The monitoring of ammonium ion concentration has gained the attention of researchers from multiple fields since it is a crucial parameter with respect to environmental and biomedical applications. For example, ammonium is considered to be a quality indicator of natural waters as well as a potential biomarker of an enzymatic byproduct in key physiological reactions. Among the classical analytical methods used for the detection of ammonium ions, potentiometric ion-selective electrodes (ISEs) have attracted special attention in the scientific community because of their advantages such as cost-effectiveness, user-friendly features, and miniaturization ability, which facilitate easy portable measurements. Regarding the analytical performance, the key component of ISEs is the selective receptor, labelled as an ionophore in ISE jargon. Indeed, the preference of an ionophore for ammonium amongst other ions (i.e., selectivity) is a factor that primarily dictates the limit of detection of the electrode when performing measurements in real samples. A careful assessment of the literature for the last 20 years reveals that nonactin is by far the most employed ammonium ionophore to date. Despite the remarkable cross-interference of potassium over the ammonium response of nonactin-based ISEs, analytical applications comprising water quality assessment, clinical tests in biological fluids, and sweat monitoring during sports practice have been successfully researched. Nevertheless, there is evident difficulty in the determination of close-to-micromolar levels of ammonium in real samples with a significant potassium background level (i.e., millimolar concentration). This fact has fostered the search for a large variety of ammonium ionophores over the years, which are critically inspected herein. Overall, we provide an easily readable state of the art accompanied by a comprehensive description of other types of ammonium electrodes, including commercially available units. We conclude that newer breakthroughs are still required in the field to reach the desired analytical applications.

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Why ammonium detection is particularly challenging but insightful with ionophore-based potentiometric sensors – an overview of the progress in the last 20 years

Von J. G. Schindler \ M. M. Schindler , K. Herna \ E. Reisinger , B. Burk ', U. Kuhlmann , J. Knaack , B. Schmidt und H. Lange 3 1 Institut fur Normale und Pathologische Physiologie, Projekt Biomedizinische Technik und Bioelektrochemische Membranelektroden, Philipps-Universitat, Marburg/Lahn, Germany 2 Schindler Biound Chemosensoren Forschungslabor, Marburg/Lahn 3 Medizinisches Zentrum fur Innere Medizin, Abteilung Nephrologie, Philipps-Universitatt Marburg/Lahn

Harnstoff-Durchflusssensor zur ammoniumselektiv-enzymatischen Prozesskontrolle der Künstlichen Niere

Harnstoff-Durchflußsensor zur ammoniumselektiv-enzymatischen Prozeßkontrolle der Künstlichen Niere

Nachdem W. Simon am 1. Oktober 1966 auf der Sommerversammlung der Schweizerischen Chemischen Gesellschaft in Soluthurn vortragen konnte, das Antibiotika vom Typ der Mak-rotetrolide in kunstlichen Membranen als aktive Komponenten fur hochselektive kationenselektive Elektrodensysteme herangezogen werden konnen [1], sind mit der Einfuhrung von Valinomycin und Nonactin als Trager-Antibiotika fur Elektrodenmembranen durch Simon et al. [1, 2, 3, 4, 5, 6] neutrale Tragermolekule par excellence zur elektroanalytischen Bestimmung von K+ [2, 5, 7, 8] und NH4 + [1, 3, 4, 6, 7, 9] verfugbar. Unter Einbeziehung von Erkenntnissen uber die antibiotischen Metallkomplexe [10] synthetisierte die Zuricher Arbeitsgruppe um W. Simon [11, 12] auf der Basis von Modellberechnungen [13] neutrale Kationen-Carrier [7] vom Typ der Dioxakorksaure- und Dioxaazelainsaurediamide fur Alkali- und Erdal-kali-Ionen. Unter Verwendung der PVC-Ionenaustauscher-Membranen nach Moody, Oke und Thomas [14] wurden die Carrier-Membran-Disk-Elektroden [15, 16, 17] entwickelt sowie ein Multimessystem zur Analyse stromender Flussigkeiten und Gase mit in Reihe geschalteten Sensoren [17, 18] konstruiert.

Kationenselektive Hämoanalytik mit Carrier-Membran-Disk-Elektroden

An ammonium-selective disc-electrode is described; the membrane consists of a polyvinylchloride-matrix with an active phase of nonactin in tris (2-ethylhexyl)phosphate and diphenyl ether as the ion-selective plastic. One side of the membrane is in contact with the solution to be measured and the other with the front surface of a platinum wire, which is enclosed in acrylic glass. This arrangement results in a very low drift potential.

Ammoniumselektive Disk-Elektrode mit dem Carrier-Antibiotikum Nonactin. Verfahren zur kontinuierlichen enzymatischen Elektroanalyse von Harnstoff, I. Mitteilung

A flow-through system for the measurement of urea concentrations is described, using soluble urease and consecutive determination of liberated ammonium ions by a selective disc-electrode. The active component of the electrode membrane was the carrier-antibiotic nonactin which was incorporated in a polyvinylchloride matrix.

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[A NH(+4)-selective-enzymatic flow-through system. A method for the continuous enzymatic, electrochemical determination of urea, II (author's transl)].

J. G. Schindler

Papers

Ammoniumselektive Disk-Elektrode mit dem Carrier-Antibiotikum Nonactin. Verfahren zur kontinuierlichen enzymatischen Elektroanalyse von Harnstoff, I. Mitteilung

[A NH(+4)-selective-enzymatic flow-through system. A method for the continuous enzymatic, electrochemical determination of urea, II (author's transl)].