Samantha F. Douman

Label Free Poly(2,5-dimethoxyaniline)–Multi-Walled Carbon Nanotubes Impedimetric Immunosensor for Fumonisin B1 Detection

An impedimetric immunosensor for fumonisin B1 (FB1) was developed from a poly(2,5-dimethoxyaniline)-multi-walled carbon nanotube (PDMA-MWCNT) composite on the surface of glassy carbon electrode (GCE). The composite was prepared electrochemically and characterized using cyclic voltammetry. The preparation of the FB1 immunosensor involved the drop-coating of a bovine serum albumin mixture of the anti-fumonisin antibody (anti-Fms) onto the composite polymer-modified GCE. The electrochemical impedance spectroscopy (EIS) responses of the FB1 immunosensor (GCE/PDMA-MWCNT/anti-Fms) have a linear range of 7 to 49 ng·L−1, and the corresponding sensitivity and detection limits are 0.272 kΩ L·ng−1 and 3.8 pg·L−1, respectively. The limit of detection of the immunosensor for certified corn sample (i.e., certified reference material) is 0.014 ppm FB1, which is in excellent agreement with the value published by the vendors and significantly more accurate than that obtained with enzyme-linked immunosorbent assay (ELISA).

Polyanilino-Carbon Nanotubes Derivatised Cytochrome P450 2E1 Nanobiosensor for the Determination of Pyrazinamide Anti-Tuberculosis Drugs

Pyrazinamine (PZA) is one of the most commonly prescribed anti-tuberculosis (anti-TB) drug due to its ability to significantly shorten the TB treatment period. However, excess PZA in the body causes hepatotoxicity and liver damage. This, therefore, calls for new methods for ensuring reliable dosing of the drug, which will differ from person to person due to interindividual differences in drug metabolism. A novel biosensor system for monitoring the metabolism of PZA was prepared with nanocomposite of multi-walled carbon nanotubes (MWCNTs), polyaniline (PANI) and cytochrome P450 3A4 (CYP3A4) electrochemically deposited on a glassy carbon electrode (GCE). The nanocomposite biosensor system exhibited enhanced electroactivity that is attributable to the catalytic effect of the incorporated MWCNTs. The biosensor had a sensitivity of 7.80 μA/μg mL-1 PZA and a dynamic linear range of 4.92 160 ng/mL PZA.

Optoelectronics of Stochiometrically Controlled Palladium Telluride Quantum Dots

Water dispersed PdTe semiconducting nanocrystals were synthesized and stabilised with 3-mercaptopropionic acid (3-MPA). HRTEM studies revealed the formation of spherical nanoparticles of average size ~4 nm with good crystallinity. UV-visible spectral analysis and band gap measurements confirmed that the nanocrystals are indeed semiconductors. This semiconducting characteristic was supported by electrochemical impedance spectroscopy (EIS) data which gave Bode plots with absolute frequency and a maximum frequency phase angle values of 38.3° and 75°, respectively. Electroanalysis of the film on glassy carbon electrode (GCE) verified the retention of the ability of Pd to adsorb hydrogen on its surface as well as absorb hydrogen within its lattice.

An Amperometric Cytochrome P450-2D6 Biosensor System for the Detection of the Selective Serotonin Reuptake Inhibitors (SSRIs) Paroxetine and Fluvoxamine

Paroxetine is the second most prescribed selective serotonin reuptake inhibitor (SSRI) antidepressant drug, characterized by extensive inter-individual variation in steady state plasma concentrations resulting in drug toxicity amongst patinets. A nanopolymeric biosensor for studying the biotransformation of paroxetine is presented. The bioelectrode system consists of cytochrome P450-2D6 enzyme encapsulated in nanotubular poly (8-anilino-1-napthalene sulphonic acid) electrochemically deposited on gold. The biosensing procedure involved the determination of the extent of modulation of fluvoxamine responses to the P450-2D6 enzyme electrode after incubation in paroxetine standard solutions. Paroxetine inhibited the activity of cytochrome P450-2D6 (CYP2D6) resulting in a decrease in the fluvoxamine signal of the biosensor. The biosensor gave a linear analytical response for the paroxetine in the interval 0.005 and 0.05 μM, with a detection limit of 0.002 μM and a response time of 30 s. Electrochemical Michaelis–Menten kinetics of the reversible competitive inhibition of the fluvoxamine responses of the biosensor by 0, 0.05 and 0.1 μM paroxetine gave apparent Michaelis–Menten constant (KMapp) values of 1.00 μM, 1.11 μM and 1.25 μM, respectively. The corresponding value for the maximum response, IMAX was 0.02 A. The dissociation constant, KI, value evaluated from Dixon analysis of the paroxetine modulation data was estimated to be-0.02 μM while Cornish-Bowden analysis confirmed the competitive inhibitory characteristics of the enzyme.

Wireless Electrochemiluminescence at Nafion–Carbon Microparticle Composite Films

Thin films of a composite of nafion and carbon microparticles have been deposited on nonconducting substrates and their conductivity as well as their ability to generate electrochemiluminescence investigated. The films exhibit very low conductivity (<6 × 103 S m-1) for low particle loadings, but once the percolation threshold is reached (volume percentage of 71 ± 8% carbon particles), the conductivity increases dramatically and a maximum conductivity of 2.0 ± 0.1 × 107 S m-1 is achieved. The electrochemical properties of the composites, including heterogeneous electron transfer rates, were probed using cyclic voltammetry. Significantly, bipolar, or wireless, electrochemiluminescence can be generated with films that contain >65% (by volume) carbon particles using [Ru(bpy)3]2+ as the luminophore and tripropylamine as the coreactant, at an electric field of 14 V cm-1. Under these conditions, the complete film is sufficiently conducting to become polarized in the external electric field and the electrochemiluminescence intensity correlates strongly with the film conductivity. These results demonstrate the usefulness of particle arrays for the wireless generation of electrochemiluminescence at relatively low electric field strengths.

Cytochrome P450-3A4/Copper-Poly(Propylene Imine)-Polypyrrole Star Co-Polymer Nanobiosensor System for Delavirdine - A Non-Nucleoside Reverse Transcriptase Inhibitor HIV Drug

HIV and AIDS are among the world’s pandemics that pose serious concern to almost every individual in the world. With the current level of availability of anti-retroviral (ARV) drugs and the ease of accessibility of treatment in many countries such as South Africa, the disease can be controlled by suppressing the viral load of an infected individual. These anti HIV drugs such as delavirdine are metabolised by enzymes which are found in the liver microsomes, particularly those of the cytochrome P450 family. Due to the fact that the metabolic rate of a patient determines the effect of the drug, the drug could either have a beneficial or an adverse effect once it is administered. It is therefore imperative that the metabolic profile of a patient is determined to ensure proper dosing of the ARV drugs. In this study a nanobiosensor system was devised and used for the determination of the metabolism of delavirdine (DLV), a non-nucleoside reverse transcriptase inhibitor (NNRTI) ARV drug. The nanobiosensor was prepared by the entrapment of the isoenzyme CYP3A4 into a pre-formed electro active carrier matrice consisting of a dendrimeric copper generation-2 poly(propylene imine)-co-polypyrrole star copolymer (Cu(G2PPI)-co-PPy). The metallo-dendrimer was used as a host for the enzyme and provided the necessary bio-compatible environment that allowed the direct transfer of electrons between the enzyme’s active centres and platinum electrode surface. (Cu(G2PPI)-co-PPy) was prepared by the incorporation of the copper metal into the G2PPI and the electropolymerization of pyrrole onto the Cu(G2PPI). The incorporation of Cu into G2PPI was determined by Fourier transform infrared (FTIR) spectroscopy which did not show the presence of the Cu but showed an increase in the intensities of the peaks after the incorporation. The surface morphology of Cu(G2PPI-2Py) was confirmed by the use of high resolution scanning electron microscopy (HRSEM) which showed a difference in the surface morphology of the dendrimer moiety with the addition of the copper metal. The HRSEM images after Cu incorporation resulted in the change from rough surface to smooth surface with open cavities which were essential for the entrapment of the biological systems (CYP3A4). The energy band gap of (Cu(G2PPI)-co-PPy) were determined to be 3.85 eV, signifying that the copolymer is characteristic of a biocompatible semiconductive platform for applications in biosensors. The star copolymer (Cu(G2PPI)-co-PPy) was characterized using cyclic voltammetry where it was confirmed that the material was electroactive and conducting due to electron movement along the polymer chain. A diffusion co-efficient (Do) value of 8.64 x 10-5 cm2/s was determined for the material indicating a slow electron transfer kinetics within the diffusion layer. The resultant nanobiosensor parameters include a dynamic linear range (DLR) of 0.01-0.06 nM, a limit of detection (LOD) of 0.025 nM and a sensitivity value of 0.379 μA/nM.

Tin Selenide Quantum Dots Electrochemical Biotransducer for the Determination of Indinavir - A Protease Inhibitor Anti-Retroviral Drug

Biocompatibility of tin selenide quantum dots was achieved by the incorporation of 3-mercaptopropionic acid (3-MPA) as a capping agent, which also improved the stability and the solubility of the material. The UV-Vis spectrophotometric analysis of the quantum dots revealed a broad absorption band at ~ 330 nm (with a corresponding band gap, Eg, value of 3.75 eV), which is within the range of values expected for quantum dots materials. The 3-mercaptopropionic acid-capped tin selenide (3-MPA-SnSe) quantum dots were used to develop an electrochemical biosensor for indinavir, which is a protease inhibitor antiretroviral (ARV) drug. The biosensor was prepared by the self-assembly of L-cysteine on a gold electrode that was functionalised with 3-MPA-SnSe quantum dots, followed by cross-linking with cytochrome P450-3A4 (CYP3A4) using 1-ethyl-3(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS). The electrocatalytic properties of the biosensor included a characteristic cyclic voltammetric reduction peak at-380 mV, which was used to detect the response of the biosensor to indinavir. The sensor performance parameters included response time and limit of detection (LOD) values of 11 s and 3.22 pg/mL, respectively. The test concentration range studied (0.014 – 0.066 ng/mL) gave a linear calibration plot for indinavir, and it was lower than the physiological plasma concentration index (i.e. maximum plasma concentrations, Cmax,) of indinavir (5 - 15 ng/mL) normally observed 8 h after intake. This indicates that the biosensor can be very useful in the case of ultra-rapid metabolisers where very low Cmax values are expected