The present invention generally relates to molecularly imprinted materials, and, more particularly, to a cross-linked molecularly imprinted polymer material having selective binding sites for phosphate ions, phosphate containing molecules, or a combination of both. 
The concept of molecularly imprinting molecules may be traced to suppositions about the operation of the human immune system made by Stuart Mudd, circa 1930, and Linus Pauling, circa 1940.  Mudd proposed the idea of complementary structures, by which a specific antibody attaches to a specific target or antigen because the shape of the antibody provides a cavity for receiving the shape of the antigen.  The “lock and key” analogy used in explaining the action of enzymes is similarly explained, where enzymes form a “lock” for a particular chemical “key”.  Pauling postulated on how an otherwise nonspecific antibody molecule could be reorganized into a specific binding molecule.  He reasoned that shape specificity was obtained when the body assembled a new protein complement, i.e., antibody, by using a target antigen as a template in arranging the complementary shape of an antibody.  A nonspecific molecule can thus be shaped to the contours of a specific target.  When the target is removed, the shape of the target is retained, which provides an antibody with a propensity to rebind the antigen.  This process is known as molecular imprinting. 
Molecular imprinting is used to create specific recognition sites in substrate materials, for example, polymeric organic materials.  Known molecular imprinting techniques involve crosslinking materials in the presence of a functional monomer or mixture of monomers.  Reactive or coordination sites on a target molecule or complex interact with a complementary site on a functional monomer during the polymerization process, either covalently or by other interactions such as ionic, hydrophobic or hydrogen bonding.  Upon removal of the target molecule from the substrate, a “cavity” or recognition site is formed for receiving a similarly shaped molecule. 
Synthetic production of molecularly imprinted polymers with selective binding sites for a specific target cation is achieved by providing polymers with cavities lined with complexing groups or “ligands” arranged to match the charge, coordination number, coordination geometry, and size of the target cation.  Molecularly imprinted polymers with selective binding sites for anions are made in a similar manner to cations, but typically employ a trapped metal ion that has a high affinity for the target anion.  Cavity-containing polymers may be produced using a specific ion as a template around which monomeric complexing ligands will be self-assembled and polymerized.  Complexing ligands contain functional groups known to form stable complexes with the specific target ion and less stable complexes with other ions. 
A method of molecular imprinting referred to as solution polymerization results in the formation of imprinted sites that are completely encased within the polymer.  To access those sites, the polymer must be ground to produce particles that have exposed sites.  The grinding process, however, produces irregularly shaped particles and also damages the sites by adversely affecting selectivity and activity.  As an alternative method to increase accessibility to the imprinted sites is by using porogens, which are typically inert solvents, which when removed, create pores to allow access to the created binding sites.  Removal of the porogen solvent adversely affects the structural integrity of the polymer, leading to deformation of the sites and loss in specificity and activity.  Lyophilization (or freeze drying) is another way to create a highly porous polymer that allows access to imprinted sites. 
Molecular imprinting is useful in a variety of applications.  For example, the ability to remove a specific component from its environment applies to both environmental and medical fields.  According to the Environmental Protection Agency, approximately 40 percent of the waterways in the United States still do not meet water quality goals and about half of the 2000 major watersheds have water quality problems.  Phosphorus (as phosphate) and nitrogen (as nitrate and ammonia) are major pollutants that enter our waterways as runoff from sewage plants and farmland, posing a clear threat to drinking water and aquatic life.  As the nitrates increase in the environment, they act as plant nutrients, and cause an increase in plant growth.  As plant material dies and decomposes, dissolved oxygen levels decrease.  An increase in nitrates may be followed by an increase in phosphates.  As phosphates increase and the growth of aquatic plants is encouraged, algal blooms can occur.  With an increase in algal growth and decomposition, the dissolved oxygen levels further decrease, causing the death of fish and disruption to the ecosystem.  Nitrogen and phosphorous nutrients both cause increases in plant growth, algal blooms, and decreases in oxygen levels.  Presently, there are existing methods that may remove phosphate but typically require adding large amounts of chemicals to water in order to precipitate insoluble phosphate salts (iron phosphate or aluminum phosphate) out of solution. 
There remains a need for an effective and easy to use method and apparatus for removing phosphate ions from a liquid medium.  There is also a need for improved methods for removing phosphate ion that are environmentally-friendly.