2652
Langmuir 2005, 21, 2652-2655
Core-Shell Nanostructures from Single Poly(N-vinylcaprolactam) Macromolecules: Stabilization and Visualization Lyudmila M. Bronstein,*,† Maxim Kostylev,† Irina Tsvetkova,† John Tomaszewski,† Barry Stein,‡ Elena E. Makhaeva,§ Ivan Okhapkin,§ and Alexei R. Khokhlov§ Departments of Chemistry and Biology, Indiana University, Bloomington, Indiana 47405, and Physics Department, Moscow State University, Moscow, 117234 Russia Received November 18, 2004. In Final Form: February 15, 2005 Formation of core-shell poly(N-vinylcaprolactam) (PVCL) single-molecule nanostructures due to interaction of PVCL with metal ions was studied using transmission electron microscopy, 13C NMR, and light scattering. This study demonstrates that addition of CoCl2 to PVCL in its globular conformation yields unimolecular core-shell polymer particles with the core decorated with Co(II) ions. The crucial condition for formation of well-defined unimolecular nanostructures is the presence of stable globular aggregates in aqueous solution. Moreover, the metal ions should have a sufficiently high coordination number (higher than 2) to provide a cross-linking and stabilization of the core.
Introduction Recently single macromolecule nanostructures have received considerable attention due to their potential applications in nanoscience and nanotechnology. Single polymer nanostructures of conjugated polymers display strongly enhanced optical properties compared to those of macromolecular aggregates.1,2 Templating with unimolecular core-shell cylindrical polymer brushes allows a controlled fabrication of wirelike assemblies of CdS nanoparticles.3 Single-chain nanostructures obtained from protonated poly(2-vinylpyridine) on a solid substrate were used for formation of wire-shaped metallic nanoparticle assembles.4 We believe that fabrication of single macromolecule nanostructures can be carried out by interaction of thermosensitive homopolymer macromolecules with certain metal ions, allowing development of novel polymer templates or building blocks of hierarchical self-assembled nanostructures. Thermosensitive polymers are able to become dehydrated and to undergo a coil-to-globule transition upon heating. Their use in drug delivery,5 in implantation,6 and as stimuli-responsive polymers7 was reported. Poly(N-isopropylacrylamide) (PNIPAM) is the most frequently studied thermosensitive polymer.8,9 Another example of a thermosensitive polymer is poly(N-vinylcaprolactam) * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Chemistry, Indiana University. ‡ Department of Biology, Indiana University. § Physics Department, Moscow State University. (1) Mehta, A.; Kumar, P.; Dadmun, M. D.; Zheng, J.; Dickson, R. M.; Thundat, T.; Sumpter, B. G.; Barnes, M. D. Nano Lett. 2003, 3, 603. (2) Kumar, P.; Lee, T.-H.; Mehta, A.; Sumpter, B. G.; Dickson, R. M.; Barnes, M. D. J. Am. Chem. Soc. 2004, 126, 3376. (3) Zhang, M.; Drechsler, M.; Mueller, A. H. E. Chem. Mater. 2004, 16, 537. (4) Minko, S.; Kiriy, A.; Gorodyska, G.; Stamm, M. J. Am. Chem. Soc. 2002, 124, 10192. (5) Eeckman, F.; Moes, A. J.; Amighi, K. J. Controlled Release 2003, 88, 105. (6) Jeong, B.; Bae, Y. H.; Lee, D. S.; Kim, S. W. Nature 1997, 388(6645), 860. (7) Laschewsky, A.; Rekai, E. D.; Wischerhoff, E. Macromol. Chem. Phys. 2001, 202, 276. (8) Kishida, A.; Kikunaga, Y.; Akashi, M. J. Appl. Polym. Sci. 1999, 73, 2545.
(PVCL), which undergoes to a coil-to-globule transition in a temperature range 31-38 °C.10,11 In the collapsed state PVCL macromolecules possess a nonspherical conformation and tend to aggregate with formation of fractal structures.10 Recently, the mean size of the globular aggregates formed by PVCL with a molecular weight of 1.5 × 106 g mol-1 was found to be 80 nm.11 When PVCL is grafted with poly(ethylene oxide) (PEO), the copolymer becomes amphiphilic upon heating above cloud point temperature (CPT), self-assembling to form stable aggregates with PEO shell.12 Stable single-chain core-shell nanostructure with the collapsed PNIPAM chain backbone as the hydrophobic core and the grafted PEO branches as the hydrophilic shell were observed upon heating above 33 °C using a combination of static and dynamic laser light scattering.13 Surprisingly, hydrophobically modified PNIPAM produced smaller aggregates than those observed for a PNIPAM homopolymer.14 This was attributed to viscoelastic effect: when the collision time is shorter than time needed to establish a permanent chain entanglement, the colliding aggregates behave like nonadhesive balls. When hydrophobic polystyrene “stickers” were placed regularly along the PNIPAM chain using micellar polymerization,15 singleflower-like single-chain nanostructures (ordered coils) were obtained when temperature increased followed by formation of collapsed core-shell nanostructures. Using interaction of carboxylate groups of P(VCL-co-sodium acrylate) with Ca2+ ions, single molecule intrachain aggregates were obtained below CPT.16 To decrease the size of PVCL homopolymer aggregates, a coil-to-globule transition of PVCL was studied in the (9) Uludag, H.; Norrie, B.; Kousinioris, N.; Gao, T. Biotechnol. Bioeng. 2001, 73, 510. (10) Lebedev, V.; Toeroek, G.; Cser, L.; Treimer, W.; Orlova, D.; Sibilev, A. J. Appl. Crystallogr. 2003, 36, 967. (11) Laukkanen, A.; Valtola, L.; Winnik, F. M.; Tenhu, H. Macromolecules 2004, 37, 2268. (12) Verbrugghe, S.; Laukkanen, A.; Aseyev, V.; Tenhu, H.; Winnik, F. M.; Du Prez, F. E. Polymer 2003, 44, 6807. (13) Wu, C.; Qiu, X. Phys. Rev. Lett. 1998, 80, 620. (14) Wu, C.; Li, W.; Zhu, X. X. Macromolecules 2004, 37, 4989. (15) Zhang, G.; Winnik, F. M.; Wu, C. Phys. Rev. Lett. 2003, 90, 035506/1. (16) Peng, S.; Wu, C. Macromolecules 2001, 34, 6795.
10.1021/la047163b CCC: $30.25 © 2005 American Chemical Society Published on Web 03/04/2005
Letters
Langmuir, Vol. 21, No. 7, 2005 2653
diluted solution (below overlap concentration of 2.5 mg/ mL) in the presence of ionic surfactants (below critical micelle concentration).17 To the best of our knowledge, although single-chain polymer nanostructures exhibit fascinating properties and visualization of various types of polymers using atomic force microscopy (AFM) was recently described in several papers,18-20 there were no other attempts to stabilize or visualize single PVCL macromolecules or their complexes with metal ions. In this paper, for the first time we report stabilization and visualization of core-shell PVCL single-molecule nanostructures using interaction of PVCL globules with metal ions. Experimental Section N-Vinylcaprolactam (VCL, Aldrich) was twice distilled under vacuum (10 mmHg). 2,2′-Azobis-(2-methylpropionitrile) (AIBN) (Merck) was recrystallized from methanol. Benzene and diethyl ether were purchased from Aldrich and were analytical purity grade. CoCl2‚6H2O and AgNO3‚2H2O (Aldrich) were used as received. The PVCL with a molecular weight 680 000 was prepared by free-radical polymerization of VCL in benzene solutions. The polymerization was carried out at 50 °C in argon atmosphere. 2,2′-Azobis-(2-methylpropionitrile) was used as the initiator. The VCL content was 50 vol % and the initiator concentration was 0.001 wt %. After the reaction, the polymer was precipitated into diethyl ether, decanted, and dried at 50 °C. The polymer was fractionated by standard solvent/nonsolvent techniques involving the addition of diethyl ether to a benzene solution of the polymer. Molecular weight value was obtained from light scattering measurements. If not indicated otherwise, in a typical experiment the interaction of metal ions with PVCL was carried out in an aqueous solution at a PVCL concentration of 1 mg/mL after 4 h of stirring at 45 °C. CoCl2‚6H2O or AgNO3‚2H2O were loaded at 8:1 PVCL/ metal ion molar ratio and stirred at the reaction temperature overnight. Then solutions were cooled to room temperature and stirred for 24 h. Specimens for transmission electron microscopy (TEM) were prepared by placing a drop of reaction solution onto a carboncoated copper grid. Images were acquired at accelerating voltage of 60 kV on JEOL JEM1010. The 13C NMR spectra were recorded on VXR-Unity spectrometers operated at 11.74 T. Light scattering experiments were carried out with an ALV-Instruments ALV/ CGS-8F laser goniometer system and ALV-5000 multi-τ digital correlator. The light source was a JDS Uniphase helium/neon 22 mW laser operating at λ ) 633 nm. All samples for light scattering studies were clarified by filtration through Millipore GS 0.22 µm pore size filters before use. The experiments were carried out at a temperature of 25 °C. In the dynamic light scattering (DLS) experiments, time-intensity correlation functions were measured. For each sample 10 correlation functions were collected and averaged. Each correlation function was collected in the course of 30 s. The average correlation functions were analyzed using the CONTIN program to calculate hydrodynamic diameter distribution of PVCL particles. Static light scattering (SLS) experiments were performed for further determination of Mw of PVCL using the Zimm-plot method. Cl- ion concentration in aqueous solutions was measured using an Oakton Acorn Series Ion 5 ion meter and a Cole-Parmer chloride electrode, 27502-13. For measurements, the following solutions were used: (i) solutions of CoCl2 with concentrations of 0.020, 039, 0.062, and 0.08 g/L (to ensure accuracy of the measurements) and (ii) PVCL-Co solutions with PVCL concentrations of 1 and 0.5 g/L and CoCl2 concentrations of 0.062 and 0.039 g/L. (17) Makhaeva, E. E.; Tenhu, H.; Khokhlov, A. R. Macromolecules 1998, 31, 6112. (18) van den Boogaard, M.; Bonnet, G.; Van't Hof, P.; Wang, Y.; Brochon, C.; van Hutten, P.; Lapp, A.; Hadziioannou, G. Chem. Mater. 2004, 16, 4383. (19) Kiriy, A.; Gorodyska, G.; Minko, S.; Tsitsilianis, C.; Jaeger, W.; Stamm, M. J. Am. Chem. Soc. 2003, 125, 11202. (20) Kumaki, J.; Hashimoto, T. J. Am. Chem. Soc. 2003, 125, 4907.
Figure 1. TEM images of PVCL-Co (a, b, c) and PVCL-Ag (d) obtained at 45 °C. Inset in (a) shows PVCL-Co-RT obtained at room temperature. In (c) the TEM grid was stained with OsO4 for 30 min. Inset in (d) shows well-defined PVCL-Ag structures.
Results and Discussion In the PVCL repeating unit (I) the carbonyl group oxygen is a stronger donor than the adjacent tertiary nitrogen, so metal cations should coordinate to the carbonyl groups. If metal cations coordinate with PVCL globular aggregates, one might expect two simultaneous events: (i) increase of the charge of resulting complexes, increasing the aggregate hydrophilicity, and (ii) cross-linking of the PVCL units located close to each other in a globular conformation, if the ions have coordination numbers of 2 or higher.
Figure 1a shows the TEM image of PVCL treated at 45 °C with CoCl2‚6H2O at the PVCL/Co molar ratio of 8:1 (PVCL-Co). This image shows spherical and rodlike particles with diameters (or cross-sectional diameters) of 10-12 nm. Since the only high electron density species in this sample are Co(II) ions, the visible particles should contain Co. A close look at the TEM image of this sample (Figure 1b) shows that the majority of even seemingly spherical particles are slightly elongated (shown by arrows), corroborating the suggested extended conformation of the PVCL macromolecules in the collapsed phase.10,21 Calculation of a PVCL (Mw ) 680 000 g mol-1) globule size using formulas22 and taking into consideration the decreased atomic radii in the bonded state and deriving the volume of the repeating unit (0.139 nm3 for PVCL), (21) Vasilevskaya, V. V.; Khalatur, P. G.; Khokhlov, A. R. Macromolecules 2003, 36, 10103. (22) Askadskii, A. A. Computational Materials Science of Polymers; Cambridge International Science Publishing: Cambridge, 2003.
2654
Langmuir, Vol. 21, No. 7, 2005
Letters
Figure 3. Hydrodynamic diameter distributions of the colloidal particles in the PVCL-Co aqueous solution at 30° scattering angle and 25 °C. Figure 2. 13C NMR spectra of PVCL (a, b) and PVCL-Co (c, d) in D2O at 25 °C (a, c) and 45 °C (b, d).
gives a mean globule diameter of 11 nm, matching well the observed PVCL-Co particles. One might conclude that interaction with Co(II) ions yields stabilization of single polymer globules. To address this issue, we carried out an NMR examination of PVCL and PVCL-Co. As has been well documented for PNIPAM,23,24 the coilto-globule transition results in a strong decrease of the intensity of the 1H and 13C NMR signals due to line broadening when mobility of the corresponding atoms decreases in a globular conformation. In a similar way we collected 13C NMR spectra of PVCL in D2O at 25 °C and upon heating at 45 °C (Figure 2a,b). The 13C NMR spectrum of PVCL at 25 °C shows a number of signals, assignment of which is shown in Figure 2a. The assignment has been performed using simulation with the ACD/ CNMR Spectrum Generator 4.5 software and literature data for poly(N-vinylpyrrolidone).25 The CH peak splitting (2 in Figure 2a) is presumably due to tacticity of the polymer.25 At 45 °C (in a globular conformation) no signals from PVCL units are recorded for 1 h (shown in Figure 2b) and even for 24 h (not shown). The 13C NMR spectrum of PVCL-Co at 25 °C (Figure 2c) is practically identical to that of PVCL, revealing that PVCL units retain their mobility and chemical structure after interaction with Co(II) cations. Similar to pure PVCL, increase of temperature to 45 °C leads to disappearance of all NMR signals. These observations suggest that the PVCL-Co macromolecule does not stay in the globular confinement at room temperature. It should become hydrated and occupy much larger volume than that matching to the particles in panels a and b of Figure 1. To clarify this apparent discrepancy, a TEM grid with PVCL-Co was stained with OsO4 for 30 min (Figure 1c) to visualize the parts of PVCL with no coordination to Co(II) ions. This results in the appearance of loose, ostensibly spherical structures with darker cores (due to Co(II) ion presence). Formation or these core-shell particles can be explained in the following way. As it was already mentioned, coordinating of Co(II) ions with PVCL monomer units leads to two effects: increase of the charge of resulting complexes and additional cross-linking. The first effect could be partially compensated by the attraction of Cl- counterions with the formation of ion pairs and (23) Zeng, F.; Tong, Z.; Feng, H. Polymer 1997, 38, 5539. (24) Larsson, A.; Kuckling, D.; Schonhoff, M. Colloids Surf., A 2001, 190, 185. (25) Yokota, K.; Abe, A.; Hosaka, S.; Sakai, I.; Saito, H. Macromolecules 1978, 11, 95.
their subsequent aggregation in ionomeric-type multiplets in the hydrophobic polymer environment. However, this should result in the decrease of the concentration of free Cl- ions in the solution. To address this possibility, we measured Cl- ion concentration in PCVL-Co solutions with different polymer and CoCl2 loading (see Experimental Section). Our measurements showed no decrease in Cl- ion concentration in the PVCL-Co solutions compared to the CoCl2 solutions of the same concentration, revealing that there is no ion pair formation and all ions are dissociated. This suggests that the main cause of the formation of these core-shell particles is cross-linking of certain areas of the PVCL macromolecules. Presumably, when CoCl2 is added in the PVCL solution at 45 °C, the Co(II) ions can diffuse along the surface of individual globules within a globular aggregate and crosslink (and stain) only the PVCL units belonging to the globule surface. At room temperature the parts of the PVCL chain without Co(II) ions protrude through this weakly cross-linked area and form the outer layer (see structure II; free Cl- ions are not shown for simplicity). This structure resembles the flower-like nanostructure reported elsewhere,15 but for PVCL-Co, this nanostructure is stabilized at room temperature.
Light scattering experiments of the PVCL-Co aqueous solution carried out at 30°, 90°, and 150° and 25 °C show the presence of two kinds of particles, the diameters of which change slightly depending on the scattering angle. The particle size distribution obtained at 30° is shown in Figure 3. We think that smaller particles are individual macromolecules while larger particles are aggregates. On the other hand, we do not see any aggregation with TEM. As is well-known, larger aggregates scatter much stronger than small ones, so impact of larger particles on the scattering profile is stronger. Thus, even a small fraction of aggregates may influence the light scattering data, while these aggregates are not observed with TEM. When Co ions are added to PVCL solution at room temperature only a few small particles are formed, while the majority of the sample shows large irregular aggregates (Figure 1a inset), suggesting that, in a coil conformation, the interaction with Co(II) ions does not
Letters
yield unimolecular core-shell nanostructures. Addition of the CoCl2 at 35 °C (∼32 °C is the coil-to-globule temperature for PVCL) results in both well-defined structures similar to those presented in Figure 1a and in larger irregular aggregates (not shown). Evidently, the existence of well-defined globular conformation is a necessary condition for formation of single PVCL nanostructures. We also explored the influence of the metal ion type on the formation of single polymer nanostructures. As is well established, Co(II) ion is able to accommodate up to six monodentate ligands in its internal coordination sphere,26 while Ag(I) cation is able to coordinate not more than two monodentate ligands. The TEM image of PVCL treated with AgNO3 at 45 °C (Figure 1d) shows both small particles with diameters of about 11 nm (inset) and larger aggregates with sizes 40-70 nm. Obviously, incorporation of silver cations does not allow a controlled formation of single polymer nanostructures. Considering that the N-caprolactam ring is fairly bulky, the real coordination number may be smaller than the maximum capacity, thus (26) Wilkinson, G. Comprehensive coordination chemistry: the synthesis, reactions, properties, & applications of coordination compounds; Pergamon Press: New York, 1987.
Langmuir, Vol. 21, No. 7, 2005 2655
Ag(I) ions may result in bonding a single carbonyl group, providing no cross-linking between PVCL units with much higher probability that Co(II) ions are bonded to at least two carbonyl groups. Conclusion This study demonstrates that addition of Co(II) ions to PVCL in its globular conformation yields unimolecular core-shell polymer particles, the core of which is decorated with Co(II) ions. The crucial condition for formation of well-defined unimolecular nanostructures is the presence of stable globular aggregates in aqueous solution. Comparison with Ag(I) ions shows that the metal ions must have a sufficiently high coordination number to provide a cross-linking between adjacent PVCL units in the globular conformation and stabilization of the core. Acknowledgment. This work was partially supported by NATO Science for Peace Program (Grant SfP-974173). We thank Professor A. A. Askadskii for valuable discussion. LA047163B
