Highly Sensitive Measurement of Bioelectric Potentials by Boron-Doped Diamond Electrodes for Plant Monitoring

Abstract

Plants have various capabilities to detect environmental factors such as atmospheric temperature, humidity, and light intensity. The feature becomes obvious by monitoring their bioelectric potential changes. In this study, we describe a novel approach for plant monitoring which uses BDD electrodes to detect electrochemical signals in plants (Figure 1). The method was built on previous work which showed that in plants electrochemical signals change in response to changing environmental factors (e.g., light, temperature, humidity, and atmospheric pressure). We chose to specifically use polycrystalline BDD plate electrodes because they have been shown to have excellent electrochemical sensitivity and have proven suitable for in vivo electrochemical detection. For comparison, we also used commercially available Ag and Pt plate electrodes with the same size as the BDD electrodes. We tested this approach on the different trees (ground-planted) and a hybrid species in the genus Opuntia (potted). For the potted Opuntia hybrid plants, we peeled the epidermis to exposed a small rectangular areas of the green phloem tissue (at least 10 mm × 10 mm to fit each electrode). We then put each pair of sensor electrodes (BDD, Pt, and Ag plate electrodes) on the areas before the green layers changed their own color to brown. Then we fixed them with plastic tape. The electrodes are fixed at the same height with 10 mm of gap. Similarly, each pair of BDD and Pt plate electrodes was fixed onto the ground-planted trees such as Eurya japonica. We sliced off the crude bark of the trees at approximately 1 m of height from the ground and made a small flat area of exposed green phloem tissue to fit the electrodes. After fixing the electrodes, we tied them up with plastic tape and wrapped aluminum foil around the tree to avoid rain and electromagnetic noise. In this time, we artificially induced bioelectric potential changes using the surface potential of the human finger for the potted Opuntia. On the other hand, for the ground-planted trees, we measured not only naturally induced bioelectric potential changes but also environmental factors for months without replacing/removing/changing electrodes. Bioelectric potentials in the plants were collected by the pair of sensor electrodes on each individual plant and amplified by a handmade amplifier. In order to reduce instability and noise, this amplifier was constructed as a differential amplification circuit with a gain of 10,000 and a 10 Hz low pass filter. All analog signals from the plants were converted to digital signals and transferred to a personal computer through a 24bit analog/digital converter with data logger. Drastic changes of bioelectric potentials were monitored by all electrodes during a finger touch on the Opuntia hybrid surface. However, the amount of bioelectric potential change monitored by the BDD electrode is more stable than that of the other electrodes. The coefficient of variability (CV) of BDD plate electrodes were 4–7 times smaller in this detection than that of Ag or Pt plate electrodes. The difference is due to the electrochemical sensitivity of the electrodes. Recently, BDD electrodes have been shown to have excellent electrochemical sensitivity and have proven suitable for in vivo electrochemical detection, such as dopamine generation in the brain or the reduced form of glutathione for the assessment of cancerous tumors. In the present research, the bioelectric potential changes evoked by surface potential of the finger and/or the ion flow through the ion channel of the plant cells were successfully detected by BDD electrodes. A similar tendency was observed in the ground-planted trees. We found that both BDD and Pt plate electrodes detected bioelectric potential changes in all the tested trees for months, BDD plate electrodes were 5–10 times more sensitive in this detection than Pt plate electrodes. For example, for Eurya japonica, spike-like bioelectric signals can be detected on baseline patterns with high S/B ratio by BDD plate electrodes (Figure 2). These spike-like signals are thought to be the result of transpiration, movement of leaves, and circadian rhythms of plants. These results indicated that electrochemical signals in plants changed in response to changing environmental factors such as temperature and humidity as reported in the literature. Given these results, BDD electrodes placed on living plant tissue are able to consistently and effectively detect bioelectrical potential changes for a long time and serve as a monitoring system for plant growth and as a warning system for impending environmental events such as a storm. For more details: Sensors 2015, 15, 26921-26928. Figure 1