A new zinc(II) porphyrin conjugate with an appended pyrene subunit has been synthesized and shown to exhibit significant and analytical usefulness for fluorescence sensing toward imidazole derivatives. The molecular recognition was based on the bridging interaction of the imidazole ring of analyte with the zinc(II) center of the porphyrin, while the transduction signal for the recognition process was the pyrene excimer fluorescence. The sensor was constructed and applied for fluorescence assay of histidine in aqueous solution by immobilizing the sensing material in a plasticized PVC membrane. When the membrane was bathed in an alkaline solution void of histidine, zinc(II) porphyrin was present in the monomer form, and pyrene emitted monomer fluorescence at 378 and 397 nm. With the presence of histidine in the sample solution, histidine was extracted into the membrane phase and bridged with the Zn(II) center of the porphyrin, causing the monomer porphyrin to be converted to its dimeric species. Since the formation of porphyrin dimer was accompanied by the enhancement of pyrene excimer emission at 454 nm, the chemical recognition process could be directly translated into a fluorescent signal. With the optode membrane M1 described, histidine in sample solution from 6.76 x 10(-7) to 5.01 x 10(-3) M can be determined. The limit of detection was 1.34 x 10(-7) M. The optical selectivity coefficient obtained for histidine over biologically relevant amino acids and anions met the selectivity requirements for the determination of histidine in biological samples. Serum histidine values obtained by the optode membrane fell in the normal range of the content reported in the literature and were in good agreement with those obtained by HPLC.