Colorimetric-fluorometric detection of basic amino acids lysine/arginine driven by acid-base reaction

A novel colorimetric-fluorometric probe, 3-hydroxy-[1,1':4',1″-terphenyl]-4-carbaldehyde (HTPC), was synthesized for the detection of basic amino acids lysine (Lys) and arginine (Arg). The response of HTPC was driven by the acid-base reaction between HTPC and Lys/Arg. Upon addition of Lys/Arg, the HTPC acetone-H₂O solution generated new electronic absorption at ∼400 nm and turned-on a fluorescence emission at ∼500 nm, and both the absorbance and fluorescence intensities significantly enhanced. Simultaneously, the solution underwent a color change from colorless to light-yellow and emitted blue-green fluorescence under UV light irradiation. This is because Lys/Arg caused deprotonation of the phenolic hydroxyl groups in HTPC using their terminal amino groups, thereby altering the electron cloud distribution of HTPC and enabling independent existence of the fluorophore, which enhanced both absorbance and fluorescence intensity. Furthermore, Lys and Arg could be distinguished by treating the probe-amino acid solutions with Hg²⁺ and monitoring the fluorescence property. The limit of detection of Lys and Arg were 8.50 μM and 2.77 μM for colorimetric detection, and 1.92 μM and 2.35 μM for the fluorescence detection, respectively. The synthesized probe was employed to accurately quantify the Lys/Arg contents in drugs and achieved live-cell fluorescence imaging of Lys/Arg. Our synthesized colorimetric-fluorometric probe based on the acid-base reaction provides a new strategy for constructing sensitive probes of amino acids.