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Electrochemical study and detection of perphenazine using a gold electrode modified with decanethiol SAM
Talanta 61 (2003) 819 /827 www.elsevier.com/locate/talanta
Electrochemical study and detection of perphenazine using a gold electrode modified with decanethiol SAM Baizhao Zeng *, Yuxia Yang, Xiaogang Ding, Faqiong Zhao Department of Chemistry, Wuhan University, Wuhan 430072, China Received 20 April 2003; received in revised form 2 June 2003; accepted 2 June 2003
Abstract A novel method for the determination of perphenazine has been developed. The method is based on the accumulation of perphenazine at a gold electrode modified with decanethiol (DEC) self-assembled monolayer (SAM) and its oxidation at about 0.6 V (vs. saturated calomel electrode (SCE)). Because some coexistent electroactives were blocked and perphenazine was selectively accumulated by the SAM, the electrode exhibited good selectivity and sensitivity. Various conditions were optimized for practical application. Under the selected conditions (i.e. 0.05 M pH 10 sodium borate buffer, accumulation time: 120 s, accumulation potential: /0.4 V, scan rate: 100 mV s 1), the anodic stripping peak current was linear to perphenazine concentration in the ranges of 6 /10 9 /5 /l0 7 and 5 /10 7 /5 /10 6 M with correlation coefficients of 0.998 and 0.995, respectively. For a 1.0 /10 6 M perphenazine solution, the relative standard deviation of peak height was 2.3% (n /8). This method was applied to the determination of perphenazine in some drugs and the recovery was 92 /101%. In addition, it was found that in the presence of perphenazine, the SAM structure changed a little and more needle holes appeared. However, the SAM could recover the original form when perphenazine and its redox product were removed from the monolayer by repeatedly cycling the electrode in a blank solution for a minute. The modified electrode was characterized by alternating current impedance and electrochemical probe. # 2003 Elsevier B.V. All rights reserved. Keywords: Perphenazine; Decanethiol; Gold electrode; Voltammetry; Self-assembled monolayer
1. Introduction Perphenazine belongs to the phenothiazine family. Its chemical structure is shown in Scheme 1. Because of its neuroleptic and antidepressive actions, it is often prescribed for the treatment of
* Corresponding author. Fax: /86-27-8764-7617. E-mail address: [email protected] (B. Zeng).
psychotic patients. Recently, it is found to be effectual in the treatment of Parkinson’s disease [1]. As it has such function and application, some researchers have been attracted to study its characteristics and fumble simple and sensitive detection methods for it. So far, many methods have been developed for its determination, such as fluorescence [2], chemiluminescence [1], spectrophotometry [3 /5], and titrimetry [6 /8]. Every
0039-9140/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0039-9140(03)00381-3
B. Zeng et al. / Talanta 61 (2003) 819 /827
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Scheme 1. Molecular structure of perphenazine.
method has its advantages and disadvantages. Because of the electroactivity of perphenazine, it can be studied and determined electrochemically [9,10]. Several electroanalytical methods, based on its oxidation and accumulation at bare or lipidcoated carbon electrodes, have been developed [9 / 12]. Self-assembled monolayer (SAM) has become popular in recent decades [13 /15]. This technique is expected to yield a very simple, highly versatile, controllable and stable approach for tailoring electrode surfaces. The well-defined SAM films have already been proven to be extremely useful for studying ion binding, incorporating redox couples into electrochemical interfaces [16,17], exploring protein adsorption [18,19], and blocking electron transfer between redox species and electrode surfaces [20 /22]. In the published documents, alkylthiol SAMs were attempted to modify gold electrodes for the determination of phenothiazine and its derivatives including perphenazine [23]. It was found that the sensitivity of the modified electrode could be enhanced to some extent by increasing the carbon chain length of alkylthiol. Wang thought the response was related to the permeability and polarity of the SAMs. In this work, the electrochemical behavior of perphenazine at decanethiol (DEC) modified gold electrode was discussed and the influence of perphenazine on the SAM structure was studied. Some new phenomena were observed and a procedure for the voltammetric determination of perphenazine was proposed.
2. Experimental 2.1. Apparatus Voltammetric study was performed with a model CII 830 electrochemical analyzer (CH
Instrumental Co., Shanghai, China) controlled by a personal computer. A three-electrode system was used, which included a gold working electrode, a platinum wire auxiliary electrode and a saturated calomel electrode (SCE) as reference electrode. The alternative current impedance (AC impedance) measurement was carried out on a Model-273A bipotentiostat in conjunction with a lock-in amplifier (EG&G PAR Co., USA). The pH values were measured with a pHS-3C pH meter (Shanghai, China). 2.2. Reagents Perphenazine was purchased from Sigma (USA) and used as received. The stock solution of perphenazine (0.01 M) was prepared with ethanol and it was diluted with redistilled water and sodium borate buffer to prepare the working solution. The drug sample came from NO.3 Pharmaceutical Company of Nantong (Nantong, China) and Huaiyin Pharmaceutical Company (Jiangsu, China), respectively. Prior to determination, they were grinded into powder, dissolved in ethanol, filtered into a container and then diluted to certain volume. Standard addition method was used to assay them. Other reagents used were analytical or reagent grade. All solutions were prepared with double-distilled water. 2.3. Electrode preparation Before modification, the gold electrode (purity: 99.99%, 2 mm diameter, sealed in a Teflon tube) was polished with 1 mm, followed by 0.3 mm alumina slurry on a polishing pad, then rinsed with distilled water, ultrasonicated in water bath for 2 min and dried in air. The resulting gold electrode was immersed in an ethanol solution containing 1 mM DEC for certain time, then taken out and washed with double-distilled water. Thus, a DEC SAM modified gold electrode was obtained. It was recorded as DEC/Au. As the performance of the modified electrode almost kept unchanged when the soaking time was changed from 2 to 60 min, the gold electrode generally was immersed in the DEC solution for 2 min or more.
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2.4. Procedure For voltammetric determination, a preconcentration step was adopted. The preconcentration lasted 2 min at /0.4 V in a stirred solution. Following this, a linear sweep in positive direction was initiated and the voltammograms were collected. In general, the voltammograms corresponding to the first scan were recorded and the anodic peak was measured. The electrode was regenerated by repeatedly cycling between 0.2 and 0.9 V in a blank solution until the peak disap3 peared. For AC impedance measurement, 2 / /10 M potassium ferrocyanide was used as electrochemical probe and the potential was fixed at 0.20 V. All experiments were performed at room temperature.
3. Results and discussion 3.1. Cyclic voltammograms Fig. 1 shows the cyclic voltammograms (CV) of perphenazine under different conditions. As can be seen, perphenazine did not exhibit discernible peak at a bare gold electrode in the given potential range, although it was electroactive. This was because the peak of perphenazine was very small
Fig. 1. CV of bare Au electrode (a, b) and DEC/Au (c, d) in a blank solution (a, c) and in a 1.0/10 5 M perphenazine solution (b, d). Scan rate: 100 mV×/s 1, supporting electrolyte: 0.050 M sodium borate buffer (pH 10).
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and overlapped by the anodic peak of the bare gold electrode. That means the bare gold electrode is not suitable for the voltammetric determination and study of perphenazine at this case. However, the response of perphenazine could be improved by modifying the gold electrode with thiols SAM. Several thiols were tested as modifying substance, including DEC, dodecanethiol, hexadecylthiol, 3mercaptopropionie acid, cysteine, cysteamine and glutathione. The results showed that the gold electrode modified by long-chain thiols gave better performance. Among those, the gold electrode modified by decanethiol (DEC/Au) was the best one. Although the carbon chains of dodecanethiol and hexadecylthiol were longer than DECs, their performance was not so good as that of DEC in this case. This may be due to the increasing compaction of SAMs formed by them, which could block the permeating of perphenazine into the SAMs to some extent. In the presence of DEC SAM, a sharp and sensitive voltammetric peak with a shoulder peak could be observed at about 0.6 V (vs. SCE) for perphenazine. This should be attributed to the accumulation of perphenazine at the SAM. As to the anodic peak and the shoulder, they resulted from the oxidation of perphenazine, which included two 1e-steps [24]. It can be seen that the background current was greatly decreased (Fig. 1(c, d)). It was due to the block action of the SAM to other electroactive substances in the solution and to the redox of gold electrode. Therefore, such modification could improve both selectivity and sensitivity of the gold electrode and made it more favorable for determining perphenazine. As the cathodic peak was invisible, the electrochemical process was thought to be irreversible and the anodic peak was selected for further study. To examine the reproducibility and stability of the peak, continuing potential scan was performed with a DEC/Au electrode in the same perphenazine solution. As a result, the anodic peak height decreased with repeating potential scan and it almost disappeared when the repetitive cycling times exceeded five (Fig. 2). This was ascribed to the irreversible electrode reaction and the accumulation of both perphenazine and its oxidation products in the SAM. When the accumulated
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Fig. 2. Linear sweep voltammograms (LSV) of a fresh prepared DEC/Au in a 5.0/10 6 M perphenazine solution. (A) the first scan, (B) the second scan, (C) the fifth scan, (D) LSV of the first scan of the electrode (C) after being repeatedly cycled in a blank solution for a minute (until no peak is observable). Other conditions were the same as in Fig. 1.
perphenazine molecular was oxidized, the products still stayed in the SAM and hindered the accumulation and oxidation of perphenazine in bulk solution. Only part of the remained products was timely reduced and oxidized again during the repeating potential scan, thus the peak height decreased, even disappeared. Taking into account this phenomenon, the first CV was utilized for the determination, and several ways were attempted to regenerate the used electrode. It was found, after being repeatedly cycled in a blank solution for a minute (i.e. five cycles or more), the used electrode could regain the function and acted as a fresh prepared one (Fig. 2). Therefore, the electrode could be reused. Taking 1.0 /106 M perphenazine solution as an example, it was determined for eight times with the same modified electrode that was regenerated after every measurement. The relative standard deviation of peak current was 2.3%. Fig. 3 illustrates the influence of scan rate on the voltammogram, which was changed from 20 to 300 mV ×/s 1. It is clear that the anodic peak grew with scan rate increasing. Furthermore, the peak current was linear to the scan rate in the investigated range with a correlation coefficient of 0.996. This indicates the electrode process was controlled by the adsorption of perphenazine or surface
Fig. 3. Variation of cyclic voltammogram with scan rate. Scan rate: 20, 50, 100, 150, 200, 300 mV×/s 1 (from bottom to top). Perphenazine concentration: 1.0/l0 6 M, other conditions as in Fig. 1. Insert: the dependence of peak current on scan rate.
reaction. The background signal also grew with scan rate increasing, which would affect the accurate measurement of the peak height. In general, the scan rate of 100 mV ×/s 1 was used.
3.2. Influence of perphenazine and its oxidation products on DEC SAM structure Potassium ferrocyanide was used as a electrochemical probe to explore the structure change of DEC SAM under different conditions. It was found that, after the used DEC/Au electrode regenerated in blank solution, it did not exhibit visible peak in a potassium ferrocyanide solution, like a fresh prepared DEC/Au electrode. However, after being cycled in a perphenazine solution till the anodic peak disappeared, the DEC/Au could exhibit a pair of irreversible peak in the potassium ferrocyanide solution (Fig. 4). These facts indicate that both perphenazine and its oxidation products can be entrapped in the DEC SAM and they were removed from the SAMs during the regenerating procedure. When perphenazine and its oxidation products were present, the SAM was not so densely packed as they were absent and more needle holes occurred. Therefore, potassium ferrocyanide could partly transit the membrane and exhibited a pair of irreversible redox peaks.
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Fig. 4. CV of 2.0 /10 3 M potassium ferrocyanide at bare gold electrode (A) and DEC/Au (B). Fresh prepared DEC/Au (. . .), regenerated DEC/Au ( */) and the DEC/Au after being cycled in a perphenazine solution (---). Supporting electrolyte: 0.1 M KNO3.
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cyanide was almost inhibited completely. When AC impedance was measured after the DEC/Au electrode was cycled in a perphenazine solution once, the ACIS curve’s diameter (curve c) was far bigger than that of the bare gold electrode, but smaller than that of fresh prepared DEC/Au, indicating potassium ferrocyanide could transit the monolayer partially. After the used electrode was regenerated by the means mentioned above, the ACIS curve (curve d) recorded almost could overlap curve b, which indicates the detained perphenazine and its oxidation products were removed from the SAM and the monolayer recovered the original form. When the ACIS measurement was carried out after the electrode was cycled in a perphenazine solution till the anodic peak disappeared, the ACIS curve became a hemisphere whose diameter was 90.22 kV (curve e). This diameter was bigger than those of curve (a) and (c), but smaller than those of curve (b) and (d), meaning the redox products of perphenazine really stayed in the monolayer and caused more needle holes. These were in accordance with the CV results mentioned above. The change of the SAM structure was illustrated in Scheme 2. 3.3. Influence of solution pH and buffer concentration
3 Fig. 5. Electrochemical impedance spectrums. (a) 2 / /10 M 3 bare gold electrode, (b) DEC/Au, (c) 2 / /10 M DEC/Au after being cycled in a 1/10 5 M perphenazine solution once, (d) (c) after being repeatedly cycled in a blank solution for a minute, (e) (c) after being repeatedly cycled in the perphenazine solution till the anodic peak disappeared. Electrochemical 3 probe; 2 / /10 M potassium ferrocyanide, potential: 0.20 V, diameter: (a) 1.6 kV, (c) 25.6 kV, (e) 90.2 kV.
Fig. 5 shows some alternative current impedance spectrum (ACIS) recorded under given conditions. From the figure, it can be learnt that potassium ferrocyanide could easily get the gold electrode surface in the absence of DEC SAM. But, when the gold electrode was coated by DEC SAM, the ACIS (curve b) almost became a straight line, meaning the transference of potassium ferro-
Because the accumulation of perphenazine at the DEC SAM was related to the hydrophobic interaction between them, the change in lipophilicity of perphenazine made the accumulation amount vary. It is known the hydrophobicity of perphenazine varied with the solution pH due to protonization. In basic environment, perphenazine was neutral and exhibited strong lipophilicity and easily entered the hydrophobic DEC SAM. Therefore, the accumulation amount increased. On the contrary, in acidic solution, perphenazine turned into cation and the hydrophobic interaction between them thus became weak, making the accumulation amount decrease and the voltammetric peak smaller. Fig. 6 shows the influence of solution pH on the peak current and potential. As can be seen, with the solution pH increasing, the peak shifted negatively at a rate of about 26 mV per pH, implying in the electrode reaction the
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Scheme 2. Illustration of the change of the DEC SAM monolayer structure. Conditions were as in Fig. 5. PPZ stands for perphenazine.
number of electron transferred was as two times as that of proton. Meanwhile, the peak current increased and it achieved a maximum at about pH 10. Further increase in solution pH made the peak height decrease. In this work, pH 10 was chosen. When the buffer concentration was changed from 0.0050 to 0.10 M, the peak potential almost kept unchanged, but the peak current gradually increased with the buffer solution concentration rising, so did the peak area. Taking into account the small solubility of sodium borate at room temperature, 0.050 M buffer solution was chosen in this experiments.
3.4. Influence of preconcentration potential and time The preconcentration potential was changed from /0.6 to 0.4 V to explore its influence. As shown in Fig. 7, when the preconcentration potential was /0.4 V, the peak current and peak area reached the maximum values. At preconcentration potential away from /0.4 V, the peak height decreased gradually. Part of this was ascribed to the change of the SAM structure due to the influence of extra charges at the surface. The peak height increased with accumulation time
B. Zeng et al. / Talanta 61 (2003) 819 /827
Fig. 6. Influence of solution pH on peak current and peak potential. Perphenazine concentration: 5.0/10 5 M, other conditions as in Fig. 1.
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Fig. 8. Variation of peak current and peak area with accumulation time. Perphenazine concentration: 5.0 /10 7 M, accumulation potential: /0.4 V, other conditions were as in Fig. 1.
zine concentration was studied. Under the selected conditions, i.e. ta /2 min, Ea / /0.4 V and pH 10, when the concentration of perphenazine solution changed from 1 /10 9 to 5 /l0 5 M, the peak current increased gradually. Furthermore, the peak current was linear to perphenazine concentration in the ranges of 6/l0 9 /5/l07 M and 5/l0 7 /5/l06 M, respectively (Fig. 9). The regression equations were: ip (mA) /0.33/ 0.79 c (mM) for the range of 6 /109 /5/l07 M (r /0.998) and ip (mA) /0.65/0.33 c (mM) for the range of 5 /l0 7 /5/l0 6 M (r/0.995). It is Fig. 7. Influence of accumulation potential on peak current and peak area. Perphenazine concentration; 5.0/10 7 M, accumulation time: 120 s, other conditions were as in Fig. 1.
increasing until it reached a maximum. For a 1/ 106 M perphenazine solution, the increase of peak current ceased at above 120 s (Fig. 8), meaning that a saturated adsorption was reached. Here, 120 s was chosen as the accumulation time and /0.4 V as preconcentration potential. 3.5. Dependence of peak current on perphenazine concentration To examine the application feasibility of this procedure for perphenazine determination, the relationship between peak current and perphena-
Fig. 9. Dependence of peak current on perphenazine concentration. Preconcentration time: 120 s, accumulation potential: /0.4 V, other conditions were as in Fig. 1. Every point came from the average value of three measurements, and the relative standard deviation of peak current was about 1.0 /4.3% for them.
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clear that the slopes of the two calibration lines are different, which was ascribed to the change of accumulation efficiency. When the concentration exceeded 5 /106 M, the peak current almost stopped increasing, even though the perphenazine concentration was increased, indicating a saturated accumulation was achieved.
3.6. Interference of foreign substance The interference of some compounds that may be present in real samples was examined. The results showed that in the presence of 100-fold dextrose, folic acid, VB1, VB2, thiocarbamide or hypoxanthine, the peak current changed by /6.7, 3.9, 4.6, 3.6 and 2.8%, respectively. Xanthosine, ascorbic acid, Zn2 and epinephrine would make the peak current change more though they were blocked by the DEC SAM. Their interference was thought to partly come from the change of DEC SAM after perphenazine molecule permeated into DEC SAM, which made those foreign substances more easily get electrode surface and affected the response of perphenazine. On the other hand, some of them could be catalytically oxidized by perphenazine radical produced during the electrochemical process and caused catalytic current. This also made the peak current change. But their interference could be eliminated or reduced by adding some reagents into the solution, such as EDTA. In addition, adsorptive transfer stripping voltammetry could be used for such purpose.
3.7. Influence of temperature When the solution temperature changed from 8 to 51 8C, the peak potential shifted. Meanwhile, the peak current changed and got a maximum at about 20 8C. At lower temperature, the diffusion speed decreased, so did electrochemical reaction rate, which made the response of perphenazine weak. When the temperature was higher than 20 8C, the peak became smaller. This should be attributed to the reduction of accumulation amount because the extractive accumulation was weakened at this case.
4. Applications This method was applied to the determination of perphenazine in drug samples (i.e. Fennaijing). The pretreatment and determination procedure of the samples were the same as described in Section 2. The analytical results were shown in Table 1. The recovery was 92 /101%. According to Table 1, the perphenazine contents can be calculated, which were 4.1 and 1.94 mg per tablet for sample 1# (its declared content was 4 mg per tablet) and sample 2# (its declared content was 2 mg per tablet), respectively. This method was simple, sensitive and cheap in comparison with some other methods.
5. Conclusions Perphenazine can be effectively accumulated at a DEC SAM modified gold electrode due to the hydrophobic interaction between DEC SAM and perphenazine. When the potential was made move in positive direction the perphenazine exhibited a sensitive anodic stripping peak at about 0.6 V (vs. SCE). Because of the block action of the SAM to some coexistents, the electrode also showed good selectivity. The electrode could be regenerated easily by cycling in a blank solution and thus could be reused. When perphenazine was present Table 1 Measurement results of perphenazine in drugs Sample drug
#
1 (A)
l#(B) 2#(A)
2#(B)
Perphenazine (mM)
Recovery (%)
Added
Expected
Found
0 1.0 3.0 0 0.5 0 1.0 3.0 0 0.5 1.5
1.03 2.03 4.03 0.51 1.01 0.97 1.97 3.97 0.49 0.99 1.99
1.03 2.04 4.00 0.51 1.01 0.97 1.89 3.96 0.49 0.966 1.96
/ 101 99 / 100 / 92 100 / 95 98
1#: From NO.3 Pharmaceutical Company of Nantong; 2#: from Huaiyin Pharmaceutical Company; (B) was prepared with (A) by diluting.
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the DEC SAM became irregular and less compact, making the corresponding AC impedance spectrum change. But the SAM would recover the origin form soon when perphenazine and its redox products were removed from the SAM.
Acknowledgements This work was supported by the National Nature Science Foundation of China (Grant No: 20173040), the Foundation of State Education Ministry for returned oversea scholar and the Electroanalytical Open Lab of Applied Chemistry Institute of Changehun, China.
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[6] M.H. Pournaghi-Azar, K. Farhadi, Talanta 44 (1997) 1773. [7] S.M. Golabi, M. Showkati-Shishvan, Talanta 38 (1991) 1253. [8] M.H. Pournaghi-Azar, K. Farhaki, Talanta 41 (1995) 345. [9] E. Bishop, W. Hussein, Analyst 109 (1984) 229. [10] J. Wang, B. Freiha, B. Deshmukh, Bioelectrochem. Bioenerg. 14 (1985) 457. [11] J. Wang, B. Freiha, Anal. Chem. 55 (1983) 1285. [12] M. Dhadari, J.M. Dauffmann, G. Patriarche, M. Ghandour, Electroanalysis 1 (1989) 501. [13] J. Wang, B.Z. Zeng, C. Fang, X.Y. Zhou, Anal. Sci. 16 (2000) 457. [14] B.Z. Zeng, F.Q. Zhao, X.G. Ding, Anal. Sci. 17 (2001) 259. [15] B.Z. Zeng, X.G. Ding, F.Q. Zhao, Electroanalysis, 14 (2002) 651. [16] C. Chidsey, C. Bertozzi, T. Prtvinski, A. Mujsce, J. Am. Chem. Soc. 112 (1990) 4301. [17] H. Tsutsumi, S. Furumoto, M. Morita, Y. Matsuda, J. Electrochem. Soc. 139 (1992) 1522. [18] K. Prime, G. Whitesides, Science 252 (1991) 1164. [19] M. Collinson, E. Bowden, M. Tarlov, Langmuir 8 (1992) 1247. [20] M. Porter, T. Bright, D. Allara, C. Chidsey, J. Am. Chem. Soc. 109 (1987) 3559. [21] Q. Chen, A. Brajter-Toth, Anal. Chem. 64 (1992) 1998. [22] F. Malem, D. Mandler, Anal. Chem. 65 (1993) 37. [23] J. Wang, H. Wu, L. Angnes, Anal. Chem. 65 (1993) 1893. [24] Y. Ni, L. Wang, S. Kokot, Anal. Chim. Acta 439 (2001) 159.
Electrochemical study and detection of perphenazine using a gold electrode modified with decanethiol SAM Baizhao Zeng *, Yuxia Yang, Xiaogang Ding, Faqiong Zhao Department of Chemistry, Wuhan University, Wuhan 430072, China Received 20 April 2003; received in revised form 2 June 2003; accepted 2 June 2003
Abstract A novel method for the determination of perphenazine has been developed. The method is based on the accumulation of perphenazine at a gold electrode modified with decanethiol (DEC) self-assembled monolayer (SAM) and its oxidation at about 0.6 V (vs. saturated calomel electrode (SCE)). Because some coexistent electroactives were blocked and perphenazine was selectively accumulated by the SAM, the electrode exhibited good selectivity and sensitivity. Various conditions were optimized for practical application. Under the selected conditions (i.e. 0.05 M pH 10 sodium borate buffer, accumulation time: 120 s, accumulation potential: /0.4 V, scan rate: 100 mV s 1), the anodic stripping peak current was linear to perphenazine concentration in the ranges of 6 /10 9 /5 /l0 7 and 5 /10 7 /5 /10 6 M with correlation coefficients of 0.998 and 0.995, respectively. For a 1.0 /10 6 M perphenazine solution, the relative standard deviation of peak height was 2.3% (n /8). This method was applied to the determination of perphenazine in some drugs and the recovery was 92 /101%. In addition, it was found that in the presence of perphenazine, the SAM structure changed a little and more needle holes appeared. However, the SAM could recover the original form when perphenazine and its redox product were removed from the monolayer by repeatedly cycling the electrode in a blank solution for a minute. The modified electrode was characterized by alternating current impedance and electrochemical probe. # 2003 Elsevier B.V. All rights reserved. Keywords: Perphenazine; Decanethiol; Gold electrode; Voltammetry; Self-assembled monolayer
1. Introduction Perphenazine belongs to the phenothiazine family. Its chemical structure is shown in Scheme 1. Because of its neuroleptic and antidepressive actions, it is often prescribed for the treatment of
* Corresponding author. Fax: /86-27-8764-7617. E-mail address: [email protected] (B. Zeng).
psychotic patients. Recently, it is found to be effectual in the treatment of Parkinson’s disease [1]. As it has such function and application, some researchers have been attracted to study its characteristics and fumble simple and sensitive detection methods for it. So far, many methods have been developed for its determination, such as fluorescence [2], chemiluminescence [1], spectrophotometry [3 /5], and titrimetry [6 /8]. Every
0039-9140/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0039-9140(03)00381-3
B. Zeng et al. / Talanta 61 (2003) 819 /827
820
Scheme 1. Molecular structure of perphenazine.
method has its advantages and disadvantages. Because of the electroactivity of perphenazine, it can be studied and determined electrochemically [9,10]. Several electroanalytical methods, based on its oxidation and accumulation at bare or lipidcoated carbon electrodes, have been developed [9 / 12]. Self-assembled monolayer (SAM) has become popular in recent decades [13 /15]. This technique is expected to yield a very simple, highly versatile, controllable and stable approach for tailoring electrode surfaces. The well-defined SAM films have already been proven to be extremely useful for studying ion binding, incorporating redox couples into electrochemical interfaces [16,17], exploring protein adsorption [18,19], and blocking electron transfer between redox species and electrode surfaces [20 /22]. In the published documents, alkylthiol SAMs were attempted to modify gold electrodes for the determination of phenothiazine and its derivatives including perphenazine [23]. It was found that the sensitivity of the modified electrode could be enhanced to some extent by increasing the carbon chain length of alkylthiol. Wang thought the response was related to the permeability and polarity of the SAMs. In this work, the electrochemical behavior of perphenazine at decanethiol (DEC) modified gold electrode was discussed and the influence of perphenazine on the SAM structure was studied. Some new phenomena were observed and a procedure for the voltammetric determination of perphenazine was proposed.
2. Experimental 2.1. Apparatus Voltammetric study was performed with a model CII 830 electrochemical analyzer (CH
Instrumental Co., Shanghai, China) controlled by a personal computer. A three-electrode system was used, which included a gold working electrode, a platinum wire auxiliary electrode and a saturated calomel electrode (SCE) as reference electrode. The alternative current impedance (AC impedance) measurement was carried out on a Model-273A bipotentiostat in conjunction with a lock-in amplifier (EG&G PAR Co., USA). The pH values were measured with a pHS-3C pH meter (Shanghai, China). 2.2. Reagents Perphenazine was purchased from Sigma (USA) and used as received. The stock solution of perphenazine (0.01 M) was prepared with ethanol and it was diluted with redistilled water and sodium borate buffer to prepare the working solution. The drug sample came from NO.3 Pharmaceutical Company of Nantong (Nantong, China) and Huaiyin Pharmaceutical Company (Jiangsu, China), respectively. Prior to determination, they were grinded into powder, dissolved in ethanol, filtered into a container and then diluted to certain volume. Standard addition method was used to assay them. Other reagents used were analytical or reagent grade. All solutions were prepared with double-distilled water. 2.3. Electrode preparation Before modification, the gold electrode (purity: 99.99%, 2 mm diameter, sealed in a Teflon tube) was polished with 1 mm, followed by 0.3 mm alumina slurry on a polishing pad, then rinsed with distilled water, ultrasonicated in water bath for 2 min and dried in air. The resulting gold electrode was immersed in an ethanol solution containing 1 mM DEC for certain time, then taken out and washed with double-distilled water. Thus, a DEC SAM modified gold electrode was obtained. It was recorded as DEC/Au. As the performance of the modified electrode almost kept unchanged when the soaking time was changed from 2 to 60 min, the gold electrode generally was immersed in the DEC solution for 2 min or more.
B. Zeng et al. / Talanta 61 (2003) 819 /827
2.4. Procedure For voltammetric determination, a preconcentration step was adopted. The preconcentration lasted 2 min at /0.4 V in a stirred solution. Following this, a linear sweep in positive direction was initiated and the voltammograms were collected. In general, the voltammograms corresponding to the first scan were recorded and the anodic peak was measured. The electrode was regenerated by repeatedly cycling between 0.2 and 0.9 V in a blank solution until the peak disap3 peared. For AC impedance measurement, 2 / /10 M potassium ferrocyanide was used as electrochemical probe and the potential was fixed at 0.20 V. All experiments were performed at room temperature.
3. Results and discussion 3.1. Cyclic voltammograms Fig. 1 shows the cyclic voltammograms (CV) of perphenazine under different conditions. As can be seen, perphenazine did not exhibit discernible peak at a bare gold electrode in the given potential range, although it was electroactive. This was because the peak of perphenazine was very small
Fig. 1. CV of bare Au electrode (a, b) and DEC/Au (c, d) in a blank solution (a, c) and in a 1.0/10 5 M perphenazine solution (b, d). Scan rate: 100 mV×/s 1, supporting electrolyte: 0.050 M sodium borate buffer (pH 10).
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and overlapped by the anodic peak of the bare gold electrode. That means the bare gold electrode is not suitable for the voltammetric determination and study of perphenazine at this case. However, the response of perphenazine could be improved by modifying the gold electrode with thiols SAM. Several thiols were tested as modifying substance, including DEC, dodecanethiol, hexadecylthiol, 3mercaptopropionie acid, cysteine, cysteamine and glutathione. The results showed that the gold electrode modified by long-chain thiols gave better performance. Among those, the gold electrode modified by decanethiol (DEC/Au) was the best one. Although the carbon chains of dodecanethiol and hexadecylthiol were longer than DECs, their performance was not so good as that of DEC in this case. This may be due to the increasing compaction of SAMs formed by them, which could block the permeating of perphenazine into the SAMs to some extent. In the presence of DEC SAM, a sharp and sensitive voltammetric peak with a shoulder peak could be observed at about 0.6 V (vs. SCE) for perphenazine. This should be attributed to the accumulation of perphenazine at the SAM. As to the anodic peak and the shoulder, they resulted from the oxidation of perphenazine, which included two 1e-steps [24]. It can be seen that the background current was greatly decreased (Fig. 1(c, d)). It was due to the block action of the SAM to other electroactive substances in the solution and to the redox of gold electrode. Therefore, such modification could improve both selectivity and sensitivity of the gold electrode and made it more favorable for determining perphenazine. As the cathodic peak was invisible, the electrochemical process was thought to be irreversible and the anodic peak was selected for further study. To examine the reproducibility and stability of the peak, continuing potential scan was performed with a DEC/Au electrode in the same perphenazine solution. As a result, the anodic peak height decreased with repeating potential scan and it almost disappeared when the repetitive cycling times exceeded five (Fig. 2). This was ascribed to the irreversible electrode reaction and the accumulation of both perphenazine and its oxidation products in the SAM. When the accumulated
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Fig. 2. Linear sweep voltammograms (LSV) of a fresh prepared DEC/Au in a 5.0/10 6 M perphenazine solution. (A) the first scan, (B) the second scan, (C) the fifth scan, (D) LSV of the first scan of the electrode (C) after being repeatedly cycled in a blank solution for a minute (until no peak is observable). Other conditions were the same as in Fig. 1.
perphenazine molecular was oxidized, the products still stayed in the SAM and hindered the accumulation and oxidation of perphenazine in bulk solution. Only part of the remained products was timely reduced and oxidized again during the repeating potential scan, thus the peak height decreased, even disappeared. Taking into account this phenomenon, the first CV was utilized for the determination, and several ways were attempted to regenerate the used electrode. It was found, after being repeatedly cycled in a blank solution for a minute (i.e. five cycles or more), the used electrode could regain the function and acted as a fresh prepared one (Fig. 2). Therefore, the electrode could be reused. Taking 1.0 /106 M perphenazine solution as an example, it was determined for eight times with the same modified electrode that was regenerated after every measurement. The relative standard deviation of peak current was 2.3%. Fig. 3 illustrates the influence of scan rate on the voltammogram, which was changed from 20 to 300 mV ×/s 1. It is clear that the anodic peak grew with scan rate increasing. Furthermore, the peak current was linear to the scan rate in the investigated range with a correlation coefficient of 0.996. This indicates the electrode process was controlled by the adsorption of perphenazine or surface
Fig. 3. Variation of cyclic voltammogram with scan rate. Scan rate: 20, 50, 100, 150, 200, 300 mV×/s 1 (from bottom to top). Perphenazine concentration: 1.0/l0 6 M, other conditions as in Fig. 1. Insert: the dependence of peak current on scan rate.
reaction. The background signal also grew with scan rate increasing, which would affect the accurate measurement of the peak height. In general, the scan rate of 100 mV ×/s 1 was used.
3.2. Influence of perphenazine and its oxidation products on DEC SAM structure Potassium ferrocyanide was used as a electrochemical probe to explore the structure change of DEC SAM under different conditions. It was found that, after the used DEC/Au electrode regenerated in blank solution, it did not exhibit visible peak in a potassium ferrocyanide solution, like a fresh prepared DEC/Au electrode. However, after being cycled in a perphenazine solution till the anodic peak disappeared, the DEC/Au could exhibit a pair of irreversible peak in the potassium ferrocyanide solution (Fig. 4). These facts indicate that both perphenazine and its oxidation products can be entrapped in the DEC SAM and they were removed from the SAMs during the regenerating procedure. When perphenazine and its oxidation products were present, the SAM was not so densely packed as they were absent and more needle holes occurred. Therefore, potassium ferrocyanide could partly transit the membrane and exhibited a pair of irreversible redox peaks.
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Fig. 4. CV of 2.0 /10 3 M potassium ferrocyanide at bare gold electrode (A) and DEC/Au (B). Fresh prepared DEC/Au (. . .), regenerated DEC/Au ( */) and the DEC/Au after being cycled in a perphenazine solution (---). Supporting electrolyte: 0.1 M KNO3.
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cyanide was almost inhibited completely. When AC impedance was measured after the DEC/Au electrode was cycled in a perphenazine solution once, the ACIS curve’s diameter (curve c) was far bigger than that of the bare gold electrode, but smaller than that of fresh prepared DEC/Au, indicating potassium ferrocyanide could transit the monolayer partially. After the used electrode was regenerated by the means mentioned above, the ACIS curve (curve d) recorded almost could overlap curve b, which indicates the detained perphenazine and its oxidation products were removed from the SAM and the monolayer recovered the original form. When the ACIS measurement was carried out after the electrode was cycled in a perphenazine solution till the anodic peak disappeared, the ACIS curve became a hemisphere whose diameter was 90.22 kV (curve e). This diameter was bigger than those of curve (a) and (c), but smaller than those of curve (b) and (d), meaning the redox products of perphenazine really stayed in the monolayer and caused more needle holes. These were in accordance with the CV results mentioned above. The change of the SAM structure was illustrated in Scheme 2. 3.3. Influence of solution pH and buffer concentration
3 Fig. 5. Electrochemical impedance spectrums. (a) 2 / /10 M 3 bare gold electrode, (b) DEC/Au, (c) 2 / /10 M DEC/Au after being cycled in a 1/10 5 M perphenazine solution once, (d) (c) after being repeatedly cycled in a blank solution for a minute, (e) (c) after being repeatedly cycled in the perphenazine solution till the anodic peak disappeared. Electrochemical 3 probe; 2 / /10 M potassium ferrocyanide, potential: 0.20 V, diameter: (a) 1.6 kV, (c) 25.6 kV, (e) 90.2 kV.
Fig. 5 shows some alternative current impedance spectrum (ACIS) recorded under given conditions. From the figure, it can be learnt that potassium ferrocyanide could easily get the gold electrode surface in the absence of DEC SAM. But, when the gold electrode was coated by DEC SAM, the ACIS (curve b) almost became a straight line, meaning the transference of potassium ferro-
Because the accumulation of perphenazine at the DEC SAM was related to the hydrophobic interaction between them, the change in lipophilicity of perphenazine made the accumulation amount vary. It is known the hydrophobicity of perphenazine varied with the solution pH due to protonization. In basic environment, perphenazine was neutral and exhibited strong lipophilicity and easily entered the hydrophobic DEC SAM. Therefore, the accumulation amount increased. On the contrary, in acidic solution, perphenazine turned into cation and the hydrophobic interaction between them thus became weak, making the accumulation amount decrease and the voltammetric peak smaller. Fig. 6 shows the influence of solution pH on the peak current and potential. As can be seen, with the solution pH increasing, the peak shifted negatively at a rate of about 26 mV per pH, implying in the electrode reaction the
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Scheme 2. Illustration of the change of the DEC SAM monolayer structure. Conditions were as in Fig. 5. PPZ stands for perphenazine.
number of electron transferred was as two times as that of proton. Meanwhile, the peak current increased and it achieved a maximum at about pH 10. Further increase in solution pH made the peak height decrease. In this work, pH 10 was chosen. When the buffer concentration was changed from 0.0050 to 0.10 M, the peak potential almost kept unchanged, but the peak current gradually increased with the buffer solution concentration rising, so did the peak area. Taking into account the small solubility of sodium borate at room temperature, 0.050 M buffer solution was chosen in this experiments.
3.4. Influence of preconcentration potential and time The preconcentration potential was changed from /0.6 to 0.4 V to explore its influence. As shown in Fig. 7, when the preconcentration potential was /0.4 V, the peak current and peak area reached the maximum values. At preconcentration potential away from /0.4 V, the peak height decreased gradually. Part of this was ascribed to the change of the SAM structure due to the influence of extra charges at the surface. The peak height increased with accumulation time
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Fig. 6. Influence of solution pH on peak current and peak potential. Perphenazine concentration: 5.0/10 5 M, other conditions as in Fig. 1.
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Fig. 8. Variation of peak current and peak area with accumulation time. Perphenazine concentration: 5.0 /10 7 M, accumulation potential: /0.4 V, other conditions were as in Fig. 1.
zine concentration was studied. Under the selected conditions, i.e. ta /2 min, Ea / /0.4 V and pH 10, when the concentration of perphenazine solution changed from 1 /10 9 to 5 /l0 5 M, the peak current increased gradually. Furthermore, the peak current was linear to perphenazine concentration in the ranges of 6/l0 9 /5/l07 M and 5/l0 7 /5/l06 M, respectively (Fig. 9). The regression equations were: ip (mA) /0.33/ 0.79 c (mM) for the range of 6 /109 /5/l07 M (r /0.998) and ip (mA) /0.65/0.33 c (mM) for the range of 5 /l0 7 /5/l0 6 M (r/0.995). It is Fig. 7. Influence of accumulation potential on peak current and peak area. Perphenazine concentration; 5.0/10 7 M, accumulation time: 120 s, other conditions were as in Fig. 1.
increasing until it reached a maximum. For a 1/ 106 M perphenazine solution, the increase of peak current ceased at above 120 s (Fig. 8), meaning that a saturated adsorption was reached. Here, 120 s was chosen as the accumulation time and /0.4 V as preconcentration potential. 3.5. Dependence of peak current on perphenazine concentration To examine the application feasibility of this procedure for perphenazine determination, the relationship between peak current and perphena-
Fig. 9. Dependence of peak current on perphenazine concentration. Preconcentration time: 120 s, accumulation potential: /0.4 V, other conditions were as in Fig. 1. Every point came from the average value of three measurements, and the relative standard deviation of peak current was about 1.0 /4.3% for them.
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clear that the slopes of the two calibration lines are different, which was ascribed to the change of accumulation efficiency. When the concentration exceeded 5 /106 M, the peak current almost stopped increasing, even though the perphenazine concentration was increased, indicating a saturated accumulation was achieved.
3.6. Interference of foreign substance The interference of some compounds that may be present in real samples was examined. The results showed that in the presence of 100-fold dextrose, folic acid, VB1, VB2, thiocarbamide or hypoxanthine, the peak current changed by /6.7, 3.9, 4.6, 3.6 and 2.8%, respectively. Xanthosine, ascorbic acid, Zn2 and epinephrine would make the peak current change more though they were blocked by the DEC SAM. Their interference was thought to partly come from the change of DEC SAM after perphenazine molecule permeated into DEC SAM, which made those foreign substances more easily get electrode surface and affected the response of perphenazine. On the other hand, some of them could be catalytically oxidized by perphenazine radical produced during the electrochemical process and caused catalytic current. This also made the peak current change. But their interference could be eliminated or reduced by adding some reagents into the solution, such as EDTA. In addition, adsorptive transfer stripping voltammetry could be used for such purpose.
3.7. Influence of temperature When the solution temperature changed from 8 to 51 8C, the peak potential shifted. Meanwhile, the peak current changed and got a maximum at about 20 8C. At lower temperature, the diffusion speed decreased, so did electrochemical reaction rate, which made the response of perphenazine weak. When the temperature was higher than 20 8C, the peak became smaller. This should be attributed to the reduction of accumulation amount because the extractive accumulation was weakened at this case.
4. Applications This method was applied to the determination of perphenazine in drug samples (i.e. Fennaijing). The pretreatment and determination procedure of the samples were the same as described in Section 2. The analytical results were shown in Table 1. The recovery was 92 /101%. According to Table 1, the perphenazine contents can be calculated, which were 4.1 and 1.94 mg per tablet for sample 1# (its declared content was 4 mg per tablet) and sample 2# (its declared content was 2 mg per tablet), respectively. This method was simple, sensitive and cheap in comparison with some other methods.
5. Conclusions Perphenazine can be effectively accumulated at a DEC SAM modified gold electrode due to the hydrophobic interaction between DEC SAM and perphenazine. When the potential was made move in positive direction the perphenazine exhibited a sensitive anodic stripping peak at about 0.6 V (vs. SCE). Because of the block action of the SAM to some coexistents, the electrode also showed good selectivity. The electrode could be regenerated easily by cycling in a blank solution and thus could be reused. When perphenazine was present Table 1 Measurement results of perphenazine in drugs Sample drug
#
1 (A)
l#(B) 2#(A)
2#(B)
Perphenazine (mM)
Recovery (%)
Added
Expected
Found
0 1.0 3.0 0 0.5 0 1.0 3.0 0 0.5 1.5
1.03 2.03 4.03 0.51 1.01 0.97 1.97 3.97 0.49 0.99 1.99
1.03 2.04 4.00 0.51 1.01 0.97 1.89 3.96 0.49 0.966 1.96
/ 101 99 / 100 / 92 100 / 95 98
1#: From NO.3 Pharmaceutical Company of Nantong; 2#: from Huaiyin Pharmaceutical Company; (B) was prepared with (A) by diluting.
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the DEC SAM became irregular and less compact, making the corresponding AC impedance spectrum change. But the SAM would recover the origin form soon when perphenazine and its redox products were removed from the SAM.
Acknowledgements This work was supported by the National Nature Science Foundation of China (Grant No: 20173040), the Foundation of State Education Ministry for returned oversea scholar and the Electroanalytical Open Lab of Applied Chemistry Institute of Changehun, China.
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