Synthesis of NAD-functionalized organic semiconducting polymer dots for fluorometric γ-aminobutyric acid sensing

γ-Aminobutyric acid (GABA) is an important neurotransmitter that is associated with neurological disorders. In this work, a semiconducting polymer dot-based biosensor was developed to detect GABA via ratiometric fluorescence change. The surface of the semiconducting polymer dots (Pdots) was functionalized with boronic acid via N-(3-dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride/N-hydroxysuccinimide coupling reaction that could further bind to diol in nicotinamide adenine dinucleotide (NAD) to form boronic ester. GABA was converted to succinate by the enzymatic reaction using GABase. During the reaction, nonfluorescent NAD became blue-emissive, converting NAD to its reduced form NADH and was also located on the polymer dot surface. Simultaneously, the red fluorescence of Pdots decreased via electron transfer between Pdots and NADH. With this fluorescence color change during the enzymatic reaction, the concentration of GABA could be determined using the NADH formed. The limit of detection of GABA was found to be 22.0 μM, and the detection range was 0–0.20 mM of GABA concentration.

GABA was successfully quantified using enzymatic reaction in the presence of GABase and fluorescent Pdots. During the reaction, the NAD on the surface of Pdots was converted to fluorescent NADH, and red fluorescence of Pdots became decreased because the energy was transferred to blue-emissive NADH. The ratio of red and blue fluorescence can be an efficient signal of GABA concentration.