Supramolecular Fe II4L4 cage for fast ammonia sensing

Abstract

The design and construction of colorimetric ammonia (NH₃) sensors with high selectivity, great stability and short response times at room temperature are highly needed for human health and food safety. We report herein a chiral supramolecular Fe^II₄L₄ metal-organic cage (MOC-1) able to detect NH_3(g) at room temperature in less than 10 s, which we used to monitor food safety at 4 °C on fresh bacon bits. The detection is accompanied by a dramatic colour change from light brown to purple. In addition, MOC-1 exhibits superior selectivity among NH_3(g) and twelve analytes including common solvents and amines. The reproducibility was studied by five continuous cyclic tests. Simple and low-cost smartphone-based and chemometrics analytical methods were used to study the sensing performance. The thermal stability is more than 200 °C. The sensing mechanism is associated with a high-spin to low-spin transition of the Fe^II ions, as expected for a ligand field strength that meets spin crossover conditions. These results show the high potential of MOC-1 as a convenient and inexpensive NH₃ gas sensor at room temperature, which could be used in the field of food safety assessment.