Contact De-electrification of Electrostatically Charged Polymers

The contact electrification of insulating organic polymers is still incompletely understood, in part because multiple fundamental mechanisms may contribute to the movement of charge. This study describes a mechanism previously unreported in the context of contact electrification: that is, “contact de-electrification”, a process in which polymers charged to the same polarity discharge on contact. Both positively charged polymeric beads, e.g., polyamide 6/6 (Nylon) and polyoxymethylene (Delrin), and negatively charged polymeric beads, e.g., polytetrafluoroethylene (Teflon) and polyamide-imide (Torlon), discharge when the like-charged beads are brought into contact. The beads (both with charges of ±20 μC/m2, or 100 charges/μm2) discharge on contact regardless of whether they are made of the same material, or of different materials. Discharge is rapid: discharge of flat slabs of like-charged Nylon and Teflon pieces is completed on a single contact (3 s). The charge lost from the polymers during contact de-electrification transfers onto molecules of gas in the atmosphere. When like-charged polymers are brought into contact, the increase in electric field at the point of contact exceeds the dielectric breakdown strength of the atmosphere and ionizes molecules of the gas; this ionization thus leads to discharge of the polymers. The detection (using a Faraday cup) of charges transferred to the cup by the ionized gas is compatible with the mechanism. Contact de-electrification occurs for different polymers and in atmospheres with different values of dielectric breakdown strength (helium, argon, oxygen, carbon dioxide, nitrogen, and sulfur hexafluoride): the mechanism thus appears to be general.Chemistry and Chemical Biolog

Magnetic Levitation as a Platform for Competitive Protein–Ligand Binding Assays

This paper describes a method based on magnetic levitation (MagLev) that is capable of indirectly measuring the binding of unlabeled ligands to unlabeled protein. We demonstrate this method by measuring the affinity of unlabeled bovine carbonic anhydrase (BCA) for a variety of ligands (most of which are benzene sulfonamide derivatives). This method utilizes porous gel beads that are functionalized with a common aryl sulfonamide ligand. The beads are incubated with BCA and allowed to reach an equilibrium state in which the majority of the immobilized ligands are bound to BCA. Since the beads are less dense than the protein, protein binding to the bead increases the overall density of the bead. This change in density can be monitored using MagLev. Transferring the beads to a solution containing no protein creates a situation where net protein efflux from the bead is thermodynamically favorable. The rate at which protein leaves the bead for the solution can be calculated from the rate at which the levitation height of the bead changes. If another small molecule ligand of BCA is dissolved in the solution, the rate of protein efflux is accelerated significantly. This paper develops a reaction-diffusion (RD) model to explain both this observation, and the physical-organic chemistry that underlies it. Using this model, we calculate the dissociation constants of several unlabeled ligands from BCA, using plots of levitation height versus time. Notably, although this method requires no electricity, and only a single piece of inexpensive equipment, it can measure accurately the binding of unlabeled proteins to small molecules over a wide range of dissociation constants (Kd values within the range from 10 nM to 100 μM are measured easily). Assays performed using this method generally can be completed within a relatively short time period (20 min–2 h). A deficiency of this system is that it is not, in its present form, applicable to proteins with molecular weight greater than approximately 65 kDa.Chemistry and Chemical Biolog

Layer-by-layer films for tunable and rewritable control of contact electrification

Charges generated by contact of solid surfaces (contact electrification) can be hazardous or useful depending on the circumstance. This paper describes a process to design a solid surface rationally to either induce or prevent charging during contact electrification; this process coats the surface with polyelectrolytes. It is observed experimentally that a surface coated with a layer of a polymer having multiple, covalently attached positive charges (a “polycation”) develops a positive charge after contacting another surface; a surface coated with a layer of polymer having negative charges (a “polyanion”) develops a negative charge. By coating the surface using layer-by-layer (LBL) deposition, the tendency of the surface to charge either positively or negatively can be switched: adding a layer of polyelectrolyte with charge opposite to the charge on the surface switches the polarity of the surface. Through microcontact printing (μCP), the surface can be stamped to create a mosaic pattern of polycation and polyanion — and importantly, the fraction of the surface area covered with polycation and polyanion can be tuned by using stamps of different patterns. Using poly(diallyldimethylammonium chloride) (PDDA) as the polycation and poly(sodium 4-styrenesulfonate) (PSS) as the polyanion, it is found that for a surface with >75% PSS, the surface charges negatively; with <75% PSS, the surface charges positively. At [similar]75% PSS, the surface becomes non-charging. The patterns on the surface can, subsequently, be erased through coating the surface with a uniform layer of polyelectrolyte. After erasing, the surface is rewritable by depositing or patterning the surface with a desired polyelectrolyte.Chemistry and Chemical Biolog

The Rate of Charge Tunneling through Self-Assembled Monolayers Is Insensitive to Many Functional Group Substitutions

Insensitivity: A series of molecules containing a common head group and body as well as structurally varied tail groups (-R) has been used in junctions with the structure Ag/S(CH2)4CONH(CH2)2R//Ga2O3/EGaIn to study the rates of charge transport by tunneling. Changing the structure of R over a range of common aliphatic, aromatic, and heteroaromatic organic groups was found to not significantly influence the rate of tunneling (see plots; the dashed lines represent calibration standards).Chemistry and Chemical Biolog

Tilted Magnetic Levitation Enables Measurement of the Complete Range of Densities of Materials with Low Magnetic Permeability

Magnetic levitation (MagLev) of diamagnetic or weakly paramagnetic materials suspended in a paramagnetic solution in a magnetic field gradient provides a simple method to measure the density of small samples of solids or liquids. One major limitation of this method, thus far, has been an inability to measure or manipulate materials outside of a narrow range of densities (0.8 g/cm3 <  < 2.3 g/cm3) that are close in density to the suspending, aqueous medium. This paper explores a simple method—“tilted MagLev”—to increase the range of densities that can be levitated magnetically. Tilting the MagLev device relative to the gravitational vector enables the magnetic force to be decreased (relative to the magnetic force) along the axis of measurement. This approach enables many practical measurements over the entire range of densities observed in matter at ambient conditions—from air bubbles ( ≈ 0) to osmium and iridium ( ≈ 23 g/cm3). The ability to levitate, simultaneously, objects with a broad range of different densities provides an operationally simple method that may find application to forensic science (e.g., for identifying the composition of miscellaneous objects or powders), industrial manufacturing (e.g., for quality control of parts), or resource-limited settings (e.g., for identifying and separating small particles of metals and alloys).Chemistry and Chemical Biolog

Rapid fabrication of pressure-driven open-channel microfluidic devices in omniphobic RF paper

This paper describes the fabrication of pressure-driven, open-channel microfluidic systems with lateral dimensions of 45-300 microns carved in omniphobic paper using a craft-cutting tool. Vapor phase silanization with a fluorinated alkyltrichlorosilane renders paper omniphobic, but preserves its high gas permeability and mechanical properties. When sealed with tape, the carved channels form conduits capable of guiding liquid transport in the low-Reynolds number regime (i.e. laminar flow). These devices are compatible with complex fluids such as droplets of water in oil. The combination of omniphobic paper and a craft cutter enables the development of new types of valves and switches, such as “fold” valves and “porous switches,” which provide new methods to control fluid flow.Chemistry and Chemical Biolog

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.Chemistry and Chemical Biolog

Swarming in shallow waters

A swarm is a collection of separate objects that move autonomously in the same direction in a concerted fashion. This type of behavior is observed in ensembles of various organisms but has proven inherently difficult to realize in artificial chemical systems, where the components have to self-assemble dynamically and, at the same time, propel themselves. This paper describes a class of systems in which millimeter-sized components interact hydrodynamically and organize into dissipative structures that swarm in thin fluid layers. Depending on the geometry of the particles, various types of swarms can be engineered, including ensembles that rotate, follow a &quot;leader&quot;, or are pushed in front of a larger particle

Blocking of Disulfide Adsorption by Coadsorbing omega-Functionalized Alkane Thiols Revealed by Wet Stamping and Fluorescence Microscopy

When alkane thiols and disulfides coadsorb onto gold, they do not necessarily create a mixed monolayer. In particular, when thiols are terminated in groups capable of hydrogen bonding, they can altogether eliminate adsorption of disulfides. Such elimination can be observed directly by using fluorescently labeled disulfides and monitoring their adsorption (or lack of) by fluorescence microscopy. These experiments suggest a mechanism in which adsorption of thiols is facilitated by hydrogen bonding

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