Flexible Boron-Doped Diamond Films for Bio-Potential Sensing in Plants
Authors: Tago Shoko , Ochiai Tsuyoshi , Hayashi Mio , Kondo Takeshi , Fujishima Akira 
Published: February 2020

Introduction
Recently, it has become obvious that the bioelectric potential of plants is influenced by environmental factors such as atmospheric temperature, humidity, and light intensity. In general, two different methods, being used to measure electric potentials in plants, are namely extracellular and intracellular measurement. On the other hand, conductive boron-doped diamond (BDD) has been investigated for the superior electrochemical properties including a low background current and a wide potential window. Then, as an extracellular potential measurement, we reported a highly sensitive measurement system involving BDD electrodes for plant monitoring1. Although the BDD electrodes seemed to be promising materials to measure the bioelectric potential for plant monitoring, further research is needed for the development of inexpensive and flexible film as an electrode of sensor elements to fit on the various plants. Therefore, in the current study, low cost and flexible BDD electrodes, using BDD powder 2 (BDDP) and resins (Nafion (R) or Vylon-KE1830 3), were prepared for plant monitoring.
Experimental
BDDP was prepared by depositing BDD onto the surface of natural diamond powder through microwave plasma-assisted chemical vapor deposition. When preparing the BDD/Nafion electrodes, adding 2-propanol to an ion-exchange polymer dispersion (20 wt % Nafion (R), Sigma-Aldrich Japan Co., Ltd.), a dispersion with a 2-propanol/20 wt % Nafion (R) ratio of 1:1 was achieved and used to prepare the BDD/Nafion ink. Pt foils (10 mm × 10 mm, Nilaco Co., Ltd.) were painted with the ink and heated in an oven at 60 °C for 1 h and then at 120 °C for 10 min to form flexible BDD electrodes (BDD/Nafion film). Next, for the preparation of the BDD/Vylon electrodes, BDDP, Vylon (amorphous co-polyester, TOYOBO Co., Ltd.), KE-1830 (Silicon resin, Shin-Etsu Chemical Co., Ltd.), and toluene were taken in the weight ratio of 5:4:1:100. The prepared BDD/Vylon ink was then painted and heated under the same conditions as those used in the preparation of the BDD/Nafion film electrodes. Polycrystalline BDD plates (10 mm × 10 mm, Electrochemistry Polycrystalline, Element Six Ltd.) were also used as sensor electrodes. Each BDD plate and film-type electrode was attached with conductive carbon tape onto 3M™ Red Dot™ electrocardiogram (ECG) monitoring electrodes (Figure (a)). Each pair of electrodes was attached to the leaves of a potted Aloe. Bioelectric potential changes in the plant were detected as changes in the potential gaps between the pairs of sensor electrodes and the signal was amplified using a handmade amplifier. An AC/DC converter (ADC-24, Pico Technology Ltd.) was set up to record the amplified potential gap data. The BDD electrodes were assessed for their response reproducibility against a bare finger touch stimulus at least three times on every recording time. To maintain constant conditions for the potted Aloe, it was watered and moved into a room in which the temperature was maintained between 19 and 23 °C for at least 30 min before any finger touch stimulus was given.
Result and discussion
Statistical analyses of the results were performed as follows. The coefficient of variability (CoV) for the signal measured by each electrode was calculated using the mean and standard deviation (SD) of the bioelectric potential change (Figure (b)). The calculated means (absolute value) and SD/Means of potential changes between each pair of electrodes on the Aloe for a finger touch stimulus are plotted on Figure (c) and (d), respectively. The potential gap between a pair of electrodes attached to the plants changes as the plants are stimulated artificially with a finger touch, suggesting that the bioelectric potentials in the plant also changed, manifesting as changes in the potential gap between the electrodes. The BDD electrodes were assessed for their response reproducibility to a finger stimulus for 140 days.
Conclusions
The results of the study allow us to conclude that BDD electrodes can be used to detect biopotential changes on a potted Aloe as changes in the potential gap between a pair of electrodes. In addition, the flexible and inexpensive BDD electrodes consisting of BDD powder and resins, successfully proved the stability for long term use.
Acknowledgements
The authors are grateful to Seitaro Suzuki for his help to preparing handmade amplifiers. This work was supported by JSPS KAKENHI Grant Number JP17K05958.
Reference
1 T. Ochiai, S. Tago, M. Hayashi, A. Fujishima, ECS Transactions
2016, 75, 233.
2 T. Kondo, H. Sakamoto, T. Kato, M. Horitani, I. Shitanda, M. Itagaki, M. Yuasa, Electrochemistry Communications
2011, 13, 1546.
3 S. Tago, T. Ochiai, S. Suzuki, M. Hayashi, T. Kondo, A. Fujishima, Sensors
2017, 17, 1638.

Figure 1