Abstract
We investigate the optical properties and surface-enhanced Raman scattering (SERS) characteristics of metal-coated silica aerogels. Silica aerogels were fabricated by easily scalable sol-gel and supercritical drying processes. Metallic nanogaps were formed on the top surface of the nanoporous silica network by controlling the thickness of the metal layer. The optimized metallic nanogap structure enabled strong confinement of light inside the gaps, which is a suitable property for SERS effect. We experimentally evaluated the SERS enhancement factor with the use of benzenethiol as a probe molecule. The enhancement factor reached 7.9 × 107 when molecules were adsorbed on the surface of the 30 nm silver-coated aerogel. We also theoretically investigated the electric field distribution dependence on the structural geometry and substrate indices. On the basis of FDTD simulations, we concluded that the electric field was highly amplified in the vicinity of the target analyte owing to a combination of the aerogel’s ultralow refractive index and the high-density metallic nanogaps. The aerogel substrate with metallic nanogaps shows great potential for use as an inexpensive, highly sensitive SERS platform to detect environmental and biological target molecules.
Highlights
Low-cost, large-area plasmonic sensing platforms have been intensively studied for high-throughput chemical and biological analysis
When a silica aerogel is attached to a semi-infinite metal, most of the electric field concentrates at the interface between the aerogel and metal with a long plasmon fringing field depth toward the silica aerogel
We found that the surface-enhanced Raman scattering (SERS) enhancement factor for benzenethiol exceeded 7.9 × 107 owing to optimization of the density and size of the metallic nanogaps
Summary
To investigate the effect of the LSPR on the SERS signal, Raman spectra of BZT were obtained with a Raman spectrometer operating with He-Ne (633 nm) laser excitation. If we increase the excitation laser power into four times and the data integration time into ten times, the Raman signals from the monolayer benzenethiol on silver-coated glasses was observable, as shown in Supplementary Fig. S7. The number of excited benzenethiol molecules in the monolayer on the SERS substrate could be calculated in a similar way. By multiplying the area of the laser spot, the surface density of the benzenethiol monolayer and the surface factor by the geometry of the SERS substrate, we calculated the number of the excited molecules on the SERS substrate. NSERS and NlBZT are the number of excited molecules on the SERS substrate and in the liquid benzenethiol, respectively. For 30 nm Ag-coated aerogels, the experimental EF and theoretical maximum EF values from FDTD showed the same order of magnitude of 107
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