Metal-organic frameworks (MOF) are a class of inorganic-organic hybrid materials. They can be used for sensing various target molecules, including explosives, metal ions, and small molecules. Luminescent MOFs are especially attractive for these sensing applications. However, their application as an enzyme activity sensor is still limited.

Wen-Juan Ruan and colleagues, Nankai University, Tianjin, China, have developed a luminescent MOF, La-atp (atp = 2-amino-terephthalic acid), for sensing the activity of the enzyme polyphenol oxidase (PPO). The researchers characterized La-atp using X-ray diffraction, scanning electron microscopy, and fluorescence spectroscopy. The team used La-atp to measure PPO activity with its substrate, catechol. PPO oxidizes catechol to o-benzoquinone, which, in turn, covalently reacts with the amino groups in La-atp. This reaction leads to a decrease in the fluorescence intensity of La-atp (pictured).

The team compared their results to the widely used ascorbic acid method and showed that La-atp is more sensitive at low concentrations. Finally, they tested La-atp with food extracts to show 100 % recovery of PPO-spiked samples, indicating that a complex sample matrix does not interfere with La-atp fluorescence.

• Luminescent Metal-Organic-Framework-Based Label-Free Assay of Polyphenol Oxidase with Fluorescent Scan,
  Yue Li, An Guo, Lan Chang, Wen-Juan Li, Wen-Juan Ruan,
  Chem. Eur. J. 2017.
  DOI: 10.1002/chem.201605992