Smartphone-assisted colorimetric detection of phosphate ion based on CeFe bi-metal-organic framework nanozyme

Phosphate ion (Pi) is a critical indicator for eutrophication of environmental water, and it is important to develop sensitive and accurate method for Pi detection. In this work, a signal-off colorimetric sensor for Pi detection was developed based on CeFe bi-metal-organic framework nanozyme (MIL-88A_Ce/Fe NPs). The MIL-88A_Ce/Fe NPs were simply synthesized at room temperature, and exhibited peroxidase (POD)-like, oxidase-like, superoxide dismutase-like and phosphatase-like activity. The synergistic effect of Ce³⁺/Ce⁴⁺ and Fe²⁺/Fe³⁺ redox pairs endowed MIL-88A_Ce/Fe NPs with higher POD-like activity. The specific activity was calculated to be 2.246 U mg⁻¹. The catalytic mechanism was ascribed to nanozyme-promoted generation of hydroxyl radical. The POD-mimicking activity followed Michaelis-Menten kinetics. The Michaelis constant (K_m) and maximum velocity (V_max) were 0.2788 mM and 10.88 × 10⁻⁸ M s⁻¹ for TMB, while 0.4598 mM and 12.07 × 10⁻⁸ M s⁻¹ for H₂O₂. MIL-88A_Ce/Fe NPs retained 85 % of catalytic activity after stored for 65 days, showing high stability. The Ce species in MIL-88A_Ce/Fe NPs served as the recognition sites for Pi, and the interaction with Pi inhibited the POD-like activity of MIL-88A_Ce/Fe NPs. Thus, a signal-off colorimetric assay for Pi detection was proposed. The method exhibited low detection limit, short assay time, good selectivity and strong anti-interference ability. By proceeding the sensing system on test paper, a smartphone-assisted portable colorimetric platform was constructed to achieve on-site detection. Moreover, the method was used to determine Pi in real water samples, good recoveries (99.27 %-102.86 %) and low relative standard deviations (≤4.64 %) were obtained, showing great potential for environment monitoring.