# Inkjet-Printed and Paper-Based Electrochemical Sensors
## 4 Electrochemical Detection and Its Applications
Various electrochemical methods have been utilized for developing paper-based sensors including potentiometry, amperometry, and voltammetry. Generally, potentiometry involves the measurement of voltage between the working and reference electrodes. [PERSON] et al. have reported on a paper-based potentiometric ion sensor. The sensor was fabricated by inkjet-printing gold nanoparticles followed by sintering to improve the conductivity of the electrodes. The working electrode was then modified with a K\({}^{+}\)-selective membrane to form an ion selective electrode [35]. Amperometry is another electrochemical sensing method that has gained interest in paper-based sensors. During amperometry measurements, a fixed potential is applied to the working electrode with respect to the reference electrode, and the current resulting from the surface reaction and diffusion of redox molecules is measured with respect to time. [PERSON] et al. have reported a fully inkjet-printed electrochemical device on paper using carbon nanotube ink using amperometry to determine dopamine levels [15]. Another paper has reported a working electrode modified with silver nanoparticles as an electrocatalyst and demonstrated a non-enzymatic hydrogen peroxide sensor [17]. [PERSON] et al. also used amperometry for hydrogen peroxide detection [36]. Graphene was inkjet-printed to form electrodes, and UV-pulsed laser irradiation was employed to increase the conductivity of the electrodes. Another very common electrochemical sensing technique is voltammetry. This method is an effective electrochemical tool that can provide information about the concentration of an electroactive target analyte. Cyclic voltammetry is widely used to determine analyte concentrations and characterize the kinetic energy of the electron transfer at the surface of the electrode. In this method, the working electrode potential is swept forward and backward, and the current is recorded. Current peaks are observed at the oxidation and reduction potentials. These current peaks are proportional to the concentration of the target analyte. [PERSON] et al. have employed cyclic voltammetry to characterize their paper-based ultrathin gold film electrodes modified with inkjet-printed conductive polymer [37]. In order to achieve even better selectivity and sensitivity for inkjet-printed paper-based electrochemical sensors, it is also possible to functionalize the electrodes using enzymes. For example, [PERSON] et al. modified the electrode surface using glucose oxidase to produce a highly selective glucose biosensor [14]. Alternatively, one group employed electrochemiluminescence as the sensing mechanism to enhance selectivity and sensitivity [50]. This particular method combines electrochemical and chemiluminescence approaches to enhance the sensor's performance. [PERSON] et al. achieved dual signal amplification using a graphene oxide-chitosan/gold nanoparticles (GCA) immunosensing platform in conjugation with a functionalized nanoporous silver signal amplification label. Considering each sensing mechanism, it is quite clear that electrochemical sensing is the most effective method for paper-based sensors. Of course, there are certainly circumstances that require other sensing mechanisms due to specific limitations or constraints, but in the majority of cases, electrochemical sensing provides advantages that far outweigh the disadvantages. Electrochemical detection is very well understood, and there are many resources available for implementing this method properly and interpreting the results correctly. It also provides accurate and selective detection results, enabling the production of high-performance sensors at minimal cost. Table 1 below provides a comprehensive table of paper-based electrochemical sensors for comparison between various approaches. As more and more groups investigate inkjet-printed paper-based sensors, it is likely that the most will focus on electrochemical sensors as the current majority indicates.