Abstract

Imaging probes serve as unique diagnostic tools in biomedical applications offering high sensitivity particularly in their ability to image cells and tissues. These tools are crucial for early detection and disease diagnostics especially with an increase in numbers of an aging population and the requirement for more efficient health care. Recent advancements in the field of nanomaterials have propelled research groups into investigating these nanoparticles for various biological applications including drug delivery, biosensing and bioimaging, among others. Recently, carbon dots have garnered significant attention because they can be prepared from simple synthetic routes using inexpensive precursors, offering tunable optical properties, low cytotoxicity and good biocompatibility. Their inherent fluorescent nature not only allows for fluorescence imaging, but also for sensing environmental changes, which can provide additional insights for the development of novel diagnostic applications.
Herein, carbon dots are synthesized using a one-step microwave- and solvothermal-assisted reaction. The prepared dots fluoresce simultaneously in both the blue and the red regions of the electromagnetic spectrum. The dots’ physico-optical properties are thoroughly studied to shed light on their fluorescence mechanism, which remains of topic of debate in the literature. It is demonstrated that the fluorescence of the nanoparticles is tailored through manipulation of key synthesis parameters to determine the underlying effect on the observed optical signature. The dots’ unique optical properties are believed to derive from the carbon core- and molecular-states fluorescence mechanism. In brief, the blue fluorescence stems from the core, while the red counterpart originates from the molecular states that are typically localized on the surface of the dot.
With their unique fluorescence properties and our understanding of this phenomenon, these dots offer the possibility of sensing changes in temperature and pH, using ratiometric approaches. Both temperature- and pH-sensing measurements translate well from the cuvette to the cellular model. The change in physiological parameters are in agreement with the change in emission using epifluorescence and confocal microscopy. The ability to glean such information renders these nanoparticles into versatile diagnostic nanotools with the ability to shed new insights on disease mechanisms and can be foreseen as future tools for in vivo sensing applications.