Surfactant-Free Solution-Based Synthesis of Metallic Nanoparticles

DOI 10.1515/ntrev-2012-0079 Nanotechnol Rev 2013; 2(1): 5–25

Hideya Kawasaki * Surfactant-free solution-based synthesis of metallic nanoparticles toward efficient use of the nanoparticles’ surfaces and their application in catalysis and chemo-/biosensing

Abstract: The choice of stabilizer and the stabilizer-to- 1 Introduction precursor ions molar ratio during metal nanoparticle synthesis are important for controlling the shape, size, Metal nanoparticles usually range from 1 to 100 nm in and dispersion stability of the nanoparticles. However, size. The high surface area, high surface energy, and the active sites on the nanoparticles surfaces may be quantum-confined nature of metal nanoparticles result in blocked by the stabilizing agents used, resulting in a displaying unique physiochemical properties with respect less-than-effective utilization of the surfaces. In this to optics, magnetics, and chemical reactions (catalysis). review, various surfactant-free solution-based methods These nanoparticles also have lower melting points and of synthesizing metal nanoparticles are described, along electric/thermal conductivities than those of their bulk with the applications of such nanoparticles in catalysis counterparts. The physicochemical properties of metal and sensing. “ Surfactant-free ” synthesis does not imply nanoparticles change as their size decreases, and the truly bare metal nanoparticles synthesis but implies number of atoms present at their surfaces becomes signifi- one where the metal nanoparticles are prepared in the cant. In addition to these size-based effects, the morpho- absence of additional stabilizing agents such as thiolate logy (i.e., the shape and dimensionality), the composition and phosphine compounds, surfactants, and polymers. (i.e., whether the nanoparticles are of an alloy or a metal), These metal nanoparticles are stabilized by the solvents and the agglomeration of the nanoparticles are impor- or the simple ions of the reducing agents or low-molecu- tant, too, as the physical/chemical properties of the nano- lar-weight salts used. Surfactant-free synthesis of metal particles are also affected by these factors. Owing to the nanoparticles via photochemical-, ultrasonochemical-, development of controllable solution-based techniques and laser ablation-mediated synthesis methods is also for synthesizing metal nanoparticles, research on metal described. Because of the effective utilization of their nanoparticles is attracting increasing scientific interest. surfaces, metal nanoparticles prepared without sur- These nanoparticles are also being used in a wide variety factants, polymers, templates, or seeds are expected of potential applications, including biomedical [1] , optical to exhibit high performance when used in catalysis [2, 3] , magnetic [4] , catalysis [5 – 7] , sensing [8 – 10] , energy (synthetic catalysis and electrocatalysis) and sensing [11] , and electronic applications [12 – 14] . In addition, (surface-enhanced Raman scattering (SERS)), surface- hybrids of metal nanoparticles and other materials such assisted laser desorption/ionization-mass spectrometry as synthetic polymers, biomolecules, and semiconduc- (SALDI-MS)). tors have also been developed [15 – 17] . Recent advances in the solution-based synthesis of metal nanoparticles have Keywords: catalysis; nanoparticles; SALDI-MS; SERS; given rise to a new class of metal nanoclusters (NCs, such surfactant-free synthesis. as those of Ag or Au) that are < 2 nm in size. These metal NCs are of significant interest because they provide the *Corresponding author: Hideya Kawasaki, Faculty of Chemistry, missing link between atomic and nanoparticle behavior Department of Chemistry and Materials Engineering , Materials and of metals. The sizes of these metal NCs are comparable Bioengineering, Kansai University, 3-3-35 Yamate-cho, Suita-shi, to the Fermi wavelength of electrons, resulting in mole- Osaka 564-8680, Japan , e-mail: [email protected] cule-like properties including discrete electronic states, size-dependent fluorescence, and specific catalytic activity. The synthesis, structures, and properties of atomically precise Au NCs with strict stoichiometry (denoted 6 H. Kawasaki: Surfactant-free solution-based synthesis of metallic nanoparticles

as Aun (SR)m , where SR refers to thiolate) have become hot This review is divided into two main sections: the first research topics [18 – 23] . section reviews various surfactant-free synthesis methods, Over the past 20 years, the development of solution- and the second section describes their applications. In based techniques for synthesizing metal nanoparticles, the first section, various methods of obtaining solvent- such as the chemical reduction of precursor ions in solu- stabilized metal nanoparticles are described, where the tion by a reducing agent, has allowed the size and the solvents play three roles: that of the reaction medium, the shape of the nanoparticles to be controlled. The solution- reducing agent, and the stabilizer. Then, simple ion-stabi- based synthesis of metal nanoparticles usually makes use lized metal nanoparticles are described. The simple ions of a soluble metal salt, a reducing agent, and a stabiliz- work as a reducing agent, a protecting agent, and a shape- ing agent. Reducing agents such as sodium borohydride, directing agent. Next, the surfactant-free synthesis of citrate ions, and alcohols are commonly used for the fab- metal nanoparticles via photochemical-, ultrasonochemi- rication of metal nanoparticles. Stabilizing agents such cal-, and laser ablation-mediated synthesis techniques is as thiol, phosphine, and amine compounds, surfactants, detailed. Finally, the applications of the thus-obtained and polymers bind to the surface of the nanoparticle and metal nanoparticles in catalysis (synthetic catalysis and prevent further growth or aggregation. In addition, the electrocatalysis) and sensing (surface-enhanced Raman stabilizing agent not only stabilizes the metal nanopar- scattering (SERS), surface-assisted laser desorption/ioni- ticles but also adds functionalities such as turning the zation mass spectrometry (SALDI-MS)) are described. The hydrophilic/hydrophobic particle surfaces, coupling bio- metal nanoparticles prepared without any surfactants, materials for biological applications (recognition, deliv- polymers, templates, or seeds being used exhibited a high ery, and manipulation), and producing building blocks performance during catalysis and sensing because of the for assembly (devices). effective utilization of the surfaces of the nanoparticles. The following two methods are the most commonly used solution-based techniques for synthesizing metal nanoparticles. The first one is a water-based method called the “ Turkevich method” and involves the reduc- 2 Solvent-mediated synthesis tion of gold ions by citrate ions in aqueous media [24] . The second is an organic solution-based method called the 2.1 N , N-dimethylformamide (DMF)-mediated “ Brust method ” and entails the reduction of gold ions by synthesis NaBH4 in organic media in the presence of a phase-transfer agent [25] . The choice of the stabilizer and the molar ratio DMF is used widely as a solvent for preparing metal col- of the stabilizer to the precursor ions during the synthesis loids. It is also used in chemical reactions involving determine the size and shape of the metal nanoparticles. organic compounds because of its higher boiling point. In some cases, however, the active sites on the surfaces In addition, it exhibits good chemical/thermal stability of the nanoparticles might be blocked by the stabilizing and high polarity. Finally, it is an excellent solvent for agent, resulting in the less-than-effective utilization of the both organic and inorganic compounds. Liz-Marz á n and nanoparticle surfaces. For example, the blocking of the coworkers have pioneered the use of DMF for reducing active sites may result in a loss in catalytic activity. metal ions such as those of silver and gold at high temper- In this review, we describe the surfactant-free solu- atures [26] . The point to note is that DMF can work as both tion-based synthesis of metallic nanoparticles. We also a solvent and a reducing agent in the preparation of metal describe the various applications of such nanoparticles. colloids. The ability to reduce metal ions (i.e., their reduc- The term “ surfactant free ” does not imply that bare metal tion rate) increases significantly at a temperature > 100° C. - 0 nanoparticles are synthesized. Surfactant-free solution- The ability of DMF to reduce AuCl4 ions to Au has been based synthesis entails that the metal nanoparticles are reported to be lower than that for Ag + ions [26] . It has been prepared in the absence of additional stabilizing agents suggested that catalysis by either polyvinylpyrrolidone such as thiolate and phosphine compounds, surfactants, (PVP) or by the seeds of the metal, itself, is required for and polymers. Such metal nanoparticles are stabilized the reduction of gold ions, although the formation of Au by solvents or the ions of the reducing agents or salts. nanoparticles both in the absence and in the presence of Although truly bare metal nanoparticles can be obtained the metal seeds has been reported [27, 28]. Therefore, it only through methods that involve the vapor phase, has been proposed that the use of AuCl3 in place of HAuCl 4 the synthesis of nanoparticles in the vapor phase is not may be preferable for the synthesis of Au nanoparticles covered in this review. via the DMF-based reduction method [26, 29]. This is H. Kawasaki: Surfactant-free solution-based synthesis of metallic nanoparticles 7

because AuCl3 can be more readily reduced than HAuCl 4 , values, such as methanol and water for at least a month. owing to the difference in the reduction potentials of Au3 + In addition, they also exhibited a higher photochemical - and AuCl4 [29] . stability than that of CdSe quantum dots (QDs, Figure 1 ). Several mechanisms have been proposed for the The DMF- stabilized Au NCs were just as stable at 150° C reduction of gold and silver ions by DMF. However, all of as are thiolate-stabilized Au NCs. these mechanisms involve the formation of carbamic acid Xie et al. further analyzed the DMF-stabilized Au NCs and H + ions. For example, Tom et al. proposed the follow- using a reverse-phase high-performance liquid chroma- ing reaction, which is similar to that for silver ions [30] : tographic (RP-HPLC) separation technique coupled with mass spectrometry [34] and showed that the as-prepared 3HCONMe + 2AuCl- + 3H O → 2 Au0 + 3Me NCOOH 2 4 2 2 DMF-stabilized Au NCs consisted of Au + , Au , Au , Au , + 6H + + 8Cl- (1) 10 10 11 12 Au 13 , and Au14 NCs stabilized using different numbers of The resultant carbamic acid is unstable in DMF and DMF ligands. readily decomposes into (CH3 )2 NH and CO2 . Kawasaki further extended the DMF-based reduction Recently, the surfactant-free synthesis of gold nano- method and was able to obtain surfactant-free Pt NCs with clusters (Au NCs) via DMF-based reduction has been blue emission [35] . The synthesis process was similar to reported by several groups [31 – 33] . In the synthesis of that used for preparing Au NCs, except that a different

Au NCs, there is no need to protect ligands such as thio- precursor (H 2 PtCl6 ) was used. By ligand exchange with late and phosphine compounds, surfactants, and poly- 2-mercaptobenzothiazole (MBT) for the as-prepared DMF- mers. Liu et al. first deve loped a surfactant-free method stabilized Pt NCs, MBT-stabilized Pt5 NCs were identified for synthesizing highly fluorescent blue light-emitting using matrix-assisted laser desorption/ionization-mass Au NCs by using DMF-based reduction methods [31] . spectrometry (MALDI-MS) (Figure 2 ). Duchesne et al. In a typical synthesis process, an aqueous solution of investigated the local structure and electronic properties

HAuCl 4 was mixed with DMF at room temperature, and of DMF-stabilized Pt NCs using X-ray absorption spec- the mixture was refluxed at 140° C for 4 h under vigor- troscopy and suggested that the NCs were non-metallic ous stirring. The obtained DMF-stabilized Au NCs were in nature (i.e., they lacked metallic bonds) and contained highly fluorescent, and they could be further function- only a few Pt atoms [36] . alized using various ligands. Moreover, Kawasaki et al. Highly dispersed spheroidal metallic silver nanopar- synthesized DMF-stabilized Au NCs using a modified ticles were prepared in surfactant-free N ,N- dimethylacet- version of the method developed by Liu et al. [32] . The as- amide (DMA) solutions by Costa et al. [37] . DMA acted as prepared DMF-stabilized Au NCs consisted of a mixture both the reduction agent and the surface stabilizer. In a of nanoclusters of various core sizes, with each NCs con- typical synthesis process, solutions containing 5 mm < taining 20 gold atoms. The DMF-stabilized Au NCs were AgNO3 in DMA were prepared and kept at room tempera- highly soluble in various solvents, having different pH ture in glass flasks in a dark environment. After 1 h, the

Figure 1 (A) Normalized photoluminescence intensities at 445 nm of DMF-protected Au clusters and those at 505 nm of CdSe QDs as a function of the UV irradiation time (356 nm, 1.3 mW/cm2 ). Photographs of the DMF-protected Au clusters and the CdSe QDs in toluene (B) before and (C) after the UV irradiation for 2 h. Reprinted from Ref. [32] with permission from the American Chemical Society. 8 H. Kawasaki: Surfactant-free solution-based synthesis of metallic nanoparticles

Figure 2 (A) UV-Vis spectrum of the MBT-stabilized Pt5 NCs and (B) their MALDI-MS spectra. Reprinted from Ref. [35] .

solutions became pale yellow, indicating the formation of usually used during polyol synthesis as a stabilizing agent, Ag nanoparticles with sizes of 4.6 ± 0.59 nm. The Ag nano- such as PVP and amine compounds that exhibit coordina- particles obtained using DMA had a smaller average size tion ability, resulting in electrostatic and steric repulsions than those obtained using DMF. This difference in size between the nanoparticles. In addition, the preferential was attributed to the interaction of DMA with the Ag(I) adsorption of the stabilizing agents in the solutions onto ions being stronger than that of DMF. different crystal faces influences the growth rates along the different crystal axes, resulting in an anisotropic metal nanostructure. A number of anisotropic metal nanostruc- 2.2 Ethylene glycol (EG)-mediated tures have been successfully synthesized using this polyol synthesis process, such as nanowires, nanocubes, and nanosheets [45 – 47] . Ethylene glycol (EG) is a convenient, versatile, and low- In the surfactant-free preparation of metal nanopar- cost solvent for synthesizing metal nanoparticles on a ticles using the polyol process, Wang et al. pioneered the large scale [38] . In EG-mediated synthesis, which is also use of alkaline EG to reduce metal ions such as platinum, called polyol synthesis, EG acts both as a solvent for the rhodium, and ruthenium [48, 49] . precursors and as a reducing reagent during the reaction. in situ ⎯⎯⎯⎯⎯⎯⎯()1NaoH,pH12> in glycol→ Typically, EG is oxidized at temperatures that are HPtCl26 () ° “unprotected” Pt nanocluster (5) 2 160C, N2 flow close to the boiling point of EG (197.6° C). The resultant oxidation product, acetaldehyde, reduces the precursor Stable noble metal NCs (Pt, Rh, Ru) with an average metal ions to metal nanoparticles through reactions (2) diameter of 1– 2 nm could be prepared using solutions and (3) [39 – 41] . A high concentration of NaOH results in of metals in EG with concentrations of 0.3– 3.7 g/l and EG dehydrating and turning into an aldehyde more readily containing NaOH, even in the absence of usually used [42, 43] . protective agents. For synthesis using the EG method, a high concentration of NaOH (pH > 12) is essential for pro- HOCH CH OH → CH CHO + H O (2) 2 2 3 2 ducing these stable metal nanoparticles; precipitates of the bulk metal were obtained when the solution pH 2/nMn + + 2CH CHO → CH COCOCH + 2/nM + 2H + (3) 3 3 3 was not high enough. It was suggested that the stabil- However, alternative pathways have been proposed by Xia ity of the metal NCs formed using the alkaline EG syn- et al. [44] . The heating of EG in air results in its oxidation thesis method was attributable to the adsorption of EG to glycolaldehyde, and glycolaldehyde can reduce ions of and OH - ions on the surfaces of the nanoparticles, pre- noble metals. venting the metal NCs from aggregating even though no organic stabilizing agents were added. The alkaline EG 2HOCH CH OH + O → 2HOCH CHO + 2H O (4) 2 2 2 2 2 synthetic method has been also applied for synthesizing To prevent the aggregation of metal nanoparticles in a bimetallic nanoparticles such as those of Pt/Ru and Pt/ solution, an excessive amount of an organic compound is Rh [48, 49]. H. Kawasaki: Surfactant-free solution-based synthesis of metallic nanoparticles 9

Kawasaki et al. have reported the synthesis of fluores- solution containing sodium naphthalenide to form Au cent Cu nanoparticles about 2 nm in size via a microwave- nanoparticles 2 nm in size [52] . In this synthesis method, assisted alkaline polyol synthesis method using NaOH no additional stabilizing surfactant was necessary because [50] . This method did not require any additional protec- the Au nanoparticles were stabilized by the solvent (i.e., tive or reducing agents. In a typical synthesis procedure, diglyme). The solvent-stabilized Au nanoparticles could a mixed solution of CuCl2 and NaOH in EG was placed in be easily replaced by other ligands with stronger binding a microwave oven and stirred vigorously for 30 min at a groups, such as thiols or amines, without there being a ° reaction temperature of 185 C in a N2 atmosphere. The change in size (Figure 4 ). transmission electron microscopy (TEM) images of the Cu nanoparticles showed that distinct and non-aggregated nanoparticles with sizes of 2.3 ± 0.25 nm were formed 2.3 Benzyl alcohol-mediated synthesis (Figure 3 ). The TEM diffraction analysis of the Cu nanoparticles showed single crystals of metallic copper, indicating Benzyl alcohol has been used successfully to synthesize the suppression of the surface oxidation of Cu during the the nanoparticles of various metal oxides without the alkaline EG synthesis method. need for additional surfactants [52 – 58] . Niederberger and Networks of highly branched 2D Pd nanowires were coworkers reported that highly crystalline metal oxide prepared by Feng et al. via a template- and surfactant- nanoparticles, such as those of CoO, ZnO, Fe3 O4 , MnO, free method using an EG/dimethyl sulfoxide (DMSO) Mn3 O4 , and BaTiO3 , could be synthesized by reacting mixture under mild conditions [51] . In a typical synthe- metal alkoxides, acetates, or acetylacetonates with benzyl sis procedure, 0.0224 g of Pd(CH3 CO)2 was added to 2 ml alcohol under microwave heating [57] . As for the metal of DMSO and 23 ml of EG in a three-necked flask and nanoparticles, there have been a few reports on them refluxed at 120° C for 3 h. The as-prepared Pd networks had being formed via benzyl alcohol-mediated synthesis [59, lengths ranging from hundreds of nanometers to several 60]. Zhang and coworkers modified the benzyl alcohol- micrometers. mediated synthesis process by employing benzyl alcohol Diethylene glycol diethyl ether (diglyme) is an organic as both the solvent and the reducing agent for the synthe- solvent with a high boiling point (180– 190 ° C). Jansen et al. sis of platinum nanostructured networks via microwave reported that gold (III) ions could be reduced by a diglyme irradiation in the absence of any surfactants, templates,

A B

100 C dav=2.30±0.25 80

40 Number of particles 20

0 1.6 1.8 22.2 2.4 2.6 2.8 3 3.2 Diameter (nm)

Figure 3 (A) Low-magnification TEM image of dried Cu nanoparticles, (B) high-magnification TEM image and TEM diffraction pattern of the Cu nanoparticles, and (C) histogram showing the particle size distribution. Reprinted from Ref. [50] . 10 H. Kawasaki: Surfactant-free solution-based synthesis of metallic nanoparticles

5 nm

A B C 10 nm 10 nm 10 nm

Figure 4 TEM images of different samples of dodecanethiol-capped gold nanoparticles of varying sizes: (A) 1.9 ± 0.4 nm, (B) 3.9 ± 0.7 nm, and (C) 5.2 ± 0.7 nm. Reprinted from Ref. [52] with permission from the American Chemical Society. seeds, or assemblers [59] . The prepared Pt nanostructured Pd nanoparticles had an average diameter in the range of networks were tens of micrometers long and consisted of 8 – 10 nm, and the particle sizes were almost independent connected secondary nanoparticles, which were actually of the concentration of the precursor Pd ions, which was formed by the aggregation of Pt nanocrystals approxi- between 0.1 and 1.0 m m . It was suggested that high-dipole- mately 3 nm in size. It was suggested that benzyl alcohol exhibiting solvents tend to solvate the palladium parti- is crucial for the formation of the Pt nanostructured net- cles and that this steric effect (solvation) stabilizes the Pd works and acted not only as the reducing agent and the nanoparticles in MIBK. Extending the work of Esumi and solvent but also as a special assembler. co workers, Chen et al. prepared solvent-stabilized Pd nano- Shivashankar and coworkers used the benzyl alcohol- particles with the sizes of around 33 nm, using microwave mediated method for the synthesis of crystalline copper irradiation (900 W, 2.45 GHz) to decompose Pd acetate in nanostructures through the microwave irradiation of a MIBK solution in the presence of KOH [62] . The TEM and a solution of copper acetylacetonate in benzyl alcohol X-ray diffraction (XRD) analysis demonstrated that the without adding any additional surfactants [60] . Interest- solvent-stabilized Pd nanoparticles (30 – 40 nm) consisted ingly, the copper structures were found to be stable against of clusters agglomerated from hundreds of smaller palla- oxidation in ambient air for several months. High-resolu- dium crystals with sizes of 3– 4 nm. tion electron microscopy, scanning electron microscopy (SEM) and TEM images showed that the copper structures comprised nanospheres of about 150 nm in diameter, with each nanosphere being made of copper nanocrystals with 3 Simple ion-mediated synthesis a size of 7 nm. 3.1 Citrate-medi ated synthesis (Turkevich method) 2.4 Methyl isobutyl ketone (MIBK)- mediated synthesis Citrate ions, which are commonly used as a reductant in the synthesis of metal colloids via the oxidation of Esumi and coworkers investigated the synthesis of palla- citrate into acetonedicarboxylic acid, interact with the dium nanoparticles via the thermal decomposition of bis surfaces of gold or silver nanoparticles, as described (2,4-pentanedionato) palladium(II) Pd(acac) in various by Turkevich et al. [24] . In the citrate reduction-based organic solvents in the absence of additives, such as chlo- synthesis of gold colloids, citrate ions play the dual role roform, 1,4-dioxane, toluene, MIBK, p-xylene, oxylene, of a reducing agent and a stabilizer. The citrate ions form bromobenzene, and DMSO [61] . The preparation method a charged layer around the surfaces of the nanoparticles, was simple. A known amount of Pd(acac) was dissolved resulting in electrical repulsive forces between them in in an organic solvent, and the resulting solution was aqueous media. Strong interactions between the amino refluxed at its boiling point for 20 min to 2 h. Of all the or thiol group and the surfaces of the gold nanoparticles various solvents used, a stable dispersion of Pd nanopar- displace the weaker bound citrate ions from the surfaces ticles was obtained in only MIBK, with MIBK acting both of the metal nanoparticles. In a typical synthesis proce- as the solvent and the stabilizing agent. The as-prepared dure, gold ions in an aqueous medium can be reduced H. Kawasaki: Surfactant-free solution-based synthesis of metallic nanoparticles 11 by citrate at 100° C, as has been reported by Turkevich 3.2 Amino acid-mediated synthesis et al. [24] . The ratio of the number of gold ions to that of the reducing agent influences the particle size. In general, Amino acids, the building blocks of proteins, can be the smaller the ratio was, the smaller was the size of the considered as ideal biocompatible capping agents. gold nanoparticles. There have been numerous reports on They have been successfully used for the synthesis of the synthesis of metal nanoparticles in aqueous media metal nanoparticles in the absence of an additional sur- through modified methods based on that of Turkevich factant. This amino acid-based method is similar to that et al. These have aimed to control the sizes and shapes involving the use of a citrate, with the amino acids also of the metal nanoparticles [63 – 71] . Kimling et al. reported serving the dual role of a reductant and a stabilizer. A that Au nanoparticles could be produced in a wide range number of amino acids can be used for the synthesis of sizes, from 9 to 120 nm, via the citrate-mediated syn- of metal nanoparticles, including aspartic acid (ASP) thesis method [64] . The lowest absolute concentration [72, 73], glutamic acid [74] , histidine [75] , tryptophan of gold ions for obtaining stable nanoparticles was [76] , phenylalanine [77] , cysteine [78] , and methionine < 2 m m . At concentrations lower than 1 m m , the suspen- [79] . Tan et al. reported the shape-controlled synthesis sions were stable even after a few months, whereas at of anisotropic Au nanostructures (nanoplates, nanorib- higher concentrations, precipitation on the walls of the bons, and nanowires) at room temperature using ASP container occurred within days after the preparation of as the reducing agent, the particle-stabilizing agent, the solutions. More recently, monodispersed sub-10 nm and the shape-directing agent. The ratio of ASP to that gold (5.7 ± 0.8 nm) [70] and silver (average size of 1.6 nm) of the gold precursor was found to be the critical factor [69] nanoparticles could be obtained via modified Turk- affecting the shape of the nanostructures (Figure 5 ) [73] . evich methods. In addition, the reduction capabilities of all 20 amino

R Low High

[HAuCI4] R=0.2 R=0.5 R=1.0 R=5.0 R=20 High AB -3 (1×10 M)

-4 5×10 M

500 nm 500 nm

-4 1×10 M

Low

-5 200 nm 200 nm 5×10 M

Figure 5 Two-dimensional maps of the gold nanostructures synthesized using different combinations of gold precursor concentrations and = R ([Asp]/[HAuCl4 ]) ratios with the TEM images showing the reaction products synthesized at the selected reaction conditions: (A) R 0.5 and ======[HAuCl4 ] 0.50 m m , (B) R 5.0 and [HAuCl4 ] 0.50 m m , (C) R 0.5 and [HAuCl4 ] 0.10 m m , and (D) R 5.0 and [HAuCl4 ] 0.10 mm . Reprinted from Ref. [73] with permission from the American Chemical Society. 12 H. Kawasaki: Surfactant-free solution-based synthesis of metallic nanoparticles acids were studied under the same reaction conditions resulting in the dispersion being stable for weeks at room and ranked on the basis of the time taken to transform temperature. On the other hand, the Au nanoparticles, the gold precursor solution (HAuCl 4 ), which was yellow- synthesized using a higher concentration of KI, were not ish in color, into a solution of Au nanoparticles, which stable for a long time. The presence of silver ions resulted was pinkish [80] . Among the 20 amino acids, trypto- in a significant increase in the growth of the dendritic Au phan was found to be the fastest reducing agent. Yang nanostructures. et al. reported the fabrication of water-soluble Au 10 NCs via a simple reaction in which histidine served as both a reducing agent and a protecting ligand [81] . The as-pre- 3.4 Good ’ s buffer (HEPES)-mediated pared Au 10 NCs fluoresced high-intensity bluish-green synthesis light. 2-[4-(2-Hydroxyethyl)-1-piperazinyl] ethanesulfonic acid (HEPES) is a zwitterionic organic chemical buffering 3.3 Iodide-mediated synthesis agent and 1 of the 12 Good ’ s buffers. Xie et al. used HEPES as a weak reducing, protecting, and shape-directing Iodide has been widely used in the surfactant-assisted agent for the high-yield surfactant-free synthesis of Au shape control of metal nanoparticles such as anisotropic nanoflowers that exhibited good size monodispersity Au nanoparticles because of its specific interaction with [83] . In a typical synthesis procedure, 1 ml of 100 m m the surfaces of the metal nanoparticles. Raj et al. showed HEPES (pH 7.4) was mixed with 9 ml of deionized water. - μ that the iodide-mediated reduction of AuCl4 ions could Then, 250 l of 20 m m HAuCl4 solution was added to result in the surfactant-free synthesis of single-crystal- the mix. The as-prepared Au nanoflowers were quasi- line polyhedral Au nanostructures. This was achieved spherical, consisting of three-dimensional branched by the reduction of a Au(III) complex by iodide ions to nanoparticles with more than 10 tips (gold nanoflow- a metastable Au(I) complex and the subsequent dispro- ers) with the average length of the tips being 88 ± 12 nm. portionation of Au(I) at room temperature to metallic It was proposed that the formation process of the gold Au(0) [82] . The shape and surface structure of the Au nanoflowers consists of three stages: the reduction of nanoparticles depended on the concentration of iodide the Au(III) ions to primary Au nanocrystals (stage 1), ions and the presence of Ag ions. The surfaces of the the agglomeration of the primary Au nanocrystals into Au nanoparticles, obtained at a low concentration of KI intermediate agglomerates (stage 2), and the anisotropic (10 μm ), were oriented along the (111) direction, and the growth of the agglomerates into flower-like nanostruc- unreduced Au (III) ions stabilized the Au nanoparticles, tures (stage 3) (Figure 6 ).

A 8 min 12 min 20 min 24 min B

60 min 24

st 2nd stage 3rd stage C 16 1 stage Absorbance (a.u.) 12 Anistropic 8 Agglomeration growth 4 0 400 600 800 Agglomerates Wavelength (nm) Primary crystals Nanoflowers

Figure 6 (A) UV-Vis spectra as a function of time of the reaction between aqueous a AuCl 4 solution (0.5 m m ) and HEPES (10 m m ). (B) Representative TEM images of the products harvested at 8, 12, 20, and 24 min into the reaction. All scale bars are 20 nm. (C) Schematic illustration of the proposed mechanism for the formation of the Au nanoflowers in HEPES solution. Reprinted from Ref. [83] with permission from the American Chemical Society. H. Kawasaki: Surfactant-free solution-based synthesis of metallic nanoparticles 13

3.5 Tetrakis(hydroxymethyl)phoephonium aqueous solution takes a few minutes to complete, and the chloride (THPC)-mediated synthesis nanoparticles remain stable for months. The particle size can be tuned by changing the intensity of the illuminat- ing radiation: the higher the UVA irradiance and the more Baiker and coworkers reported water-soluble Au NCs with uniform the light source, the smaller and more monodis- average size of 1.5 nm by using alkaline THPC, which can persed the particles. Exposure to room lights for 3 days work as both a reducing and a stabilizing agent, without the resulted in particles of 150– 300 nm in diameter. Irradia- need for large organic stabilizing agents [84] . The mixing tion with light intensities of 7, 40, and 100 W/m2 produced of equimolar amounts of partially hydrolyzed THPC with the Au nanoparticles of 40 ± 10, 12 ± 3, and 8 ± 2 nm in size, chloroauric acid to 1 mm concentration produced a clear respectively. These Au nanoparticles could be easily func- dark orange-brown solution from the THPC-mediated Au tionalized with water-soluble thiols such as 3-mercapto- NCs within 3 s after the addition of the chloroauric acid. It 1-propane sulfonic acid (MPSA), a sodium salt. was suggested that this synthesis is accompanied by the following reactions:

P(CH OH) + + OH- → P(CH OH) + CH O + H O (6) 2 4 2 3 2 2 4.2 Sonochemically mediated synthesis

P(CH OH) + 2H O → O P(CH OH) + 2H (7) 2 3 2 2 2 3 2 Studies on the sonochemical treatment of water have This suggests that THPC is also a powerful reducing suggested that hydroxyl radicals (- OH) and hydrogen agent to reduce water to hydrogen in alkaline conditions. atom radicals (H •) as the primary sonolysis products were formed. The sonochemically generated H • radicals are considered to act as reductants [86] . The addition of organic additives (e.g., 2-propanol) can produce a second- 4 Physical process-mediated ary radical species that significantly increases the reduc- synthesis methods tion rate of metal ions. 2AuIII Cl - + H • → 2AuII Cl - + Cl • + HCl (8) 4.1 Photochemically mediated synthesis 4 3 2AuII Cl - → AuIII Cl - + AuI Cl - (9) The photochemical synthesis of stable, unprotected gold 3 4 2 nanoparticles without the need for any of the conventional AuI Cl + H • → Au0 + HCl + Cl- (10) (S, N, or P) stabilizing ligands was reported by McGilvray 2 et al. [85] . In this synthesis method, 1-[4-(2-hydroxyeth- n Au0 → (Au0 ) (11) oxy) phenyl]-2-hydroxy-2-methyl-1-propane-1-one (Irga- n cure-2959) was used. On being excited with radiation with The sonochemically mediated synthesis of nanostruc- a wavelength of 350 nm, I-2959 yields ketyl radicals via tured noble metals (e.g., Au, Ag, Pt, and Pd) have been Norrish type I R-cleavage; these ketyl radicals function as studied by several groups [87 – 91] . Sakai et al. reported reducing agents that are capable of reducing Au3 + to Au 0 , the surfactant- and reducer-free synthesis of Au nanopar- resulting in the formation of Au nanoparticles (Figure 7 ). ticles in aqueous solutions by the sonochemically medi- The photochemical synthesis of Au nanoparticles in an ated synthesis method using high-frequency sound waves

O OH HO O

hν

H3C · OH H3C Au(III) Au(0)

Figure 7 The photochemical-mediated synthesis of stable, unprotected Au NPs using Irgacure-2959 in the absence of conventional (S, N, or P) stabilizing ligands. Reprinted from Ref. [85] with permission from the American Chemical Society. 14 H. Kawasaki: Surfactant-free solution-based synthesis of metallic nanoparticles

Figure 8 Photographs showing laser ablation in a liquid. The photographs provided by Dr. Takeshi Tsuji, Kyushu University, Japan.

- - - (950 kHz) [92] . Higher AuCl4 concentrations resulted in gold surfaces could react efficiently with Cl and OH ions larger nanoparticles and plate formation. The formation to augment their net surface charge. Owing to electrostatic of gold plates increased with the addition of NaCl or HCl repulsion, there was a significant reduction in their size owing to the specific adsorption of Cl- ions onto certain when different salts were used (7 ± 5 nm for 10 m m KCl, facets of the crystals (e.g., (110) facets). Spherical Au nano- 5.5 ± 4 nm for 10 m m NaCl, and 8 ± 5 nm for NaOH at a pH particles with a diameter of 20 – 60 nm were formed from of 9.4). It has been reported 3.3– 6.6 % of the gold atoms on - > ° a 0.1-m m AuCl4 aqueous solution at temperatures 50 C, the surface were negatively charged [100] . More recently, while triangular plates coexisting with the spherical nano- Co and Au nanoparticles have been synthesized by fem- particles were formed at temperatures lower than 50° C. tosecond-long laser ablation in various liquids (n-hexane, diethyl ether, toluene, 2-propanol, acetone, and methanol) in the absence of a surfactant to investigate the effect of the 4.3 Laser ablation-mediated synthesis liquids on the size of the nanoparticles [102] . It was suggested that the growth of the nanoparticles in the absence The laser ablation of a solid noble metal target immersed of surfactants occurs owing to light absorption by the col- in a liquid medium has been used to produce colloidal loids through diffusion coalescence and that the growth metallic nanoparticles, such as those of gold and silver. can be controlled by controlling the solvent polarity, the Various solvents have been used, such as water, ethanol, processing time, and the power of the laser. and n-hexane [93 – 101] . Metal clusters with fewer metal atoms are ablated from the solid metal by irradiation with a laser and are aggregated into metal clusters sufficiently 5 Applications of metallic large in size (Figure 8 ). It has been reported that the metal nanoparticles prepared by laser ablation are stable nanoparticles obtained against aggregation in water, even though they are not sta- using surfactant-free solution bilized by surfactants [93 – 101] . This approach can be used to produce metal nanoparticles with bare surfaces. synthesis methods The surfaces of the Au nanoparticles produced by laser ablation in water have been investigated via X-ray photo- 5.1 Synthetic catalysis electron and infrared (IR) spectroscopies, mass spectrometry, and ζ -potential measurements [97] . It was found that As the catalytic process takes place on metal surfaces, the Au nanoparticles were negatively charged because the the presence of strongly bound protective organic particles’ surfaces were partially oxidized. The oxidized layers around metal NPs may make them unsuitable for H. Kawasaki: Surfactant-free solution-based synthesis of metallic nanoparticles 15 catalytic applications. Therefore, metal nanoparticles The high cost of precious metals limits their use in formed using surfactant-free synthesis methods would industrial applications. Cu is widely used industrially as an be more suitable for catalytic applications. Although alternative as it is an inexpensive metal, and considerable supported metal nanoparticles (i.e., those used in het- efforts have been devoted to exploring its use in catalysis erogeneous catalysis) have been extensively investigated [105 – 107] . Obora and coworkers developed a surfactant- [5 – 7] , studies of such nanoparticle are outside the scope free synthesis method for synthesizing Cu nanoparticles of the present review. This review focuses only on unsup- with a size of about 2 nm using the DFM-based reduction ported metal nanoparticles (i.e., colloidal metal nanopar- method. These Cu nanoparticles were used as catalysts in ticles) obtained using surfactant-free synthesis methods the Ullmann coupling reaction [108] . The Cu nanoparti- and used as synthetic catalysts. Heterogeneous catalysis cles showed a high catalytic activity and exhibited a TON using supported metal nanoparticles from surfactant-free of up to 2.2 × 10 4 in the Ullmann-type cross-coupling of aryl synthesis methods will be described in the section on halides with phenols under ligand-free conditions. electrocatalysis (section 5.2). Kawasaki and coworkers investigated the catalytic Obora and coworkers reported the surfactant-free syn- properties of DMF-stabilized Au NCs in the reduction of thesis of Pd NCs for use in catalytic cross-coupling reactions 4-nitrophenol (PNP) to 4-aminophenol by NaBH4 [109] , [103] . The Pd NCs, which had a size of 1– 1.5 nm, were syn- a well-known model reaction that is catalyzed by metals thesized by the DFM-based reduction method and showed [110] . The DMF-stabilized Au NCs showed a high catalytic good dispersion in various polar solvents because of being activity even when used in small quantities (10-7 g), with × -2 stabilized by the DMF solvents (Figure 9 ). The DMF-stabi- the pseudo-first-order rate constant (kapp ) being 1.0 10 for lized Pd NCs were tested as catalysts for the Suzuki-Miyaura 1.5 μ m of the gold catalyst at 25° C. It was proposed that the and Mizoroki-Heck cross-coupling reactions, which are ver- restructuring of the DMF layer (essentially a form of acti- satile methods for forming carbon-carbon bonds during vation) was the key to achieving a high catalytic activity- organic synthesis. The Pd NCs showed a high catalytic exhibiting DMF-stabilized Au NCs. activity during the Suzuki-Miyaura and Mizoroki-Heck Mandal and coworkers demonstrated a surfactant- cross-coupling reactions. The Pd NCs had a high turnover free wet-chemical method of preparing metal nanoparti- number (TON), which was as high as 6.0 × 10 8 in the pres- cles. They also used these nanoparticles as synthetic cata- ence of 10-7 mol% of the Pd NCs. The catalytic activity of the lysts [111 – 113] . Sponge-like Au nanoparticles with a size of Pd NCs was higher than that of the subnanometer Pd NCs 20 – 140 nm were prepared by using ammonium bismuth (∼ 0.7 nm) supported on polymer micelles, which showed a citrate as the reducing and stabilizing agent [111] . The higher TON, of up to 2.8 × 10 5 [104] . Moreover, the research- sponge-like Au nanoparticles had pores with diameters ers developed a method for recycling the catalyst at least ranging from 2.4 to 6.0 nm and had a high surface area five times during the Suzuki-Miyaura cross-coupling reac- (42 m 2 /g). These sponge-like Au nanoparticles showed tion by using a liquid-liquid extraction method. a pronounced catalytic activity in the reduction of PNP × -3 -1 by NaBH4 , exhibiting a rate constant kapp of 2.1 10 s (Figure 10 ). In addition, polygonal Au nanoparticles were prepared by using ferric ammonium citrate as the reducing agent in the absence of a surfactant or polymeric template [113] . The catalytic activity of the polygonal Au nanoparticles in the reduction of nitrophenol was higher than that of spherical Au nanoparticles synthesized by the Turk- evich method by a factor of 300 – 1000. A similar increase in the activity was also observed in the aerobic oxidation of different d -hexoses. The higher catalytic activity of the polygonal gold nanoparticles was attributed to them having a greater number of sharp edges and corners. Freund and coworkers reported the catalytic activity of citrate-stabilized Au nanoparticles in the reaction between ferricyanide and thiosulfate ions [114] . The catalytic rate Figure 9 DMF-stabilized Pd NCs, which showed high catalytic activity in the Mizoroki-Heck reaction and exhibited a high TON, of up to increased with an increase in the concentration of the Au 6.0 × 108 in the presence of 10-7 mol % of the Pd NCs. The schematic nanoparticles, suggesting that the catalytic rate was propor- figure was provided by Dr. Yasushi Obora, Kansai University, Japan. tional to the surface area of Au nanoparticles. The authors 16 H. Kawasaki: Surfactant-free solution-based synthesis of metallic nanoparticles

A B 0.8

Time (s) 0.4 2.0 0 0 180 InA 300 540 -0.4 660 -0.8 1.5 780 0 200 400 600 800 Time (s) Absorbance 1.0

0.5

200 400 600 800 Wavelength (nm)

Figure 10 (A) TEM micrograph of sponge-like Au nanoparticles. The left and right insets show a magnified image and the electron diffraction (ED) pattern of a single sponge-like Au nanoparticle, respectively. (B) Successive UV-Vis absorption spectra of the reduction of PNP by NaBH 4 in the presence of sponge-like Au NCs. The inset shows the plot indicating the variation in lnA (A: absorbance of PNP) with time. Reprinted from Ref. [111] with permission from the American Chemical Society.

proposed that this reaction occurred on the surfaces of the are in the form of metal nanoparticles [116 – 119] . Electro- Au nanoparticles and was not diffusion controlled. catalysts play an important role in transferring electrons Rossi and coworkers reported the aerobic oxidation of between the electrode and the reactants, with the aim of glucose in aqueous media under strongly basic conditions obtaining a current density close to the equilibrium poten- with bare Au nanoparticles with a mean diameter of 3.6 nm tial. As the rate of reaction is correlated with the number as the catalyst, allowing for a 21% conversion of glucose of sites available for the absorbed species, the current within the first 200 s [115] . By using Au nanoparticles with density depends on the “ active ” surface area of the elec- sizes of 3– 6 nm, the authors found that the catalytic activity trode. Several electrocatalytic reactions, such as those for was inversely proportional to the diameter of Au nanopar- oxygen reduction, methanol oxidation, CO oxidation, and > ticles. In contrast, Au nanoparticles with a diameter 6 nm H2 O2 reduction, to name a few, have been demonstrated did not show a linear increase in catalytic activity, and a using electrode-supported metal nanoparticles [116 – 119] . sharp cutoff was observed at a diameter of approximately Despite these successes, in most cases, the synthesis of 10 nm. This indicated the discontinuity of the catalytic acti- metal nanoparticles with controllable sizes and shapes vity at the nanoscale. The authors also compared the catalytic requires the use of surfactants and polymers. This is not activity of unsupported Au nanoparticles (i.e., those in a conducive with the use of the nanoparticles as catalysts. colloidal dispersion) and Au nanoparticles supported on Being difficult to remove completely, the surfactants, and carbon using the same amount of gold. The initial reaction polymers block the catalytic surfaces and lower the cata- rates of the two types of nanoparticles in the oxidation of lytic activity. In this section, the electrocatalytic applica- glucose were quite similar during the first 100 s. However, tion of metal nanoparticles synthesized using the sur- the colloidal Au nanoparticles were not stable and, eventu- factant-free solution-based synthesis method is described. ally, underwent deactivation owing to the tendency of Au Zhou et al. reported the fabrication of Pt nanopar- nanoparticles to grow and agglomerate in the solution. ticles with an average diameter of about 2.9 nm by a surfactant-free polyol process. The nanoparticles were supported on carbon with Pt loading being as high as 5.2 Electrocatalysis 40 wt % [120] . The as-synthesized Pt/C electrocatalyst showed a better electrocatalytic activity as a cathode Extensive research efforts are currently being made for during the reaction for oxygen reduction in direct metha- developing efficient and selective electrocatalysts that nol fuel cells (DMFCs) than do commercial catalysts. H. Kawasaki: Surfactant-free solution-based synthesis of metallic nanoparticles 17

Li et al. reported the fabrication of multiwalled carbon H2 conversion. Yin et al. developed a novel strategy to nanotube-supported Pt (Pt/MWNT) nanocomposites grow ultrafine Au NCs on rGO sheets while not requiring prepared by both the aqueous solution reduction of a Pt an additional protecting molecule or reductant by mixing salt (HCHO reduction) and the reduction of a Pt-ion salt a HAuCl4 solution and rGO [124] . The average diameter of in an EG solution. These nanocomposites were used as the Au NCs on the graphene oxide sheets was 1.8 nm, and cathode catalysts in DMFC [121] . The Pt nanoparticles, the dispersion was narrow at 0.2 nm (Figure 11 ). This strat- which had a size of 2 – 5 nm, were homogenously dis- egy was applicable for obtaining clusters of other metals persed in the case of Pt/MWNTs by using the surfactant- (Pt and Pd) on the rGO sheets as well. The as-synthesized free polyol method. The Pt/MWNT nanocomposites Au NC/rGO hybrids displayed excellent performance as displayed a significantly higher catalytic performance catalysts in the oxygen reduction reaction when used as than that of the commercial catalyst Pt/XC-72 (Vulcan the cathode in fuel cells. The hybrids displayed a high CS-72, Cabot Corporation, MA, USA) having a Brunauer- onset potential, superior methanol tolerance, and excel- Emmett-Teller (BET) surface area of 237 m2 /g. Thus, the lent stability. preparation of the small metal nanoparticles by the Wang et al. reported a one-pot method of synthesiz- surfactant-free polyol process is an attractive way of ing AuCu intermetallic nanoparticles (iNPs) supported on obtaining heterogeneous catalysts [122] . high-surface-area carbon, with the method consisting of Kundu et al. prepared reduced graphene oxide (rGO)- two steps: the reduction of the precursors and the subse- supported Pt nanocomposites (2– 3 nm diameter) by a sur- quent annealing of the reduced precursors in glycerol at factant-free/microwave-assisted polyol synthesis method 300 ° C [125] . To prevent the active sites of the AuCu iNPs for use as electrocatalysts in methanol oxidation and from being blocked, a surfactant was not used during the

H2 conversion [123] . The methanol oxidation potential, synthesis process. It was found that the AuCu iNPs exhib- current, and the tolerance of the rGO/Pt nanocomposites ited a catalytic activity superior to that of the ordinary Au were determined, with the results showing that the cata- nanoparticles, when used in oxygen reduction in an alka- lysts exhibited both high electrocatalytic activity and high line medium (Figure 12 ).

Au clusters/graphene

1 Pt/C Au/rGO AuNPs/rGO OH-

) rGO Au clusters 1.9 nm 2 0

O2 -1

-2

-3

ORR -4 Current density (mA/cm Au clusters/graphene -5 -0.8 -0.6 -0.4 -0.2 0.0 Potential (V vs. Ag/AgCI)

Figure 11 TEM images of Au/rGO hybrids, and RDE curves of commercial Pt/C, Au/rGO hybrids, Au NP/rGO hybrids, rGO sheets, Au clusters in O2 -saturated 0.1 m KOH at a scanning rate of 50 mV/s at 1600 rpm. Reprinted from Ref. [124] with permission from the American Chemical Society. 18 H. Kawasaki: Surfactant-free solution-based synthesis of metallic nanoparticles

0.3 with SEL (electrochemical active surface of Pt on carbon 2 -1 paper) being 99 m g . In contrast, the value of SEL for a 2 -1 0.2 commercial electrode was 97.6 m g . This indicated that a greater amount of Pt was available electrochemically on the surface of the Pt nanoflower-carbon paper than at the 0.1 surface of the commercial electrode.

0.0 Current (mA) 5.3 Surface-enhanced Raman scattering -0.1 (SERS)

-0.2 The advancements in the synthesis of metal nanoparticles 0.0 0.2 0.4 0.6 0.8 1.0 1.2 have resulted in the development of new analytical tools Potential (V vs. RHE) that can be used as chemo/biosensors for life sciences, environmental monitoring and food safety applications. SERS Figure 12 CV curves of AuCu iNPs in a deaerated 1.0-m KOH solution. The potential was scanned using different upper limits is one such promising tool that makes use of metal nano- at a rate of 20 mV s-1 . The dashed line is the CV curve of Au NPs in particles, as they can be used for a label-free analysis and the same solution and shows no features in the potential range exhibit high sensitivity, surface selectivity, and multiplex- investigated. Reprinted from Ref. [125] . ing capabilities [127 – 129] . In SERS, the enhancement factor (EF) when Au or Ag nanoparticles are used can reach to as much as 1014 or 1015 , which is significantly higher than that Besides single-nanosized Pt nanoparticles or NCs when metal nanoparticles are not used. SERS substrates ( < 3 nm), anisotropic Pt nanostructures such as nanowires, can be fabricated through many routes, including elaborate nanotubes, nanorods, and nanodendrites have also been lithographic techniques, colloidal lithography, deposition found to exhibit an enhanced catalytic performance [118] . methods, electrochemical roughening, and solution-phase Jia et al. fabricated Pt nanostructured networks having supe- synthesis methods, to name a few. In this review, the appli- rior electrochemical activity and stability via the chemical cation of metal nanoparticles formed via surfactant-free reduction of chloroplatinic acid with benzyl alcohol under solution-based synthesis methods is described. If the nano- microwave irradiation while not using any surfactants, particles were to be formed in the absence of a surfactant templates, or seeds [59] . The catalytic activity of the Pt and an additional capping agent, it would advantageous nanostructured networks during methanol oxidation was as there would be little interference from the layers of such evaluated in order to evaluate their potential for application adsorbed organic species during the SERS analysis. in DMFCs. The magnitude of the peak anodic current during Wen et al. reported the high yield ( > 85% ) synthesis the forward scan at approximately 0.65 V was proportional of Ag dendrites via a simple surfactant-free method using to the amount of methanol oxidized on the Pt network. The a suspension of zinc microparticles as a heterogeneous mass current density of the Pt nanostructured networks reducing agent [130] . The stems of the silver dendrites acting as an electrode was 0.32 A mg-1 , which was 2.7 times were 20 – 30 nm in diameter and 5 – 50 μ m in length. The use that of a commercial Pt/C electrode (0.12 A mg-1 ). of the silver nanodendrites in an electrochemical glucose Sun et al. demonstrated the large-scale synthesis biosensor resulted in an increase in sensitivity as high as of 3D flower-like platinum nanostructures via a simple 1 – 2 orders of magnitude. Han et al. prepared hierarchically chemical reduction of hexachloroplatinic acid with formic arranged nanostructures of silver via a surfactant-free acid at room temperature, while using neither a template acetone-based mixed solvent route at room temperature nor a surfactant [126] . The growth of the Pt nanoflow- [131] . In this surfactant-free synthesis method, acetone ers occurred gradually after the mixing of the solutions played the key role in controlling the nucleation, growth, of H2 PtCl6 and HCOOH. The as-prepared Pt nanoflowers conversion, and assembly of the Ag nanoparticles. The SERS consisted of large quantities of single-crystal nanowires, performance of adenine using the hierarchical Ag nano- which were formed by the growth of {111} planes. Cyclic structure was similar to that of triangular Ag nanoplates. voltammetry (CV) was performed on Pt nanoflowers In addition, the hierarchical Ag nanostructure resulted deposited on carbon paper, with the paper being placed in in a much higher SERS performance than did polyhedral

H2 SO4 with a pH of 1. The electrochemically active surface Ag nanoparticles. Xie et al. used flower-like Au nanopar- of the Pt nanoflowers on carbon paper was determined, ticles prepared by the Good ’ s buffer (HEPES)-mediated H. Kawasaki: Surfactant-free solution-based synthesis of metallic nanoparticles 19 synthesis method for SERS. The flower-like nanoparticles LDI-assisting nanomaterials is a soft ionization technique exhibited strong SERS effects and resulted in a > 10-fold that results in minimal fragmentation of the analytes [133 – increase in SERS intensity when compared to spherical 141] . In contrast to the traditional matrix-assisted laser Au nanoparticles (Figure 13 ) [83] . The application of these desorption/ionization (MALDI)-TOF-MS using organic Raman-active tags in living cells was also demonstrated matrices, SALDI-MS utilizes UV-absorbing nanomaterials by using the RAW264.7 macrophage cell line. as the LDI-assisting substances, in place of organic matri- Besides gold and silver nanoparticles, other nanostruc- ces such as α -cyano-4-hydroxy-cinnamic acid (CHCA) and tured metals such as those of copper, palladium, and nickel 2,5-dihydroxybenzoic acid (DHB) (Figure 14 ). SALDI-MS have been demonstrated to be effective substrates for SERS. affords several advantages such as the ability to detect Dar et al. evaluated the SERS activity of crystalline copper small molecules ( < 500 Da), easy sample preparation, low nanostructures prepared through microwave-surfactant noise background, high salt tolerance, and fast data col- synthesis in benzyl alcohol [60] . The synthesized stable lection without the use of an organic matrix. The SALDI copper nanostructures were demonstrated to be effective technique was originally described by Tanaka et al., who SERS substrates by employing 4-mercaptobenzoic acid as used a suspension of cobalt nanoparticles, 30 nm in the model analyte, with its concentration ranging from 10-3 size, to analyze proteins and synthetic polymers using a to 10-6 m . Feng et al. reported that Pd nanowire networks pulsed UV laser [133] . Sunner referred to the technique of prepared by a surfactant-free method in an EG/DMSO using graphite particles in glycerol solutions as a matrix mixture were effective SERS materials as well [51] , with in SALDI-MS [134] . The use of metal nanoparticles syn- the detection limit for 4-mercaptopyridine being as low thesized using surfactant-free synthesis methods as LDI- as 10 -8 m . Krishnadas et al. prepared nickel nanowires by assisting nanomaterials in SALDI-MS is advantageous, as the reduction of nickel chloride in EG with a concentrated one can avoid the interference peaks in the mass spectra hydrazine hydrate solution acting as the reducing agent. resulting from the organic stabilizers present on the sur- This was done without using any surfactants, templates, faces of the nanoparticles. Herein, the application in or an external magnetic field [132] . The nickel nanowires SALDI-MS of metal nanoparticles obtained via surfactant- showed SERS activity when crystal violet was used as the free synthesis methods is summarized. probe molecule. The EF was estimated to be about 1.7 × 10 4 . Su et al. analyzed small neutral carbohydrates by SALDI-MS using bare Au nanoparticles obtained via surfactant-free synthesis [142] . In comparison to citrate- 5.4 Surface- assisted laser desorption/ capped or cationic-surfactant-stabilized Au nanoparti- ionization-mass spectrometry cles, the bare Au nanoparticles could effectively capture (SALDI-MS) analytes on their surfaces. Therefore, small neutral carbohydrates, which are difficult to ionize using MALDI-MS, SALDI-time-of-flight (TOF)-MS using metal or metal could be detected efficiently using SALDI-MS and bare Au oxide nanoparticles and nanostructured surfaces as the nanoparticles.

(Surface-Assisted Laser Desorption/Ionization) 1648 1530 1360

1282 1510 LDI process Laser

RhB@AuNPs 1200

RhB@AuNPs

SERS intensity (a.u.) Nanoparticles or RhB Nanostructured surfaces : Analyte 500 1000 1500 Sample plates Raman shift (cm-1)

Figure 13 Typical SERS spectra of pure RhB (powder, black line), RhB on Au NPs 65 nm in diameter in an aqueous solution (blue line), Figure 14 A schematic picture of SALDI-TOF-MS using nanoparticles and RhB Au nanoflowers (NFs) 88 nm in size in an aqueous solution and nanostructured surfaces as the LDI-assisting nanomaterials, (red line). Reprinted from Ref. [83] with permission from the allowing a soft ionization technique that features minimal fragmen- American Chemical Society. tation of analytes. 20 H. Kawasaki: Surfactant-free solution-based synthesis of metallic nanoparticles

Yonezawa et al. systematically investigated the feasi- This was because of thin projections on the surfaces of the bility of performing SALDI-MS on peptides using various nanoparticles. stabilizer-free bare metal (Cu, Ag, Au, and Pt) nanoparticles [143] . In order to avoid the influence of the organic molecules on the nanoparticles, stabilizer-free bare nanoparticles of Ag, Au, Cu, and Pt were prepared by laser abla- 6 Concluding remarks tion. Among these metal nanoparticles, the nanoparticles of Pt exhibited the highest performance in SALDI-MS In this review, various surfactant-free solution-based owing to their lower heat conductivity and higher melting methods of synthesizing metallic nanoparticles are sum- point, resulting in the rapid temperature increase for the marized. In the surfactant-free solution-based synthesis effective desorption of the analyte (Figure 15 ). In addition, methods, metal nanoparticles are prepared in the absence the Pt nanoparticles were stable under laser irradiation of additional stabilizing agents such as thiolate and phos- and formed fewer clusters. Tsuji et al. prepared substrates phine compounds, surfactants, or polymers. These metal for SALDI-MS via the electrophoresis of gold nanoparticles nanoparticles are stabilized by the solvents or the simple produced by laser ablation in liquids [144] . The Au nano- ions of the reducing agents or salts. The solvents or simple particles were deposited more uniformly using the electro- ions present on the surfaces of the nanoparticles can be phoresis technique than by dropping off in the solution, exchanged with other ligands, providing functionalities resulting in an improvement in the reproducibility of the such as turning the hydrophobic particle surfaces hydro- SALDI measurements. philic, coupling biomaterials for biological purposes, and Kawasaki and coworkers developed surface-cleaned producing building blocks for assembling devices. DMF, Pt nanomaterials with thin projections (petals) on the sur- alkaline EG, MIBK, and benzyl alcohol-based methods faces of the Pt nanoparticles, called Pt nanoflowers, for are effective solvent-mediated surfactant-free synthesis the SALDI-MS of biomolecules [145, 146] . The Pt nanoflow- techniques, where the solvents have three functions: to ers, which were prepared via the reduction of H2 PtCl6 with act as the reaction media, to be the reducing agent, and

NaBH4 in the absence of any organic stabilizers, showed to play the role of the stabilizer. In the case of simple ion- a good performance with both high sensitivity and high stabilized metal nanoparticles, they work as reducing molecular weight during the SALDI-MS of various biomol- agents, protecting agents, and shape-directing agents. ecules, including peptides, proteins, and phospholipids. Citrates, amino acids, iodide, THPC, and HEPES are possi- ble candidates for use in simple ion-mediated surfactant- free synthesis methods. It should be noted that both these A solvents and small ions work as reducing agents, resulting in the formation of oxidation products. In some cases, metal nanoparticles may be stabilized by the resultant 1000 1050 1100 1150 1200 1250 1300 1350 m/z oxidation products. Processes such as photochemical-, ultrasonochemical-, and laser ablation-mediated synthe- B [M+H]+ Au sis methods can result in the surfactant-free synthesis of [M+Na]+ 1240 1250 1260 1270 1280 1290 1300 1310 m/z metal nanoparticles in solution.

1000 1050 1100 1150 1200 1250 1300 1350 The effective utilization of the surfaces of the formed m/z C Cu metal nanoparticles is an advantage associated with nanoparticles formed via surfactant-free synthesis methods. Such nanoparticles can be used in a number of appli- 1000 1050 1100 1150 1200 1250 1300 1350 100 cations, including in catalysis (synthetic catalysis and m/z 80 D [M+H]+ 60 Pt electrocatalysis) and sensing (SERS and SALDI-MS). 40 [M+Na]+ In particular, supported metal nanoparticles obtained 20 1240 1250 1260 1270 1280 1290 1300 1310 m/z 0 via surfactant-free synthesis methods would be an 1000 1050 1100 1150 1200 1250 1300 1350 m/z extremely promising way for use in sensing and catalysis applications. Figure 15 SALDI-MS spectra of angiotensin I (500 fmol) obtained The emphasis of future investigations of such metal using (A) Ag, (B) Au, (C) Cu, and (D) Pt nanoparticles as the SALDI matrices. The power of the irradiating laser (LP) is shown. The Ag nanoparticles should be the following: (1) better control and Cu nanoparticles could not ionize angiotensin I even at higher over the size, shape, and morphology of the nanoparti- LP values (LP = 100). cles; (2) elucidation of the mechanism of formation of the H. Kawasaki: Surfactant-free solution-based synthesis of metallic nanoparticles 21 nanoparticle, with a particular focus on the function of of the open sites on the surfaces of the metal nanoparti- the solvents and small ions used during the surfactant- cles formed using stabilizers (polymers, surfactants, sol- free synthesis methods; and (3) an exploration of the real- vents, and small ions) and the sizes of these sites may be world applications of these metal nanoparticles. the keys to properly utilizing the surfaces for catalysis and Finally, it should be noted that this does not necessar- sensing applications. ily imply that the presence of stabilizing agents such as Acknowledgements: This research was partly supported thiolate and phosphine compounds, surfactants, or poly- by Kansai University’ s Overseas Research Program for the mers hinders the efficient use of the surfaces of the nano- year of 2012. particles. In fact, polymer-stabilized Au nanoparticles and atomically precise thiolate-Au NCs have been reported to Received November 26, 2012; accepted December 13, 2012; previously show a high catalytic activity [147, 148] . The architectonics published online January 19, 2013

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Hideya Kawasaki earned his Bsc (1993) and Msc (1995) degrees from Mie University and his DSc degree from Kyushu University in 1998. After working as a postdoctoral research fellow for 2 years, he was appointed as an assistant Professor at Kyushu University. He is currently an associate Professor at Kansai University, a position he had held since 2007. He has published over 120 research articles and has received several awards, including the Matsuura Award of Kyushu affiliate from the Division of Colloid and Surface Chemistry of the Chemical Society of Japan (1999), an Incentive award from the Division of Colloid and Surface Chemistry of the Chemical Society of Japan (2003), the Excellence Paper award of the Mass Spectrometry Society of Japan (2007), and an Incentive award of the Mass Spectrometry Society of Japan (2011). His current research interests include the synthesis of metal nanoclusters and nano particles with anisotropic shapes and the efficient use of the surfaces of such nanoparticles in applications such as catalysis, optics, electronics, and sensing.