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Theses and dissertations (Applied Sciences)

Permanent URI for this collectionhttp://ir-dev.dut.ac.za/handle/10321/6

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Start date: 2023Subject: search.filters.subject.Biosensors

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    Development of electrochemical biosensors for sweeteners using engineered nanomaterials supported by computational modelling
    (2024-05) Hloma, Phathisanani; Bisetty, Krishna; Sabela, Myalowenkosi Innocent; Uwaya, Gloria Ebube
    Electrochemical immunosensors are a powerful tool in analytical applications. The current methods for the isolation and detection of artificial and natural sweeteners suffer from challenges in sample preparation and a lack of specificity. However, electrochemical immunosensors offer a sensitive, economical, and selective analytical solution to analyse these commonly used sweeteners, such as aspartame. The author of this work developed electrochemical immunosensors for the food and beverage industries to use in the detection and measurement of aspartame, a non-nutritive sweetener, and rebaudioside A, a natural sweetener. Most artificial sweeteners are low-calorie options that are suggested for ailments linked to health. These sweeteners' ability to remain stable at even high temperatures has greatly expanded the range of meals that can use them. Aspartame and rebaudioside A have not been linked to any health risks, although regulation is still necessary because of their extensive use in the food industry. The developed immunosensors for the detection of aspartame and rebaudioside A were achieved and presented as three case studies in this study. In the first case study, the immunosensor was achieved by fabricating green synthesized PVP capped silver nanoparticles (PVP-AgNPs) with functionalized multi-walled carbon nanotubes (fMWCNTs) and immobilizing the human sweet tase receptor T1R2 in a glassy carbon electrode (GCE), resulting in GCE/PVP-AgNPs/fMWCNTs/T1R2. The electrochemical assessment of aspartame was achieved using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and differential pulse voltammetry (DPV), respectively, under optimum pH 8 in a 0.1 M phosphate buffer with reference to the Ag/AgCl reference electrode. The electro-oxidation of ASP was noticed by a well-defined oxidation peak potential at 1.4 V. The immunosensor sensor showed a linear dynamic range of 2.89 to 27.61 μM (R 2 = 0.9170) based on differential pulse voltammetry, with limits of detection (LOD) and quantification (LOQ) (S/N = 3) of 0.40 μM and 1.34 μM, respectively. The second case study focused on the indirect electrochemical detection of rebaudioside A in the presence of ferro/ferricyanide as a redox probe. The immunosensor was developed by fabricating GCE with zeolitic imidazolate framework-67 (ZIF-67) in combination with fMWCNTs and the immobilization of the T1R2 receptor. The qualitative and quantitative analysis of rebaudioside A was done using CV, EIS, and DPV utilizing a 5 mM [Fe (CN)6] 3-/-4 redox probe. The stable electrode had an exponential dynamic range of 0.9901 µM to 8.2569 µM (R2 = 0.9996). The LOD and LOQ were computed to be 1.10 µM and 3.33 µM, respectively. This case study also used Patch Dock and PyRx to better understand the interactions between Reb A and T1R2. The final case study employed a platinum electrode (PtE) as the working electrode (WE) for the electrochemical immunosensing of aspartame. The modification of PtE involved the utilization of a nanocomposite consisting of PVP-AgNPs and reduced graphene oxide (rGO), with T1R2 immobilized. The electrochemical detection of aspartame was achieved under optimized conditions at pH 8 in a 0.1 M phosphate buffer, utilizing CV, EIS, and DPV as electrochemical tools. The PVP-AgNPs/rGO/T1R2 was used to fabricate Pt and the electrode performed well with a linear increase in oxidation peak currents as aspartame concentrations were increased from 2.38 µM to 25.78 µM (0.9529). The LOD and LOQ were calculated to be 5.85 µM and 17.73 µM, respectively. The synthesized nanoparticles and nanocomposites (PVP-AgNPs/fMWCNTs, ZIF 67/fMWCNTs, and PVP-AgNPs/rGO) were characterized using conventional techniques such as UV-Vis spectroscopy, thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), field flow fractionation (FFF), single particle inductively coupled plasma mass spectrometry (sp-ICPMS), energy-dispersive X-ray spectroscopy (EDS), and scanning electron microscopy (SEM). In addition to the experimental results, computational chemistry methods were undertaken. These included adsorption assessments, density functional theory (DFT), and molecular docking techniques. These techniques were all aimed at achieving a deeper molecular-level understanding of the interactions among the analytes (Aspartame and Reb A), T1R2, and the nanocomposites employed in the modification of the working electrodes (GCE and Pt-E)
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    Development of electrochemical sensors for the detection of mycotoxins in food matrices using functionalised nanocomposites
    (2024-05) Naidoo, Lyndon; Bisetty, Krishna; Meier, Florian; Uwaya, Gloria Ebube
    The analysis of pathogens in foods is of critical importance to ensure consumer safety and quality assurance, as contaminants pose serious risks to public health. Mycotoxins are naturally occurring carcinogenic toxins that arise from specific strains of fungi as they contaminate food. They are found in a wide variety of grains, cereals, and dairy products, causing cancer in both humans and animals. Thus, there is a growing demand for simple, sensitive and inexpensive sensors for mycotoxin detection in lieu of conventionally employed large-scale instrumentation. In this study, the development of electrochemical sensors for the detection of aflatoxin B1 (AFB1), zearalenone (ZEN) and ochratoxin A (OTA) in foods was investigated and presented as three case studies, respectively. In the first case study, an ultrasensitive aptasensor was developed for the indirect detection of AFB1 in the presence of a ferri/ferrocyanide ([Fe(CN)6]3-/4-) redox probe solution. The sensor was constructed by immobilizing an anti-AFB1 aptamer (Apt) to a carboxylated multiwalled carbon nanotube (cMWCNT) and iron oxide (Fe3O4) nanoparticle (NP) composite using a glassy carbon electrode (GCE). This resulted in the development of the GCE/cMWCNTsFe3O4 NP/Apt sensor. An electrochemical response was exhibited from AFB1 applying cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and differential pulse voltammetry (DPV), respectively, utilizing a [Fe(CN)6]3-/4- redox probe prepared in phosphatebuffered saline (PBS) solution with reference to the Ag/AgCl reference electrode under optimized conditions. DPV findings reported very low limits of detection (LOD) and quantification (LOQ) of 0.43 fg mL-1 and 1.44 fg mL-1 respectively in comparison to current literature, over a calibration range of 0.50 fg mL-1 to 5.00 fg mL-1. For real sample analysis, excellent spike recoveries from 95% to 105% were obtained for corn and rice flour. Density functional theory (DFT) was used to propose a reaction scheme by ascertaining the electronic properties of the redox-active functional groups of AFB1. This supported the experimental anodic response findings of DPV. The second case study focused on how PEGylated Fe3O4 NPs and cMWCNTs fabricated on a GCE could be used for the design of an electrochemical sensor for ZEN analysis. The qualitative and quantitative analyses of ZEN were completed using CV, EIS and DPV, respectively, under optimized conditions in a sodium phosphate buffer solution. The developed sensor reported significantly low LODs and LOQs of 0.34 fg mL-1 and 1.12 fg mL-1 respectively, over a calibration range of 1.00 fg mL-1 to 10.00 fg mL-1 by DPV. Excellent spike recoveries ranging from 92% to 106% were obtained for rice and corn flour. The Monte Carlo (MC) adsorption simulation studies predicted the strong interaction of ZEN with the constructed sensor. In the final case study, an OTA electrochemical sensor was designed using a nickel metalorganic framework (Ni-MOF) and carboxylated reduced graphene oxide (cRGO) on a GCE. The detection of OTA was achieved under optimized conditions in PBS solution with the developed GCE/Ni-MOF/cRGO electrode, employing CV, EIS and DPV as electrochemical tools. Applying DPV, the sensor reported very low LODs and LOQs of 3.29 fg mL-1 and 10.97 fg mL-1 respectively, over a calibration range of 10.00 fg mL-1 to 90.00 fg mL-1. Regarding real sample analysis, excellent spike recoveries from 95% to 105% were obtained for corn and rice flour. Molecular dynamics (MD) studies predicted that the Ni-MOF exhibited a strong electrostatic interaction with the OTA analyte, in agreement with the experimental findings. The synthesized nanomaterials (cMWCNTs-Fe3O4 NP, PEG-Fe3O4 NPs/cMWCNTs, and NiMOF/cRGO) utilized in this study were characterized by an array of techniques, including single particle inductively coupled plasma-mass spectrometry (spICP-MS), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), multidetector asymmetrical flow field-flow fractionation (AF4), and Fourier transform infrared spectroscopy (FTIR). Finally, computational modelling studies were undertaken to establish a synergy with the experimental approaches employed in each case study. These methodologies included DFT, docking studies, MC adsorption and MD simulations, which were aimed at predicting and assessing the atomic and molecular interactions between the mycotoxins and their respective electrode systems.