Layer-by-Layer Self-Assembly under High Gravity Field - Langmuir

May 28, 2012 - In the present article, we have developed a facile and rapid method to fabricate a polyelectrolyte multilayer under high gravity field ...
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Layer-by-Layer Self-Assembly under High Gravity Field Lanxin Ma,† Mengjiao Cheng,† Guijin Jia,‡ Youqing Wang,‡ Qi An,*,§ Xiaofei Zeng,† Zhigang Shen,† Yajun Zhang,† and Feng Shi*,† †

State Key Laboratory of Chemical Resource Engineering & State Key Laboratory of Organic Inorganic Composites, and ‡College of Information Science and Technology, Beijing University of Chemical Technology, Beijing 10029, China § MESA Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands S Supporting Information *

ABSTRACT: In the present article, we have developed a facile and rapid method to fabricate a polyelectrolyte multilayer under high gravity field and investigated the difference of mass transfer in the diffusing process between LbL self-assembled technique under high gravity field (HG-LbL) and dipping assembly. Herein, we have employed polyethyleneimine and zinc oxide nanoparticles, which is a wellknown UV blocking material with typical absorption properties in the range of 300− 400 nm, as building blocks and applied hydrogen bonding as the driving force to construct the multilayer under HG-LbL and dipping assembly. The results show that, compared with dipping assembly, HG-LbL can highly improve the utilization and adsorption efficiency of building blocks by hastening the diffusing process, and meanwhile the resulting multilayer films still achieve comparable quality as those prepared from dipping assembly.

1. INTRODUCTION Layer-by-layer (LbL) self-assembly has been improved to be a versatile and powerful technique to fabricate layered thin films with predesigned compositions and tailored functionalities.1 Iler2 originally reported in the 1960s that multilayers could be obtained by alternately depositing in two kinds of colloidal particle solutions with opposite charges. Later in the 1990s, numerous other nanostructures were constructed via the LbL method based on the rediscovery of Decher3 on the selfassembly of oppositely charged and electrostatically stabilized polymer layers on surfaces. Various building blocks, such as nanoparticles,4 carbon nanotubes,5 dendrimers,6 enzymes,7 or polymer micelles,8 were assembled on surfaces via the LbL procedure using charged polymers9 or oligocationic molecular units10 as electrostatic bridges. Other driving forces, such as interlayer hydrogen bonds11 or ligand coordination,12 were developed to extend the LbL systems from aqueous solutions to organic solutions and stabilize the layered structures. With the development of building blocks and driving forces, different applications of functional multilayer systems were proposed, including their use as sensors and biosensors.13 Other studies also proposed their functions in enzyme immobilization,7 hollow capsule generation,14 photoelectrochemically active electrode fabrication,15 surface patterning,16 preparation of separation membranes17 and microporous films,18 fabrication of light-emitting diodes,19 surface imprinted multilayers,20 erasable films,21 and nanocomposites.22 Some studies also proposed their roles in the design of superhydrophobic coatings23 and biocompatible coatings.24 Recent related research is not restricted to fundamental aspects but is extended from © 2012 American Chemical Society

laboratories to commercial productions, such as coatings for contact lenses, antibacteria films, and conductive rubbers. The conventional LbL method is simple and has been widely used. However, it is time-consuming, and the thickness of the film depends on the number of bilayers, which restricts its further application as commercial products. One possible way to solve this problem is to develop new methods for the rapid construction of multilayer films, such as spin and spray LbL,25 electrically driven LbL,26 roll-to-roll process,27 dewetting LbL,28 dynamic LbL,29 spray-spin LbL,30 and agitated-dipping LbL.31 These methods have remarkably improved the efficiency of the LbL fabrication process and promoted the potential application in industrial production and further commercialization. However, the self-assembled process in rapid construction is still not clear, and the preparation processes are limited on laboratory instruments. Some authors attributed rapid fabrication to accelerating the rearrangement of the preabsorbed polyelectrolytes, which can only affect the surface morphology instead of the saturated adsorption capacities. In the present study, high gravity technology, which is a wellestablished technique for chemical engineering process intensification, is used to study the mass transfer in the diffusing process of LbL self-assembled technique, clarify the mechanism of the rapid construction, and promote the LbL self-assembly from laboratory instruments to industrial equipments. Received: April 17, 2012 Revised: May 27, 2012 Published: May 28, 2012 9849

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High gravity technology is an effective industrial method to intensify mass transfer and heat transfer in multiphase systems (Scheme S1 in the Supporting Information), which has been studied since 1979.32 The rate of mass transfer between a gas and liquid in high gravity equipments is 1−3 orders of magnitude larger than that in a conventional packed bed. In the high gravity equipment, the fluids going through the packing are spread or split into very fine droplets, a thread, and thin film under the high shear field, which results in intense micromixing between the fluid elements and leads to several industrial applications (Figure S1 in the Supporting Information).33,34 Under the high gravity field, when the solution of building blocks is continuously pumped into the cavity, it sprays through the nozzle. The formed mist flow affects the rotating substrates and immerses the surfaces at a high centrifugal force, which has integrated featured properties of dipping assembly, spin-andspray LbL. To date, studies on the combination of high gravity technology and LbL self-assembly are lacking. Herein, we have developed a facile and rapid method to fabricate a polyelectrolyte multilayer under high gravity field and investigated the difference of mass transfer in the diffusing process between LbL self-assembled technique under high gravity field (HG-LbL) and dipping assembly. In the present study, we have employed polyethyleneimine and zinc oxide nanoparticles, which is a well-known UV blocking material with typical absorption properties in the range of 300−400 nm, as building blocks and applied hydrogen bonding as the driving force to construct the multilayer under HG-LbL and dipping assembly. The results show compared with dipping assembly that HG-LbL can highly improve the utilization and adsorption efficiency of building blocks by hastening the diffusing process and meanwhile the resulting multilayer films still achieve comparable quality as that prepared from dipping assembly.

Scheme 1. Illustration for the HG-LbL Procedures of PEI and ZnO Nanoparticlesa

a

Building blocks for HG-LbL self-assembly: PEI and ZnO nanoparticles.

form the self-assembled monolayer terminated with −NH2 functional groups at the exposed surface. Subsequently, the quartz or silicon substrate was rinsed with deionized water and dried for further experiments. The amino units provided the donor to construct the assembly by cooperating bonding. 2.3. HG-LbL Process of PEI/ZnO Multilayers. The stepwise assembly under high gravity field was conducted using the high gravity equipment, as shown in Scheme 1. The PEI-modified substrates were rotated at 2400 rpm. The ZnO nanoparticles suspension in ethanol (0.1 mg/mL) was pumped into the cavity at 41.6 mL/min for 1 min, and then the electrical machine was raced for 30 s. Ethanol was pumped into the cavity to rinse the substrates for 1 min, and then the electrical machine was raced for 30 s. An ethanol solution of PEI (0.1 mg/mL) was pumped into the cavity at 41.6 mL/min for 1 min, and then the electrical machine was raced for 30 s; the substrates were again rinsed for 1 min. Multilayer assemblies were generated by repeating the last three steps in a cyclic fashion. In the controlled experiment, the multilayer was formed via a similar procedure but at different rotating rates (600, 1200, 1800, and 3000 rpm). The concentrations of ZnO nanoparticles (0.01 and 0.005 mg/ mL) were also investigated.

2. EXPERIMENTAL SECTION 2.1. Materials and Instruments. The following chemicals were used as supplied: polyethyleneimine (PEI, Mw = 1800 g/mol) from Alfa Aesar and H2SO4 (98%), H2O2 (30%), and ethanol from Sinopharm Chemical Reagent Beijing Co., Ltd., Beijing, China. Zinc oxide (ZnO) nanoparticles, which were capped by a certain polymer with carboxylic acid groups, were purchased from Nanomaterials Technology Pte. Ltd., Singapore. The diameters of ZnO nanoparticles were about 10 nm according to our former report.35 The high gravity equipment for HG-LbL is made by ourselves as shown in Scheme 1. The inner diameter of the rotator is 20 mm and the outer diameter 50 mm. The internal axial width of the rotator is 10 mm, and the external width is 17 mm. Four slots (14 mm ×12 mm ×1 mm) are set in four directions inside of the rotator. The distributor consists of two pipes (5 mm ×1 mm), with each having a hole at a diameter of 1 mm. The rotator is installed inside the fixed casing with a diameter of 100 mm and rotates at the speed of several hundreds to thousands rpm. The solutions of the PEI and suspension of the ZnO nanoparticles are pumped into the cavity by a peristaltic pump (BT100-2J) from Baoding Longer Precision Pump Co., Ltd. UV−vis spectra were obtained on a Hitachi U-3900H spectrophotometer. Surface morphologies of polyelectrolyte multilayers were characterized with atomic force microscopy (AFM, Dimension 3100) from Veeco, U.S.A. 2.2. Substrate Preparation. A quartz or silicon substrate was immersed into a fresh piranha solution (30% H2O2/98% H2SO4, v/v = 1:3; CAUTION: Piranha solutions are very aggressive and corrosive; appropriate safety precautions should be utilized, including the use of acid-resistant gloves and adequate shielding.) and heated until no bubbles were released. The substrate was carefully rinsed with deionized water and dried with nitrogen. The substrates were then immersed in an ethanol solution of PEI (0.1 mg/mL) for 30 min to

3. RESULTS AND DISCUSSION 3.1. Sequential Adsorption of PEI/ZnO via HG-LbL. The stepwise assembly of the multilayer film under high gravity field with a rotating rate of 2400 rpm, which employed hydrogen bonding as the driving force, is characterized by UV− visible absorption spectroscopy. As shown in Figure 1, a sudden strong absorption occurred from 340 to 360 nm. This result can be attributed to the absorption of ZnO nanoparticles. The absorption values at 340 nm versus the corresponding number of bilayers are analyzed. An almost linear increase in the absorbance intensities of ZnO nanoparticles is observed upon the build-up of the multilayer structure (inset of Figure 1). This result implies an identical content of the building blocks in the different layers. In the controlled experiments, the conventional dipping LbL assembly has been carried out under the same condition except for the involvement of high gravity field as shown in Figure S2. The absorbance of the PEI/ZnO nanoparticle multilayer fabricated under HG-LbL reaches 9850

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Figure 1. UV−visible spectra of PEI/ZnO multilayer films fabricated from ethanol solutions and measured after the deposition of each layer. Inset: full squares correspond to the dependence of the absorbance of ZnO at 340 nm.

Figure 2. UV−visible absorption of ZnO at 340 nm versus the deposition time with ZnO nanoparticles suspension concentrations of 0.1 (dot), 0.01 (square), and 0.005 mg/mL (triangle) under high gravity field.

similar values as that fabricated by dipping. This phenomenon demonstrates that, although high gravity equipment provides a vigorously turbulent condition, the building blocks can still diffuse onto the surface of the substrates, and LbL self-assembly can proceed successfully. 3.2. Effect of Concentration on HG-LbL. The diffusing process of ZnO nanoparticles in LbL self-assembly can be described by Fick’s first law,36 as shown in eq 1. Fick’s first law has been widely used to calculate the molar flux in a chemical engineering process. In eq 1, JA is the molar flux of diffusing component A, DAB is the molecular diffusing coefficient of component A though component B without eddy diffusion, which is a physical constant and concerns with temperature, pressure, and component ratio, εM is the eddy diffusing coefficient independent of the solution property and correlating to rotating rate, and dcA/dz is the concentration gradient in the direction of diffusing, whose value is negative because of the diffusion process toward the direction of decreasing concentration. JA = −(DAB + εM)

dcA dz

3.3. Effects of High Gravity Field on the LbL SelfAssembled Process. Molar flux JA increases with increasing εM value. Thus, another way to accelerate the LbL procedure is to intensify the turbulence of the diffusing process. Compared with dipping LbL assembly without turbulence at zero εM, high gravity field provides a turbulent condition with a high εM value and should remarkably hasten the LbL process. To clarify the hypothesis, the absorption kinetics of ZnO nanoparticles suspension with different concentrations under high gravity field and dipping process were compared. In Figure 3a, the absorption time to reach the equilibrium is shortened from 4 min in dipping LbL to 45 s under HG-LbL with a rotating rate of 2400 rpm and 0.1 mg/mL ZnO nanoparticles concentration, which has improved the diffusing efficiency by more than five times. For the concentration of 0.01 mg/mL, the deposition time decreases from 35 to 3 min (Figure 3b), nearly diminishing by 12 times. The deposition time remarkably decreased from 100 to 7 min when the concentration is 0.005 mg/mL (Figure 3c). These results demonstrate that the condition of high gravity field evidently affects the rate of LbL procedure. Compared with the conventional dipping LbL self-assembly, high gravity field condition exhibits high diffusing efficiency, especially at low concentrations, which may improve the utilization of building blocks and be beneficial for commercial application. Moreover, the HG-LbL method can not only be used to accelerate diffusing process of nanoparticles but also can extend to other polyelectrolyte systems (Figure S3 in the Supporting Information). Several scholars studied the rapid construction of multilayers and hypothesized that their methods can help the substrate absorb more building blocks than the dipping method. Before equilibrium adsorption, more building blocks can surely be absorbed onto substrates under intensified HG-LbL than that by under dipping LbL (Figure 3). However, all of the saturated adsorption capacities under different conditions are similar. This result suggests that the enhanced methods only speed the diffusing process of the building blocks rather than the equilibrium of the adsorption and desorption. In other words, when the adsorption equilibrium is reached, the status of the absorption between PEI and ZnO nanoparticles should be identical either under HG-LbL or in dipping LbL, which can be proved via atomic force microscope (AFM) characterization.

(1)

From eq 1, we can observe clearly that the molar flux (JA) can be increased by increasing the concentration gradient or by intensifying the turbulence to increase εM. To investigate the effect of concentration on HG-LbL, we have conducted absorption kinetics of ZnO nanoparticles suspension under HG-LbL at a rotating rate of 2400 rpm with three concentrations (0.1, 0.01, and 0.005 mg/mL). In Figure 2, the blue line with triangles is the UV−visible absorption of 0.005 mg/mL ZnO nanoparticles at 340 nm versus the deposition time, which presents a typical absorption kinetic curve, and the time to reach adsorption equilibrium is approximately 7 min. When the concentration is increased to 0.01 and 0.1 mg/mL, the absorption kinetic curve becomes sharp, and the time for equilibrium is shortened to 3 min and 45 s, respectively. In this experiment, DAB can be regarded as a constant in the applied concentration range, and εM does not change because of the identical rotating condition. Thus, JA is only correlated with concentration gradient. Furthermore, the thickness of the diffusing area remains the same. Thus, JA is only dependent on concentration. These phenomena indicate that the process of LbL self-assembly can be accelerated by increasing the solution concentration. 9851

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Langmuir adsorption equation with Lambert−Beer’s law, the relationship between the absorbance and C can be correlated as below: Abs =

A mC km + C

(3)

In eq 3, Am is the UV−visible absorbance when the adsorption equilibrium has been reached, and Am is also the maximum of Abs. km is a constant. Equations 3 and 2 are correlated and lead to eq 4 ⎛ ∂Abs ⎞2 ∂Abs ∂ 2Abs 2D ⎜ ⎟ =D + ∂t A m − Abs ⎝ ∂x ⎠ ∂x 2

Matlab has been employed to solve this partial differentiation equation described in eq 4 to get theoretical data. Then the theory curve and the experimental curve in the same experiment conditions have been compared in Figure 4. From the kinetics absorption curves of ZnO nanoparticles under HG-LbL at a rotating rate of 2400 rpm, we can obtain that the molecular diffusing coefficient increases with the concentration of ZnO nanoparticles. When ZnO nanoparticles is at the concentrations of 0.1, 0.01, and 0.005 mg/mL, the DAB + εM values are 1.25 × 10−14, 1.15 × 10−15, and 9.25 × 10−16 m2/s, respectively. Under the condition of dipping process, the molecular diffusing coefficient DAB become 6.25 × 10−17, 5.55 × 10−17, and 4.00 × 10−17 m2/s, corresponding to the concentrations of ZnO nanoparticles at 0.1, 0.01, and 0.005 mg/mL (Figure S5 in the Supporting Information). From Figure 4, we can see clearly the theoretical curves match well with the experimental curves, especially under the high gravity field. 3.4. Surface Morphologies of PEI/ZnO Multilayers under High Gravity Field and by Dipping Assembly. We have employed AFM to characterize the surface morphologies of one PEI/ZnO layer and study the adsorption status of ZnO nanoparticles after equilibrium (Figure 5). From AFM height images, the surface is totally covered by a layer of PEI/ZnO with a firm and homogeneous appearance, and the ZnO nanoparticles are uniformly distributed with few large aggregates. Besides, surface morphologies after the adsorption equilibrium showed few differences for three different concentrations (0.1, 0.01, and 0.005 mg/mL) under HG-LbL at 2400 rpm or in dipping assembly. Further measurements of surface roughness show that all the roughness values are at approximately 3 nm. The AFM analyses match well with the adsorption kinetics data. This result suggests that after adsorption equilibrium intensified HG-LbL methods and dipping LbL have few differences in adsorption status and essential. Therefore, the increased efficiency in HG-LBL should contribute to the enhanced diffusing process of the building blocks instead of adsorption/desorption equilibrium. 3.5. Mechanism for the Diffusing Process under High Gravity Field. In static surface self-assembly (v0 = 0), a diffusing layer remains between the interface and the bulk solution, in which a gradient of concentrations exists and drives the mass transfer (Scheme 2a). The diffusing velocity depends on the components of the solutions and their concentrations. When the flow velocity is under a criteria value of turbulent flow (0 < v0 < vc1, where vc1 is the first criteria velocity), the flow systems is laminar flow, which can be regarded as an ideal model in hydrodynamics. The diffusing conditions are the same as that under static cases because molecular diffusing dominates

Figure 3. Adsorption kinetic curves of ZnO nanoparticles after deposition at different concentrations: (a) 0.1, (b) 0.01, and (c) 0.005 mg/mL. The data points with full dots and squares are obtained under high gravity field and dipping assembly, respectively.

Moreover, the kinetics absorption curve of ZnO nanoparticles suspension with different concentrations under high gravity field fits well with that of obtained by theoretical simulation. In order to introduce the variable of the time for assembly, we applied Fick’s second law to simulate the diffusing process. The Fick’s second law can be presented as follows: ∂C ∂ 2C =D 2 ∂t ∂x

(4)

(2)

In eq 2, D is the is the molecular diffusing coefficient. In the high gravity, D equals DAB + εM, and in dipping process, D should be DAB, C is the parameter which is varied with time (t) and diffusing thickness (x). In this experiment, the relationship between the absorbance and adsorption equilibrium can be described by Langmuir adsorption isotherm. Combining the 9852

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Figure 5. AFM height images for one bilayer of PEI/ZnO after adsorption equilibrium at concentrations of 0.1 mg/mL (a) by HGLbL and (b) by dipping LbL; 0.01 mg/mL (c) by HG-LbL and (d) by dipping LbL; 0.005 mg/mL (e) by HG-LbL and (f) by dipping LbL. All of the scan sizes are 5 μm × 5 μm.

Scheme 2. Schematic Illustration of Relationships between the Diffusing Process and Flow Behaviora

a (a) Static fluid, (b) laminar flow, and turbulent flow (c) before and (d) after the diffusing process is disturbed by the flow behavior.

Figure 4. The kinetics adsorption curves of ZnO nanoparticles at different concentrations: (a) 0.1, (b) 0.01, and (c) 0.005 mg/mL under high gravity field. The red dotted lines are the simulated results while the blue solid lines are experimental data.

we have studied the absorption kinetics of ZnO nanoparticles suspension under HG-LbL at rotating rates of 600, 1200, 1800, 2400, and 3000 rpm, whose related centripetal accelerations are 98.6, 394.4, 887.4, 1577.5, and 2464.9 m/s2 correspondingly. As shown in Figure 6, the time for equilibrium decreases with increasing rotating rates. Compared with the dipping assembly, the time for equilibrium with a rotating rate of 600 rpm is not obviously shortened. This result suggests that the thickness value of the residual laminar flow is still larger than that of the diffusing layer and the diffusing process is dominated by molecular diffusing. From 600 to 1800 rpm, the time to reach the absorption equilibrium decreases remarkably. Raising the rotating rates of the high gravity equipment leads to increasing turbulence with larger εM. value, which further diminishes the thickness of the laminar boundary layer and decreases the thickness of diffusing layer. Thus, the adsorption process is accelerated because of a thinner diffusing layer. At 1800 rpm, small differences on the adsorption time are observed with increasing rotating rates. This result might be explained that the

in the laminar flow, and the thickness of the diffusing layer does not change (Scheme 2b). With further increasing flow velocity, the flow behavior gradually transfer from laminar flow to turbulence started with the center of the bulk solution (vc1 < v0 < vc2, where vc2 is the second criteria velocity), as shown in Scheme 2c. Under this condition, there remains a layer of residual laminar flow near to the substrate. In this initial stage, the thickness value of the residual laminar flow is larger than that of the diffusing layer, and the diffusing process remains the same. When the velocity is larger than vc2 in Scheme 2d, a highly turbulent flow is achieved, and the diffusing layer is disturbed with decreasing thickness of the laminar boundary layer, which should hasten the diffusing process of ZnO nanoparticles. This should be the mechanism why high gravity field can speed up the LBL process. 3.6. Effect of Rotating Rates on HG-LbL. To demonstrate the hypothesis for the mechanism of HG-LbL, 9853

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; E-mail: [email protected]. Author Contributions

L.X.M. and M.J.C. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant No. 21074008), Beijing Nova Program of China (Grant No. 2009B011), Program for New Century Excellent Talents in University (Grant No. NCET-100211), the Fundamental Research Funds for the Central Universities (ZZ1104), the Fok Ying Tung Education Foundation (131013), and Open Project of State Key Laboratory of Supramolecular Structure and Materials (SKLSSM201213).

Figure 6. Adsorption kinetic curves of ZnO nanoparticles fabricated via HG-LbL with different rotating rates: 600, 1200, 1800, 2400, and 3000 rpm.

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thickness of the laminar boundary layer on the substrate almost reaches its lower limit and could be no thinner at 1800 to 3000 rpm. Hence, the experimental data correspond well to our previous hypothesis on the relationship between the diffusing layer and the laminar boundary layer.

4. CONCLUSIONS In the present study, we have developed a facile and rapid method to fabricate a polyelectrolyte multilayer under high gravity field, which can highly improve the utilization and adsorption efficiency of building blocks. In contrast to conventional self-assembly, the diffusing process is hastened, especially at low concentrations of building blocks, and the time to reach adsorption equilibrium has been shortened by more than five times. With the faster self-assembly process, the resulting multilayer films still achieve comparable quality as that prepared from dipping assembly. Moreover, we have discussed the mechanism for the diffusing process under high gravity field, which suggests that the LbL process can be hastened by raising the concentration gradient or by intensifying the turbulence. The latter diminishes the thickness of the laminar boundary layer and decreases the thickness of the diffusing layer. Moreover, the HG-LbL can also be used to improve the quality of the multilayer formation for LbL assembly concerned with two polyelectrolytes as building blocks (Figure S4 in the Supporting Information). We believe that the HG-LbL can be widely used to accelerate surface absorption process on various substrates, especially for LbL self-assembly. In addition, this method is promising in promoting the LbL self-assembly in industrial productions and commercial applications.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

Description of the high gravity technique; dipping LbL assembly of PEI/ZnO nanoparticles; effects of HG field on the LbL assembled process of PAH-Por and PAA; AFM images of (PDDA/PSS)20 multilayers deposited by HG-LbL and conventional dipping LbL; and comparison of the simulated and experimental curves of ZnO nanoparticles kinetics adsorption. This material is available free of charge via the Internet at http://pubs.acs.org. 9854

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