Graphene quantum dots derived from hollow carbon nano-onions

Herein, carbon nano-onions （CNOs） with different structures have been investigated as precursors for the synthesis of graphene quantum dots （GQDs）. It was found that hollow CNOs yield GQDs with a uniform size distribution, whereas metal encapsulation in the CNO structure is disadvantageous for the same. Furthermore, the hollow CNOs are also advantageous for the synthesis of GQDs with a yellow-green hybrid luminescence and long-ranged excitation wavelength （λex）-independent photoluminescent （PL） behavior, in which the λex upper limit was 480 nm. These features enable safe sensing and cell tracking applications with a longer excitation wavelength in the visible light region. The potential applications of the synthesized GQDs as fluorescent sensing probes for detecting Cu（II） ions and non-toxic cell imaging under visible light excitation have been demonstrated. This means that sensing and bioimaging can be accomplished in the natural environment with no need for UV excitation. This work provides a reference to researchers in tailoring CNO structures in terms of their inner space to synthesize GQDs with the desired luminescence behavior.