Keele Research Repository

Chemical sensors have gone through a lot of optimisation renaissance over the years to have come from a typical bench-top tool for measurements of ions in standard solutions, to a more promising analytical technique capable of measuring the activities of free un-complexed nutrients in environmentally and biologically important samples. With various health issues arising due to the increasing anthropogenic contributions of man to the environment amongst other factors, and due to the need for more routine analysis especially do-it-yourself (DIY) of several analytes in physiological samples for clinical purposes, it is therefore imperative that simple, cheap, but robust sensors are developed, and optimised to meet these emerging needs.

First part of this work involved fabricating solid-contact ion selective electrodes (SC-ISEs) based on the mechanical abrasion of graphite on easily modified acetate paper. Similarly results from the impedance spectra and the water layer test for electrodes with or without a conducting polymer (CP) layer indicated the suitability of the procedure. The fabricated paper electrodes importantly showed fast response time and great potential stability over the course of fourteen days. The same fabrication methodology was used to produce stable and functional solid-state paper reference electrode, and then, combined with other graphite-based paper ISEs to yield a single-strip solid contact electrode for simultaneous measurements of nitrate and ammonium in environmental samples. Satisfactory results from the comparison of measured concentrations by potentiometry and standard reference methods indicated this simplified electrode platform, designed from household materials can be used as a cheaper alternative to other solid contact electrodes.

Subsequent works in this thesis involved the application of the fabricated SC-ISEs in optimization of measurement procedures of environmentally important nutrients and biologically important analytes.

Routine monitoring of reactive nitrogen Nr (majorly NH4+ and NO3-) in environmental samples including soils from major land types is important to soil management system. In achieving a relatively rapid turn-around time of analysis, inorganic N species were extracted from various soil types using a single extracting solution (0.1 M MgSO4). Extractable-NH4+ and -NO3- in soils, and bioavailable NH4+ and NO3- in water samples measured concurrently using paper ISEs showed similar results to standard analytical methods.

The analysis of urinary iodine (UI) is important to the public health due to serious health issues attached to its deficiency. As a result, iodide-selective electrodes based on [9] Mercuracarborand-3 (MC3) and [12] Mercuracarborand-4 (MC4) as ionophores, and NPOE and DOS as plasticizers were developed with a view to evaluate the concentration of iodide in urine.

However, ion-selective electrodes (ISEs) are one of the very few experimental techniques whose limit of detection (LOD) is not defined as signal-to-noise ratio. As a result, the Bayesian model was applied to estimate the activities of nitrate and ammonium in soil and water samples, and iodide in urine samples. While the Bayesian estimates for nitrate and ammonium ions were satisfactory, significant discrepancy of estimated results for iodide in urine shows more work needs to be done in relation to designing more selective ionophores to complement non-linear approaches.