Directed gold nanoparticle synthesis with plasma-activated water

• Argon jet plasma transforms water into reactive oxygen species-rich medium. • Plasma-activated water enables gold nanoparticle synthesis without other reagents. • Water treated for more than 8 min reduces 0.10–1.00 mM Au 3+ in less than 10 min. • Gold nanoparticles are faceted and size-controlled between 35 and 80 nm. Plasma-activated water (PAW) enables surfactant-free synthesis of gold nanoparticles (AuNPs) at room temperature and acts as a reaction medium with tuneable reactive oxygen and nitrogen species (ROS and RNS), decoupled from the plasma discharge. Treating deionised water with an argon atmospheric-pressure plasma jet for 1–16 min increased H 2 O 2 concentration from 0 to 8 mM, lowered pH from 7.0 to 3.5, and raised the oxidation–reduction potential from 0 to 400 mV, while maintaining negligible RNS levels. These changes directly influence Au 3+ reduction and AuNP growth. At 1.00 mM Au 3+ , increasing PAW activation time from 1 to 16 min shortened the reduction time tenfold and shifted the LSPR maximum from 610 to 538 nm, with particle size decreasing from 120 nm to about 80 nm. Using the most reactive PAW (16 min), decreasing Au 3+ from 1.00 to 0.10 mM reduced the mean AuNP size from 80 to 35 nm, yielding uniform, well-faceted nanoparticles. Weakly activated PAW (1–2 min) yields slower, partially incomplete reduction, broader size distributions, and more planar nanoparticles. These results establish PAW as a sustainable method for directed AuNP reduction kinetics, size, morphology, and optical response without external reducing or capping agents.