Smartphone-assisted off─on photometric determination of phosphate ion based on target-promoted peroxidase-mimetic activity of porous CexZr1-xO2 (x≥0.5) nanocomposites.

Given the level of phosphate ion (Pi) is a significant indicator of eutrophication in environmental waters, it becomes quite important to develop efficient methods for its monitoring. In this research, we developed a smartphone-assisted off─on photometric approach for Pi analysis based on the analyte-promoted peroxidase-mimicking catalytic activity of porous CexZr1-xO2 (x ≥ 0.5) nanocomposites. The Ce4+/Ce3+ redox pair in CexZr1-xO2 endowed it with certain activity to catalyze the 3,3',5,5'-tetramethylbenzidine (TMB) color reaction with the participation of H2O2, and both the existing Zr4+ and Ce4+ species enabled the nanozyme to specifically recognize Pi. It was observed that the bonded Pi could greatly promote the peroxidase-like activity of the CexZr1-xO2 nanocomposite towards positively charged TMB. According to the new finding, high-performance sensing of Pi with wide detection range, high sensitivity and good selectivity was realized, giving a detection limit down to 0.09 μM. Further, a 3D-printed smartphone-based signal reading system was designed and coupled with the sensing method, enabling the rapid, convenient, in-field and instrument-free analysis of Pi for environmental monitoring.