**1. Introduction**

Water contamination caused by toxic heavy metals has always been one of the greatest threats to public health due to the aversion of heavy metals to natural degradation. Such non-biodegradable properties enable heavy metals to move along the food chain, which eventually accumulate inside the human body, resulting in severe detrimental effects on

**Citation:** Wang, N.; Bora, M.; Hao, S.; Tao, K.; Wu, J.; Hu, L.; Liao, J.; Lin, S.; Triantafyllou, M.S.; Li, X. Hyaluronic Acid Methacrylate Hydrogel-Modified Electrochemical Device for Adsorptive Removal of Lead(II). *Biosensors* **2022**, *12*, 714. https:// doi.org/10.3390/bios12090714

Received: 25 July 2022 Accepted: 31 August 2022 Published: 2 September 2022

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human health. Among the different kinds of heavy metals, lead (Pb(II)) is considered the most hazardous contaminant owing to its strong toxicity. For instance, Pb(II) is capable of restraining the formation of hemoglobin, which is an essential component of red blood cells [1,2]. Other pathological symptoms, such as abdominal pain, arthralgia, anemia, and cognitive deficit, may also occur when exposure rises to a certain limit [3]. More worryingly, the intake of Pb(II) may cause permanent learning and behavioral disorders in infants and children [4–6]. Hence, the development of devices to detect Pb(II) has been of considerable interest to both academia and industry. Over the past decade, many researchers have reported a variety of successfully designed chemical/electrochemical [7–10], fluorescent [11–13], as well as biological devices [14–16] for Pb(II) detection. However, research on developing compact devices for the removal of Pb(II) is still scanty, although the importance of this research area has been reiterated in recent years because of the water crisis [17].

Conventional approaches to achieving the removal of Pb(II) from aqueous solutions include adsorption, chemical precipitation, ion exchange, reverse osmosis, coagulation, and membrane filtration. Among all these approaches, adsorption has proved to be the most practical method due to its simplicity of design and operation, high efficiency, and economical advantage [18–20]. Different kinds of polymer materials, such as poly(pyrrole methane) [21], poly(allylamine-co-methacrylamide-co-acrylic acid) [22], poly(*N*,*N*-dimethylacrylamide-co-2-hydroxyethyl methacrylate) [23], poly(acrylamide-coitaconic acid) [24], melamine-formaldehyde-diaminohexane [25], poly(*trans*-aconitic acid/2 hydroxyethyl acrylate) [26], polyisoprene-*b*-polystyrene-*b*-poly(*N*,*N*-dimethylacrylamide) [27], nanochitosan/polyurethane/polypropylene glycol [28], etc., have been functionalized or directly utilized as effective adsorbents. Hyaluronic acid (HA), a carbohydrate polymer with repeated disaccharide units of glucuronic acid and N-acetylglucosamine alternatively linked by β-1,3 and β-1,4 glycosidic bonds, is widely present inside the human body (e.g., muscular connective tissues, epithelial tissues, and extracellular matrices). The excellent biocompatible, nontoxic, and biodegradable characteristics of HA permit it to be extensively adopted for clinical, surgical, and biomedical applications, such as (1) the formation of a surgical glue with a higher shear strength for tissue adhesion [29], (2) the development of an implantable macroporous scaffold with degradation regulatability to control tumor microenvironments [30], (3) the manufacture of a pH-triggered nanogel system for tumortargeted drug delivery [31], and (4) the functionalization of a contact lens surface with improved wettability, water retention, and reduced protein binding [32], to name a few. However, the modification of HA for analytical devices for the adsorptive removal of heavy metal ions is yet to be explored.

In this work, we have developed an electrochemical device that incorporates a layer of methacrylated HA hydrogel for in situ adsorption of Pb(II) in solutions. The electrochemical device is configured to have three electrodes, i.e., one working electrode, one counter electrode, and one reference electrode. The primary polymer is chosen as HA and methacrylic anhydride (MA) is used as the precursor. The adsorption performance of the HAMA hydrogel-modified electrochemical device is theoretically analyzed and further comprehensively investigated through a series of experiments. The practical application of the HAMA hydrogel-modified electrochemical devices for Pb(II) removal from tap water is demonstrated. The use of electrochemical devices for the detection of heavy metal ions has been extensively studied in the past [7–10]. However, the utilization of electrochemical devices for the adsorptive removal of heavy metal ions has rarely been reported. This work paves the way for the development of compact electrochemical devices for in situ removal of heavy metal ions.

## **2. Materials and Methods**

#### *2.1. Chemicals and Reagents*

The chemicals and reagents used throughout this study were of analytical grade. Standard Pb(II) and bismuth (Bi(II)) stock solutions (1000 mg/L) were purchased from

Sigma-Aldrich and Merck, respectively. Ultrapure water (18.2 MΩ·cm) collected from a Milli-Q system was used to dilute the stock solutions. Acetate buffer (pH 4.6) was added into diluted Pb(II) solutions as a supporting electrolyte. HA (molecular weight, 1 MDa) was purchased from Samich (HK) Limited, China. Sodium chloride (NaCl), sodium hydroxide (NaOH), MA, and absolute ethanol were used without purification. Nafion 117 solution (5 wt% in a mixture of water and lower alcohols) was diluted by absolute ethanol. Photoinitiator Irgacure I2959 (I2959) was purchased from Ciba Specialty Chemicals, Switzerland.
