Preparation of Nitrate Bilayer Membrane Ion-Selective Electrode Modified by Pericarpium Granati-Derived Biochar and Its Application in Practical Samples

Abstract

In this study, a pericarpium granati-derived biochar with phosphoric acid activation (PGCP) was prepared, characterized, and applied together with polypyrrole (PPy) to modify a glass carbon electrode (GCE) to construct a bilayer membrane nitrate ion-selective electrode (ISE, PPy/PGCP/GCE). The morphological results showed that PGCP possessed a porous structures. X-ray photoelectron spectroscopy analysis indicated that phosphorus added in PGCP was in the forms of P-O and P–C. The electrochemical impedance spectrum of the fabricated nitrate ISE showed a very low impedance, suggesting that PGCP can be used as an effective electron transfer mediator. The open circuit potential experiments indicated that the fabricated nitrate ISE exhibited a good linear potentiometric response to nitrate over a wide concentration range of 1 × 10⁻⁵ to 5 × 10⁻¹ mol·L⁻¹ with Nernstian slope of 50.86 mV·dec⁻¹ at pH range of 3.5–9.5 and a short response time of less than 7.3 s. Its limit of detection (LOD) was determined to be 4.64 × 10⁻⁶ mol·L⁻¹. Both detection range and LOD are comparable or better than those of reported similar modified electrodes. The fabricated nitrate ISE exhibited a high selectivity with a good repeatability and stability. The selectivity sequence was determined as NO₃⁻  > NO₂⁻  > Cl⁻  > H₂PO₄⁻  > CH₃COO⁻  > CO₃²⁻  > SO₄²⁻. The fabricated nitrate ISE was validated by the nitrate detections of real samples from Shenzhen OCT wetland and laboratory wastewater, respectively, with the obtained detection discrepancy of less than 4% (RSD). This study provides useful reference information for the development of novel ISE in clinical laboratories and environmental monitoring.