Enhanced Ultrasensitive Photoelectrochemical Probe for Phosphate Detection in Water Based on a Zirconium–Porphyrin Framework

Excessive phosphate poses a serious ecological and human health risk, and thereby, monitoring its trace concentration is of great significance to environmental protection and human health. In this work, a zirconium–porphyrin framework (PCN-222) with excellent stability and unique luminescence properties was designed to modify the surface of the indium tin oxide electrode, which was first used as a photoelectrochemical (PEC) probe for phosphate detection. The PCN-222-modified PEC probe demonstrated an excellent selectivity and stability and indicated a linear response to phosphate in the range of 0–106 nM with a limit of detection (LOD) as low as 1.964 nM. To the best of our knowledge, this is the phosphate probe with the lowest LOD, and this is also the first signal-on PEC probe toward phosphate based on PCN-222. More importantly, the PEC probe can be validated for the good applicability of trace phosphate detection in real water samples, indicating a good application prospect. Finally, a series of electrochemical and spectroscopic studies have proved that phosphate can bind to the indium tin oxide (ITO)/PCN-222 electrode, which shortens the distance of the space charge region while reducing the bandwidth and thus facilitates the transfer of photogenerated electrons across the energy band barrier to reduce O2 in the electrolyte, producing an enhanced cathodic photocurrent signal. The proposed strategy of the highly sensitive PEC probe provides a promising platform for more effective label-free phosphate monitoring in the environment and organisms.