Solvent-free and dispersant-free synthesis of narrow full width at half maximum carbon dots via sole precursor of p-phenylenediamine

Narrow band gap emissive carbon dots (CDs), especially those with full width at half maximum (FWHM) below 40 nm, is extremely important for biological imaging application due to higher color purity and better resolution. The synthesis of narrow FWHM emissive CDs usually requires the use of specific-structured molecular precursors with a large π conjugated domain. In comparison, the most common precursors for CDs, for instance p-phenylenediamine, are usually believed unsuitable for preparing CDs with FWHM emission below 40 nm. In this work, we challenged this stereotype by using p-phenylenediamine precursor to synthesize CDs with a small FWHM of only 29 nm. Instead of using traditional solvothermal method, a facile solvent-free method was developed to synthesize CDs with the powder of p-phenylenediamine in the autoclave under a reaction temperature of 200 °C. After dispersed in solution, CDs show a red emission around 615 nm with FWHM of 29 nm and a photoluminescence quantum yield (PLQY) of 12.8%. Through experiments and theoretical calculation, we found that the FWHM emission is positive correlated to the content of amino groups on the surface of CDs. In comparison to traditional solvothermal method, the current solvent-free method can promote the synthesis of CDs with less amino groups on the surface, reducing fluorescence center types, and hence realizing smaller FWHM emission. In addition, as-prepared CDs were used for biological imaging, and realized a rapid no-wash cell imaging.