Printed Electrode for Measuring Phosphate in Environmental Water

Abstract

Phosphate is a major nonpoint source pollutant in both the Louisiana local streams as well as in the Gulf of Mexico coastal waters. Phosphates from agricultural run-off have contributed to the eutrophication of global surface waters. Phosphate environmental dissemination and eutrophication problems are not yet well understood. Thus, this study aimed to monitor phosphate in the local watershed to help identify potential hot spots in the local community (Mississippi River, Louisiana) that may contribute to nutrient loading downstream (in the Gulf of Mexico). An electrochemical method using a physical vapor deposited cobalt microelectrode was utilized for phosphate detection using cyclic voltammetry and amperometry. The testing results were utilized to evaluate the phosphate distribution in river water and characterize the performance of the microsensor. Various characterizations, including the limit of detection, sensitivity, and reliability, were conducted by measuring the effect of interferences, including dissolved oxygen, pH, and common ions. The electrochemical sensor performance was validated by comparing the results with the standard colorimetry phosphate detection method. X-ray photoelectron spectroscopy (XPS) measurements were performed to understand the phosphate sensing mechanism on the cobalt electrode. This proof-of-concept sensor chip could be utilized for on-field monitoring using a portable, hand-held potentiostat.

Figure 1

Cyclic voltammetry (CV) results using the phosphate sensor. (a) Cyclic voltammograms with a cobalt wire as the working electrode, Ag/AgCl as the reference electrode, and Pt wire as the counter electrode in a 25 mM KHP buffer and 1 mM KCl with KH₂PO₄ in a concentration range of 1 to 100 μM at a scan rate of 50 mV/s. (b) Phosphate sensor calibration curve showing a linear range of detection from 1 to 100 μM with bulk cobalt wire. The current signal obtained from the buffer was subtracted from all sample peak currents. (c) Effect of scan rate from 25 to 1350 mV/s on the performance of the phosphate sensor at a constant phosphate concentration (∼10^–6 M). Inset shows I vs scan rate^0.5. (d) E vs scan rate^0.5.

Figure 2

Current–time response profile of the phosphate sensor. (a) Chronoamperograms showing the phosphate sensor response at different phosphate concentrations. (b) Phosphate sensor calibration curve showing a linear range of detection from 5 × 10^–7 to 5 × 10^–4 M with bulk cobalt wire. The current signal obtained from the buffer was subtracted from all sample peak currents. The intra-assay coefficient of the variation percentage (CV%) (n = 8) was calculated to be 4.4%. (c) Evaluation of cobalt sensor response in the pH range of 2 to 8. Square wave pulsed voltammograms show the characteristic cathodic current of 10^–4 M aqueous KH₂PO₄ at various pH ranges from 2 to 8 in a 25 mM KHP buffer and 1 mM KCl at a scan rate of 50 mV/s. (d) Evaluation of the effect of dissolved oxygen on the cobalt phosphate sensor. Inset: Difference in current response measured at the cathodic peak potential (−1.05 V). (e) Evaluation of the electrochemical stability of the cobalt-based phosphate sensor at a fixed phosphate concentration (∼10^–5 M). Inset: Variation in the current response measured at the cathodic peak potential (−1.05 V) for 10 measurements. (f) Evaluation of the cobalt sensor response in the presence of various interfering cations. The bar graph shows the change in the current response measured at the cathodic peak potential (−1.05 V). Left to right: nitrate (0.05, 0.5, and 5 mM); sulfate (0.09, 0.9, and 9 mM); iodide (0.05, 0.5, and 5 mM); acetate (0.05, 0.5, and 5 mM); carbonate (0.05, 0.5, and 5 mM); and humic acid (5, 0.5, and 0.25 mM).

Figure 3

Deconvoluted XPS peaks of cobalt electrode. (a) Peaks for cobalt (Co), (b) peaks for carbon (C), and (c) peaks for oxygen (O). S1: bare cobalt wire, S2: KHP buffer (after 10 cycles of CV), S3: KHP buffer + KH₂PO₄ (after 10 cycles of CV), and S4: KHP buffer + KH₂PO₄ (after 20 cycles of CV).

Figure 4

(a) Nyquist curve at different phosphate ion concentrations in the range of 0 to 500 μM. The inset shows the equivalent circuit. (b) Plot of charge transfer resistance at different phosphate ion concentrations.

Figure 5

Phosphate detection in Mississippi (MS) water samples. (a) Electrochemical detection method. Cyclic voltammograms of cobalt wire as the working electrode with an Ag/AgCl reference electrode and Pt wire as a counter electrode in a 25 mM KHP buffer and a 1 mM KCl with KH₂PO₄; pH ∼ 4.2. (b) Photometric detection method. Absorbance recorded from a flow injection system at (i) 0, (ii) 0.1, (iii) 0.25, (iv) 0.75, (v) 2, (vi) 5, and (vii) 10 μM phosphate concentrations. Inset: Calibration curve obtained by the signals recorded from the flow injection analysis (n = 2). (c) Performance comparison of electrochemical versus photometric phosphate concentration.

Figure 6

Printed electrodes on the glass substrate. (a–b) Image showing the sensor. Here, C = counter electrode, R = reference electrode, and W = working electrode. (c) Phosphate sensing using the printed electrodes. Cyclic voltammograms of Co-metal as the working electrode with Au-metal as the reference and counter electrode in a 25 mM KHP buffer and 1 mM KCl with KH₂PO₄ in the concentration range of 10^–10 to 10^–2 M at a scan rate of 50 mV/s. Inset: Phosphate sensor calibration curve showing a linear range of detection in 10^–10 to 10^–2 M. Note: The current signal obtained from the buffer was subtracted from all sample peak currents.