NANO LETTERS
Surface Dispersion and Hardening of Self-Assembled Diacetylene Nanotubes
2005 Vol. 5, No. 11 2202-2206
Sang Beom Lee,† Richard R. Koepsel,‡ and Alan J. Russell*,§ Department of Bioengineering, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260, Department of Chemical and Petroleum Engineering, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260, and Department of Surgery, McGowan Institute for RegeneratiVe Medicine, Suite 200, 100 Technology DriVe, Pittsburgh, PennsylVania 15219 Received July 15, 2005; Revised Manuscript Received September 28, 2005
ABSTRACT We describe here the first method for dispersion of individual self-assembled diacetylene nanotubes on surfaces. Complete polymerization by UV exposure was achieved as demonstrated by nanotubes that were resistant to aggressive organic solvents and temperatures well above the melting point of the monomer. The polymerized tubes displayed reversible thermochromic and mechanochromic properties.
It is well established that lipid diacetylenes can be deposited on surfaces as Langmuir-Blodgett (LB) films.1,2 The deposited films can be produced as mono- or multilayer structures that can undergo subsequent polymerization. Such films have been proposed as sensors for viruses,3 glucose,4 and other biological entities5,6 due to their unique blue to red color transition upon perturbation of the surface during a binding event. The utility of this type of sensor is limited because of the necessity of forming them on a surface and their fragile nature.1 A more robust sensor could be produced from diacetylene nanostructures, such as the liposomes, multilayered laminates, and nanotubes.7-9 A limitation to these types of structures is that their formation most often results in a mixture of structures that have strong affinity for each other.10 Because they tend to stick together, polydiacetylene micro- and nanostructures are not easily dispersed across surfaces. In this paper we show that homogeneous preparations of diacetylene nanotubes can be produced and then dispersed evenly on surfaces. Once the surface is coated, the nanotubes can be polymerized to yield materials with interesting properties. We have previously described the remarkable ability of an achiral lipid diacetylene with a secondary amine salt headgroup to form absolutely uniform self-assembled nanotubes.11 This diacetylene, which we now designate NFM-1 (Figure 1), is the first member of a new class of nanotubeforming monomers. Although the uniformity of the nanotube formation is unprecedented, almost all potential applications
Figure 1. Polymerization of self-assembled diacetylene nanotubes.
* Corresponding author. E-mail:
[email protected]. † Department of Bioengineering, University of Pittsburgh. ‡ Department of Chemical and Petroleum Engineering, University of Pittsburgh. § Department of Surgery, McGowan Institute for Regenerative Medicine.
depend on the structure being durable under harsh conditions, something not normally associated with self-assembled nanotubes. Diacetylenes, when perfectly aligned, can be polymerized with ultraviolet (UV) light. Polymerization of
10.1021/nl0513582 CCC: $30.25 Published on Web 10/14/2005
© 2005 American Chemical Society
diacetylenes has the additional effect of turning the colorless monomer into a dark blue polymer. When the backbone of the polymerized structure is stressed in some way, there can be a dramatic color change from dark blue to bright red. Under many circumstances the color change is reversible upon removal of the stressor. Unfortunately, until now, diacetylene-based nanostructures have not been polymerizable because the alignment of the monomeric groups within highly disperse nanostructures is imperfect. The colorimetric sensitivity of polymerized diacetylenes makes them well suited to serve as the synthetic foundation for a broad array of biosensors.3,4,12,13 Polydiacetylenes are conjugated polymers produced by 1,4-addition via UV or γ-ray initiated free radical polymerization of lipid diacetylene monomers. The polymerization reaction is initiated through the induced conversion of a monomer into a diradical.14,15 Propagation proceeds via radical-radical couplings between the diradicals to produce dicarbenes resulting in more stable forms such as oligomers and higher polymers. Extended exposure to UV light causes an additional reaction that apparently reduces the stability of the polymer chains. The nature of this degradation reaction is currently under investigation.16 Nano- and microtubules of lipidic diacetylenes have been seen previously.17-21 These structures have several distinct differences from the nanotubes described herein and are composed of starting compounds which are either two-chain lipids, chiral, or both. The resulting nanostructures have distinct helical character and form populations of related but nonidentical structures. It has been recently proposed that many of these structures form from collapsed or flattened tubules resulting in this diverse population.22 These heterogeneous populations of nanostructures are not polymerizable. The nanotubes we have described can be prepared from NFM-1 as a monodisperse population. These tubes are composed of five bilayers and are seamless when observed under the SEM.11 Unlike the polymerization of monolayer LB film systems, the multilayered nature of the structure adds some complexity to an attempted polymerization reaction because UV light may not penetrate the multiwall structure evenly. In addition, as with many systems for production of nanotubes, aggregation can complicate polymerization.23,24 Therefore, the complete polymerization of diacetylene nanotubes requires a well-controlled polymerization method. Herein we demonstrate that one way to overcome these problems and to minimize polymerization time is by dispersing individual nanotubes (NTs) on surfaces and then exposing the dried NTs to UV light. Furthermore, we observed remarkable reversible thermochromic and mechanochromic properties of the resultant polymerized nanotubes and nanotube-embedded polyurethane elastomer composites. Dried NFM-1 (10 mg) was dissolved in 1 mL of dichloromethane in a 20 mL vial by heating, and 2 mL of hexane was added to the dichloromethane solution. The solution was rotary evaporated at 40 °C to yield a white powdered preparation of pure prepolymerized nanotubes which was then used for the polymerization experiments described below. Nano Lett., Vol. 5, No. 11, 2005
Figure 2. (A) Nanotubes on glass slides: (left) prepolymerized nanotubes; (middle) polymerized nanotubes; (right) plain glass slide. (B and C) SEMs of nanotubes deposited on slides by dipping followed by sonication; (D and E) SEMs of nanotubes deposited by dipping without sonication. Nanotube stability as a function of polymerization time: (F) chloroform treatment of unpolymerized nanotubes; (G) chloroform treatment after 3 min of polymerization; (H) chloroform treatment after 5 min of polymerization. Scale bar, 1 µm.
First, for solution polymerization, a nanotube solution (10 mg of nanotubes in 100 mL of either hexane or water) was placed in a beaker (80 mm in diameter and 40 mm in height). The beaker was placed in the UV cross-linker and agitated with a mechanical stirrer. At 1 min intervals, a 1.2 mL aliquot was removed from the beaker and the absorbance spectrum was measured on the UV spectrophotometer. The loss of solvent during the polymerization was compensated for by adding fresh hexane or water. Dispersions of NT precursor in hexane or water were exposed to 254 nm light under constant stirring in glass containers. The extent of polymerization was followed by measuring the increase in optical density at 625 nm. As has been shown previously,25 there are counteracting effects of UV light on the polymerization in that extended exposure causes a breakage and rearrangement of the cross-linking bonds. This reaction can be measured by an increase of absorption at 525 nm at the expense of that at 625 nm.25 The OD625 reaches a maximum and then decreases, whereas the peak at 525 nm begins to appear after a few minutes and continues to increase. Polymerization of NTs in solution reaches a peak after approximately 10 min of exposure to UV, after which the disruptive reaction begins to dominate. 2203
Figure 3. (A) DSC scans of prepolymerized, polymerized nanotubes, and precursor lipid. Heat resistance of unpolymerized (B) and polymerized (C) nanotubes. Scale bar, 1 µm.
Additionally, observation of the suspension showed that there was a noticeable clumping of the NTs during polymerization. The integrity of solution-polymerized NTs was assessed via exposure to chloroform. The solvent melted these sparsely polymerized NTs, confirming that polymerization was incomplete and unable to dramatically harden these remarkable structures. To overcome the limitations of aggregationinhibited polymerization in solution, we sought to understand whether NTs that were uniformly coated onto surfaces could be efficiently polymerized. Naturally, the first step was to design a generic surface coating technique that would deposit dry thin films of the nanotube on any given surface. For polymerization on surfaces, hexane (20 mL) was added to a vial containing dried diacetylene nanotubes (10 mg) and the vial was sonicated in a sonic bath at room temperature for 5 min to detach the NTs from inside of the vial. Next, the hexane solution was transferred to a 250 mL glass bottle and fresh hexane (230 mL) was added followed by another 5 min of sonication at room temperature to disperse the nanotubes. A glass slide (25 mm × 75 mm) was placed in the nanotube solution and sonicated for 5 min to coat the glass slide with nanotubes. After the sonication was complete, the glass slide was again transferred to a 250 mL glass bottle containing fresh hexane (230 mL). The slide was rinsed for 5 min and the dried overnight in a vacuum oven at room temperature. Glass slides (25 mm × 75 mm) coated with nanotubes, as described above, were placed in a UV cross-linker (Spectrolinker XL-1000 Spectronics Corp., Westbury, NY). One 2204
side of the slide was wiped clean with tissue paper soaked with methanol so that only one side of the slide had nanotubes on the surface. The nanotube-containing surface faced a 6100 µW/cm2 low-pressure mercury lamp, with maximum emission at 254 nm, positioned 15 cm above the glass slide. At 1 min intervals, the absorbance spectrum of the nanotubes was measured on a Perkin-Elmer spectrophotometer (model Lambda 45). Our experience has shown that the NTs formed by NMF-1 will aggregate in solution complicating the process of polymerization. To reduce the aggregation of the NTs, we have employed principles of layer-by-layer coating techniques to generate conditions that allow dispersal of individual NTs on glass slides.26 The method is based on the strong ionic interaction between positively charged coating materials and negatively charged substrates and is widely used for coating ionic polymers on silica surfaces.27 Here, glass slides are thoroughly washed with a 1:2 solution of hydrogen peroxide and sulfuric acid (piranha solution) to introduce a negatively charged oxide layer. Prepolymerized NTs dispersed in hexane are then coated onto the glass slides by placing the slide into the hexane solution containing the NT precursor. Gentle sonication of the slides in the hexane solution was used to further disperse the NTs, resulting in evenly distributed NTs on the slides (herein we describe results with glass slides cleaned with piranha, but the technique is more broadly applicable). We found that simply dipping surfaces into hexane solutions of NFM-1 followed by brief sonication and a pure hexane rinse resulted in wellNano Lett., Vol. 5, No. 11, 2005
Figure 4. Thermo- and mechanochromisms of PNTs. Thermochromism of PNTs. PNTs: (A) dried on a glass slide; (B) in polyurethane. Mechanochromism of PNTs embedded in polyurethane elastomer: (C) UV absorption before and after elongation; (D) reversible on-off behavior of the nanotubes embedded in polyurethane elastomer.
dispersed nonpolymerized nanotubes coating the surface (Figure 2A). The tendency of the NTs to clump is still evident during dip coating without sonication (panels D and E of Figure 2) but is significantly reduced when accompanied by sonication (panels B and C of Figure 2). Glass slides (25 mm × 75 mm) coated with nanotubes were placed in a UV cross-linker, which delivers 254 nm light from 15 cm above the sample. At 1 min intervals, the absorbance spectrum of the nanotubes was measured with a spectrophotometer. Polymerization of the sonicated and well-dispersed nanotubes was achieved without causing loss of the tubular nanostructure and was completed in 5 min, which corresponds to the typical polymerization time of LB films.4 The polymerization of the tubes shows a time-dependent increase in the resistance of the polymerized nanotubes (PNTs) to disruption by chloroform (Figure 2F-H). Prepolymerized nanotubes are completely dissolved by chloroform (Figure 2F). After 3 min the NTs have obtained a small level of resistance to chloroform (Figure 2G), and at 5 min of UV exposure the PNTs are fully chloroform resistant (Figure 2A (middle) and Figure 2H). Another measure of the vitality of the hardening process is to determine the thermal stability of PNTs. Using differential scanning calorimetry28 (Figure 3), we measured the melting temperature of prepolymerized NTs (108.9 °C). By comparison, the diacetylene parent compound to NFM-1 has a melting point of 63.6 °C and the secondary amine, which is the immediate precursor to NFM-1, has a melting Nano Lett., Vol. 5, No. 11, 2005
point of 59.4 °C. The PNTs do not show a melting temperature and resist a change in structure upon heating. The peak at 79.5 °C in the differential scanning calorimetry (DSC) of prepolymerized NT is due to intermolecular hydrogen bonding.29 The fact that this peak disappears when the nanotube is polymerized is likely because lipid molecules in a tightly packed diacetylene tube do not break the intermolecular hydrogen bonding upon a temperature increase. We further observed the thermal stability of PNTs via scanning electron microscopy before and after heat exposure. Polymerization hardens the tubes to such a degree that exposure to 140 °C does not change the structure (Figure 3C), whereas the prepolymerized tubes after exposure to this same temperature disassociate into an amorphous mass (Figure 3B). PNTs are intensely colored, typically a deep blue, as seen in Figure 4A (top). We analyzed the thermochromism of the isolated PNTs and PNT-coated glass to determine whether they would be responsive to their environment. The bluecolored PNTs showed a remarkable thermochromism on a glass slide. When the PNTs are heated above 44 °C, they immediately transition to a bright red color, returning to blue upon cooling. The thermochromic behavior is so stable that one can cycle the temperature between 20 and 60 °C without any loss in the speed or intensity of the color change. We presume that the reversibility6 is enhanced by the intra- and intermolecular interaction of four possible hydrogen bonding sites (two from the amide bond and one each from the hydrogen and the bromine in the headgroup). 2205
Since we have now hardened PNTs and observed that they do indeed respond to changes in their environment, we sought to embed them in a common polymer (polyurethane) and determine whether they retained their structure and properties, thereby imparting multifunctionality to a previously nonresponsive material. Polyurethane elastomer films were prepared from poly(tetramethylene glycol) and 1,4-hexamethylene diisocyanate with 1,4-butane diamine as a chain extender. Polyurethane (0.3 g) was dissolved in 1,4-dioxane (10 mL), and the polymer solution was added to powdered, dried polymerized nanotubes (100 mg) in a 100 mL beaker. The polymer-nanotube solution was sonicated for 5 min at 25 °C in a sonic water bath, poured onto aluminum foil (2.5 cm × 2.5 cm), and allowed to dry for 24 h at 30 °C to obtain a smooth surfaced polymer film approximately 0.5 mm thick. Thermochromism was conducted on a heated stage equipped with a thermocouple and temperature controller (Warner Instruments, Hamden, CT). Polymer films were placed on the stage, and temperature was cycled between 20 and 50 °C. Figure 4 shows the thermochroism of the nanotubes embedded in polyurethane (Figure 4B). The blue to red reversible thermochromic transitions of the nanotubes embedded in polyurethane were clearly evident in the polymer matrixes. Temperature is not the only means by which we can induce a shift in the color of PNT-containing elastomers. We were interested in whether physical manipulation and stretching would change the color of these combination materials. We placed the PNT-containing polyurethane elastomer in a tensile stretcher and repeatedly stretched and then relaxed the sample. Polymer films (2.0 cm × 1.0 cm) were set into a custom-designed sample stretcher. Both ends of the sample were fixed in holders, and the sample was strained by moving the one of the holders by careful rotation of the screw. The sample was placed under tension at a constant strain, at which time the absorbance measurements were made. The nanotubes in polyurethane underwent a remarkable mechanochromism visually as well as spectroscopically when tensile stress is cycled between 0 and 68% elongation (4.3 MPa strain). The change in the ratio of OD625 to OD545 was recorded spectrophotometrically. The data show that these elastomers are mechanochromic and the process is completely reversible (we performed the blue to red stretching cycle over 100 times without any measurable loss in mechanochromic properties (Figure 4D)). In conclusion, we have shown that the dispersion of selfassembled diacetylene nanotubes on glass surfaces allows facile polymerization by UV exposure. The polymerization resulted in nanotubes that were resistant to chloroform and displayed reversible thermochromic and mechanochromic properties. These results allow a broad range of applications including a new foundation for surface coating with nanomaterials, polymer-nanomaterial composites, biosensors for self-decontaminating materials, artificial retinas, neuronal networks, and nanoelectronics. Acknowledgment. This work was supported by the DoD Multidisciplinary University Research Initiative (MURI) (DAAD19-01-1-0619) program administered by the Army Research Office. We also acknowledge Sara Wargo for the 2206
skilled preparation of the polyurethane. The authors have equity in a company, NanoSembly, LLC, that is commercializing the subject technology of this paper. References (1) Langmuir-Blodgett Films; Roberts, G., Ed.; Oxford University Press: Oxford, U.K., 1990. (2) Huo, Q.; Russell, K. C.; Ringsdorf, H. Langmuir 1999, 15, 39723980. (3) Charych, D. H.; Nagy, J. O.; Spevak, W.; Bednarski, M. D. Science 1993, 261, 585-588. (4) Cheng, Q.; Stevens, R. C. AdV. Mater 1997, 9, 481-483. (5) Morigaki, K.; Baumgart, T.; Jonas, U.; Offenhausser, A.; Knoll, W. Langmuir 2002, 18, 4082-4089. (6) Ahn, D. J.; Chae, E.-H.; Lee, G. Su.; Shim, H.-Y.; Chang, T.-E.; Ahn, K.-D.; Kim, J.-M. J. Am. Chem. Soc. 2003, 125, 8976-8977. (7) Kim, J.-M.; Ji, E.-K.; Woo, S. M.; Lee, H.; Ahn, D. J. AdV. Mater. 2003, 15, 1118-1121. (8) Ma, Z.; Li, J.; Liu, M.; Cao, J.; Zou, Z.; Tu, J.; Jiang, L. J. Am. Chem. Soc. 1998, 120, 12678-12679. (9) Rangin, M.; Basu, A. J. Am. Chem. Soc. 2004, 126, 5038-5039. (10) Shimizu, T.; Masuda, M.; Minamikawa, H. Chem. ReV. 2005, 105, 1401-1443. (11) Lee, S. B.; Koepsel, R.; Stolz, D. B.; Warriner, H. E.; Russell, A. J. J. Am. Chem. Soc. 2004, 126, 13400-13405. (12) Okada, S.; Peng, S.; Spevak, W.; Charych, D. Acc. Chem. Res. 1998, 31, 229-239. (13) Carpick, R. W.; Sasaki, D. Y.; Marcus, M. S.; Eriksson, M. A.; Burns, A. R. J. Phys.: Condens. Matter 2004, 16, R679-R697. (14) Patel, G. N.; Chance, R. R.; Turi, E. A.; Khanna, Y. P. J. Am. Chem. Soc. 1978, 100, 6644-6649. (15) Bloor, D.; Chance, R. R. Polydiacetylenes; Martinus Nijhoff Publishers: Boston, MA, 1985. (16) Bloor, D.; Worboys, M. R. J. Mater. Chem. 1998, 8, 903-912. (17) Yager, P.; Schoen, P. E. Mol. Cryst. Liq. Cryst. 1984, 106, 371381. (18) Schnur, J. M. Science 1993, 262, 1669-1676. (19) Thomas, B. N.; Safinya, C. R.; Plano, R. J.; Clark, N. A. Science 1995, 267, 1635-1638. (20) Oda, R.; Huc, I.; Schmutz, M.; Candau, S. J.; Mackintosh, F. C. Nature 1999, 399, 566-569. (21) Lu, Y.; Yang, Y.; Sellinger, A.; Lu, M.; Huang, J.; Fan, H.; Haddad, R.; Lopez, G.; Burns, A.; Sasaki, D. Y.; Shelnutt, J.; Brinker, C. J. Nature 2001, 410, 913-917. (22) Mishra, B. K.; Garrett, C. C.; Thomas, B. N. J. Am. Chem. Soc. 2005, 127, 4254-4259. (23) Bandyopadhyaya, R.; Nativ-Roth, E.; Regev, O.; Yerushalmi-Rozen, R. Nano Lett. 2002, 2, 25-28. (24) Landi, B. J.; Ruf, H. J.; Worman, J. J.; Raffaelle, R. P. J. Phys. Chem. B 2004, 108, 17089-17095. (25) Alekseev, A. S.; Viitala, T.; Domnin, I. N.; Koshkina, I. M.; Nikitenko, A. A.; Peltonen, J. Langmuir 2000, 16, 3337-3344. (26) Jisr, R. M.; Rmaile, H. H.; Schlenoff, J. B. Angew. Chem., Int. Ed. 2005, 44, 782-785. (27) Schmitt, J.; Grunewald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; Losche, M. Macromolecules 1993, 26, 7058-7063. (28) Thermal stability was analyzed using a differential scanning calorimeter (DSC). NTs polymerized for various times and prepolymerized nanotubes (1.5 mg) were dispersed in hexane, placed in an aluminum cell for the DSC, and allowed to dry in a vacuum oven for 24 h at 25 °C. The DSC cell was placed on a differential scanning calorimeter (Shimadzu; DSC 60) under a helium purge. Scanning rates of 10 °C/min were used over a temperature range of 25-140 °C. To determine the stability of the nanotubes at high temperature, the prepolymerized and polymerized nanotubes were placed on a hot stage (150 °C) for 10 s and then sputter coated with a 3.5 nm coating of gold/palladium (Cressington Auto 108, Cressington, Watford, U.K.). Samples were viewed in a JEOL JEM-6335F scanning electron microscope (JEOL, Peabody, MA) at 10 kV. For determination of solvent stability, chloroform was added to the prepolymerized and polymerized nanotubes on glass slides prior to scanning electron microscopy. (29) Huggins, H. E.; Son, S.; Stupp, S. I. Macromolecules 1997, 30, 5305-5312.
NL0513582 Nano Lett., Vol. 5, No. 11, 2005
