Green Approach for the Effective Reduction of Graphene Oxide Using Salvadora persica L. Root (Miswak) Extract
© Khan et al. 2015
Received: 20 April 2015
Accepted: 23 June 2015
Published: 3 July 2015
Recently, green reduction of graphene oxide (GRO) using various natural materials, including plant extracts, has drawn significant attention among the scientific community. These methods are sustainable, low cost, and are more environmentally friendly than other standard methods of reduction. Herein, we report a facile and eco-friendly method for the bioreduction of GRO using Salvadora persica L. (S. persica L.) roots (miswak) extract as a bioreductant. The as-prepared highly reduced graphene oxide (SP-HRG) was characterized using powder X-ray diffraction (XRD), ultraviolet-visible (UV-vis) spectroscopy, Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, X-ray photoelectron (XPS) spectroscopy, and transmission electron microscopy (TEM). Various results have confirmed that the biomolecules present in the root extract of miswak not only act as a bioreductant but also functionalize the surface of SP-HRG by acting as a capping ligand to stabilize it in water and other solvents. The dispersion quality of SP-HRG in deionized water was investigated in detail by preparing different samples of SP-HRG with increasing concentration of root extract. Furthermore, the dispersibility of SP-HRG was also compared with chemically reduced graphene oxide (CRG). The developed eco-friendly method for the reduction of GRO could provide a better substitute for a large-scale production of dispersant-free graphene and graphene-based materials for various applications in both technological and biological fields such as electronics, nanomedicine, and bionic materials.
KeywordsGraphene Graphene oxide Natural product Green chemistry Spectroscopy
Among various carbonaceous materials, graphene has attracted tremendous attention of scientists and technologists, due to its stable 2D morphology and exceptional electronic properties related to its crystal structure [1–3]. Indeed, graphene has revolutionized the field of nanotechnology and has emerged as a promising new nanomaterial for a variety of exciting applications, despite being discovered recently. Remarkable thermal, electrical, and mechanical properties of graphene have been extensively exploited in various fields, including sensors , solar cells , nanoelectronics , energy storage , functional nanocomposites , biomedicine , and catalysis [10, 11]. Commonly, graphene is obtained from graphite , which is a naturally occurring material and has been in use for centuries . The free-standing single-layer of graphene was first obtained in 2004 by the isolation of graphene from graphite via micromechanical cleavage . This fascinating approach of peeling off graphene layers from graphite can only be useful for fundamental science, and is not suitable for the large-scale production of graphene . Therefore, tremendous attention is being paid to explore various alternative approaches for the low-cost and bulk production of graphene.
Several methods, including chemical vapor deposition (CVD) , arc discharge , epitaxial growth on SiC , chemical conversion, liquid phase exfoliation, and sequential oxidation reduction of graphite, have been reported . Among all these methods, sequential oxidation and reduction of graphite has attracted significant attention, because it is benign, less expensive, and is more suitable for the bulk production of graphene [20, 21]. Although, the flakes of graphene-like sheets obtained from such methods are not defect free, these nanosheets are highly processable, described as highly reduced graphene oxide (HRG), and have been extensively applied for the preparation of various graphene-based functional bio- and nanocomposites .
Among various reduction methods (thermal, electrochemical, or chemical) [23–25] of graphite oxide (GO) or graphene oxide (GRO), chemical reduction is the most promising method and is extensively applied for the large-scale production of HRG . Several reducing agents, such as hydrazine , ammonia borane complex , sodium hydride , and hydrohalic acid , have been used for the reduction of GRO to obtain HRG . However, despite several advantages, the chemically reduced HRG has limited applications, as it tends to agglomerate strongly due to interlayer attractive van der Waals forces . Therefore, further chemical stabilizers, such as porphyrins and pyrenebutyric acid, are frequently required to prevent these kinds of agglomerations . Majority of the chemicals involved in the reduction and functionalization of GRO are highly toxic in nature, hazardous, and harmful to both environment and human life . In addition, the presence of trace amount of highly toxic reducing agents on the surface of HRG could seriously alter several properties of HRG and has adverse effects on its biological applications .
However, in comparison to chemical reduction, the green reduction of GRO involves biocompatible ingredients under physiological conditions of temperature and pressure [36, 37]. To date, several green reductants extracted from microorganisms, marine organisms, or plant extracts have been applied for the preparation of HRG, including gallic acid, fluorescent protein, melatonine, ascorbic acid, and wild carrot roots [38–42]. Among these green reductants, plant extracts have been significantly exploited due to their low cost, bulk availability, and biocompatibility [43–46]. Recently, extracts of various plants were used both as reducing and stabilizing agents during the preparation of metallic nanoparticles and in some cases for the reduction of GRO [47–51].
In this study, we have applied Salvadora persica (SP) L. (miswak) extract as a bioreductant. ‘Miswak’ is an Arabic word which literally means ‘tooth-cleaning stick or chewing stick’ . It is widely used as an oral hygiene tool in most parts of the world. Approximately, 182 plant species have been used as chewing sticks; however, roots of S. persica L. are the most common one and frequently used for this purpose. Salvadora persica L. is a glabrous tree or shrub belonging to family Salvadoraceae [53–55]. S. persica L. is found in many parts of the world. In Saudi Arabia; it is very widely spread, especially in the southern regions of Saudi Arabia. Because of the Sunnah (practices of prophet MuhammadPBUH) of prophet of Islam MuhammadPBUH, use of miswak obtained from roots of S. persica L. is very common and popular in the Muslim world. Besides having several benefits for oral hygiene, various parts of S. persica L. are also used in folk medicine for the treatment of several diseases. For example, it is reported to have diuretic, antiscorbutic, anthelmintic, and analgesic properties .
The roots of S. persica L. growing in Jizan, Southern region of Saudi Arabia were purchased from a local herbal market at Batha, Riyadh, Saudi Arabia. The identity of the plant material was confirmed by a plant taxonomist from the Herbarium Division of the College of Science, King Saud University, Riyadh, Saudi Arabia. A voucher specimen was retained in our laboratory with the voucher specimen number KSUHZK-302. Graphite powder (99.999 %, _200 mesh) was purchased from Alfa Aesar (USA). Concentrated sulfuric acid (H2SO4 98 %), potassium permanganate (KMnO4 99 %), sodium nitrate (NaNO3 99 %) and hydrogen peroxide (H2O2 30 wt %) and all organic solvents were obtained from Aldrich Chemicals (USA) and used without further purification.
Preparation of S. Persica L. Root (Miswak) Extract
First, fresh roots of S. persica L. were cut into small pieces. The resultant pieces (1.3 kg) were soaked in deionized water (3000 mL) and refluxed for 4 h. Then, the aqueous solution obtained after reflux was filtered and dried at 50 °C under reduced pressure in a rotary evaporator to give a dark brownish gummy extract (30.0 g) which was stored at 0–4 °C for further use.
Preparation of Highly Reduced Graphene Oxide (SP-HRG)
Graphite oxide (GO) required for the preparation of SP-HRG was synthesized according to our previously reported method . Initially, as-prepared graphite oxide or GO (200 mg) was dispersed in 40 mL of DI water and sonicated for 30 min to obtain graphene oxide (GRO) sheets. The resulting suspension was taken in a round bottom flask mounted with a cooling condenser, which is heated to 100 °C. Subsequently, 10 mL of an aqueous solution of the root extract (0.1 g mL−1) was added to the suspension, which was then allowed to stir for 24 h at 98 °C. After this, black powder of highly reduced graphene oxide (PE-HRG-1) was collected by filtration, which was further washed with DI water several times to remove the excess root extract residue and redistributed into water for sonication. This suspension was centrifuged at 4000 rpm for another 30 min. Subsequently, the supernatant was thrown out and the precipitate was collected and dried in vacuum.
A PerkinElmer lambda 35 (USA) UV-vis spectrophotometer was used for the optical measurements. The analysis was performed in quartz cuvettes using DI water as a reference solvent. Stock solutions of SP-HRG and GRO for the UV measurements were prepared by dispersing 5 mg of sample in 10 mL of DI water and sonicating for 30 min. The UV samples of GRO and SP-HRG were prepared by diluting 1 mL of stock solution with 9 mL of water.
XRD diffractograms were collected on a Altima IV (Rigaku, Japan) X-ray powder diffractometer using Cu Kα radiation (λ = 1.5418 °A).
Transmission Electron Microscopy
TEM was performed on a JEOL JEM 1101 (USA) transmission electron microscope. The samples for TEM were prepared by placing a drop of primary sample on a holey carbon copper grid and drying for 6 h at 80 °C in an oven.
Fourier Transform Infrared Spectrometer
FT-IR spectra were measured on a PerkinElmer 1000 (USA) Fourier transform infrared spectrometer. In order to remove any free biomass residue or unbound extract to the surfaces of SP-HRG sheets, the SP-HRG nanosheets were repeatedly washed with distilled water, and then the product was centrifuged at 9000 rpm for 30 min and dried. The purified SP-HRG nanosheets were mixed with KBr powder and pressed into a pellet for measurement. Background correction was made using a reference blank KBr pellet.
X-ray Photoelectron Spectroscopy
XPS spectra were measured on a PHI 5600 Multi-Technique XPS (Physical Electronics, Lake Drive East, Chanhassen, MN) using monochromatized Al Kα at 1486.6 eV. Peak fitting was performed using the CASA XPS Version 2.3.14 software.
Results and Discussion
Reduction of GRO was carried out at an elevated temperature under reflux conditions using the root extract of miswak. Upon completion of reduction process, the brown color of GRO dispersion changed to dark black, which indicated the formation of SP-HRG. It is worth mentioning that no color change was observed in the absence of miswak root extract, under similar conditions. Although, 10 mL aqueous root extract (100 mg/mL) was found to be sufficient for the complete reduction of GRO, three different samples of SP-HRG (prepared by increasing the concentration of root extract) were also used to further investigate the effect of concentration of root extract on the dispersion quality of SP-HRG. The dispersion quality of the as-prepared samples of SP-HRG was also compared with chemically reduced GRO (CRG) using hydrazine as a reducing agent. The samples of SP-HRG were synthesized by using 10 mL (100 mg/mL) (SP-HRG-1), 20 mL (SP-HRG-2), and 50 mL (SP-HRG-3) of miswak root extract with concentration (100 mg/mL) while the amount of GRO was kept constant.
Similarly, pure miswak root extract also exhibited up to ~80 % rapid weight loss between 100 and 900 °C in various steps (orange line in Fig. 3). However, SP-HRG exhibited only ~12–15 % weight loss between this temperature range, which was much lower than that of pure GRO and miswak root extract (purple line in Fig. 3). This is attributed to the significant decrease of oxygen-containing functional groups, which clearly indicates the reduction of GRO. Notably, with increasing concentration of miswak root extract, the weight loss of SP-HRG also increased, for instance, SP-HRG-1 showed a weight loss of ~12–15 % (purple line), SP-HRG-2 exhibited 22–25 % (green line), where as in SP-HRG-3 up to ~45 % of weight loss was observed (red line). This points towards the presence of phytomolecules of miswak root extract on the surface of SP-HRG.
The large-scale production of graphene obtained via solution-based sequential oxidation-reduction processes usually suffer from poor dispersibility in water and other organic solvents, due to their strong hydrophobic nature . The irreversible agglomeration of graphene nanosheets is usually prevented by the addition of various external surfactants and stabilizers, including polymers and dendrimers , which have undesirable effects on the properties of graphene. However, during the green reduction of GRO using plants extracts, additional surfactants or stabilizers are not required, wherein the plant extracts themselves act as both reducing as well as stabilizing agents . The dispersion quality of HRG obtained using root extract of miswak (SP-HRG) is investigated and also compared with chemically obtained HRG (CRG). For this purpose, various samples of SP-HRG are prepared by increasing the concentration of miswak root extract. For instance, SP-HRG-1, SP-HRG-2, and SP-HRG-3 are prepared by using 10, 20, and 50 mL of miswak root extract (100 mg/mL), respectively.
Graphene oxide is reduced using an eco-friendly route, i.e., root extract of S. persica (miswak) as both a bioreductant and stabilizer. The as-prepared bioreduced SP-HRG exhibited excellent dispersibility compared to the chemically reduced graphene oxide (CRG). The concentration of S. persica (miswak) root extract played a critical role in the dispersibility of SP-HRG, whereas SP-HRG prepared using a high concentration of miswak root extract (SP-HRG-3) demonstrated superior dispersion in DI water. This clearly indicates that the miswak root extract acted not only as a bioreductant but also as a capping ligand, which was confirmed by various spectroscopic techniques. Therefore, the protocol presented here for the bioreduction of GRO can be potentially applied for the large-scale production of dispersant-free graphene nanosheets. Indeed, the highly oxidized nature, abundancy, and low cost of miswak root extract can be further exploited for the up-scaling of graphene and graphene-based materials for various technological and biological applications.
This project was supported by NSTIP Strategic technologies programs, number (11NAN1860-02) in the Kingdom of Saudi Arabia.
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