*2.4. Device Modification*

Hyaluronic acid was modified with methacrylic anhydride and then photocrosslinked to obtain the HAMA hydrogel [34]. Briefly, the HA solution was prepared in ultrapure water and MA (20 mol/L) was added to it. The pH of the solution was adjusted to 8 using 1 M NaOH. After 2 h of reaction (as illustrated in Figure 2), the HAMA solution was incubated at 4 ◦C for 24 h and dialyzed (Spectra/Por 6 dialysis tubing, 10 kDa molecular weight cutoff) against 0.1 M NaCl solution, 25% (*v*/*v*) ethanol and ultrapure water for 48 h at room temperature. The modified HAMA solution was freeze-dried for 72 h and used to prepare the HAMA hydrogel. An amount of 1% (*w*/*v*) of HAMA was first dissolved in ultrapure water and then 1% (*w*/*w*) of I2959 photoinitiator (prepared in Nafion 117) was added to it and mixed thoroughly. Thereafter, each electrochemical device was contained in the polycarbonate mold, which had inlets for wires and cutout slots to hold a cap on both sides of the mold to keep the swollen hydrogel intact. The HAMA solution was spread uniformly over the device inside the mold without making any air bubbles and then exposed to UV light for 5 min to complete the crosslinking process. The HAMA hydrogel over the device was swollen in ultrapure water for 24 h to form a transparent gel layer, as shown in the dashed red rectangle in Figure 1b.

**Figure 2.** Schematic representation of the synthetic route of HAMA hydrogel.

*2.5. Investigation of Morphological and Chemical Structures of the Synthesized HAMA Hydrogel* The HAMA hydrogels were prepared using a similar approach to that mentioned earlier with slight modifications. The hydrogels were formed inside 3D-printed disc-shaped polycarbonate molds (internal diameter 18 mm, outer diameter 20 mm, and depth 2.5 mm). The HAMA solution together with the photoinitiator was spread uniformly into these molds and exposed to UV light for crosslinking. After crosslinking, the hydrogels were carefully removed from the molds and transferred to Petri dishes for swelling in ultrapure water. For the swelling capacity study, the weights of all the wet hydrogel samples were first recorded. Subsequently, the weights of all the dried hydrogels were measured upon baking the samples inside an oven at 50 ◦C for 6 h. For scanning electron microscopy (SEM) imaging, the hydrogel samples were quenched in liquid nitrogen for a few seconds and then immediately lyophilized for 72 h. The dried samples were coated with Au and imaged under an SEM microscope (JSM 6360A Jeol, Tokyo, Japan) at a 10 kV acceleration voltage. For Fourier transform infrared spectroscopy (FTIR) analysis, after 24 h of swelling, the ultrapure water was replaced by either a 0.1 M acetate buffer or 100 μg/L Pb(II) solution. Before conducting FTIR analysis, the hydrogel samples were dried inside an oven at 50 ◦C for 6 h. The FTIR experiments were carried out using a Nicolet iS10 FTIR Spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) in a frequency range between 400 and 4000 cm−1. A total number of 32 scans with a resolution of 2 cm−<sup>1</sup> were averaged for each spectrum.

#### *2.6. Investigation of Adsorption Performance of the Modified Device*

The adsorption performance of the HAMA hydrogel-modified electrochemical devices was evaluated by conducting a series of square-wave anodic stripping voltammetry (SWASV) in the diluted Pb(II) solutions. The output of the devices was recorded on a CHI 600 C electrochemical workstation (CH Instruments, Austin, TX, USA). The SWASV measurements were initiated by applying a deposition potential of −1.0 V to the working electrode for 120 s while the test solution was stirred (800 rpm). This deposition step was aimed at the collection of Pb(II) ions that were available in the vicinity of the working electrode. After an equilibration time of 2 s, the voltammograms were recorded under quiescent conditions in a potential range from −1.2 to 0 V with a frequency of 50 Hz, amplitude of 50 mV, and a step potential of 5 mV. This stripping step was proportionally related to the deposition step, i.e., the higher the stripping current, the more the amount of Pb(II) ions that were collected during the deposition step. Hence, a higher concentration of Pb(II) should be present in the test solution. Prior to the next measurement, a conditioning potential of −0.1 V was provided for 120 s in order to electrochemically clean the working electrode. Herein, all the potentials applied or measured during the SWASV experiments were with respect to the potential of the fabricated Ag/AgCl reference electrode.
