1 Introduction

Norepinephrine (NE) belongs to the class of catecholamine neurotransmitters, which plays pivotal role in the functioning of central nervous system [1, 2]. The behavioral and physiological conditions in humans are controlled by neurotransmitters [3,4,5]. The neurotransmitters are involved in many neurobiological processes within the human body, such as memory, sleep, appetite and learning [3,4,5]. It is now well-established that defects in the secretion or absorption of neurotransmitters can lead to diseases such as Depression Syndrome, Neurological Diseases, and Drug Addiction [3,4,5].

One of the most important neurotransmitters is NE, which controls mood disorders, blood pressure, and emotional arousal. NE converts glycogen to glucose and also facilitates the conversion of fats into fatty acids, which in turn increases energy production. NE increases heart rate, opens the airways, and narrows blood vessels, therefore the human body can function well in stressful situations [3,4,5]. Analysis of published data from many laboratories involving more than 800 supine, resting, healthy subjects indicated an average circulating level of venous NE of 260 pg mL−1 [6]. According to the points mentioned above, knowing the concentration of NE in blood plasma is useful for diagnosing and evaluating the therapeutic and pharmacodynamic effects of psychiatric, neurological, and cardiovascular disorders.

A major problem in selectively determining NE is the presence of ascorbic acid (AA) and uric acid (UA) that are major interferents [7]. Therefore, the development of easy and sensitive methods that are also selective is crucial for the direct determination of NE.

Until now different analytical methods have been used for the determination of NE such as electrochemical techniques [7,8,9,10,11,12,13,14,15,16,17], gas chromatography [18], high performance liquid chromatography-mass spectrometry [19], ion chromatography with direct conductivity detection [20] and spectrophotometry [21]. Among the reported methods for measuring NE, the electrochemical technique has received more attention due to its advantages such as high sensitivity, high selectivity, ease and speed [7]. However, using unmodified electrodes, selective determination of NE in the presence of UA and AA is not possible because the oxidation potential of these substances is close to the oxidation potential of NE [7, 16].

The nanochemistry is applied in various areas of technology and science such as advanced materials, chemistry, optics, physics and biomedicine [22, 23]. Recently, with the development of new nanoparticles (NPs), the field of nanomedicine has expanded widely in the fields of diagnosis, monitoring and treatment of several diseases such as Parkinson, cancer and Alzheimer [24].

There are various methods of signal amplification with the aid of loading nanosphere, nanotube, nanosheet and nanoparticle on the electrode to fabricate electrochemical sensors [24,25,26,27,28,29,30,31]. The nanomaterials have excellent catalytic and electronic traits to convert bio recognition processes into electrochemical reaction, and also have large surface area to increase the speed of electron exchange [16, 32,33,34,35,36,37,38,39].

Recently, metal oxide NPs have received more attention in different biological applications due to their diversity, stability and sensitivity [40]. Bimetallic NPs compared to monometallic NPs due to their magnetic, catalytic, optical and electrical properties have attracted more attention of researchers [41, 42].

In this research, a manganese ferrite nanoparticle (MF) was synthesized and then along with benzoyl ferrocene (BF) were used to modify the carbon paste electrode. The figure of merit of the BF-MF -modified carbon paste electrode (BFMFCPE) developed in the present work are the wider linear range [2, 3, 8, 11,12,13,14] and lower detection limit [2, 9, 11,12,13,14] compared to other reported electrodes for NE measurement.

Finally, the applicability of the modified electrode for measuring norepinephrine in human urine and serum samples was investigated.

2 Experimental

2.1 Instrument and reagents

The instruments used in the present work were similar to the instruments used in the previous work [43]. Norepinephrine, benzoyl ferrocene and the other reagents with analytical grade were purchased from Merck (Darmstadt, Germany). The phosphate buffer solution (PBS) was prepared from concentrate H3PO4 and their salts. All solutions were prepared using deionized water. The MF NPs were synthesis according to previous reports [44].

2.2 Preparation of electrode

10 mg BF, 100 mg of MF and 890 mg graphite powder were mixed using a mortar and pestle for 10 min, and then 0.7 mL of paraffin was added to it and remixed for 10 min. Then, the resultant paste inserted into a glass tube (long = 10 cm; i.d. = 3.4 mm) and from the other side a copper wire is inserted to establish the electrical connection. After that, to get a smooth and polished surface, we pressed additional paste out of the glass tube and use a piece of paper to polish it. However, the resulting electrode is displayed with BFMFCPE.

Furthermore, the BFCPE without MF, MF paste electrode (MFCPE) without BF, as well as the bare CPE where there were not MF and BF were constructed for comparison.

2.3 Preparation process of the real samples

Urine samples were transferred to a refrigerator immediately after collection. In the next step, a 0.45 µm filter was used to filter them and then, 10 mL of each sample transferred to a 50.0 mL volumetric balloon and diluted with PBS (pH 3.0). Since there was no NE in the urine sample, an appropriate amount of standard NE was added to the collected urine before centrifugation and filtration. The human serum samples were prepared from a Hospital and ten-fold diluted with PBS (pH 3.0).

2.4 Synthesis of manganese ferrite nanoparticle

The MF NPs were synthesis according to previous reports [44]. For this synthesis, 13.4 g iron nitrate and 4.9 g manganese nitrate were added to 70 mL of aqueous solution containing 3.0 mL ethylene diamine and 4.2 g NaOH. The resulting solution was heated to 85 °C and kept at this temperature for 60 min to complete the reaction. The precipitate was filtered off and placed in an oven at 85 °C for 1 h. In the following, the obtained material was calcined for 60 min in an alumina crucible at 500 °C.

3 Results and discussion

3.1 Characterization of MF NPs

The surface morphology of MF NPs was assessed using scanning electron microscope image and shown that MF NPs are relatively uniform and have the spherical morphology (Fig. 1).

Fig. 1
figure 1

The SEM picture of MF NPs

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The FT-IR analysis data of the MF NPs in a frequency ranging between 400 and 4000 cm−1 is given in  Fig. 2. The absorption peak around 3450 cm−1 could be attributed to the O–H stretching vibration and absorption peak around 500–600 cm−1 could be attributed to the O–metal vibration. The band that appears at 1025 cm−1 could be attributed to vibration of Si–O–Metal [45]. The peak appeared in around 3435 cm−1 could be attributed to the N–H(stretching vibration) and the peak appeared in around 2927 cm−1 could be attributed to the –CH2– asymmetric vibration. The peak appearing at 2864 cm−1 can be due to the –CH2– symmetric vibration and the peak appearing at 1560 cm−1 can be due to the C–N (amine) bending vibration. Also, the peak appearing at 1120 cm−1 can be due to the C–N stretch (aliphatic amine).

Fig. 2
figure 2

FT-IR of MF NPs

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The phase identification and crystalline structures of the MF NPs are shown in Fig. 3. The shown diffraction peaks at 30.2°, 35.5°, 43.2°, 53.6°, 57.1° and 62.6° belonging to the miller planes (220), (311), (400), (422), (511) and (440) indicate that the nanoparticles synthesized is MF NPs having spinel FCC form and corresponded to values from JCPDS file No.10-0319 [46,47,48]. The crystalline size was calculated by Scherer Eq. (1).

Fig. 3
figure 3

XRD pattern of the synthesized MF NPs

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$$D=\frac{0.9\lambda }{\beta {\text{Cos}} \theta }$$
(1)

In this equation, the half maximum of the main diffraction peak (311) is β and the λ is 0.15406 nm [48]. The calculated size of crystallites was in the range of 27.6 nm that confirmed the nanocrystalline structure of the given material.

Figure 4 shows EDX spectra of the MF NPs, and as can be seen existence of Fe, Mn, and O elements with weight ratios of Fe (56.0%), Mn (28.1%) and O (15.9%) was confirmed.

Fig. 4
figure 4

EDX spectrum of the MF NPs

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3.2 Characterization of BFMFCPE

The morphology of BFMFCPE electrode was characterized by SEM (Fig. 5) and showed that both materials (BF and MF) are present on the surface of the BFMFCPE. Also, as shown in the Fig. 5, spherical MF nanoparticles with an average diameter of 41 nm are well distributed on the electrode surface.

Fig. 5
figure 5

SEM of BFMFCPE electrode

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3.3 Electrochemical behavior of BFMFCPE

The BFMFCPE was initially fabricated and then its electrochemical features were investigated in 0.1 M of PBS (pH 3.0) by CV (Fig. 6). Attained test findings indicated the reproducible anodic and cathodic peaks with Epa, Epc and E°′ of 625, 535 and 580 versus Ag/AgCl/KCl (3.0 M), respectively. The calculated peak separation potential, \(\Delta E_{{\text{p}}} = \left( {E_{{{\text{pa}}}} - E_{{{\text{pc}}}} } \right)\) of 90 mV, was higher than 59/n mV, as expected for a reversible system [48], this shows a quasi-reversible behavior of the redox couple of BF in BFMFCPE in aqueous medium.

Fig. 6
figure 6

CV for BFMFCPE in 0.1 M PBS (pH = 3.0) with the scan rate of 50 mVs−1

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3.4 Electrochemical impedance spectroscopy studies

Modification of electrode with MF NPs and BF was assessed by electrochemical impedance spectroscopy (EIS) method in a solution containing 0.5 mM [Fe(CN)6]−3/4− (Fig. 7). As can be seen, the value of charge transfer resistance at the surface of the bare electrode (curve a) is much higher than the modified electrode (curve b). This point confirms highly conductivity of MF NPs at the surface of the electrode.

Fig. 7
figure 7

Nyquist diagrams of bare SPE (a) and BFMFCPE (b) in the presence of 0.5 mM [Fe(CN)6]−3/4− pH 3.0

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3.5 Effective surface area of electrodes

In order to illustrate that the MF NPs could increase the electrode surface area in comparison to the surface of unmodified CPE, real surface area of all electrodes was determined by the Randles–Sevcik (Eq. 2) [49]:

$$I = \left( {2.69 \times 10^{5} } \right){\text{ACD}}^{{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}}} n^{{{3 \mathord{\left/ {\vphantom {3 2}} \right. \kern-\nulldelimiterspace} 2}}} \nu^{{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}}}$$
(2)

In the Eq. 2, “A” is the effective surface area of each electrode (cm2), “n” is the number of electrons taking part in charge transfer process, “D” is the diffusion coefficient of the analyte in the solution and “C” is the concentration of the analyte in the solution. The values of n and D for K3Fe(CN)6 are 1 and 7.6 × 10−6 cm2s−1, respectively.

The results showed that the real surface area of CPE, BFCPE, MFCPE and BFMFCPE were 0.09, 0.09, 20.7 and 20.7 cm2, respectively. Therefore, as expected, the surface area of MFCPE increases with the addition of MF NPs to the matrix of the carbon paste.

3.6 Electrochemical behaviour of norepinephrine at the BFMFCPE surface

Figure 8 represents CVs for electrochemical oxidation of 120.0 μM NE at the bare CPE (i.e., curve c), MFCPE (i.e., curve d), BFCPE (i.e., curve e), and finally, BFMFCPE (i.e., curve f). Also, the CVs responses for the bare CPE and the BFMFCPE in 0.1 M PBS (pH 3.0) were presented in curve 8a and 8b, respectively.

Fig. 8
figure 8

The CVs of the bare CPE (c), the MFCPE (d), the BFCPE (e) and the BFMFCPE in the presence of 100.0 μM NE into 0.1 M PBS (pH of 3.0); and also, the cyclic voltammograms of the unmodified CPE (a) and the BFMFCPE (b) into 0.1 M PBS (pH of 3.0). Scan rate equaled 50 mV s−1 in all cases

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As can be seen, potential of peak (Ep) for NE is 630 mV and 700 mV at the BFMFCPE surface and the bare CPE, which reflects the capability of BF and MF as good mediators. Compared to the unmodified electrode, the Ep is shifted by about 70 mV to negative values. However, compared to the unmodified electrode, the peak current (Ip) in 120.0 μM NE solution (curve d) considerably elevated, which was caused by the probable electrocatalytic impacts of BFMFCPE on NE.

Based on these results, we propose an EC′ catalytic mechanism, shown in scheme 1, to describe the electrooxidation of NE at the surface of BFMFCPE. In this scheme, NE oxidized in the catalytic reaction (C), by the oxidized form of BF produced at the electrode surface via an electrochemical (E) reaction.

Scheme 1
scheme 1

The electrooxidation oxidation of NE at the surface of BFMFCPE

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On the other hands, the electrochemical behavior of NE is dependent on the pH of the electrolyte solution, while the electrochemical properties of the BF (Ferocen/Ferocen+ redox couple) is pH-independent. Therefore, investigating the effect of electrolyte solution pH in order to determine the optimal pH is one of the necessities of research. For this reason, the LSV was used to examine the electrochemical behavior of NE at different pHs (2.0 < pH < 9.0) (Fig. 9). Data have shown higher utility of the electrocatalytic oxidation of NE at the BFMFCPE surface at acidic circumstances than the basic or neutral media (Inset of Fig. 9). Therefore, we chose pH of 3.0 as an optimum pH to oxidize NE at the BFMFCPE surface.

Fig. 9
figure 9

LSVs of BFMFCPE in 0.1 M PBS with different pH values (2–9) containing 120.0 µM NE. Inset: Variation Ip with pH

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In the following, the effect of scanning speed on the Ip of NE was assessed using LSV (Fig. 10). The observations showed that the higher the scan speed, the higher the Ip and the Ep shifted towards higher positive potentials. As can be seen (Inset of Fig. 10), the Ip vs. the square root of the scanning rate (υ1/2) varied linearly, therefore, the oxidation process of NE at the BFMFCPE is controlled by diffusion [49].

Fig. 10
figure 10

LSVs of the BFMFCPE into 0.1 M PBS (pH of 3.0) containing 120.0 µM NE at different scan rates. 1–10 denoted to 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 mVs−1. Inset: Variations in the anodic peak current versus the square root of the scan rates

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In the next step, the LSV obtained at the scan rate of 10 mVs−1 was used for draw the TOEFL diagram (Fig. 11). The ascending part of the voltammogram is called the TOEFL region that under the influence by the electron transfer kinetic between NE and the BFMFCPE.

Fig. 11
figure 11

A Tafel plot extracted from LSV of BFMFCPE into 0.1 M PBS (pH of 3.0) containing 120.0 µM NE at a scan rate equal to 10 mV s−1

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Due to the slope of the TOEFL curve, which corresponds well to the involvement of 1 electron in the electrode rate-determining step [49], a charge transfer coefficient (α) of 0.45 was obtained for NE. In addition, a charge transfer coefficient (α = 0.24) was obtained for NE on the surface of unmodified CPE. As can be seen, the value of α has been improved by modifying the electrode surface.

3.7 Chronoamperometric measurements

In the next step, the chronoamperometry of NE at the surface of BFMFCPE was assessed. For perform of this study, we adjusted the working electrode potential at 0.75 V versus Ag/AgCl/KCl (3.0 M) using diverse concentrations of NE into PBS at a pH of 3.0 (Fig. 12). It should be mentioned that we used Cottrell equation (Eq. 3) to describe the current generated for the electrochemical oxidation of a material under conditions where mass transfer is limited. In this case, the material has a diffusion coefficient that is denoted by D [49]:

$$I = nFAD^{{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}}} C_{b} \pi^{{{{ - 1} \mathord{\left/ {\vphantom {{ - 1} 2}} \right. \kern-\nulldelimiterspace} 2}}} t^{{{{ - 1} \mathord{\left/ {\vphantom {{ - 1} 2}} \right. \kern-\nulldelimiterspace} 2}}} ,$$
(3)

here Cb and D imply the bulk concentration (mol cm−3) and diffusion coefficient (cm2 s−1). We also applied experimental plots of I versus t−1/2, with the best fits for diverse concentrations of NE (Fig. 12a). In the next step, we plotted the slope of the resultant straight lines versus (Fig. 12b) concentration of NE. Using the Cottrell equation and considering the slope of the curve, the average of D for NE was obtained as 2.4 × 10–6 cm2/s.

Fig. 12
figure 12

The obtained chronoamperograms at the BFMFCPE into 0.1 M PBS (pH of 3.0) containing different concentrations of NE. It is notable that 1–5 are corresponding to 0.0, 0.1, 0.4, 0.9 and 1.5 mM of NE. Insets: The plot of I versus t−1/2 obtained from the chronoamperograms 1–4 (a), the slope of straight lines against concentration of NE (b)

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3.8 Repeatability and stability

We used DPV to assess the repeatability of BFMFCPE. The obtained results showed that RSD for ten measurements of 100.0 μM NE equaled 3.2%, which demonstrates a reasonable repeatability of sensor. Moreover, stability of BFMFCPE was assessed by storage of modified electrode for 21 days for investigating storage stability of the NE sensor. According to the outputs, the current signal after 21 days was 96.7% of its initial value, which illustrates a high stability. It is necessary to mention, after each measurement, the BFMFCPE was polished.

3.9 Calibration plot and detection limit

Preliminary investigations showed that, the Ip of NE can be used to detect NE in solution. It has been found that DPV benefits from the enhanced sensitivity and more acceptable features for analytical utilizations; therefore, this method was used to investigate the linear range of the BFMFCPE (Fig. 13). For this purpose, the BFMFCPE was placed in a series of NE solutions with different concentrations and peak current was measured. According to the findings, electrocatalytic peak current of the NE oxidation at the BFMFCPE surface has two linear dependence on the NE concentration in ranges from 0.3 × 10–7–1.0 × 10–5 to 1.0 × 10–5–5.0 × 10–4 M. Detection limit of NE based on 3 s was 20.0 nM (see Table 1).

Fig. 13
figure 13

The DPV of the BFMFCPE into 0.1 M PBS (pH of 3.0) containing different concentrations of NE (0.03, 0.5, 2.5, 7.5, 10.0, 30.0, 70.0, 100.0, 200.0, 300.0, 400.0 and 500.0 μM). Inset: The peak current plots versus concentration of NE in ranges from 0.03–10.0 to 10.0–500.0 μM

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Table 1 Comparison of the figure of merit of the BFMFCPE compared to other electrodes used in the measurement of NE
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3.10 Interference effect

In the analytical chemistry, the selectivity of sensor is a very important factor. For this assesses, the effect of some annoying species on the measurement of 100.0 μM NE was assessed by DPV under optimum conditions. The possible interfering species were selected in such a way that they could be present either in the pharmaceutical’s product or in the biological fluids accompanying the NE. The obtained data (Figs. 14, S1) showed that tenfold KCl, NaCl, glucose, ascorbic acid, dopamine, uric acid, acetaminophen and tyrosine show no disturbance in the measurement of NE and the change in Ip due to interfere was less than 5%. Therefore, can be said that the electrode has a good selectivity.

Fig. 14
figure 14

The Ip of the BFMFCPE to 120 µM of NE, which is 1.2 mM relative to each disturbing species

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3.11 Real-sample analysis

To evaluate the applicability of the BFMFCPE in determining the true samples, the pre-prepared sample was transferred to electrochemical cell and the amount of NE was measured using a DPV. The standard addition method was used to increase the accuracy of the measurement. The results are given in Table 2 and showed that the recovery percentages are in the range of 96.8–103.6%. Also, the RSD percentages of the method were 3.3% or less, which indicates the high reliability of the BFMFCPE in real samples.

Table 2 Application of the BFMFCPE to detect NE in the real specimen (n = 5)
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4 Conclusions

In this research, a new modified CPE was constructed for the measurement of NE. Modification of the electrode surface using BF and MF nanocomposite increased the electrode response to NE and thus increased the sensitivity of the measurement. The BFMFCPE was successfully applied for the measurement of NE in urine and serum samples. Simplicity, portability and cost-effectiveness are some of the advantages of the developed BFMFCPE.