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Carbon Quantum Dots (CQDs), also known as carbon dots or carbon nanodots, are zero-dimensional carbon nanomaterials composed of ultrafine, dispersed, quasi spherical, and carbon nanoparticles with a size below 10 nm. CQDs, as an emerging class of quantum dots (QDs) with good photoluminescence (PL) properties, simple and economical preparation and synthesis routes, inexpensive initial preparation materials, good water solubility, non-toxicity, small particle sizes, tunable emission spectra, high fluorescence stability, good chemical properties, good biocompatibility, and ease of functionalization, have received widespread attention in recent years. Due to the fascinating chemical, optical, electrical and biological properties of CQDs, the sensitivity, detection limit and selectivity in multi-species detection of biosensors can be dramatically improved. Currently CQD-based biosensors have been applied to the detection of different species and scenarios, such as metal ions, acids, proteins, DNA and RNA, blood, drugs, vitamins and other compounds, and scenarios such as environmental gases and water quality monitoring, cellular imaging, drug labelling, and biomedical testing. It can be seen that CQDs-based biosensors are expected to be the leading sensors with high performance and high accuracy in different fields. This review presents the main methods reported in the scientific literature for the synthesis of CQDs, surface modifications, as well as describing different types of applications on optical biosensors based on the properties of CQDs and their working mechanisms.
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