Electrodeposition of gold nanoparticles on a pectin scaffold and its electrocatalytic application in the selective determination of dopamine

Abstract

A simple electrochemical deposition strategy is proposed for the preparation of gold nanoparticles (Au NPs) at the electrode surface using biopolymer pectin as stabilizing agent. The formation of the nanoparticles was confirmed by scanning electron microscopy (SEM), UV-visible spectroscopy and X-ray diffraction (XRD) studies. A pectin-stabilized, gold nanoparticle film-modified glassy carbon electrode (pectin–Au NP/GCE) was prepared, which exhibited excellent electrocatalytic ability towards oxidation of dopamine (DA). At the pectin–Au NP/GCE, the redox couple corresponding to the redox reaction of DA was observed at the formal potential of 0.20 V with highly enhanced peak currents. A thin layer of nafion coating was applied on the pectin–Au NP composite to improve its selectivity. Two linear ranges of detection were found: (1) 20 nM to 0.9 μM with a limit of detection (LOD) of 6.1 nM, (2) 0.9 μM to 1 mM with a LOD of 0.64 μM. The fabricated sensor selectively detects DA even in the presence of high concentrations of interferents. Moreover, practical feasibility of the sensor was addressed in pharmaceutical samples, which presented appreciable recovery results. The main advantages of the sensor are its very simple and green fabrication approach, roughed and stable structure, and fast and highly reproducible detection of dopamine.

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1. Introduction

Over the past few decades, gold nanoparticles (Au NPs) have played a significant role in nanoscience and nanotechnology due to their high stability, excellent electron conductivity and unique surface chemistry.¹ Specific size and morphology of the Au NPs have been the focus of intensive research because of their potential applications in the field of electronic, optical, optoelectronic and magnetic devices.² To date, numerous methods such as chemical,^3,4 electrochemical,^5,6 irradiation⁷ and microwave-assisted methods^8,9 have been employed for the synthesis of Au NPs. Among the aforementioned methods, electrochemical techniques are simple, eco-friendly, low cost and able to prepare uniform and size-controllable nanoparticles.¹⁰ In recent years, synthetic routes using novel protectors such as polymers, surfactants, ionic liquids and green agents have been designed for the synthesis of Au NPs to prevent nanoparticle agglomeration. Due to the excellent surface chemistry of Au NPs, they play a significant role in many scientific fields such as sensors,¹¹ biosensors,^12,13 immunosensors,¹⁴ nanodevices¹⁵ and biomedicines.¹⁶

Generally, the capping of reagents with a functional group such as NH₂, COOH, SH and OH has been explored for the synthesis of Au NPs.¹⁷ In particular, research on green agent-stabilized Au NPs has intensified due to their long-term stability, solubility, lower toxicity and amphiphilicity. Functionalization with sugar polymers could be one of the facile ways to tailor the electronic and catalytic properties of the Au NPs.¹⁸ Pectin (polygalacturonic acid) is a naturally occurring sugar polymer present in cell walls of plants. It is negatively charged, highly biocompatible, biodegradable, non-toxic and finds widespread applications in food, pharmaceutical and biomedical industries.^19,20 Remarkably, pectin contains –OH and –COOH functional groups that can be used to support the nanoparticles. However, until now only our reports are available in the literature employing pectin as the stabilizing agent in Au NPs,²¹ and very few reports are seen for the chemical synthesis of other nanoparticles.^21,22

Finding new approaches for the preparation of metal nanoparticles and exploring their electrochemical applications are ongoing research interests of our research group.^23,24 Recently, we have reported a fast, microwave-assisted chemical reduction method for the preparation of Au NPs on graphene nanosheets using polyethyleneimine as stabilizing agent.²³ However, this method requires microwave irradiation and a reducing agent. In order to overcome these issues, herein we are reporting a simple and green electrochemical deposition route for the preparation of gold nanoparticles at the electrode surface utilizing pectin as the scaffold and stabilizing agent (Scheme 1).

Scheme 1 Schematic representation for the preparation of pectin-stabilized Au NPs.

Dopamine (DA) is one of the important catecholamine-based neurotransmitters that transport signals from the central nervous system to the brain and play a vital role in mammalian central nervous systems. Despite its valuable role in biological functions, abnormal concentrations of DA result in brain disorders such as Parkinson's disease and schizophrenia,^25–27 and therefore, determination of DA is of great significance in biological diagnoses. Electrochemical techniques provide an excellent platform for the detection of DA in biological diagnoses compared to conventional methods due to their simplicity, selectivity, sensitivity and portability. However, the electrochemical signal of DA is often associated very closely, and overlaps with, that of ascorbic acid (AA) and hence suffers from serious interference.^28,29 Biological samples often contain higher concentrations of AA than DA (100 to 1000 fold higher), and consequently, overcoming the interference of AA is a challenging task in the electrochemical determination of DA.²⁹ Several chemically modified electrodes have been employed in the literature to eliminate the interference from AA. In the present work, we prepared a pectin-stabilized, Au NP-modified electrode for the selective determination of DA. A thin layer of nafion film was coated in order to eliminate the interference of AA. Therefore, the final sensor exhibits high electrocatalytic effect and the required selectivity even in the presence of high concentrations of AA. We have compared the performance of our work with earlier Au NP-based electrochemical sensors; only two reports show a similar limit of detection (LOD) for DA comparable to our work; however, none of the Au NP sensors show a wide linear range of DA detection.^23,30,31

The main aim of the present work is to prepare highly stable Au NPs using pectin as scaffold and explore its electrocatalytic applications. The prepared nanoparticles are uniform and highly stable, and they exhibited excellent electrocatalytic ability towards the determination of DA. The preparation of this modified electrode is very fast (one-step electrodeposition), green (does not involve any toxic reducing agents), simple, highly reproducible and stable.

2. Experimental

2.1 Reagents and apparatus

LM-pectin (DE 35%, genu pectin LM 12 CG-Z) and gold(III) chloride trihydrate (>99%, HAuCl₄·3H₂O), DA, AA and nafion (Nf) were purchased from Sigma-Aldrich and used as received. The supporting electrolyte used for all the electrochemical studies was 0.05 M phosphate buffer solution (PBS), prepared using NaH₂PO₄ and Na₂HPO₄. The commercial sample of dopamine hydrochloride (easy dopa injection) was acquired from O-Smart Company, Taiwan (1.6 mg mL⁻¹, 8.44 mM) and diluted to the required concentrations in PBS (pH 7). Prior to each experiment, all the solutions were deoxygenated with pre-purified N₂ gas for 15 min unless otherwise specified.

The electrochemical measurements were carried out using the CHI 611A electrochemical work station. Electrochemical studies were performed in a conventional three-electrode cell using a BASi glassy carbon electrode (GCE) as a working electrode (area = 0.071 cm²), Ag|AgCl (saturated KCl) as a reference electrode and Pt wire as a counter electrode. Amperometric measurements were performed with an analytical rotator AFMSRX (PINE Instrument Company, USA) with a rotating disc electrode (RDE) having a working area of 0.24 cm². Scanning electron microscope (SEM) studies were performed using a Hitachi S-3000H scanning electron microscope. Ultraviolet-visible (UV-vis) spectroscopy studies were performed using a U-3300 spectrophotometer. X-ray diffraction (XRD) studies were carried out using an XPERT-PRO diffractometer with Cu Kα radiation (k = 1.54 Å).

2.2 Electrodeposition of pectin-stabilized Au NPs on GCE

The GCE surface was polished with a 0.05 μm alumina slurry using a Buehler polishing kit, then washed with water, ultrasonicated for 5 min, and allowed to dry. After pre-cleaning, the GCE surface was transferred to the electrochemical cell to perform the electrodeposition of Au NPs. Ten consecutive cyclic voltammograms (CVs) were swept at a scan rate of 25 mV s⁻¹ between the potential ranges of +1.40 V to −1.20 V in 0.1 M KNO₃ containing 3 mg ml⁻¹ pectin and 0.3 mg ml⁻¹ HAuCl₄. The as-prepared pectin-stabilized Au NP (pectin–Au NP)-modified electrode was rinsed with water and dried. Finally, 2 μL of 1.5% nafion (optimized concentration) was drop-casted on the pectin–Au NP/GCE, and the resulting modified electrode was denoted as GCE/pectin–Au NP/Nf.

3. Results and discussion

3.1 Characterization of pectin–Au NPs

Fig. 1A shows the electrochemical deposition of pectin–Au NPs in the potential range between 1.40 V to −1.20 V. During the first scan, a large cathodic peak was observed at the potential of +0.40 V corresponding to the reduction of Au³⁺ ions and nucleation of Au nanoparticles on the electrode surface. The evolution of hydrogen at the electrode surface was started at the potential of −0.70 V, thereby generating OH⁻ ions at the electrode–electrolyte interface. At this region, the cathodic electrodeposition of pectin has been taking place through electrophoretic deposition. In the second cycle, the reduction peak current of Au NPs is doubled with a shift in the reduction peak potential to 300 mV on the more positive side (+0.70 V), which indicates the growth of Au NPs. This catalytic behavior observed in the second cycle must be due to the deposition of Au NPs, which is taking place on the pectin-modified Au NP/GCE surface, not on the bare GC, thereby decreasing the aggregation of gold nanoparticles. Hence, the interaction between –COOH and –OH with Au³⁺ ions may lead to preconcentration of Au³⁺ ions in the electrode–electrolyte interface. In the reverse scan, a sharp anodic peak observed at the potential of +1.10 V could be ascribed to the oxidation of Au nanoparticles. During the continuous electrochemical cycling process, the growth of the reduction and oxidation peak currents reveals the successful formation of Au NPs.³² The size and thickness of the pectin–Au NP film have profound impact on its electrocatalytic efficiency, which can be controlled by regulating the scan rate and the number of cycles during electrodeposition. Therefore, we have optimized the number of cycles required to get optimum thickness of pectin–Au NP film to give maximum electrocatalytic ability towards DA.

Fig. 1 (A) Electrochemical deposition of pectin-stabilized Au nanoparticles in 0.1 M KNO₃ containing 3 mg ml⁻¹ of pectin and 0.1 mM of HAuCl₄ at the GCE for 10 cycles. Scan rate = 50 mV s⁻¹. (B) UV-visible spectra of pectin (a) and pectin–Au NPs (b). (C) SEM image of the pectin–Au nanocomposite. (D) XRD pattern of the pectin–Au NPs.

The electrocatalytic oxidation of DA (0.1 mM) was studied using the pectin–Au NP/GCE-modified electrode by controlling the electrodeposition cycles from 1 to 12 in PBS (pH 7) at the scan rate of 0.05 V s⁻¹ (inset to Fig. 1A). Since maximum electrocatalytic response of the modified electrode for the oxidation of DA has been observed at 10 cycles of deposition, we chose 10 as the optimized number of cycles for further studies. Furthermore, the electrodeposition of Au NPs was taken without employing pectin scaffold (Fig. S1†) as control. From this figure, we can see that the difference in cathodic peaks of the first and second cycles of the Au NPs is observed as 50 mV (but 300 mV for pectin–Au NPs), and the anodic and cathodic peaks of Au NPs are saturated after 4 cycles. In order to evaluate the stability of the pectin–Au NP/GCE, 200 successive CVs were recorded at the pectin–Au NP/GCE in PBS (pH 7) (Fig. S2†). Only 8.3% of the initial peak currents decreased even after 200 cycles, which clearly reveals the excellent stability of the pectin–Au NP/GCE. However, 14.3% of the initial peak currents decreased after 200 consecutive scans at the Au NP/GCE (control), which is attributed to the instability of Au NPs formed without the aid of pectin scaffold, which indicates the significant role of pectin in improving the stability of Au NPs.

Fig. 1B displays the UV-visible spectra of pectin (a) and pectin–Au NPs (b). The UV-visible spectrum of pectin exhibited a sharp absorption peak at 290 nm and a broad shoulder peak at 380 nm, which arose due to the free carboxyl group of pectin. However, these absorption peaks completely disappeared in the spectrum of the pectin–Au NP composite, indicating that these free carboxyl groups were committed to accommodating Au NPs during electrochemical reduction. Meanwhile, the appearance of a new absorption peak at 560 nm revealed the successful formation of Au NPs.³³ The SEM image of pectin–Au NPs depicts the uniform distribution of Au nanoparticles onto the interconnected network of the pectin scaffold (Fig. 1C). The Au NPs' size, ranging from 15 to 40 nm, validates the successful formation of Au NPs. However, the SEM image of Au NPs prepared without pectin exhibited the morphology of heavily aggregated Au NPs (Fig. S3†). This result clearly reveals that the presence of pectin is necessary for the formation of stable Au NPs without aggregation. Fig. 1D displays XRD patterns of the pectin–Au NPs. The observation of four diffraction peaks at the 2θ angles of 38.3°, 44.46°, 64.78° and 77.96° can be attributed to the (111), (200), (220) and (311) reflections of the face-centered cubic structure of metallic Au NPs, respectively (JCPDS, card no. 04-0784).³⁴

3.2 Electrocatalysis of DA at various modified electrodes

Fig. 2A shows the CVs obtained at unmodified (a), pectin–Au NP(b), Au NP/Nf (c) and pectin–Au NP/Nf (d) film-modified GCEs in PBS (pH 7) at the scan rate of 25 mV s⁻¹. Electrochemical parameters of the redox reaction of DA at these modified electrodes, such as the anodic peak current (I_pa), cathodic peak current (I_pc), formal potential (E°′) and peak potential separation value (ΔE_p) are given in Table 1.

Fig. 2 (A) CVs obtained for unmodified (a), pectin–Au NP (b), Nf/Au NP (c) and pectin–Au NP/Nf (d) film-modified GCEs in PBS (pH 7) containing 0.1 mM DA at the scan rate of 25 mV s⁻¹. (B) CVs obtained at GCE/pectin–Au NP/Nf in PBS (pH 7) containing 0.1 mM DA at different scan rates from 0.01 V s⁻¹ to 0.1 V s⁻¹. Inset: plot of ν^1/2 vs. I_p.

Table 1 Electrochemical parameters for the redox reaction of DA at various modified electrodes

Electrode	Epa (V)	Epc (V)	E°′/V	ΔEp/V	Ipa/μA	Ipc/μA
Unmodified GCE	0.290	0.111	0.201	0.179	2.04	1.08
GCE/pectin–Au NP	0.263	0.121	0.192	0.142	6.77	2.12
GCE/Au NP/Nf	0.225	0.106	0.166	0.119	5.52	5.23
GCE/pectin–Au NP/Nf	0.203	0.120	0.162	0.083	7.84	6.14

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The electrocatalytic ability of these modified electrodes towards the oxidation of DA are in the order of GCE/pectin–Au NP/Nf > GCE/Au NP/Nf > GCE/pectin–Au NP > unmodified GCE. Among the above modified electrodes, GCE/pectin–Au NP/Nf exhibited maximum electrocatalytic ability, whereas bare GCE exhibited poor electrocatalytic ability. Evidently, large ΔE_p value and high overpotential observed at the bare GCE revealed a sluggish electron transfer kinetic process for DA at this electrode. However, a pair of reversible redox peaks with highly enhanced peak currents and very low ΔE_p has been observed with the pectin–Au NP/Nf. Here, the anodic peak is attributed to the oxidation of DA to o-dopamine quinone, while the cathodic peak is due to the reduction of o-dopamine quinone back to DA.²³ Low ΔE_p and high peak currents observed at the GCE/pectin–Au NP/Nf indicates the fast electron transfer kinetics and promising electrocatalytic ability of the modified electrode towards electrocatalysis of DA. Interestingly, Au NPs prepared without employing a pectin scaffold showed comparatively less electrocatalytic ability than those prepared with the aid of pectin as scaffold, revealing that pectin acts as an excellent stabilizing agent and partially assists in the electrocatalysis of DA. Obviously, pectin acts as a unique scaffold and stabilizing agent, which provides excellent stability to the Au NPs, which in turn provide stable DA electrocatalytic ability. Au NPs prepared without pectin have shown poor stability caused by the aggregation of Au NPs, which result in comparatively decreased electrocatalytic ability towards DA. Overall, the outstanding electrocatalytic ability of the GCE/pectin–Au NP/Nf should be ascribed to the high surface area and high electrical conductivity of the Au NPs and also to interactions between the negatively charged functional groups present in the pectin and nafion film with the positively charged DA.

Here, the purpose of employing a thin layer of nafion coating is to block the electrochemical signal of AA, since Nf is a polymer that has the ability to hinder AA via electrostatic repulsive interaction between negatively charged Nf and negatively charged AA at pH 7.³⁵⁻³⁷ However, Nf selectively allows the movement of DA through attractive electrostatic interaction between negatively charged Nf and positively charged DA. Also, we optimized the concentration of Nf required to inhibit the interference of the maximum amount of AA (Fig. S4A†). We found that upon increasing the percentage concentration of Nf from 1% to 1.5%, the response current for DA increases and becomes stable at 1.5%. Therefore, we used 1.5% as the optimized concentration of Nf to make a thin layer coating on the pectin–Au NP-modified GCE.

We have studied the oxidation of AA for different concentrations of GCE/pectin–Au NP/Nf (Fig. S4B†). Upon addition of AA from 1 mM to 5 mM, only background current was increased, whereas no obvious response currents were observed for AA. The percent interference of each concentration of AA at the GCE/pectin–Au NP/Nf were calculated in terms of changes in the ratio of the signal to the blank signal and are presented in Table S1,† which shows negligible interference of AA (less than 5%). Fig. S4C† shows a comparison between the electrocatalytic response of GCE/pectin–Au NP/Nf towards oxidation of 2 mM AA and 0.5 mM AA. As can be seen from the figure, GCE/pectin–Au NP/Nf exhibited highly enhanced peak currents to the oxidation of DA, whereas it did not show obvious peaks for the addition of AA. Thus, the Nf coating acts as gateway at the pectin–Au NP electrode surface by permitting DA and preventing the major portion of AA from reaching the electrode surface.

The effect of scan rate (ν) towards the redox reaction of DA at the GCE/pectin–Au NP/Nf has been investigated in PBS (pH 7) containing 0.1 mM DA at the scan rate (ν) range of 0.01 to 0.1 V s⁻¹ (Fig. 2B). Both I_pa and I_pc increased as the scan rate increased from 0.01 to 0.1 V s⁻¹. Upon the increase in scan rate, I_pa shifts to the positive potential side, whereas I_pc shifts to the negative potential side. Moreover, a plot of square root of scan rates (ν^1/2) versus I_pa and I_pc exhibited a linear relationship, indicating that the redox behavior of DA at GCE/pectin–Au NP/Nf is controlled by diffusion (inset to Fig. 2B). The corresponding linear regression equation can be expressed as: I_pa (μA) = 50.15ν^1/2 (V s⁻¹)^1/2 − 1.48, R² = 0.993 and I_pc (μA) = −67.56ν^1/2 (V s⁻¹)^1/2 + 5.07, R² = 0.993.

3.3 pH studies

Fig. 3A shows the various pH values of the supporting electrolytes towards redox peaks of DA at the GCE/pectin–Au NP/Nf investigated in PBS (pH 7) containing 0.1 mM of DA. Both E_pa and E_pc of the redox peak were shifted towards the negative direction of the potential upon increasing pH from 1 to 9, indicating that the redox reaction of DA occurring at this modified electrode is pH-dependent. A plot of E⁰′of the redox peaks of DA versus various pH values rendered a linear plot with the slope value of −52.0 mV/pH. The slope value is in close agreement with the theoretical value of −59 mV/pH at 25 °C for a reversible electron transfer reaction involving the transfer of equal numbers of protons and electrons.³⁸

Fig. 3 (A) CVs obtained for the GCE/pectin–Au NP/Nf in PBS of various pH solutions (pH 1–9) in the presence of 0.1 mM DA at the scan rate of 25 mV s⁻¹. Inset: plot of E° V⁻¹ vs. pH. (B) CVs obtained at GCE/pectin–Au NP/Nf in the absence (a) and presence of DA from 10 μM to 200 μM (curves b to u; each 10 μM addition) in PBS (pH 7) at the scan rate of 25 mV s⁻¹. Inset: plot of I_p vs. [DA].

3.4 Electro-oxidation of DA

Fig. 3B shows the CVs obtained at GCE/pectin–Au NPs/Nf in the absence (curve a) and presence of DA (curves b to u; each addition of 10 μM) in PBS (pH 7). Upon the addition of 10 μM DA into the PBS solution, obvious redox peaks are observed; further, the peak current increases linearly upon the addition of DA from 10 to 200 μM. The linear increase in the I_pa and I_pc reveals the occurrence of efficient electrocatalysis at the electrode without any fouling effect. A plot of I_pa and I_pc versus concentration of DA exhibited a linear relationship (inset to Fig. 3B) with the linear concentration range of 10–200 μM.

3.5 Rotating disc electrode studies

The electrocatalytic activity of the pectin–Au NP/Nf-modified electrode towards the oxidation of DA was evaluated by rotating disc electrode (RDE) experiments. Fig. 4A shows the current–potential curves at the GCE/pectin–Au NP/Nf in PBS (pH 7) in the absence (a) and presence of DA (each 50 μM addition, from b to k). Well defined voltammograms with mass transport-limited current were observed upon each addition. The disc current (I_d) increases linearly with the increase in concentration of DA. A plot between I_d and concentration of DA exibited a linear relationship with slope 0.148 μA μM⁻¹ (inset to Fig. 4A).

Fig. 4 (A) RDE voltammograms obtained at the GCE/pectin–Au NP/Nf in the absence of DA (a) and presence of each 50 μM addition of DA (b–k) in PBS (pH 7) at the rotation speed of 1500 RPM. (B) RDE voltammograms of GCE/pectin–Au NP/Nf in the presence of 0.5 mM DA at different rotation rates (a) 200, (b) 400, (c) 600, (d) 900, (e) 1200, (f) 1600 (g) 2500 and (h) 3000 RPM.

Fig. 4B presents the current–potential curves at RDE/pectin–Au NPs/Nf for different rotation rates such as (a) 200, (b) 400, (c) 600, (d) 900, (e) 1200, (f) 1600 (g) 2500 and (h) 3000 RPM in PBS (pH 7) containing 0.5 mM DA. A Levich plot was drawn from the data obtained from RDE voltammograms and given as inset to Fig. 5B. The Levich plot is found to be linear, which indicates that the oxidation of DA at RDE/pectin–Au NPs/Nf is mass transport-limited. The relationship between the limiting current and rotating speed of the electrode can be shown by the Levich eqn (1).³⁹

I_L = I_LEV = 0.620nFAD^2/3₀γ^−1/6ω^1/2C₀ (1)

where D₀, γ, ω and C₀ are the diffusion coefficient, kinematic viscosity, rotation speed and bulk concentration of the reactant in the solution, respectively. The remaining parameters in the equation stand for their conventional meanings. By substituting all the values in the above eqn (1), the value of D₀ is calculated to be about 5.71 × 10⁻⁶ cm² s⁻¹, which is quite comparable with the values obtained by the previous reports for the electrocatalysis of DA.⁴⁰

Fig. 5 (A) Amperometric i–t response obtained at the pectin–Au NP/Nf film-modified rotating disc GCE upon each addition of 20 nM DA into continuously stirred PBS (pH 7) at the rotation speed of 1500 RPM. E_app = +0.20 V. Inset (a) and (b): plot of [DA] vs. I_p. (B) Amperometric response of pectin–Au NP/Nf film-modified rotating disc GCE for the 100 nM addition of DA in the presence of 2 mM of (a) AA (b), uric acid (c) arterenol (d), histidine (e) and tyrosine (f).

3.6 Amperometric determination of DA

Fig. 5A shows the amperometric i–t response of the pectin–Au NP/Nf film-modified rotating disc GCE upon sequential injection of 20 nM DA into PBS (pH 7) at regular intervals of 50 s into continuously stirred PBS (pH 7) at the rotation speed of 1500 RPM. The applied potential (E_app) of the electrode was held at +0.20 V. For each addition of DA, a quick and stable amperometric response was observed. The amperometric response current reaches 95% of the steady-state current within 5 s, indicating fast electrocatalytic oxidation of DA at the GCE/pectin–Au NP/Nf. A plot between the concentration of DA and peak current exhibited a linear relationship, and the sensor's working linear range was found to be between 20 nM and 0.9 μM (inset a, Fig. 5A). The respective linear regression equation is expressed as I_p/μA = 0.007 [DA]/μA nM⁻¹ − 0.037; R² = 0.99. Sensitivity of the sensor is calculated to be 0.033 μA nM⁻¹ cm⁻² and the limit of detection (LOD) is calculated to be 6.1 nM. The LOD of the sensor was calculated by using the formula, LOD = 3s_b/S (where, s_b = standard deviation of the blank signal and S = sensitivity).⁴¹ A second linear range was observed in the higher concentrations of DA between 0.9 μM and 1 mM (inset b, Fig. 5A), and the respective linear regression equation is expressed as: I_p/μA = 0.062 [DA]/μA μM⁻¹ + 1.067 (±1.23); R² = 0.986. The sensitivity and LOD at this linear range was calculated to be 0.2952 μA μM⁻¹ cm⁻² and 0.64 μM, respectively. The analytical performance of the proposed sensor towards the determination of DA is superior over the other reports in terms of wide linear ranges, high sensitivity and low LOD (Table 2).

Table 2 Comparison of analytical parameters for the determination of DA at GCE/pectin–Au NP/Nf nanocomposite film-modified electrode with other film-modified electrodes

Electrode	Linear range/μM	Limit of detection/μM	Sensitivity	Ref.
Au NPs@SiO2−; molecularly imprinted polymers	0.048–5	0.02	—	25
Graphene/polyethylene imine/Au NPs	2–48	0.13	2.635 μA μM^−1 cm^−2	23
Fe3O4@ molecularly imprinted polymers/GS-chitosan	0.5–500	0.02	—	30
Gold nanoparticle-coated polystyrene/reduced graphite oxide microspheres	0.05–20	5 × 10^−3	3.44 μA μM^−1 cm^−2	42
Au@ carbon dot–chitosan composite	0.01–100.0	1 × 10^−3	—	31
Graphene sheet and Au NP modified carbon fiber electrode	0.59–43.96	0.59	—	43
Pectin–Au NP/Nf	0.02–0.9	6.1 × 10^−3	0.033 μA nM^−1 cm^−2	This work
	0.9–1000	0.64	0.2952 μA μM^−1 cm^−2

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The selectivity of the pectin–Au NP/Nf modified electrode in detecting DA in the presence of common interferences was investigated (Fig. 5B). The operating potential of the electrode was held at +0.20 V, while the rotation speed was kept at 1500 RPM. The modified electrode exhibited well defined amperometric response to the addition of 100 nM DA (a), whereas no recognizable responses were observed for the addition of 2 mM AA (b), uric acid (c), arterenol (d), histidine (e) and tyrosine (f). However, notable amperometric response was observed for the addition of 100 nM DA into the same PBS solution containing all the aforementioned interferences. Therefore, the pectin–Au NP/Nf film-modified electrode has the ability to selectively access DA even in the presence of 5000 fold excess concentrations of AA, uric acid, arterenol, histidine and tyrosine, revealing the outstanding selectivity of the modified electrode. As explained in the previous section, electrostatic repulsion between the negatively charged modified electrode surface and the negatively charged aforementioned interferences assisted in repelling and eliminating the interferences.

3.7 Stability, repeatability and reproducibility studies

In order to determine the storage stability of the modified electrode, the electrocatalytic response of the GCE/pectin–Au NP/Nf towards 0.1 mM DA was monitored every day. The electrode was stored in PBS (pH 7) at 4 °C when not in use. During a one-month storage period, the fabricated sensor presented well defined catalytic response without any shift in the peak potential. Moreover, 93.15% of the initial I_pa was retained over one month of its continuous use, revealing the good storage stability of the sensor. Furthermore, the operational stability of the modified electrode was investigated upon continuous rotation of the pectin–Au NP/Nf-modified GCE at the rotation speed of 1500 rpm in PBS (pH 7). Stable amperomerometric response was observed with the addition of 100 nM DA. Only 7.2% of the initial response current is decreased even after continuous rotation for 3500 s, revealing the good operational stability of the modified electrode. Repeatability and reproducibility of the proposed sensor was evaluated in PBS (pH 7) containing 0.1 mM DA at the scan rate of 25 mV s⁻¹. The sensor exhibits appreciable repeatability with a relative standard deviation (R.S.D) of 2.08% for 10 repetitive measurements carried out using a single electrode. In addition, the sensor exhibits a promising reproducibility of 1.92% for the five independent measurements carried out in five different electrodes.

3.8 Real sample

The practical feasibility of the sensor was assessed in a commercially acquired dopamine hydrochloride injection sample (8.44 mM). The concentration of the injection sample was diluted to the final concentrations of 1 μM and 100 nM. The amperometric experiments were performed using GCE/pectin–Au NP/Nf by following the optimized experimental conditions used for the analysis of lab samples. The results are presented as Table 3. The appreciable found and recovery results reveal that the pectin–Au NP/Nf-modified GCE exhibits promising practical feasibility in determining the concentration of DA present in real samples.

Table 3 Determination of DA present in pharmaceutical samples using GCE/pectin–Au NPs/Nf

Real sample	Sample	Concentration samples (added)	Found	Recovery	RSD
Dopamine hydrochloride injection	1	100 nM	98.2 nM	98.2	3.1
	2	1 μM	0.98 μM	98	2.4

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4. Conclusions

We have described a simple electrochemical deposition strategy for the preparation of Au NPs using pectin as a stabilizing agent. The pectin backbone acts as a versatile scaffold for the formation of highly decorated Au NPs. The successful formation of nanoparticles was confirmed by CV, SEM, UV-visible spectroscopy and XRD studies. GCE/pectin–Au NP/Nf exhibited excellent electrocatalytic ability for DA determination. The amperometric sensor presented excellent analytical parameters towards the detection of DA. Two linear ranges were found: one from 20 nM to 0.9 μM with LOD of 6.1 nM, while a second linear range was observed between 0.9 μM to 1 mM with a LOD of 0.64 μM. The sensor has exhibited high selectivity and shown promising practical feasibility in pharmaceutical samples.

Acknowledgements

This work was supported by the National Science Council and the Ministry of Education of Taiwan (Republic of China).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra08818b

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