A THIAMINE-SELECTIVE OPTICAL SENSOR BASED ON MOLECULAR RECOGNITION

A thiamine-selective optrode based on the use of new lipophilic esters of tartaric acid as thiamine carriers and a lipophilic fluorescent pH indicator dissolved in a PVC membrane is presented. The carrier transports thiamine dication into the PVC membrane, and one or two protons are released simultaneously from the dye which thereby undergoes a color change. The sensor is fully reversible, and responds to thiamine in the concentration range from 0.01 to 100 mM. Interferences by other nitrogen heterocycles out of the vitamin B group and of various purines and amines have been investigated, and the optical selectivity coefficients for vitamins B2, B3, B4, B6 and some purines and alkaloids were calculated. Cinchonidine, N,N-dimethylaniline, triethylamine and riboflavine interfere strongly, the latter due to its strong intrinsic fluorescence. The riboflavin interference can be overcome by shifting both the excitation and emission to longer wavelengths. All other substances tested can be tolerated up to an about hundred-fold excess. The method has been applied to determine thiamine in multivitamin formulations and the results were found to be satisfactory. In contrast to the majority of sensing schemes for bio-organic molecules, the one presented here does not involve an enzyme or any other biological component. Hence, the sensor does not suffer from the typical disadvantages of protein-based sensors.