The paper introduces a rational porphyrin-based ionophore design for improved perchlorate selectivity in ion-selective electrodes (ISEs).
Behind the pretty, lit-up displays shot up into the night sky is a high-concentration perchlorate wastewater produced via firework and firecracker manufacturing. The chemical liquid poses serious risks to drinking water sources and ecosystems.
Perchlorate, a persistent and highly soluble inorganic ion, typically enters surface and groundwater through industrial leakage from sectors like aerospace, manufacturing, defense, and fireworks.
In China’s Xiangjiang River Basin, for example, perchlorate levels in wastewater have reached over 1,000 mg/L.
Frequent and effective detection of this chemical is essential, as ingesting it can disrupt thyroid function and hormone production.
However, conventional detection methods, such as ion chromatography and mass spectrometry, are costly and complex, and lengthen the detection process. The need for portable, user-friendly tools for on-site, rapid screening to meet increasingly stringent global and national drinking-water standards is clear.
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The Role of ISEs
ISEs could be a promising option for portable/continuous on-site monitoring on firework production sites.
Although they can be designed as liquid-contact/solid-contact devices, in all cases, their key performance indicators, like selectivity, detection limit, sensitivity, and linear range, are governed by the ion-selective membrane (ISM).
This ISM includes a plasticizer, ionophore, polymer matrix, and an ionic additive. The selectivity of traditional ionophores for various anions often follows the Hofmeister series. Tailoring ionophore structures can improve compatibility with perchlorate.
Porphyrins/porphyrin metal complexes are particularly attractive as molecular recognition elements owing to their rigid molecular frameworks and presence in different structural forms.
In addition, they allow precise ring substituent positioning control and enable steric environment and interaction directionality modulation. Further investigation is required to understand how ring substituents affect porphyrin hydrophobicity and hydrophilicity, as well as the role of various coordinated metal ions.
Investigating and Detecting Perchlorate
In this work, researchers introduced a liquid-contact ISE containing novel porphyrin-based ionophores for sensitive, selective, and rapid perchlorate detection.
The electrode featured a poly(vinyl chloride) (PVC)-based membrane/ISM optimized using ortho-nitrophenyloctyl ether (NPOE) as the plasticizer, tridodecylmethylammonium chloride (TDMACl) as the ionic additive, and iron(III) meso-tetraphenylporphine chloride (Fe[III]TPPCl) as the ionophore.
This electrode offers strong anti-interference capabilities and a broad linear dynamic range, making it suitable for fast and straightforward perchlorate determination.
A range of metalloporphyrins, ionic additives, plasticizers, and high-molecular-weight PVC were used as starting materials.
The ion-selective membranes were fabricated by dissolving the ionophores, plasticizers, PVC, and ionic additives in tetrahydrofuran. The mixture was then cast into a glass dish and left to evaporate tetrahydrofuran overnight in an electric thermostatic drying oven. A 0.8 mm thick elastic membrane was produced from the mixture, from which 7.0 mm disks were punched for assembly.
The ISE body was made from a polyoxymethylene tube, with an ISM disk fixed to one end and a silver/silver chloride wire sealed in the other. The electrode was filled with 10-2 M sodium perchlorate monohydrate internal solution and conditioned in the same solution for at least 24 hours prior to measurement.
The ISEs were subsequently evaluated and tested via electromotive force measurements.
Selectivity and Sensitivity of the Electrode
The researchers successfully synthesized and optimized a high-performance perchlorate-selective electrode using a Fe(III)TPPCl ionophore.
Screening results revealed that Fe(III)TPPCl outperformed other metalloporphyrins like Mn(III)TPPCl, Co(II)TPP, and substituted analogs like Fe(III)TClPPCl, providing stronger perchlorate selectivity, a sensitivity of –68.42 ± 0.91 mV decade-1, and a low detection limit (approximately 10-5 M).
The anti-Hofmeister behavior observed was attributed to dominant axial coordination between perchlorate and the Fe(III) center, with electron-donating phenyl groups further strengthening this interaction.
Membrane optimization identified NPOE as the best plasticizer at a 2:1 NPOE/PVC ratio, enabling a linear range of 10-5 to 10-1 M. TDMACl further improved sensitivity and membrane uniformity.
Field testing in fireworks wastewater and surface water yielded quantitative recoveries. A remarkable 96.4 % recovery in wastewater and 104.3 % in spiked surface water, consistent with ion chromatography and realized without pretreatment.
The electrode exhibited a rapid response (< 5 s), strong pH tolerance (2 to 10), and a low fabrication cost (below USD 2), supporting disposable, on-site monitoring. However, reliance on a liquid-contact design may limit long-term stability due to potential internal-solution leakage and sensitivity to temperature fluctuations.
The Future of Firework Chemical Detection
In conclusion, the study demonstrated a field-ready, cost-effective sensor for perchlorate detection in complex environmental matrices, addressing crucial regulatory compliance needs.
Future work should focus on extending electrode lifespan and transitioning to solid-contact ISE configurations to enhance field durability.
Journal Reference
Li, B. et al. (2025). Rational Design of Porphyrin-Based Ionophores for Enhanced Perchlorate Selectivity in Ion Selective Electrodes: Application to Fireworks Wastewater Analysis. Energy & Environment Nexus, 1(1). DOI: 10.48130/een-0025-0007
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