Au/Pd Nanocatalysts on Silica Nanoparticle-Coated Indium Tin Oxide for Colorimetric Sensing of Ascorbic Acid

Nanomaterials of gold and palladium have received research attention due to their excellent catalytic properties. Their recovery after the catalytic reaction still remains a challenge rendering the catalytic application of these nanomaterials for single use. Herein, we report a method of fabricating nanomaterials as thin nanoparticle (NP) films on catalytically inert surfaces, while allowing for their recovery and reuse is investigated. Indium tin oxide (ITO) glass slides were first hydroxylated (ITO-OH) and used to attach the silica NPs (SiO₂NPs) to form ITO-SiO₂NPs. The ITO-SiO₂NPs surface was used as an adsorbent for gold and palladium mono- and bimetallic NPs (Au/PdNPs) to form ITO-SiO₂-prS-Au/PdNPs, which was evaluated for the enzyme-like catalytic activity. The fabricated film with bimetallic NPs, ITO-SiO₂-prS-Au/PdNPs, exhibited excellent peroxidase-like activity for the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) with H₂O₂ when compared with monometallic thin NP films, ITO-SiO₂-prS-AuNPs, and ITO-SiO₂-prS-PdNPs. A typical Michaelis–Menten enzyme-like behavior of the bimetallic NP film, ITO-SiO₂-prS-Au/PdNPs, was observed for H₂O₂ and TMB with Michaelis–Menten constants (K_m) of 0.33 and 3.46 mM, respectively. A simple, sensitive, and selective colorimetric assay for ascorbic acid (AA) detection was fabricated based on the reduction effect of AA on the oxidized TMB. The fabricated AA colorimetric sensor exhibited a good linear range from 0.50 to 30.0 μM, with the limit of detection of 0.50 μM and the limit of quantification of 1.65 μM. Finally, our proposed colorimetric sensor, ITO-SiO₂-prS-Au/PdNPs, successfully detected AA in new-born calf serum samples, as a real sample representative.