Ammonium selective fluorosensor based on the principles of coextraction

The overall objective of this study was to develop an optical fluorosensor (optode) for realtime 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 0-700mM). 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 100-700mM, with optimal performance at lowest Itot evaluated (100mM); (iv) has a response time of less than 4min; (v) has the ability to reversibly sense NH4+ even at µM levels. Sensor dynamic range was estimated as 10µM-100mM NH4+ in a sample matrix with ionic strength of 100mM. 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.