Experimental section

Materials and physical experiments

All solvents were purchased from Merck. There was no need for further purification because all chemicals purchased for the experiment were of reagent-grade purity. Sigma-Aldrich provided ortho-phenylenediamine, sodium thiocyanate, copper(II) chloride, and nickel(II) nitrate. Using a Perkin-Elmer FT-IR spectrophotometer model spectrum two, FT-IR spectra were measured from 4000 to 400 cm−1 using KBr discs. To analyse absorption spectra, the Optizen-view 2120 UV plus spectrometer was run at room temperature.

The minimum inhibitory concentration (MIC) of an antimicrobial agent is determined using a broth microdilution assay. Serial twofold dilutions of the test compound are prepared in a suitable growth medium in microtiter plates and inoculated (Shimadzu) with a standardized suspension of gentamicin and amikacin. The plates are incubated at 35 to 37 °C for 24 h and the concentrations were determined by the spectrophotometer Optizen 2120 UV plus, made in Korea. The MIC is defined as the lowest concentration of the compound required to prevent visible microbial growth relative to the growth control.

Electrochemical tests were conducted to assess the effectiveness of the proposed sensor in detecting IRN using an Autolab/PGSTAT302N potentiostat/galvanostat from Metrohm, The Netherlands. For electrochemical testing, the commercial SPGEs (DS-110, DropSens, DRP-110, Asturias, Spain) were employed to construct three electrodes on a single planar ceramic substrate: a carbon working electrode (WE), an Ag pseudo-reference electrode (RE) and a carbon counter electrode (CE).

Synthesis of [Cu(opd)2(H2O)(μ-SCN)Ni(opd)(SCN)3]

In separate beakers, 0.09 g (0.50 mmol) CuCl2·2H2O and 0.11 g (1.0 mmol) orthophenylenediamine (opd) were dissolved in minimal methanol to form the cationic portion. The opd ligand solution was then added dropwise to a round-bottom flask containing the copper(II) chloride solution, which was stirred and refluxed at 60 °C for five hours. In separate beakers, 0.15 g (0.50 mmol) of nickel(II) nitrate, 0.16 g (2.0 mmol) of sodium thiocyanate salt and 0.05 g (0.50 mmol) of opd were dissolved in a minimal methanol solution to make the anionic portion. The round-bottom flask was then filled with the nickel(II) nitrate solution. The opd ligand solution and sodium thiocyanate solution was then added dropwise while stirring, and the flask was refluxed for five hours at 60 °C. Afterward, the cationic and anionic solutions were mixed and refluxed for 5 h. The beaker's contents were then filtered and allowed to dry entirely at room temperature. The complex structure is shown in Figure 1.

Figure 1.: Structures of [Cu(opd)2(H2O) (μ-SCN)Ni(opd)(SCN)3]

SPGE modification using Cu-Ni nanocomplex

To make a homogenous solution for the SPGE modification procedure, 1.0 mg of the Cu-Ni nanocomplex was first dissolved in 1.0 mL of deionized water (1 mg mL-1 suspension). Subsequently, 3.0 μL of the Cu-Ni nanocomplex aqueous solution was applied to the WE in the SPGE, and the solvent was progressively removed under ambient conditions. They were considered Cu-Ni/SPGEs.