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

Abstract In this work, a plasma‐assisted and solvent‐free synthesis of gold (Au) and gold oxide (Au ₂ O ₃ ) nanoparticles (NPs) is explored. Accordingly, only the precursor (i.e., HAuCl ₄ powder) is exposed to the N ₂ :O ₂ plasma gas mixtures, selected to promote its reduction to Au (HAuCl ₄ ‐to‐Au ⁰ ) and/or favor its further oxidation to Au ₂ O ₃ . As a result, the production of highly crystalline spherical Au ultra‐small NPs (USNPs) of 2.7 ± 0.7 nm and Au ₂ O ₃ 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 H ₂ O ₂ 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), Au ₂ O ₃ NPs stand out for its high analytical sensibility toward H ₂ O ₂ (153.1 and 16.8 µA m m ⁻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 × 10 ³ CFU mL ⁻¹ ). Herein, a green, easy, and fast method is proposed to produce stable Au USNPs and Au ₂ O ₃ NPs, showing great potential for the catalytic and biomedical fields.