Mixed-Valence Cobalt(II/III) Metal–Organic Framework for Ammonia Sensing with Naked-Eye Color Switching

The
construction of colorimetric sensing materials with high selectivity,
low detection limits, and great stability provides a significant way
for facile device implementation of an ammonia (NH3) sensor.
Herein, with excellent alkaline stability and exposed N sites in molecule
as well as with naked-eye color switching nature generated from changeable
cobalt (Co) valence, a three-dimensional mixed-valence cobalt­(II/III)
metal–organic framework (FJU-56) with tris-(4-tetrazolyl-phenyl)­amine
(H3L) ligand was synthesized for colorimetric sensing toward
ammonia. The activated FJU-56 demonstrates a limit of
detection of 1.38 ppm for ammonia sensing, with high selectivity in
ammonia and water competitive adsorption, and shows outstanding stability
and reversibility in the cyclic test. The NH3 or water
molecules binding to the exposed N sites with the hydrogen-bond are
observed by single-crystal X-ray diffraction, determining that the
attachment of guest molecules to the FJU-56 framework
changes the valence of Co ions with a naked-eye color switching response,
which provides an ocular demonstration for ammonia capture and a valuable
insight into ammonia sensing.