Controlled Growth of N-Doped and Large Mesoporous Carbon

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Controlled Growth of N‑Doped and Large Mesoporous Carbon Spheres with Adjustable Litchi-Like Surface and Particle Size as a Giant Guest Molecule Carrier Wenquan Huang, Hui Xie, Yong Tian,* and Xiufang Wang* School of Pharmacy, Guangdong Pharmaceutical University, Guangzhou 510006, China S Supporting Information *

ABSTRACT: N-doped mesoporous carbon nanospheres (NMCNs) with tunable particle size, pore size, surface roughness, and inner cavity are extremely important for the future development of new carriers for nanoencapsulation, high-performance giant molecule transport, and cell uptake. However, constructing such a multifunctional material via a simple method still remains a great challenge. Herein, a controlled growth technology was developed for the first time to synthesize such NMCNs based on the initial reaction temperature (IRT) and solution polarity. In this strategy, the IRT not only can adjust the micelle aggregation to obtain NMCNs with large mesopores but also can make the F127 micelle more lyophobic to prepare hollow N-doped mesoporous carbon spheres, which is a great breakthrough. Inspiringly, by varying the solution polarity to make nonuniform growth of nanoparticles, the litchi-like rough surface of NMCNs was obtained, which could significantly improve the cell uptake performance of NMCNs. The current understanding of nucleation and growth mechanism of nanospheres was further extended and realized the development of NMCNs with large mesopores and litchi-like rough surface, which provided a new and interesting fundamental principle for the synthesis of NMCNs. The mesoporous structure of NMCNs was successfully reverse-replicated by nanocasting of tetraethylorthosilicate to obtain mesoporous silica spheres (MSNs), revealing the easy transformation between NMCNs and MSNs. Insulin as a peptide drug cannot be directly administered orally. But it can be used as oral preparation after being loaded into NMCNs, which has never been reported before. Interestingly, the results of the animal experiment showed an excellent in vivo hypoglycemic activity. This finding provides a new paradigm for the fabrication of structurally well-defined NMCNs with a great promise for drug carriers. KEYWORDS: N-doped, nucleation and growth mechanism, mesoporous carbon spheres, hypoglycemic activity, insulin

1. INTRODUCTION Recently, constructing carbon nitride materials with a porous structure has attracted worldwide attention because they display the combined properties of mesoporous carbon (large pore volumes and high surface areas) and nitrogen-doped carbon framework (improved basicity and enhanced surface polarity1). With these characteristics, carbon nitride materials have been extensively used in catalysis, biomedical applications, and industrial adsorption and separation. 2−10 However, the performance of traditional carbon nitride materials in the adsorption and loading of giant molecules is not optimal because of their small pore size, which only allows the delivery of relatively small molecules at a low rate.5,11,12 For example, micropores (less than 2 nm) are too small to effectively load and fast transport large-size guest molecules (e.g., RNA, gene, and dye) and nanoparticles (e.g., noble metal nanoparticles).13 For decades, creating large mesopores (>10 nm) is high pursuit to load this substances.14−16 In addition, for the purpose of efficient loading and fast mass transport, one should prepare materials not only with large pore size but also with regular morphology. In this sense, if the spherical morphology and © XXXX American Chemical Society

rough surface could be introduced in such a large mesoporous structure, interesting applications might further arise by providing short pathways for mass transport and enhancing adhesion toward “hairy” objects and cell uptake properties.17,18 Nevertheless, to date, developing a novel synthesis route to gain such N-doped mesoporous carbon nanospheres (NMCNs) with large mesopores (>10 nm) and rough surface still remains a challenge but highly desirable for the improvement of adsorption, loading, and transport performance. Thus, the precise design and controlled synthesis of NMCNs are of considerable significance. Traditionally, mesoporous silica and specialized diblock copolymers have been used as templates to prepare large mesopores.19−23 The former typically involves complicated, time-consuming multistep procedures;24 whereas the latter templates are generally not commercially available and require the use of strong organic cosolvents [such as tetrahydrofuran Received: February 3, 2018 Accepted: April 23, 2018

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DOI: 10.1021/acsami.8b02040 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

time, the synthesis was carried out at an IRT as low as 5 °C to adjust the mesoporous size through controlling the selfassembly of F127 micelles. Simultaneously, by adjusting the IRT to control the nucleus growth process, the particle size increased with the increase of IRT. Inspiringly, at relatively high IRT, the hollow N-doped mesoporous carbon spheres (HNMCSs) were harvested. Moreover, the surface morphology was tunable by varying the volume ratio of ethanol/water (E/ W) to guide the aggregation process of the template and carbon precursor. Under the highly acidic condition, both F127 and phloroglucinol were protonated. This would enhance the Coulombic interactions between the templates and carbon precursors through the I+X−S+ mechanism of F127+−Cl−− (phloroglucinol−urea−formaldehyde)+ (F127+−Cl−−PUF+). More importantly, preliminary studies on the transport performance of the well-designed NMCNs showed high bioavailability of insulin by direct oral administration (note that insulin is a protein drug that cannot be administered orally). It is expected that this system of NMCNs/insulin will reduce the pain of traditional twice-daily subcutaneous injections of insulin and show promising potential in diabetes treatment.

(THF) and benzene], which restrict their scalable production.14,25−27 It is therefore desirable to fabricate NMCNs with large mesopores by using traditional and low-cost commercial templates (e.g., F127). However, so far few research studies have succeeded in using this method. The difficulties mainly lie in the molecular composition limitation of F127 and the enormous shrinkage of phenolic resin frameworks during pyrolysis, which would result in the formation of only small pores (∼2.0 nm).28 A few years ago, Liu reported that using phloroglucinol as the carbon sources was benefited to obtain ultralarge mesoporous carbon with commercially available F127 as a template.29 Unfortunately, such ultralarge mesopores were mainly formed by the packing of nanoparticles but not really by the removal of F127. Moreover, the morphology was not spherical. Similarly, Dai used phloroglucinol to fabricate mesoporous carbon but with a nonspherical shape and small mesopores.30 On the basis of the careful analysis for these earlier studies, we found that ethanol was often used as a radical trapping agent to effectively control the polymerization rate of carbon precursors. Yet, there was no notable improvement in the mesoporous size because ethanol also acted as a cosolvent to reduce the micelle sizes by decreasing the aggregation number.31 Therefore, are there any possibilities to obtain large mesoporous carbon nanospheres by simply using phloroglucinol and F127? The answer is still a suspense. It is known that phloroglucinol has highly active sites toward electrophilic aromatic substitution, and it is difficult to control the reaction rate to synthesize the targeted mesoporous and nanospherical structures. Therefore, phloroglucinol, up to now, is rarely used to prepare NMCNs compared with phenol and resorcinol. Within this context, if we could find a new and appropriate strategy (but no utilizing cosolvent) to control the polymerization of phloroglucinol and simultaneously adjust the micelle size of F127, there would be a possibility to obtain large mesoporous NMCNs by using phloroglucinol and commercially available F127. In addition, as a giant molecular carrier, only possessing large mesopores is far from enough for NMCNs. To enhance cell uptake and drug delivery, NMCNs with adjustable morphology (particle size and rough surface) are indispensable. From the theory of nucleation and growth, the final morphology results from the competing effects of the growth of the nucleated domains.32 Therefore, in order to tailor their particle size and surface morphology, it is important to control the nucleation growth of NMCNs.33 For this, under highly acidic condition, the Coulombic force plays a key role in the growth of the nucleated domains. Note that in acid, both the PEO block of F127 and phloroglucinol (pKa is around 9.834) can be protonated and the self-assembly of F127, Cl−, and resin can be driven by long-range Coulombic interactions through the I+X−S+ mechanism34,35 (where I+ is a protonated resin moiety, X− is the halide anion, and S+ is the cationic surfactant). Therefore, a novel morphology (with adjustable particle size and rough surface) can be produced by exploiting Coulombic force to control the nucleation dynamics. To achieve these goals, we developed an effective strategy to adjust the surface roughness, particle size, and mesoporous size of NMCNs through controlling the initial reaction temperature (IRT) and polarity of the system under highly acidic condition. At low IRTs, the micelles can be larger (compared with those at high temperatures) because of their more flexible shapes and less tight aggregation. Simultaneously, the polymerization reaction of phloroglucinol can be controlled.29 For the first

2. EXPERIMENTAL SECTION 2.1. Chemicals. F127 (Mw = 12 600, PEO106PPO70PEO106) was purchased from BASF. Fluorescein isothiocyanate (FITC), 3-(4,5dimethylthiazole)-2,5-diphenyltetrazolium bromide assay (MTT), Dulbecco’s modified Eagle’s medium (DMEM), glutaraldehyde, 1,3,5-trimethylbenzene (TMB), fetal bovine serum (FBS), phosphate-buffered saline (PBS), HCl (38%), ethanol, formaldehyde (37%), anhydrous THF, phloroglucinol, urea, and tetraethylorthosilicate (TEOS) were from Guangzhou Chemical Corp. (China). All reagents were used as received without further purification. 2.2. Synthesis of NMCNs with Large Mesopores. The synthesis of NMCNs was carried out at low temperature and in highly acidic solution. In a typical reaction, 0.72 g of phloroglucinol, 1.2 g of F127, and 18 mL of HCl were added into 120 mL of deionized water and stirred at vigorous IRTs (e.g., 25, 15, or 5 °C) for 20 min. After that, 0.9 g of urea was added and stirred for 10 min. Next, 0.42 mL of formaldehyde was added dropwise to the solution. Then, the solution was kept stirring at IRT for 10 h and at 60 °C for 24 h. During the reaction, the color of the aqueous solution changed from colorless to milk white and finally to pink as the degree of cross-linking increased. The resulting polymer nanospheres (PNs) were collected by centrifugation at 14 000 rpm for 10 min and washed with water three times. Then, the PNs were dried at 45 °C for 4 h and 100 °C for 2 h. Carbonization was carried out under a N2 atmosphere at 350 °C for 2 h and then at 600 °C for 2 h at a heating rate of 1.5 °C/min, leading to the formation of NMCNs. As a comparison, a series of carbon materials were obtained by using different molar ratios of phloroglucinol/formaldehyde (P/F) from 0.87 to 1.41. The corresponding synthesis parameters are listed in Table S1. To tune the surface roughness, the E/W volume ratios (15:105, 30:90, and 60:60) were varied during the synthesis process, whereas other parameters were kept constant. Note that ethanol was added before HCl during the synthesis process. 2.3. Synthesis of Hollow and N-Doped Mesoporous Carbon Spheres. Interestingly, the synthesized material can be controlled to form HNMCSs by simply varying the IRT to 40 °C during the synthesis process, whereas the other procedures remain the same as those of NMCNs. 2.4. Preparation of Mesoporous Silica Nanospheres. The NMCNs with large mesopores were used as the template to prepare mesoporous silica spheres (MSNs) by nanocasting. NMCNs (0.2 g) were added into a mixture containing 3.4 mL of ethanol and 0.2 mL of TEOS in a closed container. After 12 h, the lid was taken off to allow natural evaporation of ethanol for 8 h and then dried at 100 °C for 2 h. B


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Figure 1. Representative SEM images (a,b) and the corresponding AFM topographies (c,d) of NMCNs with a litchi-like surface. The panel (a) inset is the photograph of litchi. The height profile of the white line in (c) is depicted in panel (e). Finally, the obtained products were calcined at 600 °C for 2 h at a heating rate of 1 °C/min in air. 2.5. Cytotoxicity Assays of NMCNs. The cytotoxicity of NMCNs was evaluated using L929 as cell lines according to the MTT assay method. The cells were grown in culture flasks in a complete medium consisting of DMEM supplemented with 10% (v/v) FBS and placed in a standard incubator at 37 °C in an atmosphere of 5% CO2 and 95% relative humidity. After that, the cells (1 × 104) were seeded into 96-well plates and allowed to attach to the wells for 24 h. Then, the medium was removed, and the cells were washed twice by using fresh PBS. The NMCN solution (100 μL, containing fresh medium) at different concentrations (1000, 100, 10, 1, and 0.1 μg/ mL) was added to the cells and incubated for 48 h. Next, the medium was removed. In addition, 100 μL of MTT (0.5 mg/mL), consisting of DMEM and 10% FBS, was added to each well and incubated at 37 °C for 4 h. At the end of the assay, the obtained formazan granules were dissolved in dimethyl sulfoxide. The measurements were taken by a microplate reader at 570 nm. Cell viability (%) was calculated by the following equation: cell viability (%) = (mean optical density of the treatment group/mean optical density of control) × 100%. 2.6. Cell Uptake Assay. FITC-grafted NMCNs were fabricated according to a literature report.28 Prior to the graft, NMCNs were treated with H2O2 (30%) overnight. Next, 5 mg of dried NMCNs and 7 mg of FITC were dispersed in 10 mL of THF and stirred at room temperature in the dark for 48 h. The products were collected by centrifugation and washed with PBS solution three times. Thus, the FITC-grafted NMCNs were obtained. The L929 cells (1 × 104) were seeded in a culture dish consisting of DMEM supplemented with 10% (v/v) FBS and maintained at 37 °C in an atmosphere of 5% CO2 and 95% relative humidity. After 48 h of incubation, the cells grew with attaching on the culture dish. Next, 3 mL of complete medium containing FITC-grafted NMMS (0.83 wt %) was added into the dish. After incubation in 5% CO2 at 37 °C for 1.5 h, the cells were treated with trypsin, centrifuged, and washed with PBS three times. Finally, the cells were fixed with 400 μL of paraformaldehyde (4% in PBS solution). The samples were analyzed using the flow cytometry. About 10 000 events were obtained for each sample. For the scanning electron microscopy (SEM) studies, L929 cells (1 × 104) were incubated with NMCNs (0.86 mg/mL) containing fresh medium in the culture dish for 1 h. Next, the medium was removed.

Then, products were fixed by 2.5% of glutaraldehyde for 1 min. The dehydration process was carried out by using ethanol with concentrations of 30, 50, 70, 80, 90, 95, and 100%. The culture dishes were cut into small pieces and pasted in the SEM specimen stage. After spraying with platinum, the particle-cell adhesion was observed by SEM. 2.7. Hypoglycemic Activity of NMCNs/Insulin by Oral Delivery. 2.7.1. Building of the Diabetic Rat Model. To build a diabetic rat model, the healthy rats were treated with streptozotocin (in 0.1 mol/L citric acid/sodium citrate buffer) by intraperitoneal injection at a dose of 50 mg/kg. Because of the instability of streptozotocin in high temperature and light, this special medicine must be freshly prepared each time. After 2 h, the rats were fed with glucose (5 wt %) to prevent rat death because streptozotocin would destroy pancreas islet B cells to release an amount of insulin into the blood. After 7 d, a glucometer was used to test fasting glucose levels (after fasting for 6 h). If this value is greater than 11.1 mmol/L, the diabetic rat model would have been successfully obtained. 2.7.2. Sample Preparation. NMCNs (74.0 mg) and insulin (3.7 mg, containing 100 IU of insulin) were dispersed in 19.0 mL of PBS solution (pH = 7.4). Then, 1.0 mL of HCl (1.0 mol/L) was added. Next, the samples were shaken for 30 min and rested at 4 °C overnight. The obtained oral suspension (the insulin concentration was 5 IU/mL) was labeled as the treatment group. For the blank control group, the sample was prepared similar to that of the treatment group but without the addition of NMCNs. For the positive control group, the amount of insulin was reduced to 0.37 mg with no NMCNs, whereas the other condition was remained the same as the treatment group. 2.7.3. In Vivo Hypoglycemic Activity Studies. The Sprague-Dawley rats were provided by Guangdong Medical Laboratory Animal Center. To detect the hypoglycemic activity, the rats were fasted for 2 h. For the treatment group and the blank control group, the diabetic rats (n = 10) were treated with oral suspension (at a dose of 50 IU/kg) by gavage. For the positive control group, the diabetic rats (n = 10) were treated with suspension at a dose of 5 IU/kg by hypodermic injection. After that, the glucose levels were tested every 2 h.

3. CHARACTERIZATION SEM measurements were carried out with a Merlin highresolution field emission electron microscope (Germany). C


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polarity might play a critical role in guiding the nucleation and aggregation process of F127+−Cl−−PUF+ particles. Under this hypothesis, we speculated that the convex structure should come from one unit of the F127+−Cl−−PUF+ particle and the PN (∼140 nm) should be consisted of a number of such units. To confirm this hypothesis, a detailed polarity-dependent study was conducted where the amount of ethanol was varied. As expected in Figure 2, the surface morphology strongly

Transmission electron microscopy (TEM) measurements were conducted on a 200 kV electron microscope (JEOL 2100F). Atomic force microscopy (AFM) studies were performed with a Bruker MultiMode 8 instrument (Germany). X-ray photoelectron spectroscopy (XPS) analysis was performed by using an ESCALAB 250 photoelectron spectrometer (Thermo Fisher Scientific, USA). The Brunauer−Emmet−Teller (BET) surface area was determined by using a volumetric adsorption analyzer (Micromeritics Tristar 3020) with nitrogen adsorption. The pore size distribution (PSD) curves were calculated by density functional theory (DFT)-based approaches. Fourier transform infrared spectra were acquired with a Bruker VERTEX 70 infrared spectrometer at a resolution of 4 cm−1. Dynamic light scattering (DLS) was conducted at 25 °C with a ZS Nano S (Malvern Inc., England) instrument. The absorbance was detected by a microplate reader (Multiskan FC, Thermo Fisher Scientific Corporation). The cellular uptake was measured by flow cytometry (BD FACScalibur).

4. RESULTS AND DISCUSSION The NMCNs were synthesized via a controlled growth process under highly acidic conditions. From the SEM images (Figure 1a,b), it can be found that the NMCNs were decorated with a number of nanoparticle bumps (10−20 nm), forming a litchilike morphology (inset to panel a). Consistent results were obtained in AFM topographic measurements (Figure 1c,d) and line scan (Figure 1e), where the roughness factors Rq and Ra were estimated to be 11.2 and 8.89, respectively.36 Though such an unusual structure has been reported for many potential applications,18,37,38 the formation mechanism remains largely elusive. Therefore, the understanding of such a formation principle seems so crucial to develop this kind of new materials. For this, there are three hypotheses to explain this phenomenon: (1) the removal of F127 micelles led to the formation of the bumpy surface; (2) the severe shrinkage of the carbon structure resulted in a rough surface during the carbonization process; and (3) the solution polarity might guide the nucleation process of nanospheres and subsequently directed the aggregation of F127+−Cl−−PUF+ particles in an unusual way (deposition on the nucleus of PN). With the doubt raised by this observation, we started to explore the forming principle of NMCNs in detail. When we attempted to apply the first hypothesis to the polymer synthesized without calcination, we found a conflict. The synthesized PN without carbonization (Figure S1) also possessed a rough surface similar to the NMCNs, suggesting that the rough morphology was not a result of F127 removal during pyrolysis. To gain more insight into the formation principle, a control experiment without adding F127 was conducted. As demonstrated in Figure S2, the rough surface of the polymer before carbonization was clearly observed, which further indicated that the bumpy surface was not caused by the removal of F127. Taken together, these results suggest that the first hypothesis is unlikely. Moreover, in Figure S1, this was only a small difference in the particle size between PN (∼145 nm) and NMCNs (∼120 nm), which showed no indication of severe shrinkage of nanospheres. This observation suggested that the second hypothesis also did not hold. On the basis of careful observation from the SEM images of PN (Figure S1), one can see that the convex size was about 25 nm. This value was similar to the hydrodynamic radius of F127−HCl−phloroglucinol−urea composites (Figure S3a). It inspired us to focus on the third hypothesis that the solution

Figure 2. NMCNs with tunable surface roughness prepared at different E/W ratios: (a) 0:120; (b) 15:105; (c) 30:90; and (d) 60:60.

depended on the polarity of the solution. When the E/W volume ratio reached the upper limit of 60:60, the obtained nanospheres showed a smooth surface (Figure 2d), whereas at the low limit of 0:120, a bumpy surface was formed (Figure 2a). Both phenomena implied that higher polarity was probably essential to obtain rougher surfaces. To reveal the source of the nanoparticles at the NMCN surface, DLS measurement was used to monitor the size of nanoparticles. At 25 °C, the hydrodynamic radius detected with solution b (consisted of F127, HCl, and water) was about 7.1 nm (Figure S3b). After the addition of phloroglucinol and urea, this value increased to ∼25 nm (Figure S3a), which was similar to the size of nanoparticles at the surface of NMCNs synthesized with the IRT at 25 °C. This finding provided an evidence that the bump-like nanoparticles were one unit of F127+−Cl−−PUF+. The above speculation is clearly shown in Figure 3, which was also called the extended nucleation and growth mechanism. Because of the fast polymerization and self-assembly of F127+− Cl−−PUF+, a rapid burst of nuclei occurred. To reduce the surface free energy, F127+−Cl−−PUF+ in a thermodynamically unstable state was deposited on the surface of a nucleus spontaneously.39 A continuous cascading of nuclei led to the growth of the nanoparticles.40 At a high HCl concentration, fast polymerization and self-assembly would lead to the nonuniform growth in a short time40 and hence a rough surface. Upon the addition of ethanol, the rate of condensation was controlled because the polar protic solvents might decelerate the condensation rate by deactivating the nucleophile through hydrogen-bonding interactions.41 Thus, the uniform growth of the nanospheres occurred and then smooth surfaces were harvested. From the SEM images of NMCNs (Figure 4), one can see that the particle size is dependent on the IRT. The higher the IRT is, the larger the particle size. With the increase of IRT from 5 to 25 °C, the particle sizes increased from ∼75 to ∼400 D


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Figure 3. Schematic illustration of (a) NMCNs with adjustable surface roughness. The nucleus consists of F127+−Cl−−PUF+. Step 1: further growth. Step 2: carbonization. (b) Structure of F127+−Cl−−PUF+ localized polymerization in the PEO domain.

form more nuclei, consequently resulting in a smaller particle size.42−44 In fact, there was no reason to doubt the control of smaller particle size at higher IRT reported by Lu et al.43 However, for the samples prepared under our conditions, another intriguing regularity for the particle growth affected by the IRT was found, which may further supplement and perfect the heterogeneous nucleation and growth theory. It can be conjectured that the major difference between Lu’s and our synthesized condition was the catalyst. In Lu’s synthesis, 1,6diaminohexane was used as a weak basic catalyst (the pKa value was 9.8345), resulting in a relatively low nucleation rate.43 For our synthesis process with a high concentration of HCl (1.3 mol/L) and highly active site of phloroglucinol, the catalysis rate was so high that a rapid burst of nuclei formed immediately. Therefore, it seemed that the nucleation stage was not IRT dependent and the number of nuclei at different IRTs was the same. However, during the growth stage, the IRT seemed more important, where higher IRT would facilitate particle collisions and accelerate the growth of nanospheres. As a result, larger nanospheres were formed. In addition, as shown in Figure 4, the particle size became increasingly uniform with decreasing initial temperature, as manifested in the particle size histograms (Figure S4), which further supported the role of initial temperature in the control of the particle growth. The mesoporous structures of the as-prepared samples were then characterized by high-resolution SEM measurements (Figure 5a−e). For the sample synthesized at 5 °C, the formation of a large mesoporous structure was obvious and the mesopores with different shapes, such as spherical (Figure 5b,c), crescent-like (Figure 5d), and rodlike (Figure 5e), were harvested. TEM studies further showed the large mesopores with different shapes and pore sizes (Figure 5f,g). The pore size analysis suggested that the mesopores were predominantly 18.7

Figure 4. SEM images of NMCNs prepared at different IRTs. (a,b) 25; (c,d) 15; and (e,f) 5 °C.

nm, which implied that the IRT was another important factor for the nanospherical growth. Confusedly, this result conflicted with the heterogeneous nucleation and growth theory, which clarified that higher temperature would facilitate nucleation and E


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were formed (Figure 6c). At 15 °C, rodlike mesopores appeared (Figure 6f). With a further decrease of the temperature to 5 °C, crescent-like mesopores were formed (Figure 6i). The change of mesoporous shapes implied that lower IRT led to larger and looser aggregation of F127 micelles. Moreover, at 5 °C, the curvature of F127 micelles might be changed, leading to the formation of crescent-like mesopores. From the Helfrich bending energy model, it was noted that micellar structures possessed a spontaneous curvature in three dimensions.33 Because of the fact that the presence of nanospherical surface restricted micellar motion, in our synthesized condition, the shape of micelles became crescentlike. Moreover, to further investigate the density and porosity gradients across the NMCNs, we synthesized MSNs replica structures of NMCNs (synthesized at 15 °C) by nanocasting. The MSNs were synthesized by natural evaporation of ethanol to induce capillary force to cast TEOS into the interior voids of NMCNs. After carbonization in air, the pore wall of NMCNs would become the mesopores of MSNs. The TEM images (Figure S6) showed that the MSNs were successfully replicated from NMCNs, implying that the isolated mesopores were interconnected by micropores. Note that the mesopores (5.0 nm) were smaller compared with that of NMCNs (9.1 nm). Nitrogen adsorption analysis was used to test the mesopores of mesoporous silica (Figure S7). The PSD curve showed that the average pore size was 5.0 nm, suggesting a relatively narrow pore wall of NMCNs. This would be another strong evidence of the large mesopores of NMCNs. Nitrogen sorption was further used to investigate the influence of the IRT on the mesoporous structure. As shown in Figure 7, all of the samples displayed type IV adsorption isotherms, indicating mesoporous characteristics with capillary condensation.47,48 Notably, the samples synthesized at 25 and 15 °C had only one hysteresis loop within the relative pressure (P/P0) range of 0.40−0.90, but the sample synthesized at 5 °C showed two (one at 0.40−0.75 and another at 0.75−0.90). This implied that the former had only one kind of mesopore size and the latter had two. PSD plots also clearly showed this trend,

Figure 5. High-resolution SEM (a−e) and TEM images (f,g) of large mesoporous NMCNs synthesized at 5 °C.

nm (Table S1). This showed marked disparity compared with previous reports where the mesopore size obtained from F127 was about 2 nm and the mesoporous shape was spherical under alkaline or acidic conditions.28,46 To the best of our knowledge, this is the first report on the preparation of relatively small NMCNs (∼80 nm) with large mesopores (18.7 nm) without the use of any pore-activating reagents. Interestingly, no pore expansion was noticed when TMB (pore swelling agent) was added into the mixture solution during the synthesis (Figure S5), suggesting that only a small fraction of apolar TMB penetrated into the micelle cores of the F127 template. To understand the formation mechanism of such large mesopores, we carried out a systematic TEM investigation of the micelle assembly process of F127 as a function of IRT. As shown in Figure 6, the IRT indeed had a great influence on the mesopore size and shape. With the decrease of IRT, the mesopore became larger, along with a variation of the mesoporous shape. At the initial temperature of 25 °C, only small spherical mesopores

Figure 6. TEM images of large mesoporous NMCNs prepared with different IRTs: (a−c) 25, (d−f) 15, and (g−i) 5 °C. F


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Figure 7. Nitrogen adsorption−desorption isotherms (a) of NMCNs synthesized at different IRTs and (b) the corresponding PSD profiles.

Figure 8. TEM images of HNMCS (a−c), and the image of (c) is the high-resolution image of (b).

where one can easily distinguish the corresponding single pore system and the dual pore system (Figure 7b). Moreover, it was found that the slope of the adsorption branch at P/P0 of 0.4− 0.75 was smooth, whereas the slope at P/P0 of 0.75−0.90 was steep with the decrease of IRT, suggesting that the mesopore size became larger and the PSD became narrower. In addition, the molar ratio of P/F also played an important role in the PSD. With the increase of the P/F molar ratio, the PSD became narrower and simultaneously the slope of the adsorption branch at P/P0 of 0.75−0.90 increased significantly (Figure S8). However, it should be noted that the higher molar ratio would result in uncontrollable cross-linking of nanospheres (Figure S9a,c,e) and wormlike mesopores (Figure S9b,d,f). The adsorption properties of the NMCNs are listed in Table S1. With the decrease of IRT from 25 to 5 °C, the pore size increased from 7.0 to 18.7 nm, whereas the BET specific surface area declined from 517.3 to 475.9 m2/g. It is well-known that the mesoporous carbon spheres with a solid interior can be synthesized by using F127 as a mesoporous-forming agent and phenolic resin as the carbon precursor.28,49 However, to synthesize hollow carbon spheres, the template (such as silica or nanospheres) for the formation of hollow cavity is always needed.50−52 So far, there is no report on the synthesis of HMCS by simply using the abovementioned raw material (just using F127 and phenolic resin) but without any templates for the cavity. As mentioned above, the IRT could control the growth of PN and higher temperature favored the formation of larger particles. It easily came to our mind: how about the higher IRT? Would a larger particle size with solid inner cavity inside be obtained? In an attempt to provide additional insight into this area, we undertook a more systematic study at even higher IRT. Interestingly, as shown in TEM (Figure 8), hollow and bowllike structures of the carbon spheres were obtained. The strong contrast between the dark edge and the bright center indicated that the particles were not solid but hollow.53 The particle size and the shell thickness were about 2.6 μm and 198 nm,

respectively. Moreover, from Figure 8c, the formation of abundant mesopores in the shell was obvious. The N2 adsorption analysis showed that the porosity of this carbon shell exhibited a high specific surface area of 685 m2/g and a large total pore volume of 0.67 cm3/g. The PSD calculated by the DFT method showed that the mesopores in the shell were about 6.78 nm (Figure S10). It can be speculated that the hollow structures might come from a unique micelle assembly process of F127 (the model is shown in Figure S11) because the higher IRT would make the F127 micelle more lyophobic (more details below). The bowl-like carbon spheres appeared because the water escaped from the spherical polymer during the drying process. The TEM images (Figure S12) of the bowllike polymer precursors (prior to carbonization) gave strong evidence of this conclusion. Moreover, at lower HCl concentrations, hierarchically mesoporous carbon microspheres (Figure S13) were produced. Unfortunately, it was very difficult to balance the reaction rate, resulting in a wide particle size distribution. Elemental mapping analysis was then carried out to examine the distribution of element N (Figure 9a−d). As expected, element N was uniformly distributed in the skeleton of the resulted NMCNs. This homogeneous doping of element N was attributed to the molecular-level self-assembly of urea but not the accumulation. Because of the representative carbon nanospheres, the content of elements C and O was obviously higher than that of element N. To further determine the elemental compositions, XPS measurements were carried out. As shown in Figure 9e,f, the peaks centered at 284, 398, and 532 eV can be assigned to C 1s, N 1s, and O 1s electrons, respectively. In agreement with elemental mapping, N content was estimated to be 5.5% lower than that of C (87.6%) and O (9.8%). The N 1s spectrum of NMCNs could be deconvoluted into three peaks, at around 398.5, 400.5, and 401.3 eV, corresponding to pyridinic N, pyrrolic N, and graphitic N, respectively.54 In summary, both elemental mapping and XPS G


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volume ratios.57 As shown in Figure S11, a water droplet was surrounded by a number of small F127 micelles. After adding the carbon precursor (Figure 10 step 1), F127+−Cl−−PUF+ nuclei aggregated surrounding the water droplet and then the polymer with a droplet inside was obtained. After drying and carbonization, HNMCS was obtained (Figure 10 step 2). The cell viability measurements by MTT revealed that the cell viability remained above 96% after 48 h incubation (Figure 11). Even at a relatively high concentration of NMCNs (from 100 to 1000 μg/mL), only 1−2% L929 cells died. These results suggested that the synthesized nanospheres were not highly cytotoxic. To investigate their potential biological applications, L929 cells were incubated with NMCNs-RS. To track the location of NMCNs, it was grafted with FITC. Fluorescence spectra (Figure S14) indicated that FITC−NMCNs were successfully prepared. Quantitative analysis of cellular uptake was carried out by flow cytometry measurements. As shown in Figure 12, the cell uptake percentages of NMCNs-SS and NMCNs-RS were 97.2 and 99.9%, respectively. Surprisingly, the geometric mean of fluorescence intensity of NMCNs-RS (769) was different from that of NMCNs-SS (527). These results indicated that the rough surface of NMCNs might be exploited for enhancing the interactions between particles and cell membranes.37 To further observe the cell-attachment ability, SEM studies were conducted. L929 cells were incubated with NMCNs in culture dishes containing culture medium and FBS. After dehydration and spraying with platinum, the particle-cell adhesion was clearly observed. As shown in Figure 13b,c, there were few NMCNs-SS adhered on the surface of the cell because the smooth surface of this particle provided a limited contact area for interfacial interaction. Compared with NMCNs-SS, the increased particle number of NMCNs-RS adhered on the cell was obvious (Figure 13d−f). Notably, it was found that the NMCNs-RS were engulfed into the cell (Figure 13e,f), which was related to the strength of attractive interaction between the cell membrane and the particles.37 This implied that this material with a rough surface would be a potential drug carrier. Nowadays, millions of people suffer from diabetes. However, insulin, as the most efficient medicine for diabetes, exhibits low bioavailability by direct oral administration because it is easily digested by enzymes in the gastrointestinal tract. Moreover, such polypeptide with a large molecular mass is difficult to load into the small pore material, which is a major challenge in the pharmaceutical industry. In fact, the reason for the failure is the lack of an appropriate carrier, which has high mechanical strength, high surface area, large mesopore size, and monodispersed nanospherical structure. In this work, such a nanocarrier (NMCNs) was successfully synthesized and used as an insulin carrier to measure the transport performance. As shown in Figure 14, the highest percentage variation of blood glucose levels was 7.3% (at 4 h) by direct oral administration, suggesting that insulin without loading into NMCNs had no significance in vivo hypoglycemic effect by direct oral administration. Inspiringly, after insulin was loaded into NMCNs, this percentage variation reached up 79.7%, indicating that the insulin maintained activity in NMCNs. Moreover, the largest percentage variation extended to 6 h compared with 4 h of without loading, suggesting that insulin was released from NMCNs at a slow rate. This was probably because of the π−π conjugate interaction between insulin and NMCNs. Raman spectroscopy studies showed that the NMCNs possessed a

Figure 9. Elemental maps (a−d), XPS survey spectrum (e), and highresolution scan (f) of N 1s electrons of NMCNs.

measurements suggested that element N was successfully doped into the carbon structure. To elucidate the formation mechanism of NMCNs and HNMCS with rough surfaces, we proposed a controlled growth mechanism for the nanoengineering of carbon nanospheres by using different IRTs and E/W ratios (Figure 10). At the beginning, F127 self-assembled to form spherical micelles in the HCl/water solution. For the route of adding ethanol to control the reaction rate, the particles uniformly grew to form NMCNs with a smooth surface (NMCNs-SS). In contrast, without the addition of ethanol, the particles showed a nonuniform growth to form NMCNs with a rough surface (NMCNs-RS). Under these conditions, the micellar shape was highly IRT-dependent. At 25 °C, small micelles were obtained. With the decrease of temperature (from 25 to 5 °C), larger micelles with less tight aggregation were formed.55 With the addition of carbon precursors, the PUF resin was distributed throughout the entire PEO block of the F127 micellar to form F127+−Cl−− PUF+ nucleus by Coulombic interactions.34,56 After carbonization, larger micelles with less tight aggregation (at 15 and 5 °C) led to the formation of larger mesopores. Similarly, smaller micelles (at 25 °C) led to the production of smaller mesopores. Moreover, with the decrease of temperature, the particle became smaller and more uniform. Different from these relatively lower temperature range, when the IRT was elevated to 40 °C, the PEO block of F127 became more hydrophobic in water, resulting in a decrease of the hydrophilic/hydrophobic H


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Figure 10. Schematic illustration of different shape evolution paths for NMCNs and HNMCSs via controlled growth of nuclei. Step 1: addition of phloroglucinol, urea, and formaldehyde. Step 2: carbonization.

interaction toward aromatic molecules (insulin) via πstacking.2,4,58 These results suggested that NMCNs could be used as a potential oral delivery carrier of insulin to avoid enzymatic digestion in the gastrointestinal tract and finally enhance the bioavailability.59−71 It is expected that this NMCNs/insulin can be used to treat diabetes mellitus and reduce the pain of traditional twice-daily subcutaneous injections of insulin.72−79

5. CONCLUSIONS A controlled growth strategy was developed to synthesize NMCNs with adjustable particle size, surface roughness, large mesopore, and hollow cavity. In the synthesis process, the IRT played an important role in controlling the mesopores and particle sizes. When the IT was raised to 40 °C, the HNMCSs were obtained. Moreover, the surface roughness can be

Figure 11. Cell viability at different NMCN concentrations.

defective graphitic structure with well-defined D and G bands at 1347 and 1591 cm−1, respectively, which was beneficial to the

Figure 12. Quantitative analysis of cellular uptake by flow cytometry: (a): control group, (b): treat with NMCNs-SS, and (c): treat with NMCNsRS. GMFI: geometric mean of fluorescence intensity and CUP: cell uptake percentages. I


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Figure 13. SEM images of NMCNs with different rough surfaces adhered on the L929 cell surface. (a−c) NMCNs-RS and (d−f) NMCNs-SS.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

The authors acknowledge the financial support from the National Natural Science Foundation of China (21373061), the Project for Natural Science Foundation of Guangdong Province (2016A030313733), the Planned Project of Science and Technology of Guangzhou City (201707010131), and the Project for Innovation and Strengthen University of Guangdong Pharmaceutical University (2016KZDXM040 and 2015cxqx213).

Figure 14. Time course of the in vivo activity of different insulin formulations, expressed as the percentage variation of rat blood glucose levels after treatment.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the technical help from Haorong Lin and Pro. Linquan Zang for help with testing hypoglycemic activity of NMCNs/insulin. We are also grateful to Linquan Zang group for sharing their instrumentation.

adjusted by varied E/W. The current understanding of nucleation and growth mechanism of nanospheres was further extended and realized the development of NMCNs with large mesopores and litchi-like rough surface, which provided a new and interesting fundamental principle for the synthesis of NMCNs. As nanocarriers, NMCNs showed a low cell cytotoxic and high cell-attached ability. Most importantly, NMCNs were used as an insulin carrier in diabetes rats and showed a powerful hypoglycemic effect, which would open a new route for designing highly efficient insulin carrier.

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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.8b02040. Textural characteristic, SEM and TEM images, nitrogen physisorption isotherms, DLS profiles, particle size distribution, and the model of water in oil (PDF)

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REFERENCES

(1) Wang, G.-H.; Cao, Z.; Gu, D.; Pfänder, N.; Swertz, A.-C.; Spliethoff, B.; Bongard, H.-J.; Weidenthaler, C.; Schmidt, W.; Rinaldi, R.; Schüth, F. Nitrogen-doped ordered mesoporous carbon supported bimetallic PtCo nanoparticles for upgrading of biophenolics. Angew. Chem., Int. Ed. 2016, 55, 8850−8855. (2) Wang, Y.; Wang, K.; Zhang, R.; Liu, X.; Yan, X.; Wang, J.; Wagner, E.; Huang, R. Synthesis of core-shell graphitic carbon@silica nanospheres with dual-ordered mesopores for cancer-targeted photothermochemotherapy. ACS Nano 2014, 8, 7870−7879. (3) Zhuang, X.; Wan, Y.; Feng, C.; Shen, Y.; Zhao, D. Highly efficient adsorption of bulky dye molecules in wastewater on ordered mesoporous carbons. Chem. Mater. 2009, 21, 706−716. (4) Chen, Y.; Xu, P.; Wu, M.; Meng, Q.; Chen, H.; Shu, Z.; Wang, J.; Zhang, L.; Li, Y.; Shi, J. Colloidal RBC-shaped, hydrophilic, and hollow mesoporous carbon nanocapsules for highly efficient biomedical engineering. Adv. Mater. 2014, 26, 4294−4301. (5) Wickramaratne, N. P.; Xu, J.; Wang, M.; Zhu, L.; Dai, L.; Jaroniec, M. Nitrogen enriched porous carbon spheres: attractive materials for supercapacitor electrodes and CO2 adsorption. Chem. Mater. 2014, 26, 2820−2828. (6) Khani, H.; Rofouei, M. K.; Arab, P.; Gupta, V. K.; Vafaei, Z. Multi-walled carbon nanotubes-ionic liquid-carbon paste electrode as a super selectivity sensor: application to potentiometric monitoring of mercury ion(II). J. Hazard. Mater. 2010, 183, 402−409.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.T.). *E-mail: [email protected] (X.W.). ORCID

Xiufang Wang: 0000-0003-0822-2890 J


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ACS Applied Materials & Interfaces (7) Jain, A. K.; Gupta, V. K.; Singh, L. P.; Raisoni, J. R. A comparative study of Pb2+ selective sensors based on derivatized tetrapyrazole and calix[4]arene receptors. Electrochim. Acta 2006, 51, 2547−2553. (8) Gupta, V. K.; Singh, A. K.; Kumawat, L. K. Thiazole Schiff base turn-on fluorescent chemosensor for Al3+ ion. Sens. Actuators, B 2014, 195, 98−108. (9) Gupta, V. K.; Mergu, N.; Kumawat, L. K.; Singh, A. K. Selective naked-eye detection of Magnesium (II) ions using a coumarin-derived fluorescent probe. Sens. Actuators, B 2015, 207, 216−223. (10) Goyal, R. N.; Gupta, V. K.; Chatterjee, S. Voltammetric biosensors for the determination of paracetamol at carbon nanotube modified pyrolytic graphite electrode. Sens. Actuators, B 2010, 149, 252−258. (11) Ghimire, P.; Gunathilake, C.; Wickramaratne, N. P.; Jaroniec, M. Tetraethyl orthosilicate-assisted synthesis of nitrogen-containing porous carbon spheres. Carbon 2017, 121, 408−417. (12) Liang, C.; Li, Z.; Dai, S. Mesoporous carbon materials: synthesis and modification. Angew. Chem., Int. Ed. 2008, 47, 3696−3717. (13) Wei, J.; Sun, Z.; Luo, W.; Li, Y.; Elzatahry, A. A.; Al-Enizi, A. M.; Deng, Y.; Zhao, D. New insight into the synthesis of large-pore ordered mesoporous materials. J. Am. Chem. Soc. 2017, 139, 1706− 1713. (14) Deng, Y.; Cai, Y.; Sun, Z.; Gu, D.; Wei, J.; Li, W.; Guo, X.; Yang, J.; Zhao, D. Controlled synthesis and functionalization of ordered large-Pore mesoporous carbons. Adv. Funct. Mater. 2010, 20, 3658− 3665. (15) Zhang, J.; Deng, Y.; Wei, J.; Sun, Z.; Gu, D.; Bongard, H.; Liu, C.; Wu, H.; Tu, B.; Schüth, F.; Zhao, D. Design of amphiphilic ABC triblock copolymer for templating synthesis of large-pore ordered mesoporous carbons with tunable pore wall thickness. Chem. Mater. 2009, 21, 3996−4005. (16) Deng, Y.; Yu, T.; Wan, Y.; Shi, Y.; Meng, Y.; Gu, D.; Zhang, L.; Huang, Y.; Liu, C.; Wu, X.; Zhao, D. Ordered mesoporous silicas and carbons with large accessible pores templated from amphiphilic diblock copolymer poly(ethylene oxide)-b-polystyrene. J. Am. Chem. Soc. 2007, 129, 1690−1697. (17) Tang, J.; Liu, J.; Li, C.; Li, Y.; Tade, M. O.; Dai, S.; Yamauchi, Y. Synthesis of nitrogen-doped mesoporous carbon spheres with extralarge pores through assembly of diblock copolymer micelles. Angew. Chem., Int. Ed. 2015, 54, 588−593. (18) Yue, Q.; Zhang, Y.; Jiang, Y.; Li, J.; Zhang, H.; Yu, C.; Elzatahry, A. A.; Alghamdi, A.; Deng, Y.; Zhao, D. Nanoengineering of core-shell magnetic mesoporous microspheres with tunable surface roughness. J. Am. Chem. Soc. 2017, 139, 4954−4961. (19) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Ohsuna, T.; Teraski, O. Synthesis of new, nanoporous carbon with hexagonally ordered mesostructure. J. Am. Che. Soc. 2000, 122 (43), 10712−10713. (20) Gupta, V. K.; Jain, A. K.; Maheshwari, G.; Lang, H.; Ishtaiwi, Z. Copper(II)-selective potentiometric sensors based on porphyrins in PVC matrix. Sens. Actuators, B 2006, 117, 99−106. (21) Gupta, V. K.; Jain, A. K.; Kumar, P.; Agarwal, S.; Maheshwari, G. Chromium(III)-selective sensor based on tri-o-thymotide in PVC matrix. Sens. Actuators, B 2006, 113, 182−186. (22) Gupta, V. K.; Jain, A. K.; Kumar, P. PVC-based membranes of N,N′-dibenzyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane as Pb(II)selective sensor. Sens. Actuators, B 2006, 120, 259−265. (23) Gupta, V. K.; Chandra, S.; Mangla, R. Dicyclohexano-18-crown6 as active material in PVC matrix membrane for the fabrication of cadmium selective potentiometric sensor. Electrochim. Acta 2002, 47, 1579−1586. (24) Tian, H.; Lin, Z.; Xu, F.; Zheng, J.; Zhuang, X.; Mai, Y.; Feng, X. Quantitative control of pore size of mesoporous carbon nanospheres through the self-assembly of diblock copolymer micelles in solution. Small 2016, 12, 3155−3163. (25) Deng, Y.; Wei, J.; Sun, Z.; Zhao, D. Large-pore ordered mesoporous materials templated from non-Pluronic amphiphilic block copolymers. Chem. Soc. Rev. 2013, 42, 4054−4070.

(26) Deng, Y.; Liu, J.; Liu, C.; Gu, D.; Sun, Z.; Wei, J.; Zhang, J.; Zhang, L.; Tu, B.; Zhao, D. Ultra-large-pore mesoporous carbons templated from poly(ethylene oxide)-b-polystyrene diblock copolymer by adding polystyrene homopolymer as a pore expander. Chem. Mater. 2008, 20, 7281−7286. (27) Dunphy, D. R.; Sheth, P. H.; Garcia, F. L.; Brinker, C. J. Enlarged pore size in mesoporous silica films templated by pluronic F127: use of poloxamer mixtures and increased template/SiO2 ratios in materials synthesized by evaporation-induced self-assembly. Chem. Mater. 2015, 27, 75−84. (28) Fang, Y.; Gu, D.; Zou, Y.; Wu, Z.; Li, F.; Che, R.; Deng, Y.; Tu, B.; Zhao, D. A low-concentration hydrothermal synthesis of biocompatible ordered mesoporous carbon nanospheres with tunable and uniform size. Angew. Chem., Int. Ed. 2010, 49, 7987−7991. (29) Liu, L.; Wang, F.-Y.; Shao, G.-S.; Ma, T.-Y.; Yuan, Z.-Y. Synthesis of ultra-large mesoporous carbons from triblock copolymers and phloroglucinol/formaldehyde polymer. Carbon 2010, 48, 2660− 2664. (30) Liang, C.; Dai, S. Synthesis of mesoporous carbon materials via enhanced hydrogen-bonding interaction. J. Am. Chem. Soc. 2006, 128, 5316−5317. (31) Tan, B.; Rankin, S. E. Interfacial alignment mechanism of forming spherical silica with radially oriented nanopores. J. Phys. Chem. B 2004, 108, 20122−20129. (32) Kiefer, J.; Porouchani, R.; Mendels, D.; Ferrer, J. B.; Fond, C.; Hedrick, J. L.; Kausch, H. H.; Hilborn, J. G. Macroporous thermosets via chemically induced phase separation. MRS Online Proc. Libr. 1996, 431, 527−532. (33) Yao, N.; Ku, A. Y.; Nakagawa, N.; Lee, T.; Saville, D. A.; Aksay, I. A. Disorder-order transition in mesoscopic silica thin films. Chem. Mater. 2000, 12, 1536−1548. (34) Wang, X.; Liang, C.; Dai, S. Facile synthesis of ordered mesoporous carbons with high thermal stability by self-assembly of resorcinol-formaldehyde and block copolymers under highly acidic conditions. Langmuir 2008, 24, 7500−7505. (35) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. Nonionic triblock and star diblock copolymer and oligomeric surfactant syntheses of highly ordered, hydrothermally stable, mesoporous silica structures. J. Am. Chem. Soc. 1998, 120, 6024−6036. (36) Li, X.; Du, X.; He, J. Self-cleaning antireflective coatings assembled from peculiar mesoporous silica nanoparticles. Langmuir 2010, 26, 13528−13534. (37) Song, H.; Ahmad Nor, Y.; Yu, M.; Yang, Y.; Zhang, J.; Zhang, H.; Xu, C.; Mitter, N.; Yu, C. Silica nanopollens enhance adhesion for long-term bacterial inhibition. J. Am. Chem. Soc. 2016, 138, 6455− 6462. (38) Ming, W.; Wu, D.; van Benthem, R.; de With, G. Superhydrophobic films from raspberry-like particles. Nano Lett. 2005, 5, 2298−2301. (39) Yu, C.; Fan, J.; Tian, B.; Zhao, D. Morphology development of mesoporous materials: a colloidal phase separation mechanism. Chem. Mater. 2004, 16, 889−898. (40) LaMer, V. K.; Dinegar, R. H. Theory, production and mechanism of formation of monodispersed hydrosols. J. Am. Chem. Soc. 1950, 72, 4847−4854. (41) Artaki, I.; Zerda, T. W.; Jonas, J. Solvent effects on the condensation stage of the sol-gel process. J. Non-Cryst. Solids 1986, 81, 381−395. (42) Bin, D.-S.; Chi, Z.-X.; Li, Y.; Zhang, K.; Yang, X.; Sun, Y.-G.; Piao, J.-Y.; Cao, A.-M.; Wan, L.-J. Controlling the compositional chemistry in single nanoparticles for functional hollow carbon nanospheres. J. Am. Chem. Soc. 2017, 139, 13492−13498. (43) Wang, S.; Li, W.-C.; Hao, G.-P.; Hao, Y.; Sun, Q.; Zhang, X.-Q.; Lu, A.-H. Temperature-programmed precise control over the sizes of carbon nanospheres based on benzoxazine chemistry. J. Am. Chem. Soc. 2011, 133, 15304−15307. (44) Song, J.-S.; Tronc, F.; Winnik, M. A. Two-stage dispersion polymerization toward monodisperse, controlled micrometer-sized copolymer particles. J. Am. Chem. Soc. 2004, 126, 6562−6563. K


Research Article

ACS Applied Materials & Interfaces (45) Baba, T.; Kobayashi, A.; Kawanami, Y.; Inazu, K.; Ishikawa, A.; Echizenn, T.; Murai, K.; Aso, S.; Inomata, M. Characteristics of methoxycarbonylation of aromatic diamine with dimethyl carbonate to dicarbamate using a zinc acetate catalyst. Green Chem. 2005, 7, 159− 165. (46) Wei, J.; Liang, Y.; Zhang, X.; Simon, G. P.; Zhao, D.; Zhang, J.; Jiang, S.; Wang, H. Controllable synthesis of mesoporous carbon nanospheres and Fe-N/carbon nanospheres as efficient oxygen reduction electrocatalysts. Nanoscale 2015, 7, 6247−6254. (47) Song, Y.; Ye, G.; Wu, F.; Wang, Z.; Liu, S.; Kopeć, M.; Wang, Z.; Chen, J.; Wang, J.; Matyjaszewski, K. Bioinspired polydopamine (PDA) chemistry meets ordered mesoporous carbons (OMCs): a benign surface modification strategy for versatile functionalization. Chem. Mater. 2016, 28, 5013−5021. (48) Song, Y.; Ye, G.; Wang, Z.; Kopeć, M.; Xie, G.; Yuan, R.; Chen, J.; Kowalewski, T.; Wang, J.; Matyjaszewski, K. Controlled preparation of well-Defined mesoporous carbon/polymer hybrids via surfaceinitiated ICAR ATRP with a high dilution strategy assisted by facile polydopamine chemistry. Macromolecules 2016, 49, 8943−8950. (49) Liu, J.; Qiao, S. Z.; Liu, H.; Chen, J.; Orpe, A.; Zhao, D.; Lu, G. Q. M. Extension of the Stober method to the preparation of monodisperse resorcinol-formaldehyde resin polymer and carbon spheres. Angew. Chem., Int. Ed. 2011, 50, 5947−5951. (50) Chai, G. S.; Shin, I. S.; Yu, J.-S. Synthesis of ordered, uniform, macroporous carbons with mesoporous walls templated by aggregates of polystyrene spheres and silica particles for use as catalyst supports in direct methanol fuel cells. Adv. Mater. 2004, 16, 2057−2061. (51) Fang, Y.; Zheng, G.; Yang, J.; Tang, H.; Zhang, Y.; Kong, B.; Lv, Y.; Xu, C.; Asiri, A. M.; Zi, J.; Zhang, F.; Zhao, D. Dual-pore mesoporous carbon@silica composite core-shell nanospheres for multidrug delivery. Angew. Chem., Int. Ed. 2014, 53, 5366−5370. (52) Qiao, Z.-A.; Guo, B.; Binder, A. J.; Chen, J.; Veith, G. M.; Dai, S. Controlled synthesis of mesoporous carbon nanostructures via a “silica-assisted” strategy. Nano Lett. 2013, 13, 207−212. (53) Park, S. H.; Kim, J.; Lee, W.-E.; Byun, D.-J.; Kim, M. H. Onestep synthesis of hollow dimpled polystyrene microparticles by dispersion polymerization. Langmuir 2017, 33, 2275−2282. (54) Yu, J.; Guo, M.; Muhammad, F.; Wang, A.; Zhang, F.; Li, Q.; Zhu, G. One-pot synthesis of highly ordered nitrogen-containing mesoporous carbon with resorcinol-urea-formaldehyde resin for CO2 capture. Carbon 2014, 69, 502−514. (55) Fan, J.; Yu, C.; Lei, J.; Zhang, Q.; Li, T.; Tu, B.; Zhou, W.; Zhao, D. Low-temperature strategy to synthesize highly ordered mesoporous silicas with very large pores. J. Am. Chem. Soc. 2005, 127, 10794− 10795. (56) Florent, M.; Xue, C.; Zhao, D.; Goldfarb, D. Formation mechanism of cubic mesoporous carbon monolith synthesized by evaporation-Induced self-assembly. Chem. Mater. 2012, 24, 383−392. (57) Liu, D.; Lei, J.-H.; Guo, L.-P.; Qu, D.; Li, Y.; Su, B.-L. One-pot aqueous route to synthesize highly ordered cubic and hexagonal mesoporous carbons from resorcinol and hexamine. Carbon 2012, 50, 476−487. (58) Zhang, C.; Zhao, Q.; Wan, L.; Wang, T.; Sun, J.; Gao, Y.; Jiang, T.; Wang, S. Poly dimethyl diallyl ammonium coated CMK-5 for sustained oral drug release. Int. J. Pharm. 2014, 461, 171−180. (59) Srivastava, S. K.; Gupta, V. K.; Jain, S. Determination of lead using a poly(vinyl chloride)-based crown ether membrane. Analyst 1995, 120, 495−498. (60) Prasad, R.; Gupta, V. K.; Kumar, A. Metallo-tetraazaporphyrin based anion sensors: regulation of sensor characteristics through central metal ion coordination. Anal. Chim. Acta 2004, 508, 61−70. (61) Jain, A. K.; Gupta, V. K.; Singh, L. P.; Khurana, U. Macrocycle based membrane sensors for the determination of cobalt(II) ions. Analyst 1997, 122, 583−586. (62) Gupta, V. K.; Singh, A. K.; Mehtab, S.; Gupta, B. A cobalt(II)selective PVC membrane based on a Schiff base complex of N,N′bis(salicylidene)-3,4-diaminotoluene. Anal. Chim. Acta 2006, 566, 5− 10.

(63) Gupta, V. K.; Singh, A. K.; Al Khayat, M.; Gupta, B. Neutral carriers based polymeric membrane electrodes for selective determination of mercury (II). Anal. Chim. Acta 2007, 590, 81−90. (64) Gupta, V. K.; Prasad, R.; Kumar, P.; Mangla, R. New nickel(II) selective potentiometric sensor based on 5,7,12,14-tetramethyldibenzotetraazaannulene in a poly(vinyl chloride) matrix. Anal. Chim. Acta 2000, 420, 19−27. (65) Gupta, V. K.; Mergu, N.; Kumawat, L. K.; Singh, A. K. A reversible fluorescence “off-on-off” sensor for sequential detection of aluminum and acetate/fluoride ions. Talanta 2015, 144, 80−89. (66) Gupta, V. K.; Jain, S.; Chandra, S. Chemical sensor for lanthanum(III) determination using aza-crown as ionophore in poly(vinyl chloride) matrix. Anal. Chim. Acta 2003, 486, 199−207. (67) Gupta, V. K.; Jain, A. K.; Singh, L. P.; Khurana, U. Porphyrins as carrier in PVC based membrane potentiometric sensors for nickel(II). Anal. Chim. Acta 1997, 355, 33−41. (68) Gupta, V. K.; Jain, A. K.; Maheshwari, G. Aluminum(III) selective potentiometric sensor based on morin in poly(vinyl chloride) matrix. Talanta 2007, 72, 1469−1473. (69) Gupta, V. K.; Jain, A. K.; Agarwal, S.; Maheshwari, G. An iron(III) ion-selective sensor based on a μ-bis(tridentate) ligand. Talanta 2007, 71, 1964−1968. (70) Gupta, V. K.; Goyal, R. N.; Sharma, R. A. Anion recognition using newly synthesized hydrogen bonding disubstituted phenylhydrazone-based receptors: poly(vinyl chloride)-based sensor for acetate. Talanta 2008, 76, 859−864. (71) Goyal, R. N.; Gupta, V. K.; Bachheti, N. Fullerene-C60-modified electrode as a sensitive voltammetric sensor for detection of nandrolone−an anabolic steroid used in doping. Anal. Chim. Acta 2007, 597, 82−89. (72) Srivastava, S. K.; Gupta, V. K.; Dwivedi, M. K.; Jain, S. Caesium PVC−crown (dibenzo-24-crown-8) based membrane sensor. Anal. Proc. 1995, 32, 21−23. (73) Srivastava, S. K.; Gupta, V. K.; Jain, S. PVC-Based 2,2,2Cryptand Sensor for Zinc Ions. Anal. Chem. 1996, 68, 1272−1275. (74) Gupta, V. K.; Singh, L. P.; Singh, R.; Upadhyay, N.; Kaur, S. P.; Sethi, B. A novel copper (II) selective sensor based on Dimethyl 4, 4′ (o-phenylene) bis(3-thioallophanate) in PVC matrix. J. Mol. Liq. 2012, 174, 11−16. (75) Karthikeyan, S.; Gupta, V. K.; Boopathy, R.; Titus, A.; Sekaran, G. A new approach for the degradation of high concentration of aromatic amine by heterocatalytic Fenton oxidation: Kinetic and spectroscopic studies. J. Mol. Liq. 2012, 173, 153−163. (76) Gupta, V. K.; Kumar, S.; Singh, R.; Singh, L. P.; Shoora, S. K.; Sethi, B. Cadmium (II) ion sensing through p-tert-butyl calix[6]arene based potentiometric sensor. J. Mol. Liq. 2014, 195, 65−68. (77) Yola, M. L.; Gupta, V. K.; Eren, T.; Şen, A. E.; Atar, N. A novel electro analytical nanosensor based on graphene oxide/silver nanoparticles for simultaneous determination of quercetin and morin. Electrochim. Acta 2014, 120, 204−211. (78) Karimi-Maleh, H.; Tahernejad-Javazmi, F.; Atar, N.; Yola, M. L.; Gupta, V. K.; Ensafi, A. A. A Novel DNA Biosensor Based on a Pencil Graphite Electrode Modified with Polypyrrole/Functionalized Multiwalled Carbon Nanotubes for Determination of 6-Mercaptopurine Anticancer Drug. Ind. Eng. Chem. Res. 2015, 54, 3634−3639. (79) Dehghani, M. H.; Sanaei, D.; Ali, I.; Bhatnagar, A. Removal of chromium(VI) from aqueous solution using treated waste newspaper as a low-cost adsorbent: Kinetic modeling and isotherm studies. J. Mol. Liq. 2016, 215, 671−679.

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Controlled Growth of N-Doped and Large Mesoporous Carbon

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