NANO LETTERS
Hydrophobic Nanocrystals Coated with an Amphiphilic Polymer Shell: A General Route to Water Soluble Nanocrystals
2004 Vol. 4, No. 4 703-707
Teresa Pellegrino,†,‡ Liberato Manna,*,†,§ Stefan Kudera,† Tim Liedl,† Dmitry Koktysh,† Andrey L. Rogach,† Simon Keller,† Joachim Ra1 dler,† Giovanni Natile,‡ and Wolfgang J. Parak*,† Department of Physics & Center for Nanoscience, Ludwig-Maximilians UniVersita¨t Mu¨nchen, Mu¨nchen, Germany, Department of Chemistry and Pharmacology, UniVersity of Bari, Bari, Italy, and National Nanotechnology Lab of INFM, Via Arnesano, Lecce, Italy Received December 12, 2003; Revised Manuscript Received February 9, 2004
ABSTRACT A general strategy is described which allows for transferring hydrophobically capped nanocrystals from organic to aqueous solution by wrapping an amphiphilic polymer around the particles. In particular, high quality CoPt3, Au, CdSe/ZnS, and Fe2O3 nanocrystals have been water-solubilized in this way. Analysis with transmission electron microscopy, gel electrophoresis, and fluorescence correlation spectroscopy demonstrates that monodispersity of the particles is conserved upon phase transfer to aqueous solution.
Introduction. Colloidal inorganic nanocrystals are promising materials because of their unique size-dependent properties.1-3 For many materials nanocrystals can be either synthesized in aqueous solution (e.g., Au,4 CdTe and CdSe,5 Fe3O4 6) or in organic solvents (e.g., Au,7 CdTe and CdSe,8 Fe3O4 9). However, for some materials such as Co10 or for improved shape control,11 highly crystalline and monodisperse nanocrystals so far need to be synthesized in organic solvents at high temperature. This involves their surface to be coated with surfactants that render them hydrophobic.12 Several methods exist for converting hydrophobic nanocrystals into hydrophilic particles, which is a prerequisite for biological applications.13-16 Most methods rely on the exchange of the surfactant coating with ligand molecules that on one end carry a functional group that is reactive toward the nanocrystal surface and on the other end a hydrophilic group (see, for example, refs 13, 17-19). Appropriate ligand molecules have to be individually chosen for each material. Therefore, other strategies have been developed which, instead of exchanging the original hydrophobic surfactant layer with a hydrophilic one, are based on the addition of a second ligand layer. The second layer consists of amphiphilic molecules * Corresponding authors. E-mail:
[email protected]; wolfgang.
[email protected] † Ludwig-Maximilians Universita ¨ t Mu¨nchen, Germany. ‡ University of Bari, Italy. § National Nanotechnology Lab of INFM, Lecce, Italy. 10.1021/nl035172j CCC: $27.50 Published on Web 03/26/2004
© 2004 American Chemical Society
that can intercalate the first hydrophobic surfactant layer with their hydrophobic portion and that ensure water solubility of the nanocrystal with their hydrophilic groups. Interdigitated bilayers have been formed, for instance, by the addition of n-alcanoic acids20 or cetyltrimethylammonium bromide.21 Alternatively, cyclodextrin has been used for the formation of the second ligand layer, whereby the hydrophobic cavity of the cyclodextrin is penetrated by the hydrophobic tails of the first layer and its hydrophilic surface is pointing toward the solution.22,23 In these examples, the second layer is solely stabilized around the first layer by hydrophobic interactions. For improved stability a method has been suggested recently to coat hydrophobic CdSe/ZnS nanocrystals with a crosslinked amphiphilic polymer,24 in which the hydrophobic tails of the polymer intercalate with the surfactant molecules of the nanocrystal and form an additional coating. The water solubility of the polymer-coated nanocrystals is ensured by the hydrophilic groups located on the outer region of the polymer shell. Finally, the polymer shell is stabilized by cross-linking. Based on this scheme we have developed a simple and general strategy for decorating hydrophobic nanocrystals of various materials (CoPt3,25 Au,7 CdSe/ZnS,26 and Fe2O3 27) with a hydrophilic polymer shell by exploiting the nonspecific hydrophobic interactions between the alkyl chains of poly(maleic anhydride alt-1-tetradecene) and the nanocrystal
Scheme 1.
Polymer Coating of the Nanocrystals
Scheme of the polymer coating procedure. Several reports (e.g. ref 35) suggest that the surfactant chains for hydrophobically capped nanocrystals are pointing away from the nanocrystal surface, in a brush-like arrangement. The following plausible configuration is then assumed for the polymer coating process: The hydrophobic alkyl chains of the polymer intercalate with the surfactant coating. The anhydride rings are located on the surface of the polymer-coated nanocrystal. The amino end groups of the cross-linker molecule open the rings and link the individual polymer chains. The surface of the polymer shell becomes negatively charged, stabilizing the particles in water by electrostatic repulsion. A structural analysis aimed at determining the detailed conformation of the cross-linked polymer shell is in progress.
surfactant molecules. Addition of bis(6-aminohexyl)amine results in the cross-linking of the polymer chains around each nanoparticle (Scheme 1). The nanocrystals become soluble in water upon hydrolyzation of the unreacted anhydride groups (which effectively leads to an amphiphilic polymer shell) and can be further processed according to a universal protocol that relies solely on the chemistry of the outer polymer shell. Experimental Details. Poly(maleic anhydride alt-1-tetradecene) was purchased from Aldrich and had a number average molecular weight (Mn) ∼7300, corresponding to roughly 25 monomer units per polymer chain, and a monodispersity value (Mn/Mw) equal to ∼1.23, as declared by the vendor. Such polymer becomes amphiphilic upon hydrolyzation of the anhydride functional groups. A solution of poly(maleic anhydride alt-1-tetradecene) in chloroform and a solution of monodisperse nanocrystals in chloroform (100 polymer units per nm2 of nanocrystal surface) were 704
mixed and stirred for 2 h at room temperature. After evaporation of the solvent, bis(6-aminohexyl)amine in chloroform was then added to cross-link the polymer shell that had formed around each nanocrystal. The ratio of added cross-linker molecules to polymer units was 1:10. The solution was sonicated for 20 min, the solvent was evaporated again, and the solid was dissolved in a diluted TBE buffer solution (pH 8-9). After sonicating for five minutes, the nanocrystals dissolved completely and the solution was filtered to remove the excess unbound polymer. The buffer was exchanged with water by two rounds of dilution and reconcentration through a centrifuge filter. Any residual unbound polymer was removed by two consecutive purification steps on a size-exclusion column. After purification the nanocrystals solutions were optically clear. The particles were characterized by transmission electron microscopy (TEM), gel electrophoresis, and fluorescence correlation spectroscopy (FCS). The solutions were stable for months (i.e., no Nano Lett., Vol. 4, No. 4, 2004
Figure 1. TEM images of polymer-coated nanocrystals of four different core materials: Fe2O3 (9.2 nm average diameter), Au (4.0 nm), CoPt3 (8.0 nm), and CdSe/ZnS (7.0 nm) nanocrystals.
precipitation occurred and the bands during gel electrophoresis kept their narrow shape). Results and Discussions. In water the nanocrystals retained their major physical properties (such as the fluorescence of CdSe/ZnS and the magnetic moment of CoPt3). TEM analysis (Figure 1) showed no large aggregates of particles, besides the formation of monolayers. Such patterns are due to particle-particle interactions (van der Waals, magnetic) and are also observed on grids prepared from solutions of hydrophobic nanocrystals. However, the presence of small aggregates, such as dimers and trimers, cannot be distinguished from such monolayers and so it cannot be excluded. The polymer-coated nanocrystals were investigated with agarose gel electrophoresis28 (Figure 2). The bands on the gels are remarkably narrow, and since the particle mobility on gels depends on both charge and size, we estimated that our polymer-coated particles have a rather homogeneous Nano Lett., Vol. 4, No. 4, 2004
distribution of sizes and charges. The mobility of the polymer-coated nanocrystals is comparable to that of phosphine stabilized Au particles,28 indicating that there is no cross-linking between several particles. Any aggregates should in fact migrate much slower through the gel. We can estimate the sensitivity in assessing particle diameters by gel electrophoresis by devising a calibration curve of the particle mobility versus particle diameter, as reported in a previous study.29 The study showed that the mobility of particles with diameters between 3 and 20 nm scaled roughly linearly with the particle diameter. In the present study, the band of 4.0 nm diameter polymer-coated gold nanoparticles (Figure 2) has a spread of (0.03 if the mobility of the middle of the band is normalized to 1, allowing us to distinguish between particles with mobility of 1.0 and 0.97 (or 1.0 and 1.03), respectively. From the calibration curve,29 a decrease in 705
Figure 2. Gel electrophoresis of polymer-coated nanocrystals corresponding to the samples of Figure 1. The bands were observed either by their fluorescence (CdSe/ZnS) or by their absorption color. For each sample the left lane (lanes 1, 3, 5, 7) corresponds to particles as obtained after the last column purification step. The right lanes (lanes 2, 4, 6, 8) correspond to an additional purification step where the fastest migrating band of nanocrystals in a first gel run has been extracted.
mobility of 0.03 corresponds to an increase in effective particle diameter of only 1.2 nm, which cannot be due to an aggregation effect between particles having an average diameter of 4.0 nm. Any significant interparticle agglomeration, if present, would clearly cause a much larger band spread. We conclude then that no significant agglomerates are present. Only in the case of CdSe/ZnS nanocrystals could a small amount of aggregates be seen, since the gel electrophoresis bands showed slow migrating tails (Figure 2, lane 1). However, after extracting the fastest migrating band and rerunning it with gel electrophoresis, these aggregates were removed (Figure 2, lane 2). In the case of Fe2O3, CoPt3, and Au, no remarkable band tails were present already after the first run. It is worth noting that in this second run no stripping of the polymer shell from the particles was observed in any sample, since no tail of particles with reduced mobility was found. This is an indication that the polymer layer that formed around the nanocrystals is significantly stable. To get an estimate of the effective particle diameter in solution we used fluorescence correlation spectroscopy (FCS)30,31 (see Supporting Information). For green and red fluorescent hydrophobic CdSe/ZnS particles dissolved in chloroform, we obtained hydrodynamic diameters of 5.7 ( 0.5 nm and 11.6 ( 2.8 nm, respectively. After the polymer coating procedure, hydrodynamic diameters of 19.2 ( 2.0 nm and 23.6 ( 2.0 nm, respectively, were measured for the particles dissolved in water. Clearly an increase in size due to the formation of the polymer shell could be observed, but this is larger than expected if one considers only the 706
contribution from the polymer layer. The significant increase in the hydrodynamic diameter as determined from FCS measurements could partly result from the change in particle-solvent interaction, although the formation of small aggregates as dimers or trimers cannot be excluded. Considering the TEM, gel electrophoresis, and FCS results, it seems that no significant interparticle cross-linking occurs during the synthesis. The minimization of interparticle aggregation during the polymer coating procedure has been achieved by working with enough diluted solutions (see Supporting Information). Conclusions. The described procedure for transferring hydrophobic nanocrystals into aqueous solution should be extendable to any nanocrystal system in which hydrophobic tails are exposed to the external environment. For example, it has also been successfully applied to as complex objects as CdTe tetrapods32 (data not shown). This method therefore is quite general and could be extended to systems for which phase transfer to aqueous solution has not been possible so far with other methods. In principle, the polymer shell could be further improved regarding its functional groups and its stability. Incorporation of poly(ethylene glycol) groups, for example, should improve the stability in electrolytic solution (such as making the solubility of nanocrystals less dependent on electrostatic repulsion) and should also minimize nonspecific interactions. One promising application of the polymer-coated nanocrystals could be the construction of DNA-mediated hybrid materials,33 such as for instance the formation of dimers between fluorescent CdSe/ZnS and magnetic CoPt3 nanoNano Lett., Vol. 4, No. 4, 2004
crystals. One requisite for this goal is to precisely control the number of biological molecules (as oligonucleotides) that are attached per particle.28,34 The strategy described here yields particles with different intrinsic properties (such as fluorescence or magnetism) but identical surface chemistry and with good homogeneity in charge and size. These nanocrystals then should be promising candidates for the controlled fabrication of more sophisticated nanoscale structures. Acknowledgment. The authors are grateful to Dr. Elena Shevchenko, Dr. David Gittins, and Eric Dulkeith for help with the cobalt-platinum and the gold syntheses, to Monika Rusp for the assistance in TEM measurements, and to Prof. Dr. Hermann Gaub, Dr. Markus Seitz, Prof. Dr. Jochen Feldmann, and Prof. Dr. Roberto Cingolani for helpful suggestions. This work was funded by the Emmy Noether program of the German research foundation DFG (W.J.P.), the Fonds der Deutschen Chemischen Industrie (W.J.P.), and was also supported by the Center for Nanoscience in Munich and by the Italian INFM-NNL and FIRB funds. T.P. is grateful to Marie Curie training program of the European Union, and D.K. is grateful to the Alexander von Humboldt Foundation. Travel costs between Germany and Italy were in part supported by the Vigoni foundation/Deutscher Akademischer Austauschdienst (DAAD). Supporting Information Available: Description of the various nanocrystal syntheses and polymer coating procedures, gel electrophoresis, and fluorescence correlation spectroscopy setups. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Alivisatos, A. P.; Barbara, P. F.; Castleman, A. W.; Chang, J.; Dixon, D. A.; Klein, M. L.; McLendon, G. L.; Miller, J. S.; Ratner, M. A.; Rossky, P. J.; Stupp, S. I.; Thompson, M. E. AdV. Mater. 1998, 10, 1297-1336. (2) Efros, A. L.; Rosen, M. Annu. ReV. Mater. Sci. 2000, 30, 475-521. (3) Moriarty, P. Rep. Prog. Phys. 2001, 64, 297-381. (4) Schmid, G.; Lehnert, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 780781. (5) Rogach, A. L.; Talapin, D. V.; Weller, H. Semiconductor Nanoparticles. In Colloids and Colloid Assemblies; Caruso, F., Ed.; WileyVCH: Weinheim, 2004; pp 52-95. (6) Berger, P.; Adelman, N. B.; Beckman, K. J.; Campbell, D. J.; Ellis, A. B.; Lisensky, G. C. J. Chem. Educ. 1999, 76, 943-948. (7) Fink, J.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. Chem. Mater. 1998, 10, 922-926.
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