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Langmuir 2008, 24, 1750-1755
Unexpected Effect of Light on Colloidal Crystal Spacing Qing Zhao, Jianming Zheng, Binghua Chai, and Gerald H. Pollack* Department of Bioengineering, Box 355061, UniVersity of Washington, Seattle, Washington 98195 ReceiVed October 30, 2007. In Final Form: NoVember 29, 2007 Colloidal crystals were formed from microsphere suspensions via a simple and novel approach using gel beads. The microspheres self-assembled not only around each bead but also between beads in an ordered pattern. The crystals shrunk under incident light, with the effect of blue (wavelength 450 to 500 nm) being the most profound. The results shed new light on the fundamental issue of self-assembly and colloid science.
Introduction Colloidal crystals are three-dimensional periodic structures formed from charged particles suspended in solution. They first attracted interest in the 1960s, when studies of light scattering on latex crystals and coexistence of disordered fluid region and crystal regions were carried out.1 The light-scattering technique permitted microspheres to be tracked collectively rather than individually. Colloidal crystals now enjoy many important technological applications, including optical filters,2 switches,3 and materials with photonic band gaps.4 Yet, the principles underlying their formation are only incompletely understood. Recent interest in these crystals has surged because of their importance in self-assembly, structural phase transitions, and in understanding the interaction potential between charged latex particles.5-14 Ise’s pioneering work in the 1980s15-21 and Grier’s reports in the 1990s22-24 inspired much additional research on how and why microspheres self-assemble into ordered patterns. Standard DLVO theory predicts a purely repulsive electrostatic interaction, plus a very short-range van der Waals attraction. However, a number of results published in the past two decades have shown evidence of an additional, long-range attractive force, which could contribute to a better understanding of crystal * Author to whom correspondence should be addressed. Phone: 206685-1880 Fax: 206-685-3300. Email address:
[email protected]. (1) Luck, W.; Klier, M.; Wesslau, H. Ber. Bunsen Ges. 1963, 67, 75. (2) Kamenetzky, E. A.; Mangliocco, L. G.; Panzer, H. P. Science 1994, 263, 207. (3) Chang, S.; Liu, L.; Asher, S. A. J. Am. Chem. Soc. 1994, 116, 6739. (4) Tarhan, I. I.; Watson, G. H. Phys. ReV. Lett. 1996, 76, 315. (5) Gast, A. P.; Russel, W. B. Physics Today 1998, 51, 24. (6) Larsen, A. E.; Grier, D. G. Nature 1997, 385, 230. (7) Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: London, 1989. (8) Yethiraj, A.; van Blaaderen, A. Nature 2003, 421, 513. (9) Blaaderen, A.; van Ruel, R.; Wiltzius, P. Nature 1997, 385, 321. (10) Gasser, U.; Weeks, E. R.; Schofield, A.; Pusey, P. N.; Weitz, D. A. Science 2001, 292, 258. (11) Savage, J. R.; Blair, D. W.; Levine, A. J.; Guyer, R. A.; Dinsmore, A. D. Science 2006, 314, 795. (12) Wickman, H. H.; Korley, J. N. Nature 1998, 393, 445. (13) Kepler, G. M.; Fraden, S. Phy. ReV. Lett. 1994, 73, 356. (14) Durand, R. V.; Franck, C. Phys. ReV. E 2000, 61, 6922. (15) Ito, K.; Nakamura, H.; Yoshida, H.; Ise, N. J. Am. Chem. Soc. 1988, 110, 6955. (16) Ise, N.; Matsuoka, H.; Ito, K.; Yoshida, H.; Yamanaka, J. Langmuir 1990, 6, 296. (17) Matsuoka, H.; Kakigami, K.; Ise, N.; Kobayashi, Y.; Machitani, Y.; Kikuchi, T.; Kato, T. Proc. Natl. Acad. Sci. 1991, 88, 6618. (18) Ito, K.; Yoshida, H.; Ise, N. Science 1994, 263, 66. (19) Yoshida, H.; Ise, N.; Hashimoto, T. J. Chem. Phys. 1995, 103, 10146. (20) Rao, G. V. R.; Konishi, T.; Ise, N. Macromolecules 1999, 32, 7582. (21) Ise, N. Proc. Jpn. Acad. 2002, 78, Ser. B 129. (22) Crocker, J. C.; Grier, D. G. Phys. ReV. Lett. 1994, 73, 352. (23) Crocker, J. C.; Grier, D. G. Phys. ReV. Lett. 1996, 77, 1897. (24) Crocker, J. C.; Grier, D. G. J. Colloid Interface Sci. 1996, 298, 179.
formation. A net attractive interaction was implied by the microscopically observed inhomogeneity of particle distribution in initially homogeneous suspensions, reported by Ise et al.18 In Grier’s studies,6 the reported structure and dynamics of metastable colloidal crystallites also showed evidence for strong, long-range attractions between similarly charged spheres; average interparticle distances were on the order of several micrometers. These results were deemed qualitatively inconsistent with the longaccepted DLVO theory, but did not give a complete answer to the fundamental question of colloidal science: when and why do like-charged colloidal spheres attract each other. Numerous approaches have been employed to form colloidal crystals. They include controlled solvent evaporation, electrophoresis, dip-coating, spin coating, natural growth of void structures with time in highly purified polymer latex dispersions, crystallization in physically confined cells, and template-directed colloidal crystallization.9,18,23,25 Many of these methods either involve complicated procedures or need critical solution concentrations, which are not always easy to control. In light of the fundamental importance of understanding colloidal systems, here we report a novel and simple method of forming ordered colloidal crystal patterns in aqueous solution. Surprisingly, these crystals show unexpected sensitivity to incident light. Experimental Methods The experimental chamber was constructed from a rectangular plastic cuvette, closed and sealed at the open end, and laid flat. A hole was drilled through both the cuvette’s upper and lower sides. The bottom hole was sealed with a no. 1 microscope coverslip (150 µm thick), which was used for observing the sample. The upper hole was used to input the required suspension. All surfaces were cleaned thoroughly with ethanol and deionized water before each experiment. The suspension consisted of mixed-bed ion-exchange resin beads (Bio-Rex MSZ 501(D) resin), microspheres, and distilled, deionized water. The ion-exchange resin beads were 600 ( 100 µm in diameter. The beads came in two types: anion and cation. The anion resin contains an irreversibly bound blue dye, which turns to gold when the exchange capacity is exhausted. Prior to use, beads were first washed with ethanol, and then washed again several times with deionized water. The water was from Barnstead D3750 Nanopure Diamond purification system, type I HPLC grade (18.2 MΩ) 2 µm polished. The principal type of colloidal particle used in this study was surfactant-free sulfate white polystyrene latex microspheres with diameter 0.47 µm (product number 1-500, Invitrogen, Eugene, OR). Particles of this size undergo vigorous Brownian motion in water and are sufficiently large to be imaged with a conventional light (25) Gu, Z. Z.; Wang, D.; Mohwald, H. Soft Matter 2007, 3, 68.
10.1021/la703387m CCC: $40.75 © 2008 American Chemical Society Published on Web 01/26/2008
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Figure 1. (a) “Shell crystal” built of 0.47 µm sulfate microspheres, formed around one positively charged gel bead. The dark sphere is the ion-exchange resin bead. “+” means the charge type of the bead. (b) “Exclusion zone” formed around a negatively charged bead. microscope. The spheres are synthesized with a large number of sulfate salt groups chemically bound to their surfaces. These groups dissociate in water, giving a single negative charge per group with compensating positively charged counterions in solution. Carboxylate microspheres (Polybead carboxylate 0.5 micron Microspheres, Polysciences, Inc. Warrington, PA) and amidine microspheres (surfactant-free white Amidine latex, d ) 0.52 µm, Invitrogen, Eugene, OR) were also used, mainly for testing whether the microspheres’ surface functional group affected the self-assembly process. Immediately prior to each experiment, the desired suspension was introduced into the chamber. The initial microsphere-volume fraction was approximately 0.08. Approximately 10-20 mixed positively and negatively charged ion exchange beads were added to the chamber via the cuvette’s upper hole. The beads quickly settled to the bottom due to their high density. Then the chamber was put on the sample stage of an inverted Zeiss Axiovert-35 optical microscope with an attached video camera, used in the bright-field mode. 10 × , 20 × and 40 × objective lenses were used in different experiments. For the optical experiments, attempts were made to reduce any influence of IR and UV radiation. An IR filter (TECH SPEC Heat Absorbing Glass, Edmund Optics) was placed below the microscope condenser lens to block out incident infrared light in the range of 800 nm to 4 µm. In addition, a small beaker with an aqueous CuCl2 solution (0.3 mol/L) was placed in the incident light path to cut additional infrared light coming from the illuminating system.26 A germanium detector sensor (Newport 818-FA, 800-1800 nm sensitivity range) was used to measure the intensity of IR incident on the sample, the result showing less than 5 µW IR coming through the filters. Hence, background IR should not be a significant factor in these experiments. To exclude any UV-induced effects, a UV filter (NT54-054, Edmund Optics) was also put in the incident light path. A set of optical cast plastic color filters (Edmund Optics) were used in the optical experiments to illuminate the sample with a series of wavelengths. In all optical experiments, a Newport 1815-C optical power meter with attached silicon detector sensor (Newport 818-SL, 400-1100 nm sensitivity range) was employed to maintain the incident light intensity the same for each color. Some adjustment of the microscopepower supply voltage was typically required for each wavelength.
Results Basic Observations. After the suspension was poured into the chamber and the latter was placed on the microscope stage, four to five microsphere layers could be clearly seen around the surface of each positively charged ion exchange bead. The microspheres had self-assembled in an ordered packing mode. Please see Figure 1a. Marks “+” and “-” in figures stand for the charge type of the ion-exchange resin beads. Around negatively charged beads, by contrast, there was a clear “exclusion zone,” devoid of any microspheres. Please see Figure 1b. (26) Arnold, B. Y.; Callaghan, T. S. Liquid optical filter and method for the near infrared light, U. S. Patent 4,717,220.
Figure 2. (a) Circumferential spacing in each layer as a function of layer number, starting from the bead surface. (b) Radial spacing between each layer versus layer number, starting from the bead surface.
Exclusion-zone size was approximately 200 µm, similar to what has been reported before.27,28 Henceforth, we refer to the self-assembled layers formed around the bead as the “shell crystal”. Shell crystals built of negatively charged microspheres formed readily around the positively charged bead. Within each microsphere layer, the nearest neighbor center-to-center separation was approximately five microsphere diameters, or ∼2.5 µm. This spacing was slightly larger in the outer layers than in the inner layers. This result is shown in Figure 2a, where the circumferential spacing averaged over the first four layers was 2.45 µm, and 2.64 µm over the next four layers. Similarly, the layer-to-layer (radial) spacing increased from the inside layer to the outside layers. This can be seen in Figure 2b. The radial spacing averaged over the entire crystal was 2.0 µm, slightly smaller than the circumferential spacing (2.5 µm) averaged over all layers. Within 1 min of the initial observation, additional microspheres drawn from the suspension could be seen to build an additional layer around the positively charged beads. These microspheres tended to fill any vacancies in the existing outside layer, completing the layer before starting to build a new one. The layer-building process slowed down after about 2 h, and became progressively slower with additional time. After 12 h, almost all microspheres from the suspension had distributed themselves around one or another of the positively charged beads. (27) Zheng, J. M.; Pollack, G. H. Phys. ReV. E 2003, 68, 031408. (28) Zheng, J. M.; Chin, W. C.; Khijniak, E.; Khujniak, Jr. E.; Pollack, G. H. AdV. Colloid Interface Sci. 2006, 127, 19.
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Figure 3. (a) “Shell crystal” of 0.5 µm carboxylate microspheres around one positively charged bead. (b) “Shell crystal” of 0.52 µm amidine microspheres around one negatively charged bead.
These shell crystal structures were quite labile. After mildly shaking the chamber, the microspheres would come free, and the self-assembly process would start over again from the initial stage. The ultimate result appeared to be fully reproducible. After approximately two weeks in the chamber, the positively charged ion-exchange beads turned from blue to gold, implying loss of charge. Following that time, the crystal had disappeared. Similarly, if old beads that had already lost their charge were used, no crystal structures could form. Variation of Conditions: Microsphere Type, Size, and Concentration. To test the influence of surface functional groups on crystal formation, different types of microspheres were used. Like the negatively charged sulfate microspheres, carboxylate microspheres also carry negative charge at neutral pH, whereas amidine microspheres carry positive charge. Figure 3a shows a representative image of carboxylate microsphere layers disposed around a positively charged bead. The shell crystal appears very much like that formed by the sulfate microspheres, albeit less regular. Figure 3b shows layers formed by positively charged amidine microspheres around a negatively charged bead. Again, crystals did form, although they were again less regular than in the standard situation. Amidine microspheres formed no crystal around positively charged beads; instead, they formed an exclusion zone. Thus, negatively and positively charged entities behaved in an entirely complementary manner. To test for any size-dependent differences in self-assembly, sulfate microspheres of different sizes were studied. Microspheres smaller than the standard 0.5 µm were difficult to observe due to their small dimension. The microspheres larger than standard (1 µm and 2 µm) did form crystals, but the patterns were not as regular as with the standard (0.5 µm) microspheres. Further, they grew more slowly. Compared to the 0.5-µm microspheres, which took approximately 1 min per layer to form, the 1-µm spheres took approximately 5 min per layer, and the 2-µm spheres took approximately 1 h per layer. Microsphere concentration was also varied, as several groups have reported that certain critical concentrations were required for self-assembly in water.22-24 Concentrations were increased and decreased by a factor of 10. Crystals readily formed with volume fractions ranging between 0.008 and 0.8. With the higher concentrations, more layers were seen at the onset of observation, and the new layers formed more rapidly. For example, with volume fraction at 0.3, there were 15-20 layers at the start, with new layers forming every 10 s. For volume fraction of 0.01, only one layer was found initially, and about 15 min were required to form additional layers. All of these concentrations gave highly ordered crystal structures, although the images became fuzzy when the concentration exceeded 0.3. Bridge Area Between Beads. In addition to the observed shell crystals, crystals could also be found bridging the gap between beads. We refer to such structures as “bridge crystals.” Bridge crystals could be seen under two conditions. In the first,
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Figure 4. (a) “Plus-plus bridge crystal” between two positively charged beads. (b) “Plus-minus bridge crystal” between one positively charged bead and one negatively charged bead.
Figure 5. Incidence of bridge formation as a function of the surface to surface distance between two beads.
a bridge area made of regularly situated microspheres could be seen between two positively charged beads (Figure 4a). Such crystals were quite clearly visualized, with fairly regularly spaced elements. Bridge crystals formed spontaneously and did not require any complex procedures. Their presence depended on the distance between beads: When the beads came sufficiently close to one another, the outer shell layers joined one another to form an interconnecting bridge. Meanwhile, additional microspheres from the suspension joined to build more bridge layers. Bridge crystals formed not only between two positively charged beads (labeled “plus-plus”), but also between one positively and one negatively charged bead (“plus-minus”), as shown in Figure 4b. For the latter case there was a surprising observation: while the negatively charged bridge could extend all the way to the negatively charged bead’s surface, other regions of the bead surface remained devoid of microspheres. Where any microspheres remote from the bridge area could be seen, none came within 200 µm of the bead. Hence, except for the bridge area, the bead retained a microsphere-exclusion zone. The reason for this anomaly remains unclear. Numerous observations were made under a variety of conditions to determine whether any bridge crystals could form between two negatively charged beads. None could be found. The result was the same when positively charged microspheres were used. That is to say, when microspheres were positively charged, no bridge crystal would form between positively charged beads. Bridge formation did not occur also when beads were situated more than a critical distance apart. Figure 5 shows the relation between the number of times that bridge-crystal could be detected, and surface-to-surface distance between two beads. Bridges became increasingly rare when beads were separated by more than approximately 50 µm. Beyond 100-µm separation, bridges could not be found at all.
Unexpected Light Effect on Colloidal Spacing
Figure 6. (a) Colloidal crystal formed between two touching ionexchange beads. (b) Colloidal crystal formed between three ionexchange beads.
Another interesting phenomenon is that when the surfaceto-surface distance fell to 5 µm or less, the two beads tended to come closer and touch one another; hence, the absence of data points in Figure 5 in the bin 0-5 µm. The first column in the figure refers to the case in which two beads touched one another, which happened often. An example is shown in Figure 6a. Although the two beads touch, the microspheres nevertheless remain highly ordered in the intervening crevices near the contact point. Occasionally, such crystals could also form among three or more beads, as shown in Figure 6b. Effect of Light. Surprisingly, we found that in the presence of light, microspheres drew closer to one another. In other words, light shrunk the crystal. Shrinkage occurred pervasively: It was evident both in total crystal size, as well as in the separation of microspheres within shell crystals and bridge crystals (plus-plus and plus-minus). To investigate the dynamics of light-induced condensation, we abruptly increased the microscope-lamp power-supply voltage from 5 V to a higher level (8 V), or from 5 V to the maximum (12 V). Videos were made for a duration of 5 min both at 8 and 12 V, with 10 s between each frame. ImageJ software was used to measure the time course of microsphere separation. During a 5-min period of 8 V illumination, we found that within the shell crystal, the inter-microsphere radial spacing diminished by 5.5%, from 2.18 ( 0.15 µm to 2.06 ( 0.13 µm (n ) 10). The circumferential spacing shrunk only by 2.8 ( 0.1% (n ) 10). Thus, the spacing decreased less in the circumferential than in the radial direction. This is understandable as the microspheres are more constrained circumferentially than radially. Under stronger light (12 V illumination), the shell crystal shrunk more. For example, the radial spacing in shell crystal decreased by 6.4 ( 0.2% (n ) 5) and the circumferential spacing shrunk by 3.6 ( 0.4% (n ) 5). During the same 5-min period of 8 V illumination, the spacing also diminished in the bridge crystal. Although the beads themselves did not move closer to one another, inter-microsphere spacing diminished both in the axial direction and in the lateral direction. It is noteworthy to mention that with increased light, some microspheres migrated from bridge crystal to the suspension, that is to say, some microspheres were lost from the original crystal. When the power supply voltage was returned to the original 5 V, the lost microspheres (or others) returned to join the crystal, the process coming to completion again within approximately 5 min. Figure 7 shows two frames chosen from among the videos as a demonstration of the light-induced spacing shrinkage in the plus-plus bridge crystal. Under 8 V microscope light illumination, after 90 s, the migration away from the crystal is apparent. The microsphere spacing diminution is less apparent because the change percentage was relatively small and it is really hard to detect by naked eyes. The spacing in the bridge crystal also shrunk more when the voltage was increased to a higher level (12 V). Figure 8
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Figure 7. Time course of light-induced “plus-plus bridge crystal” shrinkage under 8 V microscope light for 90 s.
Figure 8. Time course of spacing shrinkage under bright (12 V) and dim (8 V) light.
Figure 9. Microsphere spacing shrinkage as a function of wavelength. Inset: absorption spectra of the four color filters used.
summarizes the dynamic changes in the bridge crystal. During the first 2.5 min at 8 V, the axial spacing shrunk from 2.0 µm to 1.8 µm, almost 14%. Under brighter illumination (12 V vs 8 V), it shrunk more quickly, from 2.0 µm to 1.6 µm. The total shrinkage obtained with the maximum available light (12 V) was as high as 24%. Effect of Wavelength. The light-induced effects raised the question whether the increase of inter-microsphere attractive force might show spectral dependence. To test for such dependence, we first identified a spot in the field that contained a bridge crystal, and then turned off the microscope-light power supply for 15 min. (The microscope itself was situated in a darkened room.) This procedure set the “dark” baseline. Then, the lamp was turned on to approximately 10 V, and a video was immediately made at the same spot. The experiment was repeated with each color filter (spectra given in the inset of Figure 9). Due to the presence of the filters, the incident light intensity was much less than that used for the broadband illumination. Using
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ImageJ software, we measured the spacing between neighboring microspheres in the first and last frame of the video, an interval of 5 min. We found that blue light made the crystal shrink most, green next, and yellow and red light very little. Quantitative results are shown in Figure 9. The figure refers to the plus-plus crystal. Blue light caused shrinkage of approximately 6.4%, green light 2.7%, and yellow and red light less than 1%. In the plus-minus crystal, for comparison, blue light gave 8.8% shrinkage, slightly more than the plus-plus crystal. Controls. Some concern arose because the size of the microspheres was close to the wavelength of the blue light used to generate shrinkage: both are around 480 nm. Hence, the large shrinkage caused by blue light could have something to do with the particular microsphere size. Microspheres of similar composition but different size (1 µm and 2 µm instead of 0.47 µm) were used to check this issue. Although shrinkage was less with the larger spheres (1 µm: 2.8%; 2 µm: 1.3%), the same wavelength dependence was foundsblue light still causing maximum crystal shrinkage compared to the other wavelengths. Thus, the coincidental similarity of wavelength and microsphere size did not cause the light-induced shrinkage, although size apparently does play some role in determining the absolute magnitude. Another potential issue is the temperature rise that occurs with incident illumination. To check the magnitude, an Omega HH306 thermometer with a TJC 36 stainless steel joint thermocouple probe was used to detect any temperature rise during light irradiation. The probe’s tip size was approximately 250 µm, and resolution was 0.1 °C. The probe tip was immersed into the suspension and positioned as close to the bead surface as practicable. During the 5 min period of illumination, the temperature increased by less than 0.1 °C. Hence, the observed shrinkage was apparently not the result of temperature increase. Another concern was that the bead color might somehow have been related to the blue-light effect, because the positively charged beads themselves are blue. Therefore, another type of mixed bed resin with different colors (AG 501-X8, Bio-Rad Laboratories) was substituted. In this resin, both positively and negatively charged beads are yellow. The results, checked in five experiments, were similar: 9.5% shrinkage under blue light, 2.5% for green, and less than 1% for yellow and red light. Thus, the spectral dependence of microsphere spacing shrinkage was unrelated to bead color. Another piece of evidence for the blue-light effect was found when blue and white light were used at the same overall intensity. Under those conditions, blue light produced more shrinkage than white light (8.8 ( 0.3% vs 6.5 ( 0.4% (n ) 5)), indicating that the principal agent was indeed blue light: more blue light yields more spacing decrease. This conclusion was affirmed by using incident blue light at different intensities. As summarized in Figure 10, the results, at least over the limited light power range studied, show a fairly linear effect of light intensity on the degree of shrinkage.
Discussion Novel Formation of Colloidal Crystals. The method used to generate colloidal crystals is both simple and unique. Microspheres self-assemble spontaneously in water around the gelbead surface. They also form crystal bridges between the beads. In both situations, the crystals are readily observed in an ordinary light microscope, and are amenable to study. Less clear than the behavior itself is the question why the microspheres form such highly ordered structures. It is natural
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Figure 10. Microsphere spacing shrinkage as a function of blue light intensity.
to think that the large (600 µm) positively charged bead attracts small (0.5 µm) negatively charged microspheres due to an electrostatic force, but the reason for crystal growth beyond the initial layer is not immediately obvious. Concentration did not appear to be a critical factor. We tried a range of microsphere concentrations over 2 orders of magnitude and found that microspheres could self-assemble in all cases, which implies that microsphere-microsphere interaction is not a critical feature of crystal nucleation. Nucleation appears to arise out of some long-range interaction between the ion-change resin bead, the microspheres, and perhaps also the water. Thus, it is reasonable to suppose that the positively charged bead attracts the negatively charged microspheres by an electrostatic force, achieving a minimum energy state. Once the microspheres form the first layer, the question of the secondlayer and additional layers arises: why might similarly charged microspheres attract one another? According to standard DLVO theory, there should be no attraction because the inter-microsphere spacing greatly exceeds the Debye length; this should ensure that the second and additional layers never form. In fact, additional layers do form, separated from one another by approximately 2 µm, or roughly four microsphere diameters. The fact that microspheres do not touch one another implies that in addition to the repulsive force between microspheres, there must also be a counteracting attractive force, to maintain the crystalline structure. Long ago, Feynman delivered a lecture,29 suggesting that “likes like likes” through the intermediary of unlikes. Thus, the attractive force may lie in counterions situated in between the charged surfaces. This line of thinking sheds light on the possible basis of self-assembly of similarly charged particles: through intermediary counterions. This kind of attractive force was also mentioned in a recent review.21 It is detected by measurement of the interaction potential of colloidal particles: substantial at high charge density, while absent for particles of low charge density, where only repulsion is found. These observations lead to the supposition that an electrostatic attraction may exist between like-charged particles through the intermediary of their counterions. Thus, the observed crystals probably formed in a manner similar to that of other colloid crystals: a balance between repulsive and attractive forces. The charged bead apparently acts as a nucleating surface. From the bead outward, the crystal grows based on the same principles that govern crystal growth in other colloid systems. Exclusion Zones and Counterions. When the same negatively charged microspheres were exposed to negatively charged beads, (29) Feynman, R. P.; Leighton, R. B.; Sands, M. The Feynman Lecture on Physics; Addison-Wesley: Reading, MA, 1964; Chapter 2, p 2.
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crystals failed to form. Instead, exclusion zones were found around each beadsshell-like zones that were devoid of microspheres (shown in Figure 1b). Such exclusion zones are not unusual. They were recently reported to occur around many gel and other hydrophilic surfaces.27,28 Their physical features differ from those of bulk water, and as a result one hypothesis that has arisen is that microsphere exclusion is caused by layers of water molecules growing from the respective surface in an organized manner. These layers could build upon one upon another, beginning at the nucleating surface and extending outward, excluding solutes as they grow. Thus, the presence of exclusion zones observed here Figure 1b is not surprising. On the other hand, understanding the basis of exclusion zones may lead to better understanding of how counterions mediate the required attractive force. If exclusion zones form around the beads, then it is reasonable to suppose that they form around the microspheres as well, for the microsphere surfaces, albeit smaller, are also negatively charged. Since such exclusion zones are themselves negatively charged,28 abundant positive counterions (protons) are anticipated to lie in regions beyond. Such protons could act as the intermediate unlikes, as Feynman put it 40 years ago. The unlikes would attract additional negatively charged microspheres to come and join to the array, building the colloidal crystal layer by layer. The main point of this line of thinking is that the counterions would be unexpectedly abundant. If protons were indeed in abundance between microspheres, then one ought to be able to detect their presence. Pilot experiments were carried out with a pH-sensitive dye (Riedel-de Haen, 36803, pH 3.0-10.0, Universal Indicator), but consistent results could not be obtained, mainly because of the high demand for spatial resolution: microsphere separation is on a micron-size scale. Salts were also explored in an effort to learn more about counterion effects, and it was found that very low concentrations (
