Plasma‐Assisted and Solvent‐Free Ultra‐Fast Synthesis of Gold and Gold Oxide Ultra‐Small Nanoparticles and its use as Multi‐Sensing Platforms

In this work, a plasma-assisted and solvent-free synthesis of gold (Au) and gold oxide (Au₂O₃) nanoparticles (NPs) is explored. Accordingly, only the precursor (i.e., HAuCl₄ powder) is exposed to the N₂:O₂ plasma gas mixtures, selected to promote its reduction to Au (HAuCl₄-to-Au⁰) and/or favor its further oxidation to Au₂O₃. As a result, the production of highly crystalline spherical Au ultra-small NPs (USNPs) of 2.7 ± 0.7 nm and Au₂O₃ edged pyramidal-like NPs of 18 ± 5 nm is obtained in very short times (i.e., 10 s^-1 min), showing excellent stability without the need to add any ligands. To investigate the potential features of the synthesized particles, they are loaded into alginate hydrogels and tested as multi-detection sensor platforms of H₂O₂ and NADH in front of several interferents. Although both samples present excellent electrochemical sensing activity (as compared to Au NPs synthesized through the Turkevich method), Au₂O₃ NPs stand out for its high analytical sensibility toward H₂O₂ (153.1 and 16.8 µA mm^-1 cm^-2 for single and multiplexing detection, simultaneously); and capacity to measure NADH in real human urine and low detection limits produced by Escherichia coli (7.5 × 10³ CFU mL^-1). Herein, a green, easy, and fast method is proposed to produce stable Au USNPs and Au₂O₃ NPs, showing great potential for the catalytic and biomedical fields.