Chloroauric acid

Chloroauric acid (Strem
Chemicals, 99.8%); palladium chloride (Sigma-Aldrich, >99.9%);
poly(vinyl
alcohol) (Sigma-Aldrich, M_w 9000–10,000,
80% hydrolyzed); sodium borohydride (Sigma-Aldrich, 99.99%); 5-hydroxymethyl-2-furancarboxylic
acid (Carbosynth, >97.0%); distilled water millipore (18.2 MΩ·cm
at 25 °C); 5-hydroxymethylfurfural (Sigma-Aldrich, >99.0%);
5-formyl-2-furancarboxylic
acid (Fluorochem); 2,5-furandicarboxylic acid (Sigma-Aldrich, 97%);
molecular O₂ (BOC, >99.95%); Nafion (Sigma-Aldrich,
5 wt
% in lower aliphatic alcohols and water, contains 15–20% water);
sodium hydrogen carbonate (Fisher Scientific, >99.5%); sodium hydroxide
(Fisher Scientific); Carbon Vulcan XC-72R (Cabot Corporation); ABTS
(2,2′-azinobis [3-ethylbenzothiazoline-6-sulfonic acid]-diammonium
salt) (Sigma-Aldrich, ≥98%); horseradish peroxidase (Sigma-Aldrich,
141.9 U/mg solid); hydrogen peroxide (Fisher Scientific, 30 wt %).

The 5′-CTCAGGACGACGGGTTCACAGTCCGTTGTC-3′ aptamer was obtained from Shenggong Bioengineering Technology (Shanghai, China). Chloroauric acid and sodium citrate were purchased from Strem Chemicals, Inc. The polydiene dimethyl ammonium chloride (PDDA) solution was purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). The 0.22 μm polypropylene (PP) Syringe Filter was obtained from Agela Technologies (Tianjin, China). Drinking water quality control and rice quality control samples were purchased from the Chinese Academy of Inspection and Quarantine Test Evaluation Center. The cadmium standard solution was from the National Nonferrous Metals and Electronic Materials Analysis and Testing Center. Ultrapure water was obtained from Milli-Q Element (Millipore, Boston, MA, USA). All chemicals were of analytical grade or higher.