A dip-and-read capacitive electrochemical sensor for orthophosphate monitoring

Phosphorus (P) is a finite, essential resource critical for agriculture, yet its unsustainable management through excessive fertilizer application leads to significant environmental degradation, including water pollution. This directly impedes progress towards UN Sustainable Development Goals (SDGs) 2 (Zero Hunger) by wasting resources vital for food security and SDG 6 (Clean Water and Sanitation) by polluting aquatic ecosystems. Here, we develop a reagent-free orthophosphate chemosensor based on a sorbent material containing graphene oxide (GO) and diallyl-dimethylammonium chloride (PolyDADMAC), termed GO-PDDA. Laser-induced graphene electrodes were coated with GO-PDDA material using four different grafting or drop-cast techniques. Electrochemical testing showed that grafted GO-PDDA electrodes were more efficient than drop-cast GO-PDDA or DADMAC grafted electrodes. Developed GO-PDDA sensor is applied for sensing orthophosphate in aqueous samples at pH 7–9. Equivalent circuit modeling indicated that capacitive behavior coincides with Frumkin/Melik-Gaykazyan adsorption theory, where tetrahedral oxyanions increase low-frequency capacitance in thin films. The sensor achieves a detection limit of 20 ± 4 ppb with a rapid 5 min response time, covering concentrations relevant to natural waters. It exhibits high selectivity, being 97% selective for divalent ortho-P over common interferents, 93% over chloride/nitrate, and 87% over sulfate. The chemosensor is reusable, showing less than 5% performance change after regeneration. Validated against EPA Method 365.3 in urban creek water ( R ^2 = 0.92), it offers a faster, reagent-free alternative for direct ortho-P quantification. This work marks the first use of PolyDADMAC as a recognition material in an electrochemical sensor, positioning it as a promising tool for sustainable P management, directly supporting SDG 2, SDG 6, and SDG 12 (Responsible Consumption and Production).