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• W3112876237 abstract "•In vivo, real-time monitoring of sugars fluctuations in trees with OECTs for 48hr•OECTs reveal previously uncharacterized diurnal sucrose fluctuations in aspen•Multienzyme functionalization of OECT for detection of sucrose•Operation of sensors with low-cost portable unit Bioelectronic devices that convert biochemical signals to electronic readout enable biosensing with high spatiotemporal resolution. These technologies have been primarily applied in biomedicine while in plants sensing is mainly based on invasive methods that require tissue sampling, hindering in-vivo detection and having poor spatiotemporal resolution. Here, we developed enzymatic biosensors based on organic electrochemical transistors (OECTs) for in-vivo and real-time monitoring of sugar fluctuations in the vascular tissue of trees. The glucose and sucrose OECT-biosensors were implanted into the vascular tissue of trees and were operated through a low-cost portable unit for 48hr. Our work consists a proof-of-concept study where implantable OECT-biosensors not only allow real-time monitoring of metabolites in plants but also reveal new insights into diurnal sugar homeostasis. We anticipate that this work will contribute to establishing bioelectronic technologies as powerful minimally invasive tools in plant science, agriculture and forestry. Bioelectronic devices that convert biochemical signals to electronic readout enable biosensing with high spatiotemporal resolution. These technologies have been primarily applied in biomedicine while in plants sensing is mainly based on invasive methods that require tissue sampling, hindering in-vivo detection and having poor spatiotemporal resolution. Here, we developed enzymatic biosensors based on organic electrochemical transistors (OECTs) for in-vivo and real-time monitoring of sugar fluctuations in the vascular tissue of trees. The glucose and sucrose OECT-biosensors were implanted into the vascular tissue of trees and were operated through a low-cost portable unit for 48hr. Our work consists a proof-of-concept study where implantable OECT-biosensors not only allow real-time monitoring of metabolites in plants but also reveal new insights into diurnal sugar homeostasis. We anticipate that this work will contribute to establishing bioelectronic technologies as powerful minimally invasive tools in plant science, agriculture and forestry. Bioelectronics enable electronic interfacing with the biological world as means for monitoring or stimulating biological processes. The bioelectronics field is highly driven by applications in biomedicine, specifically finding new solutions for diagnosis and therapy (Berggren and Richter-Dahlfors, 2007Berggren M. Richter-Dahlfors A. Organic bioelectronics.Adv. Mater. 2007; 19: 3201-3213Crossref Scopus (501) Google Scholar; Zeglio et al., 2019Zeglio E. Rutz A.L. Winkler T.E. Malliaras G.G. Herland A. Conjugated polymers for assessing and controlling biological functions.Adv. Mater. 2019; 31: e1806712Crossref PubMed Scopus (77) Google Scholar). Organic electronic devices can be advantageous when applied in the biological milieu since organic electronic materials support sufficient electronic and ionic transport (Paulsen et al., 2020Paulsen B.D. Tybrandt K. Stavrinidou E. Rivnay J. Organic mixed ionic–electronic conductors.Nat. Mater. 2020; 19: 13-26Crossref PubMed Scopus (164) Google Scholar), in a highly coupled manner, and thus enable efficient signal transduction. While the majority of efforts lie within the animal kingdom, applying bioelectronics to other biological organisms has emerged with successful demonstrations of sensing and actuation in bacteria (He et al., 2012He R.-X. Zhang M. Tan F. Leung P.H.M. Zhao X.-Z. Chan H.L.W. Yang M. Yan F. Detection of bacteria with organic electrochemical transistors.J. Mater. Chem. 2012; 22: 22072-22076Crossref Scopus (86) Google Scholar; Pitsalidis et al., 2018Pitsalidis C. Pappa A.M. Porel M. Artim C.M. Faria G.C. Duong D.D. Alabi C.A. Daniel S. Salleo A. Owens R.M. Biomimetic electronic devices for measuring bacterial membrane disruption.Adv. Mater. 2018; 30: 1-8Crossref Scopus (22) Google Scholar; Zajdel et al., 2018Zajdel T.J. Baruch M. Méhes G. Stavrinidou E. Berggren M. Maharbiz M.M. Simon D.T. Ajo-Franklin C.M. PEDOT:PSS-based multilayer bacterial-composite films for bioelectronics.Sci. Rep. 2018; 8: 1-12Crossref PubMed Scopus (39) Google Scholar; Demuru et al., 2019Demuru, S. et al. (2019) ‘Flexible Organic Electrochemical Transistor with Functionalized Inkjet-Printed Gold Gate for Bacteria Sensing’, 2019 20th International Conference on Solid-State Sensors, Actuators and Microsystems and Eurosensors XXXIII, TRANSDUCERS 2019 and EUROSENSORS XXXIII. IEEE, 1(June), pp. 2519–2522.not available in PubMed,CrossrefGoogle Scholar; Di Lauro et al., 2020Di Lauro M. la Gatta S. Bortolotti C.A. Beni V. Parkula V. Drakopoulou S. Giordani M. Berto M. Milano F. Cramer T. et al.A bacterial photosynthetic enzymatic unit modulating organic transistors with light.Adv. Electron. Mater. 2020; 6: 1-5Crossref Scopus (14) Google Scholar) and plants (Stavrinidou et al., 2015Stavrinidou E. Gabrielsson R. Gomez E. Crispin X. Nilsson O. Simon D.T. Berggren M. Electronic plants.Sci. Adv. 2015; 1: e1501136Crossref PubMed Scopus (130) Google Scholar, Stavrinidou et al., 2017Stavrinidou E. Gabrielsson R. Nilsson K.P. Singh S.K. Franco-Gonzalez J.F. Volkov A.V. Jonsson M.P. Grimoldi A. Elgland M. Zozoulenko I.V. et al.In vivo polymerization and manufacturing of wires and supercapacitors in plants.Proc. Natl. Acad. Sci. U S A. 2017; 114: 2807-2812Crossref PubMed Scopus (52) Google Scholar; Coppedè et al., 2017Coppedè N. Janni M. Bettelli M. Maida C.L. Gentile F. Villani M. Ruotolo R. Iannotta S. Marmiroli N. Marmiroli M. Zappettini A. An in vivo biosensing, biomimetic electrochemical transistor with applications in plant science and precision farming.Sci. Rep. 2017; 7: 1-9Crossref PubMed Scopus (21) Google Scholar; Poxson et al., 2017Poxson D.J. Karady M. Gabrielsson R. Alkattan A.Y. Gustavsson A. Doyle S.M. Robert S. Ljung K. Grebe M. Simon D.T. Berggren M. Regulating plant physiology with organic electronics.Proc. Natl. Acad. Sci. U S A. 2017; 114: 4597-4602Crossref PubMed Scopus (37) Google Scholar; Bernacka-Wojcik et al., 2019Bernacka-Wojcik I. Huerta M. Tybrandt K. Karady M. Mulla M.Y. Poxson D.J. Gabrielsson E.O. Ljung K. Simon D.T. Berggren M. Stavrinidou E. Implantable organic electronic ion pump enables ABA hormone delivery for control of stomata in an intact tobacco plant.Small. 2019; 15: 1-9Google Scholar; Janni et al., 2019Janni M. Coppede N. Bettelli M. Briglia N. Petrozza A. Summerer S. Vurro F. Danzi D. Cellini F. Marmiroli N. et al.In vivo phenotyping for the early detection of drought stress in tomato.Plant Phenomics. 2019; 2019: 1-10Crossref Scopus (16) Google Scholar; Kim et al., 2019Kim J.J. Allison L.K. Andrew T.L. Vapor-printed polymer electrodes for long-term, on-demand health monitoring.Sci. Adv. 2019; 5: eaaw0463Crossref PubMed Scopus (31) Google Scholar; Vurro et al., 2019Vurro F. Janni M. Coppede N. Gentile F. Manfredi R. Bettelli M. Zappettini A. Development of an in vivo sensor to monitor the effects of vapour pressure deficit (VPD) changes to improve water productivity in agriculture.Sensors (Switzerland). 2019; 19: 4667Crossref Scopus (11) Google Scholar; Diacci et al., 2020Diacci C. Lee J.W. Janson P. Dufil G. Mehes G. Berggren M. Simon D.T. Stavrinidou E. Real-time monitoring of glucose export from isolated chloroplasts using an organic electrochemical transistor.Adv. Mater. Tech. 2020; 5 (1900262): 1-6Crossref Scopus (23) Google Scholar). Recently, we presented an implantable organic electronic ion pump for in vivo delivery of abscisic acid, one of the main hormones involved in plant stress responses (Bernacka-Wojcik et al., 2019Bernacka-Wojcik I. Huerta M. Tybrandt K. Karady M. Mulla M.Y. Poxson D.J. Gabrielsson E.O. Ljung K. Simon D.T. Berggren M. Stavrinidou E. Implantable organic electronic ion pump enables ABA hormone delivery for control of stomata in an intact tobacco plant.Small. 2019; 15: 1-9Google Scholar), and subsequently the electronic control of physiology in intact plants. Others demonstrated conformable electrodes based on conducting polymers that were directly printed on plant leaves for long term bioimpedance monitoring (Kim et al., 2019Kim J.J. Allison L.K. Andrew T.L. Vapor-printed polymer electrodes for long-term, on-demand health monitoring.Sci. Adv. 2019; 5: eaaw0463Crossref PubMed Scopus (31) Google Scholar). In another work, a yarn-based organic electrochemical transistor (OECT) has been used for electrolyte monitoring in tomato plants in physiological conditions (Coppedè et al., 2017Coppedè N. Janni M. Bettelli M. Maida C.L. Gentile F. Villani M. Ruotolo R. Iannotta S. Marmiroli N. Marmiroli M. Zappettini A. An in vivo biosensing, biomimetic electrochemical transistor with applications in plant science and precision farming.Sci. Rep. 2017; 7: 1-9Crossref PubMed Scopus (21) Google Scholar), while in following works, the same concept was used to monitor drought stress (Janni et al., 2019Janni M. Coppede N. Bettelli M. Briglia N. Petrozza A. Summerer S. Vurro F. Danzi D. Cellini F. Marmiroli N. et al.In vivo phenotyping for the early detection of drought stress in tomato.Plant Phenomics. 2019; 2019: 1-10Crossref Scopus (16) Google Scholar) or changes in vapor pressure deficit (Vurro et al., 2019Vurro F. Janni M. Coppede N. Gentile F. Manfredi R. Bettelli M. Zappettini A. Development of an in vivo sensor to monitor the effects of vapour pressure deficit (VPD) changes to improve water productivity in agriculture.Sensors (Switzerland). 2019; 19: 4667Crossref Scopus (11) Google Scholar). Our group coupled an OECT directly with isolated chloroplasts to monitor in real-time the glucose export from the plant organelles with unprecedented time resolution (Diacci et al., 2020Diacci C. Lee J.W. Janson P. Dufil G. Mehes G. Berggren M. Simon D.T. Stavrinidou E. Real-time monitoring of glucose export from isolated chloroplasts using an organic electrochemical transistor.Adv. Mater. Tech. 2020; 5 (1900262): 1-6Crossref Scopus (23) Google Scholar). The OECT is a three terminal device where a gate electrode modulates the current, via reduction-oxidation switching of a conducting polymer-based channel (Nilsson et al., 2002Nilsson D. Chen M. Kugler T. Remonen T. Armgarth M. Berggren M. Bi-stable and dynamic current modulation in electrochemical organic transistors.Adv. Mater. 2002; 14: 51-54Crossref Google Scholar; Rivnay et al., 2018Rivnay J. Inal S. Salleo A. Owens R.M. Berggren M. Malliaras G.G. Organic electrochemical transistors.Nat. Rev. Mater. 2018; 3 (17086): 1-14Crossref Scopus (561) Google Scholar). The OECTs operate in aqueous environments and when the gate electrode is functionalized with an enzyme the OECT is converted to an enzymatic biosensor (Tang et al., 2011Tang H. Yan F. Lin P. Xu J. Chan H.L.W. Highly sensitive glucose biosensors based on organic electrochemical transistors using platinum gate electrodes modified with enzyme and nanomaterials.Adv. Funct. Mater. 2011; 21: 2264-2272Crossref Scopus (191) Google Scholar). When the analyte is present in the solution, an electrochemical reaction takes place at the gate, which becomes amplified through the modulation of the channel current. Signal amplification of the OECTs is important particularly for miniaturized devices where high signal to noise ratio is challenging. Although OECTs have received a lot of attention as amplification biosensors, their application in complex biological environments has been very limited. So far, most of the enzymatic OECT sensors have been validated in test solutions for the detection of metabolites and neurotransmitters (Shim et al., 2009Shim N.Y. Bernards D.A. Macaya D.J. Defranco J.A. Nikolou M. Owens R.M. Malliaras G.G. All-plastic electrochemical transistor for glucose sensing using a ferrocene mediator.Sensors (Basel). 2009; 9: 9896-9902Crossref PubMed Scopus (80) Google Scholar; Kergoat et al., 2014Kergoat L. Piro B. Simon D.T. Pham M.C. Noël V. Berggren M. Detection of glutamate and acetylcholine with organic electrochemical transistors based on conducting polymer/platinum nanoparticle composites.Adv. Mater. Weinheim. 2014; 26: 5658-5664Crossref PubMed Scopus (122) Google Scholar; Liao et al., 2014Liao C. Zhang M. Niu L. Zheng Z. Yan F. Organic electrochemical transistors with graphene-modified gate electrodes for highly sensitive and selective dopamine sensors.J. Mater. Chem. B. 2014; 2: 191-200Crossref PubMed Google Scholar; Berto et al., 2018Berto M. Diacci C. Theuer L. Di Lauro M. Simon D.T. Berggren M. Biscarini F. Beni V. Bortolotti C.A. Label free urea biosensor based on organic electrochemical transistors.Flexible and Printed Electronics. 3. IOP Publishing, 2018: 24001Google Scholar; Pappa et al., 2018Pappa A.M. Ohayon D. Giovannitti A. Maria I.P. Savva A. Uguz I. Rivnay J. McCulloch I. Owens R.M. Inal S. Direct metabolite detection with an n-type accumulation mode organic electrochemical transistor.Sci. Adv. 2018; 4: 1-8Crossref Scopus (95) Google Scholar). Few demonstrations have been reported where glucose is monitored from natural samples such as sweat (Scheiblin et al., 2015Scheiblin G. Aliane A. Strakosas X. Curto V.F. Coppard R. Marchand G. Owens R.M. Mailley P. Malliaras G.G. Screen-printed organic electrochemical transistors for metabolite sensing.MRS Commun. 2015; 5: 507-511Crossref Scopus (32) Google Scholar) saliva (Liao et al., 2015Liao C. Mak C. Zhang M. Chan H.L. Yan F. Flexible organic electrochemical transistors for highly selective enzyme biosensors and used for saliva testing.Adv. Mater. Weinheim. 2015; 27: 676-681Crossref PubMed Scopus (186) Google Scholar; Pappa et al., 2016Pappa A.M. Curto V.F. Braendlein M. Strakosas X. Donahue M.J. Fiocchi M. Malliaras G.G. Owens R.M. Organic transistor arrays integrated with finger-powered microfluidics for multianalyte saliva testing.Adv. Healthc. Mater. 2016; 5: 2295-2302Crossref PubMed Scopus (100) Google Scholar), or cell media (Curto et al., 2017Curto V.F. Marchiori B. Hama A. Pappa A.-M. Ferro M.P. Braendlein M. Rivnay J. Fiocchi M. Malliaras G.G. Ramuz M. Owens R.M. Organic transistor platform with integrated microfluidics for in-line multi-parametric in vitro cell monitoring.Microsyst. Nanoeng. 2017; 3: 1-12Crossref Scopus (53) Google Scholar; Strakosas et al., 2017Strakosas X. Huerta M. Donahue M.J. Hama A. Pappa A.M. Ferro M. Ramuz M. Rivnay J. Owens R.M. Catalytically enhanced organic transistors for in vitro toxicology monitoring through hydrogel entrapment of enzymes.J. Appl. Polym. Sci. 2017; 134: 1-7Crossref Scopus (26) Google Scholar) and one example focused on epidermal patches for on-body detection (Parlak et al., 2018Parlak O. Keene S.T. Marais A. Curto V.F. Salleo A. Molecularly selective nanoporous membrane-based wearable organic electrochemical device for noninvasive cortisol sensing.Sci. Adv. 2018; 4: eaar2904Crossref PubMed Scopus (215) Google Scholar). Until now, there is no demonstration of an implantable OECT enzymatic sensor for monitoring an analyte directly within the in vivo environment. Sugars are produced by photosynthesis and play a central role in plant growth and development. Several of the primary sugar metabolic pathways, responsible for carbon allocation in plants, are relatively well described. One of the current challenges in the field is to understand how the metabolic pathways of sugar metabolism are regulated, and how changes in sugar flux or concentration are adjusted. In order to address these questions, methods allowing for spatial and temporal real-time quantification of sugar levels are needed. The development of Förster resonance energy transfer (FRET)–based nanosensors for sugars was the first step toward in vivo measurements of sugar pools (Deuschle et al., 2006Deuschle K. Chaudhuri B. Okumoto S. Lager I. Lalonde S. Frommer W.B. Rapid metabolism of glucose detected with FRET glucose nanosensors in epidermal cells and intact roots of Arabidopsis RNA-silencing mutants.Plant Cell. 2006; 18: 2314-2325Crossref PubMed Scopus (129) Google Scholar; Chaudhuri et al., 2011Chaudhuri B. Hörmann F. Frommer W.B. Dynamic imaging of glucose flux impedance using FRET sensors in wild-type Arabidopsis plants.J. Exp. Bot. 2011; 62: 2411-2417Crossref PubMed Scopus (54) Google Scholar). Genetically encoded FRET sensors enable the analysis of steady-state concentration of sugar and dynamic changes in living tissue with high temporal and even subcellular resolution. However, the use of FRET sugar sensors is limited to cells and tissues, which can be monitored using a microscope. Cells buried deep in tissues, as is the case of vascular system, for example, are not accessible. Therefore, sugar analysis in plants, is usually performed by invasive methods that have poor spatial and temporal resolution and lead to disposal of the organism or tissue after sampling. Furthermore, sample analysis requires extraction/processing followed by sugar level determination based on enzymatic assays (Graf et al., 2010Graf A. Schlereth A. Stitt M. Smith A.M. Circadian control of carbohydrate availability for growth in Arabidopsis plants at night.Proc. Natl. Acad. Sci. U S A. 2010; 107: 9458-9463Crossref PubMed Scopus (412) Google Scholar), mass spectrometry (Jorge et al., 2016Jorge T.F. Mata A.T. António C. Mass spectrometry as a quantitative tool in plant metabolomics.Philos. Trans. A. Math. Phys. Eng. Sci. 2016; 374 (Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences): 20150370PubMed Google Scholar) or high-performance liquid chromatography (Mayrhofer et al., 2004Mayrhofer S. Heizmann U. Magel E. Eiblmeier M. Müller A. Rennenberg H. Hampp R. Schnitzler J.P. Kreuzwieser J. Carbon balance in leaves of young poplar trees.Plant Biol. (Stuttg). 2004; 6: 730-739Crossref PubMed Scopus (19) Google Scholar). All of these methods have high accuracy and low detection limit, but do not enable in vivo real time sugar level monitoring and therefore impede kinetic studies and analysis of biologically relevant events within the living plant. In several tree species, including the model tree aspen, sucrose is the predominant form of transported carbon (Rennie and Turgeon, 2009Rennie E.A. Turgeon R. A comprehensive picture of phloem loading strategies.Proc. Natl. Acad. Sci. U S A. 2009; 106: 14162-14167Crossref PubMed Scopus (267) Google Scholar). Sucrose is primarily transported in the phloem to different parts of the plant, but sucrose is also transported within the xylem transpiration stream (Heizmann et al., 2001Heizmann U. Kreuzwieser J. Schnitzler J.-P. Brüggemann N. Rennenberg H. Assimilate transport in the xylem sap of pedunculate oak (Quercus robur) saplings.Plant Biol. 2001; 3: 132-138Crossref Scopus (49) Google Scholar; Mayrhofer et al., 2004Mayrhofer S. Heizmann U. Magel E. Eiblmeier M. Müller A. Rennenberg H. Hampp R. Schnitzler J.P. Kreuzwieser J. Carbon balance in leaves of young poplar trees.Plant Biol. (Stuttg). 2004; 6: 730-739Crossref PubMed Scopus (19) Google Scholar). It was estimated that 9–28% of the carbon delivered to leaves in 3-month-old Populus trees over a diurnal cycle was derived from sugars transported in the transpiration stream (Mayrhofer et al., 2004Mayrhofer S. Heizmann U. Magel E. Eiblmeier M. Müller A. Rennenberg H. Hampp R. Schnitzler J.P. Kreuzwieser J. Carbon balance in leaves of young poplar trees.Plant Biol. (Stuttg). 2004; 6: 730-739Crossref PubMed Scopus (19) Google Scholar). Several tree species also use the xylem pathway to transport sugars during flowering and bud flush in the spring (Sauter and Ambrosius, 1986Sauter J.J. Ambrosius T. Changes in the partitioning of carbohydrates in the wood during bud break in betula pendula roth.J. Plant Physiol. 1986; 124: 31-43Crossref Scopus (51) Google Scholar). Xylem sap composition is typically analyzed from exudate secreted from a cut stem or leaf petiole. Sometimes, root pressure is sufficient to push out the xylem sap from the cut, but often a pressure chamber is required to squeeze out the sap and there is always a concern that xylem sap may mix with phloem sap or other cell contents at the cut surface. Furthermore, these invasive methods disrupt the transpiration stream, and do not allow monitoring of the sap composition over time. In this work, we overcome the above limitations by developing implantable glucose and sucrose OECT-based sensors that enable in vivo real time monitoring in plants (Figure 1). As a demonstration of the proof-of-concept and the kind of biological insights that this technology enables, we observed previously uncharacterized diurnal changes in sucrose levels in the xylem sap of greenhouse-grown hybrid aspen (Populus tremula x tremuloides). The OECT-based glucose and sucrose sensors were fabricated on a 125-μm-thick polyethylene naphthalate (PEN) substrate using standard microfabrication techniques as described in the Transparent Methods section. Ti/Au is used for source, drain, gate electrodes, and for wiring while the channel is based on the conducting polymer poly(3,4-ethyl-enedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The gate electrode is coated with a PEDOT:PSS thin film in order to increase its capacitance for efficient modulation of the channel conductance and is then further functionalized with enzymes and PtNPs. The PtNPs were electrodeposited on the gate while the enzymes were immobilized with the help of a chitosan matrix that is drop-casted onto the gate. The PEDOT:PSS-based OECT operates in the depletion mode with the channel initially at the high conductance state. When a positive bias is applied at the gate, cations from the electrolyte will penetrate into the channel, compensating the PSS polymer dopants resulting in PEDOT de-doping and a decrease in the channel current (Khodagholy et al., 2013Khodagholy D. Rivnay J. Sessolo M. Gurfinkel M. Leleux P. Jimison L.H. Stavrinidou E. Herve T. Sanaur S. Owens R.M. Malliaras G.G. High transconductance organic electrochemical transistors.Nat. Commun. 2013; 4: 1-6Crossref Scopus (568) Google Scholar). When the analyte is present in the solution an enzymatic reaction will take place at the gate that will result in the generation of H2O2. The H2O2 will then be oxidized on the PtNPs on the gate, which is associated with a transfer of electrons that change the effective gate potential and consequently induce a further decrease in the channel current (Bernards et al., 2008Bernards D.A. Macaya D.J. Nikolou M. DeFranco J.A. Takamatsu S. Malliaras G.G. Enzymatic sensing with organic electrochemical transistors.J. Mater. Chem. 2008; 18: 116-120Crossref Google Scholar) (Figure 2A). Typical transfer curve and transconductance of the OECT are shown in Figure 2B. The transconductance is considered the figure of merit of OECT sensor devices as describes the change in the drain current over the change in gate potential, gm = ΔID/ΔVG. Therefore, operation of the device at the high transconductance regime will result in small changes in the gate potential to induce large changes in the channel current. The analytes possible to detect by the enzymatic electrochemical sensors are limited by the availability of enzymes that can take part in redox reactions. Glucose oxidase is an oxido-reductase enzyme that catalyzes the oxidation of β-D-glucose to hydrogen peroxide and D-glucose1,5-lactone. Sucrose on the other hand does not have a corresponding oxidoreductase enzyme. In order to detect sucrose enzymatically, we overcame this limitation by incorporating successfully three enzymes within the chitosan matrix enabling a cascade of reactions to take place in a confined space. First, invertase hydrolyzes sucrose into fructose and α-D-glucose, mutarotase then catalyzes the conversion of α-D-glucose into β-D-glucose and finally β-D-glucose reacts with the glucose oxidase enzyme, Figure 2A. The performance and sensitivity range of the OECT-based glucose and sucrose sensors were assessed by monitoring the relative change of the channel current in solutions containing increasing concentration of sugars. To achieve high sensitivity, we operated the transistor at voltages across the gate, VGS = +0.5 V and channel, VDS = −0.4 V (source grounded), where the OECT has high transconductance, and thus exhibiting high amplification of the sensor signal. In order to compare the response of different devices and extract the characteristic calibration curve of the sensor we calculated the normalized drain current response of the device for each analyte concentration (ΔI/I = −(I[Μ]−I0)/I0), where I[Μ] is the drain current at concentration M and I0 is the base drain current. In Figure 2C, we show the calibration curves of the sucrose and glucose sensors represented as the mean of the response of 8 and 5 different devices, respectively. We observe that the sensors have similar, close to identical, characteristics with a dynamic range within 100 μM - 1 mM, as a result of the same concentration of glucose oxidase enzyme in the sucrose and glucose sensor. Furthermore, our biofunctionalization strategy allows us to tune the sensitivity and dynamic response of the sensor by changing the concentrations of the enzymes within the chitosan matrix. As shown in Figure S1, the sensor becomes sensitive to higher concentrations of sucrose when we reduce the enzymes concentrations. Additionally, we tested the sensitivity of the sucrose sensor to glucose. Indeed, we observe that the sucrose sensor is sensitive to glucose as it is expected, Figure S2. Therefore, for the in vivo detection of glucose and sucrose both independent sensors were used simultaneously. Finally, we evaluated the stability of the sensor in the complex biochemical environment of the plant. Sucrose sensor devices (n = 3) were implanted in the bark of hybrid aspen trees for 48hr and after being removed from the tree their response to 100μM and 300μM of sucrose test solutions was assessed as shown in Figure 2D. We observe a small decrease in the sensor response in comparison with the standard calibration curve of the sucrose sensors but the difference is not statistically significant. Next, we proceed to develop a portable OECT measurement unit set-up based on a low-cost Arduino platform that allow us to perform the sensing experiment in the growth environment of plants. In this case, the measurements were performed inside the greenhouse while these devices can be also operated in growth chambers or even in field conditions (Figures 3A and 3B ). As the platform uses uni-polar analog-to-digital and digital-to-analog converters it was operated using a common drain configuration (i.e. drain was grounded) in order to avoid negative voltages (Figure 3C). Two identical but separate circuits were used to source gate and source voltages (VGDin andVSDin) and simultaneously measure gate and source currents (IG and IS). Each circuit was composed of a voltage output (VGDout orVSDout) connected to one end of a precision resistor (RGor RS). The other end of the resistor was connected to a voltage input (VGDin orVSDin) and to the gate or source terminals. The drain terminal was connected to the ground of the microcontroller. Externally the gate, source and drain terminals were connected to the OECT using a ZIF connector and a ribbon cable (Figure 3B). Two proportional-integral-derivative (PID) controllers were used to control the voltages applied to the gate and source terminals of the OECT. Upon application of Vout, the voltage loss over R reduces the voltage at the gate/source terminal. The PID compensates for this by measuring Vin and modulating Vout so that Vin matches the desired gate/source voltage. The voltage over R, obtained from the difference between Voutand Vin, was used to calculate IG and ISthrough Ohm's law using the known value of R. In order to perform meaningful in vivo measurements with the OECT sensors it is important to minimize wound responses caused by the sensor insertion. Wounding responses may interfere with the measurements by altering the physiological processes of interest. Therefore, we first evaluated the response to the insertion of the sensor. We used hybrid aspen (Populus tremula x tremuloides) as our model system and mature xylem as the tissue of interest. The OECT design was optimized for this specific biological system. The sensors were fabricated on a flexible and thin PEN substrate with a thickness of 125 μm to ensure enough mechanical stability that enables easy insertion while decreasing the footprint to minimize invasiveness. Moreover the device was encapsulated with an SU-8 layer in order to ensure that only the active area of the transistor, gate and channel are exposed to the plant environment. The length of the implanted part of the sensor was chosen to be 3 mm to guarantee that the active sensor site reaches the mature xylem tissue and transpiration stream and the width chosen to 1 mm. An initial incision with a scalpel was performed to allow sensor insertion to the correct site at 3 mm depth from the epidermis (Figure 4A). This ensured that the sensor gate and channel are located within the mature xylem and are in contact with the transpiration stream. Any local wound response is unlikely to have a substantial effect on the composition of the xylem sap flowing past the sensor, but tissue repair responses may eventually lead to the isolation of the sensor from the transpiration stream. Therefore, wound responses due to the OECT insertion was analyzed over a time course of 1, 2, and 5 days using an optical light microscope. The visual changes in the insertion site were recorded using a camera and analyzed in more detail by preparing 60 μm thick cross sections across the" @default.
• W3112876237 created "2020-12-21" @default.
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• W3112876237 date "2021-01-01" @default.
• W3112876237 modified "2023-10-14" @default.
• W3112876237 title "Diurnal in vivo xylem sap glucose and sucrose monitoring using implantable organic electrochemical transistor sensors" @default.
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