Silver nitrate

Silver nitrate (Daejung Chemicals and Metals, Siheung, Korea); C343 and DCM (Sigma-Aldrich, St Louis, MO, USA); DNBP (Tokyo Chemical Industry, Tokyo, Japan); and all other chemicals were used as received without further purification. Homogeneous silver colloidal nanoparticles with diameters of 59–219 nm were synthesized via a kinetically controlled seeded-growth method.^{40 (link)} The SCFs were prepared by attaching the silver nanoparticles to glass slides that were pre-coated with poly(diallyldimethylammonium chloride).^{17 (link)} Half of the SCFs were removed using a dilute nitric acid solution for the evaluation of the fluorescence enhancement. Then, 1.0% (w/v) polystyrene (PS; avg. m.w. 208 000; Wako Pure Chemical, Osaka, Japan) solutions in tetrahydrofuran (Samchun Chemicals, Seoul, Korea) containing fluorophores (C343, DCM, or DNBP in the concentration of 2 × 10⁻⁴ M) were spin-coated on the SCFs. The thicknesses of the spin-coated PS films were determined as 180 nm from the interference patterns in the UV-vis absorption spectrum (see Fig. S2†).^{42,43 (link)} The steady-state extinction spectra of the SCFs were measured using a Mega-900 UV-vis spectrophotometer (Scinco, Seoul, Korea), and scanning electron microscope (SEM) images of the SCFs were obtained using a JSM-7500F field emission SEM (Jeol, Tokyo, Japan).

BA standards (tyramine hydrochloride, 2-phenylethylamine hydrochloride, putrescine dihydrochloride, cadaverine dihydrochloride, histamine dihydrochloride, tryptamine hydrochloride, spermidine trihydrochloride) and amino acid standards (L-Asp, L-Glu, L-Ser, L-Gly, L-His, L-Thr, L-Arg, γ-aminobutyric acid [GABA], L-Ala, L-Pro, L-Tyr, L-Val, L-Met, L-Iso, L-Leu, L-Phe, L-Trp, and L-Lys) were all purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA). Dansyl chloride, perchloric acid, sodium hydrogen carbonate, potassium dichromate, and silver nitrate were purchased from Daejung Chemical Co. (Siheung, Korea). Distilled water, acetone, and acetonitrile (HPLC grade) were purchased from Tedia Co. (Fairfield, OH, USA).