Influence of the sequential purification of biomass-derived carbon dots on their colloidal and optical properties

Abstract

The purification of photoluminescent carbon dots has a strong impact on their applications, making rigorous protocols necessary to isolate them. In this work, a sequential filtration process was implemented on photoluminescent carbon dots synthesized by a microwave-assisted hydrothermal treatment of an agricultural waste, achieving good effectiveness and higher yields compared to chromatographic purification approaches. Filtration membranes of different cut-offs were carefully selected considering the by-products from the biomass decomposition. The synthetic route allowed the preparation of carbon dots with average particle sizes of 3.5 nm, composed of a graphitic-like core decorated with oxygen and nitrogen moieties. Dynamic light scattering measurements of the suspensions revealed higher aggregation of the nanoparticles in the least purified samples. This also influences the photoluminescence emission properties due to self-absorbing and quenching effects. The thermal characterization of the solids recovered after each filtration step has shown (for the least purified samples) the presence of fragments assigned to molecules arising from an incomplete hydrothermal transformation of the precursor. The aqueous suspension of the carbon dots recovered after the most purified protocol displayed a notorious emission upon excitation at 375 nm, confirming that the carbon dots are responsible for the observed optical features. These results highlight the importance of an adequate purification process of carbon dots obtained from complex precursors (such as biomass), to avoid bias interpretation of their photoluminescence properties.