Sensor system study targeted to microorganism detection under impedance change

Because of some important reasons such as growing population on the world, agriculture or energy production, climate change, industrial pollution etc., usable water sources have reduced daily. According to World Water Development Report of United Nations, some descriptions of governments, total population of the world is suffering from water scarcity at least one month per year. It is known obviously that most people in the world have to be forced to use low quality water because water sources are very few and not suitable for use. Especially people living in countries like India, Indonesia, Nigeria, Pakistan, today 1 billion people cannot access the safe drinking water. Unsafe drinking water is the major problem on the spreading of infectious diseases such as dengue fever around the world. Because of these, not only these countries had a safe water problem but also all of the world is responsible from water quality. Water quality is a major problem on human health not only in underdeveloped countries but also in developed countries. In developed countries, drinking water or tap water can have a contamination that affect human life quality and health even there is a disinfection application. For example, chlorine disinfection is most popular way to disinfect tap water on many countries. However, this disinfected water can be contaminated on the distribution lines, which are old and neglected. In addition to this, biofouling on the outlet pipes of water-cooling / heating systems may be reason for drinking water contamination. Even these systems are generally disinfected by ozonation on certain periods, if biofouling layer is constructed on the pipe surface it is hard to remove microorganisms from there. This situation can be the most common reason for contamination of drinking water. However, these systems are very popular and commonly used in many place. Because of this, it can pose a risk for human health. When all these situations are taken into the consideration, it is inevitable for people to worry about the quality of the water they consume. In the light of this problem it is possible to say that there is an urgent need for technology that can be offered to people in order to get rid of existing concerns or to provide security at risk areas. The most basic and widely used method for determining the microbial contamination of a water sample is classical microbiological techniques. This practice can only be carried out by a specialist, also required special equipment and materials. In addition, the results are obtained after about 1-2 days in these applications. For this reason, there is still no technique to be applied by people to determine the safety of drinking water they consume in their daily lives. Biosensor systems, which have been a lot of work carried out in recent years, are likely to offer a solution in this area because they are both very versatile and offer fast and accurate measurements. In this thesis, it is aimed to develop a sensor system which can provide an easy-touse, fast and precise measurement, portable or integrable. When this system will be presented to the consumer, microorganism determination can be made easily from water. As a result of the literature and market research, impedimetric sensor system was found as most popular and suitable sensor system for microbial contamination detection. In 1899 it was discovered that microorganism concentration was correlated with changes of electrical parameters on the blood sample. Based on this relationship, "Impedance Microbiology" area was developed. In this field it is considered that microorganisms are very dynamic life forms which exchange some materials used on their metabolic reactions by their environments. By these exchanges, low charged materials such as glucose are taken from environment, used on the inside of the cell and more charged end products are obtained and removed from outside of the cell. In addition to these reactions, microorganisms have ion exchange by their ion channels. All of these situations cause significant changes of some electrical parameters of the environment. By following these changes, scientists aim to estimate level of microbial contamination of the tested sample on impedance microbiology studies. These studies are very common and excessive especially in the 70's, media, conductivity, temperature and other effects on these aspects were well characterized in this period. By the time this approach is used to develop detection of microorganisms by sensor system. Since these systems have a simple design, it is easy to evolve for different types of samples such as milk, ice cream, and water. When this technology is compared to conventional microbiological methods, they offer lower cost and ease of use, by which these systems shows suitability for use in laboratory. According to the literature, today there are many technologies depended on impedance microbiology. Devices such as BacTrac (Sy-Lab, Purkersdorf, Austria) that are available on the market aim to make this technique as an alternative to laboratory testing. Here, we have aimed two sensor systems designed on the basis of classical impedance microbiology, which have been tested for detection of bacterial contamination in water and liquid media samples. Different electrode designs and materials were tested and optimum parameters were determined by samples. Effects such as frequency and voltage, duration, temperature, etc. were tested. By all the tests, it was gained an idea about that usefulness of the system, yield, state of hope for future studies. It was proved that these systems have some disadvantages such as giving results within hours, high limit of detection, affecting much from environmental changes. Because of these, many researches were studied in the literature to improve these systems. One of these improvements are also made in electrode design. Especially interdigitated electrodes established by the system appears to be quite successful. Because of this, it was decided in this thesis study that interdigitated electrodes were obtained commercially and tested to have more sensitive sensor system. The interdigitated electrode is a special electrode design consisting of nested microelectrode fingers like hand fingers. Thanks to this design, it is possible to take measurements with higher precision using much less volume of sample. It is also possible to provide interdigitated electrodes for different features from companies such as Dropsens (Asturias, Spain), Micrux (Asturias, Spain), AB Tech (Richmond, Virginia). In addition to all of these properties, sensor systems are suitable to construct microfluidic systems. Although it is possible to access a wide variety of electrode designs commercially, many researchers in the literature have preferred to produce themselves by designing their electrodes for their own purposes. In this thesis, two types of gold interdigitated electrodes, which have 5 and 10 μm spacing between microelectrode fingers were provided by Dropsens (Asturias, Spain). These electrodes were connected to the LCR meter and the microorganism detection sensor system was installed. In the beginning of the study, optimum parameters such as frequency, voltage, volume, duration etc. were determined by working with pure water. In all tests, water samples with different E. coli concentrations were studied in this system with a maximum of 100 μL volume. This volume is important to get enough signal from system because it covered all microelectrode area. The other main idea of these study was construction of the sensor system for making microorganism detection on diverse water samples which have different conductivity. For these water samples and different microbial concentrations, optimum parameters of the system had been determined. However, the system had low efficiency at the high microbial contamination and high water conductivity. Because of this, dielectrophoresis application was used to improve system, which was preferred to obtain high accuracy with short measurement time. Dielectrophoresis is a force providing movement of partially charged particles on the nun- uniform electric field under alternating current. Microorganism cells that have a partially negative charge have been pulled on the electrode surface by the effect of positive dielectrophoretic force. Under these force, microorganism cells have been sequenced like pearl chain. By the way, the bridge is constructed by microorganisms between the microelectrode fingers. Because of these pearl chain form of microorganisms, rapid impedance change is obtained from system and detection is done. It is possible to find many studies which made rapid detection with high accuracy even in low microbial concentrations by using dielectrophoresis on interdigitated electrode systems. In addition to this, microfluidics systems, smart phone controlled systems and also portable systems was successfully studied according to the literature sources. The usage of dielectrophoresis on the system constructed by using interdigitated electrode provides opportunity to improve yield of the system. Low detection time and detection limit was obtained, as the system has a high accuracy to detect microorganisms. However, the system is very sensitive against conditional changes such as temperature, conductivity, pH, etc. Since the main idea of this study is rapid detection microorganism from drinking water, conductivity of tap water or drinking water is the major obstacle on the system. As a result, in this thesis study it is aimed to detect microbial contamination of drinking water with high sensitivity and speed. The system, which was designed by using both interdigitated electrode and dielectrophoresis, provided maximum efficiency. As the detection limit of the system was determined as 103 cfu / mL, targeted high measurement accuracy was not achieved. The system shows about 70% impedance change in case of 7 log/mL microorganism contamination. These results have been proof that the system works with the required accuracy. Based on all these data it is possible that the sensor system can be turned into a device that is capable of microorganism detection in contaminated drinking water. However, it has been observed that the electrodes used in the sensor design can be used for maximum 8 applications. In addition to this limited usage of electrodes, 200 μS / cm is the upper sample conductivity amount because decrease in yield was observed in proportion to the increased conductivity. This upper limit, which includes packaged water, is enough for providing of market needs. However, an additional solution to the system must be provided for other samples such as surface water, municipal water or wastewater which have higher conductivity value. All in all, by improvement of these three types impedimetric sensor systems, it is mainly aimed construction of sensor system that have a high accuracy, low detection of limit, portable or entegrable, miniaturized, long-lasting, useful on different application. It was seen that this system can be constructed by using both interdigitated electrodes and dielectrophoresis. Even there are some problems about short-life electrode, high sample conductivity and limit of detection, all of these can be solved by using the solutions such as creating usage package and designing kits. It is obvious that the sensor system is convenient to solve a very common water safety concern problem in many part of the world.