Development and application of an all-solid-state Mg²⁺ microneedle sensor for crop in-situ detection

A microneedle all-solid-state selective electrode (ASS-ISE) was developed to rapidly in-situ detect Mg²⁺ in the crop stems. The electrode consists 3 parts from the inner to the outer layer, consisting of stainless-steel microneedle, solid contact and ion ion-selective membrane (ISM). This sensor was prepared as follows. Firstly, a stainless-steel microneedle was used as the conductive substrates. The graphene conductive layer was coated on the tip of the microneedle after drying for 12 h. And then, a solid-state contact of Poly (3,4-ethylenedioxythiophene) / Poly (styrene sulfonate, PEDOT/PSS) was electroplated onto the graphene surface; After the second drying for 12 h, an ion selective membrane (ISM) was coated on the tip of the microneedle; The sensor was fabricated after the third drying time of 12 h and activation. A series of the accurate measurements were carried out on the sensor. Initially, the sensor was calibrated using a standard concentration of MgCl₂ solution. The performance of the graphene-PEDOT solid-state contact was tested using scanning electron microscopy (SEM), water layer test, and electrochemical impedance spectroscopy (EIS). A comparison test was then made on two types of sensors, the needle-graphene-PEDOT-ISM and the needle-ISM sensors. Finally, the sensor was applied to detect the Mg²⁺ in wheat seedling stalks. Both pure agar and deionized water were used to verify the reliability of the detection. The potentiometric calibration demonstrated that the sensor exhibited a rapid response with a response time of 7.0 s, and a detection limit as low as 1.0 × 10^-7 mol/L. The better detection was achieved with a Nernst slope of 27.04 mV/dec and a lifetime of at least 30 days. The SEM test demonstrated that the graphene-PEDOT solid-state contact was substantially improved in the low-frequency capacitance between the ion-selective membrane and the stainless-steel substrate. The water layer test demonstrated that the graphene-PEDOT solid-state contact was effectively inhibited the formation of the water layer for the high stability of the ion-electron transfer of the sensor, thus reducing the signal drift. The EIS fitting test showed that there were the similar equivalent circuit diagrams of the two types of sensors. In addition, the graphene-PEDOT solid-state contact structure was effectively reduced the charge transfer resistance on the sensor surface. The stability of the response potential was improved during the process. The sensor was performed best to detect the concentration of Mg²⁺ in the stems of the wheat seedlings in the range of about 1.0 ×10^-3.42-10^-3.03 mol/L. In summary, a wide range of the promising prospects was also gained in the low detection limit, the stable potential response, in-situ detection of Mg²⁺ concentration in the crop stalks.