Controlled growth of uniform noble metal nanocrystals: Aqueous-based synthesis and some applications in biomedicine

Colloids and Surfaces B: Biointerfaces 88 (2011) 1–22

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Review

Controlled growth of uniform noble metal nanocrystals: Aqueous-based synthesis and some applications in biomedicine Thai-Hoa Tran a , Thanh-Dinh Nguyen b,∗ a b

Department of Chemistry, Faculty of Sciences, Hue University, Hue, Viet Nam Department of Chemical Engineering, Laval University, Quebec G1K 7P4, Canada

a r t i c l e

i n f o

Article history: Received 29 January 2011 Received in revised form 5 July 2011 Accepted 6 July 2011 Available online 18 July 2011 Keywords: Size and shape control Noble metal Binary Nanoparticles Nanocrystals Biomedicine

a b s t r a c t Aqueous-dispersed single and binary noble metal nanocrystals have attracted much attention as key materials in many fields, especially in biomedicine, catalysis, etc. Controlled growth of the metal nuclei allow for the manipulation of uniform morphology of final products. This behavior would tailor their unique physiochemical and electronic properties and follows by their practical applications. This review presents an overall picture of kinetic formation of a particle and then summarizes an overview of recent progress in many research groups concerning aqueous- and/or polyol-based syntheses of many types of aqueous-dispersed single metallic and bimetallic nanocrystals with controlled shape. The main advantages in these synthetic approaches for the shape-controlled metal nanocrystals are simple, versatile, environmentally friendly, low cost, pure and single-crystalline products, and high yield. The formed products can be easily dispersed in water medium and compatible for biotechnological field. Particularly the biomolecule (antibody including protein and/or DNA)-conjugated gold nanocrystals have been utilized as an active agent for a broad range of biomedical applications. We expect that this review will have a high potential towards novel materials fabrication and nanotechnological fields. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

4.

5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precursor mechanism of nucleation, growth, and aging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Single metal nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Ag nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Au nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Pd nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Pt, Rh nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bimetallic nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Au–Ag nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Pd–Pt nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Au–Pd nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Au–Pt, Ru–Pt, Ag–Ag2 S nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Some applications in biomedicine of biomolecule-conjugated gold nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Single and binary noble metal nanocrystals as a class of novel materials have shown unique physiochemical and electronic

∗ Corresponding author. Tel.: +1 418 271 1079. E-mail address: [email protected] (T.-D. Nguyen).

1 3 4 4 5 7 8 9 10 14 16 16 18 20 20

properties depending on their size and shape. They are often substantially different from their small molecules, constituents or bulk counterparts and have been extensively investigated due to their fundamental and technological scientific importance [1–3]. The development of the synthetic routes for desired single and binary noble metal nanomaterials has been a great major task for both theoretical studies and practical applications. In the past decades, two broad synthetic strategies “bottom-up” and “top-down” have been devoted to such materials [4]. The nanoscale materials

0927-7765/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2011.07.017

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synthesized from atomic precursors that aggregate together to form clusters and subsequently nanocrystals are referred to as “bottom-up” synthesis. Conversely, when the nanometer sizes are reached by physically tearing down large building blocks, the process is referred to as “top-down” synthesis. The advantage of the physical top-down method is the production of a large quality of pure nanocrystals, however the synthesis of uniform nanocrystals and their size/shape control are very hard to achieve. “Bottomup” synthetic route is of primary interest to materials researchers because the fundamental building blocks are atoms; thus colloidal chemical methods can be used to synthesize uniform nanocrystals with controlled particle size and shape. For example, polyol reactions of precursor monomer solution flourished by Xia group are a successful strategy for shape control of a variety of metal nanocrystals [5]. Sau and Rogach [1] provided a review of colloidchemical synthetic routes and morphology control of nonspherical noble metal nanoparticles. Yang et al. [2] suggested an overall picture of shaped metal nanoparticles in aqueous solution. The author mentioned the key parameters such as crystallographically selective adsorbates and seeding processes that effect on the final shape of products. Niemeyer et al. [6] revealed that the evolutionary optimized biomolecules (e.g., nucleic acid, proteins, etc.) as a capping agent are utilized in the production of metal nanostructures with high potential applications in biomedicine. Accordingly, Jones et al. [7] has carried out the self-assembly of the shape-directed crystallization of DNA-capped metal nanoparticles as building blocks into colloidal ordered superlattices. Wei et al. [8] demonstrated that in the presence of a single lysozyme protein crystal to template the in situ growth of gold nanoparticles slows down the fast kinetics of gold nanoparticle formation. Herein we have focused on the bottom-up strategy including aqueous-based routes often referred to as colloid-chemical approach involving the synthesis of nanoscale particles within a continuous solvent to form a colloidal sol. A wide range of noble single-metallic and bimetallic nanomaterials has been synthesized by these colloid-chemical pathways due to their simple, inexpensive, and versatile means. Noble metal nanocrystals are probably the most extensively researched nanomaterials and a frequent tool in nanotechnology because of their peculiar electronic and optical properties and useful applications in many fields such as catalysis, optics, sensors, biocompatibility/conjugation with proteins/DNA antibodies, and medical therapy [9–13]. Particularly the Surface Plasmon Resonance features of Au nanocrystals have enabled a wide variety of applications in biomedical researches, such as cancer diagnosis, cancer treatment, drug delivery, and DNA analysis [13]. The antibody-conjugated Au nanoparticles were utilized as “lightactivated nanoscopic heaters”. In principle, control of the particle shape and size by simply tuning the reaction parameters would allow for the generation of particles with new properties from the same materials. For example, the excellent catalytic performances of 5 nm-sized Au nanoparticles for reduction of nitric oxide and oxidation of carbon monoxide and hydrogen at low temperature were recognized in comparison with bulk gold species [14]. The Pt nanocrystals could selectively catalyze different types of chemical reactions, with {1 0 0} and {2 1 0} facets being most active for reactions involving H2 and CO, respectively [15]. This behavior is well-established that the catalytic performance of the nanocrystals has a strong correlation with the shape of singlecrystalline nanoparticles, depending on the percentage of active facets exposed. In the case of localized Surface Plasmon Resonance (LSPR) and Surface-Enhanced Raman Scattering (SERS), Au and Ag nanocrystals often exhibit unique optical properties in the visible region. Symmetric spherical crystals generally have a single scattering peak, however the rods, triangular prisms, cubes expose more corners/edges as compared with spherical crystals, displayed multiple scattering peaks [16].

Bimetallic (alloy or intermetallic) nanostructures forming from two single components have been drawing much attention because of their superior or tunable properties in comparison with the single-component species [17]. Deciphering the bimetallic structures (core–shell or dumbbell) attributes of these metal–metal interfaces have been proposed to be originated from one or more of three contributions: (i) charge transfer between the metal and support; (ii) presence of low coordinative metal sites; (iii) quantum size effects. These unique combinations of specific properties would make the multi-metal materials to be potentially useful for catalytic, optical, electronic, magnetic, biomedical applications. For example, Pt–Cu alloys displayed compositiondependent electrocatalytic activity for formic acid oxidation [18]. Some reviews mentioning the aqueous-solution approaches for controllable synthesis of uniform bimetallic nanocrystals have been published recently. Wang and Li [19] reviewed the liquid-phase synthesis and catalytic applications of bimetallic nanocrystals. The author indicated that the organic surfactants play several key roles along the course of bimetallic nanocrystal formation. There are mostly three main types of formed bimetallic structures including core/shell, heterostructure, and alloy. In general, two main routes have typically been used for the controlled growth of bimetallic nanocrystals: (i) direct heterogeneous nucleation and growth of the metal precursors onto the surfaces of preformed seeds for the generation of core/shell or heterostructures; (ii) alloys are a homogeneous mixture of two metals and generated by formed metal–metal bonds. Xia group also reviewed their published results towards a variety of bimetallic nanocrystals with highly branched morphologies in polyol- and/or aqueoussolution [20]. Such formed branch-shaped nanocrystals definitively based on kinetically controlled overgrowth, aggregation-based growth, heterogeneous seeded growth, selective etching, and template-directed methods. In summary, rational shape-controlled syntheses of single-metallic and bimetallic nanocrystals are of vital importance to understand the growth mechanism and size/shapedependent properties that are an essential requirement for the practical applications. In-depth and comprehensive understanding growth behavior and morphology evolution are crucial for efficient synthesis and quality control of inorganic nanocrystals. Among the nanostructures, single-crystalline nanocrystals have received much attention. In principle, the shape of single-crystalline face-centered-cubic metal nanocrystals enclosed completely by equivalent {1 0 0}, {1 1 1}, {1 1 0} facets, corresponding to cube, octahedron, and rhombic dodecahedron, respectively [2]. In bottom-up synthesis, the growth process of nuclei is described by classical Ostwald ripening mechanism, in which the growth of larger particles at the expense of smaller ones is driven by surface energy reduction. This phenomenon is extensively used to explain the formation of thermodynamically stable nanocrystals with nearly spherical morphologies. For the controlled self-assembly of nanoparticles into well-defined anisotropic nanostructures, organic capping reagents usually play critical roles in reducing the activity of the nanocrystal surface to promote or tune the ordered self-assembly. An oriented attachment mechanism could offer as an additional tool to design advanced materials with anisotropic properties and could be used for the controllable synthesis of more complex crystalline one-dimensional structures. In addition, the sterically diffusive kinetics and selective binding or nonbinding of surfactant molecules to different faces of the growing nanocrystal can also control the product’s morphology due to the possibility of breaking the limitations of crystal growth dynamically [21–23]. In some cases, the formation of intrinsic anisotropic nanocrystals is found to be a highly kinetics-driven process, which occurs far away from the thermodynamic equilibrium, and must be overdriven by high precursor monomer concentrations [24].

T.-H. Tran, T.-D. Nguyen / Colloids and Surfaces B: Biointerfaces 88 (2011) 1–22

The reaction solvent is crucial in the aqueous-based approaches. Compared to organic-phase synthesis, an aqueous-based system should provide a more environmentally route to the production of noble metal nanocrystals because it does not involve toxic organic solvents such as toluene, diphenyl ether, and oleic acid/oleylamine. In organic solvent systems, expensive organometallic precursors, toxic and environmentally unfriendly organic solvents are not often compatible with biomedical applications. Using water as an environmentally friendly solvent with the most abundant resources and most metal nitrates and chloride salts as starting materials that can overcome these barriers. The reduction of a metal precursor can be readily achieved by introducing various reducing agents (e.g., citric acid, l-ascorbic acid, and alcohol). It seems to be easily to manipulate the reduction kinetics and different particle shapes by using chemicals with different reducing powders. Further, due to the high solubility of metal salt precursors in aqueous medium, the aqueousbased routes can be synthesized the pure products in high yield and large-scale production. For these purposes, our review has concentrated on the water-based system as a mostly comfortable pathway to the shape-controlled synthesis of noble metal nanocrystals and the biomedical applications. In this review, based on our recent endeavors, we have focused on the general aqueous-based methods to synthesize single and binary noble metal nanostructures and the possible biomedical applications of functionalized gold nanoparticles as a representative sample. The numerous adaptations to the synthetic procedures and the controlled growth of nuclei based on kinetic and thermodynamic conditions and the selection of capping agents will be discussed towards controlling the size and shape of single and binary noble metal nanostructures with the unique dependent properties.

2. Precursor mechanism of nucleation, growth, and aging Growth rate of the seed plays a major role in determining the final nanoparticle morphology. A comprehensive understanding formation mechanism of monodisperse nanocrystals is very important because it will help us to develop the advanced synthetic methods that can generally be applied to synthesize various kinds of materials. In this work, we describe the nucleation and growth mechanism for the crystallization of monodisperse inorganic (metal) nanocrystals. According to LaMer plot for the crystallization nucleation-growth process [25], colloidal metal nanocrystal growth processes comprise the following three steps: (i) seed formation initiated by increasing the monomer concentration in the bulk solution to supersaturation level, (ii) monomers continuously aggregate onto the seeds, which leads to gradual decrease in the monomer concentration, and (iii) surface stabilization of the resultant nanocrystals by surfactants. The dependence among these reaction parameters are described in detail below. The zero-charge nanosized crystalline precursors are firstly produced from reducing precursor molecules in bulk solution and then nucleated to generate clusters such as dimers, trimers, and tetramers, in supersaturated solution and finally the initially formed seeds aggregate into larger secondary particles. The products were capped by organic molecules, resulting of the restriction of the particle growth as well as the good dispersibility of the product in reaction solvent. Nucleation will increase when the concentration of growth units falls below the minimum supersaturation level. Nucleation involves the formation of a solid phase and the creation of a surface. The free energy change due to nucleation, G, can be expressed by: G = n(S − L ) + A

(1)

3

where n is the number of moles in the nucleus, S and L are the chemical potentials of the solid and the dissolved phase, respectively, A is the surface area, and  is the surface energy of the solid–liquid interface. The chemical potentials can be described as follows: S − L = −RT ln

CL = −RT ln S CS

(2)

where CL and CS are the solubility of bulk crystals and crystals, respectively, R and T are the gas constant and absolute temperature, and S is the supersaturation. Combination of Eqs. (1) and (2) leads to: 2 3 G = −nRT ln S + (36n2 Vm  )

1/3

(3)

where Vm is the molar volume of the solid material. The radius of a critical nucleus and the energy required to form a critical nucleus (G* ) is obtained at the number of moles where ı(G)/ın = 0. Hence, G* is given by: G∗ =

2 3 16Vm

(4)

3(RT ln S)2

Eq. (4) demonstrates that the energy required to form a nucleus is strongly dependent on the surface energy and the supersaturation. The nucleation rate, JN , can be expressed in terms of G* and a pre-factor (J0 ) which not only depends on the frequency of collisions, but also on which type of reaction occurs: JN = J0 exp

 −G∗  RT



= J0 exp

2 3 −16Vm

3(RT )3 (ln S)2



(5)

It can be concluded that the nucleation rate increases strongly with decreasing surface energy and increasing supersaturation. The supersaturation increases with decreasing solubility of the metal precursors, and thus, the metal properties have a strong influence on the nucleation rate. Nucleation and growth occur until the monomer precursor concentration has equilibrated with the metal, which is determined by the solubility. After nucleation and growth, the particle size can change by aging process, during which the total amount of solid material remains constant. The two main aging processes for the evolution of particles are composed of coarsening (Ostwald ripening) and aggregation (oriented attachment or lateral aggregation). Coarsening is described by the Gibbs–Thomson equation [26]: Cr = Cr=∞ exp

 2V  m

RTr

(6)

where r is radius of the crystal. Eq. (6) is called the critical nuclei in the given monomer solution because the solubility of particles with this size equals exactly the monomer concentration in the bulk solution. Some particles smaller than this critical size should have higher solubility, and thus should be unstable in the solution. Conversely, larger particles possess low solubility and are stable under the given conditions. The equilibrium shape (Gibbs–Wulff theory) of a crystal is one that minimizes surface energy for a given enclosed volume. If the surface energy is isotropic, the equilibrium shape will be spherical as the sphere has the minimum surface area. In the case of crystalline solids, the surface energy is anisotropic. The aggregation of particles into 1D, 2D, 3D nanostructures often occur in a random fashion or oriented assembly including oriented attachment or lateral aggregation mechanisms. The aggregation depends strongly on the surface charge of the nanoparticles relative to the pH solution and the presence of adsorbing molecules or ions, resulting of the diverse shapes of final products. The coarsening and aggregation of inorganic nanoparticles often lead to spherical particles as this represents the lowest possible surface energy. These mechanism features can also be important clues to the micro/mesostructural formation from primary building unit

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Fig. 1. Effect of the etchant concentration on the controlled etching reactions. SEM images following the etching progress of the Ag octahedral-shaped nanoparticles: (a) octahedra-shaped starting material showing regular size and shape; (b) using a small amount of etchant, the edges and corners can be selectively etched leaving gaps of 5–10 nm; (c) when exposed to a slightly higher concentration of etching solution, eight distinct arms develop; and (d) at relatively high concentrations of etching. All scale bars shown represent 1 ␮m. Reproduced with permission from Ref. [30]. Copyright 2010, American Chemical Society.

assembly. As a result, during these nucleation and growth stages, the control of growth parameters is a critical key in determining the final size and shape of nanocrystals. The surfactant-assisted synthetic methods provide convenient and powerful pathway for the reproducible controlled synthesis of metal nanocrystals. These methods could allow for the metal nanocrystals to be precisely adjusted in terms of their size, shape, composition, and phase structure on the nanometer scale. Further it can be modified by chemical hybridization with other functional materials for potential applications. Noble metal nanocrystals obtained by the surfactant-assisted route, in general, exhibit excellent crystallinity and monodispersity. Because of the rather strong bond between the organic surfactants and metals, it was proposed that the surfactants changed the relative surface energies and thus determined the growth rates of specific crystal faces. These routes generally have the advantageous features of versatility and greater possibility as compared to the surfactant-free routes. Because of without surfactants, wet-chemical syntheses become more difficult to control the morphology. In the following sections, we would present a variety of the representative noble single-metal and bimetallic nanocrystals that obtained from the capping agentassisted methods in aqueous medium, examples for the discussion of each species, synthetic pathways, and corresponding reaction mechanisms. 3. Single metal nanocrystals In seed-mediated growth procedure, several reaction parameters are simultaneously responsible for the final shapes of the noble metal nanocrystals. Hence several controllable experiments are conducted to elucidate the growth mechanisms of the metal nanocrystals. In the nucleation stage, the size and crystal structure of seeds are proved to be important for the formation of singlecrystalline structures of the metal nanocrystals. In the growth stage, precursors, surfactants, adsorbates, reducing reagents, solvents, additives, growth kinetics (reaction time and temperature), are found to be effected on the determination of the final shapes and sizes of the resultant single-crystalline metal nanocrystals. The most popular method to control the size and shape of pure noble metals is the ligand-based polyol and/or aqueous synthetic routes [5]. Additives, such as surfactants (e.g., cetyltrimethylammonium bromide (CTAB), cetylpyridinium chloride (CPC), poly(vinyl pyrroli-

done) (PVP), sodium dodecyl sulfate, biomolecule ligand as amino acid) and foreign ions (Cl− , Br− , I− ) present in the reaction medium, play important roles in controlling the morphology of particles produced. Surfactants or polymers were commonly added as stabilizers to impart stability to nanoparticles against aggregation, since colloidal particles tend to aggregate to decrease the overall surface area and energy. The typical route for solution-phase synthesis of colloidal metal nanoparticles is the simple reduction of metal salts by reducing agents such as NaBH4 , ascorbic acid or ascorbate, citric acid or citrate, H2 , hydrazine, and ethylene glycol. Depending on the reduction potentials of the metal precursor and the reducingagent systems, reduction can occur at room temperature or at elevated temperatures. The main advantages of the PVP, CTAB, CPC, biomolecule ligands, etc. are nontoxic and water-dissolve reagents and can generate water-dispersed nanoparticle colloids that can be compatible for the biological and medical applications [27]. 3.1. Ag nanocrystals A first great progress for the shape-selective synthesis of wellshaped monodisperse Ag nanocrystals in high yield was made by Xia group. Silver nanocubes with controllable dimensions were synthesized by the polyol reduction of silver nitrate with ethylene glycol in the presence of PVP ligand. The single-crystalline Ag cubes with the mean edge length of 175 nm and standard deviation of 13 nm had slightly truncated shape bounded by {1 0 0}, {1 1 0}, {1 1 1} facets. The presence of PVP and PVP/silver nitrate molar ratio both played important roles in determining the geometric shape of the final product that could be optimized their optical, electrical, and catalytic properties. Further uniform truncated cubic shaped-Au nanoboxes were also synthesized by reacting the Ag nanocubes as a sacrificial templates with an aqueous HAuCl4 solution [28,29]. Recent work from Yang’s group has shown that single-crystalline silver nanocrystals are attractive candidates for optical labeling, near-field optical probing, and SERS applications [30]. The oscillation of electrons at the metal dielectric interface is strongly dependent on the size, symmetry, and proximity of the nanoparticles. By tuning component and the ratio of NH4 OH/H2 O2 /chromic acid etchant mixtures that yielded single- crystalline Ag octahedral-shaped nanoparticles with different edges and corners (Fig. 1). The etching process produces intraparticle gaps, which introduce modified plasmonic

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Fig. 2. Study of absorption and scattering plasmonic optical properties of colloidal Ag nanoparticles with different shapes using UV–vis absorption spectroscopy. (A) Photos of the aqueous reaction solution containing Ag nanoparticle colloids show the color change from light yellow to green (a-l). (B) Normalized absorbance of UV–vis absorption spectra (a-l) of the colloids of Ag nanoparticles with different shapes. Reproduced with permission from Ref. [31]. Copyright 2010, The Royal Society of Chemistry.

characteristics and significant scattering intensity in the nearinfrared, indicating that these etched particles can be a function as highly sensitive SERS substrates. Multi-shaped Ag nanocrystals including spherical, rod, triangular, and cookie shapes were synthesized in an aqueous solution of AgNO3 /PVP/sodium citrate/H2 O2 [31]. The colors of colloids were tunable by controlling the sizes/shapes of single nanocrystals. The unique size- or shapedependent properties of noble metal nanocrystals exhibited the large optical field enhancements, resulting in the strong scattering and absorption of light, as shown in Fig. 2. The results revealed that the various sizes/shapes exhibited the corresponding different maximum absorptions of scattering LSPR spectra due to the different distribution of active atoms on the Ag surface. The 50 nmsized Ag nanorices were synthesized on a large scale by polyol reaction of the AgNO3 /PVP/polyethylene glycol solution [32]. Some rapid routes to synthesized Ag nanostructures through the oxidation etching were published recently. As the byproduct HNO3 agent plays as an oxidative etchant to block the homogeneous nucleation and prevents the single-crystalline seeds from evolving into twinned species for the formation of 30–200 nm-sized Ag nanocubes/spheres [33]. HNO3 formed during the synthesis from the dissolution of CF3 COOAg and AgNO3 precursors in ethylene glycol medium. The formed nitric acid performed the oxidative etching for the seed-mediated growth of Ag nanocubes and PVP could selectively adsorb onto {1 0 0} facets of Ag crystals to grow into nanocubes. The edge length of the resultant Ag nanocubes can be readily controlled by varying both the amount of Ag seeds used and the amount of AgNO3 added. The dependence of LSPR and SERS properties on the well-controlled particle size of the Ag nanocubes was also reported in detail. 3.2. Au nanocrystals The polyhedral-shaped Au nanocrystals are generally yielded by using polyol reduction method in the presence of strong reduc-

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ing agent [34]. In some cases, Au nanoplates with twin defects or stacking faults are introduced from the aggregation of small clusters in the low precursor monomer concentration via Ostwald ripening process [35]. Carbó-Argibay et al. [36] indicated that the Au nanocrystals could form octahedrons with an increase in the growth rate of gold atom on {1 0 0} facets via the selective adsorption of PVP on either {1 1 0} or {1 1 1} facets. On the other hand, the recent simulation results have shown that PVP can adsorb onto both {1 0 0} and {1 1 1} facets of metal crystal, and its adsorption can reduce the difference in surface energy between {1 0 0} and {1 1 1} facets [37]. Interestingly, single-crystalline tetrahexahedral gold nanocrystals (∼300 nm in diameter) bound to 24 high-index {2 1 0} facets were synthesized in PVP/N,N-dimethylformamide (Fig. 3) [38]. Moreover, the novel Au nanocages were typically prepared by means of the galvanic replacement reaction between Ag nanocubes and HAuCl4 in the presence of thermosensitive polymers in water [39]. The strong binding between gold and thiol groups made them straightforward to attach poly(N-isopropylacrylamide) to the surface of the gold nanocages by using a disulfide initiator. Gold nanocages represented a class of nanostructures with hollow interiors and porous walls. When the gold nanocages were irradiated with a laser and the strongly absorbed light was converted into heat through the photothermal effect. By using smart polymers, it is possible to create drug delivery systems that not only deliver their payload to a specific place, but also deliver them in response to outside stimulus. El Sayed et al. [40] reported that the photodegradation of methyl orange was found to take place very efficiently using hollow Au nanocages. They indicated the very high surface plasmon fields and excellent near-infrared sensors of the gold nanocages/frames [41]. The Au nanoplates obtained from the aqueous solution of HAuCl4 /sodium dodecyl sulfonate/CTAB at below room temperature. The belt-like shape was relative to a cooperative effect of the mixed surfactants [42]. The uniform 40 nm Au nanocrystals with systematic shape evolution from truncated cubic to cubic, trisoctahedral, and rhombic dodecahedral structures were yielded through the precise control of the reaction parameters including the bromide concentration and the amount of reducing agent ascorbic acid (Fig. 4) [43]. The particle sizes of the nanocubes and rhombic dodecahedra can be tuned from 30 to 75 nm by varying the amount of the seeds. Absorption band blue shift was observed as particle size decreases. On the other hand, the longitudinal plasmon resonance energy can be systematically controlled by varying the Au nanorod length [44]. The foreign ions effect on the particle shape, in order to control the bromide concentration in the growth solution, a combination of using cetyltrimethylammonium chloride (CTAC) surfactant and a very small amount of NaBr was performed to generate the Au nanocubes. Nanocubes and rhombic dodecahedra with controlled sizes of 30–75 nm were prepared by adjusting the volume of the seed solution added to the growth solution. As a variant of these seeded growth methods, Huang and coworkers had extended the heteroepitaxial method to prepare Au nanorods. Besides Au rods, Au nanocrystals including cubes, decahedrons, and octahedrons had also been formed (Fig. 5) [45]. In seed-mediated growth of penta-branched gold nanocrystals which resembled the shape of a star fruit but with sharp ends. By replacing CTAB with CTAC surfactant, and controlling the concentration of bromide ion originating from the CTAB in the solution, gold nanostars with five symmetrical branches were formed. It was found that silver ions are critical to promoting the development of side branches. By adding AgNO3 into the growth solution, pentagonal bipyramid-shaped nanocrystals were transformed into the pentagonal bipyramidshaped nanocrystals. Side growth over the twin boundaries results in the formation of five elongated branches. An aqueous-based method was used to selectively synthesize single-crystalline rhombic dodecahedral, octahedral, and cubic gold nanocrystals in high

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Fig. 3. SEM (a), TEM (b), HRTEM (c), SAED (d) of the single-crystalline tetrahexahedral-shaped Au nanocrystals (∼300 nm in diameter) with 24 high-index {2 1 0} facets were synthesized in PVP/N,N-dimethylformamid. Reproduced with permission from Ref. [38] Copyright 2010, American Chemical Society.

yield (Fig. 6) [23]. The effect of important reaction parameters including surfactants, growth kinetics, and adsorbates, on the final shapes of the gold nanocrystals have been studied systematically. The gold nanorods possessed {1 1 0}-exposed facets and these {1 1 0} facets disappeared during overgrowth in the presence of CTAB or PVP. The selective interactions of cetylpyridinium chloride (CPC) with different gold facets concerned in the alteration relative to the surface energy and growth kinetics. The rhombic dodecahedral gold nanocrystals formed in the presence of CPC because these molecules can selectively stabilize the {1 1 0} facets of gold. At the relatively high CPC concentrations, octahedral gold nanocrystals with {1 1 1}-exposed facets produced, suggesting that the enhanced capping of CPC promoted the formation of {1 1 1} facets of gold. Nanotechnology applied to biomedical fields has become one of the most important and challenging focuses in the past decades [46–49]. Functionalized, inert, and biocompatible noble metal nanomaterials have attracted considerable attention due to their potential applications in biomedical analysis, especially in cancer diagnosis and therapy. Noble metal nanocrystals have been utilized in the hyperthermia and photodynamic therapy of malignant tumors. However, imparting the noble metal nanocrystals with target-recognizing, water-soluble, and precise biomedical functions still needs much more efforts. Inspired by the studies of biomineralization and biomolecules have been extensively utilized in the synthesis and assembly of nanomaterials. Amino acids play a crucial role in protecting the aggregation of noble metal ions through –COOH/–NH2 groups in water medium [50]. In particular, amino acid molecule has free –COOH and –NH2 groups to provide a hydrophilic interface and a handle for further reactivity with other functional biomolecules. Amino acid is thus considered as an ideal capping biomolecule in the synthesis of noble metal nanocrystals for the biomedical applications. Some novel biomolecule-assisted

routes have been used to synthesize noble metal nanocrystals in aqueous medium for many past years. The amino acid-capped Au nanochains were synthesized by reducing aqueous AuCl4 − with NaBH4 in the presence of the amino acid (glutamic acid and histidine) as a capping agent. After synthesis, the amino groups of amino acid molecules capped on the surface of Au nanoparticles and their surface has hydrophilic character because the free uncoordinated carboxylic acid groups of capping biomolecules outwarded to water medium. The interaction between hydrophilic-surfaced groups (–COOH and –NH2 ) could result in the oriented-attachment process for the formation of the Au nanochains from 10 to 15 nm nanospheres, as illustrated by the cartoon in Fig. 7 [51]. This synthetic approach was also achieved recently by our research group [52,53]. Peptide-capped Au nanospheres with ∼10 nm in diameter were recently synthesized by Aizawa et al. [54] The products can be compatible for biomedicine. Besides, a multifunctional peptidemediated synthesis of Pd nanocrystals in aqueous environment with controllable size in the sub-10 nanometer regime by using different concentrations of the external NaBH4 reducing agent [55]. Also using the biomolecule method, Shankar et al. [56] synthesized 200–500 nm Au nanotriangles. Further Pd nanocrystals with tunable sizes from 2.6 to 6.6 nm were controlled by using the variable particle sizes of nuclei as seed and by introducing different amounts of precursors [55]. Wu et al. [57] showed the synthesized glutathione-capped Ag nanocrystals with tunable size were bound to bifunctional biomolecule including bovine serum albumin as a model protein to apply for the anticancer potential of Ag nanoparticles. Au nanowires also recently obtained by a seeded growth process using protein lysozyme fibrils as biotemplates [58]. The use of amino acid lysine biomolecules for the synthesis of the lysine-stabilized gold nanoparticles, resulting of water-redispersible nanoparticles have important implications for the formation of other medically bioconjugates [59].

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Fig. 4. SEM images and the corresponding drawings showing the morphological evolution of the gold nanocrystals synthesized by varying the amount of ascorbic acid added to the reaction solution. All scale bars represent 50 nm. Reproduced with permission from Ref. [43]. Copyright 2010, American Chemical Society.

3.3. Pd nanocrystals In order to preserve the single-crystalline nature of the seeds during the seed-mediated growth process, one has to judiciously select appropriate adsorbents or manipulate the growth kinetics. The addition of a small amount of anion such as Br− and I− can strongly influence the final particle shape through selective adsorption on certain crystal facets or changes in their relative surface energy. For example, Xu et al. [60] used the versatile seedgrowth method for the selective synthesis of single-crystalline rhombic dodecahedral, cubic, and octahedral Pd nanocrystals, as shown in Fig. 8. The addition of small amounts of different salts can strongly influence the final particle shape through selective adsorption on certain Pd facets or changes in their relative surface energy. A series of Pd nanocrystals with varying shapes obtained through the manipulation of the concentration of KI and the reaction temperature. The {1 0 0}-exposed faced Pd nanocubes were formed without KI. While increasing the temperature from low to high (from 30 to 40, 50, 60, 80 ◦ C), the {1 0 0} Pd facets were converted into the {1 1 0} Pd facets. The correlated shape transformations were explained in terms of surface-energy and growth kinetics. The cubic, multi-armed, dendritic Pd nanoparticles were selectively formed simply by changing the injection order of ascor-

bic acid reductant and CTAB surfactant (Fig. 9) [61]. The prepared nanocatalysts exhibited shape-dependent electrocatalytic properties towards formic acid oxidation. Huang et al. [62] synthesized the concave tetrahedral/trigonal bipyramidal-shaped Pd nanocrystals were enclosed by reactive {1 1 1} and {1 1 0} facets (Fig. 10). The degree of concavity of the polyhedral {1 1 1}- and {1 1 0}exposed Pd nanocrystals depended on the reaction temperatures. The particles with a smaller concave fraction yielded at higher temperatures. The existence of reactive {1 1 0} facets in the concave nanocrystals exhibited the high catalytic activity for the electrocatalytic oxidation of formic acid. Uniform Pd nanorods with average lengths of 200–300 nm synthesized in an aqueous solution of Na2 PdCl3 /CTAB/copper acetate (Fig. 11) [63]. The shape evolution of Pd nanorods from short to long occurred when adding a small volume of copper acetate solution during the nanorod growth stage. Extensively branched Pd nanocrystals were generated by slightly increasing the volume of the copper acetate solution. The author also demonstrated the highly efficient and recyclable catalytic activity of both the long Pd nanorods and branched nanocrystal catalysts for catalyzing a Suzuki coupling reaction. Xia and colleagues had used a water-based or a mixed polyol/water system in the presence of PVP to make a variety of Pd nanostructures such as nanobars, short nanorods, nanocubes, octahedra, icosahedra, and

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Fig. 5. Shape evolution from polyhedral gold nanocrystals to nanostars and penta-branched nanocrystals with increasing bromide ion concentration in the reaction solution. Reproduced with permission from Ref. [45]. Copyright 2009, American Chemical Society.

nanoplates [64]. Triangular- and hexagonal-shaped Pd nanoplates selectively synthesized by manipulating the reduction kinetics of Na2 PdCl4 by ethylene glycol in the presence of PVP [65]. Both triangular and hexagonal Pd nanoplates with sharp corners and edges serve as active substrates for surface-enhanced Raman scattering and surface plasmon resonance peaks in the visible region. The author also reported on the synthesis of shape-controlled Pd nanocrystals by reducing a Na2 PdCl4 salt with citric acid in aqueous solution in the presence of PVP. A range of concentrations for both Na2 PdCl4 (5.8–17.4 mM) and citric acid (0.13–0.39 M) determined the optimal conditions for the design of different-shaped Pd nanocrystals [66]. Highly crystalline rectangular Pd nanoparticles were synthesized by Sun et al. [67] via the reduction of K2 PdCl4 by ascorbic acid in the presence of cetyltrimethylammonium bromide. Either cubic or rod shape of Pd nanocrystals depended on the concentration of trisodium citrate. Bisson et al. [68] elucidated spontaneously two mechanisms of oriented-attachment and directed-aggregation for the formation of Pd nanostructures with multishapes in seed-mediated synthesis. The influence of CTAB surfactant and seed/nuclei ratio, solvent, temperature on two mechanisms were found definitively. Shimizu et al. [69] reported one-spot synthesis of unprecedented 1-D necklace-like chains consisting of hybrid Pd nanocrystals embedded in spheres of a peptidic lipid. The hybrid materials self-assembled into structured spheres which attach each other one-by-one to form necklace-like chains by coordination of negatively charged carboxylic groups with Pd. Huang et al. [55] reported on a peptide molecule-mediated synthesis of Pd nanocrystals in aqueous solution with controllable size in the sub-10 nanometre regime. The Pd nanocrystals with tunable sizes from 2.6 to 6.6 nm obtained by using the variable sizes of nuclei as seeds and by introducing different amounts of precursors. 3.4. Pt, Rh nanocrystals Platinum nanocubes and nanopolyhedra dominantly enclosed in six {1 0 0} facets and with tunable size from 5 to 9 nm were synthesized by controlling the reducing rate of metal precursor in

the same polyol process. The nanocubes showed a higher selectivity to n-butylamine than that of nanopolyhedra due to their enhanced ring-opening ability [70]. Yu et al. [71] synthesized Pt concave nanocubes enclosed by high-index {5 1 0}, {7 2 0}, {8 3 0} facets by using a reduction-aqueous solution route. The facilitate selective overgrowth of Pt seeds from corners and edges was controlled by using Pt pyrophosphato complex as a precursor and with Br− serving as a capping agent to block the {1 0 0} facets. Pt concave nanocubes exhibited enhanced electrocatalytic activity per unit surface area towards oxygen reduction reaction compared with those of Pt cubes, cuboctahedra, and commercial Pt/C catalysts. High-index facets of nanocatalysts have a much higher density of low-coordination-number stepped atoms that would exhibit high activity and selectivity for catalytic reactions. Using the chemical composition of H2 PtCl4 /PVP/sodium lauryl sulfate/NaBr, high-quality 8 nm Pt nanocubes had also synthesized in a single phase aqueous solution by Wang et al. [72] This route was extended to synthesize bimetallic Pd–Pt heterostructured ultrathin nanowires. Metal nanomaterials are widely used as a novel catalyst in a rich variety of reactions such as hydrogenation, hydroformylation, hydrocarbonylation, hydrogen generation, and CO oxidation. Fig. 12 shows the monodisperse 6.5 nm-sized Rh nanopolyhedron and nanocubes were synthesized by Somorjai group via the seedless polyol reduction in PVP/ethylene glycol [73]. The {1 1 1}-oriented Rh nanopolyhedra containing 76% {1 1 1}-twinned hexagons were obtained as using [Rh(Ac)2 ]2 precursor, whereas the {1 0 0}-oriented Rh nanocubes obtained with 85% selectivity as using RhCl3 precursor in the presence of alkylammonium bromide because of the selectively chemical adsorption of Br− ions from alkylammonium bromides onto the {1 0 0} facet. The CO oxidation catalytic performance relative to the shape of the active nanocatalysts were shown. The growth parameters such as temperature and precursor type both play important roles in controlling the morphology of the Rh nanocrystals, in which five-fold twinned and starfish-like Rh nanocrystals were produced by the polyol reaction of (Rh(CF3 COO)2 )2 precursors in poly(vinyl pyrrolidone)/ethylene glycol. The Au catalysts exhibited the high catalytic performance for the CO oxidation, depending on the size of the nanocrystals.

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Fig. 6. SEM images of the cetylpyridinium chloride (CPC)-capped single-crystalline Au nanocrystals with 41.3 nm in diameter and different shapes synthesized through manipulation of the growth kinetics and selection of appropriate adsorbates: (a) nanorods, (b) spherical-shaped seeds, (c) rhombic dodecahedron, (d) octahedron, (e) cubes, (f) mixed octahedron and rod. Reproduced with permission from Ref. [23]. Copyright 2009, American Chemical Society.

4. Bimetallic nanocrystals Bimetallic (alloy and core–shell/dumbbell) nanocrystals have received increasing interest because they could provide a new system with tunable catalytic and optical properties sought for practical applications such as catalysis, diagnosis, plasmonics, and surface-enhanced Raman spectroscopy [17]. The physiochemical properties of a bimetallic nanocrystal can be tailored by controlling their particle size, shape, elemental composition, as well as their internal and surface structures. Recently, the complexity of nanomaterials can be enhanced further by the formation of multimetallic nanostructures (e.g., core–shell and dumbbell). Synthetic

parameters such as reducing, capping agent, metal ion, and reaction temperature also play an important role in the overgrowth process [74]. The heterogeneous core–shell nanostructures can often be achieved by the direct and/or indirect deposition of a metal on the core surface of another metal or a galvanic replacement reaction between the core and a salt precursor as a second metal. For example, the tips and the side surface of Au nanorods have been coated with other metals such as Pd, Pt, Ag, Ni, and Au to create new functions to the Au nanorods [75–79]. The development of these specific structures has been motivated, in which they would provide a new system with tunable unique collective properties.

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Fig. 7. (a, b) TEM/HRTEM images of the glutamic acid-capped Au nanoparticles synthesized using aqueous HAuCl4 /glutamic acid/NaBH4 solution. (c) The formation mechanism of the Au nanochains through dipole–dipole interaction due to the zwitterionic nature of amino acids. Reproduced with permission from Ref. [51]. Copyright 2008, Nanotechnology.

4.1. Au–Ag nanocrystals Fig. 13 displays the uniform Au@Ag core–shell materials with Ag crystals covering Au rods in the center were successfully synthesized by Xia group [80]. The dimensions of the core–shell nanostructures were easily tuned by using Au nanorods with different aspect ratios and/or by controlling AgNO3 amount added into the reaction solution. With an increase in the AgNO3 amount, the metallic Ag started to grow faster on the side surface of the Au nanorod and the shape evolved into an octahedron. PVP and CTAB ligands affected both the deposition of Ag and the final shape of the core–shell nanostructure. When no PVP was added, small Ag particles still deposited on Au nanorods to form octahedral nanocrystals via a homogeneous nucleation process. In the case without adding CTAB, Ag could not nucleate and grow on the Au nanorods. These were Au@Ag core–shell nanostructures showing one broad peak at ∼460 nm and a shoulder peak at ∼350 nm. Ag shell formation on Au nanorod in hexadecyltrimethylammonium chloride (CTAC) and hexadecyltrimethylammonium bromide (CTAB) mixed micellar solution was also achieved. It found that the Ag shell formation in CTAC solution was much faster than that in CTAB solution [81]. The uniform Au–Ag core–shell nanorods were obtained by retarding the silver shell growth on gold nanorods. The colloidal solutions

of the Au–Ag core–shell nanorods showed the four extinction bands that originated from the anisotropic silver shell formation. The thickness of the anisotropic silver shells could be controlled by the relative amounts of silver ions and gold nanorods. The extinction spectra, photographs of the dramatic changes in color from orange to green of the reaction solutions, and TEM images of the elongated shape of the anisotropic Au–Ag core–shell nanocrystals using AgNO3 solutions at the different Ag/Au molar ratios are shown in Fig. 14 [82]. Ag@Au core–shell nanocrystals were prepared by coating Au nanodumbbells with silver in an aqueous solution containing CTAB, AgNO3 , NaOH, ascorbic acid. The morphology observation showed that Ag grew on top of the Au dumbbell core rather than growing on the side of Au. The Au@Ag core–shells serve as excellent SERS substrates, and significantly higher enhancement factors were expected for silver as compared to Au [83]. Photochemical route to the CTAC and sodium dodecyl sulfate-assisted synthesis of Au–Ag alloys and core–shells was also reported by Scaiano et al. [84] Ag@Au core–shell triangular bifrustum nanocrystals were synthesized in aqueous solution using seed-mediated approach. The formation of the Ag layer on the Au nanoprism seeds led to structures with highly tunable dipole and quadrupole surface plasmon resonances [85]. Very recently, Xia group still utilized the seedmediated growth process to synthesize the well-defined Au@Ag

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Fig. 8. SEM images of polyhedral palladium nanocrystal samples synthesized under different conditions (scale bar: 200 nm). In columns A–E, the reaction temperatures are 30, 40, 50, 60, and 80 ◦ C, respectively. In rows 1–5, 5 ␮L of 100 mM, 5 ␮L of 10 mM, 25 ␮L of 1 mM, 5 ␮L of 1 mM, and 5 ␮L of 0.1 mM KI solutions were added to the growth solutions, respectively. In row 6, no KI was added. Reproduced with permission from Ref. [60]. Copyright 2010, American Chemical Society.

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Fig. 9. Cubic, multi-armed, dendritic Pd nanoparticles were selectively prepared by changing the injection sequence of reductant and surfactant under otherwise identical experimental conditions. The scale bars indicate 50 nm. Reproduced with permission from Ref. [61]. Copyright 2010, The Royal Society of Chemistry.

Fig. 10. (a) Large-area and (b) enlarged TEM, (c) STEM, (d) SEM images of the as-synthesized concave tetrahedral/trigonal bipyramidal Pd nanocrystals. (e) High-magnification TEM image of a single concave tetrahedron. Top-right and bottom-right insets show the corresponding SAED pattern and the ideal structure model of the concave tetrahedron, respectively. (f) HRTEM image of the squared area indicated in (e). Reproduced with permission from Ref. [62]. Copyright 2009, American Chemical Society.

core–shell nanocubes [86]. The synthetic procedure of the Au seeds was composed of two steps: (i) 2–3 nm Au nanoparticles were prepared by reducing HAuCl4 with NaBH4 in the presence of CTAB, and (ii) these Au particles grew into 11 nm Au nanocrystal seeds with cubo-octahedral shape in HAuCl4 /ascorbic acid/CTAC. The Au@Ag core–shell nanocubes were formed by depositing Ag on the asprepared CTAC-capped Au seeds via adding AgNO3 , ascorbic acid, CTAC into an aqueous suspension containing the Au seeds at 60 ◦ C. The Ag shell thickness could be finely tuned from 1.2 to 20 nm

by varying the AgNO3 /Au molar ratio. The LSPR properties of the Au@Ag core–shell nanocubes were found as a function of the Ag shell thickness. The edge length of the cubes increased from 24.4 to 50.4 nm when the concentration of the Au seeds was reduced from 14.6 to 1.46 mg/L. When CTAC was replaced by CTAB as capping agent, the resultant core–shell nanocrystals with controlled cube shape became difficultly, possible due to the primary formation of AgBr precipitates. The Ag shells of the hybrid nanocubes could be transformed into porous Au shells with slightly enlarged

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Fig. 11. Controlled growth of the Pd nanocrystals with diverse shapes. The final products obtained with the addition of different amounts of Cu2+ ions in the Pd seed solutions. Reproduced with permission from Ref. [63]. Copyright 2009, American Chemical Society.

Fig. 12. Synthetic scheme of shape-controlled Rh nanocrystals in sub-10 nm sizes synthesized in ethylene glycol medium under an Ar atmosphere: (a) {1 1 1}-faceted Rh nanopolyhedra, 0.3125 mM [Rh(Ac)2 ]2 , 12.5 mM PVP, 185 ◦ C, 2 h; (b) {1 0 0}-faceted Rh nanocubes, 10 mM RhCl3 , 50 mM TMAB, 200 mM PVP, 185 ◦ C, 1.5 h. Reproduced with permission from Ref. [73]. Copyright 2010, American Chemical Society.

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Fig. 13. (A) A schematic image showing the formation of Au@Ag core–shell nanocrystals. (B, C) TEM images of (B) the Au nanorods (19.8 nm × 38.3 nm in width and length, respectively) and (C) 64.8 ± 5.9 nm-sized Au@Ag core–shell nanocrystals. The inset in Fig. 1C shows a magnified TEM image of the Au@Ag nanocrystals (scale bar: 30 nm). (D) Top: SEM image of the Au@Ag nanocrystals. Bottom: SEM images of the Au@Ag nanocrystals viewed from different angles. (E) UV–vis spectra of the Au nanorods and corresponding Au@Ag nanocrystals. Reproduced with permission from Ref. [80]. Copyright 2010, Wiley-VCH Verlag GmbH & Co.

dimensions while the Au cores were kept inside the shells via a galvanic replacement process because the reduction potential of Ag is the lower than that of Au. 4.2. Pd–Pt nanocrystals Xia group synthesized 40 nm-sized Pd–Pt alloy star-shaped decahedrons and truncated triangular nanoplates with truncation at twin boundaries through the co-reduction of Na2 PdCl4 and K2 PtCl4 with poly(vinyl pyrrolidone) in aqueous medium [87]. HRTEM image of the star-shaped decahedron clearly revealed that it was composed of five single-crystalline domains with a twin-based adjoining plane between two neighboring domains (Fig. 15). Multiply twin defected structures can result in the coalescence of both small single-particles. The crystal structure of the alloy nanocrystals could be controlled by manipulating the reduction kinetics by using different reducing agents such as PVP and EG. Alloy nanos-

tructures made of Pd and Pt can be of particular interest owing to their wide applications in catalytic and electrocatalytic reactions [87]. Recently, monodisperse and highly selective sub-10 nm Pd–Pt heterostructured ultrathin nanowires have also been synthesized through using Pd nanocubes as seeds in aqueous medium [72]. Pd–Pt bimetallic nanodendrites are consisted of a dense array of Pt branches on the core of 9 nm truncated octahedral Pd seeds by reducing K2 PtCl4 by l-ascorbic acid in an aqueous medium. Another interesting feature of this unique nanostructure is an epitaxial relationship between the palladium core and the platinum branches, which can be attributed to the close lattice match between these two metals (Pd and Pt have a lattice mismatch of only 0.77%). The Pt branches supported on faceted Pd nanocrystals exhibited relatively large surface areas and particularly active facets towards the oxygen reduction reaction. These Pd–Pt bimetallic nanodendrites can also find use as catalysts beyond fuel cell applications [88]. Yang and co-workers rationally designed and used cubic Pt nanocrystals

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Fig. 14. Anisotropic Au–Ag core–shell nanorods synthesized by using different AgNO3 amounts. (a) Extinction spectra at 180 min after adding AgNO3 solutions at 60 ◦ C. Ag/Au molar ratios of 57 (i), 28.5 (ii), 19 (iii), 14.2 (iv), 9.5 (v), 7.1 (vi), and silver ion concentration unchanged (0.25 mM). (b) Photographs of corresponding reaction solutions. (c) TEM images of the corresponding elongated Au–Ag core–shell nanorods. The scale bars indicate 50 nm. Reproduced with permission from Ref. [82]. Copyright 2010, The Royal Society of Chemistry.

Fig. 15. (a) TEM image of Pd–Pt alloy nanocrystals synthesized by coreduction of Na2 PdCl4 and K2 PtCl4 with PVP through twin-induced growth. The synthetic reaction was conducted at 80 ◦ C for 18 h, with the molar ratio of Na2 PdCl4 to K2 PtCl4 at 1:1. Magnified TEM images of (b) star-shaped decahedrons and (c,d) triangular nanoplates tilted at different angles. Reproduced with permission from Ref. [87]. Copyright 2009, Wiley-VCH Verlag GmbH & Co.

as seeds for the conformal shape-controlled epitaxial overgrowth of Pd and the anisotropic growth of Au. They also showed that lattice mismatch (0.77% for Pt/Pd versus 4.08% for Pt/Au) plays a critically important role in the overgrowth of the secondary metal and high lattice mismatch prevents the conformal overgrowth. Further control of Pt overgrowth on the surface of Pd nanocrystals

with well-defined morphologies is expected to provide a promising route to the development of Pt-based catalysts or electrocatalysts with greatly improved activity and cost-effectiveness [89]. Very recently, Xia group described a systematic study on the epitaxial overgrowth of Pt on well-defined Pd nanocrystals with different shapes with exposed facets, including regular octahedrons,

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Fig. 16. TEM images and simulated structures of epitaxial overgrowth of Pt on well-defined Pd nanocrystals. (a) Pd–Pt core–shell octahedrons synthesized by reducing K2 PtCl4 with citric acid in the presence of the octahedral Pd seeds. (c) Pd–Pt core–shell octahedrons synthesized by reducing K2 PtCl4 with citric acid in the presence of the truncated octahedral Pd seeds. (e) Pd–Pt nanocrystals synthesized by reducing K2 PtCl4 with citric acid in the presence of the cubic Pd seeds. (b, d, f) Schematic drawing of the Pd–Pt core–shell nanostructures with the corresponding shapes. Reproduced with permission from Ref. [90]. Copyright 2010, The Royal Society of Chemistry.

truncated octahedrons, and cubes (Fig. 16). They clearly demonstrated that the mode of epitaxial growth of Pt on Pd nanocrystals can be controlled by employing a surface capping agent and manipulating the kinetics for Pt reduction. The thickness of the Pt shell was extremely thin, on the scale of 1-2 nm, which epitaxial overgrowth of Pt on the {1 1 1} faces of Pd octahedrons [90]. Pd@Pt core–shell nanostructures with controllable composition synthesized via the reduction of K2 PtCl4 and PdCl2 solutions with different molar ratios in the presence of CTAB upon microwave heating condition. The Pd@Pt electrocatalysts showed higher catalytic activity than pure Pd and pure Pt catalysts for the oxygen electro-reduction reaction and methanol electro-oxidation reaction, and the highest activity obtained at the Pd@Pt electrocatalyst with 1:3 molar ratio of Pd/Pt [84]. 4.3. Au–Pd nanocrystals For bimetallic Au and Pd system, it would be interesting to use Au nanocubes as the structure-directing cores for the overgrowth of Pd shells. Because of a significant lattice mismatch between gold and palladium, the resulting Au–Pd core–shell nanocrystals may exhibit unusual morphologies and high-index facets. Such nanocatalysts with high catalytic activities may obtain. The Pd–Au core–shell nanocubes and spherical nanostructures were formed by reducing HAuCl4 with l-ascorbic acid in the presence of cubic Pd seeds in an aqueous medium. The dark-field TEM result shows a clear contrast between the Pd core and the Au shell, confirming the core–shell structure. The Pd cores retained their original cubic shape, indicating that they were intact during the deposition of the Au shells. The synthesized Pd–Au bimetallic nanocrystals combine the individual properties of both Pd and Au, can generate the novel catalytic properties and potential applications including in situ monitoring of catalytic reactions via surface-enhanced Raman scattering [91]. Two-step seed-mediated growth method for synthesizing single-crystalline 41.5 nm-sized Au@Pd nanocubes was performed in high yield via the overgrown on the octahedral Au cores with 30 nm in diameter by reducing H2 PdCl4 with ascorbic acid under the assistance of CTAB surfactant (Fig. 17). The different

deposition of metal shell on the core for the overgrowth core–shell process was revealed by Ostwald ripening mechanism. Recently, Huang et al. [92] synthesized the Au–Pd core–shell heterostructures possessing an unusual tetrahexahedral morphology with {7 3 0} facets in high yield via an aqueous-based method. Tetrahexahedral Au–Pd core–shell nanocrystals with eight progressively increasing dimensions from 56 to 124 nm were conveniently yielded as using different sized-cubic cores and adjusting the volume of the gold core solution added. In addition, by lowering the reaction temperature and increasing the reaction time, novel concave octahedral and octahedral Au–Pd core–shell nanocrystals with both {1 1 1} and {1 0 0} facets exposed could be synthesized. The tetrahexahedral Au–Pd core–shell catalyst with entirely high-index {7 3 0} facets exhibited a higher electrocatalytic activity for the oxidation of ethanol than that of the other two Au–Pd heterostructures with low-index facets. Lee et al. [93] synthesized Au–Pd rhombic dodecahedra nanocrystals with high-index {1 1 0} facets by coreduction of Au and Pd precursors with ascorbic acid in the presence of cetyltrimethylammonium chloride. The Au–Pd nanocrystals exposed high-energy {1 1 0} surfaces exhibited higher SERS and electrocatalytic activities than {1 1 1}-faceted nanoparticles. Lee et al. [94] synthesized Au@Pd nanocrystals with variable high-index facets through a controlled heteroepitaxial growth process on preformed Au trisoctahedral nanocrystal islands. Recently, Han et al. [95] demonstrated the heteroepitaxial growth of the Pd nanoshells on the high-index-faceted tetrahexahedral and trioctahedral Au nanocrystals in aqueous medium. These high-index-faceted Pd nanoshells exhibited higher catalytic activities than Pd nanocubes with low-index {1 0 0} facets for the Suzuki coupling reaction. 4.4. Au–Pt, Ru–Pt, Ag–Ag2 S nanocrystals Au@Pt nanocolloids with dendritic Pt shells were synthesized by reducing both H2 PtCl6 and HAuCl4 species in the aqueous solution containing low-concentration Pluronic F127 surfactant as a structural-directing agent. The Pt shell thicknesses on Au cores can be easily tuned by controlling the Pt/Au molar ratios.

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Fig. 17. An epitaxial growth of bimetallic core–shell nanocrystals: from Au nano-octahedra to Pd and Ag nanocubes. (a, b) SEM and TEM images of the overall morphology of Au@Pd core–shell nanocubes. (c) STEM images of the octahedral Au core seed within a cubic Pd shell and cross-sectional compositional line profiles of a Au@Pd nanocube. (d) High magnification TEM image of a Au@Pd core–shell nanocube. The inset is the SAED pattern taken from individual nanocube. (e, f) SEM images of Au@Pt nanospheres. The dashed frames indicate the core area of particles. Reproduced with permission from Ref. [74]. Copyright 2008, American Chemical Society.

The Au@Pt core–shell nanocatalysts exhibited enhanced activity for the methanol oxidation reaction [96]. The bimetallic Au–Pt nanorods were grown in situ and embedded into thermosensitive core–shell microgel particles by a novel two-step approach. Firstly, 6.6 nm × 34.5 nm Au nanorods had been homogeneously embedded into the shell of poly-N-isopropylacrylamide networks. Then platinum was preferentially deposited onto the tips of Au rod-shaped seeds embedded in microgel particles to form dumbbell-shaped Au–Pt nanocrystals. These nanocatalysts had the high catalytic performance for 4-nitrophenol reduction [97]. The synthesized Ru@Pt core shell catalyst for preferential oxidation of carbon monoxide in hydrogen was published by Alayoglu et al. [98] Xia and co-workers have introduced a general approach to

produce hollow nanocages and nanocubes of metals by using the galvanic-replacement reaction principle [99]. The products show similar visible-NIR tunability by control of the wall thickness and void size of the particle. This approach has been used to produce a variety of hollow/porous Au-, Pd-, and Pt-based nanoparticles from Ag nanoparticles. The hollow/solid Ag2 S/Ag heterodimers were synthesized at room temperature through synthesis of monodisperse Cu2 O solid spheres, conversion of the Cu2 O to CuS hollow spheres, and ion-exchange of the resultant CuS to Ag2 S hollow spheres. The Ag@Ag2 S nanohybrids were formed via the photocatalytic reduction of the metallic silver phase on the resultant Ag2 S heterodimer nanocrystals in the existence of the organic ligand ethylenediamine. Such highly asymmetric heterodimers exhibited

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strong bactericidal effects on Escherichia coli K-12 upon UV light irradiation [100].

5. Some applications in biomedicine of biomolecule-conjugated gold nanocrystals The fields that have recently been impacted by the advancement in nanostructured materials are biology, biophysics, and medicine [101–103]. The ability to integrate noble metal nanoparticles into biological systems has had the greatest highlight in biology and medicine due to the unique interaction of them with light. Remarkably, gold nanoparticles with diverse shapes are considered excellent nanosources of heat, so they can release a drug upon heating and be used in anticancer drug delivery systems and photothermal cancer treatment [104]. Therefore, many studies on the fundamental interactions and effects of noble metal nanomaterials on living systems for cancer treatment were reported recently. In this section, we describe the interesting optical properties of Au nanostructures and discuss recent research advances in the biomedical applications. The gold nanomaterials are collected as typical samples for the biomedical applications which were qualitatively published recently. The dependence of the size, shape, composition of metal nanostructures on the plasmonic properties for biomedical applications are discussed. The examples of the applications of gold nanostructures provided herein can be readily generalized to other areas of biology and medicine because the plasmonic nanomaterials exhibit great range, versatility, and systematic tunability of their optical attributes. Surface modification of Au nanoparticles is essential for cellular targeting. Such modification of antibodies and other targeting moieties is usually achieved by the adsorption or chemical conjugation of the ligand to the gold surface. Various surface-modification methods were recently developed for the gold nanomaterials and would endow compatibility in the biological environment. Using potential strategies such as antibody–antigen or ligand–receptor interaction, one can actively target and deliver the metallic gold nanostructures to specific cancer tissues [105]. For example, the hydrophilic polymers were end-capped with a disulfide could be efficiently grafted onto metal nanoparticles in aqueous medium. Some other authors have used polyethylene glycol (PEG) ligand to attach the lysine-capped Au nanoparticles through lysine-terminated PEG link [106]. The targeted delivery of gold nanoparticles to solid tumors is one of the most important and challenging problems in cancer nanomedicine. It was recently observed that colloidal gold nanoparticles were found in dispersed and aggregated forms within the cell cytoplasm and provide anatomic labeling information. The anti-EGFR antibody conjugated nanoparticles homogeneously bind to the surface of the cancer type cells with greater affinity than to the noncancerous cells [13]. These results were detected by using SPR scattering imaging and SPR absorption spectroscopy, in which a relatively sharper SPR absorption band with a red shifted maximum compared to that observed on noncancerous cells. Remarkably, gold nanoparticles could serve as light-activated nanoscopic heaters useful for the selective laser photothermolysis of cancer cells. Gold species conjugated to antibodies can be selectively targeted to cancer cells without significant binding to healthy cells. In general, the routes for nanoparticle delivery are mainly based on an “active” mechanism and a “passive” mechanism. In the active mode, molecule ligands including antibodies or peptides are used to recognize specific receptors on the tumor cell surface. In the passive mode, nanoparticles without targeting ligands are accumulated and retained in the tumor interstitial space mainly. In both mechanisms, the nanoparticles in the bloodstream must first move across the tumor blood vessels.

As an example, Huang et al. [107] revealed that gold nanospheres conjugated to anti-EGFR antibodies specifically target the cancer cells as shown by the dark-field imaging (Fig. 18). The cancer cell surface was confirmed by strong LSPR scattering from gold nanoparticles bound specifically to the EGFR antibodies on the cancer cells. Because the gold nanoparticles dispersed randomly due to nonspecific binding, cancer cells could be easily identified from the healthy cells. This approach is quite general to identify the cancer cells in liver bodies. Recently, anti-EGFR antibodyconjugated nanocages and PEGylated nanocages were synthesized and then incubated with U87MGwtEGFR cells, in which the twophoton microscopy images of the gold nanocages with anti-EGFR significantly increased the cellular binding and uptake. These results might even propose a new method, because Au nanoparticles can be used alone as an anticancer therapeutic material if conjugated to the proper nuclear-targeting ligands. The cytokines are arrest observed here should be a general effect for other types of nanoparticles [108]. In all these cases, the gold nanoparticles were excited by a broad white-light source, but only light frequencies corresponding to the LSPR are scattered strongly. The nanoparticles were seen as bright spots with a color corresponding to the LSPR frequency on a dark background. The dark-field microscopy technique can be utilized very effectively for the molecular-specific imaging of biomolecules by integrating the gold nanoparticles with specific targeting molecules. Gold nanocages can be prepared through a galvanic replacement reaction between Ag nanostructures and HAuCl4 precursors. By controlling the amount of HAuCl4 added, one can tune the surface plasmon resonance peaks of the Au nanocages into the NIR. The optical properties of Au nanocages for use as contrast agents in optical coherence tomography and as transducers for the selective photothermal ablation of cancer cells have been tailored. The results showed an improved optical coherence tomography image contrast when Au nanocages were added to tissue phantoms. Also, the selective photothermal destruction of breast cancer cells in vitro had been detected when immunotargeted Au nanocages were used [109]. The heat generated from photothermal effect can also be used directly for therapy due to a process known as hyperthermia. Irradiation of the cancer cells selectively labeled with the nanoparticles with a laser of frequency overlapping with the LSPR absorption maximum of the nanoparticles. This resulted in selective heating and destruction of cancer cells at much lower laser powers than those required to destroy healthy cells to which nanoparticles do not bind specifically. Xia group illustrated that the cancer cells were exposed to a temperature above 42 ◦ C for several minutes and then they can be irreversibly damaged due to protein denaturing and membrane disruption [39]. A comparison of the temperature increase with time when the tumor of a mouse injected with either a suspension of nanocagesor or a sham treatment of saline solution was irradiated with a NIR continuous wave laser. It is clear that in the presence of highly absorbing Au nanocages the laser irradiation resulted in a significant increase in temperature, while almost no change was observed for the sham treatment. The targeted delivery of nanoparticles to solid tumors is a key task in the development of cancer nanomedicine for in vivo molecular imaging and targeted therapy. The antibody-linked Au nanoparticles generating from 30 nm Au nanoparticles coated with polyethylene glycol were bioconjugated with an arginine–glycine–aspartic acid peptide. The products were incubated with cancer cell samples for nuclear targeting of gold nanoparticle. Fig. 19 displays typical snapshots of the movies taken of cancer cell division. In the absence of Au nanoparticles, cancer cells begin the process of cytokinesis at 45 min. After complete cleavage furrow contraction, daughter cells were linked together by a cytoplasmic bridge, which was extended over time with the midbody at its center. Abscission occurred after 2 h, and the two daughter cells separated completely to form independent

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Fig. 18. Using rod-shaped Au nanocrystals and covalently conjugated peptide ligands to study a reexamination of active and passive tumor targeting. Dark-field imaging of Au nanorods binding to cultured A549 cancer cells. The data showing the binding of peptide-conjugated Au nanorods to cultured A549 lung cancer cells, and negligible binding of nontargeted particles to the same tumor cells. Cells were incubated with 1 nM Au nanorods for 2 h at 37 ◦ C in the culture medium. Reproduced with permission from Ref. [107]. Copyright 2010, American Chemical Society.

Fig. 19. Real-time images of cancer cell division showing an apparent cytokinesis arrest (B4) followed by binucleate cell formation (B6, B7) in the presence of 0.4 nM nucleartargeting gold nanoparticles (RGD/NLS-AuNPs). This phenomenon was not observed in untreated cancer cells (A1-7) or cancer cells under other conditions. Red stars indicate the nuclei. Scale bar: 10 ␮m. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Reproduced with permission from Ref. [110]. Copyright 2010, American Chemical Society.

cells. The results show an evidence that nanomaterials localized at the cell nucleus can specifically affect cellular function. These observations have implications in understanding the basic interactions between nanomaterials and live systems and have a huge impact on the fields of biology and medicine [110]. The CTAB-capped Au nanorods with an aspect ratio of ∼3.9 were modified by negatively charged poly-4-styrenesulfonic acid (PSS) and a subsequent deposition of positively charged polyethyleneimine (PEI). The PEI/PSS-modified Au nanorods were conjugated with anti-EGFR for specific targeting of the A549 cells. The cancer cells were treated with equal amounts (8.7 × 1010 particles/mL) of anti-EGFR-conjugated Au nanostructures by irradiation of 808 nm continuous-wave laser. The result showed their high efficacy in NIR photothermal destruction of

cancer cells and multiphoton imaging contrast [111]. Shi et al. [112] utilized the prepared drug-loaded poly(lacticco-glycolic acid)-Au half-shell nanoparticles to investigate the tumor-specific delivery of heat and drugs in human cervical cancer cell line. Naked gold nanoparticles were functionalized with a thiolated poly(ethylene glycol) (PEG) monolayer capped with a carboxylate group. The active [Pt(1R,2R-diaminocyclohexane)(H2 O)2 ]2 NO3 component was added to the PEG surface to yield a supramolecular complex for improved anticancer drug delivery [113]. Significantly greater cell killing was observed when doxorubicin-loaded hollow Au nanospheres (∼40 nm) was incubated into MDA-MB-231 cells under irradiated with NIR light that attributable to both hollow Au nanospheres-mediated photothermal ablation and cytotoxicity of released free doxorubicin [114]. By functionalizing the

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Fig. 20. Anti-EGFR antibody conjugated Au nanoparticles applied to oral cancer diagnostics. Light scattering images of HaCaT noncancerous cells, HOC cancerous cells, and HSC cancerous cells without gold nanoparticles. Two different images of each kind of cells were shown to test reproducibility. The weak greenish scattered light from the cells shows large difference in the sizes and shapes of the three different types of cells. Scale bar: 10 ␮m for all images. Reproduced with permission from Ref. [118]. Copyright 2005, American Chemical Society.

surface of Au nanoparticles with a prostate-specific membrane antigen (PSMA) RNA aptamer, Jon et al. [115] established a targeted molecular imaging system capable of specific imaging of prostate cancer cells that express the PSMA protein. The resulting PSMA aptamer-conjugated gold nanoparticles showed more 4-fold greater computed tomography intensity for a targeted LNCaP cell than that of a nontargeted PC3 cell. In addition, the PSMA aptamerconjugated Au nanoparticles after capping of doxorubicin were significantly more potent against targeted LNCaP cells than against nontargeted PC3 cells. The use of plasmonic Au nanostructures can reduce the laser energy necessary for selective tumor cell destruction. The quantitative active and passive tumor uptake studies for elongated gold nanocrystals covalently conjugated to tumor-targeting peptides were recently published by El-Sayed et al. [107]. Both SPR scattering imaging and SPR absorption spectroscopy from antibodies conjugated gold nanoparticles were found to distinguish between cancerous and noncancerous cells. This makes either technique potentially useful in cancer diagnostics. Huang et al. [116] also found that the aggregated gold nanoparticles made to decrease the use of laser energy and increase the efficiency of cancer cell destruction. The aggregated Au nanoparticles were formed via the conjugation of anti-EGFR antibodies to Au nanoparticles. The biocompatible aggregated Au nanoparticles using for targeting the HSC 3 cancer cells were responsible for enhanced photothermal destruction of the cells. Lu et al. [117] recently revealed a simple colorimetric and highly sensitive two-photon scattering assay for highly selective and sensitive detection of breast cancer using a multifunctional oval-shaped gold-nanoparticle-based nanoconjugate. Moreover, the surface plasmon resonance scattering and absorption of anti-EGFR antibody conjugated gold nanoparticles in cancer cells were also investigated to diagnose the oral cancer, as illustrated in Fig. 20 [118]. It believes that the assay had enormous potential for application of cancer cell detection from clinical samples.

6. Concluding remarks This article provides an overview of very recent progress in many research groups relative to kinetic formation model and

aqueous-based synthetic approaches of many types of size/shapecontrolled single and binary noble metal nanocrystals. Compared to traditional organic methods, aqueous-based approaches were proved to be an efficient pathway for well-shape and perfectly crystal structure of the aqueous-dispersed single-crystalline anisotropic metal nanocrystals in high yields, which favors for the biotechnological applications. The effects of the reaction parameters (including precursor, surfactant/additive, foreign ion, reducing agent, temperature and time reaction, etc.) on the nucleation and growth kinetics of particle size/shape control were found definitively in the surfactant-assisted synthesis. Interestingly, the aqueous-dispersed noble metal nanocrystals would become an active agent in biomedicine as they are conjugated with antibodies (protein and/or DNA). Furthermore, the ability to manipulate the morphology, surface modifications, molecular targeting of antibody-conjugated metal nanocrystals precisely make them ideal for a variety of interesting biomedical applications. Remarkably, the antibody-conjugated Au nanocrystals serve as light-activated nanoscopic heaters useful for photothermal cancer treatment agents and further in optical imaging probes and drug delivery. With respect to the fast improvement of ability in the fabrication of biomolecule-multifunctionalized metal nanomaterials, we may expect that the impact of the nanotechnological field on our daily lives would grow perfectly in the near future.

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