Simple, sensitive, colorimetric detection of pyrophosphate via the analyte-triggered decomposition of metal-organic frameworks regulating their adaptive multi-color Tyndall effect

Abstract

This paper describes initially the application of the Tyndall effect (TE) of metal-organic framework (MOF) materials as a colorimetric signaling strategy for the sensitive detection of pyrophosphate ion (PPi). The used MOF NH₂-MIL-101(Fe) was prepared with Fe³⁺ ions and fluorescent ligands of 2-amino terephthalic acid (NH₂-BDC). The fluorescence of NH₂-BDC in MOF is quenched due to the ligand-to-metal charge transfer effect, while the NH₂-MIL-101(Fe) suspension shows a strong TE. In the presence of PPi analyte, the MOFs will undergo decomposition because of the competitive binding of Fe³⁺ by PPi over NH₂-BDC, resulting in a significant decrease in the TE signal and fluorescence restoration from the released ligands. The results demonstrate that the new method only requires a laser pointer pen (for TE creation) and a smartphone (for portable quantitative readout) to detect PPi in a linear concentration range of 1.25-800 μM, with a detection limit of ~210 nM (3σ) which is ~38 times lower than that obtained from traditional fluorescence with a spectrophotometer (linear concentration range, 50-800 µM; detection limit, 8.15 µM). Moreover, the acceptable recovery of PPi in several real samples (i.e., pond water, black tea, and human serum and urine) ranges from 97.66 to 119.15%.

Keywords: Colorimetric assay; Equipment-free quantification; Metal–organic framework; Pyrophosphate; Tyndall effect.