Gold Electrode Incorporating Corrole as an Ion-Channel Mimetic

Jul 28, 2009 - as ascorbic acid, creatinine, creatine, and uric acid. The detection limits observed ... disease, and also to the HIV infection.1-3 Dop...
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Anal. Chem. 2009, 81, 7397–7405

Gold Electrode Incorporating Corrole as an Ion-Channel Mimetic Sensor for Determination of Dopamine Katarzyna Kurzatkowska,† Eduard Dolusic,‡ Wim Dehaen, Karolina Sieron´-Stołtny,§ Aleksander Sieron´,§ and Hanna Radecka*,† Institute of Animal Reproduction and Food Research of the Polish Academy of Sciences, Tuwima Street 10, 10-747 Olsztyn, Poland, University of Leuven, Chemistry Department, Celestijnenlaan 200F, B-3001 Leuven, Belgium, and Silesian Medical University, Department of Internal Medicine, Batorego Street 15, 41-902 Bytom, Poland Here, we report on an ion-channel mimetic sensor using self-assembly monolayers deposited onto gold electrodes for electrochemical determination of dopamine. The different compositions of the modification solution consist of corrole-SH and other thiol derivatives used as the “background compounds” such as 1-dodecanethiol (DDT), 6-mercapto-1-hexanol (HS(CH2)6OH), or 11-mercapto1-undecanol (HS(CH2)11OH) were explored to find the best self-assembled monolayer (SAM) suitable for dopamine determination. Among them, the mixed SAM consisting of corroles with the -SH group and 6-mercapto-1-hexanol (HS(CH2)6OH) in the molar ratio 1:10 was the most sensitive. The signals generated by the formation of a complex between the corrole host and the dopamine guest were measured by Osteryoung square-wave voltammetry (OSWV) and electrochemical impedance spectroscopy (EIS) with [Ru(NH3)6]3+ as an electroactive marker. The developed sensor was free of interferences of components of human plasma such as ascorbic acid, creatinine, creatine, and uric acid. The detection limits observed by EIS in buffer solution and in the presence of centrifuged human plasma 80 times diluted with a 0.9% NaCl containing 0.01 M borate buffer solution of pH 7.0 were 3.3 × 10-12 and 5.3 × 10-12 M, respectively. Dopamine (DA) is one of the most significant neurotransmitters because of its role in the functioning of the cardiovascular, renal, hormonal, and central nervous system. An abnormal dopamine transmission has been linked to several neurological disorders, e.g., schizophrenia, Huntington’s disease, Parkinson’s disease, and also to the HIV infection.1-3 Dopamine is also used * Corresponding author. Phone: +48895234636. Fax: +48895240124. E-mail: [email protected]. † Institute of Animal Reproduction and Food Research of the Polish Academy of Sciences. ‡ University of Leuven. § Silesian Medical University. (1) Cooper, J. R.; Bloom, R. H.; Roth, R. H. The Biochemical Basis of Neuropharacology; Oxford University Press, Oxford, UK, 1982. (2) Damier, P.; Hirsch, E. C.; Agid, Y.; Graybiel, A. M. Brain 1999, 122, 1437– 1448. (3) Ali, S. R.; Ma, Y.; Parajuli, R. R.; Balogun, Y.; Lai, W. Y. C.; He, H. Anal. Chem. 2007, 79, 2583–2587. 10.1021/ac901213h CCC: $40.75  2009 American Chemical Society Published on Web 07/28/2009

as the therapeutic drug to reduce the risk of renal failure by increasing renal blood flow4,5 and to increase of the splanchnic blood flow and splanchnic oxygen uptake in patients with septic shock.6 Hence, there is a great interest in the monitoring of the concentration of dopamine using a reliable method with good sensitivity and selectivity. Spectrophotometry7-9 and chromatography10-13 are the most frequently applied. While these methods require sophisticated and expensive instrumentation, methods based on electrochemical measurements offer advantages in that they are simple, rapid, and easy to apply, still providing enough sensitivity to detect submicromolar concentrations of analytes. Ion-selective electrodes modified with hexahomotrioxacalix[3]arene,14 crown ether,15 or β-cyclodextrin16 were applied for potentiometric determination of dopamine. The main disadvantage of these methods is the sensitivity in the range of 0.01-0.1 mM. Therefore, the ISEs could be applied mainly for determination of DA in drugs. The majority of electrochemical sensors for dopamine determination exploit its ease of oxidation. However, the oxidative approaches suffer from interferences caused by other electroactive substances existing in the physiological samples. One of the main interferences is ascorbic acid (AA). The concentration of DA is (4) Bellomo, R.; Chapman, M.; Finfer, S.; Hickling, K.; Myburgh, J. Lancet 2000, 356, 2139–2143. (5) Brienza, N.; Malcangi, V.; Dalfino, L.; Trerotoli, P.; Guagliardi, C.; Bortone, D.; Facond, G.; Ribezzi, M.; Ancona, G.; Bruno, F.; Firore, T. Crit. Care Med. 2006, 34, 707–714. (6) Meier-Hellmann, A.; Bredle, D. L.; Specht, M.; Spies, C.; Hannemann, L.; Reinhart, K. Intens. Care Med. 1997, 23, 31–37. (7) Nagaraja, P.; Vasantha, R. A.; Sunitha, K. R. Talanta 2001, 55, 1039–1046. (8) Nagaraja, P.; Vasantha, R. A.; Sunitha, K. R. J. Pharm. Biomed. Anal. 2001, 25, 417–424. (9) Baron, R.; Zayats, M.; Willner, I. Anal. Chem. 2005, 77, 1566–1571. (10) Ji, Ch.; Li, W.; Ren, X.; El-Kattan, A. F.; Kozak, R.; Fountain, S.; Lepsy, Ch. Anal. Chem. 2008, 80, 9195–9203. (11) Carrera, V.; Sabater, E.; Vilanova, E.; Sogorb, M. A. J. Chromatogr., B 2007, 847, 88–94. (12) Yoshitake, M.; Nohta, H.; Ogata, S.; Todoroki, K.; Yoshida, H.; Yoshitake, T.; Yamaguchi, M. J. Chromatogr., B 2007, 858, 307–312. (13) Yoshitake, T.; Yoshitake, S.; Fujino, K.; Nohta, H.; Yamaguchi, M.; Kehr, J. J. Neurosci. Methods 2004, 140, 163–168. (14) Saijo, R.; Tsunekawa, S.; Murakami, H.; Shirai, N.; Ikeda, S.; Odashima, K. Bioorg. Med. Chem. Lett. 2007, 17, 767–771. (15) Othman, A. M.; Rizka, N. M. H.; El-Shahawi, M. S. Anal. Sci. 2004, 20, 651–655. (16) Lima, J. L. F. C.; Montenegro, M. C. B. S. M. Mikrochim. Acta 1999, 131, 187–190.

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extremely low (0.01-1 µM) while that of AA is as high as 0.1 mM in biological systems.3,17 Moreover, at almost all electrodes materials, DA and AA are oxidized at nearly the same potential, which results in overlapped voltammetric response. Therefore, it is very important to eliminate AA and other interfering compounds, or to use a special modification layer allowing one to simultaneously detect both DA and AA at different potentials. In order to solve this problem, various modified electrodes have been constructed such as a carbon fiber electrode,18-20 electrodes modified with polymers,17,21-24 electrodes modified with nanomaterials,3,25-29 and electrodes modified with biomolecules such as DNA or peroxidase,30-34 just to name a few. Among these numerous methods, the use of self-assembled monolayers (SAMs) carrying different functional groups were identified as the very useful approach for the modification of the electrode surface, destined for the simultaneous electrochemical determination of DA and AA. Pioneering work in this field was done by Malem and Mandler.35 They applied ω-mercaptocarboxylic acids SAM to separate the oxidation potential of DA and AA. A COOHterminated SAM formed by 3,3′-dithiopropionic acid on the gold electrode displaying a detection limit in the micromolar range was suitable for DA determination in pharmaceutical formulations.36 Raj et al. reported a cationic SAM modified gold electrode containing 2,2′-dithiobisethaneamine (CYST) and 6,6′-dithiobishexaneamine (DTH) for the square-wave voltammetric detection of DA in the presence of AA.37 The voltammetric behavior of DA on a gold electrode modified with SAM of N-acetylcysteine has been investigated by Liu et al.38 They observed that the oxidation peak current increased linearly with the concentration of DA in the range of 1.0 × 10-6 to 2.0 × 10-4 M. Hu et al. used an L-cysteine self-assembled modified electrode for the detection of DA by the chronoamperometric method in the presence (17) Ghita, M.; Arrigan, D. W. M. Electrochim. Acta 2004, 49, 4743–4751. (18) Gonon, F. G.; Fomarlet, C. M.; Buda, M. J.; Pujol, J. F. Anal. Chem. 1981, 53, 1386–1389. (19) Heien, M. L. A. V.; Phillips, P. E. M.; Stuber, G. D.; Seipel, A. T.; Wightman, R. M. Analyst 2003, 128, 1413–1419. (20) Hermans, A.; Keithley, R. B.; Kita, J. M.; Sombers, L. A.; Wightman, R. M. Anal. Chem. 2008, 80, 4040–4048. (21) Tu, X.; Xie, Q.; Jiang, S.; Yao, S. Biosens. Bioelectron. 2007, 22, 2819– 2826. (22) Lin, X.; Zhang, Y.; Chen, W.; Wu, P. Sens. Actuators, B 2007, 122, 309– 314. (23) Yi, S.-Y.; Chang, H.-Y.; Cho, H.; Park, Y. C.; Lee, S. H.; Bae, Z.-U. J. Electroanal. Chem. 2007, 602, 217–225. (24) Kumar, S. A.; Tang, C.-F.; Chen, S.-M. Talanta 2008, 74, 860–866. (25) Ali, S. R.; Ma, Y.; Parajuli, R. R.; Balogun, Y.; Lai, W. Y.-C.; He, H. Anal. Chem. 2007, 79, 2583–2587. (26) Boo, H.; Jeong, R.; Park, S.; Kim, K. S.; An, K. H.; Lee, Y. H.; Han, J. H.; Kim, H. C.; Chung, T. D. Anal. Chem. 2006, 78, 617–620. (27) Bustos, E.; Garcia Jimenez, M. G.; Diaz-Sanchez, B. R.; Juaristi, E.; Chapman, T. W.; Godinez, L. A. Talanta 2007, 72, 1586–1592. (28) Huang, X.; Li, Y.; Wang, P.; Wang, L. Anal. Sci. 2008, 24, 1563–1568. (29) Alarco´n-Angeles, G.; Pe´rez-Lo´pez, B.; Palomar-Pardave, M.; Ramı´rez-Sliva, M. T.; Algret, S.; Merkoc¸i, A. Carbon 2008, 46, 898–906. (30) Castilho, T. J.; del Pilar Taboada Sotomayor, M.; Kubota, L. T. J. Pharm. Biomed. Anal. 2005, 37, 785–791. (31) Moccelini, S. K.; Fernandes, S. C.; Vieira, I. C. Sens. Actuators, B 2008, 133, 364–369. (32) Lin, X.; Kang, G.; Lu, L. Bioelectrochemistry 2007, 70, 235–244. (33) Lin, X. Q.; Lu, L. P.; Jiang, X. H. Microchem. Acta 2003, 143, 229–235. (34) Lu, L. P.; Lin, X. Q. Anal. Sci. 2004, 20, 527–530. (35) Malem, F.; Mandler, D. Anal. Chem. 1993, 65, 37–41. (36) Codognoto, L.; Winter, E.; Paschola, J. A. R.; Suffredini, H. B.; Cabral, M. F.; Machado, S. A. S.; Rath, S. Talanta 2007, 72, 427–433. (37) Raj, C. R.; Tokuda, K.; Ohsaka, T. Bioelectrochemistry 2001, 53, 183–191. (38) Liu, T.; Li, M.; Li, Q. Talanta 2004, 63, 1053–1059.

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of AA.39 The detection limit obtained with this modification was 2.0 × 10-8 M DA. Although these sensors allow determination of DA based on its redox property, none of them are totally free from interferences coming from AA or other electrochemically active substances. Also, detection limits are still insufficient for the direct determination of DA occurring in the physiological samples at very low concentration 0.01-1 µM. Here, we report a different approach for the detection of DA based on an ion-channel mimetic sensor. Binding of analytes to receptors immobilized on electrode surfaces facilitates or suppresses the access of an anionic (cationic) marker ion to the modified surface due to electrostatic attraction or repulsion of the marker and/or distortion of the modification layer arrangement. This leads to the changes of the electron transfer rate between the marker and electrode surface through the modification layer. Because the working principle of these sensors is similar to that of ligand gated ion-channel proteins in biomembranes, they are named “ion-channel”. These types of electrochemical sensors have been introduced and developed by Umezawa and co-workers.40,41 Gold electrodes modified with macrocyclic polyamines, working as “ion-channel” sensors have been successfully applied for the voltammetric detection of adenine nucleotides42,43 and voltammetric discrimination of maleic and fumaric acids.44 It has been already reported that corrole derivatives form complexes with dihydroxybenzene derivatives through a NH · · · OH hydrogen bond.45 Thus, this compound has been selected as a suitable receptor for dopamine in the study presented. Scheme 1 illustrates the working principle of the sensor proposed. The corrole host molecules have been covalently attached on the surface of gold electrodes through Au-S bonds. In the measuring condition (pH 7.0), corrole exists as an uncharged molecule.45 The monolayer created on the electrode surface is quasi permeable for a redox marker existing in the aqueous solution. Upon addition of dopamine, the corrole-DA complex is formed and the monolayer gained the positive charge, which repealed the positive charged redox marker. This leads to the decreasing of the electron transfer rate between the marker and electrode surface through the modification layer (Scheme 1). The electroanalytical signals generated based on the corrole-DA complex formed at the electrode surface were explored using Osteryoung square-wave voltammetry (OSWV) and electrochemical impedance spectroscopy (EIS) in the presence of [Ru(NH3)6]3+ as an electroactive marker, which does not interfere with the oxidation of the DA or AA. (39) Hu, G.; Liu, Y.; Zhao, J.; Cui, S.; Yang, Z.; Zhang, Y. Bioelectrochemistry 2006, 69, 254–257. (40) Sugawara, M.; Kojima, K.; Sazawa, H.; Umezawa, Y. Anal. Chem. 1987, 59, 2842–2846. (41) Umezawa, Y.; Aoki, H. Anal. Chem. 2004, 76, 321A–326A. (42) Szyman´ska, I.; Radecka, H.; Radecki, J.; Pietraszkiewicz, M.; Pietraszkiewicz, O. Comb. Chem. High Throughput Screening 2000, 3, 509–519. (43) Radecka, H.; Szyman´ska, I.; Pietraszkiewicz, M.; Pietraszkiewicz, O.; Aoki, H.; Umezawa, Y. Anal. Chem. (Warsaw, Poland) 2005, 50, 85–102. (44) Radecki, J.; Szyman´ska, I.; Bulgariu, L.; Pietraszkiewicz, M. Electrochim. Acta 2006, 51, 2289–2297. (45) Radecki, J.; Stenka, I.; Dolusic, E.; Dehaen, W.; Plavec, J. Comb. Chem. High Throughput Screening 2004, 7, 375–381.

Scheme 1. Schematic Illustration of Amperometric Response of a Gold Electrode Coated with HSCOR/ HS(CH2)6OH SAM Generated in the Presence of Dopamine

EXPERIMENTAL SECTION Synthesis of Corrole. The corrole (HSCOR) was synthesized in adaptation of our reported procedures46-48 from 4-(10-undecenyloxy)benzaldehyde. 4-(11-Thioacetoxyundecyl)benzaldehyde. Aldehyde 4-(10-undecenyloxy)benzaldehyde (7.450 g; 27.15 mmol), thioacetic acid (4 mL; 56.23 mmol), and azo-bis-isobutyronitrile (229 mg; 1.37 mmol) were dissolved in 90 mL of toluene (p.a.). The mixture was degassed with a stream of argon and then refluxed for 6.5 h. The reaction was quenched with 5% aqueous NaHCO3 (400 mL) and extracted three times with ethyl acetate. The combined organic layers were washed with 5% aqueous NaHCO3 and then brine (46) Asokan, C. V.; Smeets, S.; Dehaen, W. Tetrahedron Lett. 2001, 42, 4483– 4485. (47) Rohand, T.; Dolusic, E.; Ngo, T. H.; Maes, W.; Dehaen, W. Arkivoc 2007, 307–324. (48) Maes, W.; Ngo, T. H.; Vanderhaeghen, J.; Dehaen, W. Org. Lett. 2007, 9, 3165–3168.

and dried over MgSO4. Upon filtration and removal of the solvent under reduced pressure, the yellow solid residue was chromatographed on silica eluting with a gradient of 5:1 to 1:1 light petroleum ether/ethyl acetate. The crude product obtained was recrystallized from methanol to afford 4.086 g (43%) of the title aldehyde as a white powder. 1H NMR (300 MHz) 9.88 (s, 1H, aldehyde H), 7.83 (d, J 8.6, 2H, aryl H), 6.99 (d, J 8.6, 2H, aryl H), 4.04 (t, J 6.6, 2H, undecenyl 1-H), 2.86 (t, J 7.3, 2H, undecenyl 11-H), 2.32 (s, 3H, CH3), 1.81 (quintet, J 7.3, 2H, undecenyl 2-H), 1.46-1.28 (m, 16H, 8CH2). meso-5,15-Bis(2,6-dichlorophenyl)-10-(4-(11-thioacetoxy-1-dodecyloxy) phenyl)corrole. meso-(2,6-Dichlorophenyl)dipyrromethane (1.522 g; 5.23 mmol) and the previous aldehyde (606 mg; 1.73 mmol) were dissolved in 146 mL of dichloromethane. The reaction flask was wrapped in aluminum foil and placed in an ice bath. Argon was bubbled through the solution for 15 min. TFA (10 µL; 0.13 mmol) was added, and the mixture was stirred under an argon atmosphere for ∼48 h. The ice bath was removed, and 935 mg (3.77 mmol) of p-chloranil was added. After an additional 1 h at room temperature, the mixture was evaporated with silica and chromatographed twice in a 1.5:1 mixture of dichloromethane and n-heptane to afford 452 mg (29%) of the title corrole. 1H NMR 9.00 (d, J 4.1, 2H, 2-H and 18-H of corrole), 8.62 (d, J 4.7, 2H, 8-H and 12-H of corrole), 8.53 (d, J 4.7, 2H 7-H, and 13-H of corrole), 8.42 (d, J 4.1, 2H, 3-H and 17-H of corrole), 8.08 (d, J 8.5, 2H, aryl o-H), 7.78 (d, J 7.7, 4H, dichlorophenyl m-H), 7.66 (dd, J1 7.7, J2 8.7, 2H, dichlorophenyl p-H), 7.27 (d partially overlapped with CHCl3, J 8.5, 2H, aryl m-H), 4.24 (t, J 6.5, 2H, OCH2), 2.89 (t, J 7.3, 2H, SCH2), 2.32 (s, 3H, CH3), 1.98 (quintet, J 7.4, 2H, β-CH2), 1.62-1.36 (m, 16H, 8CH2), -2.30 (br. s, 3H, NH); δC 196.0, 158.9, 138.6 (quaternary C), 137.4 (quaternary C), 135.5 (CH), 134.0 (quaternary C), 130.3 (dichlorophenyl p-C), 128.0 (dichlorophenyl m-C), 127.2 (pyrrole CH), 125.7 (pyrrole CH), 120.8 (C-3 and C-17 of corrole), 116.2 (C-2 and C-18 of corrole), 113.2 (CH), 111.7 (quaternary C), 108.8 (quaternary C), 68.3, 30.6, 29.6, 29.5, 29.1, 28.8, 26.2; UV-vis 410.1 (1.35 × 105), 566.1 (0.22 × 105); ESI-MS 907 (MH+). meso-5,15-Bis(2,6-dichlorophenyl)-10-(4-(11-mercapto-1-dodecyloxy)phenyl)corrole. Acetyl protected corrole (130 mg; 0.14 mmol) was dissolved in 7.5 mL of THF and 3 mL of methanol. The flask was placed in an ice bath, and 1 mL of a 1.65 M solution of CH3ONa in methanol (1.65 mmol of CH3ONa) was added. The reaction mixture was stirred at 0 °C for 30 min and then poured into diluted aqueous HCl (∼0.02 M). This solution was extracted with ethyl acetate (some brine had to be added for a better separation) and then washed with brine and distilled water and dried over MgSO4. Upon filtration and removal of the solvent under reduced pressure, 26 mg (21%) of the product was isolated by chromatography in mixtures of CH2Cl2 and hexane (1.5:1, column and 1:1, preparative plate). 1H NMR 8.97 (d, J 3.9, 2H, 2-H and 18-H of corrole), 8.61 (d, J 4.5, 2H, 8-H and 12-H of corrole), 8.51 (d, J 4.5, 2H, 7-H and 13-H of corrole), 8.39 (d, J 3.9, 2H, 3-H and 17-H of corrole), 8.07 (d, J 8.2, 2H, aryl o-H), 7.72 (m, 4H, dichlorophenyl m-H), 7.59 (dd, J1 8.3, J2 8.3, 2H, dichlorophenyl p-H), 7.24 (d partially overlapped with CHCl3, J 8.2, 2H, aryl m-H), 4.20 (t, J 6.4, 2H, OCH2), 2.50 (t, J 6.8, 2H, SCH2), 1.93 (quintet, 2H, β-CH2), 1.60 (m, 4 H, 2 Analytical Chemistry, Vol. 81, No. 17, September 1, 2009

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9.5 × 10-11 1.7 × 10-11

Y1 ) -3.6 log(x) + 59.7 r2 ) 0.968, σ ) 0.96 Y2 ) 10.0 log(x) + 104.1 r2 ) 991, σ ) 0.54 -11 -7 10 )-(1.0 × 10 ) Y1 ) -3.6 log(x) + 61.5 r2 ) 0.993, σ ) 0.66 Y2 ) 13.1 log(x) + 138.2 r2 ) 0.983, σ ) 1.31 -10 -6 10 )-(1.0 × 10 ) 10-11)-(5.6 × 10-9) 10-12)-(3.2 × 10-10) Y1 ) -4.5 log(x) + 43.8 r2 ) 0.868, σ ) 1.07 Y2 ) 16.2 log(x) + 193.1 r2 ) 0.906, σ ) 1.15 10-9)-(1.0 × 10-5) Y1 ) -6.3 log(x) + 24.9 r2 ) 0.959, σ ) 0.44 Y2 ) 28.6 log(x) + 309.6 r2 ) 0.929, σ ) 13.89 10-9)-(1.0 × 10-5) (1.0 × 10 )-(1.0 × 10 )

(1.0 ×

(1.0 ×

(1.0 × (1.0 × (3.2 ×

0.01

0.01 0.05 0.10

2

3 4 5

1.0 7

6

a

0.01 1

1.0

1.0

1.0 1.0

1.0

1.0

(3.2 ×

-9

The linear relationship: Y1 ) OSWV response (ip/i0) × 100%; i0, OSWV peak current in the absence of analytes, ip, OSWV peak current in the presence of a given concentration of analytes. Y2 ) EIS response (Ri - R0/R0) × 100%; R0, EIS resistance in the absence of analytes, Ri, EIS resistance in the presence of a given concentration of analytes. x ) concentration of analyte; r2, correaltion coefficient; σ, average standard deviation, n ) 5. b Human plasma after filtration and centrifugation for 30 min at 14 000g rcf with Millipore Microcon Ultracel YM-3 and was diluted 80 times with 0.9% NaCl + 0.01 M borate buffer pH 7.0; the volume in electrochemical cell: 2.0 mL.

5.3 × 10-12 Y1 ) -15.0 log(x) - 94.8 r2 ) 0.912, σ ) 1.44 Y2 ) 56.1 log(x) + 695.6 r2 ) 0.985, σ ) 3.10

3.3 × 10-12

9.5 × 10-11 6.1 × 10-11

8.3 × 10 2.6 × 10

2.4 × 10-12

1.5 × 10-12

EIS OSWV EIS

-10 -10

OSWV HSCOR DDT HS(CH2)6OH HS(CH2)11OH no. [mM] [mM] [mM] [mM]

linear range [M]

-5

buffer solutiona

human plasmaa,b

detection limit [M] in buffer solution detection limit [M] in human plasmab composition of modification solution

Table 1. Composition of the Chloroform Solutions Used for the Gold Electrode Modification and Their Responses Towards DA

CH2), 1.39-1.26 (m, 13H, 6CH2 and SH), ∼-1.9 (br. s, 3H, NH);13C NMR 158.8 (CO), 142.1 (9-C and 11-C of corrole), 139.8 (6-C and 14-C of corrole), 138.5, 137.4, 135.5, 134.6, 134.0, 130.7, 130.3, 128.0 (3-C and 5-C of 5,15-dichlorophenyl), 127.1, 125.7 (8-C and 12-C of corrole), 120.8 (A and D ring β-pyrrole C), 116.2 (A and D ring β-pyrrole C), 113.2 (A and D ring β-pyrrole C), 111.7 (5-C and 15-C of corrole), 108.9 (10-C of corrole); ESI-MS 865 (MH+). Reagents and Materials. 1-Dodecanethiol (DDT), 6-mercapto1-hexanol (HS(CH2)6OH), 11-mercapto-1-undecanol (HS(CH2)11OH), [Ru(NH3)6]Cl3, dopamine hydrochloride (DA), creatinine, creatine, uric acid, chloroform were purchased from SigmaAldrich (Poznan´, Poland). KOH, KNO3, NaCl, borate buffer, L(+)ascorbic acid (AA), ethanol were obtained from ABChem, Gliwice, Poland. All aqueous solutions were prepared with deionized and charcoal-treated water (resistivity of 18.2 MΩ cm) purified with a Milli-Q reagent grade water system (Millipore, Bedford, MA). Electrode Preparation. Gold disk electrodes of 2 mm2 area (Bioanalytical Systems (BAS), West Lafayette, IN) used for the experiments were polished with wet 0.3 and 0.05 µm alumina slurry (Alpha and Gamma Micropolish, Buehler, Lake Bluff, IL) on a flat pad for at least 10 min and rinsed repeatedly with water and finally in a sonicator (30 s). The polished electrodes were then dipped in 0.5 M KOH solution deoxygenated by purging with argon for 15 min, and the potential was cycled between -400 and -1200 mV (versus Ag/AgCl reference electrode) with a scan rate of 100 mVs-1 until the CV no longer changed. The electrochemically polished electrodes were dipped into different modification solutions in chloroform at room temperature for 30 min (Table 1). The modification solutions were put into the tubes (8 mm diameter, with no flat bottom). After the electrodes were dipped, the tubes were sealed with Teflon tape in order to avoid the solvent evaporation. The modified electrodes were stored in buffer solution (0.1 M KNO3, 0.01 M borate buffer, pH ) 7.0) until used. Electrochemical Measurements. All electrochemical measurements were performed with a potentiostat-galvanostat AutoLab (Eco Chemie, Utrecht, Netherlands) with a three-electrode configuration. Potentials were measured versus the Ag/AgCl electrode, and a platinum wire was used as an auxiliary electrode. Osteryoung square-wave voltammetry (OSWV) was performed with a potential from 0 to -500 mV, and with a step potential of 5 mV, a square-wave frequency of 100 Hz, and an amplitude of 50 mV for 1.0 mM [Ru(NH3)6]Cl3. Electrochemical impedance spectroscopy (EIS) measurements were recorded within the frequency range of 0.1 Hz to 10 kHz at the formal potential of the redox couple [Ru(NH3)6] 3+/2+ (-0.20 V) with an amplitude of 10 mV. The dependence of the Osteryoung square-wave voltammograms and electrochemical impedance spectra on the pH was measured in a solution containing 0.1 M KNO3 and 0.01 M borate buffer. The pH of this solution was adjusted with 0.1 M KOH or 0.1 M HNO3. Determination of Dopamine in Diluted Human Plasma. Gold electrodes modified with HSCOR/HS(CH2)6OH (modification no. 5, Table 1) were used for voltammetric and impedimetric determination of DA in human plasma. Natural human plasma

Figure 1. The absorption spectra of 1.0 × 10-5 M HSCOR in DMSO in the presence of different concentrations of DA: (a) 0, (b) 2.5 × 10-5, (c) 1.0 × 10-4, (d) 5.0 × 10-4, and (e) 7.5 × 10-4 M.

obtained from the Regional Center of Blood-Donation in Olsztyn was filtered with a Millipore Micron Ultracel YM-3 and centrifuged for 30 min at 14 000g rcf in order to remove proteins with a molecular weight above 3 kDa. The filtered human plasma was diluted 80 times with a 0.9% NaCl + 0.01 M borate buffer solution of pH 7.0. A 2.0 mL amount of thus prepared human plasma samples were spiked with a known amount of DA and analyzed with OSWV and EIS. In order to check the influence of the centrifugation and filtration on the dopamine concentration, 500 µL of human plasma was spiked with 0.1 mM DA. This sample was centrifuged for 30 min at 14 000g rcf. The concentration of DA in the supernatant was determined by UV-vis spectroscopy. UV-Visible Confirmation of Dopamine-Corrole Complex Formation. A stock solution of 1.0 × 10-3 M was prepared by dissolving DA in DMSO. The working solution of HSCOR for spectrophotometric measurements in DMSO was 1.0 × 10-5 M. UV-vis spectra of 1.0 × 10-5 M of HSCOR in the presence of different concentrations of DA: (a) 0, (b) 2.5 × 10-5, (c) 1.0 × 10-4, (d) 5.0 × 10 -4, and (e) 7.5 × 10-4 M were recorded with an UV mini-1240 spectrophotometer (Shimadzu, Kyoto, Japan). RESULTS AND DISCUSSION Formation and Characterization of Corrole Incorporating SAMs. It has been already discovered that corrole forms complexes with the hydroxy function of phenols via the formation of a hydrogen bond.45 The corrole-dopamine interaction was proven by UV-vis measurements (Figure 1). Therefore HSCOR was selected as the host molecule for the electrochemical determination of DA and was used for chemical modification of gold electrodes via Au-S bond formation. In order to show the influence of the SAM composition used for the gold electrode modification on the sensitivity of DA determination, five different mixed SAMs were explored (Table 1). Mixed SAMs were used in order to prevent mutual interactions between corrole macrocycles. The same strategy was applied for the creation of

macrocyclic polyamine SAMs43,44 and dipyrromethene SAMs.49 1-Dodecanethiol (DDT), 6-mercapto-1-hexanol (HS(CH2)6OH), and 11-mercapto-1-undecanol (HS(CH2)11OH) were applied for “dilution” of corrole in mixed monolayers. HSCOR has a longer alkyl chain than HS(CH2)6OH; therefore, in these type of SAMs, corrole “heads” were distributed over the surface of HS(CH2)6OH, which creates a well ordered monolayer through hydrogen bond between -OH groups (Table 1, modification nos. 2, 4, and 5). The lengths of the alkyl chains of DDT, HS(CH2)11OH, and HSCOR are similar, so the corrole “heads” were surrounded by OH or CH3 groups in modification nos. 1 and 3 (Table 1). The corroles, because of their unusually high N-H acidity relative to porphyrins, could exist, depending on the pH, in a neutral, deprotonated and protonated form.45,50 The binding of protons by corroles’ receptors, in relation to the pH of the aqueous phase, changes the surface charge from neutral (pH 8.0) to positive (pH 4.0), thereby suppressing the access of positively charged [Ru(NH3)6]3+ and the subsequent electron transfer rate between the electrode and maker. The OSWV and EIS performed at pH 4.0 and at pH 8.0 were used as a simple test for checking the SAMs quality for every type of modification. All of the modifications incorporating HSCOR gave similar results. A typical pH test for HSCOR-HS(CH2)6OH SAM no. 5 is shown in Figure 2. The pH tests for modification nos. 1 and 3 are shown in the Supporting Information (Figure S-1,A,B). Only these electrodes that displayed good response toward protons have been used for the determination of DA. A similar pH test was used for confirmation of the presence of macrocyclic polyamine43,44 and dipyrromethene49 on the surface of gold electrodes. The responses toward pH were also recorded for SAMs consisting only of DDT (modification no. 6) and HS(CH2)6OH (modification no. 7; Table 1). For these SAMs, pH responses were not observed. The representative OSWV and EIS are illustrated in the Supporting Information, Figure S-1,C,D. Response of Corrole Mixed SAMs toward Dopamine. The sensing of DA molecules by gold electrodes modified with corrole has been examined with Osteryoung square-wave voltammetry and electrochemical impedance spectroscopy techniques. The measurements were done in 0.1 M KNO3 and 0.01 M borate buffer (pH 7.0). Among the following marker candidates: [Fe(CN)6]3-, [Ru(NH3)6]3+, and [IrCl6]2-, the most suitable for study of the interactions between neutral corrole host immobilized on the gold electrode surface and DA guest existing in the aqueous solution as the protonated species was [Ru(NH3)6]3+. The oxidation potential for DA and AA on every type of modification using a mixed monolayer was at 0.380 ± 0.015 and 0.060 ± 0.012 V, respectively. Ruthenium complex oxidation was observed at -0.204 ± 0.01 V. Thus, when [Ru(NH3)6]3+ was used as the redox marker in the sample solutions, oxidation of DA as well as AA did not occur. (49) Szyman´ska, I.; Orlewska, C.; Janssen, D.; Dehaen, W.; Radecka, H. Electrochim. Acta 2008, 53, 7932–7940. (50) Mohammed, A.; Weaver, J. J.; Gray, H. B.; Abdelas, M.; Gross, Z. Tetrahedron Lett. 2003, 44, 2077–2079.

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Figure 2. The OSWV and EIS of HSCOR/HS(CH2)6OH (1:10) modified gold electrode in the electrolyte solution (a) pH 5.0 and (b) pH 8.0. The electrolyte composition: 0.1 M KNO3, 0.01 M borate buffer, 1.0 mM [Ru(NH3)6]Cl3; the pH of the solution was adjusted with 0.1 M NaOH or 0.1 M HNO3. The OSWV was performed with step potential: 5 mV, square-wave frequency 100 Hz, and square-wave amplitude 50 mV. The EIS measurements were recorded within the frequency range of 0.1 Hz to 10 kHz at the formal potential of the redox couple [Ru(NH3)6]3+/2+ (-0.20 V) with ac amplitude of 10 mV.

The responses toward DA of all modified gold electrodes (Table 1, Figure 3) were checked in order to find the most suitable one. On the basis of the results obtained, the best compound for dilution of HSCOR in a mixed SAM is HS(CH2)6OH. Therefore, for this type of modification, the molar ratio between HSCOR and HS(CH2)6OH was optimized. Among the compositions studied (Table 1; modification nos. 2, 4, and 5), this one with a HSCOR/HS(CH2)6OH molar ratio equal to 1:10 was the most sensitive toward DA. With an increasing concentration of DA in the range from 3.2 × 10-12 to 3.2 × 10-10 M, the peak currents of OSWV decreased and the peak potentials shifted more negatively (Figure 3A). Taking into account a signal-to-noise ratio of 3, the detection limit was estimated as 2.4 × 10-12 M. The responses of the gold electrodes modified with HSCOR/ HS(CH2)6OH were also measured using the EIS technique. The measurements were carried out under the same conditions as for OSWV. Representative EIS curves obtained for Au-HSCOR/ HS(CH2)6OH electrodes in the presence of DA are illustrated in Figure 3A. The diameter of Nyquist circles increases with the increasing concentration of DA. All the impedance spectra were fitted to the Randles equivalent circuit in order to obtain the values of charge transfer resistance for each concentration of DA studied. In order to compare the different electrodes measured in the same conditions, the relative values: [(Rct - Rct(0))/Rct(0)] × 100% were used as analytical signals. The detection limit was estimated as 3.3 × 10-12 M, defined from a signal-to-noise ratio of 3. EIS is more sensitive in comparison to OSWV. The slope of the calibration curves obtained for modification no. 5 with OSWV and EIS were 4.5 [%] M-1 and 16.2 [%] M-1, respectively (Table 1). Representative OSWV and EIS curves obtained for Au-HSCOR/HS(CH2)11OH (modification no. 3, Table 1) are shown in Figure 3B. This modification was less sensitive toward DA. The surrounding of corrole macrocycle by tightly packed OH groups makes the binding sites more rigid and less accessible for DA. This is probably the main reason for the lower response. The corrole macrocycles were surrounded by hydrophobic CH3 groups in the case of modification no. 1 (Table 1). The sensitivity of this modification toward DA was quite good (Table 7402

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1), but unfortunately, this modification was not selective. A control experiment proved that responses of DDT SAM (no. 6) toward DA are even better than the one obtained using Au-HSCOR/DDT (Table 1; Figure 3C,E). In this case, unspecific hydrophobic interactions between DDT SAM and DA occurred. Therefore, DDT is not a suitable compound for the formation of mixed monolayers with HSCOR destined for DA determination. A similar control experiment was performed with a HS(CH2)6OH SAM (no. 7). This monolayer showed no response toward DA in the buffer solution (Figure 3D). HS(CH2)6OH molecules formed well-packed monolayers through hydrogen bonds between OH groups. This eliminates unspecific interactions with DA. The above results allow us to state that among the modifications studied, this with HSCOR/HS(CH2)6OH in a molar ratio of 1:10 was the most suitable for determination of DA. The gold electrodes modified with this composition, after measuring the DA response, were restored by conditioning in the buffer solution. They could be used five times to explore DA responses with the same sensitivity. Also, storing of this type of electrode in buffer solution in the refrigerator during 1 month has no influence on its sensitivity toward DA. Therefore, this modification was used for the future experiments. Effect of Human Plasma Components on Dopamine Determination by Gold Electrode Modified with HSCOR/ HS(CH2)6OH SAM. In order to demonstrate the applicability of the sensor proposed, the influence of main components of human plasma on the electrochemical determination of DA with Au-HSCOR/HS(CH2)6OH were checked. The following compounds were explored: ascorbic acid (AA), creatinine, creatine, and uric acid. With 1.0 × 10-4 M AA in the buffer solution, the data points in the calibration curve are parallel to points in the curve obtained for DA in the buffer solution without AA (Figure S-2, Supporting Information). These results demonstrate the nearcomplete elimination of interference from ascorbic acid. Similar results were obtained in the presence of other components of human plasma. Thus, it might be concluded that the Au electrode

Figure 3. The OSWV and EIS responses of gold electrodes modified with (A) HSCOR/HS(CH2)6OH, (B) HSCOR/HS(CH2)11OH, (C) HSCOR/ DDT, (D) HS(CH2)6OH, and (E) DDT toward DA in buffer solution. The electrolyte composition: 0.1 M KNO3, 0.01 M borate buffer pH 7.0, 1.0 mM [Ru(NH3)6]Cl3; concentration of DA: (A) (3.2 × 10-12)-(3.2 × 10-10); (B-E) (1.0 × 10-10)-(1.0 × 10-6) M. The parameters of OSWV and EIS measurements, see Figure 2. Analytical Chemistry, Vol. 81, No. 17, September 1, 2009

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Figure 4. The OSWV (A) and EIS (B) response of gold electrodes modified with HSCOR/HS(CH2)6OH (1:10) toward DA in an 80-fold diluted human plasma solution. Concentrations of DA: (a) 0, (b) 3.2 × 10-12, (c) 5.6 × 10-12, (d) 1.0 × 10-11, (e) 1.8 × 10-11, (f) 3.2 × 10-11, (g) 5.6 × 10-11, (h) 1.0 × 10-10, (i) 1.8 × 10-10, and (j) 3.2 × 10-10 M. The solution composition: human plasma centrifuged and 80-fold diluted with 0.9% NaCl, 0.01 M borate buffer pH 7.0, 1.0 mM [Ru(NH3)6]Cl3. The volume in the measuring cell was 2.0 mL (see Experimental Section). The parameters of OSWV and EIS measurements, see Figure 2. Table 2. Recovery Test Performed with HSCOR/HS(CH2)6OH Gold Electrode (Modification Number 5) in the Presence of 80-Fold Diluted Human Plasmaa OSWV

EIS

dopamine added (mol/L)

dopamine determined (mol/L)

recovery (%)

dopamine determined (mol/L)

recovery (%)

7.4 × 10-12 1.5 × 10-11 2.2 × 10-11

7.0 × 10-12 (±0.13) 1.4 × 10-11 (±0.03) 2.2 × 10-11 (±0.03)

94.6 (±1.8) 93.3 (±1.8) 100.0 (±1.5)

8.0 × 10-12 (±0.07) 1.6 × 10-11 (±0.02) 2.3 × 10-11 (±0.04)

108.1 (±1.0) 106.7 (±1.4) 104.5 (±1.9)

a

Measuring conditions, see Table 1; n ) 3).

modified with HSCOR/HS(CH2)6OH SAM with [Ru(NH3)6]3+ as the redox marker in the sample solutions could be used for direct electrochemical determination of DA in human plasma. Determination of Dopamine in Diluted Human Plasma. The sensitivity of gold electrodes modified with HSCOR/ HS(CH2)6OH in a molar ratio of 1:10 is sufficient for controlling the safe concentration level of DA or its overdose in human plasma.3-6 The applicability of the proposed sensor was tested using the OSWV and EIS techniques. Before measurements, the samples of human plasma were passed through a Millipore Micron Ultracel YM-3 and centrifuged for 30 min at 14 000g rcf in order to remove the proteins with molecular weight over 3 kDa. In order to check the influence of the centrifugation and filtration on dopamine concentration, 500 µL sample of human plasma was spiked with 0.1 mM of dopamine and centrifuged for 30 min at 14 000g rcf. The concentration of dopamine in the supernatant, determined by UV-vis spectroscopy, was the same as the concentration of dopamine added to the plasma. This result confirms that the centrifugation and filtration does not influence the dopamine concentration. The presence of centrifuged and undiluted human plasma caused a strong matrix influence. Therefore, the voltammetric and impedimetric responses of the modified electrode studied toward DA were explored in the presence of 80 times diluted human plasma with 0.9% NaCl containing 0.01 M borate buffer pH 7.0 and 1.0 mM [Ru(NH3)6]Cl3. The appropriate dilution degree was found experimentally. Representative OSWV and EIS curves are shown in Figure 4A,B. The presence of the 7404

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diluted human plasma influenced electrode sensitivity toward DA very little. The linear range of response was from 3.2 × 10-12 to 3.2 × 10-10 M. The detection limit estimated in the presence of the human plasma defined from a signal-to-noise ratio of 3 was 1.5 × 10-12 and 5.3 × 10-12 M, respectively, for the OSWV and EIS measurement techniques (Table 1). The ion channel mimetic sensor having HSCOR as the sensitive element displayed better sensitivity, in the 10-12 M range, toward DA in comparison to electrochemical sensors based on separation of the oxidation peaks for DA and AA.17-29 Also, the proposed sensor is free from the interferences coming for the components of human plasma. The applicability of the proposed sensor was checked by performing the recovery test. A volume of 2.0 mL of human plasma was centrifuged, 80 times diluted with 0.9% NaCl containing 0.01 M borate buffer pH 7.0 and 1.0 mM [Ru(NH3)6]Cl3 (see Experimental Section), and spiked with a known amount of DA. Next, the DA concentration was determined by Au-HSCOR/HS(CH2)6OH using both techniques, OSWV and EIS, on the basis of a calibration equation obtained in the presence of human plasma prepared as described above. The recovery test performed for DA determination in the presence human plasma (Table 2) indicated that the gold electrode coated with mixed HSCOR/ HS(CH2)6OH SAM could be applied for the direct electrochemical determination of DA in physiological samples in the very low concentration range 10-12-10-11 M.

To our knowledge, the only ion channel mimetic approach for DA determination has been reported by Shervedani et al.51 The DA accumulated on the surface of a gold electrode modified with 4-formylphenylboronic acid covalently attached to the amino group of the cysteamine was determined by cyclic voltammetry in the presence of redox marker [Fe(CN6)]3-/4-. The detection limit observed in the PBS buffer was in the range 10-9 M. Measurements in the presence of human plasma were not performed. Mechanisms of Amperometric Response of HSCOR/ HS(CH2)6OH Coated Gold Electrode toward Dopamine. In the present study, the sensing of DA was performed based on the formation of the DA-HSCOR complex on the electrode/ aqueous interface. OSWV and EIS measurements were carried out in the presence of a solution of 0.1 M KNO3 + 0.01 M borate buffer pH 7.0 or in the presence of centrifuged human plasma 80 times diluted with 0.09% NaCl + 0.01 M borate buffer pH 7.0. All sample solutions contained 1.0 mM [Ru(NH3)6Cl3. At pH 7.0, DA, because of the presence of the amino group, is present in the cationic form (pKa ) 8.9),36 whereas corrole molecules immobilized on gold electrode are still neutral.45,50 The partial positive charging of the neutral surface of Au-HSCOR/ HS(CH2)6OH upon interaction with the cationic form of DA increases repulsion between the electrode and the positively charged redox marker [Ru(NH3)6]3+ (Scheme 1), as could be expected according to the general idea of ion-channel mimetic sensors.40-44 This caused a decrease of the electron transfer rate between the marker and the electrode surface, which was linear related to increasing DA concentration. The interaction between dopamine and corrole molecules within one phase, in DMSO solution, was also proven by UV-vis spectrophotometry. Upon an increase in the DA concentration, the peaks of corrole absorption spectra at 430, 449, and 631 nm decreased. On the other hand, the peaks at 588 and 758 nm increased and slightly shifted toward higher value (Figure 1).

CONCLUSIONS A thio-derivative of corrole has been immobilized on the surface of gold electrodes via covalent Au-S bonds acting as a selective receptor for dopamine. The complex formation on the electrode surface between the corrole host and dopamine guest via hydrogen bonding was detected by Osteryoung square-wave voltammetry and by electrochemical impedance spectroscopy using [Ru(NH3)6]Cl3 as the redox marker. Under the measuring conditions, within the potential window from 0 to -500 mV, the oxidation of dopamine and ascorbic acid, the main interfering compounds, does not exist. The proposed sensor was effective regarding the following parameters: very good sensitivity toward DA (detection limit in 10-12 M range by using both EIS and OSWV techniques), very good selectivity (human plasma components at 80-fold dilution have no influence on dopamine determination), very simple, and quick procedure of electrode preparation. Therefore, the gold electrodes modified with mixed HSCOR/HS(CH2)6OH work as “ion-channel mimetic sensors” in the presence of [Ru(NH3)6]Cl3 as the redox marker and could be applied for the determination of dopamine in clinical analysis.

(51) Shervedani, R. K.; Bagherzadeh, M. Electroanalysis 2008, 5, 550–557.

AC901213H

ACKNOWLEDGMENT This work was supported by a grant from the Polish Ministry of Science and Higher Education Grant No. 105/6.PR UE/ 2007/7 and the statutory fund of the Institute of Animal Reproduction and Food Research of Polish Academy of Sciences, Olsztyn, Poland. SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

Received for review June 4, 2009. Accepted July 13, 2009.

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