Florian Bender

Development of a preconcentration unit for a SAW sensor micro array and its use for indoor air quality monitoring

Abstract A new surface acoustic wave (SAW) sensor system for continuous monitoring of air quality was developed. The system employs a miniaturized array of eight polymer coated SAW sensors, a preconcentration unit (‘trap’), and methods of pattern recognition. Care was taken to minimize both the response times of the sensors and the gas volume of the sensor array. Thus, a small trap with low heat capacity can be used, resulting in low power consumption and rapid thermal desorption. The capabilities of the system are demonstrated by successful discrimination of closely related aromatic compounds (BTXE) in the low- and sub-ppm ranges. Design considerations are made with particular emphasis on the necessities arising from the interplay between sensors, coatings, trap, gas fluidics, and pattern recognition software.

Development of a combined surface plasmon resonance/surface acoustic wave device for the characterization of biomolecules

It is known that acoustic sensor devices, if operated in liquid phase, are sensitive not just to the mass of the analyte but also to various other parameters, such as size, shape, charge and elastic constants of the analyte as well as bound and viscously entrained water. This can be used to extract valuable information about a biomolecule, particularly if the acoustic device is combined with another sensor element which is sensitive to the mass or amount of analyte only. The latter is true in good approximation for various optical sensor techniques. This work reports on the development of a combined surface plasmon resonance/surface acoustic wave sensor system which is designed for the investigation of biomolecules such as proteins or DNA. Results for the deposition of neutravidin and DNA are reported.

Identification and Quantification of Aqueous Aromatic Hydrocarbons Using SH-Surface Acoustic Wave Sensors

A need exists for compact sensor systems capable of in situ monitoring of groundwater for accidental releases of fuel and oil. The work reported here addresses this need, using shear horizontal surface acoustic wave (SH-SAW) sensors, which function effectively in liquid environments. To achieve enhanced sensitivity and partial selectivity for hydrocarbons, the devices are coated with thin chemically sensitive polymer films. Various polymer materials are investigated with the goal of identifying a set of coatings suitable for a sensor array. The system is tested with compounds indicative of fuel and oil releases, in particular, the BTEX compounds (benzene, toluene, ethylbenzene, and xylenes), in the low milligrams/liters to high micrograms/liters concentration range. Particular emphasis is placed on detection of benzene, a known carcinogen. It was observed that within the above concentration range, responses to multiple analytes in a mixture are additive, and there is a characteristic response time for each coating/analyte pair, which is largely independent of concentration. With the use of both the steady-state and transient-response information of SH-SAW sensor devices coated with three different polymer materials, poly(ethyl acrylate), poly(epichlorohydrin), and poly(isobutylene), a response pattern was obtained for benzene that is easily distinguishable from those of the other BTEX compounds. The time courses of the responses to binary analyte mixtures were modeled accurately using dual-exponential fits, yielding a characteristic concentration-independent time constant for each analyte/coating pair. Benzene concentration was quantified in the aqueous phase in the presence of the other BTEX compounds.

Deposition of high quality coatings on SAW sensors using electrospray

Electrospray has long been used as an ion source for mass spectrometry and for coating of conducting surfaces. However, it has only recently been introduced as a novel method to deposit polymer layers and other coating materials on sensor devices. In this work, the adaptation of electrospray deposition for this particular application is described, including the development of an apparatus for polymer deposition on nonconducting surfaces, optimization of deposition parameters, and a discussion of different solvent mixtures suitable for dissolving and spraying of a variety of polymers. Results are presented on both the apparent quality of electrospray-deposited polymer layers and their performance in analytical sensing applications.

Probing Mechanical Properties of Liposomes Using Acoustic Sensors

Acoustic devices were employed to characterize variations in the mechanical properties (density and viscoelasticity) of liposomes composed of 1-oleoyl-2-palmitoyl- sn-glycero-3-phosphocholine (POPC) and cholesterol. Liposome properties were modified in three ways. In some experiments, the POPC/cholesterol ratio was varied prior to deposition on the device surface. Alternatively, the ratio was changed in situ via either insertion of cholesterol or removal of cholesterol with beta-cyclodextrin. This was done for liposomes adsorbed directly on the device surface and for liposomes attached via a biotin-terminated poly(ethylene glycol) linker. The acoustic measurements make use of two simultaneous time-resolved signals: one signal is related to the velocity of the acoustic wave, while the second is related to dissipation of acoustic energy. Together, they provide information not only about the mass (or density) of the probed medium but also about its viscoelastic properties. The cholesterol-induced increase in the surface density of the lipid bilayer was indeed observed in the acoustic data, but the resulting change in signal was larger than expected from the change in surface density. In addition, increasing the bilayer resistance to stretching was found to lead to a greater dissipation of the acoustic energy. The acoustic response is assessed in terms of the possible distortions of the liposomes and the known effects of cholesterol on the mechanical properties of the lipid bilayer that encloses the aqueous core of the liposome. To aid the interpretation of the acoustic response, it is discussed how the above changes in the lipid bilayer will affect the effective viscoelastic properties of the entire liposome/solvent film on the scale of the acoustic wavelength. It was found that the acoustic device is very sensitive to the mechanical properties of lipid vesicles; the response of the acoustic device is explained, and the basic underlying mechanisms of interaction are identified.

Selective detection of HFC and HCFC refrigerants using a surface acoustic wave sensor system

Halogenated hydrocarbons are the generic base of most refrigerants. They are known to be greenhouse gases; some of them are even suspected to have an ozone depleting potential. Thus, an urgent need exists to detect and identify these compounds. However, refrigerants usually have very low boiling points as well as low electrochemical activities. The latter problem is a serious obstacle to the development of appropriate electrochemical sensors. It is circumvented by using mass sensitive methods of detection. To solve the former problem, the analytical system has to be optimized with regard to effective collection and detection of the refrigerants. In this work, a system for detection and identification of refrigerants is presented, based on a surface acoustic wave sensor array. The system is using a two-step analyte preprocessing unit containing a molecular sieve filter to minimize humidity and a carboxen trap for preconcentration of the analytes. Refrigerants R22, R134a, and R507 have been selected for ana...

Analysis of binary mixtures of aqueous aromatic hydrocarbons with low-phase-noise shear-horizontal surface acoustic wave sensors using multielectrode transducer designs.

The present work investigates a compact sensor system that provides rapid, real-time, in situ measurements of the identities and concentrations of aromatic hydrocarbons at parts-per-billion concentrations in water through the combined use of kinetic and thermodynamic response parameters. The system uses shear-horizontal surface acoustic wave (SH-SAW) sensors operating directly in the liquid phase. The 103 MHz SAW sensors are coated with thin sorbent polymer films to provide the appropriate limits of detection as well as partial selectivity for the analytes of interest, the BTEX compounds (benzene, toluene, ethylbenzene, and xylenes), which are common indicators of fuel and oil accidental releases in groundwater. Particular emphasis is placed on benzene, a known carcinogen and the most challenging BTEX analyte with regard to both regulated levels and its solubility properties. To demonstrate the identification and quantification of individual compounds in multicomponent aqueous samples, responses to binary mixtures of benzene with toluene as well as ethylbenzene were characterized at concentrations below 1 ppm (1 mg/L). The use of both thermodynamic and kinetic (i.e., steady-state and transient) responses from a single polymer-coated SH-SAW sensor enabled identification and quantification of the two BTEX compounds in binary mixtures in aqueous solution. The signal-to-noise ratio was improved, resulting in lower limits of detection and improved identification at low concentrations, by designing and implementing a type of multielectrode transducer pattern, not previously reported for chemical sensor applications. The design significantly reduces signal distortion and root-mean-square (RMS) phase noise by minimizing acoustic wave reflections from electrode edges, thus enabling limits of detection for BTEX analytes of 9-83 ppb (calculated from RMS noise); concentrations of benzene in water as low as ~100 ppb were measured directly. Reliable quantification of BTEX analytes in binary mixtures is demonstrated in the sub-parts-per-million concentration range.

5.4.2 Quantification of Benzene in Groundwater Using SH-Surface Acoustic Wave Sensors

A need exists for compact sensor systems capable of in-situ monitoring of groundwater for fuel and oil contamination. The work reported here addresses this need using shear horizontal surface acoustic wave (SH-SAW) sensors, which function effectively in the liquid phase. To achieve enhanced sensitivity and partial selectivity for hydrocarbons, the devices are coated with thin chemically sensitive polymer films. Various polymer materials are investigated with the goal of identifying a set of coatings suitable for a sensor array. The system is tested with compounds indicative of fuel and oil contamination, in particular, BTEX (benzene, toluene, ethylbenzene and xylenes), at relatively low concentrations. Of particular importance is benzene, a known carcinogen. Using responses of the SHSAW sensor devices coated with three different polymer materials, benzene was quantified in the aqueous phase in the presence of other aromatic interferents. It is shown that various concentrations of BTEX in water can be identified and quantified by evaluation of both steady-state and transient response information.

Influence of ambient parameters on the response of polymer-coated SH-surface acoustic wave sensors to aromatic analytes in liquid-phase detection

This work addresses the importance of ambient conditions in the design of a sensor system for real-time monitoring of contaminated water sites and groundwater in situ for fuel and oil spills. The sensor system is based on shear-horizontal surface acoustic wave devices with chemically sensitive polymer coatings for analyte sorption. In addition to the use of a sensor array, evaluation of response time (absorption time constant) is discussed as a tool for analyte identification. The influence of relevant ambient parameters (temperature, pH, salinity) on the sensor response is described, and resulting requirements for the design of the sensor system are discussed.

A concentration dependent study of acoustic plate mode immunosensor response using antigen/antibody systems with different binding ability

Acoustic plate mode sensors have been used to monitor immunochemical reactions as a function of antigen concentration. In the studies, antibodies were covalently linked to the gold-coated sensing surface via mercaptoethanol, aminosilane, and glutaraldehyde. Two antigen/antibody model systems that differ in their ability to mutually bind one another have been used. For sensor operation at about 150 MHz, a detection limit of approximately 0.5 /spl mu/g/ml was obtained in both cases. No significant difference between the two systems was found for the value of the binding constants. They amount to about 1/spl middot/10/sup 8/ 1/mole and fall well into the range of binding constants reported for homogeneous immunoassays. A comparison of the sensor response obtained for the two model systems shows that about 70% of the immobilized antibodies are active.