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Graphene-Rhodamine nanoprobe for colorimetric and fluorimetric Hg2+ ion assay Anju Mohan, and Renuka Neeroli Kizhakayil ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03904 • Publication Date (Web): 19 May 2016 Downloaded from http://pubs.acs.org on May 25, 2016

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ACS Applied Materials & Interfaces

Graphene-Rhodamine nanoprobe for colorimetric and fluorimetric Hg2+ ion assay Anju Mohan and Renuka Neeroli Kizhakayil* Department of Chemistry, University of Calicut, Kerala – 673 635, India. KEYWORDS: Graphene-Rhodamine array; Mercury ion; Colorimetry; Fluorimetry; Logic gate ABSTRACT: This article reveals the first ever prospective application of Graphene-Rhodamine array (GRH) as a color2+ imetric and fluorimetric sensor for Hg ions. The duality of Graphene to undergo π-π and dispersive interactions with 2+ Rhodamine as well as to act as a selective adsorbent for Hg is conceptualized in this study. These interactions lead to decrease in absorbance of the dye in presence of graphene, 2+ which is restored when kept in contact with Hg ions. The feasibility of the mechanism has been proved using EDTA as the coordinating ligand. It is noteworthy that all the optical variations occurred in the visible scale of the electromagnetic spectrum. The GRH array exhibited higher sensitivity towards the target ion with a limit of detection of 2 ppb. A 2+ perfect linear variation of absorbance at 554 nm with Hg concentration was observed in 0 – 1000 nM range, enabling the use of the system as a quantitative sensor for the test ion. 2+ The commendable selectivity of the array towards Hg ion has been investigated by observing the optical response in presence of other environmentally relevant metal ions. A reversible turn off and turn on INHIBIT logic gate has been proposed which extends the scope of the designed array for the development of automated chemical systems. The Fluorescence Resonance Energy Transfer (FRET) ability of graphene paves the backbone for the fluorimetric detection. Fluorimetric strategy yielded a much lower limit of detection of 380 ppt using this probe, which makes a significant ad2+ vance in trace detection of Hg ions.

1. Introduction

E

ver since the isolation of free standing graphene (GN) in 1 2004 by Geim and co workers , this 2D super carbon entity continues to lure the scientific and technological world owing to its incredible physiochemical and structural proper2-6 ties. GN has exemplified itself as an indispensible tool for sensors, wherein its unique features such as electrical (high mobility of charge carriers, high capacitance, high electron transfer rate), optical (ability to alter absorption characteristics, quench fluorescence), structural and mechanical (high surface to volume ratio, flexibility) properties have been exploited. GN is widely being employed as a physical, electro7-9 chemical and biological sensor. Currently, GN and GN based systems are widely investigated for optical sensing of

materials. In this regard, GN, more precisely GN derived by chemical reduction, exhibits outstanding optical properties. GN based materials possess high transparency, high surface to volume ratio and can interact with many molecules using non covalent interactions. These suggest the potential of GN materials in optical detection of a variety of chemical species. Quite a good quantum of research has been devoted to the use of GN and its composites as fluorimetric sensors based 10-14 Reon Fluorescence Resonance Energy Transfer (FRET). cently, the union between GN and inexpensive, label free organic dyes have opened up an area with outstanding and 15-16 versatile applications in various fields. In the present study, we aim to fabricate a simple GN based optical (colourimetric and fluorimetric) sensor for the detection of heavy 2+ metal ion, Mercury (Hg ). 2+

Mercury ion (Hg ) is one of the topmost toxic heavy metal 17 ions, which in any form is poisonous to both human and aquatic systems. Mercury causes undesirable effect to central nervous system, brain, lungs, kidney etc. If ingested by pregnant women, mercury can cause developmental delays in 2+ children. Hg ion has a tendency to accumulate in vital organs of humans and animals that will lead to hematological destruction. Besides being the most common contaminant among heavy metals, the ecological effects and biomagnification of Mercury ion through the food chain have 2+ led to a demand for proper sensing and removal of Hg ion. 2+ The obvious limitation for direct Hg ion detection by spectroscopic technique is due to the lack of significant optical 2+ response from Hg ion, on account of its closed d- shell configuration. Past few decades have witnessed remarkable pro2+ 18-21 gress in the development of Hg sensors. Many attempts 2+ were directed to design portable Hg ion sensors, which includes colorimetric, fluorescent and electrochemical meth22-23 24-27 ods. Probes based on organic molecules, polymeric 28-31 materials, Au and Ag nanoparticles etc. have been extensively investigated for this purpose. Poor solubility in aqueous solution and tedious preparation and purification steps limit the use of organic and polymer probes, whereas the high cost of noble metals holds back their widespread application. In this direction, GN based systems have received considerable attention for the detection of heavy metal ions, 32-34 notably Mercury ion. Colorimetric approaches reported 2+ for Hg ion detection mainly employ Au and Ag nanoparticles. Nay Ming Huang and co-workers designed Graphene 35 oxide based silver system for this purpose. Dithia and Diaza

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stabilized Au nanoparticle and Tween 20-stabilised Au na37 2+ noparticles have also been reported for Hg ion detection. However, high cost of these systems limits their practical applications, and demands alternatives for them. The proposed detection through optical methods is based on 2+ the interaction of GN between the dye and the Hg ions, which relies on the layman’s phenomenon, adsorption. We have opted Rhodamine (R6G) as the dye, as it is a well acclaimed surface active compound for colorimetric sensing due to its π-π interaction and other noncovalent interactions. R6G based probes exhibit excellent variations in absorption and fluorescence intensity on interaction with metal ions. Besides, it is readily available and is quite convenient to 38 monitor its absorption spectra. This study exploits the 2+ unique ability of GN in selectively adsorbing Hg over Rhodamine (R6G). It is a well known observation that the interaction of GN reduces the intensity of absorbance maxima of R6G centered at 554 nm. Our efforts are directed to break this weak physical interaction between GN and R6G with 2+ Hg ion, an entity capable of replacing R6G by entering into coordination with GN. This forms the basis of colorimetric detection. Generally, colorimetric sensors are established to be inferior to the ones based on fluorescence. But the system projected in this study could even beat some of the lumines2+ cent based Hg ion sensors. The selectivity of the system towards other interfering ions was also examined. In addition 2+ to this, a reversible INHIBIT logic gate dependant on Hg ions and EDTA has also been designed. Further advance in this work is made by adopting the GRH array for determin2+ ing Hg via fluorimetric method. The Fluorescence Resonance Energy Transfer (FRET) ability of GN paves the backbone for the fluorimetric detection route. To the best of our knowledge, no study has been made so far on fluorescence 2+ based detection of Hg ion using Graphene-rhodamine unit via FRET. 2. Experimental Graphite powder was procured from Acros organics. Stock solutions of the desired ions (Mercury, Lead, Magnesium, Calcium, Sodium, Potassium, Iron, Copper, Beryllium, Manganese, Aluminium, Strontium, Barium, Cesium, Cadmium, Cobalt, Zinc, Nickel and Chromium (all obtained from Merck) were prepared by dissolving the required amount of respective salts in deionised water and were used for the investigation. All the other chemicals employed in this study were of analytical grade and was used as received.

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transmission electron microscopy (FEI TECNAI 30 G2, 300 kV). Raman spectroscopy was performed at room temperature using a Raman Microprobe with 532 nm Nd:YAG excitation source(LabRam HR-Horiba Jobinyvon Spectrometer).Ultraviolet visible (UV-vis) spectroscopy measurements were performed on Jasco V-550 spectrometer. Fluorescence emission spectra were recorded on Perkin Elmer Fluorescence spectrometer LS 55; excitation wavelength: 550 nm; slit: 5 nm 2.2 Mercury Sensing R6G solution (10μM) was prepared by dissolving the required amount of R6G in deionised water. 1.5 mL of R6G solution was taken in a glass cuvette followed by the addition of 20,40,60,80,100,120, 140μL of GN solution. UV-VIS spectra were recorded after each addition by measuring the UV-Vis absorbance of R6G at 554nm. The selectivity towards 2+ Hg was checked in presence of other competing metal ions 2+ 2+ 2+ 3+ 2+ 3+ + 2+ 2+ 2+ + 2+ 3+ (Cd ,Pb ,Cu ,Fe ,Zn ,Cr ,Na ,Ca ,Be ,Mg ,K ,Sr ,Al , 2+ 2+ 2+ 2+ 2+ Mn ,Cs ,Ba ,Ni and Co ). The reversibility of the system was examined by addition of EDTA (150μL) as coordinating ligand. pH of all the stock solutions was maintained at 6.7± 0.4, the normal pH of the solution. For fluorescence study, aliquots of GN were added to a definite volume of R6G solution and the fluorescence spectra 2+ were recorded after each addition. Titration with Hg ion against the GRH array was carried out and consequently the spectra were noted. 3. Result and Discussions 3.1 Characterisation of Graphene GN was obtained by chemical reduction of Graphene oxide (GRO) using Hydrazine Hydrate. The brownish yellow GRO solution changed to black during the process. The successful reduction of GN was monitored and affirmed by various characterization techniques. Figure 1 (a) shows the UV-Vis spectra of the systems. GRO shows a maximum absorption at 230 nm which can be attributed to two types of electronic transition, (i) π-π* of atomic C-C bonds and (ii) π-π* of the – C=O group of carboxylic acids. This peak on reduction undergoes a red shift to 256 nm in GN, which is in accordance 38 with earlier report .

2.1 Synthesis of Graphene Graphite Oxide was prepared by the well known Hummers 39 method as reported elsewhere. About 100 mg of Graphite oxide was dispersed in 100 mL of distilled water via magnetic stirring and ultrasonication to obtain a brownish yellow solution of Graphene oxide (GRO). Hydrazine Hydrate was used for the chemical reduction route. 20μL of Hydrazine Hydrate and 104 μL of ammonia solution were added to GRO solution 0 and was heated to 95 C for 1 hour. The black dispersion obtained was filtered to obtain a stable black solution of GN. The obtained GN solution was quite stable and no precipitation was observed for one month. Jasco FTIR-4100 spectrometer was used for recording the FTIR spectra of catalyst samples using KBr disc method. The morphology of the synthesised GN was investigated using


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Absorbance

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Figure 3 shows the TEM image of as-prepared GN sheets. Paper like appearance unique to GN sheets is well portrayed in the TEM image, which also indicated that the prepared system is composed of multi layered GN sheets, typical observation for GN obtained via chemical reduction.

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Figure 3 TEM image of Graphene sheets Raman spectroscopy is very useful for the characterisation of Carbon materials. Figure 4 shows the Raman spectra of GRO -1 and GN. The spectra show two prominent peaks at 1311 cm -1 and 1598 cm corresponding to the well documented D and G bands respectively. The intensity ratio (ID/IG) increases from 1.20 to 1.35 on moving from GRO to GN, confirming the successful reduction of GRO.

Figure 2 depicts the FTIR spectra of GRO and GN. The FTIR spectrum of GRO shows distinct bands which can be indexed to the presence of various functional groups on it. The band -1 near 3400 cm can be correlated to the stretching and bending vibrations of –OH groups. The symmetric and antisymmetric stretching vibrations of –CH2 groups give rise to -1 -1 -1 bands at 2927 cm and 2843 cm . Bands near 1690 cm can be ascribed to the stretching vibration of C=O group of carboxylic acid group. A steady decrease in the intensity of IR bands of GRO was noticed upon reduction. Notably, the -1 band at 830 cm corresponding to C-O stretching mode of epoxy groups in GRO has completely vanished on reduction. However, it is impossible to reduce all the functional groups on GRO via chemical reduction to obtain pristine GN sheets 40 .

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Figure 1(a) UV-VIS spectra of Graphene oxide and Graphene, & (b) Photographs of Graphene oxide (GRO) & Graphene (GN) aqueous dispersions.

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Figure 2 FTIR spectra of Graphite Oxide and GN.

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Figure 4.Raman spectra of GRO and GN. 3.2 Graphene –Rhodamine array based colorimetric Hg sensing strategy

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ion

The presence of Graphene brings about significant changes in the absorption spectrum of the Rhodamine dye, which 2+ paves the back bone for the colorimetric detection of Hg in presence of this entity. Figure 5(a) illustrates the shift in absorption spectrum of rhodamine upon the addition of graphene. The interaction between GN and R6G leads to a steady decrease in the absorbance maxima of free R6G molecule (Turn-Off state) along with a red shift in the absorption band. There was a shift of 4 nm to the red region in the visible scale in accordance with addition of aliquots of GN (140μL of GN), as evident from the Figure 5(a). The red shifted bands hint to the formation of a complex GN…R6G according to the equation: nR6G + GN

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the free dye in the solution decreases, leading to the decrease in the intensity of absorption maximum (Turn-Off state). A 60 % decrease in the absorbance maximum was seen on the addition of 140μL of GN solution. The possible interactions between GN and R6G are presented in Figure 6. These interactions do not alter the geometry of R6G even after it binds to GN.

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Figure 5 (a) UV-VIS spectra of R6G on addition of GN (R6G (10μM) + GN (140μL)) & (b) Trend in absorbance maximum with increase in concentration of GN.

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Figure 7. (a) Restoration of Absorbance maxima of GRH array 2+ 2+ on addition of Hg ions. [Hg ] from bottom to top, 0 ppb, 30 ppb, 60 ppb, 91 ppb, 1 ppm, 3 ppm, 6 ppm, 9 ppm, 12 ppm (b) Variation of intensity of absorbance of R6G in presence of various species. (c) Influence of EDTA (150μM, 100μL) 2+ on absorbance in presence of Hg ions.

Figure 6. Possible modes of interaction between GN and R6G.

The mercury ion sensing ability of GRH array nanoprobe is examined from the variation of absorbance intensity of the combination in presence of mercury ions. The intensity of absorbance at 554 nm significantly enhanced upon addition 2+ of Hg ions. Figure 7(a) presents the restoration of absorb2+ ance maxima of GRH array in presence of Hg ions (Turn2+ On state). On titrating Hg ion solution against the GRH array, the absorption spectra of R6G restored its original 2+ path(160μL of 100μM Hg ), indicating that colorimetric strategy can be successfully adopted for detecting the pres2+ ence of Hg ions. As far as our knowledge, no report exists 2+ on the selective detection of Hg based on GRH array with


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the aid of UV-VIS spectroscopy. It was quite a credible observation that the absorption spectra of R6G were almost completely restored [Figure 7(b)]. pH of the GRH array under the proposed experimental condition also favours the 2+ 42-45 adsorption of Hg ion. The removal of R6G from Gra2+ 2+ phene by Hg ions was checked by eliminating Hg ions using Ethylene DiammineTetraacetic acid (EDTA) as the coordinating ligand, which due to its strong affinity towards 2+ 2+ metal ions, remove Hg ions from Hg /GN system. Thus in turn enables the reunion of GN and R6G (Turn-Off state), thus lowering the intensity of absorbance maxima again (Figure 7c). Nitrogen atoms on EDTA have a greater tendency to form metal complexes by donating the lone pair of electrons when compared to the oxygen functional groups on GN. Moreover, according to the trend in electronegativity, formation of complex will be more facile from Nitrogen atom 2+ than oxygen atom. Hence, EDTA preferentially binds to Hg 46 ion, thus releasing the ion from GN. A survey of literature reveals that in the pH range 6.7±0.4 as maintained in this work, mercury would exist as divalent species and it can accept electron pair from Oxygen or Nitrogen.

The plausible route of the sensing mechanism is the displacement of the R6G molecules from the surface of GN by 2+ Hg ion, owing to the inherent selective adsorption of 2+ Hg ion by GN. It is established that the phenyl ring hinders the formation of strong bond between R6G and GN. As the 2+ selective adsorption of Hg ion disrupts this weak interaction between GN and R6G, the absorption spectra of R6G is restored as the concentration of free dye increases with in2+ crease in concentration of Hg ions. We hypothesized that Hg(II) being a soft acid can easily bind to the oxygen containing functional groups on GN sheets, thus displacing the dye [Figure 8]. In order to probe the sensitivity of the GRH array towards 2+ Hg , we examined the change in absorbance of the solution 2+ in presence of different concentrations of Hg (160μL of 100 nM to 10 μM), maintaining all other experimental parameters the same. The variations in intensity of the absorbance maximum at 554 nm were carefully investigated to ascertain the sensitivity and detection limit of the array. We found that a quantitative relation exists between the intensity of absorb2+ ance maximum of R6G and the Hg concentration [Figure 9 (a)]. Absorbance of the solution varied linearly with the con2+ centration of Hg ion in the solution, which enabled estima2+ tion of Hg via the usual colorimetric approach. The proposed array exhibited a perfect linear variation in absorbance 2+ over Hg concentration of 0 nM to 1000 nM enabling its use 2+ as a quantitative tool for determination of Hg at very low concentrations.[Figure 9( a)]. The limit of detection (3δ/slope of the line; where ‘δ’ is the standard deviation) was found to be as low as 2 ppb which is highly commendable 2+ 15, when compared to similar systems for Hg ion detection. 47-49 Further investigation using higher amount of graphene in rhodamine solution of the same concentration (so as to lower the absorbance maximum of the dye solution) indicated that this linear variation is valid in higher concentration range also [Figure 9(b)]. However, the value of regression 2 coefficient, R in this case was found to be less (0.976) when compared to the lower concentration range (0.999).The higher detection limit under the given experimental conditions was ̴ 2000ppb. It is obvious that the said value can be further enhanced by maintaining a higher concentration of GRH array, which in turn will lead to further reduction in absorbance maximum of the dye, leading to optical response 2+ in presence of higher amount of Hg ions.

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Figure 8. Hg interacts with GN via soft- soft interaction and Cationic-π interaction displacing R6G.



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Figure 10. Selectivity of GRH sensor towardsHg ion over other metal ions and in a mixture of metal ions. Amax represents the absorption maxima of the restored curve of GRH 2+ array after addition of Hg . A represents the absorbance maxima after addition of metal ions (160 μL, 100 μM).

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2+ Co Mn2+ 3+ Al 2+ Sr + K Mg2+ 2+ Ba 2+ Be + Cs2+ Ca + Na 2+ Ni 3+ Cr 2+ Zn 2+ Cu3+ Fe 2+ Pb 2+ Cd 2+ Hg GRH

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Figure 9. Variation in absorbance of GRH and concentration 2+ of Hg ion (a) 0 nM to 1000 nM & (b) 1μM to 10 μM. 2+

The specificity of the GRH array towards Hg over other interfering metal ions was monitored as follows. The study was repeated with some environmentally relevant metal ions 2+ 2+ 2+ 3+ 2+ 3+ + 2+ 2+ 2+ + viz., Cd , Pb , Cu , Fe , Zn , Cr , Na , Ca , Be , Mg , K , 2+ 3+ 2+ 2+ 2+ 2+ 2+ Sr ,Al ,Mn ,Cs ,Ba ,Ni and Co individually as well as 2+ in a mixture of ions including Hg ions [Figure 10]. The concentration of each of these metal ions (total 18 in number) in the mixture was kept ten-fold when compared to that of 2+. 2+ Hg The weight percentage of Hg in the mixture of ions was kept at 16.4%, so that excess concentration of competent ions is assured in the solution. Among the studied metal 2+ ions, only Pb could affect a small restoration of absorption 2+ + maxima of R6G. It was observed that Cd and K further decreased the absorbance maximum of R6G, while the other ions did not produce any noticeable optical response. The small restoration of absorbance maxima of the mixture with2+ 2+ out Hg ion may be probably due to the presence of Pb ion in the mixture. The coordinating ability, appropriate ionic 2+ radius, and charge density of Hg ions are suggested to be 50 decisive behind this highly selective response . The selectivity of the proposed system is found to be commendably superior as when compared to similar systems for mercury 16 sensing, which further increases the relevance of the study.

In addition to this, a reversible INHIBIT logic gate dependant 2+ on Hg and EDTA has also been designed, which will be usable for extending the potential of the proposed system. The advent of molecular logic gates has set the way for mo51 lecular scale computers and automated chemical systems . In this article, we propose a molecular logic inhibit gate using Boolean algebra. The INHIBIT gate has two inputs (A, B, 2+ which in the present case represents Hg and EDTA) and an output (here, the absorbance maximum of GRH). The criterion for the INHIBIT logic gate is that when either input is on, the gate is off. Switching on input A turns the gate on. Turning on input B alone does not switch the gate on. In addition, switching both inputs on leaves the gate in the off position. Thus, input B inhibits the gate from responding to 2+ input A. Here, Hg (input 1) and EDTA (input 2) are the two inputs which function through the GRH array. The presence of the inputs is defined as 1 while their absence as 0.The absorbance was defined as the output (1 or 0, corresponding to maximum absorbance and minimum absorbance) [Figure 11 (a)]. The truth table for the proposed INHIBIT logic gate is shown in Figure11 (b). The four possible input combinations 2+ for Hg and EDTA are (0,0), (1,0), (0,1) and (1,1). Without 2+ Hg or EDTA (corresponding to (0,0),the absorbance maximum (Absmax of GRH array (output) is 0 (Absmax decreas2+ es).On addition of Hg alone to the GRH array (1,0) the Absmax increases sharply leading to an output (1)[Turn-On]. Addition of EDTA alone (0, 1) does not increase the absorbance maxima of GRH array, leading to an output value (0).On subjecting the two inputs simultaneously (1, 1), the output was zero. Thus, the requirements for an INHIBIT gate are met with. The reversibility of the GRH logic gate was 2+ checked by alternate addition of Hg and EDTA for 4 cycles Figure11(c) depicts the repeated switching behaviour on al2+ ternate addition of Hg and EDTA. A marginal decrease in the performance was seen. Presumably, this may be due to the dilution effect and the clogging of adsorption sites on GN.


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be 380 ppt (parts per trillion) through this method, which is highly significant when compared to similar systems report2+ 52-53 ed previously for trace detection of Hg ions.

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Figure 11 (a) An INHIBIT logic gate system based on GRH 2+ using both Hg and EDTA as inputs,(b) Truth table for the inhibit logic gate &(c) Reversible switching of the GRH array 2+ between On and Off states on alternate addition of Hg and EDTA. 2+

3.3 Graphene –Rhodamine array for fluorimetric Hg sensing strategy

ion

The fluorescence spectrum of rhodamine is displayed in Figure 12(a). Presence of GN reduces the intensity of florescence of the dye through FRET, as evident from the figure. As ex2+ pected, Hg ions restore the fluorescence, indicating that the array can perform as a fluorimetric sensor also for the concerned ion. The system exhibited very high selectivity towards the ion of our interest, and the trend in selectivity towards other metal ions chosen followed the same order as that of colourimetric approach. The intensity of fluorescence of Graphene-rhodamine array varied linearly with increase in concentration of mercury ions, enabling the use of the system for quantitative analysis (Figure 12(c). The lowest detection limit (3δ/slope route) for mercury ions was observed to

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Figure12. (a) Fluorescence spectra of rhodamine indicating 2+ the variation of intensity in presence of graphene and Hg ions. (b) Restoration of fluorescence intensity of GRH array 2+ 2+ on addition of Hg ions. R6G (1μM,1 mL), GN (140μ L),[Hg ] from bottom to top 0 ppt, 870 ppt ,3.87 ppb, 6.1 ppb, 9.23 ppb, 12.01 ppb, 15.20 ppb, 18.33 ppb, 21.0 ppb. (c) Variation of fluorescence intensity of GRH with and concentration of 2+ Hg ion. 4. Conclusion Briefly, we have developed a novel Graphene-Rhodamine (GRH) array that can function as a reversible optical probe for facile, cost effective, sensitive and selective detection of 2+ Hg ion. We have revealed interesting interactions between


ACS Applied Materials & Interfaces 2+

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GN and R6G, and have established that Hg ions can modulate and modify these interactions. The GRH array has several merits to its credit. Specifically, (i) it can act as a highly 2+ sensitive and selective sensor for Hg ion when coexisting with other metal ions and (ii) it does not demand chemical modifications. The colourimetric route exhibits a lower detection limit of 30 ppb of mercury ions, which is much superior to similar established probes operating through colorimetric strategy. The concept of molecular logic system demonstrated herein extends the scope of this array. A comparison of this method is made with that of fluorimetry based technique using the same probe, which yielded a better sensitivity towards the test ion, viz., 380 ppt.

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resce upon Hybridization. J. Am. Chem. Soc. 2008, 130, 8351– 8358. (11) Li, F.; Huang, Y.; Yang, Q.; Zhong, Z.; Li, D.; Wang, L.; Song, S.; Fan, C. A Graphene-Enhanced Molecular Beacon for Homogenous DNA Detection. Nanoscale 2010, 2, 1021–1026. (12) Wen, Y.; Xing, F.; He, S.; Song, S.; Wang, L.; Long, Y.; Li, D.; Fan, C. A Graphene-Based Fluorescent Nanoprobe for Silver (I) ions Detection by Using Graphene oxide and a Silver-Specific Oligonucleotide. Chem.Commun. 2010, 46, 2596–2598. (13) Bo, Y.; Yang, H.; Hu, Y.; Yao, T.; Huang, S. A Novel Electrochemical DNA Biosensor Based on Graphene and Polyaniline Nanowires. Electrochim.Acta 2011,56 ,2676–2681. (14) Jung, J.H.; Cheon, D.S.; Liu, F.; Lee, K.B.; Seo, T.S. A Graphene Oxide Based Immuno-biosensor for Pathogen Detection. Angew. Chem.,Int. Ed. 2010, 49, 5708-5711.

AUTHOR INFORMATION Corresponding Author *Email:[email protected]; Fax: +91 494 2400269; Tel: +91 0494 2407413 Notes The authors declare no competing financial interests.

(15) Zhao, X.-H.; Kong, R.-M.; Zhang, X.-B.; Meng, H.-M.; Liu, W.-N.; Tan, W.; Shen, G.-L.; Yu, R.-Q. Graphene-DNAzyme based Biosensor for Amplified Fluorescence “Turn-On” De2+ tection of Pb with a High Selectivity. Anal.Chem. 2011, 83, 5062–5066.

Anju Mohan gratefully acknowledges the financial assistance received from UGC and also University of Calicut for providing research facilities.

(16) Huang, W.T.; Shi, Y.; Xie, W.Y.; Luo, H.Q.; Li, N.B. A Reversible Fluorescence Nanoswitch based on Bifunctional 2+ Reduced Graphene Oxide: Use for Detection of Hg and Molecular Logic Gate Operation. Chem. Commun. 2011, 47, 7800–7802.

REFERENCES

(17) Bings, N.H.; Bogaerts, A.; Broekaert, J.A.C. Atomic Spectroscopy. Anal. Chem. 2006, 78, 3917–3945.

ACKNOWLEDGEMENTS

(1) Novoselov, K.S.; Geim, A.K.; Morozov, S.V.; Jiang, D.; Zhang, Y.; Dubonos, S.V.; Grigorieva, I.V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666–669.

(18) Leopold, K.; Foulkes, M.; Worsfold, P. Methods for the Determination and Speciation of Mercury in Natural Waters—A Review. Anal.Chim.Acta 2010, 663, 127−138.

(3) Allen, M.J.; Tung, V.C.; Kaner, R.B. Honeycomb Carbon: A Review of Graphene. Chem. Rev. 2010, 110, 132–145.

(19) Karunasagar, D.; Arunachalam, J.; Gangadharan, S. Development of a ‘Collect and Punch’ Cold Vapour Inductively Coupled Plasma Mass Spectrometric Method for the Direct Determination of Mercury at Nanograms per Litre Levels. J. Anal.At.Spectrom. 1998, 13, 679−682.

(4) Craciun, M.F.; Russo, S.; Yamamoto, M.; Tarucha, S. Tunable Electronic Properties in Graphene. Nano Today 2011, 6, 42–60.

(20) Mahajan, R.K.; Kaur, I.; Lobana, T.S. A Mercury (II) IonSelective Electrode Based on Neutral Salicylaldehyde Thiosemicarbazone Talanta 2003, 59,101−105.

(5) Rao, C.N.R.; Biswas, K.; Subrahmanyam, K.S.; Govindaraj, A. Graphene, the New Nanocarbon. J. Mater. Chem. 2009, 19, 2457–2469.

(21) Puk, R.; Weber, J.H. Determination of Mercury( II), Monomethylmercurycation, Dimethylmercury and Diethylmercury by Hydride Generation, Cryogenic Trapping and Atomic Absorption Spectrometric Detection. Anal.Chim.Acta 1994, 292, 175−183.

(2) Bai, S.; Shen, X. Graphene–Inorganic Nanocomposites. RSC Adv. 2012, 2, 64–98.

(6) Singh, V.; Joung, D.; Zhai, L.; Das, S.; Khondaker, S.I.; Seal, S. Graphene Based Materials: Past, Present and Future. Prog.Mater.Sci. 2011, 56, 1178–1271. (7) Wang, Y.; Yang, R.; Shi, Z.; Zhang, L.; Shi, D.; Wang, E.; Zhang, G. Super-Elastic Graphene Ripples for Flexible Strain Sensors. ACS Nano 2011, 5, 3645–3650.

(22) Zhu, Z.; Su, Y.; Li, J.; Li, D.; Zhang, J.; Song, S.; Zhao, Y.; Li, G.; Fan, C. Highly Sensitive Electrochemical Sensor for Mercury(II) Ions by Using a Mercury-Specific Oligonucleotide Probe and Gold Nanoparticle-Based Amplification. Anal. Chem. 2009, 81, 7660–7666.

(8) Shan, C.; Yang, H.; Song, J.; Han, D.; Ivaska, A.; Niu, L. Direct Electrochemistry of Glucose Oxidase and Biosensing for Glucose Based on Graphene. Anal. Chem. 2009, 81, 23782382.

(23) Singh, A.; Kaur, S.; Singh, N.; Kaur, N. Fluorometric 2+ sensing of Hg Ions in Aqueous Medium by Nano-aggregates of a Tripodal Receptor. Org.Biomol. Chem., 2014, 12, 23022309.

(9) Alwarappan, S.; Erdem, A.; Liu, C.; Li, C. - Z. Probing the Electrochemical Properties of Graphene Nanosheets for Biosensing Applications. J.Phys. Chem. C 2009, 113, 8853-8857.

(24) Yang, Y.-K.; Yook, K.-J.; Tae, J. A Rhodamine-Based Fluorescent and Colorimetric Chemodosimeter for the Rapid 2+ Detection of Hg Ions in Aqueous Media. J. Am. Chem. Soc. 2005,127, 16760–16761.

(10) Yang, R.; Jin, J.; Chen, Y.; Shao, N.; Kang, H.; Xiao, Z.; Tang, Z.; Wu, Y.; Zhu, Z.; Tan, W. Carbon NanotubeQuenched Fluorescent Oligonucleotides: Probes that Fluo-


Page 9 of 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(25) Huang, W.; Zhu, X.; Wua, D.; He, C.; Hu, X.; Duan, C. Structural Modification of Rhodamine-Based Sensors toward Highly Selective Mercury Detection in Mixed Organic/Aqueous Media. Dalton Trans., 2009, 48, 10457–10465. (26) Kumari, N.; Dey, N.; Bhattacharya, S. Rhodamine Based Dual Probes for Selective Detection of Mercury and Fluoride Ions in Water Using Two Mutually Independent Sensing Pathways. Analyst 2014, 139, 2370-2378. (27) Lee, S.H.; Parthasarathy, A.; Schanze, K.S. A Sensitive and Selective Mercury (II) Sensor Based on Amplified Fluorescence Quenching in a Conjugated Polyelectrolyte/SpiroCyclic Rhodamine System. Macromol. Rapid Commun. 2013,34, 791−795. (28) Feng, L.; Sha, J.; He, Y.; Chen, S.; Liu, B.; Zhang, H.; Lü, C. Conjugated Polymer and Spirolactam Rhodamine-B Derivative Co-Functionalized Mesoporous Silica Nanoparticles as the Scaffold for the FRET-Based Ratiometric Sensing of Mercury (II) ions. Microporous Mesoporous Mater. 2015, 208, 113-119. (29) Geng, T.-M.; Wang, Y.; Huang, R.-Y. Fluorescence Sen2+ sors for Selective Detection of Hg Ion Using a WaterSoluble Poly(vinyl alcohol) Bearing Rhodamine B Moieties. J.Fluoresc. 2014, 24, 1207–1213. (30) Geng, T.-M.; Wu, D.-Y. Water-soluble Polymeric 2+ Chemosensor for Selective Detection of Hg in Aqueous Solution Using Rhodamine-Based Modified Poly(acrylamide– acrylic acid). Luminescence 2015, 30, 1263–1268.

(38) Lunn, G.; Sansone, B. E. Destruction of Hazardous Chemicals in the Laboratory; John Wiley & Sons Publication: rd New York, 3 edition,2012. (39) Hummers, W.S.; Offeman, R.E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339-1339. (40) Zhanga, X.-F.; Li, F. Interaction of Graphene with Excited and Ground State Rhodamine Revealed by Steady State and Time Resolved Fluorescence. J. Photochem. Photobiol., A 2012, 246, 8– 15. (41) Anju, M.; Renuka, N.K. A Novel Template Free Synthetic Strategy to Graphene–Iron Oxide Nanotube Hybrid.RSC Adv. 2015, 5, 78648-78654. (42) Vieira, R.S.; Oliveira, M.M.; Guibal, E.; Castellon, E.R.; Beppu, M.M. Copper, Mercury and Chromium Adsorption on Natural and Crosslinked Chitosan Films: An XPS Investigation of Mechanism. Colloids Surf., A. 2011, 374, 108–114. (43) Kushwaha, S.; Sreedhar, B.; Padmaja, P. Sorption of Phenyl Mercury, Methyl Mercury, and Inorganic Mercury onto Chitosan and Barbital Immobilized Chitosan: Spectroscopic,Potentiometric,Kinetic, Equilibrium, and Selective Desorption Studies. J. Chem. Eng. Data. 2010, 55, 4691–4698. (44) Jin, L.; Bai, R. Mechanisms of Lead Adsorption on Chitosan/PVA Hydrogel Beads. Langmuir, 2002, 18, 9765–9770. (45) Arencibia, A.; Aguado, J.; Arsuaga, J.M. Regeneration of Thiol-Functionalized Mesostructured Silica Adsorbents of Mercury. Appl. Surf. Sci. 2010, 256, 5453–5457. (46) Erdemir, S.; Kocyigit, O.; Malkondu, S. Detection of 2+ Hg ion in Aqueous Media by New Fluorometric and Colorimetric Sensor Based on Triazole–Rhodamine. J. Photochem. Photobiol., A. 2015, 309 ,15–21.

(31) Lee, J.; Jun, H.; Kim, J. Polydiacetylene–Liposome Microarrays for Selective and Sensitive Mercury (II) Detection. Adv. Mater. 2009, 21, 3674–3677.

(47) Shafeekh, K.M.; Rahim, M.K.A.; Basheer, M.C.; Suresh, C.H.; Das, S. Highly Selective and Sensitive Colorimetric De2+ tection of Hg Ions by Unsymmetrical Squaraine Dyes. Dyes Pigm. 2013, 96, 714- 721.

(32) Cui, X.; Zhu, L.; Wu, J.; Hou, Y.; Wang, P.; Wang, Z.; Yang, M. A Fluorescent Biosensor Based on Carbon DotsLabeled Oligodeoxyribonucleotide and Graphene Oxide for Mercury (II) Detection. Biosens.Bioelectron. 2015, 63, 506– 512.

(48) Amer, A.G.; Hamid, A.A.; Al-Khateeb, M.; Tahat, Z.A.; Qudah, M.; Obeidat, S.M.; Rawashdeh, A.M. A Selective Chemosensor for Mercuric Ions Based on 4Aminothiophenol-Ruthenium (II) Bis(bipyridine) Complex. Int.J.Inorg.Chem. 2011, 2011, 1-6.

(33) Zhang, T.; Cheng, Z.; Wang, Y.; Li, Z.; Wang, C.; Li, Y.; Fang,Y. Self-Assembled 1-Octadecanethiol Monolayers on Graphene for Mercury Detection. Nano Lett. 2010, 10, 4738– 4741.

(49) Santhana Krishna Kumara, A.; Jiang, S.-J. Preparation and Characterization of Exfoliated Graphene Oxide–LCystine as an Effective Adsorbent of Hg (II) Adsorption. RSC Adv. 2015, 5, 6294-6304.

(34) An, J.H.; Park, S.J.; Kwon, O.S.; Bae, J.; Jang, J. Highperformance Flexible Graphene Aptasensor for Mercury Detection in Mussels. ACS Nano 2013, 7, 10563–10571.

(50) Luo, J.; Jiang, S.S.; Qin, S.H.; Wu, H.Q.; Wang, Y.; Jiang J.Q.; Liu, X.X. Highly Sensitive and Selective Turn-On Fluo2+ rescent Chemosensor for Hg in Pure Water Based on a Rhodamine Containing Water-Soluble Copolymer. Sens. Actuators, B 2011, 160,1191–1197.

(35) Kamali, K.Z.; Pandikumar, A.; Jayabal, S.; Ramaraj, R.; Lim, H.N.; Ong, B.H.; Bien, C.S.D.; Kee, Y.Y.; Huang, N. M. Amalgamation Based Optical and Colorimetric Sensing of Mercury (II) Ions with Silver@Graphene Oxide Nano Composite Materials. Microchim.Acta 2016,183, 369–377. (36) Chansuvarn, W.; Imyim, A. Visual and Colorimetric Detection of Mercury (II) ion Using Gold Nanoparticles Stabilized with a Dithia-Diaza Ligand. Microchim.Acta 2012,176, 57–64. (37) Lin, C.-Y.; Yu, C.-J.; Lin, Y.-H.; Tseng, W.-L. Colorimetric Sensing of Silver (I) and Mercury (II) Ions Based on an Assembly of Tween 20-Stabilized Gold Nanoparticles Anal. Chem. 2010, 82, 6830–6837.

(51) Park, K.S.; Jung, C.; Park, H.G. Illusionary Polymerase Activity Triggered by Metal Ions: Use for Molecular LogicGate Operations. Angew. Chem., Int. Ed. 2010, 49, 9757-9760. (52) Wang, L.; Yao, T.; Shi, S.; Cao, Y.; Sun, W. A label-Free 2+ Fluorescent Probe for Hg and Biothiols Based on Graphene Oxide and Ru-Complex. Sci.Rep. 2014, 4, 1-6. (53) Wang, Z.; Palacios, M.A.; Anzenbacher, P.J. Fluorescence Sensor Array for Metal Ion Detection Based on Various Coordination Chemistries: General Performance and Potential Application. Anal.Chem. 2008, 80, 7451-7459.



Table of Contents

1 .0

R 6G

0 .8 Absorbance

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R 6G +G N +H g

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0 .6 R 6G +G N

0 .4 0 .2 0 .0 500

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540

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W a v e le n g th (n m )


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Graphene-Rhodamine Nanoprobe for Colorimetric ... - ACS Publications

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