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. 2019 Oct 24;4(19):17947–17955. doi: 10.1021/acsomega.9b01222

Novel Design of a Layered Electrochemical Dopamine Sensor in Real Samples Based on Gold Nanoparticles/β-Cyclodextrin/Nafion-Modified Gold Electrode

Nada F Atta 1,*, Ahmed Galal 1, Dalia M El-Said 1
PMCID: PMC6843716  PMID: 31720498

Abstract

graphic file with name ao9b01222_0012.jpg

Change in the level of dopamine (DA) concentration in the human body causes critical diseases such as schizophrenia and Parkinson’s disease. Therefore, the determination of DA concentration and monitoring its level in human body fluids is of great importance. An electrochemical sensor based on modification of the gold electrode surface with Nafion (NF), β-cyclodextrin (CD), and gold nanoparticles (AuNPs) was fabricated for the determination of DA in biological fluids. Combined impact of all the modifiers enhances the catalytic activity of the sensor. Gold nanoparticles increase the surface area of the sensor and enhance the electron transfer rate. CD plays a main role in enhancing the accumulation of protonated DA and forming stable complexes via electrostatic interactions and hydrogen bond formation. In addition, extra preconcentration of positively charged DA is achieved through ionic selectivity of NF. High electrocatalytic activity was achieved using the modified sensor for determination of DA in real urine samples in a wide concentration range, 0.05–280 μM with a low detection limit of 0.6 nM in the small linear dynamic range, 0.05–20 μM. Furthermore, common overlapped oxidation peaks of DA in presence of biologically interfering compounds at the gold electrode were resolved by using the modified sensor. Excellent recovery results were obtained using the proposed method for determination of DA in real urine samples.

1. Introduction

Dopamine (3,4-dihydroxy-phenyl-ethyl amine), is a main neurotransmitter and belongs to catecholamine’s family in the human central nervous system. Some serious diseases have arisen due to changes in the level of dopamine (DA) concentration in the human body such as schizophrenia and Parkinson’s disease.1 Therefore, the determination of DA concentration and monitoring its level in human body fluids is of great importance. There is an increase in the number of elderly people suffering from psychiatric disorders, sadness, and depression, still the effective treatment of such diseases remains a clinical challenge.1,2 The simultaneous detection of DA, ascorbic acid (AA), and uric acid (UA) has high impact in the field of diagnostic and pathological research, and it is of significance in neurochemistry and biomedical chemistry. One major problem facing the simultaneous determination of DA, AA, and UA at unmodified electrodes is overlapping of their electrochemical oxidation responses, besides adsorption of the oxidative products at the surface of the electrode which causes surface fouling.3 Thus, there is always need to improve a modified electrochemical sensors to detect a wide range of biological compounds.3,4 Clinical diagnosis and medical treatment involving DA require sensitive sensing with low limit of detection. This can be realized by an adequate design of surface modification for DA electrochemical determination. Many electrochemical sensors were applied for the sensitive determination of DA such as mercapto carboxylic acid monolayer-modified electrode,5 β-cyclodextrin (CD) graphene-modified glassy carbon electrode,6 carbon nanotubes-modified microelectrode,7 organic polymer-modified electrodes,8,9 and nanoparticles-modified electrodes.10,11

Nafion (NF) (per-fluorinated sulfonated cation exchanger) is a synthetic polymer with ionic properties which are called ionomers. It has an ionic selectivity; its permeability is high to cations and almost nil to anions. NF has intrinsic properties such as chemical and thermal stability, mechanical strength, chemical inertness, good electrical conductivity, and proper adhesion on the electrode surface.12,13 Also, the hydrophilic negatively charged sulfonate groups in the NF polymer structure helps in the accumulation of the positively charged molecules via an electrostatic interaction, resulting in enhanced sensitivity of the measurements.14−16

The outer surface of the CD structure is hydrophilic in nature because of the presence of several hydroxyl groups, whereas its cavity has hydrophobic character.6 CDs are cyclic compounds and belong to the family of oligosaccharides. CDs are used as inactive fillers in pharmaceutical formulations.17 Insoluble hydrophobic compounds form water-soluble complexes with CDs because of their exceptional structure.18 Therefore, CDs increase the aqueous solubility and the stability of drugs with their hydrophobic nature.18 The formation of a complex between CD and specific molecule is through host–guest inclusion resulting in remarkable molecular selectivity and enantioselectivity.19 Therefore, CDs are used as electrode modifiers to host specific organic and biological molecules to form stable nanostructures that allows high selectivity and low detection limits. Also, it is important to increase the bioavailability of the drugs and minimize their foul smelling.6,17,18 Atta and co-workers constructed electrochemical sensors-based β-CD modified electrodes for the sensitive analysis of some compounds recognized as neurotransmitters as well as medication.20,21

Nanostructured materials have been widely used as electrode materials for electrochemical sensing.22,23 An important feature of nanostructured materials is the relative increase in surface area compared to other surfaces.23 Among the widely used nanomaterials in electrode modification are gold nanoparticles. Au nanoparticles possess several important characteristics such as size/surface area, biocompatibility in biological environments, conductive nature, molecular recognition, and high surface activity.24−28 Thus, gold nanoparticles have been used in applications such as electrocatalysis, sensing, and biosensing.22,23

Therefore, it is important to examine the combining effect of these materials when applied to a gold substrate for the electrochemical determination of DA effectively and with considerable simplicity. In this work, we report for the first time a novel electrochemical sensor constructed from the following elements: NF, β-CD, and Au nanoparticles. The current sensor was tested for the determination of DA in human urine samples. The possible effects of interfering compounds on the electrochemical signal were also examined.

2. Results and Discussion

2.1. Optimization of Modified Surface

The modified surface was fabricated and optimized with respect to each modifier. The best response was obtained by using concentration of the NF solution 2% (optimized). CD immobilization was achieved using electrochemical deposition of CD. The best current response of DA oxidation was obtained when the film formed by bulk electrolysis (BE) at potential 1.2 V for 3 min in 10–3 M of CD/0.05 M LiClO4 solution.21 Furthermore, two electrochemical methods, namely, cyclic voltammetry (CV) and BE were employed for the deposition of gold film at the Au/NF/CD electrode surface. The resulting films were examined in 1 mM DA/0.1 M phosphate-buffered solution (PBS) at pH 7.40 (Figure 1A). In case of CV, the potential was cycled between −0.8 and +0.4 V in (6 mM HAuCl4/0.1 M KNO3) at a scan rate 50 mV s–1 for 10, 15, 20, and 30 cycles, and the best result was obtained when applying 20 cycles (Figure 1B). In case of BE, the applied potential was held constant at −0.4 V for 5, 10, and 15 s, and the current response obtained by this method was lower than the CV method (Figure 1C). Thus, CV measurements were recorded with our fabricated electrode (Au/NF/CD/AuNPs), where AuNPs deposited by the CV method showed higher current response for oxidation of DA than when the AuNPs were deposited by the BE method (Figure 1A).

Figure 1.

Figure 1

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(A) CVs of 1 mM DA/0.1 M PBS/pH 7.40 at Au/NF/CD/AuNPs (BE) (black line), Au/NF/CD/AuNPs (CV) (red line). (B) Relation between the anodic peak current of DA (μA) and number of CV cycles for electrodeposition of gold. (C) Relation between the anodic peak current of DA (μA) and the time of BE for electrodeposition of gold.

2.2. Surface Morphology

Scanning electron microscopy (SEM) was used to investigate the surface morphology of different modified surfaces. Figure 2A,B shows the SEM of Au/NF/CD and Au/NF/CD/AuNPs, respectively. The morphology of Au/NF/CD (Figure 2A) exhibits a distinctive morphology with porous surface resulting in greater surface area and enhancement of the contact area with the analyte. The SEM image of Au/NF/CD/AuNPs (Figure 2B) shows that gold nanoparticles are distributed over the Au/NF/CD surface with the formation of some clusters exhibiting a large surface area (Figure 2C shows higher magnification of the surface). The combining effect of modifiers, NF, CD, and gold nanoparticles, affected the conductivity level of the composite and increased the electrocatalytic activity of the resulting composite. Figure 2D shows the energy-dispersive X-ray analysis of Au/NF/CD/AuNPs confirming the presence of AuNPs.

Figure 2.

Figure 2

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SEM micrograph of (A) Au/NF/CD, (B) Au/NF/CD/AuNPs, (C) Au/NF/CD/AuNPs (higher magnification), and (D) EDX of Au/NF/CD/AuNPs.

2.3. Comparison between Different Modified Surfaces

Figure 3 shows cyclic voltammograms (CVs) of electro-oxidation of 1 mM DA/0.1 M PBS at pH 7.40, recorded at five different surfaces, namely, bare Au, Au/AuNPs, Au/NF/AuNPs, Au/CD/AuNPs, and Au/NF/CD/AuNPs. A summary of the oxidation potential and current values of DA at all of the studied surfaces is presented in Table 1. A poorly defined oxidation peak was obtained at the bare gold electrode at 442 mV. For all surfaces studied, the current response increases upon inclusion of gold nanoparticles. A high current response (90.3 μA) is observed at Au/NF/CD/AuNPs at 210 mV compared to the other studied surfaces (Table 1). The oxidation current of DA increased by 11.2, 3.7, 12.6, and 20.9 folds at Au/AuNPs, Au/NF/AuNPs, Au/CD/AuNPs, and Au/NF/CD/AuNPs, respectively, compared to bare Au electrode which confirmed the catalytic DA oxidation at the sensor surface. Therefore, the proposed sensor showed remarkable improvement in both the current response and the oxidation potential towards the electro-oxidation of DA compared to the bare Au electrode. Improvement in the catalytic activity for the proposed sensor was observed. NF film increases the electrical conductivity of the composite and enhances the surface preconcentration of DA by ion selectivity and accumulation of DA in the hydrophilic regions.24,29 Stable inclusion complex was formed between CD and DA because of hydrogen bonds formation.30−34 Besides, extra advantages were offered by gold nanoparticles because of their unique properties that results in improving the electron-transfer process and current response.24,31 Surface roughness also contributed in enhancing the current signal as the surface area of the electrode increases upon modification. We used a redox probe, K3[Fe(CN)6], and the Randles–Ševćik equation to estimate the surface area for each electrode. The estimated electroactive surface areas are 0.0435, 0.109, and 0.917 cm2 for Au/NF, Au/NF/CD, and Au/NF/CD/AuNPs, respectively, compared to 0.0177 cm2 for the Au electrode.

Figure 3.

Figure 3

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CVs of 1 mM DA/0.1 M PBS/pH 7.40 recorded at different working electrodes; bare Au, Au/AuNPs, Au/CD/AuNPs, Au/NF/AuNPs, and Au/NF/CD/AuNPs, scan rate 50 mV s–1.

Table 1. Values of Anodic Peak Current Ipa, Anodic Peak Potential Epa, Potential Peak Separation ΔE, and Diffusion Coefficient Dapp (1 mM DA/0.1 mol L–1 PBS/pH 7.4, Scan Rate 50 mV s–1) at Different Electrodes.

electrode Epa (mV) Ipa (μA) ΔE (mV) Dox (cm2/s)
bare Au 442 4.33 404 0.213 × 10–5
Au/AuNPs 203 48.5 58 26.7 × 10–5
Au/NF/AuNPs 549 16.0 592 2.91 × 10–5
Au/CD/AuNPs 206 54.6 63 33.9 × 10–5
Au/NF/CD/AuNPs 210 90.3 74 92.7 × 10–5
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2.4. Effect of Scan Rate

The scan rate was changed between 10 and 100 mV s–1, and its effect on the current response was examined for 1 mM DA/0.1 M PBS/pH 7.4 at the Au/NF/CD/AuNPs using the CV technique (Figure 4). The results indicate a linear relation between Ip and ν1/2 (Figure 4; inset), illustrating a diffusion-controlled process. The corresponding eq 1 is

2.4. 1

Figure 4.

Figure 4

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CVs of 1 mM DA/0.1 M PBS/pH 7.40 at Au/NF/CD/AuNPs at different scan rates (10–100 mV s–1). Inset: Linear relation between the anodic peak current of DA (μA) and the square root of the scan rate (mV s–1)1/2.

The correlation coefficient is r2 = 0.995.

Diffusion coefficient (D, cm2 s–1) values were calculated using Randles–Ševćik equation for a quasireversible process22 (eq 2), where (n = 2), (A = 0.0177 cm2), and (C = 1 × 10–6 mol/cm3)

2.4. 2

Dapp values were 0.213 × 10–5 and 92.7 × 10–5 cm2·s–1 at the gold electrode and Au/NF/CD/AuNPs, respectively, displaying the highest value at Au/NF/CD/AuNPs.

2.5. Effect of Solution pH

The influence of pH of the supporting electrolyte on the electrochemical response of Au/NF/CD/AuNPs toward DA compound is investigated (Figure 5A). The study showed that the oxidation potential of DA shifted to less positive values by the increase in solution pH (from 2.0 to 11) and the oxidation potential versus the pH displayed linear relation (Figure 5B). This study indicated protonation/deprotonation steps for the DA oxidation at Au/NF/CD/AuNPs. The corresponding eq 3 is

2.5. 3

Figure 5.

Figure 5

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(A) CVs of 1 mM DA/0.1 M PBS with different pH values (2–11) at Au/NF/CD/AuNPs. (B) Linear relation between the anodic peak potential of DA (mV) and the pH value, scan rate, 50 mV s–1. (C) Relation between anodic peak current and pH values.

With a correlation coefficient of 0.997.

The slope value of −58 mV/pH over the pH range (2.0–11) was close to the Nernstian theoretical slope value of −59 mV, indicating that there is an equal number of electrons and protons involved in the DA oxidation process. The protons number involved was predicted to be 2, as the oxidation of DA is a two-electron process. This indicates a 2e–/2H+ process (Scheme 1).

Scheme 1. Oxidation Mechanism of DA.

Scheme 1

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The present study was performed at pH 7.4 as the maximum current value was obtained at pH 7.0 (Figure 5C).

2.6. Characteristics of Au/NF/CD/AuNPs Sensor

2.6.1. Stability, Repeatability, and Reproducibility of the Au/NF/CD/AuNPs

In order to investigate the stability of Au/NF/CD/AuNP-modified electrode, the cyclic voltammetry of 1 mM DA/0.1 M PBS at pH 7.4 was studied for 30 repeating cycles (Figure 6). Excellent current stability was obtained with a slight decrease in the current response. Thus, the electrode has fouling resistance. Repeatability was examined by performing four runs in 1 mM DA/0.1 M PBS/pH 7.4 using the same Au/NF/CD/AuNPs electrode. Also, reproducibility was investigated by applying three independent measurements in 1 mM DA/0.1 M PBS/pH 7.4 using three similarly prepared Au/NF/CD/AuNPs electrodes. Low RSD values were obtained; 0.75 and 0.69%, respectively, indicating that the sensor has good repeatability and reproducibility.

Figure 6.

Figure 6

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Repeated cycle stability of 1 mM DA/0.1 M PBS/pH 7.40 at Au/NF/CD/AuNPs, 30 repeated cycles, scan rate 50 mV s–1.

2.6.2. Robustness

The stability of the current response upon small variations in the experimental parameters was examined. Low RSD values 0.62 and 0.76% were obtained for minor changes in the NF content and pH value, respectively, indicating the robustness of the suggested method.

2.6.3. Precision

Also, the proposed method for DA determination at the Au/NF/CD/AuNP-modified electrode was examined by intraday and interday precisions. The intraday precision represented the investigation of the same surface in 1 mM DA/0.1 M PBS/pH 7.4 solution four times while the interday precision represented the investigation of four similar sensors made independently in four separated DA/0.1 M PBS/pH 7.4 solutions with the same concentration four times. Low RSD values were obtained; 0.91 and 0.95%, respectively, approving good precisions of the suggested method.

2.7. Analytical Performance

2.7.1. Calibration Curve for DA in Real Samples

Standard addition method was applied for the analysis of DA at Au/NF/CD/AuNPs sensor in real urine sample. In this respect a stock DA solution of 1 mM was prepared by dissolving DA in 0.1 mM phosphate buffer solution (PBS)/(pH 7.4) and standard additions were carried out from DA solution in 10 mL of diluted urine (the urine sample was diluted 20 times by PBS). Differential pulse voltammograms (DPVs) of DA (0.05–280 μM) at Au/NF/CD/AuNPs sensor are shown in Figure 7A. The oxidation current increased linearly with increasing concentration of DA in the range of (0.05–280 μM). Figure 7B shows the calibration curve of oxidation current values of DA at the sensor in the linear dynamic range of (0.05–20 μM) with linear regression equation (eq 4)

2.7.1. 4
Figure 7.

Figure 7

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(A) DPVs of DA in urine at Au/NF/CD/AuNPs for concentrations from (0.05 to 280 μM). (B) Calibration curve of DA in urine in the linear range (0.05 → 20 μM) at Au/NF/CD/AuNPs. Inset: Calibration curve of DA in the linear range (40 → 280 μM) at Au/NF/CD/AuNPs.

Figures of merit were as follows: sensitivity of 0.886 μA/μM, detection limit (DL) of 0.6 nM, and quantification limit (QL) of 2.0 nM.

Inset of (Figure 7) shows the calibration curve of oxidation current values of DA at the Au/NF/CD/AuNPs sensor in the linear dynamic range from (40 to 280 μmol L–1) with linear regression equation (eq 5)

2.7.1. 5

(DL) and (QL)20,22 were calculated using the following eqs 6 and 7, respectively)

2.7.1. 6
2.7.1. 7

where “s” is the standard deviation and “b” is the slope of the calibration curve.

Table 2 illustrates a comparison for the determination of DA at Au/NF/CD/AuNPs with other modified electrodes mentioned in the literature.30,36−44 Reasonable sensitivity, linear concentration range, and lower DL were the achievements of the proposed electrode.

Table 2. Comparison of Au/NF/CD/AuNPs Electrode with Different Modified Electrodes Mentioned in Literature for DA Determinationa.
electrode LDR (μM) LD (μM) refs
MWCNT/β-CD/GCE 10–80 6.7 (30)
graphene/Pt/GCE 0.03–8.13 0.03 (36)
PEDOT/FC/PEDOT···SDS 6–300 0.069 (37)
GCE-CD-PNIPAM 0.1–60 0.033 (38)
RGO-pd-NPs 1–150 0.233 (39)
graphite/NF/SPCE 0.5–70 0.023 (40)
GR/β-CD/SPCE 0.1–58.5 0.011 (41)
GR-CS 0.03–20.06 0.0045 (42)
graphite-CMF/SPCE 0.06–134.5 10 nM (43)
GO-CMF/pdSPs 0.3–169.3 23 nM (44)
Au/NF/CD/AuNPs 0.05–20 0.6 nM this work
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a

LDR; linear dynamic range, MWCNT: multiwall carbon nanotubes, β-CD: β-cyclodextrin, GCE: glassy carbon electrode, FC: ferrocene, PEDOT: poly(3,4 ethylenedioxy thiophene), PNIPAM: poly(N-isopropyl acrylamide), RGO-pd-NPs: reduced graphene oxide + palladium nanoparticles, SPCE: screen printed carbon, GR-CS: grafted graphite-chitosan, GO–CMF: graphene oxide–cellulose microfiber, pdSPs: palladium nanostructures.

2.7.2. Accuracy and Precision

To evaluate the accuracy and precision of the proposed method for the determination of DA in urine sample, four different concentrations were repeated five times (Table 3). Suitable recovery results were obtained in the range of 99.9–101.3% with low SD values in the range of 0.229 × 10–7 to 5.0 × 10–7. Good agreement was realized between the obtained results and those obtained using other reported methods indicating that the validation for DA quality control analysis was achieved by this method.35

Table 3. Evaluation of the Accuracy and Precision of the Proposed Method for Determination of DA in Urine Sample.
sample concentration of DA added (μM) concentration of DA found (μM)a recovery (%) standard deviation ×10–7 standard error ×10–7
1 0.6 0.608 101.3 0.710 0.410
2 20 19.98 99.9 0.229 0.132
3 140 141 100.7 5.0 2.89
4 280 280.5 100.2 1.53 0.882
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a

Average of five determinations.

2.8. Study of the Inclusion Complex of DA with β-CD by Infrared

The IR spectrum of DA reveals the presence of two peaks at 3344.93 and 3215.72 cm–1 assigned to NH2 symmetric and antisymmetric stretching vibration modes45Figure S1A. The spectrum of β-CD is characterized by intense peak at 3423.99 due to O–H stretching vibration, whereas the vibration of the −CH and −CH2– groups appears in the region (2800–3000 cm–1)46Figure S1B. Upon complexation, the DA peaks at 3344.93 and 3215.72 cm–1 were not identified any more in the IR spectrum and the OH band of β-CD was slightly shifted toward a lower wave number 3391.21 because of the presence of host–guest interactions as shown in Figure S1C. These suggest the possibility of formation of hydrogen bonds between the hydroxyl groups of the host cavities β-CD and the DA hydroxyl groups.

2.9. Interference Study

2.9.1. Simultaneous Detection of DA with Common Interfering Compounds

Determination of DA in the presence of AA and ST (interfering compounds) is very important for patients under medical treatment from anxiety and depression.3,4,47 Acetaminophen (APAP) is a common drug for treating pain and reducing high temperature of the body.22,24 Also, if APAP is taken in high doses, it can cause serious liver damage.48 Therefore, the detection of DA in presence of these compounds was investigated.

Figure 8A shows the DPV of 0.5 mM DA, 3 mM AA, and 2 mM UA/0.1 M PBS/pH 7.4 at Au/NF/CD/AuNPs. Three defined peaks were obtained at 160, −42, and 431 mV respectively. Figure 8B shows the DPV of 0.5 mM DA, 3 mM AA, and 2 mM APAP/0.1 M PBS/pH 7.4 at Au/NF/CD/AuNPs. Three defined peaks were obtained at 164, −40, and 428 mV, respectively. A comparable trend was obtained in a binary mixture of DA and ST. Figure 8C shows the simultaneous determination of 1 mM DA, and 1 mM ST/0.1 M PBS/pH 7.4 at Au/NF/CD/AuNPs. Two well-defined oxidation peaks were obtained at 152 and 324 mV with current values 48 and 42 μA for DA and ST, respectively. Thus, high current response and good potential peak separation were achieved for simultaneous determination of DA in the presence of interfering compounds.

Figure 8.

Figure 8

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(A) DPV of 0.5 mM DA in the presence of 2 mM UA and 3 mM AA at Au/NF/CD/AuNPs. (B) DPV of 0.5 mM DA in the presence of 3 mM AA and 2 mM APAP at Au/NF/CD/AuNPs. (C) DPV of 1 mM DA and 1 mM ST at Au/NF/CD/AuNPs.

2.9.2. Calibration Curves of (DA and APAP) and (DA, ST, and TY) in Real Urine Samples

Further study was achieved by changing the concentration of DA and APAP in real urine samples in the following ranges (0.05 → 15 μM) and (0.05 → 15 μM), respectively (Figure 9A). The linear relations for DA and APAP (insets) are represented by the following equations

2.9.2. 8
2.9.2. 9
Figure 9.

Figure 9

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(A) DPVs for binary mixture DA and APAP/diluted urine with increasing concentrations of DA (0.05 → 15 μM) and APAP (0.05 → 15 μM). (Insets) Plots of peak current vs the concentration for DA and APAP. (B) DPVs for the ternary mixture of DA, ST, and TY/diluted urine with increasing concentrations of DA (0.2 → 15 μM), ST (0.5 → 30 μM) and TY (0.5 → 30 μM) using Au/NF/CD/AuNPs. (Insets) Plots of peak current vs the concentration for DA, ST, and TY.

The sensitivity of the sensor toward the electro-oxidation of DA was very close to the value mentioned in Section 2.7.1. This ascertained the independent electrochemical oxidation behaviors of these compounds at the Au/NF/CD/AuNPs sensor.

Considering the coexistence of DA, ST, and tyrosine (TY) in biological fluids, thus developing an effective method that can separate and simultaneously determine DA, ST, and TY in real urine samples, is useful for diagnosis and treatment purposes (Figure 9B). The calibration curves for DA, ST, and TY in real urine sample (insets) are represented by the following equations

2.9.2. 10
2.9.2. 11
2.9.2. 12

The sensitivity of the sensor for electro-oxidation of DA was very close to the value mentioned earlier in Section 2.7.1. Three well-defined oxidation peaks were separated. Thus, the Au/NF/CD/AuNPs sensor possessed excellent anti-interference capability for simultaneous determination of DA, ST, and TY.

3. Conclusions

A novel electrochemical sensor based on modification of a gold electrode with two successive layers of β-CD and gold nanoparticles over a thin film of NF; Au/NF/CD/AuNPs was fabricated. The sensor showed good electrocatalytic activity for the determination of DA compared to other studied electrodes. The synergism existed between gold nanoparticles, β-CD and NF, resulting in formation of a conductive matrix, large surface area, and enhancement of the catalytic process. Furthermore, the NF film facilitated the surface preconcentration of DA by ion selectivity and accumulation of DA in the hydrophilic regions, and CD formed a stable host–guest inclusion complex with DA and enhanced the electron-transfer kinetics. The calibration curves for DA in urine were linear in the ranges of (0.05–20 μM) and (40–280 μM) with R2 of 0.995 and DL of 0.6 nM in low concentration range. The proposed method demonstrated that it is possible to discriminate DA from AA and UA or AA and APAP at physiological pH. Furthermore, the sensor possessed excellent anti-interference capability for simultaneous determination of DA, ST, and TY in biological fluids.

4. Experimental Part

4.1. Chemicals and Solutions

Chemicals, namely, β-CD, acetonitrile, lithium perchlorate (LiClO4), NF, ethyl alcohol, gold chloride (AuCl2), potassium nitrate (KNO3), DA, UA, AA, TY, APAP, and serotonin (ST) were supplied by Aldrich Chem. Co. (Milwaukee, WI, USA). 0.1 M PBS (1 M K2HPO4 and 1 M KH2PO4) was used as the supporting electrolyte.

4.2. Apparatus

The voltammetry measurements were carried out with a BAS Epsilon electrochemical analyzer in a three-electrodes cell. The electrodes were Ag/AgCl, a platinum wire, and a gold electrode. A Quanta 250 FEG instrument and Gamry-750 system was used for SEM measurements and electrochemical impedance spectroscopy.21

4.3. Fabrication of the Au/NF/CD/AuNPs Sensor

Preparation of the Au/NF/CD/AuNPs sensor was performed as follows: In the first step, a layer of 10 μL from 2% NF was casted on a Au-electrode surface, and then, the electrode was left to dry in open air for 15 min. In the second step, a layer of CD was electrodeposited from a solution containing 10–3 M CD/0.05 M LiClO4 by BE method at +1.2 V for 3 min. The last layer of gold nanoparticles was electrodeposited from a solution of (6 mM HAuCl4/0.1 M KNO3), over the Au/NF/CD surface by the CV technique, and the potential was cycled between −0.8 and +0.4 V for 20 cycles at a scan rate 50 mV s–1. The electrode was represented as Au/NF/CD/AuNPs (Scheme 2).

Scheme 2. Schematic Representation of the Modified Electrode; Au/NF/CD/AuNPs Used for the Electrochemical Oxidation of DA.

Scheme 2

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Acknowledgments

The authors acknowledge the research facilities provided by Cairo University.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b01222.

  • IR spectra for β-CD; DA; inclusion complex of DA with β-CD; EIS plots of Au/NF/CD/AuNPs and Au electrodes; and EIS data fitting corresponding to Figure S2 (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao9b01222_si_001.pdf (384KB, pdf)

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