Cathodic Electrografting Research Articles

Electrografting of in situ generated pyrrole derivative diazonium salt for the surface modification of nickel

This work reports for the first time on the modification of nickel surfaces by cathodic electrografting of in situ generated diazonium. An original diazonium salt (the 4-pyrrolylphenyldiazonium called Py-PD hereafter) was electrografted on nickel after its generation from 4-(1H-pyrrol-1-yl)aniline (Py-A) in presence of three acidic conditions (1, 2 and 10equiv. of HClO4/Py-A) has been investigated by UV–vis spectroscopy. Results show that the potentiostatic electrografting of Py-PD is concomitant with nickel and proton reduction. This electrografting leads to the formation of multilayered films in each of the studied in situ generation conditions. The use of 1equiv. of HClO4/Py-A for the in situ generation results in the formation of inhomogeneous and irreproducible coatings while 2 and 10equiv. lead to the formation of highly covering, homogeneous and reproducible films. These films present good electrochemical barrier properties toward the ferri/ferrocyanide couple. The use of gentle stoichiometric acidic conditions for in situ diazonium generation widens the application field of this one-step procedure to the surface modification of oxidizable materials presenting an unstable oxide layer.

Surface Electroinitiated Emulsion Polymerization (SEEP): A Mechanistic Approach

As recently reported, the SEEP process (surface electroinitiated emulsion polymerization) is a new grafting method that provides covalently grafted polymer films on conducting or semiconducting surfaces by radical polymerization in aqueous dispersed media. It relies on cathodic electroinitiation, which creates radical species able to start a radical polymerization. Contrary to the formerly described cathodic electrografting of vinylic polymers (CE), which also delivers submicrometer-thick and stable polymer films on conducting substrates but requires strictly anhydrous conditions and organic aprotic solvent, SEEP brings a major improvement in switching from a purely anionic mechanism to a radical one by adding an aryldiazonium salt in the reaction mixture, while retaining the same polymer films characteristics. Moreover, SEEP is not restricted to water-soluble monomers but can be performed even with hydrophobic ones, such as n-butyl methacrylate (BMA). In such cases, a surfactant is necessary to stabilize the monomer in water emulsion. From this one-pot electrografting process performed in water at room temperature, in a few minutes, without restrictions on vinylic monomer water solubility, results a polymer coating strongly grafted to the substrate. This article aims at completing our first one and focuses on mechanistic aspects of SEEP to eventually establish a possible “grafting onto” mechanism. To achieve that goal, we have analyzed grafted polymer films obtained by SEEP on gold substrate from BMA in water as a miniemulsion by IR-ATR, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), and atomic force microscopy (AFM).

Cathodic Electrografting of Alkyl Nanopatterns on Silicon (100)

Nanoscale patterns are created on silicon (100) surfaces via cathodic electrografting of alkyls. Self-assembled polystyrene microspheres on silicon surfaces can serve as dielectric masks. Cathodic potentials are applied to masked substrates in liquid hexynoic acid or phenylacetylene leading to the formation of nanoscale islands with monolayers of covalently bound functional groups on silicon. Silicon surfaces functionalized with islands of hexynoic acid are demonstrated using a biotin reaction. Cyclic voltammetry, scanning electron microscopy, and atomic force microscopy are used to characterize the patterned surfaces. The method provides a facile and high throughput process for spatially defining the local reactions on silicon with nanometer-scale resolution.

Comparison of two “grafting from” techniques for surface functionalization: Cathodic electrografting and surface-initiated atom transfer radical polymerization

The grafting of poly n-butyl methacrylate (PBuMA) on gold surface has been achieved via cathodic electrografting and by surface-initiated atom transfer radical polymerization (SI-ATRP). Both polymerizations, that are classically referred to as “grafting from” methods, can be carried out in similar conditions for the synthesis of grafted organic coatings on conducting surfaces. The aim of this study is thus to compare those two techniques in terms of film thickness and time of polymerization. Experimental conditions are compared and discussed. Both polymerizations were followed in situ by QCM. The resulting grafted polymers were characterized by Fourier transform infrared and XPS spectroscopies. Both methods appear to be valuable tools for the synthesis of grafted organic coatings on conducting surfaces, but with different scopes of applications.

Stainless Steel Grafting of Hyperbranched Polymer Brushes with an Antibacterial Activity: Synthesis, Characterization, and Properties

Two strategies were used for the preparation of hyperbranched polymer brushes with a high density of functional groups: (a) the cathodic electrografting of stainless steel by poly[2-(2-chloropropionate)ethyl acrylate] [poly(cPEA)], which was used as a macroinitiator for the atom transfer radical polymerization of an inimer, 2-(2-bromopropionate)ethyl acrylate in the presence or absence of heptadecafluorodecyl acrylate, (b) the grafting of preformed hyperbranched poly(ethyleneimine) onto poly(N-succinimidyl acrylate) previously electrografted onto stainless steel. The hyperbranched polymer, which contained either bromides or amines, was quaternized because the accordingly formed quaternary ammonium or pyridinium groups are known for antibacterial properties. The structure, chemical composition, and morphology of the quaternized and nonquaternized hyperbranched polymer brushes were characterized by ATR-FTIR reflectance, Raman spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. The peeling test confirmed that the grafted hyperbranched polymer films adhered much more strongly to stainless steel than the nongrafted solvent-cast films. The quaternized hyperbranched polymer brushes were more effective in preventing both protein adsorption and bacterial adhesion than quaternary ammonium containing poly(cPEA) primary films, more likely because of the higher hydrophilicity and density of cationic groups.

Protein Immobilization onto Silicon Nanowires via Electrografting of Hexynoic Acid

Electrografting provides a method for the direct (Si-C) covalent functionalization of silicon surfaces. In this work, cathodic electrografting of hexynoic acid to silicon nanowire surfaces and subsequent immobilization of bovine serum albumin (BSA) is demonstrated. Hexynoic acid was cathodically electrografted to dense arrays of silicon nanowires approximately 50-100 nm in diameter and 10 μm in length. Protein immobilization was achieved via amidization reactions with carboxylic moieties of electrografted hexynoic acids. Results include chronoamperometric data from the electrografting process and verification of attachment using attenuated total reflectance Fourier transform infrared spectroscopy (FTIR) of electrografted hexynoic acid and immobilized BSA. In addition to FTIR analysis, covalently immobilized BSA was verified using fluorescence microscopy and solution-phase fluorescence.

Cathodic Electrografting of Versatile Ligands on Si(100) as a Low‐Impact Approach for Establishing a SiC Bond: A Surface‐Coordination Study of Substituted 2,2′‐Bipyridines with CuI Ions

Three distinct wet chemistry recipes were applied to hydrogen-terminated n- and p-Si(100) surfaces in a comparative study of the covalent grafting of two differently substituted 2,2'-bipyridines. The applied reactions require the use of heat, or visible light under a controlled atmosphere, or a suitable potential in an electrochemical cell. In this last case, hydrogen-terminated silicon is the working electrode in a cathodic electrografting (CEG) reaction, in which it is kept under reduction conditions. The resulting Si--C bound hybrids were characterized by a combination of AFM, dynamic contact-angle, and XPS analysis, with the help of theoretical calculations. The three distinct approaches were found to be suitable for obtaining ligand-functionalized Si surfaces. CEG resulted in the most satisfactory anchoring procedure, because of its better correlation between high coverage and preservation of the Si surface from both oxidation and contamination. The corresponding Si-bipyridine hybrid was reacted in a solution of CH3CN containing CuI ions coordinatively bound to the anchored ligands, as evidenced from the XPS binding-energy shift of the N atom donor functions. The reaction gave a 1:2 Cu-bipyridine surface complex, in which two ligands couple to a single CuI ion. The surface complex was characterized by the Cu Auger parameter and Cu/N XPS atomic-ratio values coincident with those for pure, unsupported CuI complex with the same 2,2'-bipyridine. Further support for such a specific metal-ligand interaction at the functionalized Si surface came from the distinct values of Cu2p binding energy and the Cu Auger parameter, which were obtained for the species resulting from CuI ion uptake on hydrogen-terminated Si(100).

First Insights into Electrografted Polymers by AFM-Based Force Spectroscopy

The very first characterization of the structural properties of polymer films obtained by electrografting is reported. AFM-based force spectroscopy was used to investigate poly-N-succinimidyl acrylate (PNSA) layers electrografted directly a silicon substrate. Quantitative analysis of compression profiles obtained in a good solvent and single molecule bridging interaction, in light of the Alexander-de Gennes model, gave access to the density and degree of polymerization. A high swelling capacity has been evidenced. This report is the first evidence that polymers obtained by cathodic electrografting are in fact brush systems, and consequently the first evidence that a polymer brush can be obtained a direct grafting from method, without any intermediate layer.

An AFM investigation of oligonucleotides anchored on unoxidized crystalline silicon surfaces

Carboxylic terminated monolayers have been covalently attached on phosphorous doped crystalline (1 0 0) silicon surfaces using a cathodic electro grafting technique. The functionalization concentration and efficiency have been evaluated with different techniques. In particular, topographic images, performed with an atomic force microscope, were used to optimize the protocol in order to obtain a surface whose characteristics of uniformity and reproducibility are ideal for a bio-electronic device. Phase lag images of the functionalized surfaces were also performed, and show non-topographic structures that have been interpreted as areas of different molecule self-orientation. Poly-thymine oligonucleotides have been anchored on such a surface to form a nano-biosensing device capable to react selectively with a specific target molecule, a poly-adenine oligonucleotide. AFM images of high density (∼3 × 10 12 mol/cm 2) single strand and double strand covered samples show toroidal shaped structures formed by the self-assembly of the oligonucleotides on the silicon surface.

Electrografting of thin polymer films: Three strategies for the tailoring of functional adherent coatings

Cathodic electrografting is an efficient technique to impart adhesion to poly(meth)acrylate coatings onto inorganic conducting surfaces. Although this technique was restricted for many years to very few monomers ((meth)acrylonitrile and (meth)acrylates) and to deposition of very thin polymer films, recent developments have overcome these limitations. First of all, classical controlled/living polymerization techniques have been combined with cathodic electrografting as a powerful strategy for tuning thickness, properties and reactivity of the chemisorbed organic films. Secondly, thanks to the successful electrografting of a new reactive monomer bearing an activated ester, electrografted surfaces are now available for further derivatization by a wide variety of nucleophiles. Finally, the electrografting process has been extended to the direct electrografting of reactive polymers, i.e. preformed polymers bearing pendant acrylic functions, which opens the way to the grafting of, e.g., polycondensates.

Mask‐free Localized Grafting of Organic Polymers at the Micrometer or Submicrometer Scale on Composite Conductors or Semiconductor Substrates

AbstractThis work presents a new one‐step process enabling the mask‐free localized functionalization by organic polymers of the conducting or semiconducting parts of composite surfaces at the micrometer and submicrometer scale. The functionalization is carried out via cathodic electrografting of suitable precursors of composite cathodes, which gives an insulating polymer film strongly grafted to selected parts of the composite electrodes and guarantees that the resolution is that of the pre‐existing pattern, even when the whole surface is dipped into the reaction medium. The presumed mechanism is based on a regioselective extraction of electrons from the composite surface according to its apparent local work function in solution.

Carboxylic acid terminated monolayer formation on crystalline silicon and silicon nitride surfaces. A surface coverage determination with a fluorescent probe in solutionElectronic Supplementary Information (ESI) available: analytical data of the new compounds and general information on the instruments used for their characterization. See http://www.rsc.org/suppdata/jm/b3/b312273e/

Carboxylic acid terminated (–COOH) monolayers (ML) have been covalently anchored on the surface (S) of crystalline silicon (Si) and silicon nitride (Si3N4) by using wet-chemistry methods. Their concentration has been determined adopting a stepwise procedure with a fluorescent probe in solution. First, 7-amino-4-methylcoumarin (H2N–C10H7O2, AMC) is covalently bonded to the monolayer on the surface through an amidation reaction forming an amide-terminated surface monolayer, S–(CH2)n–CO–NH–C10H7O2, then the surface is intensely washed in order to remove any physisorbed species, finally AMC is detached from the surface via hydrolysis of the –CO–NH– bond and quantified by a fluorescence analysis in water solution. The yields of both the amidation and hydrolysis reactions have been evaluated. By this method, submonolayers ≤0.1 ± 0.03 ML can be determined. It has been possible to evaluate the different extent of surface functionalization changing the substrate between Si and Si3N4 and for Si changing the monolayer formation reaction: heat- (Δ) or light- (hν) promoted or via cathodic electrografting (CEG). It came out that submonolayers are formed in any case, Si3N4 is functionalized to a larger extent than Si on heating (0.52 vs. 0.15 ML) and Si is best functionalized via cathodic electrografting: Δ, 0.15; hν 0.20; CEG, 0.44 ML. Surface topography of some monolayers on Si and Si3N4 substrates was investigated by Atomic Force Microscopy (AFM). On clean Si, AFM showed topographical variations of 0.3–0.4 nm while for the clean Si3N4 the corrugation was around 3–4 nm. After functionalization the two surfaces were uniformly covered and the corrugation increased in both samples with a corrugation in topography of 1–2 nm for Si and 5–6 nm for Si3N4.

Nanopatterning of Alkynes on Hydrogen-Terminated Silicon Surfaces by Scanning Probe-Induced Cathodic Electrografting

The electrochemical cathodic electrografting reaction, previously demonstrated on bulk silicon surfaces, can be patterned on the nanoscale utilizing conducting probe atomic force microscopy (CP-AFM). Alkyne electrografting is a particularly useful chemical technique since it leads to direct covalent attachment of conjugated alkynes to silicon. In addition, application of a forward bias during the reaction renders the surface less sensitive to oxidation and the resulting monolayers are very stable in air and basic aqueous solution. Alkyne monolayer lines can be drawn down to 40 nm resolution using a Pt-coated AFM tip, and the heights of the monolayers scale with the molecular length of the alkyne. The tip is biased (+) and the surface is biased (-) to drive the cathodic electrografting reaction under ambient conditions. The resistance of the monolayers to fluoride, as well as friction force microscopy, indicate that the alkynes are covalently bonded to the surface, not oxide-based, and hydrophobic. The reaction does not work with alkenes, and therefore hydrosilylation is not the primary mode of reaction. Wider lines (300 nm) can be produced using broadened Pt-coated AFM tips. This reaction could be important for the interfacing of conjugated molecules directly to silicon in a spatially controlled fashion.

Polymer Coating of Steel by a Combination of Electrografting and Atom-Transfer Radical Polymerization

Cathodic electrografting of poly(2-chloropropionate ethyl acrylate) (poly[cPEA]) onto steel followed by the styrene grafting-from by atom transfer radical polymerization (ATRP) is an efficient strategy to impart strong adhesion to polystyrene films onto the electrically conductive substrate. Electrografting of poly(cPEA) chains at an appropriate potential and persistence of the activated chloride in the grafted chains were confirmed by XPS. Polystyrene deposition by ATRP with a ruthenium-based catalyst was analyzed by scanning electron microscopy and Raman spectroscopy. Adhesion of the polystyrene layer to the substrate is so strong that it cannot be detached by standard Scotch brand tapes. Moreover, local thermal analysis showed a loss of mobility for the PS chains tethered at the surface.

Full Electrochemical Synthesis of Conducting Polymer Films Chemically Grafted to Conducting Surfaces

This paper reports on the full electrochemical synthesis of electrically conducting polymers chemically grafted to conducting surfaces (e.g., glassy carbon, stainless steel, nickel, gold). It is based on new functional acrylate monomers, i.e., 3-(2-acryloyloxyethyl)thiophene and N-(2-acryloyloxyethyl)pyrrole, whose the synthesis is reported in this work. The polymerization process consists of two electrochemical steps. The first one is the cathodic electrografting of polyacrylate chains bearing a precursor of the conducting polymer in the ester group, either thiophene or pyrrole. In the second step, this precursor is polymerized under anodic polarization, in the presence or not of additional unsubstituted monomer in the electrochemical bath. Cyclic voltammetry was used to confirm that the two-component film is conducting and electrochemically active (reversible doping and dedoping). The chemical composition and the microscopic morphology of these composites were characterized by X-ray photoelectron spectr...

Three Methods for Stabilization and Functionalization of Porous Silicon Surfaces via Hydrosilylation and Electrografting Reactions

Three reactions are discussed which accomplish a high degree of substitution of the silicon hydride bonds on the porous silicon (pSi) surface with silicon–carbon bonds. Lewis acid mediated (LAM) and white light-promoted (LP) hydrosilylation of alkynes and alkenes gives surfaces with alkenyl and alkyl residues, respectively. Evidence for the silicon–carbon bonds is given by solid-state NMR spectroscopy of free-standing pSi material. The extent of the substitution of the surface bonds is termed reaction efficiency (E) which is measured by the change in the integrated area of the silicon hydride stretch region in FTIR spectra (2000–2200 cm—1). The LAM hydrosilylation gives a higher E value than the LP reaction. The E value of the LAM reaction is believed to be limited by the efficiency of the catalyst diffusion throughout the porous structure. In the case of the LP reaction, it is limited by the relative quantity of visible light-exposed material in comparison to the entire amount of IR-apparent silicon hydride bonds. The cathodic electrografting (CEG) reaction gives directly attached alkynyl residues via reduction of a silicon hydride bond to generate alkynyl carbanions, which then react with silicon–silicon bonds.

Effects of Organic Monolayer Formation on Electrochemiluminescence Behavior of Porous Silicon

The effects of various organic monolayers on the surface of porous silicon on the electrochemiluminescence (ECL, also referred to as electroluminescence in the literature) characteristics were investigated. Surfaces were terminated with alkyne, alkene, and alkyl functionalities through cathodic electrografting (CEG), Lewis acid-mediated hydrosilylation (LA), and anodic electrografting (AEG), respectively. ECL was induced through the known formic acid/sodium formate electron injection system. Alkyl-terminated surfaces produced through AEG yielded the brightest emission of any of the functionalized surfaces although ECL emission was about half as intense as that from the underivatized Si−H-terminated surface. The lifetime, however, was extended by a factor of 2, and these surfaces demonstrate an unprecedented recharging phenomenon. When ECL ceases, a brief 10 s application of a cathodic bias restores most of the ECL emission intensity. This process can be cycled about 10 times, and results in a substantially greater light output than that from any of the surfaces examined here, including the Si−H-terminated surface. Dodecenyl-terminated surfaces, produced through Lewis acid-mediated hydrosilylation of 1-dodecyne, show the greatest lifetimes, an order of magnitude longer than that of an Si−H-terminated surface, but their emission intensity is unfortunately very low. The induction times for light emission to occur decreased for all the functionalized surfaces, as compared to the native Si−H surface. Explanations for the effects of organic monolayer formation on ECL observed here are described.

Anodic and cathodic electrografting of alkynes on porous silicon†

A versatile electrochemical grafting reaction connects conjugated molecules to porous silicon surfaces with either a positive or negative bias.