An ammonium selective fluorosensor based on the principles of coextraction

• Published in: Analytica chimica acta 2001-09, Vol.443 (2), p.215-225
• Main Authors: Strömberg, Niklas, Hulth, Stefan
• Format: Article
• Language: English
• Subjects: Analytical chemistry; Chemistry; Coextraction; Exact sciences and technology; General, instrumentation; Ionic strength; K; NH 4; Optode; Selectivity coefficient
• ISSN: 0003-2670; 1873-4324
• DOI: 10.1016/S0003-2670(01)01221-1

The overall objective of this study was to develop an optical fluorosensor (optode) for real-time measurements of ammonium concentrations in natural liquid samples. An NH 4 + optode that can be used at such conditions should be analyte specific and independent of ionic strength and pH within a range normally found in such samples (pH 6–8; I tot 0–700 mM). Additional desired sensor feature is a short response time (preferably minutes or less). This work presents an NH 4 + fluorosensor that utilizes similar principles previously described for a K + selective fluorosensor, i.e. coextraction of a nonactin–ammonium complex and a solvatochromic dye in a hydrogel–ether emulsion. The sensor was tested for its selectivity for ammonium ions compared to other solutes from which interferences frequently have been reported, e.g. K + and H +. Furthermore, sensor performance at different ionic strengths ( I) was evaluated. The ammonium fluorosensor was found to be (i) analyte specific with a selectivity coefficient ( K NH 4 +,K + opt) of 7×10 −2, i.e. sensor membranes are about twice as selective to ammonium relative potassium compared to previous studies of electrochemical sensors; (ii) pH independent within the interval 6.0–7.8; (iii) dependent on ionic strength within the interval 100–700 mM, with optimal performance at lowest I tot evaluated (100 mM); (iv) has a response time of less than 4 min; (v) has the ability to reversibly sense NH 4 + even at μM levels. Sensor dynamic range was estimated as 10 μM–100 mM NH 4 + in a sample matrix with ionic strength of 100 mM. Construction, configuration and composition of sensor membranes make the ammonium fluorosensor several times reusable, with a sensor shelf life time of at least 8 months.