Sensors on paper have been proposed and fabricated to identify gas or vapors (chemicals). Traditional sensors are based on hard substrates such as silicon. Sensors fabricated on paper are cheaper, foldable, flexible, and bio - degradable. Paper electronics is an emerging area. Logic devices, memory, RFID (radio-frequency identification) tags, etc. have been demonstrated. Sensors on paper will be another building block to achieve complete, true paper electronics.
This innovation deposits single-walled carbon nanotubes (SWCNTs) onto a paper substrate. The measured variable is resistance, as this is the easiest to measure, and the associated electronics and hardware are simple, straightforward, and can be acquired off the shelf.
The gas sensors are classified according to transduction method into capacitor, transistor, resistor, microbalance, and fiber optic. Each class has its own strengths and weaknesses, but the resistive type sensors are characterized by their simple structure, low fabrication cost, and simple readout circuitry. Here, a resistor-type sensor was fabricated with a network of cross-linked SWCNTs with purity over 99%. An ordinary cellulose paper used for filtration was employed as the substrate. Other types of papers can be used as well. The roughness and porosity of the paper tend to hamper the performance of some electronic devices such as transistors and diodes; on the contrary, the roughness and porosity are attractive here because they increase the contact area with the ambient air and promote the adhesion to CNTs. No heat treatment or vacuum environment were involved in the course of device processing.
The SWCNTs were functionalized with carboxylic acid (COOH) to render them hydrophilic, thus increasing the adhesion with the substrate. The functionalized SWCNTs were dispersed in dimethylformamide solution. The film, composed of networks of cross-linked CNTs, was formed using drop-cast coating followed by evaporation of the solvent. The 10-cm filter paper was conformally coated with COOH-functionalized SWCNTs, and the flexible paper can be custom cut to any size. Imaging results show that the nanoscale CNTs are firmly entangled with the microscale cellulose fibers. Adhesive copper foil tape was used for contact electrodes, and the distance between the two electrodes was about 2 mm. This distance can be adjusted to any desirable length. The sensor has been demonstrated for the detection of ammonia down to 10 ppm. In principle, the sensor can be used to detect a variety of gases and vapors in a range of applications in environmental monitoring, biomedical, and other areas.