The development of enzyme based printable glucose sensors

The main goal of this thesis is to develop an easy to fabricate and sensitive biosensor based on organic materials capable of monitoring saliva glucose concentration in people with diabetes. In Chapter 3 we focus on designing, fabricating and characterising flexible organic thin film transistor- (OTFT-) based sensors suitable for salivary glucose sensing. We employed different device architectures utilising poly-3-hexylthiophene (P3HT) as the semiconductor layer, dielectric layers of either poly(vinyl-pyridine) (PVPy) or poly(vinyl-phenol) (PVP) and poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) as a gate material, to produce trial OTFTs. Our results demonstrated that compared with the initial architecture of ITO/P3HT/PVPy/PEDOT:PSS, when PVPy was replaced with PVP the off current was increased. Nafion was chosen as an appropriate replacement for PEDOT: PSS in the final (ITO/P3HT/Nafion:GOX) sensor device due to the acidity of PEDOT:PSS, with the dielectric layer being removed to improve device response time. The mechanism of signal transduction in these devices is via protonic doping of the P3HT channel and thus acidic PEDOT:PSS leads to a large off current in the device. Upon the replacement of PEDOT:PSS by the Nafion:GOX mixture, a working prototype sensor was produced of architecture ITO/P3HT/PVP/Nafion:GOX. Chapter 4 focuses on the establishing the mechanisms behind the formation and the effect of mixed interlayers between the Nafion proton transport layer and P3HT semiconductor material. Surprisingly high conductivity was obtained for P3HT/Nafion bilayers, in excess of the native conductivity of either pristine material, due to intermixing of the materials and doping of the P3HT. Our results suggested that the annealing condition giving the best device performance is a postproduction treatment at 50 degrees Celsius. A full study of the effect of thermal annealing and the addition of water to pristine P3HT and Nafion and bilayers was undertaken. Chapter 5 explores the use of a porous capping layer to encapsulate GOX in the device and control the volume and location of added analyte. The phase inversion technique was used to produce the porous polyacrylonitrile (PAN) films for this purpose. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques were employed to investigate the resultant membrane morphology of the PAN films. Our results show that the PAN films are highly porous and suitable for the capping application and ITO/P3HT/Nafion:GOX/PAN devices showed improved sensitivity to glucose in to the range of salivary glucose levels (SGL) in humans. Finally, in Chapter 6, the device architecture was redesigned to incorporate a non-GOX containing reference sensor in an attempt to mitigate device-based variation. Sources of sensor output variation is discussed and we observe that the addition of a reference sensor seems to merely add an additional source of variance. Chapter 7 summarises results and discusses potential further studies.