**4. Conclusions**

In summary, our work presents a facile and effective shape-controlled synthesis of Au nanostructures and their photothermal therapeutic effect. The described procedure involves the simple mixing of HAuCl4 and EDTA tetrasodium salt in an aqueous solution at room temperature, without additional ligands or toxic reagents. Adjusting the molar ratios of HAuCl4 to EDTA tetrasodium salt enables effective morphology control of Au nanostructures from spheres to branched forms and nanowire networks. Detailed control experiments revealed that the four deprotonated carboxylic acids of the EDTA tetrasodium salt provided effective growth control and stabilization. The Au nanowire networks showed strong absorption in the NIR region and hence were suitable for photothermal therapy. Under NIR irradiation, the Au nanowire networks allowed for selective destruction of cancerous U87MG cells by local heating, generated by the NIR absorption. This work demonstrates the development of a simple synthetic route to NIR-active Au nanostructures, which can be extended to other applications including optical sensing and surface-enhanced Raman scattering.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/8/4/252/s1, Figure S1: (a) Photograph of mPEG-SH stabilized Au nanowire networks kept in ambient conditions for one month, showing good stability, (b) absorption spectra of as-synthesized Au nanowire networks (black curve) and mPEG-SH coated Au nanowire networks (red curve), and (c) TEM image of Au nanowire networks after mPEG-SH coating, Figure S2: Absorption spectra of Au nanostructures taken at different stages of the reaction, Figure S3: TEM images of Au aggregates generated by using (a) EDTA and (b) EDTA disodium salt, Figure S4: The cell viability of Au nanowire networks by MTT assay, Table S1: The pH values at different reaction conditions.

**Acknowledgments:** Y.P. was supported by the Center for Integrated Smart Sensors, funded by the Ministry of Science, ICT and Future Planning, Republic of Korea, as a Global Frontier Project (CISS-012M3A6A6054186). Y.I.P. was supported by the National Research Foundation of Korea (NRF) gran<sup>t</sup> funded by the Korean governmen<sup>t</sup> (Ministry of Science, ICT & Future Planning) (No. 2016R1A4A1012224).

**Author Contributions:** Y.J., Y.I.P. and Y.P. conceived and designed the experiments; Y.J. and Y.P. performed the synthesis experiments; Y.I.P. performed the photothermal therapy experiments; N.L. performed the MTT assay experiments; J.H.K. executed the article editing; Y.J., Y.I.P. and Y.P. wrote the paper.

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