**4. Conclusions**

The one-pot method of loading graphene quantum dots (GQDs) was proposed as a means to modify nanoparticles for fluorescent imaging. GQDs were loaded into the optical-magneto Fe3O4@PPy nanoparticles instead of on the surface. The morphology, components, optical and magnetic properties, biocompatibility and biomedical application of the GQD-loaded optical-magneto nanoparticles were investigated. The GQD-loaded optical-magneto nanoparticles performed as an excellent tacking agent, highlighting the potential application of GQDs in tracking the real-time distribution and metabolism of nanoparticles *in vivo*.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1944/12/13/2191/s1, Figure S1: Hydrodynamic size of GQDs-NPs nanocomposites.

**Author Contributions:** Conceptualization, writing—review and editing, R.Z. and Q.C.; Writing—original draft preparation, Y.W.; Material preparation and characterization Y.W., Y.H.; Cell experiment, N.X., In vivo experiment D.W., Q.G.; Figures, J.W.; Data analysis S.X., Y.L.

**Funding:** This research was funded by National Key Research & Development Program of China (2016YFC0304502).

**Acknowledgments:** We gratefully acknowledge the measurement support from State key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China.

**Conflicts of Interest:** The authors declare no conflict of interest.
