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Title: Biosynthesis and antibacterial activity of silver and gold nanoparticles from the leaf and callus extracts of Amaranthus dubius, Gunnera perpensa, Ceratotheca triloba and Catharanthus roseus
Authors: Patel, Naazlene 
Issue Date: 17-Sep-2013
The biosynthesis of NPs has many advantages over the tedious, expensive and toxic
physical and chemical methods of synthesis. Plants are stocked with valuable metabolites
that are capable of reducing metal salts to form NPs. In this study, aqueous leaf extracts of
A. dubius, G. perpensa, C. roseus and C. triloba were reacted with AgNO3 and HAuCl4 to
determine the plants reducing abilities and hence synthesis of Ag and Au NPs capabilities.
The synthesis reactions were carried out at different temperatures and extract
concentrations for optimization. The goal was to form NPs within the specific wavelength
range. Polar solvents: methanol and ethyl acetate extractions were carried out at the
optimized conditions to evaluate the best solvent for the extraction of phytochemicals from
the plants. The plant leaf extracts that were successful (A. dubius, G. perpensa and C.
triloba) in synthesizing NPs were then micropropagated to form callus cultures. The
reducing abilities of these callus cultures extracts were determined by varying temperature
and concentration parameters. Characterization of the NPs formed by the different extracts
was performed using UV-vis, TEM and FTIR. UV-vis spectrophotometry was used as a
confirmatory as well as characterizing tool. TEM analysis was able to provide a description
on the size and shape of the NPs whereas FTIR provided information on the biomolecules
responsible for synthesis and capping of NPs. The stability of the NPs was determined by
UV-vis scans over a period of 30 days which allowed observation of the alteration in peak
shape and absorbance and hence condition of particles. Phytochemical tests were
performed on the leaf extracts of the four plants to elucidate possible phytochemicals
responsible for the reduction of metal salts. Antibacterial activity of the NPs was evaluated
by using the disk diffusion assay and MICs were determined by the broth dilution method
against pathogenic bacteria.
A. dubius, G. perpensa and C. triloba were capable of synthesizing Ag NPs and Au NPs
which were indicated by yellowish orange and reddish purple colour changes respectively.
G. perpensa was able to spontaneously form Ag and Au NPs without any addition of heat
whereas A. dubius and C. triloba required heat to form Au NPs. As the temperature of the
reactions increased, the absorbance and possibly the number of NPs produced, increased.
When the concentration of the extract was doubled, the absorbance was seen to decrease.
C. roseus did not produce any Ag or Au NPs with any of the leaf extracts. Only A. dubius
and C. triloba callus extracts were investigated for NP synthesis and it was found that A.
dubius callus extracts were unsuccessful in synthesizing NPs and C. triloba callus extracts
were able to form unstable Ag and Au NPs.
The spherical Ag NPs that were formed from aqueous extracts of A. dubius were slightly
larger than the methanolic Ag NPs. The Ag NPs produced by G. perpensa were in the
same size range for aqueous and methanolic extracts. C. triloba Ag NPs formed from the
methanolic extract were closer in size to A. dubius aqueous Ag NPs but the C. triloba
aqueous extract produced much larger Ag NPs than the other extracts. The Ag NPs
produced from A. dubius aqueous and methanolic extracts as well as C. triloba methanolic
extracts exhibited the longest stability of 30 days. Ag NPs from G. perpensa aqueous
extracts had the least stability.
G. perpensa did not form any hexagonal Au NPs and the spherical and triangular Au NPs
were smaller unlike in A. dubius and C. triloba Au NPs. The Au NPs formed by the
aqueous extracts of A. dubius and C. triloba were larger in comparison to their methanolic
counterparts. The Au NPs produced from G. perpensa aqueous and methanolic extracts as
well as A. dubius and C. triloba methanolic extracts exhibited the longest stability of 30
days. Au NPs were stable for longer in comparison to Ag NPs. FTIR provided evidence
that Ag and Au NPs have a chemical bond with the amide group in amino acids. However
the intensities of biomolecules for Au NPs are more pronounced compared to the Ag NPs.
It was also found that the Ag NPs synthesized by methanolic leaf extracts have slightly
higher intensities than Ag NPs synthesized from aqueous leaf extracts. Phytochemical
screening showed the absence of tannins in the C. roseus leaf, A. dubius and C. triloba
callus extracts and presence in the other three plants.
C. triloba methanolic extract Ag NPs showed the highest activity against Gram-positive S.
aureus. Aqueous and methanolic Ag NPs from G. perpensa and C. triloba as well as A.
dubius methanolic Ag NPs had activity against all fourteen bacteria. A. dubius aqueous Ag
NPs had no activity against Enterobacter spp. and a strain of Klebsiella pneumoniae. G.
perpensa Ag NPs had better antibacterial activity and lower MICs against Gram-positive
and Gram-negative pathogenic bacteria compared to A. dubius and C. triloba. There was
no antibacterial activity seen with Au NPs.
The size and shape of NPs are the keys to their biomedical properties. Green synthesis of
NPs is a feasible way for the future. This study showed that NPs can be synthesized very
easily and economically. A key finding of this study is that different plants produce
varying sizes and aggregation of NPs.
Submitted in complete fulfilment for the Degree of Master of Technology: Biotechnology, Durban University of Technology, Durban, South Africa, 2013.
Appears in Collections:Theses and dissertations (Applied Sciences)

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