Nanotechnol Rev 2017; 6(3): 271–278

Future of nanotechnology contribution

Jae-Seung Lee* Deep eutectic 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 ; gold; nanomaterial; nificant role in the synthesis and fabrication of various nanoparticle; silver. nanomaterials, such as zeolite analogs [8], carbon nano- materials [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 proper- ties, 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 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 nano­ particles 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 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 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 nano- structures [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]. To the best of my knowledge, every DES reported to in the controlled shape of the gold nanoparticles [27, 28]. date contains a halide anion, indicating that it would take a certain amount of time until this incompatibility issue in silver nanomaterial synthesis is solved.

3 DESs with water (2) Thermal instability: DESs are known to exhibit suffi- cient chemical stability to allow for their use in industrial The presence of water in DESs during the synthesis of noble applications. At elevated temperatures with particular metal nanoparticles not only determines the reaction rate combinations of HBDs and halide salts, however, their but also the morphology of the nanoparticles such as their thermal stability decreases as a function of reaction size and shape. For example, our group has synthesized time. For example, urea, one of the most frequently used gold microstructures with a large structural diversity with HBDs to form DESs, has been observed to decompose into 274 J.-S. Lee: Deep eutectic solvents as versatile media

Figure 3: Gold nanostructures having various structures synthesized in DESs with water content of 10% (A), 20% (B), 50% (C), 80% (D), 90% (E) and 100% (F). Reprinted with permission from Ref. [31]. Copyright 2016 Royal Society of Chemistry. ammonia at 80°C for an elongated reaction time period physical methods such as stirring, vortexing, or rotating. in the presence of choline chloride, the most frequently More importantly, however, there is a chemical condi- used halide salt, or other polyols [37, 40]. It is important to tion that precedes the physical methods: low viscosity of note that pure urea is highly stable and does not decom- the solvent. For complete homogenization and dissolu- pose under the same conditions. Its participation in the tion of noble metal precursors, reductants, and SDAs in formation of DESs leads to destabilization and thermal a DES, the viscosity of the DES must be as low as pos- decomposition when heated. In our group, we also have sible. Unfortunately, however, the viscosity of DESs is, observed that the color of DESs turns brown and brown- in general, several 100 times as high as that of water at ish black after heating for hours during nanomaterial syn- room temperature [4]. This high viscosity could be one thesis (data not shown). In fact, the extremely low vapor of the reasons why only a few DESs have been demon- pressure of DESs prevents them from boiling and releas- strated for the synthesis of noble metal nanoparticles to ing heat. Therefore, unlike other conventional solvents, date, despite the huge chemical library of halide salts and DESs will thermally decompose when overheated. This HBDs. The viscosity of DESs can be reduced at elevated thermal instability of DESs can be an issue when noble temperatures. However, this is countered by the DESs metal nanomaterials are synthesized at a high tempera- becoming thermally unstable. ture over a lengthy reaction time (which is typical when water is not used) because the decomposition of urea will (4) Overcoming the limitations: There could be poten- change the reaction conditions significantly. For example, tial solutions to overcome the aforementioned limitations − the generation of NH3 will dramatically increase the pH of DESs. For example, monovalent anions such as NO3 or − of the reaction mixture, which can dramatically affect the CH3COO could be substituted for halide ions to make DESs nucleation and growth of the noble metal nanomaterials comparable with silver. The thermal stability of DESs could [41, 42]. The decomposition of one of the components of a be enhanced using component molecules having stronger DES will eventually lead to the phase change of the DES, atomic bonds such as double or triple bonds. However, it after which the DES may no longer be a solvent. is still a question if non-halide salts can form DESs with conventional hydrogen bond donors (HBDs). Moreover, (3) High viscosity: Synthetic researchers in nanosci- molecules having double or triple bonds could be thermally ence are always in pursuit of high monodispersity of polymerized at a higher temperature that is required for the noble metal nanomaterials. To achieve this goal, the formation of DESs [43]. In fact, because the chemical nature homogeneity of the reaction mixture is often ensured by of DESs, itself, causes their intrinsic problems discussed J.-S. Lee: Deep eutectic solvents as versatile media 275

Figure 4: (A) Palladium and (B) platinum nanostructures synthesized in a DES. Reprinted with permission from Refs. [46] and [47]. Copyright 2012 and 2013 American Chemical Society.

above, it would be highly difficult to overcome the limita- surface of noble metal nanomaterials by subse- tions in the near future. quent polymerization in a DES, preferably in a one-step process, would be one of the next goals to achieve in this area of research.

5 Future of DESs for Nobel (3) Synthesis of platinum and palladium nanoma- ­nanomaterial synthesis terials: In addition to gold and silver nanomaterials, a few other noble metal nanomaterials such as those made In addition to the aforementioned advantages and disad- from platinum and palladium have been synthesized in a vantages, there are a few more attractive points associated DES (Figure 4) [46–50]. In contrast to the well-developed with the use of DESs as the reaction medium for noble aqueous synthesis of platinum and palladium nanoparti- metal nanomaterial synthesis, which are as follows: cles, there are only a few reports demonstrating the syn- thesis of these nanoparticles in a DES. The platinum and (1) Excellent solvent: The high solubility of various palladium salts were reduced by electrochemical reactions, types of materials such as metal salts, metal oxides, solid which resulted in only deposited, not dispersed, nano- halogens, small organic molecules, , and even particles on a solid support, likely because of the limited gaseous molecules in DESs substantially enlarges their reducing power of chemical reductants in the DES. Con- pool of potential reactants [4, 44]. By conducting noble sidering the structural control of nanoparticles, including metal nanomaterial synthesis using noble metal precur- sors and other metallic and non-metallic starting materi- als, all dissolved together in a DES, one can expect that 60 highly functional nanomaterials could be synthesized 50 with improved chemical stability, increased catalytic

lications 40 properties, and more distinctive optical properties. 30

(2) Green media for nanofabrication using polymers: 20

After synthesis, noble metal nanomaterials are often Number of pub 10 coated with polymeric materials to enhance their stabil- 0 ity by protecting their surface and to introduce chemical 2008 2009 2010 2011 2012 2013 2014 2015 2016 groups for surface functionalization. In fact, DESs are Year efficient “green” solvents for both radical and conden- sation polymerization reactions, in general [45]. To the Figure 5: Number of publications concerning the application of DESs in nanotechnology as a function of year from 2008 to 2016. best of my knowledge, however, post-synthesis fabrica- Reprinted with permission from Ref. [16]. Copyright 2015 Elsevier B. tion of noble metal nanomaterials using polymers in a V. The data points of 2015 and 2016 were additionally obtained by DES has been rarely investigated to date. Synthesis and the author. 276 J.-S. Lee: Deep eutectic solvents as versatile media

Table 1: A summary of noble metal nanostructures synthesized in DESs to date and their properties and applications.

Element Shapes Size (nm) Properties and applications Reference number Au Concave rhombic dodecahedron, concave cube, ~ 300 Electrochemical catalysis [35] octopod, cuboctahedral box, hollow octahedron Au Flower 30–500 SPR, SERS substrate [32] Au Flower 200–1000 Catalysis [30] Au Quasi-sphere 100–3000 Catalysis, surface [29] modification Au Star, dentist, quasi-sphere 200–2000 Electrochemical catalysis [31] Au Foam 100–200 (pore) Catalysis [33] Au Quasi-sphere 25 X-ray contrast agent [51] Au Star 300–900 Electrochemical catalysis [34] Au Sheet 500–3000 Electrochemical catalysis [36] Au, Ag, Pd Mesoporous film ~ 30 (grain diameter) SERS [50] Ag Quasi-sphere > 100 or < 5 N/A [38] Ag Porous film 20–40 (grain diameter) Wettability [39] Pd Quasi-sphere 10–20 N/A [46] Pt Triambic icosahedron 158 Electrochemical catalysis [48] Pt Flower 200 Electrochemical catalysis [49] Pt Concave tetrahexahedron 62–370 Electrochemical catalysis [47] their shape, size, and monodispersity, DES-based synthe- References sis of platinum and palladium nanoparticles is still in a preliminary stage and requires further investigation. [1] Abbott AP, Capper G, Davies DL, Munro HL, Rasheed RK, ­Tambyrajah V. Preparation of novel, moisture-stable, Lewis- acidic ionic liquids containing quaternary ammonium salts with functional side chains. Chem. Commun. 2001, 2010–2011. [2] Abbott AP, Boothby D, Capper G, Davies DL, Rasheed RK. Deep 6 Conclusions eutectic solvents formed between choline chloride and carbox- ylic acids: versatile alternatives to ionic liquids. J. Am. Chem. The reaction medium in a solution-phase chemical reac- Soc. 2004, 126, 9142–9147. tion is an important consideration regardless of what is [3] Francisco M, van den Bruinhorst A, Kroon MC. Low-transition- being synthesized. The solvent must be selected care- temperature mixtures (LTTMs): a new generation of designer fully and judiciously to achieve the highest reaction solvents. Angew. Chem. Int. Ed. 2013, 52, 3074–3085. [4] Smith EL, Abbott AP, Ryder KS. Deep eutectic solvents (DESs) efficiency. DESs are an emerging class of reaction media and their applications. Chem. Rev. 2014, 114, 11060–11082. for the synthesis of noble metal nanomaterials, and are [5] Electroplating Market: Automotive and Electrical & Electron- of great promise, exhibiting a number of examined and ics End Use Industry Segments Estimated to Hold About 65% potential advantages. Although there are some limita- of the Total Market Value Share by 2026 End: Global Industry tions that need to be overcome to enable their wider Analysis and Opportunity Assessment, 2016–2026, Future Market Insights; 2016. use, the chemistry of DESs, which has been thoroughly [6] del Monte F, Carriazo D, Serrano MC, Gutierrez MC, Ferrer investigated over the last 15 years, will contribute to their ML. Deep eutectic solvents in polymerizations: a greener advancement as reaction media. The number of publi- alternative to conventional syntheses. Chemsuschem 2014, 7, cations on both DESs and DES-based nanotechnology 999–1009. grows every year (Figure 5), which shows great poten- [7] Tang BK, Row KH. Recent developments in deep eutectic tial for their widespread application as versatile media solvents in chemical sciences. Monatsh Chem. 2013, 144, 1427–1454. for the synthesis of noble metal nanomaterials [7, 16]. To [8] Cooper ER, Andrews CD, Wheatley PS, Webb PB, Wormald P, review the types, sizes, properties, and applications of Morris RE. Ionic liquids and eutectic mixtures as solvent and the noble metal nanomaterials, which are synthesized in template in synthesis of zeolite analogues. Nature 2004, 430, DESs to date, a summary is provided in Table 1. 1012–1016. [9] Carriazo D, Gutierrez MC, Ferrer ML, del Monte F. Resorcinol- based deep eutectic solvents as both carbonaceous precursors Acknowledgment: This work was supported by the and templating agents in the synthesis of hierarchical porous National Research Foundation of Korea, funded by the carbon monoliths. Chem. Mater. 2010, 22, 6146–6152. Korean government (NRF-2015R1C1A1A01053865, NRF- [10] Chakrabarti MH, Manan NSA, Brandon NP, Maher RC, Mjalli 2016R1A5A1010148, and NRF-2016R1E1A202073). FS, AlNashef IM, Hajimolana SA, Hashim MA, Hussain MA, Nir J.-S. Lee: Deep eutectic solvents as versatile media 277

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