Reducing Properties of Polymers in the Synthesis of Noble Metal ...

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Reducing Properties of Polymers in the Synthesis of Noble Metal Nanoparticles b

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An-Qi Zhang , Ling-Jian Cai , Li Sui , Dong-Jin Qian & Meng Chen

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Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials and Advanced Materials Laboratory, Fudan University, Shanghai, P. R., China b

Department of Materials Science, Fudan University, Shanghai, P. R., China c

School of Medical Instrument and Food Engineering, University of Shanghai for Science and Technology, Shanghai, P. R., China Version of record first published: 18 Apr 2013. To cite this article: An-Qi Zhang , Ling-Jian Cai , Li Sui , Dong-Jin Qian & Meng Chen (2013): Reducing Properties of Polymers in the Synthesis of Noble Metal Nanoparticles, Polymer Reviews, 53:2, 240-276 To link to this article: http://dx.doi.org/10.1080/15583724.2013.776587

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Polymer Reviews, 53:240–276, 2013 Copyright © Taylor & Francis Group, LLC ISSN: 1558-3724 print / 1558-3716 online DOI: 10.1080/15583724.2013.776587

Reducing Properties of Polymers in the Synthesis of Noble Metal Nanoparticles AN-QI ZHANG,2 LING-JIAN CAI,1 LI SUI,3 DONG-JIN QIAN,1 AND MENG CHEN1 Downloaded by [Fudan University], [Meng Chen] at 18:39 18 April 2013

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Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials and Advanced Materials Laboratory, Fudan University, Shanghai, P. R., China 2 Department of Materials Science, Fudan University, Shanghai, P. R., China 3 School of Medical Instrument and Food Engineering, University of Shanghai for Science and Technology, Shanghai, P. R., China The use of different polymers as both reducing agent and stabilizers has gained wide attention for its simplicity and impressive efficiency when being applied to synthesis noble metal nanoparticles. This article concludes reports published recently on specific polymers used to reduce metal precursors and protect metal nanoparticles. The control of the size and morphology of nanoparticles tuned by changing the polymer structure and experiment conditions has been concluded and the possible redox mechanisms of different polymer-metal systems have been illustrated. Keywords noble metal nanocomposite, polymers, nanoparticle synthesis, reduction mechanism

1. Introduction Nanoparticles, defined as artificial mini-particles with a diameter of 10–100 nm, serve to bridge the gap between macro objects and simple molecules. The physical properties of macro objects are proved to be unconcerned with particle dimensions, while properties of nanoparticles are strictly related to their shapes and dimensions. Metal nanoparticles, in particular, have attracted intense attention of researchers worldwide owing to their unusual characteristics concerning optics, electricity, magnetics, physics, chemistry, and thermotics.1–2 The most frequently investigated metals include noble metals such as Ag, Au, Pt, Pd, and it can be attributed to the following reasons:3–6 • Ag, Au, Pt, and Pd are among the most stable metals, thus they tend to maintain physical properties and can be preserved for a long time. • their surface chemistry characteristics allow organic layers, such as polymers, to ground. Received September 17, 2012; accepted January 7, 2013. Address correspondence to Meng Chen, Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials and Advanced Materials Laboratory, Fudan University, Shanghai 200433, P. R., China. E-mail: [email protected]

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• Ag, Au, Pt, and Pd are relatively inert so that their nanoparticles are resistant to chemical erosion.

Due to their potential application in various scientific fields in the near future, it is of vital importance to develop synthetic methods that are capable of controlling the size, shape, and structure of noble metal nanoparticles. Up to now, intensive efforts have been made to find effective methods to generate and characterize metal nanoparticles of highly-uniform size and high purity. Applied methods, including using chemical reductants such as sodium borohydride, hydrogen, sodium citrate and alcohol, gas condensation, laser radiation, have successfully synthesized metal nanoparticles of a wide range of dimensions.7 In most scientific fields to which metal nanoparticles will be applied, like biochemistry and the medicine field, nanoparticles are required to be soluble in aqueous solution, thus chemical reductions are the most applicable of methods. For example, Au nanoparticles can be generated by adding the aqueous solution of sodium citrate into the boiling HAuCl4 solution.8–9 However, in single-phase reduction with small organic molecules acting as both the reactants and the stabilizers, most of the obtained metal nanoparticles are stable in organic solution such as toluene and diglyme, and not easily dispersed in water.7,10 Typically, scientists use reducing agents to generate metal nanoparticles and dressing agents to stabilize them. When nanoparticles are distributed uniformly in aqueous solution without a protecting layer, they tend to crash into each other to form larger aggregates.11 Generally, the major advantage of using polymers as capping agent is that they stabilize the surface layer of nanoparticles and prevent agglomeration at the same time.12–13 Moreover, a large variety of polymers with different properties are available for metallic nanoparticles synthesis, and researchers have proved that some polymers can be used to fabricate block polymers with more desired properties.4,14 For example, Zhang and coworkers15 have reported the use of a copolymer, which means a linkage between two different polymers; one section of it connects with the nanoparticle, and the other one serves to promote its watersolubility. This way, the composite polymer possesses both advantageous properties and generates more suitable products. Similarly, studies on three-section composite polymers were also conducted.16 Therefore, it is possible to choose a most efficient, non-toxic, and even environmentally-friendly polymer to synthesize the products. Also, the structure of metal particles varies from single component to different dual/poly metal composites, such as core-shell nano-sized particles, consisting of a gold core coated by a silver layer. However, some factors affecting the quality of the products cannot be ignored. For example, excess ions added during the reducing process cannot be fully eliminated; moreover, the reducing agents and the dressing agents might require different experiment conditions. These limitations will reduce the purity and homogeneity of the obtained nanoparticles. Thus, a substitute method of one-step polymer-mediated synthesis has been developed. If we use a polymer with reducing capacity as both the reducing agent and the stabilizer, covering the nanoparticles right after they are synthesized, then highly-stabilized nanoparticles can be produced and the limitations mentioned above can be avoided as well.17 This one-step method introduces no more reducing agents into the reaction system except the polymer, so the product tends to be of high purity. Besides, by adjusting the polymer structure, reaction temperature, reactant ratio, and reacting time, it is possible to control the shape and size of metal nanoparticles. According to some previous experiments, metal nanoparticles synthesized this way are surprisingly stable and homogeneous. Although this one-phase process has just been put forward, much wider application can be forecasted for its low cost and relatively high efficiency.

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There have been a number of literature reviews that discuss the preparation and study of colloidal metal nanoparticles. Some of them focused on the synthesis, characterization, and applications of polymer-coated metal nanoparticles.4,12,14,18–20 In this feature article, we mainly discuss the preparation and study of noble metal nanoparticles with a dual functional reductant and stabilizer. To our knowledge, this is the first and only literature review to date on the reducing properties of polymers in the preparation of metal nanoparticles.


2. Preparation of Polymer-Reduced and -Protected Nanoparticles To investigate reaction mechanisms of different polymers, we roughly divide the researched polymers into four categories: nonelectrolyte homo-polymers, polyelectrolytes, dendrimers, and block copolymers. One polymer might be both a polyelectrolyte and a block copolymer, and this polymer will only be included in one section, though. 2.1 Nonelectrolyte homo-polymers 2.1.1 PVP. PVP is a white or off-white water-soluble solid at room temperature and is rather stable under ambient condition without contacting with strong oxidizing agents. More importantly, PVP is non-toxic and non-irritating to human skin and eyes. Also, PVP is a homo-polymer with polyvinyl backbone, terminal hydroxyl groups, and a large amount of C O double bond; the O atoms of PVP have a strong adhesive force to metal nanoparticles, so it can be easily attached to the surface of nanoparticles. Thus, PVP is an ideal polymer to stabilize metal nanoparticles. As Fig. 2 shows, PVP molecules disperse in a solution and cobridge a surface layer onto Au clusters during particle growth. At first, PVP was only considered as the most widely used stabilizer to protect metal nanoparticles from agglomeration or the capping agent to direct the growth of the product.21–26 However, experiment results have shown that when PVP was used solely as the stabilizer in processes involving another reducing agent, PVP can interfere itself in this reducing reaction.27 So here we only discuss the nanoparticle synthesis methods where PVP is used as both the reducing agent and the capping layer without adding normal reducing agents.27–38 Up to now, three possible mechanisms concerning the reducing capacity of PVP have been concluded, including free radical mechanism, and the oxidation mechanism of the hydroxyl end groups,32,35–36 and C O double bond.39 These mechanisms will be illustrated in the mechanism section. It is also worth noting that the mild reducing capacity and therefore the slow reducing rate of PVP contributes to the formation of plate-like nanoparticles, enabling kinetic control over the nucleation process and growth of complex morphologies with great homogeneity.35 Variation of the molar ratio of PVP repeating units to metal ions, molecular weight (MW) of PVP, reaction time, concentration, temperature, and metal precursors allow for the formation of gold and silver hydrosols of different sizes and shapes. To investigate the influence of varying reaction conditions on products, several studies introduced a series of experiments under changing conditions, from which we can conclude that: 1. Shape, size, and optical properties of the metal particles can be controlled by varying the PVP/metal salt ratio and MW of the polymer. Higher molar ratio of PVP and metal precursor leads to higher yield, narrower size distribution, and greater stability of nanoparticles. TEM and SEM images suggest that the PVP/metal molar ratio has an important effect on the morphologies of the obtained particles. The gradual decrease of the ratio lead to increasing amounts of polyhedral and

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Figure 1. Molecular structures of (1) PVP, (2) PEG/PEO, (3) PAAm, (4) PPY, (5) PVA, (6) PDF, (7) PMHS, (8) PMAA, (9) PDDA, (10) PEI (linear), (11) PSA, (12) PANI, (13) POMA, (14) PoPD, (15) PSSMA, (16) PAMAM, (17) PPI, (18) PEI (hyper-branched).

plate-like structures.27 Also, larger MW of the polymer results in smaller particle size and higher yield of nanoplates in the size-selective synthesis of PVP-stabilized Au NPs under MW heating.28,35,37


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Figure 2. Adsorption and cobridging of (a) PVP molecules in (b) a surface layer via Au-atoms on a gold particle.2 Reproduced from Ram and Fecht29 with permission from American Chemical Society (Color figure available online).

2. Varying reaction time results in different shapes of nano-plates and well-controlled dimensions. For example, if the reactant ratio and MW of PVP are kept stable, TEM images show that Ag nanoparticles evolve from circulars to hexagons and then triangles and lateral dimensions increase throughout the whole process. 3. The relationship between the reaction temperature and size distribution is not a simple linear one. At lower temperatures, the slower reduction rate helps to control the formation of nanoparticles. While an extremely low temperature might continuously produce Ag atoms, which hinders the Ostwald ripening process, resulting in lower productivity.31 Thus, it is necessary to conduct several pre-experiment researches to find the proper reaction temperature. Jiang and co-workers33 examined the effects of changing temperature on gold nanoparticles by varying the preheating time of 10, 30, 45, and 60 minutes, the average sizes of gold nanoparticles were 9.8 ± 2.8 nm, 8.1 ± 2.5 nm, 7.9 ± 2.9 nm, and 13.6 ± 3.5 nm, respectively. Thus, a preheating time of 45 minutes is most suitable. 4. Different metals precursors generate nanoparticles of different morphologies and require corresponding reaction conditions. In general, Pd, Au, and Pt present a similar behavior with Ag. For more active metals, including Fe, Co, Ni, PVP is too weak to be used as the reducing agent.31 Therefore, to synthesize a high-quality product, which means nanoparticles of small dimension, high productivity, homogeneous shape distribution, and good stability, a high molar ratio of polymers and metal precursors, polymers of larger MW, and suitable reaction time and temperature should be guaranteed. Instead of liquid-phase reactions, Debnath and coworkers30 have introduced a solventfree solid-phase synthesis method. Surprisingly, in contrast with normal routes in which PVP shows a mild reducing capacity, this vibration milling synthesis has been proved to be extremely rapid at room temperature. Several other studies have investigated the formation of silver coat on other solid matrix, such as the generation of SiO2 @Ag coreshell nanoparticles,36 and Ag-graphene composite nanosheets.38 The main advantage of the deposit sites is that they prevent the agglomeration of metal nanoparticles, thus increasing their stability. 2.1.2 Poly-(Ethylene Glycol) (PEG) and Poly-(Ethylene Oxide) (PEO). PEG or PEO, refers to a polymer of ethylene oxide (EO). The two names are chemically synonymous,



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while PEG often refers to polymers with a MW below 20,000 g/mol, PEO to polymers with a MW above 20,000 g/mol. PEG and PEO have found use in different applications in the chemical industry for a long time, and numerous studies have examined different reducing capacity of the polymer with different MWs and segmented polymers made up of the two polymers. Compared with widely used reducing agents such as DMF and sodium borohydride, ethylene glycol enjoys an obvious advantage of its environmentally-friendly feature. In addition, the reducing ability of PEO can be tuned by changing its MW. A higher MW leads to higher reducing capacity, smaller sizes of nanoparticles, higher reactivity, and a lower stability capacity.40–41 These results are due to the reason that, as the stabilizing agent, PEO can form inter-molecular and inner-molecular cavities to cover the surface of metal nanoparticles and to bind metal ions. With an increasing MW, the reaction rate becomes faster with an increasing binding of the metal ions to pseudo-crown ether cavities formed by PEO.42–43 Therefore, PEO acts as the reducing agent and the dressing agent in both polar and non-polar solvents and no extra reducing chemicals will be needed in the whole process. Usually, the original mixture needs to be heated to around 100◦ C. To find the most suitable reaction condition of the greatest efficiency and stability, researchers40–41,44–45 have tried various MWs of polymers and different metals including Au, Ag, and Pd. TEM images suggest that PEG-Au measures around 25 nm, and PEG-Ag around 10–20 nm. In these experiments conducted on PEG, a narrowly-distributed size of metal nanoparticles can be achieved. Moreover, the obtained products keep stability for a long time at room temperature. Reducing groups-terminated PEO has different reducing capacity to produce metal nanoparticles. Iwamoto and coworkers41 have investigated the reducing capacity of PEONH2 . In their experiments, the solution of HAuCl4 is mixed with poly-ethyleneoxide diamine terminated (PEO-NH2 ) or PEO-OH, respectively, in the air at around 100◦ C, without any other solvent or reducing agent added. The polymer PEO-NH2 has a three-fold function as a reducing agent, capping agent, and solvent. The TEM confirmed the formation of gold nanoparticles with a mean diameter of 16.3 nm. However, under the experiment condition, the polymer PEO-OH cannot reduce the gold ion by the OH group itself, which means that the end group NH2 increased the reducing capacity of PEO. When PEO is used to synthesize silver nanoparticles in organic solution (acetone and benzene), [Ag(NH3 )2 ]OH should be taken as the precursor, instead of the most frequently used AgNO3 . This is because in benzene solution, simply using AgNO3 as the precursor leads to a failure for no hydroxide ions to appear in the solution to initiate the reducing process.43,46 However, when [Ag(NH3 )2 ]OH serves as the metal precursor in the organic solution, the Ag-N bonds break, and produce Ag2 O. 2Ag+ + 2OH− → Ag2 O + H2 O Then, silver nanoparticle formation can be initiated by adsorption of Ag+ ions onto the surface of the Ag2 O core, the end group –OH of PEO acts as the reductant, changing Ag+ ions on the particle surface into Ag atoms. This reducing process involves metalsemiconductor interactions between the Ag shell and the Ag2 O core, due to the semiconducting property of Ag2 O. The interaction can be detected by UV-vis spectra.47 2.1.3 Other Nonelectrolyte Homo-Polymers. Apart from PVP and PEO/PEG, some other nonelectrolyte homo-polymers have also been studied, including PAAm,48–50 PPY,51–53 PAM,54 PVA,55 PDF,56 PMHS,57 POMA,58–60 and PoPD.61–63 Each polymer has a special

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Figure 3. PAAm-induced synthesis and stabilization of Au nanoparticles in water. Reproduced from Sardar et al.48 with permission from American Chemical Society (Color figure available online).

reducing mechanism and plays an important role in the formation and stabilization of metal nanoparticles. PAAm is a water-soluble polymer generally used to trap nanoparticles and promote solubility in water.64–65 When the polymer alone is used to reduce metal ions and stabilize metal nanoparticles, PAAm shows a similar behavior with PPI, which will be introduced in the next section. Sardar and coworkers48 have reported the use of PAAm as a reducing and stabilizing agent to synthesize stable gold and silver nanoparticles at room temperature. The particle size can be tuned by varying the concentration of the polymer. TEM images of gold nanoparticles show that higher polymer concentration allows for smaller, and thus more stable products, whose dimension varies around 1.7 nm. In addition, the PAAm-coated gold or silver nanoparticles can be used in ligand-exchange with a variety of water-soluble functionalized alkylthiols, such as alcohol-, amine-, and biotin-terminated alkylthiols, which help to prepare functionalized nanoparticles. Figure 3 illustrates the process of PAAm-induced synthesis of gold nanoparticles in aqueous solution. Recently, a facile and environmentally-friendly approach to generate PAA-coated Au nanoparticles deposited on grapheme nano-sheets was reported.49 Figure 4 shows the preparation method of graphene/PAA-gold nanoparticles. In this experiment, dried graphene oxide (GO) was exfoliated in water and treated by a 15-min ultrasonication. Subsequently, ◦ gold precursors and PAA were added into the aqueous suspension and stirred at 95 C for 3 hours. The gold nanoparticles dispersed on the graphene measures around 4.2 nm in average diameter, and nearly no isolated Au nanoparticles can be observed, which means that the strong bonding between Au particles and graphene effectively prevents the aggregation of nanoparticles and helped to obtain highly-uniform nanoclusters homogeneously distributed on the sheets. The nano-composites have also been proved to be of excellent sensitivity towards electric activity and mild reducing capacity. Hebeish and coworkers50 fabricated a copolymer, that is, β-cyclodextrin grafted with poly acrylic acid (βCD-g-PAA) to synthesis silver nanoparticles. βCD is a cone-like molecule with the end hydroxyl groups, providing hydrophobic cavities that contribute to the water-solubility of the molecule. PAA still performs the dual functions, both as a reducing agent and stabilizer. The negatively-charged solubilized βCD helped to trap positively-charged silver ions and increase the water-solubility of the silver nanoparticles at the same time. In this experiment, silver films were deposited on cotton fibers to generate functional textiles. The chemical bond between the silver atoms and OH groups on cotton fibers results in the adherence between the cotton fabrics and the metal nanoparticles. The obtained nanofibers have been proved to be effective in the antibacterial activities.



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Figure 4. Illustration of the preparation of graphene/PAA-Au composites and chemical structure of PAA. Reproduced from Zhang et al.49 with permission from Elsevier (Color figure available online).

PPY, normally an irregular black powder, has extraordinary tensile strength and stability at room temperature. Manohar and coworkers66 have reported the spontaneous redox reaction between PPY and noble metal ions, including Ag+ and Au3+, under mild aqueous conditions. Moreover, pyrrole (PY) could also be oxidized by HAuCl4 , thus generating PPY and Au atoms at the same time. Several studies conducted by Selvan and coworkers51–52 have demonstrated the fabrication of fine gold nanoparticles. At first, HAuCl4 was added into di-block copolymer micelles, that is, polystyrene-block-poly-(2-vinylpyridine) in toluene solution. Afterwards, pyrrole (PY), instead of its polymer PPY, was added into the micellar solution and the Au precursor was reduced to be Au atoms at room temperature by stirring the mixture for four days, together with the polymerization of PY. TEM images of gold nanoparticles clearly showed the capping layer formed by PPY. By changing the reactant ratio, monodisperse gold nanoparticles with the diameter around 7 and 9 nm were synthesized. It is worth noting that by adopting vapor-phase polymerization, “dendritic” nanostructures (Au-PPY structure) could be observed. Munoz-Rojas and coworkers53 used solid Ag2 O powder and PY to synthesis Ag–PPY nanofibers under mild heating condition or at room temperature. The morphologies and sizes of the nanosnakes were tuned by changing the experimental conditions and the ratio of reactants. Figure 5 illustrates the synthesis method of PPY protected Ag nanofibers. By stirring the mixture of Ag2 O and pyrrole solution, polypyrrole was attached onto the surface of the Ag2 O particles. The followed thermal treatment initiated the reducing process between pyrrole and Ag2 O, with pyrrole acting as the reductant. In our previous work54 studying PAM-stabilized silver nanoparticles, we found that PAM exhibited similar behavior with PPY. Acrylamide (AM) could be oxidized by Ag+, thus the reduction of Ag+ and the polymerization of AM occurred at the same time without


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Figure 5. Scheme of the synthesis of Ag-PPY nanofibers. Reproduced from Munoz-Rojas et al.53 with permission from Wiley-VCH (Color figure available online).

any additional reductant added. Moreover, the comparison between the light scattering simulation and UV-Vis absorption curve suggested that the obtained clusters were the mixture of Ag and Ag2 O nanoparticles. One possible mechanism was suggested, that is, the reduction of silver ions by the radicals produced in the polymerization of AM. PVA, a water-soluble white powder, has reducing capacity and its aqueous solution is of different viscosity varying with its MW. Porel and coworkers55 investigated the synthesis of Au nanoparticles in PVA films through mild thermal treatment. This method generated free-standing nanoparticle-embedded PVA films. It was worth noting that the solid polymer matrix provided facile control on the particle morphologies, leading to triangular, square, rectangular, pentagonal, and hexagonal gold nano-plates and other patterns of uniform orientation, according to the TEM images of Au-PVA films of different experiment conditions. Also, the sizes of particles varied according to different shapes, the side lengths of the pentagons, hexagons, triangles, and squares/rectangles measured 70–85, 20–35, 15–45, and 10–25 nm, respectively, notably smaller than the typical polyhedral gold nanocrystals. This method was proved to be simple, efficient, and environmentally-friendly. Simply mix PVA and HAuCl4 aqueous solutions and treat the mixture with an extra mild heating, the reduction would be initiated efficiently. The nanoparticles obtained showed highly-regular shapes determined by varied reaction temperature, Au/PVA ratio, and reaction time. PVA here acted as both the reducing and stabilizing agent, as well as the solid matrix for highly-oriented growth. PDF, consisting of electron-donating unit dithiafulvene (DF), was able to reduce Pd, Au, and Pt precursors because of its π -conjugated structure and electron-donating properties. Zhou and coworkers56 explored the synthesis and properties of stable Pd, Au, and Pt nanoparticles coated by PDF at room temperature. From the red-shifting effect of absorption spectrum of the synthesized gold nanoparticles, they concluded that it indicated the presence of the positively charged PDF polymer, which also demonstrated its strong electron-donating properties. With the increasing amount of the PDF concentration and reaction temperature, the dimensions of the nanoparticles also increased. All the



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products obtained this way showed well-controlled size distribution and good thermal stability. PMHS was used to generate and stabilize functional silver nanoparticles in toluene.57 In this experiment, Ag(OAc) was used as the silver precursor. To accelerate the reaction rate, an amine catalyst trioctylamine (TOA) was added into the organic solution. Because the amine group of TOA can polarize the Si-H bond of PMHS, the efficiency can be greatly increased. After being stirred gently at room temperature, a change of spectra result showed the formation of silver nanoparticles. The authors also developed a synthetic method to fabricate functionalized nanoparticles by tailoring of the surfaces properties of nanoparticles. According to TEM images, PMHS was dissociated from the nanoparticle surface when TOA was added into the solution. With an excess of TOA (6 equiv to AgOAc amine), only individual nanoparticles of 3 nm coated by TOA could be observed. The synthetic process of Ag-TOA particles is presented in Fig. 6. The reduction mechanism of PMHS is still unknown at present. PANI is a conducting polymer with advantages of a reversible doping/dedoping process and reducing capacity. Guo and coworkers67 have reported the synthesis of gold nanoparticles uniformly deposited on surfaces of PANI nanofibers. By introducing doping acid to the main chain of PANI, the uniformity of metal-polymer composites could be further improved. POMA, a polyaniline derivative, exhibits electrochemical properties similar to polyaniline, but presents an improved water-solubility. The repeating unit of POMA is shown in

Figure 6. Synthetic strategy to polysiloxane-stabilized silver sols and their surface grafting. Reproduced from Chauhan and Sardar57 with permission from American Chemical Society (Color figure available online).


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Figure 7. (a) Oxidation of POMA (EB) to POMA (PB) by silver ions, and (b) Stabilization of Au@Ag nanoparticles by POMA (PB) chains. (a) Reproduced from Dawn et al.58 with permission from American Chemical Society. (b) Reproduced from Mukherjee and Nandi59 with permission from Elsevier.

Fig. 1(13), where the value of y ranges from 1 for the fully oxidized polymer (pernigraniline), to 0.5 for the half-oxidized polymer (emeraldine), and to 0 for the fully reduced polymer (leucoemeraldine).68–69 When 0 < y < 1, POMA has reducing and oxidizing capacity at the same time. When metal ions react with the emeraldine base (EB), the form of POMA changes to the pernigraniline base (PB), and POMA (PB) can stabilize metal nanoparticles, as Fig. 7 illustrated. Thus, POMA could be the reducing agent as well as the stabilizer at the same time. The group of Prof. Nandi58–60 has successfully conducted several experiments on the dual function of POMA to synthesize mono-dispersed Ag, Au, Au@Ag nanoparticles of different dimensions, respectively. PoPD, another derivative of polyanilines, is a redox-active conducting polymer. It can be formed by electro-oxidation of oPD in acidic solutions. There are three possible chemical structures of PoPD in Fig. 1(14).70 Han and coworkers61–63 have examined the reducing and stabilizing abilities of PoPD to synthesize gold nanoparticles of different morphologies, including nanofibers and well-rounded nanospheres. The synthetic procedure of the synthesis of metal nanospheres is shown in Fig. 8. At first, HAuCl4 was added to the solution of PoPD hollow microspheres, and the reducing groups on inner and outer surfaces of PoPD gradually reduced metal ions to metal nanoparticles. Then, the strong bonding between π electrons as well as amine groups of PoPD and gold nanoclusters provided an efficient stabilizing effect. 2.2 Electrolyte Polymers Electrolyte polymers, including cationic ones such as PDDA2,71–73 and linear PEI,74–83 and anionic ones such as PSA84 and PSSMA,85 enjoy good water-solubility. Therefore,



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Figure 8. (a) Synthetic procedure of the synthesis of PoPD-gold nanoparticles, (b) FE-SEM images of PoPD submicrospheres and (c) FE-SEM images of PoPD submicrospheres decorated with gold nanoparticles. Reproduced from Han et al.62 with permission from American Chemical Society (Color figure available online).

electrolyte polymers with reducing capacity are good choices when synthesizing metal nanoparticles in aqueous solution. PDDA is a cationic quaternary ammonium polyelectrolyte, whose monomer is of low volatility, great adhesive force, flexibility, hardness, and abrasive resistance. The PDDAprotected nanoparticles exhibit great stability due to the electro-steric effect of the polymer.86–87 At room temperature, PDDA is a transparent liquid so that it can be mixed adequately with the metal precursors. According to the experiments conducted by Chen and coworkers,2 PDDA, and HAuCl4 was mixed in aqueous solution and maintained at 373 K until a red solution occurred, which indicated the formation of gold nanoparticles. From the TEM image and particle size distribution histogram, it can be concluded that the PDDA-protected Au nanoparticles have great stability and distributed within a narrow diameter range around 12 nm. It should be noted that the pH value has significant influence on the reducing capacity of PDDA. Further experiments developed a novel method to polyelectrolyte-multilayers film containing gold nanoparticles. After mixing PDDA and HAuCl4 aqueous solutions adequately, an indium tin oxide (ITO) glass with polymer layers on the surface was immersed into the solution. An extra heating to 473 K for 30 min initiated the reduction process and resulted in gold nanoparticles contained in PDDA layers.72 Recently, a novel method using PDDA to prepare mono-dispersed gold nanoparticles on the surface of graphene nanosheets (GNs) has been reported.73 The nanoparticles were of highly-uniform size around 4.1 nm, and the distribution on the nanosheets helped to prevent aggregation of the products. In addition, the Au@GNs hybrids exhibited high sensitivity


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to electrochemical activity. The authors believed that the hydroxyl groups on GO provided anchoring sites for gold ions, and then they were reduced by PDDA. Other noble metals, such as silver, platinum, and palladium could be synthesized in a similar way. AgNO3 , H2 PtCl6 , and PdCl2 (the precursors of Ag, Pt, Pd, respectively) were mixed respectively with PDDA. It can be inferred from the reaction conditions that PDDA is a mild reactant, reacting at a rather slow rate so that it is an ideal polymer to generate stable nanoparticles, especially during nucleating. Different metal nanoparticles have different sizes and shapes, such as Ag nano-cubes, Pt and Pd nano-polyhedrons, and Au nano-plates.71 The reduction reaction conducted by PDDA has indicated that this kind of polymer does not necessarily need reducing groups. Although the reducing mechanism of PDDA is still unknown, a new peak in the FTIR spectra proved that C C and C N bonds are produced after PDDA was oxidized by HAuCl4 .2 Another explanation to consider is the possible presence of a reducing impurity or side products in the polymer. The reduction reaction conducted by PDDA has indicated that this kind of polymer does not necessarily need reducing groups, and broadened the range from amine to quaternary ammonium. Linear PEI, a pale yellow transparent liquid, is soluble in aqueous solution and is of good viscidity. Commercial PEI is a hyper-branched polymer containing primary, secondary, and tertiary amino groups, so in basic solution, PEI remains a neutral polymer. On the other hand, although uncharged in its molecular structure, linear PEI always acts as a cationic polyelectrolyte in acidic solution.88 At first, PEI was used as solely a protecting agent of gold nanoparticles and was proved to be of low efficiency when served to stabilize. At present, PEI functions not only as the stabilizer, but also the reducing agent. Its reducing effect, apparently, attributes to its amino groups. Sun and coworkers74–75 studied the onestep synthesis of PEI-protected gold nanoparticles and quasi one-dimensional nano-wires under different experiment conditions, using the polymer to serve as both the reducing and stabilizing agent. They also proved that a highly concentrated reactant was also essential to the success of the experiment. The size of products ranged from 10 nm to 100 nm under different experiment conditions in concentrated solutions. While when the experiment was conducted in a diluted solution, most of the gold nanoparticles obtained were of an irregular shape. To investigate the reducing effect of PEI’s amino groups, mono- and di-alkylated poly-ethylenimines (PEI-1R and PEI-2R) (Fig. 9) was synthesized to act as the reducing agents, respectively.76 PEI-1R and PEI-2R, defined as alkylated PEIs, and non-alkylated PEI, were added into the same amount of Au solutions, respectively.

Figure 9. The molecular structures of PEI-1R and PEI-2R. Reproduced from Chen et al.89 with permission from American Chemical Society.



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Figure 10. Aggregate generated by PEI, PEI-1R, and PEI-2R, respectively. Reproduced from Kuo et al.76 with permission from American Chemical Society.

It can be observed that three kinds of polymers are of similar structures and properties. However, compared with the irregular structures of PEI, the aggregates of PEI-1R and PEI-2R chains tend to be better-structured with the dimension around 150 nm; the micelles of PEI-2R even present a well-rounded shape.76 Similarly, the nanoparticles generated by PEI are less compact, while PEI-2R and PEI-1R contribute to well-structured nanostructures. (Fig. 10) This is because non-alkylated PEI cannot provide enough protection to gold nanoparticles as alkylated ones. Also, highly-alkylated PEIs tend to result in smaller structures. Considering the reaction efficiency, gold nanoparticles can be synthesized more efficiently by using alkylated PEI as the reducing agent. In addition, excess amount of PEI also restraints the growth rate of nanoparticles. Thus, the particle size could be tuned by changing the molar ratio of PEI to gold precursors.77,79–81 The PEI-capped gold nanoparticles were even reported to be suitable for carrying si-RNA without any cellular toxicity.82 Sin and coworkers83 have studied the dual-functional role of PEI in reducing AgCl salt in aqueous solution under mild heating conditions. As a polyelectrolyte, PEI, together with poly-(acrylic acid) (PAA), can form a polyelectrolyte multilayer (PEM) to synthesize Au-Ag alloy nanoparticles embedded in the polymer multilayer. The process illustrated in Fig. 11 showed that gold and silver precursors could be dispersed homogenously in the multilayers. An extra heating afterwards changed these precursors into Au-Ag alloy nanoparticles embedded in the PEM.78 Poly-(sodium acrylate) (PSA), an anionic electrolyte, formed by acrylate, a white powder and an unsaturated organic, is of mild reducing capability. PSA has long been used as the stabilizer for silver nanoparticles generated by the reduction of silver precursors by additional reducing agents or gamma radiation.90–91 However, Hussain and coworkers84 reported the use of PSA as both the stabilizing and reducing agent. Gold nanoparticles were generated by SA and PSA, respectively. An aqueous solution of SA and PSA was


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Figure 11. Scheme of the PEM build-up and subsequent reduction process. Reproduced from Shang et al.78 with permission from American Chemical Society (Color figure available online).

added into HAuCl4 solution respectively and stirred at a temperature of 50–60◦ C, until a red solution occurred. The products using SA are quasi-spherical and of highly-uniform size distribution. While PSA-capped nanoparticles exhibited less uniform size distribution, they exhibited great stability. The advantage of PSA over SA was that PSA enabled the synthesis of composite gold-polyelectrolyte film. Recently, we have reported the use of the polyelectrolytes PSSMA, also an anionic polymer, to synthesize different metal nanoparticles without any additional reducing agent. To study the most suitable reaction conditions, different temperatures and polymer MW were experimented. The schematic illustration in Fig. 12 showed the synthetic procedure of PSSMA-coated metal nanoparticles. Different metal precursors were added into the aqueous solution of PSSMA, and the reduction process was initiated by mild thermal treatment. The obtained nanocomposite were of excellent stability, thus indicating potential applications in the biomedical, catalytic, and self-assembly fields.85 2.3 Dendrimers Dendrimers92–93 are viewed as monodisperse, hyper-branched polymers of 3D structures with large amount of surface functional groups. When dendrimers are used to act as the stabilizing agent, they exhibit all the advantages in controlling the size, stability, and solubility of nanoparticles. Besides, the steric effects of dendrimers confine the growth of nanoparticles, and their branches have selective effects of small molecules.18,20 When dendrimers are being synthesized, a small molecule is needed to be a core and repeating units are to be grafted to the core to form a hyper-branched, macro-molecule architect. As the generation of the dendrimer increases, its branches gradually spread out to form a star-like structure, its diameter increases linearly, while the surface end groups



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Figure 12. Schematic illustration of the procedure for the formation of PSSMA-coated metal nanoparticles. Reproduced from Cai et al.85 with permission from IOP Publishing Ltd.

increase exponentially. One obvious advantage of dendrimers over linear polymers is the high efficiency provided by large amount of reactive end groups such as OH and –NH2 functional groups. In addition, a large amount of polar end groups are useful in solubility enhancement. In this section we will discuss hyper-branched PAMAM, PPI, PEI, and other segmented dendrimers including reducing end groups.94–99 PAMAM is the most frequently-investigated dendrimer at present; it has a precise molecular structure, large amount of carboxyl and amino groups, intra-molecular cavities, and good controllability of MW.100–101 When used as the dressing agent, PAMAM promotes the solubility and stability of nanoparticles, due to the effect of functional groups. Figure 13 represents the synthesis of the fourth-generation, PAMAM dendrimers. Ethylene diamine (EDA) was used as the core, and methyl acrylate (MA) the repeating unit.20,102 Zhang and coworkers94 explored the synthesis of Ag nanoparticles by amine-terminated hyper-branched poly-(amidoamine) (HPAMAM-NH2 ) as both the reducing and capping agent in aqueous solution at room temperature. (Fig. 14) They investigated the relationship between the size distribution of silver nanostructures and the reactant ratio of HPAMAMNH2 /Ag. By comparing HPAMAM-NH2 with other dendrimers without amino groups, it was proved that the surface amino groups reduced Ag+ to Ag atoms. HPAMAM-NH2 is affluent of amino groups capable of reducing Ag precursors, so that it is extremely effective when producing small silver nanoparticles. Silver nanoparticles synthesized this way exhibited excellent antibacterial effects at a low Ag concentration. Advantages of this method are obvious: 1. The reaction can be conducted under mild experiment conditions, and no extra heating is needed, so that it is a rather environmentally-friendly process; 2. No extra reducing agent is needed; 3. The obtained silver nanoparticles enjoy extraordinary properties, including relatively small particle sizes, highly-uniform size distribution, and great stability in aqueous solution. Similarly, Poly-(propyleneimine) (PPI) is white powder under normal condition. PPI has good water-solubility owning to its amino groups and stability when kept away from


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Figure 13. Synthesis and structures of fourth-generation, amine-terminated (G4-NH2), and hydroxyl-terminated (G4-OH) PAMAM dendrimers. Reproduced from Scott et al.20 with permission from American Chemical Society (Color figure available online).

Figure 14. The process of the synthesis of PAMAM-covered silver nanoparticles. Reproduced from Zhang et al.94 with permission from American Chemical Society (Color figure available online).

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Figure 15. The synthesis process of PPI dendrimer. Reproduced from Gupta et al.103 with permission from Pharmaceutical Press.

strong oxidizing agents, and can be used as cohesion agent or solidification agent. As Fig. 15 illustrating the synthesis of 5.0 G-PPI dendrimer showed, Ethylene diamine (EDA) was used as the core, and acrylonitrile the repeating unit.103 Sun and coworkers95–96 have reported the one-step synthesis PPI-protected gold and silver nanoparticles. A third-generation PPI


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(G3-PPI) dendrimer acted as the reducing agent as well as the stabilizer, and a mild heating condition was needed to initiate the reaction. From the TEM images of the products, it could be observed that the metal nanoparticles distributed among a narrow range of size distribution. By changing the temperature, they found that higher temperature within the proper range would contribute to larger particle size. Those synthesized nanoparticles retained high stability for several months, suggesting that this dendrimer was a good stabilizer. In the last section, the reducing and stabilizing capacity of linear PEI was introduced, while some studies have conducted experiments to study the use of branched PEI. Note and coworkers97 focused on the role of the polymer as reducing and stabilizing agent to generate gold nanoparticles in complex system, that is, water-in-oil (w/o) micro-emulsion. At a temperature of 373 K, the branched PEI was proved to be a highly-efficient reductant. The obtained gold particles were of dimensions below 10 nm. Kim and coworkers98 synthesized branched PEI-capped Au nanoparticles at room temperature. It was proved that the size of Au nanoparticles can be controlled effectively by varying the amount of PEI added into same amount of HAuCl4 solution. They also observed that PEI-Au nanoparticles aggregated into stable 2D arrays at the toluene-water interface. Thus, microscopically smooth Au films could be formed at the interface. Hyper-branched PEI has a similar structure with the PPI dendrimer; thus their reaction mechanisms resemble each other. The copolymer HBPO-star-PDEAEMA was synthesized with a HBPO core and cationic PDEAEMA branches. The amino group on the dendrimer surface could reduce gold ions to nanoparticles at room temperature; PDEAEMA arms provide abundant grounding sites for gold molecules and guarantee great stability. The hyper-branched structure of HBPO-star-PDEAEMA is shown in Fig. 16. When the reactant molar ratio N/Au was increased to 20, Au nanoparticles with homogeneous diameters of ∼4 nm and great stability can be obtained. It was also worth noting that HBPO-star-PDEAEMA proved to be easier to prepare than other dendrimers, such as PAMAM.99 2.4 Block Copolymers Block copolymer, a structure of two or more polymers connected together, with one segment reacting with metal ions, the other one or ones promoting water solubility, sometimes the middle segment was added to link those two sections. As stated in the introductory section, polymers adopted by one-step reactions should have reducing capability, stability, and water-solubility as well. This limitation narrowed the scope of suitable polymers. To concentrate several advantageous properties in one polymer, researchers looked into the possibility of composite polymers. It has been proved that these block copolymers present greater reduction capacity and stability, and usually, no heating procedure is needed to accelerate the reaction. The design of copolymer structure helps to improve surface chemistry characteristics and control solubility and dimensions according to our needs. PEO and other similar polymers have been widely applied in the assembly of block polymers. For example, PEO-b-PMAA was used to produce silver nanowires by Zhang and coworkers.15 Carboxylic acid groups of PMAA made major contribution to the reduction process, while PEO reacted much more slowly than the PMAA section, so it mainly enhanced water-solubility of silver nanoparticles. The synthesized Ag nanoclusters had a tendency to form a structure of nanowires. Recently, PPO-PEO or PEO-PPO-PEO block polymers have been utilized as templates for nanoclusters synthesis and organization.104–109 At first, it was believed that in the use of PEO/PPO copolymers as the reducing/stabilizing agent, PPO does not contribute to the reduction of the metal ions; it serves as the connector

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Figure 16. Preparation processes of the macroinitiator and HBPO-star-PDEAEMAs (only six terminal units are shown for simplicity). Reproduced from Zhai et al.99 with permission from Springer (Color figure available online).

with gold nanoparticles to obtain the greatest efficiency and simplicity.110 While Sakai and coworkers have proved that both PEO and PPO sections had direct contribution to the reducing ability, a possible mechanism was proposed about the formation of gold nanoparticles:

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1. the reduction of gold ions by the ether-like polymers, similar to the reacting mechanism between pure PEO and silver ions; 2. the copolymer PEO-PPO-PEO was adsorbed onto the surface of gold particles and more gold ions were trapped in the polymer cavities and 3. further growth and stabilization of gold nanoclusters. (Fig. 17) To examine the role of PEO and PPO sections in the reducing process, PEO-PPO-PEO with different PEO block lengths and the same PPO block lengths, as well as the copolymers with different PPO block lengths and the same PEO block lengths were studied, respectively. The absorption efficiency at 240 nm (AuCl4 −) and 540 nm (Au NPs) was recorded for all the experiments. It was found that with the increasing length of PEO and PPO, the absorbance at 540 nm increased, indicating that both PEO and PPO parts contributed to the reduction of gold ions. Further study showed that through the whole synthesis process, the absorbance at 240 nm was reduced linearly, while the absorbance at 540 nm increased slightly at first, and then grew rapidly. This result was consistent with the fact that the crown-like structure PEO made more contribution to the accommodation and reduction of metal precursors, while the PPO section mainly contributed to the surface absorption. Thus, PEO determined the growth of nanoparticles, while PPO limited overgrowth and affected stability.104 Their works have been reviewed by Alexandridis.110–111 (PEO)m -(PSO)n (m, n stand for two integers)112 also exhibited excellent reducing and stabilizing capability. In this experiment, spherical or quasi-spherical Au particles were obtained under ambient conditions. The dimension and dispersity of gold nanoparticles could be well-tuned by controlling the copolymer block length and reactant concentration.

Figure 17. Steps of the synthesis of gold nanoparticles Reproduced from Sakai and Alexandridis104 with permission from American Chemical Society.



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Figure 18. Synthesis process of Ag, Au, and Pd nanoparticles. Reproduced from Kohut et al.113 with permission from Wiley-VCH.

In the copolymer PEO-PM, PEO section (hydrophilic) of the copolymer could reduce the metal ions to atoms, and PM section (hydrophobic) served as the protecting layer of Ag, Au, and Pd nanoparticles. The size and shape of the metal nanoparticles could be tuned by the size and shape of the hydrophilic cavities of PEO. This nanosystem was proved to be stable in both polar and non-polar solutions. (Fig. 18) The DLS data showed that the particle sizes were distributed among a narrow range around 10 nm, corresponding to the structure of PEO section.113 The copolymer Biotinyl-PEG/PDEAEMA was able to reduce the gold precursor at room temperature without any other reducing agent added into the reaction system. In this copolymer, the PDEAEMA segment, with tertiary amino groups, served as the reducing agent, while the PEG part, with its less intense reducing capacity, mainly served as the anchoring agent grafted onto the surface of nanoparticles. The obtained mono-dispersed gold nanoclusters enjoyed great stability even when the salt was of very high concentration. Figure 19 illustrates the synthesis route of Biotinyl-PEG/ PDEAEMA and polymer-metal nanoclusters.114 After changing the ratio of PEO and PPO to (EO)20 -(PO)70 -(EO)20 , Zhang and coworkers46 developed a novel synthetic method, using the copolymer to reduce [Ag(NH3 )2 ]+ ions in ethanol initiated by ambient light illumination. The silver atoms successfully deposited on TiO2 nanoparticles to generate Ag@TiO2 core-shell nanoparticles. The reaction occurred at normal temperature and a simple shaking by hand could initiate the reduction. The growth process was suggested about the reaction in Fig. 20. At first, [Ag(NH3 )2 ]+ ions were trapped into the interior crown-like cavities of PEO and/or PPO, then the silver precursor was reduced to form larger particles. Most of the copolymers with reducing capacity have PEO/PEG sections. Up to now, three copolymers without PEO/PEG section have been studied. The tri-block copolymer polystyrene-oligothiophene-polystyrene (PS-11T-PS), was synthesized by linking two polymers to the ends of oligothiophene, thus the solubility of the polymer increased greatly


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Figure 19. Synthesis of gold nanoparticles by biotinyl-PEG/PAMA. Reproduced from Ishii et al.114 with permission from American Chemical Society.

Figure 20. One possible scheme of the synthesis of silver nanoparticles. Reproduced from Zhang et al., 200346 with permission from American Chemical Society (Color figure available online).



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Figure 21. Synthesis of polystyrene-oligothiophene-polystyrene. Reproduced from Hempenius et al.16 with permission from American Chemical Society.

(Fig. 21). A solution of PS-11T-PS in toluene was stirred with HAuCl4 to generate gold nanoparticles and a color change from red to blue indicated the formation of nanoparticles. The obtained gold nanoparticles are easy to form a smooth film, as shown by SFM images.16 The group of professor Leiva115–116 synthesized the copolymer PCL-PVP and PVPPCL-PVP to generate gold nanoplates by direct reaction between the copolymer and KAuCl4 in water. The structure of PVP-PCL-PVP is shown in Fig. 22. Well-defined hexagonal and triangular gold nanoplates were obtained at room temperature. The use of some natural plant polymers provides an alternative method to generate polymer-coated metal nanoparticles. Up to now, various natural polymers, including spider silk,117 starch,118 polysaccharide alginate gel,119 Epigallocatechin-3-gallate (EGCG),120

Figure 22. Molecular structure of PVP-PCL-PVP. Reproduced from Leiva et al.115 with permission from Elsevier.


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Figure 23. Schematic illustration of the proposed formation mechanism of silver nanoparticles obtained in the mixture of oleylamine and liquid paraffin. Reproduced from Chen et al.10 with permission from American Chemical Society.

and Bayberry tannin (BT)121 have been applied to synthesis of metal nanoparticles. The biggest advantage of natural polyphenol was its environmentally-friendly process, that is, the minimization of chemical waste and the realization of sustainable development.

3. Formation Mechanisms of the Polymer-Reduced and -Protected Metal Nanoparticles Among the polymers mentioned above, most of them contain reducing groups, such as amino or hydroxyl, while some of them do not contain any reducing groups. So the reaction mechanism between polymers and metal ions might be different. 3.1 Amino-Containing Mechanisms Several polymers mentioned above contain amino as functional group, such as PEI (both linear and hyper-branched), PAAm, PAMAM, PPI, PANI, POMA, and PoPD. The oxidative dehydrogenations of amines and alcohols can be promoted by their coordination to transitional metals, such as Ag, Au, Pt, and Pd. The first step is the one-electron

Figure 24. Reducing mechanism of synthesizing amino-containing polymer-coated metal nanoparticles by (a) primary amines and (b) secondary amines.


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Figure 25. Proposed mechanism of reducing reaction initiated by HBPO. Reproduced from Zhai, N. et al.99 with permission from Springer.

oxidation of the metal center, while the mechanism of the second step is still ambiguous; either involves free-radical intermediate or alternative two-electron oxidation.122 To be specific, we have proposed a possible reaction mechanism for the formation of silver nanoparticles and the production of nitriles and imines (Fig. 23). The mechanism involves the formation of stable complexes of Ag(I) salts with amines and then a one-electron transfer of the amines to the Ag(I) species at relatively high temperature and the formation of amino radicals. The second step is the deprotonation from the amino radicals to imines or from imine to nitriles. All of these amino groups serve to stabilize the silver nanoparticles.10 For monodentate primary amines, there are no steric constraints on the further oxidation to nitrile. (Fig. 24a) Similarly, the oxidation of secondary amines by transitional metal ions can be illustrated by Fig. 24b.122 Then metal atoms aggregate together to form positively-charged metal nanoclusters, and excess amines coordinate to the particle surface and promote their stability. In the work of producing gold nanoparticles coated by HBPO-star-PDEAEMA, the author speculated a mechanism of the oxidation of tertiary amine groups. At first, the tertiary amine groups in the PDEAEMA arms are coordinated with Au (III) ions, and then the nitrogen atoms donate electrons to Au (III) ions and become monovalent cations (Fig. 25). After gold atoms are formed, they tend to aggregate into larger nanoparticles.

3.2 Hydroxy-Containing Machanisms The oxidation mechanism of alcohols resemble that of amines, the initial product is the corresponding aldehyde or ketone (Fig. 26), subsequent conversion might occur depending on the nature of metal ions and nearby ligands. The noble metal precursors of all the experiments above are two-electron oxidants, such as Ag(I), Au(III), Pd(II), or Pt(II). So the possible redox mechanism might be (i) oneelectron transfer via a radical followed by another one-electron transfer; (ii) a concerted two-electron transfer accompanied by two deprotonations of the alcohol; (iii) deprotonation followed by a hydride transfer to the metal ion.122

Figure 26. Reducing mechanism to synthesis hydroxyl-containing polymer-coated metal nanoparticles.

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Figure 27. Hydroxy-reducing mechanism of PVP. Reproduced from Xiong et al.31 with permission from American Chemical Society.


Among the polymers with reducing capacity, PEO and PVA follow the typical oxidation mechanism of alcohols. However, in the first possible oxidation mechanism of PVP, structural change of the OH-terminated commercially-available PVP is involved (Fig. 27).31–32 3.3 Free Radical-Containing Mechanisms Some redox mechanism between polymers and metal ions include free radical intermediate. In these cases, it is assumed that the metal ions are strong enough to break the covalent bond between carbon atom and hydrogen atom. For example, the second possible mechanism (Fig. 28) of PVP assumed that hydrogen from PVP could be abstracted by metal ions directly.27–28,33 In one proposed oxidation mechanism of PEO, as Fig. 29 shows, the silver ions first interact with a hydrogen atom of PEO fragment in the presence of hydroxyl ions to form PEO macro-radicals. Then Ag+ receives an electron from the macro-radical and is reduced to silver atom, and the macro-radical is transferred into a carbocation. The hydroxyl ion reacts with the carbocation and converts into a hydroxyl group. In the next step, another α-H also goes through such reactions and eventually forms carbonyl compound. We have proposed a possible mechanism in our previous work concerning the synthesis of PSSMA-coated metal nanoparticles. During the reversible thermal degradation- addition, PSSMA can be broken into two free radicals, which are capable of reducing noble metal ions to metal atoms (Fig. 30). 3.4 Other Mechanisms The third possible mechanism of the oxidation of PVP by gold precursors is illustrated in Fig. 31. The C O group is supposed to be the reducing group. The AuCl4 − and the C O

Figure 28. Free radical-related mechanism of the oxidation of PVP. Reproduced from Hoppe et al.27 with permission from American Chemical Society.



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Figure 29. Possible mechanism of the formation of PEO-reduced silver nanoparticles. Reproduced from Voronov et al.43 with permission from American Chemical Society.

group can form the coordinated complex, and then a mild thermal treatment initiates the redox process. However, according to the solid-phase experiment conducted by Debnath and coworkers,30 the FTIR spectra of the obtained silver nanoparticles indicated that the C O double bonds did not change after the reaction, this discovery was in sharp contrast with the possible mechanism stated above; whether the reducing mechanisms vary according to phases or not has not been substantiated yet. PDF does not contain any functional reducing groups, while it contains π -conjugated electron-donating unit dithiafulvene (DF), which can donate electrons to Pd, Au, and Pt precursors and form coordinated complex at the same time (Fig. 32). PPY, PSA, and PAM are different from other polymers serving as both the reducing agent and the stabilizing agent. These polymers have no reducing ability or weak reducing ability, while their units, PY, SA, and AM are capable of reducing metal ions to atoms, and form the corresponding polymer simultaneously. For example, when the solution containing gold precursors was treated with pyrrole (PY), polymerization occurred, yielding PPY and Au nanoparticles (Fig. 33).

Figure 30. Schematic illustration of the proposed mechanism for the formation of PSSMA-coated metal nanoparticles. Reproduced from Cai et al.85 with permission from IOP Publishing Ltd.


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Figure 31. Proposed chemical reaction steps in reducing gold ions via a polymer complex from dispersed PVP molecules. Reproduced from Ram and Fecht29 with permission from American Chemical Society.

Figure 32. Schematic illustration of the formation of the PDF-protected metal nanoparticles. Reproduced from Zhou et al.56 with permission from American Chemical Society.

Figure 33. Mechanism of the formation of gold nanoparticles by using PY. Reproduced from Selvan et al.52 with permission from Wiley-VCH.


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4. Conclusions and Perspectives Successful productions of noble metal nanoparticles by one-step reduction provide a novel method to produce high-purity nanoparticles with great efficiency and stability. The simplicity and flexibility associated with the one-pot method of fabricating metallic nano-materials have resulted in increasing research efforts leading to the optimization of the double use of polymers and the realization of their potential applications. In this work, the use of nonelectrolyte homo-polymers, polyelectrolytes, dendrimers, and block copolymers as both the reducing agents and stabilizers have been reviewed, and the redox mechanisms of polymer-metal nanoparticles are illustrated. From previous experiments, it has been proved that morphology, dimension, and optical properties of the metal particles can be tuned by changing the polymer structure, polymer/metal salt ratio, reaction time and temperature. Also, different reaction medium may require different metal precursors. In addition to linear homo-polymers, researchers also introduced dendrimers to the synthesis of nanoparticles. As the generation of the dendrimer increases, its branches gradually spread out from the core to form a hyper-branched, macro-molecule structure, with its surface groups increasing exponentially, which provide considerable improvement in the synthesizing efficiency. Later on, more studies looked into the possibility of block polymers, which combine reducing capability, stability, and water-solubility of different polymers into one architect. This method undoubtedly breaks the limit in choosing suitable polymers in one-step reactions. Thus, it is necessary to conduct much pre-experiment research to find the proper reaction conditions. In the one-step synthesis of metallic nanoparticles, most of the polymers contain reducing groups, such as amino ( NH2 ) or hydroxyl ( OH), while some of them do not contain any reducing groups, so the reaction mechanism between polymers and metal ions are different. The reducing capacity of these polymers might attribute to the oxidative dehydrogenations of amines and alcohols, free-radical intermediate or π -conjugated electrondonating unit, and the polymerization process. Much progress has been made in these areas, especially in synthesis of single-metal nanoparticles and efficiency enhancement, but there is still quite a bit of room for improvement in the synthesis of dual/poly-component nanoparticles and non-spherical nanoparticles. Also, knowledge concerning the oxidation mechanism of some polymers is still limited. Although it has been proved that polymers with reducing capacity do not necessarily contain any reducing groups, their mechanisms are still not available, and further research on their properties should be conducted. As many more varieties of polymers being used as the dual-functional reductant and stabilizer, metal nanoparticles with different dimensions, morphologies, structures, and optical properties are likely to be easily synthesized according to our needs. Therefore, potential applications of polymer-coated nano-composites can be quite promising in future therapeutic applications and bio-molecular manipulations, including detection, drug-transferring, and genetic engineering,

Acknowledgements Financial support from the National Science Foundation of China (20871031, 51073039, 11179015, and 51173108), Innovation Program of Shanghai Municipal Education Commission (12ZZ143), and Hui-Chun Chin and Tsung-Dao Lee Chinese Undergraduate Research Endowment (CURE) are gratefully acknowledged.

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List of Abbreviations HBPO PAAm PAM PAMAM PANI PCL PDDA PDEAEMA PDF PEG PEI PEO PEO-b-PMAA PEO-PM PEO-PPO-PEO PMHS POMA PoPD PPI PPY PS-11T-PS PSA PSO PSSMA PVA PVP

Hyper-branched poly-(3-ethyl-3-oxetanemethanol) Poly-(allylamine) Poly-(acrylamide) Poly-(amidoamine) Poly-(aniline) Poly-(ε-caprolactone) Poly-(diallyl dimethylammonium) chloride Poly-(2-(N,N-diethylamino) ethyl methacrylate) Poly-(dithiafulvene) Poly-(ethylene glycol) Poly-(ethylenimine) Poly-(ethylene oxide) Poly-(ethylene oxide)-block-poly-(methacrylic acid) Poly-(ethylene oxide)-poly-(methylene) Poly-(ethylene oxide)-poly-(propylene oxide)-poly-(ethylene oxide) Poly-(methylhydrosiloxane) Poly-(o-methoxy aniline) Poly-(o-phenylenediamine) Poly-(propyleneimine) Poly-(pyrrole) Poly-(styrene)-oligothiophene-poly-(styrene) Poly-(sodium acrylate) Poly-(styrene oxide) Poly-(4-styrenesulfonic acid-co-maleic acid) Poly-(vinyl alcohol) Poly-(N-vinyl-2-pyrrolidone)

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