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43453-AC10
Site-specific chemistry on colloidal particles by “particle lithography”
Colloids and nanocolloids have historically been used for single purposes, such as polymer colloids forming films or TiO2 particles scattering light. In recent years more complex particles have been formed that can perform controlled drug delivery or imaging. Future demands will require even more complex, multi-purpose “colloidal devices and machines”. Such assemblies might have a central function like remediation, sensing, or controlled release, but could also be moved magnetically or imaged fluorescently. Various methods exist for assembling complex particles; however, these methods have not provided a general and scalable way to assemble particles of various materials, sizes, and chemical functionalities.
Our lab group seeks to assemble colloidal particles of various materials (e.g., polymer colloids, metals, oxides, semiconductors) and shapes (e.g., spheres, rods) to form colloidal devices and even machines. In order to do this, we pattern individual particles site-specifically using the “particle lithography” method. The essence of the method is that we use surfaces or particles to mask other particles. The patterning can be done with nanoscale precision, and regions of chemistry can be placed on individual particles of micron or roughly 100 nm size. This means that individual colloidal particles can be treated like “colloidal atoms”, and larger “colloidal molecules” can be built.
This past year of the PRF grant has given three significant achievements, consistent with the original objectives of this research. 1) The first is the accurate development of site-specific annulus regions on particles. These regions are formed using the particle lithography process, using two different sizes of coating particles on a larger core particle. The size of the annulus region is quite close to the size predicted from a simple model. This research has been published in Langmuir, and is now leading to applications in controlled release.
A second area of advance concerns forces between nonuniformly-charged colloidal particles, formed using the particle lithography process, as well as the resulting time required for particles to aggregate. The modeling reveals that the critical coagulation concentration of salt is greatly reduced if even small, nanoscale patches of opposite charges are place on particles. Experiments have been done, and they are consistent with the model. This work will soon be submitted for publication. Furthermore, in a separate work, a Brownian dynamics simulation has revealed the time (and scaling) required for particles with small patches to aggregate. Thus, if a particle has a small enough patch, even though it can aggregate if the patch aligns with a second particle just right, the time required for the alignment can be orders of magnitude larger than the rapid flocculation time.
The third advance concerns separation of the particles. We have worked to identify volume fractions of particles for which single particles could be separated from doublets or other aggregates in density gradient centrifugations. The experiments revealed a rich science of sedimentation, and an extensive amount of experiments and modeling have been opened up. This third area represents ongoing research.
The research has been done by graduate students and undergrads working with Professor Velegol. The students have learned various characterization techniques, including field emission scanning electron microscopy (FESEM), confocal microscopy, video microscopy, zeta potential measurements, and many other techniques. This has been supplemented by their learning a great deal of colloidal physics, particularly with regard to interparticle forces (e.g., electrostatic, diffusion-limited aggregation).
Petroleum Research Fund support of our research into site-specific chemistry has greatly advanced our assembly process and our ability to scale production. We have identified separations as a key limiting factor in scale up, and we expect that a combination of modeling, experiments, and new techniques will be required for efficiently and rapidly separating particles in future work.
