A Fully Inkjet-Printed Carbon Nanotube Electrochemical Sensor on Paper

We present a fully inkjet-printed electrochemical sensor on paper, which uses inkjet-printed carbon nanotubes as the working, counter, and reference electrodes. Although paper-based electrochemical sensors have been demonstrated previously, our method of fabrication is both faster and cheaper and fully inkjet-printed. Due to its superior patterning ability, inkjet printing allows for the fabrication of on-demand paper-based electrochemical sensors without the use of templates or post-deposition photolithography. Additionally, in combining inkjet printing with an exciting material like carbon nanotubes rather than common metals (i.e. gold and silver), the fabrication cost is further reduced, and the possibility for mechanically flexible sensors becomes a reality.

The use of paper as a substrate offers many advantages over other possible rigid substrates as well as flexible polymer substrates. Of course, paper is indeed a renewable resource and thus is a very cost-effective substrate for fabrication of disposable sensors. Additionally, paper is naturally flexible, preventing damage to the sensor upon external stress during use. Finally, paper is a great candidate for microfluidic applications due to its superb ability to transport liquids.

The fabrication process is remarkably simple. The electrode design was developed using any drawing software or CAD.  Multi-walled carbon nanotubes (MWCNTs) were dispersed in deionized (DI) water using sodium dodecyl sulfate (SDS), an anionic surfactant.  More specifically, a bottle containing 10 mg/ml of MWCNTs and 8 mg/ml of SDS was sonicated for 30 minutes and centrifuged at 12,000 rpm for 5 minutes. The carbon nanotube ink was then injected into a clean ink cartridge and printed using an office inkjet printer.

In order to ensure proper functionality, a low electrode resistance was necessary. This was achieved by printing the carbon nanotube ink multiple times to reduce the sheet resistance. We were able to achieve less than 2 k-ohm/square with 20 prints on paper. The sensor was tested with various concentrations of FeSO₄ in 0.1 M H₂SO₄. A simple potentiostat circuit was used along with a LabView FieldPoint module for voltage control and current measurement. Potential step voltammetry was employed and the saturation current was used as a means to determine analyte concentration.