Deep Eutectic Solvents As Versatile Media for the Synthesis of Noble Metal Nanomaterials

Nanotechnol Rev 2017; 6(3): 271–278 Future of nanotechnology contribution Jae-Seung Lee* Deep eutectic solvents as versatile media for the synthesis of noble metal nanomaterials DOI 10.1515/ntrev-2016-0106 because of their low vapor pressure, low cost, non-flam- Received December 8, 2016; accepted February 6, 2017; previously mability, and easy preparation. The global electroplating published online March 20, 2017 market was estimated to be approximately 14.5 billion US$ in 2016 and is expected to continue expanding, indicating Abstract: Deep eutectic solvents (DESs) were developed the potential importance of DESs in industry [5]. In addi- 15 years ago and have been used for various purposes tion to metal processing, there is also a growing interest based on their unique chemical and physical properties. in utilizing DESs as tunable media for organic chemical Recently, they have been highlighted as versatile media syntheses, polymerization, and organic extraction and for the synthesis of noble metal nanomaterials. Although separation [6, 7]. Theoretically, an unlimited number of there are a few limitations, their vast chemical library of possible combinations of halide salts and HBDs (Figure 1) hydrogen bond donors and excellent solubility show great could be used to design a DES, resulting in a large number potential for their future applications for the synthesis of of suitable media for such inorganic and organic reactions. noble metal nanoparticles. It has also been demonstrated that DESs play a sig- Keywords: deep eutectic solvent; gold; nanomaterial; nificant role in the synthesis and fabrication of various nanoparticle; silver. nanomaterials, such as zeolite analogs [8], carbon nanomaterials [9, 10], micro- and nanostructured semicon- ductors [11–13], and DNA nanostructures [14]. As a result, DESs have proved to be versatile media for nanoscale syn- 1 Introduction thesis based on their advantageous chemical and physical properties [15, 16]. Despite the widespread impact of DESs, Deep eutectic solvents (DESs) are an emerging class of however, their role in the synthesis of noble metal nano- low-transition-temperature mixtures (LTTMs), which are materials, particularly gold and silver nanomaterials, has liquids formed by hydrogen-bonding interactions of mul- been rarely reported and discussed to date [17]. Consider- tiple starting materials having higher melting tempera- ing the importance of noble metal nanomaterials in nano- tures [1–3]. The melting temperature of a DES can be as technology, a discussion on the history of their synthesis low as 12°C, indicating that it is in the liquid phase even at in DESs and a future perspective on the use of DESs in this room temperature [2]. DESs are often mistakenly consid- area are needed. ered to constitute a subtype of ionic liquids (ILs) owing to their similar physical properties. Their chemical properties, however, are significantly different from those of ILs because the starting mixtures of DESs typically include a 2 How do DESs control the halide salt and a hydrogen bond donor (HBD), in contrast to a simple cation and anion pair for ILs [4]. Since the first structures of gold nanomaterials? report of DESs in 2001 [1], they have been highlighted as In 2008, Sun et al. reported the synthesis of gold nan- environmentally benign media with advantageous char- oparticles with various unique shapes such as stars, acteristics and have been investigated mainly in metal snowflakes, and thorns using a DES composed of choline processing applications, such as metal electrodeposition chloride and urea with a mole ratio of 1 : 2 (Figure 2) [18]. and electropolishing. In particular, DESs are highly attrac- In spite of the unusually lengthy synthetic procedure tive for large-scale industrial electroplating processes (1 week), the presence of high-index facets enabled the *Corresponding author: Jae-Seung Lee, Department of Materials nanoparticles to be used in the electrocatalytic reduction Science and Engineering, Korea University, 145 Anam-ro, Seongbuk- of H2O2. This pioneering work demonstrated three impor- gu, Seoul 02841, Republic of Korea, e-mail: [email protected] tant points that require consideration in the chemical 272 J.-S. Lee: Deep eutectic solvents as versatile media Figure 1: Chemical structures of halide salts and hydrogen bond donors (HBDs). Reprinted with permission from Ref. [4]. Copyright 2014 American Chemical Society. Figure 2: Various shapes (snowflakes, stars, and thorns) of gold nanoparticles synthesized in a DES. Reprinted with permission from Ref. [18]. Copyright 2008 WILEY-VCH Verlag GmbH & Co. design of DES-based synthesis of noble metal nanoma- chemical interactions of specific functional groups at the terials: (1) the DES exhibited dual functions as a reac- interface of a DES and nanoparticle. tion medium and a shape-directing agent (SDA); (2) the Research into the role of DESs in nanoparticle synthe- dry synthesis (free of H2O) using the DES required sig- sis is still in its infancy, and therefore, there is still a long nificantly longer reaction time compared to the reaction way to go to fully understand their chemical functions. For under aqueous conditions; and (3) the size of the nano- example, the shape-controlled synthesis of silver nanoparticles was not (able to be) controlled. The discovery materials by polyvinylpyrrolidone is still under active inves- that DESs can act as an SDA could lead to advancement tigation since the first report of this synthesis 15 years ago in the synthetic strategies for nanomaterials. Further- [19]. The kinetically unfavorable synthetic reaction in a DES more, the vast chemical library of HBDs as a potential emphasizes the importance of conventional solvents, par- component of DESs could be useful for exploring the ticularly H2O, to facilitate fast reactions. The reduction of a J.-S. Lee: Deep eutectic solvents as versatile media 273 − gold precursor (typically AuCl4 ) by ascorbic acid inevitably respect to their diameter and surface roughness by simply involves deprotonation of ascorbic acid [20] and, thus, is varying the ratio of water and DES in the mixture [29]. significantly enhanced in a protonatable solvent. Although There are several follow-up reports, which demonstrate most of the HBDs have protonatable chemical groups such as that the water content is one of the most important para- hydroxyl or carboxyl groups, they would not be expected to meters for the precise morphological control of noble metal participate in the deprotonation of ascorbic acid because of nanostructures in DESs (Figure 3) [30–33]. In addition to their mutual hydrogen-bonding interactions to form the DES, the water content, other factors such as temperature, ratio − leading to the deactivation of ascorbic acid. This specula- of reductant to AuCl4 , additional SDAs, and electrochemi- tion is conversely supported by the accelerated reaction rate cal potential have been investigated as effective variables (reaction time of a few hours) achieved by the addition of to control the morphology of the nanostructures synthe- trace amounts of H2O (e.g. at a concentration of 5000 ppm). sized in DESs [29, 34–36]. Despite the considerable effect In fact, most studies report a reaction time of just several of water, however, the addition of water to DESs needs to hours even without the intentional addition of water, which be more carefully reviewed because water, itself, is both is likely because of the hygroscopic nature of the DESs [21, a hydrogen bond acceptor (HBA) and HBD. Water mole- 22]. There is still room to systematically control the size of cules can interact with individual components of DESs by noble metal nanoparticles during their synthesis in DESs by forming multiple hydrogen bonds, leading to the breakage separating the nucleation and growth stages. Adaptation of of the halide salt-HBD interactions of DESs [22, 37]. This the seed-mediated growth method to the DES media would breakage is the major difference between the use of DESs be a reliable and reasonable starting point. and ILs. Therefore, it is difficult to determine whether a Three potential routes of structural control of DESs mixture composed of, for example, water and Reline (a on the shape of nanoparticles can be predicted. First, the common DES consisting of choline chloride and urea) at a halide ions in DESs, depending on the type and concen- 90 : 10 ratio is a true mixture of water and the DES or just tration, may exhibit the shape-directing properties as they an aqueous solution of choline chloride and urea. do in aqueous phases for various anisotropic gold nanostructures [23, 24]. Although the exact role of halide ions in the aqueous synthesis of gold nanoparticles still remains the subject of much debate, their effect on the surface 4 Current limitations of DESs of gold nanoparticles during the growth in DESs would be another interesting issue to investigate. Second, the (1) Incompatibility with silver: The halide anion of DESs facet-specific preferential adsorption of hydrogen bond poses the greatest barrier to their application as media for donors (HBDs) on gold nanoparticles during the growth the synthesis of noble metal nanomaterials. Specifically, is expected to result in their anisotropic shapes. In fact, a the halide anion of DESs in the reaction mixture irrevers- number of HBD molecules contain amine, hydroxyl, and ibly forms solid silver halide with Ag + , which fundamen- carboxyl groups in either aliphatic or aromatic molecular tally limits the application of DESs for the synthesis of silver frames (Figure 1), which is structurally similar to previ- nanomaterials. This limitation explains why DESs have ously investigated shape-directing agents (SDAs) used in been used mostly in gold nanomaterial synthesis to date. aqueous media for gold nanoparticles [25, 26]. Finally, the There have been a few reports on the use of DESs in silver addition of conventional SDAs such as polyvinylpyrro- nanomaterial synthesis, but they are limited to only films lidone (PVP) or bis(p-sulfonatophenyl)-phenylphosphine and particles grown or supported on a solid substrate [38, (BSPP), depending on their solubility in DESs, may result 39].

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