**1. Introduction**

To date, metal nanostructures have attracted a grea<sup>t</sup> deal of attention because of their intriguing electronic, optical, and catalytic properties [1–7]. In the past few decades, Au nanostructures of various shapes, including nanospheres [8–10], nanorods [11–19], nanowires [20–26], polyhedrons [27–32], nanoplates [33–35], nanoshells [36–38], and branched forms [39–49], have been extensively explored because of their shape-dependent surface plasmon resonance properties [50–53]. The fascinating surface plasmon resonance properties of Au nanostructures enable their effective implementation in a wide variety of biomedical applications, such as cancer therapy, bio-imaging, biological sensing, and diagnostics [54–58]. Biological targets (e.g., protein and DNA) are recognized as changes in the absorption of the functionalized Au nanostructures and an efficient bio-imaging using Au

nanostructures is achieved by their distinctive interactions with light. Photothermal therapy is one of the major therapeutic methods that requires near-infrared (NIR) light, which shows maximum penetration depths in tissues and has a low toxicity for normal cells, so that cancer cells can be killed by heat generation [56,59–70]. To obtain Au nanostructures exhibiting optical properties active in the NIR region, intricate methods including multiple steps, many reagents, or long reaction times have been applied. For example, a seed-mediated growth approach has been widely used to produce anisotropic Au nanostructures, such as nanorods, which require steps for the formation of Au seeds and their growth. In addition, a method using two (or more) appropriate capping reagents with different binding affinities has been developed to obtain kinetic control of the growth rates on various crystal planes, resulting in the formation of non-spherical Au nanostructures. Therefore, the development of Au nanostructure preparation that yields NIR absorption is of prime importance for the successful extension to practical applications.

Recently, it was reported that ethylenediaminetetraacetic acid (EDTA), a well-known metal chelating ligand, can act as a reducing agen<sup>t</sup> [71–73]. However, to the best of our knowledge, the morphology control of Au nanostructures by applying EDTA agents has not been attempted. Therefore, this is pioneering work on the preparation of Au nanocrystals exhibiting NIR activities using EDTA agents at room temperature.

Herein we present a facile and effective shape-controlled synthesis of colloidal Au nanostructures at room temperature and their effective performance for photothermal therapy. This simple procedure was achieved by reacting HAuCl4 and EDTA tetrasodium salt, exhibiting a dual function as a reductant and a stabilizing agent, without any toxic compounds such as cetyltriethylammonium bromide or additional agents (e.g., superhydride). The morphology of the Au nanostructures was readily controlled by adjusting the molar ratio of the EDTA tetrasodium salt to the HAuCl4 precursor. Reducing the concentration of the EDTA tetrasodium salt led to the formation of Au nanowire networks that are active in the NIR regime. Control experiments using several kinds of EDTA agents revealed that the four deprotonated carboxylic groups of EDTA tetrasodium salt play a key role as stabilizing agents and in effective growth control. The Au nanowire networks showed an effective and selective photothermal therapeutic effect on cancerous glioblastoma cells (U87MG) under NIR irradiation at 980 nm.
