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Title of Talk:
Synthesis, Characterization and Manipulation of {FeS-PAMAM} Dendrimer Nanocomposites

Abstract of Talk:
Synthesis, Characterization, and Manipulation of {FeS-PAMAM} Dendrimer Nanocomposites

Xiangyang Shi and Lajos P. Balogh*

Center for Biologic Nanotechnology, University of Michigan, Ann Arbor, MI 48109-0533. E-mail: baloghl@umich.edu

INTRODUCTION
Nanoparticles, because of their size, unusual crystal shapes, and lattice order, have received great scientific and technological interest in environmental remediation.1 FeS-based minerals are a particularly important viable reactive medium, which has higher reactivity than zero-valent iron in the reductive dechlorination of chlorinated hydrocarbons.2 It is expected that nano-sized FeS particles have much higher reactivity because of their higher surface area than bulk particles. Dendrimers are a novel class of polymers with close to spherical shape and narrow size-distribution that can be used as templates to form relatively monodispersed dendrimer nanocomposites 3 containing metal sulfides.4 In this study, FeS nanoparticles were synthesized using generation 4 polyamidoamine (PAMAM) dendrimers with amine (E4.NH2), hydroxyl (E4.N(Gly)OH), and carboxyl (E4.SAH) terminal groups as templates. These {{FeS-PAMAM}} nanocomposites were characterized by UV-Vis spectroscopy, zeta potential measurements, transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energy dispersive spectroscopy (EDS). To fabricate {FeS} nanocomposite films, preassembled multilayers of PAMAM/poly(sodium styrenesulfonate) (PSS) were employed as matrices to facilitate the binding of Fe2+ in polymer multilayers for subsequent FeS nanoparticle (NP) formation. The conditions for layer assembly of PAMAM/PSS were optimized by varying molecular weights of PSS, adsorption time, dendrimer generation, and dendrimer terminal groups. The formed NP films were characterized by UV-Vis spectrometry and X-ray photoelectron spectroscopy (XPS). Deposition of {FeS-PAMAM} nanocomposites onto mesoporous silica gel was also attempted. Preliminary zeta-potential measurements show that the surface charge of silica gels was reversed from negative to positive, indicating the formation of coating by {FeS-PAMAM} nanocomposites.

Uniform {FeS} nanoparticles have been successfully synthesized using E4.NH2, E4.N(Gly)OH, and E4.SAH PAMAM dendrimers as templates. The formed nanocomposite particles are polycrystalline as confirmed by high-resolution TEM and SAED. The nanoparticle morphology can be varied using different anions. The UV-Vis absorbance spectra show an increase at 250-1000 nm range proportional to the amount of FeS nanoparticles present. {FeS} nanoparticles had different absorbance profiles when PAMAMs of different termini were used as a template. Zeta potential measurements show that the formed {FeS-PAMAM} nanocomposites have the same polarity as the dendrimer templates do. To deposit FeS nanoparticulate films onto substrates, PAMAM/PSS multilayers were constructed by the LbL self-assembly method, and these preassembled multilayers of PAMAM/PSS were also employed as matrices to prepare FeS nanoparticles. The formed FeS nanoparticulate films have less absorbance than the corresponding PAMAM/PSS multilayers. X-ray spectroscopy qualitatively confirms the presence of iron and sulfur elements. In addition, deposition of {FeS-PAMAM} nanocomposites onto mesoporous silica gels was also confirmed by preliminary zeta-potential measurements, showing reversal of surface charge after the deposition of {FeS-E4.NH2} nanocomposites.

ABSTRACT:
Nanoparticles have received great scientific and technological interest in environmental remediation. It is expected that nano-sized FeS particles have much higher reactivity than FeS-based minerals because of their higher surface area. Dendrimers are a novel class of polymers with a spherical shape and narrow size-distribution that are used as templates to form monodispersed dendrimer nanocomposites containing metal sulfides. In this study, FeS nanoparticles were synthesized using generation 4 polyamidoamine (PAMAM) dendrimers with amine (E4.NH2), hydroxyl (E4.N(Gly)OH), and carboxyl (E4.SAH) terminal groups as templates. These {{FeS-PAMAM}} nanocomposites were characterized by UV-Vis spectroscopy, zeta potential measurements, transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energy dispersive spectroscopy (EDS).

SYNTHESIS AND CHARACTERIZATION OF NANOCOMPOSITES:
Upon addition of sulfide anions to the Fe(II)-PAMAM complexes, monodisperse {FeS} DNC particles form as a result of homogenous nucleation and simultaneous cluster growth. Figure 1 shows the UV-vis spectra of {FeS-E4.NH2} nanocomposites in aqueous solution.

Figure 1 UV-Vis Spectra of FeS-E4.NH2 nanocomposites containing different amount of FeS nanoparticles.

In the presence of {FeS} nanocomposites, a strong buildup in absorbance at 250-1000 nm is observed in the spectra, indicating the formation of FeS clusters. The peak at 630 nm may be attributed to interparticle interactions. In the case of {FeS-E4.N(Gly)OH} nanocomposites after the formation of FeS nano-domains, a prominent broad band develops at 250-800 nm as well as another one at 610 nm. With the increase of the {FeS} concentration, the intensity of these bands increases. The UV-Vis spectra of {FeS-E4.SAH nanocomposites are quite different from those of {FeS-E4.NH2} and {FeS-E4.N(Gly)OH}. The absorbance profile of {FeS-E4.SAH} nanocomposites is quite similar to the template E4.SAH dendrimer in the low molar concentration range. When the FeS amount is increased, a broad band profile at 250-1000 nm can also be observed. The sensitivity of FeS clusters to oxidation makes the exact assignment of the peaks in the UV-Vis spectra difficult. All the “as-prepared” {FeS-PAMAM} nanocomposites are quite stable and soluble under anaerobic conditions.

Surface charge of {FeS-PAMAM} nanocomposites was determined by zeta-potential measurements. All of the {FeS-PAMAM} nanocomposites have the same polarity as the PAMAM templates do. It was found that although the zeta potential values are fairly similar, the morphologies of {FeS-E4.NH2} prepared from different iron (II) salts are different, as demonstrated by TEM.

Figure 2 TEM images of {FeS-E4.NH2} (Sample ID: 3xys89-3) nanocomposites. (a) A low magnification TEM image; Inset: SAED pattern; (b) a high-resolution TEM image showing individual nanocomposite particles.

Figure 2 shows the TEM images of {FeS-E4.NH2} nanocomposites. It is clear that the formed particles are dominantly spherical with a diameter of d= 4-6 nm). Only a small portion of needle-shaped nanocrystals can be found in the TEM images (Figure 2a). High-resolution TEM image confirms the polycrystalline nature of the particles (Figure 2b). SAED pattern (inset of Figure 2a), which is composed of rings and bright dots, further verifies the polycrystalline phase of the formed FeS-E4.NH2 nanocrystals. It is interesting to note that the morphology of FeS-E4.NH2 nanocomposites can be tuned by varying the different anions from predominantly round-shaped {FeS-E4.NH2} nanocomposite particles to a needle-like shape with the length of 15-32 nm and diameter of about 2 nm. The composition of the FeS-PAMAM nanocomposites has been confirmed by EDS.

FABRICATION OF MULTILAYER STRUCTURES
To fabricate {FeS} nanocomposite films, either nanocomposites were deposited or preassembled multilayers of PAMAM/poly(styrenesulfonate) (PSS) were employed as matrices to facilitate the binding of Fe(II) ions. Polyelectrolyte multilayers are useful matrixes to confine inorganic nanoparticles, because the mass transfer of the nanoparticles within the multilayers is significantly limited compared with the solution synthesis, thus overgrowth and aggregation of nanoparticles can be prevented.

The conditions for layer assembly of PAMAM/PSS multilayers were optimized by varying the deposition parameters (molecular weight of PSS, adsorption time, dendrimer generation, and dendrimer terminal groups). The formed nanoparticulate films were characterized by UV-Vis spectrometry and X-ray photoelectron spectroscopy (XPS).

Figure 3 (a) UV-Vis spectra of (E4/PSS)5 multilayers before complexation with iron ions (solid line), complexed with Fe(II) ions (dashed line), and formation of FeS nanoparticulate films (dotted line). (b) UV-Vis spectra of (E5/PSS)5 multilayers (solid line) and the respective FeS nanoparticulate films (dashed line) using (E5/PSS)5 multilayers as matrixes.

The absorption peak at 227 nm was used to monitor the growth of PSS multilayers. It is clear that after each layer buildup of PAMAM, regular removal of PSS is also observed. E5.NH2/PSS multilayers release less PSS than E4.NH2/PSS multilayers do. We also have found that higher Mw of PSS facilitates the multilayer construction and that longer PSS adsorption times and short PAMAM adsorption times both decreased the PSS removal. Construction of E4.N(Gly)OH/PSS multilayers was unsuccessful.

Structure and composition of the PAMAM/PSS multilayers did not change considerably during storage at 48C within 10 days.. Figure 3 shows the UV-Vis Spectra of PAMAM/PSS multilayers before and after nanoparticle formation. An interesting observation that after the formation of FeS nanoparticles, the absorbance of PSS at 227 nm decreases significantly (Figure 3a and Figure 3b). The FeS nanoparticulate films were not stable for more than 10 days’ storage at 48C, probably due to partial oxidation of FeS due to exposure to air during UV-Vis measurements. XPS spectroscopy was used to confirm the composition of FeS nanoparticulate films.
NANOCOMPOSITE DEPOSITION ONTO SILICAGEL

Preliminary experiments to deposit {FeS} nanocomposites onto mesoporous silica gels has also been carried out. Preliminary zeta-potential measurements show that the surface charge of silica gels was successfully reversed from negative to positive, indicating the formation of coating by the{FeS} nanocomposites.

CONCLUSION
Uniform {FeS} nanoparticles have been successfully synthesized using surface-modified PAMAM dendrimers as templates. The formed nanocomposite particles were found to be polycrystalline by high-resolution TEM and SAED. The nanoparticle morphology can be varied from spherical to rod-like using different anions. The UV-Vis absorbance spectra increase in the 250-1000 nm range is proportional to the amount of FeS nanoparticles present. {FeS} nanoparticles displayed different absorbance profiles when PAMAMs of different termini were used as a template. Zeta potential measurements indicated that the formed {FeS-PAMAM} nanocomposites have the same polarity as the dendrimer templates do. To deposit FeS nanoparticulate films onto substrates, PAMAM/PSS multilayers were constructed by the layert-by-layer self-assembly method, and these preassembled multilayers were also used as host matrices to prepare {FeS} nanoparticles. The formed FeS nanoparticulate films exhibited less absorbance than the corresponding PAMAM/PSS multilayers. X-ray spectroscopy qualitatively confirms the presence of both iron and sulfur elements. In addition, deposition of {FeS-PAMAM} onto mesoporous silica gels was also confirmed by zeta-potential measurements, showing reversal of surface charge after the deposition of {FeS} nanocomposites.

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