Simultaneous Determination of Concentration and Enantiomeric Composition in Fluorescent Sensing

Fluorescent sensors have found broad applications in determining the concentrations of diverse analytes by measuring specific sensor-analyte fluorescent responses. For a chiral substrate containing varying composition of two enantiomers, both the concentration and enantiomeric composition should greatly influence the fluorescent response of an enantioselective fluorescent sensor. Thus, multiple independent measurements are normally needed to determine both the concentration and enantiomeric composition of a chiral compound. In order to facilitate the application of the enantioselective fluorescent sensors, our laboratory has developed four strategies to simultaneously determine the concentration and enantiomeric composition of various chiral substrates by a single fluorescence measurement. A mixture of a chiral BINOL-based dialdehyde and an achiral compound salicylaldehyde in the presence of Zn²⁺ is used to interact with chiral diamines, amino alcohols, and amino acids. The fluorescence enhancement at λ₁ = 447 nm due to the achiral sensor is mostly determined by the concentration of the substrates, and the fluorescence enhancement at λ₂ > 500 nm due to the chiral sensor is highly enantioselective. A 3D graph combining the fluorescence intensities at λ₁ and λ₂ can be used to determine the enantiomeric composition. A chiral conjugated polymer containing the BINOL-dialdehyde units is shown to amplify the enantioselectivity of the small molecule sensor under the same conditions. Combination of the chiral polymer with salicylaldehyde allows simultaneous concentration and enantiomeric composition determination. In a pseudoenantiomeric sensor pair of the BINOL-based amino alcohols, one sensor shows greater fluorescence enhancement with one enantiomer of chiral α-hydroxy carboxylic acid at λ₁ = 374 nm and another sensor shows greater fluorescence enhancement with another enantiomer at λ₂ = 330 nm. Using a mixture of this sensor pair allows the determination of both concentration and enantiomeric composition with one fluorescence measurement. A BINOL-based trifluoromethyl ketone is found to exhibit dual emission responses toward a chiral diamine at λ₁ = 370 nm and λ₂ = 438 nm. The fluorescence enhancement at λ₁ is mostly determined by the substrate concentration and that at λ₂ is highly enantioselective. Thus, using one sensor with one measurement gives both parameters. A BINOL-naphthyl imine compound is designed to show two different fluorescent responses toward functional chiral amines in the presence of Zn²⁺. When the naphthylamine unit is displaced off the sensor by a chiral amine substrate via imine metathesis, the emission of naphthylamine is restored at λ₁ = 427 nm, which allows determination of the substrate concentration. The fluorescence enhancement at λ₂ > 500 nm due to the formation of the new chiral imine products is highly enantioselective. The work discussed here has provided convenient methods to obtain the two important parameters of a chiral molecule by a single fluorescence measurement. They should contribute to the development of analytical tools for the rapid assay of chiral compounds.