Novel design of a flow-through potentiometric sensing device
Guagneli, Luca (2021)
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An innovative design of planar electrodes for flow-through potentiometric sensing devices is presented in this thesis. A planar and concentric potassium solid-contact ion-selective electrode (K+-SCISE) has been designed. The K+-SCISE is composed of a valinomycin-based potassium ion-selective membrane (K+-ISM), carbon cloth (CC) as ion-to-electron transducer, and a gold wire as electronic conductor. Three K+-SCISEs have been prepared and characterized. The electrodes showed stabile and reproducible response with short equilibration time. The slopes of the K+-SCISEs, calibrated in the range log[K^+ ] = -1 to -4, were 57.3 ± 0.6 and 56.3 ± 0.4 mV/dec (average ± SD, n = 3) without background electrolyte (BGE) and with 0.1 M NaCl as BGE, respectively. The slopes of three electrodes during five consecutive days were between 56.2 and 59.6 mV/dec in the range log[K+ ] = -1 to -4, with 0.1 M NaCl as BGE. The water layer test showed that the ion-selective membrane/carbon cloth interface is not subject to the formation of a water layer. The selectivity coefficients log K(i,j) for j = Ca(II), Li(I), Mg(II), Na(I) were -4.29 ± 0.14, -3.21 ± 0.09, -4.68 ± 0.05, and -3.34 ± 0.08 (average ± SD, n = 3), respectively. The electrochemical impedance spectroscopy (EIS) study showed that the resistance R of the membrane depends on its width and thickness. The resistance was in the order of 45-275 MΩ for several electrodes, with different geometries. Three different approaches of innovative planar solid-state reference electrodes (SS-REs) have been explored. Three SS-REs based on tetrabutylammonium tetrabutylborate (TBA-TBB) were tested. The reproducibility of the potential was not satisfactory. A commercial chloride ion-selective electrode (Cl--ISE) was calibrated versus the novel reference electrodes. The slopes of three calibration plots were -63.1, -61.8, and -58.5 mV/dec in the range log[Cl^- ] = -1 to -4. It was not possible to calibrate the novel K+-SCISE versus the SS-RE based on TBA-TBB, due to the loss of performances of the K+-SCISE when in close contact with the reference electrode. Two SS-REs based on silver/silver chloride (Ag/AgCl) approach were designed, one containing a polyvinyl acetate/potassium chloride (PVAc/KCl) composite membrane and the other containing a polyvinyl butyral/sodium chloride (PVB/NaCl) membrane. The layer of the PVAc/KCl composite membrane was too thick and irregular, and it did not meet the requirements of the sensing device. The PVB/NaCl membrane was too hard, and this complicated the cutting process. The two reference membranes did not ensure the isolation between the sensor and the analyte solution.
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