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Electrodeposition of the Charge-Transfer Complex Generated during Electrooxidation of o-Tolidine and the Effects of Coexisting Chondroitin Sulfate
ACTA PHYSICO-CHIMICA SINICA Volume 24, Issue 2, February 2008 Online English edition of the Chinese language journal Cite this article as: Acta Phys. -Chim. Sin., 2008, 24(2): 230−236.
ARTICLE
Electrodeposition of the Charge-Transfer Complex Generated during Electrooxidation of o-Tolidine and the Effects of Coexisting Chondroitin Sulfate Xueqin Jiang,
Zhijun Cao,
Qingji Xie*,
Shouzhuo Yao
Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research, Ministry of Education of China, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, P. R. China
Abstract:
The electrochemical quartz crystal microbalance (EQCM) technique was used to investigate the electrodeposition of the
charge-transfer complex (CTC) generated during electrooxidation of o-tolidine (o-TD) in Britton-Robinson buffers and the effects of coexisting chondroitin sulfate (CS). A V-shaped frequency response to the cyclic voltammetric switching of o-TD indicated the precipitation and dissolution of the poorly soluble CTC, an oxidation intermediate, formed at the Au electrode during the redox switching of o-TD in a neutral or a weakly acidic medium (pH=4.07−6.50). The effects of potential scan rate, solution pH, and several supporting electrolytes were examined. The depth of the V-shaped frequency curves (−∆f0V) was related to the supporting electrolyte used, with a decreasing sequence for −∆f0V as 0.20 mol·L−1 NaNO3>0.20 mol·L−1 NaClO4>0.10 mol·L−1 Na2SO4. The −∆f0V response to the redox switching of the CTC/o-TD “couple” was enhanced by the introduction of CS because of the formation of the CTC-CS adduct, as also characterized and supported by UV-Vis and FTIR spectrophotometry. The molar ratio (x) of the CTC to CS in the adduct and the electrode-collection efficiency of the CTC (η) were estimated using EQCM. The values of −∆f0V increased with the increase in CS concentration, with a linear range from 0.75 to 15.2 µmol·L−1, and a detection limit down to 50 nmol·L−1. The new method proposed for CS assay was characterized by a dynamically renewed surface of the detection electrode. Key Words:
Electrochemical quartz crystal microbalance; Charge-transfer complex; o-Tolidine; Chondroitin sulfate assay;
Dynamically renewed electrode surface
A charge-transfer complex (CTC) is formed through the charge-transfer effect between an electron donor (D) and an electron acceptor (A), the research and application of which have received great attention and interests in various scientific research areas including chemistry, materials science, physics, biology, and medicine[1−3]. From recent electrochemical research on CTC[4−6], it was found that the electro-deposition and dissolution of some insoluble CTC on the electrode surface can be modulated by an electrode potential that controls the redox state of D and/or A. A new electrochemical biosensing method for heparin with a dynamically renewed electrode surface was, thus, developed[6].
The spectroscopic and electrochemical behaviors of o-tolidine (o-TD) have been widely studied, because of its high UV-Vis absorption, fluorescence, and electrochemical activity[7−11]. o-TD, as a potential dye, has been used for the measurement of oxidase activity[12,13]. The electrochemistry of o-TD is dependent on the pH of the medium, exhibiting a quasi-reversible two-electron transfer in one step in an acidic medium[10,11] and two consecutive one-electron oxidation steps in a weakly acidic or neutral medium, with the formation of an oxidized intermediate being structurally similar to quinhydrone, a blue dimeric semiquinone imine CTC[8,9]. Chondroitin sulfate (CS), an acidic mucopolysaccharide
Received: September 17, 2007; Revised: October 22, 2007. * Corresponding author. Email: [email protected]; Tel/Fax: +86731-8865515. The project was supported by the National Natural Science Foundation of China (20675029, 90713018, 20335020), Foundations of the Ministry of Education of China (Jiaorensi[2000]26, jiaojisi[2000]65), the Educational Department of Hunan Province, China (04C383, 05A036), and the State Key Laboratory of Electroanalytical Chemistry, China. Copyright © 2008, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at www.whxb.pku.edu.cn
Xueqin Jiang et al. / Acta Physico-Chimica Sinica, 2008, 24(2): 230−236
Fig.1
The structure of chondroitin sulfate (CS)
present in animal cartilage tissues, contains rich sulfonic, hydroxyl, and carboxyl groups and is thus regarded as an anionic polyelectrolyte (Fig.1). CS plays an important role in physiological activities such as anticoagulation and lowering blood lipids and as an antiarthritic, antitumoral, and antiatherosclerotic agent, thus is widely used in treating neuralgia, angina pectoris, arthritis, ulcers, and hyperlipemia. The research and analytical determination of CS are thus significant[14,15]. The analytical methods of CS include spectrophotometry[16], high-performance lipid chromatography (HPLC)[17−20], electrophoresis[21−23], turbidometry[24], and complexometry[25,26]. Till date, the electroanalysis of CS has not been reported. In this work, the electrochemical quartz crystal microbalance (EQCM) is used to study the CTC behavior in the electroredox processes of o-TD in Britton-Robinson (B-R) buffer, the effects of supporting electrolyte anions and the coexisting CS are examined, and a new electroanalytical method of CS is developed.
1 1.1
Experimental Instruments and reagents
Electrochemical experiments were performed on a CHI 660A electrochemical workstation (Shanghai Chenhua, China). The resonant frequency and motional resistance data were collected on a research quartz crystal microbalance (RQCM, Maxtek Inc., USA) or on an HP4395A impedance analyzer. The infrared spectra were recorded on a Nicolet Nexus 670 FTIR spectrophotometer. The UV-Vis absorption spectra were acquired using the absorption accessory of the F-4500 fluorescence spectrophotometer (Hitachi, Japan) with a 1-cm quartz cell. AT-cut piezoelectric quartz crystals (PQC, 12.5 mm in crystal diameter and 6.0 mm in Au-electrode diameter, JA5B type) with a nominal frequency of 9 MHz were purchased from Beijing Chenjing Electronic Co., LTD (China). The electrode on one side of the PQC was exposed to the solution, which served as the working electrode, whereas that on the other side was exposed to air. The potential versus the reference electrode, a KCl-saturated calomel electrode (SCE), was reported in this work. The counter electrode used was a platinum sheet. o-TD hydrochloride was purchased from Shanghai Chemical Reagent Co., LTD, China. Shark chondroitin sulfate (molecular weight: 10000−30000, average weight 20000 was used for the calculation of its molar concentration) and CS oph-
thalmic solution were purchased from Jiehui Biological Technology Co., LTD and Shandong Haishan Pharmaceutical Co., LTD, China, respectively. B-R buffer solution of 0.040 mol·L−1 containing 0.1 mol·L−1 Na2SO4 (pH 5.2, except otherwise specified) was used. All other reagents were of analytical or better quality. All solutions were prepared fresh before use, and Milli-Q ultrapure water was used throughout. The experiments were conducted at room temperature at approximately 20 °C. 1.2
Procedures
The surface of the gold electrode was treated with one drop of the mixture of H2O2 and H2SO4 (volume ratio 1:3) for 30 s, followed by rinsing with copious Milli-Q ultrapure water and dried with a stream of pure nitrogen. The treatment was repeated thrice. The treated electrode was then scanned between 0 and 1.5 V at 30 mV·s−1 in 0.2 mol·L−1 HClO4 solution for sufficient cycles to obtain reproducible cyclic voltammograms. The electrode was washed again with copious Milli-Q ultrapure water and dried with a stream of pure nitrogen. The electrode was subjected to potential cycling (between −0.100 V and 0.500 V (vs SCE) at 30 mV·s−1) in an aqueous solution of 2.00 mmol·L−1 K4Fe(CN)6+0.10 mol·L−1 Na2SO4. The peakto-peak separation was generally between 60 mV and 80 mV, indicating that a clean electrode surface had been obtained[27]. In the EQCM experiments, the electrodes were inserted into the electrochemical cell, and the simultaneous responses of electrochemistry, ∆f0 and ∆R1, were recorded after the injection of the samples. In the UV-Vis spectroelectrochemical experiment, the working electrode was an Au minigrid electrode, and the UV-Vis absorption accessory was used at the synchronous scan mode of the F-4500 with identical excitation and emission wavelength values. The reflectance spectra were transformed to absorption spectra according to A=lg(I0/I), where A is the absorbance and I0 and I are the intensities of the optical beams for a blank reference and a sample, respectively. The CTC-CS adduct was deposited at 0.34 V potentiostatically on an ion-coated glass slice, with an Au film in quiescent 0.600 mmol·L−1 o-TD+0.10 mol·L−1 Na2SO4+0.040 mol·L−1 B-R buffer (pH 5.2). The electrode was taken out from the solution, gently rinsed with Milli-Q ultrapure water, and then dried with an air blower. The CTC-CS deposit was then scraped off from the electrode surface for FTIR analysis (KBr pellets).
2
Results and discussion
2.1 EQCM monitoring of the cyclic voltammetric redox switching of o-TD and the effects of supporting electrolytes and coexisting CS The simultaneous responses of ∆R1, ∆f0, and current to potential cycling for redox switching of o-TD with or without
Xueqin Jiang et al. / Acta Physico-Chimica Sinica, 2008, 24(2): 230−236
Fig.2
EQCM responses in 0.600 mmol·L−1 o-TD+0.10 mol·L−1 Na2SO4+0.040 mol·L−1 B-R buffer solution (pH 5.2) with or without CS scan rate: 10 mV·s−1
coexisting CS, in pH 5.2 B-R buffer, are shown in Fig.2. When the potential sweep window was confined between 0.1 V and 0.7 V, the electrochemistry of o-TD showed two consecutive one-electron transfer steps. The oxidation product of the first step was a blue dimeric semiquinone imine with current peaks at 0.32 V (Pa1)/0.27 V (Pc1), and the oxidation product of the second step was quinonediimine with current peaks at 0.50 V (Pa2)/0.45 V (Pc2)[7−13]. Either forward or backward scan showed a V-shaped frequency response[6]. The value of the V-shaped frequency response (−∆f0V) depth was positively correlated with the concentration of the coexisting CS. In the presence of CS, the redox peaks negatively shifted with decreased peak currents, suggesting that CS anions interacted with the positive charged CTC to form a poorly soluble adduct. The phenomena of net frequency decrease after one whole potential cycle between 0.1 V and 0.7 V might be ascribed to the electropolymerization of o-TD after experiencing the second redox step, and the presence of a yellow film on the Au electrode could not be removed by rinsing with water. −∆f0/∆R1>30 Hz·Ω−1 was obtained in the potential cycle, suggesting that mass and viscoelastic effects were both present, but the former was the main factor[4,5]. The electropolymerization might be avoided by confining the potential-sweep window, and the oxidation product could be controlled to be solely the CTC of dimeric semiquinone imine (Fig.2B). The frequencies could well recover their initial values after one whole potential cycle, suggesting that the electrode surface could be dynamically renewed. With the increase in CS concentration, the heights of peaks Pa1/Pc1 decreased gradually, negatively shifted to new peaks Pa1′/Pc1′, and then increased gradually, suggesting that there was a strong interaction between the CTC and CS. The effect of the medium pH (B-R buffer solution) was studied (Fig.3). There was only a pair of redox peaks and no V-shaped frequency response at pH 3.00 and 3.51, indicating
that negligible CTC was produced. When the solution pH was between 4.07 and 6.50, there were two pairs of redox peaks and V-shaped frequency responses became evident. With the increase in pH, the redox peaks were negatively shifted, and the ratio of reduction to oxidation peaks was degressive. The linear regression equations of peak potential versus pH were EPa1=0.628−0.0578pH (r2=0.9917), EPc1=0.587−0.0597pH (r2= 0.9921), EPa2=0.818−0.0584pH (r2=0.9928) and EPc2=0.753− 0.0587pH (r2=0.9940). All the slopes were close to the Nernstian slope, suggesting that proton- and electron-transfer numbers were equal in each of these electrode processes. The bottom right plots of Fig.3 show the effect of pH on −∆f0V. In the absence of CS, the effect of pH on −∆f0V was not evident, but in the presence of CS, the −∆f0V initially increased and then decreased with the increase in pH, reaching a maximum at pH 5.2. At pH>6.5, the frequency could not well recover after one whole potential cycle, suggesting that the oxidized
Fig.3
Cyclic voltammograms in pH-modulated 0.040 mol·L−1
B-R buffer solutions containing 0.600 mmol·L−1 o-TD+ 0.10 mol·L−1 Na2SO4 Bottom right plots show curves of −∆f0V vs pH. scan rate: 10 mV·s−1
Xueqin Jiang et al. / Acta Physico-Chimica Sinica, 2008, 24(2): 230−236
centration. The optimal supporting electrolyte (0.10 mol·L−1 Na2SO4) was chosen to restrain the background response of the small-sized supporting electrolyte. 2.2 Characterization of and discussion on the interaction between CTC and CS
−∆f0V responses to scan rate (v) in 0.600 mmol·L−1
Fig.4
o-TD+0.10 mol·L−1 Na2SO4+0.040 mol·L−1 B-R buffer solution (pH 5.2) with or without CS Inset shows plots of iPa1 vs v1/2.
intermediate of o-TD had experienced irreversible deep-degree oxidation and electropolymerization. The effects of scan rate (ν) on peak current (iPa1) and −∆f0V are shown in Fig.4. The iPa1 was proportional to ν1/2, both in the absence and presence of CS, with a larger slope in the absence of CS, showing that the electrode reaction was controlled by diffusion and affected by formation and deposition of the CTC-CS adduct. The −∆f0V increased with the decrease in ν in the presence of CS but changed little in the absence of CS. To compromise between the detection sensitivity and analytical time, we chose 10 mV·s−1 as the optimal scan rate. The effect of anions on supporting electrolytes was also studied, and the results of 0.20 mol·L−1 NaNO3 and 0.20 mol·L−1 NaClO4 are shown in Fig.5. Compared with 0.10 mol·L−1 Na2SO4, the −∆f0V and the peak-to-peak separation increased in 0.20 mol·L−1 NaNO3 and 0.20 mol·L−1 NaClO4, suggesting that the anions of NO−3 and ClO−4 had higher affinity to the CTC, yielding poorly soluble CTC-NO−3 and CTC-ClO−4 adducts that deposited onto the electrode surface significantly. Apparently, −∆f0V was less sensitive to the small-sized anions than CS anion polyelectrolyte in the scale of unit molar con-
Fig.5
Fig.6
EQCM responses in 0.600 mmol·L−1 o-TD+0.040
−1
−1
mol·L B-R buffer solution (pH 5.2) containing 0.10 mol·L Na2SO4, 0.20 mol·L−1 NaNO3, or 0.20 mol·L−1 NaClO4 scan rate: 10 mV·s
−1
The absorption spectra of the CTC and CTC-CS adduct during the o-TD potentiostatic oxidation on a minigrid Au electrode are shown in Fig.6. In the absence of CS, the absorption maximums of CTC were at 370 and 630 nm when the potentiostatic oxidation in 0.600 mmol·L−1 o-TD+0.10 mol·L−1 Na2SO4+0.040 mol·L−1 B-R (pH 5.2) at 0.4 V until the electric charge consumed was 3.5 mC. Whereas in the presence of 30 µmol·L−1 CS, the absorption maximums of CTC shifted to 360 and 600 nm at an identical charge consumed, proving that the CS interacted with CTC to form the CTC-CS adduct and the conjugation effects were slightly weakened. Similarly, for potentiostatic oxidation at 0.7 V, until the electric charge consumed was 9 mC, in the absence of CS, a new absorbance maximum at 440 nm (the second step oxidation product quinonediimine) was observed, in addition to the absorption maximums at 370 and 630 nm for the CTC. In the presence of 30 µmol·L−1 CS, under the same condition, the absorbance maximums at 370 and 630 nm were blue shifted to 360 and 590 nm, simultaneously, with a hardly discernible absorption peak for quinonediimine, proving that the CTC had formed and its further oxidation to quinonediimine was difficult. The infrared spectra of o-TD, CS, and the CTC-CS adduct are shown in Fig.7. For o-TD spectrum (a), the stretching vibration for the amino groups was at 3426.24 cm−1 and for methyl group was at 2855.44 cm−1. The aromatic characteristics were evident from the band peaks at approximately 1566.94 and 1489.12 cm−1. For CS spectrum (b), the stretching vibration for the O−H and N−H was at 3446.3 cm−1. The two peaks at 1415, 1254 cm−1 were formed by the coupling of stretching vibration for the C−O and bending vibration for O−H. The stretching vibrations for C=O and C−N at 1560
UV-Vis absorption spectra of potentiostatic electrolysis on an
Au minigrid electrode in 0.600 mmol·L−1 o-TD+0.10 mmol·L−1 Na2SO4+0.040 mol·L−1 B-R buffer (pH 5.2) with 0 and 30 µmol·L−1 CS
Xueqin Jiang et al. / Acta Physico-Chimica Sinica, 2008, 24(2): 230−236
Fig.7
FTIR spectra of o-TD (a, hydrochloric), CS (b), and CTC-CS (c)
cm−1 indicate the presence of acetyl amino groups, and the peaks at 1061.62 and 860 cm−1 show the presence of sulfonic groups. For CTC-CS spectrum (c), the characteristic absorption bands mentioned above for o-TD and CS alone were found, and the stretching vibrations for the amino groups shifted to 3346.85 cm−1 in the CTC-CS adduct, and the peak at 3217.01 cm−1 became more obvious, supporting the fact that CS interacted with CTC. The molar ratio of CTC to CS (x) for the CTC-CS adduct by EQCM was estimated. The CTC-CS adducts were precipitated on EQCM Au electrodes by anodic linear scan voltammetry (LSV, 0−0.37 V) at 2 mV·s−1 at given concentrations of CS and o-TD, and the EQCM responses are shown in Fig.8A. The electrode was gently rinsed with Milli-Q ultrapure water to carefully remove the surface electrolyte, then dried with a stream of pure nitrogen, and the stable dry frequency of the electrode (f0d1) was recorded. The precipitate on the electrode surface was removed by cathodic LSV (0.37−0 V) in B-R buffer (pH 5.2), and the EQCM response is shown in Fig.8(B).
Fig.8
The electrode was then taken out from the solution and rinsed with Milli-Q ultrapure water and dried with a stream of pure nitrogen, and the stable frequency of the PQC in air (f0d2) was recorded. The “dry” frequency shift, ∆f0d=f0d2−f0d1, was thus obtained. The “wet” frequency shift (∆f0w) could be obtained from Fig.8B. According to the Sauerbrey equation[28,29], the mass of the CTC-CS adduct (mCTC-CS in g) might be estimated from the data of ∆f0d and ∆f0w, and mCTC-CS=4.425×105∆fA/f0g2, where ∆f denotes ∆f0d or ∆f0w. According to Faraday′s law, the molar quantity of the CTC in the CS-CTC adduct (nCTC in mol) could be estimated from the charge (QRed in C) consumed during the reductive dissolution of the CTC-CS adduct in the cathodic LSV as follows: (1) nCTC=QRed/zF where z is the number of electrons transferred in the relevant process (here z=2) and F is Faraday′s constant. The x values can be calculated as follows: x=nCTCMCS/(mCTC-CS−nCTCMCTC)
(2)
−1
where MCTC (g·mol ) is the molar mass of CTC, and MCS (g·mol−1) is the average molar mass of CS. The x∆f0d values obtained from the ∆f0d data are at an average 12, and x∆f0w from ∆f0w data are at an average 4.2 (Fig.9). By considering that the molecular weight of CS is 10000− 30000 (average 20000) and the negative charge of one CS unit is 1.74 (assuming that about 74% carboxyl ionizes at pH 5.2 and the pKa (4.74) of the involved carboxyl groups is equal to that of acetic acid), the x of value 20.3−60.9 (average 40.6) is anticipated in the CTC-CS adduct, according to an electrostatic binding mechanism for the CTC-CS. The x∆f0d value (12) obtained from ∆f0d data is smaller than the theoretical value. This suggests that the electrostatic action between
EQCM responses to anodic linear voltammetric sweep (LSV) in 0.600 mmol·L−1 o-TD+0.10 mol·L−1 Na2SO4+0.040 mol·L−1 B-R
buffer (pH 5.2) (A) containing CS of different concentrations, and then the electrode modified with CTC-CS was subjected to cathodic LSV sweep in CS-free 0.10 mol·L−1 Na2SO4+0.040 mol·L−1 B-R buffer (pH 5.2) solution (B) scan rate: 2 mV s−1
Xueqin Jiang et al. / Acta Physico-Chimica Sinica, 2008, 24(2): 230−236
Fig.9
The molar ratio of the CTC to CS in the CTC-CS adduct
(x) and the electrode-collection efficiency for the CTC (η) as functions of CS concentration (cCS)
small-sized ions to CS or CTC and hydrophilic/hydrophobic interaction between the deposited CTC and CS also could influence the generated CTC-CS adduct, in addition to the electrostatic affinity between CS and CTC. Because both the viscous effect of the wet CTC-CS film and the dynamic mass change of the electrolyte entrapped in the adduct film contributed to the ∆f0w data, the difference of the theoretical value to x∆f0w value (4.2) obtained from the ∆f0w data is larger than that to x∆f0d. The values of x decreased with the increase in CS concentration, suggesting that both the electrostatic interaction of coexisting small-sized ions and the hydrophilic/hydrophobic interaction between the deposited CTC and the CS increased with the increase in coexisting CS, leading to a facilitated precipitation. The electrode-collection efficiency for the generated CTC (η) is defined as the ratios of the reductive charge (QRed) of the deposited CTC-CS adduct consumed (cathodic LSV experiment) to the oxidation charge (QOx) for the formation of the CTC-CS adducts (anodic LSV experiment). For calculations, the oxidation and reductive charges are taken as positive, and the equation is given by (3) η=QRed/QOx The η obtained from Eq.(3) is shown in Fig.9. The η monotonously increased with the increase in CS concentration, and 90.5% CTC could be captured by the electrode at a CS concentration up to 52.5 µmol⋅L−1, indicating that the coexisting CS could facilitate the precipitation of the CTC on the electrode surface. 2.3
Analytical application
Using the CTC generated during the electrooxidation of o-TD as a “receptor” for CS, a continuous and an electrodetreatment-free detection of CS was implemented at one Au electrode, and the results are shown in Fig.10. A “reversible” frequency response was observed during the redox switching of o-TD in each run, and the −∆f0V was found to be positively correlated with the concentration of CS (Fig.10C). The linear range was 0.75−15.2 µmol⋅L−1, with the linear regression
Fig.10
EQCM responses in 0.600 mmol·L−1 o-TD+0.10 mol·L−1
Na2SO4+0.040 mol·L−1 B-R buffer (pH 5.2) with different concentrations of CS along the directions of solid arrows A) cyclic voltammograms, B) time-dependent ∆f0V, C, D) −∆f0V vs CS concentration; scan rate: 10 mV·s−1
equation −∆f0V=19.9cCS+17 (r2=0.9992), and the lower limit of detection was 50 nmol·L−1 (at tripled signal-to-noise ratio of 3). The peak-current response was also dependent on the CS concentration, but its linearity was rather poor, being due to the observed current-peak-shift phenomenon. In addition, the current method could be affected by the nonfaradaic current, but the frequency method should not. Therefore, the detection of CS based on the frequency response showed some advantages. The effects of coexisting substances on the determination of 7.5 µmol⋅L−1 CS were also studied, and it was found that 10 mg⋅L−1 L-cysteine, glucose, BSA, urea, or ascorbic acid, or 10 µmol⋅L−1 Cu2+ and Zn2+only gave relative errors within 5%, proving that the selectivity of the suggested method is good. The method was used for CS assay in a synthetic sample containing 22.5 mmol·L−1 CS+10 mg·L−1 L-cysteine+10 mg·L−1 glucose+10 mg·L−1 BSA+10 mg·L−1 urea+10 mg·L−1 ascorbic acid+10 µmol·L−1 Mg2++10 µmol·L−1 Cu2+. The CS value was (21.60±0.06) mmol·L−1 with 2.5% relative standard deviation. This method was also used for assay of a commercially available CS ophthalmic solution. One milliliter of ophthalmic solution was diluted in a 10-mL volumetric flask with Milli-Q ultrapure water. Hundred microliters of this solution was taken out and added to 10.0 mL of 0.600 mmol·L−1 o-TD+0.10 mol·L−1 Na2SO4+0.040 mol·L−1 B-R (pH 5.2) for detection. The CS value was (1.45±0.02) mmol·L−1 in three repeated assays, with 3.3% relative error to the certified value (1.5 mmol·L−1).
3
Conclusions
The EQCM technique was used to study the effects of various electrolytes, pH of the medium, and potential sweep rate on electrodeposition of the CTC after electrooxidation of
Xueqin Jiang et al. / Acta Physico-Chimica Sinica, 2008, 24(2): 230−236
o-TD. It was found and verified that the coexisting CS might form a CTC-CS adduct with the resultant CTC. The molar ratio of CTC to CS and the electrode-collection efficiency for the CTC were evaluated. The EQCM frequency response was correlated with the CS concentration, with a linear range from 0.75 to 15.2 µmol·L−1 and a detection limit down to 50 nmol·L−1, thus a new electroanalysis method for CS was established. The electrochemical analysis based on CTC materials is featured by a dynamically renewed surface of the detection electrode (similar to dropping mercury electrode), which may expand the fields for CTC investigations and applications, and find further applications in electroanalysis and chemo/ biosensing.
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ARTICLE
Electrodeposition of the Charge-Transfer Complex Generated during Electrooxidation of o-Tolidine and the Effects of Coexisting Chondroitin Sulfate Xueqin Jiang,
Zhijun Cao,
Qingji Xie*,
Shouzhuo Yao
Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research, Ministry of Education of China, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, P. R. China
Abstract:
The electrochemical quartz crystal microbalance (EQCM) technique was used to investigate the electrodeposition of the
charge-transfer complex (CTC) generated during electrooxidation of o-tolidine (o-TD) in Britton-Robinson buffers and the effects of coexisting chondroitin sulfate (CS). A V-shaped frequency response to the cyclic voltammetric switching of o-TD indicated the precipitation and dissolution of the poorly soluble CTC, an oxidation intermediate, formed at the Au electrode during the redox switching of o-TD in a neutral or a weakly acidic medium (pH=4.07−6.50). The effects of potential scan rate, solution pH, and several supporting electrolytes were examined. The depth of the V-shaped frequency curves (−∆f0V) was related to the supporting electrolyte used, with a decreasing sequence for −∆f0V as 0.20 mol·L−1 NaNO3>0.20 mol·L−1 NaClO4>0.10 mol·L−1 Na2SO4. The −∆f0V response to the redox switching of the CTC/o-TD “couple” was enhanced by the introduction of CS because of the formation of the CTC-CS adduct, as also characterized and supported by UV-Vis and FTIR spectrophotometry. The molar ratio (x) of the CTC to CS in the adduct and the electrode-collection efficiency of the CTC (η) were estimated using EQCM. The values of −∆f0V increased with the increase in CS concentration, with a linear range from 0.75 to 15.2 µmol·L−1, and a detection limit down to 50 nmol·L−1. The new method proposed for CS assay was characterized by a dynamically renewed surface of the detection electrode. Key Words:
Electrochemical quartz crystal microbalance; Charge-transfer complex; o-Tolidine; Chondroitin sulfate assay;
Dynamically renewed electrode surface
A charge-transfer complex (CTC) is formed through the charge-transfer effect between an electron donor (D) and an electron acceptor (A), the research and application of which have received great attention and interests in various scientific research areas including chemistry, materials science, physics, biology, and medicine[1−3]. From recent electrochemical research on CTC[4−6], it was found that the electro-deposition and dissolution of some insoluble CTC on the electrode surface can be modulated by an electrode potential that controls the redox state of D and/or A. A new electrochemical biosensing method for heparin with a dynamically renewed electrode surface was, thus, developed[6].
The spectroscopic and electrochemical behaviors of o-tolidine (o-TD) have been widely studied, because of its high UV-Vis absorption, fluorescence, and electrochemical activity[7−11]. o-TD, as a potential dye, has been used for the measurement of oxidase activity[12,13]. The electrochemistry of o-TD is dependent on the pH of the medium, exhibiting a quasi-reversible two-electron transfer in one step in an acidic medium[10,11] and two consecutive one-electron oxidation steps in a weakly acidic or neutral medium, with the formation of an oxidized intermediate being structurally similar to quinhydrone, a blue dimeric semiquinone imine CTC[8,9]. Chondroitin sulfate (CS), an acidic mucopolysaccharide
Received: September 17, 2007; Revised: October 22, 2007. * Corresponding author. Email: [email protected]; Tel/Fax: +86731-8865515. The project was supported by the National Natural Science Foundation of China (20675029, 90713018, 20335020), Foundations of the Ministry of Education of China (Jiaorensi[2000]26, jiaojisi[2000]65), the Educational Department of Hunan Province, China (04C383, 05A036), and the State Key Laboratory of Electroanalytical Chemistry, China. Copyright © 2008, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at www.whxb.pku.edu.cn
Xueqin Jiang et al. / Acta Physico-Chimica Sinica, 2008, 24(2): 230−236
Fig.1
The structure of chondroitin sulfate (CS)
present in animal cartilage tissues, contains rich sulfonic, hydroxyl, and carboxyl groups and is thus regarded as an anionic polyelectrolyte (Fig.1). CS plays an important role in physiological activities such as anticoagulation and lowering blood lipids and as an antiarthritic, antitumoral, and antiatherosclerotic agent, thus is widely used in treating neuralgia, angina pectoris, arthritis, ulcers, and hyperlipemia. The research and analytical determination of CS are thus significant[14,15]. The analytical methods of CS include spectrophotometry[16], high-performance lipid chromatography (HPLC)[17−20], electrophoresis[21−23], turbidometry[24], and complexometry[25,26]. Till date, the electroanalysis of CS has not been reported. In this work, the electrochemical quartz crystal microbalance (EQCM) is used to study the CTC behavior in the electroredox processes of o-TD in Britton-Robinson (B-R) buffer, the effects of supporting electrolyte anions and the coexisting CS are examined, and a new electroanalytical method of CS is developed.
1 1.1
Experimental Instruments and reagents
Electrochemical experiments were performed on a CHI 660A electrochemical workstation (Shanghai Chenhua, China). The resonant frequency and motional resistance data were collected on a research quartz crystal microbalance (RQCM, Maxtek Inc., USA) or on an HP4395A impedance analyzer. The infrared spectra were recorded on a Nicolet Nexus 670 FTIR spectrophotometer. The UV-Vis absorption spectra were acquired using the absorption accessory of the F-4500 fluorescence spectrophotometer (Hitachi, Japan) with a 1-cm quartz cell. AT-cut piezoelectric quartz crystals (PQC, 12.5 mm in crystal diameter and 6.0 mm in Au-electrode diameter, JA5B type) with a nominal frequency of 9 MHz were purchased from Beijing Chenjing Electronic Co., LTD (China). The electrode on one side of the PQC was exposed to the solution, which served as the working electrode, whereas that on the other side was exposed to air. The potential versus the reference electrode, a KCl-saturated calomel electrode (SCE), was reported in this work. The counter electrode used was a platinum sheet. o-TD hydrochloride was purchased from Shanghai Chemical Reagent Co., LTD, China. Shark chondroitin sulfate (molecular weight: 10000−30000, average weight 20000 was used for the calculation of its molar concentration) and CS oph-
thalmic solution were purchased from Jiehui Biological Technology Co., LTD and Shandong Haishan Pharmaceutical Co., LTD, China, respectively. B-R buffer solution of 0.040 mol·L−1 containing 0.1 mol·L−1 Na2SO4 (pH 5.2, except otherwise specified) was used. All other reagents were of analytical or better quality. All solutions were prepared fresh before use, and Milli-Q ultrapure water was used throughout. The experiments were conducted at room temperature at approximately 20 °C. 1.2
Procedures
The surface of the gold electrode was treated with one drop of the mixture of H2O2 and H2SO4 (volume ratio 1:3) for 30 s, followed by rinsing with copious Milli-Q ultrapure water and dried with a stream of pure nitrogen. The treatment was repeated thrice. The treated electrode was then scanned between 0 and 1.5 V at 30 mV·s−1 in 0.2 mol·L−1 HClO4 solution for sufficient cycles to obtain reproducible cyclic voltammograms. The electrode was washed again with copious Milli-Q ultrapure water and dried with a stream of pure nitrogen. The electrode was subjected to potential cycling (between −0.100 V and 0.500 V (vs SCE) at 30 mV·s−1) in an aqueous solution of 2.00 mmol·L−1 K4Fe(CN)6+0.10 mol·L−1 Na2SO4. The peakto-peak separation was generally between 60 mV and 80 mV, indicating that a clean electrode surface had been obtained[27]. In the EQCM experiments, the electrodes were inserted into the electrochemical cell, and the simultaneous responses of electrochemistry, ∆f0 and ∆R1, were recorded after the injection of the samples. In the UV-Vis spectroelectrochemical experiment, the working electrode was an Au minigrid electrode, and the UV-Vis absorption accessory was used at the synchronous scan mode of the F-4500 with identical excitation and emission wavelength values. The reflectance spectra were transformed to absorption spectra according to A=lg(I0/I), where A is the absorbance and I0 and I are the intensities of the optical beams for a blank reference and a sample, respectively. The CTC-CS adduct was deposited at 0.34 V potentiostatically on an ion-coated glass slice, with an Au film in quiescent 0.600 mmol·L−1 o-TD+0.10 mol·L−1 Na2SO4+0.040 mol·L−1 B-R buffer (pH 5.2). The electrode was taken out from the solution, gently rinsed with Milli-Q ultrapure water, and then dried with an air blower. The CTC-CS deposit was then scraped off from the electrode surface for FTIR analysis (KBr pellets).
2
Results and discussion
2.1 EQCM monitoring of the cyclic voltammetric redox switching of o-TD and the effects of supporting electrolytes and coexisting CS The simultaneous responses of ∆R1, ∆f0, and current to potential cycling for redox switching of o-TD with or without
Xueqin Jiang et al. / Acta Physico-Chimica Sinica, 2008, 24(2): 230−236
Fig.2
EQCM responses in 0.600 mmol·L−1 o-TD+0.10 mol·L−1 Na2SO4+0.040 mol·L−1 B-R buffer solution (pH 5.2) with or without CS scan rate: 10 mV·s−1
coexisting CS, in pH 5.2 B-R buffer, are shown in Fig.2. When the potential sweep window was confined between 0.1 V and 0.7 V, the electrochemistry of o-TD showed two consecutive one-electron transfer steps. The oxidation product of the first step was a blue dimeric semiquinone imine with current peaks at 0.32 V (Pa1)/0.27 V (Pc1), and the oxidation product of the second step was quinonediimine with current peaks at 0.50 V (Pa2)/0.45 V (Pc2)[7−13]. Either forward or backward scan showed a V-shaped frequency response[6]. The value of the V-shaped frequency response (−∆f0V) depth was positively correlated with the concentration of the coexisting CS. In the presence of CS, the redox peaks negatively shifted with decreased peak currents, suggesting that CS anions interacted with the positive charged CTC to form a poorly soluble adduct. The phenomena of net frequency decrease after one whole potential cycle between 0.1 V and 0.7 V might be ascribed to the electropolymerization of o-TD after experiencing the second redox step, and the presence of a yellow film on the Au electrode could not be removed by rinsing with water. −∆f0/∆R1>30 Hz·Ω−1 was obtained in the potential cycle, suggesting that mass and viscoelastic effects were both present, but the former was the main factor[4,5]. The electropolymerization might be avoided by confining the potential-sweep window, and the oxidation product could be controlled to be solely the CTC of dimeric semiquinone imine (Fig.2B). The frequencies could well recover their initial values after one whole potential cycle, suggesting that the electrode surface could be dynamically renewed. With the increase in CS concentration, the heights of peaks Pa1/Pc1 decreased gradually, negatively shifted to new peaks Pa1′/Pc1′, and then increased gradually, suggesting that there was a strong interaction between the CTC and CS. The effect of the medium pH (B-R buffer solution) was studied (Fig.3). There was only a pair of redox peaks and no V-shaped frequency response at pH 3.00 and 3.51, indicating
that negligible CTC was produced. When the solution pH was between 4.07 and 6.50, there were two pairs of redox peaks and V-shaped frequency responses became evident. With the increase in pH, the redox peaks were negatively shifted, and the ratio of reduction to oxidation peaks was degressive. The linear regression equations of peak potential versus pH were EPa1=0.628−0.0578pH (r2=0.9917), EPc1=0.587−0.0597pH (r2= 0.9921), EPa2=0.818−0.0584pH (r2=0.9928) and EPc2=0.753− 0.0587pH (r2=0.9940). All the slopes were close to the Nernstian slope, suggesting that proton- and electron-transfer numbers were equal in each of these electrode processes. The bottom right plots of Fig.3 show the effect of pH on −∆f0V. In the absence of CS, the effect of pH on −∆f0V was not evident, but in the presence of CS, the −∆f0V initially increased and then decreased with the increase in pH, reaching a maximum at pH 5.2. At pH>6.5, the frequency could not well recover after one whole potential cycle, suggesting that the oxidized
Fig.3
Cyclic voltammograms in pH-modulated 0.040 mol·L−1
B-R buffer solutions containing 0.600 mmol·L−1 o-TD+ 0.10 mol·L−1 Na2SO4 Bottom right plots show curves of −∆f0V vs pH. scan rate: 10 mV·s−1
Xueqin Jiang et al. / Acta Physico-Chimica Sinica, 2008, 24(2): 230−236
centration. The optimal supporting electrolyte (0.10 mol·L−1 Na2SO4) was chosen to restrain the background response of the small-sized supporting electrolyte. 2.2 Characterization of and discussion on the interaction between CTC and CS
−∆f0V responses to scan rate (v) in 0.600 mmol·L−1
Fig.4
o-TD+0.10 mol·L−1 Na2SO4+0.040 mol·L−1 B-R buffer solution (pH 5.2) with or without CS Inset shows plots of iPa1 vs v1/2.
intermediate of o-TD had experienced irreversible deep-degree oxidation and electropolymerization. The effects of scan rate (ν) on peak current (iPa1) and −∆f0V are shown in Fig.4. The iPa1 was proportional to ν1/2, both in the absence and presence of CS, with a larger slope in the absence of CS, showing that the electrode reaction was controlled by diffusion and affected by formation and deposition of the CTC-CS adduct. The −∆f0V increased with the decrease in ν in the presence of CS but changed little in the absence of CS. To compromise between the detection sensitivity and analytical time, we chose 10 mV·s−1 as the optimal scan rate. The effect of anions on supporting electrolytes was also studied, and the results of 0.20 mol·L−1 NaNO3 and 0.20 mol·L−1 NaClO4 are shown in Fig.5. Compared with 0.10 mol·L−1 Na2SO4, the −∆f0V and the peak-to-peak separation increased in 0.20 mol·L−1 NaNO3 and 0.20 mol·L−1 NaClO4, suggesting that the anions of NO−3 and ClO−4 had higher affinity to the CTC, yielding poorly soluble CTC-NO−3 and CTC-ClO−4 adducts that deposited onto the electrode surface significantly. Apparently, −∆f0V was less sensitive to the small-sized anions than CS anion polyelectrolyte in the scale of unit molar con-
Fig.5
Fig.6
EQCM responses in 0.600 mmol·L−1 o-TD+0.040
−1
−1
mol·L B-R buffer solution (pH 5.2) containing 0.10 mol·L Na2SO4, 0.20 mol·L−1 NaNO3, or 0.20 mol·L−1 NaClO4 scan rate: 10 mV·s
−1
The absorption spectra of the CTC and CTC-CS adduct during the o-TD potentiostatic oxidation on a minigrid Au electrode are shown in Fig.6. In the absence of CS, the absorption maximums of CTC were at 370 and 630 nm when the potentiostatic oxidation in 0.600 mmol·L−1 o-TD+0.10 mol·L−1 Na2SO4+0.040 mol·L−1 B-R (pH 5.2) at 0.4 V until the electric charge consumed was 3.5 mC. Whereas in the presence of 30 µmol·L−1 CS, the absorption maximums of CTC shifted to 360 and 600 nm at an identical charge consumed, proving that the CS interacted with CTC to form the CTC-CS adduct and the conjugation effects were slightly weakened. Similarly, for potentiostatic oxidation at 0.7 V, until the electric charge consumed was 9 mC, in the absence of CS, a new absorbance maximum at 440 nm (the second step oxidation product quinonediimine) was observed, in addition to the absorption maximums at 370 and 630 nm for the CTC. In the presence of 30 µmol·L−1 CS, under the same condition, the absorbance maximums at 370 and 630 nm were blue shifted to 360 and 590 nm, simultaneously, with a hardly discernible absorption peak for quinonediimine, proving that the CTC had formed and its further oxidation to quinonediimine was difficult. The infrared spectra of o-TD, CS, and the CTC-CS adduct are shown in Fig.7. For o-TD spectrum (a), the stretching vibration for the amino groups was at 3426.24 cm−1 and for methyl group was at 2855.44 cm−1. The aromatic characteristics were evident from the band peaks at approximately 1566.94 and 1489.12 cm−1. For CS spectrum (b), the stretching vibration for the O−H and N−H was at 3446.3 cm−1. The two peaks at 1415, 1254 cm−1 were formed by the coupling of stretching vibration for the C−O and bending vibration for O−H. The stretching vibrations for C=O and C−N at 1560
UV-Vis absorption spectra of potentiostatic electrolysis on an
Au minigrid electrode in 0.600 mmol·L−1 o-TD+0.10 mmol·L−1 Na2SO4+0.040 mol·L−1 B-R buffer (pH 5.2) with 0 and 30 µmol·L−1 CS
Xueqin Jiang et al. / Acta Physico-Chimica Sinica, 2008, 24(2): 230−236
Fig.7
FTIR spectra of o-TD (a, hydrochloric), CS (b), and CTC-CS (c)
cm−1 indicate the presence of acetyl amino groups, and the peaks at 1061.62 and 860 cm−1 show the presence of sulfonic groups. For CTC-CS spectrum (c), the characteristic absorption bands mentioned above for o-TD and CS alone were found, and the stretching vibrations for the amino groups shifted to 3346.85 cm−1 in the CTC-CS adduct, and the peak at 3217.01 cm−1 became more obvious, supporting the fact that CS interacted with CTC. The molar ratio of CTC to CS (x) for the CTC-CS adduct by EQCM was estimated. The CTC-CS adducts were precipitated on EQCM Au electrodes by anodic linear scan voltammetry (LSV, 0−0.37 V) at 2 mV·s−1 at given concentrations of CS and o-TD, and the EQCM responses are shown in Fig.8A. The electrode was gently rinsed with Milli-Q ultrapure water to carefully remove the surface electrolyte, then dried with a stream of pure nitrogen, and the stable dry frequency of the electrode (f0d1) was recorded. The precipitate on the electrode surface was removed by cathodic LSV (0.37−0 V) in B-R buffer (pH 5.2), and the EQCM response is shown in Fig.8(B).
Fig.8
The electrode was then taken out from the solution and rinsed with Milli-Q ultrapure water and dried with a stream of pure nitrogen, and the stable frequency of the PQC in air (f0d2) was recorded. The “dry” frequency shift, ∆f0d=f0d2−f0d1, was thus obtained. The “wet” frequency shift (∆f0w) could be obtained from Fig.8B. According to the Sauerbrey equation[28,29], the mass of the CTC-CS adduct (mCTC-CS in g) might be estimated from the data of ∆f0d and ∆f0w, and mCTC-CS=4.425×105∆fA/f0g2, where ∆f denotes ∆f0d or ∆f0w. According to Faraday′s law, the molar quantity of the CTC in the CS-CTC adduct (nCTC in mol) could be estimated from the charge (QRed in C) consumed during the reductive dissolution of the CTC-CS adduct in the cathodic LSV as follows: (1) nCTC=QRed/zF where z is the number of electrons transferred in the relevant process (here z=2) and F is Faraday′s constant. The x values can be calculated as follows: x=nCTCMCS/(mCTC-CS−nCTCMCTC)
(2)
−1
where MCTC (g·mol ) is the molar mass of CTC, and MCS (g·mol−1) is the average molar mass of CS. The x∆f0d values obtained from the ∆f0d data are at an average 12, and x∆f0w from ∆f0w data are at an average 4.2 (Fig.9). By considering that the molecular weight of CS is 10000− 30000 (average 20000) and the negative charge of one CS unit is 1.74 (assuming that about 74% carboxyl ionizes at pH 5.2 and the pKa (4.74) of the involved carboxyl groups is equal to that of acetic acid), the x of value 20.3−60.9 (average 40.6) is anticipated in the CTC-CS adduct, according to an electrostatic binding mechanism for the CTC-CS. The x∆f0d value (12) obtained from ∆f0d data is smaller than the theoretical value. This suggests that the electrostatic action between
EQCM responses to anodic linear voltammetric sweep (LSV) in 0.600 mmol·L−1 o-TD+0.10 mol·L−1 Na2SO4+0.040 mol·L−1 B-R
buffer (pH 5.2) (A) containing CS of different concentrations, and then the electrode modified with CTC-CS was subjected to cathodic LSV sweep in CS-free 0.10 mol·L−1 Na2SO4+0.040 mol·L−1 B-R buffer (pH 5.2) solution (B) scan rate: 2 mV s−1
Xueqin Jiang et al. / Acta Physico-Chimica Sinica, 2008, 24(2): 230−236
Fig.9
The molar ratio of the CTC to CS in the CTC-CS adduct
(x) and the electrode-collection efficiency for the CTC (η) as functions of CS concentration (cCS)
small-sized ions to CS or CTC and hydrophilic/hydrophobic interaction between the deposited CTC and CS also could influence the generated CTC-CS adduct, in addition to the electrostatic affinity between CS and CTC. Because both the viscous effect of the wet CTC-CS film and the dynamic mass change of the electrolyte entrapped in the adduct film contributed to the ∆f0w data, the difference of the theoretical value to x∆f0w value (4.2) obtained from the ∆f0w data is larger than that to x∆f0d. The values of x decreased with the increase in CS concentration, suggesting that both the electrostatic interaction of coexisting small-sized ions and the hydrophilic/hydrophobic interaction between the deposited CTC and the CS increased with the increase in coexisting CS, leading to a facilitated precipitation. The electrode-collection efficiency for the generated CTC (η) is defined as the ratios of the reductive charge (QRed) of the deposited CTC-CS adduct consumed (cathodic LSV experiment) to the oxidation charge (QOx) for the formation of the CTC-CS adducts (anodic LSV experiment). For calculations, the oxidation and reductive charges are taken as positive, and the equation is given by (3) η=QRed/QOx The η obtained from Eq.(3) is shown in Fig.9. The η monotonously increased with the increase in CS concentration, and 90.5% CTC could be captured by the electrode at a CS concentration up to 52.5 µmol⋅L−1, indicating that the coexisting CS could facilitate the precipitation of the CTC on the electrode surface. 2.3
Analytical application
Using the CTC generated during the electrooxidation of o-TD as a “receptor” for CS, a continuous and an electrodetreatment-free detection of CS was implemented at one Au electrode, and the results are shown in Fig.10. A “reversible” frequency response was observed during the redox switching of o-TD in each run, and the −∆f0V was found to be positively correlated with the concentration of CS (Fig.10C). The linear range was 0.75−15.2 µmol⋅L−1, with the linear regression
Fig.10
EQCM responses in 0.600 mmol·L−1 o-TD+0.10 mol·L−1
Na2SO4+0.040 mol·L−1 B-R buffer (pH 5.2) with different concentrations of CS along the directions of solid arrows A) cyclic voltammograms, B) time-dependent ∆f0V, C, D) −∆f0V vs CS concentration; scan rate: 10 mV·s−1
equation −∆f0V=19.9cCS+17 (r2=0.9992), and the lower limit of detection was 50 nmol·L−1 (at tripled signal-to-noise ratio of 3). The peak-current response was also dependent on the CS concentration, but its linearity was rather poor, being due to the observed current-peak-shift phenomenon. In addition, the current method could be affected by the nonfaradaic current, but the frequency method should not. Therefore, the detection of CS based on the frequency response showed some advantages. The effects of coexisting substances on the determination of 7.5 µmol⋅L−1 CS were also studied, and it was found that 10 mg⋅L−1 L-cysteine, glucose, BSA, urea, or ascorbic acid, or 10 µmol⋅L−1 Cu2+ and Zn2+only gave relative errors within 5%, proving that the selectivity of the suggested method is good. The method was used for CS assay in a synthetic sample containing 22.5 mmol·L−1 CS+10 mg·L−1 L-cysteine+10 mg·L−1 glucose+10 mg·L−1 BSA+10 mg·L−1 urea+10 mg·L−1 ascorbic acid+10 µmol·L−1 Mg2++10 µmol·L−1 Cu2+. The CS value was (21.60±0.06) mmol·L−1 with 2.5% relative standard deviation. This method was also used for assay of a commercially available CS ophthalmic solution. One milliliter of ophthalmic solution was diluted in a 10-mL volumetric flask with Milli-Q ultrapure water. Hundred microliters of this solution was taken out and added to 10.0 mL of 0.600 mmol·L−1 o-TD+0.10 mol·L−1 Na2SO4+0.040 mol·L−1 B-R (pH 5.2) for detection. The CS value was (1.45±0.02) mmol·L−1 in three repeated assays, with 3.3% relative error to the certified value (1.5 mmol·L−1).
3
Conclusions
The EQCM technique was used to study the effects of various electrolytes, pH of the medium, and potential sweep rate on electrodeposition of the CTC after electrooxidation of
Xueqin Jiang et al. / Acta Physico-Chimica Sinica, 2008, 24(2): 230−236
o-TD. It was found and verified that the coexisting CS might form a CTC-CS adduct with the resultant CTC. The molar ratio of CTC to CS and the electrode-collection efficiency for the CTC were evaluated. The EQCM frequency response was correlated with the CS concentration, with a linear range from 0.75 to 15.2 µmol·L−1 and a detection limit down to 50 nmol·L−1, thus a new electroanalysis method for CS was established. The electrochemical analysis based on CTC materials is featured by a dynamically renewed surface of the detection electrode (similar to dropping mercury electrode), which may expand the fields for CTC investigations and applications, and find further applications in electroanalysis and chemo/ biosensing.
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