Development of Molecularly Imprinted Polymers for Chlorpyrifos, 18-9168

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Principal Investigators
Henry W. Oviatt
Mark Van Dyke
Michael A. Miller

Inclusive Dates: 10/01/99 - 3/15/2002

Background - Chlorpyrifos, chemical name phosphorothioic acid, O,O-diethyl O-(3,5,6-trichloro-2-pyridinyl) ester, is the insecticide commonly known as Dursban, Lorsban, or Pyrinex. Other trade names are Brodan, Detmol UA, Dowco 179, Empire, Eradex, Paqeant, Piridane, Scout, and Stipend. Registered since 1965 and in use in more than 88 countries on more than 50 different crops, this pesticide has been the mainstay of crop damage prevention from parasitic organisms to date and is the most widely used organophosphate pesticide in the world. It has also commonly been used in and around homes for termite treatment and control of other destructive pests. Pressure from the public concerned with safety in and around their homes, however, has caused this pesticide to be phased out from residential use by the end of 2001.

The ubiquitous environmental presence of Chlorpyrifos around the world and the greater concern and environmental awareness of the public of pesticides in general have made environmental monitoring a commonplace activity. Rapid and specific tests for compounds of environmental significance such as Chlorpyrifos are opportunities to be explored and exploited.

Molecular imprinting is a generic term used for the synthesis of materials that possess an enhanced affinity for specific molecules. Typically, imprint sites are formed by a polymerization reaction in the presence of a "print" molecule (which imparts structure in the forming polymer) and a solvent. As the polymerization reaction progresses, polymer forms around the print molecule. Removal of the print molecule leaves a void in the polymer with a greater affinity for the print molecule over other, similar small molecules. This method has been shown to be successful by many researchers, but typically involves numerous steps before a useful material results.

Approach -The concept of this research was to create a polymeric surface with structure on a molecular scale that would confer selectivity for a particular small molecule structure in a mixture and to make this material in the form of a small, uniform particle size. Emulsion polymerization is an ideal method for making high-surface area, small-particle size materials. In this approach, the research team's objective was to eliminate many of the steps typically encountered in the molecular imprinting process by making the "print" molecule, i.e., the molecular structure for which we intend to select, as a part of the surfactant used to make a high surface area polymeric material through emulsion polymerization. The creation of a surface exhibiting preferential binding of Chlorpyrifos was the primary objective. To this end, the team synthesized molecules with an organic, hydrophobic head as the print structure using a Chlorpyrifos analog and an aqueous soluble tail to give the molecule surfactant properties. These molecules were then used as both the print molecule and the surfactant to form emulsion polymers with high surface area. The hydrophobic head of the print surfactant molecule, in close proximity to the forming emulsion polymer particle with an oil in water emulsion, would be expected to impart some degree of surface structure into the emulsion particle.

Accomplishments - The technology and methodology developed in this program should be generally applicable to other small molecule systems where the print molecule can be made as part of the surfactant. In this program, the team identified and synthesized target molecules to be used as print surfactants and then used these surfactants to make emulsion polymers. The team accomplished the synthesis of a number of Chlorpyrifos analogs with different surfactant "tail" lengths and measured the critical micelle concentration for these compounds. Using this information, the team developed methods of emulsion polymer synthesis to form high surface area materials from emulsion polymerization methods using our "print" surfactants. The team targeted compositions of ternary polymer systems in the emulsion polymerization process using a ternary phase diagram, with the content of difunctional monomers kept above 60 percent of the composition. After isolating and washing the polymers, activity for Chlorpyrifos was measured using competitive binding and standard high-performance liquid chromatography techniques to determine the degree of specificity of the imprinted polymers toward the target molecule. Unfortunately, the team did not observe any increase in binding over control solutions. The team suspects that the lack of chirality in the print surfactant is possibly a significant factor in not observing selectivity, although the lack of C-2 symmetry in the Chlorpyrifos analog, along with the relatively large chlorine atoms on the ring, would lead one to believe that the Chlorpyrifos analogues as a print molecule could exhibit enhanced selectivity over non-chlorinated analogues. A second factor may be the ability of surfactant molecules to dynamically interact with the surface, as opposed to conventional methods of molecular imprinting where the print molecule is entrapped in a three dimensional matrix during polymerization. Thus the mobility of the surfactant at the surface of the growing polymer particle, in addition to the well known particle growth that occurs during the emulsion polymerization process, may reduce the ability of the surface to imprint the head of the surfactant sufficiently to obtain selectivity.

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