Ultrasensitive Capacitive Readout for Ion-Selective Electrodes

The main goal of this thesis was the development of capacitive readout technique by using an electronic capacitor instead of a solid-contact material to overcome the main drawbacks of constant potential coulometric readout. The capacitor can be adapted to amplify the current signal with ease, making this technique applicable to various ion sensing. Importantly, the current baseline drift is minimized owing to the ideal capacitive behavior of electronic capacitor. The concept has been extended to all-solid-state membrane electrodes. We reported the use of an electronic circuit to automate the control of the capacitive readout. Discharging the capacitor is executed by short circuiting after each chronoamperometric measurement. Furthermore, the development of a portable device for constant-potential coulometry is presented. A small potentiostat along with dedicated electronic circuits are integrated and fitted in a small box. Finally, a mathematical model is presented to describe the effects of exponential decay currents on the concentration polarization of ion-selective membranes. This approach improved sensitivity and gave higher precision than the traditional potentiometric probes

Rapid Constant Potential Capacitive Measurements with Solid-Contact Ion-Selective Electrodes Coupled to Electronic Capacitor

A constant potential capacitive readout of solid-contact ion-selective electrodes (SC-ISE) allows one to obtain easily identifiable current transients that can be integrated to obtain a charge vs logarithmic activity relationship. The resulting readout can therefore be much more sensitive than traditional open-circuit potentiometry. Unfortunately, however, comparatively long measurement times and significant baseline current drifts make it currently difficult to fully realize the promise of this technique. We show here that this challenge is overcome by placing the SC-ISE in series with an electronic capacitor, with pH probes as examples. Kirchhoff's law is shown to be useful to choose an adequate range of added capacitances so that it dominates the overall cell value. Two different ion-to-electron transducing materials, functionalized single-wall carbon nanotubes (f-SWCNTs) and poly(3-octylthiophene) (POT), were explored as solid-contact transducing layers. The established SC-ISE-based f-SWCNT transducer is found to be compatible with a wide range of external capacitances up to 100 μF, while POT layers require a narrower range of 1–4.7 μF. Importantly, the time for a charging transient to reach equilibrium was found to be less than 10 s, which is dramatically faster than without added electronic component. Owing to the ideal behavior of capacitor, the response current decays rapidly to zero, making the determination of the integrated charge practically applicable

Self-Powered Potentiometric Sensor Transduction to a Capacitive Electronic Component for Later Readout

Potentiometric sensors operate as galvanic cells where the voltage is spontaneously generated as a function of the sample composition. We show here that energy can be harvested, stored during the sensing process without external power, and physically isolated from the sensor circuit for later readout. This is accomplished by placing an electronic capacitor as a portable transduction component between the indicator and the reference electrode at the point where one would ordinarily connect the high-input-impedance voltmeter. The voltage across this isolated capacitor indicates the originally measured ion activity and can be read out conveniently, for example, using a simple handheld multimeter. The capacitor is shown to maintain the transferred charge for hours after its complete disconnection from the sensor. The concept is demonstrated to detect the physiological concentrations of K+ in artificial sweat samples. The methodology provides a readout principle that could become very useful in portable form factors and opens possibilities for potentiometric detection in point-of-care applications and inexpensive sensing devices where an external power source is not desired

Ultrasensitive Seawater pH Measurement by Capacitive Readout of Potentiometric Sensors

Potentiometric pH probes remain the gold standard for the detection of pH but are not sufficiently sensitive to reliably detect ocean acidification at adequate frequency. Here, potentiometric probes are made dramatically more sensitive by placing a capacitive electronic component in series to the pH probe while imposing a constant potential over the measurement circuit. Each sample change now triggers a capacitive current transient that is easily identified between the two equilibrium states, and is integrated to reveal the accumulated charge. This affords dramatically higher precision than with traditional potentiometric probes. pH changes down to 0.001 pH units are easily distinguished in buffer and seawater samples, at a precision (standard deviation) of 28 μpH and 67 μpH, respectively, orders of magnitude better than what is possible with potentiometric pH probes

Nanoparticle surface coverage controls the speciation of electrochemically generated chlorine

Cyclic voltammetry is used to investigate the oxidation of chloride on platinum nanoparticles. The electrochemical response of the nanoparticles dropcast on to a glassy carbon electrode is compared to that recorded at a platinum macroelectrode. The use of ‘nano’ and ‘macro’ scale Pt reveals different ratios of the electrochemically formed products, chlorine (Cl2) and the trichloride (Cl3-) anion. This difference in the speciation is attributed to the chloride oxidation being a surface reaction limited process. For the situation in which there are a limited number of active sites available on the electrode due to low nanoparticle surface coverages, the sub-diffusion limited currents result in higher chloride concentrations adjacent to the electrochemical interface. This excess chloride at the interface leads to the formation of the trichloride anion. The effect of surface oxide formation towards the chloride oxidation is also examined on both electrodes. Formation of platinum oxide serves to inhibit the rate of chloride oxidation

Nanoparticle surface coverage controls the speciation of electrochemically generated chlorine

Cyclic voltammetry is used to investigate the oxidation of chloride on platinum nanoparticles. The electrochemical response of the nanoparticles dropcast on to a glassy carbon electrode is compared to that recorded at a platinum macroelectrode. The use of ‘nano’ and ‘macro’ scale Pt reveals different ratios of the electrochemically formed products, chlorine (Cl2) and the trichloride (Cl3-) anion. This difference in the speciation is attributed to the chloride oxidation being a surface reaction limited process. For the situation in which there are a limited number of active sites available on the electrode due to low nanoparticle surface coverages, the sub-diffusion limited currents result in higher chloride concentrations adjacent to the electrochemical interface. This excess chloride at the interface leads to the formation of the trichloride anion. The effect of surface oxide formation towards the chloride oxidation is also examined on both electrodes. Formation of platinum oxide serves to inhibit the rate of chloride oxidation

Nanoparticle surface coverage controls the speciation of electrochemically generated chlorine

Cyclic voltammetry is used to investigate the oxidation of chloride on platinum nanoparticles. The electrochemical response of the nanoparticles dropcast on to a glassy carbon electrode is compared to that recorded at a platinum macroelectrode. The use of ‘nano’ and ‘macro’ scale Pt reveals different ratios of the electrochemically formed products, chlorine (Cl2) and the trichloride (Cl3-) anion. This difference in the speciation is attributed to the chloride oxidation being a surface reaction limited process. For the situation in which there are a limited number of active sites available on the electrode due to low nanoparticle surface coverages, the sub-diffusion limited currents result in higher chloride concentrations adjacent to the electrochemical interface. This excess chloride at the interface leads to the formation of the trichloride anion. The effect of surface oxide formation towards the chloride oxidation is also examined on both electrodes. Formation of platinum oxide serves to inhibit the rate of chloride oxidation