## Materials and Methods
### Design and Development of the Potentiostat  
The registers of the PCB potentiostat, paper-based potentiostat and development board potentiostat front-end were programmed using the Arduino UNO via Inter-Integrated Circuit (I2C) communication. The registers control functions such as input waveform generation and gain for the electrochemical experiment. The Reference Control register (REFCN) allocates the input waveform biasing sign and level, the internal zero and voltage source selection. A 50% biasing point was selected due to the expected reversible cyclic voltammograms (oxidation and redox current peaks have similar absolute heights) [34]. The transimpedance amplifier resistance, \(\mathrm{R_{TIA}}\) (35 k\(\Omega\)), and the load impedance (10 \(\Omega\)) were selected using the Transimpedance Control register (TIACN). The input signal was swept from a negative to a positive voltage by stepping the REFCN register from 0% to 24% of the supply voltage (Vcc) and varying the bias sign, creating a voltage window ranging from \(-\)0.6 V (\(-\)24% Vcc) to 0.6 V (\(+\)24% Vcc). The 2.5 V reference voltage (V\({}_{\mathrm{ref}}\)) created the internal zero around 1.25 V, as well as a voltage range of 1.2 V containing 48 steps, which generates an input voltage step size of 25 mV. Therefore, a 50 \(\mathrm{mVs}^{-1}\) scan rate is achieved by stepping once every 500 ms [14]. The voltage output signal (V\({}_{\mathrm{out}}\)) of the sensing chip is translated to the output current (I\({}_{\mathrm{out}}\)) using Equation (1) [31]. \[\mathrm{I_{out}}=\frac{\left(\mathrm{V_{out}}-\frac{\mathrm{V_{ref}}}{2} \right)}{\mathrm{R_{TIA}}} \tag{1}\]  
The second function of the Arduino was to communicate the resulting data to the PC for data analysis. A freeware program called Gobetwino [33] was utilized to prompt the Arduino to start the experiment, and to save and automatically plot the data in a personalized Microsoft(r) Excel template file. Figure 3 illustrates the development of the paper-based potentiostat from the laboratory-based approach using the Metrohm Autolab to the development board, the PCB potentiostat, and to the paper-based potentiostat. From the Autolab benchtop potentiostat to the development board potentiostat, the size reduction is due to the need for portability. Producing a further size reduction as illustrated in Figure 3, the Arduino Uno can perform similar functions to that of the built-in analog-to-digital converter (ADC) and electrically erasable programmable read-only memory (EEPROM) circuit on the development board, allowing the PCB and paper-based potentiostat to consist only of the LMP91000 chip, voltage reference and relevant passive components. If an external reference voltage provided by the Arduino UNO is used, the voltage reference can also be excluded, which simplifies the circuit further. With every step in the progress towards a printed paper-based potentiostat, both the size and the price are reduced, as depicted in Table 1. Although they are not included in the performance comparison of this work, for the sake of interest Table 1 also includes the price and size of two commercially available handheld potentiostats, the Palmsens 3 [35] and the DY2100 Mini potentiostat [32]. Due to uncertainty in supply-chain budgets, the costs given in Table 1 cannot be directly compared, but they do provide general cost considerations. It should be noted that all potentiostats include the total cost for all components except the PC. Furthermore, the PCB and paper potentiostat data include the work costs of a junior research engineer to manufacture the potentiostats. However, this price would decrease significantly if the paper-based potentiostat were to be mass produced. Between $6.7 to $8.9 would be added to the cost of the development board, PCB potentiostat and paper-based potentiostat if the PC is substituted with a display such as an LCD TFT 1.8-inch screen module and a 9 V battery [36; 37].