Nafion-coated cadmium pentacyanonitrosylferrate-modified glassy carbon electrode for detection of dopamine in biological samples

Introduction

Neurotransmitters (NTs) are endogenous chemicals that transmit electrical signals from one neuron to another across the synapse. This synaptic transmission proceeds through the interaction of NTs with the specific receptors onto the membrane of the target cells.¹ The interaction between NT and its receptor is a key event in the action of all NTs and is considered as one of the major modes of communication between neurons. Of important NTs, dopamine (DA) is responsible for transmitting signals in the mammalian central nervous system (CNS). Since its discovery in the 1950s, it has been shown that DA level fluctuations may lead to several CNS diseases such as Schizophrenia, Parkinson's disease^2,3 and Tourette's syndrome.^4-6 Therefore, it is important to quantify the content of DA in the clinical samples of patients with CNS disorders using a simple and rapid method of quantitative detection.

DA concentration in the biological samples is fairly low (1-100 ng/mL).⁷ Plasma dopamine is originated from a variety of tissues including sympathetic nerves and adrenal. It is present in free form in a concentration approximately equivalent to that of epinephrine and about 25% that of norepinephrine.⁸ Following some sympathetic activities such as stress, exercise, standing, or hypovolemia the plasma concentration of DA increase, however this nervous responses may have a significantly smaller magnitude than those for plasma norepinephrine and epinephrine.⁸

There still exist increasing demands for the selective and specific detections of DA in the biological samples. However, has not paid sufficiently on the assessment of DA level in the human blood samples, and the majority of previously reported sensors have been applied towards the detection of DA content in the pharmaceutical formulations. Moreover, a number of studies conducted for determination of DA quantitatively in the biological media, Direct electrochemical methods with high selectivity and specificity are capable for DA detection in the highly complex media, and therefore, are of great interest of scientists in the area of analytical chemistry. Because of the overlapping oxidation potentials of some endogenous biomolecules in the biological fluids (e.g., ascorbic acid [AA] and uric acid [UC]) they can interfere with detection of DA. Hence, the detection of DA in the presence of AA/UC through electroanalytical approaches is a challenging issue.⁹

In recent years, several strategies have been employed to improve the selectivity of DA detection such as polymer film modified electrodes.^10-13 Recently, much attention has been paid to the ion-exchange polymers due to their exclusive properties in removing interferences.¹⁴ Nafion, a negatively charged copolymer due to the presence of sulfonated groups in its own backbone, has widely been used as an ion-exchange material to eliminate interferences caused by anions.^15,16 At physiological pH,‏ DA (pK_a, 8.87) exists mostly in the cationic form, while AA (pK_a, 4.17) and UA (pK_a, 3.70) exist as anions. Thus, Nafion coated GC electrode allows detection of DA and eliminates interferences of AA and UA.¹⁵

There is a substantial interest in terms of redox properties of Prussian blue (PB) type metal hexacyanoferrates with respect to their fundamental importance and also possible applications. Preparation of some analogs of PB modified electrodes and their electrochemical behavior have been reported based upon different transition of metal hexacyanoferrates such as chromium,¹⁷ nickel,¹⁸ cobalt,¹⁹ copper,²⁰ palladium,²¹ molybdenum,²² vanadium,²³ and platinum.²⁴ Among many metal hexacyanoferrates, cobalt hexacyanoferrate showed a similar electrochemical behavior to the PB. The other PB analogs indicated only one redox center due to hexacyanoferrate (III)/(II) system.

In the present study, an amperometric DA sensor is developed by using of Nafion|CdPCNF|GC electrode to detect dopamine electrochemically with high sensitivity and selectivity.

Materials and Methods

Results

General characteristics of the modified electrode

The CdPCNF layer was deposited through CV method and it was observed that the thickness of the deposited layer increased after each cycle. After 21 cycles, while no changes in the CV spectrum were detected, we selected 21 potential cycles of CV for the final preparation of CdPCNF|GC electrode (see Fig. S1, Supplementary file 1). Moreover, we observed that the amount of deposited Nafion on the CdPCNF|GC electrode affected the performance of the modified electrode. After a series of attempts, 5 µL Nafion (0.5 wt. %) dispersion was considered as an optimal amount.

The CV spectra of GC, CdPCNF|GC, and Nafion|CdPCNF|GC electrodes in the absence and presence of DA are represented in Fig. 1. In the absence of DA, no redox peaks were observed by using the bare GC electrode in the potential range of 0-1 V (Fig. 1A ; see curve a). In the presence of DA (Fig. 1B; curve a) quasi-reversible voltammetric redox peaks were observed at 0.68 V (anodic) and 0.56 V (cathodic) using the bare GC electrode. Due to the coating of the electrode surface by oxidized products, the peak-to-peak separation presumably suggests slow electron transfer kinetics. As illustrated in Fig. 1 (panel B, curve c) , using Nafion|CdPCNF|GC electrode, a pair of symmetrical redox peaks appeared at 0.573 and 0.459 V. The mean peak potential E⁰= (E_pa+E_pc)/2 was calculated to be 0.516 V (vs. Ag|AgCl) and the peak potential separation was found to be 0.114 V at the scan rate of 0.1 V/s. This redox peak is related to the reduction and oxidation of the redox couple of Fe³⁺/Fe²⁺ in a matrix of PCNF.

Fig. 1

CV spectra of GC electrode (a), CdPCNF|GC electrode (b) and Nafion|CdPCNF|GC electrode (c) in the absence (A) and presence (B) of DA (5.0 mM) in PBS (pH 7.4) at the potential scan rate of 0.1 V/s.

×

Fig. 1.

The anodic peak current (I_pa) changes are proportional to the square root of potential scan rates, indicating that the electrochemical reaction is a reversible and diffusion controlled process. At higher scan rates the electrochemical process is found to be dominated by kinetic controlled parameters of the electrochemical reaction (Fig. S2, Supplementary file 1). The calibration plot of DA determination clearly shows that the peak current values are linearly changed with DA concentrations in the range of 1-40 mM (Fig. S3, Supplementary file 1). For concentrations more than 40 mM, the variation of peak currents deviated from the linearity. The LOD was found to be 0.12 mM, which is too large for some analytical purposes. To improve the LOD, we investigated the catalytic activity of the modified electrode using the constant potential amperometric technique in the next sections of this paper.

Nafion|CdPCNF|GC electrode was stable enough in the broad range of pH (2-10). The pH of supporting electrolyte also affected the voltammetric behavior of the modified electrode. Therefore, the anodic peak current (I_pa) was increased when the pH was incremented from 2 to 7.4 and decreased up to 12 (Fig. 2). Nafion|CdPCNF|GC electrode showed maximum electrocatalytic behavior at pH 7.4. Taken all, pH 7.4 was selected for the electrochemical determination of DA.

Fig. 2

The response of Nafion|CdPCNF|GC electrode to the addition of 5.0 mM DA at various pH ranging from 2 to 12 in B. R. buffer solutions at the scan rate of 0.1 V/s.

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Fig. 2.

As represented in Fig. S4 (see Supporting Information), among the buffers, PBS gave us the best responses and peak shape for oxidation of DA, and hence PBS was employed for the further analysis. Moreover, with an increase in the rotation speed from 0 to 1000 rpm, the electrochemical response of 50 µM DA continuously improved while no further improvement was observed above 1000 rpm. Thus, this rotation speed was chosen as the optimal value.

EIS analysis

Fig. 3 shows the EIS spectra of the bare GC electrode, CdPCNF|GC electrode and Nafion|CdPCNF|GC electrode in the frequency range of 0.01–10⁵ Hz represented by the Nyquist plot. The Randle’s circuit (Fig. 3, inset) was used to fit the obtained impedance data including the electron transfer resistance (R_ct) and the Warburg-type impedance (Z_w). The diameter of the semicircle in the Nyquist plot shows the R_ct value. As represented in Fig. 3, analyses of the bare GC electrode exhibited a very small semicircle showing low R_ct value and also straight line corresponding to the diffusion process at the low-frequency region.

Fig. 3

EIS spectra (Nyquist plot) of the bare GC electrode (a), CdPCNF|GC electrode (b) and Nafion|CdPCNF|GC electrode (c) obtained in the frequency range of 0.01-10⁵ Hz using Fe (CN)₆³⁻/Fe (CN)₆⁴⁻ (5.0 mM).

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Fig. 3.

The calibration plot and LOD

Amperometry technique was used for DA quantification using Nafion|CdPCNF|GC electrode. The current response was measured through the rotating disk electrode at the fixed potential of 0.7 V. Fig. 4 shows the amperometric response of Nafion|CdPCNF|GC electrode, while exhibiting a rapid response (less than 2 seconds) to the changes in the DA concentration. Furthermore, the electrochemical response of the electrode for changes in DA concentrations was linear in the range of 1.7 (-92.1 µM) and LOD and LOQ were found to be 0.7 and 2.3 µM, respectively. In Table 1, a number of previously reported DA sensors with their important features, including types of modifiers, LOD, linear ranges, and method of analysis are represented. With having relatively more selectivity of detection and simplicity of electrode preparation than reported DA sensors, our proposed DA sensor is capable to detect DA with fairly better LOD but undesirably narrow linear range of detection.

Fig. 4

Current-time cure for the Nafion|CdPCNF|GC electrode obtained on the addition of different amount of DA ranging from 1.7 to 150 µM into human serum sample. Inset: Calibration curve.

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Fig. 4.

Table 1. Comparison of our report with previous reports for DA detection
Electrode type	Modifier	LOD (µM)	Linear range (µM)	Method	Reference
Au-µA	PB-PEDOT	4.3	5 – 10	CV	27
GC electrode	HTAB-GO-MWCNT	1.5	5 – 500	A	28
GC electrode	ZrO2-GO	9	9 – 237	LSV	29
CCE	Ag	1.4	6.6 – 1200	A	30
GC electrode	AgNPs-rGO	5.4	10 – 800	LSV	31
ITO	AuNPs	3.45	10 – 900	SWV	32
GC electrode	Nafion|CdPCNF	0.7	1.7 – 92.1	A	This work
AgNPs: Silver nanoparticles, AuNPs: Gold nanoparticles, Au-μA: Gold disk microelectrode arrays, CCE: Ceramic–graphite composite, GCE: Glassy carbon electrode, GO: Graphene oxide, HTAB: Hexadecyl trimethyl ammonium bromide, ITO: Indium tin oxide, LSV: Linear sweep voltammetry, MWCNT: multiwalled carbon nanotubes, PB: Prussian blue, PEDOT: Poly(3,4-ethylenedioxythiophene), rGO: Reduced graphene oxide, SWV: Square wave voltammetry, ZrO2: Highly loaded zirconium oxide nanoparticles, CV: cyclic voltammetry, A: amperometry,

Interference study

CdPCNF|GC electrode showed a strong response to the presence of AA and UA, a very faint response to tyrosine (Try), tryptophan (Trp), norepinephrine (NE), epinephrine (EP), serotonin. however, it represented no response to CA, Glu, and NADH. Further, Nafion|CdPCNF|GC electrode displayed no response to the presence of any of these types of interferences, the result express the ability of Nafion to eliminate the possible interferences, completely (Fig. 5).

Fig. 5

Amperometric response of CdPCNF|GC electrode (a) and Nafion|CdPCNF|GC electrode (b) for the DA detection in the presence of interfering substances

×

Fig. 5.

Determination of DA in pharmaceutical injections and spiked human serum samples

Determination of DA content at biological samples was carried out using Nafion|CdPCNF|GC electrode based on an amperometric technique through the standard addition method. Fig. 6 represents Nafion|CdPCNF|GC electrode responses to the existing DA in spiked serum samples with the LOD and LOQ values of 0.72 and 2.40 µM, respectively (relative standard deviation (RSD) = 5.7 %, N = 6). The LOD and LOQ were unexpectedly close to that of the DA analysis (i.e., LOD and LOQ of 0.68 and 2.27 µM, respectively) performed on buffer solutions. Moreover, a nonlinear response was observed at a concentration above 100 μM.

Fig. 6

Current-time cure for the Nafion|CdPCNF|GC electrode Obtained on the addition of different amount of DA ranging from 1.7 to 150 µM into human serum sample. Inset: Calibration curve .

×

Fig. 6.

Recovery values of DA from pharmaceutical injections and serum samples is presented in Table 2. In our tests, no DA was detected in healthy serum samples. When samples were spiked with determined DA concentrations, acceptable recovery values were obtained in the range of 97.2-100.8% and also the presence of AA, UC and the other interferences did not interfere with the determination of DA. This finding implies a promising feature for the applicability of the modified electrode for the direct determination of DA in pharmaceutical and real samples.

Table 2. Determination of DA in pharmaceutical injection and human serum samples
Sample	DA (Detected)	DA (Added)*	DA (Found)*	Recovery %
Pharmaceutical injection	4.0	30	34.2±0.76**	100.5
	5.1	30	35.2±0.82	100.2
	3.8	30	33.1±0.67	97.9
	7.2	30	37.1±0.93	99.7
	5.4	30	35.6±0.137	100.5
	5.3	30	35.1±0.148	99.4
Serum samples	0	5	5.1±0.13	102.0
	0	10	9.9±0.29	99.0
	0	15	15.2±0.48	101.3
	0	20	19.8±0.38	99.0
*The unit of DA in the pharmaceutical injection and serum samples was µg/mL and µM, respectively. ** The results are expressed as 95% confidence interval for the mean of founded DA. Six replicate measurements were made on each sample.

Discussion

Dopamine plays a vital role in the mammalian CNS and its selective determination in the biological fluids is of tremendous importance in the field of biomedical sciences. AA, UA and NTs present in the biological fluids can interfere with detection of DA. Therefore, development of a modified electrode with capability for detecting DA in the presence of these interfering species appears to be significantly important. To develop DA sensor, a GC electrode was modified with CdPCNF and Nafion. A small amount of Nafion formed a thin layer on the CdPCNF|GC electrode that resulted in poor sensitivity for DA detection. While using a too much Nafion led to the formation a relatively thick film and this decreased the mass transfer rate of DA in the Nafion film. Besides, the excess deposition of Nafion profoundly decreases the mass transfer rate and limits the diffusion of the analyte through the polymer layer.

Using Nafion|CdPCNF|GC electrode the current redox peak increased 10 times in comparison with the bare GC electrode. An increase in the sensitivity of the DA detection reflected in the observation of two reduction peaks in the DA CV spectrum recorded through Nafion|CdPCNF|GC electrode. These two peaks were not observed in the CV spectrum of the bare GC electrode. It has been proposed that the reduction peaks at 0.162 V and 0.011 V arise from the reduction of dopamine quinone and leucodopaminechrome (a closed ring product formed from dopamine quinone) to DA respectively (Scheme 1).³³ It appears that on the surface of Nafion|CdPCNF|GC electrode, DA is oxidized to dopaminquinone, which was converted into leucodopaminechrome next (Scheme 2). However, it is clear that the electron transfer kinetic of the electrochemical reaction was remarkably improved after the modification of the GC electrode.

Scheme 1

Illustration of Nafion|CdPCNF|GC electrode preparation steps and electrochemical oxidation process of DA. (dopaminquinone need to be corrected to dopamine quinone, if it is the case).

×

Fig. 7.

Scheme 2

Chemicals produced in the electrochemical oxidation of DA at the surface of electrode. (dopaminquinone need to be corrected to dopamine quinone, if it is the case).

×

Fig. 8.

However, it was found that the selectivity and sensitivity of DA detection using CdPCNF|GC electrode were improved after modification with Nafion in comparing with bare and CdPCNF|GC electrodes and the Nafion modified electrode showed a sharp anodic peak for DA detection with the clear peak potential shift of 65 mV towards theanodic region. We assume that this effect is due to the cation exchanging characteristics of Nafion, by which negatively charged analytes are repelled from the surface of the electrode. When dopamine exists as a protonated form in the solution, it is attracted towards the electrode surface due to interactions between negatively charged sulfonate groups of Nafion and positively charged dopamine molecules. Sulfonate groups of Nafion are relatively fixed at the electrode surface. Therefore, the neutrality of the interface is achieved mainly through the cation’s concentration variation within the double layer and thus leading to the pre-concentration of DA at the electrode surface.

For the EIS analyses of the CdPCNF|GC electrode, the R_ct value was smaller than that for the bare GC electrode, whereas modification of CdPCNF|GC electrode with Nafion‏ led to a small increase in the R_ct value. These results indicate that deposition of Nafion on the CdPCNF|GC electrode has no significant effects on the electron transfer rate. Thus, it can be used as a selective barrier for the anionic interferences and also as a selective and passive transporter for the DA molecules.

DA (pKa= 5.13) exits mostly in the cationic form in human plasma, while AA (pK_a1= 4.17, PK_a2= 11.57) and UA (pK_a1= 5.70, pK_a2= 9.80) as the most problematic interfering species exist mainly as anions.¹⁶ Nafion is a negatively charged polymer and it can prevent the negatively-charged species that are reaching the electrode surface³⁴. In contrast, DA alone can easily pass through the interface. This implies that Nafion|CdPCNF|GC electrode can be used for detection of DA existing in human plasma (pH= 7.4).

Conclusion

Key roles of DA as a functional NT in the CNS are of paramount importance to mammalians especially humans. In this regard, quantitative determination of DA in biological matrices has recently emerged as a major issue in the bioanalytical sciences. In the current study, we aimed to develop a simple and robust sensor for sensitive and selective determination of DA existing in the biological mediums. To develop this sensor, bare GC electrodes were modified with the electropolymerized film of CdPCNF and then coated with Nafion to minimize interferences. The developed sensor exhibited satisfactory responses as compared with the bare electrode and previously reported sensors with adequate sensitivity and selectivity to the presence of DA in the biological samples. However, it successfully determined DA in the presence of interfering species, indicating that major difficulties arising from overlapping electrochemical signals of these substances can be overcome with utilizing a very thin film of Nafion deposited on the electrode. The other advantages of the present sensor (e.g., the simplicity of preparation, rapidity of determination, and low cost of production) are. suggest that it can be served for the practical applications.

Acknowledgments

In addition, we express our deepest blessing as a memorial to Ms. Maryam Harasi, who sadly passed away recently, for her valuable collaboration on this study.

Funding sources

This study was funded by Research Center for Pharmaceutical Nanotechnology (RCPN) at Tabriz University of Medical Sciences in Iran.

Ethical statement

None.

Competing interests

There is none to be declared

Authors contribution

MJA: conceptualization and experiments. PK: data handling, data analysis, data presentation and study validation. JB: provision of equipment and supervision. DA: consultation, draft preparation, draft writing, and draft reviewing. SD: study consultation, provision of study materials and equipment. MRR: project administration, draft reviewing.

Supplementary Materials

Supplementary file 1 contains Figs. S1-S4.

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