*3.2. Electrochemical Investigation of the HAMA Hydrogel-Modified Device*

To evaluate the adsorption performance of the HAMA hydrogel-modified electrochemical devices, a series of SWASV experiments were carried out. The magnitude of each stripping peak obtained in the SWASV experiments was measured with respect to its

baseline. Initially, the response was recorded in a solution with 20 μg/L Pb(II) (blue line in Figure 4a), in which a clear stripping peak was observed close to a potential −0.76 V, with an average peak magnitude of 1.059 μA (refer to Figure 4b). When the Pb(II) concentration was increased to 40 μg/L, the device exhibited a smaller stripping peak (green line in Figure 4a) with a decreased average peak magnitude of 0.848 μA. To investigate whether such a decrease in the stripping peak was caused by the reduced alloying capability of the working electrode, 400 μg/L Bi(II) was added to the test solution. According to the literature [38–40], Bi is capable of forming low-melting-temperature alloys with heavy metals, which significantly facilitates the accumulation of metal ions by the working electrode in the course of the deposition. As shown by the orange line in Figure 4a, adding Bi(II) did not improve the accumulation of Pb(II) ions for the device, which was manifested by an even smaller stripping peak with a further decreased average peak magnitude of 0.496 μA. With 400 μg/L Bi(II) in the test solution, a subsequent increase in the Pb(II) concentration to 60 μg/L (red line in Figure 4a) resulted in an indistinguishable stripping peak with a tiny average peak magnitude of 0.199 μA. The experimental results imply that the developed HAMA hydrogel-modified electrochemical device is capable of adsorbing the Pb(II) ions that are available in the solution.

**Figure 4.** (**a**) Anodic stripping voltammograms and (**b**) the corresponding stripping peak currents recorded for the HAMA hydrogel-modified electrochemical devices in different test solutions.

#### *3.3. Adsorption Mechanism of the HAMA Hydrogel-Modified Device*

To explain the observed phenomena, a hypothesis pertaining to the adsorption mechanism of the HAMA hydrogel-modified electrochemical devices is proposed. A schematic model illustrating the mechanism is depicted in Figure 5, in which red circles (with a plus sign or number) represent the dissolved heavy metal ions in the solution. The network denoted by black solid lines above the electrode layer represents the microscopic porous structure of the HAMA hydrogel. First, free-moving metal ions in the solution migrated to the vicinity of the hydrogel under the effect of diffusion due to a high concentration gradient established between the solution and the hydrogel. Thereafter, the metal ions penetrated the hydrogel and interacted with the functional groups of the hydrogel's polymer chain. Some of the metal ions (e.g., No. 1, 2, 3, and 6 in Figure 5) were directly captured by the polymer chain due to the molecular interaction.

**Figure 5.** Schematic model to illustrate the adsorption mechanism of the HAMA hydrogel-modified electrochemical device. Red circles (⊕ and -<sup>1</sup> –-6 ) represent heavy metal ions.

On the other hand, some other metal ions (e.g., No. 4 and 5 in Figure 5) reached the surface of the electrode by moving through the holes formed in the porous hydrogel. These ions were further captured by the electrode due to the electrochemical accumulation (M<sup>+</sup> + e−→M) triggered by the applied deposition potential. We suspect that most of the metal ions were collected by the hydrogel's polymer chain considering the above electrochemical investigation. If the dominant adsorption was contributed by the electrochemical accumulation, a relatively larger stripping peak should have been observed from the voltammograms when the SWASV experiment was conducted in solutions with a higher concentration. However, without the electrochemically induced adsorption, the collection efficiency of the device should be adversely affected since the high concentration gradient between the solution and the hydrogel could not be continuously maintained. Therefore, we attribute the promising adsorption capability of the HAMA hydrogel-modified device toward the Pb(II) ions to a combined effect of the molecular interaction and electrochemical accumulation.
