SNaP-C: Development of a Silver Nanoparticle Antioxidant Assay for the Selective Quantitative Analysis of Vitamin C in Beverages.

The development of a sensitive and selective silver nanoparticle assay for the quantitation of vitamin C (SNaP-C), as ascorbic acid (AA) and total ascorbic acid (TAA = AA + dehydroascorbic acid, DHAA), is described. Three assay parameters were investigated and optimized: (1) synthesis of silver nanoparticles (AgNPs) to produce a reliable enhanced localized surface plasmon resonance (LSPR) in the presence of specific added antioxidants; (2) ensuring long-term stability of AA and DHAA in aqueous solutions; and (3) SNaP-C assay conditions to allow for rapid analysis of samples (beverages) by monitoring the enhanced LSPR. The synthesis of AgNPs using soluble starch as a capping agent and d-arabinose as a reducing agent was optimized in a CEM Discover SP laboratory microwave. Given that AA and DHAA lack aqueous stability, these forms were stabilized via the addition of 1% (w/v) meta-phosphoric acid. To convert DHAA to AA, the reducing agent tris(2-carboxyethyl) phosphine hydrochloride (TCEP) was added with its concentration (2.5 mM) and reaction time (24 h) optimized. Using a Box-Behnken design for three factors, the SNaP-C assay reaction conditions of 1000 μL of AgNPs (in suspension) with 100 μL of beverage (or analytical standard) and 4.9 mL of water using the CEM Discover SP microwave were optimized, resulting in an incubation time of 90 °C for 6 min. Finally, a list of potential interferents that commonly respond to other antioxidant capacity assays, like the Folin-Ciocalteu, were investigated to demonstrate that the SNaP-C assay is selective and sensitive for ascorbic acid and gallic acid.