Molecularly imprinted layer-coated silica nanoparticle sensors with guest-induced fluorescence enhancement: theoretical prediction and experimental observation

Abstract

Molecularly imprinted fluorescence sensors operate on the basis of the recognition of imprinted sites to guest and the resultant changes of fluorescence emission have been studied. However, the origin of guest-induced fluorescence enhancement and the function of host molecule are still unclear in theory. In this work, we have first designed three isomers, 2-acrylamidoquinoline, 3-acrylamidoquinoline and 8-acrylamidoquinoline, with weak fluorescence emission, and used them as both functional monomers and signaling units in molecularly imprinted fluorescence sensors. Quantum chemical calculation within the density functional theory (DFT) framework has been introduced to accurately evaluate and predict the hydrogen bonding interaction between these monomers and the analyte melamine. As a result, the as-synthesized 2-acrylamidoquinoline exhibits a highest hydrogen bonding ability and the ideal molar ratio of monomer to template is 3 : 1 in molecularly imprinted polymers, which can greatly enhance the fluorescence emission of functional monomer after guest-host binding due to the strong hydrogen bonding restriction to the transformation of monomer conformations. The prediction is in good agreement with the experimental observation. Moreover, the imprinted nanoparticles display significant fluorescence enhancement upon titration with different concentrations of melamine in methanol. The fluorescence sensors can be applied to detect the melamine in dairy products with a low limit of quantification of 0.5 μM. The results reported herein supply an excellent model for the design of molecularly imprinted fluorescence sensors and their prediction of chemical sensitivity to nonfluorescent compounds.