Abstract
When conducting polymers were discovered, their use as low-cost electrocatalysts was investigated, but their low conductivity, poor environmental stability and poor solution processibility limit their possible applications. However, with impressive mechanical and electronic properties of carbon nanomaterials (such as graphene and carbon nanotubes), remarkable contributions have been made by researchers to fabricate conducting polymers composites by introducing carbon nanomaterials, especially carbon nanotubes into conducting polymers so as to combine synergistically the merits of individual components. One of the major key areas in biosensor field is the enhancement of electrochemical properties of composite material to make them suitable for use as an active sensing layer during sensor fabrication. Various techniques and methods have been employed when using these composite materials as a sensing layer in fabricating biosensor but the majority of these techniques rely on expensive equipment and complicated processes which would limit the sensor stability and reliability.
In this work, a novel, low-cost and high performance electrochemical biosensor prototype for selective determination and amperometric detection of tetracycline hydrochloride (THC) antibiotics in aqueous media has been developed. This is based on conducting polymer composites containing functionalised carbon nanotubes and prepared using a unique analytical amperometry method. Tetracycline was selected as the target analyte because it is one of the antibiotics most commonly used to treat infections in both human and animals and its residues in water and food products have become a global problem. Amperometry technique is adopted in this work to eliminate cost and complicated process that comes with other techniques so as to achieve highly sensitive and reproducible electroanalytical response. Also, important drawbacks like electrode fouling and high potential value associated with other techniques have been successfully eliminated using amperometric detection technique (amperometric i-t curve).
Polyaniline (PANI), a conjugated polymer, was adopted as the sensing layer for amperometric detection of tetracycline by using its excellent electrochemical sensing properties. A new method of synthesis of the material was first developed to resolve the problem of polyaniline’s poor processibility. Polyaniline was synthesised at room temperature via organic/aqueous interfacial polymerization to yield uniform, pure and template-free nanostructured polyaniline in bulk quantities, which is readily dispersed in water, invariably facilitating environmentally friendly processing and superior performance in chemical sensor applications. Subsequently, a polyaniline-based composite containing multiwalled carbon nanotubes (MWCNTs) was developed. Multiwalled carbon nanotubes were functionalised before the composite was formed so as to get a good stable dispersion of filler within the polymer matrix. Such a fabricated composite material was in a powder form which was characterised by a number of techniques including DC-conductivity measurement, thermal analysis, Fourier transform infrared spectroscopy, scanning electron microscopy, ultraviolet–visible spectroscopy, cyclic voltammetry and amperometric detection test, etc. Characterisation results show that the molecular weight, crystallinity, thermal stability and electrical conductivity of the fabricated polyaniline powder as well as the composite material were independent of the polymerisation temperature. Furthermore, the experimental result demonstrates that the composite shows dramatically improved performance compared to each of the individual components. Increase in electrical conductivity with the increase of carbon nanotube contents in the composite was observed, although the composite performance became poorer at higher loading content of carbon nanotubes.
Electrical conductive thin films were first prepared from the composite material to investigate their redox potentials and electrochemical properties. The composite was further tested for its potential applications in detecting antibiotics in aqueous media. A prototype of biosensor was fabricated by drop casting the composite solution in m-cresol on a glass carbon electrode at room temperature. M-cresol was used as the processing solvent as compared to other processing solvents based on the findings in published literature. Moreover, carboxylated multiwall carbon nanotubes played an important role in obtaining the thin and uniform coating of polyaniline resulting in the improved amperometric sensor response. The performance of the sensor was characterized electrochemically and all variables involved during fabrication process were optimized. Under optimal conditions, the synergistic properties of PANI/MWCNTs nanocomposite such as the ion-electron transducing capability, hydrophobic property and active tetracycline binding sites in polyaniline demonstrated high sensitivity, excellent detection limit, high electrochemical redox activity, fast response time, good biocompatibility and stability up to 20 days.
This research work has contributed to the development of highly sensitive and stale biosensor to detect antibiotics in fish farm water spiked with tetracycline. The great potential of this development was fully demonstrated using PANI/F-MWCNT nanocomposite through a fast, unique and easy to use analytical method (Amperometry technique). Interfacial polymerisation makes it possible to improve the porosity of the polymer, and the fabricated nanocomposite thin film is sufficiently stable under the experimental condition. The GCE electrode modified with the nanocomposite thin film was able to detect antibiotics in the fish farm water due to hydrophobic property of F-MWCNT and active redox activity of PANI. It is believed that these findings are important in the field of antibiotics detection and analysis in the future.