Galvanic Dissolved Oxygen Sensor Based from Inkjet Printed Silver Cathode and Electrodeposited Zinc Anode
Authors: Go Lance Paronda , Enriquez Erwin Paredes 
Published: February 2020

Inkjet printing has been gaining widespread application in printed electronics research. However, there are certain metals that cannot be printed easily without being oxidized in ambient air during post-print processing. One such example that has been thoroughly researched is sintering of printed copper nanoparticles or copper salts, which gets oxidized when exposed to sintering temperature in the presence of oxygen in air. There have been successful examples of workaround for printing copper inks. However, metals that are more reactive than copper, in terms of the activity series, is going to be a challenge to print and anneal. In such a case, we explore the use of electrodeposition, another well-established additive manufacturing technology.
This study investigates the use of electrodeposition of zinc metal on inkjet-printed silver seed layer for use in printed electronic applications where the chemistry of zinc is desirable, such as in galvanic dissolved-oxygen (DO) sensors. There are a couple of literature that demonstrate inkjet-printed polarographic DO sensors using silver anode and gold cathode. However, commercial polarographic DO sensors do require more frequent maintenance compared to commercial galvanic DO sensors due to tarnishing of the gold cathode, which then requires physical polishing. In the case of galvanic DO sensors, the zinc hydroxide/ zinc oxide that results from oxidation of zinc anode is porous and thus flakes off by itself requiring less maintenance. Another advantage of a galvanic DO sensor is that the dissolved oxygen is directly measured as a potential between the anode and cathode, whereas a polarographic sensor requires an external potential sweep between the anode and cathode to measure the current produced by the sensor. This necessitates a slightly more complex measuring equipment and a longer scan time for polarographic DO sensors.
The sensor was fabricated as such. Silver ink (Silverjet DGP 40LT-15C) electrodes (final silver cathode and anode seed layer), conducting wires, and contact pads were inkjet printed on flexible PEN substrate (150 microns, Dupont Teijin PQA1) followed by annealing at 180oC. The silver seed layer for the anode was then electrodeposited with zinc using zinc acetate dihydrate (Sigma-Aldrich ACS reagent, >99.0%) electroplating solution and zinc plate anode (acquired from Panasonic size D Zinc-Carbon battery). XRD pattern of printed silver coincides with diffraction peaks of face-centered cubic silver (JCPDS PDF # 01-087-0720), while XRD pattern of electrodeposited zinc matches with diffraction peaks of hexagonal zinc (JCPDS PDF # 00-004-0831). Preliminary devices have shown issues such as delamination of silver seed layer during zinc plating, or curling of electroplated zinc layer during galvanic operation. This was addressed by anchoring the silver seed layer to the PEN substrate by inkjet printing PMMA layer (Sigma-Aldrich 15,000 Mw PMMA; dissolved in NMP solvent at 15% wt/wt) prior to electrodeposition of zinc. In addition, PMMA ink was printed over the silver conducting wires to act as an insulator. Immersing the electrodes (both 2.0mm x 1.5mm in dimension) in 0.5M KOH electrolyte solution results in a galvanic micro-battery which yields an open circuit voltage of 1.43 V and short circuit current of 1.5 µA.
Lastly, we show a proof-of-concept galvanic dissolved oxygen probe using the electrodeposited zinc anode, inkjet printed silver cathode, 0.5 M KOH as the electrolyte, PTFE membrane (Hannah DO screw cap membrane; product # HI76407A/P) as the gas permeable membrane, and bonded acrylic sheets as water tight housing. This device has an output voltage of 62.5 mV when calibrated in saturated 100% humidified air and a voltage of 0.3 mV when calibrated in 0% calibration solution (Atlas Scientific DO test solution; part # CHEM-DO). This is comparable to the working potential of a commercially available galvanic DO probe (Atlas Scientific Dissolved Oxygen probe, part # ENV-40-DO), which puts out 61.8 mV when saturated with air and 0.9 mV in 0% calibration solution.

Figure 1