Catalysis and Nanomaterials Laboratory

78. K.-S. Lee, I. Kim, S. Gullapalli, M. S. Wong, and G. E. Jabbour, "Enhanced performance of hybrid solar cells using longer arms of quantum cadmium selenide tetrapods," Appl. Phys. Lett., 99, 223515 (2011). DOI:10.1063/1.3662839

We demonstrate that enhanced device performance of hybrid solar cells based on tetrapod (TP)-shaped cadmium selenide (CdSe) nanoparticles and conjugated polymer of poly (3-hexylthiophene) (P3HT) can be obtained by using longer armed tetrapods which aids in better spatial connectivity, thus decreasing charge hopping events which lead to better charge transport. Longer tetrapods with 10?nm arm length lead to improved power conversion efficiency of 1.12% compared to 0.80% of device having 5?nm short-armed tetrapods:P3HT photoactive blends.

77. S. Gullapalli and M. S. Wong, "Nanotechnology: A Guide to Nano-objects," Chem. Eng. Prog., 107(5), 28-32 (2011).

Rapid change in the field of nanotechnology can make it hard to keep track of the latest nanomaterial developments. Here's a primer on the most common shapes, sizes, and compositions of nano-objects.

76. H. G. Bagaria and M. S. Wong, "Polyamine–salt aggregate assembly of capsules as responsive drug delivery vehicles," J. Mater. Chem., 21 (26), 9454-9466 (2011). DOI: 10.1039/C1JM10712G (Feature Article)

Responsive capsular delivery systems that can partly mimic the complexity of cellular systems hold great promise for the future of medicine. Simple self-assembled systems like liposomes are already in clinical use and others like polymeric micelles are under clinical trials. Unlike these self-assembled systems, the greater flexibility and versatility offered by template-based routes will likely drive the development of sophisticated capsules. The focus of this review is to introduce one such template-based route, which is based on polyamine–salt aggregate or ‘PSA’ assembly. The basic synthesis premise involves the assembly of cationic polymer (like poly-L-lysine) by ionic crosslinking with multivalent anionic salts (like citrate) into metastable templates for cargo encapsulation and shell material deposition. The technique offers several benefits: (i) the synthesis procedure involves simple mixing at ambient conditions, (ii) the capsule size is easy to control in the sub-100 nm to micron range, and (iii) a wide range of formulations is readily available with the use of different polymer, salt, cargo, and shell-forming precursors. In this review, the current state of this technique, the materials chemistry of the capsule assembly, and the demonstrated applications, including photothermal therapy, MRI contrast agent development and protease-responsive NIR imaging, will be discussed.

75. H. G. Bagaria, M. R. Dean, C. A. Nichol, and M. S. Wong, "Self-Assembly and Nanotechnology: Real-Time, Hands-On, and Safe Experiments for K-12 Students," J. Chem. Educ., 88 (5), 609-614 (2011). DOI: 10.1021/ed100598y

What students and teachers often ask is, how are nano-sized materials made when they are so small? One answer is through the process of self-assembly in which molecules, polymers, and nanoparticles connect to form larger objects of a defined structure and shape. Two hands-on experiments are presented in which students prepare capsules in real time using simple and safe ingredients and study the materials for encapsulation and release of food coloring dye. These experiments are visual and interactive demonstrations of self-assembly (as a synthesis tool) and nanotechnology (in which nanomaterial can be made to perform useful functions), and build on the concepts of acid?base chemistry and electrostatic interaction.

74. N. X. Zhao, H. G. Bagaria, M. S. Wong, and Y. L. Zu, "A nanocomplex that is both tumor cell-selective and cancer gene-specific for anaplastic large cell lymphoma," J Nanobiotechnol, 9, 2 (2011). DOI: 10.1186/1477-3155-9-2

Many in vitro studies have demonstrated that silencing of cancerous genes by siRNAs is a potential therapeutic approach for blocking tumor growth. However, siRNAs are not cell type-selective, cannot specifically target tumor cells, and therefore have limited in vivo application for siRNA-mediated gene therapy.

73. Y. L. Fang, K. N. Heck, P. Alvarez, and M. S. Wong, "Kinetics Analysis of Palladium/Gold Nanoparticles as Colloidal Hydrodechlorination Catalysts," ACS Catal., 1 (2), 128-138 (2011). DOI: 10.1021/cs100067k

The aqueous-phase hydrodechlorination (HDC) of trichloroethene (TCE) is an important chemical reaction for water pollution control, for which unsupported palladium-on-gold and palladium nanoparticles (Pd/Au and Pd NPs) definitively show the beneficial effects of gold on palladium catalysis. The observed batch reactor kinetics can be erroneously oversimplified when concentration and mass transfer effects are neglected. A comprehensive treatment of NP catalysis is presented here using Pd-based NPs as the catalytic colloid and TCE HDC as the model reaction. Mass transfer effects were quantified for three specific compositions (Pd/Au NPs with 30% and 60% Pd surface coverages, and pure Pd NPs) by analyzing the observed reaction rates as functions of stirring rate and initial catalyst charge. The largest effect on observed reaction rates came from gas?liquid mass transfer. The TCE HDC reaction was modeled as a Langmuir?Hinshelwood mechanism involving competitive chemisorption of dihydrogen and TCE for all three NP compositions. Differences in adsorption affinities of the reactant molecules for the Pd/Au and Pd surfaces are suggested as responsible for the observed difference in TCE reaction order at high TCE concentrations; that is, first-order for Pd/Au NPs and non-first-order for Pd NPs.

72. H. G. Bagaria, S. B. Kadali, and M. S. Wong, "Shell Thickness Control of Nanoparticle/Polymer Assembled Microcapsules," Chem. Mater., 23 (2), 301-308 (2011). DOI: 10.1021/cm102472h

Organic/inorganic composite microcapsules can be produced in water through a two-step charge-driven assembly of polyallylamine, citrate anions, and 13 nm silica nanoparticles. The shell is composed of nanoparticles intermixed with polymer, and is thick enough (100s of nm) to provide structural stability before or after drying. Controlling shell thickness, however, is currently difficult to perform. Presented here is a new method in which the shell wall can be thickened by contacting the as-synthesized capsules with silicic acid. This shell thickening was observed and quantified for a moderately broad, unimodal size distribution of capsular particles, through a combination of transmission electron and confocal fluorescence microscopies. Thermogravimetric analysis confirmed the deposition of additional silica, and Coulter counter measurements showed the mean capsule diameter of 4.5 ± 2.2 ?m changed negligibly with silicic acid treatment. The shell-thickening process occurred in an inward direction, in which the nanosized silicic acid oligomers most likely diffused through the permeable capsule wall and deposited within the wall and on the inner shell wall surface. Adjustable shell wall thicknesses in hybrid microcapsules provide enhanced capability for chemical encapsulation, storage, and release applications.

71. J. M. Berlin, J. Yu, W. Lu, E. E. Walsh, L. L. Zhang, P. Zhang, Wei. Chen, A. T. Kan, M. S. Wong, M. B. Tomson, and J. M. Tour, "Engineered nanoparticles for hydrocarbon detection in oil-field rocks," Energy Environ. Sci., 4 (2), 505-509 (2011). DOI: 10.1039/C0EE00237B

Polyvinyl alcohol functionalized oxidized carbon black efficiently carries a hydrophobic compound through a variety of oil-field rock types and releases the compound when the rock contains hydrocarbons.

70. S. Leekumjorn and M. S. Wong, "Self-assembly of Nanoparticle Building Blocks" in Comprehensive Nanoscience and Technology, Volume 3 (Nanoscale Self Assembly), D. Andrews, G. Wiederrecht and G. D. Scholes, Eds., Elsevier: Oxford, 5, 203-224 (2011). DOI: 10.1016/B978-0-12-374396-1.00053-2

This chapter provides an overview of recent experimental and simulations work in the self-assembly of nanoparticles into structured materials.  The self-assembly of nanoparticle building blocks through substrate assembly, interfacial assembly, and template-assisted assembly is discussed.  Nanoparticle-assembled capsules are highlighted as an example of template-assisted assembly.  The effects of nanoparticle dimensionality, shape, and composition are discussed, with quantum dot tetrapods as a material of interest.  The computational approach to nanoparticle self-assembly is reviewed and assessed.

69. Y. L. Fang, J. T. Miller, N. Guo, K. N. Heck, Pedro. J. J. Alvarez, and M. S. Wong, "Structural Analysis of Palladium-decorated Gold Nanoparticles as Colloidal Bimetallic Catalysts," Catal. Today, 160 (1), 96-102 (2011). DOI: 10.1016/j.cattod.2010.08.010

Bimetallic palladium-decorated gold nanoparticle (Pd/Au NP) catalysts are significantly more active than palladium-only catalysts, but the mechanism for enhancement is not completely clear for most reactions, like the aqueous-phase hydrodechlorination of trichloroethene. In this study, we conducted X-ray absorption spectroscopy on carbon-supported Pd/Au NPs to obtain information about the local atomic environment (i.e., oxidation states, coordination numbers, and bond distances) of the two metals under different treatment conditions. The as-synthesized NPs were confirmed to have a Pd-shell/Au-core nanostructure, in which the Pd was found as surface ensembles. Upon exposure to room temperature in air, a portion of the Pd, but not the Au, was oxidized. In comparison, nearly the entire surface of monometallic Pd NPs was oxidized, suggesting that Au in Pd/Au NPs imparts oxidation resistance to Pd atoms. The surface Pd was found randomly distributed, presumably as a PdAu surface alloy, after reduction at 300 °C. X-ray absorption spectroscopy provides direct evidence for the Pd-shell/Au-core structure of Pd/Au NPs, and suggests that metallic Pd in the Pd/Au NPs is a source for higher catalytic activity for aqueous-phase trichloroethene hydrodechlorination.