The Graphene Ring Nanoelectrode (GRiN) and its Application as an Electroanalytical Sensor for Environmental Monitoring

Ferrer Huerta, Miriam and Boxall, Colin and Andrieux, Fabrice (2020) The Graphene Ring Nanoelectrode (GRiN) and its Application as an Electroanalytical Sensor for Environmental Monitoring. PhD thesis, Lancaster University.

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Abstract

Nanoelectrochemistry is the study of the electrochemical properties of materials with at least one dimension of nanometer size, which can be classified as Faradaic and non-Faradaic processes. In case of the former, the charge is exchanged between the electrolyte/electrode interface by electron transfer processes, causing redox reactions. In case of the latter, there is no charge transfer between the interface, and redox reactions do not occur, but this still involves the charging and discharging of the interface, the so-called diffuse double layer. These processes are exploited in applications such as metrology (electroanalytical sensors), energy storage (e.g. supercapacitors, fuel cells), scanning probe microscopy, molecular electronics (single molecule electrochemistry) and biological studies on cells. The exploitation of nanoelectrode properties depends critically upon reliable and robust electrode fabrication of well-defined electroactive area geometry whose size approaches molecular dimensions. 2D materials, such as graphene, offer new possibilities in terms of nanoelectrode design and electrode material thickness on the nano regime. Graphene, with a monoatomic layer of 0.344 nm, offers easy fabrication, high conductivity and high electrochemical activity – mainly on the edge sites. The electrochemistry of carbon-based materials is well-known and utilised for sensor and battery applications, whereas new studies on the physical properties of graphene have paved the way for its use in electronics applications, amongst other fields. The electrochemistry of graphene, however, has not been studied as extensively. Thus, the aim of the investigation presented in this thesis was four fold; to optimise graphene ring nanoelectrode (GRiN) / graphene ring microelectrode (GRiME) electrode fabrication by further decreasing the electrode thickness and exploiting the ring geometry and electrical properties of graphene; to characterise the electrode material and devices using physical and electrochemical techniques; to study the electrochemical performance of the device by investigating the electron transfer processes taking place at the nanoelectrode surface; and to test the device in the application of electroanalytical sensor for the detection of mercury. Based on the design of a thin ring nanoelectrode, the graphene ring nanoelectrode (GRiN) is a novel device which permits the exploitation of graphene edge sites as an electrode material to study electron transfer processes in the nano regime. The GRiN is also the first instance in which a solely-graphene-based nanoelectrode is applied for the detection of mercury as well as for the study of the heterogenous electron transfer rate constant (k0) for the outer-sphere redox probe of FCA-/FCA0. The fabrication and characterisation of Graphene Ring Nanoelectrodes (GRiNs) at size ranges below 10 nm is presented. Graphene is obtained from chemically exfoliated graphite oxide (GO) prepared by a modified Hummers’ method. GO is dipcoated from colloidal GO solution onto optical fibre and the consequential GO layer was reduced to graphene by both chemical (hydrazine) and thermal treatments. Subsequent electrical connections and isolation of the device were then carried out. Fabricated GRiNs are found to be conducting and, through the application of electrochemical sizing methods, were proven to have ring electrode thicknesses in the sub-5 nm range, specifically between 0.5 and 1 nm. Atomic force microscopy was used for corroboration of the physical sizing of GRiNs. Calculations of the heterogeneous electron transfer rate constant at GRiNs for the outer and inner-sphere redox probes, FCA-/FCA0 and Fe(CN)63-/Fe(CN)64-, respectively, have been measured over a different range of electrolyte concentrations and different electrode thicknesses. This demonstrates the effects of accessing the critical dimension of the nanoelectrodes on Faradaic electrochemistry. An application of GRiNs explored in this work is in the area of ion-selective electroanalytical sensors of environmentally important heavy metals, such as mercury, present in drinking water using Differential Pulse Stripping Voltammetry (DPSV). The capability of GRiNs to detect mercury at ppb levels is explored. .