## 2 Crosslinking of PEI and Reaction Mechanisms
The proposed structures of the glyoxal- and oxalic acid-crosslinked materials are shown in Figure 3.  
The adsorption capacity of a composite of crosslinked materials/MSU-F for CO\({}_{2}\) were measured and both showed lower adsorption capacities in comparison with a standard material represented by the code MSU-F/50 PEI. The lowest weight gain after the capture of CO\({}_{2}\) was observed for MSU-F/2.5 GOX-50 PEI, which was thought to be due to the lower concentration of primary amines in the glyoxal-crosslinked material. There are various coordinating molecules and polymers for Cu\({}^{2+}\) and Cu\({}^{+}\) ions, of which imidazole- and triazole-based molecules are well-known ligands. Tris[(benzyltriazole-only]methyl]amine (TBTA) is one of these ligands widely used in reactions that involve Cu\({}^{+}\)[38; 39]. With the aim of tuning the affinity of crosslinked PEI to a specific oxidation state of copper (Cu\({}^{+}\) or Cu\({}^{2+}\)), a triazole-based crosslinker (a triazole dialdehyde (TA)) and (benzyl]triazole capraldehyde (BTA) were synthesized by [PERSON] et al. to crosslink PEI [13]. The two crosslinkers were used in the presence/absence of glutaraldehyde. Imine bond formation is the mechanism proposed for the crosslinking reaction. The crosslinked materials were also treated with a reducing agent (Na(BH(OAC)\({}_{3}\)) to reduce the imine bond (Schiff base) to an amine bond (single bond). The reduction step aims at improving the stability of the crosslinked material toward hydrolysis reaction [40]. The structures of TA and BTA and the reactions are shown in Scheme 3. The materials prepared were analyzed using multiple techniques. The elemental analysis technique was used to determine the elemental composition of the materials. In this case, the authors measured the nitrogen content of the materials. The result indicated the presence of 12.6 wt%, 20.3 wt%, and 15.5 wt% N in PEI-GA, PEI-TA, and PEI-GA-BTA, respectively, consistent with the structures of TA and BTA and their contributions to the total nitrogen content of the final crosslinked materials. The IR spectra of the three materials were found to be similar and support imine bond formation, as peaks corresponding to C-N and C=N stretches were observed in the three materials. The Cu\({}^{2+}\) adsorption capacity  
Figure 3: Proposed crosslinking products of polyethylene imine with glutaraldehyde [36]. was found to be 8.1 wt% (PEI-GA), 12.2 wt% (PEI-TA), and 3.5 wt% (PEI-GA-BTA), and the adsorption capacities dwindled after the reduction of the imine bond with the reducing agent. The higher adsorption capacity of PEI-TA compared to the others is believed to be due to the formation of a porous structure (SEM study) and due to the presence of the additional copper-binding triazole ring in the structure. The materials prepared have also shown a higher selectivity for Cu\({}^{2+}\) over Zn\({}^{2+}\) in an adsorption experiment performed with a water sample containing both ions. The triazole-based crosslinked materials have also shown a strong affinity for Cu\({}^{+}\), showing the potential of the material to be used as an antifouling coating [13]. It is noted that Cu\({}^{+}\)-containing compounds are potent against the growth of biological organisms on marine surfaces. PEI is also crosslinked with other type of small molecules that possess an electrophilic carbon bonded to an electron-withdrawing elements such as chlorine. Such crosslink include dichloroethane, epichlorohydrin [41], trichlorotriazine, and epoxy-containing crosslinkers ((such as glycerol diglycidyl ether [42], ethylene glycol diglycidyl ether [43], and 1,4-butanediol diglycidyl ether [17]) (Figure 2). Carbon dioxide emission to the atmosphere from the excessive use of fossil fuels has remained a concern for the global community due to its greenhouse effect and causing the global temperature to rise.