Bright Green Photoluminescence in Aminoazobenzene-Functionalized Graphene OxideAbhisek Gupta, Bikash Kumar Shaw, and Shyamal K. Saha*

Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, West Bengal, India

*S Supporting Information

ABSTRACT: Graphene oxide (GO) enriched in oxygen functionalities is limited inoptoelectronic application because of its poor optical behavior. One of the majorstrategies for developing the optical properties of GO is functionalization. Here, GOsheets are functionalized by aminoazobenzene to achieve an intercalated type structurewith interlayer separation of 9.3 Å. Bright green emission is observed in thisaminoazobenzene-functionalized GO (AAB-GO). Remarkable enhancement in photo-luminescence via surface passivation and excited-state intramolecular proton transfer isnoticed in the AAB-GO composite. Density functional theory calculations are also carriedout to investigate the stability of the modified structure along with its interlayerseparation, the results of which agree well with the experimental results. The estimatedenergy gap (∼2.73 eV) between the highest occupied molecular orbital and lowestunoccupied molecular orbital is also in agreement with the experimental results (∼2.85eV) of UV−vis absorption data.

■ INTRODUCTION

Graphene, as a novel carbon-based material discovered in 20041

obeying Dirac physics, has attracted great attention because ofits outstanding structural, mechanical, and electronic propertiesand potential applications in nanoelectronics, energy storage,and photovoltaics.2,3 However, because of its intrinsic zeroband gap and hydrophobic properties, graphene possesseslimited applications in optoelectronic and sensing devices. Inrecent years, the chemical modification or functionalization ofgraphene has been investigated intensively to modulate itselectrical, optical, catalytic, and mechanical properties andexpand its applications in the related areas.4−6 Graphene oxide(GO), a chemically modified oxygen functionalities-enrichedderivative of graphene, has received great interest7 because ofits superior dispersion ability in water and a finite electronicband gap different than that of graphene.8 According to theliterature, graphene is not a fluorescent material; however, GOpossesses a very weak fluorescence property.9 GO exhibits avery weak photoluminescence (PL) property in the visible andnear-infrared region10 because of the presence of oxygenfunctionalities, e.g., hydroxyl and epoxy groups, which usuallyinduce nonradiative recombination as a result of transfer oftheir electrons to the holes present in sp2 clusters, producingnonradiative localized electron−hole (e−h) pairs.10,11 Recently,superior luminescent graphene or functionalized graphene withblue or red emissions have been obtained using oxidation orreduction treatments.11,12 Mei et al.13 have recently reported abright blue fluorescent GO that arises from passivation ofsurface reactive sites by amide formation and ring-openingamination of epoxides. Hence, the PL property in GOoriginates from the isolated polyaromatic structures and/orpassivated surface defects prepared at different conditions.

In our previous work,14 we have carried out edge selectivefunctionalization of GO by azo-pyridine ligand which involvesthe active carbon centers of the phenolic moieties present at theedges of GO. But in this reaction, the epoxy groups on the basalplane of GO responsible for nonradiative recombination arenot involved. To overcome this drawback, in the present work,we have functionalized GO by aminoazobezene (AAB) ligandin such a manner that the diazonium cation binds to the activecarbon centers of the phenolic moieties located at the edges,and amino groups are attached to the active carbon centers ofthe epoxy moieties on the basal plane of the GO15 nanosheetsresulting in the formation of a layered type structure. It is seenthat the as-synthesized layered type AAB-GO exhibits strongand stable green fluorescence emission via surface passivationand an excited-state intramolecular proton transfer (ESIPT)process.14,16 Density functional theory (DFT) is used toinvestigate the stability of the modified structure along with itsinterlayer separation and the estimated highest occupiedmolecular orbital−lowest unoccupied molecular orbital(HOMO−LUMO) gaps are compared to the experimentalresults. Therefore, this type of modification of graphene surfacewith different functional groups will enable researchers toengineer the chemical and physical properties of graphene forfabrication of optoelectronic devices and fluorescent sensors.

■ RESULTS AND DISCUSSION

Figure 1a shows the XRD pattern for the as-synthesized AAB-GO composite. For AAB-GO composite, the main peak

Received: December 12, 2013Revised: March 10, 2014

Article

pubs.acs.org/JPCC

© XXXX American Chemical Society A dx.doi.org/10.1021/jp412156x | J. Phys. Chem. C XXXX, XXX, XXX−XXX

appears at a 2θ value of 9.5° with the usual peak at 12.3°(interlayer separation, ∼0.72 nm) corresponding to GO(Figure S1 of the Supporting Information).The major peak at a 2θ value of 9.5° indicates the

intercalated structure with an interlayer separation of 0.93nm. For AAB-GO composite, another peak appears at a 2θvalue of 23.2°, which corresponds to the multilayered graphenewith interlayer separation 0.34 nm.15 It is also to be noted thatthe composite structure synthesized in the present case is not100% pure ; rather, a fraction of GO is still present in thesample, which needs to be purified for device applications.Here, the mean crystallite thickness of the AAB-GO

composite has been estimated using the following equation:

λ θ=t K /(fwhm cos ) (1)

where t is the mean crystallite thickness, K a dimensionlessshape factor of crystallite with a value of about 0.9, λ thewavelength of the X-ray used (λ = 1.5418 Å), θ the angularposition of the peak, and fwhm the full width at half-maximumextracted from the graph (expressed in radians). The value ofthe average crystallite thickness for GO is ∼73 Å (for detailedcalculation, see the Supporting Information) and for AAB-GOcomposite is ∼54 Å. The number of layers N, considering thelattice spacing as d, is

=N t d/ (2)

Taking t as 54 Å, the number of layers for AAB-GOcomposite has been found to be ∼6.Figure 1b shows TEM images in which individual layers are

indicated by dotted curves. In the figure, 6−7 layers, asestimated from XRD analysis, are clearly identified. It is alsonoticed that the edge of one GO sheet is connected to the basalplane of another GO sheet as proposed in the mechanism.The Raman spectra of graphene and GO exhibit two main

bands. The G band corresponds to the first-order scattering ofthe E2g phonon of in-plane vibration mode of sp2 carbon atomdomains and the D band assigned to the vibrations of sp3

hybridized carbon atoms of disordered graphene, i.e., thestructural defects and partially disordered structures of the sp2

domains.17 Figure 1c shows the Raman spectra of GO andAAB-GO composite. In the Raman spectra of GO, the D bandappears at 1356 cm−1 and the G band appears at 1602 cm−1;however, for the AAB-GO composite, the G band is red-shiftedto 1588 cm−1 and the D band is shifted upward to 1363 cm−1.Such shifting of binding energies of D and G bands has alsobeen reported in some functionalized graphitic materials.18 Thistype of shifting is ascribed to the modification of graphenesurface structure caused by the additional chemical bondsbetween carbon atoms of phenolic moieties of graphene andthe functional groups. The intensity ratio of D band and Gband (ID/IG) reveals the degree of in-plane defects and edgedefects in the carbon materials.19 In the present study, the ID/IGratio of the AAB-GO composite is calculated to be ∼0.91,which is slightly smaller than that of the initial GO (∼0.93).This observation implies the increase of the domains of sp2

carbon in the AAB-GO composite.The functionalization of GO with aminoazobenzene is

confirmed by FTIR measurements. Figure 2a shows the FTIRspectra for GO and AAB-GO composite samples. For puredried GO, the presence of different types of oxygenfunctionalities is observed. The usual peaks at 3432, 1702,1628, 1400, and 1067 cm−1 correspond to hydrogen-bonded

Figure 1. (a) XRD peaks of AAB-GO composite. The main peakappears at a 2θ value of 9.5° along with the usual peak at 12.3°. (b)TEM image at different magnifications of the AAB-GO compositematerial showing a layered type structure (inset: indication of differentGO layers). (c) RAMAN spectra of GO and AAB-GO composite.

Figure 2. (a) FTIR spectra of GO and AAB-GO composite. (b) TGAprofile of GO and AAB-GO composite.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp412156x | J. Phys. Chem. C XXXX, XXX, XXX−XXXB

O−H stretching, carbonyl (CO) stretching, CC stretch-ing, O−H deformation, and C−O stretching of the epoxides(C−O−C), respectively. In the AAB-GO composite, the broadpeak in the region centered at 3430 cm−1 is due to the presenceof both the −OH and −NH groups . The benzenoid CCvibrations are observed at 1599 and 1505 cm−1. The peak at1460 cm−1 indicates the presence of the NN group, i.e., theazo group stretching vibration.20 The peaks between 1277 and1219 cm−1 are due to C−N stretching vibrations. In addition,the band centered at 1055 cm−1 represents the C−O group.The additional peak at 742 cm−1 can be ascribed to the out-of-plane deformation of aromatic C−H bond.TGA analysis has been carried out for both the GO and

AAB-GO composites, which are shown in Figure 2b. The TGAcurve of GO shows a weight loss of ∼18% below 100 °C whichis due to the evaporation of adsorbed water molecules. Thesharper mass loss of 25% between 170° and 230 °C is due tothe decomposition of labile oxygen groups, e.g., carboxylic,anhydride, or lactone groups, and the gradual mass decreaseabove 230 °C (20%) is attributed to the removal of more stableoxygen-containing functional groups, e.g., phenol, carbonyl, andquinine.21 The TGA curve of the AAB-GO composite shows asmall weight loss (∼ 9%) below 100 °C, whereas a sharp weightloss is observed between 145° and 245 °C (∼ 37%) as a resultof decomposition of the labile oxygen and nitrogen groups.Above 250 °C, the gradual decrease in mass follows the samebehavior as that of GO.To investigate the binding energies of different functional

groups in AAB-GO composite, we have performed an X-rayphotoelectron spectroscopy (XPS) study. The XPS spectrum ofAAB-GO composite (Figure 3a) shows the characteristic C 1s,N 1s, and O 1s core-level photoemission peaks at ∼285, ∼400,and ∼432 eV, respectively. From the XPS analysis, the C:O:Nratio has been found to be approximately ∼0.60:0.32:0.08,which is close to the C:N ratio (∼0.56:0.10) estimated fromelemental analysis. For GO (XPS spectra shown in Figure S2 ofthe Supporting Information), the low-range XPS spectra(Figure S2a) show the presence of only C and O 1s core-level photoemission peaks.The high-resolution carbon 1s XPS spectrum of GO (Figure

S2b of the Supporting Information) shows three peaks at 284.8,286.8, and 288.1 eV corresponding to C−C, C−O (hydroxyland epoxide), and CO (carboxyl) groups of GO, whereas thehigh-resolution carbon 1s XPS spectrum of the compositeshown in Figure 3b is conveniently fitted using fourcomponents. The lowest binding energy peak at 284.8 eV ismainly assigned to the C−C bonds of the graphitic network.The peak at 286.1 eV originates from the C−N bonds. Anotherpeak at 286.8 eV accounts for the C−O bonds, and the bindingenergy component at 288.5 eV is typically attributed to thepresence of carboxylic groups. Figure 3c represents the high-resolution nitrogen 1s XPS spectrum of the composite whichappears at ∼400 eV. This peak arises because of the combinedeffect of the C−N bonds and the azo groups (NN bonds)present in the AAB-GO composite.On the basis of the above characterizations, we propose the

formation mechanism of the layered type AAB-GO compositestructure by functionalization of GO with o- aminodiazoben-zene molecules as shown in Scheme 1. GO nanosheets possessquite a large number of phenolic and epoxy moieties at theedges and basal plane which provide some active sites forchemical reactions. Here, the in situ grown aryl amino-diazonium cation reacts easily with the active sites on GO

through the formation of carbon−nitrogen covalent link-age.14,15

To investigate the stability of AAB-GO composite we havecarried out DFT calculations. In the case of aminoazobenzenelinking with the graphene sheet, its all-chemical properties likebond length, bond angle, and stabilization energy areaccordingly changed. We estimate the total energy andmolecular orbital (MO) calculation of the optimized compositestructure with B3LYP/3-21G, 6-31G levels of theory using thestandard program in the Gaussian 03 software package. Theoptimized structure shows the energy value of −1 848 520 kcal/mol for all the basis sets shown in Figure 4. For this optimizedstructure, the two-dimensional graphene sheets are slightlydisplaced outward from the plane because of the bonding withamino and azo groups of the ligand. The bond lengths andbond angles of the optimized doped structure are summarizedin Table 1. The energy levels of the highest occupied molecularorbital (HOMO) and lowest unoccupied molecular orbital(LUMO) are −5.75 and −3.02 eV, respectively, as shown inFigure 5, and the band gap (∼2.73 eV) of these two is in closeagreement with the value obtained from experimental study.The AAB-GO composite is further subjected to optical

measurements to understand the mode of functionalization inthe composite. Figure 6a shows the UV−vis absorption spectraof pristine GO and the AAB-GO composite. GO dispersionsshow a maximum absorption at 230 nm and a shoulderbetween ∼290−300 nm, which are assigned to the π−π*transition of aromatic CC bonds and n−π* transition of the

Figure 3. (a) Low-range XPS spectra for AAB-GO composite. (b)Deconvoluted XPS peaks for C 1s in AAB-GO composite. (c) High-resolution XPS peak of N 1s of AAB-GO composite.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp412156x | J. Phys. Chem. C XXXX, XXX, XXX−XXXC

CO bonds, respectively. For o-phenylenediamine, the UV−vis absorption spectrum exhibits two bands at ∼240 and ∼293nm corresponding to π−π* and n−π * transitions, respectively.

The UV−vis absorption spectra of the AAB-GO compositecomprises two peaks at ∼260 and 433 nm which correspond tothe π−π* and n−π * transitions of the composite, respectively.

Scheme 1. Schematic of the Formation of AAB-GO Composite

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp412156x | J. Phys. Chem. C XXXX, XXX, XXX−XXXD

This observation of shifting the π−π* transition andappearance of a new peak compared to that of GO indicatesthe successful functionalization of GO by aminoazobenzene.Figure 6b shows the PL and photoluminescence excitation

(PLE) spectra of the AAB-GO composite. In the inset of Figure6b we have shown that the PL spectra of the GO solution in thevisible range display a broad weak emission band exhibitingmaximum intensity at ∼565 nm for an excitation wavelength at416 nm.14 The reasons behind the weak PL of GO werediscussed in the introduction briefly. In the present work, forthe AAB-GO composite, the PL maximum is blue-shifted to536 nm and the fluorescence intensity of the compositeincreases remarkably by 1200% with respect to GO. The PLquantum yield (see the Supporting Information for detailedcalculation) of AAB-GO composite was measured to be∼21.2% using Rhodamine 6G dissolved in ethanol as reference.It is to be noted that the reagent used in this work, i.e., o-phenylenediamine possess excitation and emission peakwavelengths at 305 and 345 nm,22 which are different fromthe present PL emission maxima. By nucleophilic substitutionreaction of amino groups, the surface passivation removes thereactive sites such as basal epoxy groups which inducenonradiative recombination of localized e−h pairs, resultingin quenching of PL and hence improves the PL emissionefficiency of the sp2 domains of GO nanosheets. This is furtherverified by the changes in the absorption and excitation spectraof GO before and after functionalization. After functionaliza-tion, the absorption band at 228 nm is red-shifted to 260 nm,

indicating the increase of sp2 domains, and a new band appearsat 433 nm with respect to GO.14 Meanwhile, the excitationspectrum of AAB-GO composite (Figure 6b) exhibits a strongband centered at ∼430 nm. This result again indicates theformation of new luminescent centers in the AAB-GOcomposite which are basically the azo groups bonded to theα-position of the phenolic −OH groups present at the edges ofGO nanosheets. The excellent PL in the composite is generatedbecause of the ESIPT between the −OH group of the phenolmoiety and the azo group. It has been reported by Alarcon et al.that the fluorescence emission for substituted hydroxylbenzaldehydes comes only from the keto form.23 The enol−azo (EA) form of the AAB-GO undergoes rapid ESIPTresulting in the tautomeric keto−hydrazo (KH) form. This KHtautomeric form exhibits bright PL. To explain the observedspectral behavior, we have demonstrated the followingsimplified scheme which involves both the keto−enoltautomers as shown in Scheme 2. The excited EA (EA*)form undergoes a fast transformation to the excited KH form(KH*), and the emission results from the KH form. The PLproperty of the AAB-GO composite can also be tuned bycontrolling the pH value of the solution. Dilute HCl and NaOHas well as buffer solutions are used to adjust the pH of thecomposite solutions. As observed in Figure 6c, the PL intensityof the composite gradually increases with increase in the [H+]ion concentration of the solution, and any change in peakpositions have not been observed with the decrease in pH. Thisobservation suggests that the tuning in pH mainly influencesthe GO surface and not the size of the nanosheets.24 This typeof pH response of the PL is related to the passivation ofradiative defects embedded in the AAB-GO composite (shownin Scheme S1 of the Supporting Information). The carboxylic,hydroxyl, amino, and azo groups of the AAB-GO composite getprotonated as well as deprotonated because of the changes inpH which causes electrostatic doping or charging in the AAB-GO composite, resulting in shifting in the Fermi level similar tothat of carboxylate single-walled carbon nanotubes(SWCNT).25 With decrease in pH, the −OH, COOH, −NH,and −NN− groups of the composite get protonated, whichresults in the destabilization of the n−π* state and stabilizationof the π−π* state. Consequently, the energy gap between thestates gets reduced, which in turn decreases the vibronicinteraction. This increases the π−π * state symmetry andthereby decreases the Franck−Condon factor, resulting in adecrease in radiationless decay which explains the bright PLintensity of the AAB-GO composite with decrease in pH.14

We have investigated the PL of the AAB-GO composite atdifferent excitation wavelengths as shown in Figure 6d. With anincrease in the excitation wavelength from 400 to 460 nm, avery small red shift in PL peak is observed but a dramaticchange in PL intensity is noticed. With change in excitationwavelengths from 400 to 430 nm, the PL intensity increases,whereas from 430 to 460 nm, the PL intensity decreases. Thisobservation is totally different from the other GO-function-

Figure 4. Optimized structure of the AAB-GO composite system.

Table 1. Bond Lengths and Bond Angles of the Optimized AAB-GO Composite Structure

optimized structure

bond length (Å) N84−N85, N103−N104 N82−C56, N97−C10 C30−C46 C11−C65 C44−C7 C5−C58 C15−C42

1.2086 1.4871 7.2216 9.2731 9.0171 9.77 9.42bond angle (deg) C50−N104−N103 C24−N85−N84

109.61 113.98

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp412156x | J. Phys. Chem. C XXXX, XXX, XXX−XXXE

alized materials and graphene quantum dots in which the PLpeaks are excitation-dependent, i.e., a major shifting in PL peakshas been observed with change in the excitation wavelengths.Figure 7a shows the concentration-dependent PL spectra of

AAB-GO composite. It has been observed that at lowerconcentration (5−50 mg/L) the PL intensity graduallyincreases with concentration, but at higher concentrations(>50 mg/L) PL intensity gradually decreases. To investigatethe photostability of AAB-GO composite, the time-dependentfluorescence intensity (Figure 7b) was measured at an intervalof 120 s over the total period of 1200 s under constant

irradiation. No significant change in PL intensity was observedduring the experiment.To obtain insight into the excitonic dynamics, we have

measured the fluorescence lifetimes of GO before and afterfunctionalization. The average lifetimes together with thecomponent decay times obtained from the fitted decay curves(see Figure S3 in the Supporting Information), are summarizedin Table 2. Average fluorescence lifetimes τ for exponentialfitting are calculated from the decay times τi and the relativeamplitudes ai using the following relation:

Figure 5. Spatial distributions of HOMO and LUMO for the optimized AAB-GO composite structure.

Figure 6. (a) UV−VIS absorption spectra of GO and AAB-GO composite. (b) PL and PLE spectra of AAB-GO composite (insets: comparison ofPL spectra of GO and AAB-GO composite and digital image of AAB-GO composite (40 mg/L) under UV light of 365 nm). (c) PL spectra of AAB-GO composite with variation of pH of the solutions. (d) PL spectra of AAB- GO composite with different excitation wavelengths.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp412156x | J. Phys. Chem. C XXXX, XXX, XXX−XXXF

∑ ∑τ τ τ⟨ ⟩ = a a/i

n

i ii

n

i i2

From Table 2, it is seen that the average lifetime of GOincreases in the AAB-GO composite, indicating the formationof a more stable excited state. It is also to be mentioned thataverage lifetime increases with increase in pH values, suggestingthat the excitons become more stabilized at higher pH thoughthe considerable decrease in PL intensity. At these higher pHvalues comparatively less fluorescent complex is produced atthe ground state because of the formation of deprotonatedgroups (O−, COO−) in the composite surface which facilitatethe nonradiative recombination of the ions to the nearby holes.This results in the decrease in radiative recombination of the

excitons with the holes, and the excitons decay through adifferent path.

■ CONCLUSIONSThis work describes a convenient approach for the preparationof intercalated layered type structure by functionalizing GOsheets with aminoazobenzene. In this composite, the GO sheetsare attached through amino and azo moieties to achieve alayered type structure with an interlayer separation of 0.93 nm.The formation of AAB-GO composite is further supported by adetailed series of structural and optical characterizations, suchas XRD; Raman spectroscopy; FTIR, TGA, and XPS analyses;and UV−vis absorption spectroscopy. Structural stability andinterlayer separation is confirmed using DFT calculation, andthe estimated HOMO−LUMO gap is in conformation with the

Scheme 2. Schematic of the ESIPT Mechanism

Figure 7. (a) PL spectra of AAB-GO composite at different concentrations. (b) Variation of PL intensities with increase in time.

Table 2. Lifetime Values of GO and AABGO (in Different pH)

system τ1 (ns) a1 τ2 (ns) a2 τ3 (ns) a3 ⟨τ⟩ (ns)

GO 1.62 11.36 0.343 42.33 0.047 46.31 0.994AABGO (pH 1) 0.811 40.35 2.837 44.68 0.093 14.97 2.40AABGO (pH 7) 0.134 0.49 2.60 73.87 4.59 25.64 3.357AABGO (pH 10) 1.367 50.40 5.566 41.79 0.146 7.81 4.586

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp412156x | J. Phys. Chem. C XXXX, XXX, XXX−XXXG

experimental results. This intercalated layered type function-alized GO composite shows superior optical properties withbright green emission via surface passivation and ESIPT. Thepresent study provides an effective method for making GOluminescent as well as expanding the applications of GO inoptoelectronic devices, visible-light-emitting diodes, and solarcells with this functionalized GO as an active material.

■ ASSOCIATED CONTENT*S Supporting InformationFurther information regarding experimental sections, quantumyield (QY) measurement, XRD of GO, calculation of meancrystallite thickness of graphene oxide, XPS of GO, decaycurves of GO and the composite, and the schematic diagram ofthe pH effect on the composite. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: cnssks@iacs.res.in. Phone: +91-33-2473 4971 (Ext227). Fax: +91-33-2473-2805.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSA.G. and B.K.S. acknowledge CSIR, New Delhi, for awarding ofa fellowship and S.K.S. acknowledges DST, Project SR/S2/CMP-0097/2012 and DST unit of Nano Science for providingXPS facilities.

■ REFERENCES(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang,Y.; Dubonos, S. V.; Grigorieva1, I. V.; Firsov, A. A. Electric Field Effectin Atomically Thin Carbon Films. Science 2004, 306, 666−669.(2) Chen, J.; Li, C.; Shi, G. Graphene Materials for ElectrochemicalCapacitors. J. Phys. Chem. Lett. 2013, 4, 1244−1253.(3) Yu, D.; Park, K.; Durstock, M.; Dai, L. Fullerene-GraftedGraphene for Efficient Bulk Heterojunction Polymer PhotovoltaicDevices. J. Phys. Chem. Lett. 2011, 2, 1113−1118.(4) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater.2007, 6, 183−191.(5) Guo, Y. J.; Deng, L.; Li, J.; Guo, S. J.; Wang, E. K.; Dong, S. J.Hemin−Graphene Hybrid Nanosheets with Intrinsic Peroxidase-likeActivity for Label-free Colorimetric Detection of Single-NucleotidePolymorphism. ACS Nano 2011, 5, 1282−1290.(6) Zhang, L. P.; Xia, Z. H. Mechanisms of Oxygen ReductionReaction on Nitrogen-Doped Graphene for Fuel Cells. J. Phys. Chem.C 2011, 115, 11170−11176.(7) Baskey, M.; Saha, S. K. A Graphite-Like Zero Gap Semiconductorwith an Interlayer Separation of 2.8 Å. Adv. Mater. 2012, 24, 1589−1593.(8) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G.Processable Aqueous Dispersions of Graphene Nanosheets. Nat.Nanotechnol. 2008, 3, 101−105.(9) Loh, K. P.; Bao, Q.; Eda, G.; Chhowalla, M. Graphene Oxide as aChemically Tunable Platform For Optical Applications. Nat. Chem.2010, 2, 1015−1024.(10) Luo, Z.; Vora, P. M.; Mele, E. J.; Johnson, A. T. C.; Kikkawa, J.M. Photoluminescence and Band Gap Modulation in Graphene Oxide.Appl. Phys. Lett. 2009, 94, 111909.(11) Eda, G.; Lin, Y.; Mattevi, C.; Yamaguchi, H.; Chen, H.; Chen, I.;Chen, C.; Chhowalla, M. Blue Photoluminescence from ChemicallyDerived Graphene Oxide. Adv. Mater. 2010, 22, 505−509.(12) Gokus, T.; Nair, R. R.; Bonetti, A.; Bohmler, M.; Lombardo, A.;Novoselov, K. S.; Geim, A. K.; Ferrari, A. C.; Hartschuh, A. Making

Graphene Luminescent by Oxygen Plasma Treatment. ACS Nano2009, 3, 3963−3968.(13) Mei, Q.; Zhang, K.; Guan, G.; Liu, B.; Wang, S.; Zhang, Z.Highly Efficient Photoluminescent Graphene Oxide with TunableSurface Properties. Chem. Commun. 2010, 46, 7319−7321.(14) Gupta, A.; Saha, S. K. Emerging Photoluminescence in Azo-pyridine Intercalated Graphene Oxide Layers. Nanoscale 2012, 4,6562−6567.(15) Alonso, M. H.; Abdala, A. A.; McAllister, M. J.; Aksay, I. A.;Prud’homme, R. K. Intercalation and Stitching of Graphite Oxide withDiaminoalkanes. Langmuir 2007, 23, 10644−10649.(16) Kwon, J. E.; Park, S. Y. Advanced Organic OptoelectronicMaterials: Harnessing Excited-State Intramolecular Proton Transfer(ESIPT) Process. Adv. Mater. 2011, 23, 3615−3642.(17) Tuinstra, F.; Koenig, J. L. Raman Spectrum of Graphite. J. Chem.Phys. 1970, 53, 1126−1130.(18) Yan, J.; Zhang, Y.; Kim, P.; Pinczulk, A. Electric Field EffectTuning of Electron-Phonon Coupling in Graphene. Phys. Rev. Lett.2007, 98, 166802.(19) Jin, Z.; Yao, J.; Kittrell, C.; Tour, J. M. Large-Scale Growth andCharacterizations of Nitrogen-Doped Monolayer Graphene Sheets.ACS Nano 2011, 5, 4112−4117.(20) Lomeda, J. R.; Doyle, C. D.; Kosynkin, D. V.; Hwang, W. F.;Tour, J. M. Diazonium Functionalization of Surfactant-WrappedChemically Converted Graphene Sheets. J. Am. Chem. Soc. 2008, 130,16201−16206.(21) Szabo, T.; Berkesi, O.; Forgo, P.; Josepovits, K.; Sanakis, Y.;Petridis, D.; Dekany, I. Evolution of Surface Functional Groups in aSeries of Progressively Oxidized Graphite Oxides. Chem. Mater. 2006,18, 2740−2749.(22) Yang, J.; Zhang, G.; Wu, X.; Huang, F.; Lin, C.; Cao, X.; Sun, L.;Ding, Y. Fluorimetric Determination of Epinephrine with o-Phenyl-enediamine. Anal. Chim. Acta 1998, 363, 105−110.(23) Alarcon, S. H.; Pagani, D.; Bacigalupo, J.; Olivieri, A. C.Spectroscopic and Semi-empirical MO Study of Substituent Effects onthe Intramolecular Proton Transfer in Anils of 2-Hydroxybenzalde-hydes. J. Mol. Struct. 1999, 475, 233−240.(24) Kochmann, S.; Hirsch, T.; Wolfbeis, O. S. The pH Dependenceof the Total Fluorescence of Graphite Oxide. J. Fluoresc. 2012, 22,849−855.(25) Zhao, W.; Song, C.; Pehrsson, P. E. Water-Soluble and OpticallypH-Sensitive Single-Walled Carbon Nanotubes from Surface Mod-ification. J. Am. Chem. Soc. 2002, 124, 12418−12419.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp412156x | J. Phys. Chem. C XXXX, XXX, XXX−XXXH