First steps toward a novel chemical sensor based on a molecularly imprinted polymer as the recognition element and infrared evanescent wave spectroscopy as transduction principle are presented. Noncovalently imprinted polymer layers selective for the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) have been prepared on the surface of zinc selenide attenuated total reflection elements. Selective and reversible binding of the template to the imprinted polymer could be observed on-line by Fourier transform infrared (FT-IR) spectroscopy. Control experiments were performed with nonimprinted reference polymers and with a structurally related compound. The obtained selectivity data correlate well with those previously obtained with molecularly imprinted polymers using radioligand binding assays. This demonstrates the potential for constructing stable and selective sensors for low-molecular-weight organic substances based on FT-IR spectroscopy. Moreover, FT-IR spectroscopy is demonstrated to be a valuable tool for deeper investigation of the binding mechanism in molecularly imprinted polymers.

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Molecularly imprinted polymers (MIPs) for 2,4-dichlorophenoxyacetic acid were synthesized via a noncovalent approach with 4-vinylpyridine as functional monomer and ethylene glycol dimethacrylate as cross-linker in a methanol/water mixture. Templated polymers synthesized in this self-assembly approach rely on complex formation between the target analyte and functional monomers in porogenic solution prior to radical polymerization. Consequently, the achievable selectivity is governed by the nature and stability of these complexes. The nature of noncovalent interactions responsible for complex formation during imprinting of the template 2,4-dichlorophenoxyacetic acid (2,4-D) with the functional monomer 4-vinylpyridine has been investigated. Fourier transform infrared and 1H NMR spectroscopies provide the fundamental analytical basis for rationalizing the mechanisms of recognition during the imprinting process probing the governing interactions for selective binding site formation at a molecular level. Molecular modeling studies in explicit solvent (chloroform and water) corroborate the importance of hydrogen bonding in aprotic solvents and of hydrophobic interactions in protic media in agreement with the experimental spectroscopic investigations of prepolymerization solutions. Furthermore, chromatographic studies of the synthesized MIPs provided insight on the importance of size, shape, and functionality during selective 2,4-D rebinding processes confirming the results obtained during the prepolymerization studies.