Graphene-Embedded Hydrogel Nanofibers for Detection and

Mar 16, 2017 - In this work, we report the ability of graphene-embedded hydrogel nanofibers (GHNFs) to recognize aqueous phase dyes. ..... is likely t...
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Graphene-Embedded Hydrogel Nanofibers for Detection and Removal of Aqueous-Phase Dyes Kyungun Im,‡ Duong Nguyen Nguyen,‡ Saerona Kim,‡ Hye Jeong Kong,‡ Yukyung Kim,‡ Chul Soon Park,‡,§ Oh Seok Kwon,*,§ and Hyeonseok Yoon*,†,‡ †

School of Polymer Science and Engineering and ‡Department of Polymer Engineering, Graduate School, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, South Korea § BioNanotechnology Research Center, Korea Research Institute of Bioscience and Biotechnology (KRIBB), Daejeon 34141, South Korea S Supporting Information *

ABSTRACT: A facile route to graphene/polymer hydrogel nanofibers was developed. An aqueous dispersion of graphene (containing >40% bilayer graphene flakes) stabilized by a functionalized water-soluble polymer with phenyl side chains was successfully electrospun to yield nanofibers. Subsequent vapor-phase cross-linking of the nanofibers produced grapheneembedded hydrogel nanofibers (GHNFs). Interestingly, the GHNFs showed chemical sensitivity to the cationic dyes methylene blue (MB) and crystal violet (CV) in the aqueous phase. The adsorption capacities were as high as 0.43 and 0.33 mmol g−1 s−1 for MB and CV, respectively, even in a 1.5 mL s−1 flow system. A density functional theory calculation revealed that aqueousphase MB and CV dyes were oriented parallel to the graphene surface and that the graphene/dye ensembles were stabilized by secondary physical bonding mechanisms such as the π−π stacking interaction in an aqueous medium. The GHNFs exhibited electrochemical properties arising mainly from the electric double-layer capacitance, which were applied in a demonstration of GHNF-based membrane electrodes (5 cm in diameter) for detecting the dyes in the flow system. It is believed that the GHNF membrane can be a successful model candidate for commercialization of graphene due to its easy-to-fabricate process and remarkable properties. KEYWORDS: dyes, graphene, nanofibers, hydrogels, adsorption, sensors

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INTRODUCTION

The exceptional properties of graphene have been extensively explored to date.20−28 For example, it is known that a suspended graphene monolayer absorbs only 2.3% of white light and has an electron mobility of ∼2.0 × 105 cm2 V−1 s−1 and a thermal conductivity of ∼5000 W m−1 K−1.29 These properties have offered opportunities for advances in the fields of electronics, optoelectronics, energy devices, and functional composites Owing to its intrinsic hydrophobicity, however, it is very difficult to find suitable applications of graphene in an aqueous phase. Above all, it is quite hard to produce aqueous graphene.30−32 In addition, irreversible agglomeration of

Environmental pollution is a serious international issue that requires ongoing technological concern and policy support. The main organic pollutants include undesirable residual dyes from various sources (e.g., the textile industry, paper and pulp industry, dye and dye intermediate industry, and pharmaceutical industry), which can be introduced into natural water resources or wastewater treatment systems. To remove harmful chemicals, a variety of methodologies have been developed, such as chemical precipitation, ion exchange, membrane filtration, physical adsorption, chemical oxidation/reduction, and bioremoval,1−9 for which diverse functional materials consisting mainly of carbon, polymers, and hybrids have been designed and synthesized.10−19 © XXXX American Chemical Society

Received: January 23, 2017 Accepted: March 9, 2017

A

DOI: 10.1021/acsami.7b01163 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Schematic of phenyl side-chain functionalization of PVA, followed by the preparation of graphene/PVA hybrids.

graphene layers by π−π stacking and van der Waals interactions is accelerated in the aqueous phase, limiting the utilization of the characteristics of individual graphene layers. Although graphene oxides have been produced chemically for aqueousphase applications, their properties are completely different from those of graphene.33,34 Graphene-network-based gels can be produced with the aid of small-molecule linkers.35,36 Gels generally have small pores that enlarge their effective surface area. However, the pores of graphene-only architectures have a high interfacial tension against aqueous solutions, greatly reducing the effective surface area of graphene gels. The hydrophobicity of graphene is inherent and cannot be changed without altering its properties. Therefore, it is quite important to control the microenvironment surrounding graphene for its effective application in aqueous solutions.37,38 We have developed physical exfoliation approaches to produce graphene-based nanohybrids.27,28,39,40 In particular, recently, aqueous graphene solution was simply obtained with the aid of modified polymers. In this work, we report the ability of graphene-embedded hydrogel nanofibers (GHNFs) to recognize aqueous phase dyes.40 Graphene was introduced into hydrogel nanofibers to take advantage of their porosity, high surface area, and hydrophilicity. In particular, the watersoluble polymer-based hydrogel architecture surrounds the graphene with a hydrophilic microenvironment, facilitating interaction between the graphene and aqueous-phase dyes. Specifically, aromatic-ring side chains were grafted to a watersoluble polymer, poly(vinyl alcohol) (PVA), to provide chemical affinity for graphene. An aqueous-phase graphene/ PVA dispersion was obtained via a physical top-down approach and then electrospun into nanofibers. The resulting nanofibers were cross-linked by a vapor-phase cross-linking agent, yielding hydrogel nanofibers. Interestingly, the GHNFs showed outstanding ability to selectively adsorb cationic dyes, including two typical examples of industrially relevant cationic dyes that have also found uses in various areas of medicine and biology.41

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(>98%) were obtained from Tokyo Chemical Institute Co. and OCI Co. Ltd., respectively. MB (2−3 hydrate) was purchased from Junsei Chemical Co. MO and CV were purchased from Samchun Chemical Co., and DR (dye content, ∼50%) was purchased from Sigma-Aldrich Co. Graphite flakes as a graphene precursor were obtained from Sigma-Aldrich Co. Distilled water was used for all the experiments. Fabrication of GHNFs. Ph-PVA was synthesized using a previously reported method.40 Ph-PVA or PVA_only was dissolved in 10 mL of distilled water at a concentration of 10 wt %. Then, 0.2 mg of graphite flakes was added to the polymer solution. The mixture solution was sonicated for 30 min at an amplitude of 60%. The resulting graphene/polymer solution was centrifuged at 10000g for 10 min to remove unstable residues and then electrospun using a 10 mL syringe/18-gauge stainless steel needle and a tip-to-collector distance of 180 mm at a voltage of 25 kV. As-electrospun graphene/polymer nanofibers were exposed to glutaraldehyde vapor for 12 h. Glutaraldehyde solutions with four concentrations (1.12, 2.24, 3.36, and 5.60 mmol) were vaporized at 50 °C in a 100 mL chamber. As a result, graphene/polymer nanofibers, namely, GHNFs, were obtained. Adsorption and Electrochemical Tests of the AqueousPhase Dyes. To measure the adsorption capacity, 0.1 g of the GHNFs was added to 50 mL of 1 mM dye solution. UV−vis spectroscopy was used to determine the concentration of the dye in solution. The concentrations of MB, CV, MO, and DR were calculated from the intensities of the absorption peaks at 665, 585, 510, and 465 nm. The flow system was composed of a homemade Teflon cell integrated with three-electrode accessories, in which the flow rate was regulated using a peristaltic pump. Cyclic voltammetry (C/V) and electrochemical impedance spectroscopy (EIS) analyses and sensing experiments were conducted with the flow cell using a Metrohm Autolab B.V. PGSTAT101 potentiostat/galvanostat. Theoretical Calculation. The geometry of a 54-carbon graphene sheet terminated with periodic boundary conditions was optimized. ORCA 3.3 and Gaussian 09 software were used to perform DFT calculations. The calculations were performed using the B3LYP functional (20% HF exchange). To calculate the structural geometry, a balanced polarized triple-ζ Ahlrichs def2 basis set, namely, def2-TZVP, was employed. The dispersion forces were taken into account using Grimme’s DFT-D3 method for dispersion correction. The resolutionof-identity approximation was used to speed up the calculation, although it introduces a very small error. Furthermore, for comparison with a vacuum environment, continuum solvation effects were included by using the COSMO model as an implemented module.

EXPERIMENTAL SECTION

Materials. PVA (Mw = 89 000−98 000, 99% hydrolyzed) and 1,2epoxy-3-phenoxypropane (EPP, 99%) were purchased from SigmaAldrich Co. Glutaraldehyde solution (5.6 M) and sodium hydroxide B

DOI: 10.1021/acsami.7b01163 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION The water-soluble polymer PVA was employed as a precursor to obtain the hydrogels.42 It is not difficult to prepare bulk hydrogels via cross-linking of PVA. However, it was quite difficult to produce PVA hydrogel nanofibers. Sonicationassisted physical exfoliation of graphite was performed to readily obtain the graphene/polymer dispersion without degrading graphene properties, for which purpose the polymer was modified with a hydrophobic aromatic-ring side chain. Figure 1 shows the reaction steps involved in producing graphene/PVA hydrogel nanofibers. The phenyl side chain was grafted to the polymer by reacting PVA with EPP in a basic solution. The resultant PVA modified by the phenyl side chain (phenyl-functionalized PVA, Ph-PVA) was used as an intercalant for graphite exfoliation and as a stabilizer for colloidal stabilization of separated graphene layers in aqueous solution. Physical exfoliation of graphite by ultrasonication in the aqueous Ph-PVA solution produced a colloidally stable graphene/Ph-PVA dispersion (containing >40% bilayer graphene, see Figure S1).40 Considering that organic solvents such as N-methyl-2-pyrrolidone have typically been used to effectively exfoliate graphite, it was highly significant that graphite was directly exfoliated in aqueous solution. The aqueous-phase graphene/Ph-PVA dispersion was successfully electrospun into nanofibers, where the main parameters were the polymer and graphene contents of the solution. Figure 2 shows scanning electron microscopy (SEM) images of graphene/Ph-PVA nanofibers electrospun under optimized

nanofibers, whereas the surfaces of the PVA-based nanofibers were clearly smooth. The polymer should first be dissolved in water for electrospinning, and then, during electrospinning, solid-phase nanofibers are obtained as the water evaporates. Therefore, it can be assumed that hydrophobic phenyl moieties would induce thermodynamically driven phase separation in the intrinsically hydrophilic PVA. The electrospun nanofibers were finally obtained as a membrane composed of random nanofiber stacks, and as shown in Figures 3a and 3b, large-area membrane films (5 × 5 cm2 in area) were easily prepared. Cross-linking of PVA has been widely explored using various cross-linking agents, so it is not a difficult task to cross-link bulky PVA.43−46 When the dimensions of PVA decrease to the nanometer scale, however, it becomes quite hard to retain its microstructural features during the cross-linking reaction. In this work, the graphene/ Ph-PVA nanofibers were transformed to hydrogel nanofibers through the vapor-phase cross-linking reaction. The crosslinking agent, glutaraldehyde solution, was vaporized at concentrations of (1.12−5.60) × 10−2 mmol L−1 in a closed chamber. The as-electrospun graphene/Ph-PVA nanofibers changed from pale gray to black after the cross-linking reaction (Figures 3a and 3b). The morphology of the resultant crosslinked nanofibers was examined by SEM observation (Figure S2). At high glutaraldehyde concentrations, the graphene/PhPVA nanofibers were seriously agglomerated and finally formed amorphous films with no porosity. The optimal product, namely, nanofiber membranes with suitable porosity, was obtained at a glutaraldehyde concentration of 22.4 mM L−1, at which the diameter of the nanofibers increased by 150% to 307 ± 74 nm. As shown in Figure 3c, inter-nanofiber pores a few micrometers in diameter were well developed. A swelling test revealed that the cross-linked graphene/Ph-PVA nanofibers could absorb at maximum 20 times their initial dry weight in water and reach an equilibrium state in 60 min. This result demonstrates that the graphene/Ph-PVA nanofibers were successfully transformed to a hydrogel structure by vaporphase cross-linking. The hydrogel nanofiber structure can provide appropriate mechanical properties for aqueous-phase applications (see Figure S3), in which embedded graphene can interact more effectively with hydrophilic components. Considering these advantages, the use of the graphene/Ph-PVA hydrogel nanofibers (GHNFs) as an adsorbent for removal of aqueous-phase dyes was systematically investigated. Table 1 summarizes the chemical structures and properties of the dyes studied, namely, methylene blue (MB), crystal violet (CV), methyl orange (MO), and disperse red 1 (DR); the Hückel energies and polarizability values were calculated directly by density functional theory (DFT). First, the adsorption capacities of GHNFs for the four dyes were examined as a function of time (Figure 4). A piece of GHNF membrane was immersed in an aqueous dye solution (DR was dissolved in a water/glycol mixture owing to its low water solubility), and ultraviolet (UV)−vis absorption spectra were then taken at intervals of 5 min (Figures 4a−d). Interestingly, the GHNFs showed different adsorption behaviors toward the dyes. Significant decreases were observed in the absorption peak intensities of MB and CV, whereas no remarkable changes were observed in those of MO and DR. The dye concentration change in the solution and the adsorbed dye amount are plotted against time in Figures 4e and 4f, respectively. For MB and CV, the amount of dye adsorbed by the GHNFs increased

Figure 2. Representative SEM images of electrospun nanofibers: (a) graphene/Ph-PVA, (b) Ph-PVA_only, (c) graphene/PVA, and (d) PVA_only. Nanofibers (b, c, d) were prepared as controls for comparison.

conditions. The diameter of the graphene/Ph-PVA nanofibers was 122 ± 17 nm, and that of the Ph-PVA_only nanofibers was 175 ± 33 nm. In other words, adding graphene led to a reduction in the nanofiber diameter, probably because of the enhanced electrical conductivity of the polymer dispersion due to the embedded graphene. Nonfunctionalized PVA was also examined as a control. Like the Ph-PVA, the PVA_only nanofibers exhibited a reduced diameter when graphene was added. More interestingly, the surface morphology of the PhPVA-based nanofibers was completely different from that of PVA-based nanofibers. A surface morphology containing protruding particles was observed for the Ph-PVA-based C

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Figure 3. Photos of graphene/Ph-PVA nanofiber membrane (a) before and (b) after vapor-phase cross-linking, (c) SEM image of the cross-linked nanofiber membrane, and (d) swelling kinetics of the cross-linked nanofiber membrane as a hydrogel in aqueous solution.

First, MB and CV exist as cationic species, whereas MO and DR are anionic and nonionic species, respectively. As shown in Table 1, however, the calculated total π-electron energy was similar for all the dye molecules. The polarizability of MB was also similar to those of MO and DR, although the value for CV was approximately 1.5 times larger. As a result, the high adsorption capacities of GHNFs for MB and CV could not be explained in terms of the total π-electron energy and polarizability. To better understand the interaction between the dye molecules and graphene, a DFT calculation was performed. A piece of graphene consisting of 54 carbon atoms was employed as simple miniature model, and the interaction energy of graphene with dye molecules, Eint = Eab(graphene/ dye) − [Eab(graphene) + Eab(dye)], was calculated using the B3LYP functional in conjunction with the def2-TZVP basis set. Before the interaction energy was calculated, each ensemble and isolated fragment was optimized. The adsorption of dye molecules on graphene can be described as follows: The dye is first attracted to graphene, and the distance between the two decreases to an optimized, equilibrium value. During this process, the structure of graphene is gradually twisted, and the

Table 1. Major Properties of the Model Dyes dye

Mw

water solubility (g L−1)

MB CV MO DR

319.85 407.98 327.33 314.34

∼40 ∼15 ∼5 ∼8 × 10−4

a

Hückel analysis (keV) 32.2 30.9 36.2 29.0

b

polarizability (102 Å3)

c

21.9 31.9 21.9 22.8

a Found in the literature (measured at 20−25 °C). bThe total energy of molecular orbitals of π electrons; calculated at the B3LYP/def2TZVP level of theory. cThe relative tendency of the electron cloud (a charge distribution) of a molecule to be distorted by an external electric field; calculated at the B3LYP/def2-TZVP level of theory.

linearly over 30 min, resulting in removal of >85% of the initial amount; the GHNFs had a slightly higher removal efficiency for MB (93%) than for CV (86%). On the other hand, it was found that the GHNFs removed little or no MO (0%) and DR (4%). It is remarkable that the GHNFs show selective adsorption of MB and CV. There have been a few reports on the adsorption of MO or CV on carbon species. However, little is understood about why the materials exhibit selectivity toward the dyes.

Figure 4. Adsorption of dyes by GHNFs in aqueous solution (at pH 6.0 and 25 °C in a closed cell). (a−d) Time-dependent changes (interval, 5 min) in UV−vis absorption spectra of dye solutions upon addition of GHNFs (0.1 g of GHNFs/50 mL of 1 mM dye solution): (a) MB, (b) CV, (c) MO, and (d) DR [dissolved in 1/1 (v/v) ethylene glycol/water solution]. Plots of the adsorption capacity versus time calculated from (a−d): (e) relative dye concentration change, in which C0 and Ci denote the initial concentration and the instantaneous concentration at time t, respectively, and (f) the amount of dye adsorbed at time t, which was normalized by dividing it by the weight of GHNFs. D

DOI: 10.1021/acsami.7b01163 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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negative values, whereas the graphene/MO and graphene/DR ensembles had positive interaction energy values. Therefore, it is evident that only the adsorption of MB and CV on graphene, in which the aqueous medium plays a pivotal role, is thermodynamically favorable. Several intermolecular interactions can facilitate the physisorption of the dyes on graphene. A representative equilibrium distance between graphene and the dyes was calculated to be around 3.5 Å, revealing the existence of π−π stacking. From a surface charge point of view, the selective adsorption of the cationic dyes indicates that the embedded graphene would be negatively charged. It is also important to note that the surface charge is sensitive to the pH of the solution.47 The ability of GHNFs to recognize and remove the dyes was demonstrated in a flow system. A flow cell immobilizing a GHNF membrane (diameter, 5 cm; thickness, 1 mm; weight, 0.05 g) was employed (Figure 6), coupled with a three-

dye molecule bends slowly to minimize the conformational energy. Figure 5 shows the optimized geometries of the

Figure 5. Geometric configurations of graphene/dye ensembles optimized at the B3LYP/def2-TZVP level of theory (in vacuum): (a) MB, (b) CV, (c) MO, and (d) DR.

graphene/dye ensembles. The optimized molecular geometries in vacuum and water solvation conditions were found to be similar. The dye molecules were oriented parallel to the graphene surface at distances of 3.42−3.75 Å. Interestingly, the equilibrium distances were affected by the surrounding environment, as summarized in Table 2. Compared with the Table 2. Equilibrium Distances between Graphene and Dyes Calculated in Vacuum and Water Environments distancea (Å) dye

vacuum

water

water−vacuumb

MB CV MO DR

3.75 3.61 3.49 3.45

3.42 3.47 3.50 3.53

−0.33 −0.14 0.01 0.08

a Center-to-center distance was measured. bThe distance value determined in water minus the distance value determined in a vacuum.

Figure 6. Photographs showing the structure of the Teflon flow cell coupled with a three-electrode system: (a) inlet, (b) outlet, (c) reference electrode insertion point, (d) counter electrode insertion point, and (e) working electrode insertion point.

distances in vacuum, MB and CV had smaller distances from graphene in water, whereas MO and DR had slightly larger or almost the same distances. This result implies that water promotes the attractive interaction between graphene and MB or CV. The adsorption of dye molecules on graphene can be gauged by the interaction energy; the calculated interaction energies are listed in Table 3. All the graphene/dye ensembles had negative interaction energy values in a vacuum. However, in the aqueous environment, the interaction energies of the graphene/MB and graphene/CV ensembles shifted to more

electrode system for measuring the electrochemical properties. First, Figure 7 shows the dye removal capacity of the GHNF membrane in the flow system. Similar to the results in the closed cell, the GHNFs had high removal capacities of 0.43 and 0.33 mmol g−1 s−1 for only MB and CV, respectively, whereas MO and DR were not removed from the flowing solution. In fact, little research has been done on the dye adsorption using nanomaterials in flow systems. Most studies have been conducted in the batch reactor system (Table S1).48−51 Flow

Table 3. Interaction Energies of Graphene and Dyes Eab (Eh) method

ensemble

dye

graphene

graphene/dye

Eint (Eh)

vacuum

graphene/MB graphene/CV graphene/MO graphene/DR graphene/MB graphene/CV graphene/MO graphene/DR

1182.143 1133.969 1329.328 1064.345 1182.206 1134.025 1329.438 1064.345

2055.299

3237.436 3189.266 3384.569 3119.638 3237.492 3189.317 3385.422 3120.035

−0.006 −0.002 −0.058 −0.006 −0.017 −0.011 0.681 0.387

water

2055.303

E

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Figure 7. Real-time removal of dyes by GHNFs in the flow cell (flow rate, 1.5 mL s−1). (a−d) Time-dependent changes (recorded at durations of 1, 3, 5, and 10 min) in UV−vis absorption spectra of dye solutions (0.05 g of GHNFs/50 mL of 0.5 mM dye solution): (a) MB, (b) CV, (c) MO, and (d) DR [dissolved in 1/1 (v/v) ethylene glycol/water solution]. Plots of the adsorption capacity versus time calculated from (a−d): (e) relative dye concentration change, in which C0 and Ci denote the initial concentration and the instantaneous concentration at time t, respectively, and (f) the amount of dye adsorbed at time t, which was normalized by dividing it by the weight of GHNFs.

Figure 8. C/V curves of GHNF membrane using the flow cell: (a) representative C/V curves measured at a scan rate of 60 mV s−1, (b) relative change in integrated areas of C/V curves measured at scan rates of 20, 40, and 60 mV−1, (c) Nyquist plot and calculated equivalent circuit model (Rs, solution resistance; Rct, charge transfer resistance; C, capacitance; CPE, constant phase element; W, Warburg resistance), (d) time-dependent responses upon exposure to dyes at different concentrations (interval, ∼2 min), and (e) plots of dye concentration versus intensity of response [the maximum amount of change in the resistance calculated from (d)].

membrane filter for continuous dye uptake. The adsorption capacity of the GHNFs in the flow system became saturated after 10 min. The physical adsorption/desorption behavior of the dyes on graphene can be reversed, allowing reuse of the GHNFs in removing the dyes from solution. To confirm this, the GHNFs were regenerated by immersing them in excess ethanol for 30 min. As shown in Figure S4, even after five adsorption/harvesting cycles, the adsorption capacity of the GHNFs was maintained at >88% of the original level.

perturbation can cause undesirable physical desorption of adsorbed dye molecules, which might prevent efficient dye removal from the flowing solution. Nevertheless, compared with the closed cell, increased amounts (1.2- to 3.3-fold) of dye adsorbed per time were observed within the 10 min exposure period under the dynamic flowing condition. The adsorption capacity of GHNFs was 1.4−3.6-fold higher than those of carbon materials and hydrogels reported previously (Table S1),48−51 indicating the potential of the GHNFs for use as a F

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composites in the development of flow-cell-based sensors with selective recognition and in situ uptake of dyes.

It was also noteworthy that the GHNFs had electrochemical properties originating from the graphene embedded in the PVA chains. The electrochemical properties were affected by the adsorption of the dyes on graphene. As presented in Figure 8, C/V analysis of the GHNFs was performed before and after dye adsorption using the flow cell. The C/V curves of the GHNFs had a distorted shape with a large hysteresis (Figure 8a) in the potential range of −0.5 to 1.0 V, likely because of the high internal resistance. The C/V curve retained its shape at different scan rates (Figure S5). The C/V data suggest that a major element of the electrochemical behavior of the GHNFs was electrical double-layer capacitance. EIS analysis was performed for the GHNFs using the same setup to calculate an equivalent circuit model (Figure 8c). A Nyquist plot composed of a small semicircle at high frequencies followed by a diagonal line at low frequencies was obtained, confirming that the GHNFs consisted of a capacitor and resistors. The C/V curve of the GHNFs upon exposure to the dyes was monitored. Figure 8b plots the relative changes in the C/V area of the GHNFs upon exposure to the four dyes. Exposure to MB and CV dyes caused the area of the C/V curves to decrease significantly, whereas exposure to MO and DR caused almost no change. This result can be correlated with the selective adsorption of MB and CV by the GHNFs. In other words, the adsorption of MB and CV dyes on the embedded graphene would reduce the number of effective sites for electrolyte ion adsorption and in turn decrease the electric double-layer capacitance of the GHNFs. Those results indicate that the GHNF membrane has strong potential for use in detecting dye molecules in solution. The ability of GHNFs to detect dye molecules was examined using the flow cell coupled with the three-electrode system. The current flowing through the GHNF membrane was monitored at a constant applied potential. Figure 8d displays the time-dependent response of the GHNF membrane to dye molecules recorded in the flow cell. During transient exposure to MB and CV, the GHNF membrane electrode showed decreased current flow. The change in the current flow occurred instantaneously upon exposure to the dyes. The current then recovered to the original level. The response of the GHNF membrane to the dyes was concentration-dependent, as shown in Figure 8e. The sensing performance demonstrates that the GHNF membrane can be used for rapid detection as well as selective removal of dye molecules in a flowing solution.52−54

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01163. Figures S1−S5 and Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(O.S.K.) E-mail [email protected]. *(H.Y.) E-mail [email protected]. ORCID

Hyeonseok Yoon: 0000-0002-5403-1617 Author Contributions

K.I. and D.N.N. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) funded by the Korea government (MSIP) (NRF-2015R1A2A2A01007166; NRF2016M3C7A1905384) and by the KRIBB Initiative Research Program.

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CONCLUSIONS A facile and efficient synthetic route (namely, just sonication, followed by electrospinning) to GHNFs was developed, and indepth insight into the interaction between the embedded graphene and aqueous-phase dyes at the molecular level was provided. Structurally, the interaction of graphene with aqueous-phase dyes can be facilitated in the interior of the hydrogel. Only the cationic dyes MB and CV were adsorbed from aqueous solution by the GHNFs. It was found that graphene can make more thermodynamically stable ensembles with the dyes in an aqueous medium. Furthermore, the electric double-layer capacitance of the GHNFs was affected by the adsorbed dyes, allowing real-time monitoring of the dyes in a flowing solution. The combinatorial multifunctionality of the GHNFs, which can be readily prepared over large areas (centimeters scale), is likely to show strong potential for practical, commercialized application of graphene/polymer G

DOI: 10.1021/acsami.7b01163 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.7b01163 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX