Rapid Plasma-Assisted Synthesis of Tannic Acid-Capped Gold Nanoparticles for SERS Applications

Gold nanoparticles (AuNPs) are attractive substrates for surface-enhanced Raman scattering (SERS) because of their strong visible plasmon resonance, chemical stability, and biocompatibility. However, the enhancement they provide depends on how accessible the gold surface is to the analyte. Tannic acid (TA) is an effective green reductant and stabilizer for AuNPs. Conventional TA-based syntheses, however, require large doses and boiling, leaving a dense organic shell on the particles. Herein, an argon cold atmospheric-pressure plasma jet combined with a low dose of TA reduced HAuCl4 to AuNPs at room temperature without additional chemical reductants. Well-dispersed colloids of quasi-spherical, crystalline particles about 10 nm in size formed with 25-fold less TA than was required without plasma. In situ UV–Vis spectroscopy showed that low-dose TA largely removed the induction period of plasma reduction and that particle growth continued after the discharge was switched off. This result suggests that plasma supplies most of the reducing power, whereas TA stabilizes the nascent nuclei. XPS confirmed metallic gold, with a small positive Au 4f shift consistent with surface-bound TA. The lightly capped AuNPs gave stronger methylene blue SERS signals than both TA-free plasma AuNPs and AuNPs prepared with 25-fold more TA. This room-temperature route therefore provides clean colloidal AuNPs as promising SERS substrates for chemical sensing.