Microwave Enhanced Synthesis of Silver Nanoparticles Using Orange Peel Extracts from Nigeria
Ihegwuagu Nnemeka1, 5, Etuk-Udo Godwin2, Fatokun Olakunle3, Odusanya Olushola2, Omojola Moses4,
Onyenekwe Paul Chidozie2, Sha’Ato Rufus5
1Cordination Technical Research Programme, Agricultural Research Council of Nigeria (ARCN), Mabushi, Abuja, Nigeria
2Biotechnology Advanced Research Centre, Sheda Science and Technology Complex (SHESTCO), Abuja, Nigeria
3Chemistry Advanced Research Centre, Sheda Science and Technology Complex (SHESTCO), Abuja, Nigeria
4Raw Materials Research and Development Council (RMRDC), Maitama, Abuja, Nigeria
5Chemistry Department & Centre for Agrochemical Technology, University of Agriculture, Makurdi, Benue State, Nigeria
To cite this article:
Ihegwuagu Nnemeka, Etuk-Udo Godwin, Fatokun Olakunle, Odusanya Olushola, Omojola Moses, Onyenekwe Paul Chdozie, Sha’Ato Rufus. Microwave Enhanced Synthesis of Silver Nanoparticles Using Orange Peel Extracts from Nigeria. Chemical and Biomolecular Engineering. Vol. 1, No. 1, 2016, pp. 5-11. doi: 10.11648/j.cbe.20160101.12
Received: May 4, 2016; Accepted: May 20, 2016; Published: June 6, 2016
Abstract: The gamut of applications for metal nanoparticles fuels the interest for their synthesis. The hazardous, time and energy consuming nature of conventional techniques necessitates the search for alternative methods of synthesis that ensures the desired size, shape and dispersity is achieved. The use of aqueous plant extracts to reduce toxic heavy metals is a spontaneous, cost effective, eco-friendly method used in the synthesis of nanoparticles. Microwave catalysis have also gained acceptance as another green method for material synthesis due to its high reaction rates and shortened reaction times. This study was embarked on to develop a rapid and eco-friendly method for synthesizing silver nanoparticles (AgNPs), stabilized within a biocompatible matrix using orange peel extracts, starch and a microwave. Data obtained revealed from the characterization of the synthesized AgNPs revealed that the surface plasmon resonance (SPR) peaked at 408nm using a UV-Visible Spectrophotometer. The EDX spectrum of the solution containing silver nanoparticles confirmed the presence of an elemental silver signal. SEM and HR-TEM images suggest that the nanoparticles were of spherical shape. HR- TEM particle size range was 7-17.31±0.84nm. FT-IR spectroscopy analysis of synthesized AgNPs indicated a slight shift at the O-H absorption bands of starch. PXRD confirmed the reflections of silver nanoparticles at corresponding 2θ values respectively. The results obtained suggested that rapid catalysis of AgNPs using plant extracts is further boosted using microwave enhanced catalysis whereby the functionalbiomolecules from the plant extract is retained, altogether forming stable AgNPs.
Keywords: Silver Nanoparticles, Bioreduction/Synthesis, Orange Peel Extract, SPR, HR-TEM, Microwave
In recent times, functional materials like nanoparticles have attracted a lot of attention owing to the number of physical, medical, biological and environmental applications it can be used for resulting in an increase in commercial demand . With dimensions in the range of 0-100nm, nanoparticles, particularly silver nanoparticles (AgNPs) serves as a promising new candidate for biological applications due to its high surface area to volume ratio as well its unique chemical and physical properties , . It is widely accepted that AgNPs bind with high affinity to specific receptors on the negatively charged membrane surfaces of bacterial and fungal cells, thereby catalysing a series of phosphorylation and de-phosphorylation reactions, attacking the sulphur-hydril groups (-SH) etc that lead to protein denaturation and cell death , . Different methods have been employed in the synthesis of silver nanoparticles including thermal decomposition of silver ions in organic solvents, chemical reduction of silver ions in aqueous solutions with or without stabilizing agents, photo reduction, microwave and radiation chemical reduction, etc , . Due to the high costs associated with these methods, coupled with their environmentally harmful effects, research has turned to more eco-friendly approaches in which biological materials; bacteria, fungi and plants for nanoparticle synthesis . Plant-based synthesis is a desired alternative as AgNPs naturally possess anti-microbial and anti-fungal activities which would kill-off most synthesising microorganisms by targeting their respiratory chain and cell division machinery . Plant extracts represent a new source of metal nanoparticle synthesis due to its simplicity and ability to utilise one or more phyto-remediation mechanisms (rhizo-filtration, phyto-volatilization, etc). AgNPs synthesis has been achieved using several plants (extracts) including Aloe vera , Alfafa ; Cinnamomum canphora , Neem , Zinger officinale , Mangosteen , Rosa rugosa , Stevia rebaudiana . Although the precise mechanism of plant-mediated nanoparticle synthesis is not fully understood, it is believed that the presence of active bio-molecules with protective and redox potentials act as a major factor in the synthesis of nanoparticles . Citrus fruits like oranges contain many active compounds that can be used as food additives, flavouring, etc , , . Some of its these like the compound; limonene, have been reported to possess antioxidant and lympho-protective properties as it has been implicated in the inhibition of proliferating lymphoma cells in addition to assisting in the normal cell regulation of lymphocytes , , , . Orange peel extracts may present an effective plant extract that is cheap, simple and eco-friendly towards the synthesis of silver nanoparticles.
2. Materials and Methods
Silver nitrate was purchased from Finlab Scientific Company Abuja Nigeria. All reagents were of analytical grade and used without further purification. Also solutions were prepared using distilled water.
2.1. Preparation of Extracts
Fresh oranges were obtained from Sheda Science and Technology Complex (SHESTCO), Sheda, Abuja, washed under distilled water and peeled in the laboratory. The fresh peels were solvent distilled in a soxhlet extractor using methanol. The extract was evaporated with a rotary evaporator to give a crude extract .
2.2. Starch Isolation
A modified starch extraction method without bleaching was adopted  was adopted. The extracted starch was then defatted prior to its use.
2.3. Biological Synthesis of Silver Nanoparticles
5mls of the crude plant extract was added to 1ml of 1mM silver nitrate (AgNO3) solution and mixed by continuous stirring for 10 minutes with 1% starch dispersion and microwaved (Samsung 123 HCE, TDS) for 5 minutes at 100% power, 800W at 250MHZ working condition. The beaker was incubated at room temperature under dark conditions and observed for colour change.
2.4. Characterization of Synthesised AgNPs
2.4.1. UV-VIS Spectroscopy
The bio-reduction of silver ions was examined for a colour change, visible to the naked eye. Aliquots of the complex were analyzed using the UV-Visible spectra (7000 series, CECIL CE 7500 UV-VIS - spectrophotometer) of the solution between the scanning range of 300 to 600nm using distilled water as a blank reference.
2.4.2. Fourier Transform Infra-Red Analysis
The obtained silver nanoparticles (dried samples) were grinded with KBr pellets and used for Fourier Transform Infra-Red (FTIR) measurements. The spectrum was recorded in the range of 4000 - 400 cm-1 using Horizon, Model: MB 3000 spectrometer in the diffuse reflectance mode operating at resolution of 4 cm-1.
2.4.3. High Resolution Transmission Electron Microscopy (HR-TEM)
The particle sizing of the sample was carried out with High Resolution Transmission Electron Microscope (HR-TEM). Dispersed samples in absolute ethanol were dropped onto coated copper grids and allowed the ethanol to evaporate. Micrographs were obtained using a HR-TEM (FEI TECNAI 02) having software TECNAI G2. HR-TEM was equipped with an Energy-Dispersive Spectrum (EDX) which analyzed the elements in the biosynthesized nanoparticle.
2.4.4. Scanning Electron Microscope (SEM)
Further morphological analysis was performed using a ZEISS EVO 40 EP Scanning Electron Microscope (SEM).
2.4.5. Powder X-ray Diffraction (PXRD)
The synthesized nanoparticles were structurally analyzed using the Diffractometer, D8 Advance (by BRUKER AXS, Germany) and the XRD pattern analyzed according to JCPDs patterns.
3. Results and Discussion
3.1. UV-VIS Analysis
Visual indication of the formation of starch stabilized AgNPs was observed via a colour change from the initial colourless solution to a brownish coloured solution on heating for 5minutes in the microwave (figure 1). Excitation of the surface Plasmon vibrations of silver nanoparticles would account for the observed colour change as the first confirmatory test revealing the formation of colloidal AgNPs in accordance with other reports , . The UV-Vis spectra showed an absorption band at 408nm (figure 2). This indicated that orange peel extract possess active secondary metabolites which function as good reducing agents. The formation of AgNPs embedded within a starch matrix via microwave assisted synthesis corroborates with data obtained from others studies , .
3.2. FTIR Analysis
Fourier transform infra-red spectra performed to characterise the synthesised AgNPs and highlight the presence of functional groups of both the biomolecules within the plant extract and the starch involved in the reduction and stabilisation of the synthesised AgNPs. Observed peaks indicate carboxyl linkages, ether linkages, carbon-carbon groups or weak hydrocarbon bonds respectively. The spectra obtained revealed prominent bands at 1,022, 1,080, 1,150, 1,365, 1,421, 1, 641, 2,360, 2,929 and 3404 cm-1 (fig. 3, 4). Strong hydroxyl bonded (O-H) along with weak hydrocarbon (C-H) bonded stretching absorptions is usually observed between 3600-2800 cm-1. This feature was observed in our spectrum denoted by the characteristic broad and strong absorbance bands at 3404.47cm-1 and 2929.97 cm-1 which could be assigned to the hydroxyl (O-H) group and C-H stretching vibrations respectively. A shift from 3394 cm−1 to 3404 cm−1 is observed for stabilized AgNPs; this may be due to inter and intra molecular interactions of Ago with –OH group. There are peaks at 1641 cm-1, 1421 cm-1, 1365 cm-1, 1155 cm-1, 1080 cm-1, 1022 cm-1, 929 cm-1 and 875 cm-1.
3.3. HR-TEM Analysis
Morphological images of the starch embedded AgNPs is depicted in figure 5. The image revealed uniformly distributed spherical to cuboidal particles. The formation of these shapes can in part be attributed to the interaction between the quantity and types of capping agents present within the orange peel extract. This phenomenon supports the peak shifts revealed in the FT-IR chromatograms. The particle distribution in the HR-TEM micrograph gives an indication that the synthesised nanoparticles are poly-dispersed with a particle size range of 7-17.31±0.84 nm with a few between 20-30 nm. The Selected-Area Electron Diffraction (SAED) patterns given reveal bright dots (fig. 6), indicating that the nanoparticles are crystalline in nature. The data obtained corroborates other studies on microwave assisted synthesis of metal nanoparticles .
3.4. SEM Analysis
SEM analysis also confirmed the uniform distribution of the synthesised AgNPs with an average size of 46nm (fig. 7). Their formation in large numbers and almost uniform in size is consistent with the report of Vankar and Shukla, (2012) . These researchers went further to say that uniformity of size and shape considerably enhances wash fastness and consequently to anti-fungal property of AgNPs.
3.5. EDX Analysis
The EDX spectra obtained from the synthesised AgNPs revealed strong signals of silver as well as signals for oxygen and copper (fig. 8). Peaks located between 2 keV and 4 keV are directly related to the characteristic K and L orbitals of silver . The presence of oxygen may be as a result of radicals or reactive oxygen species stemming from other biomolecules possessing anti-oxidative potentials bound to the surface of the AgNPs. This peak located at about 0.5 keV could also arise from starch which acts as a stabilizing agent for silver nanoparticles. Copper peaks may be due to the same being present in the grids. It has been reported that nanoparticles synthesized using plant extracts are surrounded by a thin layer of some capping organic material from the plant leaf broth and are, thus, stable in solution up to 4 weeks after synthesis .
3.6. Powder X-ray Diffraction (PXRD) Analysis
The PXRD pattern of the biosynthesized AgNPs is depicted in Figure 9. It reveals the starch peak at 20.9 degree and four peaks at 2-theta values of 38, 44, 64 and 78 degrees corresponding to (111), (200), (220) and (311) planes of Silver respectively, according to the standard powder diffraction card of Joint Committee on Powder Diffraction Standards (JCPDS), silver file No. 04–0783 . The peak at 2-theta value of 38degrees is the sharpest. The XRD study confirms / indicates that the resultant particles are the Face Cubic Center (FCC) Silver nanoparticles.
In this study, we attempted microwave assisted synthesis as an alternative, rapid and simple approach towards the production of AgNPs using Nigerian orange peel extracts and starch as a stabilizing agent. The particles were poly-dispersed, spherical to colloidal. The colour changed observed was due to the excitation of the surface plasmon resonance (SPR) during the reaction with bioactive molecules within the peel extract and the resulting AgNPs formed were further confirmed via UV-Visible spectra, EDX, FTIR, SEM, PXRD and HR-TEM. From the data obtained, silver nanoparticles can be synthesized from and in quick succession using the microwave mediated technique along with natural polymers (starch) for use in a number of industries.