A transparency film served as the substrate to construct SPGEs. After printing, the carbongraphene ink was dried for 1 h at 60°C. Then, a silver/silver chloride (Ag/AgCl, Sun Chemical company, Milan, Italy) ink was painted on the conductive pads of the RE and dried for 1 h at 60°C. The obtained SPGE electrode was kept in dark and dry conditions when not in use to prevent the oxidization of the Ag/AgCl. The device design and integration of the microfluidic system with a smartphone are shown in Fig. .