Self-Organization of Silver-Core Bimetallic Nanoparticles and Their Application for Catalytic Reaction

Metal nanoparticles have received much attentions as a building block of advanced materials for nanoscience and nanotechnology (Bonnemann & Richards, 2001). Their optical, (Fukumi et al., 1994; Lu et al., 1999; Link et al., 1999; Shipway et al., 2000), magnetic (Sun et al., 1999; Teranishi & Miyake, 1999), and catalytic (Kiely et al., 1998; Pileni, 1998; Bradley, 1994; Harriman, 1990; Lee et al., 1995; Toshima et al., 1995; Bonilla et al., 2000; Siepen et al., 2000) properties have been reported with great interests. The character of metal nanoparticle can be altered by the addition of other metals. Bimetallic nanoparticles, composed of two different metallic elements, have been reported to show outstanding characters different from the corresponding monometallic nanoparticles (Harriman, 1990; Yonezawa & Toshima, 1993; Toshima & Hirakawa, 1997, 1999; Toshima & Wang, 1994; Lee et al., 1995). For example, catalytic activities of gold (Au)-core structured bimetallic nanoparticles, gold/platinum (Au/Pt) (Harriman, 1990; Yonezawa & Toshima, 1993; Toshima & Hirakawa, 1999), gold/palladium (Au/Pd) (Toshima & Hirakawa, 1999; Lee et al., 1995), and gold/rhodium (Au/Rh) (Toshima & Hirakawa, 1999), for hydrogenation and/or water reduction are higher than platinum (Pt), palladium (Pd), and rhodium (Rh) monometallic nanoparticles, respectively. Surprisingly, in some cases, a physical mixture of monometallic nanoparticles such as Pt and ruthenium (Ru) nanoparticles in solution shows higher catalytic activity than the corresponding monometallic nanoparticles under a certain condition (Toshima et al., 1995; Toshima & Hirakawa, 1997). This suggests that an interaction between two kinds of monometallic nanoparticles can produce novel nanoparticles. Further, it has been reported that physical mixture of silver (Ag) and other metal nanoparticles, such as Pt, Rh, and Pd, spontaneously forms the bimetallic nanoparticles with Ag-core structure in aqueous solution. This reaction can be used to construct the core-shell structured novel bimetallic nanoparticles. The formed nanoparticles demonstrate superior character for certain catalytic reactions.

In this chapter, the simple method of the preparation of core-shell structured bimetallic nanoparticles by the physical mixing and the application of the formed novel metal nanoparticles for catalytic reaction are described. The topics of the catalytic reaction presented in this chapter are the visible light induced hydrogen generation (Toshima & Hirakawa, 2003), the removal of reactive oxygen species (Hirakawa & Sano, 2009), and its application to the chemoprevention of ultraviolet induced biomolecules damage (Hirakwa et al., 2008(Hirakwa et al., , 2009).

Spontaneous formation of silver-core bimetallic nanoparticles
Much attention has been paid to bimetallic nanoparticles, especially those having a core/shell structure . From the view point of Au catalysts, bimetallic nanoparticles have received much attention recently. On the other hand, a physical mixture of monometallic nanoparticles such as Pt and Ru nanoparticles in solution shows higher catalytic activity than the corresponding monometallic nanoparticles under a certain condition Toshima & Hirakawa, 1997). Further, it has been reported that physical mixture of Ag and other metal nanoparticles, such as Pt, Rh, and Pd, spontaneously forms the bimetallic nanoparticles with Ag-core structure in aqueous solution ( Figure 1). In this section, the spontaneous formation of the Ag-core bimetallic nanoparticles is reviewed.

Siver-core/rhodium-shell bimetallic nanoparticles
The interaction between Ag and Rh monometallic nanoparticles in solution by physical mixing was reported. The main reason for using Ag and Rh nanoparticles is the reported prominent characteristics of Rh nanoparticles as a catalyst (Toshima & Hirakawa, 1999), and the expected electronic effect of Ag similar to Au upon enhancement of the catalytic activity of Rh. Furthermore, Ag is inexpensive metal compared with Au. The colloidal dispersions of Ag and Rh monometallic nanoparticles protected by poly(N-vinyl-2-pyrrolidone) (PVP), a water soluble polymer, were prepared by an alcohol reduction method (Hirai et al., 1979). Average diameters of Ag and Rh monometallic nanoparticles were 7.5 nm and 2.2 nm, respectively.

Surface plasmon absorption of siver-core bimetallic nanoparticles
Colloidal sol of Ag nanoparticles shows characteristic plasmon absorption aeound 400 nm (Henglein, 1979). The plasmon absorption band of Ag nanoparticles decreased by addition of Rh nanoparticles and was almost completely extinguished within 30 min after mixing ( Figure 2). The parts of plasmon absorption in larger wavelength region were preferentially extinguished within 10 min, suggesting that influences of Rh nanopartilces on Ag nanoparticles depend on the size of the Ag nanoparticles. When relatively smaller molar quantity of Rh to Ag was added, the plasmon absorption was not completely extinguished. More than 40 atom-mol% of Rh against to Ag was required to extinguish the plasmon absorption band completely.  increase of Rh/Ag molar ratio reduces the average diameter and the size distribution of the nanoparticles. The elemental analysis using characteristic X-ray in high-resolution TEM measurement has shown that the particles produced from their physical mixtures in 24 h are composed of Ag and Rh.
2.1.3 X-ray diffraction of the of siver-core bimetallic nanoparticles Figure 4 shows X-ray diffraction (XRD) patterns of poly(N-vinyl-2-pyrrolidone)-protected Ag and Rh monometallic nanoparticles, and their physical mixture. The sample of the physical mixture of Ag and Rh nanoparticles was prepared by drying the mixtures of their aqueous solutions under vacuum for 24 h after mixing. The XRD pattern of the mixtures of Ag and Rh nanoparticles was similar to that of Rh nanoparticle, suggesting that the surface of the particle produced by mixing Ag and Rh nanoparticles is composed of Rh. Similarly, Au-core/Pt-shell and Au-core/Pd-shell structured nanoparticles have shown the XRD pattern quite similar to that of their surface metals . These findings suggest that the aggregation of Rh particles around the Ag particle is involved in the extinction of the plasmon absorption.

Mechanism of the formation of the siver-core/rhodium shell bimetallic nanoparticles
Henglein et al. reported that lead (Pb) atoms transfer from Pb colloidal particle onto the surface of Ag colloidal particle in physical mixing of Ag and Pb colloidal sols (Henglein et al., 1992). If the extinction of the plasmon absorption is due to coating of the surface of Ag particle by Rh atoms transferred from Rh nanoparticle, at least 28 mol% of Rh to Ag is required assuming that a Ag particle (average diameter = 7.5 nm) is uniformly coated by Rh atoms in a one-atom layer. required to completely extinguish the plasmon absorption, which is reasonably supporting the above assumption. These observations suggest that the physical mixture of Ag and Rh nanoparticles spontaneously generates Ag/Rh bimetallic nanoparticles with an Agcore/Rh-shell structure. The disappearance of the XRD peak of Ag nanoparticles suggests that the core of this bimetallic nanoparticles is not complete Ag, but possibly has a partial Ag/Rh alloy structure. The driving force of the formation of this Ag/Rh bimetallic nanoparticles may be due to the larger binding energy between Ag and Rh atoms than between Rh atoms (Peiner & Kopitzki, 1998). Reduction of diameter of the nanoparticle increases not only its surface energy but also number of the binding sites between Ag and Rh atoms, which stabilizes the total energy. Therefore, the shrinking of Ag/Rh bimetallic nanoparticles might be explained by the balance between the binding energy and the surface energy. The size and the rate of formation of the bimetallic nanoparticles can be controlled by the kind and concentration of protective agents. The self-assembling formation of bimetallic nanoparticle using Ag nanoparticle is applicable to construction of novel nanoparticles.

Silver-core/noble metal-shell bimatallic nanoparticles
The above mentioned procedure can be used to prepare the Ag-core/noble metal shell nanoparticles, other than Ag-core/Rh-shell nanoparticles. The physical mixing of Ag and other metal nanoparticles, such as Au, Pt, Rh, and Pd particles, produces Ag-core bimetallic particles. The interaction rate between Ag and other metal nanoparticles was determined by the extinction of the surface plasmon absorption of Ag nanoparticle. The initial step of this reaction was investigated by isothermal titration calorimetry (Toshima et al., 2005). This study revealed that the strength of the interaction between Ag and other metals increases in the order of Rh/Ag > Pd/Ag > Pt/Ag.
The formed Ag-core/Pt-shell nanoparticle catalyzed the decomposition of hydrogen peroxide (described later). On the other hand, Au and Au/Ag nanoparticles showed an activity of photocatalytic decomposition of methylene blue (Hirakawa, 2007), although their activities were significantly smaller than that of well-known titanium dioxide photocatalyst (Fujishima et al., 2000(Fujishima et al., , 2008. The physical mixing method is simple and useful to prepare novel bimetallic nanoparticles. These nanoparticles may be used as catalyst and photocatalyst.

Application to the preparation of trimetallic nanoparticles
This method can be applied to the preparation of trimetallic nanoparticles (Toshima et al., , 2011. It has been reported that the synthesis of trimetallic nanoparticles having a Aucore structure by a combination of the preparation of bimetallic nanoparticles by coreduction with the formation of core/shell-structured bimetallic nanoparticles by selforganization in physical mixture ( Figure 5). The formation of trimetallic nanoparticles has been suggested by UV-Vis spectral change, TEM image change, FT-IR spectra of adsorbed carbon monoxide, XPS spectra and calorimetric studies. The catalytic activity of trimetallic nanoparticles in the molar ratio of Au/Pd/Rh = 1/4/20 was higher than the corresponding monometallic and bimetallic nanoparticles for hydrogenation of methyl acrylate. This high catalytic activity can be understood by sequential electronic charge transfer from surface Rh atoms to interlayered Pt atoms and then to core Au atoms (Toshima et al., 2011).

Catalytic activity of gold-core/platinum-shell bimetallic nanoparticles
The bimetallization of metal nanoparticle can improve the catalytic activity of surface metal. Especially, core-shell structured nanoparticles are important. Several study demonstrated the Au-core/Pt shell metal nanoparticles show higher catalytic activity for the visible-lightinduced hydrogen generation than Pt monometallic nanoparticles. The following study is an example of the hydrogen generation using Au/Pt nanoparticle catalyst (Yonezawa & Toshima, 1993). In this study, the Au/Pt bimetallic systems stabilized by polymer and micelle were obtained by alcohol-and photo-reduction of the corresponding metal ions in www.intechopen.com the presence of water-soluble polymers and non-ionic surfactant-micelles, respectively. The UV-Vis spectra and the transmission electron micrographs suggest that the polymerprotected Au/Pt bimetallic systems are composed of bimetallic alloy clusters, but the micelle-protected ones are mostly composed of the mixtures of the monometallic Au and Pt particles. The in-situ UV-Vis spectra during the reductions can elucidate the formation processes of the bimetallic dispersions which are different from each other depending on the protective reagent. The Au/Pt bimetallic systems can be used as the catalyst for visible lightinduced hydrogen generation. The bimetallic system stabilized by the polymer at a molar ratio of Au/Pt = 2/3 is the most active catalyst.

Application of siver-core/rhodium-shell bimetallic nanoparticles
It has been reported that the catalytic activity of the Ag/Rh bimetallic nanoparticles for visible-light-induced hydrogen generation (Toshima & Hirakawa, 1999) in an aqueous solution composed of ethylenediaminetetraacetic acid, tris(bipyridine)ruthenium(II), methyl viologen, and metal nanoparticle catalyst. The activity is clearly higher than the corresponding monometallic nanoparticles and alloy-structured Ag/Rh nanoparticles, suggesting that the Ag-core shows an electronic effect on the surface Rh as in the case of the Au-core (Yonezawa & Toshima, 1993) and enhances the catalytic activity of the surface Rh. The highest catalytic activity was observed at 1:9 ratio of Ag and Rh atoms ( Figure 7). Similar results reported on the other catalytic reactions.

Carbon dioxide reduction by visible-light-induced electron transfer system using metal nanoparticle
A photochemical reduction of CO 2 can be applied to a novel energy storage process for the utilization of solar energy in the future. The above mentioned catalytic system can be applied to CO 2 reduction. The strategy is the catalytic reduction of CO 2 using electrons gathered by an electron transfer system ( Willner et al., 1987. It has been reported that nanoparticles catalyzes the reduction of CO 2 and the generation of methane   (Figure 8).  Fig. 8. Schematic diagram of the visible-light induced CO 2 reduction using the electron transfer system and metal nanoparticle catalyst The possible reaction scheme of the CO 2 reduction is as follows: This eight-electron reduction of CO 2 is advantageous process compared with other possible CO 2 reduction process from the thermodynamic point of view. Although it is not a study using the silver-core bimetallic nanoprticles, this topic is closely related to the applications of bimetallic nanoparticles to catalytic reaction. Thus, the topic of the CO 2 reduction using metal nanoparticle catalyst is presented here.
Typical reactions were performed by the similar manner to the hydrogen generation. A 20cm 3 Pyrex Schlenk tube was charged with a 10 cm 3 aqueous solution, containing EDTA (a sacrificial electron donor), [Ru(bpy) 3 ] 2+ (a photosensitizer), MV 2+ (an electron mediator), NaHCO 3 (a pH adjuster and a CO 2 source), and colloidal dispersion of metal nanoparticles. The mixtures were degassed by freeze-thaw cycles and the tubes were then filled with 1 atm of CO 2 . The photo-irradiation was carried out for 3 or 4 h with a 500 W super-high-pressure mercury lamp through a UV cut filter (> 390 nm) in a water bath maintained at 30 ˚C. About 100 μmol of methane was detected in this system . However, it has not been confirmed that methane was actually the reduction product of CO 2 .

Strategy for the demonstration of the methane generation from carbon dioxide
In a heterogeneous system, photoreduction of CO 2 was confirmed by experiments using an isotope (Ishitani et al. 1993). To our knowledge, however, the isotopic method has not been applied to the confirmation of the photoreduction of CO 2 in a homogeneous system using the colloidal dispersion of metal nanoparticles. To confirm the above mentioned methane generation, the following study was carried out. In this study, photoreduction of CO 2 was carried out in a similar system to one reported previously , and the generation of methane from CO 2 was confirmed by isotopic experiments. As the catalysts, novel metal nanoparticles, i.e., liposome-protected Pt nanoparticles, were prepared and used in the present system. Colloidal dispersions of Pt and Ru nanoparticles were prepared by photoreduction without using ethanol . Preparation of nanoparticles without ethanol is required, because the coexisting ethanol is decomposed during the photochemical reaction, leading to the formation methane. This methane formation cannot be distinguished from the actual methane generation from CO 2 . Protective agents used for the metal nanoparticles were poly(N-vinyl-2-pyrrolidone), C 12 EO, and liposome. The products in the gas phase were analyzed with a gas chromatograph. The characterization of gaseous products was carried out with a gas chromatograph mass-spectrometer. www.intechopen.com

Methane generation from carbon dioxide reduction
The formation of methane was then clearly detected by gas chromatography (about 19 nmol in the case of the liposome protected Pt nanoparticles system). In order to confirm the methane generation from CO 2 , isotope experiments were carried out using NaH 13 CO 3 as a CO 2 source and analyzed by a gas chromatograph mass-spectrometer. Since NaHCO 3 is equilibrated with CO 2 in solution and easily treated, it was a good source of CO 2 in the present experiments. In this experiment, 13 CH 4 was clearly detected, though the produced methane was not pure 13 CH 4 and it did contain 12 CH 4 . In the same experiment, the mole ratio of 13 CO 2 to 12 CO 2 in the gas phase was about 57:43, which is nearly the same as the isotopic ratio of the generated methane. EDTA works as an electron donor in the system and is known to decompose into CO 2 . Therefore, 12 CH 4 generation possibly occurs through the reduction of 12 CO 2 generated from EDTA. The effect of EDTA on methane generation was examined in the Pt-liposome system. Methane was detected on visible-light irradiation of the system involving EDTA without CO 2 or NaHCO 3 but could not be detected in the absence of EDTA. These results suggest that the detected 12 CH 4 is generated by the reduction of 12 CO 2 originated from EDTA.

Liposome-protected metal nanoparticle catalyst
Liposome was better than other protective-colloid of Pt nanoparticles for methane generation. This is probably explained by assuming that liposome can form a larger and stronger hydrophobic region to concentrate CO 2 around a Pt nanoparticle than C 12 EO micelle and poly(N-vinyl-2-pyrrolidone). In addition, Ru-C 12 EO showed higher catalytic activity than Pt-C 12 EO. Thus, Ru-liposome was considered to be an active catalyst for methane generation in the system tested here. The synthesis of Ru-liposome was tried in a way similar to that of Pt-liposome, but the suspension of the Ru-liposome was not active as a catalyst. The resulting Ru-liposome was not as homogeneous, probably because the Ru ion is not miscible with liposome in water.

Summary of the carbon dioxide photo-reduction by metal nanoparticle catalyst
The Pt and Ru nanoparticle catalysts, which were prepared by a photoreduction method of metal salt in water without ethanol, successfully generated methane from CO 2 . The methane generation suggests that the eight-electron reduction of CO 2 easily proceeds on metal nanoparticles possibly due to a thermodynamic advantage. This is different from an electrochemical CO 2 reduction using Pt electrodes, on which CO 2 is reduced to CO with adsorbed hydrogen atoms. In the present system using metal nanoparticles, the competition reaction, i.e., the kinetically favorably hydrogen generation, inhibits the methane generation. An increase of CO 2 concentration, the electron supply rate, or both may enhance CO 2 reduction.

Catalytic decomposition of hydrogen peroxide by metal nanoparticle
The modification of biomacromolecules upon exposure to reactive oxygen species, including hydrogen peroxide (H 2 O 2 ), dioxide(1-) (superoxide O 2 •-), hydroxyl radical (HO • ), and singlet oxygen ( 1 O 2 ), is the likely initial event involved in the induction of the mutagenic and lethal effects of various oxidative stress agents (Kawanishi et al. 2001;Cadet et al., 2003; www.intechopen.com Drechsel & Patel, 2008). Therefore, the activity of reactive oxygen species generation by various chemical compounds is closely related to their toxicity, carcinogenicity, or both. For example, hydroquinone, a metabolite of carcinogenic benzene, causes DNA damage via H 2 O 2 generation . Many studies have addressed the role of antioxidants, such as vitamins (Slaga, 1995;Sohmiya et al., 2004) and catechins (Weyant et al., 2001), in protection against cancers and cardiovascular diseases. These antioxidants can scavenge reactive oxygen species and protect against cancer occurrence. On the other hand, every antioxidant is in fact, a redox agent, protecting against reactive oxygen species in some circumstances and promoting free radical or secondary reactive oxygen species generation in others. Indeed, an excess of these antioxidants elevates the incidence of cancer (Nitta et al. 1991;Omenn et al., 1996). Solovieva et al. reported that antioxidants, ascorbic acid (Solovieva et al., 2007) and dithiothreitol (Solovieva et al., 2008), exhibit cytotoxicity via H 2 O 2 generation. Relevantly, it has been reported that vitamins A (Murata & Kawanishi, 2000) and E (Yamashita et al., 1998) and catechins (Oikawa et al., 2003) induce DNA oxidation through H 2 O 2 generation during their oxidation. H 2 O 2 is a long-lived reactive oxygen species which plays an important role in biomacromolecular damage induced by various chemical compounds (Kawanishi et al., 2001;Hirakawa et al., 2002).

Metal catalyzes decomposition of hydrogen peroxide
Various studies have demonstrated the catalytic decomposition of H 2 O 2 by noble metals such as Pt (Keating et al., 1965;McKee, 1969;Bianchi et al., 1962), Pd (Keating et al., 1965;McKee, 1969;Bianchi et al., 1962;Eley & Macmahon, 1972) Ag (Baumgartner et al., 1963;Goszner et al., 1972;Goszner & Bischof, 1974), and Au (Eley & Macmahon, 1972;Goszner & Bischof, 1974). These metals themselves are hardly oxidized by reactive oxygen species, however, it is difficult to use metal powder or foils as anti-oxidative drugs. Recently, Kajita et al. reported that Pt nanoparticles catalyze the decomposition of reactive oxygen species (Kajita et al., 2007). These nanoparticles can be dispersed in water and used as homogenous solutions. Because this removal mechanism is catalytic decomposition, no oxidized product is formed through this reaction. Platinum metal is used as a food additive and is not considered to be a toxic material. This result led us to the idea that inorganic materials, in particular noble metals, rather than organic antioxidants, can be used as novel chemopreventive agents against reactive oxygen species-mediated biomolecules damage. In this section, the examination of the removal of H 2 O 2 generated from a chemical compound, hydroquione, using water-soluble polymer-protected Pt and Ag/Pt nanoparticles are reviewed.

Preparation of metal nanoparticles for reactive oxygen scavenger
Colloidal dispersions of poly(N-vinyl-2-pyrrolidone)-protected Pt, Pd, Rh, and Au nanoparticles were prepared using an alcohol reduction method (Hirai et al., 1979). 50 mL of water/ethanol (1/1, v/v) solution containing 1 mM metal salts and 40 mM poly(N-vinyl-2pyrrolidone) (monomer unit) was refluxed for 2 h, resulting in the formation of typical colored sols of metal nanoparticles. The solvent was removed by vacuum evaporation, and the nanoparticles were dispersed into water to prepare 1 mM/atom (atomic concentration) metal colloidal sols. An aqueous solution of poly(N-vinyl-2-pyrrolidone)-protected Ag nanoparticles ( Shiraishi & Toshima, 1999) was prepared from reduction of 1 mM AgNO 3 with NaBH 4 in the presence of 40 mM poly(N-vinyl-2-pyrrolidone). The obtained Ag colloidal dispersion was purified with an ultra-filter.

Method of the detection of hydrogen peroxide
The generated H 2 O 2 was measured by a previously reported method using folic acid (Hirakawa, 2006). This assay is based on the fluorescence enhancement of less-fluorescent folic acid via oxidative decomposition by H 2 O 2 and copper(II) ion into strong-fluorescent 2amino-4-oxo-3H-pterine-6-carboxylic acid (Figure 9). The concentration of H 2 O 2 ([H 2 O 2 ]) can be determined using a calibration curve. A reaction mixture containing folic acid, copper(II) chloride, and the H 2 O 2 sample (or H 2 O 2 generator 4 ) with or without the metal nanoparticle in a sodium phosphate buffer (pH 7.6) was incubated in a microtube for 30 min. After incubation at 37 ˚C, the fluorescence intensity of the reaction mixture at 450 nm was measured using a fluorescence spectrophotometer with 350-nm excitation.

Platinum nanoparticles effectively scavenge hydrogen peroxide
Platinum nanoparticles effectively scavenged H 2 O 2 in a dose-dependent manner and showed the highest activity among the metal nanoparticles used in this study ( Figure 10)

Preparation of silver-core bimetallic nanoparticles for hydrogen peroxide scavenger
The catalytic activity of Pt and its modified particles with Ag (Ag/Pt) on the decomposition of H 2 O 2 generated from chemical compounds was evaluated, since Pt showed the highest activity. The Ag/Pt nanoparticles were prepared from the following procedure. The absorption spectrum of the sol of Pt nanoparticles is a flat curve (Figure 11), indicating the formation of homogenous particles. Ag nanoparticles exhibited a typical yellow color due to surface plasmon absorption around 400 nm. It has been reported that a physical mixture of Ag and Pt nanoparticles spontaneously forms bimetallic nanoparticles, possibly Ag-core/Ptshell structured particles (Toshima et al., 2005). The time-course of the absorption spectra of this physical mixture showed the extinction of Ag surface plasmon absorption, and the absorption was completely extinguished within 24 h (Figure 11), suggesting that the surface of the formed bimetallic nanoparticles is composed of Pt atoms. Typical TEM images showed the formation of relatively small particles of Pt and large particles of Ag ( Figure 12). TEM photographs showed that the large Ag particles disappeared through interaction with Pt particles, resulting in the formation of bimetallic particles smaller than the parent Ag particles (Figure 12). A similar result has been observed in the case of Ag/Rh bimetallic nanopaticles (Toshima & Hirakawa, 2003). These findings suggest the formation of selforganized Ag/Pt bimetallic nanoparticles. These metal nanoparticles are stable in water for several months. The Ag/Pt (Ag-atom/Pt-atom, 1/1) bimetallic nanoparticles were prepared using a self-organization method to mix Pt and Ag monometallic nanoparticles according to previous reports (Toshima & Hirakawa, 2003;Toshima et al., 2002Toshima et al., , 2005Matsushita et al., 2007).

Hydrogen peroxide formation from hydroquinone and its removal by metal nanoparticles
Hydroquinone, which is a metabolite of carcinogenic benzene, was used as H 2 O 2 source. This compound can generate H 2 O 2 through autooxidation ( Figure 13) (Hirakwa et al., 2002). Under these experimental conditions, hydroquinone generated H 2 O 2 in a dose-dependent manner ( Figure 14). Twenty units/mL catalase effectively removed H 2 O 2 generated from this system, and 10 μM/atom (2 μg/mL) Pt nanoparticles exhibited a comparable activity to that of this catalase. Silver nanoparticles showed apparently weaker activity for H 2 O 2 removal than Pt nanoparticles. The bimetallization of Pt with Ag apparently suppressed the catalytic activity per unit atom.   Figure 15 shows the removal activity of H 2 O 2 generated from a high concentration of hydroquinone (50 μM) by metal nanoparticles. These metal nanoparticles and catalase scavenged H 2 O 2 in a dose-dependent manner. The activity of the 10 μM/atom (2 μg/mL) Pt nanoparticles was comparable to that of 20 units/mL catalase, and Pt completely scavenged H 2 O 2 over 20 μM/atom (4 μg/mL). The activity per atom of the Ag/Pt bimetallic nanoparticles was almost the same as that of the Ag monometallic nanoparticles.

Activity of silver-core/platinum-shell nanoparticles on hydrogen peroxide decomposition
To investigate the effect of Pt nanoparticles on H 2 O 2 generation through the autooxidation of hydroquinone, NADH consumption during this autooxidation was measured. The consumption of NADH during the autooxidation of hydroquinone was measured by a previously reported method (Oikawa et al., 2003). A sample solution containing 100 µM NADH, 50 µM hydroquinone, and 20 µM copper(II) chloride was incubated at 37 ˚C in the absence or presence of 20 µM/atom Pt nanoparticles. The concentration of NADH was determined by the measurement of absorbance of NADH at 340 nm using a microplate absorbance reader. The oxidized form of hydroquinone can be reduced into the parent hydroquinone by NADH (Hirakwa et al., 2002). The concentration of NADH was gradually decreased through the redox of hydroquinone and Pt nanoparticles hardly inhibited NADH consumption (data not shown). This result indicated that Pt nanoparticles do not inhibit the H 2 O 2 generation itself, because H 2 O 2 is produced through the autooxidation of hydroquinone.

Summary and possible mechanism of hydrogen peroxide decomposition by metal nanoparticles
Poly(N-vinyl-2-pyrrolidone)-protected metal nanoparticles, in particular Pt nanoparticles, exhibited a removal effect on H 2 O 2 generated through autooxidation of hydroquinone ( Figure 16). The removal of H 2 O 2 by these metal nanoparticles can be explained by a catalytic reaction similar to that by catalase, which decomposes H 2 O 2 into H 2 O and O 2 . The formation of H 2 O 2 during autooxidation of hydroquinone is through O 2 •-, which is generated from a reduction of O 2 by hydroquinone . Because the lifetime of O 2 •-, which dismutates into H 2 O 2 through reaction with H + , is short (~ 0.1 ms), the scavenging of O 2 •-by a metal nanoparticle can be negligible. The H 2 O 2 removal activity per metal atom of these metal nanoparticles occurred in the following order: Pt > Ag ≈ Ag/Pt. The activities of H 2 O 2 decomposition per metal atom consisting of these metal nanoparticles (μM-H 2 O 2 /μM-nanometal) have been estimated, and the resulting values are 4.2, 12.2, and 3.8 for Ag, Pt, and Ag/Pt, respectively. Further, the activity on the surface area of the Ag/Pt nanoparticles (17 μM-H 2 O 2 /cm 2 -nanometal) was also smaller than that of Pt (49 μM-H 2 O 2 /cm 2 -nanometal). These findings showed that the Pt nanoparticles have the highest catalytic activity for H 2 O 2 decomposition in the metal nanoparticles used in this experiment and the activity of Pt nanoparticles is suppressed by modification with Ag.  Fig. 16. Hydrogen peroxide generation from an autooxidation of chemical compound and its catalytic decomposition by metal nanoparticle H 2 O 2 is a long-lived reactive oxygen species and plays an important role in DNA damage (Kawanishi et al., 2001. Indeed, various chemical compounds, including carcinogens, generate H 2 O 2 during redox reaction (Kawanishi et al., 2001. Molecular oxygen is easily reduced by various compounds, leading to the formation of O 2 •-. Formed O 2 •-is rapidly dismutated into H 2 O 2 . Although H 2 O 2 itself is not a strong reactive species, it can generate highly reactive HO • through a Fenton reaction or a Haber-Weiss reaction. Furthermore, H 2 O 2 can penetrate a cytoplasm membrane and be incorporated into the cell nucleus. Therefore, H 2 O 2 is considered to be one of the most important reactive species or a precursor participating in carcinogenesis. The removal of H 2 O 2 is an effective method for cancer chemoprevention. Furthermore, protective agents against H 2 O 2 are important to treat acatalasemia, a genetic deficiency of erythrocyte catalase inherited as an autosomal recessive trait. Antioxidants, such as vitamins A and E, are effective protective agents. However, the oxidized products of antioxidants or these molecules themselves promote the formation of secondary H 2 O 2 (Yamashita et al., 1998;Murata & Kawanishi, 2000). Indeed, an excess of these antioxidants elevates the incidence of cancer (Nitta et al., 1991;Omenn et al., 1996). A catalyst consisting of an inorganic stable material is not oxidized and does not generate secondary reactive oxygen species. Water-soluble nanoparticles of noble metal may become novel protective agents against reactive oxygen species.
In summary, Pt, Ag, and Ag/Pt nanoparticles effectively scavenge H 2 O 2 generated from autooxidation of a highly concentrated hydroquinone. Platinum nanoparticles exhibited the highest catalytic activity among these nanoparticles. Pt is a very stable metal against various chemical compounds and permitted as a food additives. The noble metal nanoparticles may be used as novel chemopreventive agents for cancer or other non-malignant conditions induced by chemical compounds through H 2 O 2 generation.

Application of metal nanoparticles to prevention of ultraviolet radiation induced biomolecules damage
Exposure to solar ultraviolet radiation is undoubtedly linked to skin carcinogenesis and phototoxic effect. Photosensitized reaction by ultraviolet radiation, especially ultraviolet-A (UVA) radiation (320~400 nm), is considered to cause toxic effect through oxidative biomolecules damage including DNA damage (Hiraku et al., 2007). Photosensitized formation of reactive oxygen species, such as hydrogen peroxide, superoxide, hydroxyl radicals, and singlet oxygen, is involved in UVA-induced biomolecules damage. As mentioned above, the application of metal nanoparticles to scavenge reactive oxygen species through catalytic decomposition.

Traditional methods of chemoprevention to biomolecules damage by ultraviolet radiation and its problem
Many studies have addressed the role of antioxidants, such as vitamins and catechins, in protection against cancers and cardiovascular diseases. These antioxidants can scavenge reactive oxygen species and protect against cancer occurrence. On the other hand, every antioxidant is, in fact, a redox agent, protecting against reactive oxygen species in some circumstances and promoting free radical or secondary reactive oxygen species generation in others. Indeed, an excess of these antioxidants elevates the incidence of cancer. It has been reported that antioxidants, ascorbic acid and dithiothreitol, exhibit cytotoxicity via H 2 O 2 generation, and their toxic effects are significantly enhanced by vitamin B 12 . H 2 O 2 is a longlived reactive oxygen species which plays an important role in biomacromolecules damage induced by various chemical compounds.

Preventive action of metal nanoparticles on ultraviolet-sensitized oxidation of molecules
As mentioned above, metal nanoparticles catalyze the decomposition of reactive oxygen species. Because this removal mechanism is catalytic decomposition, no oxidized product is formed through this reaction. Platinum metal is used as a food additive and is not considered to be a toxic material. This result led us to the idea that inorganic materials, in particular noble metals, rather than organic antioxidants, can be used as novel chemopreventive agents against UVA-induced biomolecules damage.
Recently, it has been reported that the removal of reactive oxygen species generated from a photocatalytic reaction of titanium dioxide (TiO 2 ) particles using water-soluble polymerprotected Pt, Rh, and Pt/Ag bimetallic nanoparticles. Silver, a relatively inexpensive noble metal, is also used as a food additive, and bimetallization with Ag may improve the catalytic activity of other metal nanoparticles.

Preparation of metal nanoparticles for ultraviolet protection
The colloidal dispersions of poly(N-vinyl-2-pyrrolidone)-protected Pt and Rh nanoparticles were prepared from an alcohol reduction. The size (particle diameter) of these nanoparticles is about 2 nm. The aqueous solution of poly(N-vinyl-2-pyrrolidone)-protected Ag nanoparticle was prepared from a reduction of silver nitrate by sodium borohydride in the presence of poly(N-vinyl-2-pyrrolidone). The Ag-core/Pt-shell (Ag-atom/Pt-atom, 1/1) bimetallic nanoparticle was prepared using a physical method to mix Pt and Ag monometallic nanoparticles according to the previous reports (Toshima et al., 2005).

Evaluation model for the biomolecules damage by ultraviolet radiation
TiO 2 (anatase) and methylene blue were used as a model of the UVA-induced reaction. The sample solution containing methylene blue and TiO 2 dispersion in sodium phosphate buffer (pH 7.6) with or without metal nanoparticle was irradiated with a UVA lamp (365 nm, 1 mW cm -2 ). The decomposition of methylene blue was evaluated by absorption measurement at 659 nm. TiO 2 is a well-known photocatalyst (Fujishima et al., 2000(Fujishima et al., , 2008. When exposing to UVA light, the reduction-oxidation activity of TiO 2 has a significant biological impact, as is exemplified by its bactericidal activity. Photo-irradiated TiO 2 effectively decomposed methylene blue (Figure 17). Various reactive oxygen species contribute to the photocatalytic reaction of TiO 2 . Especially, hydrogen peroxide is long-lived reactive oxygen species and plays an important role in oxidative biomolecules damage. Molecular oxygen is reduced by photoexcited materials, leading to the formation of superoxide. Formed superoxide is rapidly dismutated into hydrogen peroxide. Although hydrogen peroxide itself is not a strong reactive species, it can generate highly reactive hydroxyl radicals through a Fenton reaction or a Haber-Weiss reaction. Furthermore, hydrogen peroxide can penetrate a cytoplasm membrane and be incorporated into the cell nucleous. Therefore, hydrogen peroxide is considered to be one of the most important reactive oxygen species participating in UVA carcinogenesis and phototoxicity. Since other reactive oxygen species, such as directly produced hydroxyl radicals (Hirakawa et al., 2004) and singlet oxygen (Hirakawa & Hirano, 2006), rapidly quenched in aqueous solution, hydrogen peroxide should be key reactive species in this experiment. The TiO 2 and methylene blue could be used as a simple model of UVA-induced oxidation. Methylene blue Fig. 17. UV-Vis absorption spectra of methylene blue photocatalyzed by TiO 2 . The sample solution containing 10 μM methylene blue and indicated concentration of TiO 2 in 10 mM sodium phosphate buffer (pH 7.6) was irradiated (Ex = 365 nm, 1 mW cm -2 ) for 30 min.

Preventive action of metal nanoparticles on ultraviolet radiation induced biomolecules damage
Poly(N-vinyl-2-pyrrolidone)-protected metal nanoparticles, in particular, the Pt nanoparticle, inhibited the methylene blue decomposition photocatalyzed by TiO 2 ( Figure  18). Poly(N-vinyl-2-pyrrolidone) itself did not inhibit the methylene blue decomposition. The UV-Vis absorption spectra of these metal nanoparticles were hardly changed by the photocatalytic reaction, suggesting that the noble metal nanoparticles are stable for reactive oxygen species and UVA irradiation. Organic antioxidant undergoes oxidation in the removal process of reactive oxygen species, leading to the formation of various oxidized products and may produce secondary reactive oxygen species. In the case of noble metal catalyst, these effects can be negligible. The sample solution containing 20 μg mL -1 metal nanoparticle, TiO 2 , and 10 μM methylene blue in 10 mM sodium phosphate buffer (pH 7.6) was irradiated (Ex = 365 nm, 1 mW cm -2 ) for 30 min.

Summary of the ultraviolet protection by metal nanoparticles
Pt, Rh, and Pt/Ag nanoparticles effectively inhibited the methylene blue decomposition photocatalyzed by TiO 2 . TiO 2 photocatalytic system was used as a UVA-induced reactive oxygen species generation. The most important reactive oxygen species in this photocatalytic reaction is H 2 O 2 , because of its long lifetime in aqueous solution. This inhibitory effect of metal nanoparticle can be explained by the removal of H 2 O 2 . Unexpectedly, the activity of Pt nanoparticle was not improved by the bimetallization with Ag. Platinum is a very stable metal against various chemical compounds and is used as food additive. A poly(N-vinyl-2-pyrrolidone)-protected Pt nanoparticle may be used as a novel preventive agent for UVA-induced biomolecules damage through reactive oxygen species generation.

Conclusion
Physical mixture of Ag and other metal nanoparticles, such as Pt, Rh, and Pd, spontaneously forms the bimetallic nanoparticles with Ag-core structure in aqueous solution. These monometallic nanoparticles can be easily prepared from an alcohol reduction of the corresponding metal ions in the presence of water-soluble polymer such as poly(N-vinyl-2-pyrrolidone), a protective colloid. Aqueous sol of Ag nanoparticles exhibits the surface plasmon absorption around 400 nm. The surface plasmon absorption was diminished through interaction with other metal nanoparticle in the physical mixture of these nanoparticles. This phenomenon was explained by that the Ag nanoparticle was coated by other metal. The transmission electron micrograph and X-ray diffraction measurement confirmed the formation of the Ag-core bimetallic nanoparticles. This reaction can be used to construct the core-shell structured novel bimetallic nanoparticles. The formed nanoparticles act superior character for certain catalytic reactions. The catalytic activity of the silver/rhodium bimetallic nanoparticles for visible-light-induced hydrogen generation in an aqueous solution was examined. This system composed of an electron source, a photosensitizer, an electron relay, and metal nanoparticle catalyst. The activity is clearly higher than the corresponding monometallic nanoparticles, suggesting that the silver-core enhances the catalytic activity of the surface rhodium. On the other hand, the catalytic activity of the decomposition of hydrogen peroxide was decreased by this bimetallization. Platinum nanoparticle effectively catalyzes hydrogen peroxide decomposition. The Ag-core/platinum shell bimetallic nanoparticle, which was prepared by the physical mixing of Ag and Pt nanoparticles, demonstrated lower activity of the decomposition of hydrogen peroxide than the monometallic Pt nanoparticle. Metal nanoparticles can be applied to various catalytic reactions. The bimetallic and trimetallic nanoparticles demonstrate superior activity in the certain reaction. The self-assembly formation of Ag-cored nanoparticle may be convenient method to prepare novel metal nanoparticle catalyst.

Acknowledgments
The author wish to thank Professor Naoki Toshima (Tokyo University of Science, Yamaguchi) for his helpful discussion and Professor Kenji Murakami (Research Institute of Electronics, Shizuoka University) for his helpful advice on TEM measurement. These works were supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government.
Methods. Journal of Molecular Catalysis, Vol.83, No.1-2, (July 1993), pp.167-181, ISSN 1381-1169 In the last few years, Nanoparticles and their applications dramatically diverted science in the direction of brand new philosophy. The properties of many conventional materials changed when formed from nanoparticles. Nanoparticles have a greater surface area per weight than larger particles which causes them to be more reactive and effective than other molecules. In this book, we (InTech publisher, editor and authors) have invested a lot of effort to include 25 most advanced technology chapters. The book is organised into three well-heeled parts. We would like to invite all Nanotechnology scientists to read and share the knowledge and contents of this book.