Significant research has been focused toward the study of ion-selective electrodes (ISE) based on ionophore-doped polymer sensing membranes. One of the most substantial properties of these ISEs is their capability to measure target ion concentrations using a simple potentiometer.¹ Little investigation has been focused on the development of ISEs targeting Zn²⁺ ions, which in excess can be harmful to both human and plant cells.²   Zinc concentrations have been measured at the cellular level by other methods such as anodic stripping voltammetry, atomic absorption spectrometry, and atomic emission spectrometry. However, these methods are invasive to the cellular environment, costly, and require significant sample manipulation.   As a result, environmental samples provide an opportunity for using ISEs in agricultural applications because ionic activity can be measured at the cellular level without disruption to the biological environment. Additionally, ISEs provide a cost effective, accurate, and fast method for  measurement of zinc ion concentration. It is known that the components of the membrane greatly influence the response of the sensor³. As a result, the membrane should be optimized towards obtaining an electrode with Nernstian response, good sensitivity and selectivity. For optimization of the composition of the membrane, the stoichiometric relationship between the ionophore and Zn²⁺ were analyzed using GaussView 5 computational software. The effect of the membrane components on the potential response and the selectivity coefficients of the zinc-selective electrodes were investigated. The equilibrium constant that describes the formation of a complex between an ion and a ligand is called the complex formation constant, which  was calculated using the sandwich membrane method⁴. Additionally, the equilibrium ion exchange process  was enhanced resulting in a shorter conditioning time. Preliminary results show response times of about 10s, a near-Nernstian response, and a good linear response ranging from concentrations of 10^-6M  to 10^-2M. Optimization of the zinc ISE will allow for the measurement of ion concentration without perturbing the biological environment for agricultural applications providing an unprecedented level of specificity and sensitivity.

1. P. Buhlmann, L. D. Chen, Supramolecular Chemistry: From Molecules to Nanomolecules, 2012, 5, 2539

2. A. K. Singh, S. Mehtab, U. P. Singh, V. Aggarwal, Anal. Bioanal. Chem., 2007, 388, 1867-1876

3. Bakker, E., P. Bühlmann, and E. Pretsch. Chemical Reviews, 1997. 97(8): p. 3083-3132

4. Bakker, E., Willer M, Lerchi M, Seiler K, Pretsch E.  Anal Chem. 1994. 66(4):516-21