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

Noble metal colloids, 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 its ability to increase the magnitude of the SERS signal, especially in in-solution SERS. The common method of reducing the 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 (AuNPs) and gold nanostars (AuNSs) to maintain dispersibility and stability. The gold-to-gold distance was narrowed by using a crosslinker, 3-aminopropyltrimethoxysilane (APTMS), which provided binding sites on silica. Au nanoparticles to SiO₂ number ratios (NRs) were optimized using mercaptobenzonitrile (MBN). The SERS performance of Au-coated SiO₂ with the optimal NR was compared with dispersed Au nanoparticles. The enhancement 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 enhancement factor of AuNS-SiO₂.