Functionalization of carbon dots and their applications in food safety

Almost two decades have passed since the discovery of carbon nanodots (CDs). As a promising family of optical nanomaterials, CDs have high emission efficiency, excellent water solubility, and good bio-compatibility; in addition, they are inexpensive and can be produced by a facile synthesis process. Because of these advantages, CDs have drawn tremendous attention for use in the development of novel optochemical sensors. However, the application scope of chemical sensors based on pristine CDs is restricted because the synthetic methods and starting materials for the CDs give rise to a great limitation of their surface chemical structures. Therefore, pristine CDs need to be functionalized so that they can be employed in sensing applications, with high sensing capability. Typically, functionalization can be classified into two types: doping and surface modification. Doping, as an effective method for introducing new elements to the skeleton of CDs, not only helps manipulate the energy bang-gap of pristine CDs, but also brings the specific acceptor of the target analyte to the CD matrix. Surface modification is a conventional functionalization method that allows one to manipulate the surface chemical structure of CDs in a covalent or non-covalent fashion, so that the selectivity and sensitivity of pristine CDs can be sequentially improved. In this paper, we review the research progress in the functionalization of CDs over the last eight years based on the publications from Web of Science, and systematically summarize the applications of CD-based sensors in the area of food safety. Although this review is incomprehensive due to space restrictions, it can shed fresh light on the development of CD-based rapid sensors for food safety applications in the near future.