Traffic lights inspired colorimetric sensing platform for cadmium/lead ions: PO43- induced structural change of porphyrin metal-organic framework

Developing rapid and user-friendly visual techniques for distinguishing and identifying heavy metal ions is a critical pursuit in food and environmental analysis. In this work, a zirconium-based metal organic framework (MOF) known as PCN-222 has been synthesized, serving as a colorimetric nanoplatform for the differentiation and detection of Cd2+ and Pb2+ in the presence of PO43-. The remarkable characteristic of PCN-222 stems from its reaction with PO43-, where the robust bonding between PO43- and Zr nodes triggers the collapses of PCN-222, leading to the release of ligands (TCPP) with visible alteration in solution color to light red. When exposure to Cd2+ and Pb2+, shifts to green and yellow, respectively, are discernible. These changes can be ascribed to differences in the size, atomic configuration, and electronegativity of the two metal ions, which impact their interaction with TCPP. The innovative concept of a nanomaterial traffic light is introduced based on these color transitions. Accurate quantification of Cd2+ and Pb2+ can be efficiently accomplished both individually and concurrently using UV spectroscopy and a mobile phone APP, providing adaptability in method selection depending on on-site conditions. Moreover, the platform effectively tackles selectivity challenges associated with ligand-based detection methods, showcasing high sensitivity, distinct visual indications, and simplicity in instrumentation requirements.