schematic enclosed in red rectangle below).  Furthermore, they saw an effect of nanoparticle probe size on absolute signal indicating either larger enhancements with larger particles or increased number of sample molecules.  The ability to enhance Raman scattering from molecules outside the gap junction by using functionalized nanoparticles provides a way to increase sensitivity in TERS experiments from complicated biological samples by providing another platform for preparing complex biological molecules that can be probed by the ultrasensitive TERS method.