Particle Sensor Using Solidly Mounted Resonators

This paper describes the development of a novel particle sensing system employing zinc oxide based solidly mounted resonator (SMR) devices for the detection of airborne fine particles (i.e., PM2.5 and PM10). The system operates in a dual configuration in which two SMR devices are driven by Colpitts-type oscillators in a differential mode. Particles are detected by the frequency shift caused by the mass of particles present on one resonator with while the other acts as a reference channel. Experimental validation of the system was performed inside an environmental chamber using a dust generator with the particles of known size and concentration. A sensor sensitivity of 4.6 Hz per μg/m3 was demonstrated for the SMRs resonating at a frequency of 970 MHz. Our results demonstrate that the SMR-based system has the potential to be implemented in CMOS technology as a low-cost, miniature smart particle detector for the real-time monitoring of airborne particles

GUMBOS- and Ionic Liquid-Coated Quartz Crystal Microbalance Sensors for Detection and Molecular Weight Determination of Organic Vapors

There has been an ever-increasing demand for the development of high-performance sensing devices for detection and discrimination of volatile organic compounds (VOCs) present in different environments. Among a number of sensing devices currently available, sorption-based sensors are particularly attractive because they are simple and inexpensive, require low power, and are appropriate for fabrication of multisensor arrays. A sorption-based sensor is comprised of a chemically active coating immobilized on the surface of a physical transducer. The chemically active film interacts with analytes, and the transducer converts the binding event into an electrical signal. This dissertation is focused on a sorption-based sensor prepared by using ionic liquids (ILs) and a group of uniform materials based on organic salts (GUMBOS) as the sensing materials and the quartz crystal microbalance (QCM) as the transducer. ILs are defined as organic salts which melt below 100 °C, and similar organic salts with melting point between 25 and 250 °C are defined as GUMBOS. In this research, a series of films comprising binary blends of an IL (or GUMBOS) and polymer are deposited onto the QCM surface in order to evaluate their vapor-sensing characteristics. The QCM sensors on exposure to organic vapors displayed a change in frequency and motional resistance, and both of these parameters were simultaneously measured. Examination of the data revealed an interesting relationship between the QCM parameters and the molecular weight of the absorbed vapors. The initial findings are reported in Chapter 2 of this dissertation. Additional studies were conducted in an effort to fully understand the interesting behavior of this type of material. More elaborate studies along with the theoretical rationale for the relationship between the QCM parameters and the molecular weight of vapors are presented in Chapter 3. Another important aspect of this dissertation is the design of highly sensitive materials for vapor-sensing applications. Toward this end, two representative GUMBOS were synthesized using porphyrin and phthalocyanine derivatives. The QCM device coated with these GUMBOS exhibited a rapid response and high sensitivity toward different organic vapors. Altogether, these studies demonstrate the true potential of this type of materials for vapor-sensing applications

Design and Implementation of QCM Virtual Sensing Schemes for Analyses of Volatile Organic Compounds

Sensor arrays have evolved as powerful approaches for providing detection and discrimination of volatile organic compounds (VOCs) as required across numerous analytical applications. Such systems typically comprise a number of cross reactive sensor elements, which generate analyte specific response patterns upon exposure to VOCs, and are known as multisensor arrays. When evaluated using statistical methods, these response patterns facilitate classification of VOCs. As an alternative, a single dynamically operated sensor could also be used to generate analyte specific response patterns. This approach is known as a virtual sensor array (VSA) and can exhibit significant advantages when compared to MSAs. Some advantages include lower power consumption, sensor drift, material cost, and experimental preparatory time. Furthermore, several dynamically operated sensors could be used in tandem (using the MSA and VSA scheme in a complementary fashion) to fabricate virtual multisensor arrays (V-MSAs). Such systems would exhibit greater data density than either the MSA or VSA, and are promising for samples that are particularly challenging to discriminate. Among the various systems utilized for VOC discrimination, sorption based systems hold considerable promise because they are simple and inexpensive yet highly effective. This dissertation is focused on the development of array sensing schemes using ionic liquids (ILs), a group of uniform materials based on organic salts (GUMBOS), and binary blends of either IL or GUMBOS with polymer as recognition elements and the quartz crystal microbalance (QCM) as the transducer. Towards this end, ILs, which are defined as organic salts with melting points below 100 °C, and group of uniform materials based on organic salts (GUMBOS) which extend the melting range of ILs to 250 °C to encompass similar solid phase salts, were used to design the first examples of QCM based VSAs, and V-MSAs, for pure VOC and complex mixture analyses. Furthermore binary blends of organic salts and polymer were used to fabricate the first VSA with the capability to identify and approximate molecular weight of pure VOCs. By and large, the studies presented here demonstrate the excellent potential of these materials and techniques for advancement of vapor phase measurement science

Advances in SAW Gas Sensors Based on the Condensate-Adsorption Effect

A surface-acoustic-wave (SAW) gas sensor with a low detection limit and fast response for volatile organic compounds (VOCs) based on the condensate-adsorption effect detection is developed. In this sensor a gas chromatography (GC) column acts as the separator element and a dual-resonator oscillator acts as the detector element. Regarding the surface effective permittivity method, the response mechanism analysis, which relates the condensate-adsorption effect, is performed, leading to the sensor performance prediction prior to fabrication. New designs of SAW resonators, which act as feedback of the oscillator, are devised in order to decrease the insertion loss and to achieve single-mode control, resulting in superior frequency stability of the oscillator. Based on the new phase modulation approach, excellent short-term frequency stability (±3 Hz/s) is achieved with the SAW oscillator by using the 500 MHz dual-port resonator as feedback element. In a sensor experiment investigating formaldehyde detection, the implemented SAW gas sensor exhibits an excellent threshold detection limit as low as 0.38 pg

Feature Extraction by Wavelet Decomposition of Surface

The paper presents a new approach to surface acoustic wave (SAW) chemical sensor array design and data processing for recognition of volatile organic compounds (VOCs) based on transient responses. The array is constructed of variable thickness single polymer-coated SAW oscillator sensors. The thickness of polymer coatings are selected such that during the sensing period, different sensors are loaded with varied levels of diffusive inflow of vapour species due to different stages of termination of equilibration process. Using a single polymer for coating the individual sensors with different thickness introduces vapour-specific kinetics variability in transient responses. The transient shapes are analysed by wavelet decomposition based on Daubechies mother wavelets. The set of discrete wavelet transform (DWT) approximation coefficients across the array transients is taken to represent the vapour sample in two alternate ways. In one, the sets generated by all the transients are combined into a single set to give a single representation to the vapour. In the other, the set of approximation coefficients at each data point generated by all transients is taken to represent the vapour. The latter results in as many alternate representations as there are approximation coefficients. The alternate representations of a vapour sample are treated as different instances or realisations for further processing. The wavelet analysis is then followed by the principal component analysis (PCA) to create new feature space. A comparative analysis of the feature spaces created by both the methods leads to the conclusion that both methods yield complimentary information: the one reveals intrinsic data variables, and the other enhances class separability. The present approach is validated by generating synthetic transient response data based on a prototype polyisobutylene (PIB) coated 3-element SAW sensor array exposed to 7 VOC vapours: chloroform, chlorobenzene o-dichlorobenzene, n-heptane, toluene, n-hexane and n-octane.Defence Science Journal, 2010, 60(4), pp.377-386, DOI:http://dx.doi.org/10.14429/dsj.60.49

Piezoelectric microsensors for semiochemical communication

Chemical communication plays vital role in the mediating the behaviour of an organism living in the “odour space”. The mechanisms by which odours are generated and detected by the organism has evolved over thousands of years and thus the potential advantages of translating this system into a fully functional communication system has opened new avenues in the area of multi-disciplinary research. This formed the basis of the Biosynthetic Infochemical Communications project – iCHEM whose central aim was to develop a new class of communication technology based on the biosynthesis pathways of the moth, S. littoralis. This novel infochemical communication system would consist of a “chemoemitter” unit which would generate a precise mix of infochemicals which after travelling through the odour space would be detected by a complementary tuned detector – the “chemoreceiver” unit comprising of a ligand specific detection element and an associated biophysical model functioning similar to the antennal lobe neuron of the moth. This combined novel system will have the capability of communicating by the help of chemicals only, in the vapour or liquid phase. For the work presented in this thesis, the novel concept of infochemical communication has been examined in the vapour and liquid phase by employing piezoelectric microsensors. This has been achieved and demonstrated throughout the thesis by employing chemo-specific acoustic wave microsensors. For vapour phase assessment, quartz crystal microbalance, were coated with different organic polymer coatings and incorporated in a prototype infochemical communication system detecting encoded volatiles. For liquid phase assessment, shear horizontal surface acoustic wave (SH-SAW) microsensors were specifically designed and immobilised within Sf9 insect cells. This GPCR based whole cell biosensing system was then employed to detect ligand specific activations thus acting as a precursor to the development of a fully functionalised OR based signalling system, thus contributing to the growing field of communication and labelling technology

Semen quality detection using acoustic wave sensors

Artificial insemination (AI) is a widely used part of the modern agricultural industry, with the number of animals inseminated globally being measured in the millions per anum. Crucial to the success of AI is that the sperm sample used is of a high Quality. Two factors which determine the quality of the sample are the number of sperm present and their motility. There are numerous methods used to analyse the quality of a sperm sample, but these are generally laboratory based, expensive and in need of a skilled operator to perform the analysis. It would, therefore be useful to have a simple and inexpensive system which could be used outside the laboratory, immediately prior to the insemination of the animal. Presented in this thesis is work developing a time of flight (ToF) technique which makes use of a quartz crystal microbalance (QCM), operating at 5 MHz, as the sensing element. Data is shown developing a device where a 50 μl sample of boar sperm is added to a liquid filled swim channel, which the sperm are allowed to self-propel down and attach to the surface of a QCM at the end. The attachment of the sperm to the surface causes a measurable frequency decrease in the QCM, aproximately 50 Hz. An average effective mass measurement was made using a QCM and gave a value of 8 ± 5 pg per sperm, which was used in conjunction with the frequency change to determine the number rate of sperm reaching the QCM

Experimental comparison of FBARs and SMRs responsitivities to mass loadings

The utilisation of thin film technology to develop film bulk acoustic resonators (FBARs) and solidly mounted resonators (SMRs), offers great potential to outperform the sensitivity and minimum detection limit of gravimetric sensors. Up to now, the choice between FBARs and SMRs depends primarily on the users' ability to design and fabricate Bragg reflectors and/or membranes, because neither of these two types of resonators has been demonstrated to be superior to the other. In the work reported here, it is shown that identically designed FBARs and SMRs resonating at the same frequency exhibit different responsitivities, Rm, to mass loadings, being the FBARs more responsive than the SMRs. For the specific device design and resonant frequency (~2 GHz) of the resonators presented, FBARs' mass responsitivity is ~20% greater than that of SMRs, and although this value should not be taken as universal for all possible device designs, it clearly indicates that FBAR devices should be favoured over SMRs in gravimetric sensing applications