© Copyright 1998 by the American Chemical Society
VOLUME 102, NUMBER 40, OCTOBER 1, 1998
LETTERS Laser-Controlled Assembling of Repulsive Unimolecular Micelles in Aqueous Solution Jun-ichi Hotta, Keiji Sasaki,* and Hiroshi Masuhara* Department of Applied Physics, Osaka UniVersity, Suita, Osaka 565-0871, Japan
Yotaro Morishima Department of Macromolecular Science, Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan ReceiVed: June 11, 1998; In Final Form: August 12, 1998
Laser manipulation techniques were applied to the control of molecular assembling in D2O. An amphiphilic random copolymer, which has hydrophilic and hydrophobic segments in a single chain and forms a monopolymer micelle through self-organization, is demonstrated to receive photon pressure of a focused near-infrared laser beam. Upon prolonged irradiation with the beam, a single microparticle is formed at a focal point, and it dissolves very quickly after switching off the laser beam. The behavior indicates that electrostatic repulsion between micelles is overcome by photon pressure. The assembling processes were examined as a function of irradiation time and polymer concentration and are discussed.
Introduction Supramolecular structures, such as micelles, vesicles, Langmuir-Blodgett films, and membranes, have generated much interest owing to their biological relevance and their potential importance as future molecular materials. Amphiphilic polymers, a class of water-soluble polymers consisting of both hydrophobic and hydrophilic segments in single polymer chains, have a tendency to self-organize and form supramolecular structures in aqueous solutions. It was reported that random copolymers of sodium 2-acrylamido-2-methylpropanesulfonate (AMPS) and methacrylamides N-substituted with bulky hydrophobic groups of cyclic structures such as cyclododecyl (Cd), admantyl (Ad), and 1-naphthyl (1-Np) groups undergo intrapolymer self-organization in aqueous solution to form unimolecular micelles.1 As this behavior is independent of the polymer concentration, interpolymer association is completely neglected, which may be ascribed to the electrostatic repulsion between the polymers. As illustrated schematically in Figure 1, amphiphilic polyanions consisting of bulky hydrophobes form hydrophobic clusters (micelle units) within a polymer chain
owing to intramolecular associations of the hydrophobes in dilute aqueous solution. However, this conformation, which is viewed as a “second-order structure”, may not be a stable conformation, because a significant portion of the surface of the hydrophobic clusters should inevitably be exposed to water. Consequently, the micelle units conglomerate to form a thirdorder conformational structure that is referred to as a unimolecular micelle, or unimer.1 This unimer is very stable and does not undergo aggregation independent of its concentration. The unimer exhibits compact structures in aqueous solution; especially, copolymers abbreviated as poly(A/Cd/Py), which were terpolymerized from sodium AMPS, N-Cd-methacrylamide, and N-(1-pyrenylmethyl)methacrylamide, are highly compact, as indicated by the ratio of molecular weight to radius of gyration (Mw ) 5.1 × 105 against RS ) 5.5 nm).1 Formation of further high-order structure is considered to be difficult; however, by applying laser manipulation techniques,2,3 this possibility is clearly demonstrated here for the first time. In the past few years, we have devoted our efforts to establish laser manipulation methods for gathering nanometer molecular
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Figure 1. Schematic representation of self-organization of an amphiphilic copolymer and of its assembling by photon pressure. The chemical structure of poly(A/Cd/Py) is also given.
systems and to elucidate photon pressure effects on molecular assembling structures. It is now well-known that a single microparticle is created in solutions of poly(N-isopropylacrylamide) (PNIPAM),4 its derivatives,5 and poly(N-vinylcarbazole)6 by a focused near-infrared laser beam, and molecular association structures of PNIPAM were confirmed to be different from those produced by thermal phase transition.7 In the case of a swelled micelle in aqueous solution, its concentration at the focal point could be increased by photon pressure so that aggregation and fusion of the micelles are induced, leading to formation of a single microdroplet.8 Here, we have demonstrated for the first time assembling of the unimer-forming amphiphilic polyelectrolyte by photon pressure of a focused near-infrared laser beam and the resultant formation of a single microparticle. This is deemed to be characteristic formation of the fourth structure of the unimer, as illustrated in Figure 1. Experimental Section All the experiments were performed at room temperature (21 °C). The sample solution was prepared by dissolving 0.060.5 wt % poly(A/Cd/Py) in D2O to avoid temperature elevation, as the overtone vibrational modes of H2O absorbs intense the laser beam and leads to local heating.4 The sample cell consisted of a thin quartz plate (thickness ) 0.35 mm) and a thick quartz plate (thickness ) 2.1 mm) with a hole (diameter ) 12 mm, maximum depth ) 0.6 mm). The sample solution was kept in the hole. A 1064-nm near-infrared laser beam (CW Nd3+:YAG laser, Spectron, SL902T) was used as an optical trapping light source, introduced to a confocal microscope (Zeiss, UMSP-50), and focused (spot size ∼1 µm) into a sample solution by an oil immersion objective lens of the microscope
(magnification 100; numerical aperture, 1.25).9 All the experiments were done between 10 and 20 µm from the upper quartz plate. The initial stages of the polymer condensation process were monitored with the backscattering image of the He-Ne laser beam and then observed by transmission image. Both images were taken by a CCD camera. For analyzing the formed microparticle, its fluorescence spectra and their rise and decay curves were measured by our fluorescence microscopy system. The excitation light source was the third harmonic of a mode-locked Ti:sapphire laser (λ ) 300 nm, pulse width ) 2 ps, 82-0.4 MHz), and the fluorescence from the sample solution was imaged on a pinhole (diameter, 80 µm) in front of a detector and analyzed by a monochromator (Zeiss, 474345) equipped with microchannel plate photomultiplier (Hamamatsu Photonics, R2809-07) for fluorescence decay curve or by an intensified multichannel spectrophotometer (Hamamatsu Photonics, PMA10) for fluorescence spectra. Conventional single-photon-counting electronics and related systems are described elsewhere.9 Results and Discussion After an irradiation of the trapping laser onto the sample solution, the backscattering of He-Ne laser was observed immediately at the focal point and increased gradually as shown in Figure 2. The scattering intensity from the particle was saturated after 20 min. After the prolonged irradiation, the particle can be seen in the transmission image of the microscope as in Figure 2f, where the diameter of the formed particle was ∼1 µm. When the laser beam was cut off, the particle disappeared very quickly in ∼1 s. This behavior in D2O is roughly similar to that observed for PNIPAM in aqueous
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Figure 2. He-Ne laser backscattering and transmission images of D2O solution of poly(A/Cd/Py) at (a) 0 min (before irradiation of the trapping laser), (b) 1 min, (c) 5 min, (d) 15 min, (e) 30 min, after starting irradiation, respectively, and (f) a transmission image of the generated particle. He-Ne and trapping infrared laser beams were focused coaxially onto the same position. The trapping laser power was 700 mW.
solution4,5,7 and poly(N-vinylcarbazole) in N,N-dimethylformamide,6 while initial formation speed and rate of disappearance here are extremely high compared to the latter polymers. Thus it is clearly demonstrated that poly(A/Cd/Py) unimers are gathered and assembled at a focal point, giving a single microparticle, although they are repulsive with each other. According to the relevant theory, photon pressure exerted on small dielectrics whose size is smaller than wavelength is expressed as follows10
1 ∂ F ) R∇E2 + R (E × B) 2 ∂t
(1)
where E and B are electric field strength and magnetic flux density, respectively, and R is polarizability of small dielectrics, namely, unimers here. The first term of eq 1 is an electrostatic force acting on the molecular dipole in the inhomogeneous electric field, which is called a gradient force. When the polarizability is positive, the gradient force attracts unimers to the focal spot. The second term is derived from the change in the direction of the Poynting vector, which is called a scattering force. This force pushes unimers along the beam direction. Since the gradient force is usually much stronger than the scattering force when a laser beam is focused onto polymer solutions, photon pressure works for trapping the molecules in the vicinity of the focal spot. The present result on poly(A/ Cd/Py) is well interpreted in terms of assembling by photon pressure. Compared to the reported behavior of PNIPAM and poly(N-vinylcarbazole), appearance of the backscattering and dissolution of the formed microparticle is very fast. As one of the authors (Y.M.) reported already, unimers are dissolved independently and not associated with each other;1 hence, it is considered that their micro-Brownian motion is very active and they diffuse in aqueous solution quite rapidly. Once the laser beam is introduced, photon pressure suppresses the Brownian motion and overcomes electrostatic repulsion between unimers, leading to an assembled structure. It is observed as a microparticle, while it is considered to be a particle consisting of a lot of unimers. Electrorepulsive unimers never associate with each other without photon pressure, so that this is the first demonstration of such assembling. Since this is the assembling
Figure 3. (A) Fluorescence spectrum of 0.5 wt % D2O solution of poly(A/Cd/Py) and (B) its fluorescence decay curves in solution (a) and of the formed microparticle (b). Decay curves of pyrene monomer fluorescence were observed at 384 ( 12.5 nm.
of the third structure already formed by self-organization, the present microparticle is now considered to be the fourth structure as shown in Figure 1. As the internal electrostatic repulsion remains in the fourth structure, the microparticle disappears very quickly after switching off the laser beam, which is quite reasonable. The single microparticle formation is proved by the backscattering from the trapping spot for the examined concentration range of 0.06-0.5 wt %. The solution with the higher concentration showed faster increase of the intensity and gave the stronger intensity at the final steady state. The polymer molecules were dispersed as unimolecular micelles in this system and do not associate with each other; hence, the number of unimers is in proportion to the concentration of the solution. The photon pressure is determined by interactions between unimers and laser light. While the latter intensity is fixed, the photon pressure becomes stronger as the number of unimers is increased. The assembled fourth structure with bigger size receives stronger photon pressure, leading to a deeper potential well. As the concentration is lower, the number of unimers trapped at the focal point is smaller, which results in shallower potential. In the steady state, trapped unimers are in equilibrium with those dispersed outside the spot, which is just consistent with conventional chemical equilibrium. The attained steady state here involves an optical effect, which is characteristic of the present work. It is noticeable that polymer micelles whose radius of gyration is 5.5 nm are efficiently trapped, and its process is interpreted in the terms of chemical and optical mechanisms. To confirm the fourth-order structure of condensed unimer micelles, fluorescence analysis was performed. Pyrene chromophore of poly(A/Cd/Py) was excited, and obtained fluorescence spectra and decay curves are shown in Figure 3. It is worth noticing that no excitation intensity dependence of fluorescence lifetime was observed. Fluorescence spectrum of a single microparticle was quite similar to that of the solution, indicating no dimer formation. Fluorescence decay curves
7690 J. Phys. Chem. B, Vol. 102, No. 40, 1998 measured at 384 nm, and fluorescence lifetime is estimated to be 250 and 220 ns in solution and the formed particle, respectively. No appreciable fluorescence spectral change and a little decrease of lifetime indicate that the photon pressure controlled assembling does not reorganize the unimer structure. This is quite different from PNIPAM and poly(N-vinylcarbazole) solutions where polymer chains associate with each other upon single microparticle formation. The shorter fluorescence from the particle compared to the dispersed solution suggests that the decay enhancement is ascribed to an increase in the radiative rate constant owing to the increase effective refractive index of the microparticle. This would be caused by an increase in local concentration of pyrene chromophore and polymer chains in D2O. Thus, it is considered that conformation and microenvironmental conditions of unimers are not changed by assembling, so that possible interpenetration of polymer chains is excluded in the fourth-order structure. To fix the latter structure without photon pressure, it is necessary to combine the present microparticle formation with local photopolymerization under the microscope. At the present stage of investigation, we consider that no appreciable association/interaction of unimers is induced, which is proved well by pyrene fluorescence spectroscopic measurement. Acknowledgment. J.H. is a research fellow of the Japan Society for the Promotion of Science. The present work is partly
Letters supported by Grants-in Aid from the Ministry of Education, Science, Sports, and Culture of Japan (3074, 94170, 07241243, 08231247, 09304067). References and Notes (1) (a) Morishima, Y.; Tominaga, Y.; Kamachi, M.; Okada, T.; Hirata, Y.; Mataga, N. J. Phys. Chem. 1991, 95, 6027. (b) Morishima, Y.; SolVents and Self-Organization of Polymers; Webber, S. E., et al., Eds.; Kluwer Academic Publishers: Netherlands, 1996; pp 331-358. (2) (a) Ashkin, A. Phys. ReV. Lett. 1970, 24, 156. (b) Ashkin, A.; Dziedzic, J. M.; Bjorkholm, J. E.; Chu, S. Opt. Lett. 1986, 11, 288. (3) Sasaki, K.; Koshioka, M.; Misawa, H.; Kitamura, N.; Masuhara, H. Jpn. J. Appl. Phys. 1991, 30, L907. (4) Ishikawa, M.; Misawa, H.; Kitamura, N.; Masuhara, H. Chem. Lett. 1993, 481. (5) (a) Hofkens, J.; Hotta, J.; Sasaki, K.; Masuhara, H.; Faes, H.; De Schryver, F. C. Mol. Cryst. Liq. Cryst. 1996, 283, 165. (b) Hofkens, J.; Hotta, J.; Sasaki, K.; Masuhara, H.; Iwai, K. Langmuir 1997, 13, 414. (6) Borowicz, P.; Hotta, J.; Sasaki, K.; Masuhara, H. J. Phys. Chem. B 1997, 101, 5900; 1998, 102, 1896. (7) Hofkens, J.; Hotta, J.; Sasaki, K.; Masuhara, H.; Taniguchi, T.; Miyashita, T. J. Am. Chem. Soc. 1997, 119, 2741. (8) Hotta, J.; Sasaki, K.; Masuhara, H. J. Am. Chem. Soc. 1996, 118, 11968. (9) Microchemistry: Spectroscopy and Chemistry in Small Domains; Masuhara, H., De Schryver, F. C., Kitamura, N., Tamai N., Eds.; Elsevier: Amsterdam, 1994. (10) Shen, Y. R. The Principles of Nonlinear Optics; Wiley-Interscience: New York, 1984; pp 366-378.
