Spatially Controlled Out-of-Equilibrium Host-Guest System under Electrochemical Control.

Self-assembly to create molecular and nanostructures is typically performed at the thermodynamic minimum. To achieve dynamic functionalities, such as adaptability, internal feedback, and self-replication, there is a growing focus on out-of-equilibrium systems. This report presents the dynamic self-assembly of an artificial host-guest system at an interface, under control by a dissipative electrochemical process using (electrical) energy, resulting in an out-of-equilibrium system exhibiting a supramolecular surface gradient. The gradient, its steepness, rate of formation, and complex surface composition after backfilling, as well as the surface compositions after switching between the different states of the system, are assessed and supported by modelling. Our method shows for the first time an artificial surface-confined out-of-equilibrium system. The electrochemical process parameters provide not only control over the system in time, but also in space.

Coupled molecular switching processes in ordered mono- and multilayers of stimulus-responsive rotaxanes on gold surfaces.

Interfaces provide the structural basis for function as, for example, encountered in nature in the membrane-embedded photosystem or in technology in solar cells. Synthetic functional multilayers of molecules cooperating in a coupled manner can be fabricated on surfaces through layer-by-layer self-assembly. Ordered arrays of stimulus-responsive rotaxanes undergoing well-controlled axle shuttling are excellent candidates for coupled mechanical motion. Such stimulus-responsive surfaces may help integrate synthetic molecular machines in larger systems exhibiting even macroscopic effects or generating mechanical work from chemical energy through cooperative action. The present work demonstrates the successful deposition of ordered mono- and multilayers of chemically switchable rotaxanes on gold surfaces. Rotaxane mono- and multilayers are shown to reversibly switch in a coupled manner between two ordered states as revealed by linear dichroism effects in angle-resolved NEXAFS spectra. Such a concerted switching process is observed only when the surfaces are well packed, while less densely packed surfaces lacking lateral order do not exhibit such effects.

Locked-in biomimetic surface gradients that are tunable in size, density and functionalization.

Tuneable and stable surface-chemical gradients in supported lipid bilayers (SLBs) hold great promise for a range of applications in biological sensing and screening. Yet, until now, no method has been reported that provides temporal control of SLB gradients. Herein we report on the development of locked-in SLB gradients that can be tuned in space, time and density by applying a process to control lipid phase behaviour, electric field and temperature. Stable gradients of charged Texas-Red-, serine- or biotin-terminated lipids have been prepared. For example, the Texas-Red surface density was varied from 0 to 2 mol %, while the length was varied between several tens to several hundreds of microns. At room temperature the gradients are shown to be stable up to 24 h, while at 60 °C the gradients could be erased in 30 min. Covalent and non-covalent chemical modification of the gradients is demonstrated, for example, by FITC, hexahistidine-tagged proteins, and SAv/biotin. The amenability to various (bio)chemistries paves the way for novel SLB-based gradients, useful in sensing, high-throughput screening and for understanding dynamic biological processes.

Electrochemically generated gradients.

This review surveys recent developments in the field of electrochemically generated gradients. The gradual variation of properties, which is a key characteristic of gradients, is of eminent importance in technology, for example, directional wetting, as well as biology, for example, chemotaxis. Electrochemical techniques offer many benefits, such as the generation of dynamic solution and surface gradients, integration with electronics, and compatibility with automation. An overview is given of newly developed methods, from purely electrochemical techniques to the combination of electrochemistry with other methods. Electrochemically fabricated gradients are employed extensively for biological and technological applications, such as high-throughput screening, high-throughput deposition, and device development, all of which are covered herein. Especially promising are developments towards the study and control of dynamic phenomena, such as the directional motion of molecules, droplets, and cells.

On-chip electrophoresis in supported lipid bilayer membranes achieved using low potentials.

A micro supported lipid bilayer (SLB) electrophoresis method was developed, which functions at low potentials and appreciable operating times. To this end, (hydroxymethyl)-ferrocene (FcCH2OH) was employed to provide an electrochemical reaction at the anode and cathode at low applied potential to avoid electrolysis of water. The addition of FcCH2OH did not alter the SLB characteristics or affect biomolecule function, and pH and temperature variations and bubble formation were eliminated. Applying potentials of 0.25-1.2 V during flow gave homogeneous electrical fields and a fast, reversible, and strong build-up of a charged dye-modified lipid in the direction of the oppositely charged electrode. Moreover, streptavidin mobility could be modulated. This method paves the way for further development of analytical devices.

Reactivity mapping with electrochemical gradients for monitoring reactivity at surfaces in space and time.

Studying and controlling reactions at surfaces is of great fundamental and applied interest in, among others, biology, electronics and catalysis. Because reaction kinetics is different at surfaces compared with solution, frequently, solution-characterization techniques cannot be used. Here we report solution gradients, prepared by electrochemical means, for controlling and monitoring reactivity at surfaces in space and time. As a proof of principle, electrochemically derived gradients of a reaction parameter (pH) and of a catalyst (Cu(I)) have been employed to make surface gradients on the micron scale and to study the kinetics of the (surface-confined) imine hydrolysis and the copper(I)-catalysed azide-alkyne 1,3-dipolar cycloaddition, respectively. For both systems, the kinetic data were spatially visualized in a two-dimensional reactivity map. In the case of the copper(I)-catalysed azide-alkyne 1,3-dipolar cycloaddition, the reaction order (2) was deduced from it.

Shape-controlled fabrication of micron-scale surface chemical gradients via electrochemically activated copper(i) "click" chemistry.

We report an electrochemical method for the shape-controlled fabrication of micron-scale surface-bound chemical gradients. The approach is based on employing platinum microelectrode arrays on glass for the establishment of a Cu(i) solution gradient via local electrochemical reduction of Cu(ii) (cathodic reaction), and oxidation of the generated Cu(i) back to Cu(ii) (anodic reaction), under ambient conditions. The Cu(i) solution gradient, in the presence of an alkyne in solution and an azide monolayer on the glass surface in between the platinum electrodes, is exploited for the surface-confined gradient fabrication via the Huisgen 1,3-dipolar cycloaddition (CuAAC). Owing to the high sensitivity of the CuAAC on the Cu(i) concentration, we demonstrate here the control of the shape of the micron-scale surface gradient, in terms of steepness and surface density, as a function of the reaction conditions. The surface gradients were assessed by fluorescence microscopy and time-of-flight secondary ion mass spectrometry (Tof-SIMS). Moreover, bi-component and biomolecular gradients have been fabricated and a method for the electrochemically mediated patterning of surface chemical gradients on external azide-functionalized substrates has been developed for the implementation of bi-directional 2D surface gradients.