Development of Novel Multifunctional Nanocomposites for Antimicrobial Efficiency in Water Treatment

Water pollution is a major concern worldwide. Bacteria, viruses and fungi present in drinking water cause various diseases as a result of poor hygienic conditions in developing countries. Similarly, presence of microorganisms in drinking water is a threat to public health in developing world due to poor hygienic condition. Numerous disinfectants and biocides are used for inhibiting the growth of pathogenic microbial contamination, producing carcinogenic by-products which are dangerous to human health. This work involved the synthesis, characterisation and application of novel multifunctional nanocomposites by the modification of cost effective available materials for antimicrobial treatment of contaminated water and the detection of specific DNA associated with water-borne bacteria. A series of multifunctional nanocomposites composed of commercially available carbon (activated charcoal and multi-walled carbon nanotubes), and silica-based materials such as diatomeous earth, celatom-80 and celatom-14 were modified with silver and iron oxide nanoparticles via a simple one-pot synthesis protocol in order to incorporate antimicrobial and superparamagnetic properties. The resultant materials have been tested for antimicrobial efficiency using model water system containing Gram-negative Escherichia coli (E. coli) K12 and Gram-positive Staphylococcus. aureus (S. aureus) microorganisms. It was found that all materials ranging from 10 to 200 μg/mL produced excellent inhibition of S. aureus and E. coli. All nanocomposites have been fully characterised by several physico-chemical techniques such as Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), X-ray Fluorescence (XRF), Energy Dispersive X-ray Analysis (EDAX), Fourier Transform Infrared Spectroscopy (FT-IR), Nitrogen gas adsorption and (BET) surface area analysis. Surface area of the materials measured in range of 5 to 560 m2/gm. XRF along with EDAX/SEM analyses have been used for the confirmation of silver and iron oxide presence in the nanocomposite materials. TEM images showed nano-sized silver particles with an average diameter of 15-17 nm and iron oxide (magnetite) nanoparticles with an average diameter of 30 nm embedded into the nanocomposites. FT-IR spectroscopy measurement confirmed the presence of Fe–O bonding of iron oxide nanoparticles due to a characteristic stretching vibration at 570 cm-1. Powder X-ray Diffraction (XRD) measurements confirmed the crystalline structure of the iron oxide nanocomposite mostly magnetite (Fe3O4). Nitrogen gas adsorption-desorption experiments suggests the presence of average pore diameter 28 to 79 A, micropore volume: 0.01 to 0.16 cm³/g, and surface area 5 to 560 m2/g. Gram-negative E. coli K12 and Gram-positive S. aureus bacteria were used for anti-bacterial activity study where the nutrient agar was used for the growth of the bacteria. The antimicrobial effect of the nanocomposites was quantified by counting the number of colonies (colony forming unit, CFU/mL) grown on the media compared with a blank solution. Different concentrations (0.2 µg/mL to 300 µg/mL) of the nanocomposite materials were used for this study. MBC of QM1-3 and QM2-3 was found 10 µg/mL for the S. aureus and 30 µg/mL for E. coli K12, while other samples of QM3-3, QM4-3 and QM5-3 were higher such as 30 µg/mL for the S. aureus and 100 to 200 µg/mL for E. coli. All experiments were performed in triplicate and the data presented are the mean values of triplicate experiments ± standard deviation. Detection of water-borne microorganisms is the second application of the developed nanocomposites via surface modification with specific oligonucleotides sequences of E. coli gene followed by hybrid capture with complementary sequence. It was observed that multi-walled carbon nanotubes, activated charcoal and diatomeous earth gave good and satisfactory results (0.384 to 0.400 nmol/mg) in hybrid capture of complementary oligonucleotides sequences in model assay. Surface modified optimum materials (carbon nanotubes and activated carbon) with efficient hybrid capture were also efficient in detecting amplicon of 97 base pairs (bp) of E. coli specific genome by PCR experiment.