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Voltammetric sensing of the fuel dye marker Solvent Blue 14 by screen-printed electrodes
Sensors and Actuators B 138 (2009) 257–263
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Voltammetric sensing of the fuel dye marker Solvent Blue 14 by screen-printed electrodes Magno Aparecido Gonc¸alves Trindade, Maria Valnice Boldrin Zanoni ∗ São Paulo State University, UNESP, Department of Analytical Chemistry, Institute of Chemistry-Araraquara, UNESP, Rua Francisco Degni, s/n Bairro Quitandinha, 14800-900, Araraquara, SP, Brazil
a r t i c l e
i n f o
Article history: Received 14 November 2008 Received in revised form 19 January 2009 Accepted 22 January 2009 Available online 31 January 2009 Keywords: Fuel marker Solvent Blue 14 Screen-printed carbon electrode Voltammetric method Surfactant in electrochemistry
a b s t r a c t A voltammetric method was developed to detect Solvent Blue 14 (SB-14) dye marker in fuel ethanol and kerosene samples. The device is based on a screen-printed carbon electrode (SPCE) operating in a Britton–Robinson buffer with N,N-dimethylformamide (7:3, v/v) and 5.50 × 10−4 mol L−1 of Dioctyl sulfosuccinate sodium (DSS) where the anthraquinone group of the dye is reduced at −0.40 V. The peak intensity is triplicated in the presence of the surfactant, which forms a charged film on the electrode surface acting as an anti-fouling agent. Using the best experimental conditions, the electroanalytical method presents a linear response for 2.00 × 10−7 to 2.00 × 10−6 mol L−1 SB-14 dye (r = 0.9986) and a detection limit of 9.30 × 10−8 mol L−1 . The developed method was successfully utilized for the quantification of SB14 dye in alcohol samples without any pre-treatment and in kerosene (after rapid clean-up procedure) with recoveries of 82.00–99.00%, respectively. The values are in good accordance with other reference methods based on spectrophotometric analysis. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The dye 1,4-Bis(pentylamino)anthraquinone also called Solvent Blue 14, Fig. 1, is one of the anthraquinone dyes commonly used as visible markers purposely added to fuels as a method of quality control. In general, the amount of the dye added to the fuel depends on a diversity of factors. On the other hand, a minimum of 5.00 ppm is recommended in the finally tagged liquid petroleum fuel [1–6]. Therefore, sensitive and rapid analytical methods are required to determine its presence in petroleum products, since this dye can be used to distinguish the origin and quality of several types of fuels. The analytical methods described in literature for the determination of dyes in fuel samples are based on techniques that employ high-performance liquid chromatography (HPLC) equipment coupled to ultraviolet absorption and diode array detectors [7–10] and, some cases, derivative spectroscopy [11] is also used. In most cases, these methods require long analysis times and long sample pretreatment steps. Studies found in literature have shown that the electrochemical technique is highly recognized as a favorable tool in the quantification of several types of dye carrying the azo and the anthraquinone groups [12–16]. However, these studies involve only the classes of dyes that are soluble in aqueous medium, which cannot be generally applied to other classes of dyes, such as those used
∗ Corresponding author. Tel.: +55 16 3301 6619; fax: +55 16 3322 7932. E-mail address: [email protected] (M.V.B. Zanoni). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.01.043
as markers in fuels. Recently, we have been successfully studying the possibility of using electrochemical methodology to quantify quinizarin [17] and SB-14 [18] on glassy carbon electrodes in fuel samples. Although, as can be seen in other electrochemical studies involving the reduction of anthraquinone compounds [19–22], the electrode process is complicated due to strong adsorption of the dye and/or electrochemical products onto the electrode surface, and these promote gradual passivation of the electrode surface compromising the success of the electroanalytical method. Taking into consideration the need to propose a simpler procedure for the determination of dye in fuel samples, the present work investigated the possibility of using screen-printed carbon electrode devices as a way of miniaturizing the set of electrodes conventionally used in electrochemical procedures and to propose a simple voltammetric method to monitor dye markers. The use of screen-printed electrodes has seen great progress, since they are versatile and disposable for various electroanalytical applications [23–30]. The advantage of this type of electrode set has to do with its modest cost, portability, simplicity of operation, reliability, and the small instrument footprint of the arrangement containing the working electrode, auxiliary and reference electrodes, which are printed directed onto a polymeric/plastic foil. The use of surfactants has also been described in literature [31–40] as successful strategies for overcoming the adsorption interferences. It was recently shown [33,36,38,40] that surfactants could be used to stabilize voltammetric response by protecting the electrode surface from fouling and enhancing the electrochemical
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Fig. 1. Structure of the dye Solvent Blue 14 (SB-14).
signal. The satisfactory response depends on the concentration and nature of the surfactants, which can influence not only the shape of electrochemical waves but also the parameters such as potential, electron-transfer rate, diffusion and transfer coefficients, stability of the intermediate species and the minimization of adsorption effects. Therefore, the aim of the present work was to develop a simple method for SB-14 dye detection in fuel samples. The method combines the simplicity of a screen-printed carbon electrode with the use of Dioctyl sulfosuccinate sodium (DSS) as the anionic surfactant to develop a simple and rapid analytical method able to offer a fast and sensitive alternative for the analysis of SB-14 in samples of kerosene and fuel alcohol using the square-wave voltammetric technique. 2. Experimental 2.1. Instrumentation Electrochemical analysis was performed using an Autolab PGSTAT-30 (Eco Chemie) connected to a microcomputer controlled by General Purpose Electrochemical System (GPES 4.9) software for data acquisition and experimental control. The measurements were performed on a conventional electrochemical cell (maximum capacity, 10.0 mL) specially adapted to insert the screen-printed carbon electrode (Oxley Developments UK). The design of the screen-printed carbon electrode (SPCE) used in all the electrochemical experiments consists of an alumina ceramic base 45.00 mm long, 10.00 mm wide and 0.80 mm thick, with the working (W), reference (R) and the auxiliary (A) electrodes exposed on the surface (Fig. 2). All the electrodes are made of conducting carbon ink. A contact at the end is connected with the active part of each electrode by internal conducting carbon parts (C) covered by a dielectric protection layer. The SPCE was connected via a cable to the potentiostat and used without any pre-treatment. The Ag/AgCl(KCl 3 mol L −1 )
(inside a Luggin capillary containing 3.00 mol L−1 KCl) was used as the reference electrode substituting the SPCE in order to minimize problems of adsorption and contamination of the surface. The spectrophotometric measurements were carried out using an HP spectrophotometer (model 8453) operating in the range of 220–700 nm with a 1.00-cm-path-length quartz cell. All pH measurements were made using a combined glass electrode (Orion, Thermo Electron Corporation) connected to the digital pHmeter (Orion, Thermo Electron Corporation) and are expressed as pHapparent (pH*). The deionized water used to prepare the supporting electrolyte solutions was purified with a Milli-Q system and had a resistivity of 18 M cm−1 (model Simplicity 185, Millipore). 2.2. Reagents and solutions All reagents were of the highest grade available and used without further purification. Stock solutions of SB-14 (obtained by Acros organics) (1.00 × 10−3 and 1.00 × 10−2 mol L−1 ) were prepared by dissolving the solid product in acetonitrile (J.T. Baker). Diluted working standard solutions were then prepared daily with acetonitrile just prior to use. The working solutions had shown sufficient stability over storage times of up to three months (the time of the evaluation). Britton–Robinson (B–R) (0.08 mol L−1 ) buffer having a pH between 2.0 and 9.0 was used as the supporting electrolyte solution and was prepared in the usual way: a mixture of 0.08 mol L−1 acetic acid (Merck), 0.08 mol L−1 boric acid (Merck) and 0.08 mol L−1 orthophosphoric acid (Merck) with the appropriate amount of 1.0 mol L−1 sodium hydroxide (Merck) solution to adjust the pH to the required value. The N,N-dimethylformamide (DMF) (Mallinckrodt) used as supporting electrolyte solution was mixed with B–R and was of analytical grade. The surfactant, Dioctyl sulfosuccinate sodium (DSS) 5.50 × 10−2 mol L−1 from (Sigma–Aldrich), was prepared from stock solution in deionized water. Samples of unmarked fuel (kerosene and alcohol) were spiked with 2.00 mg L−1 of SB-14 dye using an appropriate volume of the stock solution. Before the SB-14 dye was added, the fuel was certified that there was no evidence of this dye. 2.3. Procedure for voltammetric analysis An aliquot of 5.00 mL of the supporting electrolyte solution was added to the electrochemical cell (specially adapted to insert the screen-printed carbon electrode) and then deaerated with pure nitrogen for approximately 15 min to eliminate dissolved oxygen. Afterwards, 100 L of the surfactant solution (5.50 × 10−2 mol L−1 ) was added to the cell under stirring for 2 min. The stirring was stopped and after a 15-second rest, cyclic or square-wave voltammograms were registered. After the background voltammogram had been recorded, the aliquot of the dye solution was added to the cell and the voltammogram was then recorded for the same screenprinted carbon electrode (SPCE). Although, the SPCE is described as a device with disposable properties, it was possible to use the same SPCE repeatedly for at least 10 measurements. Therefore, before each series of measurements, the SPCE was left to rest for approximately 35 s, which was long enough to re-establish the surface. 2.4. Analysis of SB-14 dye in an alcohol sample
Fig. 2. Screen-printed carbon electrode on ceramic substrate used in the voltammetric measurements. (C) Electrical connections, (W) working, (R) reference and (A) auxiliary electrodes.
The quantification of SB-14 dye in an alcohol sample was performed by mixing an aliquot containing 0.35 mL of the commercial fuel ethanol sample (collected from a gas station in Araraquara city, SP, Brazil) spiked to 2.00 mg L−1 of dye in a volumetric flask of 5.00 mL containing the supporting electrolyte solution (B–R buffer
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pH 3.0 + DMF, 7:3, v/v) and 1.40 × 10−3 mol L−1 of the DSS surfactant. The resulting mixture was transferred to the electrochemical cell, submitted to deaeration with nitrogen for 15 min. It was then directly analyzed without any previous treatment by recording the square-wave voltammetry data. Then, the SB-14 dye in the matrix was quantified using the standard addition method and the quantitative results obtained were expressed as the total content of the dye in the sample. 2.5. Analysis of SB-14 dye in kerosene samples 0.50 mL aliquot of the aviation kerosene sample (collected in Araraquara city, SP, Brazil) spiked to 2.00 mg L−1 of SB-14 dye was transferred to a solid-phase extraction cartridge containing 0.50 g of silica previously conditioned with approximately 6.00 mL of hexane. Using a syringe attached to the top of the cartridge, the kerosene sample was slowly forced thorough the cartridge (flow rate approximately 2.00 mL min−1 ). After this, the cartridge was washed with approximately 10.00 mL of the hexane to remove all hydrocarbons and contaminants from the matrix. Then the dye was eluted from the cartridge using approximately 4.50 mL of the Britton–Robinson (B–R) buffer (pH required) and DMF mixture in the ratio of 7:3 (v/v). The eluent was collected in a 5.00 mL volumetric flask and after the addition of the DSS surfactant (125.0 L of 5.50 × 10−2 mol L−1 solution), the volume was completed with the mixture B–R:DMF 7:3 (v/v). The resulting solution was submitted to the square-wave voltammetric analysis using the best conditions previously optimized. The quantification of the SB-14 dye was performed using the standard addition method as previously described in Section 2.4. 2.6. Analysis of SB-14 dye in fuel sample by UV–vis spectrophotometry An analytical curve for SB-14 dye using spectrophotometry was plotted by registering the spectrum for aliquots of 20.00 to 120.00 L of the SB-14 dye stock solution (1.00 × 10−3 mol L−1 ) previously prepared by dilution in acetonitrile. The absorbance for standard solution was measured following the maximum signal at 590 and 640 nm after certifying that there was no peak in the blank (acetonitrile) measured. The quantification of SB-14 dye in commercial ethanol fuel was performed after diluting 4.00 mL of the sample spiked to 5.00 mg L−1 of SB-14 dye in a 10.00 mL calibrated flask with acetonitrile. The resulting mixture was submitted to spectrophotometric analysis recording the UV–vis spectrum. The quantification of the SB-14 dye in the fuel alcohol samples was performed using the external standard method. The quantification of the SB-14 dye in kerosene samples was performed by transferring 4.00 mL of the sample spiked to 5.00 mg L−1 of SB-14 dye to a solid-phase extraction cartridge containing 1.00 g of silica previously conditioned with approximately 6.00 mL of hexane. After washing with 10.00 mL of hexane to remove all hydrocarbons and contaminants from the matrix, the dye (10.00 mL) was eluted with acetonitrile and submitted to spectrophotometric analysis following the above procedure. 3. Results and discussion 3.1. Electrochemical behavior of SB-14 on the screen-printed carbon electrode (SPCE) The stability and solubility of the SB-14 dye solution is improved in micellar medium containing Dioctyl sulfosuccinate sodium (DSS). Although other surfactants were tested, the best voltammetric response was obtained for the reduction of 1.00 × 10−5 mol L−1
Fig. 3. Cyclic voltammograms for the SPCE indicating the electrochemical reduction of SB-14 dye in 0.08 mol L−1 B–R buffer (pH 2.50) containing 30% DMF for (curve A) the absence of SB-14 and (curve B) for 1.00 × 10−5 mol L−1 SB-14 in the presence of 5.50 × 10−4 mol L−1 DSS. Scan rate of 100 mV s−1 .
SB-14 dye in a mixture of B–R buffer (pH 2.5) and DMF (7:3, v/v) + 5.50 × 10−4 mol L−1 DSS on the screen-printed carbon electrode (SPCE). Fig. 3 shows a typical cyclic voltammogram for SB-14 in the presence of 5.50 × 10−4 mol L−1 DSS (curve B), where a cathodic peak at −0.49 V vs. (pseudo reference from screen-printed carbon electrode) is observed. This peak was attributed to the reduction of the central quinone group to hydroquinone derived after a two-electron process [19–22]. For the reverse scan, it is possible to detect the occurrence of two waves with lower intensities due to hydroquinone and semiquinone oxidation as the main products previously generated [19–22], indicating that the reductive mechanism is not modified in this experimental condition. Cyclic voltammograms were also compared under the same experimental conditions, but changing the pseudo reference electrode of the screen-printed carbon electrode set for an external reference electrode of Ag/AgCl(KCl 3.0 mol L −1 ) . Except for a potential shifting of 200 mV to a less negative potential in relation to the SPCE set, there was no other modification in the wave form, whose current intensity and position were constant in all the voltammograms recorded for each new electrode and thus, could consequently be used for analytical purposes. Successive cyclic voltammograms obtained for 5.00 × 10−5 mol L−1 SB-14 in 0.08 mol L−1 B–R buffer (pH 2.4) containing 30% DMF at a scan rate of 100 mV s−1 exhibit a decrease in the cathodic peak current after the first scan, suggesting that the product of electrode reaction is adsorbed onto the SPCE surface [21,43]. However, when the study is repeated in the presence of an anionic surfactant, DSS, the voltammogram can be re-established (10 repetition) after an approximate 35 s rest before each series of measurements. This procedure indicates that the surfactant presents an anti-fouling capacity under this experimental condition, which improves the feasibility of the SPCE to monitor SB-14 dye without adsorption complications. In order to improve the sensitivity, the effect of the DSS addition on the peak current intensity was monitored by recording square-wave voltammograms (SWV) instead of linear or cyclic voltammograms. Characteristic SWV curves of SB-14 dye reduction in 0.08 mol L−1 B–R buffer (pH 2.4)/DMF (7:3) in the absence and presence of DSS are compared in Fig. 4 (curves A and B). An intense peak is observed at around −0.40 V, whose height is almost three times higher than that given without surfactant. This result
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Fig. 4. Square-wave voltammograms for the SPCE indicating the reduction of 5.00 × 10−6 mol L−1 SB-14 dye in 0.08 mol L−1 B–R buffer (pH 2.40) containing 30% DMF for (curve A) the absence of DSS and (curve B) the presence of 8.00 × 10−4 mol L−1 DSS. Conditions: frequency (f) = 60 Hz, step potential (Es ) = 4 mV and pulse amplitude (Esw ) = 50 mV. Inset: influence of DSS concentration on square-wave voltammetric response under the conditions presented above.
indicates that the addition of surfactant could offer an analytical potential for the determination of SB-14 dye in fuels, improving the detectability and solubility of the dye sample in a mixture of water/DMF. The influence of the surfactant on the peak current intensity was investigated monitoring the reduction of 5.00 × 10−6 mol L−1 SB-14 dye in DSS changing the concentration from 2.00 × 10−4 to 2.70 × 10−3 mol L−1 . The effects can be visualized in Fig. 4 (inset). The peak current increases up to 1.00 × 10−3 mol L−1 DSS, but reaches a plateau above this value. The extraordinary increase in the peak current on adding the DSS can be attributed to three effects: they are (i) the anionic surfactant could be adsorbing onto the electrode surface forming an oriented film of negative charges. Since the anthraquinone group of SB-14 dye is usually in the protonated form in an acidic condition, it could be pre-concentrated due to electrostatic interaction; (ii) the reduction of SB-14 dye molecules could be occurring via vertically oriented molecules inside the DSS film, which could alter its electron transfer rate [31–40]; or (iii) the DSS could solubilize the generated products avoiding the passivation of the electrode. Above 1.00 × 10−3 mol L−1 of DSS, the system reaches the critical micellar concentration (CMC) and there is no oriented film on the electrode surface, since it is formed only at lower concentrations. Thus, films of DSS could be used to strategically improve the detectability of dyes bearing anthraquinone groups.
potential shifts to more negative potential when the pH increases, and the linear relationship presented a slope of −67.30 mV per pH unit, indicating that the electrode process is influenced by preprotonation reactions [21] of the anthraquinone group [19–22]. These results are consistent with a mechanism involving two electrons and two protons, similarly verified by other previous studies conducted for anthraquinone reduction and indicating that surfactant films do not interfere markedly in the electrodic mechanism. Nevertheless, maximum peak current intensity is obtained at pH <3.0, which decreases continuously when the pH increases. Taking into consideration that probably quinone groups are present in a protonated form under very acidic conditions [18], there is a preponderant electrostatic interaction occurring between the anionic surfactant (DSS) and the SB-14 dye at pH ≤3.0. Therefore, the voltammetric signal is enhanced preferentially due to electrostatic pre-concentration of the protonated dye on films of anionic surfactant spontaneously organized on the electrode surface. The apparent pH around 2.50 was selected as optimum for SB-14 detection, which offered an improved interaction between the dye and the anionic surfactant, leading to the enhancing of the analytical applicability. A typical reduction of SB-14 on the screen-printed carbon electrode (SPCE) is presented in Fig. 6, using a frequency of 60 Hz, where contributions of forward current (Fig. 6, curve a), backward current (Fig. 6, curve b) and resultant current (Fig. 6, curve c) are discriminated. This type of response indicates that the SB-14 dye is reduced following a reversible electron transfer process [41,42] when DSS is added to the solution and the adsorption effects of the dye molecule are suppressed. The influence of different frequencies on the peak potential and peak current was studied from 10 to 250 Hz. The effect of frequency on the ratio of forward and backward currents (Ibackward /Iforward ) taken from the square-wave voltammograms obtained for SB-14 is shown in Fig. 6 (inset). The ratio (Ibackward /Iforward ) increases from 10 to 60 Hz. Above this value the ratio reaches a constant value close to 1.0, clearly indicating that the system is controlled by a simple reversible process [41,42]. These results are confirmed by the linear relationship between peak currents and the square root of the frequency observed for frequency values between 10 and 150 Hz. The
3.2. Effect of pH The effect of pH on the reduction of SB-14 (5.00 × 10−6 mol L−1 + 1.00 mmol L−1 of DSS) was investigated on the screen-printed electrode surface for electrolyte (3:7, v/v) DMF/B–R buffer varying pH values between 2.0 and 9.0. The square-wave voltammograms obtained for acidic and neutral medium are shown in Fig. 5 (curves a–d). Studies at pH values higher than 9.0 were not conducted because the reduction potential of SB-14 occurs quite close to the electrolyte/electrode discharge on the SPCE and the peak current could not be measured accurately. Fig. 5 (inset) illustrates the effect of pH variation on the peak current intensity (curve A) and peak potential (curve B). The peak
Fig. 5. Square-wave voltammograms registered for the SPCE indicating the electrochemical reduction of 5.00 × 10−6 mol L−1 SB-14 dye in 0.08 mol L−1 B–R buffer containing 30% DMF and the presence of 1.00 × 10−3 mol L−1 DSS at: (a) pH 2.0, (b) pH 3.0, (c) pH 6.0 and (d) pH 7.0. Inset: influence of pH on the peak potential (A) and peak current (B) response for the pH range between 2.0 and 9.0. Parameters: f = 60 Hz, Es = 2 mV and Esw = 25 mV.
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Fig. 6. Square-wave voltammogram registered in 0.08 mol L−1 B–R buffer (pH 2.50) with 30% DMF and presence of DSS surfactant (1.40 × 10−3 mol L−1 ) for the reduction of 5.00 × 10−6 mol L−1 SB-14. (a) Forward current, (b) backward current and (c) resultant current. Parameters: f = 60 Hz, Es = 2 mV and Esw = 25 mV. Inset: variation of the ratio (Ibackward /Iforward ) versus frequency. Conditions as ascribed above.
electrons transferred during the electrodic process were also evaluated from the peak potential relationship vs. frequency, as shown the equation [41,42]: Ep −2.303RT = 2nF logf where: R is the gas constant, T the temperature, n the number of electrons, and F is the Faraday constant. The values of Ep versus logf1/2 obtained from the shifting of the peak potential in relation to the frequency shows a slope of the 36.00 mV/decade. From the above equation n = 0.82 is obtained, indicating that the SB-14 reduction in micellar medium involves the transfer of one electron.
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Fig. 7. Square-wave voltammograms registered in 0.08 mol L−1 B–R buffer (pH 3.0) with 30% DMF and 1.40 × 10−3 mol L−1 DSS surfactant for an increasing concentration of SB-14 (2.00 × 10−7 to 4.0 × 10−6 mol L−1 ). Inset: analytical calibration function for the dependence of the peak current on the SB-14 concentration. Parameters: f = 60 Hz, Es = 4 mV and Esw = 50 mV.
4.00 × 10−6 mol L−1 SB-14 using the same screen-printed carbon electrode and an intermediate step of 35 s rest before each measurement. The peak current increased linearly from 2.00 × 10−7 to 2.00 × 10−6 mol L−1 , as shown in Fig. 7 (inset), following the equations Ipc (A) = −0.20 + 2.90 × 106 × C (mol L−1 ) with r = 0.9986. The limits of detection (LOD) and quantification (LOQ) were computed, respectively, as 3 × Sd/m and 10 × Sd/m, where Sd is the standard deviation of the peak current for the blank (measured at the same potential as the dye) and m the slope of the analytical curve [44]. The values obtained were 9.30 × 10−8 and 3.0 × 10−7 mol L−1 for LOD and LOQ, respectively. 3.4. Analytical application
3.3. Analytical curve Using the optimal conditions, f = 60 Hz, Es = 4 mV and Esw = 50 mV (similarly investigated in previous work [18]) and pH 3.0, the method was applied to detect SB-14 in different concentrations of the dye marker in a mixture of B–R buffer (pH 3.0)/DMF (7:3, v/v) + 1.40 × 10−3 mol L−1 DSS surfactant. Fig. 7 illustrates the respective voltammograms recorded from 2.00 × 10−7 to
Using the best experimental conditions, the voltammetric method was applied to the determination of SB-14 dye in commercial alcohol and kerosene samples using the standard addition method. Firstly, samples of the collected ethanol fuels do not present any signal of SB-14 dye as marker in commercial matrices. Then, the samples were spiked to 2.00 mg L−1 of the dye as previously mentioned in the experimental section (Section 2.4) and
Table 1 Results of recovery assay to accuracy and precision for added SB-14 dye in kerosene and alcohol samples using proposed method and reference method. Analysis method Electroanalytical method
Samples a
Alcohol
Kerosenea
Spectrophotometric method
a b
( = 590 nm) Kerosenea Alcohola
Added (mg L−1 )
Found (mg L−1 )
Recovery (%)
2.00 2.00 2.00 2.00 Average 2.00 2.00 2.00 2.00 Average
1.93 1.91 1.90 1.97 1.92 1.86 1.93 1.98 1.65 1.85
96.50 95.50 93.00 98.50 95.90 93.00 96.50 99.00 82.00 92.50
5.00 5.00
5.42 5.26
108.40 105.20
Average of four determination and five measurements for each determination; R.S.D.: relative standard deviation. Confidence level of 95% (tcritical = 3.18).
R.S.D. (%)
tb
2.40
3.10
8.30
1.95
3.15 3.10
4.20 2.60
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well its quantification after the standard addition method is shown in Fig. 8 (lines b–e). From this Figure, it is possible to suggest that all hydrocarbons and other interferents present in fuel samples were effectively removed in the clean-up stage and the analytical signal presents a well-defined peak for the dye used. The recoveries of SB-14 in these kerosene samples, based on the average of four determinations, are listed in Table 1 and were found to be between 82.00 and 99.00%. The accuracy of the SB-14 assay was determined from labeled data (Table 1), which was expressed as %R.S.D. The statistic values obtained are acceptable and suggest that the proposed procedure could be successfully applied to the quantification of SB-14 dye in samples of kerosene fuels. The results were also compared by applying Student’s t [44], whose theoretical value of t for a 95% confidence level, where the experimental value (t calculated) did not exceed the theoretical value (critical value 3.182) for the electroanalytical method, as can be seen in Table 1, confirming no significant difference between the added value (spiked) and the value found. Therefore, the statistical calculations for the assay suggested good precision and accuracy of the proposed method for both commercial samples. Fig. 8. Square-wave voltammograms obtained for the determination of SB-14. (a) Kerosene sample (containing 0.20 mg L−1 SB-14) after extraction procedure as mentioned in Section 2.4, with successive additions of SB-14 (37.80 mg L−1 ) (b) 20 L and (c–e) 10 L. Conditions as described in Fig. 7.
submitted to direct square-wave voltammetric analysis. Application of the proposed method was successful, since it was possible to quantify the dye marker in commercial matrices without any preliminary treatment. The obtained results are summarized in Table 1. The obtained values are in good agreement with the spiked value, indicating that the proposed method is a good alternative for the analytical determination of this dye in the sample. The statistical treatment of the results is also summarized in Table 1. Thus, the values obtained for the recovery and %R.S.D. are acceptable and are in good agreement with the spiked value, which suggests that the proposed method is a good alternative for the analytical determination of this dye in fuel alcohol. The result was compared with a reference based on the spectrophotometric method (Table 1) following the procedure described in the experimental section (Section 2.6). A linear relationship for SB-14 dye was constructed from a concentration of 2.00 × 10−6 to 1.00 × 10−5 mol L−1 , monitoring maximum absorbance at 590 nm. Te equation: A = 0.049 + 18,850 × C(mol L −1 ) . A detection limit of 7.20 × 10−7 mol L−1 and limit of quantification of 2.40 × 10−6 mol L−1 were obtained. Mean recoveries for the alcohol and kerosene samples were found to be between 105.20 and 108.40%. The values obtained for the analyzed samples by the proposed method are in agreement with those obtained by spectrophotometry, as shown Table 1. These results are a good evidence of the accuracy of the proposed method demonstrating that it can be successfully employed for the reliable determination of marker dye in fuel ethanol without any pre-treatment. Hence, the comparison of the obtained results using the electroanalytical method with those obtained using the spectrophotometric method (Table 1) shows that the most satisfactory recoveries were achieved using the proposed method. The direct determination of SB-14 in the kerosene sample was not possible, due to the interference of hydrocarbons and other components, which suppress the voltammetric signal. Thus, a simple clean-up procedure was used. The best one consisted in a pre-treatment of the matrix with a solid-phase extraction (SPE) cartridge column (containing silica) as described in Section 2.5. The typical square-wave voltammograms obtained for SB-14 dye in the kerosene sample after the clean-up procedure as described in the experimental section (Section 2.5) and shown in Fig. 8 (line a), as
4. Conclusions The use of an anionic surfactant (DSS) leads to the solubility of the dye to improve in a mixture of water/DMF 70:30 (v/v) and propitiates an anti-fouling capacity of the screen-printed carbon electrode, which is able to take up to 10 measurements of SB14 dye using the same device. The method offers an alternative to improve the detectability of the dye directly in ethanol samples without any pre-treatment. Besides, the anionic surfactant and square-wave detection were fundamentally important in the accomplished studies. In addition, the following parameters were observed: (i) displacement of the peak potential to a less negative value decreasing the overall voltage of reduction of this dye, (ii) amplification of the peak current, increasing the analytical application, (iii) stability of the current for various measurements and (iv) improvement of the reversibility of the process, which also contributes to amplify the sensitivity of the SWV technique. The methodology proposed can be successfully applied to determine dye in kerosene samples using a fast and simple procedure after a simple step of sample pre-treatment. The technique offers lower detection limits when compared with the spectrophotometric method and it is less expensive than the chromatographic analysis. Acknowledgements The authors are grateful to the Fundac¸ão de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (contract no. 05/00382-4), CAPES and CNPq for their financial support. References [1] The hydrocarbon oil (marking) regulations 2002, Statutory Instrument 2002, n. 1773. http://www.opsi.gov.uk/SI/si2002/20021773.htm, [Access in August 2008]. [2] European refining & marketing, fuels refining & marketing in Europe and The Former Soviet Union. http://www.process-nmr.com/pdfs/ERM-V1-10-02.pdf, [Access in August 2008]. [3] A.V. Nowak, Analyzing marker dyes in liquid hydrocarbon fuels, US Patent 4918020, 1990. [4] M.R. Friswell, R.B. Orelup, Markers for petroleum, method of tagging, and method of detection, US Patent 5156653, 1992. [5] R.B. Orelup, Colored petroleum markers, US Patent 4735631, 1988. [6] Agência Nacional do Petróleo. http://www.anp.gov.br.[Access in August 2008]. [7] V.J. Barwick, S.L.R. Ellison, M.J.Q. Rafferty, R.S. Gill, The evaluation of measurement uncertainty from method validation studies. Part 2: the practical application of a laboratory protocol, Accred. Qual. Assur. 5 (2000) 104–113.
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Biographies Maria Valnice Boldrin Zanoni was born in Tupi Paulistacity, São Paulo, Brazil in 1957. She graduated from Institute of Chemistry of São Paulo University (USP) with Ph.D in Physical Chemistry, 1989. Her academic position started at University of São Paulo State (UNESP), Brazil as an associate professor in Analytical Chemistry since 1987. Her main research interests are electroanalysis and environmental electrochemistry, investigating analytical methodologies and new treatment methods for dye. Recently, she expanded her research interests to electrochemical sensors applied to dye and pharmaceutical compounds detection Magno Aparecido Gonc¸alves Trindade He Graduated (2002) in Chemistry and got his master degree in Chemistry (2005) from Federal University of Mato Grosso do Sul. Currently he is Ph.D. Student in Analytical Chemistry from University of São Paulo State, UNESP, Brazil. He has being working in chemistry, focusing on electrochemistry, acting on the following subjects: electroanalytical methods and HPLC methods for dyes
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Voltammetric sensing of the fuel dye marker Solvent Blue 14 by screen-printed electrodes Magno Aparecido Gonc¸alves Trindade, Maria Valnice Boldrin Zanoni ∗ São Paulo State University, UNESP, Department of Analytical Chemistry, Institute of Chemistry-Araraquara, UNESP, Rua Francisco Degni, s/n Bairro Quitandinha, 14800-900, Araraquara, SP, Brazil
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Article history: Received 14 November 2008 Received in revised form 19 January 2009 Accepted 22 January 2009 Available online 31 January 2009 Keywords: Fuel marker Solvent Blue 14 Screen-printed carbon electrode Voltammetric method Surfactant in electrochemistry
a b s t r a c t A voltammetric method was developed to detect Solvent Blue 14 (SB-14) dye marker in fuel ethanol and kerosene samples. The device is based on a screen-printed carbon electrode (SPCE) operating in a Britton–Robinson buffer with N,N-dimethylformamide (7:3, v/v) and 5.50 × 10−4 mol L−1 of Dioctyl sulfosuccinate sodium (DSS) where the anthraquinone group of the dye is reduced at −0.40 V. The peak intensity is triplicated in the presence of the surfactant, which forms a charged film on the electrode surface acting as an anti-fouling agent. Using the best experimental conditions, the electroanalytical method presents a linear response for 2.00 × 10−7 to 2.00 × 10−6 mol L−1 SB-14 dye (r = 0.9986) and a detection limit of 9.30 × 10−8 mol L−1 . The developed method was successfully utilized for the quantification of SB14 dye in alcohol samples without any pre-treatment and in kerosene (after rapid clean-up procedure) with recoveries of 82.00–99.00%, respectively. The values are in good accordance with other reference methods based on spectrophotometric analysis. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The dye 1,4-Bis(pentylamino)anthraquinone also called Solvent Blue 14, Fig. 1, is one of the anthraquinone dyes commonly used as visible markers purposely added to fuels as a method of quality control. In general, the amount of the dye added to the fuel depends on a diversity of factors. On the other hand, a minimum of 5.00 ppm is recommended in the finally tagged liquid petroleum fuel [1–6]. Therefore, sensitive and rapid analytical methods are required to determine its presence in petroleum products, since this dye can be used to distinguish the origin and quality of several types of fuels. The analytical methods described in literature for the determination of dyes in fuel samples are based on techniques that employ high-performance liquid chromatography (HPLC) equipment coupled to ultraviolet absorption and diode array detectors [7–10] and, some cases, derivative spectroscopy [11] is also used. In most cases, these methods require long analysis times and long sample pretreatment steps. Studies found in literature have shown that the electrochemical technique is highly recognized as a favorable tool in the quantification of several types of dye carrying the azo and the anthraquinone groups [12–16]. However, these studies involve only the classes of dyes that are soluble in aqueous medium, which cannot be generally applied to other classes of dyes, such as those used
∗ Corresponding author. Tel.: +55 16 3301 6619; fax: +55 16 3322 7932. E-mail address: [email protected] (M.V.B. Zanoni). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.01.043
as markers in fuels. Recently, we have been successfully studying the possibility of using electrochemical methodology to quantify quinizarin [17] and SB-14 [18] on glassy carbon electrodes in fuel samples. Although, as can be seen in other electrochemical studies involving the reduction of anthraquinone compounds [19–22], the electrode process is complicated due to strong adsorption of the dye and/or electrochemical products onto the electrode surface, and these promote gradual passivation of the electrode surface compromising the success of the electroanalytical method. Taking into consideration the need to propose a simpler procedure for the determination of dye in fuel samples, the present work investigated the possibility of using screen-printed carbon electrode devices as a way of miniaturizing the set of electrodes conventionally used in electrochemical procedures and to propose a simple voltammetric method to monitor dye markers. The use of screen-printed electrodes has seen great progress, since they are versatile and disposable for various electroanalytical applications [23–30]. The advantage of this type of electrode set has to do with its modest cost, portability, simplicity of operation, reliability, and the small instrument footprint of the arrangement containing the working electrode, auxiliary and reference electrodes, which are printed directed onto a polymeric/plastic foil. The use of surfactants has also been described in literature [31–40] as successful strategies for overcoming the adsorption interferences. It was recently shown [33,36,38,40] that surfactants could be used to stabilize voltammetric response by protecting the electrode surface from fouling and enhancing the electrochemical
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Fig. 1. Structure of the dye Solvent Blue 14 (SB-14).
signal. The satisfactory response depends on the concentration and nature of the surfactants, which can influence not only the shape of electrochemical waves but also the parameters such as potential, electron-transfer rate, diffusion and transfer coefficients, stability of the intermediate species and the minimization of adsorption effects. Therefore, the aim of the present work was to develop a simple method for SB-14 dye detection in fuel samples. The method combines the simplicity of a screen-printed carbon electrode with the use of Dioctyl sulfosuccinate sodium (DSS) as the anionic surfactant to develop a simple and rapid analytical method able to offer a fast and sensitive alternative for the analysis of SB-14 in samples of kerosene and fuel alcohol using the square-wave voltammetric technique. 2. Experimental 2.1. Instrumentation Electrochemical analysis was performed using an Autolab PGSTAT-30 (Eco Chemie) connected to a microcomputer controlled by General Purpose Electrochemical System (GPES 4.9) software for data acquisition and experimental control. The measurements were performed on a conventional electrochemical cell (maximum capacity, 10.0 mL) specially adapted to insert the screen-printed carbon electrode (Oxley Developments UK). The design of the screen-printed carbon electrode (SPCE) used in all the electrochemical experiments consists of an alumina ceramic base 45.00 mm long, 10.00 mm wide and 0.80 mm thick, with the working (W), reference (R) and the auxiliary (A) electrodes exposed on the surface (Fig. 2). All the electrodes are made of conducting carbon ink. A contact at the end is connected with the active part of each electrode by internal conducting carbon parts (C) covered by a dielectric protection layer. The SPCE was connected via a cable to the potentiostat and used without any pre-treatment. The Ag/AgCl(KCl 3 mol L −1 )
(inside a Luggin capillary containing 3.00 mol L−1 KCl) was used as the reference electrode substituting the SPCE in order to minimize problems of adsorption and contamination of the surface. The spectrophotometric measurements were carried out using an HP spectrophotometer (model 8453) operating in the range of 220–700 nm with a 1.00-cm-path-length quartz cell. All pH measurements were made using a combined glass electrode (Orion, Thermo Electron Corporation) connected to the digital pHmeter (Orion, Thermo Electron Corporation) and are expressed as pHapparent (pH*). The deionized water used to prepare the supporting electrolyte solutions was purified with a Milli-Q system and had a resistivity of 18 M cm−1 (model Simplicity 185, Millipore). 2.2. Reagents and solutions All reagents were of the highest grade available and used without further purification. Stock solutions of SB-14 (obtained by Acros organics) (1.00 × 10−3 and 1.00 × 10−2 mol L−1 ) were prepared by dissolving the solid product in acetonitrile (J.T. Baker). Diluted working standard solutions were then prepared daily with acetonitrile just prior to use. The working solutions had shown sufficient stability over storage times of up to three months (the time of the evaluation). Britton–Robinson (B–R) (0.08 mol L−1 ) buffer having a pH between 2.0 and 9.0 was used as the supporting electrolyte solution and was prepared in the usual way: a mixture of 0.08 mol L−1 acetic acid (Merck), 0.08 mol L−1 boric acid (Merck) and 0.08 mol L−1 orthophosphoric acid (Merck) with the appropriate amount of 1.0 mol L−1 sodium hydroxide (Merck) solution to adjust the pH to the required value. The N,N-dimethylformamide (DMF) (Mallinckrodt) used as supporting electrolyte solution was mixed with B–R and was of analytical grade. The surfactant, Dioctyl sulfosuccinate sodium (DSS) 5.50 × 10−2 mol L−1 from (Sigma–Aldrich), was prepared from stock solution in deionized water. Samples of unmarked fuel (kerosene and alcohol) were spiked with 2.00 mg L−1 of SB-14 dye using an appropriate volume of the stock solution. Before the SB-14 dye was added, the fuel was certified that there was no evidence of this dye. 2.3. Procedure for voltammetric analysis An aliquot of 5.00 mL of the supporting electrolyte solution was added to the electrochemical cell (specially adapted to insert the screen-printed carbon electrode) and then deaerated with pure nitrogen for approximately 15 min to eliminate dissolved oxygen. Afterwards, 100 L of the surfactant solution (5.50 × 10−2 mol L−1 ) was added to the cell under stirring for 2 min. The stirring was stopped and after a 15-second rest, cyclic or square-wave voltammograms were registered. After the background voltammogram had been recorded, the aliquot of the dye solution was added to the cell and the voltammogram was then recorded for the same screenprinted carbon electrode (SPCE). Although, the SPCE is described as a device with disposable properties, it was possible to use the same SPCE repeatedly for at least 10 measurements. Therefore, before each series of measurements, the SPCE was left to rest for approximately 35 s, which was long enough to re-establish the surface. 2.4. Analysis of SB-14 dye in an alcohol sample
Fig. 2. Screen-printed carbon electrode on ceramic substrate used in the voltammetric measurements. (C) Electrical connections, (W) working, (R) reference and (A) auxiliary electrodes.
The quantification of SB-14 dye in an alcohol sample was performed by mixing an aliquot containing 0.35 mL of the commercial fuel ethanol sample (collected from a gas station in Araraquara city, SP, Brazil) spiked to 2.00 mg L−1 of dye in a volumetric flask of 5.00 mL containing the supporting electrolyte solution (B–R buffer
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pH 3.0 + DMF, 7:3, v/v) and 1.40 × 10−3 mol L−1 of the DSS surfactant. The resulting mixture was transferred to the electrochemical cell, submitted to deaeration with nitrogen for 15 min. It was then directly analyzed without any previous treatment by recording the square-wave voltammetry data. Then, the SB-14 dye in the matrix was quantified using the standard addition method and the quantitative results obtained were expressed as the total content of the dye in the sample. 2.5. Analysis of SB-14 dye in kerosene samples 0.50 mL aliquot of the aviation kerosene sample (collected in Araraquara city, SP, Brazil) spiked to 2.00 mg L−1 of SB-14 dye was transferred to a solid-phase extraction cartridge containing 0.50 g of silica previously conditioned with approximately 6.00 mL of hexane. Using a syringe attached to the top of the cartridge, the kerosene sample was slowly forced thorough the cartridge (flow rate approximately 2.00 mL min−1 ). After this, the cartridge was washed with approximately 10.00 mL of the hexane to remove all hydrocarbons and contaminants from the matrix. Then the dye was eluted from the cartridge using approximately 4.50 mL of the Britton–Robinson (B–R) buffer (pH required) and DMF mixture in the ratio of 7:3 (v/v). The eluent was collected in a 5.00 mL volumetric flask and after the addition of the DSS surfactant (125.0 L of 5.50 × 10−2 mol L−1 solution), the volume was completed with the mixture B–R:DMF 7:3 (v/v). The resulting solution was submitted to the square-wave voltammetric analysis using the best conditions previously optimized. The quantification of the SB-14 dye was performed using the standard addition method as previously described in Section 2.4. 2.6. Analysis of SB-14 dye in fuel sample by UV–vis spectrophotometry An analytical curve for SB-14 dye using spectrophotometry was plotted by registering the spectrum for aliquots of 20.00 to 120.00 L of the SB-14 dye stock solution (1.00 × 10−3 mol L−1 ) previously prepared by dilution in acetonitrile. The absorbance for standard solution was measured following the maximum signal at 590 and 640 nm after certifying that there was no peak in the blank (acetonitrile) measured. The quantification of SB-14 dye in commercial ethanol fuel was performed after diluting 4.00 mL of the sample spiked to 5.00 mg L−1 of SB-14 dye in a 10.00 mL calibrated flask with acetonitrile. The resulting mixture was submitted to spectrophotometric analysis recording the UV–vis spectrum. The quantification of the SB-14 dye in the fuel alcohol samples was performed using the external standard method. The quantification of the SB-14 dye in kerosene samples was performed by transferring 4.00 mL of the sample spiked to 5.00 mg L−1 of SB-14 dye to a solid-phase extraction cartridge containing 1.00 g of silica previously conditioned with approximately 6.00 mL of hexane. After washing with 10.00 mL of hexane to remove all hydrocarbons and contaminants from the matrix, the dye (10.00 mL) was eluted with acetonitrile and submitted to spectrophotometric analysis following the above procedure. 3. Results and discussion 3.1. Electrochemical behavior of SB-14 on the screen-printed carbon electrode (SPCE) The stability and solubility of the SB-14 dye solution is improved in micellar medium containing Dioctyl sulfosuccinate sodium (DSS). Although other surfactants were tested, the best voltammetric response was obtained for the reduction of 1.00 × 10−5 mol L−1
Fig. 3. Cyclic voltammograms for the SPCE indicating the electrochemical reduction of SB-14 dye in 0.08 mol L−1 B–R buffer (pH 2.50) containing 30% DMF for (curve A) the absence of SB-14 and (curve B) for 1.00 × 10−5 mol L−1 SB-14 in the presence of 5.50 × 10−4 mol L−1 DSS. Scan rate of 100 mV s−1 .
SB-14 dye in a mixture of B–R buffer (pH 2.5) and DMF (7:3, v/v) + 5.50 × 10−4 mol L−1 DSS on the screen-printed carbon electrode (SPCE). Fig. 3 shows a typical cyclic voltammogram for SB-14 in the presence of 5.50 × 10−4 mol L−1 DSS (curve B), where a cathodic peak at −0.49 V vs. (pseudo reference from screen-printed carbon electrode) is observed. This peak was attributed to the reduction of the central quinone group to hydroquinone derived after a two-electron process [19–22]. For the reverse scan, it is possible to detect the occurrence of two waves with lower intensities due to hydroquinone and semiquinone oxidation as the main products previously generated [19–22], indicating that the reductive mechanism is not modified in this experimental condition. Cyclic voltammograms were also compared under the same experimental conditions, but changing the pseudo reference electrode of the screen-printed carbon electrode set for an external reference electrode of Ag/AgCl(KCl 3.0 mol L −1 ) . Except for a potential shifting of 200 mV to a less negative potential in relation to the SPCE set, there was no other modification in the wave form, whose current intensity and position were constant in all the voltammograms recorded for each new electrode and thus, could consequently be used for analytical purposes. Successive cyclic voltammograms obtained for 5.00 × 10−5 mol L−1 SB-14 in 0.08 mol L−1 B–R buffer (pH 2.4) containing 30% DMF at a scan rate of 100 mV s−1 exhibit a decrease in the cathodic peak current after the first scan, suggesting that the product of electrode reaction is adsorbed onto the SPCE surface [21,43]. However, when the study is repeated in the presence of an anionic surfactant, DSS, the voltammogram can be re-established (10 repetition) after an approximate 35 s rest before each series of measurements. This procedure indicates that the surfactant presents an anti-fouling capacity under this experimental condition, which improves the feasibility of the SPCE to monitor SB-14 dye without adsorption complications. In order to improve the sensitivity, the effect of the DSS addition on the peak current intensity was monitored by recording square-wave voltammograms (SWV) instead of linear or cyclic voltammograms. Characteristic SWV curves of SB-14 dye reduction in 0.08 mol L−1 B–R buffer (pH 2.4)/DMF (7:3) in the absence and presence of DSS are compared in Fig. 4 (curves A and B). An intense peak is observed at around −0.40 V, whose height is almost three times higher than that given without surfactant. This result
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Fig. 4. Square-wave voltammograms for the SPCE indicating the reduction of 5.00 × 10−6 mol L−1 SB-14 dye in 0.08 mol L−1 B–R buffer (pH 2.40) containing 30% DMF for (curve A) the absence of DSS and (curve B) the presence of 8.00 × 10−4 mol L−1 DSS. Conditions: frequency (f) = 60 Hz, step potential (Es ) = 4 mV and pulse amplitude (Esw ) = 50 mV. Inset: influence of DSS concentration on square-wave voltammetric response under the conditions presented above.
indicates that the addition of surfactant could offer an analytical potential for the determination of SB-14 dye in fuels, improving the detectability and solubility of the dye sample in a mixture of water/DMF. The influence of the surfactant on the peak current intensity was investigated monitoring the reduction of 5.00 × 10−6 mol L−1 SB-14 dye in DSS changing the concentration from 2.00 × 10−4 to 2.70 × 10−3 mol L−1 . The effects can be visualized in Fig. 4 (inset). The peak current increases up to 1.00 × 10−3 mol L−1 DSS, but reaches a plateau above this value. The extraordinary increase in the peak current on adding the DSS can be attributed to three effects: they are (i) the anionic surfactant could be adsorbing onto the electrode surface forming an oriented film of negative charges. Since the anthraquinone group of SB-14 dye is usually in the protonated form in an acidic condition, it could be pre-concentrated due to electrostatic interaction; (ii) the reduction of SB-14 dye molecules could be occurring via vertically oriented molecules inside the DSS film, which could alter its electron transfer rate [31–40]; or (iii) the DSS could solubilize the generated products avoiding the passivation of the electrode. Above 1.00 × 10−3 mol L−1 of DSS, the system reaches the critical micellar concentration (CMC) and there is no oriented film on the electrode surface, since it is formed only at lower concentrations. Thus, films of DSS could be used to strategically improve the detectability of dyes bearing anthraquinone groups.
potential shifts to more negative potential when the pH increases, and the linear relationship presented a slope of −67.30 mV per pH unit, indicating that the electrode process is influenced by preprotonation reactions [21] of the anthraquinone group [19–22]. These results are consistent with a mechanism involving two electrons and two protons, similarly verified by other previous studies conducted for anthraquinone reduction and indicating that surfactant films do not interfere markedly in the electrodic mechanism. Nevertheless, maximum peak current intensity is obtained at pH <3.0, which decreases continuously when the pH increases. Taking into consideration that probably quinone groups are present in a protonated form under very acidic conditions [18], there is a preponderant electrostatic interaction occurring between the anionic surfactant (DSS) and the SB-14 dye at pH ≤3.0. Therefore, the voltammetric signal is enhanced preferentially due to electrostatic pre-concentration of the protonated dye on films of anionic surfactant spontaneously organized on the electrode surface. The apparent pH around 2.50 was selected as optimum for SB-14 detection, which offered an improved interaction between the dye and the anionic surfactant, leading to the enhancing of the analytical applicability. A typical reduction of SB-14 on the screen-printed carbon electrode (SPCE) is presented in Fig. 6, using a frequency of 60 Hz, where contributions of forward current (Fig. 6, curve a), backward current (Fig. 6, curve b) and resultant current (Fig. 6, curve c) are discriminated. This type of response indicates that the SB-14 dye is reduced following a reversible electron transfer process [41,42] when DSS is added to the solution and the adsorption effects of the dye molecule are suppressed. The influence of different frequencies on the peak potential and peak current was studied from 10 to 250 Hz. The effect of frequency on the ratio of forward and backward currents (Ibackward /Iforward ) taken from the square-wave voltammograms obtained for SB-14 is shown in Fig. 6 (inset). The ratio (Ibackward /Iforward ) increases from 10 to 60 Hz. Above this value the ratio reaches a constant value close to 1.0, clearly indicating that the system is controlled by a simple reversible process [41,42]. These results are confirmed by the linear relationship between peak currents and the square root of the frequency observed for frequency values between 10 and 150 Hz. The
3.2. Effect of pH The effect of pH on the reduction of SB-14 (5.00 × 10−6 mol L−1 + 1.00 mmol L−1 of DSS) was investigated on the screen-printed electrode surface for electrolyte (3:7, v/v) DMF/B–R buffer varying pH values between 2.0 and 9.0. The square-wave voltammograms obtained for acidic and neutral medium are shown in Fig. 5 (curves a–d). Studies at pH values higher than 9.0 were not conducted because the reduction potential of SB-14 occurs quite close to the electrolyte/electrode discharge on the SPCE and the peak current could not be measured accurately. Fig. 5 (inset) illustrates the effect of pH variation on the peak current intensity (curve A) and peak potential (curve B). The peak
Fig. 5. Square-wave voltammograms registered for the SPCE indicating the electrochemical reduction of 5.00 × 10−6 mol L−1 SB-14 dye in 0.08 mol L−1 B–R buffer containing 30% DMF and the presence of 1.00 × 10−3 mol L−1 DSS at: (a) pH 2.0, (b) pH 3.0, (c) pH 6.0 and (d) pH 7.0. Inset: influence of pH on the peak potential (A) and peak current (B) response for the pH range between 2.0 and 9.0. Parameters: f = 60 Hz, Es = 2 mV and Esw = 25 mV.
M.A.G. Trindade, M.V.B. Zanoni / Sensors and Actuators B 138 (2009) 257–263
Fig. 6. Square-wave voltammogram registered in 0.08 mol L−1 B–R buffer (pH 2.50) with 30% DMF and presence of DSS surfactant (1.40 × 10−3 mol L−1 ) for the reduction of 5.00 × 10−6 mol L−1 SB-14. (a) Forward current, (b) backward current and (c) resultant current. Parameters: f = 60 Hz, Es = 2 mV and Esw = 25 mV. Inset: variation of the ratio (Ibackward /Iforward ) versus frequency. Conditions as ascribed above.
electrons transferred during the electrodic process were also evaluated from the peak potential relationship vs. frequency, as shown the equation [41,42]: Ep −2.303RT = 2nF logf where: R is the gas constant, T the temperature, n the number of electrons, and F is the Faraday constant. The values of Ep versus logf1/2 obtained from the shifting of the peak potential in relation to the frequency shows a slope of the 36.00 mV/decade. From the above equation n = 0.82 is obtained, indicating that the SB-14 reduction in micellar medium involves the transfer of one electron.
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Fig. 7. Square-wave voltammograms registered in 0.08 mol L−1 B–R buffer (pH 3.0) with 30% DMF and 1.40 × 10−3 mol L−1 DSS surfactant for an increasing concentration of SB-14 (2.00 × 10−7 to 4.0 × 10−6 mol L−1 ). Inset: analytical calibration function for the dependence of the peak current on the SB-14 concentration. Parameters: f = 60 Hz, Es = 4 mV and Esw = 50 mV.
4.00 × 10−6 mol L−1 SB-14 using the same screen-printed carbon electrode and an intermediate step of 35 s rest before each measurement. The peak current increased linearly from 2.00 × 10−7 to 2.00 × 10−6 mol L−1 , as shown in Fig. 7 (inset), following the equations Ipc (A) = −0.20 + 2.90 × 106 × C (mol L−1 ) with r = 0.9986. The limits of detection (LOD) and quantification (LOQ) were computed, respectively, as 3 × Sd/m and 10 × Sd/m, where Sd is the standard deviation of the peak current for the blank (measured at the same potential as the dye) and m the slope of the analytical curve [44]. The values obtained were 9.30 × 10−8 and 3.0 × 10−7 mol L−1 for LOD and LOQ, respectively. 3.4. Analytical application
3.3. Analytical curve Using the optimal conditions, f = 60 Hz, Es = 4 mV and Esw = 50 mV (similarly investigated in previous work [18]) and pH 3.0, the method was applied to detect SB-14 in different concentrations of the dye marker in a mixture of B–R buffer (pH 3.0)/DMF (7:3, v/v) + 1.40 × 10−3 mol L−1 DSS surfactant. Fig. 7 illustrates the respective voltammograms recorded from 2.00 × 10−7 to
Using the best experimental conditions, the voltammetric method was applied to the determination of SB-14 dye in commercial alcohol and kerosene samples using the standard addition method. Firstly, samples of the collected ethanol fuels do not present any signal of SB-14 dye as marker in commercial matrices. Then, the samples were spiked to 2.00 mg L−1 of the dye as previously mentioned in the experimental section (Section 2.4) and
Table 1 Results of recovery assay to accuracy and precision for added SB-14 dye in kerosene and alcohol samples using proposed method and reference method. Analysis method Electroanalytical method
Samples a
Alcohol
Kerosenea
Spectrophotometric method
a b
( = 590 nm) Kerosenea Alcohola
Added (mg L−1 )
Found (mg L−1 )
Recovery (%)
2.00 2.00 2.00 2.00 Average 2.00 2.00 2.00 2.00 Average
1.93 1.91 1.90 1.97 1.92 1.86 1.93 1.98 1.65 1.85
96.50 95.50 93.00 98.50 95.90 93.00 96.50 99.00 82.00 92.50
5.00 5.00
5.42 5.26
108.40 105.20
Average of four determination and five measurements for each determination; R.S.D.: relative standard deviation. Confidence level of 95% (tcritical = 3.18).
R.S.D. (%)
tb
2.40
3.10
8.30
1.95
3.15 3.10
4.20 2.60
262
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well its quantification after the standard addition method is shown in Fig. 8 (lines b–e). From this Figure, it is possible to suggest that all hydrocarbons and other interferents present in fuel samples were effectively removed in the clean-up stage and the analytical signal presents a well-defined peak for the dye used. The recoveries of SB-14 in these kerosene samples, based on the average of four determinations, are listed in Table 1 and were found to be between 82.00 and 99.00%. The accuracy of the SB-14 assay was determined from labeled data (Table 1), which was expressed as %R.S.D. The statistic values obtained are acceptable and suggest that the proposed procedure could be successfully applied to the quantification of SB-14 dye in samples of kerosene fuels. The results were also compared by applying Student’s t [44], whose theoretical value of t for a 95% confidence level, where the experimental value (t calculated) did not exceed the theoretical value (critical value 3.182) for the electroanalytical method, as can be seen in Table 1, confirming no significant difference between the added value (spiked) and the value found. Therefore, the statistical calculations for the assay suggested good precision and accuracy of the proposed method for both commercial samples. Fig. 8. Square-wave voltammograms obtained for the determination of SB-14. (a) Kerosene sample (containing 0.20 mg L−1 SB-14) after extraction procedure as mentioned in Section 2.4, with successive additions of SB-14 (37.80 mg L−1 ) (b) 20 L and (c–e) 10 L. Conditions as described in Fig. 7.
submitted to direct square-wave voltammetric analysis. Application of the proposed method was successful, since it was possible to quantify the dye marker in commercial matrices without any preliminary treatment. The obtained results are summarized in Table 1. The obtained values are in good agreement with the spiked value, indicating that the proposed method is a good alternative for the analytical determination of this dye in the sample. The statistical treatment of the results is also summarized in Table 1. Thus, the values obtained for the recovery and %R.S.D. are acceptable and are in good agreement with the spiked value, which suggests that the proposed method is a good alternative for the analytical determination of this dye in fuel alcohol. The result was compared with a reference based on the spectrophotometric method (Table 1) following the procedure described in the experimental section (Section 2.6). A linear relationship for SB-14 dye was constructed from a concentration of 2.00 × 10−6 to 1.00 × 10−5 mol L−1 , monitoring maximum absorbance at 590 nm. Te equation: A = 0.049 + 18,850 × C(mol L −1 ) . A detection limit of 7.20 × 10−7 mol L−1 and limit of quantification of 2.40 × 10−6 mol L−1 were obtained. Mean recoveries for the alcohol and kerosene samples were found to be between 105.20 and 108.40%. The values obtained for the analyzed samples by the proposed method are in agreement with those obtained by spectrophotometry, as shown Table 1. These results are a good evidence of the accuracy of the proposed method demonstrating that it can be successfully employed for the reliable determination of marker dye in fuel ethanol without any pre-treatment. Hence, the comparison of the obtained results using the electroanalytical method with those obtained using the spectrophotometric method (Table 1) shows that the most satisfactory recoveries were achieved using the proposed method. The direct determination of SB-14 in the kerosene sample was not possible, due to the interference of hydrocarbons and other components, which suppress the voltammetric signal. Thus, a simple clean-up procedure was used. The best one consisted in a pre-treatment of the matrix with a solid-phase extraction (SPE) cartridge column (containing silica) as described in Section 2.5. The typical square-wave voltammograms obtained for SB-14 dye in the kerosene sample after the clean-up procedure as described in the experimental section (Section 2.5) and shown in Fig. 8 (line a), as
4. Conclusions The use of an anionic surfactant (DSS) leads to the solubility of the dye to improve in a mixture of water/DMF 70:30 (v/v) and propitiates an anti-fouling capacity of the screen-printed carbon electrode, which is able to take up to 10 measurements of SB14 dye using the same device. The method offers an alternative to improve the detectability of the dye directly in ethanol samples without any pre-treatment. Besides, the anionic surfactant and square-wave detection were fundamentally important in the accomplished studies. In addition, the following parameters were observed: (i) displacement of the peak potential to a less negative value decreasing the overall voltage of reduction of this dye, (ii) amplification of the peak current, increasing the analytical application, (iii) stability of the current for various measurements and (iv) improvement of the reversibility of the process, which also contributes to amplify the sensitivity of the SWV technique. The methodology proposed can be successfully applied to determine dye in kerosene samples using a fast and simple procedure after a simple step of sample pre-treatment. The technique offers lower detection limits when compared with the spectrophotometric method and it is less expensive than the chromatographic analysis. Acknowledgements The authors are grateful to the Fundac¸ão de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (contract no. 05/00382-4), CAPES and CNPq for their financial support. References [1] The hydrocarbon oil (marking) regulations 2002, Statutory Instrument 2002, n. 1773. http://www.opsi.gov.uk/SI/si2002/20021773.htm, [Access in August 2008]. [2] European refining & marketing, fuels refining & marketing in Europe and The Former Soviet Union. http://www.process-nmr.com/pdfs/ERM-V1-10-02.pdf, [Access in August 2008]. [3] A.V. Nowak, Analyzing marker dyes in liquid hydrocarbon fuels, US Patent 4918020, 1990. [4] M.R. Friswell, R.B. Orelup, Markers for petroleum, method of tagging, and method of detection, US Patent 5156653, 1992. [5] R.B. Orelup, Colored petroleum markers, US Patent 4735631, 1988. [6] Agência Nacional do Petróleo. http://www.anp.gov.br.[Access in August 2008]. [7] V.J. Barwick, S.L.R. Ellison, M.J.Q. Rafferty, R.S. Gill, The evaluation of measurement uncertainty from method validation studies. Part 2: the practical application of a laboratory protocol, Accred. Qual. Assur. 5 (2000) 104–113.
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Biographies Maria Valnice Boldrin Zanoni was born in Tupi Paulistacity, São Paulo, Brazil in 1957. She graduated from Institute of Chemistry of São Paulo University (USP) with Ph.D in Physical Chemistry, 1989. Her academic position started at University of São Paulo State (UNESP), Brazil as an associate professor in Analytical Chemistry since 1987. Her main research interests are electroanalysis and environmental electrochemistry, investigating analytical methodologies and new treatment methods for dye. Recently, she expanded her research interests to electrochemical sensors applied to dye and pharmaceutical compounds detection Magno Aparecido Gonc¸alves Trindade He Graduated (2002) in Chemistry and got his master degree in Chemistry (2005) from Federal University of Mato Grosso do Sul. Currently he is Ph.D. Student in Analytical Chemistry from University of São Paulo State, UNESP, Brazil. He has being working in chemistry, focusing on electrochemistry, acting on the following subjects: electroanalytical methods and HPLC methods for dyes