Resonance Raman Detection and Estimation in the Aqueous Phase

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Resonance Raman Detection and Estimation in the Aqueous Phase using Water Dispersible Cyclodextrin: Reduced-Graphene Oxide Sheets Bharathi Konkena, and Sukumaran Vasudevan Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac400457h • Publication Date (Web): 14 Apr 2013 Downloaded from http://pubs.acs.org on April 16, 2013

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Analytical Chemistry

Resonance Raman Detection and Estimation in the Aqueous Phase using Water Dispersible Cyclodextrin: Reduced-Graphene Oxide Sheets Bharathi Konkena and Sukumaran Vasudevan* Department of Inorganic and Physical Chemistry Indian Institute of Science, Bangalore 560012, INDIA * Author to whom correspondence may be addressed. E-mail: [email protected]. Tel: +91-80-2293-2661. Fax: +91-80-2360-1552/0683;

ACS Paragon Plus Environment

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ABSTRACT Resonance Raman spectroscopy is a powerful analytical tool for detecting and identifying analytes, but the associated strong fluorescence background severely limits the use of the technique. Here we show that by attaching

-cyclodextrin ( -CD) cavities to reduced

graphene-oxide (rGO) sheets we obtain a water dispersible material ( -CD: rGO) that combines the hydrophobicity associated with rGO with that of the cyclodextrin cavities and provides a versatile platform for resonance Raman detection. Planar aromatic and dye molecules that adsorb on the rGO domains and non-planar molecules included within the tethered -CD cavities have their fluorescence effectively quenched. We show that it is possible using the water dispersible -CD: rGO sheets to record the resonance Raman spectra of adsorbed and included organic chromophores directly in aqueous media without having to extract or deposit on a substrate. This is significant, as it allows us to identify and estimate organic analytes present in water by resonance Raman spectroscopy.


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One of the main challenges in environmental sciences is the development of analytical methodologies that enable in situ detection and estimation of organic contaminants in water, and involve only simple sample preparation and measurement procedures. Raman spectroscopy, discovered in 1928,1 is an important and powerful analytical tool whose inherent strength is its ability to provide

structural information of greater detail as

compared to fluorescence based techniques. An intrinsic hurdle, however, is that Raman signals are weak, typically ~ 10 to 14 timers weaker than fluorescence.2 Surface enhanced Raman spectroscopy (SERS)3,4 and resonance Raman spectroscopy 5,6 offer two different routes to overcome this problem, but both come with their own set of limitations. The former requires a suitable substrate and the latter suffers from fluorescence background. Resonance Raman works by exciting the analyte with incident radiation corresponding to the electronic absorption bands.7 This causes an augmentation of the emission up to a factor of 106 in comparison to non-resonant Raman and thus allows Raman spectra to be generated with sample concentrations as low as 10−8 M. This is in contrast to conventional Raman spectroscopy, which usually requires concentrations greater than 0.01 M. The main disadvantage of resonance Raman is that since the excitation coincides with UVvisible absorption, fluorescence backgrounds can be significant and more problematic than in non-resonant Raman scattering. The problem can be so severe so as to make resonance Raman detection of organic dye chromophores and poly-aromatic hydrocarbons almost impossible because the intense fluorescence background obscures the Raman signal. Graphene sheets, one-atom-thick, two-dimensional layers of carbon atoms, have gained enormous importance over the past few years due to their unique attributes - high electronic and thermal conductivities and exceptional mechanical strength. 8,9 Considerable progress has been made in new applications ranging from chemical sensors to


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transistors.10,11,12 Among the many exciting applications of graphene discovered in recent years is the use of graphene as a substrate in surface-enhanced Raman spectroscopy,13 and in resonance Raman scattering to suppress the fluorescence background.14 Enhancement factors ranging from 2-17 have been reported for planar dye molecules on single layer graphene substrates.13 In graphene enhanced Raman spectroscopy a chemical effect, rather than an electromagnetic effect, has been proposed as the explanation for the dependence of the enhancement on number of graphene layers.13 Much larger enhancements ranging from 550-750 have been reported recently by combining flat graphene sheets with Au and Ag nanostructures to produce a novel substrate.15 Chemical enhancement of Raman signals of Rhodamine 6G on mildly reduced graphene oxide has also been reported. 16 Graphene as a substrate has also been used in resonance Raman measurements by exploiting the fact that graphene sheets are efficient fluorescence quenchers

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and can

therefore be used to suppress the fluorescence background.18 These reports follow earlier studies that had shown that Raman spectra of normally fluorescent materials, like Rhodamine 6G and the near-IR laser dye IR125, could be recorded by suppressing the fluorescence through adsorption on carbon surfaces.19 The resonance Raman spectra of the dye molecules Rhodamine 6G, protoporphyrin and pthalocyanins deposited on graphene sheets have been reported.13,14 In most of these studies using graphene or reduced graphene oxide as a substrate, either in graphene enhanced Raman spectroscopy or resonance Raman measurements, the analytes have been planar molecules usually exhibiting extended

-electron conjugation. This is understandable, as it allows for

favourable stacking interactions with the flat sp2 graphene network. The other common feature of the Raman studies on graphene and graphene oxide reported so far is that the


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Amino -CD

Scheme 1 spectra were recorded in the solid-state although in many cases the analyte was deposited from solution. Recently we had reported the synthesis of water dispersible reduced graphene oxide (rGO) sheets formed by covalently linking -cyclodextrin ( -CD) cavities to the edges of the sheets via an amide bond (Scheme 1).20 Shaped like a lampshade, cyclodextrins (CDs) are water-soluble, biodegradable, cyclic oligosaccharides whose hydrophobic cavity can act as a host for a variety of non-polar organic molecules. The presence of hydrophilic – OH groups on the rim of the CD cavities renders the -CD: rGO sheets water dispersible. The functionalized -CD: rGO sheets are dispersible over a wide range of pH values (213); the aqueous dispersions being stable upward of 90 days.20 In contrast, rGO sheets are dispersible in aqueous media only above a pH value of 8. 21,22 A well-documented property of reduced graphene oxide is the presence of hydrophobic patches that correspond to the sp2 network that is restored on reduction of graphene oxide.23 This property of rGO is retained by the water dispersible -CD: rGO

sheets. In addition, as the integrity of the

cyclodextrin cavities is preserved on linkage, the host-guest chemistry of the CD cavities is also manifested by the -CD: rGO sheets. The -CD: rGO sheets combine the


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Pyrene λmax ~ 318 nm

Rhodamine-6G λmax ~522 nm

Nile Red λmax ~585 nm

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Sudan-IV λmax ~522 nm

C H N O

trans Azobenzene λmax ~318 nm

Scheme 2 hydrophobicity associated with the rGO along with the hydrophobicity of the cyclodextrin cavities in a single water dispersible material over a wide range of pH values.20 The presence of hydrophobic patches and pouches on the

-CD: rGO sheets dispersed in

aqueous media is an important consideration for the present study. Planar aromatic and dye molecules can stick to the sp2 rGO domains via favorable -stacking interactions and have their fluorescence effectively quenched while non-planar molecules that can be accommodated in the hydrophobic cavities of the anchored

-CD, too, can have their

fluorescence quenched. Here we show that it is possible using the water dispersible -CD: rGO sheets to record the resonance Raman spectra of adsorbed planar and included nonplanar organic chromophores directly in aqueous media without having to extract them or deposit on a substrate. This is significant, as it allows us to identify and quantitatively estimate the organic analytes present in water by resonance Raman spectroscopy. The molecules that we have investigated are the planar pyrene and rhodamine 6G molecules while the non-planar molecules studied are the dye Nile Red and Sudan IV and transazobenzene (Scheme 2). All these molecules exhibit intense fluorescence making resonance Raman measurements under normal circumstances rather difficult.


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EXPERIMENTAL SECTION Materials: Pyrene, rhodamine 6G, Nile Red, Sudan IV and trans-azobenzene were purchased from Sigma Aldrich and graphite powder from Alfa-Aesar

Preparation of -cyclodextrin functionalized reduced graphene oxide Graphene oxide (GO) was prepared from graphite powder by using a modified Hummers method.24 ,25 The synthesis of the -CD: rGO hybrid was carried out using 1-ethyl-3-(3Dimethylaminopropyl)-carbodimide hydrochloride (EDC) and N-hydroxy succinimide (NHS) linkage reaction for covalently attaching -CD-NH2 to the –COOH groups located at the edges of the GO sheet via amide bond formation. 20 The covalently linked

-

cyclodextrin: GO sheets were subsequently chemically reduced to give -CD:rGO sheets In a typical experiment 100mg of GO was dispersed in 100 mL of water, sonicated for 1 hour,

followed by addition of 115mg

of EDC and 35mg

of NHS. After further

sonication for 30 minutes 50mg of -CD-NH2 was added and the solution stirred for 48 hours at room temperature. After the reaction was complete, the solution was centrifuged and the precipitate washed with water. The reduced with 5.0 µL of hydrazine hydrate

-CD: GO colloid (0.05 wt %) was then at 95oC for an hour. During this time the

solution turned black signifying reduction of the GO sheets. The material was centrifuged, washed with distilled water and dried under vacuum. Measurements Raman spectra were recorded on a Horiba Jobin Yvon Raman system with a 514nm argon ion laser and a 325nm HeCd laser. Spectra were collected with a CCD detector (1024 x 256 pixels). The laser power was typically 1.5 mW for 514nm and 15mW for 325nm and the spot size was approximately 1 μm2 at the liquid sample.


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Acquisition times were typically 15 sec resulting in total scan time of ~ 10 minutes. Raman spectra of the solutions were recorded by placing a 20 µL drop spread on a glass slide. Fluoroscence spectra were recorded in quartz cuvettes on Horiba Jobin Yvon fluoromax400 spectrometer.

RESULTS AND DISCUSSIONS Reduced graphene oxide sheets were functionalized by covalently linking

-

cyclodextrin cavities to the sheets by a recently reported procedure.20 The synthesis involved linking amino - cyclodextrin to the carboxylic groups located at the edges of GO sheets via an amide bond followed by chemical reduction of the GO using hydrazine hydrate (see supporting information). 20 The concentration of the -CD cavities tethered to the edges of the rGO sheets (Scheme 1) is 0.0093 mol/g as estimated by thermogravimetric analysis. Figure 1 shows the quenching of the fluorescence of an aqueous solution of pyrene (18.6 M) on incremental addition of the water dispersible -CD: rGO. The fluorescence spectra were recorded on excitation at 325 nm corresponding to the absorption maxima of pyrene. It may be seen that addition of

-CD: rGO quenches the fluorescence, with

complete quenching occurring at ratio of 0.43 mg/ml of -CD: rGO to 1 milli mole of pyrene. Measurements at different values of pH gave similar results. The results may be understood from the fact that the non-polar pyrene molecules present in water prefer to stick to the hydrophobic sp2 domains of the

-CD: rGO sheets. Once adsorbed their

fluorescence is quenched either as a consequence of fluorescence resonance energy transfer18 or photoinduced electron transfer26 to the sp2 network of the -CD: rGO sheets.


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(a) (b)

Figure 1. (a) Fluorescence quenching studies using -CD: rGO aqueous dispersions. The fluorescence spectra of pyrene in water with increasing addition of -CD: rGO. The ratio of the weight (mg) of -CD: rGO to -moles of pyrene in the dispersion is indicated. (b) The resonance Raman spectra (

ex

= 325nm) of the dispersions at the extreme

composition ratios. The right extreme panel shows a schematic of the photo processes for pyrene in water in the presence and absence of -CD: rGO. In the presence of -CD: rGO sheets the pyrene fluorescence is quenched.

Once fluorescence is quenched it should, in principle, be possible to detect the resonance Raman signal of pyrene in the aqueous dispersion; this was indeed found to be so. The resonance Raman spectra (excitation wavelength 325nm) of the aqueous pyrene solution in the absence and on addition of -CD: rGO sheets are shown in Figure 1. In the absence of -CD: rGO the spectra is dominated by the intense pyrene fluorescence background and only the 1160 cm-1 Raman C-H bending mode of pyrene is seen. In contrast, on addition of -CD: rGO the fluorescence is quenched and the Raman spectrum of pyrene is clearly observed (the position and assignment of the Raman bands in Figure 1 are provided as part of the supporting information).


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30

20

(b)

15

0

8

12

16

20

Conc. of Pyrene ( M)

5.2 3.9 3.13 2.35 1.5 1.24 0.93

80

Intensity

16.7 15.2 13.0 11.1 9.3 7.4

30

Intensity

18.6 (a)

Intensity

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40

0 1

2

3

4

5

6

Conc. of R6G ( M)

10

0 1240

1260

1280

1300

-1

1640

1650

1660

1670 -1

Raman shift (cm )

Raman shift (cm )

Figure 2 (a) The 1255 cm-1 Raman C-H bending mode of pyrene for differing concentrations of pyrene in the aqueous solution. Each spectra were recorded after addition of 0.0625 mg of -CD: rGO per ml of the pyrene solution. The inset shows the variation of the resonance Raman (

ex

= 325nm) intensity with concentration of pyrene in

solution. (b) The 1647 cm-1 aromatic C=C stretching mode of Rhodamine 6G

for

differing concentration of the dye in solution. Each spectra were recorded after addition of 0.0625 mg of -CD: rGO per ml of the Rhodamine 6G solution. The inset shows the variation of the resonance Raman (

ex

= 514 nm) intensity with concentration of

Rhodamine 6G in solution. The concentrations ( M) are indicated in the individual panels.

It may be emphasized that the resonance Raman spectra shown in Figure 1 were recorded for pyrene molecules in the aqueous dispersion without having to adsorb or deposit them on a solid substrate. It was found that once the pyrene fluorescence was quenched on addition of -CD: rGO the intensity of the pyrene vibrational bands in the resonance Raman spectra showed a linear dependence on the concentration of pyrene in solution. Figure 2a shows the 1255 cm-1 C-H bending band of pyrene for different


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concentration of pyrene in solution. The spectra were recorded on addition of a fixed volume (0.0625 mg/ml) of

-CD: rGO dispersed in water for each ml of the pyrene

solutions. The intensity of the 1255 cm-1 shows a linear variation with the concentration of pyrene in solution. In the present experimental setup the detection limit for pyrene in solution was 7 M. The other Raman bands of pyrene, too, show a linear variation with pyrene concentration in solution (see supporting information). Figure 2 clearly shows that it is possible to use the resonance Raman spectra, recorded on addition of -CD: rGO to the solution, as a quantitative analytical tool to estimate trace amounts of pyrene or other polyaromatic hydrocarbons that may be present in water. These observations are not limited to pyrene, Rhodamine 6G dye molecules in solution too have their fluorescence quenched on addition of the water dispersible -CD: rGO sheets to the solution. The quenching is a consequence of the non-polar Rhodamine 6G dye molecules sticking to the hydrophobic sp2 domains of the

-CD: rGO sheets by

adopting a flat planar geometry. Once fluorescence is quenched the resonance Raman spectra of the solution can be recorded (see supporting information) and as in the case of pyrene the Raman intensities show a linear variation with concentration of Rhodamine 6G in solution. The Raman bands of rGO are unaffected by the prescence of the analyte (see supporting information). Figure 2b show the variation in the intensity of 1647 cm-1 aromatic C=C Raman stretching mode of Rhodamine 6G with the concentration of the dye in solution. The spectra were recorded on addition of 0.0625 mg of -CD: rGO per each ml of the Rhodamine 6G solutions, using a 514 nm laser for excitation; the absorption maxima of Rhodamine 6G is 522 nm. Under non-resonant conditions the Raman signals are not detectable. The intensity of the 1647 cm-1 shows a linear variation with


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(a)

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(b)

Figure 3. (a) The fluorescence spectra of Nile Red in water with increasing addition of -CD: rGO. The molar ratio of tethered -CD cavities to Nile Red is indicated. (b) The resonance Raman spectra (

ex

= 514 nm) of the dispersions at the extreme composition

ratios. The right extreme panel shows a schematic of the photo processes for Nile Red in water in the presence and absence of

-CD: rGO. In the presence of -CD: rGO sheets

Nile Red molecules are included in the tethered

-CD cavities and their fluorescence

quenched.

concentration of Rhodamine 6G in solution (Figure 2b) suggesting that the technique can provide quantitative information of the concentration of the dye in aqueous solutions. In the present setup the detection limit for Rhodamine 6G in aqueous solutions is 1 M. The host-guest chemistry of the

-CD cavities tethered to the edges of the water

dispersible -CD: rGO sheets can also be exploited. Fluorescing non-polar molecules that can form inclusion complexes with the hydrophobic -CD would have their fluorescence effectively quenched by proximity to the sp2 rGO domains of the -CD: rGO sheets.20 The


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resonance Raman spectra of the included molecule could then, in principle, be recorded. The inclusion complexes of -CD are, of course, not restricted to planar guest molecules unlike the molecules that stick directly to the sp2 domains of rGO. This is important for it extends the range of molecules whose resonance Raman spectra can be detected in aqueous media. The fluorescence spectra of an aqueous solution of Nile Red on incremental addition of the water dispersible -CD: rGO to the solution is shown in Figure 3a. On addition of -CD: rGO the fluorescence is quenched with complete quenching occurring at a ratio of 3.7×10-5 milli moles of anchored -CD in -CD: rGO to 3.5 ×10-5 milli moles of Nile Red. This concentration corresponds to a molar ratio of Nile Red to tethered cyclodextrin cavities of unity. The resonance Raman spectra (excitation wavelength 514 nm) of the aqueous Nile Red solution in the absence and on addition of CD: rGO sheets are shown in Figure 3b. In the absence of -CD: rGO the spectra are dominated by the intense Nile Red fluorescence background and no Raman scattering modes are observed. However, once fluorescence is quenched by addition of -CD: rGO the resonance Raman spectrum of Nile Red in solution is clearly seen. The position and assignment of the Raman modes of Nile Red in Figure 3b are provided as part of the supporting information. The results may be understood from the fact that on addition of the

-CD: rGO dispersed in water to the aqueous solution, the nonpolar Nile Red

molecules prefer to locate in the hydrophobic interior of the tethered -CD cavities. On inclusion the fluorescence of Nile Red is effectively quenched and the resonance Raman signal detected. As in the earlier examples the intensity of the Raman modes of Nile Red increases linearly with concentration of Nile Red in solution. Figure 4a shows the intensity of 1489


13


140

(a) Intensity

7.5 6.5 5.5 4.5 3.5 2.5

(b)

150

75

2 3 4 5 6 7 Conc. of Nile Red ( M)

160

5.85 5 4.29 3.51 2.7 1.9

Intensity

210

Intensity

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80

2 3 4 5 6 Conc. of Sudan IV ( M)

70

0 1480

1500

1520 -1

Raman shift (cm )

1470

1500

-1

1530

Raman shift (cm )

Figure 4 (a) The 1489 cm-1 Raman mode of Nile Red for differing concentrations of Nile Red in the aqueous solution. Each spectra were recorded after addition of 0.0625 mg of -CD: rGO per ml of the Nile Red solution.

The inset shows the variation of the

resonance Raman ( ex = 514 nm) intensity with concentration of Nile Red in solution. (b) The 1484 cm-1 ring dilation Raman mode of Sudan Red for differing concentration of the dye in solution. Each spectra were recorded after addition of 0.0625 mg of -CD: rGO per ml of the Sudan Red solution. The inset shows the variation of the resonance Raman ( ex = 514 nm) intensity with concentration of Sudan Red in solution. The concentrations ( M) are indicated in the individual panels. cm-1 mode of Nile Red at different concentrations of the Nile Red. The spectra were recorded on addition of 0.0625 mg of -CD: rGO per each ml of the Nile Red solutions. The Raman spectra were measured using a 514 nm laser for excitation; the absorption maximum of Nile Red is 585 nm. For all concentrations the Nile Red fluorescence is almost completely quenched by the -CD: rGO. The linear variation of intensity clearly


14

200

(b) 150

100

60

34.4 28.8 24.7 20.6 16.5 12.3 8.2

20

Intensity

(a)

(c) Raman Intensity

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15 10

14 5 10

20

30

Conc.of AB ( M) 7

40

0 500

1000

1500

2000

Raman shift (cm-1)

1560

1600

1640

1680

1720

Raman shift (cm-1)

Figure 5. (a) The fluorescence spectra of trans azobenzene in water with increasing addition of

-CD: rGO. The molar ratio of tethered

indicated. (b) The resonance Raman spectra (

ex

-CD cavities to azobenzene is

= 325 nm) of the dispersions at the

extreme composition ratios. (c) The 1612 cm-1 aromatic C-C stretching Raman mode of trans azobenzene for differing concentrations of trans azobenzne in the aqueous solution. The concentrations ( M) are indicated. The spectra were recorded after addition of 0.0625 mg of -CD: rGO per each ml of the azobenzene solutions. The inset shows the variation of the resonance Raman intensity with concentration of azobenzene.

suggests its potential as an analytical tool. In the present experimental setup the detection limit for Nile Red in aqueous solution was 2.5 M. Similar experiments were carried out with aqueous solutions of the dye Sudan IV, a strongly fluorescing chromophore whose resonance Raman spectra is generally impossible to record. On addition of the water dispersible -CD: rGO the Sudan IV molecules form inclusion complexes with the tethered -CD cavities. The fluorescence of the included dye is effectively quenched by its close proximity to the rGO sp2 domains and it resonance


15

Intensity

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Raman spectra can be recorded (see supporting information). Figure 4b shows the variation of the intensity of 1484 cm-1 ring dilation vibration mode in the resonance Raman spectra (excitation 514nm; Sudan Red absorption maxima 522 nm)

with increasing

concentration of the dye in solution after addition of -CD: rGO to the solution. The intensity varies linearly with concentration; the detection limit was 1.9 M. The resonance Raman spectra of aqueous solution of other fluorescing dye molecules that have their absorption maxima at different wavelengths, too, can be recorded, provided they can form inclusion complexes with the tethered -CD cavities in -CD: rGO. The fluorescence and resonance Raman spectra of trans azobenzene in water on excitation at 325nm is shown in Figure 5. It may be seen that it is only on addition of the

-CD: rGO

sheets to the solution that the resonance Raman signal can be detected (Figure 5b). As in the previous examples the intensity of the resonance Raman signal varies linearly with concentration of azobenzene in the solution. The detection limit for azobenzene in water using the present experimental setup is 8.2 M. In conclusion we have shown that the water dispersible -CD: rGO sheets, formed by covalently linking - cyclodextrin cavities to the edges of reduced graphene oxide sheets, via an amide linkage, are a versatile platform for resonance Raman detection of organic analytes directly in the aqueous phase. We do so by exploiting the presence of both hydrophobic patches and cavities in the water dispersible sheets. The former corresponds to the sp2 network that is partially restored on chemical reduction of GO and the latter to the interior of the tethered -CD cavities. On addition of the water dispersible -CD: rGO sheets non-polar molecules that may be present in the aqueous media would, depending on their size and geometry, either adsorb on the hydrophobic patches or be included within


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the -CD cavities. Molecules that adopt a flat geometry would adsorb on the sp2 domains of the rGO sheets while non-planar molecules

would

be sequestered wthin the -CD

cavities. In either situation the fluorescence of the organic molecule is quenched and its resonance Raman signal detectable. The two more

significant features of the results

reported here, as compared to earlier reports, is the fact that the resonance Raman spectra is recorded directly in the aqueous media without having to deposit or extract the organic analyte and secondly it is not limited to planar molecules as there are no such restriction, except size considerations, for molecules to be included within the cyclodextrin cavity. While the later extends the range of molecules that can be detected, the former allows for a straightforward procedure for quantitative estimation. We have shown here for a number of different analytes that the intensity of the resonance Raman signal varies linearly with concentration of the analyte in solution. In the present experimental setup the detectin limit was typically less than 10 M. The method reported here has the potential for impact in environmental investigations for identifying and estimating organic pollutants in water. Current procedures usually require either extraction or partitioning of the organic into a non-polar solvent or by adsorption on to a hydrophobic column and subsequent estimation by conventional analytical techniques. The procedures are involved and not usually amenable for modification for field studies. The use of the water dispersible

-CD: rGO sheets as

platform for resonance Raman detection circumvents the problem of extraction as it allows for the organic molecules to be identified and estimated directly in the aqueous solution. The adventage of portable Raman spectrometers with multiple excitation wavelengths and increasing detector sensitivity could make this the method of choice in field studies.


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ASSOCIATED CONTENT Supporting Information: (S1) Resonance Raman spectra of aqueous Pyrene solutions of differing concentrations on addition of

-CD: rGO. (S2) Resonance Raman spectra of

aqueous Rhodamine-6G solutions of differing concentrations on addition of

-CD: rGO.

(S3) Resonance Raman spectra of aqueous Nile Red solutions of differing concentrations on addition of

-CD: rGO. (S4) Resonance Raman spectra of aqueous Sudan Red

solutions of differing concentrations on addition of

-CD: rGO. (S5) Resonance Raman

spectra of aqueous azobenzene solutions of differing concentrations on addition of

-

CD: rGO. (S6) Comparison of the resonance Raman spectra of -CD: rGO in the absence and presence of dye molecules. This material is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: +91-80-2293-2661. Fax: +91-80-2360-1552/0683. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We thank the Centre for Nano Science and Engineering (CeNSE), IISc, for help with the Raman measurements.

REFERENCES (1) Raman, C.V.; Krishnan, K.S. Nature 1928, 121, 501.


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TOC Graphic

Water dispersible cyclodextrin functionalized graphene sheets are shown to be a versatile platform for resonance Raman detection in aqueous media. Organic molecules in water can be identified and estimated without the need to extract or deposit on a substrate.


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Resonance Raman Detection and Estimation in the Aqueous Phase

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