# Inkjet-Printed and Paper-Based Electrochemical Sensors
## 4 Electrochemical Detection and Its Applications  
Among all of the paper-based biological and chemical sensors, various sensing mechanisms have been used, each with their own respective advantages and disadvantages. Although the focus of this review article is electrochemical sensing, it is important to briefly discuss other sensing mechanisms as a comparison. Each of the sensing mechanisms typically falls under two categories, namely optical detection and electrical detection. Optical detection methods include colorimetric detection, fluorescence, and chemiluminescence. Electrical detection methods include chemiresistive sensing and electrochemical detection. As an example of optical detection, one of the simplest and most popular detection methods is colorimetric detection [8; 11]. In colorimetric detection, certain chemical or enzymatic reactions  
Figure 8: Demonstration of surface tension and how it can be affected with an ionic surfactant: (**a**) High surface tension ink with a large contact angle; (**b**) Low surface tension ink with a small contact angle. Reproduced from [44]. are utilized to produce a color change based on the presence of the target analyte. If the analyte concentration is higher, the color change will be more noticeable. This method has been successfully used for the detection of proteins [45], enzymes [46], and metals [11; 47]. The main drawback of the colorimetric analysis method is that the measurement results are usually qualitative or semi-quantitative, and the presence of relatively complicated optical detectors and image processing units is required. Fluorescence detection is another method that has been widely used in uPADs to detect various analytes [48; 49]. The fluorescence method requires external instrumentation such as excitation light and optical detectors. Furthermore, the primary disadvantage of this method is the presence of a high background signal which originates from the chemicals used in the paper-bleaching process [4]. Yet another method demonstrated in paper-based analytical devices is chemiluminescence detection [31]. In this approach, the light intensity emitted from a chemical reaction is used to determine the target analyte concentration. Although this particular optical technique can provide simple and fast measurements, it often suffers from notable drawbacks including decreased accuracy due to ambient light noise and inconsistency in the optical detection system. Additionally, dust or insoluble compounds in the sample can easily affect the results. As an example of electrical detection, chemiresistive sensing has also been used for paper-based analytical devices. This particular technique relies on measureable changes in the conductivity of a sensing material due to the presence of the target analyte. After calibration, the target analyte concentration can be quickly calculated by measuring the change in resistance of the sensing material. This method of detection has been demonstrated for gas detection with functionalized carbon-based electrodes [32; 33; 34]. However, because the conductivity of the sensing material can be affected by a variety of chemicals, the sensor selectivity can be negatively affected. Although each of the above sensing mechanisms have been successfully used by many groups, electrochemical analysis remains one of the most common sensing mechanisms due to its simplicity, quantitative results, high accuracy, high selectivity, and low cost [3; 5; 12; 17; 18; 19; 20; 17; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30]. To be clear, chemiresistive sensing and electrochemical detection are both forms of electrical detection that measure current, but they differ in their sensing mechanism. Chemiresistive sensing relies on material conductivity changes that occur due to chemical adsorption or desorption. Electrochemical detection, on the other hand, relies on electron transfer between the electrode materials and the solution.