Fine Particle Monolayers Made by a Mobile Dynamic Thin Laminar Flow

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Langmuir 1998, 14, 3710-3715

Fine Particle Monolayers Made by a Mobile Dynamic Thin Laminar Flow (DTLF) Device† G. Picard‡ Banderali, Cso A. Tassoni, 56, 10144 Turin, Italy Received November 20, 1997. In Final Form: February 24, 1998 This paper presents an innovative device for monolayer preparation that takes full advantage of the dynamic thin laminar flow method (DTLF). The small cylinder system provided the opportunity to prepare monolayers continuously and transfer them onto a solid substrate while moving. Hydrophilic and hydrophobic surfaces were coated with fine polystyrene particles. Multilayers were also made by simply moving the mobile DTLF device back and forth over the same surface. Technically, this was achieved by controlling the cylinder rotation and translation speeds above the substrate with a set of electrical motors. The injection of various suspensions into the thin liquid film was performed with syringes to make the fluids emerge near the rotating glass surface. The whole assembly was mounted on a mobile platform. Three fine latex particles were selected to make the primary tests of the new DTLF device. In all cases mono- and multilayers were obtained. This device opens a new avenue for the deposition of monolayer strips onto m2 large surfaces.

1. Introduction The dynamic thin laminar flow (DTLF) method for making monolayers and fine particle 2D crystals based on an original principle has recently been published.1-3 Essentially, it consists of reducing the particle electrostatic forces in a 5-µm thick laminar flow driven by a rotating glass cylinder until adsorption occurs at the air-water interface. Thus the thin flow carries the adsorbed particles for several millimeters up to compression against a growing monolayer, where self-organization at the molecular scale yields the expected 2D formations. However, the transfer onto a solid substrate is executed through a delicate process of laterally sliding the newly prepared monolayer over a hydrophilic surface coated with a thin liquid film, which restricts the use of this method to protein 2D crystal formation.2 The principle of this method was discovered using a rudimentary apparatus devised for other purposes. During the first attempts all procedures were manual and the control of the unique electrical motor was only approximate. The cylinder was confined to a narrow hemicylindrical trough, and the production was kept immobile on the cylinder top. To make an important step forward in the quality control of the preparations, an innovative device is here presented. It was designed to take full advantage of the main experimental characteristics of the DTLF method, which are namely a microliter range subphase volume and micrograms of the material used in the preparation. In fact, just microgram amounts of particles are enough to prepare square centimeters of monolayer within seconds. The assembly was designed so it could move easily above a solid substrate thanks to the small cylinder (centimeter scale) and supports. The structure rigidity and cylinder positioning were also mechanically undemanding. This was achieved by controlling the translation and rotation † ‡

Italian Patent No. TO97A000466. Tel and fax: +39.11.437.6775. E-mail: [email protected].

(1) Picard G. In High Dilution Effects on Cells and Integrated Systems; Taddei-Ferretti, Cloe, Ed.; World Scientific: London, 1996. (2) Picard, G.; Nevernov, I.; Alliata, D.; Pazdernik, L. Langmuir 1997, 13, 264. (3) Picard, G. Langmuir 1997, 13, 3226.

speeds of the cylinder above the substrate with a set of electrical motors. The injection of the various solutions and suspensions, which constitute the thin liquid film, was performed with a set of syringes, thus allowing the fluids to circulate through channels and merge onto the rotating glass surface. Crystalline monolayers were prepared at the thin liquid film interface using this new device. The lateral motion of the cylinder allowed the monolayer that was being prepared to flow out onto the solid surface, either behind the cylinder (hydrophilic mode) or in front of it (hydrophobic mode). The versatility of the mobile DTLF method also allows the preparation of multilayers through a combination of the devices mentioned earlier. This DTLF method’s potential is beginning to develop as it offers thin film technology the possibility of preparing and depositing crystalline or amorphous monolayers on large and various surfaces. 2. Experimental Section A schematic view of the mobile DTLF device is presented in Figure 1. The principles governing the method have already been described.1-3 The major improvement is the ability of the DTLF method to prepare and lay down monolayers onto a solid surface while traveling over it. The manner in which the monolayer is laid down depends on the hydroaffinity of the surface. When the solid substrate is hydrophilic (see Figure 1a), the cylinder rotates in the opposite direction to its horizontal translation. The prepared monolayer at the air-water interface of the thin liquid film leaves the cylinder because of the surface pressure. The monolayer leaves by the rear side of the cylinder. The hydrophobic part of the particles emerging from the thin liquid film remains in the air after the transfer onto the solid substrate. Hydrophobic glass surfaces (see Figure 1b) on the contrary cause the cylinder to rotate in the same direction as its horizontal motion. The prepared monolayer pushed by the thin liquid film leaves the hemicylindrical surface at the front. The hydrophobic part of the particle in the air directly contacts the solid subphase under the cylinder. The crystalline monolayer is not squeezed between the cylinder and the glass plate surfaces, because of the 100-µm gap between them. Thus, the hydrophobic surfaces of both particles and slide are in direct contact. The hydrophilic part of the crystalline monolayer remains upward, coated with

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Monolayer Formation by DTLF

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Figure 2. Schematics of the mobile DTLF device. The upper view illustrates the electromechanical assembly that is connected to the glass cylinder.

Figure 1. (A) DTLF device working in the hydrophilic mode. (B) DTLF device working in the hydrophobic mode. (C) DTLF device making a multilayer. a thin liquid film, which evaporates within 1 min. In the best conditions for both cases, this horizontal motion of the glass cylinder leaves behind a crystalline strip that can be observed by means of an appropriate microscope. This ability to coat regardless of the surface hydroaffinity brings us directly to an interesting chapter: the preparation of multilayers (see Figure 1c). In fact, the alternation of the two modes of preparation and deposition by moving to and fro over the same surface led to the preparation of multilayers. It was also possible to prepare multilayers by simply using the hydrophilic mode going over the same area several times. The mobile DTLF method was mechanically devised as shown in Figure 2; that is, the cylinder goes through two soft, autolubricant and highly hydrophobic PTFE holders, which renders the cylinder rotation smooth and waterproof. The rotation of the cylinder is driven by a low-voltage dc electrical motor coupled with pulleys in order to reduce the angular velocity. With this assembly, the rotation speed can be adjusted from 0.1 to 10 H. The distance between the cylinder and the substrate is adjusted to 100 µm with verniers and is kept constant during the whole process. The cylinder travels above the fixed substrate at a predetermined height and at a constant speed ranging from 0.2 to 1 mm/s. This is possible by mounting the whole electromechanical assembly onto a mobile platform. The mobile platform, also driven by a variable speed dc electrical motor

connected to a 350:1 gear box reductor, is suspended from a rigid monorail made of anodized anticorrosive aluminum. The mobile cylinder travels over the solid substrate installed over the monorail. The glass cylinder is made hydrophilic by cleaning its surface with ethanol. The solid substrate’s 76 × 26 × 0.8 mm glass slides are manufactured by Matsunami Glass Ind., Ltd., Japan. The glass slides are made hydrophilic by cleaning them with concentrated sulfochromic acid (Carlo Erba Reagent, France). To make the injection and pumping out at the cylinder surface uniform, a special module was made. It is a single piece of Plexiglas with three channels drilled facing the cylinder: the suspension is injected into one channel, the subphase into another, and the third is used to adjust the total liquid amount. This means that as fluid is injected into a channel, it flows out through the holes and reaches the cylinder surface. A syringe connected with tubes is used to inject the fluids onto the rotating cylinder surface. The pistons are attached to a fine meshed screw in order to inject microliter amounts of fluids with precision. Variations for the introduction of particle suspension amounts were also tested. A particularly interesting one consists of depositing, through an Eppendorf pipet, microliter amounts of ready-for-adsorption particle suspensions on the glass slide, making the rotating cylinder approach them, lift them onto its surface, and process them. After the passage of the cylinder, the suspension drops on the solid surface become a particle monolayer. To widen the range of applications of the DTLF method, various substances easily available in stores, like Saran plastic films, aluminum foils, etc. were tested. In all cases, the hydrophilic mode of deposition worked well, but as far as highly hydrophobic surfaces are concerned, the hydrophobic mode proved best. The 2.967- and 0.821-µm latex bead suspensions were from Sigma Chemical Co., St. Louis, MO. The fluorescent 0.22-µm latex particles were Fluoresbrite Carboxy YG from Polysciences, Warrington, PA. The latex bead suspensions had a concentration of 10% w/w, whereas the fluorescent suspension was 2.5% w/w. The particle suspensions were delivered in pure water. The mechanism of adsorption of the particles at the air-water interface have already been studied.3,4 In this report, the conditions were adjusted to demonstrate the possibility of preparing and depositing particle monolayers at the same time. In this perspective, the mixtures that proved satisfactory were pure ethanol (Sigma) mixed with the particle suspension in a 1:10 v/v ratio, and for the subphase 4.0 M NaCl (Sigma) solution diluted in pure water (Millipore, 18 MΩ cm) in a 1:7 ratio. Increasing the salt led to aggregation in the subphase, while too much ethanol brought about aggregation in the suspension. Nevertheless, in the case of the largest particles, some 2D fractals were observed in the monolayer preparations. It was suspected that some friction problem could be at the origin of the particleparticle pseudoadsorption. By adding a slight amount of glycerol (Sigma, 99% GC) to the initial mixture, the particles seemed to slide next to one another and 2D arrays were observed. (4) Williams, D. F. Aggregation of Colloidal Particles at the AirWater Interface. Ph.D. Thesis, University Washington, 1991.

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A binocular optical microscope with a long range focus was positioned above the cylinder. Its magnification could be adjusted from 40× to 200×, which made possible the real-time visualization of the DTLF initial conditions of film homogeneity and thickness, and the process of deposition as well. It is very important to make sure the subphase in contact with the cylinder does not escape by capillarity and hydrophilicity along the cylinder out of the DTLF assembly or under the glass slide. In the same way, the subphase trapped under the monolayer should not flow out of the glass slide. The optical observation of the newly prepared monolayer was performed with a Reichert-Jung Polyvar microscope equipped with an immersion objective for a maximum magnification of 1200×. Dark field imaging revealed imperfections in the monolayers, and direct transmission allowed a better appreciation of the organization of the particles in the monolayer itself. The microscope was also equipped for fluorescence with a HBO-CS 200W/4-L1 lamp assembly. The pictures were made on a 400 ASA color film.

3. Mobile DTLF Device Mathematics The general equation describing the surface pressures generated by the DTLF method is2

Π)

ηLVC z

(1)

of the number density. This density is better expressed with the following equation:3

C)

3P 4πFr3

(6)

where P is the fraction of solids in the suspension and F is the particle relative density. Substituting C in eq 5, we obtain

VH )

3x3RC fzp Fr

(7)

Substituting VH in eq 4, we obtain

Π)

ηLRC f(2π - 3x3Pz) Frz

(8)

The injection flow can also be deduced. To sustain a horizontal speed VH of particle monolayer preparation over a substrate of a given width W, when a close-compact geometry in the monolayer is assumed, the flow must be

J ) 2πRC fWz

(9)

where η is the liquid viscosity (for water: 10 -3 kg/(m‚s)), L is the length of the monolayer, VC is the cylinder surface speed relative to the monolayer, and z is the thickness of the thin liquid film. In the case of the mobile DTLF device, the horizontal motion of the cylinder has to be taken into account. If VH is the horizontal velocity, the net velocity of the hemicylindrical surface relative to the forming but continuously withdrawn monolayer is

In this equation, one detail seems to be a contradiction of terms. The injection rate is independent of the number concentration of the suspension. In fact, this equation represents the total injection flow. However, the total flow is the combination of two independent ones: the particle suspension and the reagent flows. Thus, the suspension flow to be injected must contain enough particles to sustain the construction of the monolayer. Using eq 7, we have

V N ) VC - VH

A ) 2πWRcfzPt/Ps

(2)

The cylinder surface velocity is expressed by the simple equation

VC ) 2πRCf

(3)

where RC is the cylinder radius and f is the cylinder frequency of rotation. Substituting eqs 2 and 3 in eq 1, we obtain a working equation:

Π)

ηL(2πRCf - VH) z

(4)

In this equation, in steady-state conditions, all parameters can be held constant. To have a high-quality monolayer preparation, the cylinder horizontal speed must be equal to the monolayer rate of preparation. It was demonstrated with proteins2 and fine particles3 that particles adsorb at the air-water interface very quickly, probably within milliseconds. Thus, it can be assumed that the rate of particle injection into the thin liquid film is equal to the rate of particles going into the monolayer. This rate, in terms of meters per second, has already been expressed with an equation3

VH ) 2πRCfCsz

(5)

where s is the surface area of one particle, which is s ) 2x3r2. Practically, the 6-mm diameter cylinder rotates with a frequency of 1 H and is coated with a 5-µm thin liquid film. The particle concentration should be in terms

(10)

where Pt is the suspension concentration in the thin liquid film and Ps is the particle suspension concentration at the injection. The adsorption solution flow, B, is the difference that follows this equation:

J)A+B

(11)

The experimental conditions for the preparation of particle monolayers can be estimated. Some parameters are already fixed: the particle relative density is 1.055, the cylinder radius is 3 mm, and the particle diameters are 2.967, 0.821, and 0.22 µm. With eqs 7 and 8, surface pressures and horizontal speeds are calculated by varying the thin liquid film thickness, the cylinder rotation frequency, the monolayer length, and the particle concentration. Although the DTLF device can exert high pressures onto the monolayer and produce them at high speeds, the primary goal is to make monolayer strips. Thus, low speeds, i.e., about 1 mm/s, and moderate surface pressures, i.e., below 10 mN/m, were arbitrarily selected. The other parameters were adjusted in order to comply with these limits. 4. Observations The mobile DTLF method could make a 26 × 60 mm2 coating of micrometer particle crystalline strips over glass slides by injecting the particle suspension in a subphase under adequate conditions. In most cases, uniform monolayers were observed. The monolayers were amorphous and showed pseudoaggregation, as the particle surfaces did not slide next to one another. The purpose of this report is to simply show the feasibility of coating

Monolayer Formation by DTLF

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Figure 3. Optical microscope images of 2.9-µm polystyrene particles assembled by the DTLF method: (A, top) in crystalline monolayer; (B, bottom) holey monolayer. The image is 200 × 300 µm2 large.

surfaces with the mobile device, so this problem was not important, although the addition of glycerol in the suspension led to 2D crystallization of 2.967-µm particles, as shown in Figure 3a. Glycerol acted as a lubricant to facilitate the sliding of the particles. Nevertheless, the domain area could not surpass the limit established earlier with the DTLF method. As expected, diminishing the glycerol concentration led to an increase of 2D fractal

formations. Interestingly, the increase of glycerol concentration did not improve the formation of 2D crystals. On the contrary, the glycerol encapsulated under the particle monolayer created defects. Instead of evaporating as water usually does, glycerol remained trapped and made drops on which the particles either rested or, more often, slipped down and created open areas, or holes, in the monolayer (see Figure 3b).

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Figure 4. Crystalline 0.8-µm polystyrene spherule monolayer.

The same observation prevailed for the 0.821-µm particles (see Figure 4). Uniform monolayers were realized. The experiments confirmed the previous observations made with another DTLF setup3 that the small size of particles did not influence the yield of monolayer preparation. Indeed, the use of glycerol was not needed to make 2D arrays. However, the 2D crystal size observed was also relatively small. This limitation could originate from the same parameter limiting the formation of larger crystal with the micrometric particles. Variations in the salt and ethanol ratios were made, with the results that never went better than the typical formation illustrated in Figure 4. Particle 2D crystals were also formed on a hydrophobic surface. The original configuration of the present DTLF device meant it was possible to work in the hydrophobic mode, i.e., the cylinder translation motions being reversed (see Figure 1b). In this case, the top of the particle touched the solid surface first while the other part became exposed to the air. This strategy had to be performed over a highly hydrophobic surface. As a matter of fact, the tendency of the subphase to flow through the monolayer made the process somewhat difficult over surfaces that were not very hydrophobic. Nevertheless, a particle monolayer was prepared with this method, which confirmed the versatility of its setup. Finally, multilayers were also prepared with the 2.967µm particles. The assembly first laid down a particle monolayer over a solid surface and, when the direction of the assembly was reversed, deposited another layer over the first one. However, although successful, this operation must be performed very carefully, mainly because if the assembly returns too quickly it can detach the particle monolayer from the solid surface and push it onto the thin liquid film, causing both monolayers to collapse. A delay between preparing the first and second layers should be allowed, although this depends on air temperature and humidity in the laboratory. To overcome this problem,

another process was successfully carried out. It consists simply of making the first particle monolayer, returning the assembly to the original point, and depositing another particle monolayer above the first one. The experiment with the drop field led us to some interesting prospects. The particle suspension was pretreated in order to have the particle near the aggregation at the air-water interface, but still stable in the bulk. The arrival of the hydrophilic glass cylinder means a drastic change for the drop field. Its hemispherical shape is destroyed, and the drop spreads itself to a thin liquid film when in contact with the glass surface. The particles then proceed to the process mentioned earlier of adsorbing at the air-water interface. This process being quite fast, they are piled up into a monolayer. In this case, the only parameter to consider is the particle load onto the surface, which can be easily calculated. The fluorescent 0.22-µm particles were also tested in the process making monolayers. Their behavior was interestingly different at the beginning; however, by using the ethanol and salt concentration, uniform monolayers were finally prepared. The quantity of salt in the subphase also created some problems that did not manifest in the case of larger particles. In particular, salt crystals appeared after the preparation of the monolayers. Quite beautiful effects were noticed, in particular the apparitions of colored diamonds, due to the structural geometry of the NaCl crystal. Their thinness was the main feature. With a lower salt concentration, the thin NaCl crystals did not appear, leaving their place instead to more fractal formations (see Figure 5) over a uniform field of fluorescent latex particle. The size of the particle was not an important parameter for the preparation of the monolayer, and its authenticity was confirmed using the fluorescence microscope.

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Figure 5. Fractal formations in the fluorescent 0.22-µm polystyrene spherule monolayer. The image is 200 × 300 µm2 large.

5. Discussion Fine particle monolayers were prepared with the mobile DTLF device. The crystallinity factor depended on the concentrations of ethanol and salt present in the suspension and subphase, respectively. The presence of ethanol in the suspension of polystyrene microspheres seemed to play an important role in the success of the monolayer preparation. One possibility is that the ethanol altered the particle surface and made it more hydrophobic. As a matter of fact, when too much ethanol is added, aggregation was observed. This alteration of the polystyrene surface alone did not suffice to provoke the adsorption at the air-water interface of the subphase. The reduction of the surface electrostatic charges of the particle suppressed the energy barrier remaining between the particle and the surface. This method took the quality of the monolayer uniformity as an important criteria. The introduction of glycerol in the particle suspension was also an attempt at reaching perfection. However, where just one monolayer is needed to coat a surface, the present conditions are more than sufficient to proceed. In the perspective of industrial applications, the mobile DTLF method has fully reached its objective. The rest is a matter of using the correct proportions of components in the suspension or subphase composition in order to have the desired monolayer characteristics. Indeed, objectives other than a uniform monolayer can be desired by potential manufacturers. As an example, the fabrication of holes in the particle monolayer could also be a valuable product. This can be done by simply adding an excess of glycerol in the particle suspension. As a mater of fact, this is quite similar to the fabrication of holey-carbon-covered copper grids for the observation of organic layers. The protocol also includes the use of glycerol in the initial solutions.5 (5) Electron microscopy in molecular biology: a practical approach; Sommerville, J., Scheer, U., Eds.; IRL Press Ltd: Oxford, U.K., 1989.

The size of the particle 2D crystals did not reach spectacular limits. This indicates that a fundamental parameter governing the formation of 2D arrays, like the particle monodispersion or impurities in the suspension or on the substrate surface, could be at the origin of this limitation. An accurate study relating to monodispersion with 2D monocrystal area could be carried out. It should be noted that the quality of the particle surface used with the mobile DTLF device was not guaranteed by the distributor on delivery. As a consequence, the difference in behavior should not be overemphasized. Further studies with particles well characterized by the manufacturer will be carried out later. 6. Conclusion A new device taking full advantage of the DTLF method was designed. The primary tests with simple particles demonstrated that a continuous production of particle monolayers was achieved. The versatility of the new device concerning the solid substrate affinity was confirmed with either glass plates, plastic films, or metallic surfaces. It was possible to make multilayers as well. The mobility of the device also offers endless possibilities to the fabrication of monolayers across whatever surface dimension. A remote-controlled self-moving DTLF device can now be built with finer mechanics to move over such surfaces, or even a mobile support with the cylinder fixed can also be devised. These preliminary tests on latexes indicate that monolayers made of smaller particles can be processed now. On the basis of the previous success with ultrafine particles up to about 50 nm, further pushing the DTLF device to the subnanometer size, for example C60 particles, is the next target. Positive results would permit the users of the DTLF method to cover all particle sizes. Acknowledgment. Thanks to Dr. C. Banderali for his financial support. LA971272R