Post-imparting Brønsted acidity into an amino-functionalized MOF as a bifunctional luminescent turn-ON sensor for the detection of aluminum ions and lysine.

A novel aldehyde- and amino-functionalized luminescent metal-organic framework, Cd-TCHO, was constructed through the solvothermal reaction of 4,4',4''-tricarboxytriphenylamine, 2-amino-3-pyridinecarboxaldehyde and cadmium nitrate and was characterized. Post-synthetically oxidizing the aldehyde groups into carboxylate groups afforded a new complex, Cd-TCOOH, and this successful conversion process was confirmed by FT-IR and 1H NMR studies. With the Brønsted acidic sites inside the cavities of Cd-TCOOH, it could be used as a luminescent sensor for Al3+ detection with a high selectivity and sensitivity (LOD = 0.54 ppb), which could be attributed to the coordination between free Brønsted acidic sites and Al3+. Importantly, it could also detect Lys among 20 kinds of natural amino acids; the selectivity, sensitivity and the sensing mechanism are discussed in detail. Also, both of the sensing processes were carried out in the HEPES buffer.