Ag/ZIF-8 Substrate with Enhanced SERS via the Plasmonic Nanogap and MOF-Enabled Molecular Preconcentration Effect

Surface-enhanced Raman scattering (SERS) is an enthralling phenomenon that significantly boosts the Raman signals of molecules near "hot spots". Its sensitivity primarily relies on the design of "hot spots" and concentrating the analytes into these regions. Herein, a high-performance Ag/ZIF-8 SERS substrate is fabricated with a facile low-cost two-step method. After randomly dispersing ZIF-8 nanocrystals on the substrate, a Ag layer is sprayed. Plasmonic nanogaps as small as 10 nm are created between the Ag nanocap on the top of the ZIF-8 and the Ag layer on the substrate around the ZIF-8 crystal by optimizing the thickness of the Ag layer. Because of plasmonic coupling effects, the near-field interactions at the plasmonic nanogaps generate a greatly enhanced electromagnetic (EM) field, which is also interpreted by numerical simulations. Furthermore, the porous ZIF-8 nanoparticles' preconcentration effect can enrich analytes in the nanogap. The measurement results show that this SERS substrate has a SERS enhancement factor (EF) of up to 2.84 × 107, the lowest detectable concentration for 4-ATP of 10–9 M, and a relative standard deviation (RSD) of only about 6.99%. This work may provide a facile strategy for the construction of metal–organic framework (MOF)-based nanostructure substrates as a potential SERS platform for molecular detection.