Direct functionalization of plants with electronic materials
As part of TransPlant we took a next step on electronic functionalization of plants and used a conjugated oligomer that is not toxic to the plant and can polymerize in vivo, in the internal structure of the plant, upon distribution. The conjugated oligomer ETE-S due to its small molecular size can be distributed in every part of the vascular tissue of a rose cutting, from stem to flower and in-vivo polymerize forming long conducting wires of 10 S/cm. The polymerization is driven by the local physicochemical environment of the plant.
Plant’s anatomy upon functionalization with the conducting material provides an ideal architecture for developing supercapacitors in vivo. The long-range conducting xylem wires based on the conjugated oligomers are parallel and electronically isolated to each other while are surrounded with cellular domains and extracellular space rich in electrolyte. The xylem supercapacitor exhibits typical behavior with specific capacitance to 20F/cm3, a value in the typical range for conducting polymers. This consist the first demonstration of in-vivo polymerization of a conjugated oligomer and the formation of conductors but also the first demonstration of energy storage in plants. This work has been published in PNAS 114, 11, 2017). In addition, a patent application in the European Patent Office is pending (application- EP16178248.7).
The electronic functionalization of plants was initially performed in rose cuttings. This system was chosen due to similar anatomy of the vascular tissue of roses with the one of trees. In this way our technology can be transferred to trees later on more easily. However, advancing the in-vivo functionalization methods is one of the main goals of TransPlant. The in-vivo developed electronic components should not compromise the growth of the plant but follow its growth. To that end we focused on formation of conformable electrodes that self-organize in a growing plant and extended the functionalization in rooted plants. ETE-S in-vivo polymerized along the roots of growing bean and pea plants. The ETE-S polymer formed a conducting coatings on the surface of the roots while in some occasions the polymer entered the internal structure of the plant, reaching the vascular tissue. The polymerization, as in the case of roses is driven by the response of the plant in the presence of ETE-S. As a next step the conducting roots will be used for the construction of electrochemical devices such as electrochemical transistors and supercapacitors. In addition, we will study the effect of the electronic functionalization on the physiology of the plant. This work is currently finalized and the manuscript is under preparation.
Next, we looked more into the fundamentals of the electronic properties of ETE-S oligomers by combining theoretical calculations and experimental findings. DFT calculations along with spectroelectrochemistry and ESR show that in the oxidized state the charge carriers are mainly bipolarons. Molecular dynamics demonstrate the formation of percolative paths in ETE-S oligomers , despite the fact that the oligomers are short (6–9 rings) and crystallites are thin along the π–π stacking direction, consisting of only two or three π–π stacked oligomers. The existence of percolative paths explains the observed high conductivity in in-vivo polymerized ETE-S. This work resulted in two publications: a. Advanced Electronic Materials, DOI: 10.1002/aelm.201700096 2017 and b. Nanoscale, DOI: 10.1039/ c7nr04617k, 2017
Organic bioelectronic devices for monitoring and controlling plant functions
The second part of the project deals with the fabrication of organic bioelectronic devices for monitoring and controlling plant physiology.
Monitoring plant phycology
In order to be able to detect the sugars exported from organelles and sugars that are located within the vascular tissue of the plant we developed sensors based on organic electrochemical transistors. Conventional methods for determining sugars are based on expensive equipment such as mass spectrometry or laborious enzymatic assays that are time consuming and cannot be used for real time monitoring. The OECT based sensors were applied in-vitro and in-vivo studies.
Controlling plant physiology
We developed an implantable version of the Organic Electronic Ion Pump, an electrophoretic device that allows precise delivery of ions. The device is applied in tobacco leaves in an intact plant and delivers the hormone ABA- abscisic acid into the leaf apoplast with high spatio-temporal resolution. ABA being a stress hormone is inducing the closing of stomata. With the implantable OEIP we demonstrate the first electronic control of stomata. At the same time, we monitor the propagation of the ABA signalling through the leaf and get information on the speed of the signal.
Sensors and actuators can be used from plant biologists as new tools for studying biological processes. Manuscripts for this work is under preparation.
The work within TransPlant has been presented in many conferences with invited and contributed talks. In addition some of the findings were disseminated to the public through the press and social media.
