Synthesis of Polymer Stabilized Silver and Gold

Copyright © 2007 American Scientific Publishers All rights reserved Printed in the United States of America

Journal of Nanoscience and Nanotechnology Vol. 7, 1–17, 2007

Synthesis of Polymer Stabilized Silver and Gold Nanostructures S. K. Bajpai1 ∗ , Y. Murali Mohan2 ∗ † , M. Bajpai1 , Rasika Tankhiwale1 , and Varsha Thomas1 1

Department of Chemistry, Polymer Research Laboratory, Government Model Science College, Jabalpur 482001, India 2 Department of Polymer Science and Technology, Sri Krishnadevaraya University, Anantapur 515003, India

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This review article is focused on the various approaches that have been made to synthesize metal nanoparticles with predetermined shape, size, and fair stability. Due emphasis has been given on polymer stabilized nanoparticles. In addition use of other varieties of stabilizers like inorganic salts, organic compounds, organic solvent, biological molecules, etc. have also been discussed. Finally, formation of two and three-dimensional nanostructures like nanowires, nanodiscs, nanoprisms has also been revealed.

Keywords: Reducing Agent, Surfactant, Stabilizer, Nanostructures, Biomedical Applications.

CONTENTS 1. 2. 3. 4. 5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Nanowires and Nanorods . . . . . . . . . . . . . . . . . . . . Synthesis of Nanoprisms, Nanocubes, and Nanodiscs . . . . . . . . Nanoparticle Ring Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. INTRODUCTION In recent past the preparation and characterization of nanostructured materials have become a topic of extreme interest because of their distinctive properties and potential uses in technological applications. It is found that the optic, electronic, magnetic, and catalytic properties of these particles depend upon their size, shape, alignment, distribution, and so on. Therefore, one of the major challenges in nanoparticle synthesis is to control not only the particle size but also the particle shape and morphology as well.1 Novel metal nanocrystallities such as silver and gold provide a more interesting research field due to their close-lying conduction and valence bands in which electrons move freely. The surface plasmon band arises from the coherent existence of free electrons in the conduction band due to the small particle size effect. This band ∗

Authors to whom correspondence should be addressed. Present address: Department of Biomedical Engineering ND-20, Lerner Research Institute, Cleveland Clinic, Cleveland, OH-44195, USA. †

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is dependent on the particle sizes, chemical surroundings, adsorbed species on the surface, and dielectric constant.2–4 The unique feature of the coinage metal nanoparticles is that a change in the absorbance or wavelength provides a measure of the particle size, shape, and inter-particle properties. For small particles less than 2 nm size, the surface plasmon band is strongly damped due to low electron intensity in the conduction band. However, as particle size increases, the density of the plasmon band increases. Here, it should be remembered that both absorbance and scattering contribute to the optical property and the contribution of the latter is insignificant as compared to that of the former for small nanoparticles (≤15 nm).5–7 During the past few decades, many methods have been developed to synthesize polymer-stabilized nanoparticles. These methods include photochemical reduction, electrochemical techniques, chemical reduction, a polyol process, and radiolytic methods.8–13 Most of these methods have been successful to produce nanoparticles but their inherent property to agglomerate of particles is a major draw-back, which arises from Van der Walls interactions. In general terms, obtaining small particle sizes with narrow size distribution and good stability have remained as the most important goal of material scientists and nanotechnologists. The use of polymeric material as steric stabilizer (adsorption or steric hindrance) is one of the promising methods to obtain dispersion of nanoparticles14–16 with fair stability as shown in Figure 1. In the steric stabilization process, the adsorption of polymeric chains on the surface of the particles occurs

1533-4880/2007/7/001/017

doi:10.1166/jnn.2007.911

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Synthesis of Polymer Stabilized Silver and Gold Nanostructures

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Dr. S. K. Bajpai obtained his Ph.D. in 1997 in Polymer Science and has been working on Biopolymers for 13 years. He has published above 50 research papers in various international journals. He has been referee for a number of international journals and also contributed a chapter on “Ion-exchange resins in drug delivery” in a book being published by CRC press, Boca Raton, USA. Dr. Bajpai has worked on various research projects funded by University Grants Commission, India and presently he is working on “Hemodialysis Membranes.”

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Y. Murali Mohan obtained his Ph.D. in Polymer Science from S. K. University, Anantapur (Andhra Pradesh), India, in the year 2005. He worked as post doctoral fellow in Department of Materials Science, Gwangju Institute of Science and Technology (GIST), Gwangju, South Korea (June 2005–March 2006) and Department of Pharmaceutical Sciences, University of Nebraska Medical Center (April 2006–Jan 2007). Currently, he continuing research on Nanogels, Hydrogels, Nanocomposites, Polymer and Magnetic Nanoparticles, Polymer stabilized nanoparticles, etc. for Biomedical Applications, in the Department of Biomedical Engineering, Lerner Research Institute, Cleveland Clinic, Cleveland, OH-44195, USA. He is author for more than 35 research articles and presented/abstracted in various seminars, symposia, and conferences (15). Dr. Manjula Bajpai obtained her Ph.D. in 1993 on “Chemical Kinetics” and has published over 10 research papers in different international journals. She is also co-author in a chapter titled “Ion-exchange resins in drug delivery,” being published by CRC press, in 2007.

Ms. Rasika Tankhiwale obtained her M.Sc. Degree in chemistry in 2004 securing first division from R. D. V. V. Jabalpur (India). She has completed her research work on “Polysaccharides based polymeric hydrogel beads: synthesis, characterization, and drug delivery applications” and published five research papers in various international journals.

Ms. Varsha Thomas obtained her M.Sc. Degree in Chemistry, in 2005, securing first division from Government Model Science College, Jabalpur (India). Presently, she is carrying out her research work on hydrogel–silver nanoparticle composites from Polymer Research Laboratory, Government Model Science College, Jabalpur (India).

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highly stable nanosized particles in the three-dimensional network structures.

2. SYNTHESIS OF NANOPARTICLES

Steric stabilization of nanoparticles using polymeric chains.

where polymeric chains/coil dimensions are usually larger than the attraction forces between colloidal particles. The extensive stabilization depends on adsorbed polymer chain strength, interaction with particles, dense layers, etc.17–19 Whereas polymers, which are covalently linked onto the nanoparticles surfaces result in stronger adsorption of polymer chains and their de-sorption cannot take place at any circumstances (solvent effects or temperature variations).20 21 A typical nanostructure preparative scheme is depicted in Figure 2. In this review, we shall overview several major synthetic approaches that have been made in recent past to synthesize stable coinage-metal nanostructures under a widerange of conditions to yield different nanostructures like spherical nano-particles, nanoprisms, nanorods, nanodiscs, etc. It will also cover the recent expertise of achieving

Fig. 2.

Different nanostructures preparation using polymers as stabilizers.


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Fig. 1.

Nanoparticles of noble metals are of great interest today due to their possible applications in microelectronics.22 In recent years, with the higher integrated density of the electronic components, there are growing demands for the decrease in the thickness of the conductive films and a further narrowing of the width of printed circuits and the space between these circuits. It is thus required that the powders (to form the conductive films and printing the circuits on the basement) composing the paste should have as small diameter as possible. Therefore, synthesis of these particles is an important task. A number of chemical methods are being used to synthesize silver colloidal metal nanoparticles. In most of the available chemical methods, various reducing agents have been attempted, including hydrazine, ferric ions, ascorbic acid, etc.23–25 Depending upon the reducing power of these reagents, the synthesis reactions are carried out at various temperatures to achieve reasonable rates. Most recently, silver nanoparticles have been prepared by reducing the AgNO3 in poly-N-vinyl-2-pyrrolidone (PVP) aqueous solution by using glucose as reducing agent and sodium hydroxide (NaOH) as accelerator.26 The formation of silver nanoparticles can be confirmed easily by comparing the UV-spectra of solutions of pure silver nitrate and silver nanoparticles. It is very clear that the peak obtained at 300 nm in pure AgNO3 solution corresponds to Ag+ ions, while for the solution of silver nanoparticles the characteristic peak at 300 nm is decreased dramatically and a new peak appears at about 420 nm, thus indicating the presence of Ag nanoparticles in the solution. It has been mentioned by many researchers that the addition of alkaline ion is necessary to carry out the reduction of metallic ions.27


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Let us consider the reduction of silver ions by dextrose in the presence of OH− ions.28 The reaction may simply be written as:

N,N-dimethylformamide (DMF)36 and formamide37 can act as powerful reductants for silver or gold salts. In fact, DMF was previously studied for the reduction of Ni (IV) to Ni (II) in alkaline medium.38 This study showed that DMF could act as an active reducing agent under suitable conditions. The reduction of Ag+ ions into silver nanoparticles can easily be carried out by DMF.39 The reduction can lead to the formation of either thin film of silver nanoparticles electrostatically attached onto glass surfaces or stable dispersion of nanoparticles depending upon the stabilizer present in the system. A general scheme for reduction of Ag+ ions can be given as:

Ag+ + 21 C6 H12 O6 + 21 H2 O

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3 + → Ag + 21 C6 H11 O− 7 + 2H

When the reaction is carried out with different concentration of sodium hydroxide, it is observed that Ag+ ions are not reduced by dextrose when OH− ions are absent in the reaction system, thus indicating the necessity of OH− ions to this reduction reaction. It also suggests that there is some energy barrier to this reaction and when OH− ions are added, there occurs spontaneous increase in the silver conversion and this increase is proportional to the quantity of NaOH added. The formation of Ag nanoparticles in the presence of OH− ions may be explained as follows: When OH− ions are added to AgNO3 solution in the presence of reducing agent (say, dextrose), it is possible that at the very beginning of this synthesis process some Ag2 O is formed: 2Ag+ + 2OH− ↔ Ag2 O + H2 O The product Ag2 O is now reduced and silver particles are generated. Ag2 O + CH2 OH → CH2 OH

CHOH4 CHOH4

CHO + 2PVP COOH + 2AgPVP↓

In the process of reaction, the disperser (polymer) forms a protection layer on the surface of Ag2 O or Ag particles. Similarly, Huang et al.29 also studied the mechanism of the formation of silver in a basic 2-propanol system at a very low temperature (−45 C) and they detected the existence of Ag2 O via weak signals from electron diffraction. They used the term “autocatalytic” reaction to describe the reduction of silver in the presence of colloids. Apart from various reducing agents like dextrose, ascorbic acid, monosodium glutamate, etc.,30 a number of organic solvents can also be exploited for the synthesis of nanoparticles. In fact, several examples exist on the reduction of metallic salts by organic solvents. Probably, the most popular one is ethanol, which has been used wayback by Toshima and coworkers for the preparation of metal nanoparticles suitable for catalytic applications.31 32 Another interesting example found is the Figlarz’s polyol method, which was initially developed for the formation of larger colloidal particles,33 though later it was also used for the production of nanoparticles. Methanol has also been used for the preparation of silver colloids in basic conditions.34 Related to these processes, we can also mention the reduction of noble metal salts by non-ionic surfactants and more specifically by those with a large number of ethoxy groups, to which the reducing ability has been assigned.35 It has also recently been shown that both 4

HCONMe2 + 2Ag+ + H2 O → 2Ag0 + Me2 NCOOH + 2H+ This mechanism is supported by a measured increase of conductivity as reaction proceeds, which indicates that the larger Ag+ ions are progressively exchanged with more mobile H+ ions. Actually, the carbamic acid formed can easily decompose as: Me2 NCOOH → CO2 + Me2 NH Though the reaction is only favored at high temperature, while at room temperature the reverse reaction is preferred. A basic difference of this method as compared to the ethanol reduction method40 is that it proceeds at a reasonable rate even when performed at room temperature and in the dark. Moreover, this reaction is performed without de-gassing the solution or taking any special care with regards to the presence of oxygen. This reducing ability in the mild conditions, points towards a larger tendency of this solvent for the reduction of Ag+ than that shown by ethanol or other organic solvents. In fact, the reduction of Ag+ ions by DMF reveals some interesting facts. When the pre-calculated quantity of AgNO3 is mixed in DMF at room temperature, the color shifts from light yellow to dark brown, through orange and olive green and then starts concentrating on the glass surfaces in contact with the solution while the solution itself becomes increasing clearer indicates the number of Ag colloidal particles decreased in the solution. It means that Ag nanoparticles stick onto the glass surfaces due to electrostatic attraction between the particles with excess positive charge41 (from adsorption of unreacted Ag+ ions and negatively charged SiO2 surface). Observation of such surfaces with an atomic force microscope showed that individual metallic Ag particles are indeed attached onto the glass. Observation of such surfaces with an atomic force microscope shows that individual metallic Ag particles are indeed attached onto the glass. It is worth being mentioned here that by taking small concentration of Ag+ ions and relatively shorter deposit time, quite homogenous films of reasonably monodisperse nanoparticles can be obtained. If a suitable stabilizing agent is added to the Ag+ solution in DMF, stable silver colloids can be J. Nanosci. Nanotechnol. 7, 1–17, 2007

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Fig. 3. UV-Vis spectra of reducing reactivity of silver salt with increase of PEG chain length. Reprinted with permission from [47], C. Luo et al., J. Colloids Interf. Sci. 288, 444 (2005). © 2005, Elsevier Inc.

glycol is an environmentally benign material. Its polymer, poly(ethylene glycol) is also used to synthesize silver nanoparticles.47 The major attraction of this process is that no additives, such as solvent, surfactant, or reducing agent are needed. The size and shape of silver nanoparticles are sensitive to the reactant temperature and concentration of the precursor. A denser concentration of precursor facilitates the formation of large and highly crystalline nanoAg particles. The other metal nanoparticles, such as palladium can also be successfully fabricated as well by this technique. It is important to note that the reducing reactivity of PEG is rapidly increased with the increase of its chain length (Fig. 3). Moreover, the temperature plays a key role in governing the reduction efficacy as well as size of the nanoparticles. When the reaction is carried out at three different temperatures, 80, 100, and 120 C, the sizes of the corresponding nanoparticles formed are 10, 20, and 80 nm, respectively as shown in Figure 4.47 From the above discussion it understands that stabilization of nanoparticles is essential, no matter how they are produced. In fact, several mechanisms for the formation

Fig. 4. (a–c) TEM micrographs of silver nanoparticles prepared at different temperatures 80, 100, and l20 C for 1 h, respectively. (The concentration of AgNO3 before the reaction is 2.5 wt%.) Reprinted with permission from [47], C. Luo et al., J. Colloids Interf. Sci. 288, 444 (2005). © 2005, Elsevier Inc.


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obtained. For example, when silane coupling agent,42 3-aminopropyltrimethoxysilane (ASP) is added to AgNO3 solution in DMF, a stable colloidal solution of Ag nanoparticles is obtained due to complexation between Ag atoms and amine functionality. In this condition no adsorption of silver onto the walls of the beaker takes place, thus indicating the fair stabilization of Ag nanoparticles by APS. The major disadvantage of carrying out reduction of Ag+ ions by DMF at room temperature is that the formation of Ag particles is very slow. So, it seems more advantageous to perform the reduction at higher temperature. This effect is obvious from the observations that at 60 C the reduction of silver ions (in equimolar solutions of AgNO3 and APS in DMF) is completed in about 2 days, while at 100 C it takes just a few hours and it is basically instantaneous at reflux (152–154 C). A different approach to high-temperature reduction is the use of microwave heating. Several examples have been reported on the microwave-induced formation of nanoparticles in solution.43 The advantages of this method are based on a quick and very uniform heating of the whole reaction system with no need of stirring, so that the reaction could take place simultaneously through out the volume of the sample. However, in the case of DMF, this method is experimentally more difficult to implement due to possible harm to the oven. The reaction must be carried out in successive short exposures with intermediate cooling. This leads to colloids with more symmetrical visible spectra which points to narrower size distribution. Like DMF, formamide is also a very common solvent used for the synthesis of metal nanoparticles.44 45 Dispersions of silver nanoparticles are prepared by mixing the required concentration of stabilizers, PVP or SiO2 in formamide followed by addition of silver salt at room temperature. With time, the solution gradually turns dark brown color and a stable dispersion of silver nanoparticles will be obtained.46 As compared to the major reducing agents reported to date for the preparation of metal nanoparticles such as hydrazine, DMF, formamide and ethanol, ethylene


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of metal nanoparticles with a stabilizer are reported; these vary according to the reaction materials and the reduction methods. Zhang et al.,48 synthesized silver nanoparticles stabilized with polyvinyl pyrrolidone (PVP) by using hydrazine as a reducing agent. They suggested a mechanism of PVP protection, in which PVP promotes the nucleation of metallic silver because silver ions are easily reduced by the lone pair of electrons available on nitrogen of PVP. Henglein et al.49 suggested a different method for the growth of silver nanoparticles, in which reduction is produced by -irradiation and citrate is used as stabilizer. In their report, the authors proposed two possible growth mechanisms: the condensation of small silver clusters, and the reduction of silver ions onto silver particles via radicalto-particle electron transfer. However, they argued that the capping effects of citrate observed using their approach could not apply to other systems, particularly not to those using a polymeric stabilizer. Park and co-workers50 investigated an approach that relies on the formation of monodispersed colloids. They obtained gold nanoparticles rapidly by adding an aqueous solution of HAuCl4 into an aqueous solution of ascorbic acid. They suggested that primary particles were formed by burst nucleation, followed by their aggregation into final particles. Likewise, PastorizaSantos et al.51 prepared silver nanoparticles from AgClO4 and PVP by two different methods namely reflux and microwave. They showed that particle formation mechanism depends on the reduction method. Most recently, PVP has been employed as a stabilizer for -radiation induced formation of Ag nanoparticles,52 and a three-step mechanism has been suggested for nanoparticle formation. In the first step the silver ions interact with PVP, then in the second step the silver ions that are exposed to -irradiation are reduced to silver atoms then these silver atoms aggregate at close range. These aggregates are the primary nanoparticles. Finally, these primary nanoparticles coalesce with other nearby primary nanoparticles or interact with PVP to form larger aggregates, which are the secondary (final) nanoparticles. In addition to PVP, the stabilization of Ag nanoparticles can also be done using carbon disulphide (CS2 ). The CS2 -stabilized silver nanoparticles of ∼6 nm in diameter are prepared in aqueous solution via chemical reduction of Ag+ ions by reductant such as KBH4 in the presence of CS2 53 The product is proved to be Ag by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and electron diffraction (ED) patterns. In the UV-visible spectra, an intense surface plasmon is built-up at 350–500 nm, and it is centered at 395 nm after the reduction of Ag+ ions. This is usually assigned to a reaction product of Ag with CS2 because of the strong affinity of S on surface of Ag particles. The silver nanoparticles in the presence of CS2 remain stable for months at room temperature. It has also been reported that the biologically and environmentally friendly conditions are favorable for

the production of Au and Ag nanoparticles in water within 5–10 min (250 times faster than any other previous methods) at room temperature.54 Recently, polymer-assisted synthesis of metal nanoparticles has received considerable attention55 because of (1) the small concentrations of homopolymers and block co-polymers capable of stabilizing nanoparticles effectively by steric stabilization, (2) the polymer containing an appropriate functional group serving as both reducing and stabilizing (capping) agent, (3) the easy variation of the core size of the nanoparticles by the variation of the polymer/metal salt ratio, and (4) the ease of preparation of metal-polymer nanocomposites.

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The design and development of nanoparticles and nanostructural materials have opened new opportunities for building up functional nanostructures.56 Water-soluble polymers such as PVA, PVP, PEG, gum acacia,57 58 cellulose based polymers59 have received much attention because they can act as both reducing agent as well as stabilizer. Hydrophilic functional groups are capable to reduce the metal salts and their polymeric chain networks facilitate the stabilization process. Very recent examples include the growth of nanoparticles in hydrophilic block co-polymers,60 the use of microphase separated morphologies of block co-polymers to sequester metal particles on surface,61 the use of functional polymers to mediate the assembly of gold nanoparticles,62 controlling particle spatial distribution based on dendrimer generation,63 and the utilization of functional block co-polymer containing two structurally different blocks to fabricate nanowires consisting of [111] oriented crystalline silver.64 These methods are highly successful in creating ordered nanostructures with specific arrangements and properties, and the size and structures of nanoparticles can be manipulated in a fairly wide range. However, the cost and the complicated procedures are the emerging problems. Therefore, many recent research efforts are focused on the development of cost-effective and simple methods to fabricate nanoparticles for both industrial applications and fundamental studies.65 Recently, a novel and simple method has been proposed to fabricate Ag nanoparticles which involve the use of functional two-armed polymer with a crown ether core66 (Fig. 5). In the solution containing Ag+ and the designed polymer [poly(styrene)]-dibenzo18-crown-6-[poly(styrene)] some aggregations composed of Ag+ and polymer are formed due to the complexing effect of crown ether embedded in the polymer with Ag+ . Subsequently, the Ag+ ions in the aggregations are photochemically reduced by visible light to form Ag nanoparticles. The resulted Ag nanoparticles are also surrounded by the polymer for the complexing effect of crown ether with Ag. The size of nanoparticles is greatly influenced by the molecular weight of polymer, Ag+ concentration and J. Nanosci. Nanotechnol. 7, 1–17, 2007

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(b)

polymer concentration. Furthermore, the intensity of the photoluminescence of silver nanoparticles stabilized by the polymer is significantly increased upon photoexcitation at 343 nm. Hussain et al.67 have prepared silver and gold nanoparticles using poly(sodium acrylate) [poly(SA)] ligand in an aqueous medium. Further, the obtained nanoparticles are closely comparable with citrate-stabilized nanoparticles with regard to their shape, stability, and size distribution. It was found that in order to provide fair stability to nanoparticles, nearly 2300 sodium acrylate units must be adsorbed per particle. The formed silver and gold nanoparticles have been depicted in Figure 6. UV-irradiation of an aqueous solution of hydrogen tetrachloroaurate (III) in the presence of poly(sodium acrylate), yields ∼2–5 nm

gold nanoclusters that are protected by poly(acrylic acid) chains.68 Raveendran et al.69 reported a simple and green methodology for the synthesis of Au, Ag, and its alloy nanoparticles using glucose as reducing agent. A rigid polymer namely poly(N,N-dihexylcarbodiimide) has been recognized to give remarkable stability to gold nanoparticles through its helical rigid polymeric chains.70 Figure 7 clearly represents how this polymer stabilizes nanoparticles. A number of homopolymers and functionalized block co-polymers are useful for the preparation of functional nanoparticles.71 Poly(styrene)-block-poly(4(or 2)-vinylpyridine) (PS-b-P4(2)VP) and PEO-b-P4-(2)VP, PS-b-PEO block copolymers have been used as templates for the synthesis of nanoparticles and the nanoparticles growth in the micelle of the block copolymer.72–75 Larger size of the micelle core (≈10 nm) and nominal stability of micellular structures of the block copolymers leads to aggregate to form clusters with various shapes and sizes. Ishii et al.76 have successfully synthesized gold nanoparticles in an aqueous medium with -biotinyl-poly(ethyleneglycol)b-poly[2-(N,N-dimethylamine)ethylmethacrylate]. To get improved templeting property, unimolecular core-shell block copolymers would be another option.56 However, star block copolymer PEO-b-PCL effectively produced well-defined gold nanoparticles.77 In this study, star block copolymers with five-arm PEO core and PCL outer blocks have been used. The nanoparticles easily grow in the core of star block copolymers as shown in Figure 8. Amphiphilically modified polyethyleneimines provide hybrids of silver particles of 1 to 2 nm in size and the nanoparticles are formed, without aggregations in highly branched polymeric networks derivatives (Fig. 9).78 79 In general, polymers that contain functional groups capable of coordinating with metal particles are successfully used for stabilization of nanoparticles. However, hydrophobic polymers such as poly(styrene), poly(methyl

Fig. 6. TEM micrograph of (a) silver nanoparticles and (b) gold nanoparticles formed using sodium acrylate (SA); and (c) gold nanoparticles formed using poly(sodium acrylate), 2100 g/mol. Reprinted with permission from [67], I. Hussain et al., Langmuir 19, 4831 (2003). © 2003, American Chemical Society.


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Fig. 5. (a) Functional two-armed polymer with a crown ether core and (b) fabrication of silver nanoparticles in crown ether core of polymer. Reprinted with permission from [66], J. Gao et al., Langmuir 20, 9775 (2004). © 2004, American Chemical Society.



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Fig. 7. Formation and stabilization of gold nanoparticles by poly(N,N-dihexylcarbodiimide) rigid polymer chains. Reprinted with permission from [70], Y. Liu et al., Langmuir 18, 10500 (2002). © 2002, American Chemical Society.

methacrylate), and poly(t-butyl methacrylate) are unable to co-ordinate with metal nanoparticles (Au and Ag). Hence, a coordination group is necessary to stabilize the metal nanoparticles.80 In a recent approach, to achieve block copolymer (PS-PAA or PMMA-PAA) encapsulated gold nanoparticles; the shell cross-linking methodology is followed.81 The obtained gold nanoparticles in the core of micelle are highly stable for longer time. A clear picture depicted in Figure 10, demonstrates the complete approach to obtain stable nanoparticles. Similarly, Sakai and Alexendridis82 performed the single-step synthesis of Au nanoparticles in air saturated aqueous solution of poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (PEO-PPO-PEO). These triblock co-polymers can effectively reduce tetrachloroaurate (III) hydrate at room temperature and stabilize Au nanoparticles. Using PEO-PPO-PEO non-ionic copolymer (Pluronic), it is possible to tailor the size and shape of silver nanoparticles.83 Bae et al.84 explored the possible way to obtain Au nanoparticles through thiol-functionalized pluronic micelles stabilized by shell cross-linking (Fig. 11). Poly(N-isopropylacrylamide), terminated with dithioester groups, is employed as template whose dithioesters provides the thiol groups that are necessary

Fig. 8. Schematic illustration of stabilized gold nanoparticles (gray spheres) employing PEG-b-PCL star-block copolymers (PEG chains in gray, PCL chains in black). Reprinted with permission from [77], M. Filali et al., Langmuir 21, 7995 (2005). © 2005, American Chemical Society.

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for attaching chemically onto the surface of gold nanoparticles.85 Figure 12 illustrates a detailed schematic preparation of gold nanoparticles. This method facilitates the production of gold nanoparticles (∼13 nm) within PNIPAM chains that give potential long-term stabilization and the lack of agglomeration. Recently, the stabilization of Au nanoparticles with poly(p-methylstyrene) containing multiple thioether bonding groups has been described86 (See Fig. 12). Polyelectrolyte capsules are also used to prepare metal nanoparticles (silver, gold) on the shell constituent.87 88 In addition, a current methodology based on photo-induced synthesis of Ag nanoparticles within a polyelectrolyte capsule [Poly(styrene sulfonate) (PSS) (core)/poly(allylamine hydrochloride) (shell)] received much attention due to both practical and fundamental aspects.89 In detail, the controlled photochemical reaction of silver under the conditions shown in Figure 13 occurs mostly inside the capsule of polyelectrolyte and at the same time a minute quantity of Ag particles could also be found find on the shell portion, but it is not observed in the surrounding solution where there are no PSS polymeric chains present.

Fig. 9. Stabilization of non-aggregative silver nanoparticles by amphiphilically modified polyethyleneimines networks. Reprinted with permission from [78], C. Aymonier et al., Chem. Commn. 3018 (2002). © 2002, The Royal Society of Chemistry.


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Fig. 11. Schematic representation of nanoparticles preparation using thiolated pluronic micelle for nanostructure. Reprinted with permission from [84], K. H. Bae et al., Langmuir 22, 6380 (2006). © 2006, American Chemical Society.

Fig. 12. Schematic representation of reduction of dithioester with sodium borohydride provides the thiol group necessary for chemically attaching the thermosensitive polymer on the surface of the as-synthesized Au nanoparticle. Reprinted with permission from [85], M.-Q. Zhu et al., J. Am. Chem. Soc. 126, 2656 (2004). © 2004, American Chemical Society.


In addition to various salts like sodium citrate and various solvents like DMF, Me-OH, formamide, etc., natural polymers such as chitosan has been exploited for stabilization of nanoparticles. Most recently, Sugunan and co-workers90 have synthesized chitosan-capped goldnanoparticles for making heavy-metal ion-sensors. When gold salt solutions (AuCl3 or HAuCl4 ) are reduced by trisodium citrate, the resulting nanoparticles become negatively charged due to the adsorption of excess citrate ions. When these nanoparticles are treated with acidic solution of polycationic chitosan, the chitosan chains are adsorbed on the surface of nanoparticles thus serving a two-fold purpose, i.e., stabilization and surface functionalization. Like other hydrophilic polymers, biological templates (several peptides) are also capable of reducing silver ions into metallic silver.91 Their nucleation and growth of inorganic nanoparticles was confirmed using a PCR-mode 9

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Fig. 10. (a) Methodology for obtaining core/shell gold nanoparticles, (b) encapsulated Au nanoparticles treated with dodecanethiol and PS250b-PAA13, before purification (nanoparticles are localized at the center of the micelles), (c) purified encapsulated 12-nm Au nanoparticles, and (d) purified encapsulated 31-nm Au nanoparticles. Reprinted with permission from [81], Y. Kang and T. A. Taton, Angew. Chem. 117, 413 (2005). © 2005, John Wiley & Sons, Inc.


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Fig. 13. Schematic illustration of the photo-induced formation of silver nanoparticles inside polyelectrolyte capsules. Reprinted with permission from [89], D. G. Shchukin et al., Chem. Phys. Chem. 4, 1101 (2003). © 2003, John Wiley & Sons, Inc.

Fig. 14. Incubation of peptides with 0.2 silver nitrate solution on the laboratory bench top for 1–2 days resulting in the formation of yellowishcolored solution (nanoparticles formation) (colorless solution is silver nitrate without peptide) and TEM image of AG-35 synthesized nanoparticles. Reprinted with permission from [91], R. R. Naik et al., Adv. Funct. Mater. 14, 25 (2004). © 2004, John Wiley & Sons, Inc.

approach. Figure 14 demonstrates the use of peptides AG-4 and AG-35 for obtaining