An ammonium selective fluorosensor based on the principles of coextraction Original Research Article

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 NH4+ 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; Itot 0–700 mM). Additional desired sensor feature is a short response time (preferably minutes or less). This work presents an NH4+ 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 (KNH4+,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 Itot evaluated (100 mM); (iv) has a response time of less than 4 min; (v) has the ability to reversibly sense NH4+ even at μM levels. Sensor dynamic range was estimated as 10 μM–100 mM NH4+ 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.