Ratiometric Fluorescent Chemosensor for Zn²⁺ Ions in Environmental Samples Using Supermicroporous Organic-Inorganic Structures as Potential Platforms

We report the fabrication of supermicroporous, fluorometric chemosensors with a Zr-metal–organic framework as carrier. The fabricated ratiometric fluorescent chemosensors (RFCs) were suitable for the selective and sensitive fluorometric recognition of ultra-trace concentrations of heavy metals, such as Zn²⁺ ions, in water sources. The RFCs were designed by performing direct pressure-assisted immobilization of a hydrophobic organic probe inside supermicroporous cubic Zr-MOFs. The immobilized organic probe exhibited high sensitivity and selectivity toward Zn²⁺ in a uniform super-microchannel pore, long-range intergrowing crystal structures. Ratiometric fluorometric sensing assay of the Zn²⁺ ions revealed the high selectivity of the ions in a mixture of multiple interfering ions. This phenomenon was due to the structure of the RFCs, the pH of the system, the composition of the competitive ion system, and the Zn-to-RFCs binding process, which occurred via electron transfer. The RFCs exhibited a wide detection and monitoring range for Zn²⁺ ions. The lower limit of detection was 0.16 ppb, and a fast response within seconds was observed during complexation with the Zn²⁺ ion target. The surface functionality and long-term stability of RFCs enable the practical and multiple reuse/cycles of Zn²⁺ ion recognition.