Dynamic control of biomolecular activity using electrical interfaces

Direct electrical control of IgG conformation and functional activity at surfaces

We have devised a supramolecular edifice involving His-tagged protein A and antibodies to yield surface immobilized, uniformly oriented, IgG-type, antibody layers with Fab fragments exposed off an electrode surface. We demonstrate here that we can affect the conformation of IgGs, likely pushing/pulling electrostatically Fab fragments towards/from the electrode surface. A potential difference between electrode and solution acts on IgGs' charged aminoacids modulating the accessibility of the specific recognition regions of Fab fragments by antigens in solution. Consequently, antibody-antigen affinity is affected by the sign of the applied potential: a positive potential enables an effective capture of antigens; a negative one pulls the fragments towards the electrode, where steric hindrance caused by neighboring molecules largely hampers the capture of antigens. Different experimental techniques (electrochemical quartz crystal microbalance, electrochemical impedance spectroscopy, fluorescence confocal microscopy and electrochemical atomic force spectroscopy) were used to evaluate binding kinetics, surface coverage, effect of the applied electric field on IgGs, and role of charged residues on the phenomenon described. These findings expand the concept of electrical control of biological reactions and can be used to gate electrically specific recognition reactions with impact in biosensors, bioactuators, smart biodevices, nanomedicine, and fundamental studies related to chemical reaction kinetics.

Programmable ion-sensitive transistor interfaces. I. Electrochemical gating

Electrochemical gating is the process by which an electric field normal to the insulator electrolyte interface shifts the surface chemical equilibrium and further affects the charge in solution [Jiang and Stein, Langmuir 26, 8161 (2010)]. The surface chemical reactivity and double-layer charging at the interface of electrolyte-oxide-semiconductor (EOS) capacitors is investigated. We find a strong pH-dependent hysteresis upon dc potential cycling. Varying salinity at a constant pH does not change the hysteretic window, implying that field-induced surface pH regulation is the dominant cause of hysteresis. We propose and investigate this mechanism in foundry-made floating-gate ion-sensitive field-effect transistors, which can serve as both an ionic sensor and an actuator. Termed the chemoreceptive neuron metal-oxide-semiconductor (CνMOS) transistor, it features independently driven control gates (CGs) and sensing gates (SGs) that are capacitively coupled to an extended floating gate (FG). The SG is exposed to fluid, the CG is independently driven, and the FG is capable of storing charge Q(FG) of either polarity. Asymmetric capacitive coupling between the CG and SG to FG results in intrinsic amplification of the measured surface potential shifts and influences the FG charge injection mechanism. This modified SG surface condition was monitored through transient recordings of the output current, performed under alternate positive and negative CG pulses. Transient recordings revealed a hysteresis where the current was enhanced under negative pulsing and reduced after positive pulsing. This hysteresis effect is similar to that observed with EOS capacitors, suggesting a field-dependent surface charge regulation mechanism at play. At high CG biases, nonvolatile charge Q(FG) tunneling into the FG occurs, which creates a larger field and tunes the pH response and the point of zero charge. This mechanism gives rise to surface programmability. In this paper we describe the operational principles, tunneling mechanism, and role of electrolyte composition under field modulation. The experimental findings are then modeled by a Poisson-Boltzmann formulation with surface pH regulation. We find that surface ionization constants play a dominant role in determining the pH tuning effect. In the following paper [K. Jayant et al., Phys. Rev. E 88, 012802 (2013)] we extend the dual-gate operation to molecular sensing and demonstrate the use of Q(FG) to achieve manipulation of surface-adsorbed DNA.

Ferroelectric thin films in fluidic environments: a new interface for sensing and manipulation of matter

For decades ferroelectric thin films (FETFs) have been the focus of research and development for next-generation memory and semiconductor devices. FETFs are attractive because their polarization states are highly localized, stable, and switchable. These unique properties are also attractive for (bio)molecular sensing and separation applications. Polarization of both polymer and ceramic FETF results in the expression of a sustained high, non-Faradaic, surface charge density. If these surface charges are maintained in aqueous environments, then the resulting electrostatic forces should induce the formation of electrolyte gradients and aid in the localization of charged species to the surface. Recently, there has been a growing interest in the interfacial properties of FETFs, specifically how they interact with liquid or gaseous phases. Recent work has shown that the FETF polarization state affects adsorption from the gaseous phase, surface catalysis, and cell growth. Encouraged by these findings, the use of FETFs in aqueous environments is explored. After an introduction to FETFs, the growing body of literature on the FETF interface is reviewed, along with the limited number of studies demonstrating FETF function in gas and liquid environments. Finally, the exciting possibilities that FETFs could bring to interfacial engineering and lab-on-chip (LOC) device design is reviewed.

Micro/nanofabricated environments for synthetic biology

A better understanding of how confinement, crowding and reduced dimensionality modulate reactivity and reaction dynamics will aid in the rational and systematic discovery of functionality in complex biological systems. Artificial microfabricated and nanofabricated structures have helped elucidate the effects of nanoscale spatial confinement and segregation on biological behavior, particularly when integrated with microfluidics, through precise control in both space and time of diffusible signals and binding interactions. Examples of nanostructured interfaces for synthetic biology include the development of cell-like compartments for encapsulating biochemical reactions, nanostructured environments for fundamental studies of diffusion, molecular transport and biochemical reaction kinetics, and regulation of biomolecular interactions as functions of microfabricated and nanofabricated topological constraints.

Electronically activated actin protein polymerization and alignment

Biological systems are the paragon of dynamic self-assembly, using a combination of spatially localized protein complexation, ion concentration, and protein modification to coordinate a diverse set of self-assembling components. Biomimetic materials based upon biologically inspired design principles or biological components have had some success at replicating these traits, but have difficulty capturing the dynamic aspects and diversity of biological self-assembly. Here, we demonstrate that the polymerization of ion-sensitive proteins can be dynamically regulated using electronically enhanced ion mixing and monomer concentration. Initially, the global activity of the cytoskeletal protein actin is inhibited using a low-ionic strength buffer that minimizes ion complexation and protein-protein interactions. Nucleation and growth of actin filaments are then triggered by a low-frequency AC voltage, which causes local enhancement of the actin monomer concentration and mixing with Mg(2+). The location and extent of polymerization are governed by the voltage and frequency, producing highly ordered structures unprecedented in bulk experiments. Polymerization rate and filament orientation could be independently controlled using a combination of low-frequency (approximately 100 Hz) and high frequency (1 MHz) AC voltages, creating a range of macromolecular architectures from network hydrogel microparticles to highly aligned arrays of actin filaments with approximately 750 nm periodicity. Since a wide range of proteins are activated upon complexation with charged species, this approach may be generally applicable to a variety of biopolymers and proteins.