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Dedicated to Professor Gunter Wulff on the occasion of his 65th birthdayMolecular imprinting is now accepted as an effective tool for the preparation of polymeric materials with receptormimicking recognition sites. Numerous publications describe applications of molecularly imprinted polymers (MIPs) in chromatography, membranes, catalysis and sensor technology.[1±6] Despite the progress achieved in the area of MIP sensors, the greatest challenge remains the difficulty in transducing the binding event into the detectable signal. We report here a new ªbite-and-switchº approach for the design of an imprinted polymer-recognition material useful for the development of a sensor for creatine. In this work we explore the ability of polymerizable hemithioacetal formed by allyl mercaptan and o-phthalic dialdehyde (OPA reagents) to react with primary amines and form a fluorescent isoindole complex. To make this reaction specific for creatine we used imprinting with a methylated creatine analogue that can form a reversible complex with hemithioacetal. The practical significance of this work lies in the fact that creatine has become one of the most popular nutritional supplements for resistance-trained athletes and body builders, and its detection is necessary in sports medicine. [7] Additionally the large elevations in serum and urinary creatine and creatinine levels are basic markers of tissue degradation and/or kidney stress. [8,9] The ªbite-and-switchº approach combined with molecular imprinting can be used for the design of assays and materials for sensors, specific for any amino containing substances and because of that has general significance for analytical chemistry.The reaction between o-phthalic dialdehyde, mercapto group (OPA reagents) and primary amines is well known and used in practice for detection of amino acids.[10] We have shown recently that polymerizable thiols, such as allyl mercaptan can form stable hemithioacetal with o-phthalic dialdehyde (see Fig. 1). [11] This hemithioacetal can be copolymerized with cross-linker into highly crosslinked rigid polymer, which retained the same ability as non-polymerized monomers or soluble OPA reagents to bind primary amines forming fluorescent isoindole. Generally this reaction is non-specific because any compound containing a primary amino group can react with OPA reagents or polymerized hemithioacetal. To make a polymer specific for creatine we used molecular imprinting. Creatine itself was expected to be an ideal template in our case. Unfortunately we found that this molecule reacts irreversibly with polymerized hemithioacetal and the template cannot be removed from the synthesized polymer. Isoindole, which is the product of this reaction, could not be hydrolyzed back to the original components even when concentrated hydrochloric acid is used. This problem was overcome by using methylated analogue of creatine with blocked primary amino groups as a template (see Fig. 1). Methylated creatine still possesses a structure similar to that of creatine, but can be easily r...

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