| Phosphorus is a restrictive nutrition element attribute to the eutrophication in lakes and rivers, which is also a regular index for water monitoring. Phosphate-ion concentration has generally been determined by spetrophotometric method or by ion chromatography. However, these methods are complex, time consuming, and relying on expensive, bulky instruments, thus inappropriate for the direct measurements of phosphate activities in aqueous solutions. In order to meet the demands of quick, online phosphate detection, two cobalt-based phosphate sensors were successfully fabricated by using the screen printing technology in this work.A cobalt-based phosphate sensor which was successfully fabricated by using the screen printing technology for the determination of phosphate concentration in the aqueous solution consists of fully integrated cobalt (Co) electrode, which is a layer of carbon conductive ink (C) physically doped with Co powder, and Ag/AgCl reference electrode. At the Co/C ratio of 1:7, the cobalt-based phosphate sensor showed phosphate-selective potential response in the range of 10-5 to 10-1 mol/L, yielding a detection limit of 5×10-6 mol/L and a slope of -41.63mV/decade for H2PO4-. The proposed screen printed sensor also exhibited significant reproducibility with a small repeated sensing deviation (i.e. relative standard deviation (R.S.D.) <1% for ten measurements) on a single sensor and a small electrode-to-electrode deviation (i.e. R.S.D. <3.5% for twelve sensors).Based on the screen printing technology, another disposal cobalt-based electroplating electrode was fabricated by electroplating a layer of Co on the layer of carbon conductive ink (C). The electroplating current intensity and time are 0.5mA and 200s, respectively. The cobalt-based phosphate sensor showed phosphate-selective potential response in the range of 10-5 to 10-1 mol/L, yielding a detection limit of 5×10-6 mol/L and a slope of -30.87mV/decade in acidic solution (pH 4.0) for H2PO4-. The proposed screen printed sensor also exhibited significant reproducibility with a small electrode-to-electrode deviation (i.e. R.S.D. <3.3% for twenty sensors).After analyzing the results and the reported papers on the potential response mechanism of cobalt electrodes toward inorganic phosphate, DO and pH were found to interfere with electrode response to phosphate. The mixed potential response equation about cobalt electrodes toward phosphate was proposed. The performances of that electrode were validated for water samples from XiaoJiaHe Wastewater Treatment Plant against standard spectrophotometric methods of analysis. Compared with the cobalt-based electroplating electrodes, the cobalt-based electrodes doped with Co powder perform better and have some practicality.
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