Graduate Thesis Or Dissertation
 

Electrochemical Advancement of Solid State Ion Selective Electrodes

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  • The general theme of this dissertation is the development of new analytical sensors to quantitatively study biological and environmental samples in aqueous and non-aqueous environments. A platinum ultramicroelectrode (UME) and a unique carbon-based pH microprobe were used to design a new SECM-based electrochemical bioassay. The Pt UME was used to distinguish between different strains of acid-producing Streptococcus mutans oral bacteria based on height and morphological features. The pH microprobe was used to quantitatively measure the pH above the three different S.Mutans biofilms as a biomarker to study dental caries disease. This dissertation focuses on precisely controlling electrochemically deposited ethylenedioxythiophene (EDOT) copolymerized with an alkoxysulfonate derivative of EDOT as a stable and tunable self-doped PEDOT/PEDOT-S copolymer to select for different properties. PEDOT/PEDOT-S copolymer was used as a solid contact material for new monovalent and divalent calibration free ion-selective electrodes. PEDOT/PEDOT-S was also used to fabricate a new solid-state reference electrode with a unique in situ PTFE-AF coating for both aqueous and nonaqueous electroanalytical measurements while satisfying the need for true reference electrode nonaqueous solutions. A two-probe scanning electrochemical microscopy (SECM)-based bioassay was used to characterize the height, morphology, and pH above three live bacterial biofilms. A 25 μm Pt electrode was used as an SECM probe to quantitatively map the morphology, height, and volume of the biofilm surface. A second probe, a unique carbon-based fast response pH microprobe, was used to quantify the local metabolic activity by quantitively measuring the pH profile above the three different biofilms when fed with sucrose. S. mutans B04Sm5 wild-type (WT) was recently isolated from early childhood dental caries patients and was found to produce a new N-acyl tetramic acid called mutanocyclin. An SECM characterization of the WT immediately after sucrose feeding found that the pH of the acidic environment 50 μm above the biofilm was 4.42 ± 0.54 (N=5). A laboratory strain S. mutans UA 159 had a pH of 5.21 ± 0.22 (N=4), and a variant of S. mutans B04Sm5 (MUTD) produced a local environment pH of 4.51 ± 0.40 (N=7). The change in the pH of the local environment normalized by height for the three strains showed the WT: 0.09 ± 0.02 (ΔpH ⁄ μm) (N=3), MUTD 0.05 ± 0.01 (ΔpH ⁄ μm) (N=7), and UA 159: 0.03 (ΔpH ⁄ μm) (N=2). A comparison of WT with the laboratory strain UA 159 showed that the pH of the local environment for WT was more acidic than UA 159 (p=0.043, 95%). There was no statistical deference between the WT and MUTD strains. A unique back contact made of 3,4-Ethylenedioxythiophene (EDOT) and 4-(2,3-dihydrothieno [3,4-b][1,4]dioxin-2-yl-methoxy)-1-butanesulfonic acid, sodium salt (EDOT-S) was designed as a back contact for calibration free monovalent and divalent solid state ion selective electrodes (ISEs). The calibration free ISEs showed near Nernstian responses of 57.2 ± 0.2 mV/log [K+] and 28.5 ± 0.3 mV/log [Ca2+] while maintaining their respective selectivity against major interfering ions. The detection limits for Ca2+ and K+ ISEs were 0.45 ± 0.01 mM and 1.68 ± 0.18 mM, respectively. The charging cycles of the PEDOT/PEDOT-S back contact allowed us to fix the background potential at a desired fixed intercept value across different ionophores (K+, Ca2+). This protocol was used to determine the content of K+ and Ca2+ in creek water samples. The concentration of [Ca2+] and [K+] in a local creek was 257 ± 7.3 mM (N=6) and 28.1± 1.1 mM (N=3), respectively. Currently, there is no reference electrode that is adequate for both aqueous and organic solutions. Silver-silver chloride (Ag/AgCl) is the most widely used reference for aqueous potentiometry and amperometry experiments. This reference is particularly robust due to the half-reaction of Ag/AgCl, but the stability of the potential can change with wire strain and requires a cumbersome liquid salt bridge or complex polymer membranes. The leaching of ions can contaminate the samples, rendering them unsuitable for applications requiring a limited sample volume (e.g., wearable sensors). In nonaqueous solutions, a liquid junction of unknown potential occurs due to an unmissable aqueous salt bridge and a nonaqueous sample. Water and ion leakage into the organic phase plague Ag/AgCl electrodes in nonaqueous solutions. A self-doped PEDOT/PEDOT-S solid state reference electrode with high mechanical and chemical durability and in situ PTFE-AF coating was designed to maintain a fixed potential based on the ratio of oxidized species to reduced species in the polymer as indicated by the Nernst equation. The PEDOT/PEDOT-S reference characterized versus ferrocene in acetonitrile with 0.1 M TBAPF6 had a stable potential of 0.411 V ± 0.008 V versus ferrocene over 14 days (N=3). The single junction PTFE-AF coated PEDOT/PEDOT-S electrode showed a potential of 0.397 V ± 0.009 V characterized versus ferrocene (2100 cycles) (N=3). For nonaqueous applications where size is not limited, we designed a double junction form of the PEDOT/PEDOT-S solid state reference electrode with a potential of 0.384 V versus ferrocene that did not change over four days (3100 cycles).
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  • Koley Lab
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  • National Institute of Dental and Craniofacial Research (Grant # R01DE027999)
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  • Pending Publication
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  • 2022-06-20 to 2023-07-20

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