Smartphone assisted paper strip-based colorimetric sensing of phosphate and copper ions utilizing bi-ligand intercalated cobalt-MOF as a dual functional nanozyme

Monitoring phosphate and copper ions is vital for environmental and food safety due to associated health risks. To address this need, a new cobalt and bi-organic ligand based metal organic framework (BDC-Co-BIm MOF) has been developed. This MOF acts as a dual-function nanozyme, displaying peroxidase-like activity and selective detection abilities for PO43- ion. The Co(II) nodes and sp2 carbon within the framework exhibit excellent Fenton-like reaction, efficiently catalyzing the conversion of 3,3′,5,5′-tetramethylbenzidine (TMB) into oxidised TMB (oxTMB). Also, steady-state kinetic analysis indicated that BDC-Co-BIm MOF exhibited high affinity toward both TMB and H2O2. The Co(II) nodes and positive charge of the BDC-Co-BIm MOF capture PO43- ion through Co-O-P bonding and electrostatic interaction, limiting the electron transfer between BDC-Co-BIm MOF and H2O2. This reduces TMB oxidation and changes the color from blue to colorless. When ionic Cu2+ ion are added to the system, they react with PO43- ion to form a stable Cu3(PO4)2 complex. This reactivates the electron transfer process, allowing the H2O2 to decompose into •OH. As a result, TMB oxidation is restored and producing the blue color product again. Based on these principles, a colorimetric assay has been developed for the rapid and selective detection of PO43- and Cu2+ ions. A strong linear correlation has been established between their concentrations and the UV absorbance at 652 nm, with limit of detection (LOD) of PO43- and Cu2+ was 0.05 and 0.13 µM, respectively. These limits are notably lower than the thresholds established by the Environmental Protection Agency and World Health Organization (EPA & WHO) at 0.18 µM for PO43- and 30 µM for Cu2+. A Paper-based smartphone-assisted sensing of PO43- and Cu2+ ions was successfully achieved using intelligent Red, Green, and Blue (RGB) analysis. The platform showed high selectivity and performance with environmental water samples. This research presents a novel method for visually detecting PO43- and Cu2+ ions using versatile nanozyme, advancing portable on-site environmental safety analysis.