Introduction

Mercury ion (Hg2+) is one of the most toxic heavy metal ions. Today, with the development of the industrial efforts, the concentration of mercury increases in the environment and possesses a serious threat to the environment and human health [1]. Also, mercury is known as a chemical pollutant because it can be collected through the skin and the gastrointestinal tract animal in human organs and tissues. It must be noted that it also causes kidney and digestive damages and diseases. Therefore, utilization of the sensitive and rapid procedure for detection and measurement of Hg2+ as a toxic substance and a global pollutant is necessary and essential [2].

So far, various analytical techniques have been applied to detection and analysis of Hg2+ which comprise the atomic absorption/emission spectroscopy [3], flow injection technique (FIA), inductively coupled plasma mass spectrometry [4], colorimetry [5], luminescence [6], mass spectrometry [7], atomic fluorescence spectroscopy (AFS) [8,9,10,11] and surface-enhanced Raman scattering (SERS) [12]. While almost all these analytical methods are sensitive, selective and accurate, there are some limitations and/or disadvantages for most of these techniques, such as expensive instrumentation, require pretreatment steps, time-consuming sample preparation steps and large amounts of samples. Furthermore, they are not suitable for daily routine mercury detection. During the last decade, the aptasensor for Hg2+ detection was given special attention due to the aptamers as recognition elements were employed for the development of ultrasensitive and selectivity biosensors [13, 14]. It must be noted that the performance of aptamers led to the measurement of trace amounts of the large number of various analytes. The aptasensors have been formed the stable thymine–Hg2+ and thymine (T–Hg2+–T) complexes based on specifically bind of interaction of two DNA thymine bases with Hg2+ which play key roles in the determination of analyses. Aptasensors based on specifically bind in between two DNA thymine bases have established to form stable thymine–Hg2+−and thymine (T–Hg2+–T) complexes [15]. T–T mismatches capture can be captured Hg2+ through N–Hg covalent bonding. The metal ion-binding constant of T–Hg2+–T is much stronger than that of a nature A–T base pair. So, the (T–Hg2+–T) complexes can form DNA duplex. Therefore, based on T–Hg2+–T coordination in aptasensors architectures, this event widely applied for the determination of trace amounts of Hg2+ [16]. In recent years, researchers have been able to use a special binding with T bases of DNA thymine bases with Hg2+ in the various strategies for the sensor construction, while all of them suffer from complex structures and low probe stability [17]. In recent years, various investigations based on the sensors and biosensors have been applied using of the nanomaterials. In these procedures, it is possible to decrease the overvoltage and to overcome the kinetics of electrode processes could modify the surface of the electrode using of the nanoparticles (NPs). The modified electrodes have advantages such as low background currents, wide potential range, low cost, easy construction and fast surface reducibility. In these electrodes, various modifiers can be utilized to catalyze the oxidation–reduction processes and facilitate the electron transfer. Nanostructures owing to their supernatural physicochemical attributes have gained in the area of biosensing, recently. Among them, NFs have special features such as the large surface area per unit mass, high porosity and high mechanical flexibility.

Electrospinning is a simple, commercial and low-cost procedure for construction of NFs. In this method, NFs are synthesized using the top-down approach. It was worthy to say that NFs are fibers with a diameter of less than 100 nm [18]. The mentioned special features of these materials caused them have been developed in various fields including tissue engineering, drug delivery, membrane, base catalysts, supercapacitors and sensors. The properties of NFs are better than other materials [19, 20].

Quantum dots (QDs) due to the quantum confinement effect and the extent of their energy band gap have useful optical and electrical properties. QDs in electrochemical sensors can facilitate the electron-transfer speed. As a result, these materials owing to the unique properties are employed widely as the amplifier of signal element in electrochemical and electroluminescence sensors [21]. As well as, QDs have widespread utilization in biological and medical applications as immunoassay because they can well bind to biorecognition molecules [22, 23].

In the present work, a new and simple label-free electrochemical aptasensor was fabricated for sensitive and selective Hg2+ detection based on a combination of an electrospun NFs of polyethersulfone with quantum dots (NFs–QDs). Actually, the NFs–QDs composite was used as signal amplification in the reporter signal. In the fabricated aptasensor, the determination of Hg2+ was performed based on the binding of Hg2+ by T bases of DNA and MB. Aptamers immobilized on new platform electroactive NFs and QDs nanocomposite at the CPE surface. Next, the aptasensor was applied for the determination of Hg2+ using DPV technique. The stability, reproducibility and repeatability of the designed aptasensor using DPV procedure were applied to the monitoring of Hg2+ in fruit juice samples.

Experimental

Reagents and chemicals

The polyethersulfone (PES), methylene blue (MB), mercaptoethanol, (MCH), N-ethyl-N′-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC), N,N-dimethylformamide (DMF), potassium ferricyanide (III) and potassium ferrocyanide (II) were purchased from Sigma-Aldrich. Potassium dihydrogen phosphate (KH2PO4), potassium chloride (KCl), potassium hexacyanoferrate (III) (K3[Fe(CN)6]) and potassium hexacyanoferrate(II) (K4[Fe(CN)6]) were obtained from Merck & Co. (Darmstadt, Germany). Double distilled and deionized water was used in all experiments. DNA oligonucleotides with the following sequences were obtained from Fazabiotech Co. (Tehran, Iran):

Aptamer probe [5′-(NH2)-TTT TTT TTT TAC AGC AGA TCA GTC TAT CTT CTC CTG ATG GGT TCC TAT TTA TAG GTG AAG CTG T-3′].

Apparatus and measurements

All the CV, EIS and DPV studies were performed using an Autolab system potentiostat–galvanostat model PGSTAT 204, controlled via a Nova 1.11 software. A platinum wire and a silver–silver chloride (Ag/AgCl, 3 M KCl) electrodes were utilized as auxiliary and reference electrodes, respectively. Individually prepared sensors based on modified and naked carbon paste electrodes (CPEs) were used as working electrodes. Scanning electron microscopy (SEM) images in order to the evaluation of the various electrodes surface were acquired using a Philips instrument, Model XL-30 images. A laboratory-scale electrospinning setup contains a syringe pump with the accuracy of 0.1 ml/h, 23 gage needle, high DC voltage–power supply (0–30 kV). The modified and bare CPE was used as collector.

Synthesis of quantum dot

To the synthesis of QDs, the previous method by Safari et al. [24] has been applied. Sodium hydrogen telluride (NaHTe) was prepared by reducing Te powder with sodium borohydride (NaBH4) in deionized water under stirring conditions along with N2 purging. After 3 h, the produced NaHTe was used to prepare the particles of CdTe. In another flask, 0.3 g of CdCl2·H2O was dissolved in 40 mL of ultrapure water followed by the addition of 200 µL of TGA, while the solution was stirred strongly. The pH of the reaction was adjusted to 10 by adding dropwise of NaOH solution (1 M). In the next step, the freshly prepared NaHTe was added to a CdCl2 solution containing TGA under N2 atmosphere. After mixing, the solution was transferred into a Teflon-lined stainless steel autoclave and heated in an oven at 120 °C for 3 h. After this time, the autoclave was cooled and thiol-capped CdTe QDs could be obtained.

Preparation of electrochemical aptasensor

The carbon paste electrode was prepared by mixing graphite powder (0.7 g) with the appropriate amount of mineral oil (paraffin 0.3 g). The paste was carefully hand-mixed in a mortar, and then, a portion of the composite mixture was packed into the end of a syringe (3.0 mm diameter). Electrical contact was made by forcing a thin copper wire down into the syringe and into the back of the composite. The NFs/CPE was prepared by electrospinning procedure in a 20 wt% PES/ethanol solution. For construction of NFs–QDs/CPE, before the electrospinning process, mixture of 3 wt% PES and 1 wt% QDs was dissolved in ethanol and then the composite solution was sonicated for 30 min with a homogenizer in order to dispersion of them completely. The electrospinning process was performed under electric fields of an order of 100 kV m−1, from a 30 kV voltage applied to a 30 cm gap between the spinneret, while the collector was CPE.

Then, the electrode was immersed in 0.05 M PBS of at pH 7.0 containing 20 mM EDC and 40 mM NHS for 60 min to activate the carboxylic groups at the NFs–QDs/CPE surface. The electrode was washed quickly with PBS, and then, 5 µL aptamer (10 µM, DNA-catcher) was carefully dropped onto the pretreated modified electrode surface and allowed to dry for 1 h. The fabricated aptasensor was soaked in 20 mM MCH for 30 min in order to elimination of the possible nonspecific bands. Finally, the electrode (denoted as c-Apt/NFs–QDs/CPE) was thoroughly rinsed with deionized ultrafiltered water, dried in air and stored at 4 °C in 0.1 M PBS at pH 7.0 when not used. Due to the presence of NH2 group in the 5′ end of the aptamer probe, carboxylic groups of the QDs can form covalence bond with aptamer sequence. In the next step, prepared c-Apt/NFs–QDs/CPE was immersed in 1 mM of MB1 for 20 min at room temperature. It must be noted that the MB can interact with guanine base (Apt–MB1/NFs–QDs/CPE). Afterward, the obtained electrode was washed quickly with PBS. After each time, the prepared aptasensor was placed in solutions containing the variable concentrations of Hg2+. Again, it is immersed in a MB2 solution for 20 min for bonding between the MB and Hg2+ (c-Apt–MB1–Hg2+–MB2/NFs–QDs/CPE) (Scheme 1).

Scheme 1
scheme 1

Schematic illustration for construction of Hg2+ aptasensor

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Results and discussion

Characterization of CdTe QDs

FT-IR spectroscopy was performed to investigate the functional groups of the QDs. Figure S1.A shows the characteristic peaks of TGA-capped CdTe QDs, and the absorption peaks at 1355 and 1581 cm−1 are due to the symmetric and asymmetric stretching of the carboxylate group. Bands at 1222 and 3421 cm−1 are the stretching vibration of C–O and O–H, respectively.

As shown in Fig. S1.B, the CdTe QDs has a fluorescent emission peak at 565 nm with excitation wavelength at 310 nm.

Characterization of surface and electrochemical properties the c-Apt/NFs–QDs/CPE

Figure 1 shows the SEM of the different steps of electrode modification. The surface of the CPE (Fig. 1A) is formed of the separated layers and irregularly graphite flakes with irregularly shaped. Figure 1B shows the SEM image of NFs–QDs with a diameter about 130 nm, illustrated obviously the three-dimensional (3D) fibers with a network-like structure on the electrode surface due to the spinning step. The energy-dispersive X-ray spectroscopy (EDX) and scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX) images are shown in Fig. 1C. The EDX and EDX/SEM clearly indicate the presence of different percentages of C, O, S, Cd and Te at the prepared composite electrode.

Fig. 1
figure 1

SEM images of CPE (A) and NFs–QDs/CPE (B). EDX spectrum and mapping recorded at NFs–QDs/CPE elemental different showing signal of C, O, S, Cd and Te, respectively, as is illustrated (C)

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The NFs–QDs/CPE surface with proposed modifiers was well covered which led to increase the electroactive sites and so aptamer immobilization.

In order to the investigation of the variations have occurred between the interface of surface of electrode and solution, the EIS procedure is often utilized. In EIS measurements, mainly was used from [Fe(CN)6]3−/4− as a probe. In these measurements, the semicircle diameter of impedance is equal to the electron-transfer resistance (Rct), which controls the electron-transfer kinetics of the redox probe on the electrode surface. The changes in the value of Ret are associated with the behavior of the features of modification on the electrode surface, which is reflected in the EIS as a change in the diameter of the semicircle at high frequencies [25].

Figure 2A introduces the Nyquist plots of the electrodes in each step of the modification process. The Nyquist plot of the bare electrode displayed a relatively small electron-transfer resistance (Ret) (curve a). The NFs were placed spun on the CPE surface, and the Ret value of the CPE/NFs increased in the Nyquist plot (curve b). After modification of CPE with NFs–QDs composite, the value of Ret decreased (curve c), which indicated the conductivity of NFs–QDs and so, its electron-transfer rate and conductivity were increased at the CPE surface. In the next step, the successful immobilization of the c-Apt on the NFs–QDs/CPE was explored (Fig. 2A, curve d). It is shown upon immobilization of c-Apt, and the Ret value largely increased (Fig. 2A, curve d).

Fig. 2
figure 2

Nyquist diagrams (A) and cyclic voltammograms (B) recorded in the solution 0.1 M KCl 0.5 mM [Fe(CN)6]3/4− for CPE (a), CPE/NFs (b), CPE/NFs–QDs (c) and CPE/NFs–QDs/DNA-c (d)

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This signal enhancement could be ascribed to the covalent attachment of NH2 group in the 5′ end of the Apt probe with carboxyl groups of NFs–QDs on the surface electrode.

Also, Fig. 2B shows the CVs investigation of the electrodes surface in each step of the modification process. The CV of the CPE depicted in curve a, with the peak potential separation (ΔEp = Ep,a − Ep,c) of 300 mV, which shows the quasi-reversibility behavior of the modifier at the surface of the sensor. After placing the NFs on the CPE, the CV peak current exhibited a decline (curve b), resulting in the increased resistance. When NFs–QDs composite was placed on the CPE, due to the permeability of QDs in the composite the peak current of CV increased, which facilitated the electron transfer to the electrode surface. The peak current of NFs–QDs/CPE confirms the conductivity was increased for the suggested electrode, and ΔEp was reduced due to the increasing porous structure with a large effective surface. The increase in effective surface will increase the ability to detect and determine the immobilized elements on the modified layer and increase the c-Apt. The value of surface area for CPE/NFs–QDs was calculated according to the Randles–Sevcik equation (Eq. 1) [26]

$$I_{\text{p}} = 2.69 \times 10^{5} \,n^{2/3} AD^{1/2} v^{1/2} C_{\text{o}}$$
(1)

D and C are the diffusion coefficient (cm2 s−1) and concentration (mol cm−3) of Fe (CN) 3−/4−6 , respectively. The estimated values of surface areas (A) for the bare CPE and NFs–QDs/CPE were 0.02 and 0.12 cm2, respectively. The obtained results of NFs–QDs/CPE indicate the NFs–QDs composite can be increased the active sites and so improved the electron-transfer kinetics.

Also, as it can be seen, the curve peak current of c-Apt/NFs–QDs/CPE significantly decreased and showed that the aptamer was successfully immobilized on NFs–QDs/CPE surface (Fig. 2B, curve d).

The surface coverage (Γ) of c-Apt was estimated using the charge Fe (CN) 3−/4−6 oxidation peak in cyclic voltammograms (CVs) at scan rates (20–100 mV s−1) [according to Eq. (2)]

$$I_{\text{p}} = n^{2} F^{2} \upsilon A \, \varGamma_{\text{c}} /4RT.$$
(2)

In the above equation, the υ is the sweep rate, n is the number of electrons transferred per redox event (n = 2 For Fe (CN) 3−/4−6 ), F is the Faraday current, and A is the electrode area. The average probe coverage was 8.4 × 10−7 mol cm−2. A surface coverage amount of 3.34 × 10−9 mol cm−2 [27], 3.9 × 10−9 mol cm−2 [28], 9.67 × 10−10 [29] and 3.43 × 10−11 mol cm−2 [30] has been reported that in this work, the amount of c-Apt immobilization on NFs–QDs is higher than other reported article.

Aptasensor mechanism

MB as methylthioninium chloride in aptasensor structure was applied as the electrochemical indicator [31]. The MB has a strong tendency to connect with guanine bases and also to Hg2+ ions [32, 33]. The electrochemical behavior of the c-Apt/NFs–QDs/CPE in different solutions was investigated by DPV method. Figure 3 shows when the c-Apt/NFs–QDs/CPE was placed in the MB1 solution, the MB can be interacted with to the guanine bases. The MB peak appears at − 0.24 V (curve a), and all guanine bases are connected to MB1. When the c-Apt–MB1/NFs–QDs/CPE is placed in the MB2 solution (without the presence of Hg2+ ions), there is no significant change in curves a and b as shown in Fig. 3. When the c-Apt–MB1/NFs–QDs/CPE was immersed in the solution containing Hg2+ ions and then in MB2 solution, the DPV current of MB was increased dramatically (curve c). In addition to the thymine bases, the guanine-MB in the string of the DNA has high affinity to connect with Hg2+ ions. In general, this strategy causes increase in the Hg2+ ions receiver in the string of the DNA, and thus, the sensitivity of the sensor to mercury increases significantly. As a result, the presence of QDs in nanocomposite along with the NFs can enhance the MB signal.

Fig. 3
figure 3

DPVs of c-Apt/NFs–QDs/CPE in the absence of Hg2+ and MB2 (a), in the absence of Hg2+ and presence of MB2 (b) and in the presence of Hg2+ and MB2 (c) in a solution of 0.1 M PBS at pH 7

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Optimization of Hg2+ aptasensor conditions

For improving the aptasensor sensitivity in order to the Hg2+ ion detection, the experimental conditions were optimized using c-Apt concentration and time of reaction between Hg2+ and c-Apt. To optimize the c-Apt immobilization, the solutions containing different concentrations of the c-Apt were utilized and then NFs–QDs/CPE was immersed in these solutions for a given time. Figure 4A shows the effect of the different concentrations of immobilized c-Apt at the sensor surface using of the DPV signal to 100 nM Hg2+. As a result, the highest current in 5 µM of the c-Apt concentration was observed and then, as it can be seen, with increase in the immobilization c-Apt concentration at the surface of the electrode, the current of MB unchanged. The reason for this observation is probably the saturation of the electrode surface. So, concentration 5 µM was chosen as the optimum value for c-Apt.

Fig. 4
figure 4

DPVs of c-Apt/NFs–QDs/CPE recorded in different optimization steps. A Immobilization of probe aptamers, B, C incubation concentration and incubation time of the MB1 and the guanine base of aptamer sequence, D incubation time of 100 nM Hg2+ with the c-Apt and E incubation time of c-Apt–Hg2+ with the MB2 in a solution of 0.1 M PBS at pH 7

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The influence of MB1 incubation concentration on the aptasensor operation and response of it was investigated between 0.2 and 1.4 mM. The DPV signal was recorded in the PBS (0.1 M, pH 7). Based on the generated data (Fig. 4B), it can be found that by increasing an incubation concentration up to 1 mM, current value was enhanced. Afterward, at higher concentration, the current values were unchanged. According to the results acquired, a concentration of 1 mM was chosen as the best favorable incubation 1 mM for MB1 in subsequent investigations.

The amended electrode was dipped in a 1 mM solution of MB1 for different time durations from 10 to 30 min, and then, the DPV signal to MB1 was reported in the PBS (0.1 M, pH 7) as shown in Fig. 4B. Based on the generated data (Fig. 4C), it can be understood that the MB1 and the guanine base of aptamer sequence were contacted at the surface of sensor, and so the incubation time will be increased, up to duration of 20 min, and next at lengthy duration times stayed approximately constant. According to the results acquired, a period of time of 20 min was chosen as the best favorable incubation time of MB1 to the guanine base for subsequent investigations.

In the next investigation, the incubation time of 100 nM Hg2+ with the c-Apt was studied in the following values of 10, 20, 30 and 40 min (Fig. 4D). The experimental results show that the current of MB2 increased with the enhancement of the incubation time of Hg2+ up to 40 min, and the MB signal did not improve obviously the response with more enhancement of the response obviously with longer incubation time (Fig. 4D). So, 40 min was selected as the optimal incubation time for measurement of Hg2+.

As well as, the incubation time of c-Apt–Hg2+ with the MB2 was evaluated in the values of 10, 20, 30 and 40 min (Fig. 4E) during experiment. The evaluation outcomes are illustrated in Fig. 4D, so the current of MB2 increased with the increase in incubation time of Hg2+ up to 20 min and more enhancement of incubation time did not improve the response obviously (Fig. 4E), and this shows that after 20 min, the T–Hg2+–MB complex formation process was completed. So, 20 min was selected as the optimal incubation time for c-Apt–Hg2+ with the MB2.

Analytical performance of Hg2+ the aptasensor

To assess the quantitative analysis of Hg2+, the sensitivity of Hg2+ aptasensor was investigated by the proposed procedure electrode, under the optimized experimental conditions in PBS (pH 7, 0.1 M) by DPV procedure. Figure 5 shows the current responses of the c-Apt/NFs–QDs/CPE toward the different concentrations of Hg2+. As it can be seen, the current increased gradually with the increase in the Hg2+ concentration and so the current versus the concentration of Hg2+ in the range of 0.1–150 nM was linear and the correlation coefficient was 0.9962. Limit of detection (3Sb/m) was calculated as 0.02 nM where Sb is the standard deviation of blank (n = 6) and m is the slope of the calibration curve, which is comparable to the reported detection limit using other aptasensors.

Fig. 5
figure 5

A DPVs of c-Apt/NFs–QDs/CPE recorded in different concentrations of Hg2+ and B the linear relationship between current versus concentration of Hg2+ concentration in the range 0.1–150 nM

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These results depicted the good affinity of the offered aptasensor toward Hg2+, which found to be improved or comparable procedure than those of the previously reported electrochemical assay methods to Hg2+ (Table 1).

Table 1 Comparison of analytical properties of different assays toward Hg2+
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The defined architecture for this sensor was simple and does not need complicated interpretation in comparison with the other reported sensor, and also, it can be used for rapid detection of mercury Hg2+ ion in real samples.

Reproducibility, stability and specificity

Reproducibility of the c-Apt/NFs–QDs/CPE was also investigated. The current response of six electrodes in a PBS solution (pH 7, 0.1 M) was recorded. The relative standard deviation (RSD) of the six measurements for Hg 2+ is 2.87% detection. The results of the RSD were showed the high reproducibility of the c-Apt/NFs–QDs/CPE.

The repeatability of the aptasensor was evaluated by detecting the DPV response of the six measurements for Hg 2+ (50 nM). The RSD was obtained 2.16%. The results of the RSD were showed the high repeatability of the c-Apt/NFs–QDs modified CPE.

For investigating the specificity of the proposed aptasensor, the response was evaluated in the presence of various types of metal ions, while a sample did not contain the Hg 2+ as a blank control. No significant DPV response change was observed in the presence of metal ions (Fig. 6).

Fig. 6
figure 6

Selectivity of the constructed aptasensor. The DPV signal changes in the aptasensor in the presence of 50 nM Hg2+ and various concentrations of other metal ions (100 nM)

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This behavior can be contributed to the nature of MB, as an intercalator, and its interaction with DNA strands. In fact, as many studies have been suggested, the specific complexation of Hg2+ between thymine (and even guanine bases) bases and the MB molecules can be resulted in the maximum affinity in comparison with other metal ions [34,35,36,37,38,39]. Based on these studies, the interaction between the Hg2+ and DNA (aptamer) and MB has a strongest affinity (specificity) among cations like Fe3+, Cd2+, Pb2+, Ag+, Sn4+, Ni2+, Mn2+, Cu2+, etc. Hence, these interactions between aptamer–analytes and MB, as best electrochemical probe, have significantly increased the specificity of the as-prepared sensor toward the Hg2+.

The stability of the c-Apt/NFs–QDs/CPE was also investigated. The DPV signal of the aptasensor in a PBS solution (pH 7, 0.1 M) was recorded. After 15 days, 73.5% of its initial current response was remained compared with the current response of freshly prepared electrode. The constructed sensor was stored in a refrigerator at 4 °C during the experimental phase.

Application of aptasensor in fruit juice samples

The practical application of the proposed aptasensor in the laboratory was evaluated by measuring Hg2+ in fruit juice samples. For the preparation of the juice samples, at first, different fruits were prepared and then the fruit juice was collected by the juggernaut. Afterward, the 10 mL of juices was centrifuged for 10 min at 6000 rpm. Then, a clear layer in the glass tube was obtained. The clear solution was filtered by a microfilter. Finally, the fruit juice samples were spiked with different concentrations of Hg2+ and they were diluted for 10 times with distilled water. The concentration of Hg2+ in the fruit juice samples was obtained by the standard addition method. The results of the analysis of aptasensor are summarized in Table 2. The recovery of the Hg 2+ was obtained to be between 104.93 and 95.84%, which indicated the proposed aptasensor has a good precision for Hg 2+ monitoring in real samples.

Table 2 Determination of Hg2+ in fruit juice samples (n = 3)
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Conclusions

In this work, an efficient and new label-free electrochemical Hg2+ aptasensor was introduced for the highly accurate and precise detection of Hg2+ by using MB as a signal reporter. In this method, Hg2+ establishes a strong and special bond between T bases of DNA and MB. The introduced Hg2+ aptasensor was fabricated based on c-Apt/NFs–QDs modified carbon paste electrode. Also, the proposed modifier with the porous structure and large surface areas could improve the electrochemical behavior of the electrode to analyte detection. The MB signal was proportional to the concentration of Hg2+. The QDs in the nanocomposite have also greatly increased the electron-transfer speed. The proposed aptasensor was demonstrated with a linear range from 0.1 to 150 nM, detection limit of 0.02 nM. Also, good specificity and stability could be easily extended (Table 1).