Three-Dimensional SERS Substrates Formed with Plasmonic Core-Satellite Nanostructures

Li-An Wu,Ding-Zheng Lin,Yih-Fan Chen,Wei-En Li

doi:10.1038/s41598-017-13577-9

Abstract

We demonstrate three-dimensional surface-enhanced Raman spectroscopy (SERS) substrates formed by accumulating plasmonic nanostructures that are synthesized using a DNA-assisted assembly method. We densely immobilize Au nanoparticles (AuNPs) on polymer beads to form core-satellite nanostructures for detecting molecules by SERS. The experimental parameters affecting the AuNP immobilization, including salt concentration and the number ratio of the AuNPs to the polymer beads, are tested to achieve a high density of the immobilized AuNPs. To create electromagnetic hot spots for sensitive SERS sensing, we add a Ag shell to the AuNPs to reduce the interparticle distance further, and we carefully adjust the thickness of the shell to optimize the SERS effects. In addition, to obtain sensitive and reproducible SERS results, instead of using the core-satellite nanostructures dispersed in solution directly, we prepare SERS substrates consisting of closely packed nanostructures by drying nanostructure-containing droplets on hydrophobic surfaces. The densely distributed small and well-controlled nanogaps on the accumulated nanostructures function as three-dimensional SERS hot spots. Our results show that the SERS spectra obtained using the substrates are much stronger and more reproducible than that obtained using the nanostructures dispersed in solution. Sensitive detection of melamine and sodium thiocyanate (NaSCN) are achieved using the SERS substrates.

Highlights

Around the nanoparticles[12,13,14]
The Au nanoparticles (AuNPs) were functionalized with thiol-modified A10 DNA molecules as well as dual-labelled linker DNA molecules, which were modified with thiol on one end and biotin on the other, using a method similar to that reported by Xu et al.[62]
To improve the SERS effects of the assembled core-satellite nanostructures, we optimized the parameters for the AuNP immobilization to immobilize a high density of the AuNPs on the polymer beads, and we fine-tuned the sizes of the nanogaps between the adjacent immobilized AuNPs by adding a Ag shell to the AuNPs, which will be detailed later

Summary

Introduction

Around the nanoparticles[12,13,14]. In addition, when a DNA aptamer is used as a linker to attach two nanoparticles, an analyte-controllable SERS hot spot at the nanogap between the nanoparticles can be created[28,35,36,37]. When using either metal nanoparticles or nanostructured substrates for SERS detection of molecules, it is important to have many electromagnetic hot spots at the focal point of the Raman excitation laser to obtain good sensitivity. To obtain a high density of SERS hot spots in all three dimensions, we used a DNA-assisted assembly method to immobilize AuNPs on the surfaces of micron-sized polymer beads to create core-satellite nanostructures that had many nanogaps between the AuNPs. To maximize the enhancement factor and the number of nanogaps, we investigated the experimental parameters that could increase the density of the immobilized AuNPs, and we fine-tuned the interparticle distance through adding a Ag shell to the AuNPs. In addition, since the nanostructures synthesized in this study were dispersible in solution rather than immobilized on a fixed substrate, we were able to prepare 3D SERS substrates by accumulating the core-satellite nanostructures using a drying process. We determined the sensitivity and reproducibility of our SERS substrates by measuring SERS spectra of melamine, which was once found to be illegally added to milk products[58,59], and sodium thiocyanate (NaSCN), which is a preservative for milk[60] but could affect thyroid function when its concentration is too high[61]

Methods

Results

Conclusion