Starch Nanocomposite and Nanoparticles: Biomedical Applications - Modern Trends in Chemistry and Chemical Engineering - page 57

Problems associated with these strategies are use of (i) polar aprotic solvents that
strip critical water from enzymes lowering their activities10 and (ii) heterogeneous
reaction conditions that restrict the modifi cation of large particles and fi lms to a small
fraction of the substrate residing at the surface. To overcome the use of polar aprotic
solvents, enzymes have been incorporated within reverse micelles using the anionic
surfactant aerosol-OT [AOT, bis(2-ethylhexyl)sodium sulfosuccinate]. The AOT forms
thermodynamic water droplets surrounded by a surfactant monolayer in oil (isooctane).
Water entrapped within the reverse micelles resembles the polar pockets in cells.
Incorporation of enzymes within reverse micelles soluble in nonpolar media facilitates
productive collisions and reactions between enzymes and nonpolar substrates. Several
types of lipase-catalyzed reactions in AOT/isooctane have been studied. Dordick and
co-workers incorporated proteases from Subtilisin carlsberg and Bacillus lichenifor-
mis within AOT-coated reverse micelles. Although the enzymes within reverse mi-
celles were active for the acylation of amylose in fi lm and powder form, the inability
of AOT-coated enzyme to diffuse into the bulk of these substrates limited modifi cation
of fi lms and powders to surface regions. Nanoparticles, nanospheres, and nanogels
are used as building blocks for nanoscale construction of sensors, tissues, mechanical
devices, and drug delivery systems. For the latter, carriers with nanodimensions are
not detected by the reticuloendothelial system so they circulate for longer times. For
medical applications, nanoparticles constructed from poly(lactic acid), poly-(glycolic
acid), poly(alkyl cyanoacrylate), 2-hydroxyethyl acrylate-poly(ethylene glycol)diac-
rylate copolymers, poly(L-lysine)- g -polysaccharides, and poly- (vinylpyrrolidone)
have been reported. Starch microspheres were studied for the delivery of insulin via
the nasal system. To overcome previous diffi culties in the enzymatic esterifi cation
of polysaccharides and to prepare a family of structurally and dimensionally well-
defi ned nanoparticles from a natural material, a new idea was explored. That is, starch
nanoparticles were incorporated into reverse micelles stabilized by AOT. A key dis-
covery was that the nanodimensions of AOT-coated starch particles and their solubil-
ity in nonpolar media such as toluene allowed their diffusion to sites within the pores
of the physically immobilized lipase catalyst Novozym 435 at which esterifi cation of
starch occurred [102-110].
The acyl donors included vinyl esters of fatty acids differing in chain length, male-
ic anhydride, and € -caprolactone. Motivations for selecting these acyl donors were as
follows: (i) increase starch hydrophobicity, (ii) introduce both carboxylate side chains
and sites for free radical cross-linking, and (iii) form polyester grafts. Nuclear magnet-
ic resonance experiments gave the DS and regioselectivity of esterifi cation reactions.
The infl uence of the reaction temperature, time, and structure of the acyl donor on the
progress of esterifi cation reactions was studied. IR microspectroscopy revealed the
extent that AOT-coated starch nanoparticles diffuse through the macroporous structure
of the physically immobilized lipase catalyst Novozym 435. Dynamic light scattering
(DLS) showed the size distribution of AOT-coated reverse emulsions of starch nano-
spheres both prior to and after starch modifi cation reactions. Furthermore, DLS con-
fi rmed that the nanodimensions of modifi ed starch nanoparticles were retained after
they had been stripped of AOT and dispersed in water or DMSO.