Experimental

Apparatus and chemicals

An Autolab potentiostat/galvanostat equipped with General Purpose Electrochemical System (GPES) software (PGSTAT-302 N, Eco Chemie, The Netherlands) was used to run the electrochemical experiments. A silver pseudo-reference electrode and graphite working and auxiliary electrodes were used in the SPEs (Dropsens, DRP 110). Each solution was prepared freshly using double-distilled water. Merck (Darmstadt, Germany) supplied analytical-grade CFT, MTT, and all other reagents. Orthophosphoric acid, along with its salts, was used to prepare buffer solutions (PBS) over the pH range of 2.0 to 11.0.

Synthesis of Fe and Mg linked to a 1,4-benzene dicarboxylate ligand metal organic frameworks

At first, 2 mmol of 1,4-benzenedicarboxylic acid, also called BDC, and 2 mmol of a mix of metal salts FeCl3·6H2O together with Mg(NO3)2·6H2O, where the Fe3+ to Mg2+ molar ratio was set to 1/1, were poured into 20 mL of DMF. The whole thing was kept in constant agitation until a fairly uniform solution came together. Then, 4 mL of a 0.2 M NaOH solution was added, and the suspension was stirred for about 20 min before being transferred to an oven at 100 °C for 12 hours. Afterward, it was taken out and left to cool down to room temperature. The resulting MOFs were then dried at 80 °C for 18 hours and, in between, were repeatedly centrifuged in ethanol to remove residual or unreacted materials. Figure 1 shows the field emission scanning electron microscopy (FE-SEM) images of FeMg-BDC MOFs.

Figure 1.: FE-SEM images of FeMg-BDC MOFs

Preparation of screen-printed electrode modified with Fe and Mg linked to a 1,4-benzene dicarboxylate ligand metal organic frameworks

1.0 mg of FeMg-BDC MOFs heterostructures were ultrasonically dispersed in 1.0 mL of deionized water for 15 minutes at room temperature to provide a homogeneous suspension for the modified electrode manufacturing (FeMg-BDC MOFs/SPE). To create FeMg-BDC MOF/SPE, 3.0 μL of FeMg-BDC heterostructures suspension was drop-cast onto the SPCE surface and allowed to dry at room temperature.

Procedure of real sample preparation

Preparation of methotrexate injection

At first, a 100 mL volumetric flask was filled with 1 mL of the MTT injection (the vial containing 25 mg mL-1 injectable solution) and then brought up to the mark with PBS 0.1 M (pH 7.0). After that, a 25 mL volumetric flask was filled to the top with PBS 0.1 M (pH 7.0), but only after a selected volume of the freshly prepared solution was transferred into it. The mixture was subsequently placed into the electrochemical setup, kind of as a true specimen.

For the estimation of those species, the standard addition approach was used: the unspiked injection sample and spiked injection samples prepared with standard MTT and CFT solutions at different concentrations, were introduced into the electrochemical cell. Afterward, the differential pulse voltammetry (DPV) response was recorded. Each concentration was basically repeated five times, not once.

Preparation of calcium folinate injection

First, about 100 mL of a volumetric flask was prepared by adding 1 mL of the CFT injection, which came from a vial containing a 10 mg mL-1 injection solution. Then it was diluted until the graduation mark, using PBS 0.1 M (pH 7.0). After that, a 25 mL volumetric flask was filled to the top with PBS 0.1 M (pH 7.0), but only after a specific volume of the prepared solution was poured into it. This final mixture was then loaded into an electrochemical device as the real sample for analysis. To quantify these analytes using the standard addition approach, unspiked and spiked injection specimens were prepared. Each spiked set included standard solutions of CFT and MTT at several concentrations, and all were transferred into the electrochemical cell. The corresponding differential pulse voltammograms (DPVs) were collected and saved. For every concentration level, the whole measurement was repeated 5 times.

Urine analysis

Ten millilitres of urine samples were also centrifuged at 2000 rpm for fifteen minutes. A portion of the resulting supernatant was transferred to a 25 mL volumetric flask and, after passing through a 0.45 μm filter, diluted with PBS (pH 7.0) to the appropriate volume. Afterward, varying amounts of CFT and MTT were added to the diluted urine samples. Lastly, the conventional addition procedure was used to quantify the concentrations of CFT and MTT.