Received Date: September 12, 2016; Accepted Date: September 16, 2016; Published Date: September 23, 2016
Citation: Ghosh S, Gurav SP, Harke AN, Chacko MJ, Joshi KA, et al. (2016) Dioscorea oppositifolia Mediated Synthesis of Gold and Silver Nanoparticles with Catalytic Activity. J Nanomed Nanotechnol 7: 398. doi: 10.4172/2157-7439.1000398
Copyright: © 2016 Ghosh S, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Visit for more related articles at Journal of Nanomedicine & Nanotechnology
Biological approaches for synthesis of nanoparticles have gained prime attention in the recent decade as they not only involve lesser time and energy consumption compared to physical and chemical methods but also rapid, efficient and nontoxic. Medicinal plants with diverse phytochemistry have been considered as most promising for development and design of novel routes to synthesize metal nanoparticles. Herein, we report for the first time the synthesis of gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) using Dioscorea oppositifolia tuber extract (DOTE). The synthesis process was complete within 5 h as observed in UV-visible spectroscopy. High resolution transmission electron microscopy revealed that the bioreduced AuNPs were anisotropic exhibiting exotic shaped like nanohexagons, pentagons and blunt ended triangles. Spherical AuNPs ranging from 30 to 60 nm were also spotted. Similarly, distinctly spherical AgNPs without any agglomeration were observed with a size ranging between 17 to 25 nm. Both AuNPs and AgNPs were characterized using various techniques like energy dispersive spectroscopy, X-ray diffraction, dynamic light scattering and fourier transform infrared spectroscopy. Phytochemical analysis employing biochemical tests and GCMS/MS revealed the presence of phenolics, flavonoids, starch, ascorbic acid and citric acid that may help not only in reduction but also in stabilization of the bioreduced nanoparticles. AuNPs and AgNPs synthesized by DOTE exhibited efficient catalytic activity towards reduction of 4-nitrophenol to 4-aminophenol by NaBH4 with a pseudo-first order rate kinetics.
Dioscorea oppositifolia; Gold nanoparticles; Silver nanoparticles; High resolution transmission electron microscopy; Gas chromatography mass spectrometry; Catalysis
Nanostructured materials with tunable properties have gained attention in recent years for development of nanomedicine. Metallic nanoparticles exhibit opto-electronic, chemical and magnetic properties which are of utmost significance towards wide spread applications in catalysis, single electron tunnelling devices, nonlinear optical devices, electron microscopy markers, DNA sequencing and plasmonics [1,2]. Synthesis of gold nanoparticle (AuNPs) and silver nanoparticles (AgNPs) are accomplished traditionally, by physical and chemical methods like UV-irradiation, laser ablation, plasma synthesis, sonochemical and electrochemical reduction. However, these widely used techniques employ harsh chemicals, stringent synthesis conditions, energy and capital intensive and less productive adversities . Moreover, these methods pose environmental threats due to involvement of toxic solvents or additives and lead to overproduction of sludge. Hereby, there is a growing need to design environmentally benign, rapid, efficient, clean, nontoxic and sustainable synthesis procedures. Biological systems have proved to be economical, biocompatible and efficient for synthesis of nanoparticles . Although bacteria, virus and fungi are reported to synthesize metal nanoparticles, medicinal plants have gained much attention and preference owing to their rich phytochemical diversity [5-12]. Hence, we have reported medicinally important plants such as, Dioscorea bulbifera, Gnidia glauca and Plumbago zeylanica for their potential to synthesize various metal nanoparticles [13-19]. However, there are many unexplored medicinal plants in the tribal region which are yet to be studied for their role in synthesis of exotic metal nanoparticles. One such most significant medicinal plant is Dioscorea oppositifolia.
D. oppositifolia is used as a complementary and alternative medicine in the tribal regions of India as well as Zimbabwe. The plant tubers are used as herbal tonic which stimulates the stomach and spleen and exhibits effect on the lungs and kidneys as well. Tubers are eaten to cure poor appetite, chronic diarrhea, asthma, dry coughs, frequent or uncontrollable urination, diabetes and emotional instability. Tubers are also used for topical applications to ulcers, boils and abscesses. Allantoin present in it serves as a cell-proliferant that facilitates healing process. Diosgenin in its roots helps in synthesis of progesterone and other steroid drugs. D. oppositifolia is traditionally used as a contraceptive and for treating various disorders of genital organs as well as for arthritis [20,21]. However, there are no thorough scientific studies on the optimization and characterization of synthesis processes for nanomaterials with potent therapeutic activity from this plant.
In view of the background, it is of utmost scientific rationale to explore the detailed nanobiotechnological potential of D. oppositifolia to synthesize metal nanoparticles like AuNPs and AgNPs with therapeutic applications. Herein, we report for the first time on the synthesis of AgNPs and AuNPs using D. oppositifolia tuber extract (DOTE) which were thoroughly characterized. The conditions of the synthetic process were optimized. Further the detailed analysis of DOTE was carried out to understand the mechanism of the synthesis mediated by the key constituents present in the extract. The bioreduced nanoparticles were checked for their efficiency to catalyse chemical reaction.
Plant material and preparation of extract
D. oppositifolia tubers were collected from Western Ghats of Maharashtra, India. After chopping the tubers the thin slices were dried in shade for 2-3 days at room temperature. The dried plant material were pulverised into fine powder using an electric blender. D. oppositifolia tuber extract (DOTE) was prepared by boiling 5 g of finely ground tuber powder in 100 mL distilled water in a 250 mL Erlenmeyer flask for 5 min. After decantation and centrifugation at 3000 rpm for 10 minutes the extract was filtered through Whatman No.1 filter paper. The filtrate was collected and stored at 4°C for further use .
Synthesis and characterization of silver and gold nanoparticles
DOTE mediated synthesis of AuNPs was initiated by addition of 5 mL of DOTE into 95 mL of 1 mM HAuCl4 solution. The reaction mixture was shaken at 40°C for 5 h in a shaker incubator. Ultravioletvisible spectrometry operating at a resolution of 1 nm was used at regular intervals to confirm the bioreduction of Au3+ ions to AuNPs. Similarly for synthesis of AgNPs 5 mL of DOTE was added into 95 mL of 1 mM AgNO3 solution. Temperature optimization was carried out in a water bath at 4-50°C with reflux. Concentration optimization was performed by varying the respective salt concentration from 0.3 to 5 mM. The bioreduced nanoparticles were characterized using high resolution transmission electron microscopy (HRTEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and fourier transform infrared spectroscopy (FTIR) as per our earlier reports .
Various biochemical assays were employed to determine the total phenolic, flavonoid, starch, reducing sugar, ascorbic acid and citric acid content of the DOTE as per our earlier report [Ghosh IJN, 2012]. Similarly, GC-MS/MS analysis was used to find out the phytochemical constituents of DOTE .
The reduction of 4-nitrophenol (4-NP) was carried out using UV- vis spectroscopy at 25°C in a quartz cuvette with an optical path length of 1 cm and volume 4.5 ml. 2 mL of aqueous 4-NP solution (0.1 mM) was mixed with 1.0 mL of freshly prepared NaBH4 solution (0.1 M) and 0.5 mL of NaOH solution (0.1 M). 25 µL of DOTE synthesized AgNPs (10 mM) were added to the solution. Similar protocol was followed for DOTE synthesized AuNPs as well. The change in absorbance was measured in situ to obtain the successive information about the reaction. Completion of reaction was indicated by the change in color i.e. from yellow to colorless [15,19].
Synthesis and characterization
Synthesis of AuNPs was indicated by the development of ruby red colour on incubation of gold chloride solution with DOTE. The colour changed instantly on adding DOTE which increased upto 5 h indicated by development of intense red colour. UV-visible spectroscopy confirmed the synthesis of AuNPs by showing the peak at 550 nm which increased as a function of time (Figure 1A). Similarly, synthesis of AgNPs was indicated by development of brown colour in the reaction mixture which was initially colourless. UV-visible spectroscopy confirmed the synthesis of AgNPs by indicating the peak at 430 nm which appeared after 1 h of incubation (Figure 1B). The synthesis was completed at 5 h.
Temperature optimization studies confirmed that higher temperature facilitated the rate of synthesis in case of both AuNPs and AgNPs. Rate of synthesis steadily increased from 4°C to 50°C (Figure 2A and 2B). Thus it can be concluded that the rate of bioreduction of metal nanoparticles is temperature dependent.
Concentration optimization studies revealed that 0.5 mM of gold salt solution was optimum for synthesis of AuNPs while higher concentrations showed considerably low rate of synthesis at 550 nm (Figure 3A). However, in case of AgNPs, higher concentrations of AgNO3 solution facilitated better bioreduction. Maximum synthesis was found for 5 mM followed by 3 mM AgNO3 concentration (Figure 3B). Concentrations of respective salt solutions were found to be play critical role towards the rate of biosynthesis of respective nanoparticles.
HRTEM, EDS, DLS analysis
Exotic shapes of AuNPs were observed in HRTEM. The nanoparticles varied in shapes leading to anisotropy. As evident from Figure 4A sharp edged nanohexagons of larger dimensions were observed. Similarly, smaller pentagonal nanoparticles were observed as well beside the hexagons. Blunt ended nanotriangles were also spotted on which nanospheres ranging from 30 to 60 nm along with nanopentagons were closely adhered (Figure 4B). AgNPs were found to be very small distinctly dispersed without any agglomeration (Figure 4C). A thin film of stabilizing material was visible around the AgNPs which might contribute to their stability and dispersion (Figure 4D). AgNPs were found to be spherical in shape ranging between 17 to 25 nm. EDS analysis exhibiting the signature peaks for elemental Au and Ag in the bioreduced nanoparticles confirmed the synthesis of AgNPs and AuNPs, respectively by DOTE (Figure 5A and 5B). Size distribution of bioreduced nanoparticles observed in DLS were also found to be in close agreement with the HRTEM results (Figure 6A and 6B).
Figure 4: High-resolution transmission electron micrographs of nanoparticles synthesized by DOTE. (A) AuNPs bioreduced by DOTE, inset bar representing 50 nm; (B) Nanospheres and blunt ended gold nanotriangles bioreduced by DOTE, inset bar representing 20 nm; (C) Spherical monodispersed AgNPs synthesized by DOTE, inset bar representing 50 nm; (D) Silver nanospheres synthesized by DOTE, inset bar representing 20 nm.
The crystalline phase of the AuNPs and AgNPs was confirmed with the XRD. Figure 7A and 7B represent the X-ray diffraction (XRD) data of AuNPs and AgNPs which matches with the JCPSD (Joint Committee for Powder Diffraction Standard) data card no.04-0784 and 04-0783, respectively. Lattice planes (111), (200), (220) and (311) for the NPs show cubic crystalline phase with standard lattice constant to be 4.078 A0 and 4.086A0 for AuNPs and AgNP, respectively.
In order to understand the role of functional groups present of the phytochemicals responsible for reduction of metal ions, FTIR spectra of DOTE was recorded before bioreduction (Figure 8a) and after reduction of Au3+ (Figure 8b) and Ag 1+ (Figure 8c) into respective NPs. The peaks characteristic to key components like flavonoids and phenolics were observed in Figure 8a. Peaks observed at 1618, 1382, 1244 and 1066 cm-1 represent C=N stretch, C-N stretch, C-O stretch. The broad and strong peak at ~3380 cm-1, which is due to hydroxyl (-OH) group of phenols/alcohol, can be seen in the DOTE before and after reduction of both Au3+ and Ag1+. Another peak which remained intact after reduction of metal salts represents C=N stretching (~1618 cm-1).
DOTE exhibited the presence of higher amount of starch followed by phenolic contents. Additionally total reducing sugars ascorbic acid and citric acid were also observed (Table 1). In GCMS/MS analysis diverse groups of phytochemicals were observed major being, Isotridecanol, 2-Isopropyl-5-methyl-1-heptanol and 1-Hexadecanol, 2-methyl (Table 2).
|Sample||Total phenolic content (µg/mL)||Starch (µg/mL)||Total reducing sugars (µg/mL)||Ascorbic acid (µg/mL)||Citric acid(µg/mL)|
Table 1: Phytochemical composition of DOTE.
|Sr. No||Name of compounds||RTS||Formula||Molecular weight|
|1||Pentanoic acid, 3-methyl-||5.28||C6H12O2||116|
|2||Hexanoic acid, 2-methyl-||5.47||C7H14O2||130|
|4||Propionic acid, 3-mercapto-, isooctyl ester||9.40||C11H22O2S||218|
|18||10-Heptadecen-8-ynoic acid, methyl ester, (E)-||23.79||C18H30O2||278|
|20||3-Chloropropionic acid, heptadecyl ester||31.70||C20H39ClO2||346|
|22||Palmitic acid, (2-phenyl-1,3-dioxolan-4-yl)methyl ester||37.34||C26H42O4||418|
Table 2: Main compounds detected by GCMS/MS.
The reduction of 4-nitrophenol to 4-aminophenol by NaBH4 was chosen to study the catalytic performance of both AuNPs and AgNPs (Figure 9A and 9B). When 25 µL of DOTE synthesized AgNPs (10 mM) was used, the reaction was completed in 30 min with the time interval of 2 min. As seen from the Figure 9B, the absorption peak at 400 nm gradually decreased in intensity as the reaction proceeded. At the same time, a new peak of 4 aminophenol at 295 nm appeared and increased in intensity indicated the reduction of 4-NP. Thus, a linear relation between ln(At/A0) verses time t has been obtained as shown in the inset of Figure 9B. The reactions followed pseudo-first order rate kinetics as the concentration of NaBH4 used exceeded that of 4-NP and AgNPs or AuNPs. This demonstrates a strong catalytic potential of both AuNPs and AgNPs synthesized using DOTE.
Medicinal plants are considered to be rich source of bioactive principles which attributes to their promises in development of novel nanomedicine [23-27]. These bioactive principles are reported to have very significant role in the synthesis of nanoparticles and stabilization of the same. Moreover, being medicinal, the plant extracts are non toxic and are proven to be most biocompatible which a prerequisite for design of nanomedicines. D. oppositifolia showed complete synthesis of AuNPs and AgNPs within 5 h which is comparable to our previous report on D. bulbifera [13,14]. UV-visible spectroscopy confirmed the synthesis showing a peak at 550 nm for AuNPs and 430 nm for AgNPs which is in agreement with the previous report on synthesis using Anacardium occidentale leaf extract . Concentration of the metal salts and the reaction temperature played a major role in the synthesis process which was also reported in case of synthesis of AgNPs using Cacumen platycladi extract . HRTEM analysis showed that the exotic shapes of the nanoparticles varied from spherical to truncated triangles which are in well agreement with the report on synthesis of nanopaticles using cape Aloe. It is proposed that such shapes are generated due to slow citric acid reduction . Higher crystalline nature of AuNPs found in XRD also suggested maximum reduction of Au3+ into AuNPs, whereas small traces of organic/amorphous phase was seen in XRD of AgNPs which is in well agreement with earlier reports [31-33]. FTIR spectra confirmed the presence of C=N stretch, C-N stretch, C-O stretch and ether linkage that are reported in flavones and catechins [32,34-36]. Presence of a broad and strong peak at ~3380 cm-1, specific to hydroxyl (-OH) group of phenols/alcohol and C=N stretching (~1618 cm-1) seen in DOTE before and after reduction of both Au3+ and Ag1+ suggests the role of C=N and –OH in stabilizing the nanoparticles while rest of the groups interacting with the metal salts to reduce them into their respective metal NPs . Phytochemical estimation by biochemical assays and GCMS/MS analysis confirmed the presence of various groups of compounds similar to D. bulbifera that might help in the bioreduction like polyphenols flavonoids, ascorbic acid and citric acid. Similarly the capping agents as starch might add up to its stability [22,37-39]. Both AuNPs and AgNPs synthesized by DOTE showed efficient catalytic activity towards reduction of 4-nitrophenol to 4-aminophenol by NaBH4 with a pseudo-first order rate kinetics. Our results are in close agreement with earlier reports on Barleria prionitis, Gnidia glauca and Breynia rhamnoides [15,23].
D. oppositifolia, a traditional medicinal plant with curative effect against various ailments is reported herein, to have nanobiotechnological potential. Its tuber extract synthesized both AuNPs and AgNPs within 5 h which is considered to be rapid, efficient and environmentally benign route for synthesis of metal nanoparticles. The bioreduced nanoparticles were characterized and confirmed by various physical techniques and were found to be in a size range between 10 to 30 nm in majority. Various groups of phytochemicals estimated by biochemical methods included polyphenols, starch, reducing sugars, ascorbic acid and citric that might help in reduction of the metal ions to nanoparticles and further stabilization as well. The bioreduced AuNPs and AgNPs show chemocatalytic potential in conversion of 4-nitrophenol to 4-aminophenol.
The authors acknowledge the help extended for the use of TEM and HRTEM facilities in Chemical Engineering and CRNTS funded by the DST through Nanomission and IRPHA schemes.