Journal of Electroanalytical Chemistry 717-718 (2014) 202–205

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Journal of Electroanalytical Chemistry

journal homepage: www.elsevier .com/locate / je lechem

Designating a logical electrochemical mechanism through a combinedelectrochemical and computational method

http://dx.doi.org/10.1016/j.jelechem.2014.01.0341572-6657/� 2014 Elsevier B.V. All rights reserved.

⇑ Tel.: +98 937 8157766; fax: +98 831 4274559.E-mail address: Hadi_42810@yahoo.com

Mohammad Hadi Parvin ⇑Department of Biotechnology-Chemical Engineering, Kermanshah Science and Research Branch, Islamic Azad University, Kermanshah, Iran

a r t i c l e i n f o

Article history:Received 18 November 2013Received in revised form 24 January 2014Accepted 28 January 2014Available online 5 February 2014

Keywords:Local density approximationReduced graphene oxide (RGO)Density functional theoryHighest occupied molecular orbitalLowest unoccupied molecular orbital

a b s t r a c t

Graphene shows fascinating applications in electrochemical methods and bionanotechnology, includingdrug assay, drug delivery, and DNA sensing. In this regard, we model the adsorption of organic compoundon a reduced graphene oxide (RGO), using a first principles density functional theory–local densityapproximation method. The presence of functional groups and the exchange of nitrogen atoms with car-bon atoms can significantly alter the overall magnitude of p–p interactions between the adsorbed mol-ecules and RGO by giving rise to strong medium-range interactions involving p-orbitals of thesubstituents. In the next step, the data obtained from the modeling of the adsorbate on RGO, are coupledwith electrochemical data obtained from the reduction of adsorbate, resulting in a logical electrochemicalmechanism for the reduction of adsorbate on the electrode surface.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Specific interactions of any species presented in the electrolytewith the electrode surface leads to adsorption, which may consid-erably influence the overall process. The involvement of specificinteractions of molecules with the electrode surface (adsorption)makes the electrode process even more complex. The intensity ofsuch interactions ranges from weak (physisorption) to strong(chemical bonds formed between adsorbate and electrode). There-fore the adsorption of molecules on solid surfaces is a fundamentalprocess that has been studied intensively for many decades. Therecent advent of scanning probe microscopy (SPM) has triggeredexplosive growth of the field and has led to a much deeper under-standing of the details of molecular adsorption [1–5]. Of specialimportance have been studies of adsorption on ordered conductingsurfaces such as those of metals and graphite. Particularly informa-tive are studies on such surfaces in which multiple methods ofcharacterization and analysis are used in concert, including (1) di-rect examination of adsorption by SPM, (2) comparison of the ob-served 2D patterns with those seen in 3D structures by X-raycrystallography, (3) systematic alteration of the structure of theadsorbate to reveal how the observed 2D pattern changes, and(4) thorough theoretical analysis to help reveal the origin of theobserved adsorption. Even when these powerful tools are used incombination, predictions of how a particular compound will be

adsorbed remain unreliable, and the relative importance of themany factors that control adsorption is hard to foresee [6–8].

In this work, we coupled electrochemical and computationaldata to prove the electrochemical reduction mechanism of organiccompound. We employed computational methods to examine howthe organic compound on surface electrode was absorbed. We alsoused electrochemical methods to peruse the electron transfermechanism between the adsorbate and the surface electrode.Therefore, reduced graphene oxide (RGO) was served for theconstruction of electrode, and 4-(1-(2,3-dimethylphenyl) ethyl)-1H-imidazole (2a) (medetomidine) was used as adsorbate. Be-cause, RGO is ideally suited for implementation in electrochemicalapplications due to its reported large electrical conductivity, largesurface area, unique heterogeneous electron transfer rate, and lowproduction costs [9–12].

2. Experimental

2.1. Reagents

Paraffin oil, Graphite flakes (cat #332461, 150 lm flakes) andSodium borohydride (NaBH4) were purchased from Sigma–Aldrichfor the synthesis of RGO.

A stock solution of 1.0 mM was prepared by dissolving anappropriate amount of medetomidine in methanol. This solutionwas stored at dark and 4 �C. Other solutions were prepared bythe dilution of the stock solution. All other chemicals were of ana-lytical grade and were used without further purification. 0.1 M

M.H. Parvin / Journal of Electroanalytical Chemistry 717-718 (2014) 202–205 203

phosphate buffer with pH = 3.0 was used as supporting electrolyte.In pH < 3, we used 0.1 M HCl solution for the reduction of pH.

2.2. Apparatus

Voltammetric measurements were carried out with an Autolab(Eco Chemie B.V.) PGSTAT30 potentiostat/galvanostat. The electro-chemical cell was assembled with a saturated Ag/AgCl referenceelectrode, a Pt wire auxiliary electrode, and the prepared workingelectrodes. A Metrohm-691 pH-meter (Switzerland) was used forpH adjustments. Solutions were deoxygenated with high puritynitrogen for 5 min prior to each experiment. All measurements werecarried out at room temperature under the nitrogen atmosphere.

2.3. Preparation of reduced graphene oxide (RGO)

The graphene oxide (GO) was synthesized using a modifiedHummers method with a pre-oxidation treatment [13,14]. In a typ-ical experiment, concentrated H2SO4 (5 ml) was heated to 80 �C ina 25 ml round bottom flask, then K2S2O8 (0.15 g) and P2O5 (0.15 g)were added to the acid and stirred until fully dissolved. Nextgraphite flake (0.2 g) was added to the reaction and kept at 80 �Cfor 4.5 h. The mixture was then cooled naturally, diluted withdeionized (DI) water, filtered (Whatman, Grade No. 4) and rinsedwith additional DI water (100 ml) to remove residual reactants,and finally dried in air. The pre-oxidized graphite flake wascollected and transferred into a 50 ml round bottom flask with

Fig. 1. Optimized geometries of (A) RGO and (B) adsorption of medetomidine onRGO.

Table 1Calculated energy of adsorption of adsorbate (1–3a) on graphene.

Adsorbate Epc

1-(1-(Cyclopenta-1,4-dienyl)ethyl)-2,3-dimethylbenzene (1a) –4-(1-(2,3-Dimethylphenyl)ethyl)-1H-imidazole (2a) �04-(1-(2,3-Dimethylphenyl)ethyl)-1-nitro-1H-imidazole (3a) �0

concentrated H2SO4 (25 ml) and chilled to 0 �C using an ice bath.KMnO4 (1.0 g) was slowly added to the mixture while stirring,

Adsorption energy kcal mol�1

Before protonated After protonated

5.7 5.5.60 14.8 40.1.56 19.4 52.3

Fig. 2. (A) Isocontour plots of (a) the HOMO and (b) the LUMO of medetomidine.Both orbitals are degenerate, and the isovalue varies from �0.04 (green) to +0.04(violet). (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

204 M.H. Parvin / Journal of Electroanalytical Chemistry 717-718 (2014) 202–205

taking caution to keep the temperature below 10 �C. The flask wasthen moved to a 35 �C water bath and left for 2–12 h, and thentransferred into an Erlenmeyer flask (500 ml) in the ice bath. DI(100 ml) was slowly added to the flask while stirring, taking cau-tion so that the temperature does not rise above 55 �C. After dilu-tion, 5 ml of 30% H2O2 solution was added to the mixture, whichturned into bright yellow. [Warning: this process induced violentbubble formation, so the solution needed to be added slowly toprevent overflowing.] This mixture was centrifuged, rinsed with3.4% HCl solution (3 times) to remove residual salts, and thenrinsed with acetone (3 times) to remove residual acid. The final so-lid GO was dried in air or under vacuum for further use. The solidGO could be easily re-dispersed in water. The dispersion was thencentrifuged at low speed 3000 rpm for 30 min in order to removelarge chucks. In the reduction step, 100 mg of GO was dispersedin 100 mL of water and sonicated for 1 h. 200 mg of NaBH4 wasadded to the dispersion. The mixture was stirred for 30 min andheated at 125 �C for 3 h. During the reduction process, theyellow-brown solution gradually yielded a black precipitate. Theblack solid was isolated by centrifugation, washed with water (4times), and finally dried [14]. The Raman spectra of the graphenebefore and after reduction exhibit well defined D and G peaks, aswell as the disappearance of the peak at about 2600 cm�1

(Fig. S1). We also characterised the graphene layers used in thiswork by XRD analysis. In this analysis, our GO samples show a verystrong peak at 11.2�, whilst the RGO samples exhibit their stron-gest peak at 25.5� (Fig. S2).

Fig. 3. Big p orbital of RGO.

Fig. 4. Transport channel contrib

2.4. Preparation of working electrode

The graphene paste mixture was prepared by hand-mixing 0.5 gof RGO powder with 180 lL of mineral oil. Mixture was homoge-nized (25 min) then packed into a piston-driven graphene pasteelectrode holder (3 mm). A fresh electrode surface was obtainedby squeezing out a small portion of paste and polishing it withwet filter paper until a smooth surface was obtained.

3. Results and discussion

Calculations of electronic structure were based on the local den-sity approximation (LDA) of density functional theory (DFT), usingthe Gaussian 03program and Accelrys Materials Studio 4.3. In thecalculations, we used a fixed RGO consisting of 382 atoms of car-bon and 54 peripheral atoms of hydrogen, with CAC and CAH bonddistances set at 1.46 Å and 1.01 Å, respectively (Fig. 1A). Geometryoptimization was performed at the STO-3G and 6-31G (d) level[6,7]. During the optimization steps, all species except the RGOwere free to move.

The results of calculations at the LDA level before and after pro-tonated compound are summarized in Table 1.

For before protonated compounds and natural pH: the adsorp-tion of 1-(1-(cyclopenta-1,4-dienyl)ethyl)-2,3-dimethylbenzene(1a) estimated at the LDA level is relatively weak (5.7 kcal mol�1),indicating a physisorbed state. In contrast, the calculated value atthe LDA level for medetomidine (2a) (14.8 kcal mol�1) is signifi-cantly larger (Fig. 1B). Replacing atoms of carbon with nitrogen(2a) and adding NO2 group to the imidazole core (3a) raises theenergy of adsorption on RGO linearly. These alterations reduceelectron density on the remaining carbon atoms and thereby atten-uate p–p repulsion between the adsorbate and RGO, leading tostronger binding. We found that imidazole required less negativeapplied potentials in the process of electroreduction with electron-withdrawing group such as NO2. It is noteworthy that the reactiontolerates functional variations around the imidazole, which is inturn an additional advantage.

utes to the electron transfer.

Fig. 5. Electrostatic potential map of medetomidine.

Scheme 1. Mechanism for electroreduction of medetomidine.

M.H. Parvin / Journal of Electroanalytical Chemistry 717-718 (2014) 202–205 205

To better understand the nature of the C@NARGO interaction,we examined the electronic properties of the adsorbed states.The highest occupied molecular orbital (HOMO) and lowest unoc-cupied molecular orbital (LUMO) of medetomidine are shown inFig. 2.

The HOMO is mostly localized on nitrogen atom and corre-sponds to low energy lone pairs of nitrogen. In contrast, the LUMOis over the carbon atoms. The net charge strongly suggests thatempty orbitals on the adsorbate overlap with filled orbitals local-ized (big p orbital) (Fig. 3) on RGO, and charge transfer from RGOto the adsorbates increases the strength of adsorption and facili-tates the electron transfer between the medetomidine and theelectrode surface.

Thus, the LUMO transport channel contributes to the electrontransfer (Fig. 4). Transport channels are more located at RGO edgesbecause the electron transfer of RGO resides in its edge rather thanits side, where the former acts electrochemically akin to that ofedge plane- and the latter to that of basal plane-like sites/defectsof highly ordered pyrolytic graphite [15,16]. Though RGO has alarge basal plane composition for the adsorption of organic com-pound which adsorbs readily on basal plane sites [16,17].

For after protonated compounds and low pH: according to elec-trostatic potential map (Fig 5), in the mechanism 1a (Scheme 1),the nitrogen atom which is in azometine group has a high affinityto accept proton. So, in the first step, this nitrogen atom in mede-tomidine accepts one proton, and C1 carbocation is formed. In thenext step, C1 carbocation is overlapped with big p orbital overRGO, and charge transfer from RGO to the adsorbate increasesthe strength of adsorption and facilitates the electron transfer be-tween the medetomidine and the electrode surface. So, the emptyp orbital over C1 carbocation transport channel contributes to theelectron transfer. The data in Table 1 indicated that the adsorptionof carbocations on the RGO is stronger compared to natural com-pounds which lead to easier electroreduction of carbocations onthe GPE. Atoms such as nitrogen which have nonbonding electronsstrongly stabilize carbocation, because in the medetomidine, nitro-gen atoms have a stabilizing resonance effect.

According to the obtained results and electrochemical behaviorof medetomidine [18] the following mechanism was suggested forreduction of medetomidine (Scheme 1).

4. Conclusion

In summary, the loading of organic compound on the RGO asa substrate is important, because RGO shows fascinating

applications in electrochemical methods and bionanotechnology,including drug assay, drug delivery, and DNA sensing [19–23]. Inthis regard, we present a powerful method, which couples the elec-trochemical and the computational data in order to examine howthe organic compound on the RGO is adsorbed. This method alsohelps us to have a better understanding of the process of electrontransfer between adsorbate and surface electrode.

Acknowledgment

The author gratefully acknowledge the support of this work bythe University of Islamic Azad University, Kermanshah branchResearch Councils.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jelechem.2014.01.034.

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