Catalysis and Nanomaterials Laboratory

31. V. S. Murthy, R. K. Rana, and M. S. Wong, "Nanoparticle-Assembled Capsule Synthesis: Formation of Colloidal Polyamine-Salt Intermediates" J. Phys. Chem. B., 110 (51), 25619 -25627 (2006). DOI: 10.1021/jp061826b

There is current interest in developing new synthesis strategies for multifunctional hollow spheres with tunable structural properties that would be useful in encapsulation and controlled release applications. A new route was reported recently, in which the sequential reaction of polyamines, multivalent anions, and charged nanoparticles leads to the formation of polymer-filled and water-filled organic/inorganic micron-sized structures known as nanoparticle-assembled capsules. This technique is unique among other capsule preparation routes, as it allows the rapid and scalable formation of robust shells at room temperature, in near-neutral water, and with readily available precursors. This nanoparticle assembly synthesis route involves two steps:  the formation of polymer aggregates and the subsequent deposition of particles around the aggregates. The purpose of this paper is to understand in greater detail the noncovalent chemistry of the polymer?salt aggregation step. With poly(allylamine hydrochloride) (PAH) as the model polymer, aggregate formation was investigated as a function of charge ratio, pH, and time through dynamic light scattering, electrophoretic mobility measurements, chloride ion measurements, and optical microscopy. PAH formed aggregates by the cross-linking action of divalent and higher-valent anions above a critical charge ratio and in a pH range defined by the pKa values of PAH and the anion. The aggregates grew in size through coalescence and with growth rates that depended on their surface charge. Controlling polymer aggregate growth provided a direct and simple means to adjust the size of the resultant capsule materials.

30. M. O. Nutt, K. N. Heck, P. Alvarez, and M. S. Wong, "Improved Pd-on-Au Bimetallic Nanoparticle Catalysts for Aqueous-phase Trichloroethene Hydrodechlorination," Appl. Catal. B Env. 69, 115-125 (2006). DOI: 10.1016/j.apcatb.2006.06.005

Groundwater remediation through the catalytic breakdown of the undesired contaminants is a more effective and desirable approach than the conventional physical displacement methods of air-stripping and carbon adsorption. Palladium-on-gold nanoparticles (Pd/Au NPs) have recently been shown to catalyze the hydrodechlorination of trichloroethene in water, at room temperature, and in the presence of hydrogen, with the most active Pd/Au material found to be >70 times more active than Pd supported on alumina on a per-Pd atom basis. The potential of this catalyst as a groundwater remediation technology could be improved by synthesizing Pd/Au NPs with smaller diameters and immobilizing them on a solid support. For this study, we synthesized Pd/Au NPs with a core diameter of 4 nm and with different Pd loadings and studied them in colloidal form for aqueous-phase trichloroethene hydrodechlorination. The most active catalysts were considerably more active (>1900 L/gPd/min) than Pd NPs (55 L/gPd/min) and conventionally synthesized Pd/Al2O3 (47 L/gPd/min). Accounting for a gas–liquid mass transfer effect and converting the Pd loading to Pd surface coverage using a magic cluster model for the Pd/Au NPs, the reaction rates in terms of initial turnover frequencies were >1.4, 4.35 × 10?2, and 3.76 × 10?2 s?1, respectively. These materials exhibited volcano-like catalytic activity, in which hydrodechlorination rate was maximum near 70% Pd surface coverage. Au appeared to promote catalysis through geometric and electronic effects. Immobilization of the NPs on alumina, magnesia, and silica supports yielded active oxide-supported catalysts.

29. T. Ould-Ely, D. Prieto-Centurion, A. Kumar, W. Guo, W. V. Knowles, S. Asokan, M. S. Wong, I. Rusakova, A. Luttge, and K. H. Whitmire, "Manganese (II) Oxide Nanohexapods: Insight into Controlling the Form of Nanocrystals" Chem. Mater. 18, 1821-1829 (2006). DOI: 10.1021/cm052492q

Cross-shaped and octahedral nanoparticles (hexapods) of MnO in size, and fragments thereof, are created in an amine/carboxylic acid mixture from manganese formate at elevated temperatures in the presence of water. The nanocrosses have dimensions on the order of 100 nm, but with exposure to trace amounts of water during the synthesis process they can be prepared up to about 300 nm in size. Electron microscopy and X-ray diffraction results show that these complex shaped nanoparticles are single crystal face-centered cubic MnO. In the absence of water, the ratio of amine to carboxylic acid determines the nanocrystal size and morphology. Conventionally shaped rhomboehdral/square nanocrystals or hexagonal particles can be prepared by simply varying the ratio of tri-n-octylamine/oleic acid with sizes on the order of 35−40 nm in the absence of added water. If the metal salt is rigorously dried before the synthesis, then “flower-shaped” morphologies on the order of 50−60 nm across are observed. Conventional square-shaped nanocrystals with clearly discernible thickness fringes that also arise under conditions producing the nanocrosses mimic the morphology of the cross-shaped and octahedral nanocrystals and provide clues to the crystal growth mechanism(s), which agree with predictions of crystal growth theory from rough, negatively curved surfaces. The synthetic methodology appears to be general and promises to provide an entryway into other nanoparticle compositions.

28. J. Yu, V. S. Murthy, R. K. Rana, and M. S. Wong, "Synthesis of Nanoparticle-Assembled Tin Oxide/Polymer Microcapsules" Chem. Commun. 10, 1097-1099 (2006). DOI: 10.1039/b513901e

Tin oxide nanoparticles can be assembled into micron-sized hollow capsule structures through a simple mixing procedure based on charge-mediated polymer aggregate templating.

27. M. S. Wong, “Nanostructured Supported Metal Oxides,” in Metal Oxides: Chemistry and Applications; J. L. G. Fierro, Ed.; Taylor and Francis: Boca Raton; Chapter 2, pp. 31-54 (2006).

In this chapter, the common methods of supported metal oxide catalyst preparation are presented. The molecular and nanoscale structure (i.e., textural properties) of supported metal oxides, specifically tungstated zirconia, are also discussed. Finally, several new synthesis techniques are presented, through which simultaneous control of the molecular and nanoscale structures may be possible.

26. P. Diagaradjane, M. A. Yaseen, J. Yu, M. S. Wong, and B. Anvari, "Synchronous Fluorescence Spectroscopic Characterization of DMBA-TPA Induced Squamous Cell Carcinoma in Mice" J. Biomed. Opt. 11, Art. No. 014012 (2006). DOI: 10.1117/1.2167933

While initially confined to the epidermis, squamous cell carcinoma can eventually penetrate into the underlying tissue if not diagnosed early and treated. The noninvasive early detection of the carcinoma is important to achieve a complete treatment of the disease. Of the various non-invasive optical techniques, the synchronous fluorescence (SF) technique is considered to provide a simplified spectral profile with more sharp spectral signatures of the endogenous fluorophores in complex systems. The potential use of the SF technique in the characterization of the sequential tissue transformation in 7,12-dimethylbenz(a)anthracene–12-O-tetradecanoylphorbol-13-acetate (DMBA-TPA)-induced mouse skin tumor model in conjunction with simple statistical analysis is explored. The SF spectra show distinct differences during the earlier weeks of the tumor-induction period. Intensity ratio variables are calculated and used in three discriminant analyses. All the discriminant analyses show better classification results with accuracy greater than 80%. From the observed differences in the spectral characteristics and the ratio variables that resulted in better classification between groups, it is concluded that tryptophan, collagen, and NADH are the key fluorophores that undergo changes during tissue transformation process and hence they can be targeted as tumor markers to diagnose normal from abnormal tissues using the SF technique.