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 activ- ity. The synthesis, structures, and properties of atomi- cally 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 HAuCl4 the synthesis of nanoparticles in the vapor phase is not may be preferable for the synthesis of Au nanoparticles covered