Journal ofMaterials Chemistry A

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aCenter for Superfunctional Materials, Depa

Science and Technology, Hyojadong, Namgu

postech.ac.kr; jnt_tiw123@yahoo.co.inbDepartment of Chemistry, Inha University,

Korea

† Electronic supplementary informationelectrostatic conditions, morphology, re-dconductivity of hybrid lms. See DOI: 10.

Cite this: J. Mater. Chem. A, 2013, 1,12900

Received 15th July 2013Accepted 12th August 2013

DOI: 10.1039/c3ta12735d

www.rsc.org/MaterialsA

12900 | J. Mater. Chem. A, 2013, 1, 1

Solution-processable conductive micro-hydrogels ofnanoparticle/graphene platelets produced by reversibleself-assembly and aqueous exfoliation†

Nhien H. Le,a Humaira Seema,a K. Christian Kemp,a Nisar Ahmed,a

Jitendra N. Tiwari,*a Sungjin Parkb and Kwang S. Kim*a

Preventing the p–p restacking of graphene-based platelets is essential to advance their fundamental

attributes in a wide range of scalable chemical processes. Using macroscopic hydrogels of water-

intercalated metal-oxide/graphene platelets is a novel approach to produce microscopic hydrogels with

extraordinary surface accessibility and electronic properties. Nanoparticle decoration and surface

hydration prevent irreversible p–p stacking, paving the way for reversible self-assembly and aqueous-

phase exfoliation. The hydrophilic nanoparticle coating facilitates the colloidal stability of hybrid

microgels in aqueous and organic media without the assistance of surfactants. This allows these

materials to versatilely function as basic building blocks as well as applied nanomaterials in wet-

chemistry applications. The preservation of unique properties of SnO2-decorated graphene platelets

leads to significantly enhanced adsorptive and photocatalytic activities. By exploiting the fluorescence

quenching effect, a dye–hydrogel complex can be utilized as a supramolecular sensor for sensitive DNA

detection. This study also initiates an innovative synthetic strategy to synthesize high-quality graphene-

based nanomaterials.

Introduction

Graphene oxide (G-O) derived from chemical modication ofgraphite is recognized as a molecular two dimensional (2D)platform as well as a nanoscale building block for synthesizinggraphene-based materials due to its atomically thin structurewith multiple functionalities for interactional complemen-tarity.1–3 The restoration of the p-conjugated system and elec-trical conductivity of the graphene precursor G-O can beachieved by reducing the oxidized platelets to chemically con-verted graphene (CCG). When semiconductor nanoparticles areincorporated into CCG nanoplatelets, the resulting hybridsshow enhanced photo-electronic properties due to the syner-gistic coupling effect.4 However, aer the functionalization, dueto hydrophobic stacking some key features of CCG-basedplatelets are not retained in the reduced form, limiting theirperformance and applicability in further processing.5–7 Thespecic surface area of CCG-based materials is usually muchsmaller than the extremely high value of pristine graphene or

rtment of Chemistry, Pohang University of

, Pohang 790-784, Korea. E-mail: kim@

100 Inha-ro, Nam-gu, Incheon 402-751,

(ESI) available: Characterization ofispersibility, microscopy and electrical1039/c3ta12735d

2900–12908

fully exfoliated CCG nanoplatelets.5,8,9 Hydrophobic agglomer-ation, irreversible interlayer van der Waals stacking and p–p

interactions among aromatic p-systems can strongly inuencethe material's interactional surface and electronic proper-ties.7,10,11 Therefore, the prevention of restacking in the post-functionalization stage can preserve unique qualities ofCCG-based nanoplatelets, making subsequent engineeringmore feasible.

In wet-chemistry processes, a simple liquid exfoliation oflayered materials is a technically processable approach toobtain 2D nanoplatelets.12 In the layered structure of graphiteoxide, plentiful hydrophilic oxygenated groups on the G-Osurface allow the penetration of water molecules from aqueousenvironments into the interlayer galleries, leading to the easyultrasonic exfoliation in water.6,8 From the nature of theaqueous-exfoliation mechanism, water intercalation is highlybenecial for the processability in aqueous media. Hence,spontaneous organization of functionalized CCG nanoplateletsinto a water-intercalated 3D architecture could be an essentialsolution to preserve the fundamental nanoplatelet properties.

Chemical strategies have been applied to produce 3Dassemblies of CCG-based nanoplatelets.13 Although assembledmacrostructures of CCG-based hydrogels facilitate the accessi-bility of the exposed surface and the translocation of chemicalsinto the porous scaffolds, the nanoscale re-dispersibility of theagglomerated nanoplatelets in CCG-based macrostructures stillhas not been addressed,14–18 leaving the surface utilization and

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processability in reaction media of the functionalized nano-materials as challenges. By taking these notions into account,three dimensional (3D) architectures with stacking-preventingspacers might allow ultrasonic exfoliation into micro-assem-blies with signicant enhancements of surface reaction sites,electronic properties and processability in aqueous media.

To mimic the exfoliation mechanism of graphite oxide,sonochemically converted SnO2-decorated CCG nanoplateletswere induced to self-assemble into self-preserving 3D macro-hydrogels by chemical reduction in water at a low temperatureand atmospheric pressure. The conductive platform of SnO2/CCG hybrids with superior interfacial electron transfer andeffective charge separation is applicable to photocatalyticdegradation, solar light harvesting, electrochemical sensing,and energy storage.19–22 Hydrophilic coating of SnO2 nano-spacers plays an important role in retaining surface hydration, abio-inspired approach to prevent p–p stacking.7 Consequently,the water-intercalated architecture of a macroscopic nano-particle/CCG hydrogel (macronanogel) can be ultrasonicallyexfoliated into microscopic nanoparticle/CCG hydrogels(micronanogels). The resulting micronanogels exhibited goodcolloidal stability in aqueous and organic solvents, highly effi-cient dye adsorption and uorescence quenching effect, leadingto enhanced performances in water purication and uores-cence biosensing.

ExperimentalPreparation of G-O

Natural graphite powder was oxidized via acid-oxidationaccording to a modied Hummers method reported in ourprevious papers.23,24 In brief, a mixture of graphite, sodiumnitrate, sulphuric acid and potassium permanganate was stir-red for 1 day before the addition of water which resulted in anincrease in temperature to 98 �C. Aer the reaction had cooled,hydrogen peroxide was added to the suspension, followed bystirring for 5 days. Then the yellowish graphite oxide waswashed with 5%HCl and deionized water until a neutral pH wasobtained. The as-prepared material was further puried usingan ion exchange process. An exfoliated G-O suspension wasprepared by ultrasonic exfoliation of graphite oxide for 2 hours,followed by centrifugation at 6000 rpm for 5 minutes.

Syntheses of macronanogel and micronanogel

In a typical synthesis of sonochemically converted SnO2/CCGnanoplatelets, a 50 mL G-O suspension (1 mg mL�1) was mixedwith a 20 mL acidic SnCl2 solution (2.64� 10�2 M). The mixturewas stirred for 1 day and then sonicated for 2 hours so as tonucleate SnO2 nanoparticles onto the in situ reduced G-Osheets. The pH of the suspension was adjusted using aqueousammonium hydroxide (pH � 10). Sodium ascorbate was addedinto the suspension, and the reduction reaction was acceleratedby heating the mixture at 90 �C for 90 minutes without stirring.The resulting macronanogel was collected by ltration andwashed with ethanol and deionized water. The aqueous-phaseexfoliation of themacronanogel was conducted using ultrasonic

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cavitation to produce micronanogels in aqueous suspensions.The dispersion of micronanogels was ltered to collect thenanohybrid, which was then oven-dried at 45 �C. To investigatethe dehydration effect on the morphology and colloidalstability, thermal annealing at 200 �C for 4 hours was applied toproduce dry SnO2/CCG nanocomposite powder (drypowder).

Organic-dye adsorption and photodegradation

The organic dyes MB and RhB were used in water puricationstudies. Typically, a 5 mL suspension of the micronanogels wasadded to a 50 mL solution of dyes (0.01 g L�1). The dispersionwas kept in the dark for 1 hour followed by ltration to extractthe liquid for UV-visible spectroscopy. A spectrometer was usedto measure the concentration of dye in the extracted solutions,which was determined at l¼ 665 nm for MB and l¼ 555 nm forRhB. The collected solid was washed with water and ethanol toremove the adsorbed dyes. The rinsed adsorbents were reusedin the next adsorption cycle. Photocatalytic performances of theSnO2/CCG micronanogel and drypowder were evaluated bymonitoring the photocatalytic degradation of MB. 5 mL catalystsuspensions were added to 40 mL of MB solutions (0.01 g L�1).The suspensions were le in the dark for 1 hour to reach theequilibrium point. Then, they were exposed to visible-lightradiation. A 500 W Xe lamp was used as a light source equippedwith a 420 nm cut-off lter. The distance between the lamp andthe solution is about 10 cm. To determine the concentration ofthe remaining MB, 2 mL of the suspensions was extracted atgiven intervals and centrifuged for 5 minutes to remove thecatalyst. UV-visible absorption of the extracted solutions wasthen measured to determine the dye concentrations.

Fluorescence quenching and biomolecular sensing

A 20 mL aqueous solution of RhB (2.09 � 10�6 M) was preparedas a uorescent solution. Macronanogels of SnO2/CCG wererinsed with aqueous ammonium hydroxide and deionizedwater, followed by re-dispersion in water to form a homoge-neous suspension (1 mg L�1). A 10 mL aqueous suspension ofmicronanogels was added to the dye solution. Aer 30 minutesunder static conditions, the mixture reached the adsorptionequilibrium point to form a solution of a dye–hydrogel complex.The dye–hydrogel complex was used to measure UV-visibleadsorption and uorescent emission to conrm the chargetransfer and the uorescence quenching effect. Solutions ofbiomolecules with known concentrations were prepared.Concentrations of DNA and RNA in solutions were determinedby UV-visible spectroscopy. In typical experiments of sensing,0.95 mL aqueous solutions of biomolecules were introducedinto the 3 mL sensing solution of the dye–hydrogel complex toobtain solutions with 25 nM of the biomolecules. The solutionswere then analyzed by uorescence emission spectroscopy.

Materials characterization

X-ray diffraction (XRD) patterns were recorded on a Riguka,Japan, RINT 2500 V X-ray diffraction-meter using Cu Ka irra-diation (l ¼ 1.5406 A). Raman spectra were taken using a Sen-terra Bruker Raman microscope-spectrometer with 532 nm

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wavelength incident laser light and a power of 20 mW. Fouriertransform infrared (FTIR) spectra were recorded in KBr pelletswith a Bruker FTIR. X-ray photoelectron spectroscopy (XPS)analyses were performed with an Escalab-220I-XL (Thermo-Electron, VG Company) device. Photoemission was stimulatedby a monochromated Al Ka source (1486.6 eV). Scanning elec-tron microscopy (SEM) images were taken on a eld emissionscanning electron microscope (FESEM, JEOL, FEG-XL 30S). Toprepare SEM samples, suspensions of the materials in ethanolwere dropped onto SiO2 wafers. The samples were dried underambient conditions before SEM imaging. High-resolutiontransmission electron microscopy (TEM) analyses were con-ducted with a HR-STEM-I system (2100F with Cs CorrectedSTEM) with an accelerating voltage of 200 kV. To prepare TEMsamples, suspensions of materials in ethanol were droppedonto carbon-coated Cu grids, followed by air-drying before TEManalysis. Surface area measurements were carried out using aBelsorp mini II (Japan) facility. Before each measurement, the

Fig. 1 Schematic representation for the fabrication of microscopic nanohybrid hydSEM/TEM images of the micronanogels. The reversible self-assembly allows aqueouThe digital photographs show the macronanogel and the colloidal suspension of thebased microstructures of micronanogels were visualized using SEM (c). The high-resoand well-defined SnO2 nanoparticles on the CCG platform (f).

12902 | J. Mater. Chem. A, 2013, 1, 12900–12908

samples were heat-treated at 200 �C in a vacuum for 4 hours.BET surface areas were calculated from N2 adsorption–desorp-tion isotherms measured at 77 K. A UV-VIS spectrophotometerS-3100 was used to analyze absorption spectra. Fluorescenceemission was measured using a spectrouorophotometer RF-5301 with excitation at 555 nm. The sheet resistance wasdetermined using a four-point probe measuring instrument bya dual conguration method (DASOL ENG). The SnO2/CCG lmwas dried and then annealed at 200 �C for 1 hour and then cutinto 7 � 7 mm pieces. The electrical resistance was measuredusing the four-point probe measuring instrument ve times toobtain the average value.

Results and discussion

Well-dispersed SnO2-decorated CCG platelets in water weresynthesized by chemical impregnation and a sonochemicalmethod. The self-organization of the hybrid platelets resulted in

rogels, the self-preserving assembly process and the resulting suspension, and thes exfoliation of the macronanogel into micronanogels in colloidal suspension (a).micronanogels with the Tyndall scattering effect (b). The SnO2/CCG-nanoplatelet-lution TEM images show the hybrid nanoplatelets of the micronanogels (d and e)

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Fig. 2 Structural characterization and nanoarchitecture of the SnO2/CCGnanocomposites. (a and b) High-resolution TEM images of the sonochemicallyconverted SnO2/CCG platelets. (c and d) SEM images of the dehydrated micro-nanogels. (e and f) High-resolution TEM images of the dehydratedmicronanogels.

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a self-preserving 3D architecture, which can be exfoliated bysonication (Fig. 1). To prepare sonochemically converted SnO2/CCG platelets, SnCl2 was used as not only a precursor of SnO2

nanoparticles but also a reducing agent for G-O.25 Reactiveoxygen-based functional groups on the surface of G-O areenergetically favorable nucleation sites for growing SnO2

nanoparticles.26 Sonochemistry has been applied to supplythermal and mechanical energy to fabricate nanostructuredinorganic materials through acoustic cavitation bubblecollapses.27,28 The mixture of G-O and stannous ions wasexposed to ultrasonic irradiation that can accelerate theformation of SnO2 nanocrystals on G-O platforms and dispersethe suspension simultaneously.

In the controlled self-assembly, an aqueous ammoniumhydroxide solution and sodium ascorbate were added to theobtained SnO2/CCG suspension. The colloidal conditions,especially the ionization degree, are signicantly inuential tothe formation of agglomerated graphene-based structures.29

Ammonium hydroxide solution was used to adjust the pH toaround 10, which helps maximize the degree of ionization. Astable colloidal suspension of the nanohybrid was achieved dueto electrostatic repulsion (ESI, Fig. S1†). Then sodium ascorbatewas used as an environmentally friendly and effective reducingagent for producing 3D CCG architecture.15,30 The self-assemblyof sonochemically converted SnO2/CCG nanoplatelets occurredin a controlled manner to form a macroscopic hybrid hydrogel.

While self-assembled graphene-based hydrogels prepared bya chemical reduction or hydrothermal method have beenpreviously shown to be strongly interconnected and mechan-ically exible,15,16,29,31 the as-prepared hybrid hydrogel possessesinteresting advantages at the interfacial sites. SnO2 nanocrystalsdeposited on CCG play a role as nanospacers that effectivelyhinder the hydrophobic p–p restacking of CCG nanoplatelets.Water layers derived from the surface hydration of SnO2-deco-rated CCG nanoplatelets can prevent the cross-linking of thenanoplatelets.7,18

Consequently, the inner structure of the hybrid hydrogelmight be a loosely packed 3D scaffold, making the macro-structure available for ultrasonic architectural manipulation.

An aqueous suspension of the micro-assemblies of SnO2/CCG nanoplatelets was achieved by ultrasonic treatment of themacronanogels and the resulting suspension of micronanogelswas stable for more than three weeks. As shown in SEM andTEM images in Fig. 1, the exfoliated micronanogels are micro-assemblies of hybrid nanoplatelets, opening the accessibility ofthe electronically synergistic SnO2/CCG surface (ESI, Fig. S2 andS3†). Structures of the sonochemically converted SnO2/CCGnanoplatelets were analyzed using a high-resolution TEM. InFig. 2, the rough surface of nanoplatelets could be attributed tothe uniform distribution of nucleation sites of SnO2 crystals(Fig. 2a). Well-dened crystalline lattices of SnO2 on the CCGlayer in the TEM image (Fig. 2b) further conrm the in situformation of sonochemically converted SnO2/CCGnanoplatelets.

When the hydrogels were collected by ltration and slowevaporation at 45 �C, the nanoplatelets could undergo therestacking that enlarges hybrid aggregates. The SEM images

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(Fig. 2c and d) show the porousmorphology of thematerial aerdrying. Due to the nature of the high water content, thehydrogels encapsulate water in the 3D structure, and subse-quent drying could induce spontaneous organization of plate-lets, which created the porous surface.14 High-resolution TEMimages (Fig. 2e and f) show uniform decoration of SnO2 nano-particles on CCG platelets. Well-dened nanocrystals of SnO2

have particle sizes around 5 nm determined using a high-resolution TEM. Further calcination of the nanocomposite (at200 �C) was applied as a dehydration process to produce drySnO2/CCG nanocomposite powder (drypowder). Aer thethermal annealing, the porous morphology remained in well-arranged 3D structures (ESI, Fig. S4†).

In the FTIR spectrum (Fig. 3a), partially reduced SnO2/CCGplatelets aer the sonochemical reaction still contain severaloxygen functionalities (3150 cm�1 for the hydroxyl groups and1406 cm�1 for carboxylate symmetric stretch),32 thereby makingthe SnO2/CCG platelets negatively charged in order to electro-statically repel each other in the basic solution. The peaks in theFTIR spectrum of micronanogels can be assigned to SnO2

nanoparticles (500–600 cm�1) and oxygen functional groups ofCCG and bonded ascorbate (700–1700 cm�1).33,34 The presenceof SnO2 nanoparticles in the obtained materials was furthercharacterized by XRD, Raman spectroscopy and XPS (Fig. 3b–d).

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Fig. 3 XRD spectra (a) and Raman spectra (b) of the CCG hydrogel (black curve) and micronanogels (red curve). (c) FTIR spectra of the sonochemically converted SnO2/CCG nanocomposite (black curve) and micronanogels (red curve). XPS spectra of the micronanogels. (d) Survey of Sn 3d. (e) C 1s. (f) O 1s.

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The Sn3d5/2 and Sn3d3/2 peaks in the Sn 3d core level XPSspectrum conrm the presence of Sn species in the composite.XRD investigations also conrmed the presence of SnO2 nano-particles in the composite material (Fig. 3b). From the Ramanspectra, the bands around 475–775 cm�1 are derived from thedisorder activated surface modes of SnO2, and the strong peakat 620 cm�1 discloses the nanometric size of the SnO2 crystals.35

To clarify the architectural formation and stackinghindrance, control reactions in the absence of either the SnCl2precursor or the sodium ascorbate as the reducing agent wereconducted. Compared to the main fabrication, the controlreactions required a longer heating time (3 hours) to induce theagglomeration of reduced G-O in aqueous solutions (ESI,Fig. S5†). Hence, in the principal synthesis, we believe thatsodium ascorbate accelerates the reduction of CCG and createshydrogen-bonding interactions among the hydrated nano-platelet surfaces. Water evaporation in the drying processresulted in pore formation in the control samples; however, the

Fig. 4 Colloidal stability of SnO2/CCG hydrogels in aqueous and organic solvents. Sweeks, from left to right: chloroform, hexane, xylene, tetrahydrofuran, dimethyl sul

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degree of self-assembly might inuence their porousmorphology (ESI, Fig. S6†). Importantly, in the absence ofnanoparticle decoration, CCG platelets were strongly agglom-erated, preventing the re-dispersion of macronanogels byultrasonic irradiation (ESI, Fig. S7†).

In the investigation of colloidal properties, the macro-nanogels were ultrasonically re-dispersed in various aqueousand organic media. Due to the loose interconnection of thewater-impregnated 3D macrostructure, the ultrasonic re-dispersion proceeded easily in water and in other organicsolvents like ethanol, acetone, acetonitrile and N,N-dime-thylformamide. In contrast, the hydrated micro-hydrogels werenot miscible in non-polar solvents like xylene, toluene, chloro-form and hexane. The homogeneous suspensions in tetrahydro-furan and dimethyl sulfoxide were stable over a short period andshowed visible aggregate formation aer one day. More impor-tantly, homogeneous suspensions of micronanogels in water,ethanol, acetone, acetonitrile and N,N-dimethylformamide were

table homogeneous suspensions of micronanogels in various solvents after threefoxide, N,N-dimethylformamide, acetonitrile, acetone, ethanol and water.

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stable even aer three weeks (Fig. 4). While recent approacheshave enabled us to prepare homogeneous CCG suspensions inwater or organic solvents without the use of surfactants bycontrolling pH, sulfonating graphene or adjusting the solventsolubility,36–38 the as-synthesized hydrated micro-assemblies aredispersible into stable colloids with controllable concentrationsin various media. We found that the degree of self-assembly aswell as structural porosity inuence the colloidal stability of themicrostructures. SnO2/CCG scaffolds with lower porous organi-zation are less likely to be stable in aqueous suspensions.

The effective surface area of the exfoliated micronanogelswas measured using the methylene blue (MB) adsorptionmethod, which reveals the accessible surface area in aqueoussystems.9 The surface area of the SnO2/CCG micro-hydrogelswas determined to be approximately 1677 m2 g�1. This value isalmost comparable to that of thermally expanded graphenesheets (1850 m2 g�1),9 and much higher than that of thehydrothermal CCG-based hydrogel (964 m2 g�1).39 Aer calci-nating micronanogels at 200 �C to obtain drypowder, thenanocomposite was characterized to have a BET surface areaof 364 m2 g�1 (ESI, Fig. S8†). When the drypowder was re-

Fig. 5 Water purification capability and photocatalytic efficiency. (a) MB adsorptioncommercial activated carbon (CAC), montmorillonite (MMT) clay, and CCG sponge.38

mL MB solution) (blue curve) and the micronanogels (0.2 mg micronanogels in 45 mPhotodegradation activity based on the weight of the photocatalyst. (d) Photodegr

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dispersed in MB solution, the dye adsorption showed an effec-tive surface area of 236 m2 g�1. The dehydration of the hydrogelresulted in the stacking aggregation of SnO2/CCG platelets thatreduced re-dispersibility as well as the accessible surface area inaqueous media.

The unique attributes of CCG-based nanoplatelets werepreserved in the macronanogel and employed efficiently incolloidal suspensions of micronanogels. In water purication, itis sustainably practical that the material can be regeneratedand reused without obvious efficiency decline due to thereversible dye adsorption (ESI, Fig. S9†). As seen in Fig. 5, theadsorption equilibrium point was reached aer 1 hour inthe dark (Fig. 5b); the micronanogels showed an exceptionaladsorption capacity (685 mg g�1 for MB). The dye adsorptioncapacity surpasses other reported materials due to the largesurface area of CCG-based nanoplatelets (Fig. 5a).40–44 Aeradsorption, the suspension was irradiated with visible light toinvestigate the photocatalytic activity in organic-dye degrada-tion. In a comparison between the micronanogels and dry-powder, the photodegradation activity of the micronanogels isabout ten times higher than that of the calcinated one (Fig. 5c).

capacity of themicronanogels, metal–organic framework 235 (MOF-235), 3D G-O,–42 (b) Adsorptive and photocatalytic activity of drypowder (1 mg drypowder in 45L MB solution) (red curve) in MB solutions (dye concentration of 10 mg L�1). (c)

adation activity based on the specific surface area of the photocatalysts.

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The performance based on the accessible surface area (Fig. 5d)reveals that the micronanogels possess a greater interactionalsurface for photocatalysis. The optical band gap for the materialwas calculated using UV-VIS absorption spectra. The opticalband gap of the micronanogels was calculated to be 1.8 eV,which is much lower than the band gap of commercial SnO2

nanoparticles (3.9 eV) (ESI, Fig. S11†). This change in bandgapcan be attributed to the inclusion of oxygen vacancies intro-duced during the synthesis process as well as the inclusion ofgraphene.45 Mechanistically, in the photocatalytic degradationprocess the excitation of the system is mainly attributed to thevisible-light photosensitization of the dye rather than the band-gap transition of the SnO2/CCG composite.19 The well-preservednetwork of the CCG-based mediator can promote the chargeseparation and electron transfer to the conduction band ofSnO2 since the work function of SnO2 is higher than that ofCCG.46 The resultant movement of electrons in the system couldresult in reactive photoexcited electrons for the dye decompo-sition.47 The remarkable dye decontamination capability of the

Fig. 6 Charge-transfer quenching effect and biosensing via fluorescence recovercomplex. (b) Fluorescence spectra of solutions with different concentrations of DNAsensing solution and subsequent solutions containing biomolecules (DNA, hemoglographs of fluorescent RhB solution and fluorescence-quenched RhB–hydrogel comprecovered solutions mentioned in (c) under UV light (sensing solution, glucose, citri

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micronanogels is superior to that of other mineral materials,carbon-based materials and metal organic frameworks (ESI,Table S1†).

In relation to the optical properties of G-O or CCG, theuorescence quenching effect originates from charge-transferinteractions between the uorescent species and sp2 domainswithin graphene.48 This effect is the basis of graphene-basedoptical sensors that have been applied to detect DNA and otherbiomolecules.49,50 The as-synthesized CCG-based micro-nanogels provide unique colloidal, adsorptive and electronicproperties applicable to the off–on mechanism of quenching-uorescence sensing. When the SnO2/CCGmicrogels combinedwith organic dye rhodamine B (RhB) to form a dye–hydrogelcomplex, the orange uorescence emission of RhB was effec-tively quenched due to electronic transfers through theconductive network of the graphene-based hydrogel (Fig. 6).51

In UV-VIS spectroscopy (Fig. 6a), there is a notable red shiin absorption peaks of RhB from 555 nm for the dyesolution to 578 nm for the RhB–hydrogel complex solution. The

y. (a) UV-VIS absorption spectra of RhB, SnO2/CCG hydrogel and RhB–hydrogel(0 nM, 1 nM, 3 nM, 6 nM, 9 nM and 12 nM). (c) Fluorescence spectra of initial

bin, citric acid and glucose) at equal concentrations of 25 nM. (d) Digital photo-lex solution under UV light, sensing solution under visible light, and fluorescence-c acid, hemoglobin and DNA, respectively from left to right).

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uorescence-quenched dye–hydrogel complex was formed aerthe adsorption of RhB dye into the micro-hydrogel scaffold. Theexcellent capability of capturing dye molecules and quenchinguorescence helped obtain transparent sensing solutions.Sensing experiments were carried out using DNA sodium saltfrom salmon testes, hemoglobin from bovine blood cells, citricacid and D(+)-glucose.

The spectra in Fig. 6c show different uorescence enhance-ments for different biomolecules. Aer addition to sensingsolutions, biomolecules released RhB molecules aggregated onthemicronanogels. RhBmolecules that desorbed the compositehydrogels were delivered from the quenching effect, and theyturned on uorescence emission in the solutions. Double-stranded DNA was capable of turning on uorescence with aremarkable intensity which is about ve times higher thanthose of hemoglobin, citric acid and glucose. DNA with largemolecular size and numerous charged functional groups canattract and adsorb a larger amount of RhB. In the quantitativesensing shown in Fig. 6b, the RhB–hydrogel complex can detecta trace amount of DNA (1 nM), with uorescence recovery cor-responding to DNA concentrations in the sensing solutions.These target molecules could undergo hydrophobic stacking onthe micro-hydrogels, but sufficient complex-formation energycan allow them to detach from the hybrid particles.49 As illus-trated in Fig. 6d, the use of micronanogels as a quenchingplatform for uorescence sensing is a facile and rapid methodto quantitatively detect track amounts of DNA residue inaqueous solutions.

The water-intercalated structure of the SnO2/CCG-nano-platelet-basedmacronanogel allows the ultrasonic exfoliation ofthe material into solution-processable micronanogels.Regarding microgel-suspension stability in water and organicsolvents, the hydrophilic surface and the porous microstructureimpregnated with water can make the hybrid hydrogels sus-pended in the suspensions. The colloidal property of themicronanogels is of great importance in applications thatrequire suspensions stable enough for further processing.36,52

The creation of stable aqueous and organic dispersions ofnanoparticle-decorated CCG nanoplatelets without the assis-tance of other chemicals can enable developments in low-costsolution processing of diverse graphene-based nano-composites.36 Such metal-oxide/CCG micro-hydrogels are suit-able for the versatile wet-chemistry applications.

The combination of nanoparticle decoration and surfacehydration is a unique spacing layer that effectively prevents theinterconnection of CCG nanoplatelets.7,17,18 The strong repulsiveforce of surface hydration is a bio-inspired prevention of graphenerestacking.7 Importantly, the nanoparticle coating could helphydrophobic CCG nanoplatelets to readily maintain hydrationlayers against the hydrophobic irreversible agglomeration in thewater environment. The resulting water intercalation pavesthe way for the ultrasonic nanoscale re-dispersibility of thehydrogel structure.

By preventing stacking of CCG, the surface area and elec-tronic properties of functionalized CCG-based nanoplatelets aresuccessfully maintained. As conrmed by adsorptive and pho-tocatalytic activities, the exfoliated micronanogels were much

This journal is ª The Royal Society of Chemistry 2013

more surface-accessible and electronically efficient than theconventional nanocomposite drypowder. The large accessiblesurface area of the hybrid nanoplatelets with numerousadsorption sites for organic dyes and heavy metal ions alsomakes them some of the best candidates for water purica-tion.53 Additionally, the effective surface with extraordinaryelectronic mobility of the CCG-based hybrids allows for prom-ising catalytic activity towards chemical reactions.1 The proc-essability of micronanogels in aqueous and organic solventsopens opportunities to fabricate memory and recognitiondevices.54–56 Furthermore, micronanogels with excellent dyeadsorption and controlled dye release can be potential stimuli-responsive microcapsules for drug delivery.

Unlike macroscopic interconnected CCG-based hydrogels,the as-prepared micro-units can be applied in bottom-upapproaches to prepare subsequent coatings and polymer nano-composites with high electrical conductivity.9,57,58 A hybrid SnO2/CCG lm prepared by compressing a layer of micronanogelsshowed an electrical conductivity of approximately 350 � 10 Scm�1, which is superior to CCG thin lms (ESI, Fig. S12 and S13and Table S2†).38,52 This increased conductivity can be attributedto a decrease in the composite bandgap, as well as the chemicaldoping of graphene platelets which allows facile electron transferin the lm. Elemental compositions of the micronanogel lmwere analyzed by energy-dispersive X-ray spectroscopy. Theweight proportion of the metal oxide in the nanocomposite isabout 68%. It is believed that the electronic mobilities of SnO2

nanoparticles and CCG platelets synergistically contributetowards the high electrical conductivity of the lm.

Conclusion

We have demonstrated a novel concept of enhanced surfacehydration with the aid of metal oxide coating. The water-inter-calated 3D architecture preserves the unique features of thefunctionalized graphene nanoplatelets through preventing thep–p restacking. The strategy of reversible self-assembly andaqueous-phase exfoliation facilitates the storage and usage ofthe materials at the macroscopic and microscopic scales. Theaqueous colloidal stability, large accessible surface area andsuperior electrical conductivity are crucial properties applicableto a wide range of wet-chemistry processes.

Acknowledgements

This work was nancially supported by the National HonorScientist Program (2010-0020414) and World Class UniversityProgram (R32-2008-000-10180-0).

Notes and references

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