Solid-state P-31 NMR study of phosphonate binding sites in guanidine-functionalized, molecular imprinted silica xerogels

Phosphonate binding sites in guanidine and ammonium surface-functionalized silica xerogels were prepared via the molecular imprinting technique and ch aracterized using solid-state P-31 MAS NMR. One-point, two-point, and nonsp ecific 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 phospho nate interactions, respectively. Similar absorptions were observed with the ammonium functionalized gels. By examining the host-guest interactions wit hin the gels, the efficiency of the molecular imprinting procedure with reg ard to the functional monomer-to-template interaction could be readily asse ssed. Template removal followed by substrate adsorption studies conducted o n the guanidine-functionalized gels provided a method to evaluate the bindi ng characteristics of the receptor sites to a phosphonate substrate. During these experiments, Si-29 and P-31 MAS NMR acted as diagnostic monitors to identify structural changes occurring in the gel matrix and at the receptor site from solvent-mediated processes.