# Research on On-Line Detection Method and System for Nitrate in Groundwater
## 2 Experimental Part
### Experimental Process  
The microsensor electrode chip prepared by MEMS technology is used as the input end of excitation voltage and the receiver end of the response current of the electrochemical sensor electrode in the indoor testing room (State Key Laboratory of Sensors, China). The sensor chip integrates the working electrode (WE) made of platinum, the counter electrode (CE) made of platinum and the insulating layer on the same glass substrate, overcoming the shortcomings of the traditional discrete electrode system with its complex preparation process and poor electrode consistency. The WE is circular and its geometric area is 1 mm\({}^{2}\). The shape of CE is a 320\({}^{\circ}\) arc of a ring with an inner diameter of 2.2 mm and an outer diameter of 3.8 mm, as shown in Figure 1. The electrode structure is concentrically symmetrical, with uniform electric field distribution, which is consistent with the diffusion direction of the ions in the electrochemical reaction system. It can better suppress current noise and eliminate interference and improve the electrochemical performance of the sensor. All electrodes are connected to the PCB through gold wire. The detailed fabrication process of the microelectrode has been reported in our previous published paper [30; 31]. For NO\({}_{3}\)- detection, Na\({}_{2}\)SO\({}_{4}\) solution containing a certain concentration of Cu\({}^{2+}\) ions is used as the testing base solution (50 mL, 0.1 mol/L Na\({}_{2}\)SO\({}_{4}\) + 0.05 mol/L CuSO\({}_{4}\), pH = 2.0). Each detection cycle includes three stages:  
1. In the cleaning stage, at the dissolution potential E\({}_{1}\), the microelectrode is cleaned, which removes the copper-sensitive layer deposited on the electrode surface. 2. In the modification stage, at the deposition potential E\({}_{2}\) of the Cu\({}^{2+}\) ions, electrodeposition is added to modify the microelectrode so that a layer of copper-sensitive material is deposited onto the electrode surface. 3. In the detection stage, the response current is detected at the detection potential E\({}_{3}\) of the reduction peak of the NO\({}_{3}\)- ions. During detection, the voltage on the working electrode is changed according to the requirements of each of the three stages so that the electrochemical catalytic reaction  
Figure 1: Photo of the microelectrode fabricated by MEMS technology and the configuration of the electrochemical experimental system. occurring during each test is carried out on a newly prepared sensitive surface, thus eliminating the influence of the electrode passivation effect on NO3\({}^{-}\) detection. Based on this test principle, the change curve of the potential on the working electrode over time during one detection cycle is shown in Figure 2:  
In order to comprehensively investigate the relevant potential parameters, in 50 mL mixed solution containing 0.1 mol/L Na2 SO4, 0.05 mol/L CuSO4 and 3.5 mol/L NaNO3 (in the subsequent optimization experiment, if there is no special explanation, the test solution used is all mixed solution with this ratio), cyclic voltammetry was performed in the range of +0.5 V to \(-\)0.8 V to determine the peak potential values of relevant parameters in the three stages. The scanning results are shown in Figure 3:  
Figure 3: Cyclic voltammetry scanning curve in 50 mL mixed solution containing 0.1 mol/L Na2 SO4, 0.05 mol/L CuSO4 and 3.5 mol/L NaNO3: (a) NO3\({}^{-}\) reduction peak; (b) Cu2\({}^{+}\) reduction deposition peak; (c) Cu dissolution peak. Figure 2: Schematic representation of potential protocol in one detection cycle: relation curve of the operating voltage on the working electrode with time including: (a) cleaning stage, (b) modification stage and (c) detection stage. As can be seen from the figure, two peaks appeared in the process of voltage scanning from +500 mV to \(-\)800 mV.