Abstract:

[Objective] Rapid and accurate on-site detection of the glucose (Glu) in plant sap exudates is of practical significance for plant health assessment and precision agricultural management. However, conventional analytical methods often rely on laboratory instruments, complicated sample pretreatment and relatively large reagent consumption, which limits their application in rapid field analysis. In addition, manual operation in traditional colorimetric assays may introduce operational errors, especially when small-volume samples are used or when rapid detection is required. To address these problems, and to simplify the structure of the DMF system, improve its portability and provide a feasible technical approach for rapid plant physiological indicator detection, a single-board digital microfluidics (DMF) chip was developed in this study, and a colorimetric detection platform integrating smartphone imaging and multi-colour-space analysis was constructed for the quantitative determination of Glu in aloe sap exudates. [Methods] The developed system used an ESP32-S3 microcontroller as the main control unit. Two HV507 high-voltage driver chips were employed to realize 128-channel high-voltage output for the actuation of the electrode array. The hardware design included Type-C power supply and communication, low-voltage regulation, high-voltage boosting, digital isolation, level shifting and an 8 × 16 array electrode plate. To improve system modularity and facilitate chip replacement during experiments, the device adopted a separated structure consisting of a control board and an array electrode board. The control board was mainly responsible for power management, signal generation, voltage conversion and electrode driving, while the array electrode board provided the working area for droplet manipulation. A printed circuit board (PCB) electrode substrate was used as the bottom electrode layer, transparent adhesive tape was selected as the dielectric layer, and 5 cSt dimethyl silicone oil was introduced as the interfacial layer to reduce droplet adhesion and improve actuation stability. Based on this structure, a complete DMF operating interface was constructed. For Glu detection, droplets containing sample solution and colorimetric reagent were manipulated on the chip, merged on the electrode array and allowed to develop colour. Images of the reacted droplets were collected using a smartphone under controlled imaging conditions. Colour information was extracted from different colour spaces, and a linear model was established to evaluate the quantitative relationship between image features and Glu concentration. [Results and Discussions] The results showed that the developed single-board DMF platform could realize basic droplet deformation, directional transport and continuous movement on the array electrode plate. Stable droplet operation was achieved within the volume range of 7.5~10 μL, indicating that the designed electrode array, dielectric layer and interfacial oil layer could support reliable droplet actuation under the selected operating conditions. Compared with systems that depend on external high-voltage equipment or complex wiring, the developed platform integrated the main control, high-voltage generation and multi-channel driving functions on a compact control board, which improved the overall integration level and reduced the complexity of system assembly. The platform was further applied to Glu colorimetric detection in plant sap exudate. Glu concentration gradients of 0, 0.1, 1, 5, 10 and 20 mM were prepared for model construction and performance evaluation. Through on-chip droplet merging and colour development, followed by smartphone image acquisition and colour-space-based linear fitting, a good quantitative response was obtained within the concentration range of 0~20 mM. The coefficient of determination of the linear model reached 0.996, demonstrating that the extracted image features had a strong correlation with Glu concentration. In aloe sap exudate samples, the spiked recovery ranged from 97.15% to 105.28%, indicating that the proposed method could achieve accurate quantitative detection in real plant sap samples. These results suggest that the combination of DMF droplet manipulation, smartphone imaging and colour-space analysis can effectively reduce manual operation and reagent consumption while maintaining satisfactory quantitative performance. [Conclusions] The proposed DMF system features a simplified structure, a high degree of modularity, and good potential for on-site deployment, providing a new technical approach for the rapid detection of physiological indicators in plant xylem sap.

Key words: digital microfluidics, single-plate chip, plant sap, glucose, colorimetric analysis