Graphene–silver nanohybrids for ultrasensitive surface enhanced Raman spectroscopy: size dependence of silver nanoparticles

Abstract

The surface-enhanced Raman scattering (SERS) mechanism of graphene–silver (GE–Ag) nanohybrids was investigated by studying the Raman signals of rhodamine 6G (R6G) adsorbed onto nanohybrids with different sizes of Ag nanoparticles (NPs). The results showed that the SERS intensity ratios between GE–Ag NPs with Ag NPs with sizes of 9, 25 and 35 nm and pure Ag substrate were estimated to be 2.7, 5.0 and 8.9 respectively. GE sheets did improve the SERS activity due to their high adsorption ability, strong fluorescence quenching and additional chemical enhancement mechanism (CM)-based SERS effect. However, Ag NPs with relatively larger size (∼nm) play a major role in SERS activity of molecules adsorbed on the nanohybrids. Furthermore, the free-standing GE–Ag NP nanohybrid film was fabricated through the vacuum filtration of the optimized GE–Ag NP composite suspension followed by thermal annealing. The as-prepared film has the larger size of Ag NPs and narrower interparticle distance, which provides more "hot spots" for sensitive detection of R6G molecules, with a detection limit at 1 × 10−12 M. Additionally, the relative standard deviation (RSD) values are measured to be below 0.13, revealing a good consistency and reproducibility across the entire area of the film, suggesting this substrate holds a promise for routine SERS detection.