Electrically Responsive Surfaces: Experimental and Theoretical Investigations

Abstract

ConspectusStimuli-responsive surfaces have sparked considerable interest in recent years, especially in view of their biomimetic nature and widespread biomedical applications. Significant efforts are continuously being directed at developing functional surfaces exhibiting specific property changes triggered by variations in electrical potential, temperature, pH and concentration, irradiation with light, or exposure to a magnetic field. In this respect, electrical stimulus offers several attractive features, including a high level of spatial and temporal controllability, rapid and reverse inducement, and noninvasiveness. In this Account, we discuss how surfaces can be designed and methodologies developed to produce electrically switchable systems, based on research by our groups. We aim to provide fundamental mechanistic and structural features of these dynamic systems, while highlighting their capabilities and potential applications. We begin by briefly describing the current state-of-the-art in integrating electroactive species on surfaces to control the immobilization of diverse biological entities. This premise leads us to portray our electrically switchable surfaces, capable of controlling nonspecific and specific biological interactions by exploiting molecular motions of surface-bound electroswitchable molecules. We demonstrate that our self-assembled monolayer-based electrically switchable surfaces can modulate the interactions of surfaces with proteins, mammalian and bacterial cells. We emphasize how these systems are ubiquitous in both switching biomolecular interactions in highly complex biological conditions while still offering antifouling properties. We also introduce how novel characterization techniques, such as surface sensitive vibrational sum-frequency generation (SFG) spectroscopy, can be used for probing the electrically switchable molecular surfaces in situ. SFG spectroscopy is a technique that not only allowed determining the structural orientation of the surface-tethered molecules under electroinduced switching, but also provided an in-depth characterization of the system reversibility. Furthermore, the unique support from molecular dynamics (MD) simulations is highlighted. MD simulations with polarizable force fields (FFs), which could give proper description of the charge polarization caused by electrical stimulus, have helped not only back many of the experimental observations, but also to rationalize the mechanism of switching behavior. More importantly, this polarizable FF-based approach can efficiently be extended to light or pH stimulated surfaces when integrated with reactive FF methods. The interplay between experimental and theoretical studies has led to a higher level of understanding of the switchable surfaces, and to a more precise interpretation and rationalization of the observed data. The perspectives on the challenges and opportunities for future progress on stimuli-responsive surfaces are also presented.

Direct Extracellular Electron Transfer of the Geobacter sulfurreducens Pili Relevant to Interaromatic Distances

“…The electronic couplings between the pi-conjugated aromatic residues were calculated with DFT at the level of CAM-B3LYP/6-31G [46–48]. Electron transfer rates have a close relationship with the reaction transfer energies (Δ G °) that are greatly affected by environment [49–51]. This uncertainty disappears because we focus here on the upper limit of conductivities of the GS pili.…”

Hierarchical self-assembly of 3D lattices from polydisperse anisometric colloids

“…Fundamentally the anisometry encodes highly directional interactions and induces the formation of secondary building blocks, which enable the selection of final structures from polymorphs over an otherwise complicated free energy landscape. Beyond the conventional small molecular ligands used here, macromolecular shelling 48 or ligands that are responsive to external fields 49,50 can also be introduced to the anisometric colloids to render patchy interactions or non-equilibrium assemblies, thereby aiding self-assembly into further complicated 3D superstructures. The colloidal shapes as well as the basic quantum-confinement properties can have rich variations.…”

Elucidating the Influence of Electrical Potentials on the Formation of Charged Oligopeptide Self‐Assembled Monolayers on Gold

“…The peptide C5K is chosen as the model charged oligopeptide since it contains five lysine groups, which impart the oligopeptide with a net positive charge, and a terminating cysteine residue for binding to gold (Figure 1). Such oligopeptides are of great relevance in the field of switchable biological surfaces, with studies on their adsorption properties being essential in the design of surfaces with tunable biospecific interactions [17] . Following SAM formation under a passive incubation process, this type of oligopeptides have been previously shown to undergo conformational changes between collapsed and fully extended oligopeptide structures under an applied electrical potential [2,3] .…”