Nitrogen-doped graphene quantum dot for direct fluorescence detection of Al³⁺ in aqueous media and living cells

Graphene quantum dot (GQD) has been attractive in analytical science field due to its low toxicity, stable photoluminescence. Herein, nitrogen-doped GQD (N-GQD) was prepared by a facile solvothermal treatment of GO using dimethylformamide, and exhibited a green emission with 23.1% quantum yield. The N-GQD probe showed a selective and sensitive fluorescence enhancement response to Al³⁺, the mechanism might be the formation of a complex between Al³⁺ and N-GQD constrained the photo-induced electron transfer (PET) process of N-GQD itself. With Benesi-Hildebrand equation, the binding constant and molar ratio between N-GQD and Al³⁺ was calculated to be 4.6 × 10⁴Lmol^-1 and 1:1 respectively. The pK_a value of N-GQD was also determined to be 4.4 by capillary electrophoresis. In pH 4.0 PBS solution, there was a good linear relation between the fluorescence intensity and the logarithm of concentration of Al³⁺ in the range of 2.5-75μmolL^-1, the limit of detection (3σ) was 1.3μmolL^-1. This "Off - On" fluorescence method had been applied to accurate quantification of aluminum in hydrotalcite tablets. What's more, the fluorescence switch property of N-GQD was explored by alternate addition of Al³⁺ and EDTA. The probe was also utilized for detection Al³⁺ in living cells due to its excellent biocompatibility.