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Molecularly imprinted polymers (MIPs) have been used in a wide range of analytical applications in particular in chromatography and sensing. However, the binding properties in MIPs are typically measured only in a narrow concentration range, which corresponds to only a subset of the sites in MIPs. This limited analytical window and binding site heterogeneity of MIPs leads to inaccuracies and inconsistencies in the estimation of their binding properties. This has hampered the characterization and optimization of MIPs for analytical applications. In this study, the origins of the molecular imprinting effect were studied using the newly developed Freundlich isotherm-affinity distribution (FIAD) analysis. The analysis is able to readily calculate an affinity distribution for MIPs from the limited analytical window. The FIAD analysis also yields an estimate of number, affinity, and heterogeneity for this subset of binding sites. Consistent with previous studies, MIPs were found to have higher capacities than the corresponding nonimprinted polymers (NIPs). Interestingly, MIPs were also found to be more heterogeneous than NIPs. Examination of variables in the imprinting process including temperature, template concentration, and cross-linking percentages further confirmed these trends. Based on these observations, a model for the imprinting effect was developed. The larger population of high-affinity sites in MIPs appears to arise from a broadening of the heterogeneous distribution. This suggests that noncovalent MIPs may be ill-suited for chromatographic applications and other applications that are detrimentally affected by binding site heterogeneity and better suited to applications that are less affected by heterogeneity such as sensing.

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Demonstrated is the site-selective chemical modification (SSCM) of molecularly imprinted polymers (MIPs). In this strategy, MIPs are selectively chemically modified to improve the ratio of high-affinity to low-affinity binding sites and therefore the overall binding characteristics of the material. This was accomplished by preferentially eliminating the low-affinity binding sites by esterification with diazomethane or phenyldiazomethane. Selectivity in the esterification reaction was achieved using a guest molecule as an in situ protecting group that preferentially shields the high-affinity sites and leaves the low-affinity sites exposed toward reaction. The corresponding shifts in the populations of high- and low-affinity sites were quantified using affinity distribution analysis, which quantitatively measures the heterogeneous distribution of binding sites in MIPs as number of binding sites (N) with respect to binding affinity (K). Using affinity distribution analysis, the SSCM strategy was shown to improve the percentage of high-affinity sites in a methacrylic acid (MAA)/ethylene glycol dimethacrylate (EGDMA) matrix, imprinted with ethyl adenine-9-acetate (EA9A) in acetonitrile. The effects of different solvents and concentrations of guest molecule on the SSCM also were examined. The greatest improvements due to SSCM were observed when carried out in the imprinting solvent (acetonitrile). The demonstrated SSCM methodology is complementary to existing strategies for improving MIPs and thus can be utilized in tandem to improve the binding characteristics of MIPs.