Imagine scattering tiny sensors over large areas to detect explosives and environmental hazards. To be widely deployed, such devices must be made of cheap, reusable, self-assembling materials designed to be highly selective for target molecules such as TNT. ORNL and University of Tennessee (UT) researchers propose that a large variety of polymers be synthesized on platforms smaller than a match tip, to determine which combinations make highly sensitive, selective, and affordable sensors.
ORNL’s Phil Britt, Mike Sigman, and A. C. Buchanan, along with ORNL-UT Distinguished Scientist Jimmy Mays, all working in ORNL’s Chemical Sciences Division (CSD), have been developing the chemistry for growing ultrathin polymer coatings on microsensors. These devices include quartz crystalline microbalances, surface acoustic wave sensors, and ORNL-developed microcantilevers, which are thinner than a human hair.
These sensors vibrate at characteristic frequencies. When a target chemical adsorbs on a coated sensor or interacts with it, the additional mass or stress on the sensor platform causes a detectable shift in its frequency, indicating the presence and concentration of the target material.
Possible targets for such sensors might be explosives hidden at airports or toxic compounds introduced into drinking water supplies. Sensors distributed in groundwater and subsurface soil could detect carbon tetrachloride and other chlorinated organic pollutants present at Department of Energy sites.
A polymer is a natural or synthetic compound built from up to millions of small, simple molecules called monomers. A polymer is a long-chain molecule, and each monomer is a link in the chain.
The CSD team of scientists will be using two different methods for inducing monomers (e.g., styrene) to stick to a sensor platform and then link up with additional monomers. The linked-up monomers form a polymer coating (e.g., polystyrene), which can then be linked to monomers of a different type (e.g., isoprene forming polyisoprene). In this way, a diblock copolymer could be synthesized on a sensor platform. Add monomers of a third type and a triblock copolymer could be grown, creating pores or rods of the right size and shape to trap or plug into a specific target molecule.
Mays makes block copolymers using anionic polymerization, a labor-intensive method that requires a high vacuum in a sealed vessel. Britt and Sigman can produce block copolymers using the recently developed method called atom transfer radical polymerization, which is easier and cheaper than anionic polymerization but affords slightly less control in making the desired coating.
The silica or gold surface of a sensor would be coated with an initiator, which produces a radical that starts the polymerization reaction. The initiator would be an alkoxyamine, which is thermally unstable, or an organic halide, which is decomposed by a transition metal complex that typically contains copper. In either case, the initiator forms a molecule with an unpaired electron (called a radical) that adds to the double bond of a monomer, such as styrene, to create a new radical. This radical undergoes a reaction to reform the initiator so that the concentration of radicals is very low, reducing the probability of unwanted side reactions. Thus, the polymer chains grow by a controlled, or pseudo-living, process. After all the styrene is consumed, a second monomer, such as methyl methacrylate, could be added and polymerized to make a diblock copolymer of polystyrene and polymethyl methacrylate.
Mays and his colleagues have shown that silicon oxide or gold particles (simulating sensor surfaces) can be coated with diphenylethylene, which can be reacted with butyllithium to form a negatively charged “anionic initiator.” Mays, working with his students and collaborators, has used this initiator to graft onto these particles vertical chains made of a diblock copolymer consisting of polystyrene and polyisoprene.
“This ionic process gives us better control over the structure of the polymer we make,” Mays says. “But we plan also to try the living radical process, because it will provide a simpler process and allow us to access a wider range of structures, yet control the composition and alignment of the chains.”
“Using these methods, we can do combinatorial chemistry by trying many different combinations of many different monomers in very small samples,” Sigman says. “If the world had only eight monomers, we could combine them in up to 64 different ways to make 64 different coatings.”
“We could have a large array of sensor platforms, each with a different polymer coating,” Britt says. “We could pass a vapor over the platforms to identify the block copolymer that best attracts the molecules of the target vapor. This information could lead to a highly selective sensor.”
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