Graphene has been widely employed for the fabrication of various sensors and other new devices because of their distinct properties and high-performance range. In particular, functionalized graphene appears to be exceptionally promising for chemical and biological sensor applications.
Humidity sensors, which are extensively used in a wide variety of applications, often encounter numerous problems, such as narrow humidity detection ranges, long response and recovery times and low sensitivity. Recently, scientists from the Nokia Research Center in Cambridge reported an ultra-fast graphene oxide-based humidity sensor with unprecedented response speed thanks to the two-dimensional (2D) structure of graphene oxide and its super permeability to water molecules.
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Graphene-Based Humidity Sensor
The Nokia researchers demonstrated that the response and recovery time of their ultra-thin sensors, defined as the time required to increase the high humidity value from 10% to 90% and vice versa, is less than 100 milliseconds (ms). These sensors were found to respond at a frequency of up to 50 Hz. Furthermore, the response rate of the sensors was found to be dependent on the thickness of graphene oxide films, as demonstrated by an increase in film thickness that resulted in a reduction in the speed of the sensor.
The ultra-fast graphene oxide film developed by the Nokia team is mechanically flexible, optically transparent and, at a width of approximately 15 nanometers (nm), extremely thin as well. Researchers believe that the unique properties achieved by these films will enable the development of humidity sensors in different forms, such as band aid-type skin stickers or small wearable devices.
In their film’s design, the Nokia researchers were able to observe the modulated humid air flow in a user's breath. The research performed on this ultra-fast humidity sensor also suggests the potential application of this concept to other 2D materials, such as 2D transition metal dichalcogenides and functionalized graphene, as these types of materials have been associated with the ability to interact with gases and vapors for novel ultra-thin nanoporous sensing films. The fast response times of these humidity sensors makes it possible to evaluate moisture level while an individual is both breathing and speaking.
Wireless Monitoring for Non-Invasive Disease Diagnosis
Several wireless monitoring devices have emerged following the original research published by the Nokia team. For example, a 2018 Biosensors and Bioelectronics study reported the development of a flexible, lightweight and highly conductive porous graphene film that was incorporated into a humidity sensor for respiration monitoring. Modifications to this porous graphene network included graphene oxide and silver (Ag) colloids (AC). The researchers here found that their sensors efficiently monitored the breathing patterns for both mouth and nose respiration at normal and deeper breathing levels. Furthermore, the sensor was capable of measuring physiological activities such as water intake, skin moisture, as well as speaking and whistle rhythm.
Another 2017 study found that a wearable radiofrequency identification (RFID) device, which was equipped with a graphene oxide sensor. This facemask device was found to be capable of detecting inhalation and exhalation cycles, as well as abnormal respiration patterns by measuring changes in the resistance of the graphene oxide film.
Sensor devices with exceptional performance pave the way for new applications. This research work presents graphene oxide-based humidity and temperature sensors that exhibit unprecedented response speed besides a number of other convenient features. They are also suitable for large-scale manufacturing and Nokia has filed several patents regarding this research. Further research will focus on assessing the stability of graphene oxide films over time.
Sources and Further Reading
- “Nokia uses Graphene to develop world’s fastest humidity sensor” – Nokia Power User
- Borini, S., White, R., Wei, D., Astley, M., Haque, S., Spigone, E., et al. (2013). Ultrafast graphene oxide humidity sensors. ACS Nano 7(12); 11166-11173. DOI: 10.1021/nn404889b.
- "Nokia develops the world's fastest humidity sensor based on graphene oxide" – Graphene-Info.com
- Pang, Y., Jian, J., Tu, T., Yang, Z., Ling, J., Li, Y., Wang, X., et al. (2018). Wearable humidity sensor based on porous graphene network for respiration monitoring. Biosensors and Bioelectronics 116; 123-129. DOI: 10.1016/j.bios.2018.05.038.
- Caccami, M. C., Mulla, M. Y., Di Natale, C., & Marrocco, G. (2017). Wireless monitoring of breath by measuring a graphene oxide-based radiofrequency identification wearable sensor. IEEE Xplore. DOI: 10.23919/EuCAP.2017.7928355.
This article was updated on the 11th April, 2019.