Rapid and selective detection of Fe (III) by using a smartphone-based device as a portable detector and hydroxyl functionalized metal-organic frameworks as the fluorescence probe.

Here, a label-free fluorescent sensor was developed for detection Fe (III) by utilizing the quenching effect of Fe (III) on the fluorescence of the hydroxyl functionalized metal-organic framework MIL-53(Fe)-(OH), which was synthesized by using a one-step solvothermal method. The specific binding interaction between Fe (III) and hydroxyl facilitated the absorption of free Fe (III) to MIL-53(Fe)-(OH) which leads to rapid fluorescent intensity quenching effect. The potential quenching mechanism was proved to be the photo-induced electron transfer (PET) from electron-rich ligands of MIL-53(Fe)-(OH) to the half-filled 3d orbitals of free Fe (III) in the sample solution. For in-field applications, the fluorescence signal was detected rapidly by using a homemade 3D-printed, portable, and low-cost smartphone sensor. A commercial 365 nm UV LED light was adopted as the excitation light source, while the camera in a smartphone was utilized as the optical detector. The fluorescent signals obtained by using the smartphone sensor were in a good agreement with those obtained from a commercial microplate reader. Under the optimal assay conditions, the linear detection range of Fe (III) was 5.0-200 μM, and the limit of detection is 1.7 μM. This result is compatible with the commercial microplate reader. The developed method was successfully adopted to detect Fe (III) in human serum and environmental water samples with acceptable recovery values of 90-108.5%. The portable, low-cost, fast-response, user-friendly and sensitive fluorescent protocol based on a self-quenching fluorescent nanoprobe can be conducted at home or anywhere else without sophisticated instruments, showing a great application potential in clinical diagnosis, on-site environmental monitoring and healthcare at home.