4.1. Chemicals, Solutions, and Instruments

The alumina slurry used for electrode cleaning was from Baikowski International Corp (Charlotte, NC, USA) (0.05 µm, Baikalox 0.05CR). The supporting electrolyte for conductivity was 0.1 M pH:4 phosphate buffer solution (NaH₂PO₄–Na₂HPO₄) (Merck, Darmstadt, Germany). For electrode cleanliness, isopropyl alcohol (Sigma-Aldrich), acetone, and ethanol (both technical grade) were used. N-(1-naphtyl)-ethylenediamine dichloride (NED) (used in colorimetric sensor determination) was obtained from Fluka (St. Louis, MO, USA). The electrode coating material, 4-aminothiophenol (4-ATP), as well as the rest of the reagents, were supplied from E. Merck (Darmstadt, Germany).

Gold(III) chloride solution (99.99% trace metals basis, 30% by wt. in dilute HCl) was used for deposition of gold nanoparticles to Au electrode coated with PATP copolymer, and the 0.04% (w/v) working solution was prepared from this stock solution. The final chemical form of Au(III) in acidic solution was HAuCl₄.

Preparation of Solutions in colorimetric sensor determination of NO₂⁻: The working solutions of NO₂⁻ at 1–10 mg·L⁻¹ were prepared from the corresponding aqueous stock solution at 500 mg·L⁻¹. H₃PO₄ (0.1 M) and NED (20 mM) solutions were prepared in pure water and 4-ATP (10 mM) solution in absolute ethanol.

Voltammetric experiments were performed with a Gamry Instruments model Reference 600 potentiostat/galvanostat/zero resistance ammeter (ZRA) interfaced to a PC computer and controlled Gamry Framework software (Warminster, PA, USA). Gold (BASi stationary voltammetry electrodes; ø 1.6 mm,) was used as the working electrode. A platinum (Pt) electrode and an Ag/AgCl, 3 M KCl electrode served as the auxiliary and reference electrodes, respectively. Optical absorption measurements were carried out in matched Hellma quartz cuvettes using a Varian CARY Bio 100 UV-VIS spectrophotometer (Agilent Technologies, Santa Clara, CA, USA).