Co3O4 Based Non-Enzymatic Microneedle Glucose Sensor for Plant Health Monitoring
Authors: Yun JaeHyoung , Kim Hyemin , Kang JungMin , Kim Jaeho , Park SeungHyun , Ryu WonHyoung 
Published: August 2024

Over the last decade, various wearable sensor technologies that can monitor human health precisely in real time have extensively been developed. More recently, such wearable biosensor technologies have been employed to observe the condition of plants as well. In particular, a glucose sensor, which has been actively researched for diabetes, can also be applied to plants. A level of glucose concentration in plants indicates cellular metabolic status, biotic and abiotic stress of plants. However, unlike advanced glucose sensors for humans, early glucose sensors based on glucose oxidase are still used in plants glucose monitoring. The enzyme-based glucose sensor has several disadvantages including the stability problem due to denaturation of enzymes and difficulty of immobilizing enzymes on the electrode surface. Additionally, glucose oxidase needs a redox shuttle to transfer electrons, which complicate the sensor manufacturing process. Moreover, H2O2 is generated as a byproduct of glucose oxidase and glucose reaction, which can cause damage to plants and enzymes.
In this study, non-enzymatic microneedle (MN) glucose sensor was developed using Co3O4 as a substitute for glucose oxidase. Co3O4 have electrocatalytic properties and chemical stability, showing higher sensitivity compared to glucose oxidase, and is suitable for long-term monitoring. However, it has the disadvantage of low electrical conductivity, making it difficult to transfer electrons, which are generated from the reaction with glucose, to a sensor electrode. Therefore, carbon nanomaterials were mixed to increase electron transfer efficiency and drop casted on the working electrode of a screen-printed sensor. To optimize the ratio of Co3O4 and carbon nanomaterials, the sensitivity was observed by comparing the peak current of cyclic voltammogram and saturation currents of amperogram according to each mixing ratio. Through the analysis results, it was confirmed that 17 wt% carbon nanomaterial had the highest sensitivity of 207 μA/mM cm2. Afterwards, a MN array sensor was fabricated to measure the glucose in plant leaves. MN can penetrate the epidermis of the leaf and enable to measure the glucose from near the phloem of the leaf. MN array structure was fabricated by dispensing SU-8 into a PDMS negative mold and cured by UV exposure. The working and reference electrode of MN array were coated with Co3O4/carbon composite and pseudo-Ag/AgCl ink, respectively, using a pneumatic 3D printer. Pt sputtering was conducted for forming the counter electrode. The metabolism of plants was confirmed by measuring the glucose concentration of leaves at day and night using a MN glucose sensor.

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