Plasmonic nanogap-enhanced raman scattering using a resonant nanodome array

The optical properties and surface-enhanced Raman scattering (SERS) of plasmonic nanodome array (PNA) substrates in air and aqueous solution are investigated. PNA substrates are inexpensively and uniformly fabricated with a hot spot density of 6.25 × 10⁶ mm^-2 using a large-area nanoreplica moulding technique on a flexible plastic substrate. Both experimental measurement and numerical simulation results show that PNAs exhibit a radiative localized surface plasmon resonance (LSPR) due to dipolar coupling between neighboring nanodomes and a non-radiative surface plasmon resonance (SPR) resulting from the periodic array structure. The high spatial localization of electromagnetic field within the ∼10 nm nanogap together with the spectral alignment between the LSPR and excited and scattered light results in a reliable and reproducible spatially averaged SERS enhancement factor (EF) of 8.51 × 10⁷ for Au-coated PNAs. The SERS enhancement is sufficient for a wide variety of biological and chemical sensing applications, including detection of common metabolites at physiologically relevant concentrations. Surface-enhanced Raman scattering (SERS) sensors are integrated in a flow cell for in-line, real-time monitoring and detection of a urinary metabolite. The effect of highly confined electromagnetic fields associated with the localized surface plasmon resonance on the analyte molecules present at EM hot spots results in enhanced excitation of Raman vibrational modes and thus in a substantial increase in the SERS intensity.