Electrochemical Biosensors Based on Functionalized Zinc Oxide Nanorods
Abstract: The semi-conductor zinc oxide (ZnO), a representative of group II-VI has gained substantial interest in the research community due to its novel properties and characteristics. ZnO a direct band gap (3.4eV) semi-conductor has a stable wurtzite structure. Recently ZnO have attracted much interest because of its unique piezoelectric, semiconducting, catalytic properties and being biosafe and biocompatible morphology combined with the easiness of growth. This implies that ZnO has a wide range of applications in optoelectronics, sensors, transducers, energy conversion and medical sciences. This thesis relates specifically to biosensor technology and pertains more particularly to novel biosensors based on multifunctional ZnO nanorods for biological, biochemical and chemical applications.The nanoscale science and engineering have found great promise in the fabrication of novel nano-biosensors with faster response and higher sensitivity than of planar sensor configurations. This thesis aims to highlight recent developments in materials and techniques for electrochemical biosensing, design, operation and fabrication. Rapid research growths in biomaterials, especially the availability and applications of a vast range of polymers and copolymers associated with new sensing techniques have led to remarkable innovation in the design and fabrication of biosensors. Specially nanowires/nanorods and due to their small dimensions combined with dramatically increased contact surface and strong binding with biological and chemical reagents will have important applications in biological and biochemical research. The diameter of these nanostructures is usually comparable to the size of the biological and chemical species being sensed, which intuitively makes them represent excellent primary transducers for producing electrical signals. ZnO nanostructures have unique advantages including high surface to volume ratio, nontoxicity, chemical stability, electrochemical activity, and high electron communication features. In addition, ZnO can be grown as vertical nanorods and has high ionic bonding (60%), and they are not very soluble at biological pH-values. All these facts open up for possible sensitive extra/intracellular ion measurements. New developments in biosensor design are appearing at a high rate as these devices play increasingly important roles in daily life. In this thesis we have studied calcium ion selectivity of ZnO nanorods sensors using ionophore membrane coatings in two research directions: first, we have adjusted the sensor with sufficient selectivity especially for Ca2+, and the second is to have enough sensitivity for measuring Ca2+ concentrations in extra and intracellular media. The sensor in this study was used to detect and monitor real changes of Ca2+ across human fat cells and frog cells using changes in the electrochemical potential at the interface in the intracellular microenvironment.The first part of the thesis presents extracellular studies on calcium ions selectively by using ZnO nanorods grown on the surface of a silver wire (250 ?m in diameter) with the aim to produce proto-type electrochemical biosensors. The ZnO nanorods exhibited a Ca2+-dependent electrochemical potentiometric behavior in an aqueous solution. The potential difference was found to be linear over a large logarithmic concentration range (1?M to 0.1M) using Ag/AgCl as a reference electrode. To make the sensors selective for calcium ions with sufficient selectivity and stability, plastic membrane coatings containing ionophores were applied. These functionalized ZnO nanorods sensors showed a high sensitivity (26.55 mV/decade) and good stability.In the second part, the intracellular determination of Ca2+ was performed in two types of cells. For that we have reported functionalized ZnO nanorods grown on the tip of a borosilicate glass capillary (0.7 ?m in diameter) used to selectively measure the intracellular free Ca2+ concentration in single human adipocytes and frog oocytes. The sensor exhibited a Ca2+ linear electrochemical potential over a wide Ca2+ concentration range (100 nM to 10 mM). The measurement of the Ca2+ concentration using our ZnO nanorods based sensor in living cells were consistent with values of Ca2+ concentration reported in the literature.The third and final part, presents the calcium ion detection functionalized ZnO nanorods coupled as an extended gate metal oxide semiconductor field effect transistor (MOSFET). The electrochemical response from the interaction between the ZnO nanorods and Ca2+ in an aqueous solution was coupled directly to the gate of a MOSFET. The sensor exhibited a linear response within the range of interest from 1 ?M to 1 mM. Here we demonstrated that ZnO nanorods grown on a silver wire can be combined with conventional electronic component to produce a sensitive and selective biosensor.
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