10.2.2.2 Encapsulation
More frequently for inorganic nanomaterials, lipid or surfactants can coat their
surface to form bilayers or micelles to encapsulate those materials. The
hydrophobic tails of these molecules can form relatively stable interaction with
hydrophobic nanomaterial surface (e.g. pristine carbon nanotubes) or initial surface
coatings. The surface of nanomaterials can be further modified based on the terminal active groups from lipid/surfactant coating. For effective incorporation of
lipid/surfactant, two leading strategies, i.e. lipid film hydration and microemulsion
methods [45, 46], were actively adopted. The film hydration method usually
involves the mixture of nanomaterial and lipid/surfactant in an organic solvent,
while the solvent is subsequently evaporated with the addition of aqueous solution
(e.g. water) [47, 48]. On the other hand, the microemulsion method is to add
aqueous surfactant solution into nanomaterial-containing organic solvent (with
vigorous stirring) and form an oil-in-water microemulsion. Later, organic solvent
will be evaporated to facilitate both the interaction of lipid/surfactant-nanomaterial
and the transfer of nanomaterial into the aqueous phase [49]. Nanomaterials post
encapsulation usually possess improved stability and functionality, which makes it
easier for further modification. More recently, to further increase nanomaterial
biocompatibility, natural cell membrane was also used as a vesicle to encapsulate/
functionalize various nanomaterials [50].
10.2.2.3 p–p Interaction/Stacking
Utilization of p–p interaction/stacking is one of the most efficient method for
nanomaterials with electron-rich aromatic components inside the structure (e.g.
graphene, carbon nanotubes etc.) [51]. For example, pyrene-derived PEG molecules
can be used for surface engineering of carbon nanotubes [52], and more recently,
nano-sized metal organic framework (MOF) containing 1,4-benzenedicarboxylate
(BDC) as the bridging molecules inside the structure [53]. The in vivo pharmacokinetics of modified nanomaterials are satisfactory from these studies.
10.2.2.4 Electrostatic Interaction
Electrostatic interaction (complexation) stands as an alternative to chemical conjugation for nanomaterials bearing strong surface charges [54]. For example, in one
study, bovine serum albumin (BSA), which bears negative charges on surface, was
used to form strong interaction with cetyltrimethylammonium bromide (CTAB) on
gold nanoparticle surface to neutralize the toxicity from CTAB [55]. Another
example is that sodium thioglycolate was covered onto the surface of gold
nanoparticles to increase their interaction with neutral red [56].
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191
More frequently for inorganic nanomaterials, lipid or surfactants can coat their
surface to form bilayers or micelles to encapsulate those materials. The
hydrophobic tails of these molecules can form relatively stable interaction with
hydrophobic nanomaterial surface (e.g. pristine carbon nanotubes) or initial surface
coatings. The surface of nanomaterials can be further modified based on the terminal active groups from lipid/surfactant coating. For effective incorporation of
lipid/surfactant, two leading strategies, i.e. lipid film hydration and microemulsion
methods [45, 46], were actively adopted. The film hydration method usually
involves the mixture of nanomaterial and lipid/surfactant in an organic solvent,
while the solvent is subsequently evaporated with the addition of aqueous solution
(e.g. water) [47, 48]. On the other hand, the microemulsion method is to add
aqueous surfactant solution into nanomaterial-containing organic solvent (with
vigorous stirring) and form an oil-in-water microemulsion. Later, organic solvent
will be evaporated to facilitate both the interaction of lipid/surfactant-nanomaterial
and the transfer of nanomaterial into the aqueous phase [49]. Nanomaterials post
encapsulation usually possess improved stability and functionality, which makes it
easier for further modification. More recently, to further increase nanomaterial
biocompatibility, natural cell membrane was also used as a vesicle to encapsulate/
functionalize various nanomaterials [50].
10.2.2.3 p–p Interaction/Stacking
Utilization of p–p interaction/stacking is one of the most efficient method for
nanomaterials with electron-rich aromatic components inside the structure (e.g.
graphene, carbon nanotubes etc.) [51]. For example, pyrene-derived PEG molecules
can be used for surface engineering of carbon nanotubes [52], and more recently,
nano-sized metal organic framework (MOF) containing 1,4-benzenedicarboxylate
(BDC) as the bridging molecules inside the structure [53]. The in vivo pharmacokinetics of modified nanomaterials are satisfactory from these studies.
10.2.2.4 Electrostatic Interaction
Electrostatic interaction (complexation) stands as an alternative to chemical conjugation for nanomaterials bearing strong surface charges [54]. For example, in one
study, bovine serum albumin (BSA), which bears negative charges on surface, was
used to form strong interaction with cetyltrimethylammonium bromide (CTAB) on
gold nanoparticle surface to neutralize the toxicity from CTAB [55]. Another
example is that sodium thioglycolate was covered onto the surface of gold
nanoparticles to increase their interaction with neutral red [56].
10 Surface Modification of Radionanomedicine
191