Background-Free, Label-Free, and Reproducible SERS via Plasma-Halide-Engineered Gold Nanoparticles

Surface-enhanced Raman spectroscopy (SERS) is often limited by substrate-derived background from organic ligands (e.g., citrate), which degrades spectral fidelity, sensitivity, and quantitation. We report a measurement-oriented SERS platform based on size-tuned, microplasma-synthesized gold nanoparticles (AuNPs, 14-100 nm) that are surface-purified with bromide (Br^-). Bromide competitively displaces weakly bound citrate while remaining Raman-inactive, thereby suppressing baseline features in the 1000-1800 cm^-1 fingerprint region without introducing new peaks. An optimal Br^- window (∼1.5 mM, ∼3 h) achieves effective cleaning while avoiding surface passivation. The resulting substrates deliver strong enhancement (EF >10⁶), subnanomolar limits of detection (10^-9 M for rhodamine 6G; 10^-10 M for malachite green), and wide linear ranges (10^-9-10^-4 and 10^-10-10^-4 M, respectively). Spot-to-spot variability is low (RSD <∼8%), and activity is retained over weeks (>90% signal after storage), underscoring practical reproducibility and stability. Spectroscopic and surface charge analyses corroborate ligand displacement and improved adsorption on clean Au surfaces while preserving plasmonic response. This simple, green, and scalable plasma-halide strategy provides background-free, label-free, and reproducible SERS measurements. Driven by electrostatic complementarity, this platform offers a highly effective route for the quantitative trace analysis of cationic analytes, relevant to environmental monitoring and chemical safety.