Size controlled synthesis and fluorescence quenching behavior of N-CQDs based on molecular sieves

Tunable photoluminescence (PL) of carbon quantum dots (CQDs) strongly depends on the characteristics of particle size and edge chemistry. In this work, we report a very facile and effective method to separate nitrogendoped CQDs (N-CQDs) by molecular sieves with different pore sizes through physical-chemical adsorption method. The N-CQDs-3.7 with target sizes of 1-3 nm and N-CQDs-8 with target sizes of 4-7 nm are rapidly obtained through the screening of MCM-41 and SBA-15, respectively. The microstructure and PL performance of the separated N-CQDs are explored by transmission electron microscopy, atomic force microscopy, Raman scattering spectrum, X-ray photoelectron spectroscopy, and PL spectroscopy. The two types of N-CQDs exhibit different pH responses and fluorescence quenching behaviors to aniline compounds and benzoic acid (BA). The N-CQDs-8 emits a stable green fluorescence at pH = 2-7, and exhibits an obvious PL quenching behavior to alkaline aniline compounds, especially the aromatic compounds with di-amine. On the opposite, the N-CQDs-3.7 illuminates a stable blue fluorescence at pH = 7-10, and displays a significant PL quenching for the acidic BA, representing a good linear relationship and high sensitivity. Meanwhile, the different pH responses and PL quenching performances are also discussed.