Magnetic metal-organic framework nanocomposites for enrichment and direct detection of environmental pollutants by negative-ion matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

Nowadays, the detection and removal of environmental pollutants are particularly important, owing to their harmful influence on human beings' health. Specifically, metal-organic frameworks (MOFs) as the hot materials with great potential have been widely employed as adsorbents for pollutants removal, as well as matrixes for mass spectrometry detection. In this work, magnetic Zr-based MOF (denoted as Fe3O4@PDA@ZrMOF) with outstanding properties, such as low background interference, high desorption/ionization efficiency, excellent signal reproducibility, ultrahigh surface area and strong magnetic responsiveness, was synthesized by a modified thermal annealing method. The Fe3O4@PDA@ZrMOF not only can be utilized as adsorbent towards small molecules by π-π stacking interaction between nitrophenols and ZrMOF, but also can serve as a matrix for detection of small molecules under the negative ion mode of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). As a result, with the use of Fe3O4@PDA@ZrMOF, the detection limits of 4-nitrocatechol and 4-nitroguaiacol were estimated to be about 68 μg/ml and 25 μg/ml, respectively. In addition, enrichment of nitrophenols from PM2.5 was investigated. The detected mass concentrations of 4-nitrocatechol and 4-nitroguaiacol in collected PM2.5 samples were 0.11 ng/m3 and 0.15 ng/m3, respectively, indicating the highly efficient enrichment performance of Fe3O4@PDA@ZrMOF for low-abundance nitrophenol compounds in the environment.