Chemically modified electrodes can be prepared using two general methods, namely bulk or surface modification of the electrode using materials with suitable electrocatalytic activity. Such electrodes are electro-catalysts that can improve a desired electrochemical reaction [1] and are suitable for application as electrochemical sensors because the electrode surfaces can be customized to achieve the required selectivity or sensitivity [2]. Self-assembly is a modification procedure that can be used to prepare self-assembled monolayers (SAMs), which can be used to modify the surface of electrodes [3].
Gold nanoparticles (AuNPs) are important nanomaterials for the fabrication of modified electrodes because of their good stability and biocompatibility [4, 5], their good conductivity, and their excellent catalytic activity [6, 7]. AuNP-modified electrodes can react with organo-sulfur compounds to create SAMs via the formation of S-Au covalent bonds at the electrode surface. Several sulfur-containing compounds with different S atom positions have been reported for the preparation of SAM-modified electrodes; in some of these compounds the S atom is a simple aliphatic thiol [8], and in others the S atom is a thiophenol [9], or is in a heterocyclic ring [3].
Isoprenaline (IP) is an important neurotransmitter used for the treatment of neural disorders such as Parkinson’s disease [10]. The cardiovascular effects of IP, which are typically compared with those of adrenaline and noradrenaline, show that IP can successfully relieve nervous tension from almost every kind of smooth musculature [11]. Such effects are more conspicuous in the musculature of the bronchus and gastrointestinal tract. IP is more effective when taken via inhalation, and it can be used effectively for bronchitis, cardiac shock, and heart attacks [10, 11]. However, the excessive use of IP can lead to heart failure and arrhythmias. To date, various methods for determining IP concentration have been suggested and employed, including spectrofluorimetry [12], spectrophotometry [13], liquid and gas chromatographic methods based on fluorimetry [14], electrochemical detection (for HPLC) [15], and chemiluminescence [16]. Chemically modified electrodes have also been widely applied as sensitive analytical tools for the determination of IP [17, 18].
Abnormal levels of uric acid (UA) are a symptom of several diseases, including gout, hyperpiesia, and Lesch-Nyhan disease [19]. The concentration of UA in biological fluids can therefore be monitored to provide an early warning of the presence of these diseases. Several techniques have been reported for the analysis of UA, including the detection of chemiluminescence [20], chromatography [21], and electrochemical methods [22, 23, 24, 25]; UA is a significant interferent for the determination of other biological substances such as IP.
In this study, a novel nanocomposite electrode system was fabricated using a glassy carbon electrode (GCE) modified with AuNPs, on which a novel synthesized organo-sulfur compound (3,4-dihydroxyphenyl-azo-2-thiophenol, abbreviated as DAT) was self-assembled to produce the final GCE/AuNP-DAT electrode. We reported previously the construction of a similar electrode that used a different organo-sulfur compound as the head element of the electrode [26]. In that work, 2-(2,3-dihydroxyphenyl)benzothiazole (DPB) was used to form a SAM on an AuNP-modified GCE; DPB has an S atom in its heterocyclic ring, but the DAT used in the present study is a thiophenolic derivative. One of the aims of the present study was to build on our previous work to investigate the effect of S atoms on the formation of electro-active self-assembled layers on a gold modified-electrode. Here, we showed that this novel compound (DAT) could form electro-active SAMs on a GCE/AuNPs. This novel modified electrode was used for the first time as an electrocatalyst for the oxidation of IP, and as an electrochemical sensor for the detection of IP. The chemical hydroquinone structure of DAT in this electrocatalyst provided the ability to catalyze the oxidation of IP via an electrocatalytic mechanism that is explained in this report. The electrocatalyst was effective in eliminating any interference from UA in the determination of IP. The simultaneous determination of IP and UA was carried out at the electrode.
The electrochemical measurements were performed using a potentiostat/galvanostat (SAMA 500 Electroanalyzer System, I.R. Iran). A three-electrode cell was operated at 25 ± 1°C; the cell included a modified glassy carbon electrode (Metrohm) as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a platinum wire electrode as the auxiliary electrode. All potentials are reported here with respect to the SCE. The pH measurements were performed using a Metrohm Model 691 pH/mV meter.
All solutions were prepared using double-distilled water. IP, UA, and the other reagents were of analytical grade (Merck). The buffer solutions were prepared from orthophosphoric acid and its salts. DAT was synthesized at our laboratory. The spectroscopic data for the synthesized compound were as follows. FT-IR (ATR) ῡ/cm−1: 3430-3000, 1616, 1559, 1507, 1450, 1284, 1200, 1102, 816, 756; 1H NMR (500 MHz, DMSO-d6) (δ, ppm) (J, Hz): 7.9 (s, 1H), 7.64 (d, J = 6.6 Hz, 1H), 7.58 (s, 1H), 7.5-7.45 (m, 2H), 7.24 (brs, 1H), 6.96 (m, 1H), 6.86 (m, 1H), 6.59 (m, 1H), 6.4 (m, 1H).
The AuNPs were prepared using the citrate reduction method described by Lee and Meisel [27]. Briefly, 250 mL of HAuCl4 solution (1 mmol/L) was boiled in a 500-mL round-bottomed flask under stirring, and 25 mL of sodium citrate solution (38.8 mmol/L) was added quickly to this solution. The mixture was then refluxed for 15 min under continuous stirring. The flask was allowed to cool to room temperature and was stored in the dark at 4 °C until use.
The procedure used to fabricate the proposed GCE/AuNP-DAT electrocatalyst is illustrated schematically in Scheme 1. The GC electrode was first carefully polished with alumina on a polishing cloth, and was then subjected to ultrasonication in deionized water, and subsequently ethanol, to remove any adsorbed particles. Twenty microliters of a dispersion of AuNPs (prepared as described above) was then cast on the surface of the GC electrode, and the resulting sample was dried in air to form an AuNP film on the electrode surface (the resulting sample is denoted here as GCE/AuNPs). The GCE/AuNP-DAT was prepared by immersing the GCE/AuNPs in DAT-ethanol solution (5 mmol/L) for 24 h at room temperature. Upon removal from this solution, the electrode was thoroughly rinsed with twice-distilled water (to remove the physically adsorbed species) and was then used for the electrochemical experiments.
Before electrochemical studies were performed using the prepared electrocatalyst, its surface was characterized using scanning electron microscopy (SEM). Fig. 1 shows SEM images of the bare GCE and the GCE/AuNPs. Fig. 1(b) shows an SEM image of the GCE/AuNPs with many spherical AuNPs on the surface of the GCE, in contrast with the bare GCE in Fig. 1(a), which had a smooth and homogeneous surface. These results demonstrated that AuNPs were deposited on the electrode.
Cyclic voltammograms were measured for the GCE/AuNP- DAT in a phosphate buffer solution (0.1 mol/L) with pH = 7.0 (Fig. 2(a)), using a scan rate of 50 mV/s. The cyclic voltammogram showed an anodic peak at a potential of approximately 0.21 V in the forward potential scan, and a cathodic peak at a potential of approximately 0.07 V in the reverse scan. The ∆E (Epa - Epc) value was therefore 0.15 V vs. SCE, greater than the value expected for a reversible system, showing that this electrode process was quasi-reversible [28].
Cyclic voltammograms were measured for the GCE/AuNP- DAT at different scan rates, and the anodic and cathodic peak currents were then plotted as a function of the scan rate, as shown in Fig. 2(b). The anodic and cathodic peak current values were linearly dependent on the scan rate, with a linear relationship between the peak current and the potential scan rate, indicating a surface-confined, controlled redox process at the modified electrode. These results indicated that the DAT was attached to the electrode surface via covalent and thermodynamically favored S-Au bonds [29, 30].
The electrochemical behavior of the AuNPs-DAT at the GCE was studied in solutions with different pH values. It was observed that the anodic and cathodic peak potentials of the GC/AuNP-DAT shifted to negative values with increases in the pH (Fig. 2(c)). A potential-pH diagram was constructed by plotting the calculated E1/2 values for each cyclic voltammogram as a function of pH. As shown in the inset of Fig. 2(c), one straight line was obtained in the pH range of 4.0-11.0 with a slope of 0.053 mV/pH. This result showed that there was an equal transfer of electrons and protons in the redox reaction of DAT in this pH range [28].
The electrocatalysis of IP by the DAT molecules in the proposed GCE/AuNP-DAT electrocatalyst was investigated using CV. Fig. 3 shows cyclic voltammograms for the GCE/AuNP-DAT (curve (1)), the GCE/AuNPs (curve (4)), and the bare GCE (curve (2)) in the buffered aqueous solution (pH = 7.0) containing IP (0.05 mmol/L) measured at scan rate of 20 mV/s. For comparison, curve (3) shows the cyclic voltammogram for the GCE/AuNP-DAT in the phosphate buffer solution without IP measured under the same conditions. Where as the electrochemical signal for the oxidation of IP at the GCE/AuNPs was not very different from that measured using the bare GCE (comparing curves (2) and (4)), the signal for the electro-oxidation of IP was significantly enhanced at the GCE/AuNP-DAT. The voltammograms indicated that the IP could be electro-catalyzed by the redox couple of the DAT molecules. A comparison of curves (1) and (3) in Fig. 3 indicated that the anodic peak current of the modified electrode was greatly increased in the presence of IP, and the cathodic peak disappeared. Based on these results, an EC′ catalytic mechanism was proposed to describe the electrochemical oxidation of IP at GCE/AuNP-DAT; the mechanism is illustrated in Scheme 2. In this mechanism, IP is oxidized in the catalytic chemical reaction by the oxidized form of DAT, which is produced via an electrochemical reaction. Therefore, when the DAT was oxidized at a potential of 0.21 V, the IP could also be oxidized at this potential. The IP was thus oxidized at a potential of 0.21 V at the GCE/AuNP-DAT, while it was oxidized at a potential of approximately 0.36 V at the bare GC electrode. These results illustrated the electrocatalytic effect of the proposed electrocatalyst in improving the electro-oxidation of IP.
The effect of changing the scan rate on the electrocatalytic oxidation of IP at the modified electrode was investigated using cyclic voltammetry (Fig. 4(a)). A plot of the peak current (Ip) against the square root of the scan rate (ν1/2) was constructed (Fig. 4(b)); this plot was found to be linear, suggesting that the process was diffusion-controlled, rather than surface- controlled. The variation of the scan rate-normalized current (Ip/ν1/2) with changes in the scan rate, shown in Fig. 4(c), indicated that an electrocatalytic EC′ mechanism was responsible for the oxidation of IP by the DAT self-assembled molecules at the electrode.
Chronoamperometric measurements of IP at the GCE/ AuNP-DAT were performanced for different concentrations of IP, using a potential step of 350 mV. These chronoamperograms were used to evaluate the catalytic rate constant, k/(L mol-1 s-1), for the reaction between IP and DAT at the surface of modified electrode, based on the method of Galus [31]:
IC / IL = γ1/2[π1/2erf(γ1/2) + exp(-γ)/γ1/2] (3)
where IC is the catalytic current of IP at the GCE/AuNP-DAT, IL is the limited current in the absence of IP, and γ = kCbt is the argument of the error function (Cb is the bulk concentration of IP, and t is the time elapsed). In cases where γ exceeds a value of 2, the error function is almost equal to 1, and the above equation can therefore be reduced to:
IC / IL = π1/2 γ1/2 = π1/2 (kCbt)1/2 (4)
Therefore, k could be obtained for a given IP concentration based on the slope of the IC/IL versus t1/2 plot. From the values of the slopes, the average value of k was found to be 373.45 Lmol-1 s-1).
Differential pulse voltammetry (DPV) was used to obtain a calibration plot for the detection and measurement of IP using the GCE/AuNP-DAT. Fig. 5(a) shows DP voltammograms measured for different concentrations of IP. Fig. 5(b) and (c) show the plot of peak current versus IP concentration (calibration plot) for two linear segments with different slopes. For the concentration range of 1.0-80.0 μmol/L the sensitivity was 0.01221 μA (μmol/L)-1, and for the concentration range of 80.0-1500.0 μmol/L the sensitivity was 0.0059 μA (μmol/L)-1. The detection limit (3σ) for IP in the lower range was calculated as 0.49 μmol/L. Table 1 shows a comparison between the performance of other modified electrodes and that of the present electrocatalyst for the determination of IP [32, 33, 34, 35, 36].
UA and IP have overlapping signals on bare electrodes, and the determination of IP in the presence of UA is therefore not possible using bare electrodes. Here, the DP voltammogram for IP was recorded in the presence of UA to investigate the effect of UA on the determination of IP using the GCE/AuNP-DAT. As shown in Fig. 6, the DPV peak for IP was at a potential of approximately 0.15 V and was separated from the DPV peak for UA, which was at a potential of approximately 0.39 V. These results showed that UA, the most interferent substance in biological environments for IP, did not have any interferent effect on the determination of IP; the results also showed that the simultaneous determination of IP and UA could be achieved using the proposed modified electrode.
The function of the composite GCE/AuNP-DAT was monitored over a one-month period by recording the electrocatalytic cyclic voltammogram for IP. The response of the electrocatalyst did not change significantly in the first two weeks. In this period the peak potential for the oxidation of IP at the modified electrode did not change, and the current signals showed a decrease of less than 4.0% relative to the initial response. After three weeks the current had decreased by more than 10%, relative to its initial value.
A glassy carbon electrode modified with gold nanoparticles showed good ability for the immobilization of a self-assembled monolayer of a thiophenol-hydroquinone derivative (DAT). The fabricated GCE/AuNP-DAT electrode exhibited suitable electrochemical properties for an electrocatalytic modified electrode. This electrode displayed electrocatalytic activity for the oxidation of IP and decreased the IP oxidation overpotential. The electrode could be used to determine the concentration of IP in the presence of UA. Good selectivity, a suitable linear range, a suitable detection limit, and its ease of preparation make the proposed modified electrode a useful tool for the accurate determination of IP.
The authors wish to thank the Yazd University Research Council, IUT Research Council and Excellence in Sensors for financial support of this research.
2015, Vol. 36 Issue (8): 1273-1279
PDF (2855 KB)
2015, Vol. 36 Issue (8): 1273-1279
PDF (2855 KB)