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A New Electrochemical System Based on a Flow-Field Shaped Solid Electrode and 3D-Printed Thin-Layer Flow Cell: Detection of Pb Ions by Continuous Flow Accumulation Square-Wave Anodic Stripping Voltammetry 2+
Qianwen Sun, Jikui Wang, Meihua Tang, Liming Huang, Zhiyi Zhang, Chang LIU, Xiaohua Lu, Kenneth W. Hunter, and Guosong Chen Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b00383 • Publication Date (Web): 10 Apr 2017 Downloaded from http://pubs.acs.org on April 10, 2017
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Analytical Chemistry
A New Electrochemical System Based on a Flow-Field Shaped Solid Electrode and 3D-Printed Thin-Layer Flow Cell: Detection of Pb 2+ Ions by Continuous Flow Accumulation Square-Wave Anodic Stripping Voltammetry Qianwen Sun,a Jikui Wang,a Meihua Tang, a Liming Huang,*b Zhiyi Zhang,a Chang Liu,a Xiaohua Lu,a Kenneth W. Hunter,b Guosong Chen*a a
College of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 210009, China
b
Department of Microbiology and Immunology, School of Medicine, University of Nevada, Reno, NV 89557, USA
KEYWORDS. Flow-field shaped solid electrode; Flow electrochemical detection system; anodic stripping voltammetry; 3D-Printing; thin-layer flow cell; and Detection of Pb2+ ions
ABSTRACT: Here we describe a new and sensitive flow electrochemical detection system that employs a novel flow-field shaped solid electrode (FFSSE). The system was constructed with a 3D-printed thin-layer flow cell (TLFC) and a flat screen-printed FFSSE with USB connection. This interface facilitates continuous flow accumulation square-wave anodic stripping voltammetry (ASV). The flow distribution in the working space of TLFC was simulated using the Finite Element Method (FEM) and the shape and configuration of electrodes were optimized accordingly. We demonstrated the electrochemical determination of Pb2+ using this newly designed TLFC-FFSSE detection system without removal of oxygen from samples. This TLFC-FFSSE based system showed an attractive stripping voltammetric performance compared to a traditional ASV based method. A linear range for detection of Pb2+ was found to be 0.5 to 100 µg/L (0.5 to 100 ppb) and a detection limit of 0.2 µg/L (0.2 ppb) was achieved in the presence of bismuth as codeposition metal. The system was further applied to detect Pb2+ in biological broths of fermentation methane. The electrochemical detection results were consistent with that obtained from atomic fluorescence spectroscopy (AFS) analysis and the average recovery was found to be 95.5 ~ 106.5﹪ using a standard addition method. This new flow electrochemical detection system showed better sensitivity and reproducibility compared to a traditional ASV based method. Such a system offers great potential for on-site and real-time detection of heavy metals where compact, inexpensive, robust, and low-volume analysis is required.
Anodic stripping voltammetry (ASV) is a powerful tool for rapid, sensitive, and selective detection of trace metals.1-6 In a traditional experiment protocol, three electrodes (a working electrode, a counter electrode, and a reference electrode) have to be placed into a test sample in a container (e.g. a beaker or a reaction vessel).7,8 The metal ions presented in the test sample are gradually deposited on the electrode surface upon continuously stirring of the sample, thereby preconcentrating the analytes. The traditional method has several disadvantages including requirement of a large volume of sample due to the immersion of three electrodes, low accumulation efficiency due to exposure of the electrode surface to a small percentage of sample, and less desirable reproducibility on the stripping voltammetric peak current and analysis results due to the variation of the stirring during the deposition step.9,10 To overcome these disadvantages, flow ASV systems based on thin-layer cells have been explored by several laboratories.11,12 In this article, we describe a new flow detection system equipped with a 3D-printable thin-layer flow cell to replace the beaker (or the
reaction vessel), a solid three-electrode printed on a flat plastic board to replace three individual electrodes,13 and a peristaltic pump driving flow continuously to substitute for the stirring process. This new flow detection system has several advantages. (1) The sample volume for immersing the three-electrode in the thinlayer cell can be as low as 50 µL and the total consumption of sample per test is less than 5 mL; (2) The system is highly sensitive because the fresh solution with high concentration of the analytes is continuously flowing through the surface of the electrode, resulting in high accumulation efficiency in a short time; (3) A good reproducibility can be achieved due to the stability and fixed distance of the electrodes when exposed to the solution with a constant flow rate, resulting in a more stable stripping peak current.14,15 In addition, the use of a 3D-printable thin-layer flow cell (TLFC) could minimize the number of components used in the flow system,16-18 and the flow distribution of working area of the 3Dprinted TLFC was simulated using the Finite Element Method (FEM).19-21 We thus fabricated a novel FFSSE using a simulated
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Analytical Chemistry
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flow distribution that maximizes exposure of electrodes to the flow, increases accumulation efficiency, and improves sensitivity. The system, based on 3D-printed TLFC and FFSSE using continuous flow accumulation square-wave ASV, was further demonstrated to detect Pb2+ presented in standard solutions as well as biological broths of fermentation methane. Bismuth nitrate was used to
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Leici Instrument), a BT100-1L multi-channel peristalticpump (LongerPump Instrument), and a PHS-3C digital analytical balance (Germany sartorius Instrument). Screen-printed electrodes were selfmade (working electrodes and auxiliary electrodes were printed with carbon paste, and reference electrodes were printed with Ag/AgCl paste.).
achieve well-defined, sharp, and highly reproducible stripping peaks for low concentrations of lead.2 The electrochemical detection results
Design and preparation of 3D-printable thin-layer flow cell (TLFC). In a traditional stripping voltammetry analysis in a
were consistent with that obtained from atomic fluorescence spectroscopy (AFS) analysis and the average recovery was found to be 95.5 ~ 106.5﹪ using a standard addition method.
stirred solution, the accumulation efficiency of metals on the electrode surface is low due to slow diffusion of metal ions in a relatively large volume of sample.22,23 In contrast, the combination of a thin space cell and continuous flow of samples could significantly improve the accumulation efficiency of metals in a small sample volume. As shown in Figure 1b and 2, the thickness of the working space of the TLFC cell (0.8 mm) was designed to be similar to the diameter of the outlet and inlet in order to achieve stable flow rate and good reproducibility. A saddle-shaped space above the working surface of the electrode was designed to minimize the dead volume during the flowing process. A complete TLFC (with an outlet, an inlet, an electrode housing, and a saddle-shaped working area) was printed out using a PolyJet 3D printer using acrylate-based polymer materials. Design of flow-field shaped solid electrode (FFSSE). Since the flow distribution is not uniform in the TLFC, laying the electrode over the area with higher flow may increase the accumulation efficiency, the stripping voltammetric peak current, and sensitivity compared to having the electrode on the area with lower flow. Based on this consideration, we introduced the concept of FFSSE where the electrode was aligned to match the flow distribution. Therefore, the flow distribution inside the saddle-shaped working space of TLFC was simulated using the established FEM model of COMSOL Multiphysics software based on the Navier-Stokes equation (equation 1) and the convection diffusion equation (equation 2). (1)
EXPERIMENTAL SECTION Reagents and apparatus. Pb2+ standard solution (1000 μg/mL, AccuStandard) was diluted to prepare Pb2+ solutions with different concentrations. Bismuth nitrate (AR, Shantou Xilong Chemical Co.) was used for codeposition to enhance the ASV performance. An acetate buffer solution (HAc-NaAc, AR, Sinopharm Chemical Reagent Co., 0.1 mol/L, pH 4.7) was used as supporting electrolyte. Ag/AgCl paste (JLL-100) was purchased from Shanghai JuLong electronic technologies. All other reagents are in analytical reagent grade and used without further purification. Water used in the experiments is ultra-pure (resistivity > 18.3 MΩ·cm). Water test samples in this work were collected from fermentation tanks (stainless steel) of biological methane. The apparatuses used in the experiments are a CHI660E chemical workstation (Shanghai CH instruments), a BS 124S pH meter (Shanghai
ρ(
Figure 1. (a) Flow velocity profiles in a microfluidic TLFC with 90º and 0º injection angles. Samples were injected from the inlet on the left to the outlet on the right with an injection angle of (A) 90º and (B) 0º with respect to the flow direction inside the cell; (b) 3Dprinted TLFC; (c) 3D-printed TLFC and a long-strip shaped screenprinted three-electrode; (d) 3D-printed TLFC and a FFSSE electrode.
∂u + u ⋅ ∇u ) = −∇p + µ∇ 2 u + f ∂t
Figure 3. Picture of screen-printed three-electrode with different shape in the high flow working area (A and D), the lower flow working area (B and E), and the full working area (C and F). (2)
∂c = D∇ 2 c − u∇c ∂t
Figure 2. Schematic diagrams of a 3D-printed TLFC and its sizes (mm). (a) The internal structure of 3D-printed TLFC for FFSSE insertion (thickness 0.8 mm); (b) a complete 3D-printed TLFC.
Whereρis the fluid viscosity, μis the kinematic viscosity, p is the pressure, u is the flow rate of the sample, f is the external power, D is the analyte diffusion coefficient, and c is the analyte concentration. Figure 1a shows the FEM simulated flow velocity distribution for 90° (A) and 0° (B) flow injection. Interestingly, with a 90° injection, the flow distribution showed a S-shaped pattern. In contrast, it showed a straight
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Analytical Chemistry
line with a 0° injection. The S-shaped flow distribution pattern may be caused by the flow inertia. Based on the simulation results, curved and linear FFSS electrodes were designed and fabricated (Figure 1c and 1d). To obtain the relation between the stripping current and the surface area of the electrode, we further analyzed the variation of diffusion layer with different thickness of thin layer cell at the same velocity using FEM simulation based on the Levich equation (equation 3).24 (3) 2 3 s 0
1 2
1 2
I l = 0.62nFD c bL u v
−
1 6
Where I1 is the Levich current, n is the the number of moles of electrons transferred in the half reaction, F is the Faraday constant (C/mol), Ds is the analyte diffusion coefficient, c0 is the analyte concentration, b is the width of the electrode, L is the length of the electrode, u is the flow rate of the sample, and v is kinematic viscosity. As shown in the equation 3, the current increases as the electrode area and the flow rate of the sample increase. Since the area of a FFSSE electrode (∼25 mm2) is larger than a traditional electrode (e.g. 4 mm diameter,
