Ontwikkeling van licht-selecteerbare elektroden op basis van halfgeleidende oxiden voor lab-on-chip applicaties

This thesis covers the development of light-addressable electrodes (LAE) for space- and time-resolved electrochemical manipulation inside lab-on-a-chip (LoC) systems. LoC systems are miniaturized analysis platforms, combining various functionalities for chemical and biological processing, manipulation and monitoring, all on chip level. They are usually part of a microfluidic system. Due to their advantages associated with miniaturization, small sample volumes, their versatile design possibilities, the ability for automation, and the resulting ease of use, the research and development in this field have further increased over the recent years. In particular, point-of-care (PoC) and organ-on-a-chip (OoC) systems are currently in focus. As the term suggests, PoC instruments are intended to obtain diagnostic results directly from the patients blood or saliva, instead of waiting for results from the diagnostic laboratory. Hence, it is possible to achieve rapid, on-site results directly in the doctor's office, hospital, or at home. OoC assemblies attempt to replicate the functioning of various organs. By using these LoC setups, a more accurate simulation of the extracellular environment is possible. The final aim is to resemble the human body in vitro, making the development of drugs more reliable. Despite their various areas of use, most microfluidic systems share common components. The central component in each system is the microfluidic channel, through which the sample fluid is transported. In addition, passive and active elements for flow control, sensing and manipulation are integrated at predefined locations. Due to the trend for further miniaturization and parallelization of the microfluidic channels and the associated integrated components, the already complex fabrication process is becoming even more sophisticated. Especially, the design for biological systems is often unpredictable. Conventional systems with predefined sensor and actuator structures therefore require usually multiple design cycles which is time-consuming and costly. As an alternative approach, light-addressable technologies, which enable a time and spatial selection of the active area during runtime, can be implemented. One of these components is the light-addressable electrode, which is essentially a photoelectrochemical cell. The LAE consists of a thin-film semiconductor deposited on a conductive, transparent substrate. Local illumination leads to the generation of electron-hole pairs, triggering electrochemical reactions at the semiconductor-electrolyte interface. To enable light stimulation in this work, titanium dioxide (TiO2) as an n-type semiconducting material is deposited on fluorine-doped tin oxide via either a sol-gel strategy or pulsed laser deposition (PLD)-technique. In the first approach, sol-gel derived compact TiO2 films were prepared with different thicknesses. The resulting electrodes were characterized morphologically by scanning electron microscopy and thickness quantification as well as performing photoelectrochemical evaluations to probe the addressability. In the course of this thesis, PLD-derived electrodes replaced the sol-gel LAEs as they enable advantageous photoelectrochemical characteristics. This also led to a more detailed study of the PLD-based electrodes. The exact crystal structure after deposition, a simplified equivalent circuit and the minimal resolution of the photocurrent with a high-resolution laser setup were studied. An important parameter when discussing biological or chemical systems is the pH value. If the LAE is illuminated, the photocurrent can induce a photocatalytic water splitting reaction. This can change the pH value locally at the electrode-electrolyte interface, generating pH gradients towards the surrounding solution. These resulting pH changes have been studied for different microfluidic system designs regarding later applications in this thesis. To study pH gradients, an optical system for the excitation of the LAE under a fluorescence microscope was designed. With this setup, the pH gradient in the microfluidic channel was made visible with a pH-sensitive fluorescent dye. Subsequently, different excitation parameters (bias potential, illuminated area, illumination intensity) were varied in order to obtain information about the effect towards the formation of those pH gradients. Moreover, an electrochemical system for changing pH values and measuring them almost simultaneously inside the same microfluidic channel was developed. A light-addressable potentiometric sensor (LAPS) was additionally implemented to detect the pH change. For fabrication, a laser-cut double-sided microfluidic tape, which defines the microfluidic channel geometry, was adhered to the Al/n-Si/SiO2/Si3N4-LAPS structure and closed with the glass/SnO2:F/TiO2 LAE. To determine its potential uses, the new system was evaluated for generating and detecting static and dynamic pH changes. Hereby, the pH amplitude and width could be tailored specifically. As a final proof of concept, the catalytic conversion of penicillin (to penicilloic acid and H+ ions) by the pH-sensitive enzyme penicillinase was monitored and controlled utilizing the LAE-LAPS system. In a first step, the enzyme was immobilized on the LAPS surface using tobacco mosaic virus nanoscaffolds. Once the pH sensitivity of the catalytic reaction was proven, the pH-value changes were induced using the LAE and thus, the enzyme activity was adjusted in real time. Besides being applied as an electrochemical actuator, the LAE was also evaluated for its potential as a pH-dependent photoelectrochemical sensor. For this, the pH dependence of the photocurrent at different pH values was first studied. The local change of the pH value due to the enzymatic reaction was detectable by the LAE directly. In summary, this thesis intends to address three objectives by implementing a semiconducting light-addressable electrode. Firstly, it will be shown that different areas within a microfluidic system can be flexibly addressed with time and spatial resolution, using light-triggered photoelectrochemical reactions. In a second step, a light-addressable sensor-actuator platform that combines the LAE with the LAPS inside the same microfluidic channel has to be developed. Lastly, the use of photoelectrochemical actuators has to be highlighted in a final experiment as a versatile tool to add further functionalities to the already established flow control and sensing methods in the microfluidic system. This thesis covers the development of light-addressable electrodes (LAE) for space- and time-resolved electrochemical manipulation inside lab-on-a-chip (LoC) systems. LoC systems are miniaturized analysis platforms, combining various functionalities for chemical and biological processing, manipulation and monitoring, all on chip level. They are usually part of a microfluidic system. Due to their advantages associated with miniaturization, small sample volumes, their versatile design possibilities, the ability for automation, and the resulting ease of use, the research and development in this field have further increased over the recent years. In particular, point-of-care (PoC) and organ-on-a-chip (OoC) systems are currently in focus. As the term suggests, PoC instruments are intended to obtain diagnostic results directly from the patients blood or saliva, instead of waiting for results from the diagnostic laboratory. Hence, it is possible to achieve rapid, on-site results directly in the doctor's office, hospital, or at home. OoC assemblies attempt to replicate the functioning of various organs. By using these LoC setups, a more accurate simulation of the extracellular environment is possible. The final aim is to resemble the human body in vitro, making the development of drugs more reliable. Despite their various areas of use, most microfluidic systems share common components. The central component in each system is the microfluidic channel, through which the sample fluid is transported. In addition, passive and active elements for flow control, sensing and manipulation are integrated at predefined locations. Due to the trend for further miniaturization and parallelization of the microfluidic channels and the associated integrated components, the already complex fabrication process is becoming even more sophisticated. Especially, the design for biological systems is often unpredictable. Conventional systems with predefined sensor and actuator structures therefore require usually multiple design cycles which is time-consuming and costly. As an alternative approach, light-addressable technologies, which enable a time and spatial selection of the active area during runtime, can be implemented. One of these components is the light-addressable electrode, which is essentially a photoelectrochemical cell. The LAE consists of a thin-film semiconductor deposited on a conductive, transparent substrate. Local illumination leads to the generation of electron-hole pairs, triggering electrochemical reactions at the semiconductor-electrolyte interface. To enable light stimulation in this work, titanium dioxide (TiO2) as an n-type semiconducting material is deposited on fluorine-doped tin oxide via either a sol-gel strategy or pulsed laser deposition (PLD)-technique. In the first approach, sol-gel derived compact TiO2 films were prepared with different thicknesses. The resulting electrodes were characterized morphologically by scanning electron microscopy and thickness quantification as well as performing photoelectrochemical evaluations to probe the addressability. In the course of this thesis, PLD-derived electrodes replaced the sol-gel LAEs as they enable advantageous photoelectrochemical characteristics. This also led to a more detailed study of the PLD-based electrodes. The exact crystal structure after deposition, a simplified equivalent circuit and the minimal resolution of the photocurrent wi