Soluble, Processible Molecularly Imprinted Macromolecules
The scientific community's excitement for the use of molecularly imprinted polymers (MIPS) for sensors and filters is rising at an almost exponential rate, with the practical realization that plastics holding impressions of molecules could revolutionize many important aspects of drug discovery, medical diagnostics, chemical analysis and environmental science. Molecular imprinting is a technique for preparing synthetic polymers with recognition sites specific for a target molecule. These artificially generated recognition sites have their shapes, sizes and functionalities complementary to the target molecule, and are capable of rebinding the target molecules in preference to other closely related structures. MIPs have found uses as stationary phases in chromatography, as recognition elements in chemosensors, and as enzyme mimics in catalysis. MIPs are robust, stable and resistant to a wide range of pH, humidity and temperature. They are also relatively inexpensive to produce and can be synthesized for analytes for which no natural antibody exists. Their uses are profound and have advantageous application in nearly every industry. In typical/classical MIP synthesis, the target (or template) molecule is first allowed to interact with a functional monomer in a predetermined orientation. This monomer-template complex is then copolymerized with a crosslinker, leading to a rigid material with the imprint/template complex dispersed throughout. After the extraction of template molecules, the resulting cavities retain their ``memory'' for the target analyte. Research into imprinted materials is still in early stages. Scientists have some critical issues to resolve before the materials can become practical. For one, the density of the active sites needs to be adjustable. These sites need to be accessible to the target and be incorporated into a reporting device or sensor. The standard approach to molecular imprinting does not lend itself to the production of processable materials with these capabilities.
Although the methodologies behind molecular imprinting are well established, chemists at The Johns Hopkins University Applied Physics Laboratory have made an important advancement in the development, processing and exploitation of MIP materials. Using a specific chemical reaction, APL chemists can prepare soluble molecularly imprinted macromolecules. This new formalism addresses current MIP deficiencies by allowing more flexibility in the design and production of molecularly imprinted materials. The physical form of the soluble macromolecule is called a "star polymer" due to several straight chain polymers emanating from a central cross-linked core. The cross-linked core holds the imprinted site. The system is adjustable to most any desired target molecule and can incorporate a broad spectrum of stabilizing materials. This tremendous production flexibility allows each molecularly imprinted material to be engineered to suite the intended use. For example, an inexpensive stabilizing plastic containing toxins is avoided in food spoilage sensors while the same material may be serviceable in a sensor to detect explosives. Because the active sites can be strategically placed, fewer materials are needed overall, reducing costs and waste. A final benefit is realized by the tendency of this procedure to encourage self-assembly of the macromolecules into discrete structures. These structures can facilitate mass transfer and rapid exchange kinetics. APL has been preparing MIP sensors and filters to detect everything from rotten meat to explosives and water pollution for several years. Upon discovery of this procedure, work has been done to further develop and explore the immense possibilities of this novel procedure. The new method enables the synthesis of materials for use as coatings in applications of sequestration and sensing. The proccesible nature of these materials means techniques such as spin casting and ink jet printing can be used to apply the coatings. Materials have been coated on pipettes and used to line vessels for use in sensing and sequester metals out of water such as gold, uranium, silver and copper.
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