The future of quantum dot fluorescent labelling of extracellular vesicles for biomedical applications

Nano-sized extracellular vesicles have become an interesting target for fluorescent labelling due to their complex roles within cellular processes. Extracellular vesicles are released by most cells, including bacteria and mammalian and have demonstrated clear involvement within cellular, as well as patho-physiological processes. They contain information about the cell they originate from and communicate between cells making them promising disease biomarkers and vehicles for drug delivery. Many strategies have been used to fluorescently label extracellular vesicles to track or identify unknown cellular processes. Various targetable proteins are embedded in their membrane enabling them to be fluorescently labelled for detection and identification purposes. The fluorescent probes used for biosensing and bioimaging include organic dyes, fluorescent proteins, metal nanoparticles and quantum dots. Commonly used organic dyes and fluorescent proteins have inherent issues with stability and photobleaching. Quantum dots, however, exhibit high quantum yields, have broad excitation wavelengths and do not photobleach. The tuneable size of the quantum dots corresponds to emission wavelengths and can therefore be used in multiplexing or parallel detection of different targets. Semiconducting quantum dots can be modified with targetable proteins that make them ideal labelling agents. This review introduces extracellular vesicles and state-of-the-art use of quantum dots for bioimaging and biosensing.