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The excited state intramolecular proton transfer (ESIPT) processes of 3-(benzo [d]-thiazol-2-yl)-2-hydroxy-5-methoxybenzaldehyde (BTHMB) in four kinds of solvents were studied using density functional theory (DFT) and timedependent DFT (TDDFT) method. As the solvent dielectric constant decreases, the strength of hydrogen bonds (H-bonds) in the excited state decreases. After photoexciation, the electron density rearrangement could be affected by the solvents, while the degree of charge transfer and the hydrogen bond interaction in the excited state became lower as the solvent dielectric constant decreased. The potential energy curves displayed the complete process of ESIPT, indicating that the ESIPT process of BTHMB was affected by the solvent polarity. When the solvent dielectric constant decreased, the tendency of proton transfer followed the trend: acetonitrile (ACN) > dichloromethane (DCM) > chloroform (CHCl 3 ) > 1,4-dioxane (DOX). Therefore, the ESIPT process was affected by the solvent polarity and became more difficult to occur as the solvent dielectric constant decreased. K E Y W O R D Sexcited state intramolecular proton transfer, hydrogen bond, potential energy curves, solvent polarity

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To avoid false-positive results in immunoassays due to cross-reactivity of antibodies with structural analogues, especially metabolites of target compounds, the preparation of highly specific antibodies is crucial. Preserving the characteristic structure of a target compound when designing a hapten is important when preparing highly specific antibodies. Here, we designed a novel hapten, 4-(((1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4yl)amino)methyl)benzoic acid, named AA-BA, to improve the specificity of antibodies for detection of 4-methylaminoantipyrine (MAA), a residual marker of dipyrone, an important antipyretic-analgesic and anti-inflammatory drug. The structural features of the hapten remained almost the same as those of MAA. After experimental validation, monoclonal antibody 6A4 (mAb 6A4) was prepared with the half maximal inhibitory concentration (IC 50 ) value of 4.03 ng/mL and negligible cross-reactivity with dipyrone metabolites and other antibiotics. In addition, a specific lateral flow immunoassay (LFA) strip based on colloidal gold was developed for screening MAA with a cutoff value of 25 ng/mL in milk. The developed LFA is a useful tool for rapid and accurate detection of MAA.