The toxic heavy metals have created a challenge to the global scenario for maintaining a sustainable environment. Water-soluble heavy metal salts such as Pb, Hg, Cd, and Ni are the most prevalent contamination, whereas Hg2+, Cd2+, As3+ and Cr6+ pollution of water sources are severe in many regions of India and other countries [1-3]. The amount of heavy metals when increase over the certain limit in an ecosystem, become a pollutant and cause a very harmful effect on organism and plants as well. Hence, their effects on the environment and remediation processes have been studied extensively [4,5]. Between two different oxidation states of chromium (Cr), Cr (VI) is more toxic in nature [6,7] whereas Cr (III) is an essential metal for both human and plants [8]. Silver (Ag) contamination in water causes severe health problem like chronic toxicity, slight grayish pigmentation of the skin, eye, called argyria. Therefore, detection and removal of these toxic 1 metal ions from an aqueous environment is high in demand to prevent harmful effects on human as well as aquatic life.
    In recent years, graphene oxide (GO), a two-dimensional carbon-based nanomaterial has become popular among the researchers due to its unique properties [9-11]. GO possesses a wide range of oxygen containing functional groups (such as COOH, CO and OH) on its surface. Because of these functional groups of GO, it is hydrophilic in nature and readily dispersible into the water [12]. Moreover, GO can be functionalized with different organic ligands due to its large surface area. All these properties of GO make it an excellent material for the detection/removal of heavy metals and organic pollutants. Many works have been reported in recent years to functionalize GO using covalent interaction, non-covalent interaction, hydrothermal and solvatothermal method, etc. [13-15]. Materials of GO and functionalized GO synthesized by above methods have been utilized by researchers for the detection of toxic metal ions and pollutants. Darjee et al. have used thiacalix[4]arene for functionalization of GO for the detection of Zn2+ based on the uorescent sensor [16]. Zhu et al. reported functionalized nucleic acid-GR based sensors for the detection of Pb2+, Hg2+, and Ag+. They used two methods for the detection of metals ions viz. direct assay and ampli cation-based assay [17]. Jiang et al. prepared bio-functionalized GR based hybrid gold-nanoparticle to fabricate the biosensor for the determination of nitrite gas [18]. Yan X. et al. used GO/polyaniline hybrid material with excellent electrochemical and biocompatibility properties, which is further used as a biosensor [19]. However, very few works have been reported for the detection and removal of heavy metals. In this
work, we have used an easy method to functionalize GO using 1,3-dimethyl-5-nitroso-6-aminouracil (NO-
Ur). NO-Ur has very good ligating properties as reported in our previous works [20-24]. It has been used as a chemosensor for the detection of Cu2+ [25]. Various interaction modes are commonly found in uracil derivatives, such as H-bonding, p-p stacking, cation-p, and C-H. . . ..p. The 5-substituted uracil derivative
has many interesting properties in biochemistry and pharmacology [26-28]. Here, the synthesized material was further reduced by the hydrothermal method to prepare rGO-NO-Ur material. The materials were characterized using various spectroscopic and analytical techniques. Both GO-NO-Ur and rGO-NO-Ur were studied for their application to detect metal ions present in aqueous solution. The materials showed good sensitivity towards detection of many metal ions such as Cu2+, Fe3+, Ni2+, Zn2+, Cd2+, Ag+, Cr6+and As5+. Among them rGO-NO-Ur has shown good selectivity for Ag, Cr(VI), and Fe(III) metal ions.

2. Materials and methods

2.1. Chemicals

Graphite powder was purchased from Merck (particle size < 50m m, 99.5% pure). All solvents and
reagents including salts; cadmium chloride mono-hydrate (CdCl2.H2O), copper sulphate penta-hydrate
(CuSO4.5H2O), silver nitrate (AgNO3), ferric chloride (FeCl3), molybdenum disulphide (MoS2), nickel sul-
fate hexa hydrate (NiSO4.6H2O), zinc sulfate hepta-hydrate (ZnSO4.7H2O), mangnese chloride (MnCl2),
pottasium dichromate (K2Cr2O7) were of analytical grade chemicals and purchased from Sigma, CDH,
Merck and TCI. Ultrapure Milli-Q water was used in all the synthesis process and for preparation of solu-
tions. All chemicals were applied without further puri cation.
2.2. Instrumentation and Characterizations
Fourier-transform infrared (FTIR) spectroscopy was used to record the spectra of the samples in ATR
mode using Shimadzu, IR-Affinity-1S in the range of 4000 - 400 cm-1. X-ray powder diffraction (XRD)
study was performed on Seifert X-ray diffractometer C-3000, using Cu-Ka at a 35 kV voltage in the 2\(\theta\)
range of 5-80°. Thermo-gravimetric analysis (TGA) was done on SDT-Q600-TA instruments at a scan rate
of 10°C/min under nitrogenous condition. Scanning electron microscopy (SEM) images from FEI-NOVA
nano-400 microscope. Transverse electron microscopy (TEM) images were obtained from JEOL TEM 200
instruments with accelerating voltage 200 kV. Elemental compositions (%) of the samples were determined by using FEL-NOVA-nano 400 microscope. UV-Vis spectroscopy data were recorded using UV-Vis spectrophotometer (Thermo-Evolution 300) in the range of 200-600 nm at room temperature.

2.3. Synthesis procedures

2.3.1. Preparation of GO and NO-Ur

Graphene oxide was prepared using naturally occurring graphite powder by modifi ed Hummer's method
where H2SO4 and KMnO4 were used as oxidizing agent [30]. For the synthesize NO-Ur, 1,3-dimethyl-6-amino-uracil and aqueous sodium nitrite solution were mixed, cooled, and stirred. The mixture was acidified by the drop-wise addition of acetic acid within a period of 1 hr. The stirring was continued for a further 2 hrs at room temperature (298 K). The mixture was cooled and violet colored NO-Ur precipitated was obtained. The precipitate was filtered and washed with water. Finally, recrystallized in a 95% ethanol solution [31].

2.3.2 Synthesis of functionalized nanomaterial (GO-NO-Ur)

The GO (0.1 gm) was dispersed in distilled water (10 ml) (Milli-Q) for 30 mins using ultra-sonication
(Elma, Elmasonic P). After that NO-Ur (0.15 gm) added and stirred for 24 hrs. NaOH (0.1 gm) dissolved
in 10 mL of distilled water (Milli-Q) was added slowly during stirring. The solution was centrifuged and
washed with water followed by ethanol and dried at 60-70°C.

2.3.3. Synthesis of functionalized reduced nanomaterial (rGO-NO-Ur)

The reduction process was carried out using the hydrothermal method. The GO-NO-Ur material (50
mg) was dispersed into 50 ml of Milli-Q water for 30 min using ultra-sonication (Elma, Elmasonic P). The
reduction process was carried out at 170°C for 8 hrs in an autoclave by hydrothermal method. Finally
centrifuged and dried in a vacuum oven at 60°C for 12 hrs.

2.4. Metal ion detection

Metal ions recognition properties of GO-NO-Ur and rGO-NO-Ur were investigated. Salts (CdCl2.H2O,
CuSO4.5H2O, AgNO3, FeCl3, MoS2, NiSO4.6H2O, ZnSO4.7H2O, MnCl2, K2Cr2O7) were dissolved in ultra-
pure Milli-Q water and the resulting solution contain a 100ppm concentration of their metal ions. GO-NO-Ur and rGO-NO-Ur were dispersed in Milli-Q water (2 mg/50 mL) using ultra-sonicator. The resulting colloidal solutions were measured (4.5 mL) using measuring cylinder, followed by the addition of the solution of metal salts (10 mL) individually and volume maintained up to 5 mL by adding the same aqueous colloidal solution. The resulting solutions were run on UV-Vis spectrophotometer to investigate changes of the electronic transition state in the range between 200-800 nm at 22 ± 2°C. The interval time between sample preparation and investigation of samples were up to 20 min.

3. Results and discussion

Functionalization of GO with NO-Ur is based on covalent interaction of amine group present on NO-Ur and carboxylate group on the surface of GO via an amidation reaction. The color of the GO is golden brown and after complete reduction of GO, the color becomes completely black. The schematic diagram for the synthesis of functionalized GO and rGO as well as metal ion detection application is shown in Fig. 1.
3.1. FTIR spectroscopy
In the FTIR spectra of NO-Ur, a band appeared at 3024 cm-1 for O-H group. A peak at 3545 cm-1
con rms the presence of a primary amine in NO-Ur compound. The peak for N=O stretching and C=O
appears at 1580 cm-1 and 1710 cm-1 respectively. A band at 1600 and 1475 cm-1 is observed due to aromatic C=C bond. In the case of GO, the spectral band at 3205 cm-1 is due to the presence of O-H group. The peaks for C=O, C-O-C, and C=C groups are observed at 1716 cm-1, 1037 cm-1, and 1618 cm-1, respectively (Fig. 2). GO-NO-Ur shows peaks for both GO and NO-Ur. In functionalized GO-NO-Ur, N-H (2o amine) is shown at 3448 cm-1 and C=O at 1637 cm-1 because of the formation of amide. In addition, the shifting of peaks in the FTIR spectra of GO-NO-Ur indicates the successful interaction between the components. In the spectra of rGO-NO-Ur, a peak for C=O intensity is reduced and the peak for the epoxy group becomes negligible indicating the successful reduction of GO-NO-Ur to rGO-NO-Ur.

3.2. XRD analysis

The characteristic diffraction peak of GO was observed at 2\(\theta\) value of 10.42o in the XRD spectra (Fig. 3).
The broad peak at around 23o may be due to the incomplete oxidation of graphite. In the XRD spectra of
GO-NO-Ur, the intensity of peak at 10.42o was reduced and shifted that suggests exfoliation of GO as well
as a well interaction among the GO and NO-Ur. In rGO-NO-Ur, the peak for GO completely disappears and
a broad peak was observed around 2\(\theta\) = 26o confirming the reduction of GO. Broadening of peaks in both
GO-NO-Ur and rGO-NO-Ur may be due to the loss crystallinity arises from the interaction of molecules.

3.1. Thermal Gravimetric Analysis

TGA was done to study the thermal stability of the prepared materials. In GO, a major weight loss was
observed between 100-200oC which is due to loss of oxygen-containing functional groups. However, both
GO-NO-Ur and rGO-NO-Ur is showing comparatively less weight loss in this temperature range shown in
Fig. 4. In GO-NO-Ur, the major weight loss at 300-600oC is due to the degradation of NO-Ur. The weight
retention values of GO, GO-NO-Ur, and rGO-NO-Ur are found to be 45%, 58%, and 60% respectively. Thus
it can be stated that the thermal stability was enhanced by functionalization of GO with NO-Ur.