100 nm diameter nanospheres were formed with Ca
2+ at a 1.0:1.0 ratio. However,
a 3.0:1.0 ratio was required to obtain nanospheres of the same size with Ba
2+ .
3. The cation to mru ratio: The particles could either “grow” sequentially by adding
an extra amount of the cation or “shrink” by adding an extra amount of polymer.
For instance, addition of Ca
2+ to poly-(R)-2 or poly-(S)-2 (polymer concentration 0.1 mg/mL) at M
2+ to mru (mol/mol) ratios of 1.0:1.0, 1.2:1.0 and >1.2:1.0
generated nanospheres of 100, 160 and 200 nm, respectively. With Ba
2+ , ratios of
3.0:1.0, 4.0:1.0 and 5.0:1.0 led to nanospheres of 100, 140 and 170 nm respectively.
The usual range of sizes went from 80 to 200 nm (obtained by DLS) for welldefined and stable structures.
On the other hand, a reduction in particle size took place by the sequential addition
of extra amounts of polymer to pre-existent nanospheres. For example, nanospheres
of 160 nm obtained by addition of Ca
2+ to poly-(R)-2 [M
2+ to mru (mol/mol) ratio of
1.2:1.0, PDI 0.18] evolved after addition of an extra amount of poly-(R)-2 (1.0:3.0
ratio) into particles of 112 nm (PDI 0.20) that were further reduced into particles of
90 nm (PDI 0.20) by addition of more poly-(R)-2 (1.0:4.0 ratio).
Most uses of nanoparticles are related to processes on their surface (e.g. use on
therapeutic targets) or inside the cavity (e.g. use as nanoreactors), making the
chirality of those zones a crucial aspect. In this sense, the ability to tune the helicity
of the polymers assembling the nanospheres constitutes an especially relevant property. This ability depends not only on the starting polymer but also on its response to
mono- and divalent metal ions that, according to their valence, can play two main
roles: as chiral amplification inductors of the helical sense (both mono- and divalent
ions) and as crosslinking agents leading to aggregation (divalent ions only).
As a result of the conjunction of these above factors, nanospheres with different
helicities and chiroptical responses can be prepared by different strategies. Thus,
nanoparticles with right- or left-handed helicity could be obtained by selection of
the chirality in the pendant [(R)-2 or (S)-2]. In addition, it was also found that
appropriate use of mono- and divalent cations could cause a single polymer [poly(R)-2 or poly-(S)-2] to generate either the right- or left-handed helically oriented
nanoparticles. A more detailed explanation of these results follows.
Nanospheres made from poly-(R)-2 presented a predominant helical sense and
CD, whereas those made from poly-(S)-2 have the opposite helicity and CD sign.
So, in order to get the two axial “enantiomeric” HPMC nanospheres, both polymers
are needed. For example, positive CD responses and right-handed helical senses (P)
predominated in HPMC nanospheres made from poly-(R)-2 and divalent cations
(Fig. 7), whereas poly-(S)-2 gave nanoparticles with left-handed helicity (M ).
When the size of the particles was increased by addition of more metal, they
became progressively more “chiral” (increasing CD response) because, as the
nanostructures grew, they were composed of chains where one helicity was becoming more and more predominant. Naturally, if 1:1 mixtures of poly-(R)-2 and poly(S)-2 were used to form the nanoparticles, “racemic” HPMC nanospheres (null CD
response at any particle size) composed of polymer chains with equal amounts of
both helical senses were obtained.
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F. Freire et al.
2+ at a 1.0:1.0 ratio. However,
a 3.0:1.0 ratio was required to obtain nanospheres of the same size with Ba
2+ .
3. The cation to mru ratio: The particles could either “grow” sequentially by adding
an extra amount of the cation or “shrink” by adding an extra amount of polymer.
For instance, addition of Ca
2+ to poly-(R)-2 or poly-(S)-2 (polymer concentration 0.1 mg/mL) at M
2+ to mru (mol/mol) ratios of 1.0:1.0, 1.2:1.0 and >1.2:1.0
generated nanospheres of 100, 160 and 200 nm, respectively. With Ba
2+ , ratios of
3.0:1.0, 4.0:1.0 and 5.0:1.0 led to nanospheres of 100, 140 and 170 nm respectively.
The usual range of sizes went from 80 to 200 nm (obtained by DLS) for welldefined and stable structures.
On the other hand, a reduction in particle size took place by the sequential addition
of extra amounts of polymer to pre-existent nanospheres. For example, nanospheres
of 160 nm obtained by addition of Ca
2+ to poly-(R)-2 [M
2+ to mru (mol/mol) ratio of
1.2:1.0, PDI 0.18] evolved after addition of an extra amount of poly-(R)-2 (1.0:3.0
ratio) into particles of 112 nm (PDI 0.20) that were further reduced into particles of
90 nm (PDI 0.20) by addition of more poly-(R)-2 (1.0:4.0 ratio).
Most uses of nanoparticles are related to processes on their surface (e.g. use on
therapeutic targets) or inside the cavity (e.g. use as nanoreactors), making the
chirality of those zones a crucial aspect. In this sense, the ability to tune the helicity
of the polymers assembling the nanospheres constitutes an especially relevant property. This ability depends not only on the starting polymer but also on its response to
mono- and divalent metal ions that, according to their valence, can play two main
roles: as chiral amplification inductors of the helical sense (both mono- and divalent
ions) and as crosslinking agents leading to aggregation (divalent ions only).
As a result of the conjunction of these above factors, nanospheres with different
helicities and chiroptical responses can be prepared by different strategies. Thus,
nanoparticles with right- or left-handed helicity could be obtained by selection of
the chirality in the pendant [(R)-2 or (S)-2]. In addition, it was also found that
appropriate use of mono- and divalent cations could cause a single polymer [poly(R)-2 or poly-(S)-2] to generate either the right- or left-handed helically oriented
nanoparticles. A more detailed explanation of these results follows.
Nanospheres made from poly-(R)-2 presented a predominant helical sense and
CD, whereas those made from poly-(S)-2 have the opposite helicity and CD sign.
So, in order to get the two axial “enantiomeric” HPMC nanospheres, both polymers
are needed. For example, positive CD responses and right-handed helical senses (P)
predominated in HPMC nanospheres made from poly-(R)-2 and divalent cations
(Fig. 7), whereas poly-(S)-2 gave nanoparticles with left-handed helicity (M ).
When the size of the particles was increased by addition of more metal, they
became progressively more “chiral” (increasing CD response) because, as the
nanostructures grew, they were composed of chains where one helicity was becoming more and more predominant. Naturally, if 1:1 mixtures of poly-(R)-2 and poly(S)-2 were used to form the nanoparticles, “racemic” HPMC nanospheres (null CD
response at any particle size) composed of polymer chains with equal amounts of
both helical senses were obtained.
134
F. Freire et al.