Zinc Oxide Nanostructure Based Electrochemical Sensors and Drug Delivery to Intracellular Environments

Abstract: The nanoscale science and nanostructure engineering have well established in the fabrication of novel electrochemical biosensors with faster response and higher sensitivity than of planar sensor configurations. Moreover nanostructures are suggested and used as efficient carrier of photosensitizers for cancerous cell treatment. The semi-conductor zinc oxide (ZnO) nanostructures have attracted much interest because of its unique piezoelectric, polar semiconducting, large surface area, catalytic properties, and being biosafe and biocompatible combined with the easiness of growth. This implies that ZnO nanostructures have a wide range of applications in optoelectronics, sensors, transducers, energy conversion and medical sciences. The aim of this study is to highlight recent developments in materials and techniques for electrochemical biosensing, photodynamic therapy, design, operation, and fabrication. The sensors in this study were used to detect and monitor real changes of metal ions and glucose across human fat cells and frog cells using changes in the electrochemical potential at the interface to the intracellular microenvironments. This thesis relates specifically to “zinc oxide nanostructure based electrochemical sensors and drug delivery to intracellular environments” for biological, biochemical and chemical applications. The first part of the thesis presents extra and intracellular studies on metal ions such as Ca2+, Mg2+, and Na+…..etc selectively sensed by using ZnO nanorods grown on the tip of a borosilicate glass capillary (0.7 μm in diameter) with the aim to produce proto-type electrochemical extra/intracellular biosensors. The single human adipocyte and frog oocyte cells were used to selectively measure the intracellular free metal ions concentration. To make the sensors selective for metal ions with sufficient selectivity and stability, plastic membrane coatings containing specific ionophores were applied. These functionalized ZnO nanorods sensors showed high sensitivity and good stability with linear electrochemical potential versus a wide metal ion concentration range of interest. The measured intracellular values were consistent with values reported in the literature. Furthermore we have successfully determined that the intracellular potassium (K+) concentration decrease is not obligatory for apoptosis. The aim of this study is to show the possibility of using K+ selective microelectrode to detect and monitor intracellular changes of K+ concentration during injection of various test solution and chemically induced apoptosis in Xenopus laevis oocytes parallel with electrophysiological measurements to verify the accuracy. The second part, presents the calcium ion (Ca2+) detection using functionalized ZnO nanorods attached as an extended gate metal oxide semiconductor field effect transistor (MOSFET). The electrochemical response was coupled directly to the gate of a commercial MOSFET to study the I-V characterization. Here we verified that ZnO nanorods grown on any thin wire can be combined with conventional electronic component to produce a sensitive and selective biosensor. In the third part, we have performed the experiment to determine glucose concentration intracellularly and in airway surface liquid (ASL) with functionalized ZnO nanorod-coated microelectrodes. In this study, the GOD enzyme was immobilised electrostatically, drawing on the fact that there is a large difference in the isoelectric points of ZnO and glucose oxidase. Insulin has been found to affect the glucose uptake in human adipocytes and frog Xenopus laevis. The large size of these cells makes it possible to microinject specific reagents that interrupt or activate signal transmission to glucose. The measured glucose concentration in human adipocytes or frog oocytes and ASL using our ZnO nanorod sensor was consistent with values of glucose concentration reported in the literature by using other indirect techniques. The fourth and final part covers the application of ZnO nanorods to cancer cells for photodynamic therapy. The ZnO nanorods were conjugated with protoporphyrin for local mediated photochemistry and efficient treatment of a single cancer cell. The ZnO nanorods were used as an efficient photosensitizer carrier system and at the same time providing intrinsic white light to achieve necrosis of the cancer cell. Breast cancer cells were used to study the catalytic effect of ZnO for treatment. The grown ZnO nanorods were conjugated with protoporphyrin dimethyl ester (PPDME), which absorbs the light emitted by the ZnO nanorods and cause the cytotoxicity which appears to involve the generation of reactive singlet oxygen inside the cell.