Tunable photoacoustic and fluorescence imaging of nitrogen-doped carbon quantum dots

Photoacoustic-fluorescence imaging is an emerging medical imaging technology developed in recent years that has shown immense application potential for specialties that depend on optical diagnoses, such as ophthalmology, otorhinolaryngology, and dermatology, etc., owing to its advantages of real-time, fast, and high sensitivity. Currently, photoacoustic (PA) probes mainly accumulate in the tumor region via neovascularization leakage and poor lymphatic drainage, which phenomenon refers to as the enhanced permeability and retention (EPR) effect. However, signal-to-noise ratios (SNR) of label free PA imaging are limited by the low tumor accumulation of PA probes. Beside, the modification process of PA probes is complex. Here, we designed and synthesized a new whole series of nitrogen-doped carbon quantum dots (N-CQDs) through a molecular fusion strategy. The synthesized N-CQDs displayed narrow size distribution and average sizes of ∼1.50 nm. Meanwhile, the N-CQDs exhibited tunable optical capability via the mass ratio changes of precursor and catalyst. Finally, the N-CQDs are applied as the bioimaging probes, which demonstrated fast imaging velocity, rapid metabolism process, and great EPR effect leading to better accumulation in the tumor region. We hereby offer a guideline to design advanced N-CQDs for both fluorescence imaging and PA imaging through a molecular fusion strategy. Given this strategy, the N-CQDs may possess both optimum fluorescence intensity and PA efficiency, which allows more comprehensive, high quality and accurate tumor imaging in vivo.