Mohammad
Mazloum-Ardakani
*a,
Hadi
Beitollahi
a,
Mohammad Kazem
Amini
b,
Fakhradin
Mirkhalaf
c and
Mohammad
Abdollahi-Alibeik
a
aDepartment of Chemistry, Faculty of Science, Yazd University, Yazd, 89195-741, I.R. Iran. E-mail: mazloum@yazduni.ac.ir; Fax: +00983518210644; Tel: +00983518211670
bDepartment of Chemistry, University of Isfahan, Isfahan, 81744-73441, Iran
cSonochemistry Centre, Coventry University Coventry, CV1 5FB, UK
First published on 9th February 2011
A new carbon paste electrode modified with ZrO2 nanoparticles (ZONMCPE) was prepared, and used to study the electrooxidation of epinephrine (EP), acetaminophen (AC), folic acid (FA) and their mixtures by electrochemical methods. The modified electrode displayed strong resolving function for the overlapping voltammetric responses of EP, AC and FA into three well-defined peaks. The potential differences between EP − AC, AC − FA and EP − FA were 210, 290 and 500 mV respectively. Differential pulse voltammetry (DPV) peak currents of EP, AC and FA increased linearly with their concentration at the ranges of 2.0 × 10−7–2.2 × 10−3 M, 1.0 × 10−6–2.5 × 10−3 M and 2.0 × 10−5–2.5 × 10−3 M, respectively. The detection limits for EP, AC and FA were found to be 9.5 × 10−8, 9.1 × 10−7and 9.8 × 10−6 M, respectively.
Epinephrine (EP) is an important neurotransmitter in the mammalian central nervous systems,1 and exists in the nervous tissue and body fluid in the form of large organic cations. The changes in its concentration may result in many diseases.1 Thus; a quantitative determination of EP concentration is significant for developing nerve physiology, pharmacological research and life sciences. There are some methods applied for the determination of EP, such as high performance liquid chromatography (HPLC),2 capillary electrophoresis,3 flow injection,4 chemiluminescence,5 fluorimetry6 spectrophotometry7 and the electrochemiluminescence8 method. As an electroactive device, it can also be studied via electrochemical techniques. Some reports showed the electrochemical response of EP on different kinds of electrodes.9–12
Acetaminophen (AC) is categorized as analgesics (pain relievers) and antipyretics (fever reducers). When used appropriately, side effects are rare. The most serious side effect is liver damage due to large doses, chronic use or concomitant use with alcohol or other drugs. To date, a variety of methods such as high performance liquid chromatography (HPLC),13 spectrophotometry,14 capillary electrophoresis,15 have been developed for the determination of AC. AC is electro-active, and can be oxidized under certain conditions. Therefore, different electrochemical methods using various modified electrodes have also been proposed for the determination of AC.16,17
Folic acid (FA), is a water-soluble vitamin and can act as coenzyme in the transfer and utilization of one-carbon groups and in the regeneration of methionine from homocysteine.18 Deficiency of FA is a common cause of anaemia and it is thought to increase the likelihood of heart attack and stroke. Numerous methods for the measurement of FA are available.19–21 As FA is an electroactive component, some electrochemical methods have been reported for its determination.22,23
The electrochemical methods using chemically modified electrodes (CMEs) have been widely used as sensitive and selective analytical methods for the detection of the trace amounts of biologically important compounds.24–26
Electrode surfaces can be modified with metal nanoparticles and such surfaces have found numerous applications in the field of bioelectrochemistry, particularly in biosensors. It has also been observed that nanoparticles can act as conduction centers facilitating the transfer of electrons. In addition, they provide large catalytic surface areas. Many kinds of nanoparticles, including metal nanoparticles, oxide nanoparticles, semiconductor nanoparticles and even composite nanoparticles, have been widely used in electrochemical sensors and biosensors.27–31
Since EP, AC and FA are electroactive components they can be determined electrochemically. However, it is very difficult to distinguish their response signal at bare electrodes because of their similar potentials and interference from each other. Therefore, it is very important to develop a modified electrode to resolve their voltammetric response from each other. To the best of our knowledge, no study has been reported so far on the simultaneous determination of EP, AC and FA by using carbon paste electrodes modified with ZrO2 nanoparticles. In this study, we report the preparation and application of a carbon paste electrode modified with ZrO2 nanoparticles for the simultaneous determination of EP, AC and FA without any additional modification such as addition of electron transfer mediator or specific reagents for the first time.
All solutions were freshly prepared with doubly-distilled water. EP, AC, FA and all other reagents were analytical grade from Merck. Graphite powder and paraffin oil (DC 350, 0.88 g cm−3), both from Merck, were used as received. Phosphate buffer solutions (PBSs) were prepared from orthophosphoric acid and its salts in the pH range of 2.0–11.0. All solutions were deoxygenated with pure nitrogen gas for about 30 min prior to each electrochemical experiment.
ZrO2 nanoparticles were synthesized via the sol–gel method. In a typical procedure, ZrCl4 (2.8 mg, 12 mmol) was dissolved in water (100 mL). To this solution was added drop wise an aqueous solution of 2.5 wt.% ammonium hydroxide (200 mL) under vigorous stirring for 4 h at room temperature until a final pH of ∼9.5 was reached. The resulting gel was separated by centrifuge and washed with water (4 × 100 mL) until no chloride ion was detected by the AgNO3 test. The hydrous zirconia was dried at 120 °C for 12 h. The solid was powdered and calcined at 480 °C in air for 4 h.
A Philips model XL30 scanning electron microscope was used to determine the morphology of the synthesized zirconia sample. The morphology of the prepared ZrO2 was studied by scanning electron microscopy (Fig. 1). The SEM image shows the agglomerated ZrO2 particles with an average size of less than 100 nm.
Fig. 1 SEM of ZrO2 nanoparticles. |
Fig. 2 Cyclic voltammograms of (a) ZONMCPE and (b) bare CPE in 0.1 M PBS (pH 7.0) in the presence of 1.0 mM EP at the scan rate 20 mV s−1. |
The effect of scan rate on the electro-oxidation of EP at the ZONMCPE was investigated by cyclic voltammetry (Fig. 3). A plot of peak current (Ip) versus the square root of scan rate (ν1/2), in the range of 10–1000 mV s−1 (Fig. 3 (A)) is linear. This suggests that, at sufficient overpotential, the process is diffusion rather than surface controlled.36
Fig. 3 Cyclic voltammograms of the ZONMCPE in the presence of 1.0 mM EP at various scan rates; The numbers 1–12 correspond to 10, 20, 30, 40, 50, 60, 80, 110, 300, 500, 750 and 1000 mV s−1 scan rates, respectively. Insets: (A) The variation of the anodic peak currents vs. v1/2. (B) Tafel plot derived from the rising part of voltammogram recorded at a scan rate 10 mV s−1. |
A Tafel plot was obtained from data of the rising part of the current–voltage curve recorded at a scan rate of 10 mV s−1 (Fig. 3 (B)). This part of voltammogram, known as Tafel region, is controlled by the electron transfer kinetics between the substrate (EP) and ZONMCPE, assuming a fast proton transport step. In this condition, the number of electrons involved in the rate determining step can be estimated from the slope of Tafel plot. The Tafel slope was found to be 0.0972 V decade−1 and thus the charge transfer coefficient was calculated as α = 0.39.
I = nFAD1/2Cbπ−1/2t−1/2 | (1) |
The slope of linear region of Cottrell's plot can be used to estimate the diffusion coefficient of EP. The mean value of EP diffusion coefficient in the ranges (0.15–2.80 mM) was found to be 8.3 × 10−6 cm2 s−1.
Analyte | Linear range (M) | Regression equation | Correlation coefficient | Detection limit (M) |
---|---|---|---|---|
EP | 2.0 × 10−7–2.2 × 10−3 | I (μA) = 0.016C (μM) + 0.5934 | 0.999 | 9.5 × 10−8 |
AC | 1.0 × 10−6–2.5 × 10−3 | I (μA) = 0.0172C (μM) + 0.3823 | 0.9989 | 9.1 × 10−7 |
FA | 2.0 × 10−5–2.5 × 10−3 | I (μA) = 0.0085C (μM) + 1.6836 | 0.9985 | 9.8 × 10−6 |
Capability of separating the electrochemical responses of EP, AC and FA by a modified electrode was studied. Therefore, DPV was used for the simultaneous determination of EP, AC and FA. The importance of DPV in determination of species is based on its superior elimination of the capacitive/background current. Analytical experiments were carried out by varying the concentration of AC or FA in the presence of constant concentrations of EP in 0.1 M PBS (pH 7.0) by using ZONMCPE as the working electrode. Fig. 4A and 4B show DPVs obtained in 900.0 and 800.0 μM EP containing increasing concentrations of AC and FA, respectively. It can be clearly seen that the response of the ZONMCPE to EP is independent of both AC and FA. The utilization of the ZONMCPE for the simultaneous determination of EP, AC and FA was demonstrated by simultaneously changing the concentrations of EP, AC and FA. The DP voltammetric results show three well-distinguished anodic peaks at potentials of 250, 460 and 750 mV, corresponding to the oxidation of EP, AC and FA, respectively, indicating that the simultaneous determination of EP, AC and FA is possible at the ZONMCPE (Fig. 5). The sensitivities of the ZONMCPE towards EP, AC and FA in the individually (0.016, 0.0172 and 0.0085 μA μM−1) and simultaneous (0.0158, 0.017 and 0.0087 μA μM−1) determination are virtually the same, which further indicate that the oxidation processes of EP, AC and FA at the ZONMCPE are independent and therefore, simultaneous measurements of the three analytes are feasible without any interference.
Fig. 4 (A) Differential pulse voltammograms of ZONMCPE in 0.1 M PBS (pH 7.0) containing 900.0 μM EP and different concentrations of AC, from bottom to top, 325.0, 450.0, 600.0, 1300.0 and 2100.0 μM, respectively. Inset: plot of the peak current as a function of AC concentration. (B) Differential pulse voltammograms of ZONMCPE in 0.1 M PBS (pH 7.0) containing 800.0 μM EP and different concentrations of FA, from bottom to top: 300.0, 650.0, 1100.0, 1300.0, 1500, 1900 and 2200.0 μM, respectively. Inset: plot of the peak current as a function of FA concentration. |
Fig. 5 Differential pulse voltammograms of ZONMCPE in 0.1 M PBS (pH 7.0) containing different concentrations of EP + AC + FA, (numbers 1–17 correspond to) 100.0 + 100.0 + 30.0, 150.0 + 200.0 + 100.0, 200.0 + 300.0 + 150.0, 250.0 + 350.0 + 200.0, 300.0 + 400.0 + 300.0, 350.0 + 500.0 + 400.0, 400.0 + 550.0 + 500.0, 525.0 + 700.0 + 650.0, 600.0 + 800.0 + 750.0, 700.0 + 900.0 + 850.0, 825.0 + 1100.0 + 1000.0, 950.0 + 1300.0 + 1250.0, 1150.0 + 1500.0 + 1500.0, 1350.0 + 1750.0 + 1750.0, 1500.0 + 2000.0 + 2000.0, 1700.0 + 2200.0 + 2250.0, and 2000.0 + 2500.0 + 2500.0 μM, respectively. Insets: (B) plot of the peak current as a function of EP concentration. (C) plot of the peak current as a function of AC concentration. (D) plot of the peak current as a function of FA concentration. |
No. | EP injection (μM) | AC added (μM) | FA added (μM) | EP | AC | FA | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Found (μM) | Recovery (%) | RSD (%) | Found (μM) | Recovery (%) | RSD (%) | Found (μM) | Recovery (%) | RSD (%) | ||||
1 | 10 | 20 | 50 | 10.23 | 102.3 | 2.7 | 20.34 | 101.7 | 1.9 | 49.12 | 98.24 | 2.4 |
2 | 20 | 30 | 60 | 19.62 | 98.1 | 3.1 | 30.85 | 102.8 | 2.3 | 61.59 | 102.6 | 1.5 |
3 | 30 | 40 | 70 | 29.73 | 99.1 | 1.6 | 39.38 | 98.45 | 2.9 | 70.93 | 101.3 | 3.2 |
This journal is © The Royal Society of Chemistry 2011 |