Deoxygenation of graphene oxide using biocompatible reducing agent Ficus carica (dried ripe fig)
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To address the environmental concerns, a relatively simple, innovative and low-cost procedure has been suggested for synthesis of graphene nanosheets (GNSs). The procedure utilized here for synthesizing GNSs is derived from aqueous suspension of graphene oxide (GO). Ficus carica (FC) is known to be remarkable antioxidant. It is regarded as suitable biocompatible alternative to hazardous and toxic chemicals. This method ensures non-hazardous nature of the deoxygenating agent (FC) as well as their oxidized outcomes. Structural and morphological characterization indicates the absence of oxygen-bearing groups from graphene oxide. Characterization was done by standard techniques such as X-ray crystallography, FTIR, RAMAN, DLS, and UV–Vis spectrophotometry. Morphological investigation of resulting material was also undertaken through standard techniques by FESEM, TEM, HRTEM with SAED pattern. Thermal stability of FC derived graphene was also investigated. The investigation may initiate new routes for preparing GNSs at large scale facilitating a better research and commercial utilization.
KeywordsGreen synthesis Graphene oxide Graphene nanosheets Ficus carica extract
Graphene, the wonder material, has become popular among nanomaterial investigators. The path-breaking discovery of graphene was made by Geim and co-workers . Since then there has been active interest in graphene. Remarkable features such as large surface area (2620 m2 g−1), superior Young’s modulus (1.0TPa), exceptional thermal conductivity (~ 5100 W m−1 K−1), unique chemical strength and large electron movement (2.0 × 105 cm2 v−1 s−1) are shown by the graphene sheets . These outstanding features of graphene initiate new ways for multidimensional applications in different segments of technology. They include applications in catalyst engineering [3, 4], lithium-ion batteries , chemically derived sensors , biosensors , anti-bacterial activities , delivery of drugs , solar cells , super capacitors , touch panels , electromagnetic shielding application , water purification , and absorption of non-aqueous fluids oils, olefins, aromatic compounds, dyes and organic solvents [15, 16, 17, 18]. Due to above-mentioned applications, several methods have been investigated to synthesize graphene. Chemical reduction of graphene oxide (GO) is considered as a superior method for large scale production. The cost is affordable, and time required is of short duration. Identification of green reducing agent for this work is necessary. There onwards, many reductants have been applied for reducing GO. Hydrazine hydrate (N2H4.H2O), hydrogen bromide (HBr), hydriodic acid with trifluoroacetic acid (HI, TFA), phenol, p-toluenesulfonic acid (PTSA) and sodium borohydride (NaBH4) have been employed as deoxygenating agents. However, most of these chemicals have limitations as their by-products are hazardous for biosphere. Hence, sophistication is required in using these chemicals. The presence of harmful by-products may greatly increase production costs on industrial scale . Despite precautions the final product may contain remnants of these harmful chemicals. Its use may not be advisable in several applications such as biomedical field or water purification. To overcome these obstacles, investigators have concentrated more on identifying environmentally suitable reducing agents with higher reducing ability than chemical reducing agent. Consequently, there is an actual need for simple, environment-friendly and cost-effective method for production of graphene. These problems may be solved by green reduction. Researchers have presented various eco-friendly reductants in recent years . They include antioxidants, microorganisms, extracts of plants, vitamins, organic acids and proteins. Such reductants are termed as “green reducing agents” as they are eco-friendly and cost-effective. Further attempts should, however, continue to identify an inexpensive, efficient as well as green deoxygenating agent for production of graphene chemically at large scale.
Ficus carica commonly known as fig is nutritious fruit with useful medicinal properties. Investigation on Ficus carica becomes intensive due to its remarkable nutritional utility and therapeutic properties. Extracts of all parts of Ficus carica possess medicinal properties and may cure several illnesses including cancer , diabetes , cardiovascular disorders , and antimicrobial activity . Ficus carica has these medicinal values due to its precious chemical constituents especially present in ripe fruits. Studies have shown that fig fruit is a rich source of saccharides (mainly glucose, fructose and sucrose), organic acids (citric acid, malic acid), anthocyanin and antioxidant polyphenols [chlorogenic acid (5-O-caffeoylquinic acid), catechin, gallic acid, rutin (quercetin-3-O-rutinoside) and syringic acid] [25, 26, 27, 28, 29]. These valuable constituents generally have extraordinary binding ability to the oxygen functionalities to yield complementary oxides and various by-products, such as water molecules. These important constituents, especially carbohydrate (glucose and fructose), increase in terms of percentage on ripening of Ficus carica fruits [30, 31, 32]. Earlier studies suggested that reducing sugars such as glucose, fructose and sucrose have been used for reducing the GO [33, 34]. They work via a redox mechanism which relies upon their inherent ability to form closed-chain structures. Both reducing sugar (glucose and fructose) and its oxidized outcomes are eco-friendly. Additionally, their oxidized outcomes act as capping agents for sustaining GNS. Thus, one may conclude that FC carbohydrate content has enough potential to reduce graphene oxide into graphene nanosheets chemically.
Graphite powder, hydrogen peroxide (30% H2O2), potassium permanganate (KMnO4), sulphuric acid (98% H2SO4), and sodium nitrate (NaNO3) were procured from Merck, Sigma-Aldrich, India. The fresh and ripe fruits of Ficus carica were purchased from the local market. All aqueous solutions were prepared with extremely pure water.
Synthesis of graphene oxide
The modified Hummers’ method was used to synthesize graphene oxide . At 0 °C, 25.0 mL of sulfuric acid (93–96%) with (0.5 g) graphite powder was stirred for 20 min. After that, (0.5 g) of NaNO3 was slowly added to the solution while stirring for 1 h. After that, KMnO4 (3 g) was added gradually at 0 °C. The whole mixture was heated at 35 °C for 1 h. Then 40 mL of water was added into the mixture resulting into release of heat which was retained at 95 °C for 40 min. Again 100 mL of water was added into the solution. Finally, 30% H2O2 (∼ 10 mL) was added slowly into the solution. The warm mixture was centrifuged and washed with moderately warm water (150 mL). Subsequently, the sample was washed with plenty of water till neutral pH was obtained. Afterwards, the sample was treated with basic solution which is heated at 70 °C for 1 h. A dark brown solution is obtained which centrifuged at (12500 rpm, 30 min). The sample was re-dispersed and washed with ethanol. Then 14 mM of HCl solution was added and stirred again for 1 h at 70 °C. Again, the solution was centrifuged and washed with ethanol three times. Finally, the sample was dried in an oven at 30 °C for 3 days to yield GO.
Synthesis of Ficus carica extract
Figs were cut into small pieces. 100 g of the dry mass was immersed in 200 mL of twice-distilled water. Then the mixture was heated at 95 °C for 60 min to obtain the extract. The prepared extract was centrifuged at the speed of 10,000 rpm. The extract was drained and screened through a cheese cloth to remove impurities. The fresh extract was used immediately for reduction of graphene oxide.
Synthesis of GNSs based on FC extract reduction
For the synthesis of few layer graphene, 100 ml of FC extract was added slowly through the burette into 100 mL of uniformly distributed GO solution (0.2 mg/mL) and the solution was kept stirring for 2 h. After that 2 mL of ammonia solution is added dropwise in solution and the same is stirred for 12 h at 95 °C. Then, the incoming stable black solution was centrifuged (10,000 rpm) for 30 min. Finally, the obtained sample was washed thrice with diethyl ether and water solution. Then the resulting GNSs was re-dispersed in ethanolic solution for upcoming characterization.
X-ray diffraction (XRD) studies were performed at room temperature on a D/Max 2500 V/PC (Rigaku Corporation, Tokyo, Japan) at a scan rate of 1/min. Fourier transform infrared spectroscopy (FTIR) investigation was done over the wave number range of 4000–600 cm−1 using a Bruker (Tensor 37) spectrometer (Thermoacoustic, USA). Raman spectra of pure GO, and GNS were obtained using Nano finder 30 (Tokyo Instruments Co., Osaka, Japan) in back scattering geometry with a CCD detector, a 514-nm Ar laser and a 100 × objective mounted on an Olympus optical microscope. Ultraviolet–visible (UV–Vis) spectra were obtained using a U3900 Biochrom Cambridge, UK. The particle size of dispersions was measured by Spectroscatter 201 (Malvern Instruments, Limited, UK). The GO dispersion in water and GNS in DMSO were employed as UV–Vis samples, and their respective solvents were taken as the reference. The morphology of the prepared sample was characterized by TEM (Phillips CM-100 with a tungsten filament), scanning electron microscopy (SEM) was carried out on a S4800 scanning electron microscopy (HITACHI, Japan). Thermogravimetric analysis (TGA) of the samples was performed on (Perkin Elmer Pyris1 TGA Thermogravimetric Analyzer) at a heating rate of 10 °C min−1 under nitrogen atmosphere.
Results and discussion
UV–Vis spectra of GO and GNSs
FTIR spectrum of GO and GNSs
Size distribution analysis by DLS
Morphological characterization: TEM and SEM analyses
Thermal gravimetric analysis (TGA) study
Chemistry: mechanism of graphene oxide reduction
The present investigations suggest environment-friendly procedure for deoxygenation of GO by applying FC extract as the dynamic deoxygenating agent. The gained GNS is few layered in structure. Moreover, FC extract has several benefits over traditional reductants such as sodium borohydride and hydrazine hydrate. They are abundantly available, easily extracted from GNS, cost-effective, and suitable for eco-friendly manufacture method. Therefore, it is believed that Ficus carica extract is a powerful eliminator of the harmful chemical reducing agents. It can be utilized to develop graphene on large scale, especially for bio-materials containing graphene sheets.
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