Electrowetting on conductors: anatomy of the phenomenon.

We have recently reported that reversible electrowetting can be observed on the basal plane of graphite, without the presence of a dielectric layer, in both liquid/air and liquid/liquid configurations. The influence of carbon structure on the wetting phenomenon is investigated in more detail here. Specifically, it is shown that the adsorption of adventitious impurities on the graphite surface markedly suppresses the electrowetting response. Similarly, the use of pyrolysed carbon films, although exhibiting a roughness below the threshold previously identified as the barrier to wetting on basal plane graphite, does not give a noticeable electrowetting response, which leads us to conclude that specific interactions at the water-graphite interface as well as graphite crystallinity are responsible for the reversible response seen in the latter case. Preliminary experiments on mechanically exfoliated and chemical vapour deposition grown graphene are also reported.

Electrografting of 4-Nitrobenzenediazonium Ion at Carbon Electrodes: Catalyzed and Uncatalyzed Reduction Processes.

Cyclic voltammograms for the reduction of aryldiazonium ions at glassy carbon electrodes are often, but not always, reported to show two peaks. The origin of this intriguing behavior remains controversial. Using 4-nitrobenzenediazonium ion (NBD), the most widely studied aryldiazonium salt, we make a detailed examination of the electroreduction processes in acetonitrile solution. We confirm that deposition of film can occur during both reduction processes. Film thickness measurements using atomic force microscopy reveal that multilayer films of very similar thickness are formed when reduction is carried out at either peak, even though the film formed at the more negative potential is significantly more blocking to solution redox probes. These and other aspects of the electrochemistry are consistent with the operation of a surface-catalyzed reduction step (proceeding at a clean surface only) followed by an uncatalyzed reduction at a more negative potential. The catalyzed reduction proceeds at both edge-plane and basal-plane graphite materials, suggesting that particular carbon surface sites are not required. The unusual aspect of aryldiazonium ion electrochemistry is that unlike other surface-catalyzed reactions, both processes are seen in a single voltammetric scan at an initially clean electrode because the conditions for observing the uncatalyzed reaction are produced by film deposition during the first catalyzed reduction step.

Oxidative IR spectroelectrochemistry of copper in methanol containing carbon monoxide.

IR spectroelectrochemistry was used to examine the electro-oxidation behavior of carbon monoxide in methanol at a polycrystalline copper electrode. Under such neutral conditions copper electrodes are coated with ill-defined copper oxides and hydroxides and at the oxidative potentials can be expected to generate soluble copper species. The electrochemistry displayed complex behavior suggesting that methanol oxidation was one prominent reaction. However, the spectroscopy revealed that very little methanol oxidation had occurred and that carbon monoxide was not adsorbed to the copper electrode. Instead, the electro-oxidation generated an intense IR band at 2107 cm(-1) that was attributed to a soluble [Cu(I)CO](+) species.

Covalently anchored carboxyphenyl monolayer via aryldiazonium ion grafting: a well-defined reactive tether layer for on-surface chemistry.

Electrografting of aryl films to electrode surfaces from diazonium ion solutions is a widely used method for preparation of modified electrodes. In the absence of deliberate measures to limit film growth, the usual film structure is a loosely packed multilayer. For some applications, monolayer films are advantageous; our interest is in preparing well-defined monolayers of reactive tethers for further on-surface chemistry. Here, we describe the synthesis of an aryl diazonium salt with a protected carboxylic acid substituent. After electrografting to glassy carbon electrodes and subsequent deprotection, the layer is reacted with amine derivatives. Electrochemistry and atomic force microscopy are used to monitor the grafting, deprotection, and subsequent coupling steps. Attempts to follow the same procedures on gold surfaces suggest that the grafted layer is not stable in these reaction conditions.

Mixed monolayer organic films via sequential electrografting from aryldiazonium ion and arylhydrazine solutions.

Sequential electrografting at glassy carbon from aryldiazonium salt solutions, or an aryldiazonium salt followed by an arylhydrazine, leads to the formation of covalently attached monolayer films incorporating two modifiers. In the first step, a 4-((triisopropylsilyl)ethynyl)phenyl film is electrografted to the surface, followed by removal of the triisopropylsilyl group to give a submonolayer of phenylethynylene groups. Two general strategies can then be applied to "fill in" the sparse monolayer with a second modifier. In the first route, nitrophenyl groups are grafted to the phenylethynylene-modified surface by the oxidation of 4-nitrophenylhydrazine. Ferrocene can be coupled to the terminal alkyne groups on the surface via a click reaction with azidomethylferrocene; an electrochemical measurement of the amount of immobilized ferrocene demonstrates that the phenylethynylene layer retains close to full reactivity after the second grafting step. In the alternative strategy, ferrocene is coupled to the phenylethynylene layer prior to grafting nitrophenyl groups by the reduction of the 4-nitrobenzenediazonium ion or by the oxidation of 4-nitrophenylhydrazine. For all approaches, the optimization of the grafting conditions gives surface concentrations of ferrocene and nitrophenyl groups that are consistent with those of a mixed monolayer. The stepwise generation of mixed monolayers is also monitored by film thickness measurements by depth profiling using the atomic force microscope. Thickness values are consistent with the proposed film structure in each preparation step.

Reproducible fabrication of robust, renewable vertically aligned multiwalled carbon nanotube/epoxy composite electrodes.

We describe the reproducible fabrication of robust, vertically aligned multiwalled carbon nanotube (VACNT)/epoxy composite electrodes. The electrodes are characterized by cyclic voltammetry, impedance spectroscopy, and scanning electron and atomic force microscopies. Low background currents are obtained at the electrodes, and common redox probe molecules and NADH show excellent voltammetric behavior. When electrode performance deteriorates due to fouling, the electrode surfaces can be reproducibly renewed by mechanical polishing followed by O(2) plasma treatment. The electrochemical performance of the electrodes is maintained after more than 100 cycles of use and renewal.

Building Tailored Interfaces through Covalent Coupling Reactions at Layers Grafted from Aryldiazonium Salts.

Aryldiazonium ions are widely used reagents for surface modification. Attractive aspects of their use include wide substrate compatibility (ranging from plastics to carbons to metals and metal oxides), formation of stable covalent bonding to the substrate, simplicity of modification methods that are compatible with organic and aqueous solvents, and the commercial availability of many aniline precursors with a straightforward conversion to the active reagent. Importantly, the strong bonding of the modifying layer to the surface makes the method ideally suited to further on-surface (postfunctionalization) chemistry. After an initial grafting from a suitable aryldiazonium ion to give an anchor layer, a target species can be coupled to the layer, hugely expanding the range of species that can be immobilized. This strategy has been widely employed to prepare materials for numerous applications including chemical sensors, biosensors, catalysis, optoelectronics, composite materials, and energy conversion and storage. In this Review our goal is first to summarize how a target species with a particular functional group may be covalently coupled to an appropriate anchor layer. We then review applications of the resulting materials.

Patterning of metal, carbon, and semiconductor substrates with thin organic films by microcontact printing with aryldiazonium salt inks.

Surface modification through reduction of aryldiazonium salts to give covalently attached layers is a widely investigated procedure. However, realization of potential applications of the layers requires development of patterning methods. Here, we demonstrate that microcontact printing with poly(dimethylsiloxane) stamps inked with aqueous acid solutions of aryldiazonium salts gives stable organic layers on gold, copper, silicon, and graphitic carbon surfaces. Depending on the substrate-diazonium salt combination, the layers range from relatively irregular multilayers to smooth films with close to monolayer thickness. After printing, surface attached aminophenyl and carboxyphenyl groups retain their usual reactivity toward amide bond formation with solution species, and hence, the method is a simple route to patterned, covalently attached, reactive tether layers. Multicomponent patterned films can be prepared by printing a second modifier onto a film-coated surface. Microcontact printing using aryldiazonium salt inks is experimentally very simple and is applicable to the broad range of substrates capable of spontaneously reducing aryldiazonium salts.

Electrochemistry of ferrocenoyl beta-peptide monolayers on gold.

The electrochemistry of self-assembled monolayers (SAMs) on gold containing a lipoic acid linker, the beta-peptide sequence (beta(3)Val-beta(3)Ala-beta(3)Leu)(n) for n = 1, 2, and a terminal ferrocenyl group has been described for the first time. Circular dichroism (CD), NMR, and molecular modeling were used to evaluate the beta-peptide structure in solution, while the monolayer film organization and electron-transfer kinetics were evaluated by cyclic voltammetry, chronoamperometry (CA), and ellipsometry. The peptides were assembled from trifluoroethanol solutions, where they are linear (n = 1) or helical (n = 2) based on CD, NMR, ellipsometry, and modeling evidence. The structure of the SAMs is less well understood. There is evidence for noncompact layers that allow electrolyte ions to approach the interface. Electron-transfer rates for n = 1, 2 were found to be 2500 and 1200 s(-1), respectively, and CA evidence indicated that the transfer is based on the hopping mechanism.

Infrared (attenuated total reflection) study of propylene carbonate solutions containing lithium and sodium perchlorate.

Attenuated total reflection infrared spectroscopy was used to examine the concentration dependent solvation of LiClO4 and NaClO4 electrolytes in propylene carbonate (PC). Factor analysis and curve fitting techniques were performed on the measured spectra and the results compared with ab initio computations to provide evidence for ion-solvent solution geometries. Factor analysis of the measured data allowed the identification of the spectrum of ion-associated PC that is uniquely different from the self-associated PC spectrum. The results indicate Li+ and ClO4- ions are contact ion-paired even at relatively low electrolyte concentrations whereas Na+ and ClO4- ions are not, up to approximately 2 mol dm-3.

Diels-Alder Reaction of Anthranilic Acids: A Versatile Route to Dense Monolayers on Flat Edge and Basal Plane Graphitic Carbon Substrates.

Methods that reliably yield monolayers of covalently anchored modifiers on graphene and other planar graphitic materials are in demand. Covalently bonded groups can add functionality to graphitic carbon for applications ranging from sensing to supercapacitors and can tune the electronic and optical properties of graphene. Limiting modification to a monolayer gives a layer with well-defined concentration and thickness providing a minimum barrier to charge transfer. Here we investigate the use of anthranilic acid derivatives for grafting aryl groups to few layer graphene and pyrolyzed photoresist film (PPF). Under mild conditions, anthranilic acids generate arynes, which undergo Diels-Alder cycloadditions. Using spectroscopy, electrochemistry, and atomic force microscopy, we demonstrate that the reaction yields monolayers of aryl groups on graphene and PPF with maximum surface coverages consistent with densely packed layers. Our study confirms that anthranilic acids offer a convenient route to covalent modification of planar graphitic carbons (both basal and edge plane materials).

Multilayer nitroazobenzene films covalently attached to carbon. An AFM and electrochemical study.

Nitroazobenzene films have been grafted to pyrolyzed photoresist films by electrochemical reduction of the corresponding diazonium salt in acetonitrile solution. Two component films were also prepared by electrochemically grafting methylbenzene layers to preformed NAB films. Voltammetric investigation of the films in aqueous acid medium and the measurement of film thickness using atomic force microscopy (AFM) lead to new insights into film structure. In aqueous acid solution, the azobenzene groups have no detectable electroactivity and not all nitro groups in the films can be reduced. These findings point to a compact film structure in which proton diffusion is limited. There may also be spatial inhibition of the conformational changes that accompany azobenzene reduction. For increasingly thick NAB films, the peak for reduction of the nitro groups moves to more negative potentials and the peaks become more asymmetric in shape. These changes are interpreted in terms of the dielectric properties and the rate of proton diffusion in the films. Film thickness was measured by ploughing through the film with an AFM tip. When an NAB film prepared in acetonitrile solution is reduced in aqueous acid, the film thickness decreases by more than 50%. The changes can be partially reversed by treatment in acetonitrile-electrolyte solution and hence are attributed to ion-solvent induced swelling and shrinking. Thus, the large decrease in thickness detected by AFM after treatment of the film in aqueous acid is consistent with the compact film structure revealed by electrochemistry.

Electrochemistry of catechol terminated monolayers with Cu(II), Ni(II) and Fe(III) cations: a model for the marine adhesive interface.

The redox electrochemistry of hydroquinone and Cu2+-, Ni2+-, and Fe3+-hydroquinone complexes immobilized at the SAM interface has been studied in aqueous solutions with pH 5 to 12 using cyclic voltammetry. Self-assembled monolayers were constructed with terminal hydroquinone residues designed to model marine adhesive proteins that use the DOPA (3,4-dihydroxyphenylalanine) moiety. Coordination of metal to the hydroquinone group results in a shift to the ligand oxidation potential, with the value for Delta E p,a dependent on the solution pH and identity of the metal. Cu2+ shifts the hydroquinone oxidation by -285 mV (pH 8.8), and Ni2+ by -194 mV (pH 9.16). The hydroquinone oxidation was shifted by -440 mV at pH 5 for Fe3+ solutions examined up to pH 7. By contrast, reduction of the quinone is unperturbed by the presence of Cu2+, Ni2+, and Fe3+ ions. Implications of these results to the mechanism of marine adhesion are discussed.

Olefin cross-metathesis of a vinyl-terminated self-assembled monolayer (SAM) on Au(111): electrochemical study using a ferrocenyl redox center.

Self-assembled thiol monolayers bound to single-crystal Au(111) surfaces containing a terminal olefin have been prepared and used to monitor electrochemically the cross-metathesis (CM) between the surface and an olefin-terminated ferrocenyl (Fc) derivative from solution over time. Mixed SAM surfaces were prepared by first adsorbing a diluent for 2 days followed by the olefinic alkanethiol for known adsorption time intervals; three diluents of varying length were used. The oxidation peak areas from the voltammetry show the CM reaction yields a maximum amount of product at 100-150 min. Beyond this time, thiol desorption is apparent and the Fc oxidation peaks diminished. A kinetic simulation of the interfacial reactions involving CM and desorption reactions are described and aided in the interpretation of the voltammetric responses. The length of the diluent and the coverage of surface olefins were important factors in limiting undesirable self-CM reactions on the surface, and a model of the relationship between the diluent and surface concentration of olefin is described. This study shows that attention to monolayer formation and reaction conditions are important parameters when maximizing CM yields on surfaces.

Nanoscale patterning of flat carbon surfaces by scanning probe lithography and electrochemistry.

We report the formation of carbon surfaces patterned at the nanoscale with organic functionalities. Thin (<10 nm) films are covalently grafted to the surface via the electrochemical reduction of aryl diazonium salts. Areas of the film are removed with an AFM tip, and a second modifier is electrochemically grafted to the exposed surface. The pattern can incorporate different chemical functionalities, or alternatively topographical patterns can be assembled, where the same functionality is present throughout the pattern.

Effect of applied potential on arylmethyl films oxidatively grafted to carbon surfaces.

Arylmethyl films have been grafted to glassy carbon surfaces and to pyrolyzed photoresist films (PPFs) by electrochemical oxidation of 1-naphthylmethylcarboxylate and 4-methoxybenzylcarboxylate. Atomic force microscopy (AFM) and electrochemistry were used to characterize the as-prepared films and to monitor changes induced by post-preparation treatments. Film thickness was measured by depth profiling using an AFM tip to remove film from the PPF surface. Surface coverage of electroactive modifiers was estimated from cyclic voltammetry, and monitoring the response of a solution-based redox probe at grafted surfaces gave a qualitative indication of changes in film properties. For preparation of the films, the maximum film thickness increased with the potential applied during grafting, and all films were of multilayer thickness. The apparent rate of electron transfer for the Fe(CN)(6)3-/Fe(CN)(6)4- couple was very low at as-prepared films. After film-grafted electrodes were transferred to pure acetonitrile-electrolyte solution and subjected to negative potential excursions, the response of the Fe(CN)(6)3-/Fe(CN)(6)4- couple changed and was consistent with faster electron-transfer kinetics, the film thickness decreased and the surface roughness increased substantially. Applying a positive potential to the treated film reversed changes in film thickness, but the voltammetric response of the Fe(CN)(6)3-/Fe(CN)(6)4- couple remained kinetically fast. After as-prepared films were subjected to positive applied potentials in acetonitrile-electrolyte solution, the apparent rate of electron transfer for the Fe(CN)(6)3-/Fe(CN)(6)4- couple remained very slow and the measured film thickness was the same or greater than that before treatment at positive potentials. Mechanisms are considered to explain the observed effects of applied potential on film characteristics.

Electrochemical and atomic force microscopy study of carbon surface modification via diazonium reduction in aqueous and acetonitrile solutions.

Electrochemical reduction of the diazonium salts of 4-nitrobenzene and 4-nitroazobenzene-4'- has been investigated in aqueous acid and acetonitrile media at carbon surfaces. Using pyrolyzed photoresist films as the substrate, we have examined the deposited films using electrochemistry and atomic force microscopy (AFM). Film thicknesses were measured by scratching through the film with an AFM tip. The procedure employed two AFM cantilevers with different lengths, located on the one device. When the shorter cantilever engages the surface in tapping mode, the longer cantilever (which is not resonating) imbeds into the surface with a constant force. For both modifiers and modification media, film thicknesses increase with deposition time to a limiting value. With equivalent modification conditions, films prepared in aqueous acid medium have lower limiting thicknesses than those prepared in acetonitrile. For nitrophenyl (NP) films, the same trends are found when calculating surface coverages from the charge associated with the reduction of surface -Ar-NO2 groups. Lower limiting film thicknesses and surface coverages for films prepared in aqueous conditions is attributed to growth of inherently more blocking films and is supported by examination of the response of the Fe(CN)6(3-/4-) couple at NP-modified surfaces. Combination of voltammetrically determined surface coverage and film thickness data yields a surface coverage of -Ar-NO2 groups of (2.5 +/- 0.5) x 10(-10) mol cm(-2) for a film thickness equivalent to a monolayer of NP groups.