Exploring the electrochemical behavior of screen printed graphite electrodes in a room temperature ionic liquid

Abstract

Screen printed graphite electrodes (SPGEs) have been characterized in the room temperature ionic liquid 1-hexyl-3-methylimidazolium hexafluorophosphate [C₆MIM][PF₆] by studying pertinent electrochemical parameters of the electroactive species ferrocene (Fc), 1,4-benzoquinone (BQ), 1,4-diphenyl-9,10-anthraquinone (AQ), tetracyclone (TC) and benzophenone-3 (BZ-3). Diffusion coefficients and kinetics calculations together with Digisim simulations for comparison were performed. Additionally, the reductive cyclovoltammograms of organic carbonyl containing BQ, AQ, TC and BZ-3 derivatives provided valuable information and comparison of the electrochemical reduction of the C=O functional group in model molecules of different size, solubility and π-aryl aromaticity. Finally, potentialities for the electroanalytical measurement of BZ-3 at SPGEs via the room temperature ionic liquid [C₆MIM][PF₆] have been evaluated.

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1. Introduction

Room temperature ionic liquids (RTILs) have received considerable interest recently due to their unusual and advantageous properties such as their high ionic conductivity, ion-exchange properties, extraction and catalytic activity, thermal and electrochemical stability which allow extreme experimental conditions.¹ The electrochemical applications that involve RTILs comprise the electrochemistry of inorganic complexes,² studies of polymer deformation during electrochemical doping,³ supercapacitors,⁴ dye synthesized photoelectrochemical cells,⁵ lithium ion batteries² and nuclear waste treatments,⁶ among others.⁷ In the last few years RTILs have been actively used for basic studies of various analytes^8–12 within electroanalytical and biosensor applications.¹³

Carbonaceous materials are important in the electroanalytical and bioelectroanalytical fields for their acceptable conductivity, low cost and high versatility for electrochemical miniaturization. Recently, Pauliukaite et al.¹⁴ characterized carbon film electrodes in different RTILs and found applications for the electrochemical investigation of two ferrocene derivatives that are insoluble in water. Interestingly, RTILs media have been used by Maleki et al. which effectively act as binders for the fabrication of new carbon composite electrodes.¹⁵ Compton's group has performed an elegant comparison of electron transfer kinetics between different carbon materials such as basal and edge plane pyrolytic graphite and glassy carbon in RTILs and acetonitrile media using ferricinium ion/ferrocene (Fc⁺/Fc) as a redox couple.¹⁶

The effective performance of SPGEs has led to their widespread acceptance in environmental, biomedical and the major fields of analytical chemistry.¹⁷ Typically SPGEs are exclusively used in aqueous solutions and recent efforts have tried to make non-aqueous systems, thereby it is useful for SPGEs to operate in this type of media.¹⁸ Here, we aim to present the first investigation of SPGEs in an ionic liquid by investigating a range of probe molecules. Although we believe that this is the first such study of SPGEs in an ionic liquid, numerous other studies have successfully utilized ionic liquid layers on screen printed carbon electrodes which are then employed in aqueous systems. Hence, it is worth pointing out several works dedicated to the use of RTILs for the modification of the SPGE surface. In that respect, Ren et al.¹⁹ reported a chronocoulometric DNA sensor based on SPGEs doped with ionic liquids and polyaniline nanotubes. Further efforts have been focused on the determination of dopamine with the development of ionic liquid modified SPGEs.^20,21 In the present work, electron transfer kinetic studies are performed using a wide variety of redox couples at SPGEs in RTIL 1-hexyl-3-methylimidazolium hexafluorophosphate [C₆MIM][PF₆]. This RTIL is one of the typical ionic liquids which is reported to have large viscosity, low miscibility with water and in which organic compounds are moderately soluble.²² The main aim of this work consists of making comparisons of two SPGEs, which have electrochemical behavior similar to that of edge plane or basal plane pyrolytic graphite where the electron kinetics in the former are significantly faster than that observed at the latter. Using these electrodes termed basal SPGE and edge SPGE, using [C₆MIM][PF₆] as the solvent, the graphite film electrochemical stability window and the voltammetric behavior of Fc, BQ, AQ, TC and BZ-3 were determined. We finally present the electrochemical reduction of BZ-3 in RTIL using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in order to gain insight into the electroanalytical determination of BZ-3.

Benzophenone derivatives, particularly BZ-3 are utilized in sunscreen agents. The concentration of BZ-3 (and other relevant additives) needs to be monitored to ensure they meet current legislation since there are undesirable dermatological side effects associated with the use of sunscreens. UV filters based on benzophenone derivatives can penetrate the human body through the skin, being excreted at trace levels in urine. Thus, the development of sensitive and selective analytical methods plays an important role.²³

There are several works about the determination of benzophenone sunscreen using high-performance liquid chromatography (HPLC) ^23–25 or gas chromatography (GC) coupled with ultraviolet spectroscopy (UV)²⁶ and mass spectrometry,²⁷ in cosmetics, there are limited reports using electrochemistry. To this end, mercury film electrodes,²⁸ hanging drop mercury electrodes²⁹ as well as epoxy-carbon composite electrodes³⁰ were used for the electrochemical determination of BZ-3 in cosmetics. Recently Laranjeira et al.³¹ studied the electroanalytical determination of BZ-3 in sunscreen preparations using boron-doped diamond electrodes. Likewise Cardoso et al.²⁹ studied the determination of three simultaneous sunscreens –included BZ-3- using differential pulse polarography. Therefore, it is clear that the electroanalytical determination of BZ-3 using SPGEs has not been explored.

2. Experimental

All chemicals were of analytical grade and were used as received. Room temperature ionic liquid [C₆MIM][PF₆] was obtained from Merck (synthesis reagent) and BQ from Fluka (assay ≥99,5%). Fc, TC and BZ-3 were commercially available from Aldrich. AQ was synthesized via halogenated thiophene precursors.³² The RTIL was concentrated in vacuo overnight at 45 °C in order to remove residual water. Nevertheless, because water is highly soluble in [C₆MIM][PF₆], we used Karl Fischer titration measurements and found that typically, prior to electrochemical measurements, water content was ∼4500 ppm. Thereafter, all compounds examined in this work were weighed using a microbalance and then dissolved in 100 μL RTIL to get an established concentration.

Scanning electron microscopic (SEM) measurements of both basal and edge plane SPGE surfaces were carried out on a scanning electron microscope (JEOL JSM-6700F) at 15 kV. Raman spectroscopy of SPGE surfaces was carried out using a Fourier Transform Raman spectrometer Bruker RFS/100, with an Ar-Ne ion laser to induce the Raman spectrum. The laser beam was focused on the surface of the samples with the aid of a microscope, where three consecutive spots separated 100 μm each were illuminated for 60 s. The Raman spectrum was obtained from the average of three spectra at a basal and edge SPGE surface, respectively. XPS chemical analyses were performed with a VG-Microtech Multilab electron spectrometer, by using the Mg K⁻¹//_/(1253.6 eV) radiation of twin anode in constant analyser energy mode with a pass energy of 50 eV.

CV, chronoamperometry (CA) and DPV measurements were carried out using a μ-Autolab III (Eco Chemie, The Netherlands) potentiostat/galvanostat and controlled by Autolab GPES software version 4.9 for Windows XP. All measurements were conducted using a three electrode configuration where the working and the counter electrode consist of graphite ink with a pseudo silver/silver chloride reference electrode and were fabricated in-house.³³ The two types of fabricated SPGEs have a 3.1 mm diameter with a carbon counter and a silver/silver chloride pseudo-reference electrode on a flexible plastic base.^16,34 The type of carbon ink used in the SPGE preparation was previously described elsewhere.^24,26 For this study two types of SPGEs—named edge and basal—were used. All electrochemical experiments were carried out under a high purity argon flow (from Air-liquide) in order to keep an inert atmosphere and discard the electrochemical reduction of oxygen.

Connectors for the electrochemical connection of the SPGEs were purchased from Kanichi Research Services Ltd. (UK).³⁵ SPGEs were used as received without further conditioning of the working electrode surface. CV experiments were carried out at 293 ± 2 K. CV simulations of the ferrocene/ferrocinium couple together with theoretical calculations of diffusion coefficients (D) and the apparent rate constants (k⁰) were obtained by Digisim (Digisim Professional version 3.0b) using a charge-transfer coefficient α of 0.5. All potentials were referred to the pseudo reference Ag/AgCl.

Glassy carbon (GC, Goodfellow, UK) and boron doped diamond (BDD, Windsor Diamonds, UK) electrodes with 3.0 mm diameter, were purchased for comparison of the electrochemical behavior in the RTIL for background response and the electrochemical reduction of BZ-3. Prior to use, GC and BDD electrodes were polished with an alumina slurry of successively smaller particles (1.00, 0.30 and 0.05 μm diameter), rinsed with water, sonicated in ultra pure water, then rinsed with acetonitrile and finally, dried under argon atmosphere. For both CV and DPV electrochemical experiments 1 mL of RTIL solution was thoroughly purged with high-purity argon to remove the residual oxygen. A gold wire was used as the counter electrode and a silver wire as the pseudo-reference electrode. At the end of the experiments, Fc was added to serve as an internal probe (0.53 V vs. Ag).

3. Results and discussion

3.1. Surface characterization of SPGE

SEM images of the SPGE exhibit a more heterogeneous surface for edge SPGEs (Fig. ESI1A†) with randomly orientated graphite sheets as previously shown by Kadara et al.,³⁶ while in contrast basal SPGEs (Fig. ESI1B†) shows a more homogeneous and regular surface. Raman spectra of these electrodes, edge and basal SPGE surfaces, are shown in Fig. ESI2† where both spectra exhibit D and G bands at 1335 and 1581 cm⁻¹ respectively. The relative intensities of the two lines D (I_D) and G (I_G) depend on the type of graphitic material³⁷ with an intensity increase of band D related to a decrease in crystalline graphitic grade in surface. By contrast, our findings show more precisely that the area ratio values of the D and G bands from the Raman spectrum of edge SPGE is 1.73, whereas the D/G value is 1.86 for the basal SPGE after deconvolution of the D and G bands. From a qualitative point of view, this relatively small difference in the D/G ratio is presumably ascribed to the number of defects on the surface.³⁸

XPS spectroscopy reveals some differences in terms of functionalization of the SPGE surfaces. XPS of the basal SPGE surface, shown in the electronic supporting information, shows the presence of 87.5% atomic carbon in the basal SPGE surface (Fig. ESI3†), whereas the edge SPGE surface (Fig. ESI4†) presents a similar composition, 85.9%. Only graphitic and C–O groups at 284.2 (80.5%) and 285.6 (7.0%) respectively, are found at basal SPGEs (Fig. ESI3†). However, the de-convoluted XPS spectrum of the edge SPGE surface (Fig. ESI4†) revealed the presence of graphitic, C–O and carbonyl functional groups at 284.5 (65.3%), 285.7 (10.5%) and 286.6 (10.1%) eV. It is worth noting the remarkable increase in the oxygen functionalization at edge SPGEs which provide a more hydrophilic surface characteristic.

3.2. Characterization of [C₆MIM][PF₆]

We first consider the electrochemical characterization of [C₆MIM][PF₆] at edge and basal SPGEs as depicted in Fig. 1. We started the CV trace from the equilibrium potential, near 0 V vs. pseudo-Ag/AgCl, and then scanned to positive or negative potentials to observe how the electro-oxidation or electro-reduction at the first instance may influence the CV profile. Fig. 1A shows the basal SPGEs CVs, when the negative and positive potentials are extended up to −2.30 V and +2.70 V vs. Ag/AgCl, respectively. A potential window stability corresponding to 5 V was obtained in deoxygenated [C₆MIM][PF₆] for an established current density of 1 mA cm⁻² and the discharge of residual water or oxidation/reduction of the RTIL. The results are in agreement with previous studies performed by Compton's group using a platinum electrode.³⁹ On the negative scan (solid line) a reduction peak at −0.77 V is observed along with three oxidative redox peaks on the reverse scan at −0.50, −0.24 and +1.33 V. The presence of the cathodic peak at −0.77 V in Fig. 1A may indicate that the electrochemical system is penetrable to oxygen. Characterization of carbon films in 1-butyl-3-methylimidazolium bis(trifluoromethane) sulfonimide [C₄MIM][NTf₂], 1-butyl-3-methylimidazolium nitrate [C₄MIM][NO₃] and 1-methylpyrrolidinium bis(trifluoromethane)sulfonimide [C₄Pyr][NTf₂] have been recently investigated by CV in the presence of oxygen¹⁴ showing that O₂ reduction potentials are likely to be due to the protic nature of [BMIM][NTf₂] as previously stated by Chiappe and co-workers.⁴⁰Fig. 1B shows the CV of [C₆MIM][PF₆] at edge SPGEs. The scan direction was investigated showing a cathodic peak at −0.74 V and two anodic waves at −0.81 and +1.14 V on the reverse scan. Overall the potential window for edge surfaces between −2.20 and +2.10 V was found to be slightly narrower than the corresponding response for basal surfaces. It is worth noting that the electrochemical windows reported in most ionic liquids range between 4 and 6 V.

Fig. 1 Cyclic voltammograms of [C₆MIM][PF₆] at (A) basal SPGEs; scanning to negative potentials (solid trace) and positive potentials (dashed trace), (B) edge SPGEs; scanning to negative potentials (solid trace) and positive potentials (dashed trace). Scan rate 50 mV s⁻¹. First scan recorded.

However, the nature of the carbonaceous electrodes such as glassy carbon, carbon films or carbon nanotubes can determine the window limits^11,12,41 when using for example [C₄MIM][PF₆] and [C₄MIM][BF₄]. Unlike the basal SPGE, the edge surface presented clear evidence of double layer capacitance of about ~35 μF. This difference between the basal and edge SPGE surfaces is likely due to the micro scale surface roughness (Fig. ESI1A and ESI1B†) and the higher incorporation of site like-basal planes on the edge SPGE surface rather than on the basal surface, providing a higher surface functionality on the edge SPGE surfaces,⁴² as demonstrated in this work by the XPS results. Fig. 1A and 1B display several anodic and cathodic waves, but it is not clear whether they stem from the carbon electrode or the ionic liquid. Therefore, we performed CVs of more conventional electrodes such as GC and BDD to compare with SPGEs. Fig. ESI5A† shows the first scan of the CV trace of [C₆MIM][PF₆] at GC in the absence of oxygen and scanned first to negative potentials, it shows a similar pattern as Fig. 1A, where the stability window is close to 5 V. Anodic peaks obtained at GC at −0.89 and +0.75 V vs. Fc⁺/Fc are similar to those obtained at basal SPGEs, −0.95 and +0.82 V vs. Fc⁺/Fc. Moreover, GC electrode did not reveal a cathodic current associated to the oxygen reduction. We also investigated the electrochemical behavior at BDD in [C₆MIM][PF₆] under de-aerated conditions, as shown in Fig. ESI5B.† In this case, no dioxygen reduction wave is observed but an anodic pre-wave is observed at +1.61 V vs. the pseudo-Ag (+1.08 V vs. Fc⁺/Fc); we presume that dioxygen reduction at basal and edge SPGEs could be related to either physisorbed oxygen on the carbon or an oxygen-containing functionality on the carbon.

3.3. Ferrocene in [C₆MIM][PF₆] at SPGEs

Fig. 2 compares the electrochemical behavior of Fc⁺/Fc at edge and basal SPGEs. Fig. 2A and 2B show a well-defined quasi-reversible redox behavior for the redox couple Fc⁺/Fc 5.8 mM in [C₆MIM][PF₆]. This response at edge surfaces showed a clear double layer capacitance. Peak-to-peak (ΔE_p) separations were found to be 0.10 V (Fig. 2A) and 0.12 V (Fig. 2B). CV traces were stable with a number of scans, denoting a good stability of our Ag/AgCl reference electrode. A linear increase in peak current (I_p) vs. square root of scan rate (20–1000 mV s⁻¹) was found with slopes +18.62 μA V^−1/2 s^1/2 (r² = 0.9981) and −12.32 μA V^−1/2 s^1/2 (r² = 0.9906) for anodic and cathodic peaks with respect to the basal SPGEs denoting that the process is mostly diffusion-controlled. However, we found that the anodic-to-cathodic peak current ratio was close to an average value of 1.5 for the basal SPGEs and plots of log₁₀ of I_pvs. log₁₀ of scan rate for the cathodic and anodic current peaks both gave rise to a slope of 0.47, which is close to the theoretical value 0.5 and denote a clear diffusion controlled process. By contrast, although linear plots of I_pvs. square root scan rate for edge SPGEs were obtained, plots of log₁₀ of I_pvs. log₁₀ of scan rate presented a much more pronounced deviation value from the 0.5 indicating a combination of diffusion-adsorption process. This contribution could be associated to the inherent variability of the surface properties related to roughness and the surface oxygen functionality.⁴³ The diffusion coefficient (D) of Fc differs significantly from Fc⁺, mainly due to the different interaction of neutral and positively charged species with the RTIL.

Fig. 2 Cyclic voltammograms for the electrochemical behavior of 5.8 mM Fc in [C₆MIM][PF₆] obtained at basal SPGEs (A), and edge SPGEs (B) using 35 μL of sample solution. Scan rate 50 mV s⁻¹. First scan (solid line) and third scan (dashed line). Inset figures, calibration plots of Fc (anodic peak current).

Moreover, chronoamperometry experiments showed that the slope of plots of log₁₀ of current vs. log₁₀ of time were about 0.5 denoting that the results fitted the Cottrell equation for all tested Fc concentrations (experiments not shown). Accordingly, inset figures in 2A and 2B depict the linear plots of I_pvs. Fc concentration using the Randles–Sevcik equation⁴⁴ providing the following fitting equations I_p (μA) = (0.77 μA m³ mol⁻¹)·[Fc] − 0.20 μA and I_p (μA) = (1.31 μA m³ mol⁻¹)·[Fc] + 0.23 μA for basal and edge SPGEs, respectively. Our results confirm that D values of Fc are independent of Fc concentration in the range tested values between 1.79 and 11.75 mM. Furthermore, single step CA experiments also showed no variation of the D_Fc with concentration, as previously stated by Rogers et al.⁴⁵

Fig. 3A shows a comparison between the experimental and Digisim simulated CVs at a scan rate of 50 mV s⁻¹. D_Fc in [C₆MIM][PF₆] at basal SPGEs was calculated by CV and single step CA experiments, whereas D_Fc+ was obtained from Digisim. D_Fc was 4.5 × 10⁻¹² m² s⁻¹ using the Cottrell equation⁴⁴ and D_Fc+ was 4.0 × 10⁻¹² m² s⁻¹ obtained from Digisim. The theoretical D_Fc value using Randles–Sevcik equation was 2.0 × 10⁻¹² m² s⁻¹, which is lower than the calculated (5.9 × 10⁻¹² m² s⁻¹) using a shorter alkyl chain such as [C₄MIM][PF₆].^16,45 This difference in the diffusion coefficient could be explained by the lower viscosity of [C₄MIM][PF₆].²²

Fig. 3 (A) Experimental Fc 5.8 mM CVs in [C₆MIM][PF₆] at basal SPGEs (dashed trace) compared to simulation with Digisim model (solid trace). First scan recorded, scan rate 50 mV s⁻¹. (B) Anodic and cathodic peak potentials plots vs. log₁₀ scan rate (V s⁻¹), 5.8 mM Fc in [C₆MIM][PF₆] at basal SPGEs compared to Digisim simulation.

Fig. 3B shows the experimental E_p values for the anodic and cathodic peaks compared to Digisim values. ΔE_p separation and the kinetic parameters are in a good agreement to those obtained by Digisim. The heterogeneous rate constant (k⁰) from Digisim was 2.8 × 10⁻⁶ cm s⁻¹ which is similar to the experimental one, 2.6 × 10⁻⁶ cm s⁻¹, indicating good fitting is obtained between Digisim simulation and experimental data. Moreover, the experimental k⁰ value was in agreement with that presented by Compton's group¹⁶ where they studied the electron transfer kinetics of Fc in 1-butyl-3-methylimidazolium hexafluorophosphate [C₄MIM][PF₆], using different carbonaceous materials such as basal plane pyrolytic graphite or GC.

3.4. CVs of 1,4-benzoquinone (BQ), 1,4-diphenyl-9,10-anthraquinone (AQ) and tetraphenylcyclopentadienone (TC) in [C₆MIM][PF₆]

Here, the electrochemical behavior of three organic compounds that contain carbonyl groups were investigated at SPGEs in [C₆MIM][PF₆]. These molecules generally participate in outer sphere electron transfer reactions, and therefore no significant deviation would be expected over a wide range of carbonaceous and metallic electrode surfaces. As expected, two single electron reactions are observed. Fig. 4 shows the typical CV for the direct electron transfer of 5.2 mM BQ at basal and edge SPGEs. In Fig. 4A we assign the first reduction wave at −0.36 V with the reduction of BQ to the radical anion BQ, whereas the backward oxidation peak takes place at −0.24 V. Furthermore, a second reduction process occurs at −0.70 V, which could correspond to the dianion formation, while the forward oxidation to BQ^·− occurs at a potential of −0.56 V. Hence, the redox couple is separated by 0.30 V, close to that previously reported by Wang et al.,⁴⁶ (0.38 V) for 1-ethyl-3-methylimidazolium bis(trifluoromethane-sulfonyl)-imide [C₂MIM][NTf₂] over Au microelectrodes. We found that the ΔE_p separation increases with the scan rate, and plots of log₁₀ of the cathodic I_pvs. log₁₀ of scan rate for the first redox couples give rise to a slope of 0.39, which is close to the theoretical value 0.5 and denotes a diffusion-controlled process (Fig. ESI6†). Electrochemical reduction for BQ in [C₂MIM][NTf₂] over Au microelectrode provided D_BQ of 4.62 × 10⁻¹¹ m² s⁻¹,²² whereas our findings gave an estimated D_BQ value of 5.93 × 10⁻¹³ m² s⁻¹ obtained from the plot peak intensity of the first reduction peak (third scan) vs. square root of scan rate using Randles–Sevcik equation. The possible argument may be the higher viscosity of [C₆MIM][PF₆] 586 cP at 25 °C⁴⁷ compared to 34 cP of the [C₂MIM][NTf₂]³⁹ and the sensitivity of the electrode kinetics reflected in the use of a gold microelectrode compared to the SPGE.

Fig. 4 CVs of 5.2 mM BQ in [C₆MIM][PF₆] at basal SPGEs (A) and edge SPGEs (B). Solid line (first scan), dashed line (third scan). Scan rate 50 mV s⁻¹.

Next, we further investigated the electrochemical behaviour of other carbonyl containing compounds such as AQ. Fig. 5 depicts the voltammetric response of 5.8 mM AQ at basal and edge SPGEs in [C₆MIM][PF₆]. During the first scan in Fig. 5A, a reduction pre-wave appeared at a potential of −0.72 V which could correspond either to the electrochemical reduction of residual dioxygen dissolved in the RTIL as shown in the blank (Fig. 1A) or a pre-adsorption process of AQ. After the second scan, the reductive pre-wave disappeared and reproducible CVs were obtained. Two well-defined, chemically reversible redox couples were observed. This corresponds to the reduction of neutral AQ to the radical anion, AQ^·− (first step), and the reduction of the anion radical to AQ²⁻. This is consistent with the literature on aprotic solvents^48,49 or more recently 9,10-anthraquinone in the RTIL, 1-butyl-3-methylimidazolium hexafluorophosphate [C₄MIM][PF₆].⁵⁰ The electron transfer process was diffusion-controlled (log₁₀I_p^cat/μA = 0.40 log₁₀(v/V s⁻¹) + 0.79) as shown in Fig. ESI7,† and the estimated D_AQ for the neutral species was 2.68 × 10⁻¹³ m² s⁻¹ from the Randles–Sevcik equation. Furthermore, we also investigated the electrochemical behavior of AQ in non de-aerated solutions and the onset potential of oxygen reduction under our experimental conditions. Fig. ESI8† depicts the voltammetric response of 5.5 mM AQ at basal SPGEs in [C₆MIM][PF₆] under air-saturated conditions. It appears that the electrochemistry of AQ (Fig. 5, ESI8†) may be significantly influenced by oxygen reduction and therefore faradaic currents of oxygen reduction-oxidation are comparable with those of Fig. 1A. We have also found that under air-saturated conditions the first reduction potential wave involved an enhancement of about 1 μA compared to argon-saturated conditions and a similar increase in current was found for the second reduction wave. It is remarkable that oxygen reduction starts at similar potentials as those in Fig. 1A and this potential refers to pre-wave potential shown for the electro-reduction of tetracyclone (vide infra). Comparison of the electrochemical behavior of AQ was made to a more conventional electrode such as the BDD electrode (results not shown) in an acetonitrile/tetrabutylammonium hexafluorophosphate system to determine if the reductive pre-wave occurring at about −0.70 V vs. standard reference electrode Ag/AgCl/(3 M KCl) was stemming from the SPGE or the RTIL itself. Results revealed an identical CV pattern with a reduction pre-wave before the appearance of the first reduction peak (results not shown) suggesting it is not presumably a contribution from the RTIL.

Fig. 5 CVs of 5.5 mM AQ in [C₆MIM][PF₆] at basal SPGEs (A) and edge SPGEs (B). Solid line (first scan), dashed line (third scan). Scan rate 50 mV s⁻¹.

Fig. 6 depicts the electrochemical behavior of TC in [C₆MIM][PF₆] at basal SPGEs under de-oxygenation. Similarly to previous reports,^51–53 we found two reduction waves which could correspond to two successive one-electron processes. For the first step we surmise that the formation of the relatively stable radical anion is taking place, which is resonance-stabilized, not only by the phenyl rings conjugated into the cyclic dienone, but also by the aromaticity of the cyclopentadienyl anion (potential of −0.77 V with a ΔE_p separation of 85 mV). The second quasi-reversible electron transfer leads to a TC dianion described elsewhere⁵⁴ with a potential of −1.14 V and a ΔE_p separation of 110 mV. Separation between the redox potentials between the radical anion and the dianion was 347 mV which is similar to those presented for the observed when applied to the electrochemical reduction of BQ and AQ, see Fig. 4 and 5. As shown in Fig. 6B inset, a reduction pre-wave was observed at about −0.65 V previous to the first cathodic peak. After the fifth scan, stable CVs were obtained with a slight current reduction of the first cathodic peak. Interestingly, a successive scan up to −1.50 V revealed again the formation of that pre-wave at −0.65 V. The electron transfer process among the scan rates tested (20–400 mV s⁻¹) was diffusion controlled (log₁₀I_p^cat/μA = 0.42 log₁₀ (v/V s⁻¹) + 0.21) as shown in Fig. ESI9,† and a D_TC for the neutral species was estimated to be 9.87 × 10⁻¹⁴ m² s⁻¹ using Randles–Sevcik equation. When the potential was scanned from 0.00 V to−1.50 V and then scanned back (solid line, Fig. 6), a current decrease on the re-oxidation wave at a potential of −0.70 V, together with the appearance of a new oxidation peak at −0.31 V which is related to the second reduction peak, was observed. The nature of that species will be addressed in future work.

Fig. 6 CVs of 2.7 mM TC in [C₆MIM][PF₆] at basal SPGEs (A) and edge SPGEs (B). Solid line (first scan), dashed line (third scan). Inset figure in Fig. 6B shows five consecutive cycles between 0 and −0.9 V. Scan rate 50 mV s⁻¹.

3.5. Electrochemical response of benzophenone-3 in [C₆MIM][PF₆]

Here, the electrochemical behaviour of BZ-3 in [C₆MIM][PF₆] at basal SPGE shows a principal reduction peak at −1.85 V vs. Ag/AgCl (−2.30 V vs. Fc⁺/Fc) as shown in Fig. 7. The SPGE suffers from the fouling of the surface as a function of the number of scans (results not shown). Further, the electrochemical behavior of BZ-3 at edge-SPGE was mainly dominated by the high capacitative charge upon cyclic-voltammetry experiments. Doherty's group⁵⁵ examined the electrochemistry of a range of substituted benzophenones in two RTILs, 1-butyl-3-methylimidazolium bistriflimide ([C₄MIM][NTf₂]) and 1-butyl-1-methylpyrrolidinium bistriflimide ([Bmpyrd][NTf₂]). They observed that the protic or aprotic character was sensitive to the electrochemical response. For [C₄MIM], a protic cation, a mainly irreversible reduction peak at −2.02 V vs. Fc⁺/Fc was observed, whereas by using [Bmpyrd], a not protic cation, two consecutive one electron processes were observed, leading initially to the radical anion and subsequently to the dianion species. However, unlike Doherty's experiments, we used BZ-3 with a neighbouring hydroxyl group near the carbonyl group with a significant difference in the electrochemical behaviour leading to only an irreversible process.

Fig. 7 CVs of 6.2 mM BZ-3 in [C₆MIM][PF₆] at basal SPGEs (first scan). Scan rate 20 mV s⁻¹.

DPV was chosen as a methodological technique for the electroanalytical determination of BZ-3. Fig. 8 depicts the DPV measurements of a standard 6.2 mM solution at basal SPGE and shows two reductive peaks at −1.40 V vs. Ag/AgCl (−1.85 V vs. Fc⁺/Fc) and −1.83 V vs. Ag/AgCl (−2.28 V vs. Fc⁺/Fc) under de-aerated conditions. Furthermore, similar experiments in aprotic solvents such as acetonitrile were also made showing a reductive peak at −2.20 V vs. Fc⁺/Fc. The BZ-3 concentration dependence plot using the first reductive peak yielded a straight line (r² = 0.991) with the following equation I (μA) = −0.88 [BZ-3] (mM) − 3.88 μA within a concentration range between 1 and 20 mM BZ-3. Taking into account these results, the electroanalytical determination of BZ-3 in sunscreen is being studied.

Fig. 8 DPV of 6.2 mM BZ-3 in [C₆MIM][PF₆] at basal SPGEs. Modulation amplitude 100 mV and step potential 50 mV. Modulation time 0.05 s. Interval time 0.5 s.

4. Conclusions

The electrochemical behavior of the graphitic structure, basal or edge, of SPGEs has been studied in the RTIL [C₆MIM][PF₆] by CV. Basal SPGEs presented an electrochemical window of about 5 V under de-oxygenated conditions. On the contrary, edge SPGEs showed a narrower electrochemical window as well as a higher double layer capacitance presumably due to the roughness and oxygen functionalization of the surface. Both SPGEs figured out a quasi-reversible wave for the Fc⁺/Fc redox couple. The electron transfer process was diffusion-controlled and a good linear regression was obtained between the anodic peak current and Fc concentration. D_Fc was calculated by chronoamperometric experiments providing a value of 4.5 × 10⁻¹² m² s⁻¹, whilst D_Fc+ from Digisim provided a good fitting value of 4.0 × 10⁻¹² m² s⁻¹.

The electrochemical reduction of different carbonyl containing organic compounds such BQ, AQ and TC was applied in RTIL at SPGEs. The carbonyl derivatives presented a two successive one-electron transfer with a similar behaviour as in common organic solvents. Electrochemistry of BZ-3 at SPGE in RTIL shed light on the applicability of screen printed platforms for the electroanalytical measurements in non-volatile ionic liquids.

Acknowledgements

This work was supported by the Ministerio de Educación y Ciencia MEC Spain (projects CTQ2007-62345, CTQ2008-06730-C02-01 and CTQ2011-23968). Maria Gómez-Mingot is grateful to the University of Alicante for her Fellowship. We would like to thank Prof. Juan Manuel Pérez and Ph.D. Cristina Almansa, Verónica López and Ph.D. Fernando Coloma from the Scientific Instrumentation Area of the University of Alicante.

References

1. D. V. Chernyshov, N. V. Shvedene, E. R. Antipova and I. V. Pletnev, Anal. Chim. Acta, 2008, 621, 178.
2. T. Tsuda and C. L. Hussey, Interface, 2007, 16, 42.
3. B. J. Akle, M. D. Bennett and D. J. Leo, Sens. Actuators, A, 2006, 126, 173.
4. M. Ue, M. Takeda, A. Toriumi, A. Kominato, R. Hagiwara and Y. Ito, J. Electrochem. Soc., 2003, 150, A499.
5. T. Kato, T. Kado, S. Tanaka, A. Okazaki and S. Hayase, J. Electrochem. Soc., 2006, 153, A626.
6. T. Tsuda, C. L. Hussey, H. M. Luo and S. Dai, J. Electrochem. Soc., 2006, 153, D171.
7. H. Ohno, Electrochemical aspects of ionic liquids, Wiley, Hoboken, New Jersey, 2005.
8. M. C. Buzzeo, R. G. Evans and R. G. Compton, ChemPhysChem, 2004, 5, 1106.
9. C. A. Brooks and A. P. Doherty, Electrochem. Commun., 2004, 6, 867.
10. A. P. Doherty and C. A. Brooks, Electrochim. Acta, 2004, 49, 3821.
11. U. Schröder, J. D. Wadhawan, R. G. Compton, F. Marken, P. A. Z. Suarez, C. S. Consorti, R. F. de Souza and J. Dupont, New J. Chem., 2000, 24, 1009.
12. V. M. Hultgren, A. W. A. Mariotti, A. M. Bond and A. G. Wedd, Anal. Chem., 2002, 74, 3151.
13. Y. Liu, L. Shi, M. Wang, Z. Li, H. Liu and J. Li, Green Chem., 2005, 7, 655.
14. R. Pauliukaite, A. P. Doherty, K. D. Murnaghan and C. M. A. Brett, J. Electroanal. Chem., 2008, 616, 14.
15. N. Maleki, A. Safavi and F. Tajabadi, Anal. Chem., 2006, 78, 3820.
16. L. Xiao, E. J. F. Dickinson, G. G. Wildgoose and R. G. Compton, Electroanalysis, 2010, 22, 269.
17. P. Masawat, J. Mark and T. Slater, Sens. Actuators, B, 2007, 124, 127.
18. J. P. Metter, R. O. Kadara and C. E. Banks, Analyst, 2011, 136, 1067.
19. R. Ren, C. Leng and S. Zhang, Biosens. Bioelectron., 2010, 25, 2089.
20. J. Ping, J. Wu and Y. Ying, Electrochem. Commun., 2010, 12, 1738.
21. J. L. Chang, G. T. Wei and J. M. Zen, Electrochem. Commun., 2011, 13, 174.
22. J. G. Huddleston, A. E. Visser, W. M. Reichert, H. D. Willauer, G. A. Broker and R. D. Rogers, Green Chem., 2001, 3, 156.
23. L. Vidal, A. Chisvert, A. Canals and A. Salvador, J. Chromatogr., A, 2007, 1174, 95.
24. V. Vanquerp, C. Rodriguez, C. Coiffard, L. J. M. Coiffard and Y. D. Roeck-Holtzhauer, J. Chromatogr., A, 1999, 832, 273.
25. S. C. Rastogi and G. H. Jensen, J. Chromatogr., A, 1998, 828, 311.
26. A. Chisvert, M. C. Pascual-Marti and A. Salvador, Fresenius' J. Anal. Chem., 2001, 369, 638.
27. T. Felix, B. J. Hall and J. S. Brodbelt, Anal. Chim. Acta, 1998, 371, 195.
28. L. H. Wang, Electroanalysis, 2002, 11, 773.
29. J. C. Cardoso, B. M. L. Armondes, J. B. G. Júnior and V. S. Ferreira, Colloids Surf., B, 2008, 63, 34.
30. M. L. Chang and C. M. Chang, J. Food Drug Anal., 2001, 9, 199.
31. M. T. Laranjeira, F. de Lima, S. C. de Oliveira, V. S. Ferreira and R. T. S. de Oliveira, Am. J. Anal. Chem., 2011, 2, 383.
32. T. Thiemann, Y. Tanaka, J. Iniesta, H. T. Varghese and C. Y. Panicker, J. Chem. Res., 2009, 33, 732.
33. R. O. Kadara, N. Jenkinson and C. E. Banks, Electrochem. Commun., 2009, 11, 1377.
34. N. A. Choudry, D. K. Kampouris, R. O. Kadara and C. E. Banks, Electrochem. Commun., 2010, 12, 6.
35. http://kanichi-research.com/ .
36. R. O. Kadara, N. Jenkinson and C. E. Banks, Sens. Actuators, B, 2009, 138, 556.
37. F. Tuinstra and J. L. Koenig, J. Chem. Phys., 1970, 53, 1126.
38. C. E. Banks and R. G. Compton, Analyst, 2006, 131, 15.
39. A. M. O'Mahony, D. S. Silvester, L. Aldous, C. Hardacre and R. G. Compton, J. Chem. Eng. Data, 2008, 53, 2884.
40. C. Chiappe and D. Pieraccini, J. Phys. Chem. A, 2006, 110, 4937.
41. L. Kavan and L. Dunsch, ChemPhysChem, 2003, 4, 944.
42. M. Galinski, A. Lewandowski and I. Stepniak, Electrochim. Acta, 2006, 51, 5567.
43. R. J. Rice, N. M. Pontikos and R. I. McCreery, J. Am. Chem. Soc., 1990, 112, X1–4622.
44. G. Inzelt in F. Scholz(Ed.), Electroanalytical methods, 2nd ed., Springer, Berlin, 2005, Ch I.3.
45. E. I. Rogers, D. S. Silvester, D. L. Poole, L. Aldous, C. Hardacre and R. G. Compton, J. Phys. Chem. C, 2008, 112, 2729.
46. Y. J. Wang, E. I. Rogers, S. R. Belding and R. G. Compton, J. Electroanal. Chem., 2010, 648, 134.
47. J. Liu, Y. Chi, G. Jiang, C. Tai, J. Peng and J.-T. Hu, J. Chromatogr., A, 2004, 1026, 143.
48. P. Zuman and C. Czech, Chem. Commun., 1962, 27, 2035.
49. J. Q. Chambers in S. Patai and Z. Rappoport (Ed.), Electrochemistry of quinones, in the chemistry of the quinonoid compounds, Wiley, Toronto, 1988, p. 719.
50. V. A. Nikitina, R. R. Nazmutdinov and G. A. Tsirlina, J. Phys. Chem. B, 2011, 115, 668.
51. H. Özyörük, K. Pekmez and A. Yildiz, Electrochim. Acta, 1987, 32, 569.
52. J. Kopilov and D. H. Evans, J. Electroanal. Chem., 1990, 280, 381.
53. M. A. Fox, K. Campbell, G. Maier and L. H. Franz, J. Org. Chem., 1983, 48, 1762.
54. N. Takano, N. Takeno and M. Morita, Denki Kagaku oyobi Kogyo Butsuri Kagaku, 1982, 50, 964.
55. S. O'Toole, S. Pentlavalli and A. P. Doherty, J. Phys. Chem. B, 2007, 111, 9281.

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Footnote

† Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c2ra20927f/

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