Abstract
Food safety assurance is a major concern for both developed and developing countries. It is stringent to avoid any food safety hazards since potential food contaminants (e.g. fertilizers, pesticides) can create adverse health effects and even life-threatening for them. Indeed, contaminants like nitrate (NO3 - ) responsible for infant methemoglobinemia, gastric, bladder cancer, psychological stress, etc. and chlorpyrifos (CPF) responsible for acute neurological disorders, toxicity, and reproductive disorders become major concerns and need to be detected. Therefore, it is fundamental to realize devices that allow us to be able to detect these agents in food. In this circumstance, electrochemical sensors and biosensors are a promising class of simple and cost-effective, and portable analytical devices that can selectively detect the target agents. The thesis aims to develop portable electrochemical sensors for the detection of nitrate and CPF by selecting cost-effective materials and fabrication techniques to fulfill the requirement for low cost devices that can be used in both developing and underdeveloped countries. In addition, the aim is to eliminate the need for conventional analytical techniques and utilize active materials like copper (Cu) or carbon nanotubes (CNTs) so that guarantee that the sensors can be effectively employed not only in the laboratory but also for on-site analysis. For the detection of nitrate, a novel flexible amperometric sensor, based on a silver (Ag) working electrode modified with copper (Cu) was presented. A simple and low-cost screen-printing technique was used on top of a flexible polyethylene terephthalate (PET) substrate for fabricating the electrodes. The electrochemical deposition was used for Cu nanoclusters assembly on top of the working electrode. The electrochemical performance of our sensors was analyzed and compared to other works thanks to the increased electroactive surface area. In fact, the Cu/Ag sensors showed higher catalytic activity towards the electro-reduction of nitrate (sensitivity: 19.578 µA/mM), as well as a lower limit of detection (LOD: 0.207 nM) and dynamic linear range of detection (0.05 mM to 5 mM). To improve the sensitivity by increasing the electroactive surface area and to broaden the detection range, single-walled carbon nanotubes (SWCNTs) was spray deposited on top of the working electrode before Cu electrodeposition. This drastically improves the linear concentration range (0.5 µM to 6.0 mM) with a lower limit of detection (LOD: iii 0.166 nM). Both sensors showed their optimum performance till the 2nd week. Mechanical stress performed by a customized bending setup revealed that the SWCNTs sensor can resist up to 500 bending with minimal standard deviation. Both sensors were tested against different interferents and found very negligible effects. In real sample analysis (tap and river water), the sensors exhibited good agreement with the compared outcome of the high-performance liquid chromatography. Finally, a novel and sensitive aptasensor for the detection of chlorpyrifos was proposed. The screen-printed Ag electrode was used for the first time to develop the aptasensor in a cost effective way for the determination of this pesticide. The process of the fabrication was tuned by various characterization procedures using Fourier transform infrared spectroscopy (FT-IR), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV). The result from CV and EIS were in an agreement. The aptasensor was optimized for the ideal pH of PBS, aptamer concentration, and incubation time of chlorpyrifos where it was found that the aptamer works best at pH 7. The optimized aptamer concentration was found 1 µM as there is a possibility of intermolecular hybridization when the concentration is increased and a possibility of less conformational changes when the concentration is decreased than 1 µM. The CPF incubation time was optimized as 40 min because the CPF starts to saturate after 40 min. Under these optimal conditions, the aptasensor showed a linear range from 1 to 105 ng/mL with a low limit of detection of 0.097 ng/mL compared to the recent state-of-the-art. The aptasensor was tested against different interferents (carbofuran, dichlorvos, malathion, deltamethrin, and metamitron) and found negligible effect. Moreover, stability over time was investigated with a result of 14 days of active life. Additionally, the sensor was also tested to detect CPF in the real sample with an acceptable recovery from 97.7 % to 105.7 % for the banana sample and from 103.5 % to 104.1 % for the grape sample.