Highly sensitive and selective colorimetric sensing of nitrite based on largest surface area ZIF-67 nanostructure with enhanced peroxidase catalytic activities

The quest for the development of a convenient and effective colorimetric sensor to sensitively detect nitrite (NO₂⁻) ions in drinking water is of high concern, since large amount of NO₂⁻ ions causes severe problems to living organisms. So for this purpose, in the present research, we have mainly focused on demonstrating the largest surface area nanomaterials structure called ZIF-67, which is mostly prepared by the chemical reaction of metal ions (Co²⁺ with organic linker like 2-methylimidazole (2-mIM) rings) via a simple hydrothermal method. The as-fabricated colorimetric sensor (ZIF-67 nanostructure) was used for the sensitive detection of NO₂⁻ ions, which offers peroxidase mimetic-like characteristics, oxidizes the colorless TMB into oxidized TMB (TMB_Oxi) having dark blue color, and helps in the formation of UV–Visible peak at 652 nm called TMB_Oxi peak. However, the addition of NO₂⁻ ions will induce the TMB_Oxi and will lead to the production of diazotized TMB_Oxi (colorless), resulting in the suppression of TMB_Oxi peak at 652 nm and the formation of new peak at 448 nm, known as diazotized-TMB_Oxi peak, manifesting the successful peroxidase catalyze reaction. On the other hand, the prepared colorimetric sensor exhibited effective sensitivity towards NO₂⁻ ions with wider range of (0.001–0.540 μM). Further, it also predicts the lower detection limit (LOD) and Quantification limit (LOQ) of 0.18 ± 0.04 nM and 0.75 ± 0.04 nM, with correlation coefficient (R²) of 0.999, respectively. In addition, the proposed colorimetric sensor also shows highest selectivity towards NO₂⁻ ions, excellent cycling stability (9 cycle), outstanding long-term stability (8 weeks), better reproducible behaviors, and strong applicability behavior towards NO₂⁻ ions in real samples. These outstanding colorimetric sensing features were ascribed to extensive surface area (1540 m²/g) and highly exposed active sites of ZIF-67 nanostructure, which surely help in high adsorption–desorption rate of electrons across the surfaces. Thus, the well-astonished colorimetric sensing properties of the prepared sensor enhances the demand to be used in practical applications.