Publications (35)412.19 Total impact
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Article: When and Why Like-Sized, Oppositely Charged Particles Assemble into Diamond-like Crystals
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ABSTRACT: Like-sized, oppositely charged nanoparticles are known to assemble into large crystals with diamond-like (ZnS) ordering, in sharp contrast to analogous molecular ions and micrometer-scale colloids, which invariably favor more closely packed structures (NaCl or CsCl). Here, we show that these experimental observations can be understood as a consequence of ionic screening and the slight asymmetry in surface charge present on the assembling particles. With this asymmetry taken into account, free-energy calculations predict that the diamond-like ZnS lattice is more favorable than other 1:1 ionic structures, namely, NaCl or CsCl, when the Debye screening length is considerably larger than the particle size. A thermodynamic model describes how the presence of neutralizing counterions within the interstitial regions of the crystal acts to bias the formation of low-volume-fraction structures. The results provide general insights into the self-assembly of non-close-packed structures via electrostatic interactions.Journal of Physical Chemistry Letters 04/2013; · 6.21 Impact Factor -
Article: Charged nanoparticles as supramolecular surfactants for controlling the growth and stability of microcrystals.
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ABSTRACT: Microcrystals of desired sizes are important in a range of processes and materials, including controlled drug release, production of pharmaceutics and food, bio- and photocatalysis, thin-film solar cells and antibacterial fabrics. The growth of microcrystals can be controlled by a variety of agents, such as multivalent ions, charged small molecules, mixed cationic-anionic surfactants, polyelectrolytes and other polymers, micropatterned self-assembled monolayers, proteins and also biological organisms during biomineralization. However, the chief limitation of current approaches is that the growth-modifying agents are typically specific to the crystalizing material. Here, we show that oppositely charged nanoparticles can function as universal surfactants that control the growth and stability of microcrystals of monovalent or multivalent inorganic salts, and of charged organic molecules. We also show that the solubility of the microcrystals can be further tuned by varying the thickness of the nanoparticle surfactant layers and by reinforcing these layers with dithiol crosslinks.Nature Material 01/2012; 11(3):227-32. · 32.84 Impact Factor -
Article: Dynamic internal gradients control and direct electric currents within nanostructured materials.
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ABSTRACT: Switchable nanomaterials--materials that can change their properties and/or function in response to external stimuli-have potential applications in electronics, sensing and catalysis. Previous efforts to develop such materials have predominately used molecular switches that can modulate their properties by means of conformational changes. Here, we show that electrical conductance through films of gold nanoparticles coated with a monolayer of charged ligands can be controlled by dynamic, long-range gradients of both mobile counterions surrounding the nanoparticles and conduction electrons on the nanoparticle cores. The internal gradients and the electric fields they create are easily reconfigurable, and can be set up in such a way that electric currents through the nanoparticles can be modulated, blocked or even deflected so that they only pass through select regions of the material. The nanoion/counterion hybrids combine the properties of electronic conductors with those of ionic gels/polymers, are easy to process by solution-casting and, by controlling the internal gradients, can be reconfigured into different electronic elements (current rectifiers, switches and diodes).Nature Nanotechnology 11/2011; 6(11):740-6. · 27.27 Impact Factor -
Article: Self-division of macroscopic droplets: partitioning of nanosized cargo into nanoscale micelles.
Angewandte Chemie International Edition 09/2010; 49(38):6756-9. · 13.45 Impact Factor -
Article: Precision assembly of oppositely and like-charged nanoobjects mediated by charge-induced dipole interactions.
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ABSTRACT: The range of electrostatic interactions controls precisely the mutual orientations of assembling charged nanoobjects. For nonspherically symmetric particles, polarization effects and induced dipoles can dominate charge-charge interactions. These charge-induced dipole interactions mediate orientation-specific aggregation of both oppositely and like-charged particles.Nano Letters 06/2010; 10(6):2275-80. · 13.20 Impact Factor -
Article: Formation of Dense Nanoparticle Monolayers Mediated by Alternating Current Electric Fields and Electrohydrodynamic Flows
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ABSTRACT: Monolayers comprising oppositely charged metal nanoparticles are deposited onto conductive surfaces upon application of ac fields of frequency ranging from 0.1 to 800 kHz. It is found that surface coverage depends on the frequency and is maximal at 300 kHz, for which the deposited coatings feature large, hexagonally close packed domains. The observed trends are rationalized by a model in which ac forcing induces hydrodynamic flows around the particles. These flows, in turn, translate into additional interparticle attractions facilitating dense particle packing.04/2010; -
Article: Contact electrification between identical materials.
Angewandte Chemie International Edition 12/2009; 49(5):946-9. · 13.45 Impact Factor -
Article: Making use of bond strength and steric hindrance in nanoscale "synthesis".
Angewandte Chemie International Edition 11/2009; 48(50):9477-80. · 13.45 Impact Factor -
Article: Photoconductance and inverse photoconductance in films of functionalized metal nanoparticles
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ABSTRACT: In traditional photoconductorsNature 07/2009; 460(7253):371-375. · 36.28 Impact Factor -
Article: Nanoscale forces and their uses in self-assembly.
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ABSTRACT: The ability to assemble nanoscopic components into larger structures and materials depends crucially on the ability to understand in quantitative detail and subsequently "engineer" the interparticle interactions. This Review provides a critical examination of the various interparticle forces (van der Waals, electrostatic, magnetic, molecular, and entropic) that can be used in nanoscale self-assembly. For each type of interaction, the magnitude and the length scale are discussed, as well as the scaling with particle size and interparticle distance. In all cases, the discussion emphasizes characteristics unique to the nanoscale. These theoretical considerations are accompanied by examples of recent experimental systems, in which specific interaction types were used to drive nanoscopic self-assembly. Overall, this Review aims to provide a comprehensive yet easily accessible resource of nanoscale-specific interparticle forces that can be implemented in models or simulations of self-assembly processes at this scale.Small 07/2009; 5(14):1600-30. · 8.35 Impact Factor -
Article: Writing self-erasing images using metastable nanoparticle "inks".
Angewandte Chemie International Edition 07/2009; 48(38):7035-9. · 13.45 Impact Factor -
Article: Directing cell motions on micropatterned ratchets
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ABSTRACT: Cell motility is a process deriving from the synchronized dynamics of the cytoskeleton. In several important physiological processes—notably, cancer metastasis—the randomly moving cells can acquire a directional motility phenotype and bias their motions in response to environmental cues. Despite intense research, however, the current understanding of directional cell migration is incomplete and there is a growing need to develop systems that would enable the study and control of this process. This article demonstrates that random motions of motile cells can be rectified by asymmetric ('ratchet') microgeometries. Interactions between the cells and the imposed geometrical cues guide cell polarization and give rise to directional motility. Depending on the ratchet design, cells of different types can move either in the same or in opposite directions on the same imposed pattern. In the latter case, it is possible to partially sort mixed cell populations into different collecting reservoirs.Nature Physics 06/2009; 5(8):606-612. · 18.97 Impact Factor -
Article: Additivity of the excess energy dissipation rate in a dynamically self-assembled system.
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ABSTRACT: Despite its prevalence in biological systems and its promise as a route to adaptive and/or self-healing materials, dynamic self-assembly (DySA) far from thermodynamic equilibrium remains poorly understood. In this context, it is desirable to develop general thermodynamic relations describing the steady-state configurations of such dissipative assemblies. Here, numerical simulations and analytical methods are used to calculate the viscous energy dissipation rates in a prototypical, magnetohydrodynamic DySA system. In addition to the well-established criteria of mechanical equilibrium, it is shown that the naturally forming steady-state configurations/flows are characterized by a fundamentally different relation based on the viscous energy dissipation. Specifically, the total dissipation of the n-particle system may be expressed as a sum of pairwise "interactions" derived from the analogous two-particle system. This dissipation additivity holds despite the presence of many-body forces/torques between the particles and may prove useful in estimating the viscosities of colloidal suspensions.The Journal of Physical Chemistry B 06/2009; 113(21):7574-8. · 3.70 Impact Factor -
Article: The 'wired' universe of organic chemistry.
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ABSTRACT: The millions of reactions performed and compounds synthesized by organic chemists over the past two centuries connect to form a network larger than the metabolic networks of higher organisms and rivalling the complexity of the World Wide Web. Despite its apparent randomness, the network of chemistry has a well-defined, modular architecture. The network evolves in time according to trends that have not changed since the inception of the discipline, and thus project into chemistry's future. Analysis of organic chemistry using the tools of network theory enables the identification of most 'central' organic molecules, and for the prediction of which and how many molecules will be made in the future. Statistical analyses based on network connectivity are useful in optimizing parallel syntheses, in estimating chemical reactivity, and more.Nature Chemistry 04/2009; 1(1):31-6. · 20.52 Impact Factor -
Article: “Remote” Fabrication via Three‐Dimensional Reaction‐Diffusion: Making Complex Core‐and‐Shell Particles and Assembling Them into Open‐Lattice Crystals
Advanced Materials 03/2009; 21(19):1911 - 1915. · 13.88 Impact Factor -
Article: Mechanism of the cooperative adsorption of oppositely charged nanoparticles.
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ABSTRACT: Quartz crystal microbalance experiments were performed to study the kinetics of surface adsorption from solutions containing oppositely charged nanoparticles. A theoretical model was developed according to which formation of dense nanoparticle (NP) monolayers is driven by a cooperative process, in which the already-adsorbed NPs facilitate adsorption of NPs from solution. The kinetic rate constants change with the NP solution concentration and can be used to backtrack adsorption free energies. These energies agree with the predictions of a simple DLVO model.The Journal of Physical Chemistry A 03/2009; 113(16):3799-803. · 2.95 Impact Factor -
Article: Precipitation of oppositely charged nanoparticles by dilution and/or temperature increase.
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ABSTRACT: Mixtures of oppositely charged nanoparticles (NPs) exhibit anomalous solubility behavior and precipitate either upon dilution or upon temperature increase. Precipitation is reversible and can be explained by a thermodynamic model that accounts for changes in the electrostatic interactions due to the adsorption/desorption of counterions from the surface of the NPs. Specifically, decreasing the salt concentration via dilution or increasing the temperature causes dissociation of counterions from the NP surfaces, increasing the magnitude of electrostatic interactions between NPs and resulting in their precipitation. Model predictions of NP solubility are in quantitative agreement with the experimental observations. Such predictions are of practical importance for the preparation of "patchy" electrostatic coatings and ionic-like NP supracrystals.The Journal of Physical Chemistry B 02/2009; 113(5):1413-7. · 3.70 Impact Factor -
Article: Inside Cover: Small 1/2009.
Small 02/2009; 5(1). · 8.35 Impact Factor -
Article: Size selection during crystallization of oppositely charged nanoparticles.
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ABSTRACT: Opposites attract (selectively): Oppositely charged nanoparticles characterized by different size distributions form 3D supracrystals (see figure) only if the distributions overlap. Crystal quality decreases rapidly with decreasing degree of overlap, and, irrespective of the ratio of particle diameters/charges, no crystals are observed for non-overlapping distributions.Chemistry 02/2009; 15(9):2032-5. · 5.93 Impact Factor -
Article: Micro- and nanoprinting into solids using reaction-diffusion etching and hydrogel stamps.
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ABSTRACT: Micropatterned hydrogel stamps soaked in appropriate chemical etchants can imprint various types of micro- and nanoarchitectures into metals, conductive oxides, semiconductors, glasses, and crystals. Localized etching is mediated by a reaction-diffusion process initiated from the stamp microfeatures and gives lateral resolution down to approximately 300 nm. The method is well suited for the rapid prototyping of small-scale devices including multilevel microfluidic systems and curvilinear optical elements.Small 01/2009; 5(1):22-7. · 8.35 Impact Factor
Top Journals
- The Journal of Physical Chemistry B (4)
- Angewandte Chemie International Edition (4)
- Small (3)
- Langmuir (2)
- Science (2)
Institutions
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2005–2012
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Northwestern University
- • Department of Chemical and Biological Engineering
- • Department of Chemistry
Evanston, IL, USA
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