Simultaneous determination of dopamine and uric acid in the presence of high ascorbic acid concentration using cetyltrimethylammonium bromide–polyaniline/activated charcoal composite

Abstract

We describe a simple, low-cost and mass producible composite made up of cetyltrimethylammonium bromide (CTAB) functionalized polyaniline (PANI) and activated charcoal (CTAB–PANI/AC) for simultaneous determination of dopamine (DA) and uric acid (UA). The composite formation was verified through scanning electron microscopy, electrochemical impedance spectroscopy and electrochemical methods. The CTAB–PANI/AC composite was used to modify a glassy carbon electrode (GCE) and the resulting modified electrode displayed excellent electrocatalytic activity to DA and UA and successfully separates their overlapped voltammetric peaks. The composite completely inhibits the AA signal and does not produce any voltammetric signal for AA up to 2 mM. The DA and UA can be selectively detectable up to detection limits of 0.06 (±0.006) μM and 0.20 (±0.008) μM, respectively. The effects of kinetics, analyte concentration and pH of the supporting electrolyte were investigated and optimized. The modified electrode has appreciable stability, repeatability and reproducibility. Besides, the practical feasibility of the sensor is demonstrated in biological samples, which delivered satisfactory recovery results.

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1. Introduction

Dopamine (DA) is one of the key neurotransmitters in mammalian central nervous systems that facilities communication between brain and neurons.^1–3 An abnormal concentration of DA is directly related to the motor functions of the central nervous system, which can cause several neurological disorders.^4,5 In addition, DA is administered externally as a medication to DA-deficient patients, but excess dosage causes neurological side effects.⁶ Therefore, sensitive determination of DA is important in clinical analysis.^7–9 Furthermore, uric acid (UA) is the primary end product of purine metabolism and its abnormal levels are symptoms of several diseases, such as gout, hyperuricemia and Lesch–Nyhan disease.^10,11 It is well-known that DA, UA and ascorbic acid (AA) usually coexist in extracellular fluids; however, they are indicators for different diseases.^12,13 AA is a significant vitamin required in the human diet and usually its concentration in extracellular fluids is much higher (about 1000 times) than those of DA and UA. Hence, the detection of DA and UA always encounters strong interference from AA.^14,15 In addition, the electrode surface is susceptible to damage by the adsorption of oxidized products of AA, which leads to surface fouling and film stability problems.¹⁶ Other problems in the determination of DA and UA are overlapped voltammetric responses, poor selectivity and low reproducibility.¹⁷ The bare electrodes are not suitable for selective and/or simultaneous detection of DA and UA, and hence rationally designed chemical modifiers are required.¹⁸ The ideal modifiers should separate the voltammetric peaks of DA and UA with wide peak-to-peak separation, completely eliminate the AA interference, have high sensitivity in the presence of AA, and selectivity over coexisting electroactive species in biological fluids. So far, several modified electrodes based on nanomaterials,^7,17 conducting polymers,^1,19 metal nanoparticles^7,20 and metal oxides^21,22 have been developed; however, not all of them are ideal modifiers.²³ Xu et al. have reported Pt nanoparticle-supported reduced graphene oxide for the simultaneous detection of DA and UA, which eliminates AA interference up to 1 mM.¹⁴ Liu et al., described a poly(acrylic acid)-multiwalled carbon nanotube composite for the detection of DA and UA, which hinders the AA signal up to 0.3 mM.²⁴ Alipour et al. described a pretreated pencil graphite electrode for the detection of DA and UA that is selective up to 0.5 mM AA.¹⁷ In spite of the success of the developed electrode materials, new materials are still in demand to completely tackle the problems due to the biological significance of DA and UA.

Polyaniline (PANI) is the most commonly used conducting polymer in electrochemical sensors owing to its excellent electrocatalytic ability, conductivity, easy synthesis, high environmental stability and thermal stability.²⁵ Composites of PANI with carbon materials, such as activated carbon, graphene and carbon nanotubes, have shown improved electronic and mechanical properties compared with pure PANI, making the composites more suitable for electronic/electrochemical applications.²⁶ Among other carbon materials, activated carbon/charcoal (AC) is the cheapest and most abundant, and it has high surface area and good porosity.²⁷ In recent times, composites prepared from AC and PANI have attracted significant attention for use in supercapacitors.^27–29 Nevertheless, the electrochemical sensor applicability of AC/PANI composites has not been investigated in the literature.

Herein, we prepared an AC/PANI composite via a simple soft-template synthetic method using CTAB as the template. Previous studies proved that CTAB is a good soft-template for controlled polymerization of aniline and the resulting polymer is ordered in different aspects, such as morphology, particle size, and conductivity.³⁰ We have used potassium peroxodisulfate (PDS) as a radical initiator in PANI formation. The positively charged PANI surface binds easily with the negatively charged AC surface via electrostatic interaction, resulting in the formation of a stable composite. The morphology, elemental composition, electrode–electrolyte interface and electrochemical properties reveal that the CTAB–PANI/AC composite is highly suitable for electrochemical sensing applications. The electrochemical studies proved that the composite is highly useful for simultaneous determination of DA and UA in the presence of high concentrations of AA (up to 2 mM) (Scheme 1).

Scheme 1 Schematic representation of the preparation of CTAB–PANI/AC/GCE and its application towards simultaneous determinations of DA and UA.

2. Experimental

2.1 Chemicals and apparatus

Aniline, CTAB, AC and PDS were purchased from Merck. All the other reagents were purchased from Sigma-Aldrich and used as received. All the reagents were of analytical grade and were used without any further purification. Double distilled water was used for all the experiments. 0.1 M phosphate buffer (pH 7.0) was used as the supporting electrolyte, which was prepared from sodium dihydrogen phosphate and disodium hydrogen phosphate. Rat brain sample and human serum were acquired from Chang Gung University, Taiwan and the experimental protocols were approved by the Institutional Animal Ethic Committee. The real sample analysis performed in rat brain and human serum samples were performed in compliance with the laws and institutional guidelines of Chang Gung University, Taiwan. An informed consent was obtained for human serum collection with human subjects.

Electrochemical studies were performed in a conventional three electrode cell using a modified glassy carbon electrode (GCE) (Bioanalytical Systems, Inc., USA) as the working electrode (area 0.071 cm²), saturated Ag|AgCl (saturated KCl) as the reference electrode and Pt wire as the counter electrode. All the electrochemical measurements were carried out using a CHI 1205a electrochemical work station (CH Instruments, Inc., USA) at ambient temperature. Prior to each electrochemical experiment, the electrolyte solutions were deoxygenated with pre-purified nitrogen for 15 min unless otherwise specified. Surface morphological studies were carried out using a Hitachi S-3000 H scanning electron microscope (SEM) and a transmission electron microscope (TEM) (H-7600, Hitachi, Japan). Raman spectra were acquired using a micro-spectrometer (Renishaw inVia system, UK) with a 514.4 nm He/Ne laser. FTIR spectra were obtained using a PerkinElmer IR spectrometer. The X-ray photoelectron spectroscopy study was carried out using a PerkinElmer PHI-5702. An EIM6ex Zahner (Kronach, Germany) was used for electrochemical impedance spectroscopy (EIS) studies.

2.2 Preparation of CTAB–PANI/AC/GCE

First, a 5 mM CTAB solution was prepared in 0.5 M H₂SO₄. Next, 40 mM aniline solution was added to the CTAB solution and the mixture was stirred for 30 min using a magnetic stirrer. 1 g of AC was added to the solution and stirring was continued for an additional 30 min while the temperature was maintained below 0 °C. Afterwards, a pre-cooled solution of 50 mM PDS was added dropwise to the aniline solution and stirred for 30 min. A precipitate was formed, which was filtered and washed several times with water and acetone. The purified CTAB–PANI/AC composite was dried and redispersed in ethanol (1 mg mL⁻¹). Next, the GCE surface was polished with 0.05 μm alumina slurry using a Buehler polishing kit, then washed with water and dried. A 5 μL dispersion of CTAB–PANI/AC was dropped on the pre-cleaned GCE and dried under ambient conditions.

2.3 Sampling procedure for real sample analysis

Dopamine hydrochloride injection was obtained from a local medical hospital and directly used for the analysis. Real-time analysis was directly carried out by injecting aliquots of the dopamine hydrochloride injection sample into phosphate buffer (pH 7). In all the real-sample studies, the total volume of the electrochemical cell was kept at 1 mL. Rat brain sample and human serum were acquired from Chang Gung University, Taiwan and the experimental protocols were approved by the Institutional Animal Ethics Committee. About 1 mL of rat brain or human serum was added to 20 mL of buffer. To this solution, known concentrations of DA were spiked and analyzed using the CTAB–PANI/AC film modified electrode. A human urine sample was collected from a healthy human. About 1 mL of human urine was diluted with 50 mL of phosphate buffer and the resulting solution was DA free. To this solution, known concentrations of DA were spiked and analyzed using our method.³¹

3. Results and discussion

3.1 Characterization of CTAB–PANI/AC

The surface morphologies of the CTAB–PANI and CTAB–PANI/AC composites were studied using SEM and TEM. The SEM image of CTAB–PANI (Fig. 1a) exhibited a porous like morphology along with the presence of PANI fibers and this kind of morphology was also observed in the TEM image (Fig. 1c). The SEM image of CTAB–PANI/AC features highly porous morphology with abundant pores, cavities and randomly distributed flakes (Fig. 1b). The TEM image of the composite also displays the PANI covered with porous carbon like morphology, which is consistent with SEM results (Fig. 1d). XPS is used to measure the elemental composition of the as-prepared composite (Fig. 2a). The survey XPS spectrum indicates that the composite consists of oxygen (O 1s, 532 eV), nitrogen (400 eV) and carbon (C 1s 285 eV). The presence of nitrogen clearly indicates the successful formation of the PANI/AC composite because the nitrogen content originates from polyaniline. The enlarged XPS spectra for C 1s (Fig. 2b), O 1s (Fig. 2c) and N 1s (Fig. 2d) clearly show the typical bands and reveal the successful formation of the composite. The Raman spectrum of CTAB–PANI (curve a′, Fig. 3a) displays two sharp bands at ∼1261 cm⁻¹ and ∼1539 cm⁻¹ corresponding to the characteristic D and G bands. Furthermore, the D band to G band intensity ratio (I_D/I_G) is considerably increased in the CTAB–PANI/AC composite (curve b′, Fig. 3a) and the bands are slightly red shifted compared to those for CTAB–PANI, indicating the possible interaction between CTAB–PANI and AC. FT-IR spectroscopy was used to investigate the functionality of the as-prepared materials. The FTIR spectrum of CTAB–PANI (curve a′, Fig. 3b) displays FTIR absorption peaks, ν(N–H) = 3430, ν(aromatic C–H bond) = 2891, ν(quinoid) = 1563, ν(benzenoid) = 1490, ν(C–N stretching and bending vibrations) = 1299 and 1238, ν(C–N double bond) = 1115 and ν(C–H out of plane bending) = 801 cm⁻¹, which are characteristic stretching vibrations of PANI and this result is consistent with previous reports.^32,33 These vibration modes are observed in the CTAB–PANI/AC composite but slightly red shifted, which could be due to the composite formation (curve b′, Fig. 3b).

Fig. 1 SEM images of CTAB–PANI (a) and CTAB–PANI/AC composite (b). TEM images of CTAB–PANI (c) and CTAB–PANI/AC composite (d).

Fig. 2 (a) XPS survey spectrum of CTAB–PANI/AC composite. (b) C 1s, (c) O 1s, (d) N 1s.

Fig. 3 (a) Raman spectra of CTAB–PANI (a′) and CTAB–PANI/AC (b′). (b) FT-IR spectra of CTAB–PANI (a′) and CTAB–PANI/AC (b′).

Fig. 4 displays the EIS obtained at the bare GCE (a), CTAB–PANI (b) and CTAB–PANI/AC (c) in 0.1 M KCl containing 5 mM Fe(CN)₆^3−/4–. The Randles equivalent circuit model was used to fit the experimental data (inset to Fig. 4). The semicircles indicates the parallel combination of electron transfer resistance (R_ct) and double layer capacitance (C_dl) at the electrode surface resulting from electrode impedance. The charge transfer resistance values were obtained by fitting the Nyquist plot results with the Randles equivalent circuit model. The R_ct values of bare GCE, CTAB–PANI/GCE and CTAB–PANI/AC/GCE are 460 (±2.18) Ω, 275 (±3.05) Ω and 25 (±1.41) Ω, respectively. The R_ct value obtained at the CTAB–PANI/AC/GCE is 11 and 18 times smaller than that of the CTAB–PANI/GCE and the unmodified GCE, respectively. Thus, CTAB–PANI/AC has lower electrode resistance than the control electrodes, which is due to the high conductivity and large surface area of the composite.

Fig. 4 EIS curves of the bare GCE (a), CTAB–PANI (b) and CTAB–PANI/AC composite (c). Inset: Randles equivalent circuit model. Here, R_s is electrolyte resistance, R_ct is charge transfer resistance, C_dl is double layer capacitance and Z_w is Warburg impedance.

3.2 Electrocatalysis of DA and UA

Fig. 5 shows the cyclic voltammograms (CVs) obtained at the unmodified GCE (a), CTAB–PANI/GCE (b), and CTAB–PANI/AC (c) towards 1 mM DA and 0.5 mM UA. The scan rate is 50 mV s⁻¹. Electrochemical parameters for the electrocatalysis of DA and UA, such as anodic (E_pa) and cathodic peak potentials (E_pc), anodic (I_pa) and cathodic peak currents (I_pc), and peak potential separation (ΔE_p) at these modified electrodes, are given in Table S1.† The unmodified GCE displays poor electrocatalytic ability to oxidize DA and UA. Although CTAB–PANI/GCE displays good electrocatalytic ability, the oxidation potentials of DA and UA are closely associated and the oxidation peaks are weak. Comparatively, the CTAB–PANI/AC has shown excellent electrocatalytic ability for DA and UA, which is evident from the observation of two sharp and highly enhanced redox peaks. Compared with the control electrodes, the CTAB–PANI/AC composite showed significantly improved electrocatalytic performance due to its large electrochemically accessible surface area. Fig. 5d displays the CVs of the bare GCE (b), CTAB–PANI/GCE (c) and CTAB–PANI/AC/GCE (d) in phosphate buffer (pH 7.0) containing a mixture of 2 mM DA and 2 mM UA. As shown in the figure, the bare GCE shows an unresolved and weak oxidation peak at 0.30. Although the CTAB–PANI shows two oxidation peaks at 0.315 V and 0.40 V, the peak-to-peak separation is not sufficient. Interestingly, the CTAB–PANI/AC modified electrode features two strong and well-defined anodic peaks at 0.218 V and 0.335 V. These peaks are separated by a wide peak-to-peak gap of about 117 mV, which is enough to perform simultaneous determinations. Hence, the CTAB–PANI/AC composite is a qualified hybrid material to perform simultaneous detection of DA and UA.

Fig. 5 CVs obtained at the bare GCE (a), CTAB–PANI/GCE (b), and CTAB–PANI/AC/GCE (c) in phosphate buffer (pH 7.0) containing 1 mM DA (a) or 0.5 mM UA (b). Scan rate = 50 mV s⁻¹. (d) CVs obtained at the bare GCE (b), CTAB–PANI/GCE (c) and CTAB–PANI/AC/GCE (d) in phosphate buffer (pH 7.0) containing a mixture of 2 mM DA + 2 mM UA. CV of CTAB–PANI/AC/GCE (curve a) in the absence of DA and UA.

In order to investigate the stability of the films, 100 consecutive cyclic voltammograms of the bare GCE, CTAB–PANI/GCE, and CTAB–PANI/AC/GCE were studied in phosphate buffer containing a mixture of DA and UA. After 100 consecutive cycles (100^th cycle), the bare GCE, CTAB–PANI/GCE, and CTAB–PANI/AC/GCE retained 86.54, 90.43 and 95.90% of their initial catalytic response currents (1^st cycle), respectively. For UA detection, 85.12, 91.80 and 94.65% of the initial response currents were retained at the bare GCE, CTAB–PANI/GCE, and CTAB–PANI/AC/GCE, respectively. Apparently, the CTAB–PANI/AC/GCE retained a larger percentage of its catalytic response currents compared to the control electrodes, which indicates its good stability during electrocatalysis. Next, the influences of different scan rates (ν) on the electrocatalysis reactions of DA (Fig. S1a†) and UA (Fig. S1b†) were investigated. The oxidation peak currents of DA and UA linearly increase as the scan rate increases from 20 to 200 mV s⁻¹, which reveals a surface-confined oxidation process (insets to Fig. S1†).

The effects of electrolyte solution pH on the oxidation peak currents and peak potentials of DA and UA were examined. As shown in Fig. 6a, the anodic peak potentials of DA and UA are negatively shifted with the increase in the pH from 1 to 11. The slopes of the plots between pH values and peak potentials exhibit good linearity (Fig. 6b). The slopes are −55.3 and −56.4 (UA) pH/mV for DA and UA, respectively, which indicates that equal numbers of protons and electrons are involved in the oxidation process. The oxidation peak currents of DA and UA are predominant at low pH, while it decreases steadily when the pH changes from 1 to 11. In order to develop a sensor for biological samples, we chose pH 7 as the optimal working pH for our studies.

Fig. 6 (a) pH dependence of oxidation peak potentials of DA (2 mM) and UA (2 mM). (b) pH dependence of oxidation peak currents of DA (2 mM) and UA (2 mM).

Fig. 7a presents the voltammograms obtained at the CTAB–PANI/AC/GCE in the presence of different concentrations of DA (Fig. 7a) and UA (Fig. 7b). As shown in the figure, the modified electrode exhibited sharp oxidation peaks for each concentration of DA and UA. The oxidation peak currents linearly increased as the concentrations of DA and UA increased. For DA, the linear range is 50–500 μM and sensitivity is 0.0517 (±0.007) μA μM⁻¹ (inset to Fig. 7a). For UA, the linear range is 50–500 μM and sensitivity is 0.0169 (±0.009) μA μM⁻¹ (inset to Fig. 7b).

Fig. 7 (a) CVs obtained at the CTAB–PANI/AC/GCE in phosphate buffer (pH 7.0) containing DA (a = 50, b = 100, c = 150, d = 200, e = 250, f = 300 and g = 350 μM). (b) CVs obtained at CTAB–PANI/AC/GCE in phosphate buffer containing UA (a = 50, b = 100, c = 150, d = 200, e = 250, f = 300, g = 350, h = 400, i = 450 and j = 500 μM).

3.3 Simultaneous determination of DA and UA

The main objective of the present work was to develop an electrode with higher anti-interference ability for AA. Therefore, the catalytic response of CTAB–PANI/AC/GCE towards AA was investigated through cyclic voltammetry (CV) (Fig. S2a†) and differential pulse voltammetry (DPV) (Fig. S2b†). The modified electrode doesn't produce any measurable signal for AA up to 2 mM in both CV and DPV analyses. Notably, the CTAB–PANI/AC composite prepared without PDS showed selectivity up to 1 mM AA (figure not shown), while the composite prepared using PDS showed improved selectivity up to 2 mM AA. The plausible reasons are: (1) AC surface is negatively charged, which is unfavorable for negatively charged AA molecules, and (2) the presence of PDS makes the electrode surface negatively charged, which hinders the negatively charged AA; as a result the electrode achieved improved selectivity. Next, an electrolyte solution containing different concentrations of DA and AA in the presence of a fixed concentration of AA was investigated. Fig. 8a presents the CV curves obtained at the CTAB–PANI/AC/GCE in phosphate buffer containing 2 mM AA and different concentrations of DA and UA. As presented in the figure, the I_pa corresponding to DA and UA linearly increased as the concentrations of DA and UA increased. The plots between peak currents and respective concentrations of DA (Fig. 8b) and UA (Fig. 8c) show good linearity with slopes of 2.233 and 2.883 μA mM⁻¹, respectively. Remarkably, the coexistence of AA in the mixture didn't impose any interference on the detection of DA and UA. Therefore, the CTAB–PANI/AC composite has a high level of significance in the selective determination of DA and UA. In the presence of 2 mM AA co-existing in solution, the modified electrode showed a linear range of 50–500 μM for DA and UA.

Fig. 8 (a) CVs obtained at the CTAB–PANI/AC/GCE in phosphate buffer (pH 7.0) containing different concentrations of DA and UA mixture (a = 50, b = 100, c = 150, d = 200, e = 250, f = 300, g = 350, h = 400, i = 450, j = 500 μM). (b) I_pa/μA vs. [DA]/μM. (c) I_pa/μA vs. [UA]/μM. (d) DPVs of CTAB–PANI/AC/GCE in phosphate solution (pH 7.0) containing 1.0 mM AA and mixtures of DA (0.3, 0.5, 1.5, 2.0, 4.0, 6.0, 12.0, 15.0, 17.0, 20.0 μM) and UA (1, 3, 5, 7, 9, 11, 13, 15, 17, 20 μM). (e) I_pa/μA vs. [DA]/μM. (f) I_pa/μA vs. [UA]/μM. The optimized parameters for DPV measurements are amplitude = 0.05 V, sampling width = 0.0167 s and pulse period = 0.5 s.

In order to improve the sensitivity, DPV experiments were performed. Fig. 8d displays the DPV curves obtained at the CTAB–PANI/AC/GCE in phosphate buffer containing 2 mM AA and a mixture of DA and UA. The I_pa of the DA and UA linearly increase as the concentrations of DA and UA increases. The plots between concentrations of DA (Fig. 8e) and UA (Fig. 8f) with respective peak currents showed good linearity. The coexisting AA doesn't produce any interference, which is consistent with the CV results. For DA and UA, the regression equations were obtained as [I_p]/μA = 1.213[DA] (μA μM⁻¹) + 0.304 (R² = 0.997) and [I_p]/μA = 0.471[DA] (μA μM⁻¹) + 1.159; (R² = 0.990) respectively. For the DA determination, the linear range is 0.3–20 μM, the sensitivity is 17.08 (±0.09) μA μM⁻¹ cm⁻² and the detection limit (LOD) is 0.06 (±0.006) μM. For the UA determination, the linear range is 1–20 μM, the sensitivity is 6.63 (±0.05) μA μM⁻¹ cm⁻² and the LOD is 0.20 (±0.008) μM. The LOD was calculated using the formula, LOD = 3s_b/S (where, s_b = standard deviation of blank signal and S = sensitivity).³⁴ The linear range and LOD obtained for DA and UA detections at our modified electrode are comparable with previously reported modified electrodes (Table 1).

Table 1 Comparison of the analytical performance of the CTAB–PANI/AC composite with previously reported modifiers for the determinations of DA and UA

Electrode materials	Dopamine		Uric acid		Ref.
	Linear range/μM	LOD^a/μM	Linear range/μM	LOD/μM
a LOD = limit of detection.b RGO = reduced graphene oxide.c CNTs = carbon nanotubes.
Poly(L-leucine)/DNA	0.1–100	0.04	0.5–100	0.2	35
MoS2/RGO^b	5–545	0.05	25–2745	0.46	23
PtAu hybrid film	24–384	24	20–336	21	10
Pd NPs/carbon nanofibers	0.5–160	0.2	2–200	0.7	36
Chitosan–graphene	1–24	1	2–45	2	37
N-Doped graphene	0.5–170	0.25	0.1–20	0.045	16
Pt NPs/RGO	10–170	0.25	10–130	0.45	14
Pt NPs/polydopamine/CNTs^c	0.25–20	0.08	0.3–13	0.12	38
Pretreated pencil graphite	0.15–15	0.033	0.3–150	0.12	17
CTAB–PANI/PANI	0.3–20	0.06	1–20	0.20	This work

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Next, the selectivity of the electrode to detect DA in the presence of UA and vice versa was investigated. First, DPVs were performed using the modified electrode towards different concentrations of DA containing a fixed concentration of UA (1 mM) (Fig. 9a). As shown in the figure, the electrode shows a linear enhancement in peak currents for each addition of DA. The presence of UA in the supporting electrolyte did not produce any interference to DA oxidation. Similarly, the electrode selectively detects UA in the presence of 1 mM DA and the presence of DA has no influence on the voltammetric peak of UA (Fig. 9b).

Fig. 9 (a) DPVs obtained at the CTAB–PANI/AC/GCE in phosphate solutions (pH 7.0) with different concentrations of DA: 1, 3, 10, 15, 20, 25, 30 and 35 μM containing fixed amounts of (1 mM) of AA and UA. (b) DPVs obtained at the CTAB–PANI/AC/GCE in phosphate solutions (pH 7.0) with different concentrations of UA: 3, 7, 10, 20, 25, 30, 35 and 40 μM containing fixed concentrations (1 mM) of AA and DA.

3.4 Stability, repeatability and reproducibility

In order to determine the storage stability of the modified electrode, its electrocatalytic response towards 1 μM DA and 1 μM UA were monitored every day. The modified electrode was stored in phosphate buffer (pH 7) at 4 °C when not in use. After 15 days of storage, 92.16% and 91.05% of the initial oxidation peak currents were retained for DA and UA, respectively. Next, the repeatability of the electrode was evaluated by performing five repetitive measurements using independently prepared modified electrodes in phosphate buffer containing a mixture of DA and UA. The electrode showed satisfactory repeatability with RSDs of 4.12% and 3.87% for DA and UA detections, respectively. Similarly, the reproducibility of the electrode was evaluated for five independent measurements performed using five different modified electrodes in phosphate buffer containing a mixture of DA and UA. The electrode delivered good reproducibility for the determination of DA and UA with RSDs of 3.68 and 3.80%, respectively.

3.5 Real sample analysis

The real-time application of the modified electrode was demonstrated in rat brain, dopamine injection, human serum and urine samples. The sampling procedure is given in Section 2.3. Known concentrations of DA and UA (mixtures) were spiked into these solutions and DPV experiments were carried out. The dopamine injection sample was directly used without any dilution. For each real sample, the CTAB–PANI/AC/GCE presents quick and sensitive DPV signals, which were consistent with the results obtained in lab sample analyses. The added, found and recovery values are calculated and given in Table 2. It can be seen from the table that the modified electrode was able to detect DA and UA with a satisfactory range of recoveries (95.6–104.5%). Thus, the CTAB–PANI/AC composite has good practicality and it can be used for the real-time determinations of DA and UA.

Table 2 Determination of DA and UA in real samples using the CTAB–PANI/AC/GCE

Real samples	DA				UA
	Added/μM	Found/μM	Recovery/%	RSD^a/%	Added/μM	Found/μM	Recovery/%	RSD^a/%
a Relative standard deviation (RSD) of 3 independent experiments.
Human serum	10	10.45	104.5	3.51	5	4.78	95.6	2.54
	20	19.34	96.7	3.95	10	9.62	96.2	3.95
Urine sample	10	9.86	98.6	3.82	5	4.82	96.4	4.20
	20	19.24	96.2	3.40	10	9.81	98.1	3.18
Rat brain	10	9.61	96.1	4.22	—	—	—	—
	20	19.18	95.9	4.58	—	—	—	—
Dopamine injection	10	9.74	97.4	3.61	—	—	—	—
	20	19.37	96.85	4.08	—	—	—	—

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4. Conclusions

In summary, a sensitive and highly selective electrochemical DA and UA detection platform was developed using a CTAB–PANI/AC composite. The successful formation of the composite was confirmed by SEM, EIS and electrochemical methods. The CTAB–PANI/AC composite film-modified electrode exhibited excellent electrocatalytic ability to determine DA and UA. The electrode is highly selective for the detection of DA and UA, even in the presence of high concentrations of AA (up to 2 mM). A DPV-based sensing platform was developed and showed excellent analytical parameters for both DA and UA with detection limits of 0.06 (±0.006) μM and 0.20 (±0.008) μM, respectively. In addition, the modified electrode has satisfactory stability, repeatability and reproducibility. The composite has promising practical applicability in rat brain, dopamine injection, and human serum and urine samples.

Acknowledgements

The authors extend their appreciation to the International Scientific Partnership Program ISPP at King Saud University for funding this research work through ISPP#6. This work was supported by the Ministry of Science and Technology (MOST), Taiwan (ROC).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18740d

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