Abstract
Wearable healthcare devices are becoming increasingly important. They monitor health status indicators at molecular levels in real time, thus affording efficient health management. In this context, enzyme-based electrochemical sensors can play a pivotal role, given their simplicity, high sensitivity, and miniaturisation capability. Bioelectronics that have enzyme elements immobilized at the transducer surface are found to be potent in wearable healthcare device construction owing to their great specificities and high incompatibilities.Many different types of nanomaterials, such as osmium hydrogels, carbon nanotubes (CNTs), and metal oxides, have been used for enzyme immobilisation. The immobilisation strategies are predominantly via cross-linkage, often involving a long preparation procedure for electrode doping and modification. Although they are found effective, the efficient manufacturing procedure makes it less cost-effective and suitable for upscaling productions. This thesis presents a novel method that significantly simplifies the bioelectrode fabrication process but also exhibits promising biosensing activity based on 3D soft nanomaterials known as lipid cubic phases (LCP).
A LCP is a self-assembled structure formed by type-II amphiphilic molecules in excess aqueous conditions. It is a form of lyotropic liquid crystal (LLC), whose phase behavior depends on temperature and hydration. In this thesis, we used the LCP structure of 1-monoolein (MO) as a host medium to entrap redox-active enzymes in meso at transducer surfaces. We functionalised LCP by doping it with a redox-active surfactant (dodecyl(ferrocenylmethyl)dimethylammonium bromide (Fc12-Br) within the structures. The additional component serves as a redox mediator to catalyze the enzymatic reactions. The hydrocarbon chain of Fc12 is inserted at the lipid/aqueous interface but leaves the polar head exposed to the aqueous environment.
In Chapter 2, we first characterised the Fc12 doped LCP using electrochemical and structural methods, followed by demonstrating the feasibility of using this redox-active LCP system as a host medium for biosensing activities. A model enzyme, glucose oxidase (GOx), was employed and entrapped within the redox-active LCP. GOx entrapped within Fc12/MO followed Michaelis-Menten kinetics towards glucose with a KM and Imax of 8.9 ± 0.5 mM and 1.4 ± 0.2 μA, respectively, and a linearity range of 2-17 mM glucose.
In Chapter 3, the long-term stability of the redox-active LCP system was studied. After identifying the underlying activity decay mechanism using in-situ SANS experiments, a new analogue Fc12-PF6 was selected, forming a new redox-active LCP. The resultant system hosting GOx to undergo glucose sensing was studied. This shows significantly improved stability (>80% glucose sensing activity is maintained after 20 days) and enhances ameprometric signals: a current density four times greater than that of the system reported in Chapter 2 is obtained. Fc12-PF6/MO/GOx composites pave the way for promising applications in enzyme-based biodevices.
In Chapter 4, we developed a self-powered glucose sensor based on the Fc12-PF6/MO/GOx and a well-understood Pt nanowire network reported by the Squires group. This forms a glucose/O2 hybrid enzymatic fuel cell. The power harvesting performance at varying physiological conditions was studied, followed by testing the self-powered glucose sensing activity at the relevant conditions.
We conclude the thesis by presenting a novel smart material derived from LCP. Unlike the redox-active LCP systems reported in previous chapters, the smart material exhibits pHresponsive activity. The materials undergo reversible phase transitions with different topology and symmetry, which in turn have very different physical properties – viscosity, diffusion, and optical transparency. This is attributed to the pH-dependent phase behaviour of the oleic acid (OA) doped MO LCP. After integrating the pH switch-triggering reaction into the LCP, a temporally controlled transition between the cloudy hexagonal phase (HII) and transparent LCP is observed. This forms a proof-of-concept demonstration for a wider range of soft materials with time-programmable changes in physical properties.
| Date of Award | 26 Jun 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Adam Squires (Supervisor) & Mirella Di Lorenzo (Supervisor) |