Surface engineering of halloysite with PAMAM dendrimer via divergent and convergent synthetic routes: Quantitative and qualitative analysis

Abstract

Functionalization of halloysite nanotubes (HNT) with amine-terminated dendritic groups can modify the surface chemistry and physical properties, thereby optimizing their functionality for expanded potential applications, making them a valuable resource for various industries. In this study, hybrid HNT were successfully prepared using amine-terminated dendritic polymers via convergent and divergent synthetic routes. The functionalized HNT with γ-aminopropyltriethoxysilane (APTES) serves as a core (HNT-APTES) to grow dendritic structures in both routes. In the divergent route, the three generations of polyamidoamine dendrimers (PAMAM) were grown in a multistep process initiated by the Michael addition of methyl acrylate to the HNT-APTES, followed by amidation of the ester groups with ethylenediamine. In the convergent route, the HNT-APTES were functionalized with dicarboxylic acid as a linker between HNT and PAMAM dendrimer (HNT-COOH), then the second generation of PAMAM reacted onto HNT-COOH. The efficiency of synthesis in both methods was investigated by quantitative and qualitative analysis using X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and ζ-potential analysis. The Brunauer-Emmett-Teller (BET) method, X-ray diffraction (XRD), and Raman spectroscopy were used to characterize the physicochemical properties of the developed samples. The results confirmed the chemical interactions between HNT and dendritic polymers, which led to an increased surface charge. The growth of the dendrimer structure in each generation was examined on the outer and inner surfaces of HNT for both methods. The results reveal the possibility of growing dendritic structures into the lumen of HNT in lower generation in divergent synthetic routes while the dendritic structures only grow onto the outer surface of HNT due to lower steric hindrance. Furthermore, Acid Red 1 (AR1) was utilized as an anionic model dye for adsorption characteristics. The results confirm the higher efficiency of convergent synthesis routes compared to divergent ones in the same generation. These routes facilitated the modification of the outer surface of nanoclay for adsorption applications such as pollutants, drugs, and dyes.