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Finite element modeling simulation-assisted design of integrated microfluidic chips for heavy metal ion stripping analysis To cite this article: Ying Hong et al 2017 J. Phys. D: Appl. Phys. 50 415303
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Journal of Physics D: Applied Physics J. Phys. D: Appl. Phys. 50 (2017) 415303 (6pp)
https://doi.org/10.1088/1361-6463/aa84a3
Finite element modeling simulation-assisted design of integrated microfluidic chips for heavy metal ion stripping analysis Ying Hong1,2,3, Jianhua Zou1,3, Gang Ge1, Wanyue Xiao1, Ling Gao2, Jinjun Shao1 and Xiaochen Dong1 Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, People’s Republic of China 2 Nanjing Entry-Exit Inspection and Quarantine Bureau, 1 Guojian Road, Nanjing 211106, People’s Republic of China 1
E-mail:
[email protected] and
[email protected] Received 19 May 2017, revised 2 August 2017 Accepted for publication 7 August 2017 Published 18 September 2017 Abstract
In this article, a transparent integrated microfluidic device composed of a 3D-printed thinlayer flow cell (3D-PTLFC) and an S-shaped screen-printed electrode (SPE) has been designed and fabricated for heavy metal ion stripping analysis. A finite element modeling (FEM) simulation is employed to optimize the shape of the electrode, the direction of the inlet pipeline, the thin-layer channel height and the sample flow rate to enhance the electronenrichment efficiency for stripping analysis. The results demonstrate that the S-shaped SPE configuration matches the channel in 3D-PTLFC perfectly for the anodic stripping behavior of the heavy metal ions. Under optimized conditions, a wide linear range of 1–80 µg l−1 is achieved for Pb2+ detection with a limit of 0.3 µg l−1 for the microfluidic device. Thus, the obtained integrated microfluidic device proves to be a promising approach for heavy metal ions stripping analysis with low cost and high performance. Keywords: FEM simulation, 3D printing, microfluidic device, heavy metal ions, stripping analysis S Supplementary material for this article is available online (Some figures may appear in colour only in the online journal)
1. Introduction
stripping. Since microfluidic devices equipped with miniaturized processing chips can afford excellent detection performances with only a small amount of samples [10, 11], they can be used in various scenarios which require rapid heavy metal ions detection [12–15]. In the past few years, various electrochemical microfluidic devices have been prepared for the stripping detection of metal ions [16–20]. For example, traditional rod/wire electrodes combined with microfluidic channels were employed to fabricate integrated lab-on-chip devices [21, 22] by using photolithography, sputtering or printing approaches [21–25]. Presently, most polymer (mainly PDMS) based microfluidic cells are made by microelectronic mechanical technology.
Easy and reliable detection of hypertoxic heavy metal ions (such as Pb2+, Cd2+, and As3+) has attracted great attention, due to ever-increasing environmental pollution problems [1–5]. It is urgent to develop a simple and portable device with efficient detection capabilities. Electrochemical stripping is widely used for trace metal ions determination, owing to its low cost, rapid detection and simple implementation [6–9]. To improve the detection efficiency, a microfluidic device is a considered effective when being integrated with electrochemical 3
These authors contributed equally.
1361-6463/17/415303+6$33.00
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© 2017 IOP Publishing Ltd Printed in the UK
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J. Phys. D: Appl. Phys. 50 (2017) 415303
Figure 1. (a), (b) Picture of microfluidic heavy atom detection system and the device with S-shaped SPE. (a) Sample holder, (b) peristaltic pump, (c) 3D-PTLFC and SPE, (d) electrochemical workstation, (e) computer, (f) collector of the waste. (c) Computational domain of S-shaped SPE. (d) The analog flow field diagram of S-shaped microchannel.
Figure 2. (a) Different SPE configurations designed for the microfluidic device. (b) The analog flow field diagram of 3D-printed microchannel.
However, PDMS-based microfluidics, fabricated with bulky controlling systems, greatly limit their practical applications, due to extensive manual labor and poor repeatability. To overcome these disadvantages, 3D printing, an economical and eco-friendly technology, is taken into consideration for the fabrication of a microfluidic device with a high sensitivity, low limit of detection (LOD), as well as excellent selectivity for the stripping analysis of heavy metal ions [26–30]. In previous reports, the enrichment potential and time optimization for stripping analysis have been reported [15, 33–36]. However, the effect of diffusion features on the electron-enrichment efficiency of the microfluidic device has been rarely explored. Therefore, it is necessary to optimize the sample flow characteristics and the combined electrode configuration to obtain better microfluidic devices. Herein, an integrated microfluidic chip consisting of a 3D printed thin layer flow cell (3D-PTLFC) and an S-shaped disposable screen-printed electrode (SPE) is fabricated for heavy metal ions stripping analysis. Finite element modeling (FEM) is used to simulate the flow characteristics in 3D-PTLFC, as well as critical parameters of devices, including the integrated SPE configuration, thin-layer channel height and sample flow rate, which all have great effects on the analytical performance of devices. Based on the simulation results, an integrated detection device is fabricated for rapid detection of heavy metal ions with ultra-high sensitivity and ultra-low LOD (0.3 µg l−1). The utilization of 3D printing greatly simplifies the fabrication process of microfluidic channels, which makes it possible to realize rapid detection by continuous monitoring (without cleaning) of heavy metal ions.
the SPE are simulated using FEM simulation under various diffusion layers and thicknesses of the device cell at the same velocity. Regarding the fluid characteristics and the electrochemical reaction on the electrode surface, the Navier–Stokes equation (equation (1)) and convection diffusion equation (equation (2)) are adopted to solve the substance viscosity and the electrochemical activity, respectively, during the FEM simulation (figure 1(c)). ∂u ρ + u • ∇u = −∇p + µ∇2 u + f (1) ∂t ∂c = D∇2 c − u • ∇c (2) ∂t
where ρ represents fluid viscosity, µ represents kinematic viscosity, p represents pressure, u represents the flow rate of the fluid, f is external stress, D is the substance diffusion coefficient and c represents the substance concentration. Through FEM simulation, the analog flow field diagram of the microchannel in Matlab was also obtained (figure 1(d)). It suggests that a S-shaped velocity profile can provide samples with a longer path to the outlet, enabling more metal ions to be deposited on the SPE which increases the detection performance. To obtain appropriate FEM results, the relationship of the iteration number versus the convergence in FEM simulation were checked with different mesh numbers. The results with different mesh numbers were plotted in figure S1 (stacks.iop.org/JPhysD/50/415303/mmedia), which demonstrates that FEM simulation is independent of the mesh number.
2. Results and discussion 2.1. Device structure and FEM simulation
2.2. Shape of the electrode
Figures 1(a) and (b) show the transparent microfluidic heavy atom detection system and the device with S-shaped SPE. Based on COMSOL software, the computational domains of
To analyze the effect of the electrode shape on the anodic stripping voltammetry (ASV) detection, theoretical calculation was applied with laminar flow, following the equation [37]: 2
Y Hong et al
J. Phys. D: Appl. Phys. 50 (2017) 415303
2.3. Optimization of flow cell 2.3.1. Direction of inlet pipeline. Figure S1 shows the 3D-PTLFC and the S-shaped SPE. It is well known that a standard detection process consists of an enrichment and stripping process. Also, the enrichment of the heavy metal ions on SPEs is greatly influenced by the height of the diffusion layer, while the height is affected by the direction of the inlet. To obtain the highest enrichment efficiency, the path length of sample should be maximized to provide the longest time for metal ions in flow solution to interact with the electrode. Figures 4(a)–(c) show the connection modes between the microchannel and pipes (90°, 45° and 0°). An analog flow field diagram on the concentration profile of the S-shaped microchannel with different sample injection angles is presented in figures 4(d)–(f). By comparing the enrichment efficiency of the three connection modes, it can be concluded that the connection of the inlet and outlet pipelines on the top of both ends along the tangent direction (90°) represents the longest flow time of the solution. To verify the obtained results, the stripping and enrichment currents are measured with microfluidic devices inserted with SPEs of different shapes for the detection of Pb2+ (figure 2(a)). As shown in figures 5(a) and (b), the stripping currents of the devices increase from SPEs No. 1 to No. 5. Also, the S-shaped SPE exhibits the highest stripping currents, indicating that the S-shaped SPE presents excellent stripping and enrichment performance for Pb2+. These results are in good agreement with the FEM simulation, which further demonstrates that the S-shaped SPE provides the highest stripping current of the target heavy metal ions.
Figure 3. The printing process of S-shaped SPE.
2
1
1
1
(3) I = 0.68nFDs3 c0 bL 2 u 2 v− 6 where I is the density of the current, n is the electron numbers of the electrochemical reaction, F is Faraday constant, Ds is coefficient of diffusion, c0 is the concentration of the solution, b is the width of electrode perpendicular to the direction of the laminar flow, L is the length of electrode parallel to the direction of the laminar flow, u is the flow rate and ν is the kinematic viscosity. According to equation (3), it can be concluded that the electrolysis current increases with the accelerating of the flow rate and the increasing of the electrode area, which is useful to improve the pre-electrolysis efficiency. When the electrode area is too large, the current density will become smaller and the background noise will increase, which is harmful for the quantitative analysis. Considering the limitation of the electrode area, the shape of SPEs is adjusted for achieving higher electrolysis efficiency at a higher flow rate (figure 2(a)). Serving as an inserted SPE, the located part in the microchannel will be the effective working zone. As shown in figure 2(b), the zone consists of one ideal electrode working zone Z2 and one sub-ideal zone Z1, which is located in a simulation flow field of microchannel 22. The working zone close to inlet pipeline 221 is a sub-ideal one Z1 with the highest flow rate. Meanwhile, the working zone close to outlet pipeline 222 is the ideal one Z2, presenting high and steady flow rate. The analysis results indicate the effective working areas are S-shape, suggesting S-shaped SPEs are ideal electrodes, compared with other shapes for the transport of electrons. Based on the FEM simulation, an S-shaped SPE is fabricated. The printing process is shown in figure 3. On the insulating plastic substrate, a conductive Ag layer, a Ag/AgCl reference layer and a carbon-based working layer are printed on designed zones, respectively. At last, an insulating layer is covered on the pattern. There is a fault between the Ag/AgCl and carbon layer. It is worthwhile to point out that the resist ance of the electrode can be controlled easily by adjusting the distance between the faults.
2.3.2. Microfluidic channel height and sample flow rate. According to the simulation results of the COMSOL
Multiphysics software, the height of the microfluidic thinlayer channel also has significant influence on the stripping analysis of the detected ions. Usually, the height range of a microchannel is 0.1–2.5 mm. With the continuous increase of the thin-layer height in the range of 0.1–2.5 mm, the effect of the diffusion layer height on the stripping currents are investigated systematically under the tangent direction of inlet pipeline. Figure 5(c) shows the relationship between stripping peak currents of Pb2+ in microfluidic devices and various thin-layer channel heights. It can be found that the stripping peak currents reach maximum at the height of the microfluidic channel at 0.9 mm. Furthermore, the effect of the sample flow rate on the stripping currents are measured using the optim ized microfluidic devices with the flow rate 0.01–0.2 m s−1 controlled by the peristaltic pump, as shown in figure 5(d). The maximum stripping peak current of Pb2+can be obtained when the sample flow rate is 0.037 m s−1, which will facilitate the enrichment of Pb2+. Compared with traditional ASV devices, the optimized microfluidic devices with S-shaped SPE No. 1 can ensure the sample is enriched in a flow state and stripped in a stationary state at less dosage (less than 3 ml) with a visible process. Secondly, the SPEs can be conveniently inserted, pulled out and replaced, which facilitates the continuous detection of various samples without cross contamination. Thirdly, the 3
Y Hong et al
J. Phys. D: Appl. Phys. 50 (2017) 415303
Figure 4. Schematic diagram of three connection modes between microchannel and pipes. (a) 90°, (b) 45°, (c) 0°. Analog flow field diagram on concentration profile of microchannel with different sample injection angles. (d) 90°, (e) 45°, (f) 0°.
Figure 5. (a) The stripping voltammetry curves and (b) enrichment curves for Pb2+ at the device integrated with different shaped SPEs.
The relationship between stripping peak current of Pb2+ and (c) different microchannel heights, (d) different flow rates.
planar electrode with a flow field distribution in an S-shape will shorten the enrichment time and improve the detection sensitivity and efficiency.
Enrichment process : Pb2+ + Bi3+ + 5e → Pb (Bi) Stripping process : Pb (Bi) − 2e → Pb2+ + Bi.
2.4. Stripping detection performance for Pb2+
Figure 6(a) shows the square-wave anodic stripping voltammetry (SWASV) profiles for Pb2+ in various concentrations recorded under an enrichment potential of −1.2 V and an enrichment time of 120 s. The sharp stripping currents of the target increase proportionally with the increase of Pb2+ concentration. Figure 6(b) presents the plots of the stripping peak current versus the Pb2+ concentration, indicating a linear
When enriching the Pb2+ in the solution by plating a bismuth film in the same position of working electrode, the Bi was combined with the reduced Pb to form a bismuth alloy film like amalgam, which will be adsorbed on the surface of the working electrode. In this oxidation and stripping process, a peak current will be formed relating to the concentration of Pb2+. 4
Y Hong et al
J. Phys. D: Appl. Phys. 50 (2017) 415303
Figure 6. (a) SWASV profiles of the optimized microfluidic device for Pb2+ under different concentrations. (b) The corresponding linear
relationship between stripping current and Pb2+ concentrations.
Table 1. Comparison among 3D-PTLFC/SPE and literatures for stripping analysis of Pb2+.
Device
Electrode
Enrichment time (s)
Linear range (µg l−1)
LOD (µg l−1)
Ref.
MOHMSAa PLSb PMDc EFMd 3D-PTLFC/SPE
Bismuth Silver Bismuth Bismuth Bismuth
60 300 120 180 120
25–400 1–1000 0–100 10–100 1–80
8 0.55 2.0 0.74 0.3
[23] [15] [26] [18] This work
a
Microfabricated on-chip heavy metal sensor array. Ploymer lab-chip sensor. c Paper-based microfluidic device. d Electrochemical flow-through microcell. b
The resulting relative standard deviation of the devices is below 3%, which indicates the inserted microfluidic devices with SPEs exhibit excellent reproducibility and are suitable for real sample analysis. 3. Conclusions In summary, a transparent microfluidic device composed of 3D-PTLFC and insertable SPE has been successfully fabricated for heavy metal detection. Both the simulation and experimental results demonstrate that S-shaped SPE can significantly improve the anodic stripping behaviour and enhance its detection performance for Pb2+ with a visible process. Under optimized conditions, a wide linear range of 1–80 µg l−1 is achieved for Pb2+ detection with a limit of 0.3 µg l−1 for the microfluidic device with S-shaped SPEs. Also, the integrated microfluidic devices exhibit excellent reproducibility, continuous detection amd low cost, which presents great potential in monitoring analyses of Pb2+.
Figure 7. The reproducibility of microfluidic devices with SPEs for the detection of Pb2+.
dependency in the concentration range of 1–80 µg l−1. The sensitivity is as high as 0.14 µg l−1 according to the signal-tonoise of three (S/N = 3) protocol. Also, the calculated LOD is around 0.3 µg l−1, which is much lower than the limited value of 10 µg l−1 and 15 µg l−1 in drinking water permitted by World Health Organization (WHO) and US Environmental Protection Agency (EPA), respectively. More importantly, the integrated 3D-PTLFC/SPE microfluidic device exhibits comparable or even preferable detection performance for Pb2+ than those in the literatures, as shown in table 1. Compared with the work reported by Zhao et al [38] (Pb2+ detection limitation 5 µg l−1), the detection limit of Pb2+ was 0.3 µg l−1 and the linear range was from 1 to 80 µg l−1. To evaluate the reproducibility of the microfluidic device for Pb2+ detection, the stripping peak currents and potential are measured with the Pb2+ concentration of 50 µg l−1 (figure 7).
Acknowledgments The work was supported by the NNSF of China (61525402, 61604071), Key University Science Research Project of Jiangsu Province (15KJA430006), Natural Science Foundation of Jiangsu Province (BK20161012), QingLan Project, Research Project of General Administration of Quality Supervision, Inspection and Quarantine (2015IK129, 2016IK138, 2016KJ24, 2017KJ16). 5
Y Hong et al
J. Phys. D: Appl. Phys. 50 (2017) 415303
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ORCID iDs Jinjun Shao
https://orcid.org/0000-0001-6446-8073
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