Excitation-independent and fluorescence-reversible N-GQD for picomolar detection of inhibitory neurotransmitter in milk samples ‒ an alleyway for possible neuromorphic computing application

Abstract

N‒GQDs with an average size of ca. 20–30 nm are utilized for the picomolar detection of inhibitory neurotransmitters, glycine (Gly), in pH ca. 7.0. The crystalline nature, morphology, elemental composition, and chemical state of N–GQDs are investigated by XRD, FE-SEM, HR-TEM, XPS, and FT-IR techniques. The addition of Gly (100 × 10−9 M; 0 → 1.0 mL) steadily quenches the fluorescence intensity of N–GQD (1 × 10−6 M) at 432 nm (λex 333 nm) due to inner filter effect (IFE) through the formation of ground-state complex, N–GQD•Gly. The excitation-independent N‒GQDs showed an outstanding selectivity and sensitivity towards Gly with binding constant (Ka = 8.97 × 10−3 M−1) and LoD (21.04 pM; S/N = 3). Time-correlated single-photon counting experiment confirms the static quenching of N–GQD (8.77 → 8.85 ns) in the presence of Gly. The interference of other amino acids on the strong binding of the N–GQD•Gly complex in H2O is examined. Combinatorial Ex-OR and NOT gate logic circuits that could be useful in neuromorphic computing are developed based on the reversible fluorescence intensity changes of N–GQD upon the addition of Gly (ФF 0.54 → 0.39). The real-time application of N–GQD was investigated using commercially available relevant milk samples. Remarkably, not less than 99% cytotoxic reactivity of N–GQDs is attained against HeLa cells.