Development Of Electrochemical Sensors And Non-Covalent Monolayer Modification Of Graphene

Abstract

This dissertation is focused on two main research topics. One is the development of novel electrochemical sensors (Chapter 1-5), and the other is the non-covalently modification of graphene with monolayers of receptors, also in view of the development of chemical sensors (Chapter 6-7). Good electrode-to-electrode reproducibility is a necessity for calibration-free electrochemical sensors. Previously, our group introduced cobalt(II)/cobalt(III) redox buffers as an inner reference to improve the electrode-to-electrode reproducibility of solid-contact ion-selective electrodes (SC-ISEs), making calibration-free measurements possible. However, due to their low lipophilicity, leaching of those redox buffer from the ion-selective membrane into the aqueous sample could not be prevented, resulting in potential drifts. In this work, a new class of polymeric redox buffers was prepared by attaching redox couples to polymers through covalent bonds and using these polymeric redox buffers for the fabrication of SC-ISEs that exhibit exceptional long-term stability. A poly(vinyl chloride) (PVC)-based redox buffer was first synthesized by linking a Co(II)/Co(III) redox couple to the PVC chains through click chemistry. However, the redox capacity of this polymeric redox buffer was unsatisfactory, as measured by cyclic voltammetry. An alternative approach was adopted to attach the Co(II)/Co(III) redox couple to methacrylate copolymers containing bipyridyl ligands. Compared to coated-wire SC-ISEs, SC-ISEs using the methacrylate-based redox buffer as the inner reference exhibited a much better electrode-to-electrode reproducibility. Moreover, with its high lipophilicity, the redox buffer was compatible with ionophore-doped SC-ISEs, eliminating leaching from the sensing membranes into samples. To better understand how redox-active species control the phase boundary potential at a solid contact, a new theory was introduced. Experiments were performed to quantitatively study the effects of a Co(II)/Co(III) redox couple, Ru(II)/Co(III) redox pair, Fe(II)/Co(III) redox pair, and Os(II)/Co(III) redox pair on the electrode-to-electrode reproducibility of SC-ISEs. Perhaps surprisingly, theory predicts that there is only a minimal dependence of the phase boundary potential on the ratio of the concentrations of a pure oxidized and a pure reduced compounds if those two compounds are not a redox couple. However, even small redox-active impurities of those compounds shift the phase boundary potential drastically. Experimentally, a surprising in-batch reproducibility was observed for solid contacts prepared to contain either only the reduced or only the oxidized species of a redox couple. This can be explained by redox-active impurities and is not repeatable when different suppliers of reagents are used or long-term experiments are performed. This confirms that use of redox buffers with both the reduced and the oxidized species present in well-controlled concentrations remains the preferred approach to calibration-free sensing. In addition to calibration-free SC-ISEs, cross-linked cation-selective membrane electrodes with an inner filling solution were developed for the detection of zinc ions in methanolic samples. The cross-linked membrane was prepared by copolymerizing methyl methacrylate (polymer matrix), dodecyl methacrylate (polymer matrix), divinylbenzene (cross-linker), and tetrahexylammonium 4-styrenesulfonate (ion exchanger) under ultraviolet light. It showed near-Nernstian responses toward Na+ or Zn2+ in methanol, and the detection limit for Zn2+ was 10-5.3 M. With a view to graphene-based sensing arrays and devices, graphene was modified in this work with monolayers of pyrene, cyclodextrin, and metalloporphyrin derivatives as receptors. The receptor compounds were pyrene, pyrene derivatives with hydroxyl, carboxyl, ester, ammonium, amino, diethylamino, and boronic acid groups, benzylated α-, β-, and γ-cyclodextrins, 5,10,15,20-tetraphenyl-21H,23H-porphine manganese(III) chloride, and 5,15-bis(4-octadecyloxyphenyl)-porphyrin. Adsorption of these compounds onto graphene was quantified by contact angle measurements and X-ray photoelectron spectroscopy (XPS). Data thus obtained were fitted with the Langmuir adsorption model to determine the equilibrium constants for surface adsorption and the concentrations of self-assembly solutions needed to form dense monolayers on graphene. The equilibrium constants of all the receptors fell into the range from 102.9 to 104.6 M-1. Monolayers of 1-pyrenemethylammonium chloride on graphene were confirmed to be stable under heating up to 100 °C in a high vacuum (2×10-5 Torr), and monolayers of 1-pyrenemethylamine can be removed from graphene by immersion into toluene.