T. Wessa

New immobilization method for SAW-biosensors : covalent attachment of antibodies via CNBr

Surface acoustic wave devices (SAWs) based on horizontally polarized shear waves can be used as mass-sensitive immunosensors. If the standard material, aluminium, is used for the interdigital transducers (IDTs) of these devices a pretreatment with a shielding layer of polyimide is necessary to prevent corrosion processes. However, this layer also prevents an easy covalent attachment of the necessary receptive molecules. Therefore, we developed a new activation method of this inert polyimide surface by treating with cyanogenbromide using the established cyano-transfer-technique. With a subsequent covalent attachment of anti-glucose oxidase (anti-GOD) a very sensitive SAW-immunosensor for glucose oxidase (GOD) can be obtained.

Gas analysis with SAW sensor systems

Abstract At the institute of instrumental analysis, a family of three gas analysis instruments based on mass-sensitive gas sensor devices are developed. The SAW Aroma and Gas Analyzing Systems (SAGAS) are used to detect and analyze a large variety of gases and aromas. The common heart of these systems is a sensor head with eight differently polymer-coated surface acoustic wave (SAW) devices. These “semi-selective” sensors are used to generate typical signal pattern (“finger prints”) of analyte gases and gas mixtures. Using chemometrical data analysis, such as PCA, classification and PLS, as well as artificial neural networks, these signal patterns are used for qualitative analyte recognition and for quantitative gas mixture determination. In the following, some sensor properties are discussed and some applications of the systems are presented.

Polyimide, a new shielding layer for sensor applications

Abstract In sensor applications it is often necessary to cover the sensor surface or the interacting area of the sensor with a shielding layer. We present a well reproducible and easy method to shield a sensor surface with an aromatic polyimide layer. The shielding capacity was tested with surface acoustic wave (SAW) devices which are commercially available and furthermore meet the requirements for (bio)sensor applications in aqueous media. All experiments described here were done with these devices. One major advantage of this technique was the prevention of corrosion processes on the sensor surface, especially the damage of interdigital transducers which often consist of aluminium, because this material is known to have the best acoustoelectric properties for SAW-devices. Besides, we show that a thin polyimide film enhance the typical sensor characteristics in terms ‘of sensitivity and stability’.

Adhesives: a new class of polymer coatings for surface acoustic wave sensors for fast and reliable process control applications

Fast and reliable on-line detection of organic vapors for control of chemical processes is a challenging application for a new type of analytical instruments: sensor systems based on an array of differently selective chemical sensors. In this work we present the use of mass-sensitive polymer coated surface acoustic wave sensors (SAWs). The sensors were initially coated with a standard set of polymers consisting of a known composition. But this first approach could not meet all requirements. Therefore, a new class of commercially available polymer coating, namely adhesives, was developed. The coating procedure was optimized and the aging process of the adhesives was carefully investigated. As a result the selectivity for ambitious separation problems arising from similar polarity of the components of typical solvent mixtures could be remarkably increased. The system was then applied in a real testing environment application at a chemical plant: the fast on-line control of a preparative reversed phase process HPLC (RP-PHPLC). Data from this industrial application are shown.

Immunosensing of photoimmobilized proteins on surface acoustic wave sensors

Abstract This study describes the manufacturing and characteristics of an immunosensor with a photobonded protein on a surface acoustic wave (SAW) transducer. The protein was modified with a photoactivatable aryl diazirine and covalently immobilized on polyimide protected SAW transducer surfaces by exposure to light. The procedure is exemplified with the enzyme glucose oxidase. Probing of photobonded glucose oxidase by polyclonal anti-glucose oxidase antibodies revealed a detection limit of 2.7 nmol 1 −1 and a sensitivity of 59 Hz μg −1 antibody. Besides facile protein immobilization, the sensor system allowed fast kinetic analysis of the antigen/antibody interaction.

Validation of an industrial analytical sensor procedure realized with a SAW-based sensor system

Abstract Mass-sensitive sensors based on surface acoustic waves (Suface Acoustic Wave Devices, SAWs) can be used for qualitative as well as for quantitative determination of volatile organic compounds (VOCs). An analytical procedure for a SAW-based sensor system used for on-line process control of VOCs in pharmaceutical industry was developed. Analytical procedures applied in pharmaceutical development and production have to meet the requirements of good manufacturing practice (GMP) and therefore a validation in compliance with the International Conference on Harmonization of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH)-guidelines had to be performed. It is shown that these procedures can be successfully validated as non-chromatographic procedures according to ICH-guidelines. As a typical example, the on-line control of a HPLC gradient former was investigated. In this particular case, the sensor system has to determine the actual composition of the mobile phase and furthermore it has to discriminate possible avoidance. The following analytical performance parameters are investigated to prove the validation of the analytical procedure: ∘ Specificity, ∘ Linearity, ∘ Limit of Detection and Quantitation, ∘ Accuracy, ∘ Precision, and ∘ Robustness.

Optimization of surface acoustic wave sensor arrays and application to high performance liquid chromatography

Abstract Some issues regarding the optimization of sensor arrays are discussed. The criteria considered here are based on Lorbers figures of merit and nonlinear generalizations thereof. Especially, it is discussed how the averaging over the analyte space should be performed for nonlinear data and what optimization criterion should be used. A genetic algorithm is used as optimization algorithm. These considerations are used for sensor selection and sensor reduction of surface acoustic wave sensor devices applied in preparative process high performance liquid chromatography.

Immunosensing with Commercially Available Low-loss Surface Acoustic Wave Devices

This chapter discusses a study analyzing the immunosensing with commercially available low-loss surface acoustic wave devices. Surface transverse waves (STW) devices were used as the frequency determining element of an oscillator circuit. The adsorption of proteins from solution could be monitored directly by the resulting change in resonant frequency. When uncoated sensors were subjected to spontaneous adsorption of bovine serum albumin (BSA) in buffer solution, the relative frequency changes showed a linear relationship with respect to the operating frequency. This implies that the mass loading effect is dominant in this system. Immunosensors were prepared by coating the STW devices with antibodies. Pretreatment was found to be necessary to improve the stability of the transducer electrodes in aqueous media, while facilitating the covalent immobilization of antibodies. Sensors coated with anti-glucose oxidase showed an irreversible frequency shift when treated with a glucose oxidase solution. This shift was not observed when sensors coated with a different antibody were treated with the same solution. It is concluded that the irreversible shift arises from specific binding of the antigen.