Salt-free in-solution surface-enhanced Raman spectroscopy substrates based on gold nanoparticle-decorated silica spheres

Abstract

Noble metal colloidal, such as gold and silver, have been widely used in the SERS analysis as the substrates. The method of narrowing the interparticle distance is a popular subject, due to the ability of increasing the magnitude of the SERS signal, especially in the in-solution SERS. The common method of reducing particle-particle distance is to add salt in the colloidal solution, which introduces the substrate stability and SERS signal reproducibility problems. Herein, we prepared gold nanoparticle-coated silica spheres as the in-solution SERS substrates to avoid using salt. 100 nm silica spheres (SiO2) are used as water-dispersible substrates for gold nanospheres (AuNP) and gold nanostars (AuNS) to maintain dispersibility and stability. The gold-to-gold distance was narrowed by using a crosslinker, 3-aminopropyltrimethoxysilane (APTMS), providing binding sites on silica. Au nanoparticles to SiO2 number ratios (NR) were optimized using mercaptobenzonitrile (MBN). The SERS performance of Au-coated SiO2 with optimal NR was compared with dispersed Au nanoparticles. The increase factors of AuNP-SiO2 and AuNS-SiO2 are 2.7 and 6.0 (absolute values), respectively, compared to those of corresponding Au nanoparticles. This work shows the successful enhancement of the SERS signal by attaching Au on SiO2. The advantages of signal enhancement by using non-spherical nanoparticle as SERS substrates were also indicated by the larger increase factor of AuNS-SiO2