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0King Saud University Journal of Saudi Chemical Society
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ORIGINAL ARTICLE
Poly(m-ferrocenylaniline) modified carbon nanotubes-paste electrode encapsulated in nation film for selective and sensitive determination of dopamine and uric acid in the presence of ascorbic acid
Wongduan Sroysee a, Sanoe Chairam a'*, Maliwan Amatatongchaia, Purim Jarujamrus a, Suparb Tamuang a, Saichol Pimmongkolb, Laksamee Chaicharoenwimolkulc, Ekasith Somsookd
a Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Ubon Ratchathani University, Warin Chamrap, Ubon Ratchathani 34190, Thailand
b Department of Physics, Faculty of Science, Ubon Ratchathani University, Warin Chamrap, Ubon Ratchathani 34190, Thailand c School of Chemistry, Faculty of Science and Technology, Suratthani Rajabhat University, 272 Moo 9, Surat-Nasan Rd., Khuntale, Muang, Surat Thani 84100, Thailand
d NANOCAST Laboratory, Center for Catalysis, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, 272 Rama VI Rd. Rachathewi, Bangkok 10400, Thailand
Received 3 December 2015; revised 17 February 2016; accepted 20 February 2016
KEYWORDS
Poly(m-ferrocenylaniline); Carbon nanotubes-paste electrode; Nafion;
Electrochemical sensor;
Dopamine;
Uric acid
Abstract A nafion covered carbon nanotubes-paste electrode modified with poly(m-ferrocenylaniline), (Nf/p(FcAni)-CNTsPE), provides a novel voltammetric sensor for the selective determination of dopamine (DA) and uric acid (UA) in the presence of ascorbic acid (AA). We studied the electrochemical activity of Nf/p(FcAni)-CNTsPE toward DA, UA, and AA by differential pulse voltammetry (DPV). DA and UA anodic peaks appear at 0.30 and 0.45 V, respectively while an anodic peak for AA was not observed. DPV oxidation peak values are linearly dependent on DA concentration over the range 1-150 iM (r2 = 0.992), and on UA concentration over the range 5-250 iM (r2 = 0.997). DA and UA detection limits are estimated to be 0.21 and 0.58 iM, respectively. The modified electrode shows both good selectivity and reproducibility for
* Corresponding author. Tel.: +66 45 353 400x4137; fax:
E-mail address: sanoe.c@ubu.ac.th (S. Chairam).
Peer review under responsibility of King Saud University.
66 45 288 379.
http://dx.doi.org/10.1016/j.jscs.2016.02.003
1319-6103 © 2016 King Saud University. Production and hosting by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
the selective determination of DA and UA in real samples. Finally, the modified electrode was successfully applied for the determination of DA and UA in pharmaceutical or biological sample fluids.
© 2016 King Saud University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
Dopamine (DA), uric acid (UA), and ascorbic acid (AA) normally coexist in human biological fluids, mainly in serum, blood, and urine. These compounds play important roles in renal, hormonal, cardiovascular, and central nervous systems, and are essential for correct function of the metabolism. The neurotransmitter DA is a precursor for catecholamine synthesis, for example, of epinephrine and norepinephrine; disorders in DA synthesis can lead to Parkinson's disease [1]. UA is the major final product of purine metabolism in humans. UA is weakly soluble in aqueous media, and this can lead to problems in humans when UA levels are pathologically elevated [2]. AA (vitamin C) is a powerful reducing agent, and is an essential nutrient for humans [3]. As an antioxidant, AA provides nonspecific protection against oxidative damage, and it is an essential cofactor for monooxygenases and dioxygenases in various metabolic pathways [4]. Therefore, the determination of these compounds is very important in pharmaceutical or biological sample fluids. There are several analytical methods for the determination of DA, UA, and AA, such as chemilumi-nescence and fluorescence spectroscopy, spectrophotometry, and high-performance liquid chromatography (HPLC). However, these methods usually require expensive laboratory facilities, are time-consuming, and can be complex to perform. As an electroactive substance, they can be also determined via electrochemical techniques.
Electrochemical methods for DA, UA, and AA sensing can be performed using common electrodes. These methods generally offer greater sensitivity, and are simpler and less time-consuming than other methods available. The major obstacle to selective electrochemical determination of DA, UA, and AA is that their oxidation potentials are similar. This results in overlapping of voltammetric responses and therefore provides poor selectivity and inaccurate quantitative analysis. Most studies report a lower electrochemical oxidation potential for AA compared to DA and UA [5,6]. Thus, interference caused by AA presents a challenge for the determination of DA and UA. Attempts to overcome this problem have used various materials to modify the electrode surface, such as polymers, metal complexes, metal or metal-oxide nanoparticles, and nanocomposites [7,8].
There is great interest in the development of inexpensive, simple, and rapid methods for routine analysis of DA, UA in the presence of AA. In a recent review, Erden and Kilic
[9] identified several electrochemical approaches, including the use of modified electrodes, to separate the oxidation peaks of DA, UA, and AA, and to reduce interference by AA. As a polymeric film, nafion (Nf) exhibits good selectivity against anions and can pre-concentrate cations at the electrode surface, creating a protective coating for the electrode surface
[10]. Nafion-modified electrodes can be easily prepared by drop casting or by spin coating the polymer solution directly
onto the electrode surface. There are many reports of the successful application of nafion-coated chemically-modified electrodes for voltammetric determination of DA and UA in the presence of ascorbic acid [11,12].
In the field of analytical and bioanalytical electrochemistry, carbon paste electrodes (CPE) are one of the most common self-made electrodes because of their facile construction from a mixture of graphite powder and a pasting liquid [12]. The conductive graphite serves as the conducting electrode material. The pasting material is of insulating character and acts as an inert medium, binding individual particles into a compact mixture [13]. CPEs modified with various materials are termed ''chemically modified electrodes". These modified CPEs possess several advantages over other electrode materials, including low cost, ease of fabrication, low ohmic resistance, low background current, renew ability, and stable responses. Nanomaterials have received a great deal of attention and have a wide range of applications that exploit their unique properties. Among these materials, carbon nanotubes (CNTs), with their advantageous electrical and chemical properties, are promising materials for enhancing electrode electro-catalytic activity. CNTs have many applications in the fabrication of electrochemical sensors and biosensors [14]. Recently, CNTs-paste electrode (CNTsPE) were successfully applied to voltammetric measurements of various biological species, including dopamine, uric acid, ascorbic acid, epinephr-ine, benserazide, glutathione, acetaminophen, cysteamine, NAD, folic acid, glutathione and piroxicam [15,16].
Redox mediators are widely used to improve sensitivity, selectivity, and detection limits of electrochemical sensors and biosensors. Among these, ferrocene (Fc) is well suited to modify the electrode surface due to its excellent electron transfer properties during redox reactions [17,18]. Fc and its derivatives are effective in catalyzing both the oxidation and reduction of various electroactive compounds [19-21]. However, Fc leakage is a major problem for the modified electrode; Fc adsorbs weakly to the electrode surface due in part to its neutral charge. Substituting the Fc rings with high molecular weight compounds or polymers can resolve this problem.
We propose a new redox active ferrocene-based polyaniline chemically modified paste electrode for the detection of DA and UA. The fabrication of a poly(m-ferrocenylaniline)-carbon nanotubes-paste electrode covered with nafion film (Nf/p(FcAni)-CNTsPE) as a novel voltammetric sensor for the selective determination of DA and UA in the presence of ascorbic acid was reported. To our knowledge, this is the first report of using p(FcAni)-CNTs to improve the electrochemical activity of DA and UA sensors. The Nf/p(FcAni)-CNTsPE exhibits significant enhancement in sensor performance. The electroactive surface area and unique nanostructure of p (FcAni)-CNTs provide good conductivity and enhance electron transfer. The Nf/p(FcAni)-CNTsPE was successfully applied for the determination of DA and UA.
2. Experimental details
2.4. Preparation of working electrodes
2.1. Materials and solutions
All reagents were used as received without further purification. Multiwalled carbon nanotubes (CNTs), diameters 30 ± 15 nm, length 1-5 im and purity greater than 95% were purchased from Nanolab Inc. (MA, USA). Uric acid (UA) and graphite powder were purchased from Acros Organic (Geel, Belgium). Nafion (Nf), dopamine hydrochloride (DA) and ascorbic acid (AA) were purchased from Sigma-Aldrich (St. Louis, USA). All aqueous solutions were freshly prepared using de-ionized water (resistance P 18.2 MX cm, purified by a Nanopore Ultrapure Water System).
2.2. Synthesis of m-ferrocenylaniline
The m-ferrocenylaniline (FcAni) (1) was synthesized according to the reported method [22]. In brief, m-ferrocenylnitrobenzene was synthesized from the reaction between ferrocene and m-nitroaniline using sodium nitrite/hydrochloric acid in the presence of diethyl ether. The product was then reduced to m-ferrocenylaniline using Sn/HCl in an ice bath. The crude product was purified by column chromatography with gradient elution (hexane-ethyl acetate) to afford the ferrocene derivative. A yellow-orange crystalline solid was obtained after drying under reduced pressure at room temperature. Scheme 1 illustrates the synthetic procedure used for the preparation of m-ferrocenylaniline. 1H NMR (500 MHz, CDCl3, 298 K): d (ppm) = 3.53 (br s, NH, 2H), 3.97 (s, C5H4, 5H), 4.20 (s, C5H4, 2H), 4.51 (s, C5H4, 2H), 6.45 (d, C6H4, 1H), 6.73 (s, C6H4, 1H), 6.83 (d, C6H4, 1H), 7.01 (t, C6H4, 1H). 13C NMR (300 MHz, CDCl3, 298 K): d (ppm) = 66.5, 68.7, 69.5, 76.6, 77.0, 77.4, 85.6, 112.9, 113.1, 117.1, 129.1, 140.2, 146.1. EIMS Exact Mass: Calculated for C16H15NFe [M]+ = 277.05, Found [M + H] + = 278.06. FTIR (KBr): vmax (cm-1) = 3437, 3365, 3103, 1617, 1602, 1582, 1509, 1469, 1388, 1308, 1235, 1169, 1102, 1028, 999, 863, 820, 806, 782, 693, 519, 504, 485, 453. Elemental C:H:N Analysis (%): Calculated for C16H15Nfe: C = 69.34, H = 5.46, N = 5.05, Found: C = 69.52, H = 5.22, N = 5.06.
2.3. Synthesis of poly(m-ferrocenylaniline)
Poly(m-ferrocenylaniline) (p(FcAni)) (2) was prepared by oxidative polymerization of compound (1), following the previously reported method [23]. The resulting dark brown powder was collected by filtration and then dried in an oven at 80 0C for 24 h. Scheme 2 illustrates the synthetic procedure used for the preparation of poly(m-ferrocenylaniline).
A carbon paste electrode (PE) was constructed following our previously reported method with some modifications [24]. Graphite powder (100 mg) and paraffin oil (30 iL) were hand-mixed in a mortar to form a homogeneous paste. Subsequently, the graphite and mineral oil composite was packed into a cylindrical glass tube (2 mm diameter, 10 cm long) by pressing the open end of the tube into the paste composite to a depth of approximately 2 mm. Next, a copper wire (1.5 mm diameter, 12 cm long) was inserted into the tube from the opposite end, until the end of copper wire was buried into the graphite/mineral oil composite, forming an electrical contact between the paste and wire.
The Nf/p(FcAni)-CNTsPE was prepared by hand-mixing of 25 mg p(FcAni), 5 mg CNTs, 70 mg graphite powder and 30 iL mineral oil in a mortar and pestle. The homogeneous paste was then packed into the end of a glass tube. The surface of the p(FcAni)-CNTsPE was manually polished using weighing paper until a flat and smooth surface was obtained. Next, Nf solution (10 iL, 0.1%) was dropped onto the modified CNTsPE using a micropipette, and the electrode was left to dry at ambient temperature. The fabricated CNTsPE used for electrochemical studies were kept in a desiccator at room temperature when not in use.
For comparison purposes, the Nf/PE, Nf/p(FcAni)/PE and p(FcAni)/PE were also prepared following the same procedure.
2.5. Voltammetric procedures
All electrochemical measurements were carried out using an eDAQ potentiostat (EA161) equipped with an e-corder (210), and using e-Chem v2.0.13 software. A Metrohm 713 pH meter was used to monitor the phosphate buffer pH. Cyclic voltam-metry (CV) and differential pulse voltammetry (DPV) were used for the analysis of DA, UA, and AA. CV and DPV measurements were performed in a buffer solution over a running potential range from -0.3 to 0.9 V. A conventional three-electrode cell assembly, consisting of a platinum wire counter electrode and an Ag/AgCl (sat. 3.0 M KCl) reference electrode was used. The working electrode was either the PE or the modified-carbon nanotubes paste (CNTsPE). All solutions were purged with nitrogen gas for 5 min prior to use. DPV measurements were used for the simultaneous determination of DA and UA in the presence of AA. The electrochemical parameters for DPV measurements were: pulse amplitude = 50 mV, modulation time = 0.2 s, interval time = 0.5 s, and scan rate = 10 mV s-1.
conc.HCl/NaNCyS oC Nh Ferrocene/diethyl ether
Sn/conc.HCl
Scheme 1 Synthetic procedure for the preparation of m-ferrocenylaniline.
Fc Fc Fc Fc
1 M /H -„>%„>vS^T>4
(1) (2) Fc = Fe
Scheme 2 Synthetic procedure for the preparation of poly(m-ferrocenylaniline).
2.6. Samples
The samples to be determined were dopamine hydrochloride solutions (250 mg DA per 10 mL solution) for intravenous infusion, obtained from a local pharmacy. Urine samples used for UA determination were collected from students. All samples were centrifuged, and the supernatants filtered through a 0.45 im filter prior to analysis. All samples were diluted with buffer solution (pH 5) prior to analysis. Determinations were performed by the standard addition method.
3. Results and discussion
3.1. Characterization
The m-ferrocenylaniline (FcAni) (1) was synthesized from the reaction between ferrocene and m-nitroaniline using sodium nitrite/hydrochloric acid in the presence of diethyl ether to produce nitro-substituted ferrocene, which then was reduced to the amine using Sn/HCl. Then, poly(m-ferrocenylaniline) (p (FcAni)) (2) was prepared by the oxidative polymerization of compound (1) using ammonium persulfate, and then used in the fabrication of the modified PE. The morphology of p (FcAni) and Nf/p(FcAni)-CNTsPE was characterized using a JEOL, JSM-5910 field emission scanning electron microscope (SEM) by accelerating at a voltage of 15 kV. Fig. 1 shows SEM images of p(FcAni) and composites of Nf/p(FcAni)-CNTs paste. As shown in Fig. 1A, the synthesized p(FcAni) is spherical in shape with the average diameter of ca. 1 im. The morphological structure of the Nf/p(FcAni)-CNTs paste composites (Fig. 1B) is quite similar to the p(FcAni), but the composites has a smoother surface. Clearly, the thin film is formed for the Nf coating over the Nf/p(FcAni)-CNTs paste
surface. The p(FcAni) particles were well dispersed on the electrode surface.
3.2. Electrochemical behaviors of modified CNTsPE
The electrochemical behaviors of the modified PEs were investigated using cyclic voltammetry (CV). Fig. 2 shows the CV responses obtained from (a) Nf/PE, (b) Nf/p(FcAni)PE, and (c) Nf/p(FcAni)-CNTsPE, in a 0.1 M phosphate buffer solution (pH 7) at a scan rate of 10 mV s_1. There is no redox peak present for the Nf/PE, and the obtained current is very low (Fig. 2, curve a), indicating that the Nf/PE has no electrochemical activity over this potential range. For the plot in Fig. 2, curve b shows two redox peaks for Nf/p(FcAni)PE, at potentials of approximately 0.20 and 0.50 V. The pair of well-defined anodic and cathodic peaks arises from the Fc|Fc + redox system, which exhibits quasi-reversible behavior in aqueous solution. To investigate the effect of CNTs, Nf/p(FcAni)-CNTsPE (Fig. 2, curve c) and Nf/p(FcAni)PE (Fig. 2b) exhibit similar electrochemical characteristics, although the peak current is greater for Nf/p(FcAni)-CNTsPE because of the excellent electron transfer properties of CNTs. Barsan et al. [25] proposed that the synergistic amplification effect seen for electrochemical sensors based on redox polymer/carbon nan-otubes modified electrodes such as Nf/p(FcAni)-CNTsPE, generally have very good electron transfer rates. Nishihara and Murata [26] noted that such materials enhance sensitivity by promoting electron-transfer reactions between molecules and the electrode substrate. In our case, ferrocene-containing polyaniline provides a high electrical conductivity with convenient doping|de-doping characteristics, and offers good environmental stability. Ferrocene is an excellent electron mediator, and can increase the rate of electron transfer to provide increased current responses.
Figure 1 SEM images of (A) p(FcAni) and (B) Nf/p(FcAni)-CNTsPE.
E/V (vs. Ag/AgCl)
Figure 2 CV plots of (a) Nf/PE, (b) Nf/p(FcAni)PE, and (c) Nf/p(FcAni)-CNTsPE, in a 0.1 M PBS (pH 7) at a scan rate of 10 mV s-1.
3.3. Electrochemical behaviors of DA, UA, and AA at the Nf/p (FcAni)-CNTsPE
Yang et al. [27] demonstrated that the AA oxidation potential is very similar to those of DA and UA, and that AA coexists with DA and UA in reduced graphene oxide modified electrodes. To study the electrochemical activity of Nf/p(FcAni)-CNTsPE toward DA, UA, and AA, DPV method was
performed in a PBS (pH 7), with and without the addition of 1.0 mM DA, 1.0 mM UA, and 1.0 mM AA at a scan rate of 10 mV s_1 as shown in Fig. 3. Chen et al. [28] reported that DPV offers greater sensitivity and selectivity than CV for elec-trocatalytic oxidation of DA, UA, and AA.
After the addition of the analyte to the supporting electrolyte solution, DA and UA anodic peaks appear at 0.30 and 0.45 V, respectively (Figs. 3A and 4B). However, no anio-nic peak is seen for the AA sample (Fig. 3C). Thus, the Nf film covering the electrode eliminates the AA signal, and prevents AA interference with the detection of DA and UA. This behavior suggests that the Nf/p(FcAni)-CNTsPE might elec-trochemically catalyze the DA and UA oxidation in the presence of AA. Previous reports [29,30] provide explanations for the beneficial effects of integrating an Nf film coating. The Nf film prevents interference by AA because of its polymeric structure, which contains a hydrophobic backbone (—CF2— CF2—) and hydrophilic sulfonic acid groups (— SO3H). Nf exhibits selectivity against anions, and can pre-concentrate cations at the electrode surface. AA (pKa = 4.1) exists as the anionic ascorbate ion, so experiences strong repulsive interactions with Nf [31,32]. While negatively charged Nf-coated electrodes repel negatively charged ascorbate anions, the film allows DA and UA cations to permeate. DA (pKa = 8.8) and UA (pKa = 5.4) are in their cationic forms, and have strong attractive interactions with the negatively charged, sulfonate groups on Nf. Moreover, the anodized surface has a high affinity toward DA and UA due to hydrogen
12 10 8 6 4 2
20 18 16 14 12 10 8 6 4 2
E/V (vs. Ag/AgCl)
E/V (vs. Ag/AgCl)
0.2 0.0 0.2 0.4 0.6 0.8 1.0
E/V (vs. Ag/AgCl)
Figure 3 DPV voltammetric plots of Nf/p(FcAni)-CNTsPE in a 0.1 M PBS (pH 7) at a scan rate of 10 mV s 1 for (A) with and without 1 mM DA, (B) with and without 1 mM UA, and (C) background voltammograms (0.1 M PBS, pH 7) and 1 mM of AA.
bonding [28]. Consequently, there is no observed oxidation current from AA at the Nf/p(FcAni)-CNTsPE at pH 5, and thus, the Fc|Fc+ redox system containing polyaniline at the electrode, allows selective sensing of DA and UA in the presence of AA.
3.4. pH for the determination of DA and UA in the presence of AA
Electrolyte pH has a significant impact on electrocatalytic oxidation and on the shapes of DA and UA waves. We investigated the relationship between supporting electrolyte pH and the current response obtained from DA and UA at the Nf/p (FcAni)-CNTsPE in the presence of AA. Differential pulse voltammetry (DPV), using buffer solutions containing 1 mM of AA at various pH values from pH 4 to 8 was performed. Fig. 4A-B shows Nf/p(FcAni)-CNTsPE voltammetric responses with and without the addition of 0.05 mM DA and 0.1 mM UA, and also shows the anodic peak current (ipa), anodic peak potential (Epa), and pH for DA and UA in buffer solutions at various pH values (see Fig. 4A-B). The increasing of pH from 4 to 8 results in a negative shift of the anodic peak potential for both DA and UA. The result indicated that the electrocatalysis of DA and UA at the Nf/p (FcAni)-CNTsPE is a pH dependent reaction.
Fig. 4C shows peak potential responses to pH changes for DA and UA. The peak potentials for DA and UA are a linear function of pH over the range of pH 4-8. The linear equation
for DA is y = —0.059x + 0.602, with r2 = 0.992, while the linear equation for UA is y = —0.065x + 0.782 with r2 = 0.990. The slopes of —0.059 V pH-1 and —0.065 V pH-1 for DA and UA respectively, are close to those expected for a monoelectronic/monoprotonic electrode reaction that is -0.059 mV/pH at 25 oc, confirming that the total number of electrons and protons transferred during the DA and UA oxidation mechanisms are the same. The oxidation of both DA and UA occurs by a two-electron transfer process, and so two protons are expected to be released at the Nf/p(FcAni)-CNTsPE. The results obtained here are consistent with previous reports [33,34]. Fig. 4D shows DA and UA peak current responses to changes in electrolyte pH. The ip,a for DA and UA reached a maximum at pH 5, and then decreased with further increases in pH. Consequently, we chose pH 5 as the optimal pH value for further electrochemical determination of DA and UA in the presence of AA.
3.5. Calibration plot and detection limit for DA and UA in the presence of AA
We used DPV to enhance sensitivity toward the detection of DA and UA by the Nf/p(FcAni)-CNTsPE sensor electrode under the optimal condition (pH 5) at a scan rate of 10 mV s-1 Fig. 5 shows the peak currents for DA and UA, at 0.30 and 0.45 V, respectively. Peak current increases linearly with increasing the DA and UA concentrations. The DA and UA current responses were relatively independent, and the
■ (A) -pH 4
- -pH 7
\ pH 8
0.5 0.4 0.3 0.2 0.1 0.0
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
E/V (vs. Ag/AgCl)
(B) -pH 4
A pH 8
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
E/V (vs. Ag/AgCl)
2.0 i 1.:
Figure 4 (A) DPV plots of Nf/p(FcAni)-CNTsPE in a 0.1 M buffer solution (pH 4-8) at a scan rate of 10 mV s-1 for (A) 0.05 mM DA and (B) 0.1 mM UA. (C) Anodic peak potential (Ep a) vs. pH for DA and UA. (D) Anodic peak current (ip a) vs. pH for DA and UA.
12 10 8 6 4 2 0
0.0 0.2 0.4 0.6
E/V (vs. Ag/AgCl)
0.0 0.2 0.4 0.6
E/V (vs. Ag/AgCl)
50 100
CDA/-M
100 150 Cua/-M
Figure 5 (A) DPV plots of the Nf/p(FcAni)-CNTsPE sensor under the optimal condition (pH 5) at a scan rate of 10 mV s_1 for (A) DA (1-150 iM from inner to outer plot) and (B) UA (5-250 iM from inner to outer plot). Plots of peak current vs. (C) DA and (D) UA concentrations.
Table 1 Comparison of analytical performance of the Nf/p(FcAni)-CNTsPE sensor for determination of DA and UA, with literature reports of differently modified electrodes.
Electrode Modifier pH Linear range (|iM) Detection limit (iM) Method References
DA UA DA UA
GCE Fc@DWNTs 7.0 0.5-20 - 0.30 - Amper-ometry [19]
GCE Fc-SWNTs 7.0 5.0-30 - 0.05 - DPV [33]
GCE DNA/Pp-ABSA bi-layer 7.0 0.2-13 0.4-23 0.09 0.19 DPV [35]
GCE Nf-Fc 7.0 - 250-5000 - 22.7 Amper-ometry [36]
PE Pyrogallol red 7.0 1-700 50-1000 0.78 35 DPV [37]
Pt GNPs/CDSH-Fc/Nf 6.5 2-50.0 - 0.09 - DPV [28]
GCE GN 7.0 3.3-249.1 6.7-386.3 1.50 2.70 LPV [38]
ITO Nf-CNT-ABTS 7.0 2.2-240 3.1-400 0.84 1.75 DPV [39]
PGE - 5.0 0.2-15 0.3-150 0.03 0.12 DPV [40]
GCE NCHSs 7.0 1-400 5-350 0.30 1 DPV [41]
CNTsPE Fc 7.0 - 15-1000 - 10 SWV [42]
GCE Pt/RGO 7.0 10-170 10-130 0.25 0.45 DPV [43]
GCE Fc@ß-CD 4.0 - 5-120 - 0.08 DPV [44]
CNTsPE Nf/p(FcAni) 5.0 1-150 5-250 0.21 0.58 DPV This work
Fc-SWNTs = ferrocene and single wall carbon nanotube, Pp-ABSA = poly(p-aminobenzenesulfonic acid), Nf-Fc = ferrocene bound nafion, GNPs = gold nanoparticles, CDSH-Fc = mono-6-thio-b-cyclodextrin and ferrocene, GN = graphene nanopowder, ABTS = 2,2'-Azino-bis (3-ethylbenzthiazoline-6-sulfonic acid), CNT = carbon nanotube, NCHSs = nitrogen-doped carbon hollow spheres, Pt/RGO = platinum/ reduced graphene oxide, N-CDs/Fc@b-CD = nitrogen-doped carbon dot/ferrocene@b-cyclodextrin, N-(4-hydroxyphenyl)-3,5-dinitrobenzamide = NHPDA.
GCE = glassy carbon electrode, Pt = platinum electrode, ITO = indium tin oxide, PGE = pencil graphite electrode, PE = carbon paste electrode, CNTsPE = carbon nanotubes-paste electrode, DPV = differential pulse voltammetry, LPV = linear polarization voltammetry, SWV = square wave voltammetry.
Table 2 DA and UA concentration in different samples (n = 3) obtained from the proposed method and the reference values.
Sample Analyte Detected (iM)a Spiked level (iM) Found (iM)b Recovery (%)
D-A1 D-A2 U-S1 U-S2 DA UA 31.3 ± 0.1 31.6 ± 0.1 41.5 ± 0.6 59.6 ± 0.3 10.0 39.9 ± 0.4 10.0 41.7 ± 0.3 10.0 49.5 ± 0.1 10.0 70.1 ± 1.1 96.6 ± 1.0 101.0 ± 0.6 90.3 ± 0.3 100.7 ± 1.3
a Amount found in the samples after dilution, X ± S.D. b Amount found after spiked either 10 iM of DA or UA.
anodic peak currents (ipa) showed linear responses to changes in concentrations. For DA, the linear relationship is given by y = 0.041x - 0.125 (r2 = 0.992) over the range of 1-150 iM, while the linear relationship for UA is y = 0.053x — 0.148 (r2 = 0.997) over the range of 5-250 iM. The DA and UA detection limits, calculated from the 3-fold signal-to-noise ratio (S/N = 3) were 0.21 and 0.58 iM, respectively.
Table 1 provides a comparison of analytical performances obtained from our developed sensor with differently modified electrodes. The analytical characteristics of our sensor are comparable to, or better than, those reported for other DA and UA designs. Additionally, the detection limit for our sensor is lower, or comparable to, those in previous reports, providing the sensitive detection. The use of Nf/p(FcAni)-CNTsPE also offers a wider range of linearity compared with previously reported modified electrodes [19,33-35,38-40]. Moreover, the Nf/p(FcAni)-CNTsPE offers a simple preparation procedure and handling and storage of the sensor is uncomplicated. Clearly, the Nf/p(FcAni)-CNTsPE sensor exhibits the simplicity and high selectivity for DA and UA quantitation in the presence of AA. The oxidation peak potentials of our sensor for DA and UA are lower than [35,44], or comparable [33,37,43] to those of electrochemical sensors described in literature. In addition, the major advantages of the Nf/p(FcAni)-CNTsPE over previously reported devices are ease and fast preparation, high stability and good repro-ducibility for the determination of DA and UA.
3.6. Interference studies
We performed interference studies on the determination of DA and UA at the Nf/p(FcAni)-CNTsPE sensor using DPV. The DA and UA concentrations were maintained at 0.1 mM in 0.1 M buffer (pH 5) containing 1 mM AA. Interfering species were added to the test solution in the range of 10-100 times greater concentration than that of DA and UA. The tolerance limit was taken as the amount of substance needed to cause a signal alteration of greater than ±5%. According to our results, glucose, KCl and urea do not interfere with the DA determination studied up to 10 mM. While CaCl2 and NaNO3 produce very low interference signals at a molar concentration of 2 mM or greater (7 mM) with respect to DA. In addition, KCl also does not interfere with the UA determination studied up to 10 mM. Whereas, glucose, NaNO3 and CaCl2 produce very low interference signals at a molar concentration of 5 mM or greater (7 mM) with respect to UA. This finding indicates that the Nf/p(FcAni)-CNTsPE electrode provides an acceptable selectivity for the determination of DA and UA in real samples.
3.7. Reproducibility and stability
The reproducibility and stability of the modified PE were investigated by DPV. We used the Nf/p(FcAni)-CNTsPE in a 0.1 M buffer solution (pH 5) to determine 50 iM of DA or 100 iM of UA in the presence of 1 mM AA for 15 measurements. The calculated relative standard deviations (RSD) are 4.3% and 3.2% for DA and UA, respectively. Results from our stability study reveal that after storage for 1 week at room temperature, the Nf/p(FcAni)-CNTsPE performs well, providing 95.68% and 99.9% of its initial measurement values for DA and UA, respectively. These results demonstrate that the Nf/p(FcAni)-CNTsPE performs with high stability and good reproducibility for the voltammetric determination of DA and UA.
3.8. Real sample analysis
To evaluate the analytical applicability of the Nf/p(FcAni)-CNTsPE sensor, we performed the determination of DA or UA in pharmaceutical or biological sample fluids using the standard addition method. The proposed method was applied for the detection of DA and UA in two different samples, including dopamine hydrochloride solutions for intravenous infusion (D-A1 and D-A2) and human urine samples (U-S1 and U-S2). In order to test the reliable of this proposed sensor at a low concentration range, the samples were diluted appropriately with 0.1 M PBS. The precision of the analytical process was evaluated by the repeatability of the process. The spiked concentration of 10 iM was performed to investigate the accuracy of the method. Table 2 summarizes the analytical results. The RSDs and recoveries of the proposed method were found in the range of 0.3-1.3% and 90.3-101.0%, respectively. These results indicated that there are no significant matrix interferences in the analyzes samples as well as that this presented method is sufficiently accurate and precision and suitable for the quantification of DA and UA in the mentioned samples.
4. Conclusions
The Nf/p(FcAni)-CNTsPE provides a simple and easy approach to selectively detect DA and UA in the presence of AA. The results show that the oxidation of DA and UA is elec-trochemically catalyzed at 0.3 and 0.45-V, respectively, whereas the peak potential of AA does not appear by DPV method. The Nf/p(FcAni)-CNTsPE exhibits high selectivity and good reproducibility in measurements of DA and UA in the presence of AA. DPV oxidation peak values are linearly
dependent on DA concentration over the range 1-150 iM (r2 = 0.992), and on UA concentration over the range 5250 iM (r2 = 0.997). DA and UA detection limits are estimated to be 0.21 and 0.58 iM, respectively. The results from interference, reproducibility and stability studies demonstrate that the Nf/p(FcAni)-CNTsPE electrode provides an acceptable selectivity for the determination of DA and UA and performs with high stability and good reproducibility for the voltammetric determination of DA and UA. Finally, the Nf/p(FcAni)-CNTsPE was successfully applied for the determination of DA and UA in pharmaceutical or biological sample fluids with good results. This work offers a novel sensor for the voltammetric determination of DA and UA with a great promise toward pharmaceutical and clinical applications.
Acknowledgements
This work was supported by grants from the Thailand Research Fund (TRF; TRG5680049), the Center of Excellence for Innovation in Chemistry (PERCH-CIC), the Office of the Higher Education Commission, the Ministry of Education, and the National Research Council of Thailand (NRCT, 2558A11703003). Scholarship from The Scholarship from the Science Achievement Scholarship of Thailand (SAST) given to W. Sroysee is gratefully acknowledged. We gratefully acknowledge the help and facilities provided by the Faculty of Science, Ubon Ratchathani University (UBU).
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