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tGraphene is a two dimensional single layer carbon sheet that has attracted huge interest in various fields of science and engineering due to its distinctive electrical, optical, thermal and mechanical properties [1-3]. Solution based chemical reduction of exfoliated graphite oxide to reduced graphene oxide (rGO)  is an efficient method in the synthesis of graphene sheet due to its low cost and facile synthetic nature in a controlled, scalable and reproducible manner [4-5]. In water, GO forms stable dispersion under sonication, owing to presence of oxygen containing functional groups like hydroxyl, epoxide and carboxyl moieties [6]. These functional groups can act as nucleation centers for the attachment of nanoparticles [7]. Go or rGO sheets produce large surface area, limit the growth of nanoparticles, improve the stability and dispersion of nanoparticles on GO or rGO. This attached nanoparticles enlarge the interplaner spacing of the GO or rGO maintaining the excellent properties of individual GO or rGO sheets and avoiding the aggregation of GO or rGO sheets into graphitic structure [8-9]. Because of large specific surface area and the above advantages, GO and rGO have been used usually as supports for the attachment of nanoparticles to yield nanocomposites for various applications [10].   

Recently, rGO coated with AgNPs (AgNPs/rGO) has attracted great interest because of its optical [11], electrochemical properties [12] as well as its high catalytic activity [13-14]. A large surface area of GO nanosheet and strong Van der Waals forces between rGO and AgNPs can significantly reduce nanoparticles aggregation and an efficient interaction ensuring the stability and reproducibility of AgNPs [15]. Literatures report the preparation of Ag-rGO nanocomposite by chemical reduction method but these involves the use of hazardous reducing agents like hydrazine, NaBH4 and formaldehyde to reduce both GO and Ag+ which are exposing risks to the environment and health [16-18].  Simple sonication method is an attractive method for the synthesis of Ag-rGO nanocomposites that do not make any use of hazardous agents and this method is attractive due to its ability to produce variety of nanostructures including metals [19-20] and metal oxides [21-22]. Many researchers reported Ag-rGO nanocomposites for various applications like to enhance catalytic activity of nitroarenes, reduction of 4-NP and electrochemical sensing of tryptophan [23-27].

Mercury exists in metallic, inorganic and an organic form and is of great causes due to its harmful effects on human health and environment [28].  Mercuric ions (Hg2+), present mainly in surface water due to its high solubility and it can pose serious health problems mainly to human brain, nervous system, kidneys and endocrine system [29]. Therefore, it is critical to detect and measure the level of Hg2+ with high sensitivity and selectivity in the presence of other heavy metal ions. Therefore, the determination of Hg2+ ions using a cost effective method with a rapid response is highly recommended and the optical assay is highly attractive due to the easy determination of metal ions by spectroscopic instrumentation technique. Hence, using metal NPs as an optical sensor promising to be simple, cost effective approach with high sensitivity and rapid detection of toxic metal ions in the aqueous medium [30-31].

Herein, we report the green synthesis of Ag-rGO nanocomposites using Acacia nilotica gum extract. Acacia nilotica is a well-known medicinally important plant commonly found in the dry areas of Asia, Africa and Australia. Various parts of the plant like leaves, bark, roots and fruits are used as medicine for the treatment of asthma, meningitis, bronchitis, pneumonia, hypoglycemia, sore throat etc. Different types of plant secondary metabolites like flavanoids, tannins, triterpenoids, saponines etc. are present in the leaf extract and gum extract of Acacia nilotica. The polyphenols present in Acacia nilotica gum extract are act as reducing agent for the preparation of AgNPs [32-33]. The synthesized Ag-rGO nanocomposites were characterized using various characterization techniques and have been investigated to acts as an efficient sensor for Hg2+ ions in the aqueous solution and environment. The easy synthesis and high stability of the Ag-rGO nanocomposite makes the method very simple and easy to implement.

 2.    Experimental

2.1 Materials

Graphite powder, sulphuric acid (H2SO4, 98 %), sodium nitrate (NaNO3), potassium permanganate (KMnO4, 99.9 %), silver nitrate (AgNO3) and metal salts were purchased from sd Fine-Chem Ltd (Mumbai, India).All chemical reagents were of analytical reagent grade and used as received without further purification. The stock solutions of Ag+ and Hg2+ were prepared by dissolving AgNO3 and HgCl2 in water. Doubly distilled water was used for preparing all solutions prior to their use throughout the experiments.

2.2 Synthesis of Ag-rGO nanocomposites

The modified Hummer's method was used for synthesis of rGO from natural graphite powder [34]. In brief, the graphite powder (2.5 g) and NaNO3 (1.25 g) were added to the concentrated H2SO4 (57.5 mL) in an ice bath. KMnO4 (7.5 g) was slowly added to the solution, while maintaining the temperature below 20''C. The mixture was stirred in the ice bath for 30 min and then put in 350 C water bath for 30 min. Then 115 mL of hot water was added, followed by 25 mL hydrogen peroxide (30 wt %,) solution to terminate the reaction. The mixture was filtered and washed with deionised water many times to remove any excessive acid and inorganic salts. The resulting rGO was dried in heating mantle at 60''C. The Ag-rGO nanocomposite was prepared by reducing Ag+ ions directly on rGO with Acacia nilotica gum as the reducing and stabilizing agent. In the present typical procedure for the nanocomposite synthesis, the rGO powder (0.005 g) was dispersed in water (10.0 mL) by sonication for 30 min to form a stable rGO colloid. Then4 mmol'L-1 AgNO3was added to the 10 mL yellow colored gum extract (1%) and stirred at room temperature (R. T.). This solution was added to rGO colloid while stirring for 10 min. Finally, Ag-rGO nanocomposites were obtained and used for different characterizations.

2.3 Characterization techniques

The synthesized Ag-rGO nanocomposites were characterized by using various analytical techniques. The reduction of the pure Ag+ ions was monitored by measuring the absorbance of the reaction medium with UV'Vis'NIR spectrophotometer (Shimadzu, Model UV-3600). The X-ray diffraction pattern of synthesized Ag-rGO nanocomposites was recorded on a Philips automated X-ray diffractometer (Model PW-3710) equipped with a crystal monochromator employing Cu-K'' radiation of wavelength 1.5406 '' in 2'' range from 20'' to 80''. Transmission electron microscopy (TEM) analysis of Ag-rGO was performed with JEM- 2100 (Jeol), operated at 200 kV. The FTIR spectra of Ag-rGO nanocomposites were recorded on FTIR-4600 (Jasco, Japan). The fluorescence property of Ag-rGO nanocomposites was characterized by a Spectrofluorometer (FP 8200,Jasco,Japan).

3. Results and discussion

3. 1 Optical properties of Ag-rGO nanocomposites

Optical properties of Ag-rGO nanocomposites were determined using UV-Vis-NIR absorption spectra, FTIR spectra and photoluminescence (PL) spectra. The synthesized rGO was firstly characterized by UV'Vis-NIR absorption spectra as shown in Fig. 1A.The UV'Vis absorbance spectrum of rGO showed absorption peak at 226 nm, which corresponds to the ''-''* transitions of the aromatic C -C bonds. A UV'Vis spectra of Ag-rGO at room temperature and at 60 ''C before and after centrifugation was also shown in Fig 1B. According to the Beer's law, the absorbance is linear with the concentration of AgNPs [35]. The peaks of both Ag-rGO at room temperature and at 60 ''C showed blue shift after centrifugation due to removal of large AgNPs from rGO substrates during the centrifugation. However, the change of absorbance is apparently different for Ag-rGO at room temperature. Compared to the little decrease in absorbance for Ag-rGO at room temperature (from 0.9567 to 0.8942). Furthermore, the absorbance peak located at 440 nm for Ag-rGO at room temperature is 0.9567. This result indicates that the oxygen containing group on Ag-rGO at room temperature plays an important role in nanocomposite substrates.

The FTIR spectra of Ag-rGO at room temperature and Ag-rGO at 60''C are shown in Fig.2A. The typical peaks at 1634 cm-1 relevant to restoration of the C=C structure, 3334cm-1 corresponds to the stretching vibration of C'OH that exists in Ag-rGO nanocomposites [36].  The same peaks were observed in Ag-rGO at room temperature and Ag-rGO at 60''C.

Typical PL spectra of Ag-rGO nanocomposite were shown in Fig.2B. PL spectra of Ag-rGO nanocomposite exhibit the visible emissions covering blue to red wavelength range in the same sample under visible excitation. The maximum fluorescence emission intensity of Ag-rGO nanocomposite can be obtained at 660 nm when excited at 440 nm while the emission intensity of AgNPs was observed at 600 nm at 456 nm excitation wavelength.  Fig. 2B depicts the PL spectra of Ag-rGO nanocomposites excited by long wavelength light with the up-conversion emissions located in the range 660 nm. The Ag-rGO nanocomposite has excellent PL properties, such as the strong PL in the visible range. Compared with AgNPs, Ag-rGO nanocomposite shows sharp PL spectra.

3. 2  Structural properties of Ag-rGO nanocomposites

The structural properties of Ag-rGO nanocomposites were determined by XRD analysis. The crystalline nature of Ag-rGO nanocomposites was studied by XRD analysis.XRD pattern of Ag-rGO at R.T. (a) and 60 ''C (b) are shown in Fig. 3A.From the XRD pattern, it was observed that the characteristic rGO peak centered at 25.91'', corresponding to the (002) reflection of rGO shown in Fig. 3A [37]. Further, the diffraction peaks appeared at 38.1'', 44.1'', 64.2'' and 77.2'' correspond to the (111), (200), (220) and (311) crystal planes of face-centered cubic (fcc) Ag. The 2'' values of Ag diffraction match with the standard database values from JCPDS No. 04784. XRD peaks of Ag-rGO nanocomposites at room temperature and at 60''C are with similar patterns. The crystalline size of Ag-rGO was calculated from the highest intensity peak using the Scherer's equation [38] as the following,

D=  (0.9 '')/('' cos''' ) (1)

Where D is the crystallite size of Ag-rGO, '' is the wavelength of the X-ray source (0.1541 nm) used in XRD, '' is the full width at half maximum of the diffraction peak. The average crystallite size for as-synthesized Ag-rGO was found to be 11.9 nm. Additionally, the sharp and broaden rGO peaks (002) appearing at ~25.91'' suggest the high degree exfoliation of the rGO sheets in both Ag-rGO at room temperature and Ag-rGO at 60''C due to the insertion of AgNPs. The XRD results demonstrate that metallic AgNPs are successfully incorporated with rGO.

3. 3 Morphological properties of Ag-rGO nanocomposites

TEM was employed to visualize the size and shape of formed Ag-rGO nanocomposite. As observed in Fig.3B, SAED patterns of Ag-rGO nanocomposites showed the characteristic rings for the (111), (200), (220) and (311) planes of fcc Ag.Fig.3C shows the typical bright field TEM micrograph of the synthesized Ag-rGO nanocomposites. Figure 3C showed that the surface of rGO sheets was modified by the nanosized AgNPs. Most of the synthesized AgNPs absorbed on the rGO exhibit spherical shape, which displays a good combination between AgNPs and rGO. It is evident that there was variation in particle sizes and the average size estimated was 15 nm and the particles size ranged from 10 nm to 40 nm. Fig.3D shows the HR-TEM image of Ag-rGO nanocomposites, which shows spherical morphology of nanocomposites. Lattice fringes can be clearly seen in the HR-TEM image indicating that the d spacing for Ag [111] plane is 0.25nm respectively, which is in good agreement with results obtained from XRD. As shown in Fig. 3C exhibits numerous AgNPs because of their polar interaction with the oxygen containing group of the rGO substrate.

3.4  Detection of Hg2+ with Ag-rGO nanocomposites

The UV-Visible spectrophotometric detection of Hg2+ in aqueous solution was studied at room temperature. For a typical UV-Visible spectrophotometric analysis of metal ions with Ag-rGO nanocomposites, 1.0 mL of Ag-rGO nanocomposites and a known volume of standard Hg2+ ion solutions were added to a 10 mL standard flask. Then the solutions were diluted with water at 10 mL, mixed completely and maintained at room temperature for 5.0 min. Their absorption bands were recorded using a UV-Vis spectrophotometer. In addition, the detection of Hg2+ was also observed by the naked eye with respect to their color change from yellowish-brown to colorless.

3. 4. 1 Possible mechanism for Ag-rGO nanocomposites with Hg2+

In this study, we observed that on increasing the concentration of Hg2+ ions added to Ag-rGO nanocomposites and color of the solution changed from yellowish brown to colorless over a short period followed by a blue shift in the absorption spectra which was reflected in the UV-Vis absorption spectra. The color change was easily detected by the naked eye. The change could be due to oxidation of Ag0 to Ag+ during reduction of Hg2+ ions [39-40].

It is expected that oxidation- reduction  reaction can occur between the zero-valent Ag0 and Hg2+; the redox potential of Hg(II)/Hg(0) (+0.85 V) couple is larger than Ag(I)/Ag(0) (+0.8 V) couple, Hg(II) can oxidize the metallic Ag atoms and hence Hg atoms and Ag+ ions are produced [41]. The interaction between the Ag-rGO nanocomposites and Hg2+ was firstly determined using UV-Vis absorption spectroscopy. Different concentrations of Hg2+ were tested from one stock solution. The change in the absorption intensity of Ag-rGO nanocomposites with respect to concentration of Hg2+ is shown in Fig. 4A. The absorbance intensity of the Ag-rGO nanocomposites decreases after the successive addition of Hg2+ at a selected concentration to a solution of Ag-rGO nanocomposites at room temperature. From Fig. 4A, it is interesting to note that an increased concentration of Hg2+ induces a blue shift with quenching in the absorption spectra of the Ag-rGO nanocomposites. In this work, we observed that Hg2+ strongly reacts with Ag-rGO nanocomposites to form metallic mercury. The successive addition of Hg2+,with an accompanying blue shift in the surface plasmon absorption band which results in the color change of solution from yellowish-brown to colorless.

3. 4. 2 Calibration curve and detection limit of the method

Under the most favorable conditions, the absorption spectra of the Ag-rGO nanocomposites with increasing amounts of Hg2+ were recorded. The results are shown in Fig. 4A. The experimental data for Hg2+detection were plotted to obtain a linear relationship. Here, the developed probe gave a good linearity in the calibration graph (A0-A) at a chosen wavelength (420 nm) against concentrations of Hg2+ in the range from 0.1'1.0 ppm with a correlation coefficient of 0.9998. No further variation was observed after 1.0 ppm of Hg2+ had been added to Ag-rGO nanocomposites solution, indicating that the concentration of Hg2+ on the surface of Ag-rGO nanocomposites was reached. Interestingly, the plots of (A0-A) Vs Hg2+ concentration was shown in Fig. 4B. The limit of detection (LOD) of the method is determined to be 0.8574 ppm which was calculated by the equation, LOD = 3S/K; where, S is the standard deviation of the y-intercepts of the regression lines and K is the slope of the calibration graph.

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