Abstract
This thesis introduces the concept of using carbon nanotubes (CNT) as a novel bio-sensor and demonstrates the feasibility of incorporating these sensing elements with a Surface Acoustic Wave (SAW) device to enhance the functionality for bio-sensing applications. SAW devices have an inherently high quality factor "Q" and frequency selection properties that complement the state-of- the-art Multi Wall CNT sensors used for this application. The coupling of these two novel technologies provides an ideal platform for continuing to develop a fully integrated bio-sensor using R.F backscatter interrogation. Preliminary analysis provides us with MWCNT integrated by the way of a LC modulator to SAW devices which operate wirelessly and passively at a frequency of 2.63SGHz, where the CNT based dry electrode sensors perform with enhanced performance on porcine skin. The CNT's were precisely grown on Ni catalyst with diameters of c.a. 40nm at a rate of 260nm/min. This ensures the CNTs could punch through the dead skin layer and probe the bio-potentials as demonstrated in solution and porcine skin. The demonstration of the graphene boundary spacing of -3.38A obtained by HR-TEM and the ripples due to the boundaries at ~290-300eV as indicated by EELS confirm multiwall growth of the CNTs. The use of MWCNT based arrays for sensing facilitates the use of said sensors without the need for a Sodium Chloride based gel. The sensitivity of the MWCNT to ionic charge exchange, as expected in the human system, was enhanced by the application of an AgCl layer to the CNT. Elemental analysis of the surface of the sensors by use of EDX indicated transition peaks associated with Ag and Cl. EELS complemented the results indicating the M4 and M5 excitation edges for Ag and 2s level excitation peaks for Cl. Chemical analysis by XPS indicated AgCl formation by the binding energy peaks of the elements correlating to AgCl. The comparative studies carried out with commercial ECG/EEG sensors indicate excellent response and remains stable through the bio-potential detection frequency range of 20 - 80Hz. Thus, the work extends the current knowledge in deployable high sensitivity passive CNT nano sensors transfixed to a SAW structure, which will enable wireless connectivity. The SAW structure enabled a sensing platform merged with a unique ID capability utilising the inherent 3dB power split seen in IDT structures on a piezoelectiic substrate, which was varied using time domain VNA measurements. Medical applications in the form of ECG and EEG sensing have been the core focus of the apparatus, which can be extended for any bio-sensing need such as EMG. The sensors have a sensitivity range defined by a simple LC circuit, and thus can be extended to EEG or any other CNT based bio-sensing paradigm. The technology promises exceptional size reductions in comparison with current cutting edge wireless ECG and EEG sensors. This is accomplished by the use of an inductor, diode, SAW device, sensor and antenna in a tightly coupled passive system. Further attractions of the technology presented include the redundancy of servicing and low cost of mass production. On the negative side, the current cautionary advice issued by the HSE on the use of CNTs will need to be addressed.