Abstract
Organic π-conjugated materials are of importance for the fabrication of devices for biomedical applications due to the bio-compatibility when interfaced with biological tissues and the ability to sense gases at room temperature. Π-conjugated materials have a molecular arrangement with alternating single and double bonds. This arrangement provides the ability for electrons in the π-orbitals to delocalize. These properties provide the ability to control the optical and electronic properties of organic π-conjugated materials. Investigation of the characteristics of π-conjugated materials is of importance for the development of organic devices. In this context, sensors and prosthetic devices with organic π-conjugated materials are of interest for the use in biomedical applications. This PhD thesis demonstrates the development and characterization of devices with organic π-conjugated materials for the detection of NH3 gas and for the use in artificial retina prosthetic devices. The thesis encompasses all aspects from the fabrication to the characterization of the devices. In chapter 1 the motivation, the thesis aim and the scientific collaborations are provided. In chapter 2 the state of the art regarding π-conjugated materials (π-conjugated materials for ammonia sensing and photoactive π-conjugated materials for artificial retinas) are presented as well as the possibility to use these materials in biomedical applications by discussing the presence of ammonia gas in the body in health and disease (ammonia gas sensors), and to explore the retina in health and disease (artificial retina prosthetic devices) is included. In chapter 3 the methodology regarding the substrate preparation, the deposition methods, the characterisation of photo-active π-conjugated materials and the electrical characterisation of NH3 sensors are displayed. The first part (chapter 4) of this PhD research work focuses on the fabrication of diodes with a semiconducting polymer poly(3-hexylthiophene-2,5-diyl)(P3HT) sensing layer. Polystyrene nanoparticles were used as shadow mask to create multiple vertical diodes in an aluminum top electrode to facilitate the diffusion of ammonia gas onto the P3HT sensing layer. This devices structure was fabricated on two types of substrates (rigid glass and flexible PET). Flexible substrate was used to enable comfortable sensing for in vivo monitoring of temperature and NH3 gas. The multiple vertical diode structure was able to reach a sensitivity of 1°C in the body temperature range from 36 to 41°C. However, when detecting the response towards NH3 no clear differences in current could be obtained as high-resolution measurements would be needed to eventually distinguish the response towards NH3 from a sensor drift present during the measurement. The second part (chapter 5) of this PhD research focuses on the development of chemiresistive gas sensors consisting of screen printed silver interdigitated electrodes (IDEs) on a flexible PI substrate. A comparative study of five chemiresistive gas sensors sensing materials (namely single wall carbon nanotube SWCNTs/IDEs, and P3HT/IDEs as controls, and three configurations combining the two sensing materials in the following configurations SWCNTs/P3HT/IDEs, SWCNTs/IDEs/P3HT, and P3HT/SWCNTs/IDEs) were fabricated and investigated in terms of sensor response, recovery, and response times towards different concentrations of NH3 (5, 25 and 50 ppm). Especially, P3HT/SWCNTs/IDEs sensors showed an improvement in response towards NH3 and a complete recovery to the baseline after the exposure of NH3 was stopped. These results add a novel contribution to improve the performance of carbon nanotubes, as normally recovery to the baseline only is obtained by using external devices such as a heater to apply heat to the CNTs, facilitating the removal of NH3 gas molecules. Furthermore, during the period abroad at Surrey University, an investigation of the use of π-conjugated nature derived pigments for the use in artificial retinas was performed (chapter 6). Photo-receptor cells, which are present in the retina of the eye, can be de graded in certain age related diseases (e.g. age-related macular degeneration). Replacement of the photo-receptors with prosthetic devices containing π-conjugated materials which have a similar peak absorbance wavelength (λ) as that of one of the photoreceptor cells (blue, green, red cones) can result in a return of vision when implanted. Six chromophore pigments derived from fruits and vegetables (raspberry, matcha, red onion, black grape, rhubarb, and aubergine) were tested in a bio-electrolytic photo-capacitor devices and investigated in terms of peak absorbance, responsivity, and pulsed light illumination towards different LED wavelengths (439, 532, and 592 nm) with varying light intensity powers (10, 20 and 30 µW mm−2 ). It was found that matcha could be a possible candidate to replace the functioning of the retina blue cone photo-receptor, while red onion was found suitable as a red cone photo-receptor material. Finally, in chapter 7 a summary of the PhD work and future perspectives are given.