Article pubs.acs.org/IECR
Electrochemical Behavior of 4‑Aminophenazone Drug at a Graphite Pencil Electrode and Its Application in Real Samples Jayant I. Gowda and Sharanappa T. Nandibewoor* P.G. Department of Studies in Chemistry, Karnatak University, Dharwad-580 003, India ABSTRACT: An analytical method for the determination of 4-aminophenazone using cyclic and differential-pulse voltammetry has been developed. The oxidation process was shown to be irreversible over the pH range 3.0−11.2. The dependence of the current on pH, concentration, and scan rate was investigated to optimize the experimental conditions for the determination of 4aminophenazone. The number of electrons transferred in the oxidation process was calculated, and a probable oxidation mechanism was proposed. In the range of (1.0 × 10−6)−(1.6 × 10−5) M, the current measured by differential-pulse voltammetry as a function of the concentration of 4-aminophenazone with a detection limit of 0.458 × 10−7 M with good selectivity and sensitivity exhibited a good linear relationship. The proposed method was applied to the determination of 4-aminophenazone in a real sample.
1. INTRODUCTION The development of a simple, sensitive, rapid, and reliable method for the determination of drugs is of great importance. The graphite pencil electrode (GPE) has been successfully used as a biosensor in the modern electroanalytical field because of its high electrochemical reactivity, good mechanical rigidity, low cost, low technology, ease of modification and renewal, and low background current.1,2 The GPE has good applications in the analysis of neurotransmitters and the detection of traces of metal ions and drugs. 4-Aminophenazone (4-AP, also known as 4-aminoantipyrene and Ampyrone), with the chemical structure in Scheme 1, is scarcely administered as an analgesic, anti-
developed an electroanalytical method for its determination. This method has the advantages of fast response, easy repair, renewal of 4-AP, good reproducibility, and low detection limit. The proposed method was applied to the determination of 4AP in real samples.
2. EXPERIMENTAL SECTION 2.1. Reagents and Chemicals. Pencil-lead rods (HB, 0.5 mm in diameter and 6 cm in length) were purchased from a local bookstore. 4-AP was purchased from Sigma-Aldrich and used without further purification. A stock solution of 4-AP (1.0 mM) was prepared in Millipore water. Phosphate buffers from pH 3.0 to 11.2 were prepared according to the method of Christian and Purdy.12 Other reagents used were of analytical or chemical grade. All solutions were prepared with Millipore water. 2.2. Instrumentation and Analytical Procedures. Electrochemical measurements were carried out on a CHI 630D electrochemical analyzer (CH Instruments Inc., Austin, TX). Voltammetric measurements were carried out in a 10 mL single-compartment three-electrode glass cell with Ag/AgCl as the reference electrode, a platinum wire as the counter electrode, and a graphite pencil electrode as the working electrode. All potentials are given against Ag/AgCl (3.0 M KCl). pH measurements were performed with an Elico LI 120 pH meter (Elico Ltd., Hyderabad, India). All experiments were carried out at an ambient temperature of 25 ± 0.1 °C. The parameters for differential-pulse voltammetry (DPV) were as follows: initial potential, 0.0 V; final potential, 0.8 V; increase in potential, 0.004 V; amplitude, 0.05 V; frequency, 15 Hz; quiet time, 2 s ; sensitivity, 1.0 × 10−6 A/V. 2.3. Plasma Sample Preparation. Human blood samples were collected in dry and evacuated tubes (which contained
Scheme 1. Chemical Structure of 4-Aminophenazone
inflammatory, and antipyretic drug because of its potential side effects. 4-AP residues in the environment pose a potential threat to human health. 4-AP stimulates liver microsomes and is also used to measure extracellular water. In view of health hazards due to the presence of 4-AP, its determination is important. Because of the risk of agranulocytosis, its use as a drug is discouraged. Different methods have been reported for the determination of 4-AP, including liquid and gas chromatography, spectrophotometry,3−5 liquid chromatography/mass spectrometry,6,7 solid-phase spectrophotometry, 8 and different high-performance liquid chromatography methods.9−11 The main problems encountered in using such methods are the time-consuming extraction and separation procedures. The aim of the present study was to develop a simple, lowcost, direct-current voltammetric method for the determination of 4-AP. In the present work, we carried out the electrochemical oxidation of 4-AP at a GPE. We optimized all of the experimental parameters for the determination of 4-AP and © 2012 American Chemical Society
Received: Revised: Accepted: Published: 15936
September 15, 2012 November 7, 2012 November 23, 2012 November 23, 2012 dx.doi.org/10.1021/ie302501f | Ind. Eng. Chem. Res. 2012, 51, 15936−15941
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saline and sodium citrate solution) from a healthy volunteer. The samples were handled at room temperature and were centrifuged for 10 min at 1500 rpm for the separation of plasma within 1 h of collection. The samples were then transferred to polypropylene tubes and stored at 20 °C until analysis. The plasma samples, 0.2 mL, were deprotonized with 2 mL of methanol. After being vortexed for 15 min, the mixture was then centrifuged for 15 min at 6000 rpm, and the supernatants were collected. The supernatants were spiked with known amounts of 4-AP. Appropriate volumes of this solution were added to phosphate buffer (pH 3.0) as the supporting electrolyte, and voltammograms were then recorded.
3. RESULTS AND DISCUSSION 3.1. Electro-Oxidation of 4-Aminophenazone. The oxidation of 4-AP at a GPE was studied by cyclic voltammetry (CV) in 0.2 M phosphate as the supporting electrolyte at pH 3.0. The cyclic voltammogram obtained for 1 mM 4-AP solution at a scan rate v = 50 mV s−1 (Figure 1) shows one
Figure 2. Variation of the cyclic voltammetric anodic peak current with accumulation time.
Therefore, an optimal accumulation time of 120 s was employed in further experiments. With the change in accumulation potential, the peak current of 4-AP varied slightly. Therefore, the accumulation potential has practically no effect on the peak current of 4-AP. 3.3. Effect of Scan Rate. The effect of scan rate on the electro-oxidation of 4-AP was examined by cyclic voltammetry (Figure 3A). The influence of the square root of the scan rate on the peak current showed a linear relationship between 0.025 and 0.25 V s−1, as shown in Figure 3B. This behavior is typical of diffusion-controlled currents13 and can be expressed as Ipa (μA) = 5.123v1/2 (V1/2 s−1/2) + 11.127 (r = 0.9890)
(1)
A linear relationship was observed between log Ipa and log v (Figure 3C), corresponding to the equation log Ipa (μA) = 0.411 log v (V s−1) + 1.1707 (r = 0.9929)
(2)
The slope value of 0.41 was comparable with the theoretically expected value of 0.5 for a purely diffusion-controlled current,13 which, in turn, confirms that the electro-oxidation of 4-AP was diffusion-controlled in our experiments. With an increase in scan rate, the peak potential shifted to a positive value, and a linear relationship was observed in the range of 0.025−0.25 V s−1, as shown in Figure 3D. The relationship can be expressed as
Figure 1. Cyclic voltammogram obtained for 1 mM 4-AP on GPE in 0.2 M phosphate buffer (pH 3.0) (a) 4-aminophenazone and (b) blank at v = 50 mV s−1. Inset: Successive cyclic voltammograms obtained for 1 mM 4-minophenazone on GPE: (1) first, (2) second, (3) third, (4) fourth, and (5) fifth scans at v = 50 mV s−1.
Ep (V) = 0.0264 log v (V s−1) + 0.5457
anodic peak that occurs at Ep = +0.467 V. On scanning in the negative direction, no reduction peak was observed, showing that the oxidation of 4-AP is an irreversible process. A decrease of the oxidation current occurs with the number of successive scans and might be due to the adsorption of 4-AP or its oxidation products on the GPE surface (Figure 1 inset). 3.2. Effect of Accumulation Conditions. The two parameters of the accumulation step, namely, accumulation time and potential, were examined. Open-circuit accumulation is widely used in electroanalytical chemistry to accumulate analyte and improve the sensitivity. The influence of accumulation time ranging from 0 to 250 s on the oxidation of 4-AP at a GPE is shown in Figure 2. The current increased gradually as the accumulation time increased from 0 to 120 s. However, with a further increase in the accumulation time beyond 120 s, the peak current tended to be almost stable.
(r = 0.9860) (3)
As for an irreversible electrode process, according to Laviron,14 Ep is defined as Ep = E 0 ′ +
⎛ 2.303RT ⎞ ⎛ RTk0 ⎞ ⎛ 2.303RT ⎞ ⎜ ⎟ log⎜ ⎟ log ν ⎟+⎜ ⎝ αnF ⎠ ⎝ αnF ⎠ ⎝ αnF ⎠ (4)
where α is the transfer coefficient, k0 is the standard heterogeneous rate constant of the reaction, n is the number of electrons transferred, v is the scan rate, and E0′ is the formal standard redox potential. Other symbols have their usual meanings. Thus, the value of αn can be easily calculated from the slope of a plot of Ep versus log v. In this system, the slope was 0.0997; taking T = 298 K, R = 8.314 J K−1 mol−1, and F = 15937
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where Ep/2 is the potential when the current is at one-half the peak value. From this expression, the value of α was calculated to be 0.917. Further, the number of electrons (n) transferred in the electro-oxidation of 4-AP was calculated to be 2.44−2. The value of k0 can be determined from the intercept of the previous plot, if the value of E0′ is known. The value of E0′ in eq 4 can be obtained from the intercept of the Ep versus v curve by extrapolating to the vertical axis at v = 0.16 In our system, the intercept for Ep versus log v plot was 0.5457, so E0′ was obtained as 0.5058, and k0 was calculated to be 1.71 × 102 s−1. 3.4. Effect of pH. An electrode reaction might be affected by the pH of the medium. The electro-oxidation of 1 mM 4-AP was studied over the pH range of 3.0−11.2 in phosphate buffer solution by cyclic voltammetry, as shown in Figure 4A. The solution pH influenced the peak current considerably. The pH dependence of the peak potential and peak current obtained
Figure 3. (A) Cyclic voltammograms of 1 mM 4-AP at different scan rates (i−ix: 25, 50, 75, 100, 125, 150, 175, 200, and 225 mV s−1, respectively) in 0.2 M phosphate buffer (pH 3.0). (B) Dependence of the peak current on the square root of the scan rate. (C) Dependence of the logarithm of the peak current on the logarithm of the scan rate. (D) Relationship between the peak potential and the logarithm of the scan rate. Figure 4. (A) Cyclic voltammograms of 1 mM 4-AP at different pH values: (1) 3.0, (2) 4.2, (3) 5.0, (4) 6.0, (5) 7.0, (6) 8.0, (7) 9.2, (8) 10.4, and (9) 11.2. (B) Influence of pH on the potential of 1 mM 4-AP on GPE at a scan rate of 50 mV s−1 in phosphate buffer. (C) Variation of current with pH of 1 mM 4-AP on GPE at a scan rate of 50 mV s−1 in phosphate buffer.
96480 C mol−1, the αn value was calculated to be 2.24. According to Bard and Faulkner,15 α can be expressed as 47.7 α= mV Ep − Ep/2 (5) 15938
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when cyclic voltammetry was used is shown in Figures 4B,C. With an increase in pH of the solution, peak potential shifted to less positive value (Figure 4B), according to the equation Ep (V) = 0.5635 − 0.0359pH
(r = 0.9874)
(6)
The slope of the plot Ep versus pH was found to be 35.9 mV/pH, close to the theoretical value of 30 mV/pH. This involves two electron transfers and a proton transfer in the ratedetermining step.17−19 From the plot of Ipa versus pH (Figure 4B), it is clear that the intensity increased to a high value at pH 3.0, and then the peak intensity decreased. Because the best result with respect to sensitivity accompanied by sharper response was obtained at pH 3.0, this value was selected for further experiments. 3.5. Mechanism. In the proposed method, the electrooxidation of 4-AP involves two electron- and one protontransfer processs. The oxidation takes place at the amino group of the dipyrone ring, as reported in earlier works on dipyrone derivatives.20,21 In the first step of the mechanism, deprotonation of 4-AP takes place and forms the carbene of 4-AP. This carbene loses two electrons to form a cation that is hydrolyzed to form the final product. The probable mechanism is shown in Scheme 2. Scheme 2. Probable Electrode Oxidation Mechanism of 4AP Figure 5. (A) Differential-pulse voltammograms with increasing concentration of 4-AP in pH 3.0 buffer solution on a graphite pencil electrode with 4-AP concentration: (a) 1.0, (b) 2.0, (c) 3.0, (d) 4.0, (e) 5.0, (f) 6.0, (g) 8.0, (h) 10.0, (i) 12.0, (j) 16.0, and (k) 20.0 μM. (B) Plot of peak current versus concentration of 4-AP.
nations. The limit of detection (LOD) and quantification (LOQ) were 0.458 × 10−7 M and 1.52 × 10−7 M, respectively. The LOD and LOQ were calculated using the equations LOD = 3s /m
(8)
where s is the standard deviation of the peak currents of the blank (five runs) and m is the slope of the calibration curve.22 Sample solutions recorded after 48 h did not show any appreciable change in the assay values. The detection limits reported using different analytical methods for 4-AP-related dipyrone derivative drugs are collected in Table 1. The proposed method thus appears to be better than other reported electrochemical methods.5,21,23 The precision of the method was investigated by intraday and interday determinations of 4-AP at two different concentrations (n = 5) within the linear range. The accuracy of the methods expressed as percentage bias and percentage RSD within and between days is reported in Table 2, which indicates high precision of the proposed method. To ascertain the repeatability of the analysis, five measurements on 10 μM 4-AP solutions were carried out using a graphite pencil electrode at an interval of 20 min. The RSD value of the peak current was found to be 0.29%, which indicates that the graphite pencil electrode has good repeatability. The reproducibility between days was similar to that within one day, when the temperature was kept constant. 3.7. Effect of Excipients. For the possible analytical application of the proposed method, the effects of some common excipients used in pharmaceutical preparations were examined. The tolerance limit was defined as the maximum
3.6. Calibration Curve. To develop a voltammetry method for determining the drug, we selected the differential-pulse voltammetric mode, because the peaks are sharper and better defined at lower concentrations of 4-AP than those obtained by cyclic voltammetry. According to the obtained results, it was possible to apply this technique to the quantitative analysis of 4-AP. A phosphate buffer solution of pH 3.0 was selected as the supporting electrolyte for the quantification of 4-AP because it gave the maximum peak current at pH 3.0. Differential-pulse voltammograms obtained with increasing amounts of 4-AP showed that the peak current increased linearly with increasing concentration, as shown in Figure 5A. Using the optimum conditions described previously, a linear calibration curve was obtained for 4-AP in the range from 1.0 × 10−6 to 1.6 × 10−5 M (Figure 5B). The linear equation was Ipa (μA) = 407300C (M) + 0.4617
LOQ = 10s /m
(r = 0.9887) (7)
Deviation from linearity was observed for more concentrated solutions, as a result of the adsorption of 4-AP or its oxidation products on the electrode surface. Related statistical data for the calibration curves were obtained from nine different determi15939
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Hence, there should be no interference of phenacetin, antipyrine, and caffeine in the determination of 4-aminophenezone. 3.8. Detection of 4-AP in Spiked Human Plasma Samples. The applicability of DPV to the determination of 4AP in spiked human plasma samples was investigated. The recoveries from human plasma were measured by spiking drugfree plasma with known amounts of 4-AP. The plasma sample was prepared as described in section 2.3. A quantitative analysis was carried out by adding the standard solution of 4-AP in the detection system of the plasma sample. The calibration graph was used for the determination of spiked 4-AP in plasma samples. The detection results obtained for four plasma samples are listed in Table 4. The recovery determined was in the range from 97.4% to 101.3%, and the RSDs are included in Table 4.
Table 1. Comparison of Detection Limits for 4-AP-Related Dipyrone Derivative Drugs Using Different Methods method
LOD
ref
Dipyrone (DP), (1-Phenyl-2,3-dimethyl-5-pyrazolone-4methylaminomethanesulfonate Sodium) flow injection amperometric 2.78 × 10−4 M determination diffusion layer titration at dual3.6 μM band electrochemical cell nano-riboflavin-modified glassy 0 0.0502 μM carbon electrode (voltammetry) titanium phosphate/nickel 3.75 × 10−4 M hexacyanoferrate mod graphite electrode (voltammetry) 4-aminophenazone graphite pencil electrode 0.458 × 10−7 M (voltammetry) (LOQ = 1.52 × 10−7 M)
5 20 21 23
present work
Table 2. Analytical Precision and Accuracy of 4-AP Determination by DPV
a
added (M)
founda (M)
2.0 × 10−5 6.0 × 10−5
2.008 × 10−5 5.99 × 10−5
2.0 × 10−5 6.0 × 10−5
2.002 × 10−5 5.998 × 10−5
SD Intraday 0.0059 0.0093 Interday 0.01 0.013
Table 4. Determination of 4-AP in Human Plasma Samples
RSD (%)
bias (%)
0.39 0.155
0.4 −0.16
0.502 0.224
0.1 −0.03
human plasma sample sample sample sample a
Average of five determinations.
Table 3. Influence of Potential Excipients on the Voltammetric Response of 1.0 × 10−5 M 4-AP potential observed (V)
signal change (%)
only 4-aminophenazone citric acid + 4-AP dextrose + 4-AP glucose + 4-AP gum acacia + 4-AP lactose + 4-AP sucrose + 4-AP tartaric acid + 4-AP starch + 4-AP
0.408 0.420 0.404 0.412 0.424 0.404 0.420 0.412 0.432
0 2.94 −0.98 2.94 3.92 −0.98 2.94 0.98 5.88
founda (×10−5 M)
recovery (%)
RSD (%)
bias (%)
1.0 3.0 5.0 8.0
0.983 2.919 5.015 8.113
98.4 97.36 100.28 101.3
0.561 0.152 0.130 0.085
−1.7 −2.7 0.3 1.41
1 2 3 4
Average of five determinations.
3.9. Detection of 4-Aminophenazone in Urine Samples. The developed differential-pulse voltammetric method for 4-AP determination was applied to urine samples. The recoveries from urine were measured by spiking drug-free urine with known amounts of 4-AP. The urine samples were diluted 100 times with phosphate buffer solution before analysis without further pretreatments. A quantitative analysis was carried out by adding the standard solution of 4-AP (1.0 × 10−5 M) into the detection system of the urine samples, and the peak heights increased linearly. The calibration graph was used for the determination of spiked 4-AP in urine samples. The detection results of five urine samples are listed in Table 5. The recovery determined was in the range from 97.7% to 102.9%, and the RSDs are included in Table 5.
concentration of the interfering substance that caused an error of less than 5% in the determination of 4-AP. The effects of these excipients on the voltammetric response were studied by analyzing sample solutions containing a fixed amount of 4-AP (1.0 × 10−5 M) spiked with various excess amounts of each excipient under the same experimental conditions. The experimental results (Table 3) showed that 100-fold excesses
excipient (1.0 mM) + drug (1.0 × 10−5 M)
spike (×10−5 M)
Table 5. Determination of 4-AP in Urine Samples sample
spike (×10−5 M)
founda (×10−5 M)
recovery (%)
RSD (%)
bias (%)
1 2 3 4 5
1.0 3.0 5.0 8.0 10.0
0.99 3.03 4.99 8.23 9.77
99.82 101.09 99.86 102.94 97.75
0.089 0.040 0.065 0.100 0.046
−0.30 1.0 −0.18 2.86 −2.27
a
of citric acid, dextrose, glucose, gum acacia, lactose, tartaric acid, and sucrose did not interfere with the voltammetric signal of 4AP. However, a 100-fold excess of starch had an apparent influence on the voltammetric signal of 4-AP. We checked the possibilities of interference of other drugs that have been dispersed together with 4-aminophenazone, such as phenacetin, antipyrine, and caffeine, by cyclic and differential-pulse voltammetric studies. We obtained welldefined separate anodic peaks for each drug in the mixture (for 4-aminophenazone, Ep = +0.467 V; for phenacetin, Ep = +0.682 V; and for caffeine, Ep = +1.020 V). The same anodic peak currents were also obtained for each individual drug.
Average of five determinations.
4. CONCLUSIONS The electrochemical behavior of 4-AP at a GPE in phosphate buffer medium has been studied. The results indicated that 4AP undergoes two electron and one proton transfers and is a diffusion-controlled process. The differential-pulse voltammetric procedure can be used successfully to determine 4-AP. The method was applied in real sample analysis. This method can be a good alternative for the analytical determination of 4-AP, because it is simple, sensitive, fast, accurate, and inexpensive. 15940
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(7) Carretero, I.; Vadillo, J. M.; Laserna, J. J. Determination of antipyrine metabolites in human plasma by solid-phase extraction and micellar liquid chromatography. Analyst 1995, 120, 1729−1732. (8) Isoshi, N.; Sachico, N.; Kaori, W.; Kunio, O. Determination of Phenol in Tap Water and River Water Samples by Solid-Phase Spectrophotometry. Anal. Sci. 2000, 16, 269−273. (9) Blo, G.; Dondi, F.; Betti, A.; Bighi, C. Determination of phenols in water samples as 4-aminoantipyrine derivatives by high-performance liquidchromatography. J. Chromatogr. A 1983, 257, 69−79. (10) Damm, D. Simultaneous determination of the main metabolites of dipyrone by high-pressure liquid chromatography. Arzneimittelforschung 1989, 39, 1415−1417. (11) Liu, X. HPLC determination of paracetamol, caffein and aminophenazone in paracetamol, caffein, aminophenazone, and chlorphenamine maleate granules. Chin. J. Pharm. Anal. 2007, 27, 1487−1499. (12) Christian, G. D.; Purdy, W. C. The residual current in orthophosphate medium. J. Electroanal. Chem. 1962, 3, 363−367. (13) Gosser, D. K. Cyclic Voltammetry: Simulation and Analysis of Reaction Mechanisms; VCH: New York, 1993; p 43. (14) Laviron, E. General expression of the linear potential sweep voltammogram in the case of diffusionless electrochemical systems. J. Electroanal. Chem. 1979, 101, 19−28. (15) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2004, p 236. (16) Yunhua, W.; Xiaobo, J.; Shengshui, H. Studies on electrochemical oxidation of azithromycin and its interaction with bovine serum albumin. Bioelectrochemistry. 2004, 64, 91−97. (17) Dogan-Topal, B.; Bozal, B.; Demircigil, B. T.; Ulsu, B.; Ozkan, S. A. Electroanalytical Studies and Simultaneous Determination of Amlodipine Besylate and Atorvastatine Calcium in Binary Mixtures Using First Derivative of the Ratio-Voltammetric Method. Electroanalysis 2009, 21, 2427−2439. (18) Wang, J. Electroanalytical Techniques in Clinical Chemistry and Laboratory Medicine; VCH: New York, 1988. (19) Kissinger, P. T.; Heineman, W. R. Laboratory Techniques in Electroanalytical Chemistry, 2nd ed.; Marcel Dekker: New York, 1996. (20) Thiago, R. L. C.; Matos, R. C.; Bertotti, M. Diffusion layer titration of dipyrone in pharmaceuticals at a dual-band electrochemical cell. Talanta 2003, 61, 725−732. (21) Gopalakrishnan, G.; Manisankar, P. Stripping Voltammetric Determination of Analgesics in Their Pharmaceuticals Using NanoRiboflavin-Modified Glassy Carbon Electrode. Int. J. Electrochem. 2011, 2011, 1−11. (22) Swartz, E.; Krull, I. S. Analytical Method Development and Validation; Marcel Dekker: New York, 1997. (23) Cumba, L. R.; Bicalho, U. O.; Silvestrini, D. R.; Carmo, D. R. Preparation and Voltammetric Study of a Composite Titanium Phosphate/Nickel Hexacyanoferrate and Its Application in Dipyrone Determination. Int. J. Chem. 2012, 4, 66−78.
Furthermore, the present method could possibly be employed for pharmacokinetic studies and also in clinical and qualitycontrol laboratories.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +91 836 2770524. Fax: +91 836 2747884. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.I.G. thanks UGC, New Delhi, India, for the award of Research Fellowship in Science for Meritorious Students (RFSMS).
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NOMENCLATURE 4-AP = 4-aminophenazone C = concentration (mol dm−3) CV = cyclic voltammetry DPV = differential-pulse voltammetry E0′ = formal redox potential (V) Ep = peak potential (V) F = Faraday constant (C mol −1) GPE = graphite pencil electrode Ipa = peak current (μA) k0 = standard rate constant of the reaction (cm s−1) LOD = limit of detection (mol dm−3) LOQ = limit of quantification (mol dm−3) m = slope of the calibration curve n = number of electrons transferred R = gas constant (J K−1 mol −1) r = regression coefficient RSD = relative standard deviation s = standard deviation of the peak currents T = temperature (K) v = scan rate (mV s−1) α = transfer coefficient
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REFERENCES
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