Functionalized Multiwalled Carbon Nanotubes Multilayer

Article pubs.acs.org/IECR

Chitosan/Functionalized Multiwalled Carbon Nanotubes Multilayer Hollow Microspheres Prepared via Layer-by-Layer Assembly Technique Chao Yang and Peng Liu* State Key Laboratory of Applied Organic Chemistry and Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China ABSTRACT: Novel multilayer hollow microspheres were fabricated via the layer-by-layer (LBL) assembly technique by the electrostatic interaction between the polycation chitosan (CHI) and the anion carboxyl-functionalized multiwalled carbon nanotubes (f-MWNTs) on sacrificial polystyrene sulfonate (PSS) microsphere templates. The successful stepwise growth of the CHI/f-MWNTs multilayers in different pH media was compared by zeta potential measurement and scanning electron microscopy (SEM) analysis. The morphologies of the hollow microspheres were characterized by transmission electron microscopy (TEM) and SEM. The swelling and collapse of the hollow microspheres occurred during the template removal by dissolution and the drying process. It was found that more f-MWNTs had been deposited onto the surface of the PSS microspheres at pH 3 than at pH 5, confirmed by SEM and thermogravimetric analysis (TGA). Furthermore, this result is consistent with the differences of the electrical conductivities of the hollow microspheres obtained from the different pH media. It is also found that in the etching process, the different etching reagents, such as DMF and toluene, obviously affected the electrical conductivity of the final hollow microspheres.

1. INTRODUCTION In recent years, hollow microspheres in the scale range of nanometers to micrometers have attracted continuous interest in the context of chemistry and materials science, due to their potential applications in the fields of catalysis,1 photonics,2 solar cells,3 sensors,4 fillers,5 drug delivery,6 controlled release,7 and separation and purification processes,8 because of their high stability, uniform hollow structure, large surface areas, low toxicity, and good compatibilities with other materials. One of the frequently used strategies to prepare hollow microspheres is the template-based layer-by-layer (LBL) selfassembly technique,9,10 which is greatly popular to prepare hollow microspheres with uniform and dense layers because of its easy control on the size of hollow microspheres and thickness of shells by tuning the number of layers and the assembling conditions. These core−shell microspheres are generated by sequential deposition of inorganic or polymer layers from aqueous solutions onto a sacrificial template. Almost any type of interaction can be used as the driving force for the assembly of the multilayer shell, for example, electrostatics,11,12 hydrogen bonding,13,14 covalent bonding,15,16 host−guest interaction, 17 and van der Waals interaction.18 Moreover, the electrostatic-driven LBL selfassembly technique is predominantly and intensively researched for the preparation of hollow microspheres and membranes. The assembled materials have been rapidly developed from synthesized polyelectrolyte to natural polyelectrolyte (such as chitosan14,19 and alginate19), subsequently to functionalized nano- or macro-materials, for example, gold nanoparticles,20 Fe3O4 nanoparticles,21 and carbon nanotubes (CNTs). As is well-known, chitosan (CHI) is one of the most potential candidates for drug delivery system, with excellent biocompatible, biodegradable, and nontoxic properties. More© 2012 American Chemical Society

over, CHI/CNTs composites have been actively applied in materials science. In recent years, the CHI/CNTs scaffolds have been produced as bone graft substitutes for bone tissue engineering,22 and developed to be a highly conductive and porous composite material.23,24 Zhang et al.25 have prepared the CHI/f-MWNTs membrane for the separation of ethanol/ water mixtures by pervaporation. With good dispersibility and stability in aqueous solutions, the CHI/MWNTs nanoparticle hybrids have been synthesized for potential utilization in the immobilization of therapeutic biomolecules as protein carriers.26 CNTs have been assembled onto flat films27,28 and colloid spheres,29−38 yielding compact mono- or multilayers of CNTs. Shinkai et al.29 first reported the LbL self-assembly of the ammonium-modified cationic curdlans (CUR-N+)/SWNTs and the sulfonate-modified anionic curdlans (CUR-SO3−)/ SWNTs complexes constructed on the surface of the silica particles. CNTs treated in acid prior to their use can be incorporated onto the surface of polystyrene (PS) and poly(methyl methacrylate) (PMMA) microspheres by adding the surfactants to stabilize the nanotubes against the van der Waals force.30 By using the LBL self-assembly technique via electrostatic interaction between the negatively charged CNTs and the positively charged poly(diallyldimethyl ammonium chloride) (PDDA), Correa-Duarte et al.31−33 successfully deposited the dense mono- or multilayers of the oxidized CNTs on silica, polystyrene, and melamine microspheres as templates, showing that the relatively short CNTs wrapped the Received: Revised: Accepted: Published: 13346

June 23, 2012 September 15, 2012 September 19, 2012 September 19, 2012 dx.doi.org/10.1021/ie301666z | Ind. Eng. Chem. Res. 2012, 51, 13346−13353

Industrial & Engineering Chemistry Research

Article

Scheme 1. Scheme of the Preparation of CHI/f-MWNTs Hollow Microspheres

surface of the templates. Choi et al.34 produced large-scale SWNTs/polymeric microsphere arrays by a wet chemical selfassembly technique of the shortened SWNTs onto the surface of the mercaptoethylamine-modified PS microspheres via electrostatic interaction between the carboxylic groups of the SWNTs and the amino groups. CNTs, such as SWNTs or MWNTs, have been chosen as components to build more complicated structures than 2dimensional thin films for synthesizing 3-dimensional substratefree capsules, which might act as the new electron-conductive functional materials. Sano et al.35 successfully prepared a hollow spherical cage made of the shortened single-walled carbon nanotubes (SWNTs) onto amine-terminated silica particles as templates by the LBL self-assembly technique, relying on the nanotube−nanotube interaction. Yang et al.36 reported oxidized CNTs were adsorbed onto polyelectrolyte PDDA surfaces, forming porous hollow carbon nanotube composite cages. Moreover, they used an alternative and more effective method to stabilize the structure by incorporating inorganic materials, such as silica and titania, within the PDDA/CNTs layers. Song and co-workers37 fabricated carbon nanotube capsules from acid-modified MWNTs by nanotube−nanotube interaction via hydrogen bonding between the carboxylic groups and the hydroxyl groups, using water-in-oil emulsion technique without any additional reagent and template. The LBL self-assembly of carboxylic acid modified MWNTs and diazoresin (DR) was realized via electrostatic interaction on PS microsphere templates.38 The shells on PS cores can be partially crosslinked by the formation of covalent bonds upon UV irradiation or heating, significantly improving the stability of the DR/ MWNTs multilayers. Recently, many CNTs-based hollow microspheres have been developed; however, the unique CHI/f-MWNTs composites have not been applied as shell materials for hollow microspheres at present. Given the combination of the biocompat-

ible, biodegradable, and nontoxic properties of chitosan and the unique electrical and mechanical properties of MWNTs, the two materials were selected to fabricate CNTs-based hollow microspheres in the present work. It is firmly believed that the CHI/f-MWNTs layer-by-layer self-assembled hollow microspheres exhibit potential for application in drug delivery due to their inherent biocompatibility, or for sensors or electrochemical devices owing to their unique electrical property. Furthermore, the core/shell structured polystyrene sulfonate (PSS) microspheres coated with 5 (CHI/f-MWNTs) bilayers were prepared by deposition at pH 3 and 5, in order to investigate the effect of the media acidity on the amount of the f-MWNTs adsorbed on the PSS templates in the assembly process. The effects of different solvents (DMF and toluene) used to remove PSS templates were also investigated in the etching process, according to the contents of the f-MWNTs and conductivities of the final hollow microspheres.

2. EXPERIMENTAL METHODS 2.1. Materials. Chitosan (viscosity-average molecular weight: 6.0 × 105 and N-deacetylation degree: 96%) was purchased from Golden-Shell Biochemical Co. Ltd., Zhejiang, China. MWNTs were purchased from Shenzhen Nanotech Port Co. Ltd. (Shenzhen, China) with characteristics of 95% purity, 10−20 nm diameter, and 5−15 μm length. Styrene was distilled under vacuum before use, poly(vinylpyrrolidone) (PVP, Mw = 10 000−70 000), azobisisobutironitrile (AIBN), and other reagents (such as N,N-dimethylformamide (DMF) and toluene) were all of analytical reagent grade and obtained from Tianjin Chemicals Co. Ltd., Tianjin, China. Deionized water was used throughout. 2.2. PSS Templates. The monodisperse PS microspheres were prepared by dispersion polymerization according to Khan et al.39 The PVP steric stabilizer (1.5 g) was dissolved in 120 mL of 2-propanol in a round-bottomed flask equipped with a 13347

dx.doi.org/10.1021/ie301666z | Ind. Eng. Chem. Res. 2012, 51, 13346−13353


Article

water three times, the ultimate hollow microspheres were dried under vacuum at room temperature. The homogeneous method was used to etch the coated PSS microspheres in DMF or toluene, as summarized in Table 1.

magnetic stirrer and reflux condenser. The solution was heated to 70 °C and purged with nitrogen. Then a solution of azoisobutyronitrile (0.12 g) predissolved in a cold styrene monomer (15 g) was added with vigorous stirring and the polymerization was allowed to proceed for 24 h under N2. After being cooled to room temperature, the resulting supernatant was purified by centrifugation and washed with deionized water. After the centrifugation/redispersion procedure was repeated three times, a white fine powder (PS) was finally obtained after being dried in a vacuum oven at 50 °C for 10 h. The uniform PS particles (2.0 g) were dispersed into sulfuric acid (50 mL, 98%) with the aid of ultrasonication. The sulfonation was allowed to take place at 45 °C under magnetic stirring for 8 h. After being cooled to room temperature, the product was diluted with 100 mL of water and then neutralized with a dilute NaOH aqueous solution. When a neutral pH was reached, the product was separated by repeated centrifugation and thoroughly rinsed with water. The obtained microspheres were finally dispersed in 100 mL of deionized water. 2.3. f-MWNTs. Carbon nanotubes were oxidized by means of the following procedure. MWNTs (2.0 g) were ultrasonicated for 10 h in a mixture of H2SO4/HNO3 (3:1, 200 mL). Then the sample was magnetically stirred at 80 °C for 24 h. After being cooled to room temperature, the resulting material diluted with deionized water was neutralized with a dilute NaOH aqueous solution until the pH value reached 7, and subsequently washed with water three times by repeated centrifugation/redispersion cycles. Finally, the nanotubes were dispersed in water, obtaining a stable dispersion of the oxidized MWNTs with carboxylic groups on their walls, providing a negative-charged surface. 2.4. CHI/f-MWNTs Hollow Microspheres. The chitosan/ f-MWNTs multilayer coatings were deposited on the PSS spheres in two conditions with different pH as follows (Scheme 1). In Method 1, 50 mL of CHI solution (6.3 mg/mL, pH 3.0) containing 2% (v/v) acetic acid was added to 100 mL of PSS dispersion under magnetic stirring and stirred for 30 min. The excess CHI was removed by repeating three centrifugation/ redispersion cycles. The PSS particles coated with CHI were dispersed into 150 mL of deionized water and then 8 mL of fMWNTs dispersion (4.8 mg/mL) was added with magnetic stirring. The product was adjusted to pH 3.0 with acetic acid. An adsorption time of 30 min was then allowed, and excess fMWNTs were removed by repeating three centrifugation/ redispersion cycles. The CHI and f-MWNTs were alternately deposited five times onto the PSS templates to obtain the PSS spheres coated with 5 (CHI/f-MWNTs) bilayers. In Method 2, the adsorption was conducted in the pH 5.0 media, and the CHI solution (5.1 mg/mL, pH 5.0) was used. The other self-assembling conditions were the same as in Method 1. After the desired number of coated layers was reached, the sacrificial templates (PSS) were removed to obtain the hollow microspheres by dissolution in the etching agent (DMF or toluene). The hollow microspheres were prepared by exposing 0.5 g of PSS spheres coated with 5 (CHI/f-MWNTs) bilayers into 6 mL of the etching agent for 30 min. Then the product was collected by centrifugation, and washed five times by repeating centrifugation/redispersion in the etching agent to remove the templates completely. To confirm complete removal, the etching agent in supernatant after centrifugation was mixed with an equal volume of ethanol, ensuring the absence of any precipitate. After the products were rinsed with

Table 1. Hollow Microspheres (CHI/f-MWNTs)5-1 (CHI/f-MWNTs)5-2 (CHI/f-MWNTs)5-3 (CHI/f-MWNTs)5-4

pH value for the assembly

etching agent

3 5 3 5

DMF DMF toluene toluene

2.5. Analysis and Characterization. The zeta potentials of the PSS microspheres coated with CHI and f-MWNTs were determined in each adsorption step in the LBL self-assembly process with a Zetasizer Nano system (Malvern). The samples were dispersed in distilled water and injected into folded capillary cells. All measurements were conducted at 25 °C. The reported zeta potential value is the average of three consecutive measurements. Raman measurements were carried out with a Horiba JobinYvon LabRAM HR 800 UV apparatus using an excitation laser with a wavelength of 532 nm. Transmission electron microscopy (TEM) measurements were carried out on a JEM-1230 transmission electron microscope (JEOL, Tokyo, Japan) operating at an accelerating voltage of 100 kV. The samples were dispersed in water and dropped onto the Cu grids, followed by solvent evaporation in air at room temperature. The samples were also observed by scanning electron microscopy (SEM) on an S-4800 field emission scanning electron microscope (HITACHI, Tokyo, Japan) operating at 5.0 kV. The samples were dispersed in water and then deposited on silicon chip fixed onto a SEM stub using conductive tape. Thermogravimetric analysis (TGA) was performed under nitrogen flows from room temperature to 800 °C at a heating rate of 10 °C/min using a DuPont 1090B Thermal Analyzer. The electrical conductivities of the coated PSS microspheres and the final hollow microspheres were measured using a RTS2 four-point probe conductivity tester (Guangzhou four-point probe meter Electronic Technology Co., Ltd., Guangdong, China) at ambient temperature. The pellets were obtained by subjecting the powder samples to a pressure of 30 MPa. Each value given is an average of at least three measurements.

3. RESULTS AND DISCUSSION Since the pristine MWNTs do not carry a surface charge, they were modified by chemical oxidation using mixed acids, producing the surface carboxylic groups that are responsible for the presence of the necessary surface negative charge to be assembled on the positively charged surfaces.40 One can find from the TEM image of the functionalized MWNTs (Figure 1) that the shortening of the f-MWNTs is apparent after the acid treatment. According to the TEM measurement, the average length of the f-MWNTs is about 215 nm, with approximately 60% of them shorter than 200 nm and 90% shorter than 400 nm. The acid and ultrasonic treatment of the pristine MWNTs inevitably introduces the surface defects and sacrifices desirable electronic properties to some extent. Raman spectroscopy was employed to identify the defects of the f-MWNTs. In Figure 2, 13348



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Figure 1. TEM image (left) and length distribution (right) of the f-MWNTs.

The stepwise adsorption of CHI and the f-MWNTs on the surface of the PSS microspheres was monitored by measuring the electrophoretic mobility after deposition of each layer (Figure 3). Before the LBL self-assembly, the PSS micro-

Figure 2. Raman spectra of the pristine MWNTs and the f-MWNTs.

two peaks at 1346 cm−1 (D-band) and 1576 cm−1 (G-band) are clearly visible in the spectra of the two samples. The D-band is associated with disordered, sp3-hybridized carbon and corresponds to the defects in the walls of the MWNTs, whereas the G-band provides an indication of the ordered sp2-bonded carbon as a characteristic of MWNTs. The ratio of D-band to G-band intensity (ID/IG) for the pristine MWNTs was calculated to be 0.66. After the functionalization with mixed acids, some sp2-bonded carbons in the pristine MWNTs converted to the sp3 hybridization ones. Therefore, the ID/IG ratio of the f-MWNTs increased to 1.11. This increase in the ID/IG ratio indicates the successful introduction of the carboxyl groups onto the MWNTs’ surfaces. The LBL self-assembly technique was then used to prepare the multilayer hollow microspheres with alternate deposition of the positively charged CHI and the negatively charged fMWNTs on the sacrificial PSS templates. The whole proposal is summarized in Scheme 1. In step 1, the uniform PS particle previously prepared by dispersion polymerization was sulfonated in concentrated sulfuric acid to obtain the PSS particle as template. In step 2, CHI as the starting substance was deposited on the PSS template to make its surface positively charged. In step 3, a layer of negatively charged f-MWNTs was deposited. In step 4, the bilayers of CHI and f-MWNTs were alternately assembled in the desired cycles. In the last step, the PSS template was removed by dissolution, yielding the hollow microsphere.

Figure 3. Zeta potential variation of the microspheres after being coated every layer at pH 3 (a) and pH 5 (b).

particles exhibited a zeta potential of about −26 mV, whereas it switched between −26 to 0 mV (a) and −26 to −4 mV (b) during the assembly procedure. As expected, the zeta potential varied between positive and negative values depending on the charge on the surface of the microspheres, indicating the successful coating of each layer.41,42 However, in this case, the microspheres coated with CHI and f-MWNTs exhibited a negative surface charge from beginning to end, and each deposition step did not induce the charge reversal as expected initially. Specifically, after the adsorption step of the positively charged CHI, the coated PSS microspheres still exhibited a negative surface charge, probably due to more negative charge density on the surface of the PSS templates.43,44 Significantly found from the figure, the negative charge on the surface of the microspheres deposited at pH 3 is less than that at pH 5, probably due to the difference of the protonation degree of the amino groups of chitosan. This result accounted for the electrostatic repulsion in the process of depositing f-MWNTs, resulting in a larger value of the f-MWNTs adsorbed on the PSS templates with CHI as the outer layer at pH 3, consistent with the results confirmed by SEM and TGA. When the desired number of the (CHI/f-MWNTs) bilayers was reached, the hollow microspheres were obtained by immersing the core/ 13349



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shell microspheres in DMF or toluene to dissolve the PSS cores. The surface morphologies of the bare PSS microspheres (Figure 4) and the coated microspheres (Figure 5) were

Figure 4. SEM image of the PSS microparticles.

observed by SEM. The SEM images indicate that the nanotubes were densely adsorbed on the surface of the PSS microspheres, and provide direct evidence for the stepwise growth of the CHI/f-MWNT multilayers on the PSS microspheres. The compact and uniform packing was achieved, allowing a good control on the thickness of the shells through the number of the bilayers deposited. The uncoated monodisperse PSS microspheres showed a clean and smooth surface (Figure 4), with an average diameter of about 1.90 μm. After one fMWNTs layer was adsorbed, many thread-like nanotubes appeared on the surface of the PSS microspheres. With the increase of the assembly number, more and more f-MWNTs tightly wrapped the microspheres, indicating the uniform deposition in each cycle. After five (CHI/f-MWNTs) bilayers depositions, more f-MWNTs were densely adsorbed on the surface of the PSS microspheres in pH 3 media in comparison with pH 5 media, probably due to the difference of the electrostatic repulsion between the negatively charged microspheres (Figure 3) and the f-MWNTs when being adsorbed in the different pH conditions. It is remarkable that a large degree of flexibility is observed for these carbon nanotubes, which can be related to some loss of rigidity during the process of acid treatment. And the f-MWNTs did not wrap the microspheres completely and therefore long nanotubes can stick out of the surface, allowing them to contact various spheres at the same time. From the SEM images (Figure 5i and 5j), we can estimate that the average diameters of the microspheres with five bilayers deposited at pH 3 and pH 5 are about 2.17 and 2.04 μm, indicating that the thicknesses of the each bilayer are about 54 and 28 nm, respectively. Removal of the PSS templates by dissolution could result in the CHI/f-MWNTs hollow microspheres. The morphologies of the final products were characterized by TEM (Figure 6) and SEM (Figure 7). The high-magnification TEM images reveal that the hollow structure of the microspheres is well kept. It is anticipated that the resulting hollow microspheres could share the morphologies of their precursors. The uniform size and stable shape of the hollow microspheres prepared in this work resulted from the homogeneous PSS templates and the tight

Figure 5. SEM images of the PSS microspheres coated with one (a and c), three (e and g), and five (i and k) (CHI/f-MWNTs) bilayer(s) at pH 3, and one (b and d), three (f and h), and five (j and l) (CHI/fMWNTs) bilayer(s) at pH 5, at low magnification and at high magnification, respectively.

LBL self-assembly. The stability of the hollow structure was believed to be related to the electrostatic interaction between CHI and the f-MWNTs. However, the removal of the PSS cores caused collapse and swelling of the hollow microspheres from Figure 7. As we know, the hollow microspheres flatten 13350



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hollow microspheres were broken after ultrasonicating the sample in deionized water for several minutes, indicating the relative instability of the hollow structure as other hollow microspheres produced by LBL self-assembly. The actual contents of the f-MWNTs in the CHI/f-MWNTs hollow microspheres assembled in the two pH media were measured by thermogravimetric analysis (TGA) from room temperature to 800 °C under a nitrogen atmosphere, as shown in Figures 8 and 9. Figure 8 notes that the PSS and CHI

Figure 6. TEM images of the (CHI/f-MWNTs)5-1 (a), (CHI/fMWNTs)5-2 (b), (CHI/f-MWNTs)5-3 (c), and (CHI/f-MWNTs)5-4 (d).

Figure 8. TGA curves of the PSS microspheres coated with five (CHI/ f-MWNTs) bilayers at pH 3 (a) and pH 5 (b).

Figure 9. TGA curves of the f-MWNTs (a), (CHI/f-MWNTs)5-1 (b), (CHI/f-MWNTs)5-2 (c), (CHI/f-MWNTs)5-3 (d), and (CHI/fMWNTs)5-4 (e).

decomposed almost entirely at 460 °C, which started their decomposition at 350 °C. The mass loss upon the temperature indicates that the contents of the f-MWNTs adsorbed on the PSS microspheres at pH 3 and pH 5 are about 18.2% and 13.9%, respectively. Figure 9 presents the mass loss curves of the hollow microspheres. The weight loss of the f-MWNTs should be attributed to the decomposition process of the defects produced during the excessive oxidizing and cutting of MWNTs with the mixed acids.38 The contents of the fMWNTs in the CHI/f-MWNTs hollow microspheres can be calculated from the mass loss of CHI in the final products, as shown in Table 2. Comparing the hollow microspheres obtained from the precursors coated at pH 3 with those obtained from the precursors coated at pH 5, the contents of the f-MWNTs in the former microspheres is about 12.6% higher than that in the latter microspheres, indicating the pH effect in the deposition process. Comparing the (CHI/fMWNTs)5-1 with the (CHI/f-MWNTs)5-3 (or the (CHI/fMWNTs)5-2 with the (CHI/f-MWNTs)5-4), the contents of

Figure 7. SEM images of (CHI/f-MWNTs)5-1 (a and c), (CHI/fMWNTs)5-2 (b and d), and (CHI/f-MWNTs)5-2 (e and f) after ultrasonication for several minutes.

due to the drying of the sample on the substrate.45,46 In our case, the average diameters of the dried hollow microspheres are about 2.50 and 2.20 μm (Figure 7), slightly larger than those of the core−shell precursors (2.17 and 2.04 μm), indicating that the swelling of the hybrid shells occurs during the core dissolution process.47,48 Furthermore, some of the 13351



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Table 2. Content of the f-MWNTs and Conductivity of the Microspheres

PSS@(CHI/f-MWNTs)5pH3 PSS@(CHI/f-MWNTs)5pH5 (CHI/f-MWNTs)5-1 (CHI/f-MWNTs)5-2 (CHI/f-MWNTs)5-3 (CHI/f-MWNTs)5-4

content (wt%) of the fMWNTs

conductivity (mS/ cm)

18.2

28.9

13.9

2.95

90.4 79.3 86.1 72.1

420 59.3 61.2 6.10

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 86-931-8912582. Phone: 86931-8912516. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was granted financial support from the National Nature Science Foundation of China (grant 20904017) and Program for New Century Excellent Talents in University (grant NCET-11-0441).

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the f-MWNTs in the hollow microspheres obtained by being etched with DMF ((CHI/f-MWNTs) 5 -1 and (CHI/fMWNTs)5-2) are higher than those in the hollow microspheres being etched by toluene ((CHI/f-MWNTs)5-3 and (CHI/fMWNTs)5-4), indicating that about 5.75% of CHI with low molecular weight in the final products could be dissolved by DMF in the etching process,49 compared with toluene as solvent. The electrical conductivity of these nanotubes-adsorbed microspheres was measured by a four-probe method using pressed disk-like samples, with a procedure similar to that reported by other researchers.50 The four-probe electrical measurements gave a DC conductivity at room temperature (Table 2). The conductivities of these microspheres are much lower than that of the pure MWNTs with the value of about 23 S/cm,51 and they depend on the amount of the f-MWNTs adsorbed on the PSS microspheres or the hollow microspheres. As a result, the conductivity of the (CHI/f-MWNTs)5-1 is about 7.1 times higher than that of the (CHI/f-MWNTs)5-2, and the conductivity of the (CHI/f-MWNTs)5-3 is about 10 times higher than that of the (CHI/f-MWNTs)5-4, indicating the pH effect in the deposition process. Moreover, the conductivity of the (CHI/f-MWNTs)5-1 is about 6.9 times greater than that of the (CHI/f-MWNTs)5-3, and the conductivity of the (CHI/f-MWNTs)5-2 is about 9.7 times greater than that of the (CHI/f-MWNTs)5-4, respectively, attributed to the loss of CHI in the DMF dissolution process.49 Further detailed study on the application of the unique CHI/fMWNTs hollow microspheres in the field of materials science is still in progress.

REFERENCES

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4. CONCLUSIONS In summary, we have demonstrated that the LBL self-assembly of CHI and f-MWNTs was realized via the electrostatic interaction on the PSS microsphere templates. Removal of the PSS cores by dissolution in DMF or toluene yields the (CHI/fMWNTs)5 hollow microspheres with the collapse and swelling state. It is proved that the different pH conditions would affect the amount of the adsorbed f-MWNTs when they deposited onto the surface of the PSS microspheres. We have also found that different reagents (DMF and toluene) in the dissolution process could affect the conductivity of the hollow microspheres. With a uniform and ordered distribution of carbon nanotubes, such hollow microspheres are excellent candidates to be applied in many fields of materials science, such as microreactors, catalysis, sensors, and drug delivery. 13352



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Functionalized Multiwalled Carbon Nanotubes Multilayer

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