Covalent bonding of alkene and alkyne reagents to graphitic carbon surfaces

Assessing the Acidic and Alkaline Recalcitrance of Covalently Modified Surface Amines on Ordered Mesoporous Carbon

Ordered mesoporous carbon (OMC) scaffolds were covalently modified with primary amine groups by means of oxidation-coupling, yielding C-O-C bonds, or organometallic activation-coupling, yielding C-C bonds. The aminated OMCs were stressed by immersion in either 1 M hydrochloric acid or 1 M sodium hydroxide solutions at room temperature for 6 h and characterized by nitrogen sorption, electron microscopy, low-angle X-ray diffraction, thermogravimetric analysis, and the 4-nitrobenzaldehyde assay. Results demonstrate that aminated surfaces of OMC by butyllithium grafting are stable toward both 1 M HCl and 1 M NaOH, whereas the oxidation-aminated OMC surfaces can withstand 1 M NaOH only. This study illustrates the importance of chemical testing to supplant chemical intuition when tailoring carbon surfaces for applications where strong acids or bases are employed. This is especially emphasized for carbonaceous materials because of the surface heterogeneity among different carbon allotropes.

Fabrication of Biosensing Interface with Monolayers

Surface modification is recognized as one of the fundamental techniques to fabricate biosensing interfaces. This review focuses on the surface modification of carbon substrates (GC and HOPG) and silica with a close-packed monolayer, in particular. In the cases of carbon substrates, GC and HOPG, it was demonstrated that surface modification of carbon substrates with diazonium derivatives could create a close-packed monolayer similar to the self-assembled monolayer (SAM) formation with mercapto derivatives. Similarly, the potential of trialkoxysilanes to form a close-packed monolayer was evaluated, and modification with a close-packed monolayer tended to occur under milder conditions when the trialkoxysilanes had a longer alkyl chain. In these studies, we synthesized surface modification materials having ferrocene as a redox active moiety to explore features of the modified surfaces by an electrochemical method using cyclic voltammetry, where surface concentrations of immobilized molecules and blocking effect were studied to obtain insight for density leading to a close-packed layer. Based on those findings, fabrication of a biosensing interface on the silica sensing chip of the waveguide-mode sensor was carried out using triethoxysilane derivatives bearing succinimide ester and oligoethylene glycol moieties to immobilize antibodies and to suppress nonspecific adsorption of proteins, respectively. The results demonstrate that the waveguide-mode sensor powered by the biosensing interface fabricated with those triethoxysilane derivatives and antibody has the potential to detect several tens ng/mL of biomarkers in human serum with unlabeled detection method.

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.

Anodic Methods for Covalent Attachment of Ethynylferrocenes to Electrode Surfaces: Comparison of Ethynyl Activation Processes

The electrochemical oxidation of ferrocenes having an H- or Li-terminated ethynyl group has been studied, especially as it relates to their covalent anchoring to carbon surfaces. The anodic oxidation of lithioethynylferrocene (1-Li) results in rapid loss of Li(+) and formation of the ethynyl-based radical FeCp(η(5)-C5H4)(C≡C), (1, Cp = η(5)-C5H5), which reacts with the electrode. Chemically modified electrodes (CMEs) were thereby produced containing strongly bonded, ethynyl-linked monolayers and electrochemically controlled multilayers. Strong attachments of ethynylferrocenes to gold and platinum surfaces were also possible. The lithiation/anodic oxidation process is a mirror analogue of the diazonium/cathodic reduction process for preparation of aryl-modified CMEs. A second method produced an ethynylferrocene-modified electrode by direct anodic oxidation of the H-terminated ethynylferrocene (1-H) at a considerably more positive potential. Both processes produced robust modified electrodes with well-defined ferrocene-based surface cyclic voltammetry waves that remained unchanged for as many as 10(4) scans. Ferrocene derivatives in which the ethynyl moiety was separated from the cyclopentadienyl ring by an ether group showed very similar behavior. DFT calculations were performed on the relevant redox states of 1-H, 1-Li, and 1, with emphasis on the ferrocenyl vs ethynyl character of their high valence orbitals. Whereas the HOMOs of both 1-H and 1-Li have some ethynyl character, the SOMOs of the corresponding monocations are strictly ferrocenium in makeup. Predominant ethynyl character returns to the highest valence orbitals after loss of Li(+) from [1-Li](+) or loss of H(+) from [1-H](2+). These anodic processes hold promise for the controlled chemical modification of carbon and other electrode surfaces by a variety of ethynyl or alkynyl-linked organic and metal-containing systems.

Regeneration of pyrolyzed photoresist film by heat treatment

A simple, time-, and cost-effective procedure is described for regenerating film-modified or deactivated pyrolyzed photoresist film (PPF) surfaces. Heating for 30 min at 545 ± 25 °C in argon at a flow rate of 1 L min(-1) removes covalently bound thin organic films, attached via electrografting from aryldiazonium salt solutions. The heat-treated surfaces exhibit improved electrochemical characteristics compared to those prior to modification and can be reused for solution-based electrochemical measurements and for electrografting. The same treatment reactivates PPF electrodes that have been deactivated by exposure to adsorbates from air or solution. X-ray photoelectron spectroscopy (XPS), atomic force microscopy, and water contact angle measurements establish that the regeneration procedure does not lead to significant changes in oxygen content, roughness, or hydrophobicity of PPF surfaces. XPS measurements also confirm the complete removal of covalently attached organic films after heat treatment but reveal a specific interaction between grafted nitrophenyl films and PPF which results in a small amount of N incorporation in the surface.

Surface modification of GC and HOPG with diazonium, amine, azide, and olefin derivatives

Surface modification of glassy carbon (GC) and highly oriented pyrolytic graphite (HOPG) was carried out with diazonium, amine, azide, and olefin derivatives bearing ferrocene as an electroactive moiety. Features of the modified surfaces were evaluated by surface concentrations of immobilized molecule, blocking effect of the modified surface against redox reaction, and surface observation using cyclic voltammetry and electrochemical scanning tunneling microscope (EC-STM). The measurement of surface concentrations of immobilized molecule revealed the following three aspects: (i) Diazonium and olefin derivatives could modify substrates with the dense-monolayer concentration. (ii) The surface concentration of immobilized amine derivative did not reach to the dense-monolayer concentration reflecting their low reactivity. (iii) The surface modification with the dense-monolayer concentration was also possible with azide derivative, but the modified surface contained some oligomers produced by the photoreaction of azides. Besides, the blocking effect against redox reaction was observed for GC modified with diazonium derivative and for HOPG modified with diazonium and azide derivatives, suggesting fabrication of a densely modified surface. Finally, the surface observation for HOPG modified with diazonium derivative by EC-STM showed a typical monolayer structure, in which the ferrocene moieties were packed densely at random. On the basis of those results, it was demonstrated that surface modification of carbon substrates with diazonium could afford a dense monolayer similar to the self-assembled monolayer (SAM) formation.

Two-component mixed and patterned films on carbon surfaces through the photografting of arylazides

Organic films have been grafted to glassy carbon surfaces by the photolysis of arylazides. Atomic force microscopy and electrochemical measurements reveal that the films are loosely packed. The methodology was expanded to prepare two-component thin films incorporating either a reactive tether species and a nonreactive background film or two different reactive tethers. Strategies were developed to generate both continuous mixed films and surfaces presenting patterns of two components. For patterning, the arylazide derivative was grafted onto previously modified glassy carbon surfaces. In this case, the first modification step is not limited to photografting, which increases the scope of the methods. For all grafted surfaces, the reactivity of tether species was confirmed by coupling electroactive targets to the tethers, followed by electrochemical monitoring. The ease of preparing surfaces with spatially controlled functionality offers promise for the design of sensing platforms on graphitic carbon substrates.

Functionalization of glassy carbon with diazonium salts in ionic liquids

The paper reports on the chemical functionalization of glassy carbon electrodes with 4-bromobenzene (4-BBDT) and 4-(4'-nitrophenylazo)benzene diazonium tetrafluoroborate (4-NAB) salts in ionic liquids. The reaction was carried out at room temperature in air without any external electrical bias in either hydrophobic (1-butyl-3-methylimidazolium hexafluorophosphate) or hydrophilic (1-butyl-3-methylimidazolium methyl sulfate) ionic liquids. The resulting surfaces were characterized using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and electrochemical measurements. Electrochemical reduction of the terminal nitro groups allowed the determination of surface coverage and formation of an amine-terminated carbon surfaces. The results were compared to glassy carbon chemically modified in an aqueous solution in the presence of 1% sodium dodecyl sulfate (SDS) with the same diazonium salt. Furthermore, Raman spectroscopy coupled with electrochemical measurements allowed to distinguish between the reduction of -NO2 to -NH2 group and the -N=N- to -NH-NH- bond.

Photochemical grafting and activation of organic layers on glassy carbon and pyrolyzed photoresist films

Organic films have been grafted to polished glassy carbon (GC) and as-prepared pyrolyzed photoresist film (PPF) by photolysis of alkenes and an alkyne. The alkene or alkyne is spin-coated onto the carbon surface and photolyzed in air at 254 nm. Characterization by water contact angle measurements, depth profiling and surface roughness measurements using atomic force microscopy (AFM), and electrochemistry reveal that for most modifiers a loosely packed monolayer is grafted to the surface. Grafted layers of 1-decene were further reacted by drop-coating with oxalyl chloride and photolyzing at 254 nm in air. The procedure adds acid chloride groups to the film. Amines were attached to these films via amide bond formation, and were characterized by electrochemistry and assembly of citrate-capped gold nanoparticles. Amines were also coupled to photografted 1-undecylenic acid layers and to carboxyphenyl layers prepared by electroreduction of the corresponding diazonium salt. Quantitative analysis using electrochemistry established that the highest concentration of amines was attached to the oxalyl chloride treated film, and that a higher concentration of amines was attached via reaction with the photografted 1-undecylenic acid layer than the electrografted carboxyphenyl layer. Thus photografting and photoreaction with oxalyl chloride are simple methods for generating amine-reactive tethers on GC and PPF surfaces.