Article pubs.acs.org/Langmuir
Polyelectrolyte-Stabilized Graphene Oxide Liquid Crystals against Salt, pH, and Serum Xiaoli Zhao, Zhen Xu, Yang Xie, Bingna Zheng, Liang Kou, and Chao Gao* Ministry of Education (MOE) Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, 38 Zheda Road, Hangzhou, Zhejiang 310027, People’s Republic of China S Supporting Information *
ABSTRACT: Stabilization of colloids is of great significance in nanoscience for their fundamental research and practical applications. Electrostatic repulsion-stabilized anisotropic colloids, such as graphene oxide (GO), can form stable liquid crystals (LCs). However, the electrostatic field would be screened by ions. To stabilize colloidal LCs against electrolyte is an unsolved challenge. Here, an effective strategy is proposed to stabilize GO LCs under harsh conditions by association of polyelectrolytes onto GO sheets. Using sodium poly(styrene sulfonate) (PSS) and poly[2-(methacryloyloxy)ethyl]dimethyl(3-sulfopropyl)ammonium hydroxide (PMEDSAH), a kind of polyzwitterion, GO LCs were well-maintained in the presence of NaCl (from 0 M to saturated), extreme pH (from 1 to 13), and serum. Moreover, PSS- or PMEDSAH-coated chemically reduced GO (rGO) also showed stability against electrolyte.
1. INTRODUCTION Graphene, a one-atom-thick honeycomb lattice made of carbon atoms, combines exceptionally high mechanical strength, electrical and thermal conductivities, and many other supreme properties, promising potential applications, such as flexible electronics, photonics, energy storage, composites, biomedicine, etc.1−7 However, the poor solubility of pristine graphene8 seriously hinders its chemical manipulation, fluid assembly, and bioapplications. Alternatively, graphene oxide (GO), one of the most important precursors of graphene,9,10 can be well-dispersed in water and polar organic solvents, because of its oxygencontaining groups (−OH and −COOH).11,12 Hence, GO was widely used to prepare macroscopic papers,13 polymer-based composites, 14−17 inorganic nanoparticle−graphene hybrids,18−22 biomacromolecule−graphene complexes, etc. Because of its high solubility and large aspect ratio, GO colloids would form liquid crystals (LCs) in water23−25 and polar organic solvents.26 Benefiting from the ordering of GO LCs, high-performance graphene fibers 25 and films 26 were achieved.27 The stability of aqueous GO LC is mainly contributed by its hydrophilicity and electrostatic repulsion of negative-charged carboxylate groups.12 Even though, the contribution is not strong enough to resist an increase of ion strength, because of the screen effect of ions to the electrostatic field. For instance, 50 mM NaCl would totally make GO sheets aggregate, with no mention of the formation of LC.24 The instability of GO LCs in electrolyte solution obviously hinders their applications, especially in high-performance and multifunctional composites and bioapplications, in which GO sheets have to cooperate with other charged components. Hence, the © 2014 American Chemical Society
realization of stable GO LCs in electrolyte solution and extreme pH conditions is a fundamental issue that should be eagerly addressed. High-concentration (∼8 mg mL −1) triblock copolymer, Pluronic F127,28 was ever found to protect lowconcentration GO solution (0.1 mg mL−1) from aggregation at a low NaCl concentration (0.1 M). However, the stabilization of high-concentration GO LCs in more severe surroundings remains a challenge. Likewise, almost all of the colloidal LCs, such as montmorillonoid,29 nontronite,30 and artificial nanoparticles,31 should surmount this big problem. Here, a facile strategy is proposed on the basis of hydrophobic association of amphiphilic polyelectrolytes to stabilize aqueous GO LCs in an extremely high ion strength environment. The studied amphiphilic polyelectrolytes were featured with a hydrophobic backbone and hydrophilic ionic groups (denoted as PHBIG). In water, driven by hydrophobic forces, PHBIG chains adsorb on GO sheets and effectively decrease the interfacial tension between GO and water. With the help of PHBIG of low concentration (∼0.88 mg mL−1) in the dispersion, even ultra-large GO sheet (up to 20 μm in diameter) dispersions kept their LC state for a long time (>2 days) in the surrounding of NaCl (from 0 M to saturated) and extreme pH (from 1 to 13). Notably, the PHBIG-stabilized GO LCs were well-maintained even in the complex bioenvironment of serum. Such stability against electrolyte, pH, and serum opens the door to study the fluid properties of colloidal LCs and explore their applications under extreme conditions. Received: February 11, 2014 Revised: March 7, 2014 Published: March 20, 2014 3715
dx.doi.org/10.1021/la500553v | Langmuir 2014, 30, 3715−3722
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2. MATERIALS AND METHODS 2.1. Materials. Sodium poly(styrene sulfonate) (PSS) (molecular weight of 70 000 g mol−1) and polyethylene glycol (PEG) (molecular weight of 20 000 g mol−1) were purchased from Sigma-Aldrich. [2(Methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (MEDSAH) was purchased from Changzhou Yipingtang Company (China). The water used in this study was deionized water with an electrical resistance of 18.2 MΩ cm. 2.2. Synthesis of PMEDSAH, GO, and Heavily Oxidized GO (hGO). As-received MEDSAH was directly used for polymerization. A total of 5.586 g (20 mmol) of MEDSAH was dissolved in 20 mL of deionized water in 50 mL of Schlenk. A total of 0.028 g (0.1 mmol) of 4,4′-azobis(4-cyanovaleric acid) (ACVA) was dissolved in 2 mL of deionized water, assisted by 0.0144 g of NaOH. Under N2 protection and vigorous stirring, ACVA solution was injected. The Schlenk was then immersed in a 70 °C oil bath. After reacting for 10 h, the resultant solution was precipitated in ethanol. The precipitates were collected and resolved in water again. Then, the solution was freeze-dried to obtain solid product, PMEDSAH. GO was synthesized according to the previous protocol by oxidation exfoliation of natural graphite.32 hGO was synthesized by a further oxidization of GO by KMnO4.23 2.3. Stabilization Process. The stabilization process was simply dissolving a certain amount of PSS or PMEDSAH in GO dispersion. 2.4. Characterization. Atomic force microscopy (AFM) images of GO sheets were taken in the tapping mode on a NSK SPI3800, with samples prepared by spin coating at 1000 revolutions per minute (rpm) from dilute aqueous solutions onto freshly exfoliated mica substrates. Scanning electron microscopy (SEM) images were taken on a Hitachi S4800 field-emission SEM system. Cryo-SEM images were taken on a Hitachi S3000N equipped with a cryotransfer. Polarized optical microscopy (POM) observations were performed with a Nikon E600POL, and the liquid samples were loaded into homemade planar cells with a light path length of 1−1.5 mm. Macroscopic photos under crossed polarizers were taken by Canon 600D, with samples being placed between two slides of polarizers (10 × 10 cm). The measurement of ζ potential was performed on a ZET3000HS apparatus. An ultraviolet−visible (UV−vis) spectrum was performed on a Cary 300 Bio UV−vis spectrophotometer. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a PHI 5000C ESCA system operated at 14.0 kV, and all of the binding energies were referenced to the C 1s neutral carbon peak at 284.8 eV.
Figure 1. (a) GO dispersions (1 mg mL−1) at 0.5 M NaCl stabilized by PSS (0.88 mg mL−1), Pluronic F127 (1 mg mL−1), and PEG (20 mg mL−1, on the basis of the reported concentration in ref 34) located under (top) natural light and (bottom) crossed polarizers. (b) GO dispersions (2 mg mL−1) in the presence of 0−1.5 M NaCl, stabilized by 0.88 mg mL−1 PSS.
VR = K1 n0 exp( −K 2 n0 d)
(1)
where K1 and K2 are two positive constants, d is the distance between the two plates, and n0 is the electrolyte concentration in the solution. From eq 1, one can see that VR decreases exponentially along the increase of n0 (representative curves are shown Figure S1b of the Supporting Information). This relationship can describe the sensitivity of GO dispersion stability to ion strength. To overcome the sensitivity, we choose PHBIG, such as PSS, to play the role of stabilizer. With 0.88 mg mL−1 PSS pre-dissolved in GO dispersion, the GO LC state survived the high ion strength (0.5 M NaCl) and even the interference color was preserved (images 3 and 8 of Figure 1a). By comparison, the previously reported stabilizers of adsorbing triblock polymer Pluronic F12728 and non-adsorbing PEG34 cannot stabilize GO solution at such a high ion strength (images 4, 5, 9, and 10 of Figure 1a). As the GO concentration increased from 1 to 2 mg mL−1, 0.88 mg mL−1 PSS can still preserve the GO LC state at a NaCl concentration up to 1.0 M (Figure 1b). To directly visualize the ordered structure in the LC solution, more concentrated GO dispersion (5 mg mL−1) stabilized by 15 mg mL−1 PSS at 0.5 M NaCl concentration was characterized by cryo-SEM. As shown in Figure 2, no aggregates occurred and the GO sheets oriented well in a large scale. From Figure 2, we can see that the nematic GO LC also showed local lamellar ordering, similar to the neat GO solutions25 and other two-dimensional clay LCs.30 In the case of PSS-stabilized GO dispersion, upon the further increase of the NaCl concentration, the black brushes became vague seen under crossed polarizers (Figure 3a) but no
3. RESULTS AND DISCUSSION 3.1. Stabilization of GO Dispersions by PSS against Electrolytes. The as-prepared GO dispersion (1 mg mL−1) in pure water was homogeneous, as seen under natural light (image 1 of Figure 1a). Under crossed polarizers (image 6 of Figure 1a), the fluid shows vivid schelieren textures consisting of dark and bright brushes, indicating their nematic mesophase nature.23−26 The optical-pass difference of light would generally make a LC sample show interference colors under crossed polarizers. Indeed, the observed interference color in image 6 of Figure 1a was from red to green, indicating that the optical-pass difference for the sample in the bottle (12 mm in diameter) was about 400 nm or integral multiples of 400 nm. Upon the addition of 0.5 M NaCl, the GO sheets aggregated to form suspension of millimeter-scale particles that can be observed by the naked eye (2 of Figure 1a and Figure S1a of the Supporting Information); correspondingly, under crossed polarizers, the birefringence totally disappeared (image 7 of Figure 1a), demonstrating the sensitivity of GO dispersion and LC to ion strength. The stability of GO aqueous dispersion greatly relies on the electrostatic repulsion between GO sheets. The arealspecific electrostatic repulsive energy (VR) between two parallel charged plates can be written as follows:33 3716
dx.doi.org/10.1021/la500553v | Langmuir 2014, 30, 3715−3722
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accelerate the sedimentation of particles, the upper layer dispersions were taken out for transmittance spectrum detection. As shown in Figure 3b, PSS-stabilized GO dispersions at NaCl concentration of ≤2.5 M shared almost the same transmittance curve with the neat GO dispersion, confirming their good stability. The transmittance of the solution in 3 M NaCl was higher than the others, owing to the settling down of the aggregates during the centrifugation process. Upon further increase of the PSS concentration to 4−6 mg mL−1, the NaCl critical coagulation concentration shifts to 4 M. However, PSS cannot make GO dispersions stable at the NaCl concentration of ≥4 M, which can be ascribed to the decreased solubility of PSS in such high ion strength surroundings. 3.2. Stabilization of GO Dispersions by PMEDSAH against Electrolytes. One can expect that an appropriate kind of PHBIG with better solubility in electrolyte solution may maintain the quality of GO LC at higher NaCl concentrations. Here, we selected PMEDSAH, a polyzwitterion-natured PHBIG, to test such a deduction. Unlike the “salting out” behavior of neutral polymers and common polyelectrolytes, polyzwitterions can be well-solvated in high ion strength solutions.36 Also, because of their high hydration ability, polyzwitterions hold excellent hemo- and biocompatibility and protein resistance.37 PMEDSAH is a typical PHBIG that holds a hydrophobic backbone and zwitterionic side groups. Figure 3c shows the stabilization effect of PMEDSAH to GO LCs in saturated NaCl solution (6.2 M) with 3−4 mg mL −1 PMEDSAH and 0.5 mg mL−1 GO dispersions, showing clear birefringence of LC, indeed, much better than the case of PSS. Under even harsher conditions of centrifugation at 3000 rpm
Figure 2. Cryo-SEM images of 5 mg mL−1 GO LC in 0.5 M NaCl, stabilized by 15 mg mL−1 PSS. The scale bars are (a−c) 100 μm and (d) 50 μm.
aggregates were seen yet with close view under natural light. At NaCl concentration of ≥3 M, small aggregates emerged, although PSS-stabilized GO dispersion still showed some degree of birefringence. The aggregates could not be redispersed by either shaking or ultrasonication. We can deduce that small-sized GO sheets were still “soluble” under that condition and form the LC anisotropy.35 The aggregates grew in number and size, upon the further increase of the NaCl concentration. The instability of the colloids above the critical coagulation concentration (3 M NaCl in the PSS-stabilized case) can be magnified by the change in the transmitted spectrum curve of the dispersions. After 5 min of centrifugation at 3000 rpm to
Figure 3. (a) (Image 1) As-prepared 1 mg mL−1 GO and (images 2−10) 1 mg mL−1 GO dispersions stabled by 0.88 mg mL−1 PSS with NaCl concentrations of 0.5, 1, 1.5, 2, 2.5, 3, 5.5, 6, and 6.2 M, respectively. (b) Transmittance spectrum of the supernatants after centrifugation from PSSstabled GO at a series of NaCl concentrations. (c) (Images 1−4) GO (0.5 mg mL−1) in saturated NaCl solutions with PMEDSAH at 0, 2, 3, and 4 mg mL−1, respectively, (images 6 and 7) GO solution (2.5 mg mL−1) with PMEDSAH at 0 and 20 mg mL−1 in saturated NaCl, respectively, (image 5) as-prepared 0.5 mg mL−1 GO, and (image 8) as-prepared 2.5 mg mL−1 GO. (d) Transmittance curve of the supernatants after centrifugation from PMEDSAH-stabilized GO with a series of PMEDSAH concentrations in saturated NaCl solution. 3717
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for 5 min, 0.5 mg mL−1 GO stabilized by 4 mg mL−1 PMEDSAH in saturated NaCl solution showed the same stability as the neat GO dispersion without salt, as demonstrated by their almost overlapping transmittance curves in the UV−vis spectra (Figure 3d). Given a longer time of centrifugation (∼1 h), the PMEDSAH-stabilized GO in saturated NaCl solution still kept uniform dispersion, whereas GO sheets already settled down for the neat GO dispersion without additional salt. This indicates the ultrahigh stability of PMEDSAH-stabilized GO in saturated NaCl solution in a gravitational field, because the density (∼1.3 g cm−3) of saturated NaCl solution is close to the density of GO (1.32 g cm−3).23 We suggest this effect benefits the experimental study for a colloidal system that needs to exclude the influence of the gravity field. It is noteworthy that, in a salt-free environment, the presence of PMEDSAH would bring about slight aggregation of GO dispersion and decline of GO LC quality. Because each monomer unit has both anionic and cationic groups, polyzwitterion holds strong inter- and intrachain interactions in low ion strength, leading to their self-association. The depletion attraction force of PEMDSAH association particles is likely responsible for the GO aggregation in salt-free surroundings. Upon the increase of ion strength, their interand intrachain interactions are gradually broken and the molecular chains become solvated, isolated, and stretched. In our case, with the addition of NaCl up to 0.5 M to the PMEDSAH−GO dispersion, the aggregates can be redispersed well by hand-shaking. 3.3. Stabilization of GO LCs in 0−6.2 M NaCl Solutions. On the basis of the results aforementioned, we draw the phase diagram of GO LCs in NaCl solutions in the presence of PSS (Figure 4a) and PMEDSAH (Figure 4b). Generally, PSS can stabilize GO LCs in 0−3.5 M NaCl solution; the proper NaCl concentration for the case of PMEDSAH stabilizer is 0.5−6.2 M (saturated). The combination use of PSS (1.5 mg mL−1) and PMEDSAH (4.0 mg mL−1) rendered GO LC stability at 0 M to the saturated NaCl concentration (Figure 4c and Figure S2 of the Supporting Information). 3.4. GO LCs in Acid, Base, and Multivalent Ions and Serum. Besides NaCl solution, PSS-stabilized GO also kept good quality of LC in a wide pH range of 1−13, adjusted by the addition of hydrochloric acid or NaOH (Figure 5a). Both solutions at pH 1 and 13 were absent of aggregation with careful view. On the contrary, GO without stabilizer severely aggregated at pH 1 and 13. Upon further addition of 0.5 M NaCl to the dispersions, the PSS-stabilized GO still remained stable at pH 1 (see Figure S3 of the Supporting Information). Despite the much stronger coagulation ability of multivalent ions, PSS-stabilized GO also showed considerable tolerance to multivalent ions. As shown in Figure 5b, in the presence of 12 mM CaCl2, GO immediately flocculated, while PSS-stabilized GO showed distinct black brushes under crossed polarizers, indicating the good LC quality. The PSS-stabilized dispersion held fine LC textures with increasing the concentration of CaCl2. At a CaCl2 concentration up to 50 mM, LC birefringence was observed with slight aggregation. Furthermore, PSS also stabilized the GO LCs in the presence of trivalent ions, such as Fe3+ (
