Collagen derived carbon quantum dots for cell imaging in 3D scaffolds via two-photon spectroscopy

Biocompatible and photostable fluorescent probes are crucial yet challenging to develop for visualising and tracking biological functions and interactions that occur in living organisms. We synthesised biocompatible CQDs with 15% QY and tested them for cellular and scaffold imaging at variable depths. The CQDs, synthesised from collagen under controlled and benign conditions in a hydrothermal reactor, were characterised for their key physicochemical properties. The fluorescence characteristics were determined using two-photon microscopy, and based on our results we propose a mechanism for CQD luminescence by combining the carbogenic core and edge-effect contributions to the PL behaviour. The bioimaging of cells embedded in a luminescent 3D printed scaffold showed that CQDs enable imaging at depths of about 1500 μm under biomimetic conditions. Real-time videography and imaging tests showed differential visualisation of individual cells and scaffold. The excellent photostability and non-photobleaching characteristics of CQDs make them suitable for long-term whole cell and tissue imaging via multi-photon microscopy.