Graphene Functionalization by 1 , 6-Diaminohexane and Silver Nanoparticles for Water Disinfection

Reduced graphene (G) was prepared by chemically reducing graphene oxide (GO). For the first time, the resulting G was functionalized by 1,6-diaminohexane and decorated with silver nanoparticles (AgNPs). The resulting G and modified G were characterized by X-ray diffraction (XRD), nuclear magnetic resonance (NMR), infrared (FTIR) spectroscopies, and high resolution transmission electronmicroscopy (HRTEM).The 1,6-diaminohexane-graphene structure was ascertained from theNMR and FTIR data. AgNPs were identified with various sizes within the graphene matrix. The resulting hybrid material was used as an effective antimicrobial contact catalyst for disinfecting water from Total Coliform and Fecal Coliform bacteria. A triple action in this respect was achieved from graphene, 1,6-diaminohexane, and AgNPs without observed release of silver that causes toxicity.


Introduction
Several approaches have been introduced for water disinfection [1][2][3].For example, free chlorine has been the choice for water disinfection that is known to have a carcinogenic effect.Also ultraviolet or ozone were other choices; however they are relatively expensive.Nanotechnology has recently introduced several materials that include carbon-based structures such as carbon nanotubes and graphene.These materials allowed advancements in several applications including electronic, coatings, and pharmaceutical industries.In particular carbon-based structures possess unique properties such as large surface area, allowance for chemical functionalization, and well-known ability to counter microbial disinfection and removal of toxic organic pollutant suggested their application in water purification.
Graphene has unique structural properties in the form of one atomic layer of aromatic stacked structure of sp 2hybrid carbon atoms [4].Graphene was first introduced as a "free" standing two-dimensional structure by Novoselov and his coworkers when isolated from graphite using a "scotch" tape [5].Another advantage is the possibility of graphene using noncovalent or covalent approaches [6,7].Graphene can be modified using nanostructured metallic or polymeric materials to form hybrids or composites with more advantageous properties.
For instance, silver nanoparticles have been known to have tremendous antibacterial activities with noticeable effect over other materials [8][9][10].Therefore silver is known to be an effective biocide to fight several bacteria, fungi, and viruses [11].Silver nanoparticles were therefore the focus of several investigations for their usage as antiseptic, disinfectant, and other active ingredients in pharmaceutical formulations [12][13][14].It was cited in the literature that the size of silver nanoparticles is among the crucial factors on their antibacterial activity.Thus, it was mentioned that silver nanoparticles with sizes <10 nm proved to have relatively higher antibacterial activity because of the increase in surface area and ease of cell penetration [10,11].This explained the long time usage of silver as antibacterial agents [15,16].
The mechanism of silver antibacterial function is based on its ability to release silver ions which causes inactivation to microorganism cells that is due to cell membrane destruction [17,18].Moreover, silver nanoparticles penetrate cell membranes causing their damage and resulting in their death [10,19].
Graphene oxide (GO) is formed of stacked sheets with rich oxygen-functional groups including hydroxyl, epoxide, carbonyl, and carboxyl groups [20,21].Therefore, cell membranes of the bacteria are damaged when placed in contact with the oxygenated graphene walls.An example given in the literature is Escherichia coli bacteria that proved to be inhibited by GO [22].The outstanding specific surface area of the graphene (2600 m 2 /g) and the ease of its functionalization allowed its use as a substrate to form graphene composites [20,[23][24][25].Pt, Au, and Ag have also been cited in the literature to form inorganic hybrids with graphene and found good application in different domains including catalysis, sensors, and electronic [26][27][28][29].
Graphene has been also suggested for various applications in next generation storage and electronic devices [30][31][32][33].Graphene can be prepared using chemical vapor deposition (CVD) from methane [23,34,35], graphite oxidation and reduction [36,37], one-step graphite exfoliation [38], graphite stamping [39], and carbon nanotube unzipping [40,41].Due to the ease of preparation and cost reduction, graphene has been prepared by the oxidation of graphite and then its further reduction.GO can be reduced by either chemical, thermal, or ultraviolet assisted reduction methods, respectively [42][43][44][45][46][47].In the chemical reduction method GO in solution with large amount is converted into graphene sheets (GS) by the removal of epoxide groups present in the basal plane of GO using hydrazine as an oxidizing agent [48].
In this work, we prepared graphene by chemical reduction of graphene oxide.The resulting graphene was functionalized with 1,6-diaminohexane and further modified by silver nanoparticles.The new hybrid material was tested for the first time for its antibacterial efficiency against Total Coliform and Fecal Coliform bacteria for disinfecting wastewater of very infected water resources.

Instrumentation.
A Siemens D 5000 X-ray powder diffractometer was used for XRD analyses.The functional groups were identified by a Bruker Avance III HD proton nuclear magnetic resonance where CDCl 3 was the solvent.IR spectroscopy was done with a JASCO FT/IR-4000 where the samples were in the form of a KBr pellet.
The morphologies of the resulting graphene and hybrids were determined using high resolution transmission electron microscopy (HRTEM) using an JEOL JEM-2100F field emission electron microscope with accelerating voltage of 200 kV.The sample for HRTEM characterization was prepared by placing a drop of colloidal solution of the sample dissolved in deionized water over a carbon-coated copper grid.

Hybrid Graphene-1,6-Diaminohexane-Silver
Nanoparticles Synthesis 2.3.1.Graphene Oxide Preparation.Graphene oxide was prepared using a modified Hummers and Offeman method [49].In summary, the steps were as follows: 2 g of graphite powder was added to 300 mL concentrated sulfuric acid in an ice bath.Solid KMnO 4 (20 g) was then added slowly and followed by the addition of NaNO 3 (3 g) slowly with stirring and keeping the temperature of the reaction mixture near zero.The temperature of the reaction mixture was kept at 35 ∘ C for 30 minutes and then 300 mL deionized water was added gradually with small portions and the temperature was increased to 98 ∘ C for 40 minutes.Then, 500 mL of deionized water was added to the reaction mixture followed by addition of 20 mL 30% H 2 O 2 solution.
The solid product was separated by filtration, washed repeatedly by 5% HCl solution to remove sulfate ions, and then washed several times by distilled water until chloride ions are removed when the pH becomes neutral.The residue was kept to dry in air oven at 60 ∘ C for 3 hours and then redispersed in distilled water under ultrasonication for 1 hour to be exfoliated.Reddish brown nanosheets of graphene oxide (GO) in homogeneous suspension solution were obtained.

Conversion of GO to
Reduced Graphene (G).0.5 g of graphene oxide sheets is dissolved in 3 mL H 2 O; then 1 mL of hydrazine hydrate is added to the solution and stirred for 1 hour at room temperature and then heated to 100 ∘ C for 2 hours, then raising the temperature to 200 ∘ C for 30 min.

Graphene Functionalization with 1,6-Diaminohexane and Silver
Nanoparticles. 20 mL of distilled water solution is added to 0.2 g graphene sheets with 2 mL HCl 30% solution.15 mL g of 1,6-diaminohexane is added with continuous stirring for 1 hour at 80 ∘ C. 10 mL AgNO 3 10% is added followed by 3 mL hydrazine hydrate with continuous stirring at room temperature for 15 min and left overnight at room temperature.The precipitate is removed by centrifugation.hours and the blue colonies were counted for four sample dilutions.
For Total Coliform, SM 9221B guidelines were adopted using multiple-tube fermentation technique.In this procedure, LTB presumptive medium was incubated at 35 ± 0.5 ∘ C for 24 ± 2 hours.Brilliant green lactose bile broth was used at 35 ± 0.5 ∘ C for 24 ± 2 hours.Results were calculated using SM table 9221(IV).For all tests, control cultures were used for the microbiological tests.

Characterization of Graphene Oxide (GO) and Graphene (G).
The structure of the prepared GO was ascertained using XRD and FT-IR.Figure 1 shows the XRD pattern of GO where a characteristic peak (002 plane) appears at 2 theta = 10.9 ∘ indicating the presence of oxygen containing functional groups after liquid phase oxidation.Figure 2 shows the XRD pattern of graphene formed by chemical reduction of GO.The XRD results of Figure 2 proved the disappearance of both the peak related to GO (around 10 ∘ ) and that related to graphite (26 ∘ ).However, the XRD pattern of graphene is characterized by a characteristic peak (002 plane) that appeared at 2 theta = 25.7 ∘ .These results proved the complete conversion of GO to reduced graphene.
FT-IR measurements were also used to explore the functional groups associated with GO and graphene prepared.Figure 3 shows the IR spectrum of GO where the presence of carboxylic groups after oxidation steps was confirmed with the peaks at 1719 cm −1 and 1407 cm −1 , which correspond to the stretching of C=O (carbonyl/carboxyl) and C-O  (carboxyl) bond, respectively.The broad band at 3425 cm −1 was assigned to the stretching of O-H carboxylic groups.The stretching at 1067 cm −1 corresponds to C-O (epoxy group).The peak at 1625 cm −1 is assigned to C=C stretching of aromatic domains on the graphene surface.Figure 4 shows the IR spectrum of chemically prepared graphene.The IR results showed the reduction of the carboxyl groups (graphene-COOH) to hydroxymethyl (graphene-CH 2 OH) as indicated by the disappearance of the C=O bands (at 1719 cm −1 ) and the appearance of a band at 2925 cm −1 specific to the sp 3 C-H bond of the CH 2 OH group of the reduced graphene.
The fine structure and morphology of the graphene layers prepared by the reduction of GO were investigated by HR-TEM microscope.Thus, Figure 5 shows representative HR-TEM image of graphene sheet.Figure 5 shows that graphene sheets are represented by a very thin layer of graphene and nearly a transparent layer.The structure of graphene as depicted from Figure 5 showed well separated single layer of graphene sheets that will allow better distribution of the nanoparticles and the inclusion of 1,6-diaminohexane functionalities over the surface of the material.

Functionalization of Graphene with 1,6-Diaminohexane and Silver
Nanoparticles.The surface structure and bonding nature of graphene functionalized by 1,6-diaminohexane with silver nanoparticles were characterized by XRD, 1 HNMR, IR, and HRTEM techniques.
The AgNPs were formed in a fcc crystallinity on the surface of the graphene functionalized by 1,6-diaminohexane according to the standard values.
On the other hand, Figure 7 shows FT-IR spectrum of the 1,6-diaminohexane-silver nanoparticles functionalized graphene.The two peaks with spikes at 3781 cm −1 and 3693 cm −1 representing the formation of primary amine NH 2 group and the band at 1024 cm −1 corresponding to C-N bond can be noticed Figure 7.
1 HNMR spectrum of 1,6-diaminohexane-silver nanoparticles functionalized graphene is depicted Figure 8.The broad chemical shift at 1.5 ppm corresponds to the protons of the primary amine group NH 2 (taking symbol A) proving the of graphene by 1,6-diaminohexane.The chemical shifts at 1.26, 1.29, 1.31, 1.34, and 1.35 ppm refer to the protons on the 2,3,4,5 carbons of CH 2 group that is linked to the alkyl chain of 1,6-diaminohexane The chemical shifts at 2.48, 2.50, and 2.52 ppm are assigned to the protons on 1,6 carbons of CH 2 group that is linked to NH The main forces stabilizing the functionalization are the hydrogen bonding between the nitrogen, oxygen, and hydrogen on the composite surface, in addition to Van der Waals forces and dipolar attractions through all the hybrid material thanks to the electron rich surface of graphene and its electrostatic attraction with the amine group and silver.The morphology and size distribution were studied using HR-TEM.The composite films were characterized by HR-TEM; the images are given in Figure 9.The silver nanoparticles can be identified to be embedded within the graphene matrix.
The figures show the HR-TEM of graphene functionalization by 1,6-diaminohexane with AgNPs where the images were taken by high magnification HRTEM.The power beam is 200 KV.The figures show the presence of a mixture of  graphene layers.Most of the layers are bilayers or few layers of graphene on which AgNPs are deposited on its surface by different sizes.

Antibacterial Activity against Water
Pathogens.The main goal of this part of the work is to explore the efficiency of the synthesized hybrid material towards antibacterial activities.Therefore, four water samples were collected according to the regular protocols for water sampling from two locations around water sewage around Bahr El-Baqar and Kotchainer regions.The results were compared to the control samples before treatment with the composite material.First, the results of using graphene functionalized by 1,6-diaminohexane with silver nanoparticles for disinfecting water against Total Coliform and Fecal Coliform bacteria are presented.
Before treatment, the analysis showed that Total Coliforms were 5000000 and 2600000 CFU/100 mL for samples 1 and 2, respectively, of the first zone and 3000000 and 1200000 CFU/100 mL for samples 3 and 4, respectively, of the second zone, while those for Fecal Coliforms were 700000 and 500000 CFU/100 mL for samples 1 and 2, respectively, of the first zone and 1200000 and 300000 CFU/100 mL for samples 3 and 4, respectively, of the second zone.
The treatment was achieved by using 0.1 gram of the 1,6diaminohexane-silver nanoparticles functionalized graphene in 100 mL infected water sample.The analyses according to the protocols explained in the experimental section showed that full disinfection of all samples from Total and Fecal Coliforms was reached except the first sample where the disinfection is few colonies above zero value and about 10 and 3 colonies remain after treatment for Total and Fecal Coliform, respectively.A zero value was obtained for the other three samples as can be noticed from the data given in Table 1.

Conclusions
We synthesized successfully a new hybrid material, 1,6-diaminohexane-silver nanoparticles functionalized graphene.The synthesis followed the chemical reduction of graphene oxide that was converted to graphene.The graphene was functionalized by 1,6-diaminohexane and silver nanoparticles with different sizes.The structure of the hybrid material was ascertained from the XRD, FT-IR, and 1 HNMR analyses.The morphology of the resulting hybrid contained silver nanoparticles with adequate distribution within a stack of very thin transparent graphene films composed of few layers clusters.The new material proved exceptionally effective in removing the bacterial Coliform infection of sewage water taken from some very polluted regions.
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