Abstract
Carbon nanodots (CDs) formed by hydrothermal dehydration occur as mixtures of differently sized nanoparticles with different degrees of carbonization. Common ultracentrifugation has failed in sorting them, owing to their extremely high colloidal stability. Here, we introduce an ultracentrifugation method using a hydrophilicity gradient to sort such non-sedimental CDs. CDs, synthesized from citric acid and ethylenediamine, were pre-treated by acetone to form clusters. Such clusters “de-clustered” as they were forced to sediment through media comprising gradients of ethanol and water with varied volume ratios. Primary CDs with varied sizes and degrees of carbonization detached from the clusters to become well dispersed in the corresponding gradient layers. Their settling level was highly dependent on the varied hydrophilicity and solubility of the environmental media. Thus, the proposed hydrophilicity-triggered sorting strategy could be used for other nanoparticles with extremely high colloidalstability, which further widens the range of sortable nanoparticles. Furthermore, according to careful analysis of the changes in size, composition, quantum yield, and transient fluorescence of typical CDs in the post-separation fractions, it was concluded that the photoluminescence of the as-prepared hydrothermal carbonized CDs mainly arose from the particles’ surface molecular state rather than their sizes.
