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Title: Impact of the Diazonium Grafting Control on the Interfacial Reactivity: Monolayer vs Multilayer
Authors: Thibaud Menanteau, Sylvie Dabos-Seignon, Eric Levillain, and Tony Breton
This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article.
To be cited as: ChemElectroChem 10.1002/celc.201600710
Link to VoR: http://dx.doi.org/10.1002/celc.201600710
Impact of the Diazonium Grafting Control on the Interfacial Reactivity: Monolayer vs Multilayer
Thibaud Menanteau, Sylvie Dabos-Seignon, Eric Levillain and Tony Breton*
Abstract: A very simple strategy to prepare, in two steps, a versatile and sustainable monolayer platform for on-surface chemistry is presented. The first step consists in the electroreduction of the well- known 4-nitrobenzenediazonium in the presence of a radical scavenger, leading to a covalent monolayer surface modification.
Then, a dense reactive phenylamine monolayer is obtained by the full electroreduction of the nitrophenyl moieties. The platform thus obtained is available for post-functionalization with carboxyl derivatives via a usual peptide coupling. Attachment of a TEMPO unit, offering both redox and electrocatalytic properties, validates this approach and leads to high surface coverage and fast electron transfer. A comparison of the electrochemical properties of the modified surface with a classical multilayered post-functionalized one highlights important differences in terms of interfacial reactivity.
The results presented here justify the interest of preparing reactive monolayer platform for molecule grafting and opens the way for simple and controlled surface chemistry without the need of synthesis.
Simple and robust grafting of functional molecules onto conducting surfaces is of paramount importance to prepare sustainable nanomaterial in numerous applications.
Preparations of functional surfaces are, most of the time, achieved using a two-step procedure: the grafting of an anchoring layer, followed by its coupling with the functional group.[1] This strategy allows to limit the number of synthetic steps and to attach complex and/or fragile structures via a simple chemical coupling.[2] The quest of the material sustainability makes the diazonium electrografting a good option for the preparation of anchoring layers as robust films are usually obtained on various materials using this technique.[3] The functionalization process rests on the electrochemical production of reactive aryl radicals by reduction of a diazonium salt that covalently binds onto the surface. However, the high reactivity of radicals usually leads to the uncontrolled polymerization of aromatics incompatible with the development of well-defined nanostructures essential for applications in nanochemistry and nanotechnologies.[4] Some general strategies were developed to limit the extent of the anchoring layer by using ionic liquids to control the diffusion[5] or arylhydrazines wich can be selectively converted into diazonium cation at the substrate/solution interface.[6] Sterically hindered[7] or long chain[8] substituted diazonium salts can also be used to avoid radical attacks on grafted aromatics, thus leading to monolayer grafting. This strategy was extended to the use of diazonium cations bearing
bulky protecting substituents, allowing post functionalization after deprotection.[9] Based on our recent control strategy, which avoids the aryl polymerization by the use of a free-radical scavenger,[10] we propose to assess the opportunities afforded by dense and homogeneous reactive monolayers prepared from the commercially available 4-nitrobenzenediazonium salt (4- NBD). As previously reported, the electroreduction of this group in acidic medium should lead to NH2-tethered surfaces, able to react with carboxyl derivatives.[11] Coupling of the controlled versus uncontrolled plateform with 4-carboxy-TEMPO will be achieved to highlight the difference of electronic and catalytic properties obtained.
The grafting of the organic monolayer was obtained by potential imposition of -0.5 V/SCE during 60 minutes in a 0.1 M nBu4PF6
acetonitrile solution containing the diazonium salt (1 mM) and the radical scavenger (DPPH) (2 mM). Glassy carbon for electrochemical studies, and pyrolyzed photoresist film (PPF) for AFM imaging, were modified. Electrochemical signal of the nitrophenyl tethered surface on glassy carbon in KOH (see experimental section and Supporting Information, Figure S1a for CVs) gave access to a surface coverage of 6.2 × 10-10 mol cm-2, consistent with the formation of a single molecule layer.[12]
Reduction of the nitrophenyl groups into aminophenyl functions was achieved in sulfuric acid 0.1 M by potential cycling (see the supporting information, Figure S2 for CVs). Integration of the reduction wave (25 µC) suggests a 6 electron process, corresponding to a full conversion of the grafted nitro groups.
The disappearance of the N1s XPS signal located at 406 eV, attributed to NO2 functions[13] (see the supporting information, Figure S3), confirms this assessment.
Via AFM scratching (see experimental section), a thickness value of 0.8 nm ± 0.3 nm was estimated for the nitrophenyl layer, consistent with a monolayer coverage (Figure 1).[10b] After conversion into aminophenyl groups, the film homogeneity remained unchanged and no significant variation of the layer thickness was observed (i.e. around 0.9 nm ± 0.3 nm).
The potentialities of aminophenyl reactive monolayers were evaluated through the grafting of a TEMPO unit, offering both redox and electrocatalytic properties. After a 24 h peptide coupling of the platform with 4-carboxy-TEMPO, a thickness of 1.4 nm ± 0.3 nm was measured by AFM scratching, consistent with the expected final thickness estimated from a CPK model.
In positive direction, CVs of post-functionalized electrode exhibit a fast one-electron process (i.e. 189 ± 20 s-1, determined by ACV, see the supporting information, Figure S4), assigned to the full-reversible TEMPO/TEMPO+ system (Figure 2a). From the estimated surface coverage (i.e. 4.3 × 10-10 mol cm-2), it can be calculated that 69 % of the –NH2 functions have undergone the chemical coupling, what is explainable by considering the steric hindrance of the TEMPO unit.
Dr T. Menanteau, Dr S. Dabos-Seignon, Prof. E. Levillain, Dr. T. Breton MOLTECH-Anjou, Université d’Angers, UMR CNRS 6200
2 Boulevard Lavoisier, 49045 Angers, France E-mail: [email protected]
Supporting information for this article is given via a link at the end of the
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6 nm
-5 nm
Figure 1. AFM topography images (3.3 × 1.3 µm) and the corresponding depth profiles of the modified PPF with 4-nitrobenzene diazonium in the presence DPPH, after reduction, and after coupling with 4-carboxy-TEMPO.
The standard deviation is close to 0.3 nm for depth measurements.
In order to demonstrate the benefits provided by a monolayer platform, the same three-step grafting procedure has been applied without the use of a free-radical scavenger. In this case, the thickness of the multilayered nitrophenyl grafted film reached 5.5 nm and a surface coverage of 27 × 10-10 mol cm-2 has been calculated by cyclic voltammetry in KOH (see the supporting Information, Figure S1b). It should be noted that, as previously shown for films > 2 nm, this way of calculation leads to an underestimate of the number of immobilized nitrophenyl groups because most of them remain electroinactive.[10b] Integration of the reduction wave (i.e. 47 µC) allows to calculate that 2.6 electrons were exchanged per electroactive nitrophenyl group, suggesting a partial reduction into aminophenyl function (see the Supporting Information, Figure S2b for CVs). The XPS monitoring of this electroreduction step shows a decrease of the N1s (406)/C1s atomic ratio (from 9.2 % to 5.5 %), confirming that only 60 % of the nitrophenyl functions are converted (see the Supporting Information Figure S3). This partial reduction agrees with results reported by Creus et al. on similar multilayered nitrophenyl films.[14] Moreover, the appearance of peaks localized at 402 and 404 eV, are consistent with the presence of incompletely reduced species as hydroxylamine or nitroso groups.[11a] Those results demonstrate that the absence of thickness control obviously leads to a chemical heterogeneity
of the surface where most of the grafted molecules are useless for the post-functionalization.
After coupling with 4-carboxy-TEMPO, the thickness of the film was estimated at 0.8 nm ± 0.3 nm via AFM scratching experiments, confirming a TEMPO unit addition (see the supporting Information, Figure S5). The electrochemical surface coverage (i.e. 4.2 × 10-10 mol cm-2 at 100 mV.s-1) reaches the same value than the one estimated for the monolayered material (i.e. 4.3 × 10-10 mol cm-2), reinforcing the idea that only aminophenyl functions localized on the upper part of the film are accessible for the peptide coupling due steric constraints.
Figure 2. Cyclic voltamogramms of TEMPO grafted monolayer (top) and multilayer (bottom) platform recorded in acetonitrile 0.1 M nBu4NPF6 for various scan rates.
In contrast with the monolayered film, the increase of the peak to peak potential separation with increasing scan rate highlights the lower electronic transfer efficiency (Figure 2). ACV measurements confirm this fact, giving a transfer rate constant of 72 ± 15 s-1. However, experimental points cannot be fitted as well as in the case of the monolayer using the model (see the supporting information, Figure S4). Ion/Ioff data are clearly spreaded over a larger frequency range, reflecting the presence of several chemical environments characterized by different electron transfer rates.
By plotting the variation of the electrochemical surface coverage as function of the scan rate, a more detailed analysis of the heterogeneous electron transfer can be obtained. As shown in Figure 3, the electrochemical surface coverage of a TEMPO- based monolayer is scan rate independent between 5 and 2000 mV.s-1, demonstrating its full redox-activity whatever the
dynamic required. In contrast, the TEMPO-based multilayer undergoes a dramatic decrease of its electroactivity with increasing scan rate which falls to a minimum value corresponding to 65% of that calculated for lower scan rates.
The main assumption to explain this behaviour is to consider the coexistence of various physical and/or chemical environments for the TEMPO units which play an important role in their electroactivity. As already observed for thick layers,[15] at high scan rate, it is quite possible that the diffusion of the electrolyte solution to the less accessible redox centers becomes the limiting step of the redox process, leading to the partial electrochemical response of the film.
Figure 3. Variation of the electrochemical surface coverage extracted from CVs for the monolayered TEMPO-modified film () and multilayered TEMPO- modified film () as function of the scan rate.
Interfacial reactivity of the TEMPO-modified surfaces was studied toward the oxidation of phenethyl alcohol as model molecule. 2,6-lutidine was used as organic base to ensure the electrochemical regeneration of the oxoammonium ions via a disproportionation reaction (see the electrocatalytic regeneration cycle in the Supporting Information, Figure S6). In the presence of alcohol, a catalytic current characteristic of the TEMPO regeneration is observed for both multi- and monolayered films (Figure 4). However, the catalytic behavior (i.e. potential shift and peak intensity) is much more marked for the monolayered one as evidenced by the subtraction of the current corresponding to the first TEMPO oxidation (i.e. current recorded without alcohol in solution).
Figure 4. Top: CVs of TEMPO grafted electrodes from monolayered (left) and multilayered (right) platforms in 0.1 M nBu4PF6/CH3CN under five repetitive cycles without sec-phenethyl alcohol (grey) and with 40 mmol.L-1 of sec- phenethyl alcohol and 80 mmol.L-1 of 2,6-lutidine (black). Bottom: calculated CVs obtained by subtracting first vertex of CVs in the absence of alcohol from first vertex of CV in its presence.
This difference is attributed to a higher turnover of the immobilized redox mediator, which is probably due to the combination of a higher electron transfer rate and a better accessibility of the catalytic centers. Furthermore, the stability under electrocatalytic conditions appears also impacted. In the case of a monolayered material, the catalytic current reaches a steady-state level after four voltammetric cycles, whereas a continuous decrease is noted for the multilayered one (a total desactivation of the catalytic surface is observed after 30 cycles).
Explanation for this last point remains intricate because of the multifactorial aspect involved in the alteration of the film under electrocatalytic conditions. However, it appears logical that the 2D structure of the monolayered material reduces the negative impact of steric constraints involved in the concerted mechanism between TEMPO+, alcohol and 2,6-lutidine[16] and also favors the free diffusion of acetophenone produced.
To summarize: this work shows that a dense and robust reactive monolayer platform can be easily prepared without synthesis step via the use of a free-radical scavenger. The usefulness of this simple approach was illustrated via the preparation of a TEMPO-modified surface from the platform. This strategy leads to a full responsive material with a high interfacial reactivity compared to that observed when the post-functionalization is made on a multi-layered film. These results underline the potentialities of the controlled diazonium grafting and demonstrate that structure-properties and structure-reactivity relationships are strongly dependent on the thickness and structure of the layer. This approach opens the way for the monolayer attachment of a wide variety of molecules or biomolecules and should offer promising prospects in forthcoming research on nanomaterials.
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Experimental Section
4-Nitrobenzenediazonium tetrafluoroborate (4-NBD, Aldrich), 4-carboxy- TEMPO (Aldrich), N,N’-dicyclohexylcarbodiimide (DCC) (Aldrich), 4- dimethylaminopyridine (DMAP) (Aldrich), 2,2-diphenyl-1-picrylhydrazyl (DPPH) (Aldrich), sec-phenethyl alcohol (Aldrich), 2,6-lutidine (Aldrich), tetrabutylammonium hexafluorophosphate (Aldrich) and acetonitrile (HPLC grade, Carlo Erba) were used as received.Experimental Details.
A potentiostat/galvanostat model SP300 (from Bio-Logic) monitored by ECLab software was used for electrochemical experiments. All potentials are reported versus SCE. Glassy carbon (GC) electrodes used for cyclic voltammetry were obtained from Bioanalytical Systems Inc. (Model MF- 2012; diameter 3 mm). GC sheets (10 × 10 × 2 mm) used for XPS experiments were obtained from GoodFellow. Preparation of pyrolyzed photoresist film (PPF), used for atomic force microscopy (AFM) experiments, has been described previously.[17] The GC surface was cleaned by polishing with Buehler 1 and 0.04 µm alumina slurry. After each polishing step, the electrode was washed with Nanopure water (18.2 M cm) by sonication. Prior to each derivatization, the electrode was sonicated in acetonitrile for 1 min. All electrolytic solutions were deaerated by argon bubbling for 15 min before cyclic voltammetry (CV) or potentiostatic experiments. Modification of surfaces was achieved at a fixed potential of -0.5 V (SCE) in deaerated acetonitrile containing 0.1 M nBu4NPF6 and 1 mM diazonium salt. When present, the concentration of DPPH was 2 mM. After each derivatization, the electrode was sonicated in acetonitrile for 1 min. Reduction of nitrophenyl tethered surface was achieved by 2 voltammetric cycles at 50 mV.s-1 between 0 and -1 V(SCE) in H2SO4 0.1 M and sonicated in ultrapure water for 1 minute after cycling.
TEMPO units were immobilized on the NH2 tethered surface using DCC activated coupling with 4-carboxy-TEMPO. 36 mg of 4-carboxy-TEMPO, 32 mg of DMAP and 49 mg of DCC were solubilized in a Schlenk. After stirring under Ar(g) for 1 h, the electrode was dipped in the solution for 24 h under Ar(g) atmosphere. After reaction, the modified electrode was sonicated in acetonitrile for 1 min and dried with nitrogen.
Surface coverage of Ar-NO2 tethered surfaces was estimated from CVs recorded in KOH by summing the charges for the irreversible reduction of Ar-NO2 and the charge for Ar-NHOH reoxidation and assuming a 6 electron transfer for each nitro group.[18]
Electrocatalysis experiments were achieved in a 20 mL cell containing 0.1 M nBu4NPF6 in acetonitrile in the presence of sec-phenethyl alcohol (20 mM) and 2,6-lutidine (86 mM).
ACV experiments were carried out using the following parameters: scan rate of 5 mV.s-1, sinusoidal voltage amplitude of 25 mV with one point being acquired every 10 mV.
The electronic transfer rate constant (ks) was determined by using the method developed by Creager et coll.[19] The ratio between the maximal intensity of anodic wave (ion) and the background intensity (ioff) was plotted versus the frequency of the sinusoidal superimposed current.
Then, the experimental points were fitted with the model (1) to calculate the constant of electron transfer (ks) with equation (2)
𝑖𝑜𝑛 𝑖𝑜𝑓𝑓= ‖𝑍𝑜𝑓𝑓
𝑍𝑜𝑛‖ = √(𝐶𝑑𝑙+ 𝐶𝑎𝑑𝑠)²𝜔² − (𝑅𝐶𝑇𝐶𝑑𝑙𝐶𝑎𝑑𝑠𝜔²)² (𝐶𝑑𝑙𝜔)² − (𝑅𝐶𝑇𝐶𝑑𝑙𝐶𝑎𝑑𝑠𝜔²)² (1)
𝑘𝑠= 1
2𝑅𝐶𝑇𝐶𝑎𝑑𝑠 (2)
XPS data were collected using a Kratos Axis Ultra spectrometer on modified GC sheets (Goodfellow, model VC 551). The X-ray source was
monochromated Al K working at 1486.6 eV. Spectra were accumulated at a take-off angle of 90°, using a spot size of 0.7×0.3 mm2 at a pressure of less than 10-8 mbar. High resolution scans (N 1s, C 1s and O 1s) were carried out with 0.1 eV step size and pass energy 40 eV. All spectra were calibrated taking C 1s as a reference binding energy of 284.5 eV (graphitic carbon component of the vitreous carbon substrates), without internal standard. XPS spectra were analyzed with the curve fitting program CASA XPS and involved background subtraction using Shirley and a subsequent pseudo-Voigt function mixing Gaussian-Lorentzian functions. Atomic ratios of the surfaces were calculated from core level spectra normal area divided by number of scans and the element sensitivity factor. For the elements considered, the sensitivity factors are O 1s 2.93, N 1s 1.78 and C 1s 1.00.
Film thickness measurements were made on modified PPF working electrodes by depth profiling using an AFM instrument (Nano-Observer device from CHInstruments). Images were processed using the open acces free software Gwyddion. A section of film was removed by scratching with the AFM tip (silicon cantilever, HQ:CSC37/No Al model, µmasch) and the scratch was imaged using non-contact tapping mode (silicon cantilever ACT model, AppNano). The spikes visible on the scratch edges are caused by the accumulation of removed material by the tip.
Acknowledgements
This work was supported by the "Centre National de la Recherche Scientifique" (CNRS France), and the "Agence Nationale de la Recherche" (ANR-15-CE09-0006 RADICAL).
Keywords: post-functionalization • radical scavenger • DPPH • carbon surface • TEMPO
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