Screen printed electrodes

Screen-printed electrodes were purchased from CH Instruments, Inc. (TE100, Bee Cave, TX, USA). The SPE pattern included 3 mm diameter carbon working electrode, carbon counter electrode, and Ag/AgCl reference electrode. Hydrogen tetrachloroaurate(iii) trihydrate (HAuCl₄·3H₂O), sodium citrate, hydrochloric acid (37%), and sodium hydroxide, were supplied by ACROS Organics. Arsenic trioxide (As₂O₃) was purchased from Sigma Aldrich. All other reagents were obtained either from Sigma Aldrich or Fisher Scientific with the highest grade available and were used without further purification. All solutions were prepared using deionized (DI) water with a resistivity of 18.2 MΩ cm at room temperature (Ultrapure Water System, Millipore, and Billerica, MA, USA). A 20 mM primary stock solution of As(iii) was prepared by dissolving As₂O₃ (solubility in water at 25 °C is 20 g L⁻¹) in DI water. To produce a standard calibration curve for As(iii), different concentrations (0.001 mM, 0.01 mM, and 0.1 mM) of As(iii) solutions were prepared by diluting the primary stock solution using DI water. Tap water samples were from our laboratory at the University of Wisconsin-Madison, WI, which did not contain any visible sediments so samples were not filtered prior to use.

To enable an electrochemical readout, 50 μL of 0.5 M H₂SO₄ was used to stop the HRP/TMB reaction after 15 mins and to convert TMB to its electrochemically active form [21 (link)]. Fifty microlitres of the resulting TMB solution was added onto screen-printed electrodes (CH Instruments, USA) and electrochemical response was detected using ampometry at 150 mV over 30s on a potentiostat (CH Instruments, USA). Current response at 10 s was used for subsequent analysis.