Date

2011

Author

Soy, Esin

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Over the last decade, immobilization of proteins onto inorganic materials is becoming more crucial to extend a deep understanding of interaction between proteins and nanoparticles. With understanding of the real interaction lying under the protein-nanoparticle relations, it is possible to organize the conformation and orientation of surface and framework species of nanoparticles to generate ideal surfaces for potential biotechnological applications. Due to their unique properties such as large clean surface, tunable surface properties, adjustable surface charge, and dispersibility in aqueous solutions, zeolite and zeo-type materials are one of the remarkable classes of inorganic materials that are widely studied in the literature. These properties make zeolites promising alternative candidates for the immobilization of enzymes and incorporation into biosensing devices. In the current study, a new approach was developed for direct determination of urea, glucose, and butyrylcholine where zeolites were incorporated to the electrode surfaces of a conductometric biosensor and Ion Sensitive Field Effect Transistors were used to immobilize the enzymes. Biosensor responses, operational stabilities, and storage stabilities of the new approach were compared with results obtained from the standard membrane methods for the same measurements. For this purpose, different surface modification technique, which are simply named as Zeolite Modified Transducers (ZMTs) were compared with Standard Membrane Transducers (SMTs). During the conductometric measurements ZMT electrodes were used, which allowed the direct evaluation of the effect of zeolite morphology on the biosensor responses for the first time. It was seen that silicalite added electrodes lead to increased performances with respect to SMTs. As a result, the zeolite modified urea and glucose biosensors were successfully applied for detecting urea and glucose, which can offer improved possibilities to design biosensors. The results obtained show that zeolites could be used as alternatives for enzyme immobilization in conductometric biosensors development. Furthermore, the sensitivities of urease and butyrylcholinesterase biosensors, prepared by the incorporation of zeolite Beta crystals with varying acidity on the surface of pH-sensitive field-effect transistors (pH-FETs), have been studied and compared. In order to study exclusively the effect of zeolite acidity, highly crystalline pure zeolite Beta sample with Si/Al ratio of 17 was synthesized and subjected to different heat treatment protocols. In this way, the surface acidic OH groups were controllably altered, as confirmed by Fourier transform infrared (FTIR) spectroscopy without changing any other zeolitic properties, such as zeolite morphology and Si/Al ratio. Upon incorporation of zeolite Beta, the biosensors sensitivity towards urea and butyrylcholine increased 2 and 3 times, respectively. Operational stability and possibility to use the biosensors for inhibition analysis were also investigated. The combined ion-sensitive field-effect transistor (ISFET) and FTIR data provided evidence that urease and butyrylcholinesterase responded to changes in the nature of surface OH groups in zeolite Beta samples. Accordingly, it was found that the Brønsted acidity of zeolite Beta is important for the ultimate ISFET performance. Additionally, analytical characteristics of urease and butyrylcholinesterase based ISFET sensors were investigated by the incorporation of zeolite (70 nm zeolite beta crystals with varying Si/Al ratio, particle size, and surface charge) and zeo-type materials with varying pore diameter and surface charge for the first time. The results obtained by the zeolite modified ISFET transducers suggested that the Si/Al ratio, particle size and surface charge of the zeolite Beta crystals were strongly influenced the biosensor performances due to the electrostatic interactions between enzyme molecules, substrates, and zeolite surface as well as the nature of the enzymatic reaction.

Subject Keywords

Biosensors.

URI

http://etd.lib.metu.edu.tr/upload/12613313/index.pdf
https://hdl.handle.net/11511/20532

Collections

Graduate School of Natural and Applied Sciences, Thesis