**Abstract**

This chapter comments on the advantages of nanobiosensors using nanocrystals in improving electrochemical sensors' response and their use as theragnostic tools in biomedical applications. The use of nanomaterials to modify electrochemical sensors' surfaces to increase these devices' sensitivity and their bio-functionality enables high-quality nanotechnological platforms. Thus, graphene nanostructures and CdSe/ CdS magic-sized quantum dots (MSQDs) were shown to improve biosensor's sensitivity. In addition, the use of CdSe/CdS MSQDs and cobalt ferrite nanocrystals (NCs) as potential tools for drug delivery systems and biocompatible titanium dioxide NCs in osseointegration processes and their bio-location are also demonstrated. So, this chapter shows some impressive results on which the group has been working regarding the applications of nanocrystals to electrochemical sensors and theranostic applications.

**Keywords:** nanocrystals, luminescence, magnetic nanocrystals, magic-sized quantum dots, drug delivery systems, biosensors, biomaterials, theranostic applications
