Influence of a spin crossover iron(<scp>iii</scp>) complex on the detection of phenylenediamines of graphene-modified screen printed electrodes

We developed a sensitive voltammetric sensor for detecting aromatic phenylenediamine (PD) isomers: <i>para</i>-phenylenediamine (<i>p</i>-PD), <i>ortho</i>-phenylenediamine (<i>o</i>-PD), and <i>meta</i>-phenylenediamine (<i>m</i>-PD). This sensor uses a screen-printed graphene electrode (SPGE) modified with a new spin crossover (SCO) Fe^III-complex [Fe(salEen-5-I)₂]NCS 1. The structure of 1·0.3CH₂Cl₂·0.5H₂O has been determined by single crystal X-ray crystallography and confirmed by spectroscopic techniques. Magnetic susceptibility measurements reveal a gradual SCO around room temperature. SEM-EDX and XPS analysis confirm the smooth surface morphology and uniform distribution of the SCO molecule on the modified 1-SPGE. Cyclic voltammetry (CV) shows that the modified 1-SPGE exhibited higher sensitivity to PDs than the bare SPGE. Differential pulse voltammetry (DPV) provided linear responses for <i>p</i>-PD (0.3-150 μM), <i>o</i>-PD (0.5-100 μM), and <i>m</i>-PD (1-50 μM), with corresponding limits of detection (LODs, S/N = 3) of 0.062 μM, 0.20 μM, and 0.41 μM, respectively. Furthermore, we successfully employed 1-SPGE to simultaneously detect a mixture of all three PD isomers. Remarkably, these isomers displayed distinct and well-defined electrochemical signals, with comparable electrochemical parameters to those observed in individual measurements, allowing us to successfully use the modified 1-SPGE sensor to monitor PDs in real samples.