# Development of Screen-Printable Nafion Dispersion for Electrochemical Sensor
## 1 Introduction  
Nafion(tm) material is a perfluorosulfonic acid ion-exchange polymer that was invented in 1955 by DuPont. The polymer is produced by incorporating perfluorovinyl ether groups terminated with hydrophilic sulfonate cation-exchange sites onto a hydrophobic polytetrafluoroethylene (PTFE) backbone. The combination of sulfonate groups and a stable PTFE backbone makes Nafion a well-known ionic polymer with advantages of excellent ionic conductivity, cation selectivity, chemical inertness, and thermal stability [1]. The unique properties of Nafion have drawn great interest in a broad range of research areas and applications, including fuel cells and batteries [2; 3; 4; 5], chemical sensors [6; 7; 8; 9; 10; 11; 12; 13; 14; 15], biosensors [16; 17], and separation technologies [18]. In recent years, Nafion has been extensively investigated as a membrane or electrode modifier of electrochemical sensors for environmental and biomedical applications [6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17]. Electrochemical sensors monitor the changes in potential or current caused by electrochemical reactions on an electrode's surface. This type of technique offers portability, good sensitivity, and real-time analysis with a simple and low-cost setup. For detection of cationic species, such as heavy metal ions, Nafion membranes have been used to pre-concentrate target analytes and reduce interferences from anionic species [7; 8; 9; 10]. The membrane has good adhesion to most electrode surfaces and can protect the surfaces from fouling and deterioration [8]. [PERSON] et al. utilized a Nafion-modified bismuth film sensor and anodicstripping voltammetry (ASV) for determination of Pb(II) and Cd(II) in lake water with the presence of surfactants [9]. Nafion solution was drop casted onto the working electrode to form a 0.4-m thick membrane. The limit of detection was around 0.5 \(\upmu\)g/L. It was also found that the thickness of the Nafion membrane played an important role in the sensor's sensitivity and resistance toward surfactants. [PERSON] et al. developed a crown ether/Nafion and bismuth film-modified, screen-printed carbon electrode (SPCE) for detection of Pb(II) and Cd(II) in rice samples by sequential injection and ASV [7]. Nafion solution (0.5 wt%) as an electrode modifier was mixed with carbon ink and screen printed onto the conducting layer. [PERSON] et el. developed a screen-printed graphene electrode modified by bismuth nanoparticles and Nafion for in situ environmental monitoring of Pb(II) and Cd(II) [10]. The detection limit was reported to be 280 ng/L and 40.34 \(\upmu\)g/L for Pb(II) and Cd(II), respectively. Nafion solution was diluted to 1% (\(v/v\)) by ethanol and mixed with bismuth nanoparticles. The mixed dispersion was then drop casted onto the working electrode. Interestingly, in most of these efforts, the Nafion membrane was deposited onto the electrodes by drop casting, which is not very suitable for product development and up-scaling. Another important application of Nafion is as a solid polymer electrolyte for polymer electrolyte membrane fuel cells (PEMPC) [5] and vanadium redox flow batteries [3]. Nafion-based nanocomposites demonstrated good performance in terms of proton conductivity, MeOH permeability, and mechanical strength. However, membrane fabrication is still limited to solvent casting or drop casting. The microstructure of the membrane prepared by solvent casting is highly dependent on the solvent and annealing temperature [3], which could limit the selection of high-boiling point solvents and process temperatures. Additionally, controlling film thickness and reproducibility could be challenging for drop casting. There are several reasons that a Nafion dispersion is not suitable for large-scale fabrication techniques, such as screen printing. A commercial Nafion polymer is typically dispersed in a mixture of water and low boiling point solvent, such as 1-propanol, ethanol, or mixed ethers.