## 1 Introduction  
Heavy metals are common pollutants in the environment that present a major threat to public health, even at trace levels. Heavy metals enter the environment from different sources, which include metal plating, pickling, pigment industries, tanneries, municipal landfills, and wastewater treatment facilities [1]. Some of the heavy metals are needed by animals and plants; however, even these biologically essential metals become toxic and pose a health risk if they are found in the environment beyond a certain acceptable level. While copper is found in biological molecules such as in chlorophyll, enzymes, etc., and hence is essential for proper functioning of biological processes [2], it is also known for its toxicity when it is found above a certain limit. The presence of excess copper can inactivate enzymes and form copper-protein aggregates that disturb the normal functioning of biological processes in the organism [3; 4]. Studies also show that when the concentration of copper is above 1.3 mg L\({}^{-1}\) in drinking water, it can cause kidney failure and other ailments, which clearly signifies the importance of monitoring copper levels in the environment and taking appropriate actions to mitigate their impacts [5]. Many approaches have been studied and developed to remove heavy metals from the environment to minimize impacts, which include flocculation, membrane filtration, chemical precipitation, ion exchange, and adsorption precipitation methods [6; 7; 8]. Among them, ion exchange and adsorption are attractive options due to their simple operation and potential recovery and reuse of the metals and the adsorbents [9; 10]. One of the most widely researched area in the field of sensing and removal of heavy metals from the environment is the use of polymeric materials. In this regard, polyethylene imine (PEI)/composite-based materials are interesting and have been widely investigated. Structurally, PEI exists in both linear and branched forms; can have primary, secondary, and tertiary amine groups; and can be synthesized with various molecular weights [11] (Figure 1). PEI is a water-soluble polyamine known for its high adhesion and adsorption properties. It contains polar amino groups, allowing for strong electrostatic interactions with ions and making it effective at absorbing dyes and toxic anions [12]. The stability of PEI in water is increased by either binding the material onto insoluble support materials and/or by chemically crosslinking the polymer using reagents like glutaraldehyde and epiclorohydrin (ECH) [13; 14]. For example, glutaraldehyde is extensively used in the crosslinking of PEI to create a mechanically and chemically robust material. The large number of amine groups on crosslinked PEI makes it an ideal polymeric ligand for complex numerous heavy metal ions [15]. Crosslinked PEI coatings, for instance, were found to efficiently adsorb copper ions from water under acidic conditions [16; 17]. In another study, the highest copper adsorption was achieved using PEI cryogels crosslinked with diglycidyl ethers in alkaline media [17]. Some previous studies have shown the adsorption of copper from 2 to 200 ppb concentrations in seawater [18; 19]. More importantly, the adsorbed metal ions can be released by chemical treatment, allowing the regeneration and reuse of the crosslinked material [20; 21], and making PEI an interesting polymer for industrial uses such as waste water treatment [21]. PEI and modifications/composites have also been used in other fields, including biomedical applications. PEI exhibits pH-dependent properties due to the protonation of its amino groups, which makes it suitable for controlled drug release systems and pH-responsive drug delivery [22]. Its high cationic charge density and its interaction with negatively charged molecules or surfaces makes it useful in applications such as gene delivery and flocculation processes [23].