Rapid and sensitive melamine detection via paper-based surface-enhanced Raman scattering substrate: Plasma-assisted in situ growth of closely packed gold nanoparticles on cellulose paper

Cellulose-paper-type surface-enhanced Raman scattering (SERS) substrates have shown promise for constructing economical high-performance molecular sensors. However, conventional paper-based SERS substrate fabrication methods are complex. Therefore, in this study, dry plasma reduction (DPR) – a simple and green process – was tailored to develop a paper-based SERS substrate featuring Au-nanoparticle (AuNP)-impregnated cellulose fiber surfaces. Au ions pre-adsorbed on fiber surfaces were reduced by abundant injected electrons and grown into AuNPs by high-energy Ar-ion bombardment during DPR. Fiber surfaces of the AuNP–cellulose paper, enriched with AuNPs having nanometer-scale gaps and SERS hotspots, exhibited broadband absorption and a large SERS enhancement factor of 1.7 × 10⁷. The SERS sensitivity of the AuNP–cellulose paper was leveraged to realize label-free sensing of melamine, an illegally added milk contaminant. The AuNP–cellulose paper not only exhibited a low detection limit (23 nM (2.9 ppb)) for melamine, adulterated in milk, after sample pretreatments but also enabled rapid detection of 0.2 ppm melamine in formula and low-fat milk within 30 s without any pretreatments, with the supports of principal component analysis (PCA) method. The AuNP–cellulose paper, cost-effective and permitting low-ppb-level label-free molecular sensing, can be a feasible SERS sensor for environmental and biomedical applications.