Solid-State 31P NMR Study of Phosphonate Binding Sites in Guanidine-Functionalized, Molecular Imprinted Silica Xerogels

Abstract

Phosphonate binding sites in guanidine and ammonium surface-functionalized silica xerogels were prepared via the molecular imprinting technique and characterized using solid-state 31P MAS NMR. One-point, two-point, and nonspecific host−guest interactions between phenylphosphonic acid (PPA) and the functionalized gels were distinguished by characteristic chemical shifts of the observed absorption peaks. Using solid-state as well as solution-phase NMR analyses, absorptions observed at 15.5 and 6.5 ppm were identified as resulting from the 1:1 (one-point) and 2:1 (two-point) guanidine to phosphonate interactions, respectively. Similar absorptions were observed with the ammonium functionalized gels. By examining the host−guest interactions within the gels, the efficiency of the molecular imprinting procedure with regard to the functional monomer-to-template interaction could be readily assessed. Template removal followed by substrate adsorption studies conducted on the guanidine-functionalized gels provided a method to evaluate the binding characteristics of the receptor sites to a phosphonate substrate. During these experiments, 29Si and 31P MAS NMR acted as diagnostic monitors to identify structural changes occurring in the gel matrix and at the receptor site from solvent-mediated processes.