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Fine-Tuning the Surface Functionality of Aqueous Luminescent Nanocrystals through Surfactant Bilayer Modification Hao Zhang, Yi Liu, Junhu Zhang, Haizhu Sun, Jie Wu, and Bai Yang* State Key Laboratory of Supramolecular Structure and Materials, Jilin UniVersity, Changchun 130012, P. R. China ReceiVed August 7, 2008. ReVised Manuscript ReceiVed September 27, 2008 We report a two-step phase transfer approach to locate surfactant bilayers on water-soluble luminescent nanocrystals (NCs), through which the surface functionality of the NCs is tunable. Since the species of both inner and outer surfactants of the bilayer are alterable in wide range, the current effort provides a facile approach to anchor various functional groups on aqueous NCs. The primary results indicate that these bilayer-modified NCs are able to be incorporated with various organic and inorganic materials.
1. Introduction In the past decade, significant progress has been achieved in colloidal synthesis of highly luminescent semiconductor nanocrystals (NCs), represented by the capability to control the NC size, shape, composition, as well as its surface chemistry.1 This success makes NCs highly desired in the applications of optical displays, nonlinear optical devices, photovoltaic cells, and biological imaging.2 Among these applications, NCs are usually used as building blocks to assemble with other organic or inorganic building blocks to achieve composite materials,3 which strongly requires techniques to endow NCs with ample surface functionalities.4,5 On the basis of covalent linkage, ligand exchange has been applied to extend NC surface functionalities.6 However, this strategy requires laborious synthesis to obtain specific ligands. Moreover, ligand exchange sometimes quenches NC photoluminescence (PL).7 Thus, the current challenge in NC surface modulation is to develop general method through which to enrich the surface functionality of preformed NCs without quenching their PL.8 In comparison to NCs prepared in organic solvents, NCs prepared in aqueous solutions usually use mercapto-compounds * To whom the correspondence should be addressed. E-mail: byangchem@ jlu.edu.cn; Fax: (+86) 431-85193423. (1) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (b) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. Nature 2005, 437, 121. (c) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J.; Wang, L.; Alivisatos, A. P. Nature 2004, 430, 190. (d) Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 664. (2) Yu, W. W.; Peng, X. G. Angew. Chem., Int. Ed. 2002, 41, 2368. (b) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237. (c) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science 2000, 290, 314. (d) Chan, W. C. W.; Nie, S. Science 1998, 281, 2016. (3) Mokari, T.; Rothenberg, E.; Popov, I.; Costi, R.; Banin, U. Science 2004, 304, 1787. (b) Nirmal, M.; Brus, L. Acc. Chem. Res. 1999, 32, 407. (4) Dubertret, B.; Skourides, P.; Norris, D. J.; Noireaux, V.; Brivanlou, A. H.; Libchaber, A. Science 2002, 298, 1759. (5) Fan, H. Y.; Yang, K.; Boye, D. M.; Sigmon, T.; Malloy, K. J.; Xu, H.; Lo´pez, G. P.; Brinker, C. J. Science 2004, 304, 567. (6) Skaff, H.; Ilker, M. F.; Coughlin, E. B.; Emrick, T. J. Am. Chem. Soc. 2002, 124, 5729. (b) Kim, S.; Bawendi, M. G. J. Am. Chem. Soc. 2003, 125, 14652. (c) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Eychmu¨ller, A.; Weller, H. Nano Lett. 2002, 2, 803. (7) Wang, Y. A.; Li, J. J.; Chen, H.; Peng, X. J. Am. Chem. Soc. 2002, 124, 2293. (8) Nikolic, N. S.; Krack, M.; Aleksandrovic, V.; Kornowski, A.; Fo¨rster, S.; Weller, H. Angew. Chem., Int. Ed. 2006, 45, 6577. (b) Osaki, F.; Kanamori, T.; Sando, S.; Sera, T.; Aoyama, Y. J. Am. Chem. Soc. 2004, 126, 6520. (c) Zhang, T.; Ge, J.; Hu, Y.; Yin, Y. Nano Lett. 2007, 7, 3203. (d) Pellegrino, T.; Manna, L.; Kudera, S.; Liedl, T.; Koktysh, D.; Rogach, A. L.; Keller, S.; Ra¨dler, J.; Natile, G.; Parak, W. J. Nano Lett. 2004, 4, 703.
as ligands.9 Functional groups of mercapto-compounds spontaneously locate on NC surfaces during preparation, which are greatly useful for the conjugation of other functional objects or devices. For instance, carboxyl is introduced using 3-mercaptocarboxylic acid (MPA) as ligand, whereas hydroxyl or amine is introduced, respectively, by using 1-thioglycerol and 2-mercaptoethylamine.9 Due to the limited structures of available mercapto-compounds, however, NC surface functionality is still limited within the aforementioned three functional groups or their combination. Especially, only carboxyl and hydroxyl functionalized NCs are highly stable for assembly applications and bioconjugation,9 thus limiting the applications of these watersoluble NCs. The formation of a lipid bilayer is popular in nature; namely, biological molecules, surfactants, or amphiphilic polymers have the tendency to form bilayer structures spontaneously.10 In this context, lipophilic alkyl chains interdigitate inside a bilayer through hydrophobic van der Waals interaction, whereas hydrophilic groups spread toward to environment, leading to thermodynamically stable structures. On the basis of bilayer structures, omnifarious micelles, vesicles, and capsules have been designed to construct nano- or micrometer-sized composite materials, which are applicable in biological imaging, drug delivery, and so forth.11,12 Especially, recent success in aqueous synthesis of cationic surfactant-stabilized metal nanorods inspires researchers to explore novel methods for surface modulation of hydrophobic NCs.5 Moreover, we have demonstrated a route for transferring aqueous NCs to the organic phase through the modification of the hydrophobic surfactant monolayer, which was used to fabricate NC-polymer composites.13 Here, we demonstrate a facile method to modulate the surface functionality of water-soluble NCs through the encapsulation of the surfactant bilayer. The species of surfactants within the bilayer are alterable (9) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Hoppe, K.; Shevchenko, E. V.; Kornowski, A.; Eychmu¨ller, A.; Weller, H. J. Phys. Chem. B 2002, 106, 7177. (10) Katagiri, K.; Caruso, F. AdV. Mater. 2005, 17, 738. (b) Beaune, G.; Dubertret, B.; Cle´ment, O.; Vayssettes, C.; Cabuil, V.; Me´nager, C. Angew. Chem., Int. Ed. 2007, 46, 5421. (c) Wijaya, A.; Hamad-Schifferli, K. Langmuir 2007, 23, 9546. (d) Lin, C. J.; Sperling, R. A.; Li, J. K.; Yang, T.; Li, P.; Zanella, M.; Chang, W. H.; Parak, W. J. Small 2008, 4, 334. (11) Wang, M.; Felorzabihi, N.; Guerin, G.; Haley, J. C.; Scholes, G. D.; Winnik, M. A. Macromolecules 2007, 40, 6377. (12) Kim, J.; Lee, J. E.; Lee, J.; Yu, J. H.; Kim, B. C.; An, K.; Hwang, Y.; Shin, C.; Park, J.; Kim, J.; Hyeon, T. J. Am. Chem. Soc. 2006, 128, 688. (13) Zhang, H.; Cui, Z.; Wang, Y.; Zhang, K.; Ji, X.; Lu¨, C.; Yang, B.; Gao, M. Y. AdV. Mater. 2003, 15, 777.
10.1021/la802560p CCC: $40.75 2008 American Chemical Society Published on Web 10/24/2008
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Figure 1. Schematic illustration of the fabrication of surfactant bilayer-modified CdTe NCs. The whole procedure starts with the surface modification of MPA-stabilized aqueous CdTe NCs using DODAB to obtain chloroform-soluble CdTe/DODAB NCs, and followed by the modification of CdTe/DODAB NCs using a second surfactant to obtain bilayer-modified water-soluble CdTe NCs. R represented the functional groups of various surfactants.
in a broad category, making NCs multifunctional building blocks for the conjugation with various organic and inorganic materials.
2. Experimental Section Monolayer Modification. Aqueous CdTe NCs that were stabilized by 3-mercaptocarboxylic acid (MPA) were prepared according to our previous publication.14 The preformed NCs were transferred from water to chloroform using the typical extraction method as reported in previous publications;15 dimethyldioctadecylammonium bromide (DODAB) was dissolved in chloroform with the concentration of 10 mg mL-1. Ten milliliters of DODAB chloroform solution was added to 100 mL 1.25 mM (referring to Cd2+) CdTe aqueous solution under vigorous stirring to extract NCs from water to chloroform. The chloroform phase was separated to yield DODABcapped CdTe NCs (CdTe/DODAB NCs). Following a similar procedure, except using octadecyl-p-vinylbenzyldimethylammonium chloride (OVDAC) instead of DODAB, CdTe/OVDAC NCs were obtained. Bilayer Modification. Two milliliters of chloroform solution of CdTe/DODAB NCs was added to 10 mL water containing 0.16 g sodium dodecyl sulfate (CH3(CH2)11OSO3Na, SDS) at room temperature. The mixture was heated around 80 °C with vigorous stirring to cause the evaporation of chloroform. After the solution became completely clear, bilayer-modified CdTe/DODAB/SDS NCs were obtained. Following a similar procedure, except using 0.2 g hexadecyltrimethylammonium bromide (CH3(CH2)15N(Br)(CH3)3, CTAB), 0.26 mL tetraethylene glycol monooctyl ether (CH3(CH2)7(OCH2CH2)4OH, C8E4), or 1 mL polyethylene glycol tert-octylphenyl ether (CH3(CH2)7C6H4(OCH2CH2)10OH, C8PhE10) rather than SDS, CdTe/DODAB/CTAB, CdTe/DODAB/C8E4, and CdTe/DODAB/ C8PhE10 NCs were obtained. Similarly, using CdTe/OVDAC NCs instead of CdTe/DODAB NCs, CdTe/OVDAC/CTAB, CdTe/ OVDAC/SDS, CdTe/OVDAC/C8E4, and CdTe/OVDAC/C8PhE10 NCs were obtained. Preparation of NC-Polystyrene Microspheres. To 180 mL deionized water, 20 mL aqueous CdTe/OVDAC/C8PhE10 NCs was added. To 20 mL styrene, 0.15 g azobisisobutyronitrile (AIBN) was added. The aforementioned two solutions were mixed under vigorous stirring and deaerated using N2 for 30 min. The polymerization was operated with a water bath at 80 °C for 6 h. Characterization. UV-visible absorption spectra were recorded by using a Shimadzu 3100 UV-vis-near-IR spectrophotometer. (14) Zhang, H.; Zhou, Z.; Yang, B.; Gao, M. J. Phys. Chem. B 2003, 107, 8. (15) Tian, Y.; Fendler, J. H. Chem. Mater. 1996, 8, 969. (b) Kurth, D. G.; Lehmann, P.; Lesser, C. Chem. Commun. 2000, n/a, 949.
Fluorescence experiments were performed with the help of a Shimadzu RF-5301 PC spectrofluorimeter. The excitation wavelength was fixed at 400 nm. Transmission electron microscopy (TEM) images were recorded by a JEOL-2010 electron microscope operating at 200 kV. Scanning electron microscopy (SEM) was done on a JEOL JSM-6700F field-emission electron microscope. FTIR spectra were recorded with a Nicolet AVATAR 360 FTIR instrument. Zeta potential measurements were performed using a Zetasizer NanoZS (Malvern Instruments).
3. Results and Discussion The overall preparative procedures of surfactant bilayermodified aqueous NCs involved a two-step phase transfer, starting with negatively charged NCs (Figure 1). In experiments, MPAstabilized CdTe NCs were synthesized in aqueous solution according to our method.14 The negative charges of MPA endowed NCs with the ability to adsorb cationic surfactants through electrostatic attraction, such as DODAB,15 providing the driving force to modify surfactant monolayer on NCs (Figure 1). The capping of DODAB also related to the transfer of NCs from water to chloroform. It was attributed to the hydrophobic nature of DODAB alkyls, the encapsulation of which made the original hydrophilic CdTe NCs hydrophobic. Consequently, CdTe/DODAB NCs were transferred to chloroform (Figure 2a,b). Besides, the hydrophobic nature of DODAB alkyls outside NCs also provided a hydrophobic-hydrophobic interaction to modify the second layer of surfactant,5 thus obtaining bilayermodified NCs (Figure 1). In this context, a chloroform solution of CdTe/DODAB NCs was mixed with an aqueous solution of the second surfactant under vigorous stirring. Various anionic surfactants, for instance, SDS, cationic surfactants, for instance, CTAB, and nonionic surfactants, for instance, C8E4, were all available in this process (Figure 2a). After the evaporation of chloroform under moderate heating (around 80 °C), the alkyl chains of both DODAB and the second surfactants interacted through hydrophobic-hydrophobic interaction, whereas the hydrophilic groups of the second surfactants spread toward aqueous media, making NCs soluble again in water.5 Note that the formation of a surfactant bilayer was thermodynamically favorable, leading to an interdigitated stable structure.10 As shown in Figure 2a, the UV-vis absorption and PL spectra of bilayer-modified NCs, except for CdTe/DODAB/C8E4 NCs,
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Figure 2. (a) UV-vis absorption and PL spectra of original aqueous CdTe NCs, CdTe/DODAB NCs in chloroform, aqueous CdTe/DODAB/ CTAB NCs, CdTe/DODAB/SDS NCs, and CdTe/DODAB/C8E4 NCs. The spectra of CdTe/DODAB/C8E4 NCs were measured at 40 °C, whereas other ones were measured at room temperature. (b) Optical (upper panel) and PL (lower panel) images of CdTe NCs, CdTe/DODAB NCs, and CdTe/DODAB/CTAB NCs (from left to right). (c) TEM image of CdTe/ DODAB/CTAB NCs.
were consistent with that of the original aqueous NCs, indicating that the optical property of NCs was preserved after the two-step phase transfer. The corresponding PL images and TEM image also revealed that the emission color and size of bilayer-modified NCs had no difference in comparison to the original NCs (Figure 2b,c). Besides, the solution of CdTe/DODAB/C8E4 NCs was clear only above 40 °C. At room temperature, however, the solution was foggy due to the hydrophobic nature of CdTe/ DODAB/C8E4 NCs (Supporting Information Figure S1). Consequently, we measured its UV-vis absorption and PL spectra at 40 °C, which in turn led to the red shift of PL spectra (Figure 2a). Note that the hydrophilic-hydrophobic conversion was reversible, presenting a thermal response property of poly(ethylene glycol) (PEG) modified NCs.16 It should be mentioned that the transfer of NCs from chloroform to water was not from the removal of primarily capped DODAB but from the encapsulation of the second surfactant. As indicated in Supporting Information Figure S2, CdTe/DODAB/CTAB NCs were completely different from the mixture of MPA-stabilized CdTe NCs and CTAB. After heating to 40 °C, the green emission of CdTe/DODAB/CTAB NCs was no different in comparison to that of the original MPA-stabilized NCs (Figure 2a,b), whereas the emission color turned yellow for the mixture of MPAstabilized NCs and CTAB. This result provided direct evidence of bilayer modification. The encapsulation of CdTe NCs with the primary and secondary surfactants was also proven by FTIR spectra (Supporting Information Figure S3). For various surfactants (DODAB, SDS, and CTAB), the strong absorption peaks at 2918, 2852, and 1467 cm-1 were assigned to the -CH2stretching vibration of alkyl chains.17 Besides, the strong absorption of CTAB at 3015 cm-1 was assigned to the C-H vibration of -N(CH3)3, and the absorption of SDS at 1223 cm-1 was assigned to -OSO3- vibration. For MPA-stabilized CdTe (16) Lee, J.; Govorov, A. O.; Kotov, N. A. Angew. Chem., Int. Ed. 2005, 44, 7439. (17) Zhang, H.; Wang, C. L.; Li, M. J.; Ji, X.; Zhang, J. H.; Yang, B. Chem. Mater. 2005, 17, 4783.
Figure 3. SEM images of CdTe/DODAB/CTAB NC-embedded silica spheres (a) and CdTe/DODAB/SDS NC-embedded BaSO4 spheres (b).
NCs, the weak absorption peaks at 2918 and 2852 cm-1 were also assigned to the -CH2- vibration of MPA, whereas the absorption peaks at 1558 and 1394 cm-1 were assigned to the vibration of carboxylate of MPA on the NC surface.17 The appearance of the characteristic absorption peaks of both MPAstabilized CdTe and the surfactants for bilayer-modified NCs confirmed the successful coating of NCs with surfactant bilayer (Supporting Information Figure S3). After bilayer modification, moreover, the surface potential of NCs was significantly altered (Supporting Information Table S1). The room temperature surface potential of the original aqueous CdTe NCs was -38.4 mV, whereas it was, respectively, +65.0, -65.2, and +17.1 mV after CTAB, SDS, and C8E4 modification. This result further confirmed the encapsulation of NCs with surfactant bilayer. On the other hand, except CdTe/ DODAB/C8E4 NCs, the zeta potential of bilayer-modified NCs was almost the same as that of corresponding surfactants (Supporting Information Table S2), indicating that NC surface properties were mainly dependent on the nature of the outer surfactants. Besides, in comparison to C8E4, the surface potential of CdTe/DODAB/C8E4 NCs was slightly positive (Supporting Information Tables S1 and S2). It was attributed to the partial removal of inner DODAB to the outer layer, which should be an entropy-driven process. Especially, the functional groups of the outer surfactants were expected to induce different behavior in both the NC assembly and the surface chemical reaction. For instance, CTAB modification provided a reactive scaffold to grow a silica shell around NCs (Figures 3a and Supporting Information S4), whereas it was difficult to obtain this structure using NCs without CTAB modification. The SDS modification provided the -SO4- anchoring points on NC surface, which was applied to grow a BaSO4 shell around NCs (Figure 3b). The primary result indicated that the formation of a BaSO4 shell significantly improved the luminescent stability of CdTe NCs in acidic solution (Supporting Information Figure S5). Besides, the
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Figure 4. (a) Optical (upper panel) and PL (lower panel) images of CdTe/DODAB/SDS NCs that were obtained from the original NCs, respectively, with green, yellow, and red PL emission (from left to right). (b) PL spectra of CdTe NCs, CdTe/OVDAC/C8PhE10 NCs, and CdTe-PS composite microspheres. (c) SEM image of CdTe-PS composite microspheres. (d) Optical (left panel) and PL (right panel) images of the aqueous dispersion of the spheres.
PEG block of C8E4 was highly biocompatible and applicable for NC bioconjugation.8 Note that there were a great deal of optional surfactants for outer modification. Besides CTAB, SDS, and C8E4, various cationic, anionic, and nonionic surfactants with proper lengths of alkyl chains (C8-C16) were all available under this strategy. Through the encapsulation of outer surfactants with desired functional groups, ample surface functionalities would be endowed on aqueous NCs. Figure 4a presented the PL image of CdTe/DODAB/SDS NCs with green, yellow, and red PL emission. The size-dependent PL was well-preserved after two-step phase transfer. Supporting Information Figure S6 indicated the UV-vis absorption and PL spectra of MPA-stabilized CdHgTe and corresponding CdHgTe/ DODAB/SDS NCs, which had near-IR emission. These results meant our method was applicable to NCs prepared in aqueous solution with different sizes or species, thus providing a general strategy for the surface modulation of these NCs. Besides, the inner surfactant bilayers could also be designed for chemical reaction, for instance, using polymerizable surfactant of OVDAC rather than DODAB to achieve CdTe/OVDAC/ C8PhE10 NCs (Figure 4b). The styrene group of OVDAC made CdTe/OVDAC/C8PhE10 NCs polymerizable with free radical monomers, such as styrene, to achieve composite microspheres. SEM images indicated that the diameter of the resulting CdTe-polystyrene (PS) spheres was around 100 nm (Figure 4c), which was dramatically smaller than the corresponding spheres directly through emulsion polymerization.18 Further studies have been underway to apply these bilayer-modified (18) Yang, Y. H.; Tu, C. F.; Gao, M. Y. J. Mater. Chem. 2007, 17, 2930.
luminescent NCs as macromolecular monomers to copolymerize with various hydrophobic and hydrophilic monomers.
4. Conclusion In summary, we demonstrated a two-step phase transfer method to modify aqueous NCs with surfactant bilayers. Electrostatic attraction and hydrophobic-hydrophobic interaction were respectively applied in each step. The inner and the outer surfactant bilayer were alterable in wide range, thus greatly enriching the surface functionalities of aqueous NCs. Bilayer-modified aqueous NCs were expected to find applications in surface reaction, bioconjugation, and other surface-related chemistry. Acknowledgment. This work was supported by the National Basic Research Program of China (2007CB936402, 2009CB939701), the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (FANEDD Grant No. 200734), National Natural Science Foundation of China (Grant No. 20704014, 20534040, and 20731160002), the Program of Technological Progress of Jilin Province (20080101), and the Independent Research Program of State Key Laboratory of Supramolecular Structure and Materials. Supporting Information Available: Additional absorption and PL spectra, FTIR spectra, optical and PL images of bilayer-modified CdTe and CdHgTe NCs; zeta potentials of various surfactants and surface modified CdTe NCs; PL spectra of SiO2- and BaSO4-capped CdTe NCs; and PL image of CdTe-PS spheres. This material is available free of charge via the Internet at http://pubs.acs.org. LA802560P
