Functionalized Nanoparticles
Schematic structure of a surface-functionalized CdS nanoparticle
Surface-Activated Nanoparticles
By its simplest definition, a nanoparticle is a tiny chunk of material. But it's a tiny chunk of material in a size domain where its properties are determined by size. For many materials, their physical, and even chemical, properties are as dependent on particle size and shape as they are on chemical composition when made into nanoparticles. For example, all the vials in the picture contain the same substance, cadmium sulfide. The two vials on the left are bulk powder dry and in acetone, where it is completely insoluble. The vials on the right contains 30 angstrom diameter nanoparticle dry and in acetone, where it is completely soluble.
We made these nanoparticles by attaching small organic groups to the surface via a covalent linkage. The surface groups serve the dual purpose of preventing the particles from coalescing into bulk powder, and providing a chemically active functionality for subsequent reactions.
Natural (left) and 30 angstrom butyrate-functionalized CdS (right), dry and in acetone
Electroactive Nanoparticle Surfaces
Over the course of our research, we produced CdS nanoparticles having a wide variety of surface functionalities, including -NH2, -OH, -CHO, -Cl, -COOH, -COOR, and -CONR2, to name a few. We then used the reactivity of these groups to make interesting functional materials by attaching electroactive, polymeric, optically active, and catalytic groups, ligands for transition metal binding, and even other nanoparticles. One interesting attachment involved the esterification of a phenbipyridine carboxylic acid with statistically one surface hydroxyl group . When reacted with bis(bipyridyl)dichlororuthenium, a coordination compound was formed, attaching the bis(bipyridyl)ruthenium moiety to the CdS surface. We observed a striking correlation between the Ru(II)/Ru(III) redox potential and the Hammett parameters of other substituents on the surface. This study proved facile electronic communication between surface groups mediated by the semiconductor core.
Nanoparticles for Flow Chemistry
For over a hundred years, chemical reactions have been carried out with the familiar apparatuses consisting of reaction flasks, heating baths, reflux condensers, addition funnels, stirrers, etc. For over a hundred years, products have been "fished out" of reaction mixtures by distillation, crystallization, chromatography, and a variety of other methods. And for over a hundred years, chemical reactions have been messy, bulky, cumbersome, unreliably unscalable, and wasteful, and failed reactions have been expensive and in some cases, disastrous.
Recent years, however, have seen the advent of flow chemistry. In a flow chemical reaction, the reaction vessel is a small-bore tube, a region of which is heated or cooled. Reagents are pumped into a small mixing chamber just before flowing into the reaction tube, and exit the tube as product. Flow chemistry has several remarkable advantages over conventional synthetic methods. For example, (1) flow flushes product from the tube as it is formed, whereas in a conventional system, prolonged exposure to reaction conditions could result in deterioration of the product. (2) Flow reactions are safe. Let's say one is conducting a reaction that produces an explosive intermediate, such as a diazonium salt. Five grams of explosive compound detonating in a 500 mL flask of flammable solvent would be a disaster. However, in a flow reactor, the explosive intermediate exists only in a short length of small-bore (and thick walled) tube with a total volume of a few milliliters at most. (3) Flow reactions can be conducted at temperatures well above the normal boiling point of the solvent. The force from a simple syringe pump is enough to produce in excess of 100 atm pressure inside a capillary bore flow reaction vessel. Imagine running a Grignard reaction in diethyl ether at 250 degrees celsius. (4) It's well known that trying to scale up a conventional reaction frequently opens a can of worms. However, in a flow reaction, scaling up is enacted by simply flowing longer.
One of our research areas involves the fabrication of novel nanoparticle systems for use in flow chemistry. This research is in collaboration with Professor Michael G. Organ of the University of Ottawa, a leading pioneer of flow chemistry. His flow apparatus is used to evaluate our nanoparticle systems, and is shown below.
Photo Credit: M. G. Organ, Department of Chemistry, University of Ottawa
Amine functionalized 55 nm Fe3O4@SiO2 nanoparticles. Magnification: 200,000X
with a magnet.AVIAmine functionalized 55 nm Fe3O4@SiO2 flowing in a 100 micron bore capillary flow tube, and steered by a neodymium iron boron magnet. Magnification: 30X
Core-Shell Nanoparticles for Flow Separations
Once a product is produced in flow, it still needs to be isolated from the reaction mixture. We are researching the development of novel nanoparticles that can "fish out" product from a flowing chemical reaction. Our system is actually two nanoparticles in one, a so-called "core-shell" nanoparticle. The core is 20 nm diameter Fe3O4, and the encapsulating shell is a 60 nm diameter envelope of silica, as observed in the TEM image here. We then functionalize the silica shell with a functionality that reacts specifically and reversibly with the product. Fe3O4 nanoparticles posses the unique property of superparamagnetism. They are highly attracted to a magnet, but unlike bulk (ferromagnetic) Fe3O4, the nanoparticles do not become permanently magnetized.
Hence, when mixed into, and flowed with, the reaction mixture, the nanoparticles selective bind the product as it's formed, and can then be magnetically steered into one arm of a Y-junction in the flow tube, thus isolating the product cleanly, economically, and in-line. The nanoparticles are brought to a chamber where the product is detached, then magnetically steered back to the input of the flow tube. A video of these functionalized core-shell nanoparticles in action is presented here. The suspension of nanoparticles is admitted into the reaction mixture through the upper arm at the input of a microbore capillary flow tube (100 micron diameter). As they pass the magnet, the nanoparticles agglomerate and steer towards the magnet. They then exit the reaction capillary through the lower arm at the right of the flow tube.
