Rapid visual detection of glutathione in vegetables and distinguishing multiple sulfur-containing compounds by smartphone-assisted sensor based on calcined PPy@CuNi-ZIF-67 nanozyme with enhanced oxidase-like activity.

In this work, polypyrrole-coated Cu, Ni co-doped ZIF-67 (PPy@CuNi-ZIF-67) was designed and prepared via a facile method. Calcined PPy@CuNi-ZIF-67 exhibited highly efficient oxidase-like activity and possessed excellent capacity to oxidize colorless 3,3',5,5'-tetramethyl benzidine (TMB) into blue oxidation products (oxTMB) within 4 min. Glutathione (GSH) and sulfur-containing compounds can inhibit its oxidase-like activity. Based on this principle, an effective colorimetric method was developed in combination with a smartphone for efficient, sensitive, and visual detection of GSH. Under optimal conditions, good linearity was achieved in the range of 0.83-33.33 μM, and the visual detection limit was 0.27 μM. Moreover, based on the response difference of different sulfur-containing compounds to different chromogenic substrates, a new array sensor was constructed with three chromogenic substrates. The array sensor was proven to quickly distinguish five sulfur-containing compounds at a concentration as low as 8.3 μM in combination with principal component analysis and hierarchical clustering analysis.