Eunji Lee

Cellulose and its derivatives have evoked much attention in sensor technology as host-matrices for conducting materials because of their versatility, renewability, and biocompatibility. However, only a few studies have dealt with the potential utilization of cellulose as a sensing material without a composite structure. In this study, cellulose nanofibers (CNF) and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibers (TOCNF) extracted from rice husks by using ultrasonic-assisted methods are introduced as a potential gas sensing material with highly sensitive performance. To fabricate nanocellulose-based films, CNF, TOCNF, and TOCNF with glycerol (TOCNF/G) were dispersed in water and applied on polyimide substrate with digital electrodes to form self-standing thin films by a drop-casting method. A transparent coating layer on the surface of the plate after drying is used for the detection of water-soluble gases such as acetone, ammonia, methane, and hydrogen sulfide gases at room temperature at 52% relative humidity. The sensor prototypes exhibited high sensitivity, and the detection limit was between 1 ppm and 5 ppm, with less than 10 min response and recovery time. The results indicate that both the CNF- and the TOCNF-coated sensors show good sensitivity toward ammonia and acetone, compared to other gases. A TOCNF/G-coated sensor exhibited minimum time in regard to response/recovery time, compared to a CNF-coated sensor. In this study, nanocellulose-based sensors were successfully fabricated using a low-cost process and a bio-based platform. They showed good sensitivity for the detection of various gases under ambient conditions. Therefore, our study results should further propel in-depth research regarding various applications of cellulose-based sensors in the future.

The level of urea in urine can be a crucial biomarker to detect kidney failure, dehydration, renal function, and hepatic failure. [1] Thus, rapid and accurate sensing of urea is essential in a diagnostic clinic, especially for point-of-care testing devices (POCT). Paper-based analytical devices have been demonstrated as POCT due to its disposability, portability, low cost, and environmental-friendly quality. [2] In order to measure urea, electrochemical biosensors based on the enzyme were explored, but the application of enzyme has its limitations due to low shelf-life, complicated immobilization procedure, high cost, and complicate operating condition which is not fit to POCT application. [3] To overcome the limitations of enzyme, non-enzymatic urea sensor have attracted from researchers due to advantages such as good stability, re-usability, high sensitivity, redox flexibility, simplicity, and low cost. [4] As a non-enzymatic catalyst, nano metal oxides such as NiO, SnO₂, and Fe₂O₃, exhibited catalytic activity on urea by the formation of the redox couple on the sensor surface.[5] [6] [7] To utilize non-enzymatic catalysts with comparable properties of enzyme, the development to increase reaction surface area has been emphasized by using nanomaterial, hierarchical structure, and support matrix.[8] [9] However, those processes involve complicate fabrication steps and often need additional post-process to remove the template leading to poisoning of the catalyst. As a potential approach for template-free process, paper can be used for an electrochemical sensor substrate. Paper has a complex fiber matrix that can provide pore pathways for the electrolyte to infiltrate inside, and by distributing catalyst uniformly within the paper, which formsmore reaction sites. Therefore, enhancement of performance and simplification of the fabrication can be achieved with the use of paper. In this study, the simple, disposable paper-based device was developed for urea electro-oxidation by using paper matrix as support. The paper-based electrode was fabricated by a screen printing method with filter paper, and different combinations of nano-metal oxides, NiO, SnO₂, and Fe₂O₃. The metal oxide catalysts were loaded on the working electrode area and compared the performance. Structure and morphology of paper-based electrode were characterized by XRD and SEM. Investigation of selectivity and LOD was conducted by potentiostat. A detailed description of the fabrication of paper-based electrode and sensing performances will be presented. References [1] Boggs, Bryan K., Rebecca L. King, and Gerardine G. Botte. "Urea electrolysis: direct hydrogen production from urine." Chemical Communications 32 (2009): 4859-4861. [2] Martinez, Andres W., et al. "Patterned paper as a platform for inexpensive, low‐volume, portable bioassays." Angewandte Chemie International Edition 46.8 (2007): 1318-1320. [3] Riklin, Azalla, et al. "Improving enzyme–electrode contacts by redox modification of cofactors." Nature 376.6542 (1995): 672. [4] Nie, Huagui, et al. "Nonenzymatic electrochemical detection of glucose using well-distributed nickel nanoparticles on straight multi-walled carbon nanotubes." Biosensors and Bioelectronics 30.1 (2011): 28-34. [5] Arain, Munazza, et al. "Simpler and highly sensitive enzyme-free sensing of urea via NiO nanostructures modified electrode." RSC Advances 6.45 (2016): 39001-39006. [6] Ansari, S. G., et al. "Electrochemical enzyme-less urea sensor based on nano-tin oxide synthesized by hydrothermal technique." Chemico-biological interactions 242 (2015): 45-49. [7] Das, Gautam, et al. "NiO-Fe2O3 based graphene aerogel as urea electrooxidation catalyst." Electrochimica Acta 237 (2017): 171-176. [8] Wu, Mao-Sung, Ren-Yu Ji, and Yo-Ru Zheng. “Nickel Hydroxide Electrode with a Monolayer of Nanocup Arrays as an Effective Electrocatalyst for Enhanced Electrolysis of Urea.” Electrochimica Acta 144 (October 2014): 194–99. [9] Liang, Yanhui, et al. "Enhanced electrooxidation of urea using NiMoO4· xH2O nanosheet arrays on Ni foam as anode." Electrochimica Acta 153 (2015): 456-460. Acknowledgement This work was partially supported by Agency for Defense Development (ADD) as global cooperative research for high performance and light weight bio-urine based fuel cell (UD160050BD), and the Ocean University of China-Auburn University (OUC-AU) Grants program.

There has been considerable effort to develop wearable electronics from life-supporting devices for solders to fashion accessories such as smartwatches. The research of gas sensor has also attempted to adapt wearables with the power of nanotechnology. On the wearable platform, miniature gas sensors will provide real-time information about the atmosphere to protect each personnel from possible hazardous chemical attacks. In addition, wearable gas sensor can be facilitated to monitor human’s breath as medical applications. [1] One of the important factors to develop wearable gas sensors is to operate at room temperature with excellent sensing performance. Recently, two-dimensional (2D) nanostructured materials have been highlighted as new sensing materials due to their working ability at low temperatures. Unlike conventional metal oxides, 2D materials such as graphene are able to work at room temperature with comparable gas response to NO₂ gas. However, insufficient sensing performance, such as sluggish recovery and low gas response, need to be developed. [2] As one attempt to enhance performance, 2D material was hybridized with metal oxide. By incorporating two nanostructured materials, synergistic effect, which comes from benefits of each material, can improve sensing performance. [3] Another novel innovation is the introduction of newly discovered 2D materials. 2D transition metal carbides and/or carbonitrides (called MXenes) are a new family of 2D materials. The vast diversity in a combination of constituent elements and ordered structure in MXenes offers future possibilities to find out a new generation of sensing materials with their superior performance. [4, 5] In this study, different types of 2D nanostructured materials and their hybrids were investigated for the breath analysis of various volatile compounds. Graphene oxide (GO) was incorporated with TiO₂ nanoparticles, and the composite was photo-reduced under UV irradiation. Room temperature gas sensing was carried out against various VOC gases, and sensing performance was evaluated by comparing with pure GO. With the tailored hetero-junction at the interfaces of GO and TiO₂, the composite can identify ethanol, methanol, and acetone, and its gas response was enhanced. In addition, we introduced new 2D nanostructured materials, MXenes including Ti₃C₂T_x and V₂CT_x. 2D V₂CT_x gas sensors showed excellent gas-sensing performance in terms of high response toward non-polar gases. Compared to the sensing properties of Ti₃C₂T_x MXene, 2D V₂CT_x sensor showed higher selectivity and long-term stability. The comparative results suggest that the modification of ordered structure and constituent elements of MXenes play a pivot role in the interaction between analyte and MXenes leading to an extensive change designed to detect individual compounds. [5, 6] References [1] Patel, S., Park, H., Bonato, P., Chan, L., & Rodgers, M. (2012). A review of wearable sensors and systems with application in rehabilitation. Journal of neuroengineering and rehabilitation, 9(1), 21. [2] Choi, S. J., & Kim, I. D. (2018). Recent Developments in 2D Nanomaterials for Chemiresistive-Type Gas Sensors. Electronic Materials Letters, 1-40. [3] Meng, F. L., Guo, Z., & Huang, X. J. (2015). Graphene-based hybrids for chemiresistive gas sensors. TrAC Trends in Analytical Chemistry, 68, 37-47. [4] Yu, X. F., Li, Y. C., Cheng, J. B., Liu, Z. B., Li, Q. Z., Li, W. Z., ... & Xiao, B. (2015). Monolayer Ti2CO2: a promising candidate for NH₃ sensor or capturer with high sensitivity and selectivity. ACS applied materials & interfaces, 7(24), 13707-13713. [5] Lee, E., VahidMohammadi, A., Prorok, B. C., Yoon, Y. S., Beidaghi, M., & Kim, D. J. (2017). Room Temperature Gas Sensing of Two-Dimensional Titanium Carbide (MXene). ACS applied materials & interfaces, 9(42), 37184-37190. [6] Lee, E., VahidMohammadi, A., Yoon, Y. S., Beidaghi, M., & Kim, D. J. (2019). Two-Dimensional Vanadium Carbide MXene for Gas Sensors with Ultrahigh Sensitivity Toward Nonplar Gases. ACS Sensors, 4, 1603-1611.

This review focuses on newly emerging two-dimensional MXenes for gas sensing applications from a theoretical to an experimental view to guide future research. Various synthesis routes of 2D MXenes have been explored and recent success of various MXenes has allowed more knowledge on the relations between their structure and materials properties. We review distinctive gas sensing properties of MXenes in two aspects of theoretical and experimental view. Theoretical insight into the gas-surface interaction mechanism and experimental results of various MXenes on their sensing properties are complied and discussed. To tailor and enhance the sensing performance of MXenes, the parameters such as precursors, morphology, surface terminations, and interlayer structures are emphasized. Perspectives on challenges and opportunities are offered for further development of MXenes-based gas sensors.

Existing energy sources based on fossil fuel is an obstacle for environmental sustainability, and there is a concern regarding the depletion of fossil fuel resources [1], [2]. Therefore, it is essential to develop new alternative energy sources. As a safe and effective method of generating power, the conversion of natural biomass to electricity in the form of direct biomass fuel cell (DBFC) has attracted significant attention due to its high efficiency and low emissions [3]. Among various DBFCs, direct urea fuel cell (DUFC) using urea in wastewater and urine as fuel has recently been validated as a clean energy device [4]. The overall power response of the DUFC mainly depends on the urea oxidation reaction (UOR), so it is vital to investigate the UOR catalysts. Ni-based catalysts, which are inexpensive and commonly used as UOR catalysts, have widely utilized for replacement of noble metals. The Ni/C electrode as an anode in the reported study achieved a power density of 1.7 mW/cm² [5], and nanostructured Ni-based catalysts or bimetallic types of catalysts also have been implemented for better performances [6], [7]. However, the above studies proceeded in an alkaline medium, which makes it difficult to use actual wastewater or urine directly as a source for fuel cells application. Since the electrochemical features of nickel-based catalysts are based on the mechanism of electrooxidation from Ni²⁺ to Ni³⁺ in alkaline medium, the catalytic activity in neutral solutions is less well distinguished [8]. Thus, nickel-based catalysts essentially comprising Ni³⁺ ions will more efficiently improve the UOR performances even if a neutral atmosphere of urea solution is adopted. In this study, the catalyst with a nanostructures array based on nickel oxyhydroxide with metal (metal-NiOOH) was prepared by using the electrospinning process for DUFC. The morphology and the structure of the metal-NiOOH catalysts were observed using scanning electron microscopy, Transmission electron microscopy, and X-ray diffraction methods. The metal-NiOOH catalyst used as the DUFC anode showed excellent UOR performances and power achievements in real human urine. A comprehensive investigation on the performances of metal-NiOOH and the function of internal Ni³⁺ ions as a catalyst in DUFC will be presented. References Suranovic, Steven. "Fossil fuel addiction and the implications for climate change policy." Global Environmental Change 23.3 (2013): 598-608. Veziroğlu, T. Nejat, and Sümer Şahi. "21st Century’s energy: Hydrogen energy system." Energy conversion and management 49.7 (2008): 1820-1831. Zhao, Xuebing, and J. Y. Zhu. "Efficient conversion of lignin to electricity using a novel direct biomass fuel cell mediated by polyoxometalates at low temperatures." ChemSusChem 9.2 (2016): 197-207. Xu, Wei, et al. "Nickel-cobalt bimetallic anode catalysts for direct urea fuel cell." Scientific reports 4 (2014): 5863. Lan, Rong, Shanwen Tao, and John TS Irvine. "A direct urea fuel cell–power from fertiliser and waste." Energy & Environmental Science 3.4 (2010): 438-441. Ye, Ke, et al. "Facile preparation of three-dimensional Ni (OH) 2/Ni foam anode with low cost and its application in a direct urea fuel cell." New Journal of Chemistry 40.10 (2016): 8673-8680. Guo, Fen, et al. "Preparation of nickel-cobalt nanowire arrays anode electro-catalyst and its application in direct urea/hydrogen peroxide fuel cell." Electrochimica Acta 199 (2016): 290-296. Vedharathinam, Vedasri, and Gerardine G. Botte. "Direct evidence of the mechanism for the electro-oxidation of urea on Ni (OH) 2 catalyst in alkaline medium." Electrochimica Acta 108 (2013): 660-665. Acknowledgement This work was supported by Agency for Defense Development (ADD) as global cooperative research for high performance and light weight bio-urine based fuel cell (UD160050BD) and the Ocean University of China-Auburn University (OUC-AU) Grants program.

With the development of technology in healthcare systems, interest in a wearable, portable, and even implantable device is growing, allowing health indicators to be read by direct skin contact [1], [2]. The non-invasive sensing platform overcomes the inconvenience of the conventional monitoring system and enabling easy and quick medical diagnosis in real time [3]. Urea is a major metabolite of the human body in a form of nitrogen, a key biomarker above the renal failure, dehydration, and hepatic issues [4], [5]. Urea detection is conceivable from serum, urine, and sweat in the body. The concentration of urea in the sweat is higher than that of serum so sweat based urea sensor can substitute the current pricking test method [6]. Higher levels of urea in sweat are also essential notices because they are correlated with uremia [7]. To sense the non-electric active of urea, the present method uses an enzyme in electrochemical sensors. The enzyme-based sensor consists of an enzyme (urease) and a mediator, which distinguishes the urease-catalyzed hydrolysis products [8]. However, the enzymatic sensor is complex, unstable, and costly, so non-enzyme sensors are better suited for wearable sensor devices. Nickel-based catalysts with nanostructures, large surface area, good electrical conductivity, and low cost are the best nominees for urea detection as they oxidize urea without enzymes [9]. Recently, a polymer-type substrate has been utilized for the flexible and wearable sensor device which requires porous and conductive polymers. Herein, we investigated a facile approach to form nickel oxyhydroxide with metal (metal-NiOOH) based flexible electrochemical sensor by electrospinning method. The morphology and the structure of the metal-NiOOH catalysts were observed using transmission electron microscopy, scanning electron microscopy, and X-ray diffraction. The metal-NiOOH catalysts with polymer showed excellent flexibility and competent linear sensitivity to urea electrooxidation in the physiological concentration area. A comprehensive investigation of the performances regarding metal-NiOOH as a catalyst in a sweat-based sensor will be presented. References Feiner, Ron, et al. "Engineered hybrid cardiac patches with multifunctional electronics for online monitoring and regulation of tissue function." Nature materials 15.6 (2016): 679. Liu, Yuhao, Matt Pharr, and Giovanni Antonio Salvatore. "Lab-on-skin: a review of flexible and stretchable electronics for wearable health monitoring." ACS nano 11.10 (2017): 9614-9635. Pickup, John C., et al. "In vivo glucose monitoring: the clinical reality and the promise." Biosensors and Bioelectronics 20.10 (2005): 1897-1902. D'Apolito, Maria, et al. "Urea-induced ROS cause endothelial dysfunction in chronic renal failure." Atherosclerosis 239.2 (2015): 393-400. Thudichum, J. L. W. "On the analysis of urea in urine for clinical purposes." British medical journal 1.38 (1857): 788 Stefaniak, Aleksandr B., and Christopher J. Harvey. "Dissolution of materials in artificial skin surface film liquids." Toxicology in Vitro 20.8 (2006): 1265-1283. Kazory, Amir. "Emergence of blood urea nitrogen as a biomarker of neurohormonal activation in heart failure." The American journal of cardiology 106.5 (2010): 694-700. Liu, Yan-Ling, et al. "Flexible electrochemical urea sensor based on surface molecularly imprinted nanotubes for detection of human sweat." Analytical chemistry 90.21 (2018): 13081-13087. Mahshid, Sahar Sadat, et al. "Template-based electrodeposition of Pt/Ni nanowires and its catalytic activity towards glucose oxidation." Electrochimica Acta 58 (2011): 551-555. Acknowledgement This work was supported by Agency for Defense Development (ADD) as global cooperative research for high performance and light weight bio-urine based fuel cell (UD160050BD) and the Ocean University of China-Auburn University (OUC-AU) Grants program.

Recently emerging materials for advancing gas sensor technology have potentialized versatile applications of a sensor in the society of safety, health, environmental protection and much more [1]. In accordance with significant progress of chemical and physical approaches, gas sensing materials are not limited to metal oxide but include various 2D nanostructures such as graphene and metal dichalcogenides, etc. [2, 3]. Of these, the rise of interest on newly discovered 2D transition metal carbides and/or carbonitrides (called MXenes) have recently shown their potential as sensing materials with their intriguing surface chemistry [4]. The first MXene material introducing in chemiresistive sensor is titanium carbide (Ti₃C₂T_x). Gas sensing capabilities of Ti₃C₂T_x MXene to detect various gases were demonstrated with their sensing mechanism [5], and now, further enhancement of its sensing performance is actively studied in many academic domains. Beginning with Ti₃C₂T_x, vast diversity in a combination of constituent elements and ordered structure in MXenes offers future possibilities to find out a new generation of sensing materials with their superior performance. In this study, 2D V₂CT_x gas sensors were firstly demonstrated in chemiresistive sensing. The sensor device was fabricated with single/few-layer 2D V₂CT_x on polyimide film after the selective etching process. This device measured both polar and non-polar chemical species such as hydrogen and methane at room temperature (23 °C) with an ultra-low limit of detection of 2 ppm and 25 ppm, respectively. 2D V₂CT_x gas sensors showed excellent gas sensing performance in terms of high response toward non-polar gases, which is originated from the surface oxygen functional groups on the surface of V₂CT_x nanoflakes. Compared to the sensing properties of Ti₃C₂T_x MXene [5], 2D V₂CT_x sensor showed higher selectivity and long-term stability. This comparative result suggests that the modification of ordered structure and constituent elements of MXenes play a pivot role in the interaction between analyte and MXenes leading to an extensive change in performance. References [1] Barzegar, Maryam, and Bharati Tudu. "Two-dimensional materials for gas sensors: from first discovery to future possibilities." Surface Innovations 6.4–5 (2018): 205-230. [2] Yang, Shengxue, Chengbao Jiang, and Su-huai Wei. "Gas sensing in 2D materials." Applied Physics Reviews 4.2 (2017): 021304. [3] Lee, Eunji, et al. "Enhanced Gas-Sensing Performance of GO/TiO2 Composite by Photocatalysis." Sensors 18.10 (2018): 3334. [4] Xiao, Bo, et al. "MXenes: Reusable materials for NH3 sensor or capturer by controlling the charge injection." Sensors and Actuators B: Chemical 235 (2016): 103-109. [5] Lee, Eunji, et al. "Room temperature gas sensing of two-dimensional titanium carbide (MXene)." ACS applied materials & interfaces 9.42 (2017): 37184-37190. Acknowledgement This research was partially supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP), grant funded by the Korea Government Ministry of Trade, Industry and Energy (20158520000210), and Agency for Defense Development (ADD) as global cooperative research for high performance and light weight bio-urine based fuel cell (UD160050BD).

Urea in the urine, protein metabolism, is present in the final product of human. The level of urea is a crucial biomarker that can assess the metabolic activity of humans such as the liver and renal function. [1] A high level of urea in the urine can result in kidney failure, dehydration, and gastrointestinal bleeding, while a low level can lead to nephritic syndrome, hepatic failure, etc. [2] Therefore, accurate and rapid sensing of urea is essential in a diagnostic clinic. Recently, paper-based analytical devices (PAD) are gaining its popularity as a manageable device for point-of-care equipment due to its inexpensive, portable, disposable, and environmentally-friendly qualities.[3] The majority of electrochemical biosensors rely on enzymes, such as urease; yet, utilization of enzyme is limited due to its high cost, poor reproducibility, and most of all, thermal and chemical instability which can influence the duration of sensor that depends on storage conditions such as temperature, pH, and humidity.[4] In addition, non-enzymatic biosensors has been attracted with the advantage of simplicity, redox flexibility, stability, and low cost. As a non-enzymatic metal-based material, Ni-based nanomaterials exhibited excellent catalytic activity over urea originated from catalytic effect for the formation of the redox couple of Ni(II) and Ni(III) on the surface. [5] When compared to enzymatic-based sensors, non-enzymatic catalysts tend to present lower sensitivity and higher limit of detection corresponding to lower performances of urea biosensor. To address this limit, there have been efforts to modify nanostructures with various shapes such as nanofiber, nanoparticle, and nanoflake arrays. Here we investigated a hollow sphere structure with nanosheet building block by using a hydrothermal method. The hollow sphere structure can increase the selectivity and sensitivity for urea biosensor by providing facile transport channels through electrolyte that exploits its inner and outer surfaces as the active site with maintaining the structural stability of the catalyst. Integration of non-enzymatic hierarchical catalyst on disposable PAD will lead to a high potential for the development of point-of-care testing devices. In this study, a non-enzymatic electrochemical paper-based sensor using hollow structure NiO was developed and demonstrated for the determination of urea. PADs were fabricated by sputtering on cotton paper, and hollow NiO was synthesized by hydrothermal method. Structure and morphologies of hierarchical NiO on PAD were characterized by XRD and SEM. Electrochemical properties such as selectivity and LOD were conducted by using a potentiostat with urea solution. A detailed description of the fabrication process and discussion of structure formation and sensing performances of hollow nickel oxide catalyst will be presented. References [1] Boggs, Bryan K., Rebecca L. King, and Gerardine G. Botte. "Urea electrolysis: direct hydrogen production from urine." Chemical Communications 32 (2009): 4859-4861. [2] Srivastava, Saurabh, et al. "A self assembled monolayer based microfluidic sensor for urea detection." Nanoscale 3.7 (2011): 2971-2977. [3] Martinez, Andres W., et al. "Patterned paper as a platform for inexpensive, low‐volume, portable bioassays." Angewandte Chemie International Edition 46.8 (2007): 1318-1320. [4] Riklin, Azalla, et al. "Improving enzyme–electrode contacts by redox modification of cofactors." Nature 376.6542 (1995): 672. [5] Nie, Huagui, et al. "Nonenzymatic electrochemical detection of glucose using well-distributed nickel nanoparticles on straight multi-walled carbon nanotubes." Biosensors and Bioelectronics 30.1 (2011): 28-34. Acknowledgement This work was partially supported by Agency for Defense Development (ADD) as global cooperative research for high performance and light weight bio-urine based fuel cell (UD160050BD), and the Ocean University of China-Auburn University (OUC-AU) Grants program.

The sensitive detection of explosive and flammable gases is an extremely important safety consideration in today’s industry. However, identification of trace amounts of non-polar analytes at ambient temperatures is still a challenge due to their weak adsorption, and very few studies have been able to achieve it via chemiresistive mechanism. Herein, we demonstrate the high performance of 2D vanadium carbide MXene (V2CTx) gas sensors with ultrahigh sensitivity toward non-polar gases. The fabricated 2D V2CTx sensor devices consisting of single/few-layer 2D V2CTx on polyimide film were able to detect both polar and non-polar chemical species including hydrogen and methane with a very low limit of detection of 2 ppm and 25 ppm, respectively, at room temperature (23 °C). The performance of the fabricated V2CTx gas sensors in detection of non-polar gases surpasses that of previously reported state-of-the-art gas sensors based on other 2D materials.

By virtue of the technology in ubiquitous computing and smart textile, gas sensor with wearability has been developed to play an imperative role in improving future healthcare. [1] The sensor revolution can significantly impact on outpatient care and management of chronical diseases by offering real-time condition monitoring. The combination of many technology is on the verge of producing this dramatic advance, and the significance placed on the development of sensor performance. [2] Recently, two-dimensional (2D) nanostructured materials have been regarded as instrumental sensing materials due to their working ability at low temperature. One attractive approach to enhance performance of 2D materials-based sensor is to hybridize with metal oxide. By grafting two nanostructured materials, sensing performance can be highly advanced by hybridization effects. [3] Another innovative strategy is to adapt newly discovered 2D materials. 2D transition metal carbides and/or carbonitrides (called MXenes) have recently shown excellent properties particularly in energy storage, and water purification applications. [4] Boundless combination of constituent elements and ordered structure in MXenes provides many opportunities to tailor its properties for different applications. In this study, 2D nanostructured nanomaterials and their hybrids were explored to develop wearable chemical sensor for precise health monitoring systems. Graphene oxide (GO) and molybdenum disulfide (MoS₂) were studied to understand the effect of surface dangling bonds on gas sensing properties. To enhance sensing performance, 2D nanomaterials was combined with metal oxides. The gas sensing performance of the GO/TiO₂ hybrid was improved by decorating titanium dioxide (TiO₂). [5] After photo-reduction, the gas sensing behavior was converted from n-type to p-type with extended long-term stability. Moreover, 2D MXene was introduced as a promising room-temperature sensing material with their intriguing surface chemistry. The capability of titanium carbide (Ti₃C₂T_x) to sense an array of VOC gases was demonstrated, and its possible sensing mechanism was proposed in terms of the interaction between sensing species and the oxygen terminated surface of MXene. Another MXene material, vanadium carbide (V₂CT_x), was also investigated. 2D V₂CT_x gas sensors showed outstanding gas sensing performance including high sensitivity toward non-polar gases such as hydrogen and methane. Transformation of ordered structure and constituent elements of MXenes largely influenced on interaction between analyte and MXenes showing outstanding selectivity and limit of detection to non-polar gases. References Gravina, Raffaele, et al. "Multi-sensor fusion in body sensor networks: State-of-the-art and research challenges." Information Fusion 35 (2017): 68-80. Yao, Shanshan, Puchakayala Swetha, and Yong Zhu. "Nanomaterial‐Enabled Wearable Sensors for Healthcare." Advanced healthcare materials 7.1 (2018): 1700889. Chatterjee, Shyamasree Gupta, et al. "Graphene–metal oxide nanohybrids for toxic gas sensor: a review." Sensors and Actuators B: Chemical 221 (2015): 1170-1181. Anasori, Babak, Maria R. Lukatskaya, and Yury Gogotsi. "2D metal carbides and nitrides (MXenes) for energy storage." Nature Reviews Materials 2 (2017): 16098. Lee, Eunji, et al. "Enhanced Gas-Sensing Performance of GO/TiO2 Composite by Photocatalysis." Sensors 18.10 (2018): 3334. Acknowledgement This research was partially supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP), grant funded by the Korea Government Ministry of Trade, Industry and Energy (20158520000210), and Agency for Defense Development (ADD) as global cooperative research for high performance and light weight bio-urine based fuel cell (UD160050BD).