A group of researchers from the University of Washington have successfully integrated carbon nanotubes (CNTs) into paper material, thereby leading to a highly sensitive and conductive material that is capable of sensing the presence of water.
By eliminating any possible degradation to the sensing material, these smart papers are an economically advantageous option that has a wide range of applications for both portable electronics and sensing devices.
Combining Paper and Electronics
The recent large-scale production of lightweight and flexible materials into most portable electronic devices has inspired researchers to look at paper, a renewable, biodegradable and biocompatible alternative to these materials that is also a much cheaper option than most petrochemical substrates. Previous attempts to incorporate electrically conductive materials into cellulose, the primary fibrous material of paper, have been successful in sensing external stimuli including temperature, mechanical deformations and the presence of various liquids or gases.
Specifically, the combination of carbon nanotubes (CNTs) into paper has been achieved through two different mechanisms in an effort to utilize the exceptional electrical conductivity of this material. One challenge that has been associated with the combination of CNTs and paper is that the carbon of CNTs is devoid of hydroxyl groups and prevents interaction between cellulose fibers, which can therefore greatly limit the mechanical integrity of the paper material.
Preparation of the CNT Paper
CNTs are typically hydrophobic materials that readily interrupt the aqueous-phase required for most paper-making processes. To reduce any adverse interactions, alkali lignin (AL), a byproduct of wood pulp found in the paper industry, was used as a surfactant to ensure the stable and concentrated suspensions of CNTs within the aqueous system. A cationic polymer was then used to fix the CNTs to the pulp. The combined CNT and cellulose material was then pressed and dried into handsheets for morphometric analysis and sensing performance.
The tensile strength of the CNT-cellulose material is attributed to the incorporation of CNTs into the microfiber network of the pulp derivative, which was found to significantly reduce the breaking length of the paper as the concentration of CNTs increased. To evaluate the capability of this material to accurately detect liquid water, the researchers applied a Keithly 2400 voltage-current meter to evaluate the electrical resistance of the material both when exposed to aqueous and aerosol conditions.
The strength retention of the CNT-cellulose material was found to increase as the CNT content increased, with hydroxyl-functionalized CNTs showing greater retention rates as compared to non-functionalized groups. In fact the researchers found the hydroxyl-functionalized CNTs exhibited a wet strength that was more than half that of its dry strength.
The exceptional water detection by the CNT-cellulose material was attributed to the availability of hydroxyl groups on the material to interact with the water materials. Within the amorphous region of the material where a greater amount of free hydroxyl groups are present, the water molecules are able to readily enter or leave, thereby causing changes in the electrical resistance of the material.
Future Applications of this Smart Paper
The lack of adequate fluid leak detection systems in various industries such as water, gas or chemical suppliers can cause devastating economic losses to the company, as well as deleterious health effects to the surrounding environment in the event that these chemicals are released. Due to the polar nature of water, current detection technologies are very limited in adequately detecting the presence of these leaks until their damage has already occurred. The high sensitivity of the CNT-cellulose composite papers that were developed in this study therefore prove to be ideal for water leakage detection equipment that function at remote locations to quickly relay information to workers of the industry.
- “Smart papers comprising carbon nanotubes and cellulose microfibers for multifunctional sensing applications” A. Dichiara, A. Song, et al. Royal Society of Chemistry. (2017). DOI: 10.1039/C7TA04329E.