# Inkjet-Printed and Paper-Based Electrochemical Sensors
## 2 Fabrication Methods
### Inkjet Printing  
Inkjet printing is a very well-known deposition technique due to its prevalence in home and office environments across the world. As a result of its popularity, inkjet printing is a very low-cost technique lending itself toward fabrication of paper-based electrochemical sensors with distinct advantages. Unlike screen printing, inkjet printing does not require the pre-fabrication of a template or stencil. Instead, the pattern is designed using CAD software and sent directly to the printer which is able to precisely deposit very small ink droplets row by row to form the desired two-dimensional shape. Furthermore, due to the nature of the deposition technique, inkjet printing allows for greater control over the pattern as compared to screen printing through the selection of different printing equipment. For example, in low budget circumstances, regular office printers can be modified to be used with custom ink for paper-based electrochemical sensor fabrication ([PERSON] et al., 2016; [PERSON] et al., 2017). On the other hand, when greater precision and control are required, sophisticated material printers are available that allow for precise adjustment of ink droplet volume, ejection speed, and spacing. Additionally, many of the high-end printers even come with heated beds to allow for better control over ink dry times. As an example,  
Figure 4: The fabricated Au(III) sensor with a three-electrode electrochemical setup and colorimetric detection zone for determination of Fe(III). Reproduced with permission from ([PERSON] et al., 2016), Copyright American Chemical Society, 2017.  
the FUJIFILM Dimitax Materials Printer is a very capable inkjet printer that enables incredible control over the printing process. Given these advantages, inkjet printing has been used by many groups to fabricate paper-based microfluidic analytical devices. One of the first paper-based electrochemical sensors was developed by [PERSON] et al. [14]. A piezoelectric commercial printer (Dimitax Materials Printer) and silver and gold ink were employed to pattern electrodes on a paper substrate. Additionally, a AgCl layer was electrochemically deposited on silver strips to achieve a Ag/AgCl reference electrode. Lastly, a reaction well was formed on the paper platform using PDMS. Various electrode modifications were performed on the sensor in this report including electropolymerization of a polyaniline film on the working electrode surface using cyclic voltammetry in order to develop a potentiometric pH sensor. Furthermore, a glucose biosensor was developed by modifying the electrode surface with glucose oxidase. The schematic diagram of the paper-based sensor developed in this work is illustrated in Figure 5.  
Another well-executed demonstration of a fully inkjet-printed paper-based electrochemical sensing platform was presented by [PERSON] et al. in 2015 [15]. Custom-made carbon ink based on multi-walled carbon nanotubes was achieved by using SDS as an ionic surfactant. Next, the ink was transferred into a commercial cartridge, and the three-electrode electrochemical sensor was printed onto a paper substrate by employing a standard office inkjet printer. The printing process was repeated several times in order to yield electrodes with a lower sheet resistance. Additionally, mineral oil was printed onto the substrate to act as a hydrophobic barrier during sensing. The fabrication process and the schematic diagram of the sensor is depicted in Figure 6.  
Figure 5: The schematic diagram of the inkjet-printed electrochemical sensor on paper developed by [PERSON] et al. Reproduced with permission from [14], Copyright Elsevier, 2017. Figure 6: Illustration of the fully inkjet-printed electrochemical sensor developed by [PERSON] et al. : (**a**) Fabrication process; (**b**) Schematic diagram of the completed sensor. Reproduced from [15].