Building Fluorescent Sensors by Template Polymerization: The Preparation of a Fluorescent Sensor for l-Tryptophan

The development of fluorescent sensors for organic molecules is of great practical importance in chemical, biological, and pharmaceutical sciences. Using l-tryptophan as an example, we have studied a new way of making polymeric fluorescent sensors using template polymerization or molecular imprinting techniques. The fluorescent polymers were prepared using functional monomers with a fluorescent probe attached to it. The fluorescence of this polymer could be quenched by 4-nitrobenzaldehyde. Addition of the template molecules, l-tryptophan, increased the fluorescence intensity of the imprinted polymer/quencher mixture in a concentration-dependent fashion, presumably through the displacement of the quencher from the binding sites by l-tryptophan. This fluorescence intensity change upon mixing with l-tryptophan allows the binding event to be detected easily. The sensor also exhibited enantioselectivity for the template molecules. For example, the effect of d-tryptophan on the fluorescence intensity of the polymer is about 70% that of its l-enantiomer. Furthermore, the effect of l-phenylalanine and l-alanine on the fluorescence intensity change is much smaller than that of l-tryptophan. Because the approach used does not require the de novo design of the complementary binding site and does not rely on any specific structural features of the template molecule or prior knowledge of its three-dimensional structure, the same principle could potentially be useful for the future construction of practical fluorescent sensors for many other compounds.