Three-Dimensional-Stacked Gold Nanoparticles with Sub-5 nm Gaps on Vertically Aligned TiO2 Nanosheets for Surface-Enhanced Raman Scattering Detection Down to 10 fM Scale.

Seeking for ultrasensitive and low-cost substrates is highly demandable for practical applications of surface-enhanced Raman scattering (SERS) technology. In this work, we report an ultrasensitive SERS-active substrate based on wet-chemistry-synthesized vertically aligned large-area TiO2 nanosheets (NSs) decorated by densely packed gold nanoparticles (Au NPs) with sub-5 nm gaps. Via a multistep successive deposition process, three-dimensional-stacked Au NPs sandwiched by a 3 nm SiO2 layer were assembled onto the TiO2 NS, enabling numerous hotspots due to the formation of both ultratiny plasmonic gaps and semiconductor/metal interfaces. Experimental results show that the fabricated substrate displays a detection limit down to 10 fM (10-14 M) without involving any condensation process by using the crystal violet as probe molecules. Control experiments and electromagnetic simulations indicate that the nanogaps defined by the 3 nm spacer are essential for the obtained excellent SERS performance. With its ultrasensitive detection capability, we demonstrate that the fabricated SERS substrate can be used for the trace analysis of melamine in milk.