Preparation of noble metal nanoclusters is very important in the development of optical materials, polymer conductors, catalyst carriers, and other applications. Formation of metallic silver particles from silver salts has been in the focus of interest since the discovery of photography. To achieve optimal performance, it is fundamental to control the size and polydispersity of the silver particles.

It has been shown that dendrimers are able to form complexes with a great variety of ions and compounds and act as excellent templates for nanoparticle fabrication. Dendrimer templates offer usually better control over size, shape, and polydispersity than linear polymers.

Dendrimer nanocomposites (DNCs) are organic/inorganic hybrid materials composed of nanosized particles displaying unique physical and chemical propertiesdue to the atomic/molecular level dispersion of inorganic guest(s) and organic macromolecular hosts.1 DNCs are made by reactive encapsulation, when dendrimers are used to preorganize small molecules or metal ions, followed by an in-situ immobilization of the resulting atomic or molecular domains of inorganic guests within or on their dendrimer host. This method provides nearly uniform hybrid nanoparticles with an excellent control over size and size distribution.

Synthesis of silver dendrimer nanocomposites have been reported both by direct complexation, and by substitution of copper in the preformed {Cu(0)-PAMAM} copper nanocomposite.

Nanocomposites of metallic silver and poly(amidoamine) PAMAM dendrimers {Ag(0)n-PAMAM} are soft, nanosized spherical polymeric particles containing dispersed silver atoms and/or small silver clusters separated by the dendrimer wedges. They are stable and soluble in polar solvents, such as water and methanol. Primary structure of these nanocomposite particles (internal, external, mixed)6 depends on the particular dendrimer (generation, surface, interior, etc.) and the conditions during fabrication.1,6,10 Structurally, these materials are completely different from those when dendrimers are used as stabilizer for preformed crystalline silver colloids.

Structure of these hybrid nanoparticles is in the focus of interest since the first report on DNC materials. However, it is not a simple quest to gain reliable information on the formed structures in solution without changing the soft hybrid particles. TEM and HRTEM are often used to demonstrate the structure of a particular dendrimer nanocomposite, but sample preparation procedures used in these techniques make it impossible to obtain reliable overall information about the hybrid DNC structure existing in solution. Smallangle scattering is the method of choice to determine average particle sizes,15 but this technique grants no information about the internal structure of the metaldendrimer complexes and the nature of the connections between host and guest.

A typical procedure for synthesizing internal 无效 silver-dendrimer nanocomposites from silver complexes of Tris-terminated PAMAM dendrimers by photolysis has previously been reported.6 It was observed that the size of dendrimers usually is not altered by the metal ion complexation.16 In preliminary experiments, solutions of silver-PAMAM complexes proved to be quite insensitive to light: 1 day exposure to daylight resulted in only approximately 50％ conversion based on the increase of intensity of the 420 nm plasmon peak in the visible spectra. Long-term photolysis of the complexed silver ions generated silver(0) nanocomposites, in whichthe homogeneous distribution of silver within the dendrimerdefined nanodomains was retained upon transformation. This was also supported by the UV-visible spectra displaying only one major symmetric plasmon peak at 420 nm.16 On the basis of TEM images, it was also concluded that silver atoms did not migrate considerably in the organic template within the time scale of the investigation.9 According to the featureless and very poor XRD spectra of nanocomposites, these internalized metallic domains are small, are amorphous, and have a very low degree of crystallinity.

Our goal was to determine the structure of the silver ion complexes and nanocomposites and identify the structural differences between silver complexes and nanocomposites. Thus, we studied the ligand structure of [(Ag+)n-PAMAM] silver(I) poly(amidoamine) dendrimer complexes and the respective {(Ag0)n-PAMAM} silver(0) dendrimer nanocomposites of various PAMAM dendrimers by EPR and energy-filtering TEM.

In general, 本地 of the Ag(0) domains depends on the metal/dendrimer ratio, the structure of the template, the kinetics of the immobilizing reaction, and the fabrication procedure. It has originally been assumed that silver ions have randomly filled positions in PAMAM complexes and these positions are determined by the availability of electron donor ligands (tertiary nitrogen branching sites). When silver-DNCs formed in photolysis (i.e., in a process which does not require the diffusion of an other reactant to the silver ions), metal atoms should originally form and appear at the same 本地 where they were bound to the dendrimer structure even though they may undergo redistribution in the polymeric phase of the hybrid nanoparticles.

Electron paramagnetic resonance (EPR) technique was successfully applied before19,20 to gain insight into the detailed structure of copper(II) complexes of the amine and methylester terminated PAMAM dendrimers.

Selection of copper(II) ions as reporter ions is well justified, since their chemistry, spectral properties and ligand character have extensively been investigated. Details of Cu(II) complexation with PAMAM dendrimers have also been examined in details by UV-visible spectroscopy,2,3,22 by small-angle neutron and X-ray scattering,16 and by the combination of ultrafiltration and ion-selective concentration measurements18 both in aqueous and methanol solution.

EPR spectra were detected by adding known amount of copper(II) ions to the preformed silver-dendrimer complexes in comparable concentrations. The same procedure was applied to the solutions of silver nanocomposites made by the X-ray irradiation from the aliquot parts of the silver-PAMAM complex solutions (Scheme 1). Computer-aided analysis of the EPR spectra provided information on the formation of copper complexes in various internal or external 本地s of the dendrimers, as well as the structure of the complexes and nanocomposites as a function of the size and surface of the dendrimers, temperature, silver content, etc.

Energy-filtering TEM was also utilized to confirm and illustrate the structure of the silver nanocomposite particles. Energy-filtering TEM uses the fact that electrons can interact with a sample in a number of ways, including elastic (forward scattering, energy is unchanged) and inelastic scattering. Inelastic scattering results from interactions with the sample where electrons experience energy loss. Electrons can interact inelastically with a sample plasmon, or with atoms in the sample generating an energy loss spectrum corresponding with different aspects of the sample material experienced by the electrons. Element specific energy loss may then be quantitized and can be used to map elements and generate element specific contrast. Energy loss due to plasmon interactions can also be used to provide enhanced contrast in polymers.

Experimental Section

Materials. In this work, poly(amidoamine) dendrimers were used. Amine terminated ethylenediamine core PAMAM dendrimers were purchased from Dendritech, Midland, MI, and were used without further purification. Technical grade chemicals were used with >85％ generational dendritic purity.

Tris- and pivalate-terminated PAMAM dendrimers wereprepared according to the literature.5

These combinations of terminal groups offered cationic and strong electron-donor -NH2 and polar and weak proton donor -NH-C(CH2OH)3 groups as well as hydrophobic -NH-COC( CH3)3 terminal groups with a comparable number of nitrogen ligands and dendrimer template sizes (see Table 1).

High purity AgNO3 and methanol were purchased from Aldrich and were used without further purification. Cupric nitrate was purchased from Sigma and used as received.

Preparation of Silver-Dendrimer Nanocomposites. To reflect the contribution of architectural differences among various dendrimer templates, both the total concentration of N ligands (approximately 0.1 M) and the concentration of silver ions (6.26 _ 10-3 M) were kept constant. This silver concentration was estimated to be a 50％ capacity of the dendrimer total silver binding ability, allowing measurable amount of copper ions to bind. (Relative ratio of reporting ions was between 1.5 and 24.6 Cu2+/dendrimer molar ratio from the generation 2 to the generation 6 dendrimer complexes and nanocomposites.)

Complexation. For complexation experiments, silver nitrate was selected because of its good solubility both in water and in methanol. To ensure identical silver concentration 0.1064 g AgNO3 was dissolved in 50 mL of DI water and 5 mL aliquots were used. Thus, a typical procedure was, as follows: 5 mL of silver nitrate stock solution (10.64 mg AgNO3) was added dropwise with stirring to the weighed amount of PAMAM dendrimer stock solution (20％ w/w in methanol). The volumetric flask was filled up to 10.00 mL with methanol resulting in a 1:1 ) water/MeOH solution of sample containing 0.1 M nitrogen ligands and 5.32 mg of silver ion ([Ag+] ) 6.263 * 10-3 M) for the investigated solutions.

The metal ion/dendrimer ratio is predetermined by the ratio of metal ion moles per dendrimer moles due to the uniformity of dendrimers and the isotropic nature of the diffusion. Accordingly, individual dendrimers will form complexes with an equal and well-defined number of metal atoms per dendrimer molecule which are expressed as average numbers (Table 2).

Photolysis. To ensure complete conversion of silver ions, X-ray irradiation was used with a dose rate of 1.5 Gy/min. The vials containing silver-dendrimer complex solutions were submerged in water. The vials were irradiated from the side for appropriate buildup of dose. The applied doses were 2 _ 1000 rad to make sure that the photolysis is complete.

Exposure of the complex solutions to visible light over an extended time yielded yellow to light brown silver nanocomposite solutions, which were stable even at 10％ w/w nanocomposite solutions. DNC solutions were stored in Parafilm-sealed volumetric flasks under nitrogen in the dark at room temperature.

Preparation of the Samples for EPR Analysis. Stock solutions of cupric nitrate were prepared in dry methanol and added to the dendrimer solutions to obtain final concentrations of 0.01 and 0.005 M in Cu2+ and 0.2 M in dendrimer external surface groups (for simplicity and for a better comparison with previous works,19,20 the dendrimer concentration in the EPR analysis was calculated in external surface groups). About 100 ÌL of freshly prepared solutions were inserted in 2 mm glass tubes, sealed, and immediately processed by EPR. Indeed, aging of the solutions did not change the results for at least 1 month.

Energy Filtering TEM. TEM samples were prepared as reported previously.6 In some of the images, contrast appears inverted because we were using the energy filter to create an image using inelastically scattered electrons. Dendrimers are visible as white spots in these images (and can be virtually invisible in normal TEM). The resolution is lower in the inelastic images because (A) a lower proportion of the electrons hitting the sample was used to form the image resulting in increased speckling and (B) a fairly wide slice of the inelastic loss spectrum (i.e., electrons with a variety of energies) was used to focus and form an image, which sometimes results in chromatic aberration, etc. By selecting a “slice” of inelastically scattered electrons, we can distinguish the polymeric dendrimers from the polymeric film substrate used in sample preparation, because the two substances have different inelastic interactions with electrons (even though their elastic interactions are virtually identical). Inelastic interactions are very sensitive to the electronic structure of the sample.

The “normal” images are elastic brightfield onessthe inelastic electrons have been filtered out to maximize crispness and contrast from the elastic electrons. In an unfiltered TEM image, a high magnification image is created using electromagnetic lenses after the electron beam interacts with the sample. Inelastic interactions between the electrons and the sample result in a spectrum of electron energies in the beam after it interacts with the specimen, resulting in increased chromatic aberration which limits both contrast and resolution. In an elastically filtered image, the aberration and background from nonelastic interactions with the sample is removed, resulting in high contrast images even at high voltages.

Techniques. The starting materials and the obtained products were carefully characterized by different analytical techniques. UV-visible spectra were obtained on a Perkin- Elmer Lambda 20 spectrophotometer at room temperature between 200 and 1100 nm in a Suprasil 300 quartz cell (L ) 1 mm). 1H and 13C NMR measurements were carried out on a Bruker 500 multinuclear spectrometer equipped with a temperature controller. Size exclusion chromatography was performed on three TSK gel columns (4000, 3000, and 2000) using a Waters 510 pump with a Wyatt Technology Dawn DSP-F MALLS and Wyatt Technology 903 interferometric refractometer and a Waters 510 pump with a Waters 410 differential refractometer, respectively.

An AECL Theratron 80 high intensity cobalt-60 source was used to irradiate the silver complex solutions.

Energy-filtering TEM images were taken at Leo Electron Microscopes in Oberkochen, Germany, using Formvar coated carbon grids after appropriate dilution. A LEO 922 with a highresolution pole piece and an Omega energy filter was used to obtain energy filtered images of the dendrimers at 200 kV. Images were captured using a slow scan CCD camera. Grids were screened prior to analysis in Germany at the University of Massachusetts Amherst using a JEOL 2000 Mark II with a high-resolution pole piece operated at 200 kV and images were collected on “Kodak electron image plates” EM film. The grids used for HRTEM were carbon support film 400 mesh copper grids. The carbon films were rendered hydrophilic by plasma cleaning. Á-Irradiated silver nanocomposites were imaged. A glovebox with a dry nitrogen atmosphere was used to prevent change of oxidation state of the metals within the dendrimer nanocomposites during sample preparation.

EPR spectra were recorded by means of a EMX-Bruker 包含ument interfaced to a PC computer with the Bruker software for spectra recording and handling. The temperature was controlled by means of a Bruker ER4131VT temperature controller. Magnetic parameters were measured referring to 1,1-diphenyl-2-picryl-hydrazyl (DPPH) standard (g ) 2.0036) and 执行uated by suitable computer programs for spectral computation. Low-temperature spectra, typical of Cu(II) in a glasslike environment, were computed by means of the CU23GP program, kindly provided by Prof. Romanelli, University of Florence, Florence, Italy. Accuracy in determining the magnetic and mobility parameters, as well as the relative percentages of the spectral components, was 5％, as obtained from the fitting between the experimental and the computed signals.

Results and Discussion

Dendrimers of various generations (generation 2 to generation 6) with different surfaces (primary amine, tris(hydroxymethylamino)methane, and pivalate) were used as templates and as organic components of the nanocomposites. Complexation of metal ions depends on the metal/dendrimer ratio and the composition, generation, and structure of the PAMAM dendrimers, but it does not depend on the dendrimer concentration itself. 23,9,11 In our experiments, both the overall concentration of nitrogen ligands as well as the silver ion concentration were kept constant. Variation in the silver/PAMAM relative molar ratio therefore is expected to be the result of the variations in template architecture and generation.

The concentration of the dendrimers was kept sufficiently low (Table 2) not to favor intermolecular bonding between the templates, since aggregation of templates may occur in solution, and it has also been demonstrated that Ag-N coordination bonds can be reliably used for the construction of supramolecular networks.23 In the case of silver-PAMAM complexes, these networks could form within the dendrimers and between the complex containing macromolecules when primary amine surface groups are present that can act as ligands (as opposed to tert-butyl and Tris termini).

Dendrimer-silver complexes were transformed into nanocomposites by irradiating the [(Ag+)n-PAMAM] aqueous solutions with an excessive dose of gamma-rays to avoid the introduction of further chemicals. Scheme 1 delineates the general procedure followed. Since silver does not have a measurable signal in EPR, copper(II) ions were used as a probe. On the basis of general stability constants, copper(II) complexation with small molecules is preferred to silver(I) with otherwise identical nitrogen ligands.21 Due to the equilibrium nature of the complexation process, a redistribution of complexing sites is expected to take place when both Cu(II) and Ag(I) ions are present. In other words, Cu2+ ions will compete with Ag+ ions in the mixed complexes and they will occupy the space available in the dendrimer structure.

Cu(II) ions will not disturb the structure of the preformed silver(0) DNC, because silver is a less electronegative metal, and it has been shown that silver substitutes copper in its compounds21 and in preformed {Cu(0)-PAMAM} copper nanocomposites.14 However, Cu(II) complexation will be hindered near ligand sites preoccupied by the silver atoms in the hybrid nanocomposite particles. As a consequence, ligand structure of Cu(II) complexes of nanocomposites will provide indirect but useful structural information regarding the positioning of silver ions and metallic silver domains in the host dendrimers. In our study, the structures of Cu(II) complexes were detected and analyzed for information regarding the silver components.

Intensity of the silver plasmon resonance in the DNC solutions was found to be a function of nanoparticle size and architecture: it was observed to be increasing for increasing generations of amine-terminated PAMAMs (Figure 1). Increasing intensity suggests increasing nanoparticle size or increased concentration.25 As the overall silver concentration (and silver/nitrogen ratio) was kept constant throughout the nanoparticle fabrication process, the molar concentration of the templates was observed to be decreasing with increasing dendrimer generations and the number of silver ions per macromolecule was observed to be increasing. (Every generation roughly doubles the molecular mass of a PAMAM.) It is demonstrated in both Figures 1 and 2 that the intensity of the silver plasmon peaks clearly increases with increasing generations until the overall diameter of the DNC reaches about 4 nm. The low intensity peaks observed for {(Ag0)3.71-PAMAM•E3.P} and (Ag0)7.41-PAMAM•E4.T} nanocomposites can be explained with the formation of internal nanocomposites that contain smaller metal domains. Tris- and pivalateterminated dendrimers can form internal silver complexes only as their terminal groups do not interact.

{(Ag0)15.87-PAMAM•E5.P} displayed a surprisingly intense plasmon resonance (Figure 2). One can speculate

that in the case of a dense dendrimer surface structure, such as for PAMAM•E5.P complexes and nanocomposites may form only in the peripheral region. A more detailed investigation is in order to explain this observation.

EPR Analysis of the Cu(II) Complexes of the Silver-Dendrimer Complexes [(Cu2+)i(Ag+)n- Em.A], [(Cu2+)j(Ag+)n-Em.P], and [(Cu2+)i(Ag+)n- PAMAM•Em.T]. EPR samples have been prepared with Cu(II) at 0.01 and 0.005 M present in the samples. EPR spectra were recorded at 160, 298, and 358 K. Analysis of both room temperature and low-temperature spectra was carried out by means of suitable computer programs. The spectral computation allowed us to extract the magnetic parameters, namely the components of the g tensor for the Zeeman coupling between the electron spin and the magnetic field and the components of the A tensor for the coupling between the electron spin and the nuclear spin. The increase in A components with the correspondent decrease in the g components of powder spectra arises from a variation in Cu(II) coordination, such as substitution of oxygen ligands with nitrogen ligands, and a variation in the complex structure. By comparison of the magnetic parameters with those found in the literature for several Cu(II) complexes, the coordinating groups and the structure of the complexes are proposed. All the spectra shown in the present study are typical of Cu2+ complexes with an elongated octahedral structure (square planar, with dx2-y2 ground level), and an almost axial symmetry (gzz > gxx _ gyy). Also, the line widths are included as 无效 data in the calculation. An increase in line width is often attributable to inhomogeneous sources and spin-spin interactions. The room-temperature spectra were analyzed by means of the program of Schneider and Freed,24 which not only allows us to confirm the magnetic components 执行uated from the analysis of the spectra at low temperature and, therefore, the proposed coordination and structure of the complexes but also let us 执行uate the correlation time for the reorientational mobility, Ùc. “Slow motion” conditions correspond to 1 _ 10-9s < Ùc < 5 _ 10-7 s and led to a partial resolution of the anisotropy of the magnetic components. It was possible to 执行uate the “not rigid” components, A|¢, g|¢ and g^¢ (usually A^¢ is too small to be 执行uated). “Fast mobility” and “rigid mobility” are of course at the two outer extremes (Ùc < 1 _ 10-9 s and Ùc > 5 _ 10-7 s, respectively).