Studies On Self Assembled Monolayers On Gold By Electrochemical Methods

Author : Cherie Tollemache
Publisher :
ISBN 13 :
Total Pages : 305 pages
Book Rating : 4.:/5 (124 download)

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Download or read book Evaluation of Mixed Alkanethiolate Self-assembled Monolayers on Gold Electrodes for Biosensor Applications written by Cherie Tollemache and published by . This book was released on 2020 with total page 305 pages. Available in PDF, EPUB and Kindle. Book excerpt: It is important to develop highly selective and sensitive biosensors that can reproducibly detect a target analyte in a rapid and cost-effective way. The creation of alkanthiolate self-assembled monolayers (SAMs) is a common approach to functionalise gold electrode materials for biosensor applications. This thesis evaluates a SAM-based electrochemical biosensor design proposed by a small engineering firm (the Company) and optimise the surface functionalisation methodology and bioassay conditions towards detection of progesterone for agricultural applications. Chapter 1 presents a general introduction to the field of biosensors and the design considerations that are essential to the functioning of biosensors. Recent examples of approaches to detecting analytes in low concentration in complex biological samples using elegant biosensor designs are surveyed. The use of alkanethiolate self-assembled monolayers as a method for surface functionalisation is introduced. The impact of the methodology for SAM formation on the properties and function of the modified surface is examined. The use of this surface functionalisation strategy to create electrochemical biosensors is discussed and the objectives of this thesis are outlined. Chapter 2 describes the methodology used for the experimental work performed in this thesis. The surface characterisation techniques used in the thesis are outlined and the relevance of these techniques for determining the reproducibility, composition and quality of mixed alkanthiolate self-assembled monolayers as functional surfaces for biosensor applications are evaluated. Chapter 3 describes a series of experimental measurements designed to evaluate the sensor electrode system designed by the Company. The experiments perfomed examined the SAM deposition methodology effects on the biosensor response including experimentation with diluent chain length and ratio of adsorbates in solution. The reaction conditions for attachment of DNA probes to the SAM coated surfaces were explored and experiments regarding calibration of the sensor response and detection of progesterone in buffer were performed. It was found that the sensor design provided measurable signals that were affected by changes in the SAM deposition conditions and attachment chemistry method, however, there was an unacceptable level of variation in sensor response in every experiment. A series of issues were identified in both the biosensor architecture and the surface chemistry which led to the irreproducible bioassay results reported. These limitations in sensor design are discussed and suggested improvements are outlined. Chapter 4 describes experiments designed to examine some of the hypothesised surface chemistry contributions to the irreproducibility of sensor response discovered in Chapter 3. The limitations in device architecture were mitigated by switching to an alternative electrode design and the methodology of mixed SAM deposition and subsequent reaction chemistry is examined in detail. A systematic study of the electrochemical properties of mixed SAMs incorporating carboxylic acid or azide functional groups mixed with hydroxyl or methyl terminated diluents with different chain lengths is reported. The ratio of the functional to diluent molecule is varied and short and longer deposition times are compared. The amide bond coupling to mixed SAMs with carboxylic acid functionality and click chemistry reactions with azide terminated mixed SAMs were examined by the attachment of ferrocene derivatives to the SAMs. This was also used as a means of evaluating the compositional reproducibility of the mixed SAMs and the reproducibility of surface reactions for these different systems. Chapter 5 presents preliminary results for a novel methodology for creating mixed SAMs from thioacetyl-protected adsorbate species that occurs on a significantly faster time-scale than the methods used in Chapters 3 and 4. The purpose of this work was to evaluate this method compared to the more conventional methodology as a possible approach for creating mixed SAM modified surfaces of a controllable composition and quality for commercial biosensor applications. The new method involves coadsorptiopn of alkanethiolates generated in situ by deprotection of thioaceytl-protected adsorbates under basic conditions. This proof-of concept study examines some of the factors that influence the SAM formation under these new deposition conditions including solvent effects and the use of adsorbate molecules with hydrophilic and hydrophobic terminal groups. Evaluation of the reproducibility of this method is performed in two ways, with one researcher performing the experiments multiple times and a comparison between two different researchers performing the experiments. Chapter 6 provides a brief summary of the conclusions determined from the experimental results and literature analysis with emphasis on the broader context of the commercial objectives of this thesis. This chapter also provides a discussion on future experimentation that could be performed to improve the understanding of the mixed SAMs systems studied. Experimentation with SAM deposition methodology and reaction chemistry to further functionalise electrode surfaces towards biosensor applications did not lead to discovery of a system that is sufficiently reproducible for the intended application. The use of SAMs on gold prepared by methods not explored in this thesis may provide the reproducibility in electrochemical properties that is required for electrochemical biosensors. A company aiming to create a biosensor for commercial use would be advised to explore other methods of surface functionalisation or design a bioassay/device architecture that does not rely so heavily on precisely reproducible surface functionalisation.