Halloysite nanotubes
(HNTs) have attracted great attention in the
field of nanotechnology as natural, high value-added nanomaterials.
Despite their significant potential as carriers of active agents and
fillers in nanocomposite structures, inhomogeneity of HNTs in terms
of length and diameter along with their agglomeration tendency poses
important obstacles for the utilization of them in a wider range of
applications. Here, a facile, three-step separation protocol that
allows the sorting of HNTs into agglomeration-free, uniform size fractions
is reported. The protocol consists of coating of HNTs with polydopamine
to impart hydrophilicity and aqueous dispersibility, followed by their
ultrasonication and centrifugation at varying velocities for size-based
separation. Particle size distribution analysis by scanning electron
microscopy and dynamic light scattering has demonstrated that the
separation protocol resulted in uniform HNT fractions of varying agglomeration
states and particle sizes. The highest quality fraction obtained with
18% yield was free of agglomerations and consisted of HNTs of uniform
lengths and diameters. The polydopamine coating on HNTs which facilitated
the separation was demonstrated to be removed by a simple heat treatment
that preserved the crystal structure of HNTs. The impact of the separation
protocol on the loading and functionalization capacity of halloysites
was investigated. Highest quality HNTs presented 4.1-fold increase
in lumen loading and 1.9-fold increase in covalent surface coupling
ratios compared to the loading and functionalization ratios obtained
with raw HNTs. Similarly, sorted, high-quality HNTs were demonstrated
to be better dispersed in a polymeric matrix, resulting in polymeric
nanocomposites with significantly enhanced mechanical properties compared
to nanocomposites prepared with raw HNTs. The three-step separation
protocol presented here provides a toolbox that allows sorting of
raw HNTs into uniform fractions of different size ranges, from which
HNTs of desired qualities required by different applications can be
selected.
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