# Smartphone-Based NFC Potentiostat for Wireless Electrochemical Sensing
## 1 Introduction  
Electrochemical sensing has been developed and is widely applied in various fields including food safety and control [1, 2, 3], environmental monitoring [4, 5, 6] and clinical diagnostics [7, 8, 9], since it offers many advantages such as high sensitivity and selectivity, rapid detection and small sample volume [10, 11, 12]. Typically, a potentiostat is an electronic instrument used to perform electroanalytical measurements by controlling the voltage difference and measuring the current flow through an electrochemical cell [13, 14]. However, the commercially available potentiostats that are suitable for the laboratory are large and expensive, with a price of several thousand dollars [15, 16, 17], which largely limits their application for point-of-care testing and in resource-limited settings. For these reasons, an electrochemical workstation needs to be portable, low-cost and easy to use to meet the demand for on-site measurement. In recent years, there has been growing research in the development of low-cost and portable potentiostats. Some studies have developed inexpensive and open-source potentiostats that complement commercial laboratory potentiostats [18, 19, 20, 21], but the instrument requires a computer to operate the detection process, analyze the data and display the results. These devices limit the application for home-based testing and in resource-limited settings. Moreover, those devices cannot easily transmit the data to the cloud for storageand cannot directly share data with related users such as physicians or healthcare services for clinical applications. With the rapid growth of smartphone users worldwide, which exceeds 3.5 billion in 2020, accounting for 45.4% of the world's population and being projected to grow further in the next few years [22], the development of smartphone coupling with potentiostats is an increasing trend for electrochemical sensing systems. In these systems, smartphones can be connected with potentiostats through Bluetooth, Wi-Fi, USB or an audio port interface for supplying power, data transmission, data processing, result display and sharing [23]. There have been previous studies on the development of potentiostats interfaced with a smartphone for specific applications and techniques. [PERSON] et al. proposed an open-source \"universal wireless electrochemical detector\" (UWED) that interfaces with a smartphone using Bluetooth [24]. In addition, [PERSON] et al. presented xenSTAT, which can connect to the phone via Bluetooth and can perform most electrochemical techniques [25]. In another study, ABE-Stat, developed by [PERSON] et al., can perform electrochemical analyses including cyclic voltammetry (CV), differential pulse voltammetry (DPV), high impedance potentiometric measurements, and can connect wirelessly to a smartphone through Wi-Fi [26]. However, these devices often require wired connection with USB and audio port or need rigid batteries for wireless power through Bluetooth and Wi-Fi, which greatly limits simplification and miniaturization of the device, and also increases the cost and maintenance demands [27; 28]. Near-field communication (NFC) is short-range wireless connectivity that allows the smartphone or other device to communicate with devices containing an NFC tag. NFC is being implemented on most new generation smartphones, and is being rolled out for several applications, including identification and cashless payments [29; 30]. This technology is nowadays a high potential technology for use as a wireless sensor platform [31]. NFC sensors provide inexpensive, portable and real-time monitoring, which has been utilized in the field of food [32; 33], the environment [34; 35] and healthcare [36; 37; 38]. The advantage over other wireless standards such as Wi-Fi, Bluetooth and ZigBee is that NFC can receive and transfer power between devices with a long battery lifetime or battery-free implementation of a sensor [39; 31]. Some studies have demonstrated the integration of NFC technology with an electrochemical sensing platform.