Shell Nanoparticles

An additional method for preparing DENs can be described as the sequential reduction method. This method, shown in Fig. 4.4, involves the initial complexation and reduction of a seed metal (Ma), followed by the complexation and subsequent reduction of the second metal (Mb) to produce the MaMb system. The synthetic utility of this method is that it provides the means to access both well mixed and core-shell type DENs. When the reducing agent used for Mb is a mild reductant, such as H2 or ascorbic acid, core-shell nanoparticles with an Ma core and Mb shell can be selectively prepared (see Table 4.1). In the nomenclature for these core-shell nanoparticles, [Ma] denotes the core metal(s) and (Mb) indicates the exterior metal shell. Several bimetallic DENs have been prepared via this route, including AuAg [40, 41], [Au](Pd) [32], [Pd](Au) [32], [Au](Ag) [40] and [AuAg](Au) [40], 

UV-visible spectroscopy has also proven to be a valuable tool for characterizing DENs. Rayleigh scattering gives rise to the monotonic increase in absorption as wavelength decreases, showing the initial preparation of nanoparticles [43]. Au and Ag nanoparticles have intense surface plasmon bands that are valuable additional spectroscopic tools [43-45]. These bands, shift with particle size and 

DEN Measured mean particle diameter Calculated theoretical diameter b 

X-ray photoelectron spectroscopy (XPS) provides further evidence for the complete reduction of the metal ions to nanoparticles. A representative XPS study from Rhee s group explored the degree of metal reduction in bimetallic nanoparticles using the G4-OH(Pd Pt ) system [31]. In this study, the Pd/Pt ratio was varied while maintaining a constant metal dendrimer ratio of 55 1. The peaks corresponding to Pd(3ds/2) and Pd(3d3/2) at 337.6 and 342.7eV, respectively, were assigned to Pd. After reduction, these peaks shift to 334.9 and 340.5eV, respectively, and are consistent with Pd [46]. Comparable shifts were observed for Pt The Pt peaks at 72.5 eV (Pt(4f7/2 and 75.7eV (Pt(4fs/2)) shift to 71.3 and 74.4eV, respectively, upon reduction and are consistent with Pi . Peaks for unreduced Pd and Pt were not reported, suggesting that complete reduction of both metals occurred. 

Just as DENs particle sizes have some dishibution (albeit relatively narrow), there is surely some distribution in particle compositions for bimetallic DENs. 

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Synthesis of Ni/Pd bimetallic core/shell nanoparticles and their applications to Sonogashira coupling reactions... 

Nanoparticles of Mn and Pr-doped ZnS and CdS-ZnS were synthesized by wrt chemical method and inverse micelle method. Physical and fluorescent properties wra cbaractmzed by X-ray diffraction (XRD) and photoluminescence (PL). ZnS nanopatlicles aniKaled optically in air shows higher PL intensity than in vacuum. PL intensity of Mn and Pr-doped ZnS nanoparticles was enhanced by the photo-oxidation and the diffusion of luminescent ion. The prepared CdS nanoparticles show cubic or hexagonal phase, depending on synthesis conditions. Core-shell nanoparticles rahanced PL intensity by passivation. The interfacial state between CdS core and shell material was unchan d by different surface treatment. 

Fig. 6 shows PL spectra of CdS nanoparticles and CdS-ZnS core-shell nanoparticles. In PL spectrum of CdS nanoparticles, the emission band is seen at around 400nm. The emission band of CdS-ZnS core-shell nanoparticles is higher dian that of CdS ones at around 400nm. The PL enhancement of CdS-ZnS core-shell nanoparticl is due to passivation which means that surface atoms are bonded to the shell material of similar lattice constant and much larger band gap [9], Althou the sur ce treatment conditions are different, the ranission band of CdS-ZnS core-shell nanoparticles is same in PL spectra of Fig. 6(b). This indicates that interfacial state between CdS core and shell material was unchan d by different surfaKs treatment. 

CdS and CdS-ZnS core-shell nanoparticles were synthesized by inverse micelle method. Crystallinity of CdS nanoparticles was hexagonal structure under the same molar ratio of CM and S precursor. However it was changed easily to cubic structure under the condition of sonication or higher concentration of Cd than S precursor. The interfacial state betwran CdS core and shell material was unchanged by different surface treatment. 

Shanker et al. [120] prepared bimetallic Au-core/ Ag-shell nanoparticles by the simultaneous reduction of Au(III) and Ag(I) ions in the presence of neem (Azadirachta indica) leaf broth as an extracting agent. 

Competitive reduction of Au(III) and Ag(I) ions occurs simultaneously in solution during exposure to neem leaf extract leads to the preparation of bimetallic Au-core/Ag-shell nanoparticles in solution. TEM revealed that the silver nanoparticles are adsorbed onto the gold nanoparticles, forming a core/sheU structure. Panigrahi et al. [121] reported that sugar-assisted stable Au-core/Ag-shell nanoparticles with particles size of ca. 10 nm were prepared by a wet chemical method. Fructose was found to be the best suited sugar for the preparation of smallest particles. 

Michaelis and Henglein [131] prepared Pd-core/Ag-shell bimetallic nanoparticles by the successive reduction of Ag ions on the surface of Pd nanoparticles (mean radius 4.6 nm) with formaldehyde. The core/shell nanoparticles, however, became larger and deviated from spherical with an increase in the shell thickness. The Pd/Ag bimetallic nanoparticles had a surface plasmon absorption band close to 380 nm when more than 10-atomic layer of Ag are deposited. When the shell thickness is less than 10-atomic layer, the absorption band is located at shorter wavelengths and the band disappears below about three-atomic layer. 

Recently many core/shell nanoparticles were successfully prepared by the successive or two-step reduction. 

Yang et al. found that Ag-core/Pt-shell nanoparticles with a core/shell could only be formed by the successive reduction method using Ag nanoparticles as the seeds. Results of measurements of UV-Vis, TEM, EDX, and XPS supported the core/shell structure of the bimetallic nanoparticles. The reverse order of preparation using Pt nanoparticles as the seeds did not provide any core/shell nanoparticles while a physical mixture of Ag nanoparticles and the original Pt seeds was obtained [140]. 

Kan et al. reported preparation of Au-core/Pd-shell bimetallic nanoparticles by successive or simultaneous sonochemical irradiation of their metal precursors in ethylene glycol, respectively. In the successive method, Pd clusters or nanoparticles are first formed by reduction of Pd(N03)2, followed by adding HAUCI4 solution. As a result, Au-core/Pd-shell structured particles are formed, although Pd-core/Au-shell had been expected. In their investigations, the successive method was more effective than the simultaneous one in terms of the formation of the Au-core/Pd-shell nanoparticles [143]. 

Lee et al. [145] succeeded in preparation of Co-based bimetallic nanoparticles with core/shell structure via transmetalation reaction (Figure 3). The Co-core/Au-shell nanoparticles, e.g., were confirmed to be almost the same in particle size with the seeded Co nanoparticle, as shown in Figure 4. 

In summary, we concluded that the successive reduction method easily provides the bimetallic nanoparticles with the core/shell structure according to versatile design. For example, different reducing agents may be used for the first reduction and the second one, respectively, depending on the property of the metal. In some cases of two kinds of metals with much different redox potentials, however, inverted core/shell nanoparticles are difficult to form even in the successive reduction. The inverted core/shell structure can be realized by an... 

Figure 3. Schematic illustration of core/shell nanoparticle formation via redox transmetalation process. Metal ions (Mu) of reactant metal complexes (Mn-L ) are reduced on the surface of Mi nanoparticles while neutral Mi atoms are oxidized to Mi " by forming a Mi-ligand complex (Mi-Lj) as a resultant reaction byproduct. Repeating this process results in the complete coverage of shell layers on core metals. (Reprinted from Ref [145], 2005, with permission from American Chemical Society.)...

Figure 4. TEM and HRTEM images of (a) 6.5 nm Co nanoparticles and (b) Co-core/Au-shell nanoparticles using Co nanoparticles as the seed material. Lattice distances measured by HRTEM as well-matched to known Au lattice parameters for the (111) plane (inset). The average size of the Co-core/Au-shell nanoparticles is ca. 6.4 nm, which is similar to the initial size of the Co nanoparticles because the atom exchange process is the only operative reaction. (Reprinted from Ref [145], 2005, with permission from American Chemical...

Bimetallic Ag-core/Au-shell nanoparticles, prepared by a NaBH4 reduction method, were directly confirmed by HRTEM [124],... 

Figure 6. TEM images obtained at an accelerating voltage of 200 kV of Pt-core/Ru-shell nanoparticles prepared by sequential soni-cation of 1 mM Pt(ll) and 1 mM Ru(lll) ions at 213 kHz. Two representative particles are shown at different magnification. (Reprinted from Ref [141], 2006, with permission from American Chemical Society.)...

Chen et al. [123] examined the amount-dependent change in morphology for a series of Au/Pt bimetallic nanoparticles. The EXAFS results confirmed the formation of a core/shell structure and inter-diffusion between Au and Pt atoms. The composition of the shell layer was found to be Pt-enriched AuPt alloy. They also characterized bimetallic Ag-core/Au-shell nanoparticles by the EXAFS [124]. 

Figure 12. FTIR spectra of CO adsorbed on PVP-protected Pd-core/Pt-shell nanoparticles having a Pt-shell (a) Pt Pd = 2 1 (b) Pt Pd = 1 1 (c) Pt Pd = 1 4. [Pd] 0.1 mmol in lOmL of CH2CI2. (Reprinted from Ref [132], 1997, with permission from American Chemical Society.)...

Core-shell nanoparticles, 7 Cytochrome c, 603, 610, 637-647 Cytochrome c oxidase, 603, 621, 637-647... 

Harada et al. [62] achieved Pd core-Au shell nanoparticles by a co-reduction method. The difference in the structure was argued to be due to the difference in the reduction potentials of Pd and Au ions. When Au ions were added in the presence of Pd nanoparticles, some Pd° atoms of the nanoparticles were oxidized and reduced Auni ions, the oxidized Pd ions were reduced again by the reductants, such as, alcohols. This process led to the formation of particles with core-shell structure in the co-reduction method. 

Hodak JH, Henglein A, Giersig M, Hartland GV (2000) Laser-induced inter-diffusion in AuAg Core — shell nanoparticles. J Phys Chem B 104 11708-11718... 

Arias, J.L., Lopez-Viota, M., Ruiz, M.A., Lopez-Viota, J. and Delgado, A.V. (2007) Development of carhonyl iron/ ethylcellulose core/shell nanoparticles for biomedical applications. International Journal of Pharmaceutics, 339, 237-245. 

Hu Y, Litwin T, Nagaraja AR et al (2007) Cytosolic delivery of membrane-impermeable molecules in dendritic cells using pH-responsive core-shell nanoparticles. Nano Lett 7 3056-3064... 

These materials have possible utility in a number of specialty applications and are being explored by Guan et al. [37], They have used these catalysts, and their unique chain-walking characteristics to synthesize a variety of dendritic materials (Fig. 4), which could find potential application as processing aids, rheological modifiers, and amphiphilic core-shell nanoparticles for drug delivery and dye formulation. 

The preparation of both, the particles themselves and the protective surface layer, has direct influence on their cytotoxicity. It is common belief that in the case of core/shell nanoparticles, properly prepared, close shell or multiple shells such as ZnS/Si02-shells prevents the leakage of toxic elements and thus makes cytotoxicity unlikely. Naturally, a better solution is to avoid cytotoxic materials in the first place. QDs, for example, can be synthesized without utilization of any class A or B elements InP/ZnS QDs have photophysical properties comparable to those of CdSe-based systems [43, 93]. Principally, whenever a new approach for QD synthesis or coating is used or if the QDs are applied in an extreme environment that could compromise their integrity, it is recommended to assess their cytotoxicity. 

Tovmachenko OG, Graf C, van den Heuvel DJ, van Blaaderen A, Gerritsen HC (2006) Fluorescence enhancement by metal-core/silica-shell nanoparticles. Adv Mater 18 91-95... 

Lyon, J.L., Fleming, D.A., Stone, M.B., Schiffer, P., Williams, M E., 2004. Synthesis of Fe oxide core/Au shell nanoparticles by iterative hydroxylamine seeding. Nano Lett. 4, 719-723. 

Alayoglu S, Nilekar AU, Mavrikakis M et al (2008) Ru-Pt core-shell nanoparticles for preferential oxidation of carbon monoxide in hydrogen. Nat Mater 7 333-338... 

Tao F, Grass M, Zhang Y, Butcher D et al (2008) Reaction-driven restructuring of Rh-Pd and Pt-Pd core-shell nanoparticles. Science 322 932-934... 

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