# Inkjet-Printed and Paper-Based Electrochemical Sensors
## 2 Fabrication Methods
### Screen Printing  
Screen printing is the first and most frequently used method for fabrication of paper-based electrochemical sensors. This particular fabrication method is popular in the clothing industry where it is used to apply labels and designs to shirts and other articles of clothing. Screen printing is a relatively simple process that utilizes ink, a template or stencil, and a squeegee. The template or the screen is first designed and fabricated. Depending on complexity, the template may require computer-aided design (CAD) software for design and specialized equipment for fabrication. After fabrication, the template is placed over the substrate and long thin bead of ink is deposited along one edge of the template. Finally, the squeegee is used to spread the ink across the entire surface with constant even pressure. After removing the template, the ink is left to dry. Among the many demonstrations of utilizing screen printing to fabricate electrochemical sensors, the [PERSON] group at Colorado State University was the first group to incorporate electrochemical measurements in a paper-based microfluidic device [39]. In order to guide the sample fluid, photolithography was initially used to define channels and detection zones. Following photolithography, screen printing was performed in two separate steps to implement the electrodes and contacts. First, silver/silver chloride (Ag/AgCl) ink was screen-printed to define the reference electrode and contact pads. Next, carbon ink containing Prussian Blue was screen-printed to implement the working electrode and the counter electrode. The basic design and a fabricated electrochemical cell are shown in Figure 1. Using this system, they were able to measure the concentration of glucose, lactate, and uric acid. [PERSON] et al. demonstrated an electrochemical gas sensor using paper as the supporting material for a room temperature ionic liquid (RTIL) [12]. In this work, a three-electrode electrochemical cell was fabricated on filter paper with a thermally-laminated polyethylene (PET) backing used to prevent gas permeation. Here again, wax printing is used to form a hydrophobic barrier to contain the liquid sample. Working, counter, and reference electrodes were then created by screen-printing conductive carbon ink. The carbon ink was doped with cobalt(II) phthalocyanine due to its electrocatalytic properties toward the thiol oxidation. Next, the paper was soaked in butyl-3-methylimidazolium bis(trifluoromenthanesulfonyl)imide as the RTIL. Another example of an ePAD developed by screen printing was demonstrated by [PERSON] et al. [21]. This sensor was used to determine dopamine (DA) levels in biological samples. The microfluidic channels were patterned on paper using photolithography based on SU-8. Carbon-based ink was then used to screen-print electrodes to form a commercial electrochemical sensor for square-wave voltammetry. To improve the performance of the sensor, the filter paper as modified with sodium dodecyl sulfate (SDS) as the ionic surfactant, which facilitated the diffusion of DA between paper layers. Furthermore, an ePAD was fabricated by [PERSON] et al. for determination of cholesterol in human blood serum [22]. The working and counter electrodes were patterned using screen printing and carbon ink, and a Ag/AgCl electrode was used as the reference electrode. The working electrode was modified by a nanocomposite made from graphene, polyvinylpyrrolidone (PVP), and polyaniline (PANi). Also, cholesterol oxidase (chOx) was attached to these nanoparticles (NPs), and the chronoamperometry measurements were performed for determining the concentration of cholesterol. Using a unique form-factor, [PERSON] et al. demonstrated an ePAD for determination of beta-hydroxybutyrate (BHB) levels which is a biomarker for diabetic ketoacidosis [23]. They fabricated a three-dimensional microfluidic device that can alter the fluid direction flow or electrical connection by folding the device. Each of the three electrodes were implemented using screen printing and carbon-based ink.