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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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Author: search.filters.author.Naidoo, LyndonHas files: Yes

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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.
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    Analysis of nanoscale ingredients in commercial food and cosmetic products by field-flow fractionation and single particle ICP-MS
    (2020-09) Naidoo, Lyndon; Bisetty, Krishna; Kanchi, Suvardhan; Sabela, Myalowenkosi Innocent
    There is a growing need to disclose the possible presence of nanostructures on the labels of commercial food and cosmetic products in South Africa. Synthetic amorphous silica (SiO2) and titanium dioxide (TiO2) are the two widely used nanoparticles (NPs) selected for this study. This work was undertaken in two stages. The first part deals with an analytical method developed for the separation and characterization of TiO2 NPs capped with poly(ethylene glycol) (PEG) as a potential reference material for sunscreen analysis. The second aspect of this work focuses on SiO2, a highly attractive biomaterial widely used as a food additive (E551) to improve the flow properties of powdered food ingredients. Also, modern computational methods were implemented in both case studies, to better understand the nanoparticulate interactions with the organic substrates at an atomistic and molecular level. For the cosmetics study, asymmetric flow field-flow fractionation (AF4) coupled online to multi-angle light scattering (MALS) and dynamic light scattering (DLS) detectors were employed to assess the geometry and size of the PEGylated TiO2 NPs in terms of the evaluated radius of gyration (rg) and hydrodynamic radius (rh). Single particle inductively coupled plasma mass spectrometry (spICP-MS) and transmission electron microscopy (TEM) were implemented to provide information on the core diameter (d) of the TiO2 particle. To overcome agglomeration, Monte Carlo simulations were employed in this study to assess the effectiveness of PEG capped onto spherically constructed TiO2 anatase nanoclusters by systematically performing a series of adsorption studies. For PEG-TiO2 NPs, AF4-MALS-DLS reported rg and rh values of 28.7 nm and 40.3 nm respectively, with the shape factor (rg/rh) values generally reported in the range of 0.7 to 0.8, indicative of spherical particle geometry. SpICP-MS and TEM obtained complementary measurements of d = 32.0 nm and d = 38.4 nm, respectively. The computational modelling results demonstrated the strong binding affinity of PEG as a capping agent to TiO2, exhibiting stabilisation of TiO2 NPs in aqueous medium. Finally, the developed AF4-MALS-DLS method was applied to two commercial SPF 50 sunscreens, exhibiting promising separation and detection efficiency. These findings can contribute to regulatory measures in line with the South African National Nanotechnology Strategy for the cosmetics industry. With regards to the food application study, a multivariate method was developed for the detection and characterization of SiO2 particles also based on AF4-MALS-DLS. This analytical approach attempts to address the fate and the presence of nanoparticulate SiO2 additives (E551) in food products. The experimental design using SiO2 NP standards resulted in the following optimum conditions of the system: crossflow, 0.8 ml/min; injection time, 5 min and sonication time, 60 min. It was observed that the average geometric diameters (Dgeo) for SiO2 NPs in three selected commercial coffee creamers (A, B and C) detected by AF4-MALS were 286.7 nm, 129.8 nm and 190.7 nm respectively. Similar trends for the coffee creamers were observed for the hydrodynamic diameter (Dh) measurements using AF4-DLS i.e. 301.5 nm, 141.3 nm and 197.8 nm respectively. The rg/rh values were reported ranging from 0.7 to 0.8, indicative of spherical particle geometry. Also, the electrostatic interactions between SiO2 NPs and glucose/water mixtures, as evaluated by Monte Carlo simulations, revealed O-interactions dominating over the flat amorphous SiO2 surface. The strongest interaction observed (around - 239 kcal/mol) for the SiO2-water/glucose mixture demonstrates the hydrophilic nature of SiO2 NPs. The findings in both case studies provide fundamental information to improve the understanding of nanoparticulate interactions with additives and paves the way for the labelling of cosmetic and food products that potentially exhibit nanomaterials in complex matrices.