| Abstract |
Chemical sensors are significant analytical instruments used for the determination of a wide variety of analytes. In comparison with other analytical methods, chemical sensors excel mainly because of their ability to monitor the analyte continuously and without destroying the sample. Additionally, due to their small size, they can be easily incorporated in automated systems, such as continuous flow systems, and perform real-time monitoring. These characteristics are very important since they account for the use of chemical sensors in field analysis and in vivo monitoring. Because of that, up to now, chemical sensors have been implemented in food and water quality control, in medical diagnosis, in environmental monitoring e.t.c. Still, the applications and the interest in the market of analytical instruments are continuously increasing, revealing the intense research activity in the field which is directly related with the great significance of chemical sensors in instrumental analysis.
The use of new materials for the development of chemical sensors with improved or novel characteristics is one of the main research activities in the field. In this framework, new semiconductor materials have been studied in the last few years, for the development of chemical sensors, including the group of III-nitrides. These materials are mainly used for the development of high power and high frequency electronic devices, while they are also used in optoelectronics, since they are the only semiconductors that emit light in the blue and ultraviolet region of the spectrum. Moreover, in the last few years, this group of semiconductor materials, due to their excellent chemical inertness and also mechanical and thermal stability, has attracted great interest for their use in the development of chemical sensors.
The scope of this project is to explore the use of planar and nanostructured III-nitride crystals for the development of chemical sensors and biosensors. More specifically, in addition to the most extensively studied +c-plane GaN, it is aimed to expand the research to different crystal directions, such as the non polar a-plane GaN, and to other III-nitride materials, such as the InN and InGaN alloys. Moreover, the use of III-nitride nanocolumns for the development of chemical sensors and biosensors is also explored for the first time.
In more detail, the first part of this work concerns a comparison study between the potentiometric response to different anions and pH of an a-plane GaN, a +c-plane GaN and a +c-plane InN. From the obtained results, it is concluded that the reduced electronegativity of the In atom, compared with that of the Ga atom, improves the ability of the polar +c-plane IΙΙ-Ν to sense anions. On the other hand, the crystal direction does not influence the sensing ability of these materials in a great degree. Because of that, the +c-plane InN exhibits the highest sensitivity to anions and thus is the material of choice for such applications, while the +c-plane GaN is better for pH sensing. The a-plane GaN exhibits an interesting high sensitivity for the zwitterionic MES compound, which is most probably attributed to multi-dentate interaction between MES and the crystal surface, since the a-plane GaN bears both Lewis bases (Ga) and Lewis acids (N) on its surface. In the same section, the immobilization of enzyme urease through physical adsorption, on planar +c- and a-plane GaN crystals, is explored for the development of urea biosensors. The obtained results reveal that the crystal direction does not influence the sensitivity and the lifetime of the corresponding urea biosensor.
In the second part of this work, the potentiometric response of GaN and InN nanocolumns to ions and pH is examined. Moreover, the stability of these nanocolumns in strong acidic environments is also tested. The obtained results reveal that GaN and InN nanocolumns respond to anions as their corresponding planar crystals, but with a much lower sensitivity. On the other hand, their sensitivity to pH is quite high and very similar to that of their planar counterparts, although both kinds of nanocolumns are found to be more vulnerable to HCl etching compared to the corresponding flat crystals. This fact allows the use of these nanocolumns as pH sensors only under mild conditions, such as in biosensing systems. To confirm that, potentiometric urea biosensors based on GaN and InN nanocolumns are developed. For comparison reasons, similar urea biosensors based on GaN and InN planar crystals are also developed. The obtained results reveal that the urea biosensors based on nanocolumnar samples exhibit higher sensitivity and longer lifetime than the biosensors based on the corresponding planar crystals.
In the last part of this work, the use of planar and nanocolumnar III-nitride crystals for the development of amperometric biosensors, is explored. More specifically, besides GaN and InN crystals, also GaxIn1-xN alloys are examined. First, the ability to monitor the oxidation of Η2Ο2at the surface of these crystals is tested. The results show that only the materials with small energy gap, meaning the InN crystals and the InGaN alloys with low percentage of Ga, are suitable for this purpose. Moreover, it is revealed that nanocolumnar samples exhibit much higher sensitivity than the planar counterparts. Then, these crystals that are able to monitor the oxidation of Η2Ο2, are used for the development of amperometric glucose biosensors. Based on the performance of these biosensors, it is concluded that nanocolumnar biosensors clearly excel the planar ones due to their higher sensitivity and prolonged lifetime.
In conclusion, the results of this work show that, as far as the potentiometric sensors are concerned, the electronegativity of the metal atom, and also the crystal direction, influence the sensing abilities of the III-nitride crystal. Moreover, although III-nitride nanocolumns are more vulnerable to HCl etching compared to the corresponding flat crystals, they can be used as transducers for the development of potentiometric biosensors, because of their high pH sensitivity. The increased active surface, together with the fact that the space between the nanocolumns can act as nanocavities for the stabilization of the adsorbed enzyme, account for the superior performance of the nanocolumnar biosensors, compared with those based on the corresponding planar crystals. As far as the amperometric biosensors are concerned, the results reveal that nanocolumns are not only better stabilizing material for the enzyme, but also better signal transducers. More specifically, it is shown that it is possible to control the conductivity of the sample by changing two parameters, the content of the alloy and the dimensions of the nanocolumns. This fact broadens the possibilities to grow nanocolumns with the desirable characteristics for each
case. This advantage, along with the superior optical properties of the III-nitride nanocolumns, make these materials very promising candidates for the development of novel and more sophisticated biosensors.
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Ανάπτυξη χημικών αισθητήρων και βιοαισθητήρων σε ετεροδομές και νανοδομές ΙΙΙ-νιτριδίων
