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

Tefashe U. Comment on “Extent of conjugation in diazonium-derived layers in molecular junction devices determined by experiment and modelling” by C. Van Dyck, A. J. Bergren, V. Mukundan, J. A. Fereiro and G. A. DiLabio, Phys. Chem. Chem. Phys., 2019, , 16762

Misinterpretation of scanning tunnelling microscopy results and sampling error yielded incorrect conclusions about the flatness of a carbon electrode substrate used for molecular electronic devices.

Application of visible-light photosensitization to form alkyl-radical-derived thin films on gold

Visible-light irradiation of phthalimide esters in the presence of the photosensitizer [Ru(bpy)₃]²⁺ and the stoichiometric reducing agent benzyl nicotinamide results in the formation of alkyl radicals under mild conditions. This approach to radical generation has proven useful for the synthesis of small organic molecules. Herein, we demonstrate for the first time the visible-light photosensitized deposition of robust alkyl thin films on Au surfaces using phthalimide esters as the alkyl radical precursors. In particular, we combine visible-light photosensitization with particle lithography to produce nanostructured thin films, the thickness of which can be measured easily using AFM cursor profiles. Analysis with AFM demonstrated that the films are robust and resistant to mechanical force while contact angle goniometry suggests a multilayered and disordered film structure. Analysis with IRRAS, XPS, and TOF SIMS provides further insights.

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.

Electrochemical immobilization of a benzylic film through the reduction of benzyl halide derivatives: deposition onto highly ordered pyrolytic graphite

The reactivity of electrogenerated benzyl radicals at carbon surfaces was examined through the cathodic reduction of the corresponding bromide derivatives. 4-Nitrobenzyl bromide and benzyl bromide were reduced in N,N-dimethylformamide (DMF) on highly ordered pyrolytic graphite (HOPG) surfaces. Electroproduced films were examined using electrochemistry, scanning electron microscopy (SEM), and atomic force microscopy (AFM). Experiments show the formation of strongly adherent deposits and the occurrence of electrografting processes. They are based on radical generation and the reaction of the radical with the substrate. As expected, the thickness of the organic film increases with deposition time but the deposit displays a lower compactness than previously reported for the electroreduction of aryl diazonium salts. Interestingly for benzyl derivatives, the reduction potential required for the electrografting could be rendered much more positive by simply using an iodide-type supporting electrolyte.

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.

Nitrophenyl groups in diazonium-generated multilayered films: which are electrochemically responsive?

Various nitrophenyl-containing organic layers have been electrografted to glassy carbon surfaces using diazonium chemistry to elucidate the extent by which the layer structure influences the solvent (i.e., acetonitrile) accessibility, electroactivity, and chemical reactivity of the films. For most of these films, cyclic voltammetric and impedance spectroscopy measurements show that the electron-transfer process at the electrode is facile and independent of film thickness and structure. This is consistent with the occurrence of self-mediated electron transfers throughout the film with nitrophenyl groups serving as redox stations. Importantly, this behavior is seen only after the first potential sweep, the effect of which is to increase the porosity of the layer by inducing an irreversible desorption of nonchemisorbed material along with a reorganization of the film structure. From a kinetic point of view, the radical anions of surface-attached nitrophenyl groups are reactive toward the residual water present in acetonitrile. Thin layers (thickness of 1 to 2 nm) containing redox-active groups only in the outer part of the layer are protonated two to three times as fast as groups located in a more hydrophobic but still solvent-accessible inner layer. Hence, kinetic measurements can detect small differences in the layer environment. Finally, a deconvolution of the cyclic voltammetric response of an electrode grafted from 4-nitrobenzenediazonium discloses that roughly 25% of the overall signal can be attributed to the presence of 4-azonitrophenyl moieties introduced during the electrografting process.

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.

Steric effects in the reaction of aryl radicals on surfaces

Steric effects are investigated in the reaction of aryl radicals with surfaces. The electrochemical reduction of 2-, 3-, 4-methyl, 2-methoxy, 2-ethyl, 2,6-, 2,4-, and 3,5-dimethyl, 4-tert-butyl, 3,5-bis-tert-butyl benzenediazonium, 3,5-bis(trifluoromethyl), and pentafluoro benzenediazonium tetrafluoroborates is examined in acetonitrile solutions. It leads to the formation of grafted layers only if the steric hindrance at the 2- or 2,6-position(s) is small. When the 3,5-positions are crowded with tert-butyl groups, the growth of the organic layer is limited by steric effects and a monolayer is formed. The efficiency of the grafting process is assessed by cyclic voltammetry, X-ray photoelectron spectroscopy, infrared, and ellipsometry. These experiments, together with density functional computations of bonding energies of substituted phenyl groups on a copper surface, are discussed in terms of the reactivity of aryl radicals in the electrografting reaction and in the growth of the polyaryl layer.

Covalent photochemical functionalization of amorphous carbon thin films for integrated real-time biosensing

Recent studies have demonstrated that carbon, in the form of diamond, can be functionalized with molecular and/or biomolecular species to yield interfaces exhibiting extremely high stability and selectivity in binding to target biomolecules in solution. However, diamond and most other crystalline forms of carbon involve high-temperature deposition or processing steps that restrict their ability to be integrated with other materials. Here, we demonstrate that photochemical functionalization of amorphous carbon films followed by covalent immobilization of DNA yields highly stable surfaces with excellent biomolecular recognition properties that can be used for real-time biological detection. Carbon films deposited onto substrates at 300 K were functionalized with organic alkenes bearing protected amine groups and characterized using X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The functionalized carbon surfaces were covalently linked to DNA oligonucleotides. Measurements show very high selectivity for binding to the complementary sequence, and a high density of hybridizing DNA molecules. Samples repeatedly hybridized and denatured 25 times showed no significant degradation. The ability to use amorphous carbon films as a basis for real-time biosensing is demonstrated by coating quartz crystal microbalance (QCM) crystals with a thin carbon film and using this for covalent modification with DNA. Measurements of the resonance frequency show the ability to detect DNA hybridization in real time with a detection limit of <3% of a monolayer, with a high degree of reversibility. These results demonstrate that functionalized films of amorphous carbon can be used as a chemically stable platform for integrated biosensing using only room-temperature processing steps.