Research on self-made zinc ion microfluidic chip detection system based on all solid state ion selective electrodetrode

This paper first prepared and characterized an all solid zinc ion selective electrode using electrode deposition, and further established a microfluidic chip system integrating zinc ion ASS-ISE and a self-made potential detection device. The stability of the self-made potential detection device was verified by comparing it with traditional electrochemical workstations. The accuracy of the microfluidic chip detection system was verified through comparative testing with traditional atomic absorption spectrophotometry (AAS). The experimental results show that the accuracy and stability of the microfluidic chip system integrated with ASS-ISE and self-made potential detection device meet the requirements of on-site rapid detection.


Introduction
Due to zinc ions being an important indicator of the Surface Water Environmental Quality Standards (GB3838-2002) [1] , the detection of zinc ions has received high attention from many scholars.At present, the conventional detection methods for zinc ions at home and abroad mainly include UV visible spectrophotometry(UV-Vis), atomic absorption spectroscopy(AAS) atomic fluorescence spectroscopy(AFS), high-performance liquid chromatography(HPLC) and ion chromatography(IC) [2] .Although traditional detection methods have the advantage of high accuracy, it is difficult to achieve on-site and rapid detection of heavy metal ions.In response to the demand for online real-time detection, electrochemical sensing detection systems have attracted much attention.Due to its advantages of good selectivity, high sensitivity, and fast response, ion selective electrodes (ISE) are more suitable for on-site analysis of trace metal ions in environmental water samples.The all solid state ion selective electrode (ASS-ISE) composed of a conductive substrate layer, a solid-state conversion layer, and an ion selective membrane layer can effectively solve the problems of liquid leakage and difficulty in miniaturization in traditional liquid connected ion selective electrodes [3] .At present, there is increasing research on ASS-ISE, mainly focusing on the study of solid conductive materials [4] .This paper preliminarily investigates the application of a solid-state zinc ion selective electrode based on PEDOT/PSS in a self-made microfluidic chip system.

Preparation and Characterization of the Polymer PEDOT/PSS
Considering the shortcomings of traditional liquid connected ion selective electrode detection methods such as internal liquid leakage, low detection sensitivity, and low service life, this paper has studied a fast detection sensing chip based on all solid state electrodes to achieve fast and highly sensitive detection of zinc ions.This paper has prepared a solid-state zinc ion selective electrode and tested its sensing performance.The poly(3,4-ethylenedioxythiophene) and polystyrene sulfonate (PEDOT/PSS) conversion layer was prepared by electrode deposition.The experiment optimized the deposition time of PEDOT/PSS conductive polymer during electrode preparation process.The results showed that using a carbon electrode with a diameter of 2mm as the substrate material, the PEDOT/PSS film with an electrodeposition time of 714s had the best conductivity and capacitance performance (as shown in Figure 1).The effect of ion exchange agents on the performance of solid-state zinc ion selective electrodes was experimentally studied.The results showed that when a mass fraction of 2.1% ion exchange agent was added to the zinc ion selective membrane, the prepared solid-state zinc ion selective electrode exhibited a linear response in a zinc ion solution with a concentration of1×10 -5 mol.L -1 ~1×10 -2 mol.L -1 (as shown in Figure 2).When the solid-state zinc ion selective electrode is repeatedly placed in a zinc ion solution with a concentration of 1×10 -4 mol.L -1 and 1×10 -3 mol.L -1 , the potential fluctuation range is 1-2 mV (as shown in Figure 3).The results indicate that the prepared solid-state zinc ion electrode has good reproducibility.

Preparation of the Ion−Selective Electrode Microfluidic Chip
The integrated ASS-ISE microfluidic chip uses PET as the substrate material, and carbon paste and Ag/AgCl slurry are screen printed on the PET board, which are used as conductive substrates for all solid ion selective electrodes and reference electrodes, respectively.A fully solid zinc selective electrode was prepared by sequentially modifying polymer PEDOT/PSS membrane and ion selective membrane on a carbon conductive substrate.A PVC polymer film containing ionic liquid was modified on an Ag/AgCl conductive substrate to prepare an all solid reference electrode.In order to ensure the accuracy of the test results, this paper sets up four selective solid-state electrodes that can be measured simultaneously in the microfluidic chip.The diameter of the electrode is 2mm, the spacing between the selective electrodes is 6mm, and the spacing between the selective electrode and the reference electrode is 8.5mm.At the same time, a micro mixer was installed at the inlet of the testing pool to ensure sufficient mixing of the sample.

Construction of the Integrated ASS−ISE Microfluidic Chip Test System
This paper establishes a microfluidic chip system that integrates zinc ion ASS-ISE and a self-made potential detection device.Compared with traditional electrochemical workstations, the stability of the self-made potential detection device meets the requirements of on-site detection.In order to further verify the accuracy of the detection results, a comparative test was conducted with traditional atomic absorption spectrophotometry.The experimental results show that the RSD of the microfluidic chip system integrating ASS-ISE and self-made potential detection device is 5.62% in the concentration range of1×10 -5 mol•L -1 to 1×10 -2 mol•L -1 .

Figure 4 .
Figure 4. (a)Schematic diagram of electrodes chip;(b)Schematic diagram of PDMS mold；(c)Schematic diagram of ion-selective electrode chip

Fig 5 .
Fig 5. Schematic diagram of detection system