Controlling the supramolecular assembly of redox-active dendrimers at molecular printboards by scanning electrochemical microscopy

Adaptable and Reprogrammable Surfaces

Establishing control over chemical reactions on interfaces is a key challenge in contemporary surface and materials science, in particular when introducing well-defined functionalities in a reversible fashion. Reprogrammable, adaptable and functional interfaces require sophisticated chemistries to precisely equip them with specific functionalities having tailored properties. In the last decade, reversible chemistries-both covalent and noncovalent-have paved the way to precision functionalize 2 or 3D structures that provide both spatial and temporal control. A critical literature assessment reveals that methodologies for writing and erasing substrates exist, yet are still far from reaching their full potential. It is thus critical to assess the current status and to identify avenues to overcome the existing limitations. Herein, the current state-of-the-art in the field of reversible chemistry on surfaces is surveyed, while concomitantly identifying the challenges-not only synthetic but also in current surface characterization methods. The potential within reversible chemistry on surfaces to function as true writeable memories devices is identified, and the latest developments in readout technologies are discussed. Finally, we explore how spatial and temporal control over reversible, light-induced chemistries has the potential to drive the future of functional interface design, especially when combined with powerful laser lithographic applications.

Microcontact Printing Patterning of an HOPG Surface by an Inverse Electron Demand Diels-Alder Reaction

The chemical modification of an sp² hybridized carbon surface in a controllable manner is very challenging but also crucial for many applications. An inverse electron demand Diels-Alder (IEDDA) reaction using microcontact printing technique is introduced to spatially control the modification of a highly ordered pyrolytic graphite (HOPG) surface under ambient conditions. The covalent modification was characterized by Raman spectroscopy, XPS, and SECM. Tetrazine derivatives can effectively react with an HOPG surface and with microcontact printing methods resulting in spatially patterned surfaces being produced with micrometer-scale resolution.

Electrochemistry of Redox-Active Guest Molecules at β-Cyclodextrin-Functionalized Silicon Electrodes

Functionalization of silicon-based sensing devices with self-assembled receptor monolayers offers flexibility and specificity towards the requested analyte as well as the possibility of sensor reuse. As electrical sensor performance is determined by electron transfer, we functionalized H-terminated silicon substrates with β-cyclodextrin (β-CD) molecules to investigate the electronic coupling between these host monolayers and the substrate. A trivalent (one ferrocene and two adamantyl moieties), redox-active guest was bound to the β-CD surface with a coverage of about 10-11 mol/cm2 and an overall binding constant of 1.5⋅109 M-1. This packing density of the host monolayers on silicon is lower than that for similar β-CD monolayers on gold. The monolayers were comparable on low-doped p-type and highly doped p++ substrates regarding their packing density and the extent of oxide formation. Nonetheless, the electron transfer was more favorable on p++ substrates, as shown by the lower values of the peak splitting and peak widths in the cyclic voltammograms. These results show that the electron-transfer rate on the host monolayers is not only determined by the composition of the monolayer, but also by the doping level of the substrate.

Redox titration of gold and platinum surface oxides at porous microelectrodes

Cavity-microelectrodes were used to investigate surface oxides on supported platinum nanoparticles and nanoporous gold with the surface interrogation mode of scanning electrochemical microscopy.

Imaging Local Electric Field Distribution by Plasmonic Impedance Microscopy

We report on imaging of local electric field on an electrode surface with plasmonic electrochemical impedance microscopy (P-EIM). The local electric field is created by putting an electrode inside a micropipet positioned over the electrode and applying a voltage between the two electrodes. We show that the distribution of the surface charge as well as the local electric field at the electrode surface can be imaged with P-EIM. The spatial distribution and the dependence of the local charge density and electric field on the distance between the micropipet and the surface are measured, and the results are compared with the finite element calculations. The work also demonstrates the possibility of integrating plasmonic imaging with scanning ion conductance microscopy (SICM) and other scanning probe microscopies.

Immobilization of multivalent glycoprobes on gold surfaces for sensing proteins and macrophages

A non-covalent host–guest strategy to immobilize heptavalent glyco-β-cyclodextrin on gold-coated glass slides to study multivalent carbohydrate–protein interactions is described.

Self-assemblies of amphiphilic homopolymers: synthesis, morphology studies and biomedical applications

The need for a simplified access to supramolecular assemblies with enhanced tenability has led to the development of amphiphilic homopolymers (APHPs). This review highlights recent advances and future trends in APHP design, self-assembly, and biomedical applications.

Cyclodextrin-based molecular machines

This chapter overviews molecular machines based on cyclodextrins (CDs). The categories of CD-based molecular machines, external stimuli for CD-based molecular machines, and typical examples of CD-based molecular machines are briefly described.

Electron-transfer processes in dendrimers and their implication in biology, catalysis, sensing and nanotechnology

The extraordinary development of the design and synthesis of dendrimers has allowed scientists to locate redox sites at precise positions (core, focal points, branching points, termini, cavities) of these perfectly defined macromolecules, which have generation-controlled sizes and topologies matching those of biomolecules. Redox-dendrimer engineering has led to fine modelling studies of electron-transfer metalloproteins, in which the branches of the dendrimers hinder access to the active site in a manner reminiscent of that of the protein. It has also enabled the construction of remarkable catalysts, sensors and printboards, including by sophisticated design of the interface between redox dendrimers and solid-state devices - for example by functionalizing electrodes and other surfaces. Electron-transfer processes between dendrimers and a variety of other molecules hold promising applications in diverse areas that range from bio-engineering to sensing, catalysis and energy materials.

Electron-induced dynamics of heptathioether β-cyclodextrin molecules

Variable-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) measurements are performed on heptathioether β-cyclodextrin (β-CD) self-assembled monolayers (SAMs) on Au. The β-CD molecules exhibit very rich dynamical behavior, which is not apparent in ensemble-averaged studies. The dynamics are reflected in the tunneling current-time traces, which are recorded with the STM feedback loop disabled. The dynamics are temperature independent, but increase with increasing tunneling current and sample bias, thus indicating that the conformational changes of the β-CD molecules are induced by electrons that tunnel inelastically. Even for sample biases as low as 10 mV, well-defined levels are observed in the tunneling current-time traces. These jumps are attributed to the excitations of the molecular vibration of the macrocyclic β-CD molecule. The results are of great importance for a proper understanding of transport measurements in SAMs.

Mechanism and release rates of surface confined cyclodextrin guests

The dissociation of a cobalt bisdiphenylterpyridine, [Co(biptpy)(2)](2+), guest at mixed (γ-CD-(py)(2))-alkanethiol layers (where γ-CD-(py)(2) is di-6(A), 6(B)- deoxy-6-(4-pyridylmethyl)amino- γ-cyclodextrin) formed on platinum electrodes is reported. Cyclic voltammetry (CV) shows reversible one-electron surface confined waves consistent with the Co(2/3+) couple bound at the interface. The quantity of [Co(biptpy)(2)](3+) reduced is found to be dependent on the scan rate employed, with greater amounts at higher scan rates. This behavior is in contrast to the CD guest ferrocene, which upon oxidation to the ferrocenium ion shows little charge associated with reduction even at elevated scan rates. Chronocoulometry was conducted to systematically vary the time spent oxidizing [Co(biptpy)(2)](2+) and to measure the resulting charge associated with the reduction of [Co(biptpy)(2)](3+). It is determined experimentally that as the pulse width increases, i.e. greater time spent in the oxidizing region, the amount of charge needed to reduce [Co(biptpy)(2)](3+) decreases dramatically. This decrease, along with the CV data, suggests strongly that the [Co(biptpy)(2)](3+) dissociates from the cavity. Significantly, this dissociation of the interfacial host-guest complex occurs on a much longer timescale (the order of seconds) compared to the oxidation of [Co(biptpy)(2)](2+) to [Co(biptpy)(2)](3+), which has been measured using high speed chronoamperometry to occur with a rate contant, k(0), of approximately 10(3) s(-1). The comparison of the timescale for dissociation of the interfacial complex and for electron transfer signifies that the electron transfer step occurs before dissociation, i.e. dissociation via an EC mechanism. The dissociation mechanism of [Co(biptpy)(2)](3+) is contrasted with that of the ferrocene/ferrocenium couple.

Touching surface-attached molecules with a microelectrode: mapping the distribution of redox-labeled macromolecules by electrochemical-atomic force microscopy

We report on the development of a mediator-free electrochemical-atomic force microscopy (AFM-SECM) technique designed for high-resolution imaging of molecular layers of nanometer-sized redox-labeled (macro)molecules immobilized onto electrode surfaces. This new AFM-SECM imaging technique, we call molecule touching atomic force electrochemical microscopy (Mt/AFM-SECM), is based on the direct contact between surface-anchored molecules and an incoming microelectrode (tip). To validate the working-principle of this microscopy, we consider a model system consisting of a monolayer of nanometer long, flexible, polyethylene glycol (PEG) chains covalently attached by one extremity to a gold surface and bearing at their free end a ferrocene (Fc) redox tag. Using Mt/AFM-SECM in tapping mode, i.e., by oscillating the tip so that it comes in intermittent contact with the grafted chains, we show that the substrate topography and the distribution of the redox-tagged PEG chains immobilized on the gold surface can be simultaneously and independently imaged at the sub-100 nm scale. This novel type of SECM imaging may be found useful for characterizing the surface of advanced biosensors which use electrode-grafted, redox-tagged, linear biochains, such as peptides or DNA chains, as sensing elements. In principle, Mt/AFM-SECM should also permit in situ imaging of the distribution of any kind of macromolecules immobilized on electrode surfaces or simply conducting surfaces, provided they are labeled by a suitable redox tag.

Host-guest complexation: a convenient route for the electroreduction of diazonium salts in aqueous media and the formation of composite materials

Electrochemical grafting of a water-insoluble diazonium salt in aqueous media onto an electrode surface was achieved by host-guest complexation. 1-(2-Bisthienyl)-4-aminobenzene (BTAB) was solubilized in a water/beta-cyclodextrin solution (beta-CD). The corresponding diazonium salt was generated in situ then electroreduced. This process leads to the attachment of bithiophene or short oligothiophene groups to the electrode surface. The modified surfaces were analyzed by cyclic voltammetry (CV), scanning electrochemical microscopy (SECM), X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS), and atomic force microscopy (AFM). The electrochemical investigations show that the water-based modified surface is similar to one generated in acetonitrile without beta-CD. Thus, the attached organic layer behaves like an electrochemical switch (above some threshold potential, a soluble external probe is oxidized, but the oxidized form cannot be reduced). The modified surfaces consist of grafted bisthienylbenzene (BTB) and cyclodextrins that can be removed from the surface. This procedure may be considered as a new means of creating a surface made of submicrometric holes in an organic semiconducting layer.

Stamps, inks and substrates: polymers in microcontact printing

It's all about polymers! Polymers play a key role in the patterning and functionalization of surfaces by microcontact printing. Polymers are versatile stamps, inks and substrates and microcontact printing can provide microstructured polymer surfaces in a single printing step.

Electrochemical and surface plasmon resonance characterization of beta-cyclodextrin-based self-assembled monolayers and evaluation of their inclusion complexes with glucocorticoids

This paper describes the characterization of a self-assembled beta-cyclodextrin (beta-CD)-derivative monolayer (beta-CD-SAM) on a gold surface and the study of their inclusion complexes with glucocorticoids. To this aim the arrangement of a self-assembled beta-cyclodextrin-derivative monolayer on a gold surface was monitored in situ by means of surface plasmon resonance (SPR) spectroscopy and double-layer capacitance measurements. Film thickness and dielectric constant were evaluated for a monolayer of beta-CD using one-color-approach SPR. The selectivity of the beta-CD host surface was verified by using electroactive species permeable and impermeable in the beta-CD cavity. The redox probe was selected according to its capacity to permeate the beta-CD monolayer and its electrochemical behavior. In order to evaluate the feasibility of an inclusion complex between beta-CD-SAM with some steroids such as cortisol and cortisone, voltammetric experiments in the presence of the redox probes as molecules competitive with the steroids have been performed. The formation constant of the surface host-guest by beta-CD-SAM and the steroids under study was calculated.

Microelectrochemical modulation of micropatterned cellular environments

Patterned cell cultures obtained by microcontact printing have been modified in situ by a microelectrochemical technique. It relies on lifting cell-repellent properties of oligo(ethylene glycol)-terminated self-assembled monolayers (SAMs) by Br2, which is produced locally by an ultramicroelectrode of a scanning electrochemical microscope (SECM). After Br2 treatment the SAM shows increased permeability and terminal hydrophobicity as characterized by SECM approach curves and contact angle measurements, respectively. Polarization-modulation Fourier transform infrared reflection-absorption spectroscopic (PM FTIRRAS) studies on macroscopic samples show that the Br2 treatment removes the oligo(ethelyene glycol) part of the monolayer within a second time scale while the alkyl part of the SAM degrades with a much slower rate. The lateral extension of the modification can be limited because heterogeneous electron transfer from the gold support destroys part of the electrogenerated Br2 once the monolayer is locally damaged in a SECM feedback configuration. This effect has been reproduced and analyzed by exposing SAM-modified samples to Br2 in the galvanic cell Au|SAM|5 microM Br2 + 0.1 M Na2SO4||10 microM KBr + 0.1 M Na2SO4|Au followed by an PM FTIRRAS characterization of the changes in the monolayer system.

Nanofabrication with metal containing dendrimers

Dendrimers are versatile building blocks for "bottom-up" nanofabrication because they combine molecular structure and nanoscale dimensions. Moreover, dendrimers can be functionalized at their numerous peripheral end groups, in their core, along their branches, and in the voids of their interior. This Frontier highlights the potential of metal containing dendrimers for nanofabrication.

Review: Recent applications of scanning electrochemical microscopy to the study of charge transfer kinetics

Scanning electrochemical microscopy (SECM) has been proven to be a valuable technique for the quantitative investigation and surface analysis of a wide range of processes that occur at interfaces. In particular, there is a great deal of interest in studying the kinetics of charge transfer characteristics at the solid/liquid and liquid/liquid interface. This overview outlines recent advances and applications of SECM to the investigation of charge transfer reactions at the solid/liquid interface and liquid/liquid interface.

Free energy balance predicates dendrimer binding multivalency at molecular printboards

Self-assembled monolayers (SAMs) terminating in beta-cyclodextrin (beta-CD) cavities can be used to bind ink molecules and so provide a molecular printboard for nanopatterning applications. Multivalent, or multisite, binding strengthens the attachment of large inks to the printboard, yielding more robust patterns. We performed fully atomistic molecular dynamics (MD) simulations in bulk explicit solvent to probe the conformational space available to dendrimer and dendrite ink molecules, in both free and bound environments. We show that accurate treatment of both pH effects and binding conformations gives calculated binding modes in line with known binding multivalencies. We identify and quantify the steric frustration causing small, low-generation dendrimer inks to bind to the printboard using just a subset of the available anchor groups. Furthermore, we show that the enhanced binding energy of multisite attachment offsets the steric strain, the feasibility of a given binding mode thus determined by the relative magnitudes of the unfavorable steric strain and favorable multisite binding free energies. We use our experimentally validated model of dendrimer binding to predict the binding mode of novel fluorophoric dendrites and find divalent binding, consistent with confocal microscopy imaging of pattern formation at molecular printboards.