Abstracts for Publications 21-40

40. R. S. Krishnan, M. E. Mackay, P. M. Duxbury, C. J. Hawker, S. Asokan, M. S. Wong, R. Goyette, and P. Thiyagarajan, "Improved Polymer Thin-film Wetting Behavior through Nanoparticle Segregation to Interfaces," J. Phys. Condens. Matter, 19, Art. No. 356003 (2007) DOI:10.1088/0953-8984/19/35/356003
Abstract:
We report a systematic study of improved wetting behavior for thin polymer films containing nanoparticles, as a function of nanoparticle size and concentration, the energy of the substrate and the dielectric properties of the nanoparticles. An enthalpy matched system consisting of polystyrene nanoparticles in linear polystyrene is used to show that nanoparticles are uniformly distributed in the film after spin coating and drying. However, on annealing the film above its bulk glass transition temperature these nanoparticles segregate strongly to the solid substrate. We find that for a wide range of film thicknesses and nanoparticle sizes, a substrate coverage of nanoparticles of approximately a monolayer is required for dewetting inhibition. Cadmium selenide quantum dots also inhibit dewetting of polystyrene thin films, again when a monolayer is present. Moreover, TEM microscopy images indicate that CdSe quantum dots segregate primarily to the air interface. Theoretical interpretation of these phenomena suggests that gain of linear chain configurational entropy promotes segregation of nanoparticles to the solid substrate, as occurs for polystyrene nanoparticles; however, for CdSe nanoparticles this is offset by surface energy or enthalpic terms which promote segregation of the nanoparticles to the air interface.

39. W. C. Ketchie, Y. L. Fang, M. S. Wong, M. Murayama and R. J. Davis, "Influence of gold particle size on the aqueous-phase oxidation of carbon monoxide and glycerol," J. Catal. 250, 94-101 (2007). DOI:10.1016/j.jcat.2007.06.001
Abstract:
Carbon-supported Au particles with mean sizes ranging from 5 to 42 nm and unsupported Au powder were evaluated as catalysts in the aqueous-phase oxidation of CO and glycerol. For the aqueous-phase oxidation of CO at pH 14 and 300 K, the turnover frequency (TOF) for the 5-nm Au particles was 5 s−1, whereas the TOF for large supported Au (42 nm) and bulk Au were only 0.5 and 0.4 s−1, respectively. The observed rate of peroxide formation during CO oxidation also was much higher on the small Au particles. Oxidation of glycerol in the aqueous phase at 333 K and elevated pH over the same catalysts revealed a similar influence of particle size, with the 5-nm Au particles giving a TOF of 17 s−1 at pH 13.8 and the larger particles and bulk Au nearly an order of magnitude less active. However, large Au particles (>20 nm) were more selective to glyceric acid. The lower selectivity of small Au particles is attributed to a higher formation rate of H2O2 during glycerol oxidation, because peroxide promotes CC cleavage reaction.

38. J. Sunarso, C-Y. Chen, A. T. T. Tran, M. S. Wong and J. C. Diniz Da Costa, "Proton conductive composite membranes", Int. J. Nanotech. 4, 597-608 (2007).
DOI:10.1504/IJNT.2007.014754
Abstract:
In this work we investigated the synthesis of composite organic and inorganic membranes for proton conduction. Particles derived from metal alkoxides (M(OR)n) sol-gel processes (Ti, Zr, W with phosphoric acid) were embedded in polymeric matrices of poly-vinyl alcohol, (3-glycidoxypropyl)-trimethoxysilane and ethylene glycol. The structure of the composite membranes was complex as several IR peaks were convoluted, indicating the assignment of several functional groups. However, the peaks assigned to OH groups reduced in intensity in the composite membranes, indicating that cross-linking of hydroxyl groups in the organic and inorganic phases of the membrane may have occurred. The particles allowed for re-arrangement of the polymer matrix, as crystallinity was reduced compared to a polymer blank membrane. The composite membrane process resulted in homogeneous dispersion of nanoparticles into the polymer film. Proton conduction of the inorganic phase was mainly dominated by titania. Binary mixtures of titania phosphate (sample name TiP) resulted in proton conduction of 7.15 × 10−2 S.cm−1, one order of magnitude higher than zirconia phosphate (ZrP). The addition of Zr and W to TiP forming ternary or quaternary phases also led to lower proton conduction as compared to TiP. Similar trends were also observed for the composite membranes, though the TiP composite membrane proton conduction reduced after several hours of testing at 50°C, which was mainly attributed to acid leaching.

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:

Keeping in shape: Quaternary alkylammonium compounds promote the formation of faceted quantum dots. Their use in hot-injection synthesis chemistry provides a new means to form uniform CdSe tetrapods without a selective precipitation step (see picture; TOPSe: trioctylphosphine selenide), and introduces the prospect of nanoparticle shape control through ligand-nanoparticle charge interactions.

36. A. Tuteja, M. E. Mackay, S. Narayanan, S. Asokan, and M. S. Wong, "Breakdown of the Continuum Stokes-Einstein Relation for Nanoparticle Diffusion," Nano Lett. 7, 1276-1281 (2007). DOI:10.1021/nl070192x
Abstract:
 

Cadmium selenide nanoparticles are found to diffuse approximately 200 times faster in a polymeric liquid than predicted by the Stokes−Einstein relation. This remarkable behavior is hypothesized to be due to the nanoparticles being smaller than the entanglement mesh to create a frictional drag that does not follow continuum expectations, in line with a theoretical calculation presented before. This is one of the first demonstrations of X-ray photo correlation spectroscopy applied to polymeric liquids, which we use to explain the simultaneous 60% viscosity reduction of the mixture through a proposed constraint release mechanism.

35. W. V. Knowles, M. O. Nutt, and M. S. Wong, "Supported Metal Oxides and the Surface Density Metric," in Handbook of Catalyst Synthesis: The Science and Engineering of Catalyst Preparation; J. R. Regalbuto, Ed.; Taylor and Francis: Boca Raton; Chapter 11, 251-281 (2007).
Abstract:
Supported metal oxides (SMOs) comprise a large class of catalytic materials used in numerous industrial processes. There are many conventional approaches to preparing these materials, ranging from impregnation and equilibrium adsorption to grafting and co-precipitation. Independent of preparation methods, one of the key metrics in characterizing SMOs is surface density, which quantifies the amount of the supported metal oxide relative to the underlying support surface area. Catalytic activity is correlated to the surface density-dependent structure of the supported species. There are different definitions for surface density and different methods for its determination, though, causing some difficulties in reconciling structure-activity results reported by different researchers. Here, a rigorous analysis of the different surface density calculation methods is presented, using tungstated zirconia as an example.

34. J. Yu, M. A. Yaseen, B. Anvari, and M. S. Wong, "Synthesis of Near-Infrared-Absorbing Nanoparticle-Assembled Capsules," Chem. Mater. 19, 1277-1284 (2007). DOI: 10.1021/cm062080x
Abstract:

Indocyanine green (ICG) is an FDA-approved photosensitizer dye used in clinical settings for optical diagnostics and near-infrared laser-based therapy. However, the rapid clearance and nonspecific vascular plasma binding issues impede ICG performance. Encapsulating ICG within a colloidal matrix is a potential approach to solving these problems, but thus far, there has been limited success. A new strategy, based on the nanoparticle assembly synthesis of stable, non-liposomal nanoparticle/polymer microcapsules, to encapsulate ICG is presented. Nanoparticle-assembled capsules (NACs) are prepared at room temperature, in aqueous solution, and at neutral pH by combining a polyallylamine solution, a phosphate solution, and an aqueous sol of silica nanoparticles; ICG-containing NACs with 0.6−1.0 μm diameter are prepared by adding an ICG solution before the nanoparticle sol. ICG loading is readily controlled with an attainable maximum loading of 23 wt %. There is negligible leakage from the capsules after 24 h at room temperature in phosphate buffer saline solution, with 17% ICG leakage after 8 h at 37 °C. ICG-containing NACs are capable of heat generation in response to near-infrared laser irradiation and are stable to multiple photothermal heating cycles. Fibroblast cells exposed to these capsules remain viable after 2 days of incubation. ICG-containing NACs are a promising material for new photothermal therapy applications and are illustrative of a new approach for encapsulating organic dye compounds.

33. R. S. Krishnan, M. E. Mackay, P. M. Duxbury, A. Pastor, C. J. Hawker, B. Van Horn, S. Asokan, and M. S. Wong, "Self-Assembled Multilayers of Nanocomponents," Nano Lett. 7, 484-489 (2007). DOI:10.1021/nl062866u
Abstract:

We show it is possible to assemble nanoparticle−polymer layers in a controllable manner dictated by the difference in nano-object morphology and dielectric properties. A thin (10−100 nm) layer of the two components is spin coated onto a solid substrate and the system thermally aged to activate a cross-linking process between polymer molecules. The nanoparticles segregate to the solid substrate prior to complete cross-linking if entropic forces are dominant or to the air interface if dielectric (surface energy) forces are properly tuned. Subsequent layers are then spin coated onto the layer below, and the process is repeated to create layered structures with nanometer accuracy useful for tandem solar cells, sensors, optical coatings, etc. Unlike other self-assembly techniques the layer thicknesses are dictated by the spin coating conditions and relative concentration of the two components.

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:
Nanotechnology is having a major impact on medicine and the treatment of disease, notably in imaging and targeted drug delivery. It may, however, be possible to go even further and design 'pseudo-cell' nanofactories that work with molecules already in the body to fight disease.

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
Abstract:
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
Abstract:
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
Abstract:

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
Abstract:

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
Abstract:
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.

25. P. Diagaradjane, M. A. Yaseen, J. Yu, M. S. Wong, and B. Anvari, "Autofluorescence Characterization for the Early Diagnosis of Neoplastic Changes in DMBA/TPA Induced Mouse Skin Carcinogenesis", Lasers Surg. Med. 37, 382-395 (2005). DOI: 10.1002/lsm.20248
Abstract:
Background and Objective: Squamous cell carcinoma (SCC), the second most common skin cancer, usually remains confined to the epidermis for some time but eventually penetrates the underlying tissues, if left untreated. The non-invasive early detection of the SCC is important for appropriate therapeutic strategies. In this study, we aim to characterize the tissue transformation in DMBA/TPA induced mouse skin tumor model using autofluorescence excitation emission matrix (EEM) in conjunction with a multivariate statistical method for early detection of the neoplastic changes.

Study Design/Materials and Methods: The fluorescence EEM from experimental group (n = 40; DMBA/TPA application), control group (n = 6; acetone application), and the blank group (n = 6; no application of DMBA/TPA or acetone) were measured every week using a spectrofluorometer coupled with a fiber optic bundle. The EEM was recorded at excitation wavelengths from 280 to 460 nm at 10 nm intervals and the fluorescence emission was scanned from 300 to 750 nm. The fluorescence emission characteristics corresponding to different fluorophores were extracted from the EEM and the spectral data were used in a multiple/linear discriminant statistical algorithm.

Results: The changes in the fluorescence emission intensity were observed as early as the 1st week of tumor initiation by DMBA. Morphological changes as well as differences in the gross appearance of the skin surface were observed during the entire tumor initiation and promotion period of 15 weeks. The statistical analysis was performed for each excitation wavelength in the EEM and better classification accuracy was obtained for 280 and 410 nm excitations, corresponding to tryptophan and endogenous porphyrins, respectively. The statistical analysis of the combination wavelengths resulted in 11.6% increase in the overall classification accuracy when compared to the highest classification accuracy obtained with single wavelength analysis.

Conclusion: The intensity ratio mapping using the combination of emission intensities of key fluorophores such as tryptophan, collagen, NADH, and endogenous porphyrins from the measured EEM in conjunction with a simple multivariate statistical analysis can be used as a potential tool for the discrimination of early neoplastic changes with improved classification accuracy. Tryptophan and endogenous porphyrins may be used as biomarkers for the discrimination of early neoplastic changes when single wavelength excitations are used.

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:
Fluorescent semiconductor nanoparticles, or quantum dots, 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 for a batch of CdSe quantum dots. Thus, less expensive solvents are needed. In this paper, we identify heat transfer fluids, a class of organic liquids commonly used in chemical process industries to transport heat between unit operations, as alternative solvents for quantum dot synthesis. We specifically show that two heat transfer fluids can be used successfully in the synthesis of CdSe quantum dots with uniform particle sizes. We show that the synthesis chemistry for CdSe/CdS core/shell quantum dots and CdSe quantum rods can also be performed in heat transfer fluids. With the aid of a population balance model, we interpret the effect of different HT fluids on QD growth kinetics in terms of solvent effects, i.e., solvent viscosity, CdSe bulk solubility in the solvent, and surface free energy.

23. M. S. Wong and G. U. Lee, "Selected papers from the Fourth Topical Conference on Nanoscale Science and Engineering of the American Institute of Chemical Engineers," Nanotechnology 16, Preface (2005). DOI: 10.1088/0957-4484/16/7/E01
Abstract:
No Abstract.

22. S. Laus, B. Sitharaman, É. Tóth, R. D. Bolskar, L. Helm, S. Asokan, M. S. Wong, L. J. Wilson and A. E. Merbach, "Destroying Gadofullerene Aggregates by Salt Addition in Aqueous Solution of Gd@C60(OH)x and Gd@C60[C(COOH2)]10," J. Am. Chem. Soc. 127, 9368 - 9369 (2005). DOI: 10.1021/ja052388+
Abstract:

A combined proton relaxivity and dynamic light scattering study has shown that aggregates formed in aqueous solution of water-soluble gadofullerenes can be disrupted by addition of salts. The salt content of fullerene-based materials will strongly influence properties related to aggregation phenomena, therefore, their behavior in biological or medical applications. In particular, the relaxivity of gadofullerenes decreases dramatically with phosphate addition. Moreover, real biological fluids present a rather high salt concentration which will have consequences on fullerene aggregation and influence fullerene-based drug delivery.

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:
Microcapsules with multilayer-thick shells are synthesized by depositing negatively charged nanoparticles around cationic polyamine aggregates crosslinked by multivalent counteranions (see Figure and inside cover).

Nanoparticle (NP)-assembled microcapsules represent a new class of hollow-sphere material, in which materials properties can be programmed by way of NP and polymer compositions. Synthesis is based on a sequential self-assembly of NPs and polymer macromolecules. Reported by Wong and co-workers on p. 1145, this rapid, green synthesis technique applies to a wide spectrum of colloidal species and permits non-destructive encapsulation of water-soluble compounds. The inside cover shows a confocal microscopy image of water-filled microcapsules composed of silica NPs and a fluorescence-labelled polymer. The scale bar is 20 m. The inset shows a simplified illustration of the microcapsule assembly process.