An example apparatus is disclosed. The apparatus includes a reference electrode and an ion selective electrode (ISE). The reference electrode includes a reference electrode substrate, a reference electrode conductor formed via a printable composition on the reference electrode substrate, and a reference membrane formed on the carbon nanotube layer. The ISE includes an ISE substrate, a ISE conductor printed on the substrate via a printable composition on the ISE substrate, and an ion-selective membrane printed on the conductor via a printable membrane solution.