Title：Flexible and wearable sensor for in situ monitoring of gallic acid in plant leaves
Journal: Food Chemistry
IF: 9.8
Original link：https://doi.org/10.1016/j.foodchem.2024.140740
Reporter：Qin Li-25-master
Gallic acid (GA) is one of the main phenolic components naturally occurring in many plants and foods and has been a subject of increasing interest owing to its antioxidant and anti-mutagenic properties. This study in-troduces a novel flexible sensor designed for in situ detecting GA in plant leaves. The sensor employs a laser-induced graphene (LIG) flexible electrode, enhanced with MXene and molybdenum disulfide (MoS2) nano-sheets. The MXene/MoS2/LIG flexible sensor not only demonstrates exceptional mechanical properties, covering a wide detection range of 1–1000 μM for GA, but also exhibits remarkable selectivity and stability. The as-prepared sensor was successfully applied to in situ determination of GA content in strawberry leaves under salt stress. This innovative sensor opens an attractive avenue for in situ measurement of metabolites in plant bodies with flexible electronics.
Gallic acid, a polyphenolic compound widely present in plants and foods, holds significant importance in food processing and plant physiology research due to its biological activities such as antioxidant and antibacterial effects. However, traditional detection methods like high-performance liquid chromatography, despite their high sensitivity, rely on complex pretreatment, expensive equipment, and in vitro sampling, which can easily cause plant damage and are unable to achieve in situ real-time monitoring. With the advancement of precision agriculture, there is an urgent need to develop in situ detection technologies that can adapt to the soft and curved surfaces of plant leaves. In recent years, electrochemical sensors have gained attention due to their simple operation and rapid response. However, conventional rigid electrodes are difficult to conform to the leaf surface, limiting their practical application. The emergence of flexible electronics technology offers a potential solution to this problem. In particular, laser-induced graphene, with its high specific surface area and excellent conductivity, combined with nanomaterials such as molybdenum disulfide and MXene, can further enhance the sensitivity and adaptability of sensors, opening new avenues for in situ monitoring of metabolites in plants.
1.Design of the LIG-based electrode
Through systematic optimization, the optimal laser power was identified as 3.1 W with a scanning speed of 10 mm/s, under which the formed LIG electrodes exhibited the highest electrochemical activity. Meanwhile, comparison of serpentine structures with different connection angles revealed that when θ = 0°, the electrodes demonstrated superior conductivity and electrochemical performance, providing an ideal platform for subsequent modification and application.
2. Study of mechanical properties
Mechanical tests demonstrated that the sensor can withstand over 2.5% strain. As shown in the figure, even under repeated bending and 2% stretching, its electrochemical signal (CV curve) remained nearly unchanged. This proves its ability to adapt to the natural deformation of plant leaves, ensuring the reliability and accuracy of in-situ detection.
3. Characterization of MXene/MoS2/LIG
The successful preparation and modification of the materials were verified through multiple analytical techniques including SEM, EDS, Raman spectroscopy, and XPS. SEM images revealed that LIG possesses a continuous porous structure, with the layered structure of MoS₂ uniformly distributed within the graphene. The introduction of MXene further enhanced the material's density. EDS analysis confirmed the presence of characteristic elements such as Mo, S, and Ti. The appearance of G peak, D peak, and characteristic peaks of MoS₂ and MXene in the Raman spectra further corroborated the material composition. XPS spectra revealed the existence of chemical bonds such as C–Ti, C–C, Mo⁴⁺, and S 2p, clarifying the chemical states on the material surface. Electrochemical impedance spectroscopy showed that the charge transfer resistance of MXene/MoS₂/LIG significantly decreased to 86.3 Ω, indicating a substantial improvement in conductivity due to the synergistic effect of MXene and MoS₂, thereby laying the material foundation for the high-sensitivity detection performance of the sensor.
4.Electrochemical behavior of GA on MXene/MoS2/LIG
DPV curves demonstrated that the MXene/MoS₂/LIG electrode exhibited a significantly higher oxidation peak current response toward GA compared to electrodes modified with individual materials, indicating a strong synergistic catalytic effect. Scan rate studies revealed a linear relationship between the peak current and the square root of the scan rate, confirming a diffusion-controlled process. Furthermore, systematic optimization identified the optimal working pH as 3.0, along with the ideal modification concentrations of MXene and MoS₂ as 1.0 mg/mL and 0.6 mg/mL, respectively.
5. Performance of MXene/MoS2/LIG
The sensor exhibited a favorable linear relationship across a wide concentration range of 1–1000 μM, with a detection limit as low as 0.625 μM, outperforming most reported sensors. It also demonstrated remarkable selectivity against various common interferents, along with satisfactory reproducibility (RSD < 6.03%) and stability (retaining 84.4% of its initial activity after two weeks).
6. Real sample analysis
Under salt stress, the GA content in the leaves significantly increased from 12.99 μM to 38.65 μM, which is fully consistent with the physiological process of plants activating antioxidant defense mechanisms under adverse conditions. This result demonstrates that the sensor can effectively reflect the physiological stress state of plants and possesses practical application value.
This study successfully developed a flexible wearable electrochemical sensor based on MXene/MoS₂-modified laser-induced graphene for the in-situ monitoring of gallic acid in plant leaves. The sensor not only exhibits excellent mechanical flexibility, stretchability, and bending stability, allowing it to conform well to soft, non-planar leaf surfaces, but also demonstrates a wide detection range (1–1000 μM), high sensitivity (detection limit of 0.625 μM), as well as good selectivity, reproducibility, and stability. Through its application in salt stress experiments on strawberry leaves, the sensor successfully achieved in-situ and real-time detection of gallic acid content in plants, confirming its practical potential for analyzing plant physiological status. This research opens up a new pathway for in-situ monitoring of plant metabolites using flexible electronics technology, holding significant value for the advancement of smart agriculture. Although the current detection process still requires the addition of buffer solution, future work will explore the use of solid-state electrolytes to further enhance detection accuracy and practicality.