The reference electrode was constructed using a mask and silver ink.
For this, a mask was initially designed using CorelDraw and printed on a transparency sheet using a conventional
HP 810C printer.
The mask was cut and positioned at region (iii) of the paper based device and then the silver ink was spread over the surface of the mask.
The mask was removed and the device was heated at 70 ◦C for 30 min. After the construction of the reference electrode,
solid wax was spread over region (ii) of the PAD, to control the time of the solution in contact to the enzyme when necessary,adapting the recently reported procedure [33]. However, the integration of the system with the configuration showed in Fig. S1(c)