Continuous microflow synthesis of fluorescent phosphorus and nitrogen co-doped carbon quantum dots

•A microflow technique is applied for N/P co-doped carbon quantum dots synthesis.•The continuous microflow approach is effective for the co-doping of heteroatoms.•The properties of the co-doped carbon dots can be controlled by process parameters.•The photoluminescence performance of the carbon dots is further evaluated. Fluorescent carbon quantum dots (CQDs) doped with heteroatoms are highly promising for diverse applications ranging from bioimaging to environmental sensing. However, the ability for doping and functionalizing CQDs in a continuous, scalable, industry-relevant flow chemistry process, remains limited. Here we overcome this limitation by developing a continuous, facile and efficient method for the preparation of nitrogen and phosphorus co-doped carbon quantum dots (NP-CQDs), using microflow reactors combined with localized, energy-efficient heating of ethanolamine and phosphoric acid aqueous solution. The products are characterized by advanced microanalysis including fluorescence spectrophotometry, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) measurements. The results show ultrasmall fluorescent CQDs with narrow size distribution were successfully prepared, where N and P atoms proved to be effectively doped into the CQDs. The reaction conditions for the continuous synthesis of CQDs are investigated, including the influence of residence time and reaction temperature on the obtained CQDs (particle size, distributions, fluorescence intensity, bandgap energies, conductivity, and fluorescence lifetime) are analyzed. This study is expected to provide guidance for the continuous and controllable preparation of fluorescent CQDs with effective doping of heteroatoms.