Oxidative and Stepwise Grafting of Dopamine Inner-Sphere Redox

Article pubs.acs.org/ac

Oxidative and Stepwise Grafting of Dopamine Inner-Sphere Redox Couple onto Electrode Material: Electron Transfer Activation of Dopamine Jalal Ghilane,*,† Fanny Hauquier,‡ and Jean-Christophe Lacroix† †

Nano-Electro-Chemistry group, Univ Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR 7086 CNRS, 15 rue Jean-Antoine de Baïf, 75205 Paris, France ‡ Analyse chimique et bioanalyse, Conservatoire National des Arts et Métiers, 292 rue St Martin, 75003 Paris, France ABSTRACT: The immobilization of dopamine, a neurotransmitter, onto macroelectrode and microelectrode surfaces has been performed following two strategies. The first consists of a one-step grafting based on electrochemical oxidation of an amino group in acidic media. The second is a stepwise process starting with electrochemical grafting of diazonium, leading to the attachment of aryl layer bearing an acidic headgroup, followed by chemical coupling leading to immobilized dopamine molecules onto the electrode surface. Electrochemical, infrared (IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) analyses evidence that both methods are suitable for the immobilization of dopamine onto millimetric and micronic electrodes. The electrochemical responses of modified electrodes demonstrate that the electroactivity of the attached dopamine layer appears unaffected by the nature of the spacer, alkyl or aryl layers, suggesting that the communication, through tunneling, between the attached dopamine and the electrode is possible. More interestingly, the dopamine-modified electrode exhibits electron transfer activation toward dopamine in solution. As a result, not only does the dopamine modified electrode yield a fast electron transfer with lower ΔEp (30 mV) than the majority of pretreatment procedures but also the ΔEp is as small as that observed for more complex surface treatments.

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surfaces. This was mainly performed using a two-step procedure based on the attachment of a first layer followed by a cross-coupling reaction, such as peptide coupling and click chemistry between the attached head groups and other molecules in solution.20−23 Independently of the used grafting procedure, various studies have been devoted to the immobilization of the redox system onto the electrode surface.6,24−27 However, most of the investigated redox molecules are outer-sphere redox couples, while a little attention has been devoted to the immobilization of inner-sphere redox couples. From the electrochemical point of view, in an outer-sphere reaction, the reactants, products, and intermediates do not interact strongly with the electrode material and electron transfer occurs by tunneling across at least a monolayer of solvent, while in an inner-sphere reaction there is a strong interaction of reactant or product with the electrode surface.28 One can typically distinguish experimentally between inner and outer sphere electrode reactions because outer-sphere reactions are generally rather insensitive to the nature of the electrode material, whereas inner-sphere reactions depend very strongly on the electrode material.29

ver the last two decades considerable efforts have been devoted to the development of methods for the immobilization of thin organic layers bearing specific functional groups onto electrodes surfaces to form modified electrodes.1,2 The functionalization of electrode surfaces leads to the addition of specific properties to the materials and facilitates their use in various applications such as electrocatalysis, molecular electronic, photovoltaic, and sensing.3,4 Various strategies have been proposed to form defectless mono- and multilayered structures, leading to control of their physicochemical properties. In this context, electrochemistry has been proposed as a convenient method to produce modified surfaces. The most commonly used electrochemical procedure for derivatizing electrodes surfaces consists of the electrochemical oxidation of amines or the electrochemical reduction of aryldiazonium salts.5−9 Both processes conduce to the formation of thin organic film covalently attached onto the electrode surface, and a number of reviews were dedicated to these processes.6,9,10 The electrochemical grafting has been successfully used for the attachment of a large numbers of molecules bearing various functionalities,11−16 was applied to various electrode materials, and was performed in different electrolytic media.17−19 Recently, the grafted layers have been used as a platform for further functionalization, leading to the immobilization of various functional groups onto the electrode © XXXX American Chemical Society

Received: September 19, 2013 Accepted: October 31, 2013

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dx.doi.org/10.1021/ac402994u | Anal. Chem. XXXX, XXX, XXX−XXX

Analytical Chemistry

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Scheme 1. Schematic Illustrating the Immobilization of Dopamine onto Electrode Surfacea

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(a) Oxidative grafting and (b) stepwise grafting combining diazonium grafting and peptide coupling.

dopamine (Scheme 1b). The grafting was performed on a glassy carbon electrode, an ultramicroelectrode carbon fiber, and was analyzed by electrochemical methods. In addition, the presence of the attached dopamine was investigated by surface analysis using infrared (IR) spectroscopy and X-ray photoelectron spectroscopy. Finally, the capability of the modified electrode to activate the electron transfer of dopamine in solution was investigated.

Dopamine has been widely used and studied by means of electrochemical methods and has been demonstrated to be inner-sphere redox molecules.30,31 Recently, redox molecules presenting an inner-sphere electron transfer, such as dopamine, have been successfully used for investigating modified surfaces and highly orientated pyrolytic graphite (HOPG), enabling the revealing pinholes, defects, and active sites at graphite surfaces.32,33 Besides that, dopamine was found to be of particular interest with phenols due to its biological role. Thus, dopamine is a neurotransmitter in humans and plays a welldocumented role in the function of the nervous, cardiovascular, and renal systems.34,35 It is shown that people diagnosed with Parkinson’s disease show a significant, if not complete, depletion of dopamine in the central nervous system. The inherent electrochemical activity of dopamine promoted application of direct electrochemical schemes for real-time in vivo monitoring.36,37 Fast and sensitive in situ electrochemical detection of dopamine performed on carbon fiber microelectrodes shown to be practically useful in biomedical and neurophysiological studies.38,39 In surface chemistry, polydopamine-coating electrode materials have been investigated. Thus, inspired by the strong adhesion and cohesion behaviors of proteins of marine organisms such as mussels and other sandcastle worms mussel proteins,40 dopamine and other catechol compounds were used to perform surface coating,41,42 including electrochemical polymerization of dopamine in alkaline media.43 In this work, we investigate the grafting of dopamine, the inner-sphere redox probe, onto the electrode surface. Electrochemical oxidation of amine and diazonium reduction will be employed for the attachment of a thin layer of dopamine onto the electrode surface. Scheme 1 summarizes the followed strategies for dopamine grafting. The first procedure consists on a one-step grafting process based on electrochemical oxidation of the amino group in acidic media (Scheme 1a), while the second is based on a two-step process starting with electrochemical grafting of diazonium, leading to the attachment of an aryl-layer-bearing acidic headgroup, followed by activation with NHS/EDC and a cross-coupling reaction, peptide coupling, in the presence of

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EXPERIMENTAL SECTION Chemicals. N-hydroxysuccinimide NHS (Acros), 1-(3dimethylaminopropyl)-3-ethylcarbodiimide EDC (Acros), 3,4dihydroxyphenethylamine hydrochloride (dopamine), and 4aminobenzoic acid (Aldrich) were used as received. Perchloric acid (HClO4) and sodium nitrite (NaNO2) were purchased from Sigma-Aldrich. Sulfuric acid (H2SO4, 18M) was supplied by Prolabo. Electrochemical Measurements. For the electrochemical experiments, a conventional three-electrode cell was used. Platinum wire was used as auxiliary electrode. Saturated calomel electrode SCE was used as reference electrode. Glassy carbon electrode GCE (3 mm diameter) and ultramicroelectrode carbon fiber (30 μm diameter) were used as working electrodes. Prior to use, the working electrodes were polished using SiC-paper 5 μm (Struers) and DP-Nap paper 1 μm (Struers) with Al2O3 0.3 μm slurry (Struers), successively. After being polished, the electrode was thoroughly rinsed with ultrapure water (18.2 MΩ cm). Before any electrochemical measurements, the solutions were deoxygenated by bubbling argon gas for 30 min. During the experiment, the electrochemical cell remained under argon. The potentiostat used in this study was CHI 660C (CH Instruments, made in TX). All the experiments are performed at 25 °C. IR Spectroscopy. FT-IR spectra were obtained with a PerkinElmer spectrometer equipped with a universal ATR module (Waltham, MA). Each spectrum of modified substrate results from the accumulation of 100 scans at a resolution of 4 cm−1 and from the difference of the bare substrate spectrum in the same conditions. B



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X-ray Photoelectron Spectroscopy analyses. XPS measurements were performed using a Thermo VG Scientific ESCALAB 250 system fitted with a microfocused, monochromatic Al Kα (hν = 1486.6 eV) 200 W X-ray source. The Xray spot size was 500 μm. The samples were stuck on sample holders using conductive double-sided adhesive tape and pumped overnight in the fast entry lock at ∼5 × 10−8 mbar prior to transfer to the analysis chamber. The pass energy was set at 100 and 40 eV for the survey and the narrow scans, respectively. Data acquisition and processing were achieved with Avantage, version 4.67. Spectral calibration was determined by setting the main C(1s) component at 285 eV. Atomic percentages have been determined using this software and taking into account photoemission cross sections, analyzer transmission, and variation of electron mean free paths with kinetic energy. For surface analyses, the grafting was performed on a gold substrate and the resulting electrodes are systematically rinsed with H2O and then sonicated for 10 min to remove the weakly adsorbed molecules.

Figure 2. CV of dopamine-modified GC electrode in 0.1 M H2SO4. Scan rate = 0.1 V s−1.

related to o-dopaminoquinone transformation to dopamine. The oxidation and reduction of dopamine follows a twoelectron two-proton transfer reaction mechanism. The peak-topeak separation around 10 mV indicates that the dopamine moieties are immobilized onto the electrode surface and suggests rapid electron transfer reaction kinetics in the strong acidic condition. One has to note that the characterization of GCE in acidic media after the polarization in dopamine solution at a potential less positive than 1 V versus SCE did not show the presence of a dopamine signal. The presence of a dopamine electrochemical signal confirms the attachment of dopamine onto an electrode surface through the amine oxidation process. In order to check the stability of the modified electrode, successive CV’s were recorded in acidic solution. Similar behavior as shown in Figure 2 is obtained with a negligible current variation, less than 5% after 50 cycles. The high stability and strong immobilization of dopamine species allow for the investigation of the variation of both current and peak potential as functions of the scan rate. The obtained results are presented in Figure 3. Figure 3a shows linear variation of the peak current versus the scan rate. This linear dependence is typical for surfaceconfined electroactive species and thus demonstrates that the oxidative electrografting conduces to the attachment of dopamine species onto the electrode. Figure 3b shows the variation of the peak potential as a function of the scan rate. For a scan rate below 1 V s−1, the peak potentials are not affected by the scan rate variation. Upon increasing the scan rate, the peak potentials shift in positive and negative directions relative to the standard redox potential of dopamine, which reflects control of the voltammetry by the rate of electron transfer (ET) of the immobilized species (see Figure 3b). The apparent rate constants for electron transfer, kaapp and kcapp, were calculated using Laviron’s formalism44 based on the classical Butler− Volmer theory.

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RESULTS AND DISCUSSION Electrochemical Oxidation of Dopamine on Carbon Electrode. The first part of this work is devoted to the immobilization of dopamine through direct electrochemical oxidation of dopamine in acidic media on a glassy carbon electrode. Figure 1a shows the cyclic voltammogram (CV) of

Figure 1. Cyclic voltammogram of GCE in solution containing 10−3 M dopamine in 0.1 M H2SO4. Scan rate = 0.1 V s−1.

10−3 M of dopamine in acidic media (0.1 M H2SO4) on GCE. The grafting was realized by performing 10 successive CV’s scanning from 0.8 to 1.8 V versus SCE, and the final scan was stopped at 0 V. The voltammetry exhibits an irreversible oxidation wave at a potential around 1.3 V versus SCE assigned to the oxidation of the amine group. During the oxidation process, aminyl radicals were generated in the vicinity of the electrode and react rapidly with the electrode surface. The peak current becomes very small or even disappears after a few cycles, suggesting the formation of a thin film attached onto the electrode surface. Similar behavior has been observed when using other primary amine derivatives.6,24 Following that, the electrode was rinsed thoroughly in water, sonicated for 10 min, and then characterized by electrochemistry acidic media. Figure 2 shows the electrochemical response of the dopamine-modified electrode in 0.1 M H2SO4. The recorded CV shows a reversible redox system at 0.53 V versus SCE: the anodic peak is responsible for dopamine oxidation to o-dopaminoquinone and the cathodic peak is

ka app = (1 − αa)nFva/RT ; kc app = αcnFvc /RT

where αa and αc are the charge-transfer coefficients and va and vc are the critical scan rates obtained by extrapolating the linear portion of the Ep vs log(v) plots to the formal anodic and cathodic potentials. For this study αa and αc are found to be 0.6 and 0.4, respectively. The two apparent rate constants were averaged, and a value of about 140 s−1 was obtained. Compared to the apparent rate constant measured when other redox units are attached onto the electrode material, the attached dopamine exhibits a high electron transfer rate. Indeed, for ferrocene immobilized on silicon, a value of 50 s−1 was obtained;45 for C



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Figure 3. (a) Scan rate dependency of the anodic and cathodic peak currents. (b) Variation of the anodic and cathodic peak potentials as functions of scan rate (v).

catechol attached by diazonium reduction onto the carbon electrode, a value of 0.87 s−1 was measured,46 and for hydroquinone-terminated SAM, the apparent electron transfer change from 79 to 10−4 s−1 depended on the alkyl chain lengths.47 For benzoquinone SAMs, the apparent ET rate can be varied by several orders of magnitude when replacing saturated alkyl bridges with unsaturated ones, obtaining in some instances rate constants comparable to the present value.48 Indeed, the observed electron transfer is controlled by electron tunneling from attached dopamine to the GCE through a short insulating barrier formed by the dopamine layer. The fast electron transfer of attached dopamine could suggest that a strictly inner-sphere mechanism, observed for dopamine in solution, could be not applicable for immobilized dopamine. A direct comparison is however difficult because the ET rate is influenced by many parameters. The variation of the ET rate depends mainly on the surface density of redox species, the nature of the redox probe, the distance between the redox center and the electrode surface, and the used method for grafting. Another issue regarding the comparison of rate constants deals with the validity of the Laviron analysis for the immobilized redox system, which undergoes a proton coupled electron transfer of a more complex nature than the simple electron transfer underlying Laviron’s theoretical treatment. The reasons of the apparent discrepancies of the ET for a system involving electron transfer and proton transfer are often eluded, but few suggestions such as a possible multistep kinetics and the potential dependence of the transfer coefficient have been invoked. 49,50 It seems that the protonation reaction should be somehow considered, but this would require an independent theoretical treatment beyond the scope of the present work. Electrochemical Oxidation of Dopamine on Ultramicroelectrode. Most of the studies related to the electrochemical grafting are achieved on millimeter-sized electrodes; however, only a few works are devoted to micrometer-sized electrodes.51,52 The use of ultramicroelectrodes (UME) allows high spatial and temporal resolution of the achieved measurements and offers direct distinguishing between the electrochemical process occurring in solution (steady-state current) or at the electrode surface (peak current). In addition, carbon fiber microelectrodes have been extensively used to record extracellular/intracellular neural activity and local changes in chemical concentration.53 In this connection, the oxidative grafting of dopamine was investigated on a carbon fiber ultramicroelectrode. The modification was performed in similar way, as described above. Figure 4 shows the electrochemical response of the modified carbon fiber UME in 0.1 M H2SO4.

Figure 4. CV characterization of grafted dopamine onto ultramicroelectrode carbon fiber (30 μm diameter) in 0.1 M H2SO4. Scan rate = 0.1 V s−1.

The electrochemical characterization exhibits a reversible redox system at a potential around 0.53 V versus SCE related to the o-dopaminoquinone/dopamine redox couple. Obtaining a peak shape rather than a sigmoidal shape is characteristic of a process where the species involved do not diffuse to and from the electrode. All these results indicate the possibility to perform the oxidative electrochemical grafting onto a microelectrode and confirm the immobilization of dopamine onto the carbon surface, whatever the size of the electrode. The analysis of the electrochemical signal in Figures 2 and 4 could provide an estimation of the amount of the attached dopamine molecules. The charge of the electroactive species could be measured, and the surface concentration is calculated using the formula Γ = Q/nFA, tacking a two-electron twoproton reaction mechanism. Where Q corresponds to the charge measured by integration of the anodic peak, n is the number of the electron, F is the faraday constant, and A is the area of the electrode. In the present case, the average surface concentration was found to be in the range of 5 × 10−10 mol cm−2; this value is close to the theoretical maximum surface coverage for a monolayer. Overall, the oxidative grafting of dopamine in acidic media yielded an electroactive surface where dopamine inner-sphere centers are immobilized within a tunneling distance from the GCE. Postfunctionalization: Stepwise Process. All the results of oxidative grafting of dopamine in acidic media onto a carbon electrode confirm the immobilization of a thin dopamine layer and are not compatible with the formation of polydopamine film. The polymerization of dopamine under our experimental conditions is not possible. Indeed, it has been demonstrated that the electrochemical polymerization of dopamine does not D



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Figure 5. CV characterization in 0.1 M H2SO4 of grafted dopamine using postfunctionalization method (a) on a glassy carbon electrode and (b) on a carbon fiber UME.

Figure 6. (a) FT-IR spectra of Au-modified dopamine using direct oxidative grafting (red line) and postfunctionalization (black line). (b) FT-IR spectra of Au-modified dopamine using the postfunctionalization route (black line) and after polarization at 0.6 V vs SCE for 200 s in 0.1 M H2SO4 (red line).

performed on carbon fiber UME and Figure 5b shows the obtained electrochemical response. Again the dopamine redox reversible system was observed with a peak shape confirming the attachment of dopamine through peptidic coupling onto ultramicroelectrode surface. Similar shape of the CV was observed when compared to direct oxidative grafting. More interestingly, the presence of the aryl layer does not affect the signal of the over-grafted dopamine, suggesting that even for the inner-sphere system, the communication, through tunneling, between the dopamine and the electrode is still possible. The average surface concentration of attached dopamine was found to be in the range of 4 × 10−10 mol cm−2; this value is close to that obtained by direct oxidative grafting, confirming the formation of a thin dopamine layer onto the carbon electrode. Collectively, these results suggest that both methods are suitable for the immobilization of a thin dopamine layer, a few nanometers thick, onto millimetric and micronic electrodes. Surface Investigations. The electrochemical investigations were complemented by surface analyses. FT-IR and XPS measurements were performed on dopamine-modified electrodes generated either by direct oxidative grafting (Scheme 1a) or by post functionalization (Scheme 1b). In this part, a similar procedure as described above was followed but the grafting was performed on the Au substrate. One has to note that changing the electrode material does not change the electrochemical behavior of the grafted dopamine. Figure 6a compares the FT-IR spectra of the dopaminemodified Au surface generated by direct oxidative grafting (red line) and postfunctionalization through peptidic coupling (black line).

occur in strong acidic media (pH < 4), and this finding was supported by a quartz crystal microbalance investigation.43 To confirm this point, the attachment of dopamine was performed by a cross-coupling reaction instead of oxidative grafting. In this part of the work, dopamine was immobilized by a twostep process onto the electrode surface through a peptidic coupling reaction, as illustrated in Scheme 1b. First, the 4carboxyphenyldiazonium was generated in situ in the electrochemical cell by reaction of the corresponding amines with NaNO2 and HClO4 in acetonitrile solution. The electrochemical grafting onto the carbon electrode was performed by cyclic voltammetry (data not shown). Thus, during the reduction process, a monoelectronic irreversible wave at a potential close to 0.1 V/SCE is observed and corresponds to the diazonium reduction and the formation of its corresponding radical.6,54 Next, the attached terminal COOH groups were activated by immersing the modified surface for 2 h in a freshly prepared mixture of a deaerated solution of EDC at 0.2 M in water and a deaerated solution of NHS at 0.1 M in water. The surface was then rinsed with water, dried under an argon stream, and immersed immediately for amide formation. The attachment of dopamine units on the carbon electrode was performed by immersing the NHS-activated surface in a deaerated aqueous solution containing 10−2 M of dopamine at room temperature for 3 h. The obtained surface was rinsed with water, sonicated for 10 min, and dried under an argon stream. The modified electrode was characterized by cyclic voltammetry in a sulfuric acid aqueous solution. Figure 5a shows the presence of a redox reversible system at 0.53 V versus SCE, corresponding to the electrochemical response of dopamine. In addition, the same experiment was E



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Figure 7. (a) XPS survey scans of Au substrate (black line) and after dopamine immobilization using oxidative grafting of dopamine (red line) and postfunctionalization (blue line). (b) Compare the high-resolution XPS spectra for C(1s) core level for the three samples.

C(1s) (at 285 eV) and oxygen O(1s) (at 532 eV), which are ascribed to classical surface contamination. After dopamine immobilization, either by direct oxidative grafting or postfunctionalization, clear modifications of the Au surface chemical composition are observed. Indeed, the gold signal is strongly attenuated from 60% for virgin substrate to 45 and 26% after the modification using the oxidative grafting or the post functionalization, respectively. This first observation indicates the covering of the gold surface. Conversely, the carbon atomic percentage increases from 20 for unmodified gold to 37 (for oxidative grafting) and 55% (for postfunctionalization), respectively. This increase suggests the presence of an organic layer onto the gold surface. The C1s core level spectrum of the dopamine-modified electrode can be curve-fitted with three peak components having binding energies of 284.6 eV for the C−H species of aryl, 286.5 eV for the C−O groups, and a peak at 288.6 eV attributed to the CO species. In the case of postfunctionalization, the presence of CO groups is related to the attached arylbenzoic (see Scheme 1b). However, this peak is also observed after oxidative grafting but with lower intensity, which could be explained by the existence of odopaminoquinone produced during the oxidative grafting. Similarly, the concentration of oxygen increases from 3%, for unmodified gold, to 8 and 13% after direct oxidative grafting and postfunctionalization grafting, respectively. In addition, after grafting of dopamine, a new peak appears at 400 eV assigned to the N 1s signal of the N−H groups. The presence of this peak confirms the attachment of dopamine onto the gold electrode surface through N−H binding. A closer examination of the XPS spectra provides additional information about the thickness of the grafted organic layer. The detection of the Au XPS signals in the survey for both modified electrodes is an indication of the presence of a thin layer, a few nanometers thick, in perfect agreement with the electrochemical observations evoked in the first part. In addition, the thickness of the attached layer could be measured using the XPS results following the formula reported by Whitesides et al.57 Thus, based on the variation of the peak intensity of Au (4f7/2), the average thickness of the dopamine layer is around 1.4 nm for oxidative grafting and 3.5 nm when using the postfunctionalization route. These values are in perfect agreement with the measured thickness, using the AFM scratch experiment. Dopamine Electron Transfer Activation. Electron-transfer activation of the electrode surface is of fundamental importance to the electrochemical detection of dopamine. Various surface treatments have demonstrated that the electrode surface has a profound effect on catechol

The spectra shown in Figure 6a support the presence of dopamine groups attached on the gold electrode, whatever the method used for grafting. The evidence is clearly observed through various bands. A broad absorbance is observed between 3500 and 3100 cm−1, corresponding to N−H/O−H stretching vibrations. The strong absorbance at 2925 and 2860 cm−1 is attributed to the aliphatic C−H stretching vibration. The adsorption peak at 1600 cm−1 is assigned to the overlap of CC resonance vibration in the aromatic ring and N−H binding, while the peak observed at 1510 cm−1 is related to the N−H shearing vibration. The bands positioned between 1100 and 800 cm−1 corresponding to C−H bending modes of aromatic rings are observed. Moreover, both spectra show a peak at 1720 cm−1 ascribed to the CO stretching vibrations of the carboxyl group. The presence of this peak is expected when using the postfunctionalization route (see Scheme 1b). However, for direct oxidative grafting, the presence of the C O band may reflect the existence of some o-dopaminoquinone generated during the oxidative grafting. The shape of the IR spectra of the attached dopamine either by oxidative grafting or by stepwise processes are similar but differ from that reported on polydopamine film.43,55,56 All these results confirm the immobilization of the thin layer of dopamine and rule out the possibility of dopamine polymerization when using direct oxidative grafting in strong acidic media. Figure 6b compares the FT-IR spectra of the attached dopamine through the postfunctionalization method before and after polarization of the same sample at 0.6 V vs SCE for 200 s in acidic solution. The spectrum recorded after anodic polarization shows the disappearance of the broad absorbance between 3500 and 3100 cm−1, related to the O−H vibration accompanying increases of peak intensity at 1720 cm−1 (CO stretching vibration). These observations clearly indicate the presence of carbonyl groups and confirm the conversion of the attached dopamine layer onto o-dopaminoquinone upon anodic polarization. XPS is a versatile tool for accurately analyzing the surface chemical composition of a given material. Figure 8a compares the XPS survey scan of the bare Au electrode (black line), dopamine attached onto the Au electrode using direct oxidative grafting (red line) and postfunctionalization (blue line). Whereas, Figure 7b displays the high-resolution XPS spectra of the C(1s) core level obtained for the three samples. The spectrum of unmodified Au substrate displays mainly the Au signals which originate from Au(4p) (548 eV), Au(4d) (doublet at 336 and 354 eV), and Au(4f) (doublet at 84.2 and 88 eV), core electrons in agreement with literature reports.18 Besides that, minor peaks are also identified such as carbon F



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between cathecols and GC electrodes could be catalyzed by a layer of adsorbed quinone.71 In this connection, a GCE was electrochemically pretreated by performing 10 cycles from 0 to 2.2 V in 2 M H2SO4. The CV recorded onto GC after the ECP in dopamine solution shows the redox system with ΔEp around 75 mV (Figure 8, dashed line). In comparison to the polished GC electrode, the ECP treatment leads to an increase in the electron transfer rate of dopamine. This result is consistent with the previous report in the literature.64,69 Next, using similar conditions as described above, the oxidative grafting of dopamine was performed onto bare GCE and onto ECP-treated GCE. The CV’s responses of dopamine-modified electrodes in dopamine solution are perfectly superposed (Figure 8, solid line) and exhibit a reversible system related to the redox couple o-dopaminoquinone/dopamine. More interestingly, the peak-to-peak separation is around 30 mV, indicating fast electron transfer of dopamine onto dopamine-modified electrodes. Furthermore, the observed ΔEp for dopamine is lower than that obtained after vacuum heat treatment (48 mV)63 and comparable to that recorded after laser activation (32 mV)72 and fracturing in solution (28 mV).69 Not only does the dopamine-modified electrode yield a lower ΔEp than the majority of pretreatments procedures reported in the literature but also yields as small an ΔEp as that observed with more aggressive and complex treatments. This experiment demonstrates that whatever the initial state of the GC electrode surface, the immobilization of dopamine yields a rapid electron transfer for dopamine in solution with an ΔEp close to 30 mV. In general, the electron transfer rate of DA on the GC electrode is slow due to a number of factors, including rapid contamination of the surface or surface preparation. However, our results demonstrate that the chemical modification of the surface by an appropriate molecule, a thin film of dopamine, could help to prevent such a deleterious process.

voltammetry. Unwin et al. have shown that the electrochemical oxidation of DA, at neutral pH, on HOPG is rapidly poisoned due to a rapid contamination of the surface by the dopamine polymer.33 McCreery et al. have studied the response of dopamine and the influence of various treatments on the glassy carbon electrode.58,59 Indeed, it has been shown that the electrochemical response of dopamine on the carbon electrode depends strongly on the surface state and the used procedure to activate the electrode. This effect is correlated to the mechanism of dopamine oxidation to o-quinone, which follows the “scheme of squares” with two electron-transfer steps interspersed with fast proton transfers and requires adsorption onto the electrode surface for exhibiting fast charge-transfer kinetics.60,61 Figure 8 compares the CVs of 10−3 M dopamine in 0.1 M H2SO4 recorded on bare GCE (dot line), after electrochemical

Figure 8. CV’s of bare GCE (dotted line), electrochemically pretreated GCE (dashed line), and dopamine-modified electrode (solid line) in 0.1 M H2SO4 containing 10−3 M dopamine.

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pretreatment procedure in acidic media (dashed line) and on dopamine-modified GCE (solid line). The CV recorded on bare GC shows an oxidation and reduction peak with a peak-to-peak separation of about 260 mV (Figure 8 dotted line). This large ΔEp value indicates slow electron transfer of dopamine to o-dopaminoquinone on the electrode surface. Note that the voltammogram of the same electrode recorded in the presence of an outer-sphere redox couple (Fc) shows a reversible system with ΔEp close to 60 mV. The reason for such behavior is mainly related to the surface activity toward an inner-sphere redox probe, dopamine, which is affected by various factors such as chemi- and physisorbed species, microscopic surface area, polishing debris, and various levels of impurities and surface oxides.62 Pretreatment procedures of the carbon electrode have been employed to reach rapid electron transfer, such as heat treatment under vacuum or inert atmosphere, laser irradiation, or electrochemical pretreatment procedures (ECP).63−68 Activation by heat treatment and laser ablation of polished GC surfaces is believed to occur through a cleaning mechanism which exposes or creates active sites on the bulk carbon. Electrochemical activation also cleans the surface, but with oxidation of the carbon to an extent depending on the strength of the electrochemical treatment. Functional groups generated at the carbon surface during ECP may promote electron transfer by participating in a proton-exchange mechanism, causing electrostatic interactions with redox centers acting as catalytic sites for adsorption or electron transfer or decreasing the hydrophobicity of the carbon surface.69,70 The electron transfer

CONCLUSION In summary, the grafting of dopamine onto the electrode surface was successfully performed either by one-step electrochemical oxidative grafting or via a stepwise procedure based on peptide coupling. The electrochemical characterization of the modified electrode evidences several features. First, the electrochemical characterizations of the modified electrode in the acidic media show the presence of the o-dopaminoquinone/ dopamine redox signal, confirming the immobilization of dopamine onto millimetric and micronic electrodes. Second, our results demonstrate that the electroactivity of the attached dopamine layer appears not affected by the nature of the spacer, alkyl, or aryl layer, suggesting that the communication, through tunneling, between immobilized dopamine and the electrode is possible. Third, the surface concentration of attached dopamine moieties was found to be around 5 × 10−10 mol cm−2, indicating the formation of a thin dopamine layer, a few nanometers thick. FT-IR and XPS investigations confirm the immobilization of dopamine either by direct oxidative grafting or via a stepwise procedure. In addition, the electrochemical switching of the dopamine layer to the o-dopaminoquinone layer was successfully performed and confirmed by FT-IR analysis. As an exciting result, the dopamine-modified electrode exhibits electron transfer activation toward dopamine in solution. The dopamine-modified electrode yields a fast electron transfer with lower ΔEp (i.e., 30 mV) than the majority of other surface treatments. We anticipate that such G



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electron transfer activation could be used in the active field of analytical chemistry, especially for the determination and/or discrimination of dopamine in the presence of ascorbic and uric acids. Finally, the immobilization of the neurotransmitter onto regular electrodes and microelectrode surfaces may lead to build the brain electrode that could be used in psychoanalytical electrochemistry, neuroscience, and as analytical sensors.

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(25) Ghilane, J.; Martin, P.; Randriamahazaka, H.; Lacroix, J. C. Electrochem. Commun. 2010, 12, 246−249. (26) Adenier, A.; Chehimi, M. M.; Gallardo, I.; Pinson, J.; Vilà, N. Langmuir 2004, 20, 8243−8253. (27) Smith, R. D. L.; Pickup, P. G. Electrochem. Commun. 2009, 11, 10−13. (28) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 2001; p 116. (29) Bard, A. J. J. Am. Chem. Soc. 2010, 132, 7559−7567. (30) Hawley, M. D.; Tatawawadi, S. V.; Piekarski, S.; Adams, R. N. J. Am. Chem. Soc. 1967, 89, 447−450. (31) Anjo, D. M.; Krhr, M.; Khodabakhsh, M. M.; Nowinski, S. Anal. Chem. 1989, 61, 2603−2608. (32) Lhenry, S.; Leroux, Y. R.; Hapiot, P. Anal. Chem. 2012, 84, 7518−7524. (33) Patel, A. N.; McKelvey, K.; Unwin, P. R. J. Am. Chem. Soc. 2012, 134, 20246−20249. (34) Adams, R. N. Anal. Chem. 1976, 48, 1126A−1138A. (35) Heien, M. L. A. V.; Khan, A. S.; Ariansen, J. L.; Cheer, J. F.; Phillips, P. E. M.; Wassum, K. M.; Wightman, R. M. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10023−10028. (36) Wightman, R. M.; Kuhr, W. G.; Ewing, A. G. Ann. N.Y. Acad. Sci. 1986, 473, 92−105. (37) Zimmerman, J. B.; Wightman, R. M. Anal. Chem. 1991, 63, 24− 28. (38) Hermans, A.; Keithley, R. B.; Kita, J. M.; Sombers, L. A.; Wightman, R. M. Anal. Chem. 2008, 80, 4040−4048. (39) Venton, B. J.; Wightman, R. M. Anal. Chem. 2003, 75, 414A− 421A. (40) Waite, J. H. Nat. Mater. 2008, 7, 8−9. (41) Dalsin, J. L.; Hu, B.-H.; Lee, B. P.; Messersmith, P. B. J. Am. Chem. Soc. 2003, 125, 4253−4258. (42) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Science 2007, 318, 426−430. (43) Li, Y.; Liu, M.; Xiang, C.; Xie, Q.; Yao, S. Thin Solid Film 2006, 497, 270−278. (44) Laviron, E. J. Electroanal. Chem. 1979, 101, 19−28. (45) Fabre, B.; Hauquier, F. J. Phys. Chem. B 2006, 110, 6848−6855. (46) Nguyen, N. H.; Esnault, C.; Gohier, F.; Bélanger, D.; Cougnon, C. Langmuir 2009, 25, 3504−3508. (47) Hong, H.-G.; Park, W. Langmuir 2001, 17, 2485−2492. (48) Bouffier, L.; Lister, K. E.; Higgins, S. J.; Nichols, R. J.; Doneux, T. J. Electroanal. Chem. 2012, 664, 80−87. (49) March, G.; Reisberg, S.; Piro, B.; Pham, M.-C.; Delamar, M.; Noël, V.; Odenthal, K.; Hibbert, D. B.; Gooding, J. J. J. Electroanal. Chem. 2008, 622, 37−43. (50) Zhang, W.; Rosendahl, S. M.; Burgess, I. J. J. Phys. Chem. C 2010, 114, 2738−2745. (51) Ernst, S.; Aldous, L.; Compton, R. G. Chem. Phys. Lett. 2011, 511, 461−465. (52) Janin, M.; Ghilane, J.; Randriamahazaka, H.; Lacroix, J. C. Electrochem. Commun. 2009, 11, 647−650. (53) Kozai, T. D. Y.; Langhals, N. B.; Patel, P. R.; Deng, X.; Zhang, H.; Smith, K. L.; Lahann, J.; Kotov, N. A.; Kipke, D. R. Nat. Mater. 2012, 11, 1065−1073. (54) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Savéant, J.-M. J. Am. Chem. Soc. 1997, 119, 201−207. (55) Dreyer, D. R.; Miller, D. J.; Freeman, B. D.; Paul, D. R.; Bielawski, C. W. Langmuir 2012, 28, 6428−6435. (56) Wang, W.; Cheng, W.; Tian, M.; Zou, H.; Li, L.; Zhang, L. Electrochim. Acta 2012, 79, 37−45. (57) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 1670− 1673. (58) Sternitzke, K. D.; McCreery, R. L. Anal. Chem. 1990, 62, 1339− 1344. (59) Ranganathan, S.; McCreery, R. L. Anal. Chem. 2001, 73, 893− 900. (60) Deakin, M. R.; Wightman, R. M. J. Electroanal. Chem. 1986, 206, 167−177.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 33-1-57-27-8865. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the CNRS. The authors gratefully thank Dr. Philippe Decorse for XPS investigations and for the valuable discussions.

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REFERENCES

(1) Bard, A. J.; Statmann, M.; Rubinstein, I.; Fujihira, M.; Rusling, J.F. Modified Electrodes Encyclopedia of Electrochemistry; Wiley: Hoboken, NJ, 2007; Vol. 10. (2) Gooding, J. J.; Ciampi, S. Chem. Soc. Rev. 2011, 40, 2704−2718. (3) Buriak, J. M. Chem. Rev. 2002, 102, 1271−1308. (4) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437−463. (5) Barbier, B.; Pinson, J.; Desarmot, G.; Sanchez, M. J. Electrochem. Soc. 1990, 137, 1757−1764. (6) Downard, A. J.; Tan, E. S. Q.; Yu, S. S. C. New. J. Chem. 2006, 30, 1283−1288. (7) Barrière, F.; Downard, A. J. J. Solid State Electrochem. 2008, 12, 1231−1244. (8) Gallardo, I.; Pinson, J.; Vilà, N. J. Phys. Chem. B 2006, 110, 19521−19529. (9) Bélanger, D.; Pinson, J. Chem. Soc. Rev. 2011, 40, 3995−4048. (10) Chehimi, M. M. Aryl Diazonium Salts: New Coupling Agents in Polymer and Surface Science; Wiley: Hoboken, NJ, 2012. (11) Fave, C.; Noë l , V.; Ghilane, J.; Trippe-Allard, G.; Randriamahazaka, H.; Lacroix, J.-C. J. Phys. Chem. C 2008, 112, 18638−18643. (12) Ernst, S.; Aldous, L.; Compton, R. G. Chem. Phys. Lett. 2011, 511, 461−465. (13) Garrett, D. J.; Jenkins, P.; Polson, M. I. J.; Leech, D.; Baronian, K. H. R.; Downard, A. J. Electrochim. Acta 2011, 56, 2213−2220. (14) Tessier, L.; Deniau, G.; Charleux, B.; Palacin, S. Chem. Mater. 2009, 21, 4261−4274. (15) Deniau, G.; Azoulay, L.; Bougerolles, L.; Palacin, S. Chem. Mater. 2006, 18, 5421−5428. (16) Stockhausen, V.; Ghilane, J.; Martin, P.; Trippé-Allard, G.; Randriamahazaka, H.; Lacroix, J.-C. J. Am. Chem. Soc. 2009, 131, 14920−14927. (17) Brooksby, P. A.; Downard, A. J. Langmuir 2004, 20, 5038−5045. (18) Fontaine, O.; Ghilane, J.; Martin, P.; Lacroix, J.-C.; Randriamahazaka, H. Langmuir 2010, 26, 18542−18549. (19) McCreery, R. L. Chem. Mater. 2004, 16, 4477−4496. (20) Liu, G.; Liu, J.; Böcking, T.; Eggers, P. K.; Gooding, J. J. Chem. Phys. 2005, 319, 136−146. (21) Brooksby, P. A.; Downard, A. J. Langmuir 2005, 21, 1672−1675. (22) Corgier, B. P.; Bélanger, D. Langmuir 2010, 26, 5991−5997. (23) Leroux, Y. R.; Hui, F.; Noël, J.-M.; Roux, C.; Downard, A. J.; Hapiot, P. Langmuir 2011, 27, 11222−11228. (24) Buriez, O.; Labbé, E.; Pigeon, P.; Jaouen, G.; Amatore, C. J. Electrochem. Chem. 2008, 619−620, 169−175. H



Article

(61) DuVall, S. H.; McCreery, R. L. Anal. Chem. 1999, 71, 4594− 4602. (62) Fagan, D. T.; Hu, I.; Kuwana, T. Anal. Chem. 1985, 57, 2759− 2763. (63) Kazee, B.; Weisshaar, D. E.; Kuwana, T. Anal. Chem. 1985, 57, 2736−2739. (64) Stutts, K. J.; Kovach, P. M.; Kuhr, W. G.; Wightman, R. M. Anal. Chem. 1983, 55, 1632−1634. (65) Poon, M.; McCreery, R. L.; Engstrom, R. Anal. Chem. 1988, 60, 1725−1730. (66) Rice, R. J.; Pontikos, N. M.; McCreery, R. L. J. Am. Chem. Soc. 1990, 112, 4617−4622. (67) Kepley, L. J.; Bard, A. J. Anal. Chem. 1988, 60, 1459−1467. (68) Bowling, R. J.; Packard, R. T.; McCreery, R. L. Langmuir 1989, 75, 683−688. (69) Cabaniss, G. E.; Diamantis, A. A.; Murphy, W. R.; Linton, R. W.; Meyer, T. J. J. Am. Chem. Soc. 1985, 107, 1845−1853. (70) Allred, C. D.; McCreery, R. L. Anal. Chem. 1992, 64, 444−448. (71) DuVall, S. H.; McCreery, R. L. J. Am. Chem. Soc. 2000, 122, 6759−6764. (72) Poon, M.; McCreery, R. L. Anal. Chem. 1986, 58, 2745−2750.

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