Scx 3405

D-(+)-Glucose was purchased from Ajex Finechem. Galactose, sucrose, maltose, L-ascorbic acid, and uric acid were obtained from BDH Chemical. H2O2 (30 wt%), standard cation and anions (KCl, CaCl2, NaCl, and Fe(NO3)3) were purchased from Merck. GOx and HRP were purchased from Sigma-Aldrich. Artificial urine was obtained from Carolina Biological Supply Company (Burlington, NC, USA). All chemicals used in this experiment were of analytical reagent (AR) grade and were prepared with deionized water from a Mill-Q purification system (resistance ≥18 MΩ cm -1 ). Lastly, Whatman No. 1 filter paper was purchased from Cole-Parmer (VernonHills, IL).
UV-visible absorption spectra were measured with a UVvisible spectrophotometer (Lambda 35, Perkin15 Elmer Instruments, USA) using quartz cuvettes (1.0 cm path length) to characterize AgNPs. The morphology and size of silver nanoparticles were determined using a Transmission Electron Microscope (TEM, JEOL). Glucose and H2O2 detection were achieved under a black box to protect the decomposition of H2O2. With the completed enzymatic reaction, we recorded the results with a scanner (Scx-3405, Samsung).

The results of the colorimetric detection were recorded with a scanner (Scx-3405, Samsung) using 600-dpi resolution, and the color of the whole spot was analyzed by ImageJ (RGB color mode). 20, 21

Prior to the quantification of artificial urinary oxalate samples, the optimal conditions, including 1-methoxy-5-methylphenazinium methyl sulfate (PMS) at 1, 3 and 5 mM, 3-(4,5dime thylthiazol-4-yl)-2,5-diphenyl tetrazolium bromide (MTT) at 0.1, 0.3, 0.4 and 0.5 M, and nicotinamide adenine dinucleotide (NAD + ) at 5, 10, 25, 40 and 57 mM, were investigated at various time points, 5, 15, 30 and 45 min. The reactants were dropped on the center of the cPAD: 1 μL of each reagent was sequentially dropped, followed by 3 units/mL oxalate decarboxylase, 40 mM of NAD + , 50 units/mL formate dehydrogenase, and MTT, and the droplets were allowed to dry prior to the addition of the next reagent. During the optimization, 5 μL of 200 μM oxalate were mixed with PMS (ratio 4:1) and spotted on the loading zone. The sample or standard solutions were incubated for 30 min at room temperature. After incubation, the cPADs were recorded with the scanner (Scx-3405, Samsung) using 600-dpi resolution, and then the intensity of the colors were analyzed with ImageJ (RGB color mode) using the whole spot (3 mm). The intensity difference was calculated by the purple color intensity generated from the sample or standard oxalate solution after subtraction of the background color intensity.