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.
Highlights
Surfaces with stimuli-responsive properties have emerged as a fascinating class of biomedical and biotechnological materials for a broad spectrum of applications, ranging from cell biology research to drug delivery, tissue engineering, and regenerative medicine.[1−6] From a biological perspective, the ability to respond to stimuli is inherently present in living systems
The experiments described hitherto were conducted under limited biological conditions: artificial seawater for controlling bacterial adhesion and phosphate buffer saline (PBS) solution for regulating specific biomolecular interactions
Having established very high switching efficiencies in such media, we were interested to investigate whether the switchable surfaces that rely on electrically induced conformational changes can be used in more complex biological conditions
Summary
Surfaces with stimuli-responsive properties have emerged as a fascinating class of biomedical and biotechnological materials for a broad spectrum of applications, ranging from cell biology research to drug delivery, tissue engineering, and regenerative medicine.[1−6] From a biological perspective, the ability to respond to stimuli is inherently present in living systems. In a biomedical application context, electrically triggered activation is attractive as it provides fast response times, allows for easy creation of multiple individually addressable switchable regions on the same surface, and uses low drive voltages and fields that are compatible with biological systems.[10]. In this Account, we describe our recent progress on the development of self-assembled monolayers (SAMs) on gold substrates that respond to electrical potentials with altered molecular conformations. It follows by outlining how molecular dynamics (MD) simulations can most successfully be applied to elucidate dynamic molecularlevel events occurring on the surface in response to stimuli, being it an electrical stimulus[16,17] or other stimulus such as light[18−20] and pH.[21]
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