Influence of template basicity and hydrophobicity on the molecular recognition properties of molecularly imprinted polymers

Triazine herbicides were used as model templates for a basic study of the molecular recognition process in imprinted polymers. Five structurally related triazine herbicides (atrazine, ametryn, cyanazine, prometryn and terbutylazine) that differ in basicity and hydrophobicity were imprinted. Chromatographic evaluation of the resulting materials in an aqueous-poor mobile phase showed that the selectivity for the template increased with its Bronsted basicity whereas it did not correlate with template hydrophobicity. Thus, the highest and lowest affinity and selectivity for the template was observed using the ametryn-imprinted (pK(a)=4.1) and the cyanazine-imprinted polymers (pK(a)=1.0), respectively. In aqueous-rich mobile phases however affinity and selectivity correlated with template hydrophobicity. On a polymer imprinted with atrazine (log P(ow)=2.6), the selectivity for atrazine over the more hydrophobic prometryn (log P(ow)=3.4) decreased whereas the retention increased when going from a mobile phase containing 2.5% (v/v) water (k'(ATR)=1.7, k'(PRO)=0.6) to one containing 70% (v/v) water (k'(ATR) = 15, k'(PRO)=38). A different trend was observed using a polymer imprinted with prometryn. In this case both the selectivity and affinity dramatically increased when changing the mobile phase in the same order (k'(PRO)=1.8, k'(ATR) = 1.2 at 2.5% water, k'(PRO)=150, k'(ATR)=21 at 70% water). Depending on the hydrophobicity of the template, the selectivity in molecular imprinting may thus be enhanced by increasing the aqueous content of the medium used in the rebinding step.