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Langmuir 1997, 13, 3226-3234
Fine Particle 2D Crystals Prepared by the Dynamic Thin Laminar Flow Method† Gilles Picard* Istituto di Ricerca “Alberto Sorti”, IDRAS, Via Passalacqua 19, 10122 Turin, Italy Received January 16, 1997X A novel method to make 2D crystals of fine particles is proposed. It is based on the hydrodynamic properties of thin laminar flow. To explore the possibilities of this method, we chose two variables, the particle diameter and the surface charge density. The spherule diameter varied from 6 µm to 53 nm, and its calculated surface charge density from 1134 to 4 µC/cm2. In every case, we easily obtained a rate of monolayer preparation up to 1 mm/s. Moreover, 2D crystals were always observed. The subphase ionic condition was the key parameter providing an adsorption window wide enough to allow a controlled particleair/water interface adsorption, while maintaining particle-particle repelling electrostatic forces strong enough to avoid fractal formation during the surface compression. This report demonstrates that the DTLF method is an efficient way of producing monolayers and 2D crystals of colloids and that it could be developed to work in a continuous mode.
1. Introduction Two-dimensional assemblies of fine particles is part of current research. Commercial availability of spherules in a wide range of sizes (from 100 µm down to 5 Å) and material composition (glass, gold, polystyrene, carbon, etc.) offers vast possibilities for fundamental research in twodimensional physicochemistry. Among the many subjects drawing the attention of researchers on colloids are the adsorption mechanisms at the air/water (A/W) interface1 and the basic principles of 2D crystallization.2 The properties of small particles in monolayers or crystals are frequently spectacular. Transparent nanoparticles in monolayers creating “beautiful colors” were reported.3 In the field of application high-density optical storage on nanoparticle 2D crystals with near-field optical microscopy was recently described.4 Moreover, theoretical calculations of metallic spheres predict unusual new properties.5 Another branch of science studies the behavior of proteins at the A/W interface. However, in order to avoid the problems related to the particle denaturation at the A/W interface, solid particles are often used instead of proteins.6 One of the main obstacles, in the utilization of microto nanoparticles, is putting them in uniform monolayers. No method seems to be able to cover all sizes relevant to physicochemistry, i.e., from 10 µm to 5 Å, without extensive modifications to the instrumentation or a significant loss in efficiency. The compression of these spherules with the standard LB method makes voids because of the reduced Brownian motion and frictional forces between particles. In some areas, where particles are pressed against each other, local collapses occur upon further compression. From the physicochemical point of view, † Italian Patent Pending. * Current address: c/o BanderalisC.so A. Tassoni 56, 10144 Turin, Italy. Telephone and Fax: +39-437-6842. E-mail: picard@ scientist.com. X Abstract published in Advance ACS Abstracts, May 15, 1997.
(1) Williams, D. F. Aggregation of Colloidal Particles at the Air-Water Interface. Ph.D. Thesis, University of Washington, 1991. (2) Picard, G.; Takeda, S.; Yamaki, M.; Yoshimura, H.; Ebina, S.; Nagayama, K. Sixth International Conference on Organized Molecular Films, Trois-Rivie`res, Canada, 1993. Takeda, S.; Picard, G.; Nagayama, K. Third IUMRS-ICAM Int. Conf. on Advanced Materials, Sunshine City, Japan, 1993. Takeda, S.; Picard, G.; Yamaki, M.; Nagayama, K. 5th Annual Meeting of the Protein Engineering Society of Japan, Kyoto, Japan, 1994. (3) Dushkin, C. D.; Nagayama, K.; Miwa, T.; Kralchevsky, P. A. Langmuir 1993, 9, 3695. (4) Micheletto, R.; Fukuda, H.; Ohtsu, M. Langmuir 1995, 11, 3333. (5) Modinos, A.; Stefanou, N. Acta Phys. Pol., A 1992, 81, 91. (6) Dushkin, C. D.; Yoshimura, H.; Nagayama, K. Chem. Phys. Lett. 1993, 204, 455.
S0743-7463(97)00049-8 CCC: $14.00
there are various experimental difficulties concerning the 4-decade-colloid sizes at the water surface, which limit the fabrication of particle 2D crystals. It was then necessary to find a different approach based on a totally new strategy capable of making monolayers with equal efficiency from the largest to the smallest and providing the flexibility requested for optimizing the conditions of the operation. A new method for making protein monolayers called dynamic thin laminar flow (DTLF) was recently presented.7,8 It is based on a new approach, devised for high speed preparation. Essentially, the particles are funneled into a moving thin liquid film, which is only a few micrometers thick. The particles are forced to emerge and adsorb at the interface by a combination of motions. As the rotation of the thin liquid film is rapid, the particles are quickly compressed, firstly onto a polytetrafluoroethylene (PTFE) edge and shortly after onto the edge of the growing particle monolayer. It is demonstrated that this simple method can produce particle monolayers at a speed never reached before. Two-dimensional crystals are usually observed. Both particle monolayers and 2D crystals can be prepared by fine tuning the relevant parameters, namely the rotation speed and the subphase ionic concentration. This report presents a series of tests on fine particle monolayer preparations, where the particle diameter was varied from 6 µm down to 53 nm. In order to better understand the basic principles of the DTLF method, the particle hydroaffinity was divided into two categories: moderately and highly hydrophilic. The observations are well explained by the theories concerning the adsorption of particles at the A/W interface and by the mathematics developed specifically for the DTLF method. 2. Experimental Section All experiments were performed in a clean room. The dust level was measured by a commercial dust detector for white room quality control. The areas of operation were either R1000 (below 1000 particles per cubic foot) or R100 (below 100 particles per cubic foot). Materials. The prototype testing this new method is presented at Figure 1, in lateral and top views. The glass cylinder was 6 mm in diameter and 50 mm long. The glass cylinder surface was polished with fine abrasives for commercial lenses until no (7) Picard, G. In High Dilution Effects on Cells and Integrated Systems, Taddei-Ferretti, C., Ed.; World Scientific, London, 1996. (8) Picard, G.; Alliata, D.; Nevernov, I.; Pazdernik, L., Langmuir 1997, 13, 264.
© 1997 American Chemical Society
Preparation of Fine Particle 2D Crystals
Langmuir, Vol. 13, No. 12, 1997 3227
Figure 1. Rotating cylinder and PTFE base assembly. A glass cylinder rotates at about 300 µm above a hemicylindrical PTFE trough filled with buffers injected through channels. The cylinder is covered by a 5 µm thin liquid film and a particle suspension is injected. The particles are adsorbed at the air-water interface and compressed onto the PTFE corner. The channels are used to control the subphase composition and film thickness and clean the subphase before transferring the particle monolayer onto a solid substrate. scratch could be seen at 1000× magnification with an optical microscope. A hemicylindrical trough was obtained by cutting out and drilling a 10 × 3.5 × 0.5 cm PTFE plate. A dc electric motor with a speed control up to 3 Hz was used to drive the glass cylinder. The dc electric motor and the glass cylinder were connected mechanically by means of a thick rubber tube, in order to transmit the torque while damping the vibrations. The cylinder was held horizontally by two PTFE circular plates drilled at 2 mm from the center. The gap between the cylinder and the hemicylindrical trough could be adjusted to about 300 µm by simply rotating the circular plates. After a vertical position was found, the circular plates were clamped firmly on a rigid Plexiglas structure. The polystyrene particles were from Interfacial Dynamics Corporation (IDC), Portland, OR. The particle concentration was always 4% w/w. Only the fluorescent 0.22 µm particles were from Polysciences, Warrington, PA. The buffers and NaCl (99.99%) were from Merck. Water was distilled (Aquatron BS I) and demineralized (Elgastat UHQ II) before use. Its surface tension was higher than 72 mN m-1 and its conductivity equal to 18 MΩ cm-1. Procedures. Six channels controlling the subphase volume input, the pH, and the thin liquid film thickness above the cylinder were drilled in the lateral parts of the PTFE hemicylindrical trough. A groove was drilled at the bottom of the trough and parallel to the cylinder axis to connect all channels so the injected
fluids would mix more easily. When salt was used for the formation of the latex particle monolayer, the subphase was thoroughly rinsed by simultaneously injecting pure water and pumping out the dilution. The prototype was mounted on an optical microscope bench for in situ observation of the particle monolayers and the rinsing procedures. Details concerning the preparation are described below. (1) Each of the Six Channels Had a Specific Function. Channel 1, injector #1 for buffer or salt solution; channel 2, injector #2 for buffer or salt solution; channel 3, injector #3 for pure water (pH 5.8); channel 4, pump #1 for rough pumping out; channel 5, 2-way pump for fine liquid film thickness adjustment; channel 6, pump #2 for fine pumping out. (2) Subphase Preparation. The subphase was prepared with the injectors #1 and #2, at cylinder rotation frequency of 3 Hz. The pH or salt concentration was gradually modified and adjusted with the syringe graduations. The thickness of the thin liquid film around the cylinder was carefully controlled by the 2-way pump and by the use of a focal depth of an optical microscope. The subphase conditions are shown in Table 1 with the corresponding observations. (3) Particle Monolayer Preparation. The preparation of 6-0.6 µm particle monolayers was relatively easy, because the monolayers could be seen as a white coating material over the cylinder. Thus, the procedure just consisted of injecting the particles into the thin liquid film with a pipette and looking at the growing
3228 Langmuir, Vol. 13, No. 12, 1997
Picard
Table 1. Particle Properties, Subphase Initial Conditions, and Observation of the Monolayer Preparations diameter (µm)a
time (s)b
6.240
particle typec sulfate
surface charge (µC/cm2)d 4
[NaCl] (M) nil
30 1.170 1.010
30
carboxyl CML
19 923
carboxyl CML
1135 16 226
30 0.833 0.614 0.250
