In this master’s thesis I researched electrochemical detection of pesticide imidacloprid (IMD) in miniature fluidic system (PS). IMD is a neonicotinoid insecticide and is, among other things, toxic to aquatic environments and non-target insects like bees. For that reason, its monitoring in open waters and agricultural products is of utmost importance. IMD has redox active nitro group which makes it eligible for detection via electrochemical methods. These methods are cheaper, faster and simpler than conventionally used methods like liquid chromatography.
The aim of this thesis can be separated into three parts: first one was to study flow patterns of liquid in miniature flow system (PS) at different flow rates, second was to investigate influence of flow rate on the results of electrochemical measurements in PS in model analyte hexacyanoferrate (HCF) and third one was to research parameters of electrochemical detection of IMD in PS.
The first part included construction of computational fluid dynamics (CFD) model in PS and determination of parameters at which laminar flow regime is achieved. The results of CFD model were used for visualization of impact of system’s geometry and liquid’s flow rate on the flow patterns in PS. We determined that at flow rates higher than 2,9 ml/min circulations in PS appear. Thickness of diffusion layer and theoretical height of limiting current in cyclic voltammetry (CV) in HCF at different flow rates were also determined based on the results of CFD model. The thinnest diffusion layer and the highest limiting current were calculated at 4,9 ml/min and were 90 µm and 0,025 mA respectively.
In second part I connected results of CFD model with experimental measurements in HCF and thus I validated the CFD model. Integrated three electrode system (TES) with graphite based working electrode was used for detection of HCF. Electrochemical measurements were conducted in electrochemical cell and PS using cyclic voltammetry (CV). I demonstrated that with higher flow rates and lower scanrates mass transport transitions from more diffusion controlled to more convectively controlled one. In our system, for detection of HCF, scanrates equal or lower than 10 mV/s and flow rates equal or higher than 1,1 ml/min were needed to record convectively controlled process. Experimental results were compared to previously calculated limiting currents. For flow rates at 1,1 and 2,9 ml/min calculated limiting currents deviated for 5 % from the measured ones and at 4,9 ml/min the deviation was 25 %, which is attributed to circulation of fluid at that flow rate in PS.
The goal of third part was to evaluate suitability of miniature flow system for detection of IMD and evaluation of limit of detection and limit of quantification in this system. The measurements were carried out using CV and differential pulse voltammetry (DPV). From results of these measurements, I concluded that electrochemical response of TES in IMD depends on fluid’s flow rate and adsorption of IMD on the surface of the working electrode. The value of limit of detection in PS and in electrochemical cell was determined at optimal conditions and was at around 40 nM for both systems.
Results of master’s thesis contributed to the better understanding of influence of mass transport on electrochemical measurements in miniature flow systems and exposed possibility of IMD detection in flow systems, thus contributing to the development of flow systems for fast, simple and cheap detection of IMD and other pollutants in water.
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