Numerous reports have been published detailing a wide variety of strategies for the production of many different prototype screen-printed biosensors, hmvever, few of these devices have been developed to the commercialisation stage. There is an unquestionable need for disposable biosensors suitable for decentralised analysis that can be mass-produced at low cost by a simple process; screen-printed carbon electrodes (SPCEs) fulfil both of these criteria. Conventional methods for producing biosensors based on this technology usually involve the deposition of a biological recognition element (typically an enzyme) onto a SPCE which has been printed using an organic solvent-based ink. The removal of organic solvents from the manufacturing process is a highly desirable goal as it should result in improved health and safety and also the possibility of incorporation of enzymes directly into the ink. The latter is difficult to achieve with conventional screen-printing inks as enzymes are inactivated by both the organic solvents themselves and the elevated temperatures required in the curing step. The studies described in this thesis utilise a screen-printing ink which incorporates a water-based binder and the electro catalytic mediator cobalt phthalocyanine (CoPe.) It is demonstrated that the addition of different oxidase enzymes directly into this ink allows for the one-step manufacture of biosensors with desirable performance characteristics, notably high precision and outstanding stability. A water-based carbon ink incorporating CoPC was used to produce robust and precise SPCEs which were found to act as effective sensors for H20 2• The sensors were operated in stirred solutions at an applied potential of +0.5 V, which was shown to be a significant reduction in the potential required for H20 2 detection at an un-modified electrode. The ink was modified further by adding glucose oxidase (GOD) to its bulk prior to printing. This allowed for the one-step printing of glucose biosensors which dried at room temperature. These biosensors were investigated using amperometry in stirred solution which revealed long-term operational stability and a shelf-life of at least 18 months. The analytical signal was shown to arise from the electro catalytic oxidation of the H20 2 produced by the enzyme in the presence of glucose and O2. Using these biosensors with a background correction technique, it was possible to determine the concentration of glucose present in a bovine serum sample with a good degree of both accuracy and precision. This demonstrated that the newly-developed glucose biosensors were capable of operating reliably in a biological sample. In order to extend the linear range, the ink composition was re-formulated and chronoamperometry was used as the measurement technique. The sensitivity of these new glucose biosensors was found to be comparable to that of the earlier system, and the upper limit of the linear range was successfully extended. A simple method for interference removal was developed, which involved the use of 'dummy' sensors that did not contain any enzyme. Using this system, it was possible to quantify glucose in dilute human plasma samples which had been spiked with glucose in order to represent diabetic samples. The generic nature of the CoPC modified water-based ink was illustrated by adding a different enzyme, lactate oxidase (LOD,) which is known to be much more delicate than GOD, into the ink. Even under un-optimised conditions, the LOD-containing biosensors gave a measurable response to lactate over a clinically useful range. Owing to their unusual properties, the study of materials with dimensions smaller than 100 nm is playing an increasingly important role in the development of biosensors. In an attempt to extend the linear range of the GOD-containing biosensors to higher glucose concentrations, the possibility of using nanoscaled cobalt phthalocyanine (n-CoPC) as a mediator was investigated. It was shown that the response of the n-CoPC containing sensors towards H20 2 was superior to those incorporating the bulk mediator. GOD was added to the n-CoPC modified ink, and the resulting glucose biosensors displayed a superior sensitivity and linear range to the bulk-CoPC containing biosensors produced earlier. The increased sensitivity was attributed to the increased sensitivity of the base transducer, and further experiments were conducted to determine the reason for the extended linear range. Remarkably, it was discovered that the n-CoPC modified biosensors were capable of operating in the absence of O2, which implied that the n-CoPC must be interacting in some way with the redox centre of GOD. Such direct electron transfer has been reported for biosensors incorporating other types of nanomaterials but, as far as is known, never for CoPC. In order to investigate the possibility of determining another clinically important analyte, cholesterol oxidase was introduced into the n-CoPC modified water-based ink. Although the resulting cholesterol biosensors did not display the same Orindependent operation, a good sensitivity and a linear response up to at least 2 mM cholesterol was achieved. Free cholesterol was determined in 40 dilute human plasma samples which had been spiked with cholesterol to represent a clinically useful range of total cholesterol concentrations; the results agreed well with a standard reference method (R2 = 0.95.)
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:574410 |
| Date | January 2005 |
| Creators | Crouch, Eric |
| Publisher | University of the West of England, Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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