Abstract
Pharmaceuticals have added immense value to the health of living organisms over the years and recent reports have shown that the global annual production of pharmaceuticals is around 200 thousand tonnes. The presence of pharmaceuticals in the environment therefore aroused profound concerns amongst environmental scientists as these substances can also have unpredictable adverse effects on living organisms. Ciprofloxacin (antibiotic) and paracetamol (PAR) (painkiller) are two examples of these pharmaceutical compounds which can persist in the environment as a result of human use. They are manly prescribed together. Antibiotics are not the only problem with residual medicines in the environment. Due to the fact that many analgesics, including substituted acetanilides such as paracetamol, their residues are also found in the environment and can cause cell proliferation or restriction of reproductive disorders. The quinolone functional group in these drugs are stable towards the bacterial metabolism process, thus is degraded slower in the environment compared to other antibiotics. The lack of much information on the environmental impact of ciprofloxacin, paracetamol and the ciprofloxacin- paracetamol (CPX-PAR) mixture has warranted more research in understanding their potential risks. In view of these adverse effects and the water pollution caused by high levels of CPX and PAR, it is critical to monitor the levels of these drugs in pharmaceutical dosage forms, water and biological fluids. Numerous analytical methods for the detection of single or multiple antibiotics and analgesics have been reported such as High Performance Thin Layer Chromatography (HPTLC,) colorimetric, fluorometric, electrochemical, capillary electrophoresis and Reversed Phased High Performance Liquid Chromatography (RP-HPLC). Electrochemical techniques are the most preferred methods of detection due to the advantages such as rapid detection, portability, simplicity and low maintenance cost. It is well known that the performance of electrochemical sensors is mainly determined by the electrocatalytic activity of the working electrode (bare glassy electrode). Hence, the design of electrode materials is key for electrochemical sensors. In order to improve the electrochemical signal and the rapid response of electrochemical sensors, it is necessary to modify the working electrode surface with suitable functional materials.
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Using unmodified electrodes such as the bare glassy electrode for electro-oxidation of analytes causes surface fouling on electrode and limits the electro-detection of the analyte. Fouling blocks electron transfer and ultimately leads to deterioration of sensor performance and this is considered a major problem in electrochemical sensing.To overcome this issue glassy carbon electrodes (GCE) were modified with different nanomaterials such as graphitic carbon nitride (gCN), silver polyvinylpyrilidene (AgPVP), activated carbon banana peels, bismuth vanadate, activated carbon potato peels, silver and tin oxide hollow spheres. The Ag+/Ag redox behaviour of the electrode in 0.1 M phosphate buffer solution (PBS), pH 6.1 at potentials 0.55 and 0.68 V for the oxidation of gCN-AgPVP and at 0.17 V for the reduction of Ag cations using cyclic voltammetry (CV) confirmed the presence of AgPVP on the electrode surface. The prepared gCN-AgPVP were characterised by using scanning electron microscope (SEM), transmission electron microscopy (TEM),ultraviolet-visible spectrophotometry (UV-Vis) and X-ray diffraction (XRD). All of these characterisations confirmed that silver was present on the surface gCN-PVP. Cyclic voltammetry showed the reversible electrochemical behaviour of the platform (GCE/gCN-AgPVP) in a redox probe [Fe(CN)6]3-/4-. Electrochemical impedance spectroscopy (EIS) experiments measured the charge transfer resistance (Rct) which indicated that the addition of AgPVP improved the electro-kinetics of the system. A 68 mV/pH value obtained from pH studies proved that the redox chemistry of PAR on the gCN-AgPVP electrode was pH dependent and involved a two-electron process.. At the optimised pH of 6.1, the limit of detection of PAR was within a linear range of 0.01 to 100 μM, and was evaluated as 0.079 μM using square wave voltammetry (SWV). When compared to other values reported in the literarure this limit of detection was relatively lower. The prepared sensor was reproducible and specific to PAR especially when other interfering species such ascorbic acid, glucose, urea, sodium nitrate and potassium chloride were present. Stability studies were conducted for 10 days at 4°C. The results from these studies showed good stability where the electrode retained 94.64 % of its sensor activity. This electrochemical method of detection of PAR was validated for a real sample by analysis using UV-Vis spectroscopy. Both methods showed similar results for all samples. This implied that the prepared nanomaterial was reliable for the detection of PAR .
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Activated carbon was synthesised from agricultural waste (banana peels) and was modified with bismuth vanadate, for the electrochemical detection of ciprofloxacin. The modification of working electrode with activated carbons has characteristics like large surface area and various oxygen-containing functional groups. It is also very chemically stable, resistant to corrosion, and has good electrical conductivity. It enhances the sensitivity of the electrode towards the analyte. In addition, Ag, gCN, PVP, BiVO4, SnO2 were also applied in modification of the working electrode therefore to achieve excellent electronic band structure, stability, and conductivity (charge transfer which are the key characteristics for electrochemical sensors. Similar characterisation techniques as for gCN-AgPVP confirmed the formation of ACBP-BiVO4.The detection limit was again detemined using square wave voltammetry and was found to be 0.1279 μM. It was again noted that the electro-kinetics of the system was improved by the incorporation of ACBP-BiVO4 as indicated by the charge transfer resistance (Rct) which was determined from EIS experiments. The redox chemistry of CPX on the ACBP-BiVO4 electrode was also pH dependent and involved a two-electron process. A 39,67 mV/pH value at 25 C (pH vs E). which is closely related to the Nernstian theoretical equation, indicates this because 2e− and 2H+ are involved in the redox process (the participation of the same protons and electrons in the electrochemical process).
The linear range of CPX detection was 0.1–1 μM at the optimised pH of 12. The results obtained using the prepared sensor were reproducible and specific to CPX when other interfering species such Ascorbic acid and L-Cysteine hydrochloride were present. This sensor again demonstrated good stability where it retained 90% of its sensor activity for 10 days at 4°C. Repeatability and reproducibility were studied and the relative standard deviation (RSD) was 0.3820 μM. Thus, the ACBP-BiVO4 modified GCE showed good performance for CPX detection as evidenced by the good repeatability which was observed.
Activated carbon (AC), graphene, and carbon nanotubes have all been studied in developing carbon-based catalysts. Since carbon-based catalyst materials have benefits like a large specific surface area, good biocompatibility, stability, and high conductivity, researchers are wary of using them. Agricultural waste can be used as adsorbents for a number of purposes, including lowering
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environmental waste emissions. Agricultural wastes used as adsorbents include palm bark, rice husk, straw, pumpkin peel, coffee grounds, and palm shell. Banana and potatoes are readily available and cheaper to use. Many local governments in South Africa face problems with organic waste, and the circular economy is one of the most concerning issues for the government. Recovering banana and potato peel waste for activated carbon can reduce the quantity of organic waste entering the environment.
Bismuth, a heavy metal which is found to be inexpensive and at a reduced cost, is utilized in the synthesis of different nanomaterials with novel structure, remarkable physical and chemical properties, adjustable bandgap, notable efficiency for photothermal conversion. These characteristics have made this element desirable for various applications such as storage and conversion of energy, electronics, sensors, photocatalysis, and other biomedical applications. Bismuth vanadate possesses several unique characteristics, including the capacity to absorb solar energy, ferroelasticity, ionic conductivity and hydrogen generation.With its hydrophobic nature, greater surface area and favorable interactions of its 2D aromatic structure, AC has established a reputation for its improved capacity to absorb a variety of organic and inorganic contaminants.
Activated carbon (ACPP) was therefore synthesised from the potato peels and modified with a hollow sphere of tin oxide (SnO2-HS) as a binder. The hollow tin oxide sphere was enriched with silver nanoparticles (AgSnO2-HS) to increase the conductivity of the composite and DMF was added for signal stabilization. The composite (ACPP / AgSnO2-HS) was characterised by Fourier transform infrared spectroscopy (FTIR), diffraction spectroscopy (XRD), scanning electron microscopy (SEM), electron X-ray dispersion (EDX) spectroscopy, and transmission electron microscopy (TEM). The modified GCE- ACPP / AgSnO2-HS was again electrochemically characterised by cyclic voltammetry and electrochemical impedance spectroscopy. Differential pulse voltammetry was applied for the co- detection of CPX and PAR using a GCE / ACPP / AgSnO2-HS sensor under the pH, concentration, stability and interference studies. The limit of detection (LOD) for CPX was 0.2441 μM and its limit of quantification (LOQ) was 0.8137 μM. The limit of detection for PAR was 0.0729 μM while its limit of quantification was 0.2343 μΜ. The linear range was 0.1 to 1.0 μM. The GCE /
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ACPP / AgSnO2-HS sensor showed excellent reproducibility and stability towards simultaneous detection of CPX and PAR in wastewater and synthetic fluid samples.