Electrochemistry of metal complexes and their use in amperometric sensors

This thesis concerns the utilization of metal complexes in amperometric sensors. Chapter One provides a general introduction to the area. The electrochemical theories relating to the development and use of amperometric sensors, are described, and applications for such sensors are outlined. These include trace element analysis for environmental and clinical use and the determination of NADH for the detection of clinical analytes. In Chapter Two, the electrochemical changes occurring in a ligand upon complexation, are examined as a possible method of selective metal ion detection. Screen-printing is used to produce disposable, single-use electrodes modified with the ligand bis- cyclohexanone oxaldihydrazone. At +250 mV vs SCE, the electrodes give a linear response to copper(II) across the range 30-300 pM (r = 0.983, n= 13). The effect on the electrode response of variations in pH, temperature, ligand content and storage time are outlined; as well as the effect of competing cations. In Chapter Three, pre-formed metal complexes are used as electron-transfer mediators. Part I considers homogeneous mediation from the enzyme NADH oxidase, using the Ru(lII/IV) redox couple. A scheme for enzyme amplification of the NADH response is outlined using alcohol dehydrogenase (ADH) and NADH oxidase. Additionally, ethanol determination is performed using an ADH/NADH oxidase bilayer. In Part 11, mediator immobilisation is examined using a novel ion-exchange/hydrogel composite (Nf/PVA). The structure of the composite is investigated by following the diffusional characteristics of both hydrophobic and hydrophilic mediators, incorporated within the film. An analytical application of the Nf(PVA layer is illustrated following the co-immobilisation of a mediator with glucose oxidase. The effect of protein adsorption onto the composite is also examined. Part HI of Chapter Three considers a possible alternative to mediated electrocatalysis, by using an electro- deposited film of poly(indole-5-carboxylic acid) (PICA). The overpotential. for the oxidation of ascorbate and NADH is lowered, apparently without the action of a redox mediating species. Strategies for the development of a PICA-based biosensor are outlined. Chapter Four provides an overview and general discussion of the experimental results and suggests areas for further work. These include further improvements to the design of the screen-printed electrodes in Chapter 2; the preferable choice of mediator for the immobilisation matrix in Chapter 3 Part 11, as well as possible methods of improving the biocompatability of the matrix; and a possible route to the immobilisation of NAD,, for the PICA-based system described in Chapter 3 Part III

Straightforward Immunosensing Platform Based on Graphene Oxide-Decorated Nanopaper : A Highly Sensitive and Fast Biosensing Approach

Immunoassays are nowadays a crucial tool for diagnostics and drug development. However, they often involve time-consuming procedures and need at least two antibodies in charge of the capture and detection processes, respectively. This study reports a nanocomposite based on graphene oxide-coated nanopaper (GONAP) facilitating an advantageous immunosensing platform using a single antibody and without the need for washing steps. The hydrophilic, porous, and photoluminescence-quenching character of GONAP allows for the adsorption and quenching of photoluminescent quantum dots nanocrystals complexed with antibodies (Ab-QDs), enabling a ready-to-use immunosensing platform. The photoluminescence is recovered upon immunocomplex (antibody-antigen) formation which embraces a series of interactions (hydrogen bonding, electrostatic, hydrophobic, and Van der Waals interactions) that trigger desorption of the antigen-Ab-QD complex from GONAP surface. However, the antigen is then attached onto the GONAP surface by electrostatic interactions leading to a spacer (greater than ≈20 nm) between Ab-QDs and GONAP and thus hindering nonradiative energy transfer. It is demonstrated that this simple-yet highly sensitive-platform represents a virtually universal immunosensing approach by using small-sized and big-sized targets as model analytes, those are, human-IgG protein and Escherichia coli bacteria. In addition, the assay is proved effective in real matrices analysis, including human serum, poultry meat, and river water. GONAP opens the way to conceptually new paper-based devices for immunosensing, which are amenable to point of care applications and automated diagnostics