Insights into the Oxidation Mechanism of sp2–sp3 Hybrid Carbon

Article pubs.acs.org/Langmuir

Insights into the Oxidation Mechanism of sp2−sp3 Hybrid Carbon Materials: Preparation of a Water-Soluble 2D Porous Conductive Network and Detectable Molecule Separation Siwei Yang,†,‡ Yucheng Yang,† Peng He,† Gang Wang,*,§ Guqiao Ding,*,†,‡ and Xiaoming Xie†,‡ †

State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Science, Shanghai 200050, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China § Department of Microelectronic Science and Engineering, Faculty of Science, Ningbo University, Ningbo 315211, P. R. China S Supporting Information *

ABSTRACT: A thorough investigation of the oxidation mechanism of sp2−sp3 hybrid carbon materials is helpful for the morphological trimming of graphene. Here, porous graphene (PGN) was obtained via a free radical oxidation process. We further demonstrated the difference between traditional and free radical oxidation processes in sp2−sp3 hybrid carbon materials. The sp3 part of graphene oxide was oxidized first, and well-crystallized sp2 domains were reserved, which is different from the oxidation mechanism in a traditional approach. The obtained PGN shows excellent performance in the design of PGN-based detectable molecule separation or other biomedical applications.

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INTRODUCTION As the most promising nanomaterial, carbon nanomaterials, especially graphene, has been predicted to play a crucial role in microelectronics,1−3 new energy resources,4,5 catalysts,6 and biotechnology7,8 because of its unique two-dimensional (2D) hexagonal lattice structure and extraordinary physical properties. To realize the potential of graphene in molecule separation,9 lithium ion batteries,5 and supercapacitors,4 a porous structure of graphene with a well-controlled barrier property should be synthesized first because most of the physicochemical properties of graphene are sensitive to its barrier property.10,11 Thus, the morphological trimming of graphene (etching and cutting) is necessary for the specific application areas.12,13 By now, various approaches have been followed for the morphological trimming of graphene; in particular among them is oxidation treatment. It is well known that oxidation treatment is an effective approach to shear graphene sheets and obtain graphene sheets with a suitable lateral size [the typical product is graphene quantum dots (GQDs)14]. In the oxidation step, sp2 carbon atoms create epoxy groups that tend to form a line along a carbon lattice, causing a rupture of the C−C bonds under acidic conditions.15 The orientation of the rupture process can be attributed to the steric hindrance of the adjacent epoxy groups. Because of this © 2017 American Chemical Society

oriented rupture process, the graphene sheet is cut into small pieces. However, owing to the above-mentioned reaction mechanism, the traditional oxidation approach is still limited because it is difficult to achieve the goal of etching. Indeed, the etching method always depends on the chemical or physical pore-forming process. With the local bonding/corrosion process of metallic oxide and metal nanoparticles [such as alkalis, K2TiO3, and (NH4)6Mo7O24 nanoparticles], or highenergy techniques (such as laser and plasma), part of the graphene is removed and porous graphene (PGN) can be obtained.16,17 It is a remarkable fact that the chemical oxidation method is low cost and easy to operate and has many advantages for practical applications. Thus, the development of an etching approach based on the chemical oxidation method has a great value in practical applications. Furthermore, most large-scaleprepared graphene-based materials [such as graphene oxide (GO), GQDs, and reduced graphene] are indeed sp2−sp3 hybrid carbon materials.18 In general, the oxidizing and acidating intercalation process destroys the perfect sp2 structure Received: October 31, 2016 Revised: January 4, 2017 Published: January 6, 2017 913

DOI: 10.1021/acs.langmuir.6b03937 Langmuir 2017, 33, 913−919

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Figure 1. Characterization of PGN. (a) Transmission electron microscopy (TEM) and (b) high-resolution TEM (HR-TEM) image of PGN thus formed. (c) Selected-area electron diffraction pattern (SAED) of PGN randomly taken from (a). (d) Typical Raman spectra of PGN (black curve), GO (blue curve), and reduced graphene oxide (RGO) (black curve) for comparison. (e) High-resolution X-ray photoelectron spectroscopy (XPS) O 1s spectra of PGN. (f) Schematic diagram of the structure of PGN thus formed. PGN has a highly crystalline structure and oxygen-containing edge groups.

RGO (470 m2/g, as shown in Figure S3) obtained under the same conditions as PGN without the presence of H2O2. The HR-TEM image in Figure 1b shows a distinct crystal lattice, which indicates the crystallinity of PGN, and a lattice parameter of 0.34 nm, which represents the (100) lattice fringe of graphene.20 SAED of PGN is displayed in Figure 1c; only one set of hexagonal diffraction patterns is observed, and a single crystalline lattice structure can be inferred.21 Raman spectroscopy is used to further demonstrate the wellcrystallized structure of PGN. The representative Raman spectra of carbon materials show three primary peaks: the D peak (approximately 1350 cm−1), G peak (approximately 1590 cm−1), and 2D peak (approximately 2700 cm−1) are characteristic signals. For graphene-based materials, the G peak corresponds to the sp2 hybridized carbon atoms in the hexagonal framework and the D peak is indicative of heteroatom doping, defective sites (non-six atom ring and edging sites), and sp3 carbons.22 Previous studies reported that PGN (obtained via a chemical pore-forming process) has a higher D to G peak intensity ratio (ID/IG) than GO and RGO, which indicates that more defects and edges are introduced on graphene sheets, resulting in an increase in the amount of sp3 hybridized carbon atoms.23 However, surprisingly, the PGN in our work exhibits a smaller peak intensity ratio ID/IG (Figure 1d) compared with GO and RGO. The peak intensity ratios ID/ IG of PGN, GO, and RGO are 0.4, 1.2, and 0.9, respectively, which indicate the lower proportion of sp3 hybridized carbon atoms in PGN. Moreover, this result also indicates the possible selective oxidation and etching process. XPS analysis is performed to measure the surface chemical composition and chemical states. For PGN, the XPS results shown in Figure S4 indicate that carbon (C) and oxygen (O) signals are found on the surface. The O/C atomic ratio for PGN is 0.13, which is higher than that for RGO. Deconvolution of the high-resolution C 1s spectra (Figure S5) can be fitted

of natural graphite and introduces large amounts of oxygencontaining groups and sp3 carbon.18 The sp3 domains in these materials play important roles in the morphological trimming of graphene, but they have been little investigated because of the complex and variable forms in which they exist. Thus, an indepth study of the oxidation behavior of sp2−sp3 hybrid carbon material is beneficial for the morphological trimming of graphene based on chemical oxidation. Many researchers have noticed that the sp3 carbon of GO would be preferentially oxidized in the oxidation process, but there is little experimental evidence to support this viewpoint.14 Thus, it is important to further understand the oxidation process of sp2−sp3 hybrid carbon materials. In this paper, PGN was obtained via chemical oxidation process between hydroxyl radical (•OH) and GO. The tungsten oxide nanowire (W18O49) catalyst •OH, which is produced from the catalytic decomposition of hydrogen peroxide (H2O2), can oxidize and etch GO effectively. We demonstrated the difference between the molecular and free radical oxidation processes of graphene. Because of the high reaction activities of • OH, GO was oxidized selectively and rapidly without the oriented rupture process. Further analysis shows that the sp3 part of GO was oxidized first and that well-crystallized sp2 domains were reserved, and the findings may offer new insights both for better understanding the oxidative actions of graphene films and for better designing PGN-based detectable molecule separation or other biomedical applications.

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RESULTS AND DISCUSSION The pores formed on PGN can be confirmed using TEM. Plain view TEM images shown in Figures 1a and S1 indicate a lateral size of 1−5 μm with nanoscale pores (5−50 nm) on the sheet (Figure S2). The porosity of the obtained PGN is 62%, which indicates a loose structure with high surface area. Indeed, the surface area is 803 m2/g, which is much higher than that of 914

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Figure 2. Structure of GO and its etching process. (a) Aberration-corrected TEM image of GO. (b) Color-coded TEM image of GO. The purple features are holes and the supporting carbon film, the green features are graphitic regions, and the brown features represent the disordered regions. (c) Well- and (d) poorly crystallized areas. (e−h) TEM images of the GO sheet at different reaction times: (e) before the reaction, (f) 12 h, (g) 24 h, and (h) 36 h.

(marked green) is composed of sp2 carbon with a graphene structure. The corresponding FFT image (Figure 2c) shows a highly crystalline structure and a significant sixfold symmetry. Moreover, irregular poorly crystallized areas (marked yellow) and holes (indicated in purple) can also be observed. The highcontrast disordered regions appear in the form of domains with a lateral size in the range of 5−150 nm and an average size of 45 nm, which have a similar pore size range as the synthesized PGN. Furthermore, the size distribution of domains is also similar to the pore size distribution of the synthesized PGN (Figure S8). These high-contrast disordered regions are composed of sp3 carbon, showing no sixfold symmetry in the FFT image (Figure 2d). On the basis of the nature of poorly crystallized domains in the GO sheet, we propose that these areas are highly reactive due to the sp3 bonds with oxygen-containing groups. Owing to the unstable sp3 hybridized C atoms, these atoms were preferentially oxidized by the oxidizing agent effectively. Meanwhile, the well-crystallized graphitic regions (sp2 hybridized C atoms) were preserved. With the increased reaction time, PGNs is achieved after the sp3 hybridized C atoms are exhausted and the graphitic regions with latticed structure in GO are retained. Thus, the etching process occurred in the poorly crystallized domains in the GO sheet. Figure 2e,h shows the etching process of the GO sheet. Irregular holes appear with increased reaction time. When the reaction time is increased from 12 to 36 h, the porosity increased obviously from 9.2 to 48.5% (Figure S9). It should be noted that the average pore size of PGN increased in the first 36 h. With the increase in the reaction time from 4 to 36 h, the average pore size changed from 5 to 47 nm, as shown in Figure S10. When the reaction time is 40 h or longer, the pore size and porosity are similar to those acquired from the sample deposited at 36 h (not shown here), implying that the poorly crystallized areas have been totally etched. Obviously, this pore size matched with the average lateral size range of poorly crystallized areas.

using three curves, that is, the C−C/CC peak located at 284.98 eV, the C−O peak located at 287.00 eV, and the CO peak located at 288.49 eV.24−26 The deconvolution of the highresolution O 1s spectra (Figure 1e) can be split into three different peaks centered at 535.1 eV (C−OOH), 532.6 eV (C− O/C−O−C), and 531.5 eV (CO).27−29 X-ray diffraction (XRD) patterns of GO for different reaction times are shown in Figure S6. The typical XRD pattern of the GO sheets exhibits a sharp (001) diffraction peak at around 12.1°, corresponding to a d-spacing of 0.73 nm. This interlayer distance of GO is much larger than that of pristine graphite (0.34 nm) because of the introduction of oxygen-containing groups on the carbon basal plane. With the increase in the reaction time, the diffraction peak at around 12.1° decreased. This indicates a significant reduction in oxygen-containing groups in the GO sheets as time progresses. The XRD results coincide with the XPS results. Further, the oxygen-containing edge groups in PGN are characterized using energy dispersive spectrometer (EDS) mapping under TEM. Figure S7 displays the EDS mapping image, indicating the high distribution density of O at the edge of PGN, as schematically illustrated in Figure 1f. Therefore, this structure endows PGN with both a highly crystalline structure and good hydrophilicity. To study the detailed mechanism of the pore-forming process, aberration-corrected TEM is performed to measure the microstructure of GO (the precursor of PGN). Well- and poorly crystallized areas and a small number of holes are found clearly in the GO sheets, as shown in Figure 2a. The approximate area percentages of the well-crystallized areas, poorly crystallized areas, and holes are 41, 53, and 6%, respectively. The sum total of the area percentages of poorly crystallized areas and holes is similar to the porosity of the obtained PGN, indicating that the etched area is the poorly crystallized area of GO. Color-coded Figure 2b is the corresponding one processed using the fast Fourier transform (FFT) bandpass filter of Figure 2a. The well-crystallized area 915

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Figure 3. Mechanism of the free radical oxidation process. (a) ID/IG ratio of PGN as a function of different reaction times. (b) Photoluminescence (PL) spectra of the mixture of TA + W18O49 + H2O2 and TA + H2O2 aqueous solutions after the reaction at 200 °C for 48 h. (c) Average lateral size of graphene sheets at different reaction times using the traditional and radical oxidation approaches. (d) Molecular model of sp2−sp3 hybrid carbon material in the density functional theory (DFT) calculation. (e,f) Color-coded mapping showing ΔG of C atoms at different positions in the (e) radical and (f) traditional oxidation processes.

primary oxidizing agent in this process, a free-radical-capture experiment was carried out. Terephthalic acid (TA) was used as a PL probe for the tracking •OH because it could capture •OH and generate 2-hydroxy-terephthalic acid (TAOH),30 which emits a unique fluorescence around 435 nm (inset, Figure 3c). Figure 3c shows the PL intensity change in the mixed solution of TA, W18O49, and H2O2 aqueous solution. (The concentration of H2O2 and TA is 1.0 and 0.2 M, respectively, the dosage of W18O49 is 12 mg, and the reaction temperature is 200 °C.) After 48 h, the PL intensity increased observably, indicating that abundant •OH was produced from the catalytic decomposition of H2O2. W18O49 plays a role as the catalyst in this decomposition reaction, which is demonstrated by a contrast experiment.14 As shown by the red curve in Figure 3c, the PL intensity of the mixed solution of TA and H2O2 shows an obvious increase after 48 h (reaction conditions are the same as those of the above experiment). The extremely high reactivity of •OH ensures that GO can be oxidized in the reaction. Otherwise, H2O2 cannot oxidize GO under the same reaction conditions (Figure S11), supplying evidence that the • OH is the main oxidizing agent in our approach. On the basis of the above analysis, the radical oxidation process can be distinguished from the traditional oxidation process in thermodynamics. To further confirm the mechanism, DFT is performed to design a graphene sheet with sp3 C atoms as a measure. As shown in Figure 3d, sp3 C atoms at different loci of graphene (including interstitial lattice site, edge site, and dangling bond) were taken into consideration in the DFT calculation. The Gibbs free energy change (ΔG) of the oxidation reaction was evaluated to determine the reactivity of all C atoms in both the radical oxidation process (main oxidizing agent: •OH) and the traditional oxidation process (main oxidizing agent: ClO3−). Color-coded mapping was used to show the reactivity of these C atoms. The yellow area

All of the above results confirmed the proposed etching process. Raman spectrum results further demonstrated our hypothesis. As shown in Figure 3a, the significant reduction in the ID/ IG ratio indicates the reduction of poorly crystallized areas, and the decrease in the ID/IG ratio begins to flatten after 36 h, proving that the poorly crystallized areas have been totally etched, which matches with the result of the variation trend for the average pore size. Meanwhile, the well-crystallized areas are preserved. This selective etching process is quite different from traditional oxidation approaches. In general, traditional oxidation approaches are usually carried out in a strong acid environment (such as highly concentrated H2SO4) with strong oxidants (such as KMnO4, KClO3, and K2FeO4). The immediate effect of traditional oxidation approaches is that the obtained products are always small-sized graphene sheets. Figure 3b shows the difference between traditional oxidation approaches and our approach. GO (20.0 g) was added into a mixture of HNO3 (80 mL), H2SO4 (150 mL), and 60.0 g of NaClO3, and the mixture was maintained at 15 °C for 72 h.30 The change in these sheets is shown in Figure 3b. Obviously, the average lateral size of these sheets diminishes gradually as the reaction time is varied from 4 to 60 h. After 60 h, the GO sheets are oxidized into GQDs and no obvious pore structure could be observed in these sheets. On the contrary, in the oxidative system of our approach (H2O2 + W18O49 nanowires), the average lateral size of these sheets shows no obvious change in the first 48 h. Meanwhile, a typical pore structure is obtained. The distinct reactive result of the traditional oxidation approach and our approach demonstrates the essential distinction between them. It is well known that free radicals (such as •OH) are the main oxidizing agents in the H2 O 2 reaction system. 14 We demonstrate the existence and the important role of free radicals in the preparation process. To confirm the role of the 916

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Figure 4. Dispersity and application of PGN. (a) Photograph of PGN aqueous dispersions irradiated by a red laser beam after standing for different amounts of time, displaying an evident Tyndall effect. (b) Sedimentation behavior of PGN and RGO aqueous dispersions. (c) Digital photograph of PGN ink on paper. (d) Preparation process of flexible electroconductive coating. (e) Sheet resistance and transmittance of PGN coating obtained from different values of CPGN.

coating is further dried at 150 °C for 0.5 h. The sheet resistance of the obtained film is 600 kΩ/sq, which is higher than the RGO coating (13 000 kΩ/sq, the preparation process is the same as for the PGN coating). Moreover, the sheet resistance decreased with the increased PGN ink dosage (Figure 4e). The sheet resistance is 145, 200, 420, 515, 620, and 800 kΩ/sq when the PGN ink concentration (CPGN) is 25, 20, 14, 12, 8, and 4 mg/mL, respectively. The corresponding transmittance (T) of the PGN coating is 0.02, 0.31, 0.45, 0.51, 0.64, and 0.82 when the CPGN is 25, 20, 14, 12, 8, and 4 mg/mL, respectively. The sheet resistances of these PGN films are always higher than those of RGO films, indicating a better electrical conductivity than RGO. Finally, based on the stable PGN dispersion, PGN paper with good electrical conductivity can be obtained via the routine filtration technique.31 Typically, 100 mL of 0.5 mg/mL graphene dispersion was filtered through a 50 nm membrane (AAO, Shanghai ShangMu, China), followed by drying overnight at 80 °C. A free-standing flexible PGN paper was obtained. Figure 5a,b shows the layered structure with a uniform thickness of 2.2 μm, which demonstrated the outstretched and homogeneously dispersed PGN sheets. No obvious penetrated hole can be observed in the SEM and TEM images of the PGN paper (Figure S12). The sheet resistance is 24 kΩ/sq, and the Young’s modulus is 4.3 GPa. We further show the application of PGN paper in detectable molecule separation. Highly efficient separation membranes require a highly selective rejection for specific molecules and a high water flux. We carried out the separation test for small biological molecules (urea, uric acid, creatinine, and creatine) and biomacromolecules (hemoglobin, glycogen, glutathione, and Gly−Gly) aqueous solutions (Figure 5c). As seen in Figure 5d, PGN paper with a thickness of 2.4 μm shows high rejection ratios for biomacromolecules. The rejection ratio for hemoglobin, glycogen, glutathione, and Gly−Gly is 98, 97, 99, and 96%, respectively. However, the rejection ratio for small

indicates that the corresponding C atoms have low ΔG, which means the oxidizing reaction has a strong spontaneous tendency. On the other hand, the blue area indicates high ΔG and a weak spontaneous tendency. In the radical oxidation process, the sp3 C atoms show an extremely high reactivity (Figure 3e) and the sp2 C atoms show a relatively low reactivity. This indicates that the oxidizing reaction occurred mainly at the sp3 C atoms. However, in the traditional oxidation process, not only sp3 C atoms but also adjacent sp2 C atoms show an extremely high reactivity (Figure 3f). The highly active C atoms are arranged in a zigzag manner. Intuitively, the traditional oxidation process is indeed the shearing process. After a detailed study of the mechanism, we finally explored the application this material. The obtained PGN with high porosity, highly crystalline structure, and oxygen-containing edge groups indicates that it has both a stable dispersion and good electrical conductivity. As shown in Figure 4a, 10 min of ultrasonication of 10 mg of PGN in 50 mL of H2O resulted in a homogeneous black dispersion without obvious macroscopic agglomeration, as expected. Surprisingly, no precipitation was observed after 3 days of storage, indicating the formation of a stable dispersion of graphene in water (Figure 4a). Ultraviolet− visible (UV−vis) spectroscopy is used to further verify the stable dispersion of PGN. The absorption intensity ratio (A/A0, as shown in Figure 4b) is 0.94 ± 0.08 after 3 days of storage, which means that there is also no obvious microcosmic agglomeration of PGN. The stable PGN dispersion (PGN ink) can be used to construct a flexible electroconductive coating. Figure 4c,d shows a Meyer rod setup for a laboratory-scale coating of PGN. This can be easily scaled up using a slot die or gravure coating (500 μL). PGN ink (10 mg/mL) is dropped on a 32 cm2 (4 cm × 8 cm) polyethylene terephthalate (PET) substrate. Then, a Meyer rod (RD specialist Inc.) is pulled or rolled over the solution, leaving a uniform, thin layer of PGN ink on the substrate. After the initial drying of the film at 80 °C, the 917

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CONCLUSIONS In summary, we demonstrated the selective oxidation process of the sp3 C atoms of GO in the free radical oxidation process, which is different from the shearing mechanism in the traditional oxidation process (main oxidizing agent: ClO3−). The sp3 part of GO was oxidized first, and the well-crystallized sp2 domains were reserved. The obtained PGN has a high surface area, homogeneous pore diameter, and good conductivity, which is suited for use as a separation membrane. With the stable PGN aqueous dispersion, the obtained PGN paper shows highly selective rejection for biomacromolecules in the molecule separation process. On the basis of the concentration-dependent sheet resistance, in situ electrical measuring for molecule separation process is achieved. This work deepened the understanding of the oxidation mechanism of sp2−sp3 hybrid carbon and is helpful for the morphological control of other sp2−sp3 hybrid carbon materials.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Chemicals. GO was purchased from SIMBATT (Shanghai, China) and used as received without further purification. Other chemicals were purchased from Aladdin (Shanghai, China) and used as received without further purification. The water used throughout all experiments was purified using a Millipore system. Preparation of PGN. The preparation process is as follows: W18O49 nanowires were prepared using a modified method described previously19 (experimental details are shown in the Supporting Information). Typically, 5.0 mL of 30 mg/mL GO aqueous solution was added into 5.0 mL of 2.0 M (6 wt %) H2O2 aqueous solution. Then, 12 mg of W18O49 nanowires was added into this mixture. The obtained mixture was transferred into a 15 mL Teflon-lined autoclave and heated at 200 °C for 48 h. The obtained PGN has good dispersibility in water, and the catalyst can be removed easily by keeping it still or low-speed centrifugation (800−1000 rpm, 3−5 min). The obtained PGN aqueous solution has no byproducts. Characterization. TEM measurements were carried out on a spherical aberration-corrected TEM (FEI Titan 80−300) at 80 kV. XPS was carried out on a PHI Quantera II system (Ulvac-PHI, INC, Japan). UV−vis absorption spectroscopy was recorded using a UV5800 spectrophotometer. Raman spectroscopy was conducted using a Jobin Yvon (France) LABRAM-HR confocal laser micro-Raman spectrometer at an excitation wavelength of 514.5 nm. Fluorescent emission and excitation spectra were recorded on a Perkin-Elmer LS55 luminescence spectrometer (Perkin-Elmer Instruments, U.K.)

Figure 5. Application of PGN paper in molecule separation. (a) Photograph showing the flexible graphene paper against a metallic luster. (b) SEM image of PGN paper showing a uniform thickness (2.2 μm) and a layered structure. (c) Schematic diagram of the highly selective rejection for biomacromolecules in the molecule separation process. (d) Rejection ratio of PGN paper for small biological molecules (urea, uric acid, creatinine, and creatine) and biomacromolecules (hemoglobin, glycogen, glutathione, and Gly−Gly). (e) Sheet resistance of PGN paper under different creatinine concentrations.

biological molecules is less than 5%. This indicates that both small biological molecules and H2O2 can pass through the PGN paper (as shown in Figure 5c) with a high flux (470 L m−2 h−1 bar−1). The possible mechanism for the crossing of small molecules may be due to the loose structure and the large interlayer spacing (which can be observed in Figure S12) of the PGN paper. Figure S13 shows the rejection ratio of the PGN paper with different thicknesses for urea, creatinine, hemoglobin, and glutathione. The PGN paper shows a high rejection ratio when the thickness is larger than 5 μm. This indicates the possibility of penetrated holes on a thinner PGN paper. The dynamics model or thermodynamics model will be considered for future theoretical studies. It is well known that most small biological molecules are metabolites in physiological processes that are harmful.32 The above results indicate that the PGN paper is a potential function material for hematodialysis. It is worth noting that the sheet resistance of the PGN paper obviously changes in the above selective separation process, which indicates the possibility of in situ electrical measuring for the above-mentioned molecule separation process. As shown in Figure 5e, the sheet resistance significantly reduced with the decrease in the creatinine concentration. The sheet resistance is 27.3 kΩ/sq when the creatinine concentration is 8 × 10−7 M. However, when the creatinine concentration is 5 × 10−8 M, the sheet resistance is 3.1 kΩ/sq. This remarkable change can be due to the change in the electrolyte concentration in the pores of the PGN paper.33

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b03937. Experimental details, matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDITOFMS) result of GQDs, PL emission spectra of GQDs with different excitation wavelengths (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.W.). *E-mail: [email protected] (G.D.). ORCID

Siwei Yang: 0000-0002-5227-8210 Guqiao Ding: 0000-0001-7138-5372 Notes

The authors declare no competing financial interest. 918

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ACKNOWLEDGMENTS This work was supported by projects from the Chinese Academy of Sciences (grant no. XDA02040000) and K. C. Wong Magna Fund in Ningbo University.

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DOI: 10.1021/acs.langmuir.6b03937 Langmuir 2017, 33, 913−919