Bioinspired lotus fiber-based graphene electronic textile for gas sensing

Graphene electronic textiles (e-textiles) have attracted significant attention in various sensing applications owing to their strong advantages. During the fabrication of these textiles, there are factors to consider, such as electrical conductivity, mechanical flexibility, weight, and applicability in other practical applications. Bioinspired lotus fiber has appropriate advantages to be used as graphene e-textiles, including lightweight (< 1 mg), eco-friendliness, crease-resistant, pilling resistance, and flexibility. However, lotus fiber-based graphene e-textiles have not yet been reported. In this study, we developed a reduced graphene oxide‒coated lotus fiber (RGOLF) which was successfully fabricated by the hydrogen interaction between graphene flakes and cellulose fiber. The higher the GO concentration (~ 3 g/L) and fiber diameter (~ 300 μm), the higher the electrical conductivity of the RGOLF was measured. The RGOLF exhibited a higher electrical conductivity (4.63 ± 0.22 μS) and a remarkable sensing performance for hazardous NO2 gas molecules within a short exposure time (~ 3 min), including a low detection limit (~ 1 ppm), selectivity, and resistance to relative humidity. Moreover, we verified the mechanical flexibility and elasticity of RGOLF through a 1,000-cycle bending test, and tensile test, respectively. These results suggest that the bioinspired RGOLF could be used as a gas sensor in environmental air with a strong potential for use in various wearable applications.

graphene oxide-coated lotus fiber (RGOLF) which was successfully fabricated by the hydrogen interaction between graphene flakes and cellulose fiber. The higher the GO concentration (~ 3 g/L) and fiber diameter (~ 300 μm), the higher the electrical conductivity of the RGOLF was measured. The RGOLF exhibited a higher electrical conductivity (4.63 ± 0.22 μS) and a remarkable sensing performance for hazardous NO 2 gas molecules within a short exposure time (~ 3 min), including a low detection limit (~ 1 ppm), selectivity, and resistance to relative humidity. Moreover, we verified the mechanical flexibility and elasticity of RGOLF through a 1,000-cycle bending test, and tensile test, respectively. These results suggest that the bioinspired RGOLF could be used as a gas sensor in environmental air with a strong potential for use in various wearable applications.

Introduction
Electronic textiles (e-textiles) have attracted significant attention in various applications, including chemical gas sensors (Lee et al. , 2021a(Lee et al. , 2021b, energy harvesting (Abdelkader et al. 2017;Liu et al. 2015a), and electrochemical sensors for bio-signal detection (Kinnamon et al. 2018;Hu et al. 2020). E-textiles have many advantages, such as portability, flexibility, high conductivity, and lightweight, making them suitable for application as wearable devices (Stoppa and Chiolerio 2014;Kan and Lam 2021;Lee et al. 2021a). Specifically, graphene e-textiles have been widely developed in sensing technologies, such as electrochemical biosensors (Coyle et al. 2007), pressure sensors (Li et al. 2019), motion sensors , and electronic skin (Chun et al. 2019). Moreover, graphene e-textiles have been advanced as chemical gas sensors for detecting hazardous gases, such as nitrogen dioxide (NO 2 ) (Lee et al. 2021b), ammonia (NH 3 ) , carbon monoxide (CO) (Utari et al. 2020), and hydrogen (Zhu et al. 2021). Graphene is a well-known 2-dimensional carbon material which is consisting of a single-atom-thick layer with sp 2 -hybridized carbon atoms (Lee et al. 2018aAllen et al. 2010). The honeycomb structured graphene has outstanding intrinsic properties such as electrical/thermal conductivity, physical property, mechanical strength/flexibility, chemical stability, and biocompatibility Geim and Novoselov 2007;Afroj et al. 2019). Moreover, graphene is suitable for detecting gas molecules, due to high electrical mobility at room temperature, high carrier mobility, high signalto-noise ratio arising from low intrinsic noise, and high surface-to-volume ratio (Novoselov et al. 2012). These remarkable advantages allow graphene to be used for chemical gas sensors (Lee et al. 2021a).
To fabricate the graphene e-textiles, graphene has been attempted to coat on various 1-dimensional textiles (e.g., cotton yarn, nylon-6 yarn, and polyester yarn) (Lee et al. 2021bYun et al. 2013;Ju Yun et al. 2015). According to the literature, graphene e-textiles exhibit mechanical flexibility, washability, and the ability to sense toxic gases (Lee et al. , 2021b. Thus, graphene e-textiles have the potential to be used as wearable sensors for detecting toxic gases and even for monitoring human health from their breath. However, to improve electrical conductivity, which is an essential component of wearable sensors, textiles are required to be functionalized with adhesive molecules for attaching abundant graphene flakes. Textile surfaces have been functionalized using various biomolecules (e.g., bovine serum albumin, dopamine, globular/fibrous protein) (Ju Yun et al. 2015;Lee et al. 2020Lee et al. , 2021b, and organic molecules (Kang et al. 2018), resulting in highly conductive graphene e-textiles. Metal/metallic nanoparticles (e.g., Au and Ni) have also been composed in the fabrication of highly conductive graphene e-textiles (Yun et al. 2017;Liu et al. 2015b). However, to obtain highly conductive e-textiles, previous 1-dimensional textile requires a gluing of their surface with adhesive molecules or fabrication of the graphene composites with nanoparticles. Moreover, the yarn-based graphene e-textiles are not suitable for use as wearable sensors because of their weight (~ 22 mg) (Lee et al. 2021a).
To address these issues, graphene e-textiles were fabricated on 2-dimensional textiles (e.g., polyester sheet and nylon-6 mesh fabric) (Lee et al. 2021a;Park et al. 2018). These graphene e-textiles showed remarkable performance for sensing gas molecules, mechanical flexibility, and washability. Moreover, it has approximately 19 times lighter than the yarnbased graphene e-textiles (Lee et al. 2021a). However, they still require the adhesive glues to improve electrical conductivity that would enable the attainment of good sensing efficiency for toxic gases. Further, it is difficult to employ 2-dimensional textiles in various applications owing to their lattice structure. Therefore, we endeavored to identify a 1-dimensional textile that is appropriate for use as a wearable device and widely expandable for applications.
Lotus fiber (Nelumbo nucifera), an aquatic cellulosic fiber, could be a promising candidate as a template for e-textiles owing to its lightweight (< 1 mg), eco-friendly, crease-resistant, pilling resistant, and flexible properties (Sangita Tomar 2019). Furthermore, the functional groups on cellulose fiber could bind with graphene flakes via hydrogen interaction, which allows for its improvement of the electrical conductivity with no use of adhesive materials.
In this regard, we developed a reduced graphene oxide-coated lotus fiber (RGOLF). The physical and chemical properties of the RGOLF were analyzed microscopically and spectroscopically. We characterized the electrical properties according to the RGOLF thickness. Finally, the RGOLFs were attempted to detect toxic NO 2 gas molecules. The RGOLF exhibited good enough sensing performance, including a low detection limit, selectivity, and resistance to relative humidity. Furthermore, we confirmed that the RGOLF could be a wearable gas sensor, through mechanical flexibility up to 1,000 times of bending test and tensile test. Taken together, these results suggest that the RGOLF could form the next generation of 1D graphene e-textiles for various applications.

Materials
Graphene oxide (GO) solution was purchased from Graphene Supermarket, Inc. (USA). The optimal concentration of the GO solution was adjusted using distilled water (DW, ThermoFisher, USA). The GO solution was vigorously shacked to disperse the GO flakes in the solution by using a vortex. For chemical reduction of GO, we purchased hydroiodic acid (57 wt% in H 2 O, 99.99%), sodium bicarbonate (> 99.7%), and acetic acid solution (> 99%) from Sigma Aldrich (USA).

Preparation of the lotus fibers
Lotus fiber (LF) was obtained from the stem of Nelumbo nucifera harvested from nature (Pandey et al. 2020). Before preparing LF, the stems were washed using DW and dried at room temperature (23 °C). We obtained several strands of cellulose fibers by cutting the outer skin of the lotus peduncle and pulling it on both sides. Bare LF was prepared into a single strand (length: ~ 20 cm) by twisting and gathering several strands of cellulose fibers (Fig. 1ac). Bare LF was then dried in a fume hood which is maintained at 23 °C of room temperature and 26% of relative humidity.

Fabrication of RGOLF
Reduced graphene oxide-coated lotus fibers (RGOLFs) were fabricated using bare LF and GO solution ( Fig. 1a-c). Through a dip-coating method, the bare LFs were soaked in a 3 g/L GO solution for 2 h to uniformly coat the GO flakes onto the LF surface, resulting in a GO-coated LF (GOLF) (Lee et al. 2021b). The GOLF was dried in a fume hood (23 °C of room temperature and 26% of relative humidity) before the chemical reduction of the GO flakes. The GO flakes on the GOLF surface were transformed into reduced graphene oxide (RGO) flakes using the commonly used chemical reduction method (Lee et al. , 2021b(Lee et al. , 2021a. In detail, the GOLF was immersed in a vial containing 2 mL of hydroiodic acid solution (8.91 μM) and 5 mL of acetic acid solution (83.26 μM). For the chemical reduction process, the vial was incubated in oil bath for 2 h which was maintained at 40 °C. Subsequently, the fibers were washed several times using saturated sodium bicarbonate solution and DW to remove the residual reducing agent (Ju Yun et al. 2015;Lee et al. 2020). The brownish color of GOLF turned black after GO reduction ( Fig. 1d) (Lee et al. , 2021b. The RGO-coated LF (RGOLF) was dried overnight under a fume hood maintained at room temperature.

Characterization
The morphology of GO flakes was characterized using field-emission transmission electron microscopy (FE-TEM, JEM-2100F, JEOL, Japan) and atomic force microscopy (AFM, Multimode 8, Bruker). AFM imaging was operated in the tapping mode under ambient air. The height of GO flakes was analyzed using Nanoscope Analysis software (Bruker). Furthermore, the morphology of the fibers was characterized using a scanning electron microscope (SEM, JEOL-7610F-Plus, JEOL, Japan). For high-resolution SEM imaging, a Pt layer (~ 12 nm thick) was coated on the fiber surface using a sputter coater (108 auto, Cressington Scientific Instruments Inc., UK) with a deposition rate of 0.83 Å/s for 150 s . SEM imaging was performed at a high voltage (10 kV). A microscopic image of bare LF was obtained using an optical microscope (DS-Ri2, Nikon, Japan) to measure the diameter.
Chemical characterization of the fibers (including the GOLF, and the RGOLF) was performed to compare the chemical differences before and after the GO reduction by using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) (Ferrari and Basko 2013;Lee et al. 2021a). The Raman spectra were obtained by Raman spectroscopy (LabRam Aramis, Horriba Jovin Yvon, USA) using a 563 nm excitation laser. The XPS (K-alpha, Thermo VG, UK) analysis was carried out with a sampling area (400 × 400 μm 2 ) in a vacuum (4.8 × 10 −9 mbar), using a monochromatic Al X-ray source at 1486.6 eV. The electrical conductivity of all fibers was characterized using a commercial current-voltage (I-V) meter (Cantis Inc., South Korea) (Lee et al. 2018b). Two probes from the I-V meter were connected to both ends of the fiber (3 cm in length) . Simultaneously, the electric current was recorded on a computer. The applied voltage (2 V) and sweep mode (from -2 to 2 V) were controlled using the enclosed software. We measured the initial electrical conductivity at 2 V before the exposure of RGOLF to NO 2 . The experiment was performed at 23 °C (room temperature) and 26% relative humidity.

Exposure of the RGOLF to gas molecules
The RGOLF was placed in a gas chamber for exposure to NO 2 (i.e., the target gas). The volume (500 sccm) and concentration (0 -100 ppm) of the gas were regulated by using a home-built mass flow controller (MFC) (Lee et al. 2018b(Lee et al. , 2021b. The NO 2 concentration was controlled by mixing it with an inert pure nitrogen gas (i.e., the base gas) in the MFC. To characterize gas detectability, the electric current of RGOLF was measured at an applied voltage of 2 V after gas exposure. The gas detectability test was conducted in a laboratory where the temperature was maintained at 21 °C and 28% relative humidity.
Selectivity and relative humidity test of the RGOLF The selectivity and humidity tests of the RGOLF were conducted via various gases and relative humidity (20 -99%). Each RGOLF was exposed to various gases, including nitrogen (> 99%, N 2 ), ethanol (10 ppm, C 2 H 5 OH), carbon monoxide (10 ppm, CO), acetone (100 ppm, C 3 H 6 O), ammonia (100 ppm, NH 3 ), and NO 2 (10 ppm). For the humidity test, relative humidity (20 -99%) was controlled by changing the mixing ratio of fully humid air and dry air. Humidity was confirmed using a humidity sensor. The electric current of the RGOLFs was measured by using an I-V meter before and after exposure to various gases or relative humidity.

Contact angle
For verifying the hydrophobicity of RGOLF, we measured a water contact angle by using contact angle analyzer (Phoenix 150, SEO, South Korea) (Moon and Lee 2021). 0.17 μL of droplet was dropped on the surface RGOLF. Same volume of droplet was dropped on the GOLF surface for the comparison. The image and contact angle were obtained through the enclosed software (Surfaceware 9, SEO, South Korea).

Mechanical tests of the RGOLF
We carried out a bending test of the RGOLF (5 cm in length) using a computer-controlled actuating system (EA10.X/HD, Step Lap, Italy) (Lee et al. 2021b). One end of the RGOLF was held on a fixed stage, and the other end was held on a movable stage. The elongation and compression of the RGOLF proceeded in one cycle. The bending test was repeated for up to 1,000 cycles at a scan rate of 1 cm/s. The electric current of RGOLF was measured after every 50 cycles of bending. The tensile test of the RGOLF was proceeded by using a material testing machine (Galdabini QUASAR 50, Galdabini SPA, Italy) (El Nahrawy et al. 2021). The same length (5 cm) of the RGOLF was held on a fixed stage, and the other side of RGOLF was held on a movable stage. The tensile stress proceeded with a constant rate (5 mm/min) and tensile load (1 N). A stress-strain curve was obtained by enclosed software. The mechanical test was performed in a laboratory where the temperature was maintained at 20 °C and 25% relative humidity.

Results and discussion
The schematic illustration shows the fabrication steps for a reduced graphene oxide-coated lotus fiber (i.e., RGOLF) (Fig. 1a). We prepared lotus fiber (LF) from Nelumbo nucifera with various diameters (150 -300 μm) for use as an e-textile platform (Fig. 1b) (Wu et al. 2014). As well known, microscopic analyses have demonstrated that the LFs have pilling resistance (Fig. 1c) (Sangita Tomar 2019). The detailed information is provided in the Experimental Section. Graphene oxide-coated lotus fiber (GOLF) was fabricated by coating bare LF with GO flakes (3 g/L) through a dip-coating method (Ju Yun et al. 2015). Through chemical reduction, brownish-colored GOLF was converted to black-colored fiber (Fig. 1d) Ju Yun et al. 2015). This indicated that the GO flakes were well transformed into reduced graphene oxide (RGO) flakes, resulting in RGOLF (Lee et al. 2021b(Lee et al. , 2021a. At applied voltages ranging from -2 to 2 V, the RGOLF exhibited high electrical conductivity, whereas both bare LF and GOLF exhibited insulating properties (Fig. 1e). It also implied that the RGO flakes were well transformed and stably attached to the LF surface.
Chemical analysis of the fibers (GOLF, and RGOLF) was performed to confirm whether RGO flakes were reduced from GO flakes (height, ~ 1 nm) ( Fig. 2a and b)  . From the Raman spectrum, both fibers exhibited intrinsic graphene peaks, including the D and G peaks at 1342.34 cm −1 , and 1595.89 cm −1 , respectively (Ferrari and Basko 2013). It means that the graphene flakes (GO, and RGO) were well attached to the LF surface ( Fig. 2c) (Lee et al. 2021b(Lee et al. , 2021a. After GO reduction, the I D /I G ratio increased from 0.91 to 1.19, which is attributed to the decrease of G peak intensity (Lee et al. , 2021b(Lee et al. , 2021a. In detail, the hexagonal network of carbon atoms with defects were recovered that occurs decreases of G peak intensity (Moon et al. 2010). In XPS analysis, the GOLF and RGOLF depicted both C 1 s and O 1 s peaks, which were derived from graphene derivatives (Fig. 2d) (Ju Yun et al. 2015;Lee et al. 2021b). The intensity of the O 1 s peak at 532.24 eV was diminished by 61.55% after GO reduction, indicating the chemical reduction of GO flakes (Lee et al. 2021a). Furthermore, the intensity of the C − O peak in the XPS C 1 s spectra decreased from 34.96% to 33.95% after GO reduction, signifying that the RGO flakes on RGOLF were successfully transformed from the GO flakes on GOLF ( Figure S1 and Table S1 in Supplementary Information) (Lee et al. 2018b). The N 1 s peak was observed in both GOLF and RGOLF, which is derived from amino acids in the LF (Pandey et al. 2020;Johansson et al. 2020). Both chemical analysis results revealed that the GO flakes were well bound on the LF without any additional adhesive materials, and the RGO flakes on RGOLFs were well transformed from GO flakes on the GOLFs through chemical reduction. Interestingly, without adhesive materials, the RGOLFs (9.31 ± 0.40 μA at 2 V of applied voltage) exhibited higher electrical conductivity, compared to the conventional cotton yarn-based graphene e-textiles (3.42 ± 0.01 μA at the 2 V), as shown in our previous work (Lee et al. 2021b). This might be attributed to the hydrogen interactions between GO flakes and cellulose (i.e., LF) (Chen et al. 2018;Xu et al. 2015). Therefore, compared to the conventional cotton yarn-based graphene e-textiles, we successfully developed graphene e-textiles (RGOLF) with high electrical conductivity without any side-effects from the insulating adhesive proteins (i.e., globular proteins, and amyloid nanofibrils) (Lee et al. 2021b).
The morphological characteristics of the fibers (LF, GOLF, and RGOLF) were obtained by using SEM (Fig. 3). We observed a distinct difference between the fibers before and after the GO coating. The bare LF has no distinctive features on its surface, whereas both fibers (GOLF and RGOLF) exhibited numerous wrinkles and ripples on their surface after GO attachment (Lee et al. 2021b;Ju Yun et al. 2015). The images showed that the GO flakes were well attached to the surface of the GOLF, as well as the RGOLF. Additionally, the SEM images confirmed that the RGOLFs were appropriately fabricated using LF and graphene.
Before exposure to NO 2 , we prepared the RGOLFs with various GO concentrations (1 -3 g/L) to analyze their electrical characteristics (Fig. 4). The results indicated that the electric current of RGOLF increased as the GO concentration increased (Fig. 4a). Specifically, an electric current of RGOLF fabricated using 3 g/L of GO was measured to be 22.87 μA at 2 V. It is attributed to the difference of GO concentration that large amount of GO flakes might attach on the LF surface than the low concentration of GO (Lee et al. 2021b). Meanwhile, both bare LF and GOLF exhibited an insulating property at the applied voltage range (from -2 to 2 V). The electrical characteristics of RGOLFs were empirically investigated using RGOLFs fabricated with GO flakes (3 g/L) and various diameters (150 -300 μm) of LF (Fig. 4b). It showed that the greater the diameter of LF, the greater the electric current was measured; 1.76 ± 0.10 μA in RGOLF with 150 μm, 3.05 ± 0.32 μA in RGOLF with 200 μm, 4.70 ± 0.24 μA in RGOLF with 250 μm, and 9.31 ± 0.40 μA in RGOLF with 300 μm) (Table S2). Similarly, electrical conductance increased as the LF diameter increased. At the same applied voltage (2 V), the RGOLF fabricated with 3 g/L GO and 300 μm diameter showed the highest conductance (4.63 ± 0.22 μS) ( Table S2). The electrical conductance (G) was obtained using the following equation, where ΔV is the increment of the applied voltage (0.1 V), and ΔI is the change in the measured current when ΔV occurs.
For practical application, we exposed NO 2 to RGOLFs fabricated with the optimized concentration of GO (3 g/L) on LF (diameter: 300 μm). NO 2 gas detectability was scrutinized using the RGOLF (Fig. 5a). The RGOLFs were exposed to various NO 2 concentrations (1 -10 ppm), and their response characteristics were measured (Fig. 5b). Noted that NO 2 gas has harmful effects not only environmental pollution, such as mild dust, acid rain, and destruction of the ozone layer, but also on human health, causing nausea, throat/eye irritation, and respiratory disease. According to the literature (Lee et al. 2018a(Lee et al. , 2021b, the specific concentration (10 ppm) of NO 2 is known to be the threshold concentration that induces a fatal effect on human health. Long-term exposure to 10 ppm NO 2 causes nose/throat discomfort, transient cough, eye irritation, fatigue, and nausea (Ou et al. 2015;Elsayed 1994).
In this regard, we exposed the RGOLF to various concentrations (1 -10 ppm) of NO 2 for 3 min. As shown in Fig. 5b, the electric current increased with increasing NO 2 concentration, exhibiting linearity (R 2 = 0.997). The changes in electric current (ΔI = I−I 0 ) were measured at the applied voltage of 2 V. I 0 and I denote the electric current before and after exposure to gas, respectively. In detail, the electric current was increased to 0.24 ± 0.02 μA, 0.44 ± 0.04 μA, 0.63 ± 0.03 μA, 0.81 ± 0.11 μA, 1.10 ± 0.05 μA, and 1.34 ± 0.14 μA with 1, 3, 5, 7, 9, and 10 ppm NO 2 , respectively. The selectivity and relative humidity resistance were investigated by exposing RGOLFs to various gases or relative humidity (20 − 99%). In terms of the selectivity test, RGOLF responded well to NO 2 (10 ppm) but barely responded to other gases such as N 2 (99%), C 2 H 5 OH (100 ppm), CO (10 ppm), C 3 H 6 O (100 ppm), and NH 3 (100 ppm) (Fig. 5c). The electrical response was defined using the following equation: The electrical response of RGOLF was increased after the NO 2 exposure, due to the NO 2 molecules are acted as an electron-acceptor on the RGO flakes (Lee et al. 2018a). Besides, the RGOLF exhibited a negligibly small negative electrical response to the NH 3 , CO, C 2 H 5 OH, and C 3 H 6 O gas molecules which are acted as an electron-donor (Leenaerts et al. 2008;Lee et al. 2020). Among the electron-donor molecules, it seems that the RGOLF could be used for detecting NH 3 molecules emitted from the patient's breath (e.g., renal disease and halitosis) through the negative electrical response (Lee et al. , 2021aLi et al. 2018;Cha et al. 2018). Moreover, we confirmed that the effect of relative humidity (20 -99%) on the response to electric current was negligible (Fig. 5d). It is due to the hydrophobicity of RGOLF that has a negligible effect on response to electric current under various ranges (20 -99%) of relative humidity. For (2) Response = I − I 0 ∕I 0 × 100(%) Fig. 4 Electrical characterization of the RGOLF via (a) various concentrations (1 -3 g/L) of GO, and (b) the fiber diameter (150 -300 μm). Each data point represents triplicate measurements verifying the hydrophobicity, we measured a water contact angle on the RGOLF. As seen in the images, the GOLF surface shows hydrophilicity to the water, while the RGOLF has a hydrophobicity ( Figure S2a and b). The contact angle was measured 20.64 ± 2.29° on the GOLF surface, and 86.40 ± 7.15° on the RGOLF surface ( Figure S2c). It is attributed to the RGO flakes on the RGOLF surface have hydrophobicity (Mousavi et al. 2020).
Mechanical flexibility is an essential factor for various practical applications in wearable devices . For mechanical flexibility, 1,000 bending cycles were performed on RGOLF (Fig. 6). As shown in the schematic, the elongation and compression of RGOLF proceeded as a single bending cycle. The RGOLF exhibited no dramatic changes in the electrical response (1.14% of variation) during the 1,000 cycles of the bending test. This indicates that the RGOLF exhibited outstanding flexibility. Furthermore, we proceeded with a tensile test for verifying the elasticity and tensile strength of the RGOLF ( Figure S3). The strain-stress curve of RGOLF shows 410.62 ± 29.85 MPa of Young's Modulus and 30.63 ± 1.32 MPa of tensile strength. The values were corresponded to the previous work (Wu et al. 2014;Wang and Facchetti 2019;Zhang and Guo 2014), and has superior mechanical properties (e.g., flexibility, elasticity, and tensile strength) than other platforms The RGOLFs were exposed to various gases including N 2 (99%), C 2 H 5 OH (100 ppm), CO (10 ppm), C 3 H 6 O (100 ppm), NH 3 (100 ppm), and NO 2 (10 ppm), respectively. (d) Electrical responses of the RGOLF exposed to relative humidity (20 -99%). Each data point represents triplicate measurements (Table S3). Based on both mechanical tests, RGOLF could be a candidate for a wearable sensor based on graphene e-textiles for detecting toxic gas molecules in the environment. Also, it can be used to monitor human health (e.g., kidney disease, halitosis, and cancer) by detecting specific gas molecules from the patient's breath (Di Natale et al. 2014;Chan et al. 2020;Kahn et al. 2015).

Conclusions
We demonstrated graphene e-textiles (RGOLF) that were fabricated by coating graphene flakes on bioinspired LF. Compared to conventional graphene e-textiles, RGOLF exhibited a higher electrical conductivity (4.63 ± 0.22 μS) with use of the same concentration of graphene. Moreover, the RGOLF is based on cellulose fiber and therefore very light (< 1 mg), which would be a great advantage for wearable applications such as e-textiles. We optimized the GO concentration and diameter of the LF, which showed high electrical conductivity. The higher the GO concentration and LF diameter, the higher was the electrical conductivity of RGOLF. We verified the possibility of RGOLF as a wearable application through NO 2 gas exposure and mechanical flexibility tests. From the practical experiment, we confirmed that the RGOLF has a potential candidate for a wearable gas sensor to detect toxic NO 2 gas from environmental air. The RGOLF showed a good sensing performance to NO 2 with a low detection limit (~ 1 ppm), high selectivity, and resistance to relative humidity. This means that the RGOLF could be used as a NO 2 gas leakage alarm system in industrial areas or living environments (Video S1; see Supplementary Information). Furthermore, the mechanical flexibility test exhibited the outstanding flexibility of the RGOLF, supporting its applicability as a wearable gas sensor. Taken together, our results demonstrate that the bioinspired RGOLF has the potential to be used in various wearable applications such as gas sensors for detecting hazardous gas molecules in environmental air, and exhaled breath sensors for monitoring human health based on human breath. Overall, the RGOLF suggests an eco-friendly, cheap, disposable, and easily fabricated graphene e-textile that can be used in various wearable applications. Fig. 6 Electrical responses of the RGOLF upon repeated bending cycles (up to 1,000 times). The inserted schematics show one process cycle for the bending test. Each data point represents double measurements