Modification of carbon dots derived from biomass by exogenous nitrogen doping: Action mechanism and difference analysis

Carbon dots derived from biomass (B-CDs) is a burgeoning nanomaterial with great application potential, attracting increasing attention for its sustainability, economy, and eco-friendliness. However, most of B-CDs had low quantum yields and emitted blue fluorescence, limiting their application. In this work, B-CDs were modified through exogenous nitrogen doping in the microwaved-assisted hydrothermal process. Considering the complex composition of biomass, we selected the "cellulosic" and "nitrogen-rich" biomass as representative raw materials and two reagents with different nitrogen-containing structures as nitrogen sources. Various combinations of nitrogen sources and biomass produced different modification effects by regulating the chemical potential of C and N atoms. These differences were reflected in nitrogen-doped contents and states, and sp2 C structure, resulting in different luminescent properties. It tells suitable nitrogen dopants should be selected for different biomass. The nitrogen-doped contents were up to 21.49%. Dopamine played a role in the formation of quaternary N. Melamine facilitated the formation of pyridinic N and amine N. A higher proportion of nitrogen-doped, pyridinic N, amine N, and sp2 C could reduce the bandgap, thus improving quantum yields and generating longer wave emissions. The absolute quantum yields enhanced from 2.95% to 35.39%. Green fluorescence and a 732 nm long-wave emission appeared in the modified B-CDs. A slight increase in temperature was conducive to the optimization of luminescence. The fluorescence lifetimes of 6.17–33.20 ns were obtained due to the combination of radiative and non-radiative recombination in different proportions. The B-CDs showed good fluorescence stability.