Aqueous CS solutions at different concentrations were prepared by dissolving a predefined amount of CS in an aqueous solution of acetic acid (1 wt.%) by rigorous mixing for 5 h. For each concentration, 2 μL of the CS solution was dispensed over the WE and allowed to dry under ambient conditions. After that, 2 μL of the aqueous solution of glutaraldehyde (3 wt.%) was dispersed on the electrode surface for a few hours to crosslink chitosan. Before electrochemical measurements, the electrodes were washed 3 times with deionized water to remove impurities. Alg was dissolved in deionized water by rigorous mixing for 5 h. The coating process was similar to CS, but 2 μL of 100 mM calcium chloride aqueous solution was used as a crosslinker. Divalent calcium ions enable ionic crosslinking for anionic Alg hydrogels. Agar was dissolved in hot water (90°C) under stirring to obtain a transparent solution. Then, 2 μL of this solution was dispersed over WE. No crosslinking agent is used for agarose, because this non-ionic PSA undergoes physical crosslinking via multiple hydrogen bonding interactions.