Langmuir 2001, 17, 1807-1810
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A Simple Method To Produce Dendrimer Nanodots over Centimeter Scales by Rapid Evaporation of Solvents Masahito Sano,*,† Junko Okamura,† Atsushi Ikeda,‡ and Seiji Shinkai†,‡ Chemotransfiguration ProjectsJST, 2432 Aikawa, Kurume, Fukuoka 839-0861, Japan, and Department of Chemistry and Biochemistry, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan Received June 29, 2000. In Final Form: January 2, 2001 A simple method to produce aggregates of dendrimers in a form of nanometer-sized round dots over an area measured in centimeters is presented. The method is based on rapid evaporation of a solvent from a thin cast film of an electrolyte solution of polyamidoamine dendrimers on mica. Chemically specific adsorption, electrostatic interactions, phase transitions, and dewetting instabilities are shown not to be responsible for the dot formation. Changes in temperature and humidity have only little effect. There is a threshold evaporation rate above which the dots are formed. Once formed, the dot size and spacing are independent of the evaporation rate. The dot is also formed by different types of dendrimers and solutes, demonstrating the wide applicability of the present method.
In so-called “step-up” nanotechnology, one is interested in building up large-scale assemblies from molecular-scale objects. Due to a huge number of molecules involved in a large-scale assembly, the methods based on spontaneous self-organization are of particular interest. Many researchers have taken chemical approaches guided by supramolecular science and have been producing promising results.1 In this report, we are interested in a formation of self-organized patterns by physical, nonequilibrium processes. Those involving dissipation interactions, feedback loops, transport of matters, and hydrodynamic instabilities are some examples of processes that are known to induce ordered patterns.2,3 Here, a simple, yet complicated process of forced evaporation of solvents is applied on a system that is not self-organizing by the “natural” or left-undisturbed evaporation process. As a molecular-scale building component, we have chosen dendrimers. Recent advances in synthesis allow various functional groups to be implemented into dendrimer structures.4,5 High densities of functional groups and well-defined chemical structures make dendrimers an important element of nanotechnology. Despite these advantages, dendrimers can also display intrinsic properties which are unfavorable. Cascading branched structures cause an individual dendrimer to be compact, yet deformable. A high density of functional groups on its surface allows interfacial, rather than body, interactions6 to dominate. In fact, their aggregation states on solid surfaces are numerous, ranging from a dispersed single molecule to coalescent films.7-15 Thus, it is important to develop a †
Chemotransfiguration ProjectsJST. Department of Chemistry and Biochemistry, Kyushu University. ‡
(1) Lehn, J.-M. Angew. Chem., Int. Ed. 1990, 29, 1304. (2) Nicolis, G.; Prigogine, I. Self-Organization in Non-equilibrium Systems; Wiley: New York, 1977. (3) Cross, M. C.; Hohenberg, P. C. Rev. Mod. Phys. 1993, 65, 851. (4) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A., III Angew. Chem., Int. Ed. 1990, 29, 138-175. (5) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681-1712. (6) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic: New York, 1992. (7) Sheiko, S. S.; Eckert, G.; Ignat’eva, G.; Muzafarov, A. M.; Spickermann, J.; Ra¨der, H. J,; Mo¨ller, M. Macromol. Rapid Commun. 1996, 17, 283-297. (8) Sheiko, S. S.; Gauthier, M.; Mo¨ller, M. Macromolecules 1997, 30, 2343-2349.
method to control aggregation behaviors of dendrimers regardless of their chemical structures. A technical aspect of the present method is simply to blow a strong flow of air onto a cast solution of dendrimers on a flat substrate until dryness. Blowing of the strong air has two major consequences. The excess amount of liquid is blown off the surface, leaving only the strongly adsorbed layer. It also prevents the surrounding liquid from flowing back into the blown area. Thus, the strong air produces a very thin layer of dendrimer solution. Second, it causes the solvent to evaporate so fast that the solutes are not permitted to drift into a large aggregate. There has been a report of producing dendrimer nanodots by an application of dewetting instabilities.15 However, the present method involves a totally different mechanism on wetting systems, as inorganic salts are introduced to control wetting and electrostatic interactions of solutes. Amino-terminated generation 4 (G4) and carboxylateterminated generation 3.5 (G3.5) polyamidoamine (PAMAM) starbust dendrimers (Aldrich) were supplied as a 10 wt % methanol solution. This dendrimer solution was diluted with an electrolyte solution in water as indicated below. Inside of a thermostatically and hygrostatically controlled box, a small amount of the diluted solution was cast on the substrate, usually freshly cleaved mica (typically, 1 × 2 cm2). The solution wets the substrate surface completely, thus spreading over the entire surface area. Immediately after casting, the substrate surface was dried by blowing either air or N2 at a controlled flow rate until dryness. The different gases did not cause any difference in morphology. The dried surface was examined by atomic force microscopy (AFM, TopoMetrix) in air at 23 °C with relative humidity (RH) controlled to be less (9) Coen, M. C.; Lorenz, K.; Kressler, J.; Frey, H.; Mu¨lhaupt, R. Macromolecules 1996, 29, 8069-8076. (10) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171-2176. (11) Hierlemann, A.; Campbell, J. K.; Baker, L. A.; Crooks, R. M.; Ricco, A. J. J. Am. Chem. Soc. 1998, 120, 5323-5324. (12) Li, J.; Swanson, D. R.; Qin, D.; Brothers, H. M.; Piehler, L. T.; Tomalia, D.; Meier, D. J. Langmuir 1999, 15, 7347-7350. (13) Imae, T.; Funayama, K.; Aoi, K.; Tsutsumiuchi, K.; Okada, M.; Furusaka, M. Langmuir 1999, 15, 4076-4084. (14) Dı´az, D. J.; Storrier, G. D.; Bernhard, S.; Takada, K.; Abrun˜a, H. D. Langmuir 1999, 15, 7351-7354. (15) Hellmann, J.; Hamano, M.; Karthaus, O.; Ijiro, K.; Shimomura, M.; Irie, M. Jpn. J. Appl. Phys. 1998, 37, L816-L819.
10.1021/la000909v CCC: $20.00 © 2001 American Chemical Society Published on Web 02/15/2001
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Figure 1. AFM image of nanodots made by the rapid evaporation of a cast liquid containing 0.1 wt % G4 in 10 mM NaOH on mica. The average height, apparent diameter, and spacing in this image are 15, 70, and 200 nm, respectively.
than 40%. AFM was operated in the so-called tapping mode using a conical-shaped Si tip with a resonance frequency of around 200 kHz. The tip has a radius of 20 nm as specified by the manufacturer. The evaporation rate was measured by directly monitoring the weight loss of a pool of water over the same area as the sample surface by N2 blown with a known flow rate. Under our experimental condition, the “natural” evaporation rate without blowing N2 was 5 µg/(s‚cm2) at 23 °C. Figure 1 shows an AFM image of G4 on mica cast from 0.1 wt % solution in 10 mM NaOH, made at 23 °C, RH 38% with an evaporation rate of 57 µg/(s‚cm2). The solid
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aggregates of G4 and the inorganic salts form dispersed arrays of round dots. Typically, the dots have an average height of 15 nm. An average diameter without deconvolution of the AFM tip is about 70 nm. Although the dots are placed without notable orientational order, a power spectrum analysis indicates a radial density peak at 200 nm. There is no indication of the direction of blowing air affecting the orientation of the dots. The height, diameter, and spacing vary about 20% among independently prepared samples. These variations are definitely larger than usual errors we have seen on other experiments using different mica substrates. On a single sample surface, however, nearly the same morphology continues over the entire mica surface measured in centimeters. There is no material deposited in the spaces between the dots. We shall denote the condition to obtain the morphology shown in Figure 1 to be standard (STD). Below, we consider the formation mechanism by changing each casting parameter from STD. Control. Casting the salt solution without dendrimers produced salt crystals of various sizes and shapes randomly placed on the surface (Figure 2A). On the other hand, diluting the original dendrimer stock with methanol without salt resulted in a network of patches 2 nm high (Figure 2B). The observed height is considerably smaller than the expected diameter of G4, 4 nm. This flattening of dendrimers has been reported previously.10-13 pH and Salt. Addition of 10 mM NaCl to STD gives dispersed dots similar to those shown in Figure 1, but 20-30 nm high. Lowering the pH by casting from 10 mM HCl results in irregularly shaped flat patches less than 1 nm high. Adding 10 mM NaCl to this acid solution gives randomly placed microcrystals. These results indicate that positively charged G4 is strongly adsorbed on the anionic mica surface. The addition of salt coagulates G4 and forms salt crystals around the dendrimers. Functional Groups. Casting G3.5 from 10 mM HCl/ 10 mM NaCl yields dots identical to G4 in STD. Casting from pure water without salt produces irregularly shaped flat patches 1 nm high. Adding 10 mM NaCl to this solution
Figure 2. AFM images after rapid evaporation of (A) 10 mM NaOH without dendrimers and (B) 0.1 wt % G4 in methanol without salts. The color scales on heights are not equal in these images.
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Figure 3. The dendrimer concentration dependence of morphology. These are cast from (A) 0.5, (B) 0.002, and (C) 0.0004 wt % G4 in 10 mM NaOH, respectively.
affords dispersed dots. Thus, the dot formation is independent of chemical functional groups. Hydration. A small amount of KOH is known to induce a strong hydration force on the mica surface.16 Casting G4 from 10 mM KOH produced randomly placed, large aggregates. Thus, the repulsive interaction on mica is not responsible for the dispersion of the dots. Substrate. Casting STD solution on highly oriented pyrolytic graphite forms dots, although they are much wider than those on mica. Many dots are pinned at the graphite step edges. The initial liquid layer before blowing air is thicker than one on mica due to the presence of many step defects, causing larger dots to form. Also, the STD solution does not wet graphite as completely as mica. Nevertheless, dispersed dots are formed on graphite. Dendrimer Concentration. At 0.5 wt %, G4 forms circular patches 4 nm high that are connected by bridges (Figure 3A). At lower concentrations, patches are disconnected and smaller. The dispersed nanodots are formed between 0.15 and 0.001 wt %. Interestingly, lowering of the dendrimer concentration produces smaller dots separated by shorter spacing. For instance, 0.002 wt % gives an apparent diameter of 20 nm, separated by 100 nm (Figure 3B). Below 0.001 wt %, the morphology becomes a mixture of dots with pure salt crystals (Figure 3C). Temperature and Humidity. We could not detect any systematic variation of size and spacing of the dots, when films were cast at 9 °C (RH ) 30%), 23 °C (12%), and 30 °C (17%) within the experimental error. Low vapor pressure of water around room temperature does not influence the dot formation. This also eliminates a possible complication arising from lowering of temperature during evaporation. In contrast, increasing RH at each temperature, 9 °C (76%), 23 °C (68%), and 30 °C (57%), caused inhomogeneously sized dots to appear. Figure 4 summarizes apparent diameters measured directly from AFM images when RH is varied at 23 °C. Below 40%, the diameter distribution is well represented by a single Gaussian curve. At RH above 50%, on the other hand, both dots with smaller Gaussian distributed diameters and dots of various larger sizes are produced. We think that the larger dots are formed by a postevaporation process. At high RH, water condenses on the smaller dots that have been formed by forced evaporation. When the water content becomes large enough to soften some materials to flow, the dot spreads out due to complete wetting. Interestingly, if only the smaller dots that are (16) Pashley, R. M. J. Colloid Interface Sci. 1981, 83, 531.
Figure 4. A distribution of the dot diameters measured directly from the AFM images was fit with a Gaussian curve. The peak values are plotted against the relative humidity and denoted by filled circles. A straight line shows the best fit. Above 50%, there are many larger dots of various sizes and are indicated as vertical error bars.
assumed to be formed by forced evaporation are examined, the peak values over the entire range can be fit with a straight line with a positive slope of 1.3 nm/% RH. Since the slope is a relative increment, this value is not affected by the AFM tip convolution. Thus, humidity affects the final size, but its effect amounts to only 3 molecular widths for each 10% increase in RH. Evaporation Rate. The evaporation rate was varied from 5 to 433 µg/(s‚cm2) at 23 °C. Under the “natural” evaporation rate, the materials are not deposited evenly and fingering can be seen in some places. Figure 5 displays the film morphology seen at selected evaporation rates. At smaller rates, dots are not formed and patches of thin films connect globular aggregates. Then, there is a threshold value of 26 µg/(s‚cm2) or equivalently 260 nm/s of the layer thinning rate, which must be exceeded to form dots. This is about 5 times greater than the “natural” rate. Once formed, we could not detect significant systematic variation of size and spacing on the rate for the range examined. Since amino-terminated and carboxylate-terminated dendrimers give consistent results, the dot formation is not due to specific adsorption of dendrimers on the substrate surface. Electrostatic interactions, which are known to produce dispersed arrays of charged colloids on a surface,17 are also not responsible for the present case.
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Figure 5. AFM images (2.5 × 2.5 µm2) of the material deposited on mica after different rates of evaporation. One at 65 µg/(s‚cm2) is presented to show that there is no significant differences in size and spacing with the results of smaller rates as long as dots are formed.
Actually, the dots are formed preferably by neutral dendrimers or in high salt concentrations. The continuous change of morphology with decreasing dendrimer concentrations shows no indication of a phase transition. Dot formation by instabilities of dewetting liquids15 does not apply to the present wetting case. Very small dependence on temperature and humidity and an existence of the minimum evaporation rate signify that the dots are formed by dynamic processes. Because of rapid evaporation, only a limited time is available for the solute materials to aggregate into a dot. Since the dot size and number density are nearly independent of the evaporation rate, a uniform thinning of the solution layer over the entire surface is not likely, as such a thinning should produce smaller structures at faster drying time. In fact, the morphology seen below 26 µg/(s‚cm2) is in accord with this expectation. The simplest model to explain the observed behaviors is that above the threshold evaporation rate, the solution layer breaks into many separate domains, probably due to hydrodynamic instability of fast lateral flows against the solid surface. Because there is an indication of dendrimer concentration dependence, dendrimers may couple to the instability by regulating the domain density. Subsequent aggregation of solute materials occurs within each domain, making the size and spacing independent of the evaporation rate. In addition to the hydrodynamic flow carrying the solutes, there is a possibility of lateral capillary forces18,19 acting to gather materials into a dot shape in shorter time scales. In the case of PAMAM dendrimers, it is necessary to have inorganic salts present. Experimentally, we see that (17) Johnson, C. A.; Lenhoff, A. M. J. Colloid Interface Sci. 1996, 179, 587-599. (18) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183-3190. (19) Kralchevsky, P. A.; Nagayama, K. Langmuir 1994, 10, 23-36.
the role played by the salt is numerous. It solvates the dendrimers and makes the solution wet the surface completely. In the absence of salts, the dendrimers may aggregate in solution and the aqueous solution does not wet graphite. Salts shield unwanted electrostatic repulsion, and they help the dendrimer to be packed tightly by solidifying with the dendrimers. For general applications of this technique to form nanodots, however, it is not necessary to have any salts present. We are able to obtain dispersed dots similar to those shown in Figure 1 by casting a 5 × 10-4 wt % solution of the dihydroxybenzyl alcohol dendrimer20 1 in THF. Addition of 10 µM 4-hydroxy-4′carboxyazobenzene to this THF solution of 1 also gives dispersed dots. The result clearly demonstrates that the present method is a versatile and simple way of producing dispersed arrays of various dendrimers and solutes on flat surfaces over the centimeter scale.
LA000909V (20) Numata, M.; Ikeda, A.; Fukuhara, C.; Shinkai, S. Tetrahedron Lett. 1999, 40, 6945-6948.
