pubs.acs.org/Langmuir © 2010 American Chemical Society
Size-Controlled Fabrication of Supramolecular Vesicles for the Construction of Conjugated Polymer Sensors with Enhanced Optical Properties Gangjune Kim,† Simon Song,*,† Jung Lee,‡ and Jong-Man Kim*,‡ †
Department of Mechanical Engineering and ‡Department of Chemical Engineering, Hanyang University, Seoul 133-791, Korea Received September 30, 2010. Revised Manuscript Received November 1, 2010
Polymerizable supramolecular monomer vesicles are readily fabricated by employing a hydrodynamic focusing method on a microfluidic chip. The polymerized diacetylenene nanovesicles, generated using the microfluidic method, display an improved fluorescence property compared to those prepared by employing a conventional bulk method. The flexibility of the vesicle size control by manipulating the flow conditions is another significant feature of the new microfluidic approach.
1. Introduction Owing to their unique structural and optical properties, conjugated polydiacetylenes (PDAs) have received great attention in the fields of sensor technology and optoelectronics.1-4 Especially important is the brilliant blue-to-red color transition that takes place in PDAs upon environmental perturbation. This phenomenon has been extensively investigated as a sensory signal for the
Scheme 1. Schematic Representation of a Polydiacetylene (PDA) Supramolecule Formed from the Polymerization of Self-Assembled Diacetylenes Derived from 10,12-Pentacosadiynoic Acid (PCDA)
*Corresponding authors. E-mail:
[email protected]; jmk@ hanyang.ac.kr. (1) Wegner, G. Z. Naturforsch. 1969, 24B, 824. (2) Reviews on polydiacetylenes: (a) Yoon, B.; Lee, S.; Kim, J.-M. Chem. Soc. Rev. 2009, 38, 1958. (b) Lauher, J. W.; Fowler, F. W.; Goroff, N. S. Acc. Chem. Res. 2008, 41, 1215. (c) Reppy, M. A.; Pindzola, B. A. Chem. Commun. 2007, 4317. (d) Zhou, W.; Li, Y.; Zhu, D. Chem. Asian J. 2007, 2, 222. (e) Carpick, R. W.; Sasaki, D. Y.; Marcus, M. S.; Eriksson, M. A.; Burns, A. R. J. Phys.: Condens. Matter 2004, 16, R679. (f) Mueller, A.; O'Brien, D. F. Chem. Rev. 2002, 102, 727. (g) Jelinek, R. R.; Kolusheva, S. Biotechnol. Adv. 2001, 19, 109. (h) Okada, S.; Peng, S.; Spevak, W.; Charych, D. Acc. Chem. Res. 1998, 31, 229. (i) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Int. Ed. Engl. 1988, 27, 113. (3) Dei, S.; Matsumoto, A.; Matsumoto, A. Macromolecules 2008, 41, 2467. (4) Wu, S.; Niu, L.; Shen, J.; Zhang, Q.; Bubeck, C. Macromolecules 2009, 42, 362. (5) Charych, D. H; Nagy, J. O.; Spevak, W.; Bednarski, M. D. Science 1993, 261, 585. (6) Kolusheva, S.; Boyer, L.; Jelinek, R. Nat. Biotechnol. 2000, 18, 225. (7) Wang, C.; Ma, Z. Anal. Bioanal. Chem. 2005, 382, 1708. (8) Yoon, J.; Chae, S. K.; Kim, J.-M. J. Am. Chem. Soc. 2007, 129, 3038. (9) Gill, I.; Ballesteros, A. Angew. Chem., Int. Ed. 2003, 42, 3264. (10) Chen, X.; Kang, S.; Kim, M. J.; Kim, J.; Kim, Y. S.; Kim, H.; Chi, B.; Kim, S.-J.; Lee, J. Y.; Yoon, J. Angew. Chem., Int. Ed. 2010, 49, 1422. (11) Nie, Q.; Zhang, Y.; Zhang, J.; Zhang, M. J. Mater. Chem. 2006, 16, 546. (12) Ma, G.; Cheng, Q. Langmuir 2005, 21, 6124. (13) Song, J.; Cisar, J. S.; Bertozzi, C. R. J. Am. Chem. Soc. 2004, 126, 8459. (14) Chance, R. R. Macromolecules 1980, 13, 396. (15) Carpick, R. W.; Sasaki, D. Y.; Burns, A. R. Langmuir 2000, 16, 1270. (16) Kolusheva, S.; Molt, O.; Herm, M.; Schrader, T.; Jelinek, R. J. Am. Chem. Soc. 2005, 121, 10000. (17) Li, X.; McCarroll, M.; Kohli, P. Langmuir 2006, 22, 8615. (18) Kim, J.-M.; Lee, Y. B.; Yang, D. H.; Lee, J.-S.; Lee, G. S.; Ahn, D. J. J. Am. Chem. Soc. 2005, 127, 17580. (19) Lee, J.; Kim, H.-J.; Kim, J. J. Am. Chem. Soc. 2008, 130, 5010. (20) Ahn, D. J.; Kim, J.-M. Acc. Chem. Res. 2008, 41, 805. (21) Eo, S.-H.; Song, S.; Yoon, B.; Kim, J.-M. Adv. Mater. 2008, 20, 1690. (22) (a) Song, Y.; Hormes, J.; Kumar, C. S. S. R. Small 2008, 4, 698. (b) Tan, Y.-C.; Hettiarachchi, K.; Siu, M.; Pan, Y.-R.; Lee, A. P. J. Am. Chem. Soc. 2006, 128, 5656. (c) Jahn, A.; Vreeland, W. N.; Gaitan, M.; Locascio, L. E. J. Am. Chem. Soc. 2004, 126, 2674. (d) Zhang, H.; Tumarkin, E.; Peerani, R.; Nie, Z.; Sullan, R. M. A.; Walker, G. C.; Kumacheva, E. J. Am. Chem. Soc. 2006, 128, 12205. (e) Kim, J.-W.; Utada, A. S.; Fernandez-Nieves, A.; Hu, Z.; Weitz, D. A. Angew. Chem., Int. Ed. 2007, 46, 1819. (f) Khan, S. A.; Gunter, A.; Schmidt, M. A.; Jensen, K. F. Langmuir 2004, 20, 8604. (g) Eun, T. H.; Kim, S.-H.; Jeong, W.-J.; Jeon, S.-J.; Kim, S.-H.; Yang, S.-M. Chem. Mater. 2009, 21, 201. (h) Karnik, R.; Gu, F.; Basto, P.; Cannizzaro, C.; Dean, L.; Kyei-Manu, W.; Langer, R.; Farokhzad, O. C. Nano Lett. 2008, 8, 2906.
17840 DOI: 10.1021/la103920p
detection of diagnostically, biologically, environmentally, and chemically important analytes.5-13 More recently, the stimulusinduced fluorogenic changes that take place in PDAs14,15 have begun to receive substantial attention as an alternative output signal.16-22 In addition, the fluorogenic property of PDA allows for the fabrication of microarrayed18-20and microfluidic21 PDA sensor systems, which are almost impossible to obtain by using the conventional colorimetric method. For fluorescence-based PDA sensor systems to function effectively, they must consist of PDA supramolecules that are prepared in a quality-controlled manner (vide infra). PDA vesicles can be generated by injecting an organic solution containing diacetylene (DA) monomers into large excess of water, followed by sonication and stabilization. This leads to the spontaneous formation of DA vesicles, and UV polymerization of the DA vesicles yields PDA supramolecules (Scheme 1). This bulk method inevitably generates polydisperse PDA vesicles with size distributions that vary from one batch to another. High polydispersities may limit the quality of sensor systems because we found that PDA vesicles prepared by the bulk method show inconsistent fluorescence intensities in spite of being subjected to the same stress, although the
Published on Web 11/08/2010
Langmuir 2010, 26(23), 17840–17842
Kim et al.
Letter
fluorescence of PDA vesicles taken from the same batch varies little. Thus, the development of a reliable method for the generation of uniformly distributed PDA vesicles would be very desirable. Recently, size-controlled methods for the fabrication of nanoparticles and microparticles using microfluidic devices have been actively studied.22 As a result, a variety of nanosized and microsized functional materials, including lipid vesicles,22b,c biopolymer microcapsules,22d microgels,22e colloidal silicas,22f inorganic hollow spheres,22g and block copolymer nanoparticles,22h have been efficiently prepared using various microfluidic chip devices. Below, we describe a new a microfluidic method that can be employed in the preparation of size-controlled PDA vesicles with narrow size distributions.
2. Experimental Method
Figure 1. Schematic illustration of the microfluidic chip used for the preparation of diacetylene vesicles. The inset shows an optical microscopy image at the junction of the inlet and main channels.
2.1. Chip Fabrication. A standard soft lithography and molding technique was used to fabricate a polydimethylsiloxane (PDMS) microfluidic chip. A standard lithography procedure for making a mold for a microchannel pattern includes wafer cleaning, photoresist (PR) coating, soft baking, UV exposure, hard baking, PR development, washing, and drying. For PDMS molding, PDMS (DC-184A, Dow Corning) is thoroughly mixed with a curing agent (DC-184B, Dow Corning) in a 10:1 ratio by volume. After the PDMS mixture is poured onto the wafer mold, the mixture is degassed in a vacuum chamber and cured in an oven at 65 °C for 2 h. A microchannel-patterned PDMS substrate is obtained by peeling it off of the wafer. Chip fabrication is completed by bonding the patterned PDMS substrate on a slide glass, after treating the PDMS with UV ozone. The microfluidic chip has one main channel, one outlet channel, and three inlet channels: one for sample flow and the others for sheath flow. The cross section of the main channel is 100 μm in height and 50 μm in width. 2.2. Preparation of Polydiacetylene (PDA) Vesicles. 2.2.a. Bulk. A diacetylene monomer, 10,12-pentacosadiynoic acid (PCDA) (GFS Chemicals), is dissolved in dimethyl sulfoxide (DMSO) (11 mg of PCDA in 200 μL of DMSO). The monomer solution is added dropwise to deionized water (80 °C) with gentle shaking to yield a total lipid concentration of 1 mM. The resulting suspension is probe sonicated for 30 min and cooled to 4 °C for 4 h. Polymerization is carried out at room temperature by exposing the suspension to 254 nm UV light (1 mW/cm2). Scanning electron microscopy (SEM) and dynamic light scattering (DLS) analyses of the vesicles were carried out with a model S-4800 (Hitachi Ltd.) and a Nano ZS (Malvern Instruments Ltd.), respectively. 2.2.b. On Chip. A DMSO solution (1 mM) of PCDA and deionized water were injected into the sample and two sheath channels, respectively. Typical flow rates were 0.1 mL/h in the sample channel and 0.3 mL/h in each sheath channel. The formed PCDA vesicles were collected and kept in a refrigerator for 4 h. Polymerization was carried out at room temperature by exposing the suspension to 254 nm UV light (1 mW/cm2). 2.3. Fluorescence Monitoring. To a suspension (0.1 mM, 4 mL) of PDA vesicles, prepared using a bulk method, was added 25 mg of R-cyclodextrin (R-CD) powder, and the resulting suspension was injected into a straight microchannel (50 μm (height)50 μm (width)). The fluorescence of the vesicles in the main channel was monitored using a fluorescence microscope system (IX71, Olympus) with an excitation wavelength of approximately 530 nm. A suspension (0.2 mL) of PDA vesicles prepared from a hydrodynamic focusing method was exposed to R-CD (1 mg) prior to injection into the microchannel for the fluorescence measurement.
3. Results and Discussion In Figure 1 is shown a schematic of the experimental setup for the microfluidic method used to produce size-controlled PDA vesicles. A fabricated microfluidic chip, having three inlets and Langmuir 2010, 26(23), 17840–17842
Figure 2. SEM images of PDA vesicles generated in the bulk (A) and on a microfluidic chip (B). Comparison of size distributions of PCDA vesicles prepared in the bulk (blue) and on a microfluidic chip (red) (C). The sample and a sheath flow rates used for B and C are 0.1 and 0.3 mL/h, respectively. Mean diameter as a function of the sheath to sample flow rate ratio (D). SEM images of PDA vesicles produced on a chip with different flow rate ratios: (E) 3 and (F) 15.
one outlet with cross sections of 100 μm in height and 50 μm in width, is employed in this procedure. A dimethylsulfoxide (DMSO) solution (1 mM) of 10,12-pentacosadiynoic acid (PCDA) and deionized water were injected into the sample and two sheath channels, respectively. The formed PCDA vesicles were collected, refrigerated for 4 h, and photopolymerized using a hand-held 254 nm UV lamp (1 mW/cm2). For comparison, PCDA vesicles were also prepared by using a conventional bulk method. In Figure 2 are displayed SEM images and DLS data of PCDA vesicles generated from a conventional bulk method and the new microfluidic method. A mixture of small nanoparticles and large aggregates can be clearly observed in the SEM image of polymerized PCDA vesicles, which are generated by employing a bulk method (Figure 2A). In contrast, the microfluidic approach leads to the clean formation of PDA vesicles that have a narrow size distribution and no large aggregates (Figure 2B). DLS analysis DOI: 10.1021/la103920p
17841
Letter
Figure 3. Fluorescence intensity profiles measured across the microchannel of R-CD-exposed PDA samples obtained in the bulk (A) and on a microfluidic chip (B). Each PDA sample was prepared from six different batches (A) or microfluidic chips (B). Insets show the representative fluorescence microscopy images monitored across microchannels.
confirms this observation. The mean and standard deviations of the diameters of the PCDA vesicles produced by using the bulk method are 88 and 31 nm, and those of vesicles prepared with the microfluidic method are 39 and 12 nm (Figure 2C). The uniform size distribution of the PDA vesicles fabricated in a microfluidic system is due to the steady and stable flow conditions present in the microfluidic chip. An additional significant feature of the on-chip fabrication method is that vesicle sizes can be controlled by precisely engineering the flow conditions. For instance, in Figure 2D the effects of the ratio of sheath to sample flow rates on the PCDA vesicle size with the sample flow rate fixed at 0.06 mL/h are shown. The mean diameter of the self-assembled PCDA vesicles is found to decrease when this ratio is increased. SEM images of polymerized PCDA vesicles demonstrate the ratio dependence of size distributions. PDA vesicles with an average diameter of 42 nm are produced when the flow rate ratio is 3 (Figure 2E), and smaller PDAs (ca. 16 nm) are generated when a higher ratio (15) is employed (Figure 2F). Note that the sample stream becomes narrower in the main channel when the ratio increases or, equivalently, when the sheath flow rate increases. Thus, the generation of smaller PDA vesicles at high flow ratios is presumably a consequence of the relatively rapid diffusive mixing of the sample and sheath flows. It should also be noted that the SEM images are obtained after the polymerization of the PCDA vesicles and the DLS data are collected with unpolymerized vesicle suspensions. This technique is used in part because the polymerized PCDA provides better stability during SEM monitoring and also because shrinkage of the PDA vesicles is inevitable during sample preparation.
17842 DOI: 10.1021/la103920p
Kim et al.
The optical properties of the PDA vesicles were determined. For this purpose, PDA vesicles prepared in bulk and on a chip were exposed to R-cyclodextrin (R-CD). It is well known that R-CD is capable of disrupting molecularly assembled PDAs derived from PCDA and inducing a blue-to-red color transition.23 The mixtures resulting from the treatment of the PDA vesicles with R-CD were introduced into a straight microchannel (50 μm 50 μm in width and height). Fluorescence intensities were monitored in the middle of the channel utilizing an excitation wavelength of about 530 nm, at which the red-phase PDAs absorb. In Figure 3, fluorescence intensity profiles and images monitored across the channel are displayed. It is clear from the Figure that significant variations in fluorescence intensity exist with R-CDtreated PDAs that are prepared using a bulk method (Figure 3A). In contrast, the variations are relatively small for those obtained with the hydrodynamic focusing method (Figure 3B). The large fluctuation in the fluorescence intensity of the former PDAs is presumably caused by the fact that each batch contains heterogeneously distributed PDA vesicles. In contrast, the latter PDAs emit more uniform fluorescence owing to their uniform size distribution.
4. Conclusions The investigation described above has led to the development of a new method for the size-controlled fabrication of polymerizable supramolecular vesicles. By using this technique, uniformly distributed diacetylene vesicles are readily generated with a micro fluidic chip as a result of the hydrodynamic focusing principle. Importantly, in this procedure vesicle size can be controlled by manipulating flow conditions. More importantly, the fluorescence intensities of R-CD-perturbed PDA vesicles are observed to be more consistent from one batch to another when the monomer vesicles are produced on a chip rather than by using a bulk method because of the more uniform size distribution of vesicles. This observation highlights the significance of the new microfluidic method. Acknowledgment. We gratefully thank the National Research Foundation of Korea (NRF) for financial support through the Basic Science Research Program (nos. 2009-0076414 and 20100018438), the Center for Next Generation Dye-Sensitized Solar Cells (20100001844), and the International Research & Development Program (K20901000006-09E0100-00610). (23) Kim, J.-M.; Lee, J.-S.; Lee, J.-S.; Woo, S.-Y.; Ahn, D. J. Macromol. Chem. Phys. 2005, 206, 2299.
Langmuir 2010, 26(23), 17840–17842
