Abstract
We present for the first time a method for the preparation of magnetic halloysite nanotubes (HNT) by loading of preformed superparamagnetic magnetite nanoparticles (SPION) of diameter size ∼6 nm with a hydrodynamic diameter of ∼10 nm into HNT. We found that the most effective route to reach this goal relies on the modification of the inner lumen of HNT by tetradecylphosphonic acid (TDP) to give HNT–TDP, followed by the loading with preformed oleic acid (OA)-stabilized SPION. Transmission electron microscopy evidenced the presence of highly crystalline magnetic nanoparticles only in the lumen, partially ordered in chainlike structures. Conversely, attempts to obtain the same result by exploiting either the positive charge of the HNT inner lumen employing SPIONs covered with negatively charged capping agents or the in situ synthesis of SPION by thermal decomposition were not effective. HNT–TDP were characterized by infrared spectroscopy (ATR-FTIR), thermogravimetric analysis (TGA), and ζ-potential, and all of the techniques confirmed the presence of TDP onto the HNT. Moreover, the inner localization of TDP was ascertained by the use of Nile Red, a molecule whose luminescence is very sensitive to the polarity of the environment. The free SPION@OA (as a colloidal suspension and as a powder) and SPION-in-HNT powder were magnetically characterized by measuring the ZFC-FC magnetization curves as well as the hysteresis cycles at 300 and 2.5 K, confirming that the super-paramagnetic behavior and the main magnetic properties of the free SPION were preserved once embedded in SPION-in-HNT.
Highlights
Halloysite nanotubes (HNT) are unique natural nanomaterials composed by a double-layered aluminosilicate with a hollow tubular structure (Figure 1a) in the microrange and an inner diameter usually ranging between 15 and 50 nm (Figure 1b)
As already mentioned in the Introduction, just a few examples were reported on the loading of preformed NPs in the HNT lumen.[46−48] One of them was carried out using small negative Ag NPs.[46]
We have successfully prepared a novel magnetic halloysite nanocomposite with apolar Superparamagnetic iron oxide nanoparticles (SPION) selectively loaded in the inner lumen of prefunctionalized HNT, exploiting vacuum-N2 cycles
Summary
Halloysite nanotubes (HNT) are unique natural nanomaterials composed by a double-layered aluminosilicate with a hollow tubular structure (Figure 1a) in the microrange and an inner diameter usually ranging between 15 and 50 nm (Figure 1b). The chemical composition of the HNT inner and outer parts is different. The inner surface is made by alumina with exposed aluminum hydroxide groups (Al−OH), while the outer one is made by silica [siloxane groups (Si−O−Si)].1. The potentiality for the industrial development of HNT-based derivatives is more than probable, due to the low price, ecofriendly properties, and biocompatibility of this material, as well as facile and selective functionalization of the HNT two layers with different functional groups.[5,6]
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