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From Helixes to Mesostructures: Evolution of Mesoporous Silica Shells on Single-Walled Carbon Nanotubes Yao Wang, Hao Song, Chengzhong Yu, and Hongchen Gu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04660 • Publication Date (Web): 11 Jan 2016 Downloaded from http://pubs.acs.org on January 16, 2016

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Chemistry of Materials

From Helixes to Mesostructures: Evolution of Mesoporous Silica Shells on Single-Walled Carbon Nanotubes Yao Wang,† Hao Song,‡ Chengzhong Yu,‡,* and Hongchen Gu†,* †

State Key Laboratory of Oncogenes and Related Genes, School of Biomedical Engineering, Shanghai Jiao Tong University, Shanghai 200030, China ‡ Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane QLD 4072, Australia ABSTRACT: Constructing a novel nanoplatform by integrating single-walled carbon nanotubes (SWCNTs) and mesoporous silica is of considerable interest due to their combined advantages. However, it is difficult to coat SWCNTs with mesoporous silica shells (MSS). Conventional mechanisms used in the formation of mesostructured materials can not be simply applied in this coating process because the diameter of SWCNTs is smaller than the size of surfactant micelles. Here we report the formation mechanism of MSS on SWCNTs (SWCNTs@MSS) which involves the structural evolution from helixes to mesostructures for the first time. The evolution process mainly includes four stages. The first stage is the transition from tight surfactant helix to loose silica-surfactant composite helix, followed by a second stage of gap filling process on the silica-uncovered surfaces. Afterwards, the surfactant/silicate composite micelles further self-assemble onto nanotubes with the increase of diameter in the third stage. Finally, the silicate frameworks further condense to obtain stable mesostructures. The obtained SWCNTs@MSS with outstanding features exhibit high potential for cancer treatment and also promise future applications in other various fields.

INTRODUCTION Single-walled carbon nanotubes (SWCNTs), which can be envisioned as one layer of graphene sheet rolled-up to a tubular structure, have attracted tremendous scientific interest because of their outstanding optical, electrical, thermal, and mechanical properties.1-3 The potential applications of SWCNTs in various areas, from composite materials to nanoelectronics, from optics to biomedicine, have been extensively demonstrated during the past two decades.4-9 Mesoporous silica has also received increasing attention in recent years due to its intrinsically attractive characteristics, including stable structures, abundant mesopores, high surface area, large pore volume, ease of surface modification, and excellent biocompatibility.10,11 It is of great significance to construct a novel multifunctional nanoplatform by coating mesoporous silica on SWCNTs to overcome the limitations (e.g., toxicity or aggregation of SWCNTs) and combine the merits of both components. Encapsulation of SWCNTs in mesoporous silica shells (MSS) is able to not only improve the biocompatibility and avoid the undesirable aggregations of SWCNTs, but also expand their application ranges to adsorption, separation, catalysis, bioimaging, and drug delivery.12,13 In fact, MSS have been successfully modified on a range of nanoparticles14-17 and carbon nanotubes (CNTs).13,18-22 Compared to multi-walled carbon nanotubes (MWCNTs), empirically it is relatively difficult to coat SWCNTs with MSS (SWCNTs@MSS) as reflected by few number of re-

ports,13,18 but the underlying reason is not clear. As a cationic surfactant, cetyltrimethylammonium bromide (CTAB) is generally utilized to promote the formation of MSS via electrostatic interactions.15,16,18,19 Indeed, the organizations of some similar surfactants on SWCNTs have been investigated.23,24 Richard et al.23 found that the surfactants with a lipidic chain would form supramolecular structures made of rolled-up half-cylinders on the nanotube surface when the concentration is higher than the critical micellar concentration (CMC). Surfactants are the templates to direct the formation of mesostructured silica generally through liquid-crystal templating,10 cooperative assembly25 or evaporation-induced self-assembly26 pathways. However, these mechanisms apply only in bulk solutions or the assembly process on an infinite large substrate, or in a confined space.27,28 The diameter of SWCNTs is normally smaller than 2 nm, smaller than the diameter of conventional micelles, so the coating process of silica-micelle composites on SWCNTs is expected to be different from conventional mechanisms, which, however, has not been reported to the best of our knowledge. Herein, we demonstrate that the growth of MSS on SWCNTs experiences a structural evolution including a helix-to-helix and then a helix-to-mesostructure transition. As illustrated in Figure 1, SWCNTs@MSS were synthesized in a basic aqueous solution by employing SWCNTs as cores, CTAB as templates, and tetraethyl orthosilicate (TEOS) as a silica precursor. The structural evolution mainly goes through four stages. (I) The first

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stage is the helix-to-helix transition. The spontaneously self-assembled CTAB micelles wrap onto SWCNTs to form a surfactant helix (a) with a short pitch size of 4.22 ± 0.47 nm. With the addition of TEOS, the silicate oligomers interact with surfactant templates and co-assemble a composite helical structure (b) with a large pitch size of 14.63 ± 3.51 nm. (II) The second stage is the transition from incomplete to uniform silica coating (c) on the surface of SWCNTs by filling the uncoated gap area of composite helical structure. (III) The third stage is the thickening of MSS. The CTAB/silicate composite micelles further self-assemble onto nanotubes with increased thickness of MSS (d) in a time-dependent manner. (IV) The final stage is the framework condensation. The silicate frameworks of MSS further condense to generate stable mesostructures (e). After the template removal, SWCNTs@MSS with ordered mesopores are obtained. To our knowledge, this is the first report on the structural evolution of MSS on SWCNTs from helixes to mesostructures.

Figure 1. Illustration of the synthesis process and growth mechanism of MSS on SWCNTs.

EXPERIMENTAL SECTION Materials. High purity single-walled carbon nanotubes (SWCNTs, purity > 95%, diameter < 2 nm, bundle length 5-30 μm) were obtained from Chengdu Organic Chemicals Co., Ltd., China and used without further treatment. Cetyltrimethylammonium bromide (CTAB, AR), sodium hydroxide (NaOH, AR), tetraethyl orthosilicate (TEOS, AR), absolute ethanol (AR), and toluene (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Ammonium nitrate (NH4NO3, AR), 3aminopropyltriethoxysilane (APTES, AR), and fluorescein isothiocyanate isomer I (FITC, 96%) were purchased from Aladdin. Rapamycin (Rapa) was kindly provided by MicroPort Co., Ltd. (Shanghai, China). Dulbecco’s modified Eagle’s medium (DMEM), penicillin-streptomycin solution (5 ku/mL) and fetal bovine serum (FBS) were purchased from HyClone Laboratories, Inc. (USA). Cell counting kit-8 (CCK-8) was obtained from Dojindo La-

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boratories (Japan). 4’, 6-diamidino-2-phenylindole (DAPI) was obtained from Beyotime Institute of Biotechnology (Shanghai, China). All chemicals were used as received, and Millipore water (18.2 MΩ cm) was utilized in the preparation of all aqueous solutions. Synthesis of SWCNTs@MSS. The samples were synthesized according to the procedures as follows. In detail, SWCNTs (10 mg) were first suspended in an aqueous solution (10 mL) containing CTAB (500 mg) using a sonic bath (SCIENTZ, SB25-12DTD, 500 W, 40 kHz) for 3 h at room temperature. Then, the homogeneous solution was subsequently treated with a tip sonicator (SCIENTZ, JY92IIDN, 360 W, 20 kHz) to obtain a more stable dispersion, by applying working time of 2 s and intermittent time of 2 s with ice cooling for the whole process of 1 h. The black dispersion was diluted with water (90 mL), followed by the addition of NaOH aqueous solution (2 M, 0.1 mL) and a preheating treatment at 60 °C for 10 min. Afterwards, TEOS (0.5 mL) prediluted with ethanol in 1:4 proportion were introduced to the mixture with the aid of a mild shake. The reaction solution continued to be aged at 60 °C for 12 h to finish the coating process. The black precipitate was collected by centrifugation and washed with ethanol. Finally, the template removal was performed via a highly efficient ion-exchange method. The products were dispersed in ethanol solution (60 mL) containing NH4NO3 (60 mg) and subjected to a sonic bath for 2 h, this procedure was repeated three times to obtain the surfactantfree SWCNTs@MSS. Study of the morphology evolution of SWCNTs@MSS. Firstly, the self-assembled structures of CTAB on SWCNTs (SWCNTs@CTAB) were captured before the coating process of mesoporous silica. A droplet of the suspension of SWCNTs@CTAB was placed onto a copper grid for 15 min and the excess was drawn off with filter papers. Then the grid was negatively stained with a 2% phosphotungstic acid solution (pH 7.0) for 15 min. This sample that reflected the state of initial time (0 s) was characterized by transmission electron microscopy (TEM). Subsequently, the intermediate morphologies of SWCNTs@MSS were recorded after TEOS was introduced into the reaction solution. The holey carbon coated copper grids were dipped into the reaction mixture and then quickly frozen using liquid nitrogen at different reaction time (from 10 s to 1 h). Afterwards, the specimens were freeze-dried overnight before TEM characterization. Characterization. Transmission electron microscopy (TEM) images were taken on a JEM-2100F microscope (JEOL, Japan) operated at an accelerating voltage of 200 kV. Electron tomography (ET) was conducted on a Tecnai F30 transmission electron microscope (FEI, USA) operated at 300 kV. The tilt series for tomography study consists of 101 TEM images, which were recorded over a tilt range of -50° to +50° at an increment of 1°. After fiducialless alignment of the tilt series, the images of ET slice were obtained using IMOD software.29 Scanning electron microscopy (SEM) investigations were performed with an S-


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4800 microscope (Hitachi, Japan) operated at 10 kV. Scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX) analyses were carried out by a F20 G2 field-emission microscope (FEI, USA) operated at 200 kV with an EDX detector system. Raman spectra were obtained on a Invia-reflex Raman microscope system (Renishaw, UK) with a 532 nm excitation source. The powder samples were placed on a silicon wafer for the measurement. Thermogravimetric analyses (TGA) were conducted on a TG 209 F1 iris instrument (NETZSCH, Germany) from room temperature to 900 °C with a heating rate of 10 °C/min under air atmosphere. Nitrogen sorption experiments were carried out at 77 K by an ASAP2010 analyzer (Micromeritcs, USA). The Brunauer 30 −Emmett− Teller (BET) method was utilized to calculated the specific surface area using the data in a linear relative pressure range of 0.05 to 0.20. The pore size distribution was derived from the desorption branch of the isotherm by the NLDFT method31 using the Quantachrome Autosorb 1 software. The total pore volume was given by a single point measurement of adsorbed nitrogen amount at a relative pressure (P/Po) of 0.99.32 Small angle X-ray scattering (SAXS) measurements were conducted at BL16B1 beamline in the Shanghai Synchrotron Radiation Facility (SSRF). The d-spacing value was calculated according to the formula d = 2π/q, where q is the scattering vector. Zeta potential values were measured on a Zetasizer Nano instrument (Malvern, UK) at 298 K. Photothermal effect of SWCNTs@MSS. For photothermal effect investigation, a series of solution samples of SWCNTs@MSS in phosphate buffered saline (PBS) (pH 7.4) with different concentrations (from 50 to 200 μg/mL) were irradiated using near-infrared (NIR) laser (wavelength of 808 nm, power density of 3.2 W cm-2) for different time periods. PBS (pH 7.4) was applied as a control group. The temperature of solution was measured by a thermocouple detector. Drug loading capacity. For drug loading, dried SWCNTs@MSS (3 mg) were dispersed in toluene solution (3 mL) of Rapa with different concentrations (from 0 to 400 μg/mL). After ultrasonication for 5 min, the mixture was shaken at 25 °C for 1 h and separated by centrifugation to collect Rapa-loaded SWNCTs@MSS. The amount of loaded drug was calculated by detecting the concentration of Rapa in toluene before and after loading, determined by UV-Vis spectroscopy (NanoDrop 1000 spectrophotometer, Thermo Scientific, USA) using the adsorption peak of 280 nm. For the measurement, the standard curve was established. The drug loading capacity was then calculated by the following equation: Drug loading capacity = (weight of drug in SWCNTs@MSS) / (weight of SWCNTs@MSS). Cell culture. HeLa cells (human cervical cancer cell line) were cultured in DMEM, supplemented with 10% FBS, 1% penicillin, and 1% streptomycin in a 37 °C humidity incubator containing 5% CO2. The culture medium was changed every 2 days before experimental operation.

Cell viability assay. HeLa cells were cultured in a 96well plate at a density of 1×104 cells per well for 24 h. After removing the culture medium, the fresh medium containing SWCNTs or SWCNTs@MSS with various concentrations (from 10 to 100 μg/mL) was added. After 24 h of exposure, the CCK-8 assay was used to measure the cell viability according to the manufacturer’s protocol. Unexposed wells were regarded as control, and cell viability was calculated as the ratio of the absorbance of test and control wells. The absorbance of each well at 450 nm was read by a microplate reader (BioTek, USA). Cellular uptake study. SWCNTs@MSS were firstly labeled with the fluorescent molecules of FITC as follows. During the synthesis process of SWCNTs@MSS, TEOS (0.5 mL) was replaced with TEOS (0.45 mL) and APTES (0.05 mL), followed by the same steps as mentioned in Experimental Section “Synthesis of SWCNTs@MSS”. Then, the as-prepared amine functionalized SWCNTs@MSS (10 mg) were reacted with FITC (0.5 mg) in ethanol (10 mL) and shaken under darkness overnight. The products were collected by centrifugation and washed with ethanol three times to remove any unreacted FITC. The obtained nanomaterials were denoted as FSWCNTs@MSS. For the cellular uptake study, HeLa cells were seeded on cover slips in a 6-well plate, and cultured in 37 °C incubator for 24 h. Then, F-SWCNTs@MSS suspended in DMEM with a concentration of 50 μg/mL were added to cells. After incubation for 2 h, cells were washed with PBS three times, fixed by 4% paraformaldehyde for 15 min, and then treated by DAPI solution to stain the nuclei. The cells were then sealed and performed on a Leica confocal microscopy system (Leica TCS SP5, Mannheim, Germany).

RESULTS AND DISCUSSION The pristine SWCNTs have a high tendency to form bundles, which can be clearly observed from their transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images (Figure S1a and b). The diameter of SWCNTs is calculated to be 1.22 nm according to their radial breathing mode (RBM) region (100-350 cm-1) from Raman spectrum (Figure S1d), which is consistent with the result of high-resolution TEM (HRTEM) characterization (Figure S1c). Figure 2 displays the final morphology of SWCNTs@MSS prepared with the TEOS/SWCNTs/CTAB/H2O ratio of 46.7 mg/1 mg/50 mg/ 10 mL at 60 °C for 12 h. After the template removal process, discrete SWCNTs@MSS with improved dispersibility are obtained, and hardly any free mesoporous silica nanoparticles are found (Figure 2a). Compared with SWCNTs that precipitate out immediately after sonication, the dispersion of SWCNTs@MSS in ethanol can even keep stable after 24 h (Figure S2), which should be attributed to the coated layer of MSS. High magnification TEM and scanning transmission electron microscopy (STEM) images (Figure 2b and c) of SWCNTs@MSS exhibit a long-range ordered porous architecture along the whole length of the underlying SWCNTs. The center-tocenter pore distance is about 4.68 nm on average, and the



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pore size is roughly estimated to be 3.93 nm. Moreover, some pore openings at both edges of the individual nanocomposite have been clearly observed in the STEM image. The samples of SWCNTs and SWCNTs@MSS were also studied using thermogravimetric analyses (TGA) (Figure S3). The TGA curve of SWCNTs shows a weight loss of 98.68%, indicating the high purity of the material (Figure S3a). As for SWCNTs@MSS, a weight loss stage is observed between 600 °C and 900 °C, which corresponds to the oxidation of SWCNTs. As a result, the mass fraction of the embedded SWCNTs is calculated to be 9.17%. The decomposition temperature of SWCNTs is raised from 672 to 783 °C after coating MSS, as evidenced by the derivative thermogravimetric (DTG) results (Figure S3b), which illustrates that the thermal stability of SWCNTs has been effectively improved by silica shells. To further understand the detailed inner structure of SWCNTs@MSS, electron tomography (ET) was performed on a typical nanocomposite and selected ET slices along z-axis are given in Figure S4. The total slice number of SWCNTs@MSS (from emergence to disappearance) along z-axis is 72, and the distance between each slice is 0.32 nm, thus the z-axial width (or the diameter) of SWCNTs@MSS is 23.04 nm. From the ET slice cutting right in the middle of SWCNTs@MSS (slice-36, Figure 2d), the diameter of the channel inside SWCNT is measured to be 0.72 nm, and the thickness of the silica shells on both side is about 10.10-10.78 nm. The diameter of SWCNTs@MSS measured in the x-y plane axial width is 21.51 nm, similar to the size determined from the perpendicular direction, confirming a uniform core/shell-type and co-axial cylindrical structure of SWCNTs@MSS.

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Before coating with MSS, SWCNTs were capped with the surfactant CTAB (SWCNTs@CTAB) by ultrasonication, making them well dispersed in water. The selfassembled supramolecular structures of CTAB on SWCNTs were recorded through the negatively stained strategy (see Experimental Section). The TEM images (Figure 3a) reveals regular striations on the entire surface of SWCNTs, suggesting that CTAB molecules selfassemble onto SWCNTs and form a helical structure. In fact, our observations are similar to the supramolecular self-assembly of other surfactants on SWCNTs reported previously.23,24 The space between these parallelly arranged striations is measured to be 4.22 ± 0.47 nm, which is in agreement with the value of CTAB stripes on graphite.33 Both left-handed helix (Figure 3a-ii) and righthanded helix (Figure S5) with different rolled-up angles are found in the samples of SWCNTs@CTAB, which is probably ascribed to the chirality of SWCNTs.34

Figure 3. Morphology evolution of MSS on SWCNTs at different reaction time. (a) (i) Low magnification and (ii) high magnification TEM images of supramolecular structures of SWCNTs@CTAB obtained at 0 s. The sample in (ii) shows a left-handed helix with a rolled-up angle of 34°. (b-h) Representative TEM images of SWCNTs@MSS captured at (b) 10s, (c) 30 s, (d) 1min, (e) 1.5min, (f) 5min, (g) 10min, and (h) 1 h.

Figure 2. (a) Low magnification and (b) high magnification TEM images of SWCNTs@MSS. (c) STEM image of SWCNTs@MSS. (d) An ET slice of SWCNTs@MSS showing a clear co-axial core/shell structure.

The time right before the addition of TEOS into the reaction system is counted as the zero point. After the TEOS molecules were added, the intermediate structures were captured at different time points via a liquid nitrogen freezing method (see Experimental Section) and monitored by TEM. At the early stage (10-30 s), the hydrolyzed silicate species adsorb onto SWCNTs quickly (Figure 3b), forming intriguing helixes at 30 s (Figure 3c). The pitch of the helical composite is measured to be 14.63 ± 3.51 nm, larger than that of CTAB helixes wrapping on SWCNTs. At 1 min, the silicate species begin to fill the gaps at the uncovered sites of SWCNTs (as marked by a black arrow in Figure 3d). The naked surfaces are completely covered by silica within 1.5 min, even though some of the helical vestiges can still be observed (as indicated


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by black arrows in Figure 3e). Thereafter, the growing process continues, along with the increase of diameter and the appearance of disordered mesopores at 5 min (Figure 3f), which should be attributed to the selfassembly of CTAB/silicate composite micelles onto nanotubes. The pore structures are well developed after 10 min (Figure 3g), and become totally ordered at 1 h (Figure 3h). After an additional aging time to 12 h and a template removal procedure, the final products of SWCNTs@MSS with ordered mesostructures are obtained (Figure 2). Based on the above results, the growth mechanism of MSS on SWCNTs is illustrated in Figure 1. The concentration of CTAB utilized in our reaction solution is 13.72 mM, which is much higher than CMC.33 Under this condition, CTAB molecules form rolled-up half-cylinders with different chiralities on SWCNTs via hydrophobic interactions.23 Consequently, the zeta potential (ZP) value of the nanotubes increases obviously from + 4.82 ± 1.73 to + 56.63 ± 0.75 mV. Once TEOS molecules are added, they are rapidly hydrolyzed in the basic aqueous solution to form oligomeric silicate species with negative charges.35 The positively charged SWCNTs@CTAB (a) acting as “nucleation sites” facilitate the deposition of silicate via electrostatic attraction, as the local concentration of CTAB on SWCNTs is higher than in the bulk solution, consequently the diameter of the composite helical structure increases with reaction time in stage I as shown in Figure S6a. It is surprising to observe the composite helical structure formed on the surface of SWCNTs (b) with the pitch size three times of CTAB helixes. The transition from tight to loose helixes in stage I has an important implication. The nucleation of negatively charged silica oligomers on the surface of SWCNTs@CTAB is neither homogeneous at the molecular level, nor a faithful templating of the helical surfactant structure. The increased pitch size could be attributed to the relatively high charge density of silicate oligomers in the early stage of reactions. In addition, the higher rigidity of silica could be regarded as another account for this phenomenon. Thus, a loosely packing helical structure is adopted. When the reaction time is between 0.5-1.5 min (stage II), silicate oligomers start to deposit on uncovered areas (gap) of SWCNTs@CTAB and the diameter of composite helical structure keeps constant in this gap filling process (Figure S6a). When the gap areas are fully covered by silica, a relatively homogeneous silica coating (c) is formed. Afterwards, the CTAB/silicate composite micelles selfassemble onto nanotubes and the SWCNTs@MSS (d) is developed. In this stage (stage III), the thickness of MSS gradually increases within 30 min, which is clearly demonstrated in Figure S6b. Although the ordered mesostructures appear after 10 min (Figure 3g), the structures are not stable and rapidly disassemble into amorphous or disordered phase after template removal process if the products were collected before 12 h (Figure S7). To improve the stability, a relatively long aging time of 12 h is acquired to reach a rigid framework (stage IV) and finally obtain a stable mesostructure (e).

Understanding the growth mechanism of MSS on SWCNTs is important for the controllable synthesis of SWCNTs@MSS. The procedure for anchoring surfactant molecules on SWCNTs in this study is more facile and reproducible as the complicated centrifugation steps of SWCNTs@CTAB can be omitted compared to the previous report.18 The self-assembly of CTAB on SWCNTs plays a key role in facilitating the heterogeneous nucleation process (stage I) according to our mechanism. When the concentration of CTAB decreased to 0.27 mM (lower than CMC), no obvious mesostructures are detected even though a small amount of blurry pores can be observed surrounding SWCNTs (Figure S8a). When the CTAB concentration is 2.74 mM (higher than CMC), the mesoporous architecture appears while the arrangement of mesopores is disordered (Figure S8b). These results are ascribed to the insufficient amount of surfactants in the bulk solution as they prefer to adsorb on the surface of SWCNTs, indicating that stage III is necessary for the development of ordered mesopores. Therefore, SWCNTs@MSS with ordered mesostructure should be obtained with a much higher concentration of CTAB (e.g., 13.72 mM) as demonstrated in Figure 2. STEM-energy dispersive X-ray spectroscopy (STEMEDX) element mapping performed on SWCNTs@MSS shows that the Si and O elements are uniformly located at the periphery, whereas the C element is at the center of the nanocomposites (Figure 4a). This result is consistent with ET characterization, which further confirms the coaxial core/shell structure of SWCNTs@MSS. The nitrogen isotherm measurement for SWCNTs@MSS exhibits a type IV isotherm with a narrow pore size distribution (Figure 4b), indicating a uniform mesoporous architecture. The pore size distribution shows a sharp peak centered at 3.78 nm. BET surface area and total pore volume of SWCNTs@MSS are determined to be 871 m2/g and 2.33 cm3/g, respectively. The small angle X-ray scattering (SAXS) pattern is displayed in Figure 4c. The peak position of 1.27 nm-1 in SAXS curve corresponds to a d-spacing of 4.95 nm, which is in agreement with the TEM results. According to the diameters of SWCNTs@MSS at the reaction time of 1.5 min (~7.5 nm) and 30 min (~20 nm) (Figure S6), there should be one to two layers of CTAB/silicate composite micelles self-assembled on nanotubes during stage III. Indeed, some of the mesopores at both edges of SWCNTs@MSS have not been fully developed, which can be clearly observed in Figure 2d. Furthermore, the diameter of SWCNTs@MSS can also be tuned by simply adjusting the mass ratio of added TEOS to SWCNTs (Figure S9 and S10a). The dispersibility of SWCNTs@MSS improves with the increase in diameters (Figure S9a-i to d-i), because the naked surface areas decrease with a high ratio of added TEOS to SWCNTs, as reflected in their more negative ZP values (Figure S10b).



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cultured with HeLa cells for 2 h. As shown in Figure 5d, all cells render strong green fluorescent signals which disperses in cytoplasm surrounding the nuclei (stained with 4’, 6-diamidino-2-phenylindole (DAPI)), indicating that SWCNTs@MSS are successfully internalized by HeLa cells. All these functional tests illustrated here demonstrate that SWCNTs@MSS hold the promise to be a nanoplatform for cancer treatment due to their advantages of multifunctionality and reliable biocompatibility. It opens a large space for further exploration.

Figure 4. (a) STEM-EDX element mapping of SWCNTs@MSS: (i) carbon (C), (ii) silicon (Si), (iii) oxygen (O), and (iv) overlapping of images (i)-(iii). (b) Nitrogen sorption isotherm of SWCNTs@MSS (the inset shows the corresponding pore size distribution). (c) SAXS pattern of SWCNTs@MSS.

SWCNTs have been proven to be an effective photothermal agent as they exhibit strong optical absorption in the near-infrared (NIR) region.36-38 After coating with MSS, the photothermal effect of SWCNTs is still preserved as shown in Figure 5a. When irradiated by the 808 nm NIR laser at a power density of 3.2 W cm-2, the temperature of SWCNTs@MSS solutions increases immediately in a concentration-dependent manner. In contrast, the change in temperature is negligible for the control group of phosphate buffered saline (PBS). The superior photothermal heating effect of SWCNTs@MSS, even better than that of mesoporous silica-coated gold nanorods (Au@SiO2) reported recently,16 shows high potential for photothermal therapy. SWCNTs can also be utilized as a drug carrier. However, the loaded drugs are mainly limited to aromatic molecules via π-π interactions unless SWCNTs are subjected to a further modification.39 Coating MSS is able to endow SWCNTs with capabilities of loading various kinds of drugs. Herein, the model drug rapamycin (Rapa) was used to evaluate the drug loading capacity of SWCNTs@MSS. Rapa could form hydrogen bonds with the numerous silanol groups on the pore wall surfaces of SWCNTs@MSS. The maximum Rapa-loading capacity is calculated to be 173.67 ± 1.40 μg/mg (Figure 5b), indicating SWCNTs@MSS an excellent candidate for carrying drugs. Furthermore, the toxicity issue of SWCNTs has caused serious concern in recently years.40 The cell viability of HeLa cells (human cervical cancer cell line) exposed to various concentrations of SWCNTs or SWCNTs@MSS for 24 h was measured by the CCK-8 assay (Figure 5c). SWCNTs has exhibited considerable cytotoxicity when their concentration is higher than 50 μg/mL. However, no obvious cytotoxicity is observed for SWCNTs@MSS even at 100 μg/mL. That is, the biocompatibility of SWCNTs could be efficiently improved by coating MSS. Finally, a cellular uptake study was performed to test whether SWCNTs@MSS as a nanocarrier could be readily internalized by cancer cells. For this purpose, SWCNTs@MSS were labeled with fluorescein isothiocyanate isomer I (FITC) (F-SWCNTs@MSS), and

Figure 5. (a) Photothermal heating curves of SWCNTs@MSS solutions with different concentrations exposed to the 808 -2 nm NIR laser at a power density of 3.2 W cm . (b) Adsorption isotherm of Rapa with SWCNTs@MSS (the inset shows the formed hydrogen bonding interactions between Rapa and pore wall surfaces). A Langmuir fit shown as the red curve is used for the data. Data are presented as mean ± standard deviation (n = 3 independent samples). (c) An assessment of cell viability using CCK-8 assay in HeLa cell line 24 h after treatment with SWCNTs or SWCNTs@MSS at different concentrations. Data are presented as mean ± standard deviation (n = 3 independent samples). (d) Fluorescent confocal microscopy images of HeLa cells after cultured with F-SWCNTs@MSS for 2 h: (i) bright field image, (ii) blue fluorescent image of cell nuclei stained by DAPI, (iii) green fluorescent image of SWCNTs@MSS labeled with FITC, (iv) overlaying image of (i)-(iii).

CONCLUSIONS In summary, we report the growth mechanism of MSS on SWCNTs based on the structural evolution process from helixes to mesostructures for the first time. The development of MSS on SWCNTs mainly includes four stages: (I) the helix-to-helix transition, (II) the transition from inhomogeneous to homogeneous coating by gap filling process, (III) the thickening of MSS, and (IV) the final framework condensation. The growth mechanism of MSS on SWCNTs is of great significance for the controllable synthesis of SWCNTs@MSS. As a result, the co-axial core/shell structured SWCNTs@MSS with ordered mesostructures, tunable diameters, excellent dispersibility, outstanding photothermal effect, and reliable bio-


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compatibility are successfully obtained. These prominent features of SWCNTs@MSS promise future applications in separation, catalysis, bioimaging, hyperthermal therapy, and drug delivery.

ASSOCIATED CONTENT Supporting Information TEM and SEM images, TGA and DTG curves, diameter growth curves, and zeta potential values. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C. Y.); *E-mail: [email protected] (H. G.).

Author Contributions Y. W. performed the experiments and analyzed the data. H. S. conducted the ET characterization and analysis. C. Y. and H. G. designed and supervised the study. Y. W. wrote the paper. C. Y. and H. G. revised the manuscript. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by Grants from High Tech Program of MOST China (2013AA032203) and State Key Laboratory of Oncogenes and Related Genes (91-14-03). Y. Wang and H. Gu acknowledge the Instrumental Analysis Center of Shanghai Jiao Tong University and the Nanotechnology and Application National Engineering Research Center for assistance with the characterization of materials; Y. Chen of Shanghai Jiao Tong University for his assistance with cell experiment; F. Tian and X. Miao (BL16B1) of Shanghai Synchrotron Radiation Facility (SSRF) for SAXS measurement; and D. Wu of FEI company for assistance with STEM-EDX element mapping. H. Song and C. Yu acknowledge the Australian Research Council, the Australian Microscopy and Microanalysis Research Facility at the Centre for Microscopy and Microanalysis, The University of Queensland, and the Queensland node of the Australian National Fabrication Facility for their support.

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