Van Keuren Lab

Department of Physics
Georgetown University


Nanoparticle synthesis
We use a solvent displacement technique known as "Flash Nanoprecipitation (FNP)" or the "reprecipitation method"
to create nanoparticles in solution. This method is based on rapidly mixing a solution into a miscible non-solvent,
as shown schematically in the figure. The solubility of the solute decreases rapidly, usually inhomogeneously and
over various time scales. The molecules aggregate and depending on the conditions, may form nanoparticles/nanocrystals.
We have used this method to investigate the formation of single component nanoparticles such as naphthalene,
anthracene, Magnesium phthalocyanine and polymers such as polystyrene and PMMA. More recently, we have
expanded this method to create multicomponent nanoparticles, for example, creating nano-co-crystals of two different
molecules or by combining two different polymers into single nanoparticles. We have recently found that polymer
blend nanoparticles can be formed using nanoprecipitation. We prepared stable suspensions of nanoparticles of the
blend polystyrene/poly methyl (methacrylate) with radii below 200 nm. TEM images shown in the figure, seem to
indicate that both polymers are incorporated into single nanoparticles, with an intraparticle phase separation into
a core-shell structure. We continue to study these materials in order to answer question such as: How do components
partition into separate phases in the nanoprecipitation of multicomponent polymer systems? What is the mechanism of
the particle formation? Can rapid mixing lead to colocalization of different polymers into single nanoparticles,
even those that are immiscible? How will the glass transition affect the particle properties?






Charge transfer nano-cocrystals for organic electronics
We have been using reprecipitation to create nanocrystalline wires of various charge transfer co-crystals, in particular phenothiazine:tetracyanoquinodimethane PTZ:TCNQ). Recent theoretical calculations have shown that it could be have a useful organic semiconductor. Organic semiconductors are being heavily investigated for applications in electronics, where they could replace Silicon as the active material in transistors and related devices. While their electronic properties may not match those of Silicon, they have other advantages such as flexibility, low cost, and ease of processing. In addition, many organic semiconductors possess good mobilities for both holes and electrons, making them ambipolar, compared to doped Silicon, which is bipolar. New types of devices have been proposed using ambipolar semiconductors, for example for use radio frequency inverters. In addition, the ambipolar nature may make them useful as charge transport additives in organic photovoltaic devices. SEM images of wires that were freeze-dried following nanoprecipitation show nanowires with diameters ranging from ~100 nm to several microns, and lengths from ten to several hundred microns. We have been characterizing the optical and electronic properties of these materials in collaboration with Prof. Patrick Vora at George Mason University and Prof. Pratibha Dev at Howard University. With support from the Georgetown Environmental Initiative, we are constructing a characterization station with a solar simulator for device efficiency measurements and a wavelength tunable raster system for measuring spatial and wavelength dependence response.


Liquid-liquid Phase Separation
The key initial stage in nanoparticle formation using FNP is the phase separation of the solute/good solvent from the nonsolvent. This type of liquid=liquid phase separation (LLPS) is of interest in a wide variety of areas, including colloidal synthesis and biomlecular condensates. While LLPS has been studied in complex biological system, there is less work on simple model systems consisitng of only a few components. We are currently studying two model systems: polysthylene glycol and dextran (PEG-DEX) and polystyrene-polybutadiene. We are specfically interested in understanding how the presence of additional small molecules affects both the thermodynamics and kinetics of the phase separation. Our primary characterization of these materials is through phase-contrast microscopy, along with image analysis with methods such as differential dynamic microscopy (DDM). We have found, for example, that very small amounts of biomolecules such as urea can drastically change the temperature at which LLPS occurs, as well as the kinetics of the separation.

MRI contrast agents
We are using miniemulsion polymerization to create novel nanoparticles that show promise as contrast agents for magnetic resonance imaging. These particles are based on oxo-metallic clusters of Mn and Fe,
and possess high spin-states that result in large magnetic interactions with surrounding water molecules. In order to maintain the stability of the clusters under biologically relevant conditions, we encapsulated them in polystyrene nanoparticles using miniemulsion polymerization. Our current work involves using newer types of clusters, which require tailoring the synthetic chemistry in order for miniemulsion polymerization to be possible. These materials have shown good value of relaxivities for both T1 and T2 weighted imaging. In order to take advantage of this, a second part of this project consists of the development of image fusion algorithms. These algorithms would combine T1 and T2 weighted images, highlighting areas in which there is enhancement of both signals relative to the background. These regions are the ones that are likely to contain contrast agent, so the algorithms would be able to significantly increase their sensitivity.