© Copyright 2007 by the American Chemical Society
VOLUME 111, NUMBER 11, MARCH 22, 2007
ARTICLES Femtosecond/Picosecond Time-Resolved Fluorescence Study of Hydrophilic Polymer Fine Particles Daisuke Nanjo,† Haruko Hosoi,‡ Tatsuya Fujino,*,† Tahei Tahara,‡ and Takashi Korenaga† Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan UniVersity, 1-1 Minami-Osawa, Hachioji-Shi, Tokyo 192-0397, Japan, and Molecular Spectroscopy Laboratory, RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ReceiVed: October 2, 2006; In Final Form: December 26, 2006
Femtosecond/picosecond time-resolved fluorescence study of hydrophilic polymer fine particles (polyacrylamide, PAAm) was reported. Ultrafast fluorescence dynamics of polymer/water solution was monitored using a fluorescent probe molecule (C153). In the femtosecond time-resolved fluorescence measurement at 480 nm, slowly decay components having lifetimes of τ1 ∼ 53 ps and τ2 ∼ 5 ns were observed in addition to rapid fluorescence decay. Picosecond time-resolved fluorescence spectra of C153/PAAm/H2O solution were also measured. In the time-resolved fluorescence spectra of C153/PAAm/H2O, a peak shift from 490 to 515 nm was measured, which can be assigned to the solvation dynamics of polymer fine particles. The fluorescence peak shift was related to the solvation response function and two time constants were determined (τ3 ∼ 50 ps and τ4 ∼ 467 ps). Therefore, the τ1 component observed in the femtosecond time-resolved fluorescence measurement was assigned to the solvation dynamics that was observed only in the presence of polymer fine particles. Rotational diffusion measurements were also carried out on the basis of the picosecond time-resolved fluorescence spectra. In the C153/PAAm/H2O solution, anisotropy decay having two different time constants was also derived (τ6 ∼ 76 ps and τ7 ∼ 676 ps), indicating the presence of two different microscopic molecular environments around the polymer surface. Using the Stokes-Einstein-Debye (SED) equation, microscopic viscosity around the polymer surface was evaluated. For the area that gave a rotational diffusion time of τ6 ∼ 76 ps, the calculated viscosity is ∼1.1 cP and for τ7 ∼ 676 ps, it is ∼10 cP. The calculated viscosity values clearly revealed that there are two different molecular environments around the polyacrylamide fine particles.
Introduction Nanomaterials, which are considered to be the key to the progress of nanoscience and technology, have unique properties because of their size and structure. Among those materials, nanoto submicrometer-sized polymer particles having large surface areas and hydrophilic polymer chains at their surfaces are * To whom correspondence should be addresed. Tel.: +81-426-77-2531. Fax: +81-426-77-2525. E-mail:
[email protected]. † Tokyo Metropolitan University. ‡ RIKEN.
attracting much interest in many research areas.1-6 For example, polymer blends or alloys, chromatographic separation media, polymer-supported heterogeneous catalysis, and drug delivery systems (DDSs) utilize such hydrophilic polymer particles. Polymer particles having a diameter of several tens nanometers to micrometers can be selectively synthesized, and novel functions and characteristics can be easily introduced by choosing monomers that control the chemical properties of polymer chains and the particle core. Despite their numerous applications, however, there remain many uncertainties regarding
10.1021/jp0664871 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/01/2007
2760 J. Phys. Chem. B, Vol. 111, No. 11, 2007 their influence on the natural environment or on the human body. Therefore, a fundamental understanding of the physical and chemical properties of polymer fine particles is necessary. Spectroscopy is a valuable tool to understand the physical and chemical properties of polymer fine particles. Fluorescence spectroscopy is one of the most effective techniques because the peak intensity and wavelength of a steady-state fluorescence spectrum are highly sensitive to the environment surrounding the fluorescent probe molecules. In addition to the steady-state spectroscopies, it has become possible to investigate molecular dynamics with very high time resolution owing to recent innovations of ultrafast lasers. Femtosecond/picosecond timeresolved spectroscopy enables us to investigate the properties of short-lived excited states as well as the electronic and vibrational relaxation processes. Among the ultrafast spectroscopies, fluorescence spectroscopy is also one of the powerful tools because the time-resolved fluorescence can derive the dynamical information about the electronic excited state of interest.7-9 This technique is applicable not only to homogeneous solutions but also to supramolecular assemblies such as micelles and polymers by using fluorescent probe molecules.10-13 In this study, we conducted femtosecond and picosecond timeresolved fluorescence measurements of polymer fine particles to reveal the physicochemical properties around the polymers in liquid. Polyacrylamide (PAAm), a typical corona-sphere-type hydrophilic polymer particle, was synthesized by dispersion copolymerization of hydrophilic macromonomers. The synthesized polymer particles were dispersed in water containing a fluorescent probe molecule, Coumarin 153 (C153 hereafter), and then the solvation dynamics around the polymer particles was measured from the ultrafast time-resolved fluorescence of the fluorescent probe molecule. C153 enables us to monitor molecular environments, and its spectroscopic properties have been widely investigated mainly by time-resolved fluorescence spectroscopy.14 It was clarified that the chemical properties around the polymers were much different from those of bulk water and that the polymer fine particles formed a microscopically unique chemical environment in water. 2. Experimental Nanosized polymer particles were synthesized by the dispersion copolymerization of hydrophilic macromonomers.15 Acrylamide, N-(hydroxymethyl)acrylamide, 2-hydroxyethyl methacrylate, acrylic acid, and poly(oxyethylene) methacrylate (number-average molecular weight Mn ) 1100) were used as monomers. 2,2′-Azobis(isobutyronitrile) (AIBN) was used as an initiator. The monomers were dissolved in 1-methoxy-2propanol, and then 10% of the solution was diluted with 1-methoxy-2-propanol to prepare a 5 wt % solution. The solution was reacted at 117 °C for 30 min in the presence of AIBN while being stirred to form seed particles. By adding the remaining solution slowly, the seed particles grew and PAAm particles were finally obtained. To evaporate the solvent (1-methoxy-2propanol), the synthesized polymer particles were dried with a dryer (MDL-050, Fujisaki Electric). The dried polymers were aggregates of small particles having a cluster size of several micrometers. These aggregated polymers were dissolved in distilled water, and a monodispersed polymer solution of fine particles was obtained. The synthesized polymers were spherical, as revealed by scanning electron microscopy (JSM-6100, JEOL). The diameter of each particle was evaluated with a dynamic light scattering size analyzer (LB-550, HORIBA), and the median was ∼500 nm.16 Coumarin 153 (laser grade, Exciton) was used as received. Solvents (HPLC grade, Wako) were used without further
Nanjo et al. purification. Femtosecond time-resolved fluorescence measurements were carried out by using the apparatus for fluorescence upconversion described previously.7 The second harmonic (400 nm) of a mode-locked Ti:sapphire laser (Tsunami, SpectraPhysics) was used as the excitation light of the sample. The time resolution of this system was estimated to be ∼180 fs by the Raman scattering from solvent (water). Picosecond timeresolved fluorescence spectra were measured with the apparatus on the basis of a streak camera (C4334, Hamamatsu).17 The second harmonics (400 nm) of a Ti:sapphire laser with a regenerative amplifier (Spitfire, Spectra-Physics) was used as the excitation light. The time resolution of this system was estimated to be ∼20 ps by measuring the streak image of excitation light. To monitor the chemical properties around the polymer surface, C153 was introduced into the polymer/water solution. The C153/ PAAm/H2O(D2O) solution was prepared according to the following procedure: 0.1 g of the dried polymer was dissolved in distilled water (100 mL) containing ∼10 mg of C153, and then the solution was sonicated with increasing temperature. After filtering out the undissolved C153, the filtrate was used for the experiments. Femtosecond and picosecond time-resolved fluorescence measurements were carried out in the magic-angle polarization condition except for the anisotropy measurements of picosecond time-resolved fluorescence spectra. All time-resolved measurements were performed at room temperature. Steady-state fluorescence spectra from the solutions were also measured at room temperature with a fluorescence spectrometer (RF-5300, SHIMADZU). 3. Results and Discussion Because of the water-insoluble property of the fluorescent probe molecule, the steady-state fluorescence spectrum of the C153-saturated/H2O solution showed only very weak fluorescence with an intensity maximum at ∼550 nm. In the presence of synthesized polymer fine particles, on the other hand, solubility of C153 was increased because the polymer particles have less hydrophilicity compared with water. Thus, the C153/ PAAm/H2O solution showed an almost ∼30-fold increase in fluorescence compared with the C153/H2O solution. The steadystate fluorescence spectrum obtained from the C153/PAAm/ H2O solution clearly showed that the C153 molecules were selectively located around the polymer particle surfaces. Furthermore, it was also observed that the center wavelength of the fluorescence spectrum of the C153/PAAm/H2O solution was blue-shifted to ∼515 nm, whereas the center wavelength of the C153/H2O solution is located at ∼550 nm. C153 is one of the fluorescent probe molecules that enable us to monitor molecular environments, and the shift of the peak wavelength of its steadystate fluorescence spectrum is dependent on the polarity of the solvent used. For example, the peak wavelength of the C153 fluorescence spectrum is 455 nm for hexane (Snyder’s polarity index ) 0.218), 480 nm for toluene (2.4), 515 nm for acetonitrile (5.8), and 550 nm for water (10.2), and we can see an almost linear relationship between the peak wavelength of the fluorescence spectrum and the polarity index. Therefore, it can be said that polymer fine particles form a microscopically unique environment in water and the polarity around the polymer surface is as low as that of acetonitrile even though the polymers are dissolved in water. The femtosecond time-resolved fluorescence spectrum of C153-saturated/H2O solution observed at 480 nm is depicted in Figure 1a. Immediately after the photoexcitation, the timeresolved fluorescence showed a very rapid decay because of the solvation dynamics of water with the time constant of
