**7. Conclusion**

In this chapter, various QDs were introduced and in addition to expressing methods of synthesis, modification and cytotoxicity, the bioimaging applications (*in vitro and in vivo*) were discussed. SQDs due to having heavy metal cannot be used for clinical application. But, nontoxic CuInS2 QDs are suitable alternatives for SQDs in biological imaging. Beside these QDs, carbon-based QDs, MoS2 QDs, and BP QDs have low toxicity and fluorescence emission in the NIR region of the light spectrum. The easy synthesis and the possibility of modification are other advantages of these QDs compared to SQDs. However, since studies have shown that QDs are not toxic at low concentrations, more research is needed to evaluate their blood circulation, cytotoxicity, etc. for bioimaging application. In addition, different methods of synthesis and modifications may affect the optical properties as well as the biocompatibility of QDs, and for this reason, more investigations should be done in this field. In the end, it should be noted that the use of safe QDs in clinical imaging techniques such as CT and MRI seems very likely in the near future.
