Experimental

Chemicals and instruments

A phosphate buffer solution (PBS) was prepared using ortho-phosphoric acid and adjusted to different pH values. The pH of the solution was fine-tuned by adding an appropriate amount of sodium hydroxide solution. All chemicals used were of analytical-grade purity. In this experiment, all aqueous solutions were fabricated utilizing ultrapure water generated by a Millipore Direct-Q 8UV system (Germany; Darmstadt).

All electrochemical experiments were conducted at ambient temperature using a Metrohm PGSTAT302N potentiostat (Netherlands), operated with GPES version 4.9. A commercial screen-printed carbon electrode (DropSens model DRP-110, Spain) was used, featuring an integrated three-electrode configuration comprising a carbon working electrode, a silver pseudo-reference electrode, and a carbon counter electrode. The pH of all solutions was determined using a digital pH meter (Metrohm model 713; Switzerland).

Synthesis of NiO nanostructures

At first, NiCl2·6H2O (0.0760 g) was dispersed into 80 mL of distilled water by magnetic stirring; then 0.0582 g cetyltrimethylammonium bromide (CTAB) was added to the above solution, followed by stirring until it completely dissolved. A few drops of ammonium hydroxide solution were added dropwise to the above solution, and the mixture was continuously stirred at room temperature for 2 hours. Subsequently, the prepared solution was transferred into a Teflon-lined stainless-steel autoclave and heated under hydrothermal conditions at 160 °C for 24 h. The precipitated solid products were centrifuged and washed several times with distilled water and ethanol to remove impurities. Finally, the product was dried in an oven at 70 °C for 15 h and then calcined at 300 °C for 3 h to obtain NiO nanostructures.

NiO nanostructures modified-screen-printed carbon electrode

The SPCE was modified with NiO nanostructures. A stock suspension of NiO nanostructures was prepared by adding 1 mg of the material to the 1 mL of aqueous solution. It was sonicated for a period of 1 hour. A volume of 3 μL of the aqueous suspension of NiO nanostructures was cast onto the surface of the carbon working electrode. Finally, the solvents evaporated at room temperature, yielding the modified electrode (NiO/SPCE). To estimate the electroactive surface area (A) of bare SPCE and modified-SPCE, CV studies were performed at various scan rates in a 0.1 mol L-1 KCl solution containing 5.0 mmol L-1 [Fe(CN)6]3-/4-. Based on these studies and the Randles-Ševčik equation, the values of A were determined as 0.0321 cm2 for bare SPE and 0.086 cm2 for modified-SPCE.