Organic,Electrochemical,Transistor,based,on,Polypyrrole/Crosslinked,Chitosan/Nylon,Fibers

ZHU Rufeng,WANG Yao,TAO Yang,CHEN Yuanli,WANG Yuedan

(Key Laboratory of Textile Fiber and Products (Ministry of Education),Wuhan Textile University,Wuhan 430200,China)

Abstract: Glutaraldehyde (GA) crosslinked chitosan (CHIT) was modified on nylon fibers.Afterwards,pyrrole was in-situ polymerized on the surface of the CHIT/Nylon fiber.The SEM and FT-IR results show that the functional fiber is successfully prepared,and the obtained polypyrrole (PPy) presents nanorods morphology on the fiber surface.The mechanical properties of the fibers were studied by Instron.The organic electrochemical transistors based on PPy/Nylon fiber,PPy/CHIT/Nylon fiber,and PPy/GA-CHIT/Nylon fiber as channels were prepared and their transistors performance was compared.It is found that PPy/GA-CHIT/Nylon fiber-based transistor has great output,transfer,transient curves,and excellent transconductance of 6.8 mS,providing a new platform for the field of wearable devices.Furthermore,the study introduces chitosan material with excellent biocompatibility,which makes prepared transistors also have potential applications in the field of biosensing.

Key words: organic electrochemical transistors;chitosan;polypyrrole;fiber;wearable

Since organic electrochemical transistors(OECTs) were developed by Wrightonet alin the mid-1980s[1],many groups have carried out research on them.Common OECTs devices are composed of source,drain,gate electrode,semiconductor layer and electrolyte.The semiconductor layer is located between the source and drain.The electrolyte completely covers all three electrodes[2].The working mode of OECTs relies on the ion injection or extraction of the organic semiconductor layer in the electrolyte,which changes its doping state and thus its electrical conductivity,leading to current modulation between the source and drain[3,4].OECTs have the characteristics of flexible design,low operating voltage,miniature,and good biocompatibility.They can be used for sensing and signal amplification[5].However,most of the reports on OECTs are concentrated in planar devices,and there are few reports on flexible materials such as fibers and fabrics.Due to its light weight and good flexibility of fiber,it can be easily woven and integrated into fabrics.Fiber-based organic chemical transistors creatively combine the sensitivity of OECT with the flexibility and weaveability of fibers.In recent years,many scientific researchers have begun to focus on fiber-based organic electrochemical transistors.Since Hamediet alfirst reported fiber-based organic electrochemical transistors(FECTs),many groups have successively published relevant literature on the preparation of various FECTs for different applications using cotton,nylon and other fibers as carriers.

The semiconductor layer of OECT mostly uses organic polymer materials,such as polypyrrole (PPy),polyaniline (PANI),poly(3,4-ethylenedioxythiophene)(PEDOT),etc.Polypyrrole is a polymer obtained by polymerization of pyrrole compounds.It has high conductivity,good environmental stability and excellent biocompatibility,easy to synthesize,and relatively simple to combine with flexible substrates[6].Some groups have explored the project of combining PPy with fabrics or fibers.Boschiet alapplied the insitu polymerization method to evenly coat PPy on different silk substrates,so that silk products have good electrical conductivity and can be maintained at a good level[7].Jaouhariet alprepared PPy with a spherical microscopic morphology on flax fibers by in-situ polymerization technology,which can be used for triethylamine gas sensors[8].Nimbekaret algrafted nano-spherical PPy onto polyester fabric by chemical oxidation polymerization,resulting in fabrics that could be used for sensitive ammonia detection[9].However,the PPy layer directly generated on the fiber or fabric is mostly granular,with poor performance and uneven distribution on the fabric substrate.In addition,the structure formed by single PPy has weak mechanical strength,brittleness and rigidity.Therefore,the generation of semiconductor composite layers by doping other materials has become a mainstream solution for fibers and fabrics.Zhenget alused graphene as a support layer to prepare nanofibrous PPy,which has been well applied in the field of supercapacitors[10].Yanget algrew PPy and Mexne on cotton fibers to prepare fiber electrodes.The performance of its composite fiber capacitors is significantly better than that of cotton fiber electrodes modified only by PPy,with good mechanical strength and electrical conductivity[11].Chenet algrew PPy on Fe2O3to form a core-shell structure,which can be used in the field of catalytic degradation[12].The materials mentioned above,such as Mxene and graphene,have been widely used in the field of OECTs.However,its relative high cost limits its application.A more readily available and biocompatible material is needed to further broaden the application of composite conductive fibers.Chitosan (CHIT) has excellent properties such as biocompatibility,safety,and microbial degradability.Significant applications have been made in many fields such as medicine[13,14],food,chemicals,cosmetics,water treatment,advances in biochemical and biomedical engineering[15],metal extraction and recovery[16],etc.It has good film forming ability,high mechanical strength and hydrophilicity,which can be used as a good support layer to further improve the performance of conductive polymer layer.Chitosan and PPy composites have been widely used in biomedical,sensing and other fields.Due to the good biocompatibility of chitosan/PPy,Talebiet alprepared the conductive composite nanofiber scaffold polycaprolactone/chitosan/polypyrrole[17].Zareiet aldeveloped a conductive polypyrrole-chitosan-collagen electrospun nanofiber scaffold to accelerate the healing of damaged tissues[18].The excellent conductivity of PPy also makes it widely used in other fields.Antonyet alsynthesized polymer nanocomposites composed of polypyrrole,chitosan and zinc oxide nanoparticles through chemical oxidation polymerization and modified them on ITO to prepare electrode.The obtained electrode can be used in the field of electrochemical sensing[19].Dresvyaninaet alprepared PPy-chitosan fiber film with conductivity and mechanical properties by in-situ oxidized polymerization of pyrrole gas on the surface of chitosan fiber[20]. Although many research groups have studied the combination of chitosan and PPy,there are few researches on the modification of fiber substrate and its application in the field of OECT.

In this study,the chitosan layer was crosslinked with glutaraldehyde (GA) on nylon fibers.Then the PPy/GA-CHIT/Nylon fiber was obtained by in-situ polymerized of PPy on the treated nylon fibers.The prepared PPy has a regular nanorod shape that provide a larger specific surface area and facilitate ion migration.Different functional fiber based organic electrochemical transistor were compared.The obtained transistor based on PPy/GA-CHIT/Nylon fiber has excellent output,transfer and transient performance,providing a new platform for the field of wearable devices.Furthermore,the study introduces chitosan material with excellent biocompatibility,so that our OECTs also have potential applications in the field of biosensing.

2.1 Materials

Chitosan was purchased from Bio-sharp Co.,Ltd.Pyrrole,glutaraldehyde and sodium polystyrene sulfonate were supplied by Aladdin Co.,Ltd.Sodium Anthraquinone-2-sulfonate and polyvinyl alcohol were purchased from Sigma-Aldrich Co.,Ltd.5-sulfosalicylic acid was obtained from Macalline Co.,Ltd.Ethylene glycol and sorbitol were purchased from Sinopharm Co.,Ltd.Nylon was supplied by Jinhao sheng Textile Co.,Ltd.

2.2 Preparation of CHIT/Nylon fiber

In this process,the mass fraction of 0.5% chitosan solution was prepared.Then the purified nylon fiber is then repeatedly immersed in the chitosan solution until the fiber surface is completely covered with the chitosan,and then the obtained fiber is air-dried for later use.Since subsequent in-situ polymerization will take place in an acidic environment,cross-linked chitosan is required.Therefore,CHIT-Nylon is crosslinked in 10wt% glutaraldehyde solution for 10 hours.

2.3 Preparation of PPy/CHIT/Nylon fiber

PPy/CHIT/nylon fiber was prepared by in-situ polymerization method.1 g of sodium anthraquinone-2-sulfonate was added to 100 mL of deionized water,heated and stirred at 60 ℃ to obtain a yellow surfactant solution.10.8 g of 5-sulfosalicylic acid dihydrate was weighed and 100 mL deionized water was added to obtain the solution.Then 17.28 g of oxidant ferric nitrate nonahydrate (Fe(NO3)3·9H2O) was added to the 5-sulfosalicylic acid dihydrate solution.Shake the mixture at room temperature until the particles completely dissolve,resulting in a purple-black oxidant solution.The above-mentioned CHIT/Nylon fiber was put into the surfactant solution,followed by the addition of 1.2 g pyrrole monomer,and stirred in an ice bath for 10-20 minutes,then dropped into the oxidant solution.Finally,the reaction solution was stirred continuously for 4 hours at 600 r/min.After the fiber was taken out,the impurities were washed with a large amount of deionized water and dried to obtain the PPy/CHIT/Nylon fiber.The PPy/CHIT/Nylon conductive fiber was prepared by the same method without glutaraldehyde cross-linking treatment.

2.4 Assembly of fiber-based organic electrochemical transistors

The gel electrolyte of the organic electrochemical transistor was prepared as follows.The polyvinyl alcohol (PVA),polystyrene sodium sulfonate (PSS),ethylene glycol,sorbitol and decanter were mixed with a mass ratio of 1:3.3:1.2:0.8:10.The electrolyte was prepared in a constant temperature water bath at 90℃ for 3 hours.The conductive fibers were placed in parallel at a pitch of 0.5 mm.Both ends of the two functional fibers were coated with conductive silver paste,which were used as the source,drain,and gate electrode of the FECTs.In the channel part,gel electrolyte was dropped into the middle channel part of the two fibers to cover the two fibers to complete the FECTs assembly.

2.5 Characterization methods

Scanning electron microscope (JSM-6510LV)was used to observe the morphology of composite fibers,Instron (Instron 5967) was adopted to test the mechanical properties of composite fibers.Fourier infrared spectrometer (Vertex 70) was used to verify the successful preparation of composite fibers,and Keithley 4200-SCS semiconductor performance analyzer (Tektronix) was used to test the electrochemical performance of composite fiber-based organic electrochemical transistors.During the output measurement,the scan voltage between the source and drain is set at 0-2 V,and the gate voltage is scanned from 0 to 2.8 V at a constant voltage interval of 0.4 V.For the transfer test,a linear sweep voltage of 0-2.8 V is set at the gate,and a constant voltageVds=-1.5 V is applied between the source and the drain.During the transient test,0 and 2.8 V were alternately applied to the gate with a period of 200 s.

Scheme 1 presents the detailed preparation process of the FECTs.The nylon fiber is first coated with a CHIT layer,and then GA is added to the surface of the cotton fiber to cross-link the CHIT layer to it.The modified nylon fiber was in-situ polymerized with pyrrole.The addition of pyrrole forms a nanorodshaped PPy semiconductor layer on the surface of fiber.The composite fiber was obtained after cleaning.The fiber organic electrochemical transistor was synthesized by combining composite fiber with electrolyte and silver glue electrode.

Scheme 1 The preparation of conductive fiber and FECTs

The surface morphology of the fiber electrode was characterized by scanning electron microscope(SEM).Fig.1(a) shows the purified nylon fiber.The image shows a single fiber monofilament with a smooth surface and a large space between the filaments.The gap between monofilaments has a great influence on the subsequent polymerization of conductive polymers.Figs.1(b) and 1(c) show nylon fibers treated with chitosan but not crosslinked.Nylon fibers are clearly covered with a thin chitosan film.Fig.1(d)shows the morphology of the chitosan-nylon fiber after cross-linking treatment.The surface of the fiber is covered with a thick layer of chitosan,presenting a wrinkled shape.Fig.1(e) show the morphology of insitu polymerized polypyrrole on nylon fibers treated with glutaraldehyde-crosslinked chitosan.It can be seen intuitively that relatively regular PPy nanorods are formed on the fibers with a diameter of about 900 nm,which proves that the introduction of chitosan can promote the growth of PPy,effectively providing a continuous growth template with a large surface area,and will significantly improve the conductivity of the fiber.The nanorods are not smooth and have some nanoparticles loaded on them.The morphology of PPy/CHIT/Nylon fibers not cross-linked with glutaraldehyde was observed (Fig.1(f)).It can be seen that part of the nanowire structure forms on the fiber.But more of them form agglomerated block structures.This is because uncross-linked chitosan dissolves in an acidic environment,which affect the subsequent polymerization.

Fig.1 SEM images of (a) pristine nylon fiber bundle,(b) CHIT/Nylon fiber,(c) GA-CHIT/Nylon fiber,(d) PPy/GA-CHIT/Nylon fiber,(e)magnification of (d),and (f) PPy/CHIT/Nylon fiber

As shown in Fig.2,the resistance of different fiber increases with the length of the fiber.The resistance of PPy/Nylon fiber is about 230 Ω/cm,while that of PPy/CHIT/Nylon fiber is 300 Ω/cm,slightly higher than pure PPy layer coating due to chitosan is an insulating material.The resistance value of PPy/GA-CHIT/Nylon fiber is about 200 Ω/cm.Because chitosan provides a more stable surface through cross-linking and improves bonding with conductive polymer,subsequent PPy coatings perform better than other coatings.

Fig.2 The resistance comparison of different functional fibers

Fig.3 is the Fourier infrared (FT-IR) curve of the composite fiber.The FT-IR spectrum of the nylon fiber shows peaks at 3287,2921,and 2854 cm-1which correspond to the N-H bending vibration in primary amine,C-H in phase and C-H out of phase stretching vibrations,respectively.The bands at 1630 and 1539 cm-1are assigned to C=O amide I,N-H and C-N combination amide II stretch respectively.The characteristic peaks at 1470 cm-1and 1408 cm-1could be attributed to CH2shear vibration.The band at 1367 cm-1corresponds to vibration of amide III,and the band at 1180 cm-1corresponds to twisted vibration of CH2[21,22].For the PPy/CHIT/Nylon fiber,the peak at 3447 cm-1is attributed to -NH2and -OH groups stretching vibration.The peaks at 1637 and 1560 cm-1are attributed to the CONH2and NH2groups,respectively.The peaks at 1160 cm-1is attributed to the bridge -O-stretch,and 1085 cm-1is attributed to the C-O stretch[23,24].The spectrum of the GLA crosslinked chitosan shows different characteristic peaks than that of the pure chitosan.The peak at 1658 cm-1corresponding to the formation of imine C=N bond,which is Schiff’s base structure formed by the reaction between the amino group of chitosan and the aldehyde group of glutaraldehyde[25].As PPy/GA-CHIT/Nylon fiber shown,the peak at 1433 cm-1is attributed to N-H stretching of pyrrole ring.The very strong peaks at 1049 and 1143 cm-1are attributed to (C-H) in plane vibration.The characteristic peak at 958 cm-1corresponds to C-H out of plane bending vibrations,and the existence of these characteristic peaks confirms the successful coating of PPy on the fiber[26,27].

Fig.3 The FT-IR curves of the nylon fiber and composite fiber

Fig.4 shows the stress-strain curves of nylon fiber,CHIT/Nylon fiber,GA-CHIT/Nylon fiber and PPy/GA-CHIT/Nylon fiber.The maximum breaking stress of nylon fiber is about 0.2 N/tex,and the breaking elongation ε is 76%.After CHIT treatment,the maximum breaking stress is increased to 0.23 N/tex,and the breaking elongation ε was 63%.The maximum breaking stress of GA-CHIT/Nylon fiber is about 0.17 N/tex.The elongation at break ε is 65%.The maximum breaking stress of PPy/GA-CHIT/Nylon fiber is about 0.14 N/tex,and the breaking elongation ε is 34%.Compared with the previous fiber,the young’s modulus of PPy/CHIT/Nylon fiber has been reduced.The reason for the decrease of modulus is that our polymerization system needs to be stirred vigorously,and the reaction system is acidic,which will cause certain damage to the fiber.However,the fiber can remain flexibility and resistant during weaving.

Fig.4 The stress-strain curves of the nylon fiber and functional fiber

Figs.5(a)-5(c) show the output,transfer and transient performance of FECT based on PPy/GACHIT/Nylon fiber.The output curve of our FECT shows the characteristics of a typical p-type transistor.The on-current is about 12 mA at low gate voltage.The source and drain current decreases with the increase of gate voltage.The devices have linear and saturation behaviors,proving their semiconductor characteristic.In addition,the transistors can operate at low voltage,indicating the low power consumption for various applications.Fig.5(b) shows the transfer and transconductance performance of FECT.This curve shows that the transistor operates in a typical depletion mode.When a forward gate voltage is applied,the channel material is de-doped and the conductivity is reduced.This leads to a drop inIds.It can be seen intuitively that our transistor has an on-current close to 12 mA when the gate voltage is small,and then as the gate voltage increases,the source-drain current drops sharply between 0.5-1.6 V,indicating that the current changes fastest in this interval.After 1.6 V,the downward trend of the current becomes very gentle.At the same time,it can be observed that FECT shows a high transconductance of about 6.8 mS when the gate voltage is 1.7 V,which indicates that the FECT has excellent electrocatalytic ability at this voltage.The transient curve also shows the good stability of the FECT,with the device remaining stable at alternating switching voltages.

Fig.5 (a) Output;(b) Transfer-transconductance;(c) Transient curves PPy/GA-CHIT/Nylon fiber-based organic electrochemical transistor

Figs.6(a)-6(c) show the performance of FECT using PPy/CHIT/Nylon fiber as the electrode.In the doped state at the gate voltage of 0 V,the FECT has an initial current of 3.8 mA,and reaches the minimum current in the dedoped state at the gate voltage of 2.5 V.The transfer curve still shows the law of depletion transistors.The positive gate voltage de-dopes the channel and decreases the source and drain current.When the gate voltage is 1.7 V,the transconductance of FECT exhibits 3.4 mS.Fig.6(c) shows the cycling stability of FECTs,the performance of the device is slightly degraded under cyclic operation.Compared with the FECT based on PPy/GA-CHIT/Nylon fiber,the performance of the FECT with PPy/CHIT/Nylon fiber as channel has decreased a lot.This is because uncrosslinked chitosan dissolves under acidic conditions.This leads to subsequent irregular PPy polymerization,which degrades performance of transistors.

Fig.6 (a) Output;(b) Transfer-transconductance;(c) Transient curve of PPy/CHIT/Nylon fiber-based organic electrochemical transistor

Fig.7 (a) Output;(b) Transfer-transconductance;(c) Transient curve of PPy/Nylon fiber-based organic electrochemical transistor

Figs.7(a)-7(c) show the performance of the FECT using PPy/Nylon fiber as channel.It can be seen directly that the output on-current is only 0.8 mA,the transconductance is only 0.5 mS,and the switching curve is also very unstable.This may be attributed to the irregularity structure of PPy.The induction of GA cross linking chitosan improve the interface interaction and enhance the polymerization of PPy.

Fiber-based organic electrochemical transistor was prepared by polymerizing polypyrrole on GA crosslinked chitosan treated nylon fibers.The introduction of GA improves the solubility of chitosan under acidic conditions.After crosslinking,more chitosan increases the specific surface area of the fiber,making the nylon fiber obtain regular nanorods morphology of PPy.The performance of transistors with PPy/Nylon fiber,PPy/CHIT/Nylon fiber and PPy/GA-CHIT/Nylon fiber as channel was compared.After the introduction of glutaraldehyde,the PPy has a regular nanorod shape.Therefore,the FECT based on PPy/GA-CHIT/Nylon fiber has excellent performance,good output curve,stable transient curve,and high transconductance of 6.8 mS.In addition,chitosan is a green,environmentally friendly and low-cost materials.It has great application potential as a composite layer to induce conductive polymer nanostructures.At the same time,the transistors based on PPy/GA-CHIT/Nylon fiber has good biocompatibility,and may have good application prospects in subsequent biosensing applications.