Two-Dimensional Nanomaterials for Wearable Breath Sensors
Authors: Kim Dong-Joo , Lee Eunji , Lee Doohee , Yoon Jaesik , Beidaghi Majid 
Published: May 2020

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 Ti3C2Tx and V2CTx. 2D V2CTx gas sensors showed excellent gas-sensing performance in terms of high response toward non-polar gases. Compared to the sensing properties of Ti3C2Tx MXene, 2D V2CTx 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.