Hierarchical PS@Ag-Au plasmonic architecture for pretreatment-free wet-state SERS of weakly responsive additives in viscous e-liquids.

Conventional Ag-dominant substrates produced clear spectra for pesticides or cooling agents but gave only a faint sodium saccharin response and no discernible fingerprints for five other selected additives at 10 mg g-1, even after low-viscosity solvent deposition and drying. To address this interface-specific limitation, we developed a hierarchical polystyrene (PS)@Ag-Au architecture based on functional separation. Directional Ag deposition produced semi-shells, localized interparticle continuity, and basal nanofeatures that stabilized the three-dimensional PS scaffold; subsequent Au assembly generated a dense nanoparticle layer with distributed AuAg junctions. Finite-difference time-domain (FDTD) calculations at the fixed 785 nm excitation showed geometry-dependent field localization, with the characteristic junction-centered pattern retained in a representative wet medium (n = 1.45). The selected configuration comprised 400 nm PS spheres, a nominal 100 nm Ag layer, and approximately 46 nm Au nanoparticles. Under controlled methanol deposition and drying, all six sweetening and flavoring additives showed distinguishable multi-band fingerprints and approximately linear intensity relationships with log₁₀ concentration over 0.001-20 mg g-1, including measurable responses at 0.001 mg g-1. Direct wet-state measurements retained analyte-specific fingerprints in propylene glycol/vegetable glycerin (PG/VG) matrices and selected spiked commercial e-liquids without dilution, extraction, deliberate drying, or chromatographic separation. In a blinded controlled commercial-matrix set, all 15 sample-level classifications agreed with the coding record, and all 19 intentionally spiked analyte instances were assigned. Authentic-standard liquid chromatography-photodiode array detection (LC-PDA) provided orthogonal compositional confirmation. These results support a structure-guided platform for pretreatment-free qualitative SERS analysis of weakly responsive additives in viscous liquids.