**5. Conclusions**

We have demonstrated an optimized synthesis and application of gold nanostars as a theranostic agent for combined multimodal imaging using CT, PAI and photothermal therapy. Due to the red shift of the LSPR band and the high absorption coefficient, these nanostars could have a better theranostic outcome compared to other shaped nanoparticles. The gold nanostars created significant contrast in both CT and PAI and proved to be an effective therapeutic agent for PTT, due to a successful passive uptake by the tumor cells. Future work should focus on the intravenous delivery of gold nanostars (passively or actively) leading most likely to further improvements in the nanotheranostics field, aiming for imaging and therapy of specific tumor types, which also has the potential to target and treat tumor metastases.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/10/11/2112/s1, Figure S1: UV-Vis absorption spectroscopy of nanostars. Figure S2: UV-Vis absorption spectroscopy, DLS intensity plots and zeta-potential measurements of nanostars. Figure S3: Intracellular gold concentration using ICP-OES. Figure S4: Temperature profile, fluorescence and bright field microscopy of gold nanostars and irradiated tumor tissue. Figure S5: In vivo BLI before and after nanostar injection. Figure S6: Bright field microscopy images and histology of control and nanostar-injected tumors.

**Author Contributions:** Conceptualization, A.D., L.L. and U.H.; methodology, A.D., G.V.V., H.J., B.V., E.V., J.J., T.S. (Tim Stakenborg) and T.S. (Tom Struys); data analysis, A.D., T.S. (Tim Stakenborg), H.J. and E.V.; validation, T.S. (Tim Stakenborg), H.J., A.D., L.L. and U.H.; investigation, A.D., H.J., B.V. and T.S. (Tom Struys); resources, J.J., T.S. (Tim Stakenborg), L.L. and U.H.; data curation, A.D., L.L. and U.H.; writing—original draft preparation, A.D., L.L. and U.H.; writing—review and editing, all authors; supervision, G.V.V., H.J., L.L. and U.H.; funding acquisition, L.L. and U.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Flemish foundation for Innovation, Science and Technology for the IWT SBO 'Imagine' (080017) and IWT SBO 'NanoCoMIT' (140061), the European Commission under the 'PANA' project, Call H2020-NMP-2015-two-stage (grant 686009), and by the University of Leuven for program financing PF IMIR (10/017).

**Acknowledgments:** We are grateful for technical assistance by Jesse Trekker (IMEC, KULeuven) for particle characterization, Marc Jans (Lab of Histology, UHasselt) for ex vivo TEM measurement, Tom Dresselaers (Biomedical MRI, KULeuven) for help with MRI experiments and Joris Deconinck (IMEC) for ICP-OES experiments is highly appreciated. We would also like to thank Visualsonics for their guidance and help with photoacoustic imaging experimentation.

**Conflicts of Interest:** Jithin Jose is an employee of VisualSonics. All other authors declare no conflict of interest.
