A ratiometric fluorescence probe based on porphyrin-based MOF for phosphate ions detection

Abstract

Porphyrin-based metal-organic frameworks (MOFs) are a type of periodic network framework materials constructed from porphyrin or metalloporphyrin as the structural unit with metal ions or metal clusters. Due to their unique photoelectric property, porphyrin-based MOFs have potential application in fluorescence (FL) sensing. In this study, we have successfully prepared porphyrin-based MOF (Zn-TCPP(BPDC)) using the hydrothermal method, based on 5,10,15,20-tetra(4-carboxyphenyl)porphyrin (TCPP) as first ligand, 4,4′-biphenyldicarboxylic acid (BPDC) as second ligand and Zn²⁺ as the metal node. The structure, morphology and porosity of Zn-TCPP(BPDC) were characterized using Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and N₂ adsorption isotherms. Zn-TCPP(BPDC) exhibits a sheet-like structure with a side length of ca. 1 μm. The Brunauer-Emmett-Teller (BET) surface area is 98.5 m²/g, and the pore size is mainly concentrated at 2.5 nm. The porosity could provide channels for the adsorption and diffusion of analytes, which helps to improve sensing performance. Zn-TCPP(BPDC) exhibits high sensitivity and low detection limit towards phosphate ions (Pi), which is attributed to strong coordination of phosphate to Zn²⁺ that leads to the collapse of the layered structure of Zn-TCPP(BPDC), resulting in the release of TCPP monomers and enhanced FL emission. Additionally, FL sensing with single-wavelength is susceptible to environmental interference, which reduces the reliability. To address this, carbon dots (CDs) were introduced into the sensing system of Zn-TCPP(BPDC) aqueous solution to provide a reference FL signal, enabling visualization of the ratiometric FL sensing for Pi in aqueous solution with a low detection limit (0.13 μM), excellent anti-interference and selectivity. Finally, a portable sensing platform was constructed using a smart phone color recognizer, enabling real-time and visual ratiometric FL sensing of Pi.