Multiwalled Carbon-Nanotube-Embedded Microcapsules and Their

J. Phys. Chem. C 2009, 113, 3967–3972

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Multiwalled Carbon-Nanotube-Embedded Microcapsules and Their Electrochemical Behavior Jiwei Cui, Yaqing Liu, and Jingcheng Hao* Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education, Jinan 250100, P. R. China ReceiVed: October 4, 2008; ReVised Manuscript ReceiVed: January 3, 2009

Multiwalled carbon-nanotube (MWCNT)-embedded microcapsules were fabricated by the stepwise deposition of polyelectrolytes and oxidized MWCNTs using the layer-by-layer (LbL) self-assembly technique based on electrostatic interaction. Electrochemical behaviors of the MWCNT-embedded microcapsules were studied by cyclic voltammetry (CV). Transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and confocal laser scanning microscopy (CLSM) were used to characterize the morphology of the microcapsules. Experimental results revealed that MWCNTs were homogeneously assembled in the microcapsule shells, forming a netlike structure. CV measurements indicated that MWCNTembedded microcapsules exhibited different electrochemical behaviors by changing surrounding conditions, such as pH and salt concentration. The MWCNT-embedded microcapsules, combined with the electrochemical behaviors, are envisaged to be utilized in applications for biosensors and catalysis. Introduction The Nobel laureate Richard Smalley said that carbon nanotubes, which were brought to the world’s attention by Sumio Iijima,1 would be cheap and environmentally friendly materials.2 Certainly, carbon nanotubes, including single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs), have attracted more and more attention from scientists in different fields3 because of their novel properties, such as high specific surface area, electrical conductivity, good chemical stability, and extremely high mechanical strength.4 The properties of carbon nanotubes make them good candidates for electrochemical devices,5 nanoelectronic devices,6 electron field emission,7 composites,8 sensors,9 and so on. However, one impediment to the potential applications of carbon nanotubes is that they often entangle together with bundles of micrometerlong tubes and cannot dissolve in any solvent because of strong van der Waals interactions between carbon nanotubes. The general method to cut carbon nanotubes into shorter lengths, to improve their solubility and for further functionalization,10 uses strong acids and sonication. Progress in the development of several techniques for fabricating functionalized carbon nanotubes, for example, the patterned deposition technique for wellaligned carbon nanotubes,11 the chemical assembly of carbon nanotube arrays,12 the template synthesis of nanomaterials,13 and so on, has been expected to widen their application areas. The layer-by-layer (LbL) self-assembly method, based on electrostatic interaction, is a simple but powerful strategy for fabricating multilayers.14 This technique has been widely used to fabricate polyelectrolyte and carbon nanotube films.15 There have been abundant reports of voltammetry experiments to investigate the applications of polyelectrolyte and carbon nanotube films in the fields of biosensors, detection, and electrocatalysis.16 Fabricating multifunctional microcapsules using LbL selfassembly has become an increasingly active research topic in * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +86-531-88366074.

the last 10 years because the wall of the microcapsules can be tuned in thickness, composition, and functionality by choosing various constituents and adjusting the number of layers.17 Various components, such as polyelectrolytes,18 biopolymers,19 lipids,20 and inorganic nanoparticles,21 have been fabricated into the shells to construct functional microcapsules. Undoubtedly, functionalized carbon nanotubes containing carboxyl groups can also be good candidates for constructing microcapsules. However, most research work focuses on the assembly and arrays of carbon-nanotube-embedded multilayers or composites.22 In the present work, MWCNT-embedded microcapsules were fabricated by the stepwise deposition of polyelectrolytes and oxidized MWCNTs using the LbL self-assembly technique based on electrostatic interaction. MWCNTs could be homogeneously distributed in the microcapsules. We chose MWCNTs as shell material not only because of the higher purity but also because of the cheaper price, compared to that of SWCNTs. Then, we mainly investigated the electrochemical behavior of MWCNT-embedded microcapsules using cyclic voltammetry in different surrounding conditions. The MWCNT-embedded microcapsules, combined with its electrochemical behaviors, are envisaged to be utilized in applications for biosensors and catalysis. Experimental Section Materials and Chemicals. Poly(sodium 4-styrenesulfonate) (PSS, MW ∼70 000), poly(allylamine hydrochloride) (PAH, MW ∼70 000), polyethylenimine (PEI, MW ∼25 000), and fluorescein isothiocyanate labeled dextran (FITC-dextran) were obtained from Sigma-Aldrich Inc., USA. Commercial polyelectrolytes were used without further purification. Monodispersed weakly cross-linked melamine formaldehyde (MF) spherical particles with a diameter of 6.58 ( 0.13 µm were purchased from Microparticles GmbH, Berlin, Germany. Water used in all experiments was triply distilled, which had a resistivity higher than 18.2 MΩ · cm. MWCNTs (thin homogeneous tubes with 8-10 concentrically bent single graphene layers with an average diameter of ∼15

10.1021/jp808785j CCC: $40.75  2009 American Chemical Society Published on Web 02/18/2009

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SCHEME 1: Representation of the LbL Assembly of MWCNT-Embedded Microcapsules

nm, length of 5-50 µm, BET-surface ∼250 m2/g, electrically and thermally conductive, and purity >98%) were a gift from Future Carbon GmbH, Bayreuth, Germany. According to the literature,23 MWCNTs were treated in a mixture of concentrated sulfuric and nitric acids (3:1 v/v, 98 and 70%, respectively) under ultrasonication for 4 h at 40 °C. Then, the MWCNTs were rinsed with triply distilled water until the pH became nearly neutral. Such a procedure shortened the MWCNTs and produced oxygen-containing moieties mainly on the open ends of the nanotubes.24 The oxygen-containing MWCNTs (polydispersed, ∼300 nm) could be stably and homogeneously dispersed in water. Fabrication of MWCNT-Embedded Microcapsules. The polyelectrolyte multi-layer-coated MF particles were first prepared by LbL assembly. The positively charged MF particles were suspended in PSS aqueous solution for 20 min. The dispersion was then centrifuged for 5 min at 900g, followed by removal of the supernatant. The particles were then redispersed in water by gentle shaking. The washing process (centrifugation, removal of supernatant, and redispersion) was repeated twice before the next deposition. The PAH layers were assembled in the same way. The concentration of the polyelectrolytes was 2.0 mg · mL-1 with 0.5 mol · L-1 NaCl. The desired number of polyelectrolyte multilayers was obtained by the alternate deposition of anionic and cationic polyelectrolytes onto the MF particles using this procedure. Subsequently, the oxygencontaining MWCNT dispersion (0.5 mg · mL-1) was added in the (PSS/PAH)4 microcapsules. After the 30 min of adsorption, excess MWCNTs were removed by three centrifugation (900g, 10 min)/wash cycles. For the MWCNT-embedded hollow microcapsules, the template MF core was decomposed in 0.1 mol · L-1 HCl for 10 min. The washing process was repeated until the supernatant became nearly neutral. The detail preparation of the MWCNT-embedded microcapsules is illustrated in Scheme 1. Transmission Electron Microscopy (TEM). The MWCNTembedded microcapsules were loaded on a Formvar-film-coated copper grid and observed under a JEM-100CXII TEM (Japan) at an accelerating voltage of 100 kV and a vacuum of 10-6 Torr. Scanning Electron Microscopy (SEM). SEM analysis was performed using a JEOL JSM-6700F instrument (Japan) at an operating voltage of 3 keV. Atomic Force Microscopy (AFM). The silicon wafers (0.6 × 0.6 cm2) were cleaned in a hot piranha solution (a 7:3 v/v mixture of concentrated H2SO4 and 30% H2O2) at 70 °C for 60 min, followed by thoroughly rinsing with distilled water and drying with compressed nitrogen.25 (Piranha solution is ex-

tremely corrosive and reacts violently with organic materials. It should be handled with great care.) After this procedure, the silicon wafers were negatively charged. Then, the wafers were immersed into a 1 mg · mL-1 solution of PEI for 15 min, rinsed three times in distilled water, and dried with nitrogen. The samples were prepared by applying a drop of the microcapsule suspension (approximately 0.2 µL) on a PEI-coated silicon wafer and drying it in air. The morphology of the hollow MWCNTembedded microcapsules in the dried state was performed using a Nanoscope IIIa Multimode AFM (Digital Instruments Inc., USA) at room temperature. Silicon tips with a resonance frequency of about 269 kHz were utilized for the tapping mode at a scan rate of 1.0 Hz. Confocal Laser Scanning Microscopy (CLSM). Equal amounts of capsule suspension and FITC-dextran solution (0.04 mg/mL) were mixed together and stored for 2 h at 4 °C. The suspension was centrifuged at 2200g and washed three times in distilled water to remove the excess FITC-dextran. CLSM images were taken with an Olympus IX 81 instrument (Japan) equipped with a 60× oil immersion objective. The excited wavelength was 488 nm. Images in transmission mode were taken in parallel. Electrochemical Characterization. The GC electrode used in the experiments was a 3 mm glassy carbon disk insulated in a 7 mm diameter Teflon rod. Before the measurement, the GC electrode was polished with chamois leather and ultrasonically cleaned in distilled ethanol and water. Then, the cleaned GC electrode was modified by a PEI layer. The electrochemical behavior of MWCNT-embedded microcapsules on a GC electrode was examined using cyclic voltammetry (600B CHI electrochemistry analytical instrument) under a N2 atmosphere. All the electrochemical measurements were performed with a conventional three-electrode cell with a saturated calomel electrode (SCE) and a Pt plate as the reference and the counter electrodes, respectively. All the potentials in this paper are given relative to the SCE. Standard phosphate buffer solution (PBS) with different pH values (4.0, 6.86, and 9.18) were used as supporting electrolytes. All electrochemical experiments were conducted at room temperature (25 °C). Results and Discussion Previous investigations have shown that oxygen-containing moieties (e.g., carboxyl and hydroxyl) can be introduced mainly on the open ends of the carbon nanotubes after oxidation, which could be proved by X-ray photoelectron spectroscopy and FTIR spectra.26 Obviously, carboxyl moieties make shortened MWCNTs hydrophilic and negatively charged, which meets the require-

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Figure 1. TEM images of (PSS/PAH)4/MWCNT microcapsules before (a,b) and after (c,d) dissolving the core. (e) Hollow (PSS/PAH)4 microcapsules. SEM images of (PSS/PAH)4/MWCNT microcapsules before (f,g) and after (h) dissolving the core at different magnifications.

ments of LbL self-assembly very well. Fabricating MWCNTembedded microcapsules using the LbL technique relies on the electrostatic attraction between the charged species. Compared with the W/O emulsion technique,27 the LbL self-assembly method can produce homogeneous MWCNT-embedded microcapsules with a controllable diameter by choosing different templates. Figure 1a shows a representative TEM image of MWCNTembedded microcapsules. MWCNTs appearing like villiform protuberance can be seen on the surface of polyelectrolytecoated spheres at high magnification (Figure 1b). SEM images (Figure 1f,g) also clearly show the homogeneous distribution of MWCNTs absorbed on the spheres. The oxidized MWCNTs are not long enough to bridge neighboring microcapsules together, which can be attributed to the effective acid treatment and the LbL technique. Acid treatment can shorten the MWCNTs, and the LbL technique makes the shorter MWCNTs assemble on the spheres more easily because of the rigidity of the MWCNTs. Hollow polyelectrolyte microcapsules and MWCNT-embedded hollow microcapsules without an MF core are also intact (Figure 1c,e). The surface roughness of MWCNTembedded hollow microcapsules is greater, compared with that of hollow polyelectrolyte microcapsules. Careful observation shows that the microcapsule in Figure 1c has a higher electron contrast than the microcapsule in Figure 1e at the same brightness, which is due to the thicker shell layers of the MWCNT-embedded hollow microcapsule. Local magnification of the MWCNT-embedded hollow microcapsule in Figure 1c shows that MWCNTs intertwine together (Figure 1d). As confirmed by SEM images (Figure 1h), MWCNTs are tightly connected with PAH distributed in the shells and randomly cover the surface of the hollow microcapsules, which show no signs of rupture. To get the surface information of MWCNT-embedded microcapsules, the morphology of the hollow microcapsules was further elucidated by AFM in the tapping mode. Figure 2 compares the morphology of a (PSS/PAH)4 microcapsule and a (PSS/PAH)4/MWCNT microcapsule after dissolving the core. The polyelectrolyte microcapsules exhibit many creases and folds, which are due to the evaporation of the solvent molecules.28 However, creases and folds are not obvious for the

Figure 2. AFM images of (PSS/PAH)4 (a) and (PSS/PAH)4/MWCNT (b) microcapsules. The section analysis is the height profile along the line drawn on the microcapsule.

TABLE 1: Wall Thickness and Roughness Comparisons of (PSS/PAH)4 Microcapsules and (PSS/PAH)4/MWCNT Microcapsules microcapsule

wall thickness, nm

roughness, nm

(PSS/PAH)4 (PSS/PAH)4/MWCNT

24.5 ( 0.2 45.4 ( 2

4.1 ( 0.1 14.6 ( 0.5

MWCNT-embedded microcapsules, which is ascribed to the embedded MWCNTs making the shell layer thicker. The results of AFM measurements are presented in Table 1. The wall thickness and root-mean-square (rms) roughness of (PSS/PAH)4/ MWCNT microcapsules are greater than the corresponding values of (PSS/PAH)4 microcapsules, which is consistent with the results of TEM measurements. MWCNT-embedded hollow microcapsules can be in situ visualized under CLSM. FITC-dextran was used to label the microcapsules. From the images, we can see that the microcapsules are spherical, and no aggregation is observed (Figure 3a,b). A continuous shell structure is noticeable in both fluorescence and transmission modes, which also indicates that the microcapsules are intact.

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Figure 3. CLSM images of hollow (PSS/PAH)4/MWCNT microcapsules. Fluorescence image (a) and transmission image (b).

Figure 4. (a) Cyclic voltammograms of (PSS/PAH)4/MWCNT microcapsules on the GC electrode in phosphate buffer solution (pH ) 6.86) at different scan rates. (b) Reduction (9) and oxidation (∆) peak current as a function of scan rate. The line is a linear fit to the data. Ipc and R denote peak current and fitting degree, respectively.

Figure 5. Cyclic voltammograms of (PSS/PAH)4/MWCNTs microcapsules on the GC electrode in phosphate buffer solution with different pH values: pH ) 4.0 (a), pH ) 6.86 (b), and pH ) 9.18 (c). Scan rate: 0.1 V/s.

The electrochemical behavior of carbon nanotube films has been widely investigated.29 However, few reports are found about the electrochemical behavior of microcapsules using cyclic voltammetry.30 Figure 4a displays typical cyclic voltammograms of (PSS/PAH)4/MWCNT microcapsules on the GC electrode using phosphate buffer solution (pH ) 6.86) as the support electrolyte at different scan rates. MWCNT-embedded microcapsules show a well-defined reversible voltammogram at -0.05 V. The peak potential has a little shift with increasing scan rate. From Figure 4b, we can see that the redox peak current linearly scales with the scan rate, revealing that the redox response corresponds to a surface-confined species. However, (PSS/ PAH)4 microcapsules on the GC electrode cannot exhibit redox peaks. Obviously, it is MWCNTs embedded in the microcapsules that result in the redox process. Previous investigations have found that the redox response of polyelectrolyte/MWCNT films was attributed to the redox process of the oxygencontaining groups.31 Therefore, we can conclude that the redox

response in this study is ascribed to the oxygen-containing groups on the open ends of the carbon nanotubes. In addition, we observe that (PSS/PAH)4/MWCNT microcapsules have good stability because the cyclic voltammograms remained unchanged on a continuous redox cycling (50 cycles at 0.1 V · s-1), which also indicates no detachment of the microcapsules from the GC electrode during the electrochemical cycling experiment. Polyelectrolyte microcapsules will exhibit different behaviors (e.g., swelling, shrinking, or permeability variation) in different surrounding conditions.32 If we change the pH or ionic strength of the support electrolyte, how will the electrochemical behavior of MWCNT-embedded microcapsules change? Figure 5 shows the cyclic voltammograms of (PSS/PAH)4/MWCNT microcapsules on the GC electrode in phosphate buffer solution with different pH values. The peak potential decreases as the pH increases, which can be visually shown in a histogram (Figure 6). The reasonable explanation is that protons and electrons take part in the electrode reaction. The concentration of protons

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J. Phys. Chem. C, Vol. 113, No. 10, 2009 3971 Acknowledgment. The authors thank the NSFC (Grant No. 20625307) for financial support, and the National Basic Research Program of China (973 Program 2009CB930103). References and Notes

Figure 6. Reduction (red cubes) and oxidation (green cubes) peak potential as a function of pH concluded from Figure 5. Scan rate: 0.1 V/s.

Figure 7. Cyclic voltammograms of (PSS/PAH)4/MWCNT microcapsules on the GC electrode in phosphate buffer solution (pH ) 6.86) with increasing KCl concentration. Scan rate: 0.1 V/s.

decreases when the pH increases, which will need a low peak potential to complete the redox process. Figure 7 displays the cyclic voltammograms of (PSS/PAH)4/ MWCNT microcapsules in the presence of different concentrations of KCl. The peak potential does not change, compared with the case without KCl. From the figure, we can see that the peak current decreases only a little as the salt concentration increases. In other words, salt concentration in the support electrolyte has little influence on the peak current. The small decrease of peak current may be due to the following reason: ionic strength increases as the salt concentration increases, which will result in the slow mobility of protons and electrons to the MWCNT-embedded microcapsules on the GC electrode. Conclusions MWCNT-embedded microcapsules have been fabricated using the LbL technique based on electrostatic interaction. MWCNTs can be homogeneously distributed in the microcapsules. We also investigated the electrochemical behavior of MWCNT-embedded microcapsules using cyclic voltammetry. The MWCNT-embedded microcapsules exhibit well-defined reversible voltammograms, due to the oxygen-containing groups on the open ends of the MWCNTs embedded in the microcapsules. The peak potential decreases with increasing pH. We speculate protons and electrons take part in the electrode reaction. However, salt concentration in the support electrolyte has little influence on the peak current. The investigation of the electrochemical behavior of MWCNT-embedded microcapsules will provide a good way to enrich the application of microcapsules, for example, in the fields of biosensors and catalysis.

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