Halloysite nanotube was successfully loaded with diverse type of active agents including pharmaceutical agents, antibacterials, corrosion inhibitors, mold growth inhibitors, etc.,, being made of simple organic and inorganic substances as well as enzymes and proteins having relatively large molecular dimensions. Halloysite entraps molecules in three different ways; adsorption to the external and internal walls of the tubes, loading into lumen and intercalation of substance between layers. The most important of three is the lumen loading as this provides the highest loading capability and controlled release. Release from halloysite lumen typically lasts from several hours to days depending on molecular weight, structure and solubility of the active agent in the release medium [3, 4].
Empty lumen of the halloysite occupies about 20% of its volume, which makes it suitable for loading with about 10-15% of the active agents within internal lumen. There are some studies claiming even higher loading efficiency, which could be associated with the external adsorption of active agents along with the lumen loading.
Figure 3 describes general procedure of the halloysite lumen…show more content… Loading efficiency of the unmodified halloysite nanotube lies in the range of 5-13% for low molecular weight substances and about 1-2% for enzymes. Loading of low molecular weight actives were increased up to 20% by various surface functionalization methods. It should be noted that these values are lower than the typical loading efficiency reported for layered clays and synthetic zeolites (20-30% range) [5-11]. However, halloysite provides complete isolation of the loaded active from surrounding media, which is critical for both sustained and controlled release. Therefore halloysite is the template of choice when benefits of the nanosized container and slow release predominate over the loading