Faculty of Applied Sciences
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Item Electrochemical and molecular modelling studies to assess the photoreactive properties of Efavirenz(2022-09) Mthiyane, Thethiwe Promise; Bisetty, Krishna; Jordaan, M. A.; Uwaya, Gloria EbubeEfavirenz (EFV) is commonly used as an antiretroviral drug to treat HIV/AIDS and is known to undergo photoreactions that could be exploited for photodegradation applications. In addition, there is limited information on the photoreactivity of EFV. This work focuses on two case studies to assess the photocatalytic properties of EFV supported by experimental and molecular modelling (commonly referred to as computational chemistry). The first case study deals with the design of an innovative electrochemical sensor for the detection of EFV, using titanium dioxide nanoparticles (TiO2-NPs) doped on glassy carbon electrode (GCE) with nafion as an anchor agent (GCE/TiO2-NPs-nafion). TiO2-NPs were synthesized using Eucalyptus globulus leaf extract and characterized using Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-vis), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and energy-dispersive spectroscopy (EDS). The electrochemical and sensing properties of the developed sensor for EFV were assessed using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), differential pulse voltammetry (DPV) and chronoamperometry. The oxidation peak current response for EFV on the GCE/TiO2-NPs-nafion electrode was greater compared to the bare and modified GCE/TiO2-NPs electrodes. A linear dynamic range of 4.5 to 18.7 µM with a 0.01 µM limit of detection was recorded on the electrode using DPV. The electrochemical sensor demonstrated good selectivity as well as practicability for the detection of EFV drugs with excellent recoveries ranging from 92.0-103.9%. The density functional theory (DFT)-based quantum chemical modelling was used to establish the chemical reactivity for EFV, suggesting the benzoxazine ring as the active site. Monte Carlo (MC) simulations revealed a strong electrostatic interaction on the GCE/TiO2-NPs-nafion-EFV (substrate-adsorbate) system. The results showed good agreement between the MC computed adsorption energies and the experimental CV results for EFV. The stronger adsorption energy of nafion onto the GCE/TiO2-NPs substrate contributed to the catalytic role in the signal amplification sensing of EFV. The second case study deals with the assessment of the photocatalytic degradation of EFV in combination with green synthesized TiO2-NPs. The photocatalytic activity of TiO2-NPs was examined by the degradation of EFV in an aqueous medium and a maximum degradation efficiency of 91.77% was observed at a reaction time of 5 h. In addition, the electronic spectra of the EFV complex bound to single TiO2-NPs in a gas- and solution-phase were investigated using time-dependent density functional theory (TD-DFT) calculations. The calculated spectra obtained in this work were benchmarked against the gas-phase photodecomposition of the EFV- TiO2-NPs complex using UV-vis spectrophotometry. Overall, the results show that the biosynthesized TiO2-NPs have the potential for sensing pharmaceutical applications and their degradation. The results provide an effective way to explore the design of new 2D materials for the sensing of EFV, which is highly significant in the field of medicinal and materials chemistry.Item Development of electrochemical immunosensors for detection of Tau protein : computational and experimental studies(2019-11) Harilal, Calvin Carl; Bisetty, Krishna; Kanchi, SuvardhanTau protein is a microtubule-associated protein (MAP) found in neuronal cells of the central nervous system. In recent years it has become an important biomarker for neurodegeneration and pathologies of the nervous system, thereby necessitating novel approaches for its detection. This study involves the development of two immunosensors for the detection of tau protein. The study makes use of nanomaterials and antibody transducers as signal enhancing strategies. Both sensors rely on indirect detection of tau protein as a copper(II) complex using a Cu(II)/Cu (I) redox probe. The electrochemical immunoassay is based on the immobilisation of anti-tau antibodies onto a gold working electrode that has been modified with nanomaterials using N- Hydroxysuccinimide (NHS) binder. The first sensor makes use of gold nanoparticles (AuNPs) and the second utilises a nanocomposite of graphene oxide (GO) decorated with silver nanoparticles (AgNPs). Cyclic voltammetry (CV) was used to optimise the electrochemical signal of the tau protein, while quantitative analyses were achieved by differential pulse voltammetry (DPV) and square wave voltammetry (SWV) under the established optimised conditions. Results for the quantitative experimental studies revealed relatively low detection limits for both sensors. The lowest of these detection limits were obtained for DPV analysis of using sensor 1 which produced an LOD of 3.31 nM and an LOQ of 11.04 nM. For sensor 2 the SWV analysis produced the lowest LOD and LOQ of 1.73 nM and 5.76 nM respectively. Computational chemistry methods implemented at the DFT level were used to support the developed electrochemical sensor. The molecular docking results showed relatively good binding affinity of -4.72 kcal/mol between the NHS and the antibody. A 100 ns MD simulation showed a good free binding energy value of -20.51 kcal/mol at pH 7, in accordance with the optimum pH implemented in the experimental work. Furthermore, adsorption studies were performed between the citrate coated nanoparticles on the Au electrode and NHS/anti-tau antibody/tau complex. The energy adsorption simulations revealed the energy favoured interaction between the designed layers with the stabilizing energy changes from -- 23.74 to -142.96 kcal/mol for sensor 1 and for sensor 2 it changed from -7.6 to -127.82 kcal/mol. Overall the computational data correlated well with experimental work. The two novel immunosensors developed in this work, give new insights into electrochemical and computational methods for the detection of proteins, and could lead to the fabrication of a device for point-of-applications in early diagnosis of neurodegenerative disorders.Item An investigation of the voltammetric behaviour of antioxidants in flavonoids(2020-04) Ramsarup, Lee-Ann; Bisetty, Krishna; Kanchi, SuvardhanThe two case studies in this work involve the development and fabrication of an electrochemical biosensor using various enzymes for the evaluation of the electrochemical responses, relating to the total phenolic (TP) content and the antioxidant activities in wine and tea samples respectively. The modification of the glassy carbon electrode (GCE) was carried out using green apple as an enzymatic source of polyphenol oxidase and laccase enzyme. The experimental variables were optimized using the Box-Behnken experimental design as a predictive model, for a better understanding of the parameters and their interaction responses with each other during an electrochemical analysis. This multivariate optimization method is based on a factorial design, where the three most influential factors include the electrolyte pH, the deposition time (td) and the scan rate (sr). The design was run in a single block fashion while random order of experiment was selected to provide greater protection against the effects of outlying variables. The optimized results obtained yielded the most suitable conditions for the determination of the TP content in wine samples. They were selected as follows: phosphate buffer of pH 7.65 as supporting electrolyte, td 29.8 s and sr 25.0 mV/s respectively. The method was optimized for the current signal at a deposition potential of 0.2 V and within an oxidation potential of -0.2 V to 0.6 V. Good analytical responses were obtained with apple sensors for the detection of TP content in wine samples, with a higher concentration in red wines than in white wines. Differential pulse voltammetry (DPV) and cyclic voltammetry (CV) were used to establish and interpret the redox mechanisms of flavonoids present in alcoholic and non-alcoholic beverages. The sensor responses were evaluated by first, investigating the changes in the total phenolic (TP) content in wine samples using catechin as a standard. Thereafter, the electrochemical behaviour of rutin and ascorbic acid, antioxidant capacities (trolox reagents) were established in tea samples, yielding a positive linear correlation between the trolox equivalent antioxidant capacities (TEAC) and TP content (R2 = 0.9812 ± 0.012). DPV was applied to the laccase modified GCE, and the experimental results indicate that this sensor shows good reducing properties. The scavenging ability of 2,2’-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) a diammonium salt, was assessed in the sample extracts, which yielded half-maximal effective concentration (EC50) values of 10.80 μg/ml and 11.62 μg/ml for ascorbic acid and rutin respectively. These findings indicated that the experimental design was a convenient method to evaluate the statistical significance of the optimised parameters, and the positive linearity for the TEAC and the TP content confirms the robustness of this methodology.