Luminescent Carbon Dot-Gated Nanovehicles for pH-Triggered

May 3, 2013 - (1-8) Compared to traditional heavy-metal-based quantum dots, ... (31, 32) Despite these burgeoning achievements, the systems that ... S...
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Luminescent Carbon Dot-Gated Nanovehicles for pH-Triggered Intracellular Controlled Release and Imaging Li Zhou,†,‡ Zhenhua Li,†,‡ Zhen Liu,†,‡ Jinsong Ren,*,† and Xiaogang Qu† †

Laboratory of Chemical Biology and State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, China ‡ Graduate School of the Chinese Academy of Sciences, Beijing, 100039, China S Supporting Information *

ABSTRACT: In this paper, the use of biocompatible carbon dots (C-Dots) as caps on the surface of mesoporous silica nanoparticles (MSPs) for the design of intelligent on-demand molecular delivery and cell imaging system is described. These C-Dots-attached MSPs exhibited low cytotoxicity toward the cells and strong luminescence both in vitro and in vivo. A further loading of anticancer drug (DOX) endowed the fluorescent material with therapeutic functions. It was found that changing the pH to mildly acidic condition at physiological temperature caused the dissociation of the C-Dots@MSPs complex and release of a large number of DOX from the nanospheres. Moreover, the DOX-loaded nanocomposites exhibited a remarkably enhanced efficiency in killing cancer cells. The endocytosis and the efficient drug release properties of the system were confirmed by luminescence microscopy. Overall, we believe that the well-designed C-Dots@MSPs nanocomposites are promising for a simultaneous bioimaging and drug delivery system, which show more potential for clinical application.

1. INTRODUCTION As newcomers to the world of nanomaterials and nanolights, luminescent carbon dots (C-Dots) have attracted growing attention recently due to their great potential in biological labeling/imaging, photocatalysts, sensor design, as well as being promising building blocks for future nanodevices.1−8 Compared to traditional heavy-metal-based quantum dots, these functional carbon nanoparticles exhibit excellent photostability, biocompatibility/good water solubility, and low toxicity, and have shown great advantage in a variety of applications.1−4,9−13 For example, Sun et al. reported the potential of C-Dots passivated with PPEI-EI for two-photon luminescence microscopy bioimaging using human breast cancer MCF-7 cells.1 Despite these burgeoning achievements, the exploring of further functions of these fascinating nanomaterials still remains a big challenge in this field. Here, for the first time, we explored the use of C-Dots as pHresponsive caps for mesoporous silica nanoparticles (MSPs), and demonstrated the operability of this system for intelligent on-demand molecular delivery system and optical imaging (Scheme 1). Due to the high surface area, uniform and tunable pore structure, and diversity in surface functionalization, MSPs have been recognized as an attractive material for drug delivery.14−17 A series of MSP-based release systems have been developed that are responsive to distinct external stimuli.18−27 Acid sensitive MSP drug delivery systems have attracted particular attention in the treatment of disease because of the acidic environment of tumor and inflammatory tissues.28 Various components, such as dendrimers, nanoparticles, and supramolecular nanovalves, have been used in the © XXXX American Chemical Society

construction of end-capped MSPs for efficient delivery of therapeutic content in acidic environments.29−34 Upon the pHstimulation, these gatekeepers allow the release of the cargo from the reservoir into the environment. Of the gatekeepers previously studied, nanoparticles represent one of the most effective strategies for blocking the pores of MSPs due to their ultrafine dimensions and unique optical, electronic, and magnetic properties.19−21,31−33 For example, Au nanoparticles were attached to MSPs through an acid-labile acetal linker to construct a pH-responsive nanogated ensemble.31,32 Despite these burgeoning achievements, the systems that combined end-capped MSPs and excellent biological optical imaging for stimuli-responsive controlled drug release have not been achieved yet. The present luminescent quantum dots or organic fluorescent dyes that incorporated the MSPs as bioimaging agents have poor biochemical stability or potential toxicity, which may be potentially hazardous for the in vivo applications.35−38 In this work, the C-Dots gated nanocarrier not only responded to physiopathological pH signals to trigger selective drug release in cancerous cells, but also served as imaging agents in the transport process to their destination. Typically, fluorescent C-Dots contain many carboxylic acid moieties at their surface, thus imparting them with excellent water solubility and the suitability for further application.9 We then sought to take advantage of this feature to control the gate Received: February 4, 2013 Revised: May 3, 2013

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Scheme 1. Schematic Representation of pH-Triggered Release of Drug Molecules from the Pores of Mesoporous Silica Nanoparticles Capped with Carbon Dots

operation: the negatively charged C-Dots-COO − were anchored to the openings of aminopropyl group functionalized MSPs through electrostatic interaction and were utilized as caps for trapping the guest molecules within the pores. When partially ionized carboxylic acid species (COO−) on the C-Dots were transformed into protonated groups (COOH) by changing of the pH value, C-Dots could be separated from the surface of modified MSPs, leading to opening of the gates. We have demonstrated loading of anticancer drugs doxorubicin hydrochloride (DOX) into MSPs, and efficient intracellular drug delivery in human cancer cells. More importantly, the ability of C-Dots@MSPs as biocompatible agents for in vitro and in vivo imaging has been confirmed.

obtained by evaporating the upper yellow solution and drying the concentrated solution under vacuum at 70 °C for 15 h. Then the obtained C-Dots were redissolved in 4 mL of double-distilled water as stock solution (132 μg/mL). Synthesis of C-Dots@MSPs. The NH2-MSPs (20 mg) were dispersed in 20 mL of phosphate-buffered saline (10 mM, pH 7.4), and the solution was sonicated for 1 min. The as-prepared C-Dots stock solution with appropriate volume was added into the solution to functionalize MSPs with C-Dots. The mixture was stirred at room temperature for 6 h, and followed by centrifugation, washing with phosphate-buffered saline for several times. The free C-Dots in the supernatant were investigated by measuring the PL spectrum. DOX Loading and Release. DOX was first loaded inside MSPs as follows: the purified NH2-MSPs were incubated in the phosphatebuffered saline (10 mM, 25 mL, pH 7.4) of DOX (5 mg) for 24 h at room temperature, with loading efficiency monitored by absorption spectra measurement. After that, the mixed solution was centrifuged, washed with PBS buffer, and dried under high vacuum to produce the DOX load nanoparticles. The closure reaction was performed by mixing C-Dots stock solution with the DOX load nanoparticles in phosphate-buffered saline (10 mM, pH 7.4). The solution was stirred for 6 h, followed by centrifuging and repeated washing with phosphate-buffered saline (10 mM, pH 7.4). The C-Dots@MSPs with DOX material was then dispersed in 25 mL of phosphatebuffered saline at different pH values. Aliquots were taken from the suspension, and the delivery of DOX from the pore to the buffer solution was monitored via the absorbance band of the drug. Fluorescence Microscopy. C-Dots@MSPs exhibit intense green fluorescence under blue illumination, and DOX shows red fluorescence under green light. These properties allow the use of fluorescence for the cell imaging and for studing the distribution of CDots@MSPs inside the cells. To do the test, the concentration of HeLa cancer cells was fixed at a density of 105 cells/well in 24-well assay plates. C-Dots@MSPs were added to the cells, and the mixture was incubated at 37 °C. The cells were then washed twice with PBS, and finally examined by fluorescence microscopy. The experimental procedures were the same for the DOX-loaded C-Dots@MSPs. Cytotoxicity Assays. MTT assays were used to probe cellular viability. HeLa cells were seeded at a density of 5000 cells/well (90 μL total volume/well) in 96-well assay plates. After 24 h, drugs at the indicated concentrations were added, and cells were further incubated for 24 h to allow the uptake of nanoparticles. To determine toxicity, 10 μL of MTT solution (BBI) was added to each well of the microtiter plate, and the plate was incubated in the CO2 incubator for an additional 4 h. The cells then were lysed by the addition of 100 μL of DMSO. Absorbance values of formazan were determined with Bio-Rad model-680 microplate reader at 490 nm (corrected for background

2. EXPERIMENTAL SECTION Chemicals. Tetraethylorthosilicate (TEOS), (3-aminopropyl) trimethoxysilane (APTES), phosphate, and doxorubicin hydrochloride (DOX) were purchased from Sigma-Aldrich. N-Cetyltrimethylammonium bromide (CTAB) and EDTA-2Na·2H2O were obtained from Alfa Aesar. All the chemicals were used as received without further purification. Water throughout all experiments was obtained by using a Millie-Q water system. Synthesis and Chemical Modification of the MSPs’ Surface. N-Cetyltrimethylammonium bromide (CTAB, 0.80 g) was first dissolved in 384 mL of pure water. Sodium hydroxide (2.8 mL, 2 M) was added to CTAB solution, followed by adjusting the solution temperature to 80 °C. The mixture was stirred for 15 min and TEOS (3.88 mL) added rapidly while stirring was continued. TEOS (60 μL) and APTES (60 μL) were introduced 30 min later. The mixture was allowed to stir for 2 h to give rise to white precipitates. The solid product was filtered, washed with deionized water and methanol, and dried in air. To remove the surfactant template (CTAB), the white powder was refluxed for 16 h in a solution of 1.00 mL of HCl (37%) and 80.00 mL of methanol followed by extensive washing with deionized water and methanol. The obtained nanoparticles were harvested through centrifugation, washed with deionized water and ethanol in sequence, and then dried in vacuum at 60 °C overnight. The resulting surfactant-removed amine-functionalized MSPs (NH2MSPs) were placed under high vacuum to remove the remaining solvent in the mesopores. Synthesis of C-Dots. In a typical procedure, a quartz boat filled with EDTA-2Na·2H2O (AR, 0.5 g) was thrust into a quartz tube and calcined at 400 °C for 2 h in flowing N2. The resulting blank powders were dissolved in acetone (20 mL) and then centrifuged at a high speed (13 000 rpm) for 15 min. Pure luminescent C-Dots powder was B

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Figure 1. Representative TEM image of MSPs (a) and C-Dots@MSPs (b). The FT-IR spectra (c) and TGA analysis (d) of MSP-NH2 and C-Dots@ MSPs. The inset in panel a is the HR-TEM image of C-Dots. absorbance at 630 nm). The results were expressed as the mean values of three measurements. In Vivo Imaging Studies. Nude mice were purchased from Shanghai SLAC Laboratory Animal Co. Ltd. (Shanghai, China). All animal procedures were in accord with the guidelines of the Institutional Animal Care and Use Commottee. Before experiments, nude mice were first anesthetized by intraperitoneal injection of 10 wt % chloral hydrate. For in vivo photoluminescence imaging, 0.9 wt % NaCl solution containing MSPs-C-Dots was subcutaneously injected into the back flank area of the nude mice. The whole-body photoluminescence images of the nude mice were recorded on Kodak In-vivo Imaging System Fx Pro. Measurements and Characterizations. Fourier transform infrared (FT-IR) analyses were carried out on a Bruker Vertex 70 FT-IR spectrometer. X-ray measurements were performed on a Bruker D8 FOCUS powder X-ray diffractometer using Cu Kα radiation. Thermogravimetric analyses were carried out on a PerkinElmer Pyris Diamond TG/DTA analyzer, using an oxidant atmosphere (Air) with a heating program consisting of a dynamic segment (10 °C/min) from 373 to 1122 K. TEM images were recorded using a FEI TECNAI G2 20 high-resolution transmission electron microscope operating at 200 kV. N2 adsorption−desorption isotherms were obtained on a Micromeritics ASAP 2020 M automated sorption analyzer. The samples were degassed at 150 °C for 5 h. The specific surface areas were calculated from the adsorption data in the low pressure range using the BET model and pore size was determined following the BJH method. The ζ-potential of the nanoparticles in PBS was measured in a Zetasizer 3000HS analyzer. UV−vis spectroscopy was carried out with a JASCO V-550 UV−vis spectrometer. Dynamic light scatterer (DLS) mesurement was made by Malvern Corp, U.K. (ZEN3690). Fluorescence spectra were recorded with a JASCO FP-6500 spectrofluorometer.

that contain a honeycomb-like structure were confirmed by TEM and low-angle powder X-ray diffraction (Figure 1a and Figure S1). The surface of MSPs was then functionalized with aminopropyl groups by treatment with 3- aminopropyltriethoxysilane (APTES) to afford NH2-MSPs. N2 adsorption− desorption isotherms of NH2-MSPs showed a typical type IV curve with a specific surface area of 1301 m2 g−1, an average pore diameter of 2.5 nm, and a narrow pore distribution (Figures S2 and S3). To obtain pH-responsive caps, watersoluble C-Dots were prepared by an established strategy.40 The existence of carboxyl groups on C-Dots were clearly identified through both the broad O−H stretching vibration (3346 cm−1) and the sharp CO stretching vibration (1647 cm−1) in the FT-IR (Figure S4). The TEM results (Figure S5) showed that the C-Dots were mostly spherical dots with sizes averaging about 3.8 nm (based on statistical analyses of more than 200 dots), which suggested that they were large enough to block the 2.5 nm pores of the MSPs. The solution of the C-Dots exhibited an excitation dependent PL behavior (Figure S6), and its quantum yield was determined to be 11.0% (relative to RhB in water). To obtain C-Dots@MSPs, different concentrations of C-Dots were added to the MSPs (1 mg/mL). The production of C-Dots@MSPs were monitored by the change of the photoluminescence (PL) signal of C-Dots in the suspension. As shown in Figure S7, the binding of C-Dots by the MSPs remained above 90% from 0 to 20 μg/mL. When the concentration of C-Dots changed from 20 to 30 μg/mL, the binding of C-Dots by the MSPs decreased from 96.2% to 77%, revealing that the C-Dots used were in an excess amount. To prevent the leakage of guest molecules stored in the MSPs, a concentration of 30 μg/mL was used in our following study. The as-prepared C-Dots@MSPs were washed with PBS (pH 7.4) several times. The successful incorporation of C-Dots to the MSPs matrix was further confirmed by different methods.

3. RESULTS AND DISCUSSION Preparation and Characterization of C-Dots@MSPs. MCM-41 particles were first synthesized by the reported basecatalyzed sol−gel procedure,39 and MSPs (130 nm in diameter) C

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treated with C-Dots@MSPs after 2 h of incubation (Figure 3b). In contrast, cells incubated without the nanoparticles showed no fluorescence under similar imaging parameters and conditions (Figure 3a), which demonstrated that C-Dots@ MSPs could be used for cell labeling/imaging. Meanwhile, free C-Dots that separated from the MSPs in mildly acidic conditions after a certain time could also perform the bioimaging assignment (Figure S6). Owing to its excellent optical property for the cell labeling, the C-Dots@MSPs were used for optical imaging in vivo. The nanocomposites were subcutaneously injected into the nude mice (100 μL, 2 mg/mL) and applied for PL imaging. As expected, the corresponding injection site of the mouse displayed a clearly PL signal, and the imaging of subcutaneous tissue was effective (Figure 4), which suggested that C-Dots@ MSPs might offer great potential for the in vivo tracking the delivery of therapeutic agents. Release Studies. To investigate the stimuli-triggered release of this system, DOX was first loaded. The loading efficiency was determined to be 0.073 mmol/g by UV−vis absorbance spectroscopy. After the drug was loaded, the pores of the nanocarriers were sealed with C-Dots and the samples were immersed in phosphate buffer (25 mL, 10 mM) at different pH values. As can be seen in Figure 5a, a very clear and highly effective pH-operable gating effect was demonstrated by monitoring the absorbance of DOX (480 nm). There was a steady release with stepwise increasing from 13% to 85.6% by adjusting the pH values from 7.4 to 5.0, suggesting that the environment-sensitive orifices in this system were gradually opened. To further demonstrate the uncapping of CDots@MSPs as the decrease of pH, C-Dots@MSPs system without DOX was also dispersed in phosphate buffer with different pH values and the PL of free C-Dots in the supernatant was monitored. As shown in Figure S9, the PL signal of the supernatant increased significantly as an increase of time at pH 5.0 while only negligible PL increase occurred at pH 7.4 under the same condition. The obtained results were consistent with the mechanism of operation of the MSPs system: release of guest molecules depended on the interaction between anionic C-Dots and cationic NH2-MSPs. With the decrease of pH, more and more of the C-Dots-COO− were protonized (Figure 5c), resulting in the dissociation of C-DotsCOO−/NH2-MSPs complex, the uncapping of the pores, and releasing of the entrapped cargo. Moreover, MSPs functionalized with APTS without C-Dots or unfunctionalized MSPs with C-Dots showed much higher drug release in PBS buffer (pH 7.4) (Figure S10). These results further demonstrated that the interaction between C-Dots-COO− with cationic NH3+ groups attached on MSPs surface played a crucial role in the storage of guest molecules and were in a good agreement with the previous report that gating scaffold could be switched on/ off by the molecular electrostatic interaction.34 Furthermore, it was reported that DOX became more water-soluble under an acidic environment. This could also be a factor for accelerating its release from MSPs after the separation of C-Dots from MSPs. Figure 5b showed the dependence of the released amount for DOX on time at pH values of 5.0 and 7.4, respectively. Notably, the released amount of DOX was lower at pH 7.4. On the contrary, at pH 5.0, there was a steady release over 8 h, which indicated that the drug delivery in our system at a mildly acidic pH value might be long-lived and continuous. Taken together, these results further confirmed that the level of

TEM image investigations of C-Dots@MSPs provided direct evidence of the distribution of C-Dots on the surfaces of NH2MSPs (Figure 1b). Zeta potential analysis of the MSPs showed that the surface potential of NH2-MSPs decreased from 13.27 to −4.94 mV in PBS buffer (pH 7.4) after adding C-Dots, which indicating the efficient grafting of the negatively charged C-Dots (−22.19 mV) on the particles. The appearance of an enhanced band at 1647 and 1470 cm−1 in the FT-IR spectra of MSPs (Figure 1c) also verified the presence of C-Dots. After grafting of the C-Dots, the hydrodynamic diameter of MSPs was distinctly larger than the corresponding free MSPs (Figure S8). The quantification of the density of C-Dots anchored on NH2-MSPs was accomplished by thermogravimetric analysis (TGA) (Figure 1d), which corresponded to a maximum immobilization efficiency of approximately 22.4 mg/g MSPs. Meanwhile, the C-Dots@MSPs nanocomposites exhibited PL that was similar to that of the free C-Dots (Figure S6). In Vitro Cell Viability Assay. The stability and cytotoxicity would be important to be considered for the actual application of a potential carrier in biomedical fields. To evaluate the biocompatibility of C-Dots@MSPs, the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliu-bromide) cell assay was performed on HeLa cells. This method is based on the formation of dark red formazan by the metabolically active cells after their exposure to MTT. Cell viability is directly proportional to the amount of formazan produced monitored by the absorbance at 570 nm. As shown in Figure 2, more than 90% HeLa cell viability was observed under varying concentration range, manifesting the excellent biocompatibility of the C-Dots@MSPs in all dosages.

Figure 2. HeLa cell viability after incubating with C-Dots@MSPs for 24 h and quantitative assays by standard MTT method. The error bars represent standard deviation of five independent experiments.

In Vitro and in Vivo Photoluminescence Imaging. Comparing to the photobleaching and quenching of fluorescent organic molecules and the toxicity of semiconductor quantum dots,34−37 the C-Dot capped system could provide the possibility of cell imaging due to its fascinating PL property and nontoxicity.9 In our present work, we evaluated the capability of the C-Dots@MSPs for bioimaging by incubating them with HeLa cells in physiological conditions. By taking advantage of the excitation-dependent emission behavior of CDots, blue light was employed as irradiation for the luminescence of C-Dots@MSPs to solve the problems such as strong background fluorescence of cells caused by UV excitation. Bright green fluorescence was observed for cells D

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Figure 3. Fluorescence microscopy images of HeLa cells incubated without the C-Dots@MSPs (a) and with C-Dots@MSPs (b): (left) bright field; (middle) observed by the blue light excitation; (right) the merged images of above. The scale bar is 10 μm.

guest release from this system was strongly dependent on the pH value. In Vitro Cytotoxicity of C-Dots@MSPs-DOX against HeLa Cells. To verify the feasibility of the C-Dots@MSPs system for intracellular therapeutic effect, the MTT assay was used for testing of the viability of HeLa cells in the presence of different concentrations of free DOX, MSPs-DOX, and CDots@MSPs-DOX. Before the experiment, the C-Dots@MSPsDOX were first dispersed in fetal bovine serum (FBS) and DMEM cell medium for 12 and 24 h to determine whether there was a release of drug in biological environments. The results shown in Figure S11 suggested that the cumulative release amount of DOX was less than 15%, indicating the potential of the system for further intracellular therapeutic

Figure 4. In vivo photoluminescence image of nude mice after subcutaneous injection with C-Dots@MSPs.

Figure 5. Released amounts of DOX from the nanocomposites at various pH values for 480 min. (b) Dependence of released amounts of DOX on time from the nanocomposites. (c) pH dependence of the zeta-potential of C-Dots. (d) In vitro cell viability of HeLa cells incubated with free DOX, MSPs-DOX, and C-Dots@MSPs-DOX for 24 h. The error bars represent standard deviation of five independent experiments. E

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Figure 6. Fluorescence of HeLa cells after incubation with a suspension of C-Dots@MSPs. The endolysosomal compartment was stained with LysoTracker Red. Color scheme: green, C-Dots@MSPs (a); red, LysoTracker Red (b); yellow, merged green and red (c). The scale bar is 10 μm.

applications. For the MTT experiment, free DOX, MSPs-DOX, and C-Dots@MSPs-DOX were incubated with HeLa cancer cells for 24, 48, and 72 h. As shown in Figure 5d and Figure S12, growth inhibition of cancer cells was observed when the cells were treated with DOX, MSPs-DOX, and C-Dots@MSPsDOX with the increase of incubation time. In contrast to DOX and MSPs-DOX, the remarkably higher cytotoxic efficacy of the C-Dots@MSPs-DOX observed at all time points may be attributed to the release of DOX inside cells after uptake of the C-Dots@MSPs-DOX nanocomposites. In consideration of the biocompatibility of the nanomaterials, we concluded that the CDots@MSPs nanocomposites had potential for drug loading and delivery into cancer cells to induce cell death. The therapeutic effect by C-Dots@MSPs-DOX was also investigated by the uptake behavior in HeLa cells. To visualize the location of the nanoparticles inside the cells after the internalization, the HeLa cells were incubated with C-Dots@ MSPs for 2 h and were then treated with lysotracker red. As showed in Figure 6, the green fluorescence of C-Dots@MSPs was clearly observed within cells and colocalized with lysotracker red fluorescence. This result suggested that the particles were remarkably internalized into the cells and distributed mainly in the lysosomes, which was beneficial for release of DOX because of the pH sensitive drug release. To further investigate the intracellular drug delivery behavior, HeLa cells were incubated with C-Dots@MSPs-DOX for 5 h and washed by fresh cell medium before optical imaging. As shown in Figure 7b, bright green photoluminescence was found in the entire cell cytoplasm, thus suggesting that this material could be effectively taken up by the cancer cells and mainly localized in the cytoplasm instead of entering the nucleus. By contrast, red fluorescence (Figure 7c,d) throughout the cell nuclei indicated that DOX molecules have been released from the MSPs, passed through the nucleus membrane, and eventually assembled in nucleus to kill cells. Thus, the effective therapy may result from the enhanced intracellular delivery, the pH-sensitive release, and the protection of DOX extracellular by C-Dots@MSPs.

Figure 7. Fluorescence microscopy images of HeLa cells labeled with the C-Dots@MSPs-DOX: bright field (a); observed by the blue light excitation (b); observed by the green light excitation (c); and the merged images of above (d). The scale bar is 10 μm.

remarkably enhanced efficiency in killing cancer cells. This result makes the system reported here a potential candidate in the formulation of a pH-sensitive vehicle for in vivo delivery of therapeutic agents to low pH tissues, such as tumors and inflammatory sites. Moreover, the application of C-Dots@ MSPs nanocomposite for simultaneous in vitro and in vivo imaging indicates that the system here is promising for noninvasive in vivo tracking the delivery of therapeutic agents. In view of fascinating photoluminescent property, excellent biocompatibility, cellular uptake, and efficient intracellular drug release properties, these nanocomposites promise their potential in bioimaging/biolabeling and biomedical areas.

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4. CONCLUSIONS In summary, we have demonstrated, as a proof of concept and for the first time, the use of C-Dots as caps on the surface of mesoporous silica which provides a suitable method for the design of intelligent on-demand molecular delivery and optical imaging system. The carboxylic groups functionalized C-Dots were attached directly to the outlet of the NH2-MSPs through the electrostatic interaction and served as caps to entrap guest molecules within the mesopores. Cargo release was blocked from the hybrid materials at neutral pH and triggered at lower pH. Importantly, the C-Dots-capped nanoparticles showed a

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Additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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Corresponding Author

*Fax: (+86) 0431-85262526. E-mail: [email protected]. Notes

The authors declare no competing financial interest. F

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (Grant 2012CB720602, 2011CB936004) and the National Natural Science Foundation of China (Grants 91213302, 21210002, 21072182).

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