Nanoparticle Assembly

With the advances made in solution-phase materials chemistry in the last 20 years, nanoparticles (NPs) can now be prepared out of a wide spectrum of compositions with a high degree of particle size and shape control. NPs are an intriguing class of materials because their reduced physical dimensionality leads to the appearance of catalytic, chemical, optoelectronic, and magnetic properties not found in bulk materials. They remain rather difficult to handle for applications, though, due to their colloidal nature and their susceptibility to uncontrolled aggregation. Thus, there is a need for synthetic methodologies for creating functional materials out of NPs.

We recently discovered that, under specific solution conditions, cationic polyelectrolytes can induce negatively-charged silica NPs to form micron-sized hollow spheres rather than the randomly structured precipitate that would ordinarily result from flocculation. The polyelectrolyte (e.g., polyallylamine) forms aggregates under the crosslinking action of a multivalent salt (e.g., EDTA). These aggregates act as templates around which the NPs deposit to form a multilayer-thick NP/polymer shell. We term this form of NP assembly as "polymer aggregate templating" and the resultant nanoparticle-assembled capsules as "NACs."

Schematic of NP assembly: polymer aggregate templating

Optical (differential interference contrast) image of a microcapsule suspension.

Selected publications:

75. 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 (Abstract) (Feature Article)

51. V. S. Murthy, S. B. Kadali, and M. S. Wong, "Polyamine-Guided Synthesis of Anisotropic, Multicompartment Microparticles," Appl. Mater. Interfac., 1, 590-596 (2009). DOI:10.1021/am8001499 (Abstract)

21. R. K. Rana, V. S. Murthy, J. Yu and M. S. Wong, "Nanoparticle Self-assembly of Hierarchically Ordered Microcapsule Structures," Adv. Mater. 17, 1145-1150 (2005). (Cover Article) DOI: 10.1002/adma.200401612 (Abstract)


Nanoparticle-assembled Capsules as Advanced Encapsulation/delivery Agents

There are several methods to prepare microcapsule structures: surfactant self-assembly, sacrificial templating, and physical extrusion. Vesicles or liposomes are hollow sphere structures composed of a bilayer of surfactant molecules. They can form very easily and can readily encapsulate water-soluble compounds; however, the structure is very sensitive to fluid conditions and is difficult manipulate physically. In sacrificial templating, solid or liquid particles are enveloped by a solid shell, and the inner particles are removed through solvent extraction, chemical dissolution, or high temperatures. Uniformly sized shells result from this multi-step process, but non-destructive encapsulation is difficult to perform. Physical extrusion encompasses many methods for the scalable generation of coated liquid droplets, in which the coating contains a crosslinkable precursor that reacts or hardens around the droplet (that contains the target encapsulate). Shell sizes smaller than 1 micron are difficult to achieve, though.

Uniquely different from these methods is polymer aggregate templating, which is the simplest method yet to generate stable microcapsule structures for encapsulation purposes. This materials synthesis route is quite general, as a variety of NP compositions and polymers can be used. The nanoscale properties of the constituent NPs can be incorporated into the NACs, and the polymer molecular structure can be modified to a great extent (to increase functionality, for example) as long as the charge-driven NP assembly process is not disrupted.

SEM image of tin oxide NACs (average diameter = 620 nm) prepared from SnO2 NPs, poly(allylamine), and phosphate anions.


The mild conditions (room temperature, atmospheric pressure, near-neutral pH, water solvent) and simplicity that allow microcapsule synthesis to be a highly scalable process, also allow compounds to be encapsulated easily and without damage. The encapsulation of dye molecules, drug compounds, and enzymes, for example, can be carried out by simply contacting the target compounds with the polymer aggregates prior to shell formation (i.e., prior to addition of the shell-forming NPs).

Enzyme-containing NACs can be considered individual micro-bioreactors. The enzymes are protected by the NAC shell wall from direct contact with the external aqueous environment, but can interact with reactant molecules that penetrate the semipermeable shell wall via concentration-gradient-driven diffusion. There is no well-defined pore size or shape in the shell walls, but it is presumed that the transport pathway is defined by the interstitial spaces of the NP/polymer composite.


Representative microcapsules prepared from citrate-bridged PLL aggregates and SiO2 NPs that contain acid phosphatase enzyme, in an aqueous solution containing fluorescein diphosphate: (a) a series of time-lapse confocal microscopy images collected over the course of 30 min, (b) brightfield image, and (c) line intensity profiles along the red line shown in (b). The reactant diffuses inside the microcapsules and interacts with the encapsulated enzyme to form the fluorescent fluorescein product. Scale bars: 2 µm.

Selected publications:

73. 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 (Abstract)

62. G. C. Kini, S. L. Biswal, and M. S. Wong, "Non-LBL Assembly and Encapsulation Uses of Nanoparticle-Shelled Hollow Spheres," Adv Polym Sci, 229, 175-200 (2010). DOI:10.1007/12_2010_53 (Abstract) (Link)

58. J. Yu, D. Javier, M. A. Yaseen, N. Nitin, R. Richards-Kortum, B. Anvari and M.S. Wong, "Self-assembly Synthesis, Tumor Cell Targeting, and Photothermal Capabilities of Antibody-coated Indocyanine Green Nanocapsules," J. Am. Chem. Soc., 132 (6), 1929-1938 (2010). DOI:10.1021/ja908139y (Abstract)

56. S. E. Plush, M. Woods, Y. Zhou, S. B. Kadali, M. S. Wong and A. D. Sherry, "Nanoassembled Capsules as Delivery Vehicles for Large Payloads of High Relaxivity Gd3+ Agents,"J. Am. Chem. Soc., 131 (43), 15918–15923 (2009). DOI:10.1021/ja906981w (Abstract)

32. P. R. LeDuc, M. S. Wong, et al., "Towards an in vivo Biologically Inspired Nanofactory," Nature Nanotech., 2, 3-7 (2007). DOI: 10.1038/nnano.2006.180 (Abstract)


Palladium-on-Gold Bimetallic Nanoparticles for Groundwater Remediation


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 (Pd) catalysts are known to catalyze the hydrodechlorination (HDC) of trichloroethene (TCE) in water, at room temperature, and in the presence of hydrogen. TCE is one of the most common organic pollutants in US groundwaters today. Mostly used as a solvent to degrease metal parts in a number of industries and at numerous Department of Energy and Department of Defense facilities, it contaminates 60% of Superfund sites at concentration levels that far exceed the current EPA maximum contaminant level of 5 ppb. TCE is also one of the most hazardous pollutants, as it has been found to cause liver and kidney damage and is suspected to be a carcinogen.

We recently discovered that palladium-on-gold nanoparticles (Pd/Au NPs) can be >70 times more active than Pd supported on alumina on a per-Pd gram basis, and are currently working towards understanding the source of the catalytic enhancement and towards improving the potential of this catalyst as a groundwater remediation technology. We synthesized Pd/Au NPs with a core diameter of 4 nm and with different Pd loadings. The most active catalysts were considerably more active (1956 L/gpd/min) than Pd NPs (54 L/gpd/min) and conventionally synthesized Pd/Al 2O3(47 L/gpd/min). Accounting for a gas-liquid mass transfer effect and for the magic-cluster-like core/shell geometry, the reaction rates in terms of initial turnover frequencies were 1.43, 4.35×10-2, and 3.76×10-2 s-1, respectively. These materials exhibited a volcano-shaped activity vs. Pd surface coverage curve, in which the HDC activity peaked near 70% surface coverage.

These NPs can be immobilized on alumina, magnesia, and silica supports to yield active oxide-supported catalysts. This effort, in collaboration with environmental engineers, will lead to an engineered treatment system that can be studied for long-term operational stability and efficacy for remediation of other oxidized water pollutants. Such a system is envisioned to replace the thousands of existing, but aging, pump-and-treat systems throughout the United States.

TCE HDC reaction rate constants of Pd/Au NPs (4-nm Au NPs) plotted against Pd loadings (on metals basis)

Selected publications:

54. K. N. Heck, M. O. Nutt, P. Alvarez, and M. S. Wong, "Deactivation Resistance of Pd/Au Nanoparticle Catalysts for Water-phase Hydrodechlorination," J. Catal. 267, 97-104 (2009)DOI: 10.1016/j.jcat.2009.07.015 (Abstract)

49. M. S. Wong, P. J.J. Alvarez, Y.L. Fang, N. Akcin, M. O. Nutt, J. T. Miller, and K. N. Heck, "Cleaner Water using Bimetallic Nanoparticle Catalysts" J. Chem. Tech. & Biotech, 84, 158-166 (2009). DOI:10.1002/jctb.2002 (Abstract)

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 (Abstract)

17. M. O. Nutt, J. B. Hughes and M. S. Wong, "Designing Pd-on-Au Bimetallic Nanoparticle Catalysts for Trichloroethene Hydrodechlorination," Environ. Sci. Technol. 39, 1346 - 1353 (2005). DOI: 10.1021/es048560b (Abstract)


Improving the Synthesis Chemistry and Scalability of Quantum Dots


Fluorescent semiconductor NPs, or quantum dots (QDs), have potential uses as an optical material, in which the optoelectronic properties can be tuned precisely by particle size. Advances in chemical synthesis have led to improvements in size and shape control, cost, and safety. A limiting step in large-scale production is identified to be the raw materials cost, in which a common synthesis solvent, octadecene, accounts for most of the materials cost in a batch of CdSe quantum dots. Thus, less expensive solvents are needed. Heat transfer (HT) fluids is a class of organic liquids commonly used in chemical process industries to transport heat between unit operations, and have physical properties that suggest possible use as such alternative solvents for QD synthesis.

We found that two heat transfer fluids (Dowtherm A and Therminol 66) can be used successfully in the synthesis of CdSe quantum dots with uniform particle sizes. The synthesis chemistry for CdSe/CdS core/shell quantum dots and CdSe quantum rods can also be performed in HT fluids. We noted subtle differences in the growth patterns of the QDs using the different HT fluids, octadecene, and trioctylphosphine oxide. With the aid of a population balance model developed by Prof. Mantzaris, we concluded that these differences are due to solvent effects, specifically solvent viscosity, CdSe bulk solubility in the solvent, and surface free energy of the oleic-acid -covered QDs.

These findings on HT fluids use and QD growth kinetics should be applicable to NPs of other compositions, and can help guide the development of control strategies for liquid-phase production of high-quality NPs.


Fluorescing suspensions of CdSe quantum dots (QDs) synthesized in a heat transfer fluid, of varying particle diameters: blue (2.0 nm), green (2.5 nm), yellow (3.0 nm), orange (3.9 nm), and red (4.2 nm).


Along the same lines, the synthesis of 4-legged CdSe QDs called tetrapods are difficult to scale up. They are difficult to prepare with uniform arm lengths and widths; non-tetrapod QDs invariably get formed simultaneously; and the synthesis requires expensive and toxic phosphonic acid surfactants. We discovered that cationic ligands in the form of quaternary ammonium surfactants induces faceting in QDs, and that cetyltrimethylammonium bromide (CTAB) surfactant in particular lead to >90% selective synthesis of uniform CdSe tetrapods. The use of CTAB enables the greener and scalable synthesis of tetrapods, which are currently being studied for solar cell applications.

Schematic of new synthesis method for CdSe tetrapods

Collage of transmission electron micrograph (TEM) images, of uniform CdSe tetrapods of various sizes. Images are false-colored to indicate the emitted color of the tetrapods. Scale bar = 50 nm.

Selected publications:

60. W. Y. L. Ko, H. G. Bagaria, S. Asokan, K.-J. Lin and M. S. Wong, "CdSe Tetrapod Synthesis Using Cetyltrimethylammonium Bromide and Heat Transfer Fluids," J. Mater. Chem. 20 (12), 2474-2478 (2010). DOI:10.1039/b922145j (Abstract)

37. S. Asokan, K. M. Krueger, V. L. Colvin, and M. S. Wong, "Shape-Controlled Synthesis of CdSe Tetrapods Using Cationic Surfactant Ligands," Small 3(7), 1164-1169 (2007). DOI: 10.1002/smll.200700120 (Abstract)

24. S. Asokan, K. M. Krueger, A. Alkhawaldeh, A. R. Carreon, Z. Mu, V. L. Colvin, N. V. Mantzaris and M. S. Wong, "The Use of Heat Transfer Fluids in the Synthesis of High-quality CdSe Quantum Dots, Core/Shell Quantum Dots, and Quantum Rods," Nanotechnology 16, 2000-2011 (2005). DOI: 10.1088/0957-4484/16/10/004 (Abstract)

Structural and Catalytic Studies of Nanoparticle-Supported Metal Oxides


Supported metal oxides represent a large class of materials used in many catalytic processes. There are a number of well-established preparation methods that lead to some molecular structure control of the supported metal oxide in the form of isolated, polymerized, and crystalline species. Advances in metal oxide-forming chemistries have come about recently that provide new ability to form pure metal oxides and mixed metal oxides, with structural and compositional control at the nanometer-level. We are exploring several synthesis routes to NP-supported metal oxides. There is emerging evidence that suggests that nanocrystalline domains of the surface oxide can be active sites, based on studies of WOx/ZrO2 materials in methanol dehydration.

In addition, we have analyzed one overlooked aspect of supported metal oxides, the surface density metric. Confusion in SMO literature can arise because there is no generally accepted method for determining surface density. As the metric that characterizes the surface oxide of supported metal oxide catalysts, surface density allows one to consider the various structures of the surface oxide on a common scale, independent of total oxide content, preparation method, calcination treatment, and surface area of the support oxide. Some analogy can be made with supported metal catalysts and the metal dispersion metric.

Surface saturation and monolayer coverage are important threshold surface density values at which surface oxide crystals form and at which complete consumption of surface hydroxyl groups of the support oxide occurs, respectively. Differences in these values come about, though, because of (i) inconsistency in their definitions, (ii) difficulties in compatibilizing data from different characterization techniques, and (iii) and the use of support surface area instead of the overall composite SMO. These differences can make structural comparison of the same SMO composition difficult across different research groups. Calculated properly, however, surface density provides the most simple and useful basis for understanding the relationship between surface nanostructure and catalytic and surface properties.

Selected publications:

66. N. Soultanidis, W. Zhou, A. C. Psarras, A. J. Gonzalez, E. F. Iliopoulou, C. J. Kiely, I. E. Wachs, and M. S. Wong, "Relating n-Pentane Isomerization Activity to the Tungsten Surface Density of WOx/ZrO2," J. Am. Chem. Soc., 132 (38), 13462-13471 (2010). DOI: 10.1021/ja105519y (Abstract)

57. W. Zhou, E. I. Ross-Medgaarden, W. V. Knowles, M. S. Wong, I. E. Wachs and C. J. Kiely, "Identification of Active Zr–WOx Clusters on a ZrO2 Support for Solid Acid Catalysts," Nature. Chem, 1, 722-728 (2009). DOI:10.1038/NCHEM.433 (Abstract)

50. E. I. Ross-Medgaarden, I. E. Wachs, W. V. Knowles, A. Burrows, C. J. Kiely, and M. S. Wong, "Tuning the Electronic and Molecular Structure of Catalytic Active Sites with Titania Nanoligands," J. Am. Chem. Soc., 131, 680-687 (2009). DOI:10.1021/ja711456c

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). (Abstract)

Fundamentals of Colloidal Bimetallic Nanoparticle Catalysts


The Pd-on-Au NPs currently investigated for the catalytic reduction of organic halides dissolved in water are highly useful as a model catalytic material to study in greater detail (i) the effects of mass transfer on reaction kinetics), (ii) the bi-metal distribution of the core/shell nanostructure, (iii) the nature of the catalytically active sites, and (iv) the details of the surface reaction pathway. We have successfully addressed these questions through the development of new spectroscopic techniques (gold nanoshell-based surface enhanced Raman spectroscopy and in situ x-ray absorption spectroscopy), careful synthesis methods, and the application of classical reaction kinetics analysis (e.g., experimental identification of mass transfer resistances and Langmuir-Hinshelwood mechanistic studies). Our research team and collaborators continue to deepen our understanding of bimetallic catalysts.

Selected publications:

72. 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 (Abstract)

69. L. A. Pretzer, Q. X. Nguyen, and M. S. Wong, "Controlled Growth of Sub-10 nm Gold Nanoparticles Using Carbon Monoxide Reductant," J. Phys. Chem. C, 114 (49), 21226-21233 (2010). DOI: 10.1021/jp107945d (Abstract)

65. 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 (Abstract)

54. K. N. Heck, M. O. Nutt, P. Alvarez, and M. S. Wong, "Deactivation Resistance of Pd/Au Nanoparticle Catalysts for Water-phase Hydrodechlorination," J. Catal. 267, 97-104 (2009)DOI: 10.1016/j.jcat.2009.07.015 (Abstract)

46. K.N. Heck, B.G. Janesko, G.E. Scuseria, N.J. Halas and M.S.Wong, "Observing Metal-Catalyzed Chemical Reactions in Situ Using Surface-Enhanced Raman Spectroscopy on Pd-Au Nanoshells" J. Am. Chem. Soc. , 130 (49), 16592–16600 (2008). DOI:10.1021/ja803556k (Abstract)


Nanoparticles for Oil-Field Applications


A futuristic application of nanoparticles is the 4-D detection of petroleum deposits within a reservoir, in which nanoparticles are released into an injection well and recovered at the production well. These nanoparticles would "record" the conditions of their ensemble trajectory based on their physical and/or chemical changes, which may provide a more detailed assessment of a production site. A materials challenge that impedes such developmental work is the conventional lack of colloidal and chemical stability of nanoparticles at mild temperatures (~100 C), high pressures (<100 atm), and high salinities (ranging from seawater to brine). In a first step, we recently developed a processing method that successfully takes any solvent-suspended, bilayer-coated nanoparticles into high-salinity water. A breakthrough is the recognition that high-salinity conditions are conducive towards an outer bilayer coating with a high charge density, important to enhanced nanoparticle stability. In a second step, nanoparticles can be rationally designed to contain a marker molecule that leaches into oil deposits nearby which the nanoparicles travel. The synthesis and transport of carbon nanoreporters were successfully carried out to demonstrate the proof-of-concept.

Selected publications:

70. 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 (Abstract) (Hot Article)

68. H. G. Bagaria, G. C. Kini, and M. S. Wong, "Electrolyte Solutions Improve Nanoparticle Transfer from Oil to Water," J. Phys. Chem. C, 114 (47), 19901-19907 (2010). DOI: 10.1021/jp106140j (Abstract)

67. J. Yu, J. M. Berlin, W. Lu, L. Zhang, A. T. Kan, P. Zhang, E. E. Walsh, S. N. Work, W. Chen, J. M. Tour, M. S. Wong, and M. B. Tomson, "Transport study of nanoparticles for oilfield application," SPE 131158, 2010 SPE International Conference on Oilfield Scale, Aberdeen, UK, May 26-27, (online) DOI: 10.2118/131158-MS (Abstract)


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