Miniaturized device with a detachable three-electrode system and vibration motor for electrochemical analysis based on disposable electrodes

Miniaturized device with a detachable three-electrode system and vibration motor for electrochemical analysis based on disposable electrodes

Sensors & Actuators: B. Chemical 297 (2019) 126719 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www...

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Sensors & Actuators: B. Chemical 297 (2019) 126719

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Miniaturized device with a detachable three-electrode system and vibration motor for electrochemical analysis based on disposable electrodes Wan Wang, Yanling Fu, Qing Lv, Hua Bai, Haiyu Li, Zhijuan Wang, Qing Zhang

T



Chinese Academy of Inspection and Quarantine, 11 Ronghua South Road, Beijing, 100176, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Disposable electrode Detachable three-electrode system Vibration motor Voltammetric detection Bisphenol A

This paper reports a miniaturized device for electrochemical analysis based on disposable electrodes. The device was integrated with a detachable three-electrode system and a vibration motor. A disposable and easy to fabricate Au-film electrode was used as working electrode. The Pt wire auxiliary electrode and Ag/AgCl wire reference electrode used in the reported system were detachable and reusable. A coin vibration motor was integrated to provide convection conditions in the accumulation step to enable rapid and sensitive analysis. Using bisphenol A (BPA) as a model analyte, the performance of the proposed device was demonstrated. The linear range and limit of the detection of BPA under optimal condition were 50–1000 and 30 μg L−1, respectively. The reduction in accumulation time by 25% through the introduction of vibration greatly improved the analysis efficiency. The device was applied to quantify BPA in real samples and the results were in agreement with those obtained by UPLC–MS.

1. Introduction Recently, the use of disposable electrodes, which are inexpensive, easy to mass produce, and simple to pretreat, has become increasingly popular in electrochemical analysis [1–4]. Currently, the most widely used disposable electrodes include screen-printed electrode (SPE) [5–7] and metal thin-film electrode [8–10], both of which have sheet-like or planar shape. However, the application of these disposable electrodes continues to feature several limitations. For example, the operation of disposable electrodes in a conventional electrochemical cell equipped with a magnetic stirrer or mechanical stirrer [5,11,12] may weaken the feasibility of using disposable electrodes in practical application that requires convection. Otherwise, high sensitive electrochemical analysis will require a long diffusion time (accumulation time) in the absence of convection [13]. In this study, we developed a miniaturized device for the application of disposable electrodes. A vibration motor was utilized to generate stable convection conditions and enable rapid and sensitive measurement. Furthermore, a detachable three-electrode system that simplified instrumentation and facilitated electrode maintenance and displacement was integrated in the device. To demonstrate its effectiveness, voltammetric detection of bisphenol A (BPA) by the proposed device was optimized. BPA is an important plasticizer material and has been extensively utilized for



manufacturing plastics [14]. Nevertheless, despite its beneficial applications, BPA is an endocrine-disrupting chemical that may exert detrimental effects on humans even at trace amounts [15,16]. Hence, the sensitive determination of BPA is a major concern. Several analytical techniques, such as liquid chromatography–mass spectrometry [17], gas chromatography–mass spectrometry [18], spectrophotometric method [19], and immunoassay [20] have been developed for determination of BPA. Electrochemical analysis is highly useful as an alternative method for BPA determination because it is sensitive, easy to operate, and suitable for on-site determination [21]. Using BPA as a model analyte, the proposed device showed a limitof-detection of 30 μg L−1 with the sample solution volume of 50 μL. The accumulation time required to achieve the same sensitivity in the presence of vibration was only one quarter of that in the absence of vibration. The proposed device may serve as a useful platform for improving the applicability of disposable electrodes. 2. Experimental 2.1. Instruments and reagents All electrochemical experiments were performed by using a CHI 660C electrochemical working station (CH Instrumentation, China). Measurements were carried out with the miniaturized device that

Corresponding author. E-mail addresses: [email protected], [email protected] (Q. Zhang).

https://doi.org/10.1016/j.snb.2019.126719 Received 14 March 2019; Received in revised form 29 May 2019; Accepted 20 June 2019 Available online 21 June 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.

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working electrode connection was also assembled in the middle layer. The connection layer for electric conduction was fabricated by using a printed circuit board (PCB). Meanwhile, a coin vibration motor was installed into the bottom layer of the device. The working electrode was fabricated in accordance with our previous work [13]. As shown in Fig. 1C, Au was sputtered on an art paper substrate for 80 s at a constant current of 30 mA. The Au coated paper was then cut into pieces with the size of 2 cm long and 1.2 cm wide. The working area (10 mm diameter) on the electrode was defined with a piece of insulating tape. Thereafter, a 50 μL aliquot of the aqueous dispersion of 0.1% MWCNTs was dropped on the hole and dried at 50 °C. The as-prepared MWCNT/Au/paper electrode was covered with a piece of filter paper and then integrated into the miniaturized device prior to performing electrochemical analysis.

consisted of a disposable electrode as the working electrode, a Pt wire (0.5 mm diameter) as the auxiliary electrode, and an Ag/AgCl wire (0.5 mm diameter) as the reference electrode. The Au-film substrate was fabricated by using a 108Auto automatic sputter coater (Cressington, UK). The plastic components of the miniaturized device were manufactured by using a N2-Plus 3D printer (Raise3D, China). For comparison tests, an ACQUITY UPLC coupled with Xevo TQ MS tandem quadrupole mass spectrometry (UPLC-MS) system (Waters, USA) was used to measure BPA in water and plastic samples. BPA was purchased from Sigma (USA). An aqueous dispersion of multi-walled carbon nanotubes (MWCNTs) with a weight ratio of 9.4% and an outer diameter of 50 nm was purchased from Nanjing XFNANO Materials Tech Co., Ltd. (China) and then diluted with water to a weight ratio of 0.1% before use. The coin vibration motor (10 mm diameter; model number: 1034; construction: permanent magnet) was purchased from Leader Microelectronics Co., Ltd. (China). Whatman grade 1 filter paper was purchased from GE Whatman (Buckinghamshire, UK). All other chemicals were of analytical grade and used without further purification.

2.3. Sample pretreatment Polypropylene (PP) and polyvinylchlorid (PVC) plastic samples containing BPA were purchased from Lung Cheong Institute of Intelligent Technology (China). The plastic samples were shattered into small pieces, and then the plastic pieces (1.0 g) were mixed with 10% ethanol (20 mL). Then, the mixture was agitated for 48 h at 70 ± 2 °C, cooled, passed through a membrane filter, and diluted with 0.05 mol L−1 phosphate buffer (pH 7.0, containing 0.1 mol L−1 KCl). A water sample was collected from the lake in Beihai Park in Beijing and was analyzed no later than 24 h after collection. It was filtered by using a 0.45 μm membrane filter and then diluted with phosphate buffer to the concentration of 0.05 mol L−1. Its pH value was adjusted to 7.0.

2.2. Fabrication of the miniaturized device The schematic representation and photography of the proposed device are shown in Fig. 1A and B, respectively. This device consisted of four main components, namely, the top, connection, middle, and bottom layers (from top to bottom). The top, middle, and bottom layers were made of polylactic acid plastic by using 3D printing technology. A Pt wire auxiliary electrode and an Ag/AgCl wire reference electrode were assembled in the middle layer through jacks. A joint for the

Fig. 1. Schematic representation (A) and photography (B) of the miniaturized device. (C) Schematic illustration of the process for fabrication of the working electrode. 2

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MWCNT dispersion on the oxidation peak current of 500 μg L−1 BPA. The electrochemical response sharply increased as the MWCNTs content reached 0.1%. When the MWCNTs content was higher than 0.1%, the oxidation peak current decreased significantly. This result might be attributed to the enhancement of the interface electron resistance as a result of the increasing thickness of the MWCNT layer [24]. Thus, the MWCNT dispersion with the weight ratio of 0.1% was employed in the following experiment. The vibration condition was then optimized. The vibration strength of the coin vibration motor could be adjusted by the applied voltage. Fig. 3B shows the influence of the applied voltage of the vibration motor on the oxidation peak current of 500 μg L−1 BPA. The peak current increased linearly with motor voltage to 3 V and then decreased slightly. Therefore, the applied voltage of 3 V was used in the following experiment.

2.4. Voltammetric measurements A 50 μL aliquot of the sample solution was dropped on the working electrode, which was covered with a round piece of filter paper, and then analyzed through linear sweep voltammetry (LSV). The scan window of the potential ranged from 0.2 V to 0.8 V and the scan rate was 50 mV s−1. The open-circuit accumulation time was 60 s with continuous vibration. Before each measurement, the reference electrode, counter electrode and the joint for the working electrode connection were washed with double distilled water. 3. Results and discussion 3.1. Design of the proposed device The device is portable and cost effective given that it was only 6.5 cm long, 2 cm wide, and 2.5 cm high and weighed only 21 g. The proposed device had the following features: (i) It has a detachable counter electrode and reference electrode that are durable and can be used repeatedly. (ii) It can be generally fitted with any planar electrode, such as screen-printed electrodes, although the MWCNT-modified Aufilm paper electrode was used as the working electrode in the present work. (iii) It can enable electrochemical analysis with a sample volume of only tens of microliters through the replacement of the conventional electrochemical cell with a piece of filter paper, which can prevent the solution spreading out of the electrode. (iv) It can considerably shorten accumulation time by offering convection for voltammetric analysis.

3.3. Effect of the vibration on the voltammetric detection of BPA In voltammetric analysis, accumulation time has a considerable effect on the amount of analyte that absorbed on the surface of the electrode and, correspondingly, on the sensitivity of electrochemical detection [25–27]. Fig. 4A shows the influence of the open–circuit accumulation time on the oxidation peak currents of 500 μg L−1 BPA in the absence and presence of vibration condition. The oxidation peak current of BPA increased with the increase in accumulation time within 0–420 s when vibration was disabled in the device. No significant increase in the peak current was observed when the accumulation time exceeded 420 s, which might be attributed to the saturated adsorption of BPA on the surface of the working electrode. The peak current of BPA also increased gradually as accumulation time increased to 240 s and then leveled off in the presence of vibration. In addition, the reduction in the time required to achieve the saturated adsorption of BPA demonstrates that vibration may accelerate the mass transport of BPA from the bulk solution to the electrode surface [28]. Furthermore, the peak current with vibration was considerably higher than that without vibration with the same accumulation time. The enhancement factor (F) was defined as the ratio of the peak current with or without vibration with the same accumulation time for comparison:

3.2. Optimization of the experimental conditions First, cyclic voltammetry (CV) was performed to assess the electrochemical behavior of BPA at the working electrode. Fig. 2 displays the CVs of the working electrode in the absence and presence of 5 mg L−1 BPA solution. Upon the addition of BPA, an anodic peak at ca. 0.52 V was observed. This peak was ascribed to the oxidation of BPA. The absence of a corresponding cathodic peak indicates that the electrochemical oxidation of BPA at our working electrode is irreversible [22,23]. Then, several experimental conditions were optimized. The surface of the electrode was modified with MWCNTs given their unique properties, such as large surface area, excellent electron conductivity, and adsorption ability. Fig. 3A shows the effects of the weight ratio of the

F=

Ip (vibration) Ip (static )

(1)

Fig. 4B shows the relationship of the enhancement factor versus the accumulation time. The maximum F was obtained at the accumulation time of 60 s. The accumulation time of 60 s was selected for the following experiment in consideration of sensitivity and analytical efficiency. Here, 240 s was regarded as the accumulation time required to achieve high sensitivity in the absence of vibration. 3.4. Voltammetric determination of BPA The performance of the proposed device for BPA determination under optimized conditions was demonstrated on the basis of LSV measurements. Fig. 5A illustrates the typical LSV curves for BPA solutions with various concentrations. The oxidation peak current of BPA showed a regular increment over the concentration range of 50–1000 μg L−1. Fig. 5B presents the typical calibration plots of peak current versus BPA concentration. The regression equation is as follows: I (μA) = 0.0025C (μg L−1) + 0.2541 (R2 = 0.995). The detection limit was estimated to be 30 μg L−1 (based on the three-fold the signal-tonoise ratio), which meets the supervision requirements of some legislations [29]. The relative standard deviations (RSDs) for three replicate measurements at various concentrations were between 1.2% and 7.9%. To investigate the sensor-to-sensor reproducibility of the developed sensor, a fresh batch of twenty independent disposable working electrodes was prepared and examined. The RSDs of these electrodes for the

Fig. 2. CVs recorded at first sweep at the working electrode in the absence (curve a) and presence (curve b) of 5 mg L−1 BPA in 0.05 mol L−1 pH 7.0 phosphate buffer containing 0.1 mol L−1 KCl. Scan rate: 50 mV s−1. 3

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Fig. 3. Effects of the weight ratio of MWCNTs dispersion (A) and the applied voltage of the vibration motor (B) on the oxidation peak currents of 500 μg L−1 BPA (n = 3). Buffer solution: 0.05 mol L−1 pH 7.0 phosphate buffer containing 0.1 mol L−1 KCl. Scan rate: 50 mV s−1. Adsorption time: 60 s with vibration.

detection of 500 μg L−1 BPA was 9.7%. These results indicate that the proposed device has good reproducibility. The possibility of the proposed sensor to perform repeated assays was also evaluated. The results showed that the disposable sensor remains 82% of its activity after eight successive measurements with vibration and remains 85% of its activity after ten successive measurements without vibration. 3.5. Interference study The selectivity of the proposed device for BPA in the presence of several possible interfering substances, including K(I), Ca(II), Na(I), Mg (II), Cu(II), Fe(III), Zn(II), bisphenol AF, pentachlorophenol (PCP), pnitrophenol (p-NP), catechol, hydroquinone, and 4-nitroaniline, was investigated. As shown in Fig. 6, the peak current change observed in the determination of 500 μg L−1 of BPA caused by 50-fold of Ca(II), Na (I), Mg(II), Zn(II), p-NP, catechol, and 4-nitroaniline was lower than 5%. The peak current change caused by 50-fold of K(I), Cu(II), Fe(III), bisphenol AF, PCP, and hydroquinone was between 5% and 10%. Thus, these interfering substances do not have significant influence on the voltammetric response of BPA. 3.6. Real sample application The device was utilized to determine BPA in plastic and lake water samples to challenge the practicability of the proposed device with real samples. The results obtained by the device were compared with those obtained through the UPLC–MS method. These results are summarized in Table 1. BPA was detected in PVC and PP plastic samples. F-test revealed that the results obtained by the two different methods had no significant difference. For further validation of the method, Real samples were spiked with known amounts of BPA and analyzed to validate the method further. The obtained recoveries were in the range of 93.0%–116.4%. The RSD of three successive measurements of the same sample was below 8.2%. These results suggested that the proposed device has potential practical applications. 4. Conclusion In conclusion, we introduced a portable device for electrochemical measurements based on disposable electrodes. This device was integrated with a detachable three-electrode system and a vibration motor. It is easy to fabricate, cost effective and can be fitted with any planar electrode. The vibration function of our device could provide stable convection conditions during the adsorption step. The accumulation time in voltammetric detection of BPA with vibration decreased

Fig. 4. (A) Influence of the open-circuit accumulation time on the oxidation peak current of 500 μg L−1 BPA with and without vibration. (B) Relationship of enhancement factor versus the accumulation time. Buffer solution: 0.05 mol L−1 pH 7.0 phosphate buffer containing 0.1 mol L−1 KCl. Scan rate: 50 mV s−1.

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Fig. 5. (A) Typical LSV curves for BPA solutions with various concentrations (a–f: 50, 100, 300, 500, 700, 1000 μg L−1). (B) Calibration plots of peak current versus BPA concentration. Buffer solution: 0.05 mol L−1 pH 7.0 phosphate buffer containing 0.1 mol L−1 KCl. Scan rate: 50 mV s−1. Adsorption time: 60 s with vibration.

Fig. 6. Interference of other chemicals on 500 μg L−1 BPA. The concentration of other chemicals was 25 mg L−1. Table 1 Analytical results on real samples (n = 3). Sample

Content (μg L−1) Present method

Lake water PVC plastic PP plastic a

0 261.0 ± 21.3 2272 ± 39.0

Spiked (μg L−1)

Found (μg L−1)

Recovery

100 100 1000

110.9 ± 11.1 377.4 ± 17.7 3202 ± 74

110.9 116.4 93.0

a

(%)

UPLC-MS 0 247.7 ± 15.2 2172 ± 22.5

Results for present method.

online version, at doi:https://doi.org/10.1016/j.snb.2019.126719.

to 25% compared with that without vibration. The practical application of the developed device in the detection of BPA in real water and plastic samples was demonstrated. The proposed device provides a promising platform for rapid detection of BPA and could be further applied for other practical applications based on disposable sensors.

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