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

Electro-polymerization rates of diazonium salts are dependent on the crystal orientation of the surface

Electro-polymerization of diazonium salts is widely used for modifying surfaces with thin organic films. Initially this method was primarily applied to carbon, then to metals, and more recently to semiconducting Si. Unlike on other surfaces, electrochemical reduction of diazonium salts on Si, which is one of the most industrially dominant material, is not well understood. Here, we report the electrochemical reduction of diazonium salts on a range of silicon electrodes of different crystal orientations (111, 211, 311, 411, and 100). We show that the kinetics of surface reaction and the reduction potential is Si crystal-facet dependent and is more favorable in the hierarchical order (111) > (211) > (311) > (411) > (100), a finding that offers control over the surface chemistry of diazonium salts on Si. The dependence of the surface reaction kinetics on the crystal orientation was found to be directly related to differences in the potential of zero charge (PZC) of each crystal orientation, which in turn controls the adsorption of the diazonium cations prior to reduction. Another consequence of the effect of PZC on the adsorption of diazonium cations, is that molecules terminated by distal diazonium moieties form a compact film in less time and requires less reduction potentials compared to that formed from diazonium molecules terminated by only one diazo moiety. In addition, at higher concentrations of diazonium cations, the mechanism of electrochemical polymerization on the surface becomes PZC-controlled adsorption-dominated inner-sphere electron transfer while at lower concentrations, diffusion-based outer-sphere electron transfer dominates. These findings help understanding the electro-polymerization reaction of diazonium salts on Si en route towards an integrated molecular and Si electronics technology.

Simultaneous Photografting of Two Organic Groups on a Gold Surface by using Arylazo Sulfones as Single Precursors

Arylazo sulfones have been exploited as photoactivatable substrates for the simultaneous photografting of both aryl and methanesulfonyl groups on a gold surface. The obtained samples have been characterized by different spectroscopic techniques including ellipsometry and electrochemistry, infrared reflection absorption, surface-enhanced Raman spectroscopy, XPS, and AFM. Grafting occurs through a simple N-S cleavage and not, as usually observed with aromatic precursors, by electron transfer.

Towards molecular electronic devices based on 'all-carbon' wires

Nascent molecular electronic devices based on linear 'all-carbon' wires attached to gold electrodes through robust and reliable C-Au contacts are prepared via efficient in situ sequential cleavage of trimethylsilyl end groups from an oligoyne, Me₃Si-(C[triple bond, length as m-dash]C)₄-SiMe₃ (1). In the first stage of the fabrication process, removal of one trimethylsilyl (TMS) group in the presence of a gold substrate, which ultimately serves as the bottom electrode, using a stoichiometric fluoride-driven process gives a highly-ordered monolayer, Au|C[triple bond, length as m-dash]CC[triple bond, length as m-dash]CC[triple bond, length as m-dash]CC[triple bond, length as m-dash]CSiMe₃ (Au|C₈SiMe₃). In the second stage, treatment of Au|C₈SiMe₃ with excess fluoride results in removal of the remaining TMS protecting group to give a modified monolayer Au|C[triple bond, length as m-dash]CC[triple bond, length as m-dash]CC[triple bond, length as m-dash]CC[triple bond, length as m-dash]CH (Au|C₈H). The reactive terminal C[triple bond, length as m-dash]C-H moiety in Au|C₈H can be modified by 'click' reactions with (azidomethyl)ferrocene (N₃CH₂Fc) to introduce a redox probe, to give Au|C₆C₂N₃HCH₂Fc. Alternatively, incubation of the modified gold substrate supported monolayer Au|C₈H in a solution of gold nanoparticles (GNPs), results in covalent attachment of GNPs on top of the film via a second alkynyl carbon-Au σ-bond, to give structures Au|C₈|GNP in which the monolayer of linear, 'all-carbon' C₈ chains is sandwiched between two macroscopic gold contacts. The covalent carbon-surface bond as well as the covalent attachment of the metal particles to the monolayer by cleavage of the alkyne C-H bond is confirmed by surface-enhanced Raman scattering (SERS). The integrity of the carbon chain in both Au|C₆C₂N₃HCH₂Fc systems and after formation of the gold top-contact electrode in Au|C₈|GNP is demonstrated through electrochemical methods. The electrical properties of these nascent metal-monolayer-metal devices Au|C₈|GNP featuring 'all-carbon' molecular wires were characterised by sigmoidal I-V curves, indicative of well-behaved junctions free of short circuits.

Reversible on-surface wiring of resistive circuits

Whilst most studies in single-molecule electronics involve components first synthesized ex situ, there is also great potential in exploiting chemical transformations to prepare devices in situ. Here, as a first step towards this goal, we conduct reversible reactions on monolayers to make and break covalent bonds between alkanes of different lengths, then measure the conductance of these molecules connected between electrodes using the scanning tunneling microscopy-based break junction (STM-BJ) method. In doing so, we develop the critical methodology required for assembling and disassembling surface-bound single-molecule circuits. We identify effective reaction conditions for surface-bound reagents, and importantly demonstrate that the electronic characteristics of wires created in situ agree with those created ex situ. Finally, we show that the STM-BJ technique is unique in its ability to definitively probe surface reaction yields both on a local (∼50 nm2) and pseudo-global (≥10 mm2) level. This investigation thus highlights a route to the construction and integration of more complex, and ultimately functional, surface-based single-molecule circuitry, as well as advancing a methodology that facilitates studies beyond the reach of traditional ex situ synthetic approaches.

Advances on Aryldiazonium Salt Chemistry Based Interfacial Fabrication for Sensing Applications

Aryldiazonium salts as coupling agents for surface chemistry have evidenced their wide applications for the development of sensors. Combined with advances in nanomaterials, current trends in sensor science and a variety of particular advantages of aryldiazonium salt chemistry in sensing have driven the aryldiazonium salt-based sensing strategies to grow at an astonishing pace. This review focuses on the advances in the use of aryldiazonium salts for modifying interfaces in sensors and biosensors during the past decade. It will first summarize the current methods for modification of interfaces with aryldiazonium salts, and then discuss the sensing applications of aryldiazonium salts modified on different transducers (bulky solid electrodes, nanomaterials modified bulky solid electrodes, and nanoparticles). Finally, the challenges and perspectives that aryldiazonium salt chemistry is facing in sensing applications are critically discussed.

Controlling Grafting from Aryldiazonium Salts: A Review of Methods for the Preparation of Monolayers

Surface modification by grafting from aryldiazonium salts has been widely studied and applied to many substrates as a simple and versatile method for preparing strongly adherent organic coatings. Unless special precautions or conditions are used, the usual film structure is a loosely packed disordered multilayer; however, over the past decade, attention has been paid to establishing strategies for grafting just a monolayer of modifiers to the surface. To date, four general approaches to monolayer preparation have emerged: use of aryldiazonium ions with cleavable protection groups; use of aryldiazonium ions with steric constraints; grafting in the presence of a radical scavenger; and grafting from ionic liquids. This review describes these approaches, illustrates some of their applications, and highlights the advantages and disadvantages of each.

"Janus" Calixarenes: Double-Sided Molecular Linkers for Facile, Multianchor Point, Multifunctional, Surface Modification

We herein report the synthesis of novel "Janus" calix[4]arenes bearing four "molecular tethering" functional groups on either the upper or lower rims of the calixarene. These enable facile multipoint covalent attachment to electrode surfaces with monolayer coverage. The other rim of the calixarenes bear either four azide or four ethynyl functional groups, which are easily modified by the copper(I)-catalyzed azide-alkyne cycloaddition reaction (CuAAC), either pre- or postsurface modification, enabling these conical, nanocavity reactor sites to be decorated with a wide range of substrates to impart desired chemical properties. Redox active species decorating the peripheral rim are shown to be electrically connected by the calixarene to the electrode surface in either "up" or "down" orientations of the calixarene.

Multifunctional and Stable Monolayers on Carbon: A Simple and Reliable Method for Backfilling Sparse Layers Grafted from Protected Aryldiazonium Ions

A new strategy for preparation of robust multifunctional low nanometer thickness monolayers on carbon substrates is presented. Beginning with protected aryldiazonium salts, sparse monolayers of ethynyl-, amino-, and carboxy-terminated tethers are covalently anchored to the surface. The layers are then backfilled with a second modifier via the nucleophilic addition of an amine derivative to the surface. Through use of electroactive moieties coupled to the tethers, and an electroactive amine for backfilling, electrochemical measurements reveal that backfilling approximately doubles the surface concentration of the monolayer. Cyclic voltammetry of solution-based redox probes at the modified surfaces is consistent with the expected blocking properties at various stages of surface preparation. Fractional surface coverages of the layers are estimated using electrochemically determined surface concentrations of modifiers and computationally derived modifier footprints. Assuming free rotation of the coupled ferrocenyl or nitrophenyl groups leads to physically unreasonable fractional surface coverages, indicating that these larger modifiers must be rotationally restricted. Using a conformationally constrained model produces lower bound estimates of the total fractional surface coverage close to 0.4, with tether-only coverages close to 0.2. The backfilled tether layers constitute practical platforms for controlled construction of complex interfaces with many potential applications including sensing, molecular electronics, and catalysis.

Strategies To Achieve Control over the Surface Ratio of Two Different Components on Modified Electrodes Using Aryldiazonium Salts

Controlling the composition of an interface is very important in tuning the chemical and physical properties of a surface in many applications including biosensors, biomaterials, and chemical catalysis. Frequently, this requires one molecular component to a minor component in a mixed layer. Such subtle control of composition has been difficult to achieve using aryldiazonium salts. Herein, aryldiazonium salts of carboxyphenyl (CP) and phenylphosphorylcholine (PPC), generated in situ from their corresponding anilines, are electrografted to form molecular platform that are available for further functionalization. These two components are chosen because CP provides a convenient functionality for further coupling of biorecognition species while PPC offers resistance to nonspecific adsorption of proteins to the surface. Mixed layers of CP and PPC were prepared by grafting them either simultaneously or consecutively. The latter strategy allows an interface to be developed in a controlled way where one component is at levels of less than 1% of the total layer.

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.

Toward the Control of the Creation of Mixed Monolayers on Glassy Carbon Surfaces by Amine Oxidation

A versatile and simple methodology for the creation of mixed monolayers on glassy carbon (GC) surfaces was developed, using an osmium-bipyridyl complex and anthraquinone as model redox probes. The work consisted in the electrochemical grafting on GC of a mixture of mono-protected diamine linkers in varying ratios which, after attachment to the surface, allowed orthogonal deprotection. After optimisation of the deprotection conditions, it was possible to remove one of the protecting groups selectively, couple a suitable osmium complex and cap the residual free amines. The removal of the second protecting group allowed the coupling of anthraquinone. The characterisation of the resulting surfaces by cyclic voltammetry showed the variation of the surface coverage of the two redox centres in relation to the initial ratio of the linking amine in solution.

Cross-linking and postfunctionalization of polymer films by utilizing the orthogonal reactivity of 7,7,8,8-tetracyanoquinodimethane

Spontaneous radical copolymerization of poly(7,7,8,8-tetracyanoquinodimethane) and α-chloromethylstyrene followed by click postfunctionalization produces colored and redox-active cross-linked polymer films.

One-step formation of bifunctionnal aryl/alkyl grafted films on conducting surfaces by the reduction of diazonium salts in the presence of alkyl iodides

The formation of partial perfluoroalkyl or alkyl radicals from partial perfluoroalkyl or alkyl iodides (ICH2CH2C6F13 and IC6H13) and their reaction with surfaces takes place at low driving force (∼-0.5 V/SCE) when the electrochemical reaction is performed in acetonitrile in the presence of diazonium salts (ArN2(+)), at a potential where the latter is reduced. By comparison to the direct grafting of ICH2CH2C6F13, this corresponds to a gain of ∼2.1 V in the case of 4-nitrobenzenediazonium. Such electrochemical reaction permits the modification of gold surfaces (and also carbon, iron, and copper) with mixed aryl-alkyl groups (Ar = 3-CH3-C6H4, 4-NO2-C6H4, and 4-Br-C6H4, R = C6H13 or (CH2)2-C6F13). These strongly bonded mixed layers are characterized by IRRAS, XPS, ToF-SIMS, ellipsometry, water contact angles, and cyclic voltammetry. The relative proportions of grafted aryl and alkyl groups can be varied along with the relative concentrations of diazonium and iodide components in the grafting solution. The formation of the films is assigned to the reaction of aryl and alkyl radicals on the surface and on the first grafted layer. The former is obtained from the electrochemical reduction of the diazonium salt; the latter results from the abstraction of an iodine atom by the aryl radical. The mechanism involved in the growth of the film provides an example of complex surface radical chemistry.

Evidence of monolayer formation via diazonium grafting with a radical scavenger: electrochemical, AFM and XPS monitoring

AFM monitoring of controlled surface modification with a radical scavenger.

Light-induced organic monolayer modification of iodinated carbon electrodes

We report the modification of carbon electrodes formed from pyrolyzed photoresist films (PPF) via plasma iodination followed by the organic monolayer modification of these surfaces. The iodinated surfaces were characterized using cyclic voltammetry, atomic force microscopy, and X-ray photoelectron spectroscopy to enable the optimization of the iodination while preserving the stability and smoothness of the carbon surface. Subsequently, the C-I surface was further modified with molecules that possess an alkene or alkyne at one end through light activation with low energy (visible range λ 514 nm). The versatility of the modification reaction of the C-I surfaces is shown by reactions with undecylenic acid, 1,8-nonadiyne, and S-undec-10-enyl-2,2,2-trifluoroethanethioate (C11-S-TFA). Modification with 1,8-nonadiyne allows further modification via "click" chemistry with azido-terminated oligo(ethylene oxide) molecules demonstrated briefly to alter the hydrophilicity of the surface after attachment of ethylene oxide moieties. Furthermore, patterning of C11-S-TFA was demonstrated using a simple photolithography technique. Deprotection of the C11-S-TFA gave a free thiol allowed patterning of gold nanoparticles on the surface as verified using scanning electron microscopy (SEM). These results demonstrate that plasma iodination to form C-I is a versatile, simple, and modular approach to functionalize the carbon surface.