The pollution caused by heavy metals (HMs) may occur through both natural processes and anthropogenic activities and is found in complex media. The purpose of this review is to summarize the state-of-art of fluorescent CDs and the sensing applications in a systematic manner. This review intends to provide clues on the origin on the observed selectivity in chemiluminiscence sensors, which was until now a stated but unaddressed question, and still remains open for debate. Indeed, it is tempting to think that CDs possessing functional groups with soft bases at the surface are able to detect soft metal acids, while the opposite is to be suspected for hard acid-base pairs. However, the literature shows several examples where this trend does not hold. We found that such observation is explained by the involvement of dynamic quenching, which does not involve the formation of a non-fluorescent complex, as in the case of static quenching. We have provided an interpretation of published data that was not provided by the original authors and offer guidelines to enable the design of CDs to target ions in solution.