- Email: [email protected]
bismuth composite electrodes
Accepted Manuscript Title: A Miniaturized and Flexible Cadmium and Lead Ion Detection Sensor Based on Micro-Patterned Reduced Graphene Oxide/Carbon Nanotube/Bismuth Composite Electrodes Author: Xing Xuan Jae Y. Park PII: DOI: Reference:
S0925-4005(17)31468-5 http://dx.doi.org/doi:10.1016/j.snb.2017.08.046 SNB 22913
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
18-1-2017 27-7-2017 4-8-2017
Please cite this article as: X. Xuan, J.Y. Park, A Miniaturized and Flexible Cadmium and Lead Ion Detection Sensor Based on Micro-Patterned Reduced Graphene Oxide/Carbon Nanotube/Bismuth Composite Electrodes, Sensors and Actuators B: Chemical (2017), http://dx.doi.org/10.1016/j.snb.2017.08.046 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A Miniaturized and Flexible Cadmium and Lead Ion Detection Sensor
Based
on
Micro-Patterned
Reduced
Graphene
Oxide/Carbon Nanotube/Bismuth Composite Electrodes Xing Xuan and Jae Y. Park*
ip t
Department of Electronic Engineering, Kwangwoon University, 447-1, Wolgye-dong, Nowon-gu, Seoul,
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139-701 Republic of Korea
Abstract
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We proposed and fabricated a miniaturized, flexible, and fully integrated electrochemical sensor incorporated with micro-patterned reduced graphene oxide (rGO) and a carbon nanotube (CNT) composite working electrode on a flexible gold substrate. By in situ plating bismuth film, the fabricated
an
sensor exhibited well-defined and separate stripping peaks for cadmium (Cd) and iron (Pb) ions, respectively. The CNT was mixed with the rGO in order to improve the performance of the sensor by increasing the electrode surface area. Several experimental parameters, including an electrolyte
M
environment and electrodeposition conditions, were also carefully optimized to achieve the best stripping performance. Under optimal conditions, high sensitivities of 262 nA/ppbcm2 (Cd) and 926 nA/ppbcm2 (Pb) along with favorable detection limits of 0.6 ppb (Cd) and 0.2 ppb (Pb) were obtained.
d
The sensor exhibited good linear responses to both ion types in the concentration range of 20 ppb to
Ac ce pt e
200 ppb. Due to the enlargement of the electrode surface area, the determination efficiency towards target ions was significantly enhanced by the developed Au/rGOCNT/Bi modified electrode. The developed sensor shows high sensitivity, stability, and reliability for the detection of the target heavy metal ions. Finally, the fabricated sensor was successfully demonstrated to detect the target metal ions in drinking water samples with satisfactory results.
Keywords
Flexible sensor, sensitivity, cadmium and lead ion detection, reduced graphene oxide/carbon nanotube composite, integrated electrochemical sensor, microfabrication
*
Corresponding author. Tel.: +82-2-940-5113; Fax: +82-2-942-1502. E-mail address: [email protected] (J. Y. Park)
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1. Introduction Determining the presence of toxic heavy metals is important because of their quick accumulation and toxicity in the human body [1]. Among the heavy metals, Cd and Pb have been particularly recognized due to their human toxicity and related environmental pollution [2, 3]. Nowadays, lakes,
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rivers, and tap water are considered to be natural reservoirs supplying fresh water to humans and ecosystems. Hence, the continuous monitoring of the contamination level of heavy metal ions in crucial water supplies is needed to quickly identify pollution and minimize the risk of toxicity. In
cr
addition, on-site field measurement requires a compact sensing system that can be easily and automatically operated without complicated accessories. Therefore, the development of easy to use, miniaturized, sensitive, and flexible sensors that can be integrated into the sensing system is a
us
primary need since conventional sensing techniques such as atomic absorption spectrometry (AAS) and inductively coupled plasma atomic emission spectrometry (ICP-AES) are not suitable [4, 5].
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Furthermore, conventional sensing techniques are complicated, expensive, and require a large machine to be of use. To address these issues, electrochemical sensors that can be easily operated have been developed for the detection of heavy metal ions. Furthermore, anodic stripping
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voltammetry (ASV) has proven to be a powerful tool for sensing heavy metal ions due to its high sensitivity, easy operation, and low cost of analysis [6-8].
In most cases, mercury film-based sensors are preferred due to their excellent stripping
d
characteristics [9-11]. However, the toxicity of mercury makes it unsuitable for the previously mentioned design [12]. Bismuth, an environmentally friendly element, provides an alternative
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electrode material for the application of heavy metal detection due to its attractive electrochemical features including reproducible stripping behavior, broad linear range, and good signal-to-background ratio [13, 14]. Bismuth can be deposited on many electrode substrates, including glassy carbon electrodes, screen printed electrodes, some noble metals, and graphene-based modified electrodes [15-17]. Among these, graphene-based modified electrodes have been identified as a promising alternative due to their unique electrochemical and mechanical properties and nano-structure [18-21]. Recently, reduced graphene oxide (rGO) was introduced for constructing enhanced electrochemical sensors due to its high theoretical surface area and applicability for the production of electrochemical-sensors and biosensors [22-25]. Among different synthesis techniques, the solvothermal reduction of graphene oxide is most attractive due to its simple setup, sound scalability, and the ability to recover π –conjugation networks at a high temperature and pressure without toxic materials [26, 27]. The performance of an electrochemical sensor depends upon the surface area of the working electrode. However, a graphene-based electrochemical sensor is usually hindered by the fact that graphene sheets tend to aggregate and restack during processing and the actual accessible surface area of the electrodes is much lower when compared to the theoretical surface area. One of the effective strategies to avoid this problem is the addition of spacers such as carbon nanotubes (CNT) between the graphene sheets to prevent their restacking.
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In this work, a reliable and simple fabrication process was newly developed for a micro- patterned rGOCNT electrode modified sensor on a flexible substrate. The advantages of this process include reduced sensor size, reduced sample volume, reduced cost. In addition, geometrically identical, uniform, and well-defined electrode surface structures can be produced. For sensing heavy metals accurately, the proposed composite working electrode was micro-patterned and plated with Bi film by an in situ approach. The Bi-modified working electrode (Au/rGOCNT/Bi) was measured and
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characterized by evaluating the stripping performance for simultaneous analysis of Cd and Pb ions via square wave anodic stripping voltammetry (SWASV). Furthermore, the developed flexible sensor was
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evaluated and was demonstrated to determine Cd and Pb ions in drinking water samples.
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2. Experimental 2.1. Reagent and instruments
The standard solutions (1000 mg/L) of all of the metals (Cd ion, Pb ion and Bi ion), the graphite
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powder (44-µm size), the β-D (+) glucose, the ethanol, the sodium acetate, the carbon nanotube (multi-walled, 15-nm diameter and 3-6 µm length), and the glacial acetic acid) were purchased from Aldrich Co. (St. Louis, MO, USA). The Ag/AgCl paste was purchased from ALS Co. (Tokyo, Japan).
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Acetate buffer solution (0.1 mol L-¹) served as the supporting electrolyte for detection of the heavy metals. Nano Remover PG was purchased from MicroChem Corp. (Westborough, MA, USA). Deionized water (resistivity ≥ 18 MΩ.cm) was used for all of the experiments; all of the
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electrochemical measurements were carried out in a 20-ml cell. Electrochemical measurements were carried out using an electrochemical workstation (CHI 660E, CH Instruments Inc., Austin, TX, USA).
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The electrode morphology was characterized by a field emission scanning electron microscope (FESEM). The physical characteristics of the rGO were investigated via high-resolution X-ray photoelectron
spectroscopy
(XPS),
Raman
spectroscopy,
and
Fourier
transform
infrared
spectroscopy (FTIR).
2.2. Synthesis of rGOCNT composite suspensions The graphene oxide (GO) was prepared using a modified Hummer’s method [28]. The reduction of GO was synthesized using a modified hydrothermal reduction technique [29, 30]. GO (52 mg) was added to deionized water (25 ml) and dispersed by sonication for 30 minutes with an ultrasonic probe. Next, the mixture solution and 0.9 g of glucose were mixed together and left at room temperature for 1 hour. This solution was then sealed in a Teflon-lined autoclave and maintained at 180°C for 2 hours in a convection oven. When the process was completed, the rGO gel was dispersed again in 1 M acetic acid/aqueous solution for 5 hours. Next, the solution was washed with deionized water several times and dried overnight in a vacuum oven at 90°C. Finally, 25 mg of rGO, 12.5 ml ethanol, and 12.5 ml deionized water were mixed, followed by ultrasonication for 2 hours. In the case of the rGOCNT solution, CNT was added to the GO solution to form solutions with GO. The CNT ratios were 5:1
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(rGO: 5, CNT: 1) and 10:1 (rGO: 10, CNT: 1), respectively. After initial process, the same reduction process was carried out for rGOCNT solutions.
2.3. Fabrication of miniaturized and flexible sensors Fig. S1 shows the detailed fabrication process of the sensor. First, polyimide (VTEC 1388) flexible film of 50-µm thickness was formed in a 4-in. silicon wafer by spin-coating. After curing the wafer at
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90°C for 10 min, 110°C for 15 min, and 220°C for 70 min in a convection oven, a Cr (30 nm)/Au (200 nm) layer was formed on the polyimide (PI) film by an electron-beam evaporation system. The three electrodes were made by conventional photolithography and wet etching of the Cr/Au layer. A
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removable mask (SU-8) was made using photolithography to exactly deposit materials on the working electrode. . The material solution was then coated on the sensor using a spray-coater and dried in
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the air at room temperature for 1 hour. The sensor was next put in Remover PG to remove the SU8 mold and washed with IPA and deionized water. Finally, Ag/AgCl paste was cast on top of the reference Au electrode by using a micro steel tip and then dried at 120°C for 5 min. The flexible
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Micro-Electro-Mechanical Systems (MEMS) based electrochemical sensor along with differently shaped electrodes are shown in Fig. 1, Fig. S2, and Fig. S3. Here, the comb-shaped working
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electrode was utilized in order to improve the flexibility and stability of the sensor.
2.4. Measurement procedure, sensing principle, and real sample preparation Cyclic voltammetry was performed by immersing the fabricated working electrodes with a
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commercial Ag/AgCl reference electrode and a commercial Pt counter electrode into the solution of 2
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m mol L-¹ K3[Fe(CN)6] with 0.1 mol L-¹ KCl as a supporting electrolyte. The electrochemical analysis of Cd and Pb ions with the proposed flexible MEMS sensor was implemented by simultaneous accumulation of 400 ppb Bi (after optimization) and a different concentration of Cd and Pb onto the working electrode. The deposition step was carried out with a potential of -1.4 V (after optimization) applied to the working electrode for 150 seconds. After a 10-second equilibration time, the voltammograms were recorded as being from -1.4 V to -0.6 V under the stirring condition by SWASV with a frequency of 25 Hz, an amplitude of 50 mV, and and step potential of 5 mV. In order to remove the residual metals on the working electrode, a constant potential of -0.1 V was applied for 100 seconds after each measurement. From Fig. S4, we found that the sensor performance was decreased without stirring target solution. This result indicated that stirring during the experiment enhances the mass transfer of analytes and improves the performances. Fig.2 shows the detailed schematic drawings of the operation principle to detect heavy metal ions using the above-mentioned method. All the experiments were performed at room temperature and all solutions were used without prior de-aeration. The real water samples were collected from the connection part tube of tap water and the water dispenser. In a typical procedure, Bi, Pb, and Cd standard solutions were mixed with a real water sample to make different concentrations of Cd and Pb ions with 400 ppb Bi ions.
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3. Results and Discussion 3.1. Characterization of the synthesis materials The prepared rGO was first characterized by using FE-SEM. The morphology of rGO film (Fig. 3A) showing stacked layers of graphene sheets were most likely the result of rGO sheets bending during the deposition. Fig. 3D, a side view of the rGO electrode, shows the local folding and non-
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uniform stacking on the rGO sheets. This kind of nano structure can increase the surface area of the working electrode. However, the theoretical surface area could be reduced by using stacked rGO sheets. From the Fig. 3B, C, E F, it is clear that the amount of CNTs in the Au/rGOCNT5-1 electrode
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was increased as compared to Au/rGOCNT10-1 electrode. The cross-sectional view of the FE-SEM images shows the appearance of CNTs between the rGO sheets. This structure most likely increases
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the surface area in comparison to the rGO electrode without CNTs.
To obtain further information on the structure topology of rGO, Raman spectra measurement was carried out. As shown in Fig. 4 (A), a D-band (the vibration of sp3 – hybridized carbon atoms) at 1353
an
cm-1 and a G-band (the vibration of sp2 bonded carbon atoms) at 1599 cm-1 could be observed from the Raman spectra of the original GO film [31]. After reduction, these two bands remained at nearly the same position. However, the intensity ratio of the ID/IG was increased from 0.93 to 1.02. Such an
M
increase could be attributed to the reduction of oxygen-containing groups on the GO along with the π network restoration within the crystal structure of rGO [32]. This result indicated that the rGO had been prepared by this method. Fig. 4 (B) shows the FTIR spectra of GO and rGO. It was obvious
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that the spectrum of GO showed: (1) a broad band at 3258 cm-1, which belongs to a strong stretching mode of the OH group; (2) a peak at 1638 cm-1 due to the C=C stretching mode; and (3) the peaks at
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1736 cm-1 and 1068 cm-1 correspond to the stretching modes of C=O and CO, respectively. For rGO however, the peak in the 3258 cm-1 region disappeared. This reveals the absence of OH group after reduction. The peak at 1583 cm-1 belongs to C=C; while the peak at 1233 cm-1 existed, it was attributed to C-OH [31]. An XPS analysis was performed in order to observe the functional groups contained on the GO (Fig .4C) and rGO (Fig .4D) surfaces. The typical C1s spectrums of the GO and rGO are shown in Fig. 4C and 4D. The C1s spectrum of GO can be deconvoluted to four components corresponding to four types of carbon within GO. The peaks were centered at 284.6, 286.7, 288.3, and 289.2 eV. The peak at 284.6 eV corresponds to C-C in aromatic rings and the peak at 286.7 eV corresponds to C-O (epoxy and alkoxy) groups. Other peaks include C in C=O bonds (288.3 eV) that relate to carbonyl groups and C in O-C=O bonds (289.2 eV) that relate to carbolic acids or ester groups, respectively [33]. In Fig .4D, the C1s spectrum of rGO shows all four of these peaks. To summarize, the proportion of the C-C (284.6 eV) bond has increased after the reduction of GO; the proportion of the C-O (286.2 eV) peak has decreased after the process; and the remaining C-O groups should correspond to peripheral phenolic and carboxyl functionalities. The proportion of the C=O (288.1 eV) groups has decreased after reduction and the C in the O-C=O (289.2 eV) bonds involve carbolic acids or ester groups [34]. The XPS results confirm the FTIR results and show the
Page 5 of 22
reduction of GO after the process. These results indicated that the partial functional groups in GO had been effectively eliminated during the reduction treatment.
3.2. Electrochemical characterization of the fabricated electrodes Figure 5(A) shows cyclic voltammograms of several different modified electrodes in 2.0 m mol L-¹ K3[Fe(CN)6] with 0.1 mol L-¹ KCl solution. On the Au electrode, a pair of weak redox peaks was
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observed, indicating the sluggish electro-transfer rate at the interface. While the electrode was modified with rGO, a well-defined and enhanced redox peak was found. According to the Randles–
cr
Sevˇcik equation, this result indicated that the apparent electroactive area of the rGO-modified Au electrode was increased in comparison to the Au electrode. This improved performance could be attributed to the unique rGO nano structure and good catalytic performance [35]. On the
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Au/rGOCNT5-1 electrode, the performance was much improved in comparison to the Au/rGO electrode. This phenomenon is probably caused by the addition of spacers, since carbon CNTs inbetween rGO sheets prevent their re-stacking. The highest redox peaks appeared on the
an
Au/rGOCNT10-1 electrode. This result indicated that, after reducing the ratio of the CNTs, some rGO sheet surfaces, which were covered by CNTs, might be reduced. Accordingly, the ratio of the CNT
M
should be less than 5:1 (rGO: 5, CNT: 1), or else the CNTs will reduce the surface area of electrode. In addition, in the Fig. 5A, the peak-to-peak voltage of the rGOCNT is greater than rGO. This result indicates that the addition of CNTs may slightly reduce the electron transfer kinetics, which might be
d
the cause of worse electron junctions between CNTs and rGO in comparison with pure rGO electrode. Fig. 5B shows the square wave anodic stripping voltammograms of 300 ppb Cd and Pb ions at Bi
Ac ce pt e
film modified by Au (Au/Bi), Au/rGO/Bi, Au/rGOCNT5-1/Bi, and Au/rGOCNT10-1/Bi. As shown, two weak peaks were observed on the Au/Bi electrode. In contrast, the Au/rGO/Bi exhibited higher stripping responses toward Cd and Pb ion detection. The reason might be that the rGO improved the electron transfer property and surface area of the working electrode. After modifying with rGOCNT composites, the striping increased compared with the Au/rGO/Bi electrode. These results indicate that the CNTs reduced the re-stacking of rGO sheets so that the surface area of the working electrodes increased. The highest striping peaks were observed on the Au/rGOCNT10-1/Bi electrode. This means that the concentration of CNTs must be controlled, and that a higher concentration of CNTs is not a benefit.
For the preliminary investigation of the electrochemical behavior of the proposed flexible MEMS electrochemical sensor with an rGOCNT modified electrode, cyclic voltammetry was performed in a solution of 0.1 mol L-¹ acetate buffer without and with 5 ppm Pb, Cd, and Bi ions. As shown in Fig. 5C, the sensor showed two clear reduction and oxidation peaks when three types of ions were present in the solution. This demonstrates a high possibility of using the proposed sensor for Cd and Pb detection. Groups of CV curves were obtained by measuring the Au/rGOCNT10-1 working electrode using a fabricated and commercial reference electrode in a 0.1 mol L-¹ KCl solution containing 2 m mol L-¹ K3[Fe(CN)6]. The reference electrode was calibrated against a commercial Ag/AgCl (3 mol L-¹ NaCl) electrode. As shown in Fig. 5D, we can calculate that the standard potential shift is 110 mV
Page 6 of 22
compared to the normal hydrogen electrode. Fig. S5 shows the CV curves of the fabricated sensor at different scan rates. The shapes of all the CV curves are nearly identical and show clear redox peaks. These peaks show regular gradient slopes with increasing scan rates, and there is no obvious potential shift between the peaks. Fig. S6 shows the linear relationship between peak current and square root of scan rate. The correlation coefficient is 0.991. These results indicated that the
3.3. Optimization of deposition potential and Bi ion concentration
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fabricated sensor was stable and reliable in the electrochemical sensing platform.
In order to optimize the performance of the sensor, several important parameters pertaining to the
cr
detection process were analyzed. Fig. S5 shows the influence of the stripping response on the deposition time from 30 s to 200 s at the deposition potential of -1.4 V. The peak currents increased
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linearly with the extension of the deposition time. However, when the deposition time becomes longer than 150 s, the current curve begins to slightly increase with respect to time; this is most likely due to the working electrode-surface saturation. Considering sensitivity, a determination time of 150 s was
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selected for the deposition of the ions. The effect of pH on the determination of Cd and Pb ions was also investigated. As illustrated in Fig. S8, the pH of buffer solution has a profound influence on the formation of Bi metal alloy. The best stripping response for Cd and Pb was obtained when using
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buffer solution with a pH value of 4.5 as the supporting electrolyte. Fig. 6A and 6B demonstrate the effect of the Bi ion concentration on the stripping responses. As shown, the stripping peak of the target metal ions increased with the increased concentration of Bi ions from 0 to 400 ppb. The
d
responses of target metal ions decreased at a concentration of Bi exceeding 400 ppb. This
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phenomenon can most likely be attributed to the formation of a thick bismuth film on the electrode surface, which was not favorable for the Pb and Cd ions to diffuse out. A similar result was already discussed by Wang et al (17). Consequently, we chose 400 ppb as the optimal Bi ion concentration. The effect of the deposition potential on the stripping peak current of the heavy metal ion sensor with the rGOCNT composite modified electrode was investigated from -1.2 to -1.5 V by keeping the Bi concentration unchanged (Fig. 6C and 6D). The magnitude of the peaks’ current initially increased in the period where deposition potential became more negative. This was due to the fact that higher energy was supplied to induce more metal ions to participate in the process of electrochemical reduction. The highest peak current was observed at -1.4 V deposition potential, after which the stripping peak began to drop due to the enhanced evolution of hydrogen. Therefore, we chose -1.4 V as the optimal deposition potential.
3.4. Calibration of the fabricated flexible electrochemical sensor and real water sample analysis Calibration of the fabricated flexible electrochemical sensor in regard to Cd and Pb metal ion measurement was conducted in a 0.1 mol L-¹ acetate buffer solution. Under optimization conditions (string the solution with a magnetic bar), a series of SWASV voltammograms with increasing concentrations of Cd and Pb ions were recorded. Stripping voltammograms with well-defined,
Page 7 of 22
undistorted peaks are depicted in Fig. 7A and the corresponding calibration curve is illustrated in Fig. 7B and. 7C. The fabricated sensor displayed linear responses towards Pb and Cd ions in the concentration range of 20 to 200 ppb. The stripping peak current and the concentrations of the Cd and Pb ions exhibited a favorable linear relationship. For the Cd ions, the sensitivity is 262 nA/ppbcm2 with a correlation coefficient of 0.989. For the Pb ions, the sensitivity is 926 nA/ppbcm2 with a correlation coefficient of 0.994. The limit of detection (LOD) calculated by taking a signal-to-noise ratio
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of 3, reached 0.6 ppb for Cd and 0.2 ppb for Pb ion with a short deposition time of 150 s. The sensitivities and LOD could be improved if longer deposition time was applied. The comparison of the analytical performance of the fabricated sensor with other electrodes previously reported is s
cr
summarized in Table S1. It can be seen that our sensor possesses improved or comparable performance for the simultaneous determination of Pb and Cd ions, with the added benefit that our
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electrodes are inexpensive and easy to fabricate. The ability of the sensor was observed for practical applications by detecting of Pb and Cd ions in drinking water. A 20 ml water sample was collected from our university drinking water. Standard Pb and Cd ions were then injected in the water sample
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during measurement so that the concentration of Pb and Cd ions could be detected from the calibration curve. The results are listed in Table 1; the recovery of spiked Pb and Cd ions in the water sample was observed in the range of 94% to 104.4% (Pb) and 92% to 96.3% (Cd). The RSD was
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observed in the range of 5.3% to 6.8% (Pb) and 6.4% to 7.2% (Cd). These results indicated that the
3.5. Interference study
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fabricated sensor can potentially be used for Pb and Cd ion detection in the drinking water.
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Since the presence of non-target metals in solution can interfere with ASV determination of Pb and Cd ions, the tolerance ratio for common interfering metals was investigated [36]. Because water samples usually contain other metal ions that could interfere with our sensor, other metal ions together, along with Cd and Pb ions were used to study the possible interference of the developed sensor. The peak current decrease was calculated in Table S2 and employed for the interference evaluation. It was found that a 100-fold K+, Na+, Ca2+, Cl-, NO3-, and a 30-fold Fe3+ increase had no significant effect on the signals of Cd and Pb ions. However, Cu ions were found to reduce the response of target metal ions, most likely due to the competition between electroplating Bi and Cu on the electrode surface as a result of the close reduction potential of Cu and Bi. Thus, target water samples containing high concentrations of Cu ions need to be treated before measurement [37].
4. Conclusions In this work, a flexible and miniaturized electrochemical heavy metal ion sensor based on a micropatterned Au/rGOCNT10-1/Bi working electrode was successfully developed and further used for the simultaneous determination of Cd and Pb ions by SWASV. The developed sensor exhibited sharp and high peaks for the target metal ions due to the excellent properties of the rGOCNT10-1 film and the good stripping characteristics of Bi. The developed sensor also has some advantages over
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traditional heavy metal ion sensors. These include the large surface area of the working electrode, good sensitivity, fast response time (150 sec), and stability. In addition, this simple and green fabrication method greatly expands the possibilities for the mass production of “mercury-free” sensors for heavy metal analysis. This holds great promise for its wide application in environmental and food analysis. In the near future, we are planning to continuously monitor Cd and Pb ions contamination with extra component to inject Bi to the target solution. It is highly necessary to carry out a
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comprehensive analysis of the sensor behavior on different testing samples, such as drinking water, lake and tap water. Furthermore, we will integrate and package the sensor with read-out circuitry and
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wireless communication systems for practical applications.
Acknowledgments
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This work was partially supported by a research grant of the Kwangwoon University in 2016 and the ICT R&Dprogram of MSIP/IITP. [2016(10041876), Implantable Biosensor and Automatic Physiological
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Function Monitor System for Chronic Disease Management]. The authors are grateful to MiNDaP
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(Micro/Nano Device & Packaging Lab) group members at the Department of Electronic Engineering, Kwangwoon University, for their technical discussion and support.
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Author-contribution Statement
Ac ce pt e
X. Xuan synthesized the rGOCNT nanocomposite, and designed and fabricated the pattern for the electrochemical device. X. Xuan and J. Y. Park wrote the manuscript and discussed the results of the experiments.
Additional Information
Competing financial interests: The authors have no competing financial interests to declare.
Table
Table. 1. Investigation of heavy metal ions in a drinking water sample (n = 3)
Figure captions Fig. 1. Photographs of the fabricated miniaturized, integrated, and flexible heavy metal ion sensor with micro-patterned reduced graphene oxide (rGO) and a carbon nanotube (CNT) composite working electrode. Photo images (A, D) of a fabricated flexible heavy metal ion sensor, (B) microscope image
Page 9 of 22
of 3 electrodes, and (C) working electrode. (Gap size: 50 µm, total effective working electrode area: 1.5 mm2, total working electrode thickness: ~ 1 µm.) Fig. 2. Schematic illustration of the sensing principle occurring on the modified composite working electrode. An Au/rGOCNT functionalized electrode was employed for detection of Cd and Pb ions.
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Fig. 3. Scanning electron microscopy images showing the morphology of the fabricated electrodes. Top and cross view of: (A, D) rGO electrodes; (B, E) rGOCNT5-1 electrodes; (C, F) rGOCNT10-1
cr
electrodes;
Fig. 4. (A) Raman spectra and (B) FTIR spectra of the fabricated GO and rGO electrodes. The XPS
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spectra of (C) GO and (D) rGO,
Fig. 5. (A) Cyclic voltammograms for different electrodes in 2 mM K3[Fe(CN)6] with 0.1 M KCl. Scan
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rate: 50 mV/s. (B) SWASV responses of 300 ppb Cd and Pb ions in 0.1 M acetate buffer solution (pH 4.5) on several fabricated electrodes. Deposition time: 150 s (fitting curve). (C) Cyclic voltammograms of Au/rGOCNT10-1 electrode in 0.1 M acetate buffer solution without (black) and with (red) 5 ppm Cd,
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Pb, and Bi ions. Scan rate: 50 mV/s. (D) Cyclic voltammograms of the fabricated reference electrode and commercial Ag/AgCl (3M Nacl) electrode in 0.1 M KCl solution containing 2 mM K3[Fe(CN)6].
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Scan rate: 50 mV/s.
Fig. 6. Influence of (A, B) Bi ion concentration and (C, D) deposition potential on the stripping peak
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current of 450 ppb Cd and Pb ions on the fabricated sensor. Supporting electrolyte: 0.1 M acetate buffer solution.
Fig. 7. (A) Square wave anodic stripping voltammograms for different concentrations of Cd and Pb ions of the in situ plated Au/rGOCNT10-1/Bi in 0.1 M acetate buffer solution (pH 4.5) containing 400 ppb Bi. (B, C) The corresponding calibration curves of Cd and Pb ions. Data are represented with mean of three replicates.
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Amorphous Graphene, Nano Lett. 16 (2016) 4763-4772.
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Research highlights 1. rGO/CNT nanocomposite was successfully synthesized and micro-patterned on a
Page 14 of 22
flexible electrode (Au/PI) substrate by using a simple, low-cost MEMS fabrication method. 2. Novel rGO/CNT nanocomposite are uniformly distributed and patterned on the flexible substrate for the fabrication of miniaturized electrochemical heavy metal ions sensor. The CNTs was mixed with the rGOs to increase the surface area by preventing the rGO sheets being restacked and aggregated.
cr
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3. The influences of the mixed ratio of CNT and rGO in the composite fabrication on the electrochemical performance of integrated heavy metal ions sensor were investigated.
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4. The physical and electrochemical properties of the as-fabricated CNT/rGO/Bi composite were investigated for being used as working electrode for electrochemical determination of Pb and Cd ions.
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5. The fabricated electrochemical sensor with three electrodes (WE, CE, RE) on a flexible polyimide substrate exhibited high sensitivity, low detection limit, and wide linear range for Pb and Cd ions simultaneous detection.
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S0925-4005(17)31468-5 http://dx.doi.org/doi:10.1016/j.snb.2017.08.046 SNB 22913
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
18-1-2017 27-7-2017 4-8-2017
Please cite this article as: X. Xuan, J.Y. Park, A Miniaturized and Flexible Cadmium and Lead Ion Detection Sensor Based on Micro-Patterned Reduced Graphene Oxide/Carbon Nanotube/Bismuth Composite Electrodes, Sensors and Actuators B: Chemical (2017), http://dx.doi.org/10.1016/j.snb.2017.08.046 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A Miniaturized and Flexible Cadmium and Lead Ion Detection Sensor
Based
on
Micro-Patterned
Reduced
Graphene
Oxide/Carbon Nanotube/Bismuth Composite Electrodes Xing Xuan and Jae Y. Park*
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Department of Electronic Engineering, Kwangwoon University, 447-1, Wolgye-dong, Nowon-gu, Seoul,
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139-701 Republic of Korea
Abstract
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We proposed and fabricated a miniaturized, flexible, and fully integrated electrochemical sensor incorporated with micro-patterned reduced graphene oxide (rGO) and a carbon nanotube (CNT) composite working electrode on a flexible gold substrate. By in situ plating bismuth film, the fabricated
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sensor exhibited well-defined and separate stripping peaks for cadmium (Cd) and iron (Pb) ions, respectively. The CNT was mixed with the rGO in order to improve the performance of the sensor by increasing the electrode surface area. Several experimental parameters, including an electrolyte
M
environment and electrodeposition conditions, were also carefully optimized to achieve the best stripping performance. Under optimal conditions, high sensitivities of 262 nA/ppbcm2 (Cd) and 926 nA/ppbcm2 (Pb) along with favorable detection limits of 0.6 ppb (Cd) and 0.2 ppb (Pb) were obtained.
d
The sensor exhibited good linear responses to both ion types in the concentration range of 20 ppb to
Ac ce pt e
200 ppb. Due to the enlargement of the electrode surface area, the determination efficiency towards target ions was significantly enhanced by the developed Au/rGOCNT/Bi modified electrode. The developed sensor shows high sensitivity, stability, and reliability for the detection of the target heavy metal ions. Finally, the fabricated sensor was successfully demonstrated to detect the target metal ions in drinking water samples with satisfactory results.
Keywords
Flexible sensor, sensitivity, cadmium and lead ion detection, reduced graphene oxide/carbon nanotube composite, integrated electrochemical sensor, microfabrication
*
Corresponding author. Tel.: +82-2-940-5113; Fax: +82-2-942-1502. E-mail address: [email protected] (J. Y. Park)
Page 1 of 22
1. Introduction Determining the presence of toxic heavy metals is important because of their quick accumulation and toxicity in the human body [1]. Among the heavy metals, Cd and Pb have been particularly recognized due to their human toxicity and related environmental pollution [2, 3]. Nowadays, lakes,
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rivers, and tap water are considered to be natural reservoirs supplying fresh water to humans and ecosystems. Hence, the continuous monitoring of the contamination level of heavy metal ions in crucial water supplies is needed to quickly identify pollution and minimize the risk of toxicity. In
cr
addition, on-site field measurement requires a compact sensing system that can be easily and automatically operated without complicated accessories. Therefore, the development of easy to use, miniaturized, sensitive, and flexible sensors that can be integrated into the sensing system is a
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primary need since conventional sensing techniques such as atomic absorption spectrometry (AAS) and inductively coupled plasma atomic emission spectrometry (ICP-AES) are not suitable [4, 5].
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Furthermore, conventional sensing techniques are complicated, expensive, and require a large machine to be of use. To address these issues, electrochemical sensors that can be easily operated have been developed for the detection of heavy metal ions. Furthermore, anodic stripping
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voltammetry (ASV) has proven to be a powerful tool for sensing heavy metal ions due to its high sensitivity, easy operation, and low cost of analysis [6-8].
In most cases, mercury film-based sensors are preferred due to their excellent stripping
d
characteristics [9-11]. However, the toxicity of mercury makes it unsuitable for the previously mentioned design [12]. Bismuth, an environmentally friendly element, provides an alternative
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electrode material for the application of heavy metal detection due to its attractive electrochemical features including reproducible stripping behavior, broad linear range, and good signal-to-background ratio [13, 14]. Bismuth can be deposited on many electrode substrates, including glassy carbon electrodes, screen printed electrodes, some noble metals, and graphene-based modified electrodes [15-17]. Among these, graphene-based modified electrodes have been identified as a promising alternative due to their unique electrochemical and mechanical properties and nano-structure [18-21]. Recently, reduced graphene oxide (rGO) was introduced for constructing enhanced electrochemical sensors due to its high theoretical surface area and applicability for the production of electrochemical-sensors and biosensors [22-25]. Among different synthesis techniques, the solvothermal reduction of graphene oxide is most attractive due to its simple setup, sound scalability, and the ability to recover π –conjugation networks at a high temperature and pressure without toxic materials [26, 27]. The performance of an electrochemical sensor depends upon the surface area of the working electrode. However, a graphene-based electrochemical sensor is usually hindered by the fact that graphene sheets tend to aggregate and restack during processing and the actual accessible surface area of the electrodes is much lower when compared to the theoretical surface area. One of the effective strategies to avoid this problem is the addition of spacers such as carbon nanotubes (CNT) between the graphene sheets to prevent their restacking.
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In this work, a reliable and simple fabrication process was newly developed for a micro- patterned rGOCNT electrode modified sensor on a flexible substrate. The advantages of this process include reduced sensor size, reduced sample volume, reduced cost. In addition, geometrically identical, uniform, and well-defined electrode surface structures can be produced. For sensing heavy metals accurately, the proposed composite working electrode was micro-patterned and plated with Bi film by an in situ approach. The Bi-modified working electrode (Au/rGOCNT/Bi) was measured and
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characterized by evaluating the stripping performance for simultaneous analysis of Cd and Pb ions via square wave anodic stripping voltammetry (SWASV). Furthermore, the developed flexible sensor was
cr
evaluated and was demonstrated to determine Cd and Pb ions in drinking water samples.
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2. Experimental 2.1. Reagent and instruments
The standard solutions (1000 mg/L) of all of the metals (Cd ion, Pb ion and Bi ion), the graphite
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powder (44-µm size), the β-D (+) glucose, the ethanol, the sodium acetate, the carbon nanotube (multi-walled, 15-nm diameter and 3-6 µm length), and the glacial acetic acid) were purchased from Aldrich Co. (St. Louis, MO, USA). The Ag/AgCl paste was purchased from ALS Co. (Tokyo, Japan).
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Acetate buffer solution (0.1 mol L-¹) served as the supporting electrolyte for detection of the heavy metals. Nano Remover PG was purchased from MicroChem Corp. (Westborough, MA, USA). Deionized water (resistivity ≥ 18 MΩ.cm) was used for all of the experiments; all of the
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electrochemical measurements were carried out in a 20-ml cell. Electrochemical measurements were carried out using an electrochemical workstation (CHI 660E, CH Instruments Inc., Austin, TX, USA).
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The electrode morphology was characterized by a field emission scanning electron microscope (FESEM). The physical characteristics of the rGO were investigated via high-resolution X-ray photoelectron
spectroscopy
(XPS),
Raman
spectroscopy,
and
Fourier
transform
infrared
spectroscopy (FTIR).
2.2. Synthesis of rGOCNT composite suspensions The graphene oxide (GO) was prepared using a modified Hummer’s method [28]. The reduction of GO was synthesized using a modified hydrothermal reduction technique [29, 30]. GO (52 mg) was added to deionized water (25 ml) and dispersed by sonication for 30 minutes with an ultrasonic probe. Next, the mixture solution and 0.9 g of glucose were mixed together and left at room temperature for 1 hour. This solution was then sealed in a Teflon-lined autoclave and maintained at 180°C for 2 hours in a convection oven. When the process was completed, the rGO gel was dispersed again in 1 M acetic acid/aqueous solution for 5 hours. Next, the solution was washed with deionized water several times and dried overnight in a vacuum oven at 90°C. Finally, 25 mg of rGO, 12.5 ml ethanol, and 12.5 ml deionized water were mixed, followed by ultrasonication for 2 hours. In the case of the rGOCNT solution, CNT was added to the GO solution to form solutions with GO. The CNT ratios were 5:1
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(rGO: 5, CNT: 1) and 10:1 (rGO: 10, CNT: 1), respectively. After initial process, the same reduction process was carried out for rGOCNT solutions.
2.3. Fabrication of miniaturized and flexible sensors Fig. S1 shows the detailed fabrication process of the sensor. First, polyimide (VTEC 1388) flexible film of 50-µm thickness was formed in a 4-in. silicon wafer by spin-coating. After curing the wafer at
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90°C for 10 min, 110°C for 15 min, and 220°C for 70 min in a convection oven, a Cr (30 nm)/Au (200 nm) layer was formed on the polyimide (PI) film by an electron-beam evaporation system. The three electrodes were made by conventional photolithography and wet etching of the Cr/Au layer. A
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removable mask (SU-8) was made using photolithography to exactly deposit materials on the working electrode. . The material solution was then coated on the sensor using a spray-coater and dried in
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the air at room temperature for 1 hour. The sensor was next put in Remover PG to remove the SU8 mold and washed with IPA and deionized water. Finally, Ag/AgCl paste was cast on top of the reference Au electrode by using a micro steel tip and then dried at 120°C for 5 min. The flexible
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Micro-Electro-Mechanical Systems (MEMS) based electrochemical sensor along with differently shaped electrodes are shown in Fig. 1, Fig. S2, and Fig. S3. Here, the comb-shaped working
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electrode was utilized in order to improve the flexibility and stability of the sensor.
2.4. Measurement procedure, sensing principle, and real sample preparation Cyclic voltammetry was performed by immersing the fabricated working electrodes with a
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commercial Ag/AgCl reference electrode and a commercial Pt counter electrode into the solution of 2
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m mol L-¹ K3[Fe(CN)6] with 0.1 mol L-¹ KCl as a supporting electrolyte. The electrochemical analysis of Cd and Pb ions with the proposed flexible MEMS sensor was implemented by simultaneous accumulation of 400 ppb Bi (after optimization) and a different concentration of Cd and Pb onto the working electrode. The deposition step was carried out with a potential of -1.4 V (after optimization) applied to the working electrode for 150 seconds. After a 10-second equilibration time, the voltammograms were recorded as being from -1.4 V to -0.6 V under the stirring condition by SWASV with a frequency of 25 Hz, an amplitude of 50 mV, and and step potential of 5 mV. In order to remove the residual metals on the working electrode, a constant potential of -0.1 V was applied for 100 seconds after each measurement. From Fig. S4, we found that the sensor performance was decreased without stirring target solution. This result indicated that stirring during the experiment enhances the mass transfer of analytes and improves the performances. Fig.2 shows the detailed schematic drawings of the operation principle to detect heavy metal ions using the above-mentioned method. All the experiments were performed at room temperature and all solutions were used without prior de-aeration. The real water samples were collected from the connection part tube of tap water and the water dispenser. In a typical procedure, Bi, Pb, and Cd standard solutions were mixed with a real water sample to make different concentrations of Cd and Pb ions with 400 ppb Bi ions.
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3. Results and Discussion 3.1. Characterization of the synthesis materials The prepared rGO was first characterized by using FE-SEM. The morphology of rGO film (Fig. 3A) showing stacked layers of graphene sheets were most likely the result of rGO sheets bending during the deposition. Fig. 3D, a side view of the rGO electrode, shows the local folding and non-
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uniform stacking on the rGO sheets. This kind of nano structure can increase the surface area of the working electrode. However, the theoretical surface area could be reduced by using stacked rGO sheets. From the Fig. 3B, C, E F, it is clear that the amount of CNTs in the Au/rGOCNT5-1 electrode
cr
was increased as compared to Au/rGOCNT10-1 electrode. The cross-sectional view of the FE-SEM images shows the appearance of CNTs between the rGO sheets. This structure most likely increases
us
the surface area in comparison to the rGO electrode without CNTs.
To obtain further information on the structure topology of rGO, Raman spectra measurement was carried out. As shown in Fig. 4 (A), a D-band (the vibration of sp3 – hybridized carbon atoms) at 1353
an
cm-1 and a G-band (the vibration of sp2 bonded carbon atoms) at 1599 cm-1 could be observed from the Raman spectra of the original GO film [31]. After reduction, these two bands remained at nearly the same position. However, the intensity ratio of the ID/IG was increased from 0.93 to 1.02. Such an
M
increase could be attributed to the reduction of oxygen-containing groups on the GO along with the π network restoration within the crystal structure of rGO [32]. This result indicated that the rGO had been prepared by this method. Fig. 4 (B) shows the FTIR spectra of GO and rGO. It was obvious
d
that the spectrum of GO showed: (1) a broad band at 3258 cm-1, which belongs to a strong stretching mode of the OH group; (2) a peak at 1638 cm-1 due to the C=C stretching mode; and (3) the peaks at
Ac ce pt e
1736 cm-1 and 1068 cm-1 correspond to the stretching modes of C=O and CO, respectively. For rGO however, the peak in the 3258 cm-1 region disappeared. This reveals the absence of OH group after reduction. The peak at 1583 cm-1 belongs to C=C; while the peak at 1233 cm-1 existed, it was attributed to C-OH [31]. An XPS analysis was performed in order to observe the functional groups contained on the GO (Fig .4C) and rGO (Fig .4D) surfaces. The typical C1s spectrums of the GO and rGO are shown in Fig. 4C and 4D. The C1s spectrum of GO can be deconvoluted to four components corresponding to four types of carbon within GO. The peaks were centered at 284.6, 286.7, 288.3, and 289.2 eV. The peak at 284.6 eV corresponds to C-C in aromatic rings and the peak at 286.7 eV corresponds to C-O (epoxy and alkoxy) groups. Other peaks include C in C=O bonds (288.3 eV) that relate to carbonyl groups and C in O-C=O bonds (289.2 eV) that relate to carbolic acids or ester groups, respectively [33]. In Fig .4D, the C1s spectrum of rGO shows all four of these peaks. To summarize, the proportion of the C-C (284.6 eV) bond has increased after the reduction of GO; the proportion of the C-O (286.2 eV) peak has decreased after the process; and the remaining C-O groups should correspond to peripheral phenolic and carboxyl functionalities. The proportion of the C=O (288.1 eV) groups has decreased after reduction and the C in the O-C=O (289.2 eV) bonds involve carbolic acids or ester groups [34]. The XPS results confirm the FTIR results and show the
Page 5 of 22
reduction of GO after the process. These results indicated that the partial functional groups in GO had been effectively eliminated during the reduction treatment.
3.2. Electrochemical characterization of the fabricated electrodes Figure 5(A) shows cyclic voltammograms of several different modified electrodes in 2.0 m mol L-¹ K3[Fe(CN)6] with 0.1 mol L-¹ KCl solution. On the Au electrode, a pair of weak redox peaks was
ip t
observed, indicating the sluggish electro-transfer rate at the interface. While the electrode was modified with rGO, a well-defined and enhanced redox peak was found. According to the Randles–
cr
Sevˇcik equation, this result indicated that the apparent electroactive area of the rGO-modified Au electrode was increased in comparison to the Au electrode. This improved performance could be attributed to the unique rGO nano structure and good catalytic performance [35]. On the
us
Au/rGOCNT5-1 electrode, the performance was much improved in comparison to the Au/rGO electrode. This phenomenon is probably caused by the addition of spacers, since carbon CNTs inbetween rGO sheets prevent their re-stacking. The highest redox peaks appeared on the
an
Au/rGOCNT10-1 electrode. This result indicated that, after reducing the ratio of the CNTs, some rGO sheet surfaces, which were covered by CNTs, might be reduced. Accordingly, the ratio of the CNT
M
should be less than 5:1 (rGO: 5, CNT: 1), or else the CNTs will reduce the surface area of electrode. In addition, in the Fig. 5A, the peak-to-peak voltage of the rGOCNT is greater than rGO. This result indicates that the addition of CNTs may slightly reduce the electron transfer kinetics, which might be
d
the cause of worse electron junctions between CNTs and rGO in comparison with pure rGO electrode. Fig. 5B shows the square wave anodic stripping voltammograms of 300 ppb Cd and Pb ions at Bi
Ac ce pt e
film modified by Au (Au/Bi), Au/rGO/Bi, Au/rGOCNT5-1/Bi, and Au/rGOCNT10-1/Bi. As shown, two weak peaks were observed on the Au/Bi electrode. In contrast, the Au/rGO/Bi exhibited higher stripping responses toward Cd and Pb ion detection. The reason might be that the rGO improved the electron transfer property and surface area of the working electrode. After modifying with rGOCNT composites, the striping increased compared with the Au/rGO/Bi electrode. These results indicate that the CNTs reduced the re-stacking of rGO sheets so that the surface area of the working electrodes increased. The highest striping peaks were observed on the Au/rGOCNT10-1/Bi electrode. This means that the concentration of CNTs must be controlled, and that a higher concentration of CNTs is not a benefit.
For the preliminary investigation of the electrochemical behavior of the proposed flexible MEMS electrochemical sensor with an rGOCNT modified electrode, cyclic voltammetry was performed in a solution of 0.1 mol L-¹ acetate buffer without and with 5 ppm Pb, Cd, and Bi ions. As shown in Fig. 5C, the sensor showed two clear reduction and oxidation peaks when three types of ions were present in the solution. This demonstrates a high possibility of using the proposed sensor for Cd and Pb detection. Groups of CV curves were obtained by measuring the Au/rGOCNT10-1 working electrode using a fabricated and commercial reference electrode in a 0.1 mol L-¹ KCl solution containing 2 m mol L-¹ K3[Fe(CN)6]. The reference electrode was calibrated against a commercial Ag/AgCl (3 mol L-¹ NaCl) electrode. As shown in Fig. 5D, we can calculate that the standard potential shift is 110 mV
Page 6 of 22
compared to the normal hydrogen electrode. Fig. S5 shows the CV curves of the fabricated sensor at different scan rates. The shapes of all the CV curves are nearly identical and show clear redox peaks. These peaks show regular gradient slopes with increasing scan rates, and there is no obvious potential shift between the peaks. Fig. S6 shows the linear relationship between peak current and square root of scan rate. The correlation coefficient is 0.991. These results indicated that the
3.3. Optimization of deposition potential and Bi ion concentration
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fabricated sensor was stable and reliable in the electrochemical sensing platform.
In order to optimize the performance of the sensor, several important parameters pertaining to the
cr
detection process were analyzed. Fig. S5 shows the influence of the stripping response on the deposition time from 30 s to 200 s at the deposition potential of -1.4 V. The peak currents increased
us
linearly with the extension of the deposition time. However, when the deposition time becomes longer than 150 s, the current curve begins to slightly increase with respect to time; this is most likely due to the working electrode-surface saturation. Considering sensitivity, a determination time of 150 s was
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selected for the deposition of the ions. The effect of pH on the determination of Cd and Pb ions was also investigated. As illustrated in Fig. S8, the pH of buffer solution has a profound influence on the formation of Bi metal alloy. The best stripping response for Cd and Pb was obtained when using
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buffer solution with a pH value of 4.5 as the supporting electrolyte. Fig. 6A and 6B demonstrate the effect of the Bi ion concentration on the stripping responses. As shown, the stripping peak of the target metal ions increased with the increased concentration of Bi ions from 0 to 400 ppb. The
d
responses of target metal ions decreased at a concentration of Bi exceeding 400 ppb. This
Ac ce pt e
phenomenon can most likely be attributed to the formation of a thick bismuth film on the electrode surface, which was not favorable for the Pb and Cd ions to diffuse out. A similar result was already discussed by Wang et al (17). Consequently, we chose 400 ppb as the optimal Bi ion concentration. The effect of the deposition potential on the stripping peak current of the heavy metal ion sensor with the rGOCNT composite modified electrode was investigated from -1.2 to -1.5 V by keeping the Bi concentration unchanged (Fig. 6C and 6D). The magnitude of the peaks’ current initially increased in the period where deposition potential became more negative. This was due to the fact that higher energy was supplied to induce more metal ions to participate in the process of electrochemical reduction. The highest peak current was observed at -1.4 V deposition potential, after which the stripping peak began to drop due to the enhanced evolution of hydrogen. Therefore, we chose -1.4 V as the optimal deposition potential.
3.4. Calibration of the fabricated flexible electrochemical sensor and real water sample analysis Calibration of the fabricated flexible electrochemical sensor in regard to Cd and Pb metal ion measurement was conducted in a 0.1 mol L-¹ acetate buffer solution. Under optimization conditions (string the solution with a magnetic bar), a series of SWASV voltammograms with increasing concentrations of Cd and Pb ions were recorded. Stripping voltammograms with well-defined,
Page 7 of 22
undistorted peaks are depicted in Fig. 7A and the corresponding calibration curve is illustrated in Fig. 7B and. 7C. The fabricated sensor displayed linear responses towards Pb and Cd ions in the concentration range of 20 to 200 ppb. The stripping peak current and the concentrations of the Cd and Pb ions exhibited a favorable linear relationship. For the Cd ions, the sensitivity is 262 nA/ppbcm2 with a correlation coefficient of 0.989. For the Pb ions, the sensitivity is 926 nA/ppbcm2 with a correlation coefficient of 0.994. The limit of detection (LOD) calculated by taking a signal-to-noise ratio
ip t
of 3, reached 0.6 ppb for Cd and 0.2 ppb for Pb ion with a short deposition time of 150 s. The sensitivities and LOD could be improved if longer deposition time was applied. The comparison of the analytical performance of the fabricated sensor with other electrodes previously reported is s
cr
summarized in Table S1. It can be seen that our sensor possesses improved or comparable performance for the simultaneous determination of Pb and Cd ions, with the added benefit that our
us
electrodes are inexpensive and easy to fabricate. The ability of the sensor was observed for practical applications by detecting of Pb and Cd ions in drinking water. A 20 ml water sample was collected from our university drinking water. Standard Pb and Cd ions were then injected in the water sample
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during measurement so that the concentration of Pb and Cd ions could be detected from the calibration curve. The results are listed in Table 1; the recovery of spiked Pb and Cd ions in the water sample was observed in the range of 94% to 104.4% (Pb) and 92% to 96.3% (Cd). The RSD was
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observed in the range of 5.3% to 6.8% (Pb) and 6.4% to 7.2% (Cd). These results indicated that the
3.5. Interference study
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fabricated sensor can potentially be used for Pb and Cd ion detection in the drinking water.
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Since the presence of non-target metals in solution can interfere with ASV determination of Pb and Cd ions, the tolerance ratio for common interfering metals was investigated [36]. Because water samples usually contain other metal ions that could interfere with our sensor, other metal ions together, along with Cd and Pb ions were used to study the possible interference of the developed sensor. The peak current decrease was calculated in Table S2 and employed for the interference evaluation. It was found that a 100-fold K+, Na+, Ca2+, Cl-, NO3-, and a 30-fold Fe3+ increase had no significant effect on the signals of Cd and Pb ions. However, Cu ions were found to reduce the response of target metal ions, most likely due to the competition between electroplating Bi and Cu on the electrode surface as a result of the close reduction potential of Cu and Bi. Thus, target water samples containing high concentrations of Cu ions need to be treated before measurement [37].
4. Conclusions In this work, a flexible and miniaturized electrochemical heavy metal ion sensor based on a micropatterned Au/rGOCNT10-1/Bi working electrode was successfully developed and further used for the simultaneous determination of Cd and Pb ions by SWASV. The developed sensor exhibited sharp and high peaks for the target metal ions due to the excellent properties of the rGOCNT10-1 film and the good stripping characteristics of Bi. The developed sensor also has some advantages over
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traditional heavy metal ion sensors. These include the large surface area of the working electrode, good sensitivity, fast response time (150 sec), and stability. In addition, this simple and green fabrication method greatly expands the possibilities for the mass production of “mercury-free” sensors for heavy metal analysis. This holds great promise for its wide application in environmental and food analysis. In the near future, we are planning to continuously monitor Cd and Pb ions contamination with extra component to inject Bi to the target solution. It is highly necessary to carry out a
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comprehensive analysis of the sensor behavior on different testing samples, such as drinking water, lake and tap water. Furthermore, we will integrate and package the sensor with read-out circuitry and
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wireless communication systems for practical applications.
Acknowledgments
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This work was partially supported by a research grant of the Kwangwoon University in 2016 and the ICT R&Dprogram of MSIP/IITP. [2016(10041876), Implantable Biosensor and Automatic Physiological
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Function Monitor System for Chronic Disease Management]. The authors are grateful to MiNDaP
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(Micro/Nano Device & Packaging Lab) group members at the Department of Electronic Engineering, Kwangwoon University, for their technical discussion and support.
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Author-contribution Statement
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X. Xuan synthesized the rGOCNT nanocomposite, and designed and fabricated the pattern for the electrochemical device. X. Xuan and J. Y. Park wrote the manuscript and discussed the results of the experiments.
Additional Information
Competing financial interests: The authors have no competing financial interests to declare.
Table
Table. 1. Investigation of heavy metal ions in a drinking water sample (n = 3)
Figure captions Fig. 1. Photographs of the fabricated miniaturized, integrated, and flexible heavy metal ion sensor with micro-patterned reduced graphene oxide (rGO) and a carbon nanotube (CNT) composite working electrode. Photo images (A, D) of a fabricated flexible heavy metal ion sensor, (B) microscope image
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of 3 electrodes, and (C) working electrode. (Gap size: 50 µm, total effective working electrode area: 1.5 mm2, total working electrode thickness: ~ 1 µm.) Fig. 2. Schematic illustration of the sensing principle occurring on the modified composite working electrode. An Au/rGOCNT functionalized electrode was employed for detection of Cd and Pb ions.
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Fig. 3. Scanning electron microscopy images showing the morphology of the fabricated electrodes. Top and cross view of: (A, D) rGO electrodes; (B, E) rGOCNT5-1 electrodes; (C, F) rGOCNT10-1
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electrodes;
Fig. 4. (A) Raman spectra and (B) FTIR spectra of the fabricated GO and rGO electrodes. The XPS
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spectra of (C) GO and (D) rGO,
Fig. 5. (A) Cyclic voltammograms for different electrodes in 2 mM K3[Fe(CN)6] with 0.1 M KCl. Scan
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rate: 50 mV/s. (B) SWASV responses of 300 ppb Cd and Pb ions in 0.1 M acetate buffer solution (pH 4.5) on several fabricated electrodes. Deposition time: 150 s (fitting curve). (C) Cyclic voltammograms of Au/rGOCNT10-1 electrode in 0.1 M acetate buffer solution without (black) and with (red) 5 ppm Cd,
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Pb, and Bi ions. Scan rate: 50 mV/s. (D) Cyclic voltammograms of the fabricated reference electrode and commercial Ag/AgCl (3M Nacl) electrode in 0.1 M KCl solution containing 2 mM K3[Fe(CN)6].
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Scan rate: 50 mV/s.
Fig. 6. Influence of (A, B) Bi ion concentration and (C, D) deposition potential on the stripping peak
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current of 450 ppb Cd and Pb ions on the fabricated sensor. Supporting electrolyte: 0.1 M acetate buffer solution.
Fig. 7. (A) Square wave anodic stripping voltammograms for different concentrations of Cd and Pb ions of the in situ plated Au/rGOCNT10-1/Bi in 0.1 M acetate buffer solution (pH 4.5) containing 400 ppb Bi. (B, C) The corresponding calibration curves of Cd and Pb ions. Data are represented with mean of three replicates.
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Research highlights 1. rGO/CNT nanocomposite was successfully synthesized and micro-patterned on a
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flexible electrode (Au/PI) substrate by using a simple, low-cost MEMS fabrication method. 2. Novel rGO/CNT nanocomposite are uniformly distributed and patterned on the flexible substrate for the fabrication of miniaturized electrochemical heavy metal ions sensor. The CNTs was mixed with the rGOs to increase the surface area by preventing the rGO sheets being restacked and aggregated.
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3. The influences of the mixed ratio of CNT and rGO in the composite fabrication on the electrochemical performance of integrated heavy metal ions sensor were investigated.
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4. The physical and electrochemical properties of the as-fabricated CNT/rGO/Bi composite were investigated for being used as working electrode for electrochemical determination of Pb and Cd ions.
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5. The fabricated electrochemical sensor with three electrodes (WE, CE, RE) on a flexible polyimide substrate exhibited high sensitivity, low detection limit, and wide linear range for Pb and Cd ions simultaneous detection.
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