r-GO nanocomposites as ultrasensitive fluorescent sensor for Hg2+

Microchemical Journal 154 (2020) 104577

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Rhodamine B associated Ag/r-GO nanocomposites as ultrasensitive fluorescent sensor for Hg2+ Deepak Sahu, Niladri Sarkar, Priyaranjan Mohapatra, Sarat K. Swain

T

⁎

Department of Chemistry, Veer Surendra Sai University of Technology, Sambalpur 768018, Odisha, India

ARTICLE INFO

ABSTRACT

Keywords: Silver nanoparticle Reduced graphene oxide Rhodamine B Hg2+ ion

This work represents the designing of rhodamine B (RhB) based fluorescence sensors for detection of mercury (II) ion in aqueous media with low cost and efficient technique where silver nanoparticle decorated reduced graphene oxide (Ag/r-GO) nanocomposites act as a quencher. Transmission electron microscope (TEM) and scanning electron microscope (SEM) analysis of nanocomposites are carried out to investigate the structural properties. In this work, Ag/r-GO nanocomposite plays two important roles for sensing of Hg2+ ion i.e. (i) it interacts with Rhodamine B (RhB) through π–π interaction and dispersive interaction; (ii) it acts as adsorbent for Hg2+ ion. The fluorescence of RhB is quenched with addition of Ag/r-GO nanocomposites. After that the fluorescence spectra of quenched RhB is increased with addition of Hg2+ ion due to the soft–soft interaction, cationic–π interaction and interaction of Ag and Hg2+ ion in the nanocomposites. The present fluorescence approach shows very small limit of detection (LOD) of 2 nM at optimized pH and optimized contact time. The selectivity of the nanocomposites towards Hg2+ ion in water is carried out in the presence of other metal ions. The prepared Ag/rGO@RhB nanocomposites can be applied in all real water samples for detection of mercury.

1. Introduction Present day, monitoring of toxic pollutants such as heavy metal ions from water resources is important and challenging, since these pollutant affects not only the human health but also the environment. Among different heavy metals, mercury (Hg2+) is one of the most toxic pollutant due to its strong affinity to sulphur containing group so that it affects the enzyme and protein in the body. Exposure of mercury can cause many diseases such as Hunter–Russell syndrome, acrodynia, and Minamata disease [1-4] and also it can affect the central nervous system, kidney and lungs. According to U.S. environmental protection agency, maximum permissible value of mercury in drinking water is 2 parts per billion [5]. Therefore, development of low cost, selective and sensitive method is required for mercury detection. Since today, various methods such as atomic absorption spectrometry (AAS) [6], inductively coupled plasma mass spectrometry (ICP-MS) [7], atomic fluorescence spectrometry (AFS) [8], and surface-enhanced Raman scattering (SERS) [9] have been developed to detect Hg2+ ion. But these methods are expensive, more time consuming and difficult to handling. Although aqueous phase detection of mercury has taken into account by developing molecular probe sensors based on colorimetric and electrochemical detection but these methods are not suitable for detection of mercury due to complicated synthesis procedures, large physical size, ⁎

poor aqueous solubility, bad stability. Considering the above drawbacks, fluorescence spectroscopy techniques is chosen as appropriate tool for monitoring the metal ions in aqueous phase due to some benefits such as small response time, cost effective, high selectivity and selectivity [10, 11]. Previously fluorescent sensors have also been considered for mercury detection through a fluorescence quenching (turn-off) response because of the spin orbit coupling effect of Hg2+ ion in inter system crossing process [12-15]. However, ‘turn off’ sensing approach is not appropriate to apply in real water sample due to occurrence of unexpected quenching object. Therefore, design of “turnon” fluorescent sensors for Hg2+ ions is important in which non fluorescent molecules can be converted into fluorescent molecules. Recently, many fluorescence sensors have been designed to monitor mercury ion through a “turn-on” response [16–19] due to small response time. Rhodamine dye can be used as fluorescent material to synthesize a probe for metal ion detection due to its high molar extinction coefficient, good fluorescent quantum yields, high light stability, longer excitation and emission wavelengths and also rhodamine based sensors are used for “naked eye” detection of metal ions by changing the color of the probe as well as changing the absorbance and fluorescence intensity for specific metal ions. This is due to fact that rhodamine derivatives are non-fluorescent and which are converted to ring-opened amide forms that are pink and highly fluorescent. Many

Corresponding author. E-mail address: [email protected] (S.K. Swain).

https://doi.org/10.1016/j.microc.2019.104577 Received 30 August 2019; Received in revised form 3 December 2019; Accepted 24 December 2019 Available online 25 December 2019 0026-265X/ © 2019 Elsevier B.V. All rights reserved.

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researchers have been synthesized rhodamine based chemo sensor for mercury ions [20–22]. Some literatures are focussed on the synthesis of non-fluorescent rhodamine B (RhB) based materials (derivative of RhB) for sensing of heavy metal ions [23]. However, these have many drawbacks such as it has low sensitivity, costly and toxic chemical requires for modification and difficult reaction procedure are included in the synthesis approach. Xu et al. have synthesized rhodamine B/napthalimide chemosensor for recognition of Hg2+ ion [24]. A rhodamine B Schiff base 3′, 6′-bis (diethylamino)−2-(2-oxoethylideneamino)-spiro[isoindoline-1,9′-xanithen]−3-one (RHO) is developed for sensitive detection of Hg2+ ion [25]. A chemosensor based on rhodamine with NS2 receptor have been designed for Hg2+ ion [26]. Further, in some literature sulphur containing groups are doped to the sensor materials to improve the sensitivity, but these have some negative influences towards environment. Therefore, in this paper we have design a “turn on” fluorescent sensor for Hg2+ using Ag/r-GO@RhB nanocomposites as probe that are not only cost effective, selective and sensitive but also simplicity and easy to handling. Graphene, as allotrope of carbon, is a two dimensional sheet containing sp2 carbon atoms. It has large specific surface area, high electrical conductivity, high thermal and mechanical properties and strong fluorescence quenching properties. Hence, graphene can be used in various applications such as, conductive thin films, supercapacitors, biomedical, nanoelectronics and biosensors [27-31]. Graphene oxide (GO) has been attracted by researcher due to its specific high surface area and low cytotoxicity, which is one of the derivative of graphene. GO has a number of oxygen-containing functional group such as carboxyl, epoxide and hydroxyl groups due to which, it can be used as a template for synthesis and stabilizing of metal nanoparticles. Recently a number of noble nanometals, especially silver and gold nanoparticles are chosen for preparation of metal/r-GO nanocomposites for further application in catalysis [32], electrochemical [33], microbicide [34], and surface-enhanced Raman scattering (SERS) [35]. The mechanical strength, molecular-level chemical sensing capability and also the growth of metal nanoparticles is due to presence of oxygen functionalities groups in r-GO [36]. Furthermore, the incorporation of silver nanoparticles onto the r-GO sheet inhibits the aggregation of graphene sheets by improving the interlayer spacing between r-GO sheets [37, 38]. Now a day, Researcher have been interested in designing many optical sensors using GO as quencher for the detection of biomolecules, metal ions and GO-based fluorescence nanoprobes have been used for sensing of heavy metals ion. Lu et al. has used water soluble graphene oxide as quencher for sensitive and selective detection of DNA and human thrombin [39]. GO based fluorescence sensor is also used to detect other biomolecules such as heparin and dopamine [40, 41]. With considering different problems which arises due to increase contamination on environment through heavy metal ions, researchers are focussed on the development of sensor for sensing of toxic metal ion in aqueous system. Wang et al. [42] have prepared graphene oxide nanosheet for fluorescent detection of Fe3+ ion. The ‘turn off’ sensing of Au3+ ion is adopted using graphene based hybrid material [43]. However, these sensing strategies are not suitable for real sample measurements because false signals might be obtained as a result of the presence of unexpected quenching entities and it requires complicated synthetic route. Recently, ‘turn on’ approach is selected for sensing of heavy metal using GO based hybrid material. In the sensing approach, GO act as quencher and other material act as detection unit. Using this approach, many metal ions are detected successfully. In a literature, GO is used as quencher to quench the fluorescence emission of amino pyrene (AP)-grafted gold nanoparticles (AP−AuNPs) for the detection of heavy metal ions [44]. However, gold nanoparticles used in this approach are costlier than silver nanoparticles and low molar extinction coefficient which limits their practical applicability. Further, DNAzymes based GO hybrid material is used for sensing heavy metal ion

[45-47] But these strategies have many dis-advantages such as high cost, limited practical application and difficult reaction procedure. As it is observed that silver nanoparticles with some unique properties such as good thermal, electrical conductivity, good antimicrobial properties and high chemical stability are considered as recent research interest and it is used in various applications such as biomedical, food packaging and pharmaceutical application. Silver nanoparticles with very low concentration (nano molar) are more useful than high concentrations (micro) of silver ions [48]. Further, silver nanoparticles are used in surface coated of dressing materials and drinking water purification to save from pathogen [49, 50]. Recently, it is shown that combination of silver nanoparticles and graphene oxide play a vital role for enhancing the individual properties. Therefore, Ag/r-GO nanocomposites with synergistic properties of AgNP and GO are used in various fields such as sensor, catalyst and biomedical application [5154]. Gold nanoparticles (AuNPs) can also be used as sensors for the detection of Hg2+ ion [55-57]. Zhu et al. have synthesized DNA template gold nanoclusters for the detection of Hg2+ ion [58]. Gold nanoparticles functionalized with mercaptopyridine have been prepared by Yuan et al. for the sensing of Hg2+ ion [59]. However, AuNPs are relatively high cost with lower value of molar extinction coefficients as compared to AgNPs. Herein, we have synthesized rhodamine B based Ag/r-GO nanocomposites which are non-fluorescent using Ag/r-GO nanocomposites as quencher. When these Ag/r-GO@RhB nanocomposites interact with Hg2+, rhodamine B is separated out from Ag/r-GO with producing a highly fluorescence emission. This is due to the two facts (i) interaction of Hg2+and r-GO in nanocomposites through soft–soft interaction and π–metal interaction and (ii) interaction of Hg2+and AgNPs in nanocomposites due to different reduction potential value of Hg2+/Hg (0.85 V) and Ag+/Ag (0.8 V). The proposed detection method is based on the interaction of Ag/r-GO between the rhodamine B and the Hg2+ ions. We have studied the strong interaction of Hg2+ by Ag/r-GO over rhodamine B. It is clearly noticed that the interaction of Ag/r-GO with rhodamine B made a non-fluorescent material. Our main object to break this weak physical interaction between Ag/r-GO and rhodamine B with Hg2+ ion so that rhodamine B is free from Ag/r-GO by giving a highly fluorescence emission. To the best of my knowledge, the detection of mercury by fluorescent “turn on” method using rhodamine B based Ag/ r-GO nanocomposite is new in this literature. 2. Experimental 2.1. Materials Natural Graphite, Sulphuric acid (H2SO4), Sodium nitrate (NaNO3), Potassium per manganate (KMnO4), Tri-sodium citrate and silver nitrate (AgNO3) were purchased from Merck. Mercury Nitrate and other salt for interfering ions were brought from Sigma aldrich. Deionized water was used for the preparation of salt solution. 2.2. Preparation of Ag/r-GO nanocomposite Graphene Oxide (GO) was prepared using previous report (modified Hummers’ method) [30]. In a synthesis procedure, given amount of graphite powder, sodium nitrate and conc. H2SO4 were mixed in a 250 ml beaker and stirred at 0 °C for 15 min. After that KMnO4 was added to above mixture slowly so that the temperature of the solution not exceeds to 20°C and the solution is kept with high stirring at 35 °C for 30 min. After addition of 10 ml of water the temperature of solution is maintained to 98 °C for 15 min. Finally, the mixture was mixed with warm water upon addition of 3% H2O2 and filtered 6–7 time with warm water to remove impurity. After mixing with water the residue was sonicated and centrifuged at 5000 rpm for 30 min to get the GO. Silver Nanoparticle/r-GO nanocomposites were fabricated using “one pot” synthetic protocol. Firstly, 10 mL of GO suspension (5 mg/ 2

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mL) was dispersed into 100 mL of deionized water and mixed with required concentration of AgNO3 solution with constant stirring. After that solution mixture was heated at 70–80 °C and the required amount of sodium citrate was quickly added to the reaction mixture which was then further stirred for 1 h. Here Ag+ ion is reduced Ag (0) by sodium citrate on the surface of graphene oxide and finally suspension of Ag/ GO nanocomposites is formed which is stable for 5–10 days. The entire reaction process is schematically represented in Scheme 1.

3. Results and discussion 3.1. Structural analysis of Ag/r-GO nanocomposite Fig. 1(a) represents UV–vis spectra of GO (0.5 mg/mL) and Ag/r-GO (5 mM) nanocomposites. A maximum absorption at 235 nm is observed in UV-visible analysis of GO which are ascribed by two transitions (i) π−π* transition (C−C bonds) and (ii) n−π* transition (C=O group) [60]. In case of UV–vis spectra of Ag/r-GO nanocomposites, the maximum absorption peak at 415 nm is noticed this is due to characteristic SPR peak of silver nanoparticle. After addition of sodium citrate to solution mixture of GO and AgNO3, both GO and Ag+ is reduced. Moreover, the color of solution mixture is changed from brownish yellow to black color and the photographs are inserted in Fig. 1(a). FTIR spectra of graphene oxide, prepared from oxidation of graphite are in accordance with the previous report as shown in Fig 1(b) . Fig 1(b) shows the FTIR spectra of GO and Ag/r-GO nanocomposite which gives the interaction between Ag and RhB. In the case of GO, the broad and intense peak observed at 3356 cm−1 is ascribed to the stretching vibration of –OH groups. The peak for -C=O group and aromatic C=C bond in GO are observed at 1720 cm−1 and 1621 cm−1 respectively. The FTIR band at 1359 cm−1, 1217 cm−1 and 1050 cm−1 correspond to the C−O−H deformation, -C−H stretching and C−O stretching vibrations respectively. From the above data it is found that GO is successfully prepared from graphite containing different functional groups (–OH, –COOH) groups on the surface of GO which creates a suitable stage for the preparation of different plasmonic nanoparticles. In case of FTIR spectra of Ag/r-GO nanocomposite, the intensity of hydroxyl peak and carbonyl peak are decreased with slight

2.3. Characterization of Ag/r-GO nanocomposite Scanning electron microscope (Hitachi, japan with model No. SU3500) studied the surface morphology of as-prepared Ag/r-GO nanocomposite. TEM, Tec-nai12, Philips, operated at 120 kV was used to study the surface morphology of nanocomposites. Fourier infrared spectrophotometer with Bruker alpha II was used to study the surface functionality in the range of 500–4000 cm−1. UV–visible spectra (Shimadzu UV‐2550 ultraviolet (UV)–vis spectrophotometer) of the Ag/ r-GO nanocomposite were performed for the confirmation of formation of in situ silver nanoparticle on graphene sheet. The thermogravimetric analysis (TGA) was done using TGA apparatus with model TGA 4000 by Perkin Elmer. Zeta potential measurement and DLS analysis of the samples were taken by Zetasizer analyser (Malvern) with model number: ZEN-3690. The quenching of rhodamine B through Ag/r-GO nanocomposite and the detection of Hg2+ by the Ag/r-GO@RhB nanocomposite in aqueous media were investigated by using Fluorescence spectrophotometer (Agilent Cary Eclipse).

Fig. 1. (a) UV–vis spectra of GO (0.5 mg/mL) and Ag/r-GO nanocomposites (5 mM). (b) FTIR spectra of GO and Ag/r-GO nanocomposites. (c) XRD of GO (inset) and Ag/r-GO nanocomposites. 3

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Fig. 2. (a and b) SEM images of Ag/r-GO nanocomposites; (c and d) TEM images of Ag/r-GO nanocomposites; (e) EDS spectra of Ag/r-GO nanocomposites; (f) SAED spectra of nanocomposite.

shifting because, these groups are involved in the synthesis and stabilization of AgNPs which is accordance with a similar previous report [61]. Fig. 1(c) shows the comparative XRD patterns of GO and Ag/r-GO nanocomposites. In case of graphene oxide, the intense peak at 2θ of 12.34° corresponds to (001) plane which is the characteristic feature of GO (inset of Fig. 1(c)). This indicates that GO is prepared successfully through oxidation of graphite by modified Hummers method. However, the peak at 38.26, 44.47, 64.75° and 77.51 observed from XRD pattern of Ag/r-GO corresponds to (111), (200), (220) and (311) hkl parameter of silver nanoparticles in FCC lattice respectively. It is also noticed that, GO peak does not appear in XRD pattern of Ag/r-GO. However, a peak is observed at 2θ value of 25° which is due to the reduction of GO (rGO) by sodium citrate.

microscopy (TEM) demonstrates the presence of silver nanoparticles (black spots) onto the graphene oxide surface (Fig. 2(c and d). Silver nanoparticles in Ag/r-GO nanocomposites are uniformly dispersed throughout the graphene oxide surface and it is observed to be spherical with ranging the diameter from 20 nm to 30 nm. Therefore, GO play important role for synthesizing disperse AgNPs on its surface and prevent agglomeration because, there is the physical and electrostatical interaction between AgNPs and GO. The silver nanoparticle, graphene sheet and oxygen adsorbed on the samples reveal the formation of Ag/rGO nanocomposites. The presence of carbon, oxygen and AgNP in the Ag/r-GO nanocomposites is observed from EDS (Energy dispersive Xray spectroscopy) study (Fig. 2(e)). From EDS spectrum, the weight percentage of silver in the nanocomposites is found to be 10%. Selected area electron diffraction (SAED) graph gives the idea of crystalline nature of as-prepared nanocomposites. It is noticed that the in situ synthesized AgNPs show single crystalline nature (Fig. 2(f)). Fig. 3(a–c) shows the elemental mapping of C, O and Ag in Ag/r-GO nanocomposites whereas Fig. 3(e) displays the mapping graph of mix composition present in the nanocomposites. The location on which mapping is carried out is shown in Fig. 3(d). The silver nanoparticles are uniformly distributed throughout the surface of graphene oxide which is observed from elemental mapping study. EDS study reveals the occurrence of AgNPs in the nanocomposites (Fig. 3(f)).

3.2. Morphological analysis of Ag/r-GO nanocomposites Fig. 2(a and b) shows the SEM images of Ag/r-GO nanocomposites at different magnification. The silver nanoparticles are formed and spread onto the surface of graphene sheet which indicates the strong interaction of AgNPs with graphene oxide. These types of micrographs are also observed in earlier study [62]. Large surface area of GO and uniform distribution of AgNPs onto GO sheet are responsible for high adsorption capacity of nanocomposites. Transmission electron 4

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Fig. 3. Mapping graph of (a) C; (b) O; (c) Ag and (e) all elements; (d) SEM micrograph of Ag/r-GO nanocomposites on which location mapping are taken; (f) EDS spectra of Ag/r-GO nanocomposites.

Fig. 4. Zeta potential of (a) AgNPs; (b) GO; (c) Ag/r-GO nanocomposites.

3.3. Zeta potential and size distribution of Ag/r-GO nanocomposites

presence of negative functional groups in GO [64]. The stabilizing agent is responsible for negative surface charge of AgNP (−30.1 mV). Moreover, it is observed that Ag/r-GO nanocomposites (−44.6 mV) signify higher negative zeta potential value as compared to AgNP which confirms the higher stability of Ag/r-GO nanocomposites as compared to AgNP. The zeta potential with greater than 30 mV both negative or positive value is known to be stable in aqueous dispersion [65].

To study of the effect of surface charge and stability of Ag/r-GO nanocomposites, zeta potential measurements are taken [63]. Stability and surface charge of GO, AgNPs and Ag/r-GO nanocomposites are analysed in the Fig. 4(a–c). It is noticed that all samples in Fig 4 are in negative surface charge. The negative surface charge of GO is due to 5

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Fig. 5. Size distribution graph of (a) GO; (b) Ag/r-GO nanocomposites.

be 0.221 and 0.333 respectively. That's mean GO and Ag/r-GO nanocomposites are monodispersed sample. 3.4. Thermo gravimetric (TGA) analysis of Ag/GO nanocomposites TGA of GO and Ag/r-GO nanocomposites are measured with ranging the temperature from 30 °C to 550 °C, which are placed in Fig. 6. In case of GO, it is noticed that approximately 12 wt% weight loss is found below 100 °C, which is due to loss of adsorbed water. After that, approximately 48 wt% weight loss is observed up to 550 °C which correspond to the pyrolysis of negative functional groups of GO. In case of Ag/r-GO nanocomposites, no same weight loss is observed as compared to GO. That means GO is reduced to r-GO. The total weight loss of Ag/rGO nanocomposites (55%) up to temperature 550 °C is lower than GO (60%). Therefore, the thermal stability of Ag/r-GO nanocomposites is higher as compared to GO which is assigned for the presence of AgNP in GO and the strong interaction of Ag and r-GO. Fig. 6. TGA graph of GO and Ag/r-GO nanocomposites.

3.5. Detection of mercury ions with Ag/r-GO nanocomposite

Dynamic light scattering (DLS) technique is used to study the size of in situ formed nanoparticles. The hydrodynamic size of GO and Ag/r-GO nanocomposites are observed to be 300 d nm and 112 d nm respectively (Fig. 5(a and b)). In case of Ag/r-GO nanocomposites, two peaks are observed with hydrodynamic size of 25 d nm and 200 d nm. The polydispersity index of GO and Ag/r-GO nanocomposites are found to

Fluorescence technique is used for the detection of Hg2+ ion. In this process, rhodamine B (RhB) is considered as fluorescent material, which gives a highly orange fluorescence emission at 576 nm. Firstly, the appropriate concentration of rhodamine B is taken in a spectrometer cuvette to study the fluorescence spectra and then different concentration of dispersed Ag/r-GO is added to it and studied the effect

Fig 7. (a) Fluorescence spectra of rhodamine B in the presence of Ag/r-GO nanocomposites; (b) trend in fluorescence intensity with addition of Ag/r-GO nanocomposites. 6

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Fig 8. (a) Effect of pH towards stability and sensing of Hg2+ ion; (b) response time for sensing of Hg2+ ion using Ag/r-GO@RhB nanocomposites.

Fig. 9. (a) Fluorescence spectra of quenched RhB in the presence of Hg2+; (b) relative fluorescence intensity as a function of different concentration of Hg2+; (c) photographs of quenched RhB in the presence of Hg2+ ion.

of Ag/r-GO on fluorescence spectra of RhB on room temperature. The fluorescence spectrum of rhodamine B is decreased with increasing concentration of Ag/r-GO nanocomposite. This is because RhB is adsorbed onto the surface of Ag/r-GO nanocomposites. This indicates that RhB is strongly quenched by Ag/r-GO nanocomposites. Due to dispersion forces and π–π interaction between RhB and Ag/r-GO nanocomposites, the free dye in the solution decreases with addition of Ag/rGO nanocomposites which lead to decrease in fluorescence intensity of RhB (Turn OFF). This result is similar with earlier report which includes the quenching of rhodamine 6 G with addition of r-GO due to dispersion forces and π–π interaction [66]. After total quenching of rhodamine B, Hg2+ (aq) ion is added and again fluorescence spectrum of rhodamine B is taken. With addition of Hg2+ ion to quenched RhB, the fluorescence

emission of RhB is increased which is due to cationic–π interaction, soft–soft interaction and interaction between Ag and Hg2+ ion (Turn ON). The detection of Hg2+ ion using “OFF-ON” approach is schematically represented in the Scheme 2. Fig. 7(a) shows the fluorescence spectra of RhB with addition of different concentration of Ag/r-GO solution. It is observed that the intensity of RhB is decreased on addition different volume of Ag/r-GO solution with same concentrations of stock solution such as 20, 50, 80, 110, 140 and 180 μl. After proper optimization, it is found that, with addition of 180 μl of Ag/r-GO nanocomposites to RhB solution, maximum quenching is observed. Therefore, 180 μl of stock solution of Ag/ r-GO nanocomposites are taken for quenching of RhB at each experiments. The photographs of RhB with different concentration Ag/r-GO 7

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intensity of the quenched RhB is increased at pH 〈 7 and there is very small increase in intensity at pH 〉 7 with addition of Hg2+ ion. Because Hg2+can form Hg(OH)2 at alkaline media. Hence, the Ag/r-GO@RhB could be more useful at slightly acidic medium and neutral medium. Further, maximum increase in intensity of quenched RhB is noticed at pH 5.4. Therefore, the detection of mercury (II) ion by Ag/r-GO@RhB nanocomposites is studied at pH 5.4. In addition to that time is also important factors to find the efficiency of the sensor. From Fig. 8(b), it is observed that the fluorescence intensity of quenched RhB is increased up to 7 min and remain almost same for a wide range of time at constant pH environment (pH 5.4). Therefore, for detail study of sensing of Hg2+ion, the fluorescence spectra of Ag/r-GO@RhB nanocomposites are taken in the presence of Hg2+ ions at 7 min. After maximum quenching of RhB by 180 μl of Ag/r-GO nanocomposites, the fluorescence spectra are taken with different concentrations of Hg2+under optimized condition. Fig. 9(a) shows the fluorescence spectra of Ag/r-GO@RhB in the absence and presence of different concentrations of Hg2+ ion such as 2.5, 5.0, 7.5, 10.0, 12.5, 25.0, 30.0, 35.0, 40.0 and 45.0 ppb. With increasing concentration of Hg2+, the intensity of fluorescence spectrum of quenched rhodamine B increases. Because, the interaction of Ag/GO with rhodamine B made a non-fluorescent material and after addition of Hg2+ ion to the nanocomposites, rhodamine B is free from Ag/r-GO by giving a highly fluorescence emission. The mechanism of sensing process is based on the increase in interaction of Hg2+ towards Ag/r-GO and there is decrease in interaction between Ag/r-GO and RhB. There is different interaction between Hg2+ and Ag/r-GO such as soft–soft interaction, cationic–π interaction and Ag (0)and Hg2+ interaction due to different reduction potential value. These interactions help for breaking of weak attraction between RhB and Ag/r-GO. Therefore, RhB is free from Ag/rGO@RhB nanocomposites in the presence of Hg2+ by giving orange fluorescence emission (Turn on). Using this principle, our synthetic approach is designed for ultrasensitive detection of Hg2+ ion. In redox

Table 1. Comparison of Hg2+ ion sensor with different method. Method

Probe

LOD

References

Colorimetric Colorimetric Fluorescence Fluorescence Fluorescence Fluorescence

p-PDA/AgNP Ag/GO GO-PPV@MSN@SRh6G DNA–Ag/Pt nanoclusters Fluorescein and Rhodamine B Ferrocene and Triazole-appended Rhodamine Ag/r-GO@RhB

800 nM 338 nM 71 nM 5 nM 20 nM 25 nM

[67] [68] [69] [70] [71] [72]

2 nM

Present work

Fluorescence

solution are shown in inset Fig. 7(b) under UV light (365 nm). Our main object is to prepare non-fluorescent RhB for the detection of Hg2+ ion by taking Ag/r-GO nanocomposites as quencher without using any toxic and costly chemicals towards modification of RhB. In earlier literature, RhB derivatives (non-fluorescent) are prepared for detection heavy metal ion using complicated procedure and toxic chemicals [19]. Therefore, our sensing approach is much better than other reported approach using RhB as fluorescent material. After maximum quenching of RhB by 180 μl of Ag/r-GO nanocomposites, different metal ions are added to it. It is observed that the intensity of quenched RhB is increased only in the presence of Hg2+ ions. For sensing of Hg2+ ion, different parameter such as pH and time are to be optimized for finding better efficiency of the sensor because pH of the nanocomposites affects the stability of nanocomposites as well as interaction of Hg2+ and Ag/r-GO nanocomposites. Therefore, we can find better efficiency of our synthesized sensor at optimized condition. Further, for the analysis of different environmental samples, pH plays an important role towards Hg2+sensing. Fig. 8(a) shows the stability and sensing ability of Ag/r-GO@RhB nanocomposites towards Hg2+ ion at different pH environment (pH 2–9). It is noticed that Ag/rGO@RhB nanocomposites are stable at wide range of pH. However, the

Fig. 10. (a) The fluorescent response of Ag/r-GO@ RhB nanocomposites to different metal ions and the relative fluorescence intensity of Ag/r-GO@RhB nanocomposites in the presence of Hg2+ and other interfering ions. (b) Photographs of Ag/r-GO@RhB nanocomposites with addition different metal ions under UV light. (c) The relative fluorescence intensity of Ag/ r-GO@RhB nanocomposites in the presence of Hg2+ with and without addition of mixture of cations at different pH (pH 2–9) in 100 mM NaCl.

8

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Table 2 Assay of Hg2+ concentrations in different water samples. Sample

Detection of Hg2+

Added Hg2+(ppb)

Found Hg2+(ppb)

Recovery (%)a

RSD (%)b

Mineral water Mineral water Pond water Pond water Tap water Tap water

Not Not Not Not Not Not

10 20 10 20 10 20

9.67 19.62 9.78 19.74 9.94 20.04

96.7 98.1 97.8 98.7 99.4 100.2

0.12 0.06 0.21 0.33 0.08 0.11

a b

detected detected detected detected detected detected

Recovery (%) = 100 × (Cfound/Cadded). RSD (%) = Relative standard deviation of three determination.

reaction, Hg2+ reacts with AgNP in Ag/r-GO nanocomposites to form mercury which further reacts with AgNP to form Ag/Hg amalgam. The graph is plotted between relative intensity (-F0/F) as a function of concentration of Hg2+, where F0 and F are the fluorescence intensities of quenched RhB in the absence and presence of various concentration of Hg2+ and shown in Fig. 9(b). Fig. 9(b) represents the correlation graph of sensing of Hg2+ion where, R2 is greater than 0.99. The detection limit (LOD) of synthesized sensor is calculated to be 2 nM (LOD = (3 × Sd)/b; where ‘Sd represents the standard deviation of the ordinate intercept and b is the slope’). The limit of detection (LOD) of as synthesized nanocomposite is compared with other mercury sensor and represented in Table 1. Although rhodamine based sensor have designed for sensing of heavy metal ions in many literatures, however the sensor has limited practical application in pond water, river water and tap water samples or high limit of detection or toxic and costly chemical requires for synthesis of rhodamine derivatives as compare to present work. Fig. 9(c) shows the photographs of quenched RhB with addition of different concentration of Hg2+ ion under UV light. It is noticed that the fluorescence emission is increased with increase in mercury concentration which is due to the increase of free RhB in the solution. Fig 10(a) shows the selectivity of sensing of Hg2+ using Ag/r-GO@ RhB nanocomposites as compare to other metal ions. The fluorescence responses Ag/r-GO@RhB probe are investigated towards other interfering ions such as K+, Ca2+, Fe2+, Zn2+, Cr2+, Mn2+, Cd2+, Pb2+ and Mg2+. The fluorescence intensity ((F0 – F)/F0) of nanocomposites probe towards different cations are shown in Fig. 10(a) where, F0 and F are the fluorescence intensity of quenched RhB in the absence and presence of metal ions respectively. Other metal ions except Pb2+ ion show negligible response of sensing even at higher concentration. It is noticed that, only Pb2+ ion can able to small increase the intensity of fluorescence spectrum of quenched RhB but not as compare to Hg2+ ion. However, there is very small increase in intensity of fluorescence spectrum of quenched RhB by Cd2+ and K+ metal ions. The interference of other metal ions towards the selectivity of the Ag/r-GO@RhB nanocomposites for Hg2+ sensor is investigated by taking the fluorescence spectra of the Ag/r-GO@RhB nanocomposite in aqueous solution of Hg2+ with other metal ions and compared in Fig 10-a. The high selective response of mercury is due to the coordinating ability, appropriate ionic radius, and charge density of Hg2+ ions. The photographs of quenched RhB in the presence of different metal ions are shown in Fig. 10(b) under UV light. It is observed that fluorescent color is seen only in the presence of Hg2+ ion. Fluorescence interferance of the as-synthesized nano sensor is also checked under high ionic strength at different pH environment. Fig 10(c) shows the graph of relative fluorescence intensity of Ag/r-GO@RhB nanocomposites in the presence of Hg2+ ion with and without addition of mixture of cations (K+, Ca2+, Fe2+, Zn2+, Cr2+, Mn2+, Cd2+, Pb2+and Mg2+) at different pH in 100 mM NaCl. From Fig. 10(c), it is observed that maximum increase in relative intensity was found in slightly acidic medium and neutral medium. At pH 5.4, maximum increase in intensity of fluorescence spectra of Ag/r-GO@RhB nanocomposites in 100 mM NaCl was noticed. Nearly same results and fluorescence quenching is also observed in

distilled water. The selectivity and sensitivity for sensing of Hg2+ ion is not affected significantly by ionic strength. The synthesized Ag/r-GO@RhB nanocomposite is used to determine the Hg2+in mineral water, pond water and tap water samples. The percentage recoveries of Hg2+ ion are calculated in the water samples by adding 10 ppb and 20 ppb concentration of Hg2+. First, the two concentration of Hg2+ are added to the water sample and then filtered through a 0.22 μm membrane. The fluorescence spectra of Ag/r-GO@ RhB nanocomposite are analysed and given in Table 2. It is observed from Table 2 that the average recoveries of Hg2+ at 10 ppb and 20 ppb are between 96.7% and 100.2%, the relative standard deviations using three determinations are between 0.06% and 0.33%. The results concluded that the applications of our as-synthesized sensor to real water samples for determination of Hg2+ ion are successful. The breaking of weak interaction between RhB and Ag/r-GO through Hg2+ ion is examined by taking ethylene di-amine tetra acetic acid (EDTA) as the coordinating ligand. After addition of Hg2+ ion to Ag/r-GO@RhB nanocomposite (quenched RhB), RhB is free from Ag/rGO with formation of Ag/r-GO@ Hg2+system. However, EDTA which has strong affinity towards metal ions eliminates Hg2+ from Ag/r-GO@ Hg2+system after addition of EDTA. Therefore, the interaction of RhB and Ag/r-GO is increased with decreasing the fluorescence emission (Fig. 11). In general, the fluorescence spectra of quenched RhB is increased with addition of Hg2+ ion which is due to the soft–soft interaction, cationic–π interaction and interaction of Ag and Hg2+ ions in

Fig. 11. Effect of EDTA on fluorescence intensity in the presence of Hg2+ ion. 9

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Scheme 1. Synthesis scheme of Ag/r-GO nanocomposites.

nanocomposites. From the sensing mechanism, Hg2+ is reduced by Ag to form elementary Hg which are not binding with EDTA. However, Hg2+ ion which is attached to -COO- of Ag/r-GO nanocomposites

through soft-soft interaction is binding with EDTA. Therefore, very small quenching of fluorescence induced by Hg2+ is taking place. Similar result of quenching is also obtained in earlier literature [66].

Scheme 2. Schematic representation of sensing process of Hg2+ ion by Ag/r-GO@RhB nanocomposites. 10

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4. Conclusions

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In the present study, Ag/r-GO nanocomposites are successfully synthesized by “one pot” synthetic protocol using sodium citrate as reducing agent and are characterized by UV-visible, SEM, TEM, FTIR and XRD analysis. Zeta potential measurement analyse the surface charge and stability of Ag/r-GO nanocomposites. In a process, RhB is quenched by Ag/r-GO nanocomposites due to dispersion forces and π–π interaction between RhB and Ag/r-GO nanocomposites. Sensing mechanism of Hg2+ ion is based on increase in the fluorescence intensity of quenched RhB with addition of Hg2+ ion. This is due to breaking of weak physical interaction between RhB and Ag/r-GO by Hg2+ and forming of cation–π interaction between Ag/r-GO and Hg2+ so that rhodamine B is free from Ag/r-GO by giving a highly fluorescence emission. Further, silver forms Ag/Hg amalgam due to redox reaction of AgNPs and Hg2+. The stability of Ag/r-GO@RhB nanocomposites with wide range of pH is good. The practical application of nanocomposites can be done in real water samples such as mineral water, tap water and pond water for mercury sensor. The limit of detection of Hg2+ ion in aqueous solution is calculated to be 2 nM which makes it great important towards recognition of Hg2+ ion. It is also found that the detection of mercury (II) ion using current approach is not affected in the presence of other metal ion. The present synthetic approach shows a low cost and efficient technique which are used as a potential candidate for selective and sensitive detection of Hg2+ ion in water. CRediT authorship contribution statement Deepak Sahu: Methodology, Investigation. Niladri Sarkar: Formal analysis. Priyaranjan Mohapatra: Resources. Sarat K. Swain: Conceptualization. Declaration of Competing Interest There are no conflicts in publishing this article. Acknowledgement The authors would loke to thank Veer Surendra Sai University of Technology for providing university fellowship to Mr. Deepak Sahu for pursuing PhD degree. The TEQIP-III of Government of India is acknowledged for partial financial support. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.microc.2019.104577. References [1] P. Samanta, A.V. Desai, S. Sharma, P. Chandra, S.K. Ghosh, Selective recognition of Hg2+ ion in water by a functionalized metal–organic framework (MOF) based chemodosimeter, Inorg. Chem. 57 (2018) 2360–2364. [2] H. Yuan, W. Ji, S. Chu, Q. Liu, S. Qian, J. Guang, J. Wang, X. Han, J.F. Masson, W. Peng, Mercaptopyridine-functionalized gold nanoparticles for fiber-optic surface plasmon resonance Hg2+ sensing, ACS Sens. 4 (2019) 704–710. [3] S. Zhang, D. Zhang, X. Zhang, D. Shang, Z. Xue, D. Shan, X. Lu, Ultratrace naked-eye colorimetric detection of Hg2+ in wastewater and serum utilizing mercury-stimulated peroxidase mimetic activity of reduced graphene oxide-PEI-Pd nanohybrids, Anal. Chem. 89 (2017) 3538–3544. [4] J. Mutter, J. Naumann, C. Sadaghiani, R. Schneider, H. Wallach, Alzheimer disease mercury as pathogenetic factor and apolipoprotein E as a moderator, Neuroendocrinol. Lett. 25 (2004) 331–339. [5] USEPA, United States Environmental Protection Agency, 2009, EPA 816-F-809-004. [6] Q. Yang, Q. Tan, K. Zhou, K. Xu, X.J. Hou, Direct detection of mercury in vapor and aerosol from chemical atomization and nebulization at ambient temperature: exploiting the flame atomic absorption spectrometer, J. Anal. Atom. Spectrom. 20 (2005) 760–762. [7] B. Fong, W. Mei, T.S. Siu, J. Lee, K. Sai, S. Tam, Determination of mercury in whole blood and urine by inductively coupled plasma mass spectrometry, J. Anal. Toxicol.

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