Interfacial solvation explains attraction between like-charged objects in aqueous solution

Over the past few decades the experimental literature has consistently reported observations of attraction between like-charged colloidal particles and macromolecules in solution. Examples include nucleic acids and colloidal particles in bulk solution and under confinement, and biological liquid-liquid phase separation. This observation is at odds with the intuitive expectation of an interparticle repulsion that decays monotonically with distance. Although attraction between like-charged particles can be theoretically rationalised in the strong-coupling regime, for example, in the presence of multivalent counterions, recurring accounts of long-range attraction in aqueous solution containing monovalent ions at low ionic strength have posed an open conundrum. Here we show that the behaviour of molecular water at an interface - traditionally disregarded in the continuum electrostatics picture, provides a mechanism to explain attraction between like-charged objects in a broad spectrum of experiments. This basic principle will have important ramifications in the ongoing quest to better understand intermolecular interactions in solution.Comment: 22 pages; 5 figure

Single-Molecule Trapping and Measurement in a Nanostructured Lipid Bilayer System

The repulsive electrostatic force between a biomolecule and a like-charged surface can be geometrically tailored to create spatial traps for charged molecules in solution. Using a parallel-plate system composed of silicon dioxide surfaces, we recently demonstrated single-molecule trapping and high precision molecular charge measurements in a nanostructured free energy landscape. Here we show that surfaces coated with charged lipid bilayers provide a system with tunable surface properties for molecular electrometry experiments. Working with molecular species whose effective charge and geometry are well-defined, we demonstrate the ability to quantitatively probe the electrical charge density of a supported lipid bilayer. Our findings indicate that the fraction of charged lipids in nanoslit lipid bilayers can be significantly different from that in the precursor lipid mixtures used to generate them. We also explore the temporal stability of bilayer properties in nanofluidic systems. Beyond their relevance in molecular measurement, such experimental systems offer the opportunity to examine lipid bilayer formation and wetting dynamics on nanostructured surfaces

A charge dependent long-ranged force drives tailored assembly of matter in solution

The interaction between charged objects in solution is generally expected to recapitulate two central principles of electromagnetics: (i) like-charged objects repel, and (ii) they do so regardless of the sign of their electrical charge. Here we demonstrate experimentally that the solvent plays a hitherto unforeseen but crucial role in interparticle interactions, and importantly, that interactions in the fluid phase can break charge-reversal symmetry. We show that in aqueous solution, negatively charged particles can attract at long range while positively charged particles repel. In solvents that exhibit an inversion of the net molecular dipole at an interface, such as alcohols, we find that the converse can be true: positively charged particles may attract whereas negatives repel. The observations hold across a wide variety of surface chemistries: from inorganic silica and polymeric particles to polyelectrolyte- and polypeptide-coated surfaces in aqueous solution. A theory of interparticle interactions that invokes solvation at an interface explains the observations. Our study establishes a specific and unanticipated mechanism by which the molecular solvent may give rise to a strong and long-ranged force in solution, with immediate ramifications for a variety of particulate and molecular processes including tailored self-assembly, gelation and crystallization, as well as biomolecular condensation, coacervation and phase segregation. These findings also shed light on the solvent-induced interfacial electrical potential - an elusive quantity in electrochemistry and interface science implicated in many natural and technological processes, such as atmospheric chemical reactions, electrochemical energy storage and conversion, and the conduction of ions across cell membranes.Comment: 20 pages, 6 figure

TECHNICAL NOTE: Theoretical considerations for counting nucleic acid molecules in microdevices

We describe the theoretical design of a microchip for the amplification, detection and counting of individual nucleic acid molecules in an ensemble of non-specific molecules. The device permits access to individual molecules through the transformation of a three-dimensional amplification reaction volume into a two-dimensional area or a one-dimensional line. The nucleic acid molecules in the amplification solution can be spatially separated along a one-dimensional line or across a two-dimensional area. The arrangement of the molecules is governed by the geometry of the microreactor in a manner analogous to the concept of ‘limiting dilution’ in a volume of fluid. When PCR amplification is allowed to proceed, the target sequences on individual molecules are amplified and diffuse outward. At the end of the amplification process, the zones of concentrated DNA surrounding the individual molecules can be detected as spatially resolved signals through the use of DNA binding dyes or fluorescence resonance energy transfer (FRET) probes. These amplification peaks would serve to establish a count of the number of target molecules in a sample with a much higher accuracy than that of traditional quantification using real-time PCR. The theoretical accuracy of this technique is only limited by the Poisson statistics of sampling.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/49035/2/jmm5_1_n02.pd

Lattice diffusion of a single molecule in solution

The ability to trap a single molecule in an electrostatic potential well in solution has opened up new possibilities for the use of molecular electrical charge to study macromolecular conformation and dynamics at the level of the single entity. Here we study the diffusion of a single macromolecule in a two-dimensional lattice of electrostatic traps in solution. We report the ability to measure both the size and effective electrical charge of a macromolecule by observing single-molecule transport trajectories, typically a few seconds in length, using fluorescence microscopy. While, as shown previously, the time spent by the molecule in a trap is a strong function of its effective charge, we demonstrate here that the average travel time between traps in the landscape yields its hydrodynamic radius. Tailoring the pitch of the lattice thus yields two different experimentally measurable time scales that together uniquely determine both the size and charge of the molecule. Since no information is required on the location of the molecule between consecutive departure and arrival events at lattice sites, the technique is ideally suited to measurements on weakly emitting entities such as single molecules

Electrostatic free energy for a confined nanoscale object in a fluid

We present numerical calculations of electrostatic free energies, based on the nonlinear Poisson-Boltzmann (PB) equation, for the case of an isolated spherical nano-object in an aqueous suspension, interacting with charged bounding walls. We focus on systems with a low concentration of monovalent ions (≲10−4 M), where the range of electrostatic interactions is long (∼30 nm) and comparable to the system and object dimensions (∼100 nm). Locally tailoring the geometry of the boundaries creates a modulation in the object-wall interaction, which for appropriately chosen system dimensions can be strong enough to result in stable spatial trapping of a nanoscale entity. A detailed view of the underlying mechanism of the trap shows that the physics depends predominantly on counterion entropy and the depth of the potential well is effectively independent of the object's dielectric function; we further note an appreciable trap depth even for an uncharged object in the fluid. These calculations not only provide a quantitative framework for understanding geometry-driven electrostatic effects at the nanoscale, but will also aid in identifying contributions from phenomena beyond mean field PB electrostatics, e.g., Casimir and other fluctuation-driven forces

Innovations in DNA analysis device technology: Exploiting the effects of scale.

This dissertation focuses on the development of novel systems for miniaturized DNA analysis. The design of these systems exploits the nature of fundamental phenomena such as conductive heat transfer and fluid mechanical instability on the length scales of interest to achieve devices with enhanced functionality compared with existing technology. We describe the design and operation of a multiple reaction system for parallel analysis operations that relies on steady-state temperature gradients through the device substrate to power multiple thermal reactions in parallel. Studies were performed on the scale-down of the Polymerase Chain Reaction (PCR), a ubiquitous technique for DNA amplification, in high surface-to-volume ratio microchannels. One possible application of these high surface-to-volume ratio microchannels is the counting of individual nucleic acid molecules through amplification. In addition, we studied and developed fluidic systems using a temperature gradient driven fluid mechanical instability phenomenon, Rayleigh-Benard convection. This natural convection phenomenon presents a faster, low-power and elegant alternative to conventional thermocycling for PCR. We describe the use of polymeric devices where the steady-state vertical temperature gradient through the material serves as the driving force for the convection that drives the process of DNA amplification. Geometric modifications such as closed-loop flows for fluid pumping and reactions, droplet mixing, and prospects for further miniaturization are also developed and discussed.Ph.D.Applied SciencesBiomedical engineeringChemical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/124109/2/3121976.pd

Measured electrical charge of SiO2 in polar and nonpolar media

We present measurements of the net electrical surface charge of silicon dioxide (SiO2) in contact with solvents of dielectric constants between 5 and 80. Our experimental approach relies on observing the thermal motion of single silica particles confined in an electrostatic fluidic trap created by SiO2 surfaces. We compare the experimentally measured functional form of the trapping potential with that from free energy calculations and thereby determine the net surface charge in the system. Our findings clearly demonstrate that contrary to popular perception, even in the absence of surfactants, the net electrical charge of ionizable surfaces in contact with apolar solvents can be large enough to lead to significant repulsive forces. A charge regulation model for SiO2 surfaces with a single tunable parameter explains our measurements. This model may find general applicability in estimating the net charge of ionizable surfaces, given system parameters such as the dissociation or association constants of the ionizable groups and the pH, ionic strength, and dielectric constant of the solvent phase