Siowling Soh

The Mosaic of Surface Charge in Contact Electrification

Electrification caused by rubbing two objects creates patches of positive and negative charge on both surfaces. When dielectric materials are brought into contact and then separated, they develop static electricity. For centuries, it has been assumed that such contact charging derives from the spatially homogeneous material properties (along the material’s surface) and that within a given pair of materials, one charges uniformly positively and the other negatively. We demonstrate that this picture of contact charging is incorrect. Whereas each contact-electrified piece develops a net charge of either positive or negative polarity, each surface supports a random “mosaic” of oppositely charged regions of nanoscopic dimensions. These mosaics of surface charge have the same topological characteristics for different types of electrified dielectrics and accommodate significantly more charge per unit area than previously thought.

Maze solving by chemotactic droplets

Droplets emitting surface-active chemicals exhibit chemotaxis toward low-pH regions. Such droplets are self-propelled and navigate through a complex maze to seek a source of acid placed at one of the mazes exits. In doing so, the droplets find the shortest path through the maze. Chemotaxis and maze solving are due to an interplay between acid/base chemistry and surface tension effects.

The rate of charge tunneling through self-assembled monolayers is insensitive to many functional group substitutions.

At its conception, the field of molecular electronics promised to provide the ability to engineer the rate of charge transport, by design of the molecular structure of electronic junctions.[1] The hypothesis was that the electronic and geometrical structure of molecules in a junction would have a significant and predictable effect on the rate and mechanism of charge transport through their influence on the energetic topography of the tunneling barrier. Here we show the preparation and electrical characterization of junctions (Figure 1) of the structure AgTS/S(CH2)4CONH(CH2)2R//Ga2O3/EGaIn (AgTS = template-stripped silver surface[2]; R = tail group; EGaIn = eutectic gallium and indium alloy; Ga2O3 = a passivating metal oxide film on the surface of the EGaIn[3–5]) including a range of common aliphatic, aromatic, and heteroaromatic organic tail groups. We demonstrate that the rate of charge transport across these self-assembled monolayers (SAMs) is surprisingly insensitive to changes in the structure of the organic molecules of which they are composed. This study is based on a physical-organic design: that is, the information it provides comes from comparisons of rates of tunneling across related structures, rather than from the interpretation of the absolute values of single measurements. Figure 1 A) Schematic description of tunneling junction consisting of a template-stripped Ag bottom-electrode supporting a SAM, and contacted by a Ga2O3/EGaIn top-electrode. B) A schematic of one junction. C) The numbering system based on non-hydrogen atoms in ... Targets for shaping the tunneling barriers of molecular junctions have included electron–donor–bridge–acceptor molecules,[1a,6] molecular quantum dot systems,[7] aromatic molecules,[8] and complex organic molecules with multiple functional groups.[9] Many of these studies ostensibly shaping the tunneling barriers of molecular junctions have, however, been difficult to interpret because, when they were carried out, there were no experimental systems that generated well-characterized, statistically validated data. This paper characterizes the rates of charge transport by tunneling across a series of molecules—arrayed in SAMs—containing a common head group and body (HS(CH2)4CONH(CH2)2-) and structurally varied tail groups (-R); these molecules are assembled in junctions of the structure AgTS/SAM//Ga2O3/EGaIn. Over a range of common aliphatic, aromatic, and heteroaromatic organic tail groups, changing the structure of R does not significantly influence the rate of tunneling. In making these measurements, we utilize C12 and C18 alkanethiols as calibration standards to allow comparison with results from other types of junctions. Limited studies[4,5,10–15] of charge transport using a range of junctions have described the relation between molecular structure and the rate of tunneling. For example, Venkataraman et al.[14] reported that the rate of charge transport through a series of diaminobenzenes depends on the alignment of the metal Fermi level to the closest molecular orbital. Chiechi and Solomon et al.[15] compared the rate of charge transport through three different anthracene derivatives of approximately the same thickness, and concluded that conjugation influences the rate of charge transport. Studies exploring the correlation between molecular structure and charge transport based on systematic physical–organic measurements of the rate of charge transport over a wide range of structures are sparse. This paper describes tunneling rates through SAMs of molecules with a variety of molecular structures including aromatic, heterocyclic, and aliphatic moieties. We have previously examined ferrocene-terminated SAMs[4] and SAMs comprising odd-and even-numbered n-alkanethiolates.[5]

Dynamic self-assembly in ensembles of camphor boats.

Millimeter-sized gel particles loaded with camphor and floating at the interface between water and air generate convective flows around them. These flows give rise to repulsive interparticle interactions, and mediate dynamic self-assembly of nonequilibrium particle formations. When the numbers of particles, N, are small, particle motions are uncorrelated. When, however, N exceeds a threshold value, particles organize into ordered lattices. The nature of hydrodynamic forces underlying these effects and the dynamics of the self-assembling system are modeled numerically using Navier-Stokes equations as well as analytically using scaling arguments.

Rewiring Chemistry: Algorithmic Discovery and Experimental Validation of One‐Pot Reactions in the Network of Organic Chemistry

In 2005 and 2006, we published the first reports on the representation of all synthetic knowledge as a giant network in which molecule “nodes” are connected by reaction “arrows” (Figure 1). In these early works, we focused on the topological structure and evolution of this network and demonstrated the scale-free network topology, existence of hub molecules central to organic synthesis, exponential growth of the network in time, correlations between molecular masses, trends in reactivity based on network connectivity, and more. While our analyses had little applicability to the everyday synthetic practice, we envisioned that such a junction between network theory and synthesis would one day be achieved. Now, we are reporting, in three consecutive communications, the extension of chemicalnetwork concepts into methods directly relevant to experimental chemistry: 1) discovery of one-pot reactions; 2) optimization of multiple reaction pathways, and 3) the detection and blocking of synthetic pathways leading to dangerous chemicals. The first communication in this series addresses one of the most important challenges in organic chemistry: namely, how to “wire” individual reactions into sequences that could be performed in one pot. One-pot reactions save resources and time by avoiding isolation, purification, characterization, and production of chemical waste after each synthetic step. Sometimes, such reactions are identified by chance or, more often, by careful inspection of individual steps that are to be wired together; even this latter process, however, is invariably subjective and depends on the knowledge and intuition of any individual chemist (or group of chemists) involved. Herein, we show that the discovery of one-pot reactions can be facilitated by computational methods. We first describe algorithms that identify possible onepot reactions within the network of all known synthetic knowledge and then demonstrate that the computationally predicted sequences can indeed be carried out experimentally in good overall yields. The experimental examples are chosen to “rewire” small networks of reactions around practically important chemicals: quinoline scaffolds, quinoline-based enzyme inhibitors, and thiophene derivatives. In this way, we replace individual synthetic connections with two-, three-, or even four-step one-pot sequences. The network of organic chemistry (NOC; Figure 1) is constructed from reactions reported in the chemical literature since 1779 and nowadays stored in chemical databases. Pruning the raw data to remove catalysts, solvents, substances that do not participate in reactions, and duplicate or incomplete reactions, leaves about 7 million reactions and about 7 million substances on which further analyses are based. This dataset is translated into a network by representing chemical substances as network nodes, and the reactions as arrows directed from the reaction s substrates to products. At first glance, this giant network of chemistry might look akin to the metabolic networks of biochemical reactions. In reality, however, metabolic networks are true chemical systems comprising reactions that can, in most cases, occur concurrently within the same reaction medium (that is, in Figure 1. a) A small (ca. 5500 nodes, ca. 0.1% of the total) fragment of the network of organic chemistry (NOC), where individual nodes represent the molecules and arrows represent reactions. The representation in b) has the reaction arrows colored by the times these reactions were first reported. This representation emphasizes the fact that NOC, by itself, is not a “coherent” giant chemical system but only a repository of reactions discovered separately, without regard for their mutual compatibility. At best, it can be said that there was a “coherent” interest in certain areas of chemistry (for example, synthetic activity around the Penicillin V node in the 1960s, following the first total synthesis).

Nanoparticle Supracrystals and Layered Supracrystals as Chemical Amplifiers

Nanoparticle crystals and core–shell crystals detect and amplify the presence of chemical and enzymatic analytes. These crystals are made insoluble in water by cross-linking their surface with dithiols incorporating analyte-specific groups. Upon addition of an analyte, these groups are cut, and the “punctured” crystals liberate millions of individual, brightly colored NPs.

Dynamic internal gradients control and direct electric currents within nanostructured materials

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).

Cell motility on micropatterned treadmills and tracks

Surfaces micropatterned with disjointed cell adhesive/non-adhesive regions allow for precise control of cell shape, internal organization and function. In particular, substrates prepared by the reaction-diffusion ASoMic (nisotropic lid roetching) method localize cells onto transparent micro-islands or tracks surrounded by an opaque, adhesion-resistant background. ASoMic is compatible with several important imaging modalities ( wide-field, fluorescent, TIRF and confocal microscopies), and can be used to study and quantify various intracellular and cellular processes related to cell motility. For cells constrained on the islands, the imposed geometry controls spatial organization of the cytoskeleton, while the transparency of the islands allows for real-time analysis of cytoskeletal dynamics. For cells on transparent, linear tracks, the high optical contrast between these adhesive regions and the surrounding non-adhesive background allows for straightforward quantification of the key parameters describing cell motility. Both types of systems provide analytical-quality data that can assist fundamental studies of cell locomotion and can provide a technological basis for cell motility microassays.

Noncontact orientation of objects in three-dimensional space using magnetic levitation.

Significance We describe several noncontact methods of orienting objects in three-dimensional (3D) space using Magnetic Levitation (MagLev), and report the discovery of a sharp geometry-dependent transition of the orientation of levitating objects. An analytical theory of the orientation of arbitrary objects in MagLev explains this transition. MagLev is capable of manipulating and orienting hard and soft objects, and objects of irregular shape. Because controlling the orientation of objects in space is a prerequisite for assembling complex structures from simpler components, this paper extends MagLev into 3D self-assembly, robotic assembly, and noncontact (stiction-free) orientation of hard and soft objects for applications in biomimetics, soft robotics, and stimulus-responsive materials, among others. This paper describes several noncontact methods of orienting objects in 3D space using Magnetic Levitation (MagLev). The methods use two permanent magnets arranged coaxially with like poles facing and a container containing a paramagnetic liquid in which the objects are suspended. Absent external forcing, objects levitating in the device adopt predictable static orientations; the orientation depends on the shape and distribution of mass within the objects. The orientation of objects of uniform density in the MagLev device shows a sharp geometry-dependent transition: an analytical theory rationalizes this transition and predicts the orientation of objects in the MagLev device. Manipulation of the orientation of the levitating objects in space is achieved in two ways: (i) by rotating and/or translating the MagLev device while the objects are suspended in the paramagnetic solution between the magnets; (ii) by moving a small external magnet close to the levitating objects while keeping the device stationary. Unlike mechanical agitation or robotic selection, orienting using MagLev is possible for objects having a range of different physical characteristics (e.g., different shapes, sizes, and mechanical properties from hard polymers to gels and fluids). MagLev thus has the potential to be useful for sorting and positioning components in 3D space, orienting objects for assembly, constructing noncontact devices, and assembling objects composed of soft materials such as hydrogels, elastomers, and jammed granular media.

Measuring Binding of Protein to Gel-Bound Ligands Using Magnetic Levitation

This paper describes the use of magnetic levitation (MagLev) to measure the association of proteins and ligands. The method starts with diamagnetic gel beads that are functionalized covalently with small molecules (putative ligands). Binding of protein to the ligands within the bead causes a change in the density of the bead. When these beads are suspended in a paramagnetic aqueous buffer and placed between the poles of two NbFeB magnets with like poles facing, the changes in the density of the bead on binding of protein result in changes in the levitation height of the bead that can be used to quantify the amount of protein bound. This paper uses a reaction-diffusion model to examine the physical principles that determine the values of rate and equilibrium constants measured by this system, using the well-defined model system of carbonic anhydrase and aryl sulfonamides. By tuning the experimental protocol, the method is capable of quantifying either the concentration of protein in a solution, or the binding affinities of a protein to several resin-bound small molecules simultaneously. Since this method requires no electricity and only a single piece of inexpensive equipment, it may find use in situations where portability and low cost are important, such as in bioanalysis in resource-limited settings, point-of-care diagnosis, veterinary medicine, and plant pathology. It still has several practical disadvantages. Most notably, the method requires relatively long assay times and cannot be applied to large proteins (>70 kDa), including antibodies. The design and synthesis of beads with improved characteristics (e.g., larger pore size) has the potential to resolve these problems.