*2.3. Device Packaging*

The packaging of the electrochemical devices was performed by connecting three wires with one set of contact pads of the working, counter, and reference electrodes, respectively, using EPO-TEK® H20E conductive epoxy (Epoxy Technology, Billerica, MA, USA). After

mixing the resin part A and the hardener part B of the H20E conductive epoxy in a weight ratio of 1:1, the mixture was applied to each contact pad. Each device was then baked inside an oven at 80 ◦C for 3 h to solidify the conductive epoxy. The resistance of all three electrodes between the other set of contact pads and the wires was measured after the baking process. If the resistance of any electrode was 10% higher than the average value, that device was excluded from subsequent modification and experiment. This is due to the concern that a higher resistance value usually suggests bad connectivity of the Au layer, which can be caused by several factors, such as stripping of the metal connection line, a crack on the electrode or the contact pad, and peeling of the Au layer at certain locations. Thereafter, the EPO-TEK® H70E non-conductive epoxy mixture (resin part A and hardener part B in a weight ratio of 1:1) was drop-casted on the devices to cover all the contact pads (refer to Figure 1b) in order to provide electrical insulation. The non-conductive epoxy was cured by baking inside an oven at 80 ◦C for 1.5 h. Each packaged device was then placed into a 3D-printed polycarbonate mold for further modification. The detailed dimensions of the mold are presented in Figure S1.
