Effect of Cr substitution in ZnFe<inf>2</inf>O<inf>4</inf> nanoparticles on the electron transfer at electrochemical interfaces

In this study, we explored the effect of Cr³⁺ substitution by partially and fully replacing Fe³⁺ in the normal spinel ZnFe₂O₄ crystal structure at electrochemical interfaces. The resulting ZnCrₓFe₂₋ₓO₄ nanomaterials exhibited an average particle size between 20 and 50 nm with a spherical morphology. The materials also demonstrated energy band gaps ranging from 2.1 to 3.1 eV X-ray diffraction (XRD) analysis confirmed that all the synthesized materials maintained a normal spinel structure, attributed to the octahedral site preference energy (OSPE) of Zn²⁺, Fe³⁺, and Cr³⁺ ions. Electrochemical performance assessments revealed that the ZnFe₂O₄-based sensor achieved a sensitivity of (37.8 ± 0.2) μA/mM with a kinetic rate constant of (13.1 ± 2.8) ms⁻¹, while the ZnCr₂O₄-based sensor exhibited a sensitivity of (32.4 ± 0.5) μA/mM and a kinetic rate constant of (3.73 ± 0.55) ms⁻¹ in the detection of paracetamol, whereas ZnCrFeO4 sensor has produced the second-best sensitivity (35.7 ± 0.1 μA/mM) and the rate constant (4.53 ± 0.54 ms⁻¹) with the lowest limit of detection (1.94 ± 0.01 μM). These differences in electrochemical performance were correlated with the variations in the energy band gaps caused by the restructuring of the normal spinel structure. Our findings indicate that the ZnFe₂O₄ sensor has a higher potential for direct electron transfer, whereas the other sensors are more likely to facilitate surface-mediated electron transfer.