Department of Chemical Engineering, The Pennsylvania State University
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Shahravan, A., Matsoukas, T. Encapsulation and Permeability Characteristics of Plasma Polymerized Hollow Particles. J. Vis. Exp. (66), e4113, doi:10.3791/4113 (2012).
In this protocol, core-shell nanostructures are synthesized by plasma enhanced chemical vapor deposition. We produce an amorphous barrier by plasma polymerization of isopropanol on various solid substrates, including silica and potassium chloride. This versatile technique is used to treat nanoparticles and nanopowders with sizes ranging from 37 nm to 1 micron, by depositing films whose thickness can be anywhere from 1 nm to upwards of 100 nm. Dissolution of the core allows us to study the rate of permeation through the film. In these experiments, we determine the diffusion coefficient of KCl through the barrier film by coating KCL nanocrystals and subsequently monitoring the ionic conductivity of the coated particles suspended in water. The primary interest in this process is the encapsulation and delayed release of solutes. The thickness of the shell is one of the independent variables by which we control the rate of release. It has a strong effect on the rate of release, which increases from a six-hour release (shell thickness is 20 nm) to a long-term release over 30 days (shell thickness is 95 nm). The release profile shows a characteristic behavior: a fast release (35% of the final materials) during the first five minutes after the beginning of the dissolution, and a slower release till all of the core materials come out.
1. Preparation of Silica Nanoparticles for Deposition
2. Preparation of the Vacuum System
3. Plasma Deposition Process
4. Preparation of Hollow Particles by Dissolution of Core Material
5. Characterization of Permeability (Rate of Core Release)
Materials: Potassium chloride for core materials
6. Representative Results
We have applied this process to a variety of core materials, including oxides (silica), salts (KCl) and metals (Al), as shown in Figure 2. Transmission electron microscope has been used to confirm the radial uniformity of the films and to measure their thickness. We have successfully coated particles ranging from 37 nm to 200 nm in diameter (Figure 2) but there is no fundamental limitation on the size of particles that can be treated by this method. The rate of shell deposition is approximately 1 nm/min. This rather slow rate makes it possible to control the thickness of the films quite accurately via the deposition time. The plasma-polymerized shell is a permeable barrier, as demonstrated by the fact that the core material can removed by etching or dissolution. Figure 3 shows the hollow shells that remain after the silica core is removed. The removal of the core is complete and the radial uniformity and thickness of the films are quite high. For the purposes of evaluating the permeability through these films, we switched to KCl as the core material since the dissolution of KCl can be monitored very easily through the ionic conductivity of the solution. Figure 4 shows the release of KCl from the core for four samples with different thickness, 20 nm, 40 nm, 75 nm, and 95 nm, respectively. Coated KCl particles were suspended in water and the conductivity of the solution was followed for a period of 30 days. In addition to the four samples, a control that consists of uncoated KCl particles was also monitored. Uncoated KCl particles dissolve within a very short time of approximately 1 min. By contrast, coated KCl shows a significantly slower release rate. The release profile of the coated particles is characterized by initial burst that takes place within the first hr, followed by a much slower release that takes several days to complete, depending on the thickness of the film.
Figure 1. Schematic representation of the preparation of nanoparticles, plasma deposition, and hollow particle formation.
Figure 2. TEM images of coated (a), (b) silica particles with d=200 nm, (c) silica particle with d=37 nm, (d) aluminum with d~100 nm, and (e) KCl particle with d=100 nm
Figure 3. TEM images of hollow particles after etching the (a), (b) silica core with diameter of 200 nm, and (c) KCl core.
Figure 4. Effect of shell thickness on the release profile. The inset graph shows the release during the first hr.
One of the biggest challenges in coating nanoparticles is to provide a compatible chemistry between coating and the substrate 1,2. The methodology described here has the advantage that it is not material-specific. Plasma polymers show excellent adhesion on a variety of substrates, including hard metals (Figure 2(c)), silica (Figure 2(c)), silicon, or soft materials (e.g., polymers) without the need for any special surface modification 3,4,5. The technique has the further advantage that it is not limited by the size of the core particle and is easily adaptable to particles in the nano- and micrometer range. The thickness of the coating is controlled by the deposition time and can be easily varied from few to several hundred nm. Another level of control is provided by the organic precursor that is used to produce the coating. For example, the hydrophobic character of the coating can be varied by suitable selection of the precursor 6. One aspect of the process in need of further improvement is achieving the uniformity of coating. We estimate that approximately 70% of the particles treated in the plasma become fully coated with the remaining 30% showing partial coating. Designing and engineering a new reactor in which plasma surrounds all around the particles during the entire process can improve this.
No conflicts of interest declared.
This work was supported by Grant No. CBET-0651283 from the U.S. National Science Foundation and Grant No. 117041PO9621 from Advanced Cooling Technology.
|Silica particles||Geltech Inc.|
|Potassium chloride (crystals)||EMD Millipore|
|Isopropyl alcohol (99.9%)||Sigma-Aldrich|
|Hydrofluoric acid (48-51%)||VWR international|
|Pipes and flanges||Swagelok||diameter of ¼ and 1 inch|
|roughing pump||Edwards Lifesciences|
|liquid nitrogen trap||A&N Corporation|