Inkjet-printed bifunctional carbon nanotubes for pH sensing

The monitoring of pH values of liquids for environmental, industrial, and biological applications requires sensitive, stable, compact, and easy-to-use pH sensors. Here, we develop an inkjet printing process to deposit functionalized single-wall carbon nanotubes as both electron conduction and pH sensitive layers. Doping and de-doping of the carbon nanotubes by hydronium and hydroxide ions is the primary pH sensing mechanism. Multi-pass printing is used to deposit closely packed thin films and reduce the electrode resistance for efficient electron conduction and improved pH sensitivity. The printed electrodes on glass substrates show a reproducible pH sensitivity of 48.1mV/pH with an average response time of 7s and a small hysteresis of 4mV. In addition, similar pH sensing behaviors are obtained by carbon nanotubes printed on flexible polymeric substrates. The developed inkjet printing process and pH sensing electrodes provide a cost-effective solution for future electrochemical monitoring systems.