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Light-Driven Directed Motion of Azobenzene-Coated Polymer Nanoparticles in an Aqueous Medium Jean-Pierre Abid,† Michel Frigoli,§ Robert Pansu,‡ Jacob Szeftel,† Joseph Zyss,† Chantal Larpent,*,§ and Sophie Brasselet*,|| Institut d’Alembert, Laboratoire de Photonique Quantique et Moleculaire and ‡Institut d’Alembert, Laboratoire de Photophysique et Photochimie Supramoleculaires et Macromoleculaires, Ecole Normale Superieure de Cachan, 61 Avenue du President Wilson, 94235 Cachan, France § Institut Lavoisier UMR CNRS 8180, Universite de Versailles-Saint-Quentin en Yvelines, 45 Avenue des Etats-Unis, 78035 Versailles Cedex, France Institut Fresnel, MOSAIC group, Domaine Universitaire St. Jer^ome, 13397 Marseille, France
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bS Supporting Information ABSTRACT: Azobenzene-coated polymer nanoparticles in the 16-nm-diameter range act as phototriggered nanomotors combining photo to kinetic energy conversion with optical control through light intensity gradients. The grafted dyes act as molecular propellers: their photoisomerization supplies sufficient mechanical work to propel the particles in an aqueous medium toward the intensity minima with velocities of up to 15 μm/s. It is shown that nanoparticles can be driven over tens of micrometers by translating the intensity gradients in the plane. The analysis of the particles motion demonstrates the decisive role of photoisomerization in the transport with a measured driving force that is 3 to 4 orders of magnitude higher than optical forces.
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irected motion of nano-objects in liquid media is desirable for many applications in nanoscience, microfluidics, and biology and is therefore attracting widespread attention.1 Inspired by biological systems, chemically fueled motions of particles or rods on micrometer scales have been achieved utilizing the catalytic decomposition of hydrogen peroxide at their surfaces to create localized oxygen concentration gradients (and hence surface tension gradients) that induce translational and/or rotational movement.27 Although these systems provide the advantage of autonomous motion, the directional control of their movement is a difficult task to carry out and has been found to require the design of asymmetric and/or magnetic devices in conjunction with an external magnetic field.47 The elaboration of phototriggered devices might address this issue with the advantage of being noninvasive and allowing noncontact positioning control, which is relevant for many potential applications. Photoinduced isomerization has been employed in a variety of molecular machines to achieve the conversion of light into mechanical energy.1 Conformational changes resulting from the trans to cis photoisomerization of azobenzene dyes have been found to induce collective movements in macromolecular structures.810 It has been shown recently that surface tension gradients generated by an asymmetrical illumination can induce the translational motion of liquid droplets or micrometer beads on photoresponsive surfaces such as azobenzene-coated solid substrates, azobenzene liquid crystals, or aqueous solutions of an azobenzene surfactant.1,11,12 Surface relief gratings of azo-polymer r 2011 American Chemical Society
films have been achieved under light intensity gradients, taking advantage of mass transport toward the dark regions where photoisomerization is minimized.8,10 We show herein that this phenomenom can be used to achieve the directionally controlled motion of nanosized polymer particles in an aqueous medium, as observed by fluorescence microscopy imaging. These nanoparticles (NPs) are specifically designed to circumvent Brownian motion, which dominates mechanical behavior in the nanoscopic world, using the light-induced isomerization of attached molecular motors to provoke their motion toward the dark regions of the modulated illumination of the sample. Similar to numerous biological motors, this device also takes advantage of Brownian motion to explore active illuminated areas of the sample efficiently and therefore to lead to directed motion. Our nanomotors consist of azobenzene-dye-decorated polymer nanoparticles in the 16-nm-diameter range. They combine photomechanical energy conversion with optical control through light intensity gradients and utilize the isomerizable dyes grafted on the particle surface as molecular propellers. The photoisomerization of the grafted dyes supplies sufficient mechanical work to propel the nanoparticles, with a power-tunable velocity, over tens of micrometers toward the dark regions of the optical gradient. Received: February 23, 2011 Revised: May 11, 2011 Published: June 09, 2011 7967
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Scheme 1. (a) Synthesis of nonfunctionalized and azobenzene-coated fluorescent nanoparticles NP0 and NPazo; fluorophore F: tetrabutyltetraazaporphine (TBTP). (b) trans-cis photoisomerization of the grafted azobenzene molecules (DR1-Py)
Azobenzene-coated cross-linked polystyrene nanoparticles (NPazo) were obtained by the covalent attachment of a photoisomerizable disperse red one (DR1) derivative bearing a pyridyl residue on chlorobenzyl-functionalized particles (NP0) prepared by microemulsion polymerization (Scheme 1).13,14 The starting nanoparticles (NP0, used as reference bare NPs) and DR1-coated nanoparticles (NPazo) were doped by soaking with a hydrophobic fluorescent dye (tetrabutyltetraazaporphine, TBTP) for fluorescence imaging (Figure 1b).13 This procedure gives access to aqueous suspensions of fluorescent NPs of 16 nm diameter with about 270 DR1 dyes grafted per NP as deduced from elemental composition and spectrophotometric and TEM analyses (Figure 1a,c). The fluorophore, TBTP, has been chosen for an optimal separation between its excitation wavelength (around 620 nm) and the absorption wavelength of DR1 (around 495 nm) (Figure 1a and Figures S2S4 in the Supporting Information). The excitation wavelengths used in our experiments for TBTP fluorescence imaging in the 650670 nm spectral range (633 nm) and for DR1 photoisomerization activation (473 nm) are therefore well separated and exhibit independent effects on these two molecular components.
The particle motion was studied in a liquid medium of controlled viscosity by dilution in aqueous solutions of poly(vinyl alcohol) (PVA) at 10 or 5 wt % (corresponding to viscosity values of 38.5 and 6.8 cP, respectively). The NP motion was analyzed by particle tracking using wide-field fluorescence imaging upon excitation at 633 nm (typical average power of 2 mW over a 50-μm-diameter illumination area) through an inverted illumination microscope equipped with a CCD camera. Photoisomerization was achieved by illumination at 473 nm with average power ranging from 1 to 20 mW (over a 80-μm-diameter illumination area). These conditions correspond to light intensities ranging from 5 to 100 W/cm2. As detailed in Table S1 in the Supporting Information, these conditions correspond to a power received per azobenzene molecule on the order of what has been used in previous work on DR1 photoisomerization15 so that photoisomerization is expected to occur under our experimental conditions. The encapsulated fluorescent dye and the grafted photoisomerizable chromophore were seen to be stable under these illumination conditions because we did not observe any photobleaching during the course of our experiments. The measurement time was limited only by the 3D out-of-plane 7968
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Figure 1. (a) Absorption (continuous line) and fluorescence emission spectra (dotted line, λex = 612 nm) of azobenzene-coated fluorescent nanoparticles NPazo. (b) Wide-field fluorescent image of NPazo in a 10 wt % PVA aqueous solution (scale bar, 1 μm). (c) TEM image of NPazo (scale bar 50 nm).
motion of the particles in our experimental setup (i.e., 30 s of observation time). Time-dependent fluorescence measurements have shown that there is no photobleaching of encapsulated fluorescent dye TBTP under the applied illumination intensities. It is noteworthy that previously published experiments of a surface relief grating on azobenzene-functionalized films have demonstrated the long-time photostability of azo-chromophores under similar irradiation conditions.10 Upon uniform illumination at 633 nm or simultaneous uniform illuminations at 633 and 473 nm, both bare (NP0) and azo-dye-coated NPs (NPazo) display Brownian motion. The viscosity conditions allow the observation of particle motion in two dimensions within an observation time of 5 to 10s. The analysis of the 2D trajectories shows a linear evolution of the mean square displacement (MSD) with time as Ær2(Δt)æ = 4DΔt where D is the particle diffusion coefficient and Δt is the time interval measurement. Experimental and calculated values of the diffusion coefficients, D, are given in Table S2 in the Supporting Information. The motion of bare NPs (NP0) is not influenced by illumination at 473 nm, and similar experimental values of diffusion coefficients, in good agreement with the theoretical values, are obtained with and without illumination at the photoisomerization wavelength (Dexp = 0.73 μm2/s in 10 wt % PVA, theoretical value 0.74 μm2/s). However, the diffusion coefficient of DR1-coated NPazo was found to increase by about 20% under uniform illumination at 473 nm (0.85 and 0.69 μm2/s in 10 wt % PVA with and without illumination, respectively).16 As discussed below, the light-induced local heating in the vicinity of the particle does not exceed 2K. Local heating might contribute to the variation of the diffusion coefficient but could not account for the observed 20% increase, suggesting that the azo-dye
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photoisomerization cycles amplify the stochastic diffusion process through photomechanical energy conversion. The particle motion was then studied under a structured periodic intensity at 473 nm of either a 5 or 10 μm period, generated by transmitting the illuminating beam through a projection grid. (The average power of 120 mW was unchanged.) The polarization was chosen to be perpendicular to the gradient direction in order to favor adequate photoselection of the dye molecules that potentially contribute to directed motion along the intensity gradients. The optical gradient was found to provoke the directed and accelerated translational motion of azo-dye-coated NPs toward intensity minima. The trajectories, depicted in Figure 2a and in Figures S7 and S8 in the Supporting Information, show that the azobenzene-coated nanoparticles undergo Brownian-type diffusion behavior in the homogeneous parts of illumination and are drawn toward the darker regions of illumination once they reach the intensity gradient regions of the sample. Figure 2b illustrates the directed translational motion of an NPazo particle in the gradient region. Corresponding Movie S1 in the Supporting Information shows that when an azo-dyecoated particle reaches the intensity gradient it is propelled away with high-speed translational motion until it is trapped in a dark zone.17 The first seconds of this Movie also show that the NPs benefit from Brownian motion to explore the spatial regions of the sample and to reach a gradient region that directs their motion in a deterministic way. Note that this behavior is not observed for uncoated particles NP0: the Brownian motion of bare particles is not modified under structured illumination at 473 nm regardless of the irradiation power. As control experiments, we applied a structured illumination at 633 nm to the uncoated and azo-coated fluorescent particles that absorb at this wavelength. Both bare and azo-coated fluorescent particles NP0 and NPazo were found to display standard Brownian motion without any directional displacement, thus suggesting that pure absorption is not responsible for the directional motion. Moreover, we estimated the local heating induced by the illumination at 473 nm on absorbing NPazo particles using the procedure recently reported by Carlson et al.:18 as detailed in the Supporting Information, the calculated maximum temperature increase at the NP surface is only about 1.5 K at the maximum illumination intensity used in our experiments (250 W/cm2). Furthermore, the intensity used at 473 and 633 nm of 250 W/cm2 maximum is orders of magnitude below the values that would induce the heating of the polymer matrix.19 All of these findings strongly suggest that the observed transitional accelerated motion of azo-coated NPs under an illumination gradient is actually induced by the photoisomerization of the grafted azo dyes rather than by local temperature gradients or pure absorbing effects. Because azo-coated NPs spontaneously move toward dark zones of illumination, we studied the possibility to control the displacement of nanoparticles over long-range distances by translating the intensity gradients in the sample plane. Movie S2 in the Supporting Information and Figure 3 show that particles can indeed be driven toward a desired direction over tens of micrometers simply by translating the projection grid. The statistical investigation of a population of 500 particles detailed in the Supporting Information shows that for 90% of the measured trajectories of azo-coated NPs in the illumination gradients the MSDs are no longer linear functions of Δt but instead exhibit a close to quadratic power dependence in the gradient regions, similar to motions submitted to external flows. 7969
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Figure 3. Directed translational motion of an azo-dye-coated NP under a translating gradient of intensity (by translation of the projection grid in this plane along the diagonal, the white arrow indicates the direction of translation). The colored lines correspond to the position of the maximum of intensity at 1 s (green), 2.5 s (blue), and 4.2 s (red). Integration time: 50 ms per image. Horizontal scale: 35 μm. Average input power: 2 mW at 633 nm and 3 mW at 473 nm. See Movie S2 in the Supporting Information.17
Figure 2. Motion of azobenzene-coated NPs under an optical gradient. (a) Map of some trajectories of NPazo in 10 wt % PVA under structured illumination at 473 nm superimposed on the image of the intensity gradient (average input power at 633 nm, 2 mW; average input power at 473 nm, 1mW; horizontal image size, 30 μm). (b) Directed motion of an azo-dye-coated particle (NPazo) under an intensity gradient of illumination at 473 nm (12 mW average power) in 10 wt % PVA. The white line represents the intensity profile (max, illuminated zone; min, dark region). Horizontal size of the images: 15 μm. See Movie S1 in the Supporting Information.17 (c) Velocity v of NPazo in the gradient area, calculated from MSD time dependencies as a function of the average power of structured illumination at 473 nm in 10 wt % aqueous PVA.
The good agreement of the MSDs with the fitting law Ær2(Δt)æ = 4DΔt þ v2Δt2 shows in particular that the velocity v of the particles transported in the gradient region is constant over time, which is representative of a deterministic uniform motion in the gradient regions. Moreover, the measured velocity of the uniform translational motion increases linearly with the average power of the
illumination gradient at the photoisomerization wavelength (Figure 2c). In 10 wt % aqueous solution of PVA (38.5 cP), the measured velocities range from 2.3 to 16.9 μm/s for illumination powers ranging from 2 to 20 mW. It is worth noticing that the velocity can further be modulated by adjusting the viscosity of the medium: measurements in 5 wt % aqueous PVA (6.8 cP) lead to velocities roughly 1 order of magnitude higher. However, the faster particle motion leads to shorter 2D tracking times and therefore did not allow enough statistics for a precise estimation. These results further support the hypothesis that photoisomerization of the attached molecular motors plays a decisive role in the transport of nanoparticles. A simple model to explain the observed translational motion is that grafted azo dyes provide mechanical kicks to the particles when they undergo transcis trans excitation/relaxation cycles. An increase in light intensity leads to an increase in the number of isomerization cycles and hence results in accelerated motion. Note that this observation does not rule out the possibility of the rotational motion of the particle on itself (by symmetrical kicks over its surface); however, spatial asymmetry provided by the intensity gradient is ultimately seen to provoke directed transport. As detailed in the Supporting Information, the observed uniform accelerated motion of the NPazo particles can be understood following a simple kinetic model where mechanical kicks are applied to the particle with an instantaneous driving force f during short events (assigning the initial velocity of the object), with the particle being submitted to only the viscous damping force imposed by the medium for the rest of the time. An investigation of the uniform motion of NPs in the gradient regions provides a quantitative estimation of the time-averaged effective driving force F involved in the transport, defined as a phenomenological force that would be continuously applied during the motion. This driving force balances the dragging force imposed by viscosity (6πηav, with η denoting the medium viscosity and a denoting the particle Stokes radius) and therefore depends on both η and v, the particle velocity. The estimated values of F range from 0.012 to 0.105 pN depending on the 473 nm excitation power (2 to 20 mW) in 10 wt % PVA. This driving force is about 3 to 4 orders of magnitude higher than other forces applied to the nanoparticles (gravity and external optical forces), which range from 1011 to 105 pN as detailed in the Supporting Information. These values further confirm the 7970
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Langmuir proposed driving role of photoisomerization and shed light on the possibility of using azobenzene-coated nanospheres to achieve optical steering and transport. In conclusion, we have shown that the directed motion of nanoparticles coated with photoisomerizable dyes can be achieved in aqueous solution at high velocity under light intensity gradients. The translation of the intensity gradient affords a means to drive the particle over tens of micrometers. Compared to optical trapping devices and tweezers, this strategy applies to objects down to tens of nanometers with a refractive index close to that of the surrounding medium and furthermore does not require the use of high optical focused power.20,21 Although further photophysical studies should be performed to clarify the exact mechanism that caused the observed controlled motion, this construct of light-fueled nanomotors is hoped to be quite general and applicable to various types of nanomaterials using known surface functionalization techniques. It could open the way to the elaboration of a wide range of photoaddressable nanodevices including drug delivery nanomachinery.
’ ASSOCIATED CONTENT
bS
Supporting Information. Detailed preparation and characterization of nanoparticles, experimental setup for particle motion imaging and illumination power, detailed analyses of the particles trajectories, estimation of the temperature at the particle surface, and details of the mechanical kick model. Movies showing the accelerated motion of a particle under an illumination intensity gradient and the directional transport of a particle by translating the illumination gradient. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected];
[email protected].
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(10) Viswanathan, N. K.; Kim, D. Y.; Bian, S.; Williams, J.; Liu, W.; Li, L.; Samuelson, L.; Kumara, J.; Tripathy, S. K. J. Mater. Chem. 1999, 9, 1941. (11) Kausar, A.; Nagano, H.; Ogata, T.; Nonaka, T.; Kurihara, S. Angew. Chem., Int. Ed. 2009, 48, 2144. (12) (a) Diguet, A.; Guillermic, R.-M.; Magome, N.; Saint-Jalmes, A.; Chen, Y.; Yoshikawa, K.; Baigl, D. Angew. Chem., Int. Ed. 2009, 48, 9281. (b) Laloyaux, X.; Jonas, A. M. Angew. Chem., Int. Ed. 2010, 9, 3262. (13) (a) Frigoli, M.; Ouadahi, K.; Larpent, C. Chem.—Eur. J. 2009, 15, 8319. (b) Gouanve, F.; Schuster, T.; Allard, E.; Meallet-Renault, R.; Larpent, C. Adv. Funct. Mater. 2007, 17, 2746. (14) (a) Cannizzo, C.; Amigoni-Gerbier, S.; Frigoli, M.; Larpent, C. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 3375. (b) Larpent, C.; Amigoni-Gerbier, S.; De Sousa Delgado, A. P. C. R. Chim. 2003, 6, 1275. (15) Fukuda, T.; Matsuda, H.; Shiraga, T.; Kimura, T.; Kato, M.; Viswanathan, N. K.; Kumar, J.; Tripathy, S. K. Macromolecules 2000, 33, 4220. (16) An increase in the hydrodynamic radius of about 1.9 nm, engendered by the grafting of azo dyes, accounts for the slight decrease in D compared to uncoated NP0 in the absence of irradiation at 473 nm. As shown in Table S1 in the Supporting Information, a slight diminution of the hydrodynamic radius arising from trans-to-cis isomerization does not account for the observed increase in the diffusion coefficient under homogeneous illumination at 473 nm. (17) For the sake of clarity, the Figure and the corresponding movie present the motion of a single NPazo observed in diluted samples. As mentioned in the text, in more concentrated samples the directed accelerated motion was observed for a large number of particles. However, because of the high fluorescent background, probably resulting from some leakage of the encapsulated fluorophore, the image contrast is not good enough to present high-quality illustrative movies. (18) Carlson, M. T.; Khan, A.; Richardson, H. H. Nano Lett. 2011, 11, 1061. (19) Liu, Y.; Cheng, D. K.; Sonek, G. J.; Berns, M. W.; Chapman, C. F.; Tromberg, B. J. Biophys. J. 1995, 68, 2137. (20) Ashkin, A.; Dziedzic, J. M.; Bjorkholm, J. E.; Chu, S. Opt. Lett. 1986, 11, 288. (21) (a) Jauffred, L.; Richardson, A. C.; Oddershede, L. B. Nano Lett. 2008, 8, 3376. (b) Agarwal, R.; Ladavac, K.; Roichman, Y.; Yu, G; Lieber, C.; Grier, G. Opt. Express 2005, 13, 8906.
’ ACKNOWLEDGMENT This work was supported by CNRS, the French Ministry of Research (MESR), and the Agence Nationale de la Recherche (project ANR-05-NANO-011). ’ REFERENCES (1) Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew. Chem., Int. Ed. 2007, 46, 72 and references therein. (2) (a) Heureux, N.; Lusitani, F.; Browne, W. R.; Pshenichnikov, M. S.; van Loosdrecht, P. H. M.; Feringa, B. L. Small 2008, 4, 476. (b) Vicario, J.; Eelkema, R.; Browne, W. R.; Meetsma, A.; La Crois, R. M.; Feringa., B. L. Chem. Commun. 2005, 3936. (3) Agrawal, A.; Dey, K. K.; Paul, A.; Basu, S.; Chattopadhyay, A. J. Chem. Phys. C 2008, 112, 2797. (4) Paxton, W. F.; Sundararajan, S.; Mallouk, T. E.; Sen, A. Angew. Chem., Int. Ed. 2006, 45, 5420. (5) Sundararajan, S.; Lammert, P. E.; Zudans, A. W.; Crespi, V. H.; Sen, A. Nano Lett. 2008, 8, 1271. (6) (a) Wang, J. ACS Nano 2009, 3, 4. (b) Burdick, J.; Laocharoensuk, R.; Wheat, P. M.; Posner, J. D.; Wang, J. J. Am. Chem. Soc. 2008, 130, 8164. (7) Mirkovic, T.; Zacharia, N. S.; Scholes, G. D.; Ozin, G. A. Small 2010, 2, 159. (8) (a) Natansohn, A.; Rochon, P. Chem. Rev. 2002, 102, 4139. (b) Yager, K. G.; Barrett, C. J. J. Photochem. Photobiol., A 2006, 182, 250. (9) Ercole, F.; Davisa, T. P.; Evans, R. A. Polym. Chem. 2010, 1, 37. 7971
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