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This thesis presents the potential sensing applications of fibre Bragg gratings in polymer optical fibres. Fibre Bragg gratings are fabricated in different kinds of polymer optical fibres, including Poly methyl methacrylate (PMMA) and TOPAS cyclic olefin copolymer based microstructured polymer optical fibres and PMMA based step-index photosensitive polymer optical fibre, using the 325nm continuous wave ultraviolet laser and phase mask technique. The thermal response of fabricated microstructured polymer optical fibre Bragg gratings has been characterized. The PMMA based single mode microstructured polymer optical fibre Bragg gratings exhibit negative non-linear Bragg wavelength shift with temperature, including a quasi-linear region. The thermal sensitivity of such Bragg gratings in the linear region is up to -97pm/°C. A permanent shift in the grating wavelength at room temperature is observed when such gratings are heated above a threshold temperature which can be extended by annealing the fibre before grating inscription. The largest positive Bragg wavelength shift with temperature in transmission is observed in TOPAS based few moded microstructured polymer optical fibre Bragg gratings and the measured temperature sensitivity is 250±0.5pm/°C. Gluing method is developed to maintain stable optical coupling between PMMA based single mode step index polymer optical fibre Bragg gratings and single mode step index silica optical fibre. Being benefit from this success, polymer optical fibre Bragg gratings are able to be characterised for their temperature, humidity and strain sensitivity, which are -48.2±1pm/°C, 38.3±0.5pm per %RH and 1.33±0.04 pm/µε respectively. These sensitivities have been utilised to achieve several applications. The strain sensitivity of step index polymer optical fibre Bragg grating devices has been exploited in the potential application of the strain condition monitoring of heavy textiles and when being attached to textile specimens with certain type of adhesives. These polymer fibre Bragg grating devices show better strain transfer and lower structure reinforcement than silica optical fibre Bragg grating devices. The humidity sensitivity of step index polymer optical fibre Bragg grating devices is applied to detecting water in jet fuel and is proved to be able to measure water content of less than 20 ppm in Jet fuel. A simultaneous temperature and humidity sensor is also made by attaching a polymer fibre Bragg grating to a silica optical fibre Bragg grating and it shows better humidity measurement accuracy than that of electronic competitors.

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This thesis describes experimental work on electrochemical sensing mechanisms. Chapter 1 and Chapter 2 provide an introduction to electrochemical and surface science techniques as well as nano-carbon materials which are of interest in electroanalysis and sensing. Chapter 3 and Chapter 4 focus on electrochemical processes at liquid | liquid | electrode triple phase boundary systems. In Chapter 3 the electrochemical behaviour of CoPc (cobalt phthalocyanine) dissolved into an organic water –insoluble liquid and deposited as microdroplets on a graphite electrode is studied. Both cation and anion transfer are observed at the liquid | liquid phase boundary. Chapter 4 describes redox processes of a highly hydrophobic anthraquinone derivative where preferential transfer of protons and pH sensitivity are observed. Both systems, CoPc and anthraquinone derivative, are investigated towards CO2 sensitivity. In Chapter 3 and 4 graphite electrodes are employed, but in Chapter 5 graphitic carbon nanoaprticles are employed with a surface functionalisation to provide binding capability to DNA fragments. Layer-by-layer deposition of DNA-carbon nanoparticle composite film electrodes is demonstrated and the electrochemical properties of the films are investigated. A novel type of DNA hybridisation sensing mechanism based on a nano-gap generator – collector electrode system is proposed. Chapters 6 and 7 are dedicated to gas sensing with a novel electrochemical system based on ionomer spheres in contact to the working electrode. In Chapter 6 Dowex ionomer particles are impregnated with carbon nanoparticles which are functionalised with DOPA to provide redox activity and Faradaic current responses. The effect of ionomer type and gas composition is studied. In Chapter 7 Prussian blue nanoparticles are immobilised onto the ionomer particle surface to provide a sensing system with peroxide sensitivity. Overall, this thesis contributes to sensing of bio-molecules and of gases. By introducing new types of interfaces (triple phase boundary, ionomer contacts, carbon nanoparticle redox systems) it is shown that sensitivity and selectivity can be tailored. In future these types of sensor prototypes could be further developed for specific applications.

This thesis proposes a novel form of MEMS (micro electro mechanical system) resonant pressure sensor. The sensor incorporates a MEMS silicon resonator that is coupled to a macro scale metallic diaphragm. The combination of a large metallic diaphragm with a micro engineered MEMS resonator required an innovative selective strain coupling structure to be designed. Without the proposed coupling structure, unwanted packaging or thermally induced strain would severely degrade sensor performance, resulting in resonator misalignment, or unwanted strain loading. The feasibility of the proposed device is investigated though analysis and characterisation work performed on a prototype resonator, and a prototype selective coupling structure. These devices allowed fabrication processes to be developed, and their predicted performance to be verified. The prototype resonator demonstrated high quality factors of 28,000 - 30,000 in a vacuum, and resonant frequencies within 10% of modelled predictions. The selective coupling structure was shown to provide a reduction of resonator misalignment of 100: 1, while providing selective strain coupling of 900:1. Following this a fabrication route for producing the fully integrated resonator and selectively coupling structure was developed. This device was electrically tested to determine its resonant characteristics, and quality factors greater that 40,000 were demonstrated.

Aptamers are short single-stranded DNA or RNA oligonucleotides artificially selected against specific targets. They exhibit high target binding affinity and exquisite specificity, making them very useful in developing biosensors for a wide range of targets, such as proteins, peptides, amino acids, drugs, metal ions and even whole cells. While fluorescent semiconductor nanocrystals, also known as quantum dots (QDs), have unique size-dependent, bright and extremely photo-stable fluorescence that make them as excellent fluorescent labels for biological imaging, sensing, cell tracking/trafficking and diagnostics. Their unique optical properties (broad absorption and narrow symmetric emission) are well-suited for FRET (fluorescence resonance energy transfer) based sensing applications. By combining the advance properties of both aptamers and QDs, this project aims to develop a sensitive, specific and robust aD-DNAlaptamer FRET based biosensing technology that can be used for rapid biosensing, diagnostics and environmental monitoring. A major hurdle here is the preparation of a compact, stable and water-soluble QD-bioconjugate that can effectively resist non-specific adsorption because FRET efficiency (E) decreases dramatically with the increasing donor-acceptor distance. Hence for high sensitivity, a compact QD-bioconjugate structure is essential. In this regard, a series of highly fluorescent, water-soluble CdSe/ZnS aDs were prepared first by ligand exchange with hydrophilic thiolated ligands, such as dihydrolipoic acid (DHLA), glutathione (GSH), dihydrolipoic acid-polyethyleneglycol (DHLAPEG600) derivatives. These QDs exhibited high fluorescence quantum yields (QYs, 6-30%), comparable to commercial water-soluble aDs (ca. 30%), but having significantly smaller hydrodynamic diameters «10 v.s. > 25 nm). Building upon these, three different QD-DNA aptamer sensing systems have been developed successfully: (1) A simple self-assembled aD-DNA system: I have found that thiolated DNA can self-assemble onto DHLA capped QDs to form compact, functional QD-DNA conjugates with small donor-acceptor distances, producing efficient FRET (E > 70%) at a relatively low (target: QD) copy number of 6:1. The resulting self-assembled aD-DNA (aptamer) conjugate has been used to detect low nM levels of labelled DNA target via QD sensitised dye FRET signal. Moreover, it has been successfully used for detection of nM level of a protein target via the encoded DNA aptamer sequence, although its specificity and stability still need further improvement. (2) A more stable and sensitive aD-dual-donor FRET sensing system based on an amine-modified DNA covalently coupled to a glutathione capped aD combined with the use of specific ethidium bromide (EB) intercalation in hybridized DNA duplex. As a result, both the aD and intercalated EBs can FRET to the dye acceptor appended to the complementary DNA, leading to significantly improved the overall FRET efficiency E, and hence sensitivity in both DNA and protein target detection down to pM level. (3) A Cu-free "clicked", robust, and_specific QD-DNA aptamer sensor. A compact, functional aD-DNA conjugate was prepared via the Cu-free "click chemistry" (CFCC) between a dihydrolipoic acid -polyethylene glycol-azide (DHLA-PEG600-N3) capped aD and a cyclooctyne modified DNA. The resulting QD-DNA conjugate is highly stable in biological buffer, effectively resisting nonspecific absorption, displaying a relatively small size (hydrodynamic radius - 5 nm) and retaining almost the native ay of the parent aD. Moreover, the CFCC based DNA conjugation method is also highly efficient, leading to high DNA loading (- 15-30 DNA strands per aD is readily achieved). This system is well-suited for robust biosensing: it can quantitate pM level of complementary DNA targets with SNP (singlenucleotide polymorphism) discrimination ability in complex media, e.g. 10% human serum. It can also detect pM level of a specific protein via the encoded DNA aptamer sequence. Compared with these approaches, the self-assembled system is the most convenient to prepare, but it has the least stability and cannot resist nonspecific absorption. The dual-donor FRET sensing system shows some enhancement on the stability and resisting nonspecific absorption, but it still cannot work in complex media, such as serum. The CFCC clicked QD-DNA aptamer sensor shows the highest stability, specificity and assay robustness, and can effectively work in clinical media. It can be readily extended to design sensors against other targets by simply clicking on specific aptamer sequences against such targets. Because the CFCC clicked QD-DNAlaptamer sensor shows high stability, specificity, robustness· and sensitivity, it may have a wide range of biosensing and diagnostic applications

A wireless sensor network (WSN) consists of spatially distributed autonomous devices using sensors to cooperatively monitor physical or environmental conditions, such as pressure, temperature, sound, vibration or motion at different locations. The development of wireless sensor networks was originally motivated by military applications such as battlefield surveillance. However, wireless sensor retworks are now used in many commercial applications, including environment and habitat monitoring, healthcare applications, home automation, and traffic control. The physical size of these devices is shrinking due to advances in semiconductor technology. The main challenge is to produce low cost and miniature sensor nodes. Energy is the scarcest resource for these nodes as it determines the WSN lifetime. Since these nodes will be deployed close together to form a dense wireless network the received signaI to noise ratio at any instant of time not only depends on physical channel (i. e. path loss and fading) but also on various design parameters like CSMA/CA inhibition threshold, polarization, deployment strategy and node density. This thesis characterises the propagation channel for miniature wireless nodes. A characterization of the short range (<10cm), narrowband, wireless channel, appropriate to a dense network of wireless transceivers operating in the 2.4 GHz ISM band, is presented. Transmission loss measurements have been made in the laboratory at 2.45 GHz and a fading model derived. Aggregate interference due to neighboring carrier-sense-multiple-access (CSMA) nodes has been calculated. The resulting dependence of signl-to-interference ratio (SIR) on node density is presented to allow density dimensioning. Cumulative distributions of SIR have been used to establish performance statistics for example modulation and detection schemes. A simulation model has been developed to characterise the physical link experienced by these networks.

A saccharide sensor was fabricated using an AlGaN/GaN heterostructure structure and a boronic-acid containing receptor. Parallel to this, photoelectrochemical (PEC) etching of Gallium Nitride (GaN) was employed both as a nanostructuring technique and as a method of rapid dislocation density enumeration. The device fabrication techniques necessary for the development of devices based on n-type GaN and its alloys were successfully implemented. A robust photolithographic mask capable of producing a variety of transistor and sensor structures was designed and fabricated. Surface Charge Lithography (SCL) was studied and implemented as a technique for the nanostructuring of n-type GaN. In contrast to previous work (where patterns with feature sizes down to 100nm were created¹), several shortcomings of the technique were noted and subsequently investigated: a failure to achieve the intended minimum feature size; elongation of features in the direction of FIB instrument rastering; and the loss of thin features perpendicular to the rastering direction. A pattern design scheme to overcome these shortcomings was proposed along with experimental improvements expected to alleviate such issues. A novel receptor molecule employing a phenylboronic acid (BAT) was synthesised and used to functionalise an AlGaN/GaN FET device, thereby creating an electronic saccharide sensor device. The response of the sensor to a panel of saccharides (fructose, galactose and glucose) was investigated, with the order of response confirming previous findings (decreasing from fructose to galactose to glucose). The device was found to have good stability prior to failure, indicating that this type of sensor device shows a great deal of potential for wider use. PEC etching was used as a technique for determining the dislocation density in two distinct GaN on sapphire structures with thick AlN buffer layers. The SRI sample (100nm of GaN with 500nm of AlN) had a dislocation density of 1.9±O.2xl0⁹cm-², while the SH2 sample (500nm of GaN with lOOnm of AIN) had approximately twice this density, at 3.8±O.2xl0⁹cm-². The differences are thought to primarily arise from the difference in AlN thickness.

Online test is a technology that has great importance and value in many applications. It provides unique benefit of real-time condition monitoring, based on which early warning of system failure or degradation and potentially system self-repair can be achieved to avoid more significant impact. In the context of on-line test, embedded off-line test is still a challenging issue for MEMS device, mainly due to their micro scale and what is typically multi energy domain functionality. There are very limited methods to observe and characterize the MEMS components under test, especially for the dynamic behavior. It is also in general more difficult to verify their performance under real working conditions. This work attempts to deliver real online test solutions and methodologies which can be adapted to wide range of MEMS devices. Novel techniques of Bias-Superposition and Multi-Mode Sensing are conceived and engineered with due consideration of the availability of on board resources (e.g. signal processing power) and design/cost overhead. The proposed techniques are demonstrated on two existing MEMS systems: a capacitive MEMS accelerometer and a resistive MEMS based conductance sensor. The concepts are further studied through the design of a piezoresistive multi-functional humidity/pressure sensor. By developing the theoretical model and applying the proposed methodologies on these different structures and applications, this work pushed the State-of-the-Art of micro & nano system's online test in three areas: firstly the work potentially enabled many existing electrical induced off-line test so lutions for MEMS systems to be transfonned to on-line; secondly an on-line test solution for electro-chemical microsystems has been conceived which by traditional methods is difficu lt to implement; last but not least the work advanced from on-l ine test to fault-to lerance within NJEMS sensor systems through resilience of the sensor network.

Research into chemical sensors, and in particular colourimetric sensors, is an area of increasing interest. The instant access to important chemical information that such devices can bring is their most attractive feature. The aim of this project has been to develop such colourimetric indicators and to research their behaviour in detail such that their response to different environments can be known, and their formulation altered to suit the application.