Distance-based qualitative determination of pyrophosphate ions with a Haus-made microfluidic paper-based device using the metal–organic framework NH2-MIL-88 (Fe)

This study initially reports a distance-based quantitative assay of pyrophosphate ion (PPi) based on analyte-triggered decomposition of metal–organic framework (MOF) in microfluidic paper-based analytical devices (µPADs). The µPADs were created using a mini-cutter machine and a home-use hot-pressing machine for loading one type of MOF of NH2-MIL-88 (Fe) prepared with Fe3+ and fluorescent ligand of 2-aminoterephthalic acid (NH2-BDC). Upon sample introduction, the PPi could compete with NH2-BDC to bind Fe3+, leading to disintegration of the MOF and releasing of the fluorescent ligand in paper channel. The length (distance) of the NH2-BDC's flow trace, when irradiated by a money tester pen, is proportional to the PPi level. The results show that the developed method only requires a ruler to linearly quantify PPi in a concentration range of 0–1000 μM, with a limit estimated to be ∼0.25 μM. Furthermore, recovery of PPi in orange juice and human urine was between 95.72 and 108.11 %, further validating its high detection accuracy. The fabrication method for µPADs offers many attractive advantages kike simplicity, batch production capability and low cost. Moreover, to our knowledge, this may be the first report of a portable quantitative MOF-based PPi assay with distance-based readout in µPADs.