Laser-Induced Carbon Electrodes in a Three-Dimensionally Printed Flow Reactor for Detecting Lead Ions.

Nowadays, heavy metal pollution has attracted wide attention. Many electrochemical methods have been developed to detect heavy metal ions. The electrode surface usually needs to be modified, and the process is complicated. Herein, we demonstrate the fabrication of electrodes by direct laser sintering on commercial polymer films. The prepared porous carbon electrodes can be used directly without any modification. The electrodes were fixed in a 3D-printed flow reactor, which led to very little analyte required during the detection process. The velocities of the analyte under stirring and flowing conditions were simulated numerically. The results prove that flow detection is more conducive to improving detection sensitivity. The limit of detection is about 0.0330 mg/L for Pb2+. Moreover, the electrode has been proved to have good repeatability and stability.

Introduction

With the rapid development of economy and technology, the pollution of heavy metals such as lead ions has caused widespread concern.[1] Pb2+ is a kind of toxic heavy metal ion that is harmful to human health and can damage the nervous and digestive systems. Lead poisoning can cause intellectual and developmental problems.[2,3] For human health, the detection and analysis of heavy metal ions are critical. At present, the detection methods of lead ions mainly include absorption photometry, atomic absorption spectroscopy, atomic fluorescence spectrometry, inductively coupled plasma atomic emission spectrometry, X-ray fluorescence spectrometry, and mass spectrometry.[4−7] Although these methods are very sensitive, they have some disadvantages, such as expensive equipment, complex preprocessing, long-time consumption, and poor real-time detection.

Compared with other detection methods, electrochemical detection has the advantages of fast analysis speed, high sensitivity, simple operation, low detection cost, and no requirement of expensive large-scale instruments. It is widely used in the detection of heavy metal ions.[8−11] The most commonly used electrochemical detection technique is voltammetry, including cyclic voltammetry (CV), linear scanning voltammetry (LSV), anode stripping voltammetry (ASV), square wave voltammetry (SWV), and differential pulse voltammetry (DPV).[12−14] In ion trace analysis, the most common detection method is stripping voltammetry, which combines the constant potential analysis with the voltammetric analysis.[15] The analysis method has two steps. The first step is to make the analyte perform constant potential electrolysis in the stirred state, which will make the analyte be reduced and deposited on the cathode. This process is called the enrichment process. The second step is to apply a reverse scanning voltage to the electrode after a period of quiescence so that the analyte deposited on the cathode is oxidized and redissolved in the solution. This process is called the stripping process.[16,17] Compared with the direct voltammetric analysis of the original solution, the biggest advantage of this method is that the analyte can be pre-enriched on the electrode, so the dissolution current is not affected by the charging current and the impurity residual current.

Metal oxides, expensive metals, and carbon are often used as electrodes for detecting heavy metal ions.[18,19] But metal oxide has low conductivity. Noble metals are scarce, and their high price limits their widespread use. However, as one of the most common elements in nature, carbon has good biocompatibility and is a kind of a low-consumption material,[20,21] so it is often used to make electrodes. Traditional carbon materials are activated carbon, graphite, carbon nanotubes, and graphene. To improve material performance, researchers have introduced the concept of porosity into the construction of carbon materials. Porous carbon has many advantages, such as a large specific surface area, good conductivity, and many active sites. It is a very suitable electrode material.[22]

Herein, porous carbon electrodes were prepared by CO2 infrared laser sintering on commercial polymer films. The process is simple to operate and is a new manufacturing technology, which significantly improves production efficiency and reduces costs. Tour et al. used this technique to make polymer-written electronic and energy storage devices.[23] The application of this technology in microsupercapacitors has been widely reported,[24−26] but the application in detection is scarce. In this work, we use this technology to prepare electrochemical sensors for the detection of lead ions.

In the electrochemical detection process, lead ions were first deposited on the electrode surface and then dissolved by DPV.[27−29] The critical step to detect heavy metal ions is the ion deposition process. High deposition efficiency will improve the sensitivity of detection. The traditional method to improve the ion deposition efficiency is usually by stirring the solution. However, it has some drawbacks, such as the generation of a non-uniform and time-dependent advection, which influences the detection signal, and it usually requires a large number of analytes. Compared with stirring in batch reactors, flow detection is easy to describe and control and requires fewer analytes. Thus, a 3D-printed reactor was prepared to realize the flow detection of lead ions.[30−34] When the electrochemical analysis is performed, the analyte flows over the electrode surface, and then the corresponding electrochemical signal can be quickly displayed. Therefore, lead ions can be effectively detected in real time.

Results and Discussion

Characterization of the Laser-Induced Porous Carbon (LIPC)

When the laser irradiates the polyimide (PI) film, PI begins to depolymerize and carbonize. Because laser sintering takes place in the atmosphere, O2 in the air can quickly react with carbon at high temperatures to produce carbon monoxide and carbon dioxide gases. The thermolysis of the PI film also produces these gases to form pores. Therefore, the carbonization layer on the PI film has a porous structure.[35] The porous carbon was produced by laser irradiation at a power of 27 W. Characterization was performed by scanning electron microscopy (SEM), Raman spectrometry, X-ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) test.

The SEM image clearly shows that the laser-induced structure has a porous morphology, as shown in Figure A. The cross-sectional SEM image (Figure B) compares the LIPC structure with the PI substrate. It can be clearly seen from the figure that the structure of PI changed significantly after decomposition and carbonization. The inset in Figure A shows the confocal Raman spectra of the porous carbon. The yellow line is the Raman spectrum of porous carbon on the laser-engraved track, and the blue line is the Raman spectrum of porous carbon between the two tracks. There are three strong peaks, namely, the D peak at 1352 cm–1, the G peak at 1585 cm–1, and the 2D peak at 2701 cm–1. The D peak is induced by defects or bent sp2-carbon bonds. The G peak and 2D peak are associated with second-order zone-boundary phonons.[23] In addition, the D/G peak intensity ratio represents the graphitization degree of carbon, and the smaller the D/G peak intensity ratio is, the better the graphitization degree of carbon is. It can be seen from the figure that the graphitization degree of carbon between two laser-engraved tracks is higher than that on the laser-engraved track. This is because the sintering temperature at the laser-engraved track is very high. Excessive temperature will destroy the carbon structure, making the graphitization degree of the porous carbon in this region lower than that of the porous carbon between two laser-engraved tracks.

Figure 1

Characterization of LIPC. (A) SEM image of LIPC. The scale bar is 500 μm. The inset is the Raman spectra of carbon on the region of the laser scratch track and between the two tracks. (B) Cross-sectional SEM image of LIPC on the PI film. The scale bar is 100 μm. (C) XRD of powdered LIPC scraped from the PI film. (D) N2 adsorption–desorption isotherm of powdered LIPC scraped from the PI film. The inset is the PSD of the sample calculated from the desorption isotherm.

The XRD pattern shows two peaks at 2θ = 25.88° (002) and 42.88° (100), respectively (Figure C). The 002 peak intensity is relatively low. It indicates that the porous substance is carbon with a low degree of graphitization. Figure D shows the N2 adsorption–desorption isotherm of powdered LIPC. The inset is the pore size distribution diagram (PSD) of the sample calculated from the desorption isotherm. BET multipoint method analysis results show that the specific surface area of LIPC is 191 m2/g, the total pore volume is 0.34 mL/g, and the average hole diameter is 7.1 nm.

Optimization of the Laser-Induced Porous Carbon Electrode (LIPCE)

The power and path of laser irradiation can affect the carbonization of the PI film, which further affects the sensitivity of lead ion detection. To obtain the best electrochemical detection performance, the laser irradiating power and path were optimized.

LIPCs with powers of 15 W (LIPC15), 21 W (LIPC21), 27 W (LIPC27), and 30 W (LIPC30) were fabricated. SEM images of LIPC samples (Figure A–D) show the effect of laser power on the laser-induced carbon structure. The specific surface areas of LIPC15, LIPC21, LIPC27, and LIPC30 were tested by a specific surface area analyzer, and their specific surface areas are 261.476, 230.525, 215.033, and 314.867 m2/g, respectively. The result shows that the specific surface area of LIPC30 is much larger than those of LIPC15, LIPC21, and LIPC27. It indicates that when the laser power is 30 W, more pores are formed on the surface of the PI film. LIPC15, LIPC21, LIPC27, and LIPC30 were also tested by the confocal Raman spectrometer (Figure F). It shows that the degree of graphitization of LIPC21, LIPC27, and LIPC30 is significantly higher than that of LIPC15.

Figure 2

Optimization of the LIPCE formed at different laser powers. SEM images of LIPCs formed at different laser powers: (A) 15 W, (B) 21 W, (C) 27 W, and (D) 30 W. (E) Digital photo of the LIPCE. (F) Confocal Raman spectrometry of LIPCs with different powers. (G) CV curves of different power LIPCEs in 0.1 M KCl and 5 mM [Fe(CN)6]3–/4–. (H) DPV curves of different power LIPCEs in 0.4 mg/L Pb2+ solution (buffer: 0.1 M NaAc-HAc containing 0.1 M NaCl, pH = 4).

Then, the PI films sintered with 15, 21, 27, and 30 W laser powers were made into electrodes to detect lead ions. Figure E shows a digital photo of the LIPCE. These electrodes were tested by CV (Figure G). It shows that the performance of LIPCE21, LIPCE27, and LIPCE30 is better than that of LIPCE15. Then, the electrodes were used to detect 0.4 mg/L Pb2+ by stripping voltammetry. Figure H shows the corresponding DPV curves. Compared with LIPCE15, we can see that LIPCE21, LIPCE27, and LIPCE30 are more sensitive to lead ion detection. It shows that the highly graphitized LIPC makes the concentration of lead ions deposited on the electrode surface higher and the rate of dissolution faster. The difference of their specific surface area has no effect on the electrochemical test results. There is almost no difference between LIPCE21, LIPCE27, and LIPCE30. Here, we choose LIPCE30 for the follow-up experiments.

Then, the porous carbon electrodes prepared by six different laser sintering paths were studied. The first one (1) was perpendicular to the long side of the electrode, the second one (2) was at an angle of 45° to the long side of the electrode, the third one (3) was parallel to the long side of the electrode, the fourth one (4) was sintered by path (1) twice, the fifth one (5) was sintered by path (3) twice, and the sixth one (6) was sintered by path (1) and then sintered by path (3). SEM images are shown in Figure A. It can be seen from the figure that the porous structure of the LIPC surface sintered through paths (4) and (5) is more obvious. However, when the PI film was sintered in accordance with the designed electrode pattern, the surface of the LIPCE prepared through path (2) would be damaged. This is because the length of the sintering path is uneven and irregular, leading to uneven heating on the PI surface. This caused some porous carbon layers to warp off from the PI surface. Incomplete surfaces could negatively affect the results of electrochemical detection. Therefore, it should not be made into an electrode. The performance of other five LIPCE30 electrodes prepared by different laser irradiation paths was compared by CV (Figure B). It can be seen from the figure that the redox peak intensity of the fourth electrode is higher and the peak potential difference (ΔEP) is smaller than others. It shows that the laser irradiation path (4) makes the charge transfer on the electrode surface easier and faster. Then, the sensitivity of the five electrodes to lead ion detection was compared by DPV (Figure C). The LIPCE30 with the laser irradiation path (4) (LIPCE30 (4)) shows the highest DPV response. It indicates that the laser irradiation path (4) makes the lead ion concentration deposited on the electrode surface higher and the dissolution rate faster. The results show that LIPCE30 (4) had excellent electrochemical performance, and we used it in the subsequent experiments.

Figure 3

Optimization of LIPCE30 formed at different laser sintering paths. (A) SEM of porous carbon generated by six different laser sintering paths. (The laser sintering path of (1) was perpendicular to the long side of the electrode. The laser sintering path of (2) was at an angle of 45° to the long side of the electrode. The laser sintering path of (3) was parallel to the long side of the electrode. (4) The laser sintered the same path as (1) twice. (5) The laser sintered the same path as (3) twice. (6) The laser sintered the same path as (1) and then sintered the same path as (3).) (B) CV curves of LIPCE30 with different laser-irradiated paths in 0.1 M KCl and 5 mM [Fe(CN)6]3–/4–. (C) DPV curves of the LIPCE30 with different laser-irradiated paths in 0.4 mg/L Pb2+ solution (buffer: 0.1 M NaAc-HAc containing 0.1 M NaCl, pH = 4).

Electrochemical Characterization of the LIPCE

The electrochemical characterization of LIPCE30 (4) was performed by CV. As shown in Figure A, the CVs of 5 mM [Fe(CN)6]3–/4– were recorded using LIPCE30 (4) at different scan rates (20–200 mV/s). The CVs have two peaks: the oxidation peak and the reduction peak. ΔEP is greater than 0.058 V, and iPa/iPc is greater than 1. With the increase in the scan rate, the oxidation peak position shifts to the positive direction, and the reduction peak position shifts to the negative direction. Moreover, the peak current increases with the increase in the scan rate and is proportional to the square root of the scan rate (Figure B). It indicates that this is a quasi-reversible electrode reaction process. In the electrode reaction, electrically active substances first reached the electrode surface through diffusion and then participated in the reaction by adsorption on the electrode surface. The relationship between the peak current and the square root of the scan rate is linear, which indicates that the electrode process was mainly controlled by diffusion. The electrochemical surface area (ECSA) of LIPCE30 (4) was estimated by the Randles–Sevčik equation:[36−41]where IP is the peak current, A is the effective surface area, D is the diffusion coefficient of K3Fe(CN)6 (7.6 × 10–6 cm2/s), n is the number of electrons transferred (n = 1), v is the scan rate, and C is the concentration of K3Fe(CN)6. For LIPCE30 (4), the ECSA is about 0.3275 cm2.

Figure 4

Electrochemical performance of LIPCE30 (4). (A) CV curves of LIPCE30 (4) in 0.1 M KCl and 5 mM [Fe(CN)6]3–/4–. Scan rates from 20 to 200 mV/s. (B) Plots of peak current versus the square root of the scan rate corresponding to (A).

Optimization of Analytical Conditions

Determining the best experimental conditions was the key to obtain the highest detection sensitivity.[21] The analytical conditions were optimized before evaluating the electrochemical detection performance of LIPCE30 (4).

The enrichment process has a great influence on the detection sensitivity of lead ions, and usually, the solution must be stirred to reduce the thickness of the diffusion layer. The flow of the solution allows the lead ions to effectively approach the electrode surface and more easily adsorb on the porous carbon electrode. Here, we used a flow reactor to make the liquid flow. The velocity distribution near the electrode surface was simulated numerically when the electrode was in batch and flow reactors (Figure ). It can be seen that agitation leads to uneven dynamics, and mass transfer is mainly limited to the liquid phase far away from the solid phase (Figure B), which is not beneficial for the adsorption of the ions. However, in the flow reactor, the flow of solution can not only produce more stable advection in a controlled and predictable manner, but more importantly, it can also effectively improve surface dynamics (Figure E).

Figure 5

Numerical simulation of agitation and flow. (A) Schematic diagram of the magnetic agitator. (B, E) Velocity field distribution on the parallel surface 0.3 mm away from the electrode surface in the stirred state and flow state, respectively (unit: m/s). (C, F) i–t and DPV curves of LIPCE30 (4) in 0.4 mg/L Pb2+ solution, respectively (stirring rate: 300 r/min; flow rate: 2 mL/min). (D) Schematic diagram of the flow device.

It can be seen from Figure C that the i–t curve in the flowing state is much smoother than that in the stirring state, which indicates that the stirring of the solution will cause current fluctuations and unstable deposition of lead ions. However, the solution’s flow keeps the analyte on the electrode surface always in a new state, and lead ions can be stably deposited on the electrode surface and generate a stable current. It can be seen from Figure F that the detection sensitivity of the electrode to lead ions in the flow state is higher than that in the stirred and static states. Therefore, flow detection was adopted in the following experiments.

The deposition time from 100 to 1000 s was studied (Figure A). The peak current increases with the increase in electrodeposition time, which indicates that extending the electrodeposition time can improve the sensitivity of lead ion detection. But with the increase in electrodeposition time, the linear relationship will become worse. Therefore, we choose 200 s as the best electrodeposition time. The deposition potential from −1.8 to −0.8 V was studied (Figure B). The peak current increases significantly from −1.8 to −1.2 V and reaches a maximum at −1.2 V, and then, the current decreases. So, −1.2 V was selected as the optimal deposition potential. Similarly, the flow rate of the analyte from 1.4 to 2.4 mL/min was studied (Figure C). The DPV response peak current increases with increasing flow rate and reaches a maximum at 2 mL/min. Then, the current drops significantly. When the flow rate is less than 2 mL/min, the deposition amount of lead ions increases with the increase in the flow rate. When the flow rate is more than 2 mL/min, the lead ion cannot be deposited effectively due to the high flow rate. So, the flow rate of 2 mL/min was selected for this work. Then, the pH of the buffer was optimized. Figure D shows the effect of NaAc-HAc buffer pH on electrochemical behavior. The response peak current of the DPV curve increases with the increase in the pH value and reaches the maximum at pH = 4. Then, the current gradually decreases. When the pH of the buffer increases, the concentration of OH– in the solution also increases. OH– will react with Pb2+, leading to a decrease in the concentration of lead ions in the solution. So, the DPV response peak current goes down. Therefore, the buffer solution with pH = 4 was selected for the experiment.

Figure 6

Optimization of electrochemical analytical conditions. Plot of the DPV peak current curve versus (A) deposition time, (B) deposition potential, (C) analyte flow rate, and (D) buffer pH.

Analytical Performance of the Sensor toward Pb2+ Detection

To realize the quantitative detection of Pb2+, the sensor detected different concentrations of Pb2+ through DPV measurement. LIPCE30 (4) was used as the electrode, and the experiment was performed under the optimized conditions. Figure A shows the DPV curves for the concentrations from 0.05 to 0.35 mg/L. Figure B shows that peak currents are linearly related to the concentrations of Pb2+, and the corresponding calibration curve is y = 6.4440x – 0.2041 (R2 = 0.9849). The limit of detection (LOD) is estimated to be 0.0330 mg/L for Pb2+.

Figure 7

Analytical performance of the sensor toward Pb2+ detection. (A) DPV recordings obtained for increasing concentrations of Pb2+ (0.05–0.35 mg/L) at LIPCE30 (4) in 0.1 M NaAc-HAc containing 0.1 M NaCl (pH = 4). Deposition potential: −1.2 V; deposition time: 200 s; flow rate of the analyte: 2 mL/min. (B) The calibration curve corresponded to (A).

Reproducibility, Stability, and Anti-interference Studies

The reproducibility of LIPCE30 (4) was evaluated by 5 times DPV measurements of 0.4 mg/L Pb2+ (Figure A). The relative standard deviation (RSD) is 4.79% (Figure B). It indicates that the reproducibility of LIPCE30 (4) is good. The stability of LIPCE30 (4) was evaluated by CV measurement in a 5 mM [Fe(CN)6]3–/4- solution containing 0.1 M KCl. Figure C shows that the CV oxidation peak current changes little within 5000 sweep segments, indicating that LIPCE30 (4) has high stability. To study the anti-interference of LIPCE30 (4), some possible interfering metal ions were added into 0.4 mg/L Pb2+. These ions were 10 times more concentrated than lead ions. It can be seen from Figure D that these metal ions have little interference with Pb2+ except for Cu2+. When the interference of Cu2+ to lead ion detection was studied, the oxidation peak of Cu2+ would also appear. It indicated that Cu2+ and Pb2+ were deposited together on the electrode surface when the deposition potential was −1.2 V. Then, Cu2+ would be dissolved together with Pb2+. So, the lead ion has a lower response peak current. However, lead ions still can be detected in the presence of Cu2+. These characteristics make LIPCE30 (4) have a broad application prospect in electrochemical detection.

Figure 8

Reproducibility, stability, and anti-interference studies. (A) Five DPV recordings of Pb2+ (0.4 mg/L) at the same LIPCE30 (4) in 0.1 M NaAc-HAc containing 0.1 M NaCl (pH = 4). (B) Plot of peak current versus the cycle number corresponding to (A). (C) Plot of CV oxidation peak current versus sweep segments in 0.1 M KCl containing 5 mM [Fe(CN)6]3–/4–. (D) Anti-interference study of LIPCE30 (4) to 0.4 mg/L Pb2+ in the presence of 4 mg/L Ba2+, Co2+, Cu2+, Hg2+, Zn2+, Mg2+, Ca2+, and Cd2+.

Conclusions

In conclusion, we reported a low-cost, direct laser sintering method for electrode preparation. The electrode can detect lead ions quickly and efficiently. The laser irradiation power and path significantly affect the electrochemical performance of LIPCE. The best laser irradiation power and path were obtained through optimization. According to the electrochemical characterization, the electrode process was controlled by diffusion. The electrochemical deposition was usually carried out in the state of agitation. To compare the effects of agitation and flow on electrochemical detection, the velocity distribution near the electrode surface under stirring and flowing conditions was simulated numerically. Simulation results showed that the laminar flow in a continuous flow reactor was more beneficial to the improvement of electrode surface dynamics than stirring in a batch reactor, and electrochemical experiments also showed that flow detection had higher sensitivity. Under the optimal experimental conditions, the sensor was used for the quantitative analysis of Pb2+. The estimated LOD was 0.0330 mg/L. The study also proved that LIPCE30 (4) had good stability and repeatability. Therefore, the sensor was expected to be widely used for more electrochemical detections.

Experimental Section

Materials and Reagents

PI was obtained from Changchun Gao Qi Polyimide Material Co., Ltd. Co(NO3)2·6H2O, K3[Fe(CN)6], K4Fe(CN)6·3H2O, CH3COONa, CH3COOH, HCl, and KCl were bought from Tianjin Kemiou Chemical Reagent Corp. Mg(NO3)2·6H2O, Ba(NO3)2, and Pb(NO3)2 were received from the Shanghai Aladdin Biochem Technology Corp. Zn(NO3)2·6H2O and Ca(NO3)2·4H2O were purchased from Damao Chemical Reagent Factory (Tianjin, China). NaCl and Cd(NO3)2·4H2O were obtained from Shanghai Macleans Biochemical Technology Co., Ltd. (CH3COO)2Cu·H2O was bought from Sinopharm Chemical Reagent Co., Ltd. Hg(NO3)2·H2O was received from West Asia Chemical Technology (Shandong) Co., Ltd. The copper sheet was bought from Shanghai Lujiang Hardware. Silver wire (1.0 mm in diameter, annealed, 99.9%, metal basis) was obtained from Alfa Aesar Chemical Co., Ltd. Epoxy adhesive (Devcon 14250) was obtained from Shenzhen Darbond Technology Co., Ltd. Deionized water was used for all of the experiments.

Preparation of the Working Electrode (WE), Conference Electrode (CE), and Ag/AgCl Reference Electrode (RE)

Both WE and CE were manufactured by laser sintering on PI films (Scheme A). Using a CO2 laser (marking speed: 400 mm/s; frequency: 15 kHz; resolution: 0.001 mm; repeated positioning accuracy: 0.003 mm), the PI film was converted into a porous carbon electrode with a design pattern (7.48 × 28.05 mm). The RE was prepared by an electroplating method. First, the silver wire was ultrasonically cleaned in alcohol for 30 min and rinsed with deionized water. It was then electroplated for 30 min in a 0.1 M HCl solution by chronopotentiometry, with commercial Ag/AgCl as the RE and Pt as the CE. The silver wire was coated with AgCl at last, and we obtained the Ag/AgCl RE.

Scheme 1

Procedure of Electrochemical Analysis

(A) Laser sintering process; (B, C) schematic and digital photographs of the electrochemical sensor, respectively.

Assembly of the 3D-Printed Flow Reactor and Electrodes

We used Solidworks to design the reactor structure and imported the STL files into a 3D printer. The reactor can be accurately manufactured with the 3D printer at a high resolution of 50 μm. The flow reactor structure with the reaction channel is schematically illustrated in Scheme B. The reaction channel is 14 mm in length, 1 mm in depth, and 1 mm in width. A hole (r = 0.65 mm) for the RE was made on the longer side of the channel.

The Ag/AgCl RE was fixed in the reactor with epoxy adhesive. The PI film with the WE and CE was fixed between the silicon gasket and the higher flange. The whole device was then fixed with screws. The copper sheets, which were fixed on the edge of both WE and CE with conductive silver glue, were used to ensure good contact between the sensor and the electrochemical workstation. Scheme B,C shows schematic and digital photographs of the electrochemical sensor, respectively. The inlet and outlet of the channel were connected by hoses. The inlet hose was then connected to a syringe pump.

Electrochemical Measurements

A three-electrode system was used in all experiments. The electrodes were characterized by CV in 0.1 M KCl containing 5 mM [Fe(CN)6]3–/4–. Except for special declarations, the scanning rate of the CV test was 0.1 V/s. The lead ions were determined by DPV. The pulse width was 0.05 s, the pulse period was 0.5 s, and the pulse amplitude was 0.05 V.

Calibration Experiments

The RSD between the electrodes can be solved by calibration experiments. First, we use electrode A to obtain the standard curve and its peak current (IPA) in 0.4 mg/L Pb2+ solution. For the other electrode B, we can also obtain its peak current (IPB) in 0.4 mg/L Pb2+ solution. IPA/IPB is the correction factor (σ). When using the B electrode to detect the sample, the corresponding peak current (IPI) is obtained. IPI × σ is the peak current of the A electrode for the sample, and then the concentration of the sample is obtained from the standard curve.

Characterization

SEM images were obtained by a QUANTA 450 scanning electron microscope at 20 kV. The pore properties were surveyed by an ASAP 2010 analysis instrument at 77 K. The specific surface areas were calculated by the BET method, and the pore size was calculated using the Barrett–Joyner–Halenda (BJH) model. XRD patterns were measured on a Rigaku D/MAX-2400 X-ray powder diffractometer (Japan) operating at 40 kV and 100 mA using Cu Kα radiation. The Raman spectra were recorded with a Nicolet Almega XR Raman system with a 532 nm laser. LIPC was obtained by a CO2 laser (Coherent C series 30 W, 10.6 μm wavelength). The injector containing the analyte was controlled by a syringe pump (Leifu, TYD03-01). The electrochemical performance of the LIPCE was studied using CV and DPV on a CHI model 660D electrochemical workstation (CH Instrument, Inc.). The flow reactor was produced by 3D printing from stereolithography resin (methacrylate photopolymer resin, Formlabs Form 2, 0.05 mm layer resolution).