Ag-Nanowire Bundles with Gap Hot Spots Synthesized in Track-Etched Membranes as Effective SERS-Substrates

Elizaveta P Kozhina,Stepan N Andreev,Yuriy V Grigoriev,Sergey A Bedin,Vladimir P Tarakanov,Sergey N Podoynitsyn,Andrey V Naumov,Natalia L Nechaeva

doi:10.3390/app11041375

Abstract

This paper presents a cost-effective approach for the template-assisted electrodeposition fabrication of substrates for surface-enhanced Raman scattering (SERS) with metal nanowires (NWs) grown in pores of polymer track-etched membranes (TM). This technique allows the synthesis of NWs array with its certain surface density and diameter (from dozen to hundreds of nm). NWs length also may be varied (order of μm) by controlling deposition time. Here we grow vertical Ag-NWs which are leaning towards their nearest neighbors, forming self-assembled bundles whose parameters depend on the NW aspect ratio (length to diameter). We show that in such bundles there are “hot spots” in the nm-gaps between NWs tips. Computer simulations have demonstrated a strong enhancement of the electric field within these hot spots; thus, the Raman signal is markedly amplified for analyte molecules placed directly inside the gaps. We have experimentally proved the potential of this SERS-technique on the example of 4-Mercaptophenylboronic acid (4-MPBA). For 4-MPBA the maximal enhancement of Raman signal was found at NWs length of ~1.6 μm and diameter of ~100 nm. The effect is higher (up to twice) if “wet” substrate is used just immediately after the TM polymer removal so that the tips are brought to lean after analyte exposure. We suggest this new type of nanostructured SERS-substrates as a base of effective sensing of extremely low concentration of analytes.

Highlights

Surface-enhanced Raman scattering (SERS) is an analytical tool that can be applied to molecules, providing vibrational spectra, and which can be adsorbed on specially prepared substrates or nanoparticles in colloidal solutions for its spectral identification
After the track-etched membranes (TM) removal and following washing the substrates with the array of AgNWs, those substrates were left in distilled water
We report a simple strategy for the fabrication procedure of the SERSactive substrates with the array of metal nanowires (NWs) using a template-assisted electrodeposition method with commercial tracking membranes (TM)

Summary

Introduction

Surface-enhanced Raman scattering (SERS) is an analytical tool that can be applied to molecules, providing vibrational spectra, and which can be adsorbed on specially prepared substrates or nanoparticles in colloidal solutions for its spectral identification. As a result of the analyte adsorption and drying, the analyte molecules appear to be sandwiched between neighboring nanopillars, that is, in the hot spot area and demonstrating strong signal amplification This synthesis strategy allows varying of nanopillar length and diameter without using a template, but this strategy is costly and time-consuming. This makes it possible to adjust the position of the plasmon resonance to the experimental wavelength range, and, as a result, to increase the contribution of the resonance to the amplification of the obtained SERS-signal Such substrates with the leaning neighboring silver nanowires (Ag-NWs) amplify the signal due to the presence of a gap hot spots. The NWs length and their behavior during the evaporation were defined using Nikon LV100 optical microscope (Japan) and by scanning electron microscope (SEM) JEOL JCM-6000

Microscopy

Findings

Conclusions