Carbon quantum dots with uniform size distribution have broad application prospects in the fields of optoelectronic devices

ion detection

nano-sensors

biological imaging and catalyst's due to their good optoelectronic properties. An “ultrafiltration-nanofiltration” bi-membrane method was proposed for the separation and purification of carbon quantum dots (CQDs). The molecular weight cutoff (MWCO) of ceramic ultrafiltration (UF) and nanofiltration (NF) membranes was 2000 and 800 and the corresponding Stocks-Einstein sizes were 2.3 nm and 1.4 nm

respectively. The pure water flux of UF and NF membranes were 140 and 70 L·(m2·h)−1

respectively. The effects of pH on the fluorescence intensity and particle size distribution of CQDs were studied. At pH=3

the carbon quantum dots were well dispersed and the fluorescence intensity is high. The ceramic UF membrane could effectively reject the large grained impurities. The average size of the CQDs in the permeate was about 2 nm with good dispersion and no agglomeration. Ceramic NF membrane played a great role in retention properties to CQDs. Small molecular impurities could be further removed in the concentration and washing process by using NF membrane. After bi-membrane treatment

the emission spectrum changed from multimodal to monomodal distribution

and the peak width narrowed. The luminous purity of CQDs was significantly improved.