# Inkjet-Printed and Paper-Based Electrochemical Sensors
## 2 Fabrication Methods
### Inkjet Printing
[PERSON] and [PERSON] also reported an inkjet-printed paper-based electrochemical sensor fabricated using carbon nanotube ink and Ag nanoparticles as an electrocatalyst for hydrogen peroxide detection [17]. In order to increase the conductivity of the inkjet-printed electrodes, a sintering process was employed. [PERSON] et al. presented two notable contributions to fully inkjet-printed paper-based sensors through the use of an embossed well and chemically modified paper [41]. Unlike many of the other groups utilizing hydrophobic materials, this particular group chemically altered the paper prior to printing. Specifically, through a simple process, they were able to develop what is referred to as omniphobic \"fluoroalkylatd paper\" (\"RF\" paper\"). This treated paper is essentially hydrophobic and oleophobic, which still allows it to absorb fluids but yields a very large contact angle with ink droplets. As shown in Figure 7, this results in a much higher resolution print. In addition to chemically modifying the paper substrate, this group also physically altered the shape of the paper through an embossment process in order to produce a reservoir for the sample. To form the working, counter, and quasi-reference electrodes, they employed a piezoelectric inkjet printer and carbon-based ink. Furthermore, a repeatability demonstration was performed using multiple fabricated sensors yielding very little variation in the measurements. This is actually a very significant conclusion for inkjet-printed paper-based sensors. Due to the nature of the fabrication technique and the substrate material, there are sincere concerns over repeatability across mass-manufactured devices. Although additional demonstrations are required to support these results, the paper treatment and subsequent printing certainly indicate that high-resolution repeatable patterns are possible with paper-based substrates. Another more basic method to achieve high-resolution prints was utilized by [PERSON] et al. [42]. Instead of modifying the paper substrate, this group opted to use a standard home photo printer with a compact disc (CD) slot. Unlike simple home printers for common documents, the Epson R230 inkjet printer used by this group is specifically designed to produce high quality photo prints that maintain a high resolution. The fabrication process included two primary steps. First, gold nanoparticles were printed onto a paper substrate. Next, the substrate was placed into a plating solution for seeded growth. This produced high-resolution electrochemical sensors with good conductivity. In the final fabrication step, a drop of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF\({}_{6}\)) was placed on the sensing area and allowed to dry, resulting in a novel paper-based electrochemical oxygen sensor. Deviating from the norm, [PERSON] et al. recognized advantages of both screen printing and inkjet printing, which led to a device fabricated using both deposition techniques [43]. Screen printing was first used to form the working, counter, and reference electrodes using carbon-based ink. Next,  
Figure 7: Demonstration of change in ink contact angle and print resolution due to chemical modification of the paper substrate by [PERSON] et al. Reproduced with permission from [41], Copyright [PERSON], 2017.  
inkjet printing was used to deposit polyaniline on the working electrode for detection of ascorbic acid. This is an interesting approach that demonstrates the flexibility of inkjet printing. Without any need to fabricate a stencil or template, multiple ink cartridges can be loaded with various functional materials aimed at detecting distinct target analytes. As demonstrated by these examples, inkjet printing is a simple but sophisticated fabrication method that is very well-suited towards paper-based sensors. Thus far, screen printing has arguably been the most popular method of fabrication for paper-based sensors, but inkjet printing is quickly gaining popularity.