U.S. patent application number 12/438591 was filed with the patent office on 2010-01-14 for novel nanoparticles.
Invention is credited to Steven Armes, Jian-Jun Yuan.
Application Number | 20100009001 12/438591 |
Document ID | / |
Family ID | 37232408 |
Filed Date | 2010-01-14 |
United States Patent
Application |
20100009001 |
Kind Code |
A1 |
Armes; Steven ; et
al. |
January 14, 2010 |
NOVEL NANOPARTICLES
Abstract
The invention provides a composition comprising core-shell
nanoparticles, the nanoparticles comprising (a) cationic core
material comprising polymer; and (b) a shell material comprising
silica. Preferred core materials comprise diblock copolymer
micelles comprising one block of dialkylaminoethyl methacrylate
units which are partially or fully quaternised and one block of
dialkylaminoethyl methacrylate units that remain non-quaternised.
The invention also provides a method for the preparation of the
said composition, the method involving (a) preparing a cationic
core material comprising polymer; and (b) coating the core material
with a shell comprising silica by treating the polymer with a
silica precursor under ambient conditions. The invention also
envisages a composition comprising core-shell nanoparticles which
is adapted to facilitate controlled delivery of at least one active
agent into a system in response to controlled changes in the pH of
the system.
Inventors: |
Armes; Steven; (Sheffield,
GB) ; Yuan; Jian-Jun; (Osakidai, JP) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Family ID: |
37232408 |
Appl. No.: |
12/438591 |
Filed: |
September 5, 2007 |
PCT Filed: |
September 5, 2007 |
PCT NO: |
PCT/EP2007/007729 |
371 Date: |
August 28, 2009 |
Current U.S.
Class: |
424/490 |
Current CPC
Class: |
C09D 7/61 20180101; A61K
9/501 20130101; C08K 3/36 20130101; C08L 2207/53 20130101; C09D
7/65 20180101; A61K 9/5089 20130101; G02B 1/111 20130101; A61P
43/00 20180101; C08K 9/12 20130101; Y10T 428/254 20150115; C08L
53/00 20130101; C08K 9/02 20130101; C08K 2201/013 20130101; C08K
2201/011 20130101; Y10T 428/2998 20150115; C08K 9/00 20130101; C09D
7/70 20180101 |
Class at
Publication: |
424/490 |
International
Class: |
A61K 9/14 20060101
A61K009/14; A61P 43/00 20060101 A61P043/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 6, 2006 |
GB |
0617480.9 |
Claims
1. A composition comprising core-shell nanoparticles, wherein said
nanoparticles comprise: (a) cationic core material comprising
polymer; and (b) shell material comprising silica.
2. A composition as claimed in claim 1 wherein said core material
comprises a copolymer micelle.
3. A composition as claimed in claim 2 wherein said copolymer
micelle comprises a diblock copolymer micelle.
4. A composition as claimed in claim 3 wherein said diblock
copolymer micelle has a core comprising at least one block of a
first polymer and a corona comprising at least one block of a
second polymer wherein said second polymer is different to said
first polymer.
5. A composition as claimed in claim 2 wherein said copolymer
comprises a first polymer and a second polymer which both comprise
amino-based (alk)acrylate monomer units.
6. A composition as claimed in claim 5 wherein said (alk)acrylate
units comprise acrylate units.
7. A composition according to claim 1 wherein the polymer comprises
poly[2-(diisopropylamino)ethyl
methacrylate)-block-2-(dimethylamino)ethyl methacrylate]
(PDPA-PDMA).
8. A composition as claimed in claim 7 wherein the degree of
polymerisation of the PDPA-PDMA copolymer is controlled such that
the mean degree of polymerisation of the at least one PDPA block
falls in the range of 20-25.
9. A composition as claimed in claim 7 wherein the degree of
polymerisation of the PDPA-PDMA copolymer is controlled such that
the mean degree of polymerisation of the at least one PDMA block
falls in the range of 65-70.
10. A composition according to claim 1 wherein said shell material
comprises silica which is deposed on said core material from at
least one silica precursor.
11. A composition as claimed in claim 1 wherein said nanoparticles
have an average specific size (g) of about 300 nm or less.
12. A composition as claimed in claim 1 wherein said nanoparticles
have an average particle size is in the region of from 10-100
nm.
13. A composition as claimed in claim 1 wherein said nanoparticles
have an anisotropic rod-like morphology.
14. A method for the preparation of a composition comprising
core-shell nanoparticles as claimed in claim 1, said method
comprising the steps of: (a) preparing a cationic core material
comprising polymer; and (b) coating said core material with a shell
comprising silica.
15. A method as claimed in claim 14 wherein said polymeric core
material is prepared by group transfer polymerisation or controlled
radical polymerisation.
16. A composition adapted to facilitate controlled delivery of at
least one active agent into a system, said composition comprising
core-shell nanoparticles according to claim 1, wherein said
composition is adapted to provide said controlled delivery in
response to controlled changes in the pH of said system.
17. A coating comprising nanoparticles according to claim 1.
Description
[0001] The present invention is concerned with novel nanoparticles.
More specifically, the invention relates to core-shell
silica-copolymer nanoparticles, methods for their preparation, and
their potential uses.
[0002] There is growing academic and industrial interest in the
synthesis and applications of nanoparticles, most particularly
nanoparticles having a core-shell structure in view of their
potential use as delivery vehicles for active materials such as
drugs. Consequently, much prior art is devoted to the preparation
of nano-sized particles of this type.
[0003] Specifically, several authors have considered the potential
applications of core-shell nanoparticles comprising silica and, in
this context, attention has been devoted to the synthesis of block
copolymer-templated silica structures, and studies of their
properties and possible uses. Moreover, the presence of
core-forming materials which allowed for the possibility of
achieving triggered release of active materials from the core of
the particles could offer significant opportunities.
[0004] It is known that biomineralisation of silica, or
biosilicification, occurs in water under ambient conditions for
various biological systems, such as diatoms and sponges. Moreover,
this natural process leads to hierarchical structures and multiple
morphologies with precise nanoscale control, features which
continue to elude materials scientists. Ideally, any biomimetic
approach to silica synthesis would be both environmentally benign
and controllable, in order to allow for the generation of a range
of structures and morphologies.
[0005] Recent improvements in the understanding of
biosilicification have resulted in some studies which have
successfully demonstrated silica formation under ambient
conditions.
[0006] Furthermore, it is well known that block copolymers can
self-assemble into a wide range of nanostructures that can be used
for controlling the formation of various inorganic materials.
However, block copolymer-mediated silica formation is seldom
reported. Moreover, the production of such particles in a
chemically efficient manner that allow for morphological and
structural control remains a major challenge.
[0007] Silica-based core-shell nanoparticles have been suggested
for various bioanalytical applications, such as drug delivery,
bioimaging and biolabeling. In such cases, the particles have been
previously synthesised by coating functional cores with silica
shells either by using Stober chemistry or by means of a
microemulsion approach. Both methods do, however, require the use
of non-ideal conditions, such as elevated temperatures,
non-physiological pH values, and the presence of large amounts of
surfactants and/or organic co-solvents.
[0008] It is apparent, therefore, that there is scope for the
development of alternative nano-sized particles, which may be
obtained using convenient reaction conditions.
[0009] According to a first aspect of the present invention, there
is provided a composition comprising core-shell nanoparticles,
wherein said nanoparticles comprise:
[0010] (a) cationic core material comprising polymer; and
[0011] (b) shell material comprising silica.
[0012] Preferably, the core material comprises copolymer micelles,
more preferably diblock copolymer micelles. Most preferably, said
diblock copolymer micelle has a core comprising at least one block
of a first polymer and a corona comprising at least one block of a
second polymer, wherein said second polymer is different to said
first polymer.
[0013] Preferably, said copolymer comprises a first polymer and a
second polymer which both comprise amino-based (alk)acrylate
monomer units, more preferably tertiary amino-based (alk)acrylate
units, most preferably tertiary aminoalkyl (alk)acrylate units.
Particularly preferably, said (alk)acrylate units comprise acrylate
or, more particularly, methacrylate units.
[0014] In preferred embodiments, said tertiary aminoalkyl
methacrylate units comprise dialkylaminoalkyl methacrylate units,
especially dialkylaminoethyl methacrylate units. In a particularly
preferred embodiment, said copolymer comprises
poly[2-(diisopropylamino)ethyl
methacrylate)-block-2-(dimethylamino)ethyl methacrylate]
(PDPA-PDMA).
[0015] According to the invention, said micelles may either be
non-crosslinked or shell crosslinked (SCL) micelles based on said
polymers. Thus, especially preferred embodiments envisage
non-crosslinked or shell crosslinked micelles based on tertiary
amine methacrylate-derived block copolymers such as
poly[2-(diisopropylamino)ethyl
methacrylate)-block-2-(dimethylamino)ethyl methacrylate].
[0016] The conventional synthetic route to shell crosslinked
micelles involves covalent stabilization of the micelle coronal
chains, although polyion crosslinking has also been recently
suggested. However, there are no literature reports of micelle
shell cross-linking via biomineralization.
[0017] In the present invention, crosslinking of the micelles of
said tertiary amino-based (alk)acrylate copolymers is most
conveniently achieved by partially or fully quaternising the
tertiary amino groups of said copolymers with bifunctional
quaternising agents. Thus, in the case of the most preferred
embodiment of the first aspect of the invention, partial
crosslinking of poly[2-(diisopropylamino)ethyl
methacrylate)-block-2-(dimethylamino)ethyl methacrylate]
(PDPA-PDMA) may be achieved by selective
quaternisation/crosslinking of the PDMA chains with a suitable
bifunctional quaternising agent, for example a
bis(haloalkoxy)alkane, such as 1,2-bis-(iodoethoxy)ethane (BIEE).
In this most preferred embodiment, the PDPA chains remain
essentially unquaternised.
[0018] The invention also envisages analogous non-crosslinked
quaternised derivatives, wherein quaternisation is achieved by
means of monofunctional quaternising agents, such as alkyl halides,
in particular alkyl iodides such as iodomethane. However, it is
believed that control of the silica deposition process may be
enhanced in the case of crosslinked materials.
[0019] The degree of polymerisation of the polymer is preferably
controlled within specified limits. Thus, in the most preferred
embodiment of the invention, the degree of polymerisation of the
PDPA-PDMA copolymer is preferably controlled such that the mean
degree of polymerisation of the PDPA falls in the range of 20-25
and the mean degree of polymerisation of the PDMA falls in the
range of 65-70, with particularly favourable results having been
obtained with the PDPA.sub.23-PDMA.sub.68 copolymer, wherein the
subscripts denote the mean degrees of polymerisation of each block.
In the said embodiment, PDPA units form the cores of the micelles
and PDMA units form the coronas of the micelles.
[0020] Preferably, said shell material comprises silica which is
deposed on said core material from at least one silica precursor.
Optionally, said at least one silica precursor may comprise an
inorganic silicate, for example an alkali metal silicate, such as
sodium silicate. However, preferred silica precursors comprise
organosilicate compounds, especially alkyl silicates such as
tetramethyl orthosilicate or tetraethyl orthosilicate. Most
preferably, said silica precursor comprises tetramethyl
orthosilicate. Said treatment is found to effectively crosslink the
copolymer chains in uncrosslinked micelles, and thereby stabilise
the micelles towards dissociation.
[0021] Preferably, said nanoparticles have a particle size in the
region of from 10-100 nm, more preferably from 20-50 nm, most
preferably from 30-40 nm and, particularly preferably, the particle
size is around 30 nm.
[0022] Preferably the nanoparticles have an average specific size g
(where g=1/2.times.(length+width)) of about 300 nm or less. More
preferably the particles have an average size of about 200 nm or
less. Even more preferably the particles have an average size of
about 100 nm or less. Preferably the particles have an average size
of 1 nm or more. More preferably the particles have an average size
of about 10 nm or more.
[0023] Preferably the average specific size of the void is 1 nm or
more, more preferably 3 nm or more, even more preferably 6 nm or
more. Preferably the average specific size of the void is 100 nm or
less, more preferably 80 nm or less, even more preferably 70 nm or
less.
[0024] Preferably the shell is at least 1 nm thick, more preferably
at least 5 nm, even more preferably at least 10 nm. Preferably the
shell is 75 nm thick or less, more preferably 50 nm or less, even
more preferably 25 nm or less.
[0025] In a particular embodiment of the first aspect of the
invention there is provided a composition comprising core-shell
nanoparticles, wherein said nanoparticles comprise:
[0026] (a) cationic core material comprising a copolymer micelle;
and
[0027] (b) shell material comprising silica
wherein said nanoparticles have an anisotropic rod-like morphology.
Preferably, in said embodiment of the invention, said copolymer
micelle comprises a diblock or triblock copolymer.
[0028] According to a second aspect of the present invention, there
is provided a method for the preparation of a composition
comprising core-shell nanoparticles according to the first aspect
of the invention, said method comprising the steps of:
[0029] (a) preparing a cationic core material comprising polymer;
and
[0030] (b) coating said core material with a shell comprising
silica.
[0031] The polymeric core material may be prepared by any suitable
polymerisation technique, but particularly favourable results are
achieved when employing methods such as group transfer
polymerisation and controlled radical polymerisation. Said core
material is then coated with silica by treatment with a suitable
silica precursor.
[0032] The method according to the second aspect of the invention
is particularly suited to the preparation of the compositions
comprising core-shell nanoparticles according to the more preferred
and most preferred embodiments of the first aspect of the
invention. Thus, particularly preferred embodiments envisage the
preparation of cationic diblock copolymers by sequential monomer
addition using group transfer polymerisation of tertiary aminoalkyl
methacrylates.
[0033] Full or partial quaternisation of said copolymers may be
achieved by any of the standard quaternisation techniques reported
in the literature. Typically, therefore, treatment of said tertiary
amino-based copolymers with alkyl halides, most particularly alkyl
iodides such as iodomethane, in suitable inert solvents facilitates
the preparation of non-crosslinked quaternised derivatives, whilst
crosslinked quaternised copolymers are obtained by treatment of the
tertiary amino copolymers with bifunctional quaternising agents
such as bis(haloalkoxy)alkanes, for example
1,2-bis-(iodoethoxy)ethane, in appropriate inert solvents.
Typically, said quaternisation reactions are carried out by
treating the tertiary amino copolymers with quaternising agents at
or around ambient temperature (20-30.degree. C.), preferably about
25.degree. C., for a period of time of between 1-100 hours,
preferably between 24 and 72 hours.
[0034] Deposition of silica is carried out by simply treating the
cationic polymers with suitable silica precursors under mild
conditions. Thus, in the case of the preferred copolymer micelles,
these materials may be stirred with a silica precursor, typically
an organosilicate compound, especially an alkyl silicate such as
tetraethyl orthosilicate or, most preferably, tetramethyl
orthosilicate, for between 10 and 60 minutes at 5-30.degree. C. and
a pH of between 6.2 and 9.0. In a typical reaction, PDPA-PDMA
copolymer micelles may be treated with tetramethyl orthosilicate
for 20 minutes at 20.degree. C. and pH 7.2. The method of the
second aspect of the present invention does, in this regard, offer
significant advantages over the methods of the prior art, which
require that silica deposition procedures should be carried out at
low pH values, and typically at pH 1.
[0035] According to a third aspect of the present invention, there
is provided a composition adapted to facilitate controlled delivery
of at least one active agent into a system, said composition
comprising core-shell nanoparticles according to the first aspect
of the invention, wherein said composition is adapted to provide
said controlled delivery in response to controlled changes in the
pH of said system.
[0036] According to a fourth aspect of the present invention, there
is provided a method for facilitating controlled delivery of at
least one active agent into a system, said method comprising
introducing a composition according to the third aspect of the
invention into said system and changing the pH of the system in a
controlled manner so as to facilitate said delivery.
[0037] Preferred examples of said active agent include, for
example, drugs, dyes and catalysts, and suitable systems into which
they might be delivered include such diverse examples as human and
animal bodies, coatings and chemical reactors. In the case of the
most preferred compositions according to the first aspect of the
invention, wherein said compositions comprise copolymers which
comprise tertiary amine-based alkyl (meth)acrylate units,
controlled delivery of active agents may be achieved by introducing
said composition into a system and adjusting the pH of a system to
a value of less than 6 by addition of a suitable acidic agent.
[0038] According to a further aspect of the present invention,
there is provided a thin-film coating comprising the present
nanoparticles. As used herein, "thin-film" refers to coatings
having an average thickness of 500 nm or less.
[0039] According to a further aspect of the present invention,
there is provided an optical coating comprising the present
nanoparticles. As used herein, the term "optical coatings" refers
to coatings with an optical function as major functionality.
Examples of optical coatings include those designed for
anti-reflective, anti-glare, anti-dazzle, anti-static, EM-control
(e.g. UV-control, solar-control, IR-control, RF-control etc.)
functionalities. Preferably the present coatings have an
anti-reflective functionality. More preferably the present coatings
are such that, when measured for one coated side at a wavelength
between 425 and 675 nm (the visible light region), the minimum
reflection is about 2% or less, preferably about 1.5% or less, more
preferably about 1% or less.
[0040] It will be apparent that it may be necessary to remove some
or all of the core material from the particle in order to achieve
some of the benefits of the present particles. This may be achieved
in any suitable manner at any suitable point in the production
process. Preferred methods include, for example, thermodegradation,
photodegradation, solvent washing, electron-beam, laser, catalytic
decomposition, and combinations thereof. Therefore, the scope of
the present invention encompasses core-shell nanoparticles where
the core is present and where the core has been at least partially
removed.
[0041] Throughout the description and claims of this specification,
the words "comprise" and "contain" and variations of the words, for
example "comprising" and "comprises", means "including but not
limited to", and is not intended to (and does not) exclude other
moieties, additives, components, integers or steps.
[0042] Throughout the description and claims of this specification,
the singular encompasses the plural unless the context otherwise
requires. In particular, where the indefinite article is used, the
specification is to be understood as contemplating plurality as
well as singularity, unless the context requires otherwise.
[0043] Features, integers, characteristics, compounds, chemical
moieties or groups described in conjunction with a particular
aspect, embodiment or example of the invention are to be understood
to be applicable to any other aspect, embodiment or example
described herein unless incompatible therewith.
[0044] The invention will be described in further detail with
particular reference to the accompanying drawings, in which:
[0045] FIG. 1 schematically shows the formation of core-shell
silica nanoparticles obtained by biomineralization of tetramethyl
orthosilicate (TMOS) using either shell crosslinked (SCL) or
non-crosslinked cationic block copolymer micelles as templates.
Both routes lead to well-defined, core-shell copolymer-silica
nanoparticles. The use of non-crosslinked micelles, as shown in the
upper route, additionally leads to in situ silica crosslinking.
[0046] FIG. 2 presents TEM images of copolymer-silica
nanoparticles: (A) synthesised by directly using non-quaternised
PDPA.sub.23-PDMA.sub.68 copolymer micelles as templates; and (B)
formed using partially quaternised copolymer micelles (50% with
respect to the PDMA shell); the inset in (B) is a typical high
magnification image obtained after dispersing the same particles
directly into an acidic solution (pH 2). The scale bars are 100
nm.
[0047] FIG. 3 displays TEM images obtained for: (A) core-shell
copolymer-silica nanoparticles prepared by stirring a mixture
containing 2.0 ml of a 0.25 w/v % aqueous solution of partially
quaternised shell crosslinked micelles [30% target degree of
crosslinking for the PDMA chains] solution and 2.0 ml TMOS for 40
minutes (the top inset shows a representative hollow silica
nanoparticle after pyrolysis of the copolymer component by
calcination at 800.degree. C.; the lower inset highlights a typical
core-shell particle); (B) core-shell copolymer-silica nanoparticles
formed using partially quaternised SCL micelles (50% target degree
of crosslinking with respect to the PDMA chains) using the same
biomineralisation conditions as those employed in (A); (C)
core-shell copolymer-silica nanoparticles formed 40 minutes after
stirring an initially homogeneous solution comprising 2.0 ml of a
0.25 w/v % aqueous solution of partially quaternised SCL micelles
[30% target degree of crosslinking for the PDMA chains], 2.0 ml
TMOS and 2.0 ml methanol; and (D) core-shell copolymer-silica
nanoparticles formed 120 minutes after stirring using the same
conditions as described in (C). The scale bars are 50 nm in each
case.
[0048] FIG. 4 shows the particle size distribution of the
core-shell copolymer-silica nanoparticles prepared from the
PDPA.sub.23-PDMA.sub.68 copolymer (50% quaternised coronal PDMA
chains using iodomethane) estimated from the TEM image shown in
FIG. 2B. These particles have a TEM number-average diameter of
28.+-.3 nm and an intensity-average diameter of 34 nm, as judged
from DLS measurements.
[0049] FIG. 5 shows a transmission electron micrograph of silica
nanoparticles obtained from micelle templates prepared using the
quaternised PDPA.sub.23-PDMA.sub.68 copolymer (100% quaternisation
of the PDMA chains), using biomineralization conditions which were
the same as those used for templating micelles prepared with the
50% quaternised copolymer; in this case there appears little or no
evidence for the formation of core-shell copolymer-silica
nanoparticles, and silification appears to occur throughout the
micelle interior.
[0050] FIG. 6 shows a transmission electron micrograph of silica
nanoparticles (the same particles as shown in FIG. 2B, formed by
50% quaternised PDPA.sub.23-PDMA.sub.68 micelles) after dispersing
in acidic solution at pH 2 with the aid of an ultrasonic bath.
[0051] FIG. 7 illustrates .sup.1H NMR spectra of: (a) a molecular
solution of the PDMA.sub.68-PDPA.sub.23 diblock copolymer (50%
quaternised PDMA block using iodomethane) in D.sub.2O/DCl at pH 2
(signal G at .delta. 1.3-1.4 is due to the four equivalent methyl
groups of the protonated DPA residues); (b) micelles for the same
copolymer obtained in D.sub.2O at pH 7 (there is no longer a G
signal at .delta. 1.3-1.4 due to the DPA residues since the PDPA
block becomes deprotonated and forms hydrophobic micelle cores at
this pH; (c) silica-coated nanoparticles derived from
PDPA.sub.23-PDMA.sub.68 diblock copolymer micelles (50% quaternised
PDMA block) in D.sub.2O at pH 2 (the signal G at .delta. 1.3-1.4
corresponds to the protonated PDPA chains within the micelle
cores); and (d) the same silica-coated nanoparticles in D.sub.2O at
pH 7 (signal G at .delta. 1.3-1.4 disappears, indicating that the
PDPA chains in the micelle cores become hydrophobic due to
deprotonation).
[0052] FIG. 8 shows the TEM particle size distribution of the
hybrid silica nanoparticles (as shown in FIG. 3A; prepared using
SCL micelles at a target degree of crosslinking for the PDMA chains
of 30%); these core-shell copolymer-silica nanoparticles have a
number-average diameter of 32.+-.5 nm and an intensity-average
diameter of 35 nm from DLS measurements.
[0053] FIG. 9 shows Transmission Electron Micrographs of core-shell
copolymer-silica nanoparticles obtained by stirring a mixture
containing 2.0 ml of a 0.25 wt. % aqueous solution of partially
quaternised (50% iodomethane-quaternised with respect to the PDMA
shell) copolymer micelles and either (images A, B) 58 mg or (images
C, D) 116 mg of TMOS at 20.degree. C. for 20 minutes at pH 7.2.
[0054] FIG. 10 shows Transmission Electron Micrographs of
core-shell copolymer-silica nanoparticles obtained by stirring a
mixture containing 2.0 ml of a 0.25 wt. % aqueous solution of
partially quaternised copolymer micelles (50% target degree of
crosslinking with respect to the PDMA shell, using BIEE for
quaternisation) and either (images A, B) 58 mg or (images C, D) 116
mg of TMOS at 20.degree. C. for 20 minutes at pH 7.2.
[0055] FIG. 11 shows TEM images taken after silica deposition using
PDPA.sub.23-PDMA.sub.68 diblock copolymer micelles with higher
copolymer concentrations, wherein copolymer-silica core-shell
nanoparticles were obtained by stirring a mixture containing 1.0 ml
of either 1 wt. % or 2 wt. % aqueous solutions of copolymer
micelles 50% quaternised with iodomethane [with respect to the PDMA
chains only] with either 116 mg or 232 mg of TMOS at 20.degree. C.
for 20 minutes at pH 7.2, then diluting the particles with 40 ml
ethanol and centrifuging at 16,000 rpm for 30 minutes, and finally
redispersing in ethanol with the aid of an ultrasonic bath. This
centrifugation-redispersion cycle was repeated to ensure removal of
excess TMOS and unreacted silicic acid oligomers.
[0056] FIG. 12 presents TEM images from silica deposition processes
with PDPA.sub.23-PDMA.sub.68 diblock copolymer micelles after much
longer deposition times, wherein copolymer-silica core-shell
nanoparticles were obtained by stirring a mixture containing 2.0 ml
of a 0.25 wt. % aqueous solution of copolymer micelles 50%
quaternised with iodomethane [with respect to the PDMA chains] with
58 mg of TMOS at 20.degree. C. for 8 hours at pH 7.2, and then
subjecting the particles to two ethanol washing and centrifugation
cycles (16,000 rpm, 30 minutes).
[0057] FIG. 13 illustrates FT-IR spectra recorded for: (a) the
precursor PDPA.sub.23-PDMA.sub.68 diblock copolymer; (b)
copolymer-silica core-shell nanoparticles obtained after silica
deposition onto shell crosslinked micelles obtained from the
PDPA.sub.23-PDMA.sub.68 diblock copolymer (target degree of
crosslinking=30% using BIEE) under the stated conditions (see FIG.
3A); and (c) hollow silica nanoparticles obtained after pyrolysis
of the copolymer by calcination at 800.degree. C.; the FT-IR
spectrum of the copolymer-silica core-shell nanoparticles contains
IR bands that are characteristic of both the silica network (1080
cm.sup.-1, multiplet corresponding to Si--O stretching; 950
cm.sup.-1, Si--OH vibration mode; 800 cm.sup.-1, Si--O--Si bending;
470 cm.sup.-1, Si--O bending) and also the copolymer (the carbonyl
ester stretch at 1730 cm.sup.-1); this latter carbonyl band
disappears after calcination of the copolymer, as expected,
suggesting the formation of hollow silica particles, an eventuality
which is confirmed by the results of TEM studies.
[0058] FIG. 14 shows zeta potential vs. pH curves obtained for the
original SCL micelles prepared from the PDPA.sub.23-PDMA.sub.68
diblock copolymer at a target degree of crosslinking of 30% for the
PDMA coronal chains (circles) and the final copolymer-silica
core-shell particles synthesised using a mixture of 2.0 ml of a
0.25 wt. % SCL micelle solution (target degree of crosslinking=30%)
and 2.0 ml TMOS for 40 minutes (squares); for comparative purposes,
the zeta potential curve obtained for an ultrafine commercial 20 nm
silica sol (Nyacol 2040) is also shown (triangles).
[0059] FIG. 15 shows the transmission electron micrograph of
Au/silica nanoparticles obtained by protonating the PDPA chains in
the cores of the silica-coated micelles using HAuCl.sub.4, followed
by in situ reduction using NaBH.sub.4; this experiment confirms
that the PDPA chains remain located within the micelle cores after
silica deposition, as expected.
[0060] FIG. 16 schematically shows the synthesis of an ABC triblock
copolymer based on poly(ethylene oxide) (PEO), PDMA and PDPA,
wherein a PEO.sub.45-PDMA.sub.29-PDPA.sub.76 triblock copolymer was
synthesized by Atom Transfer Radical Polymerisation (ATRP) using a
PEO-based macro-initiator (PEO.sub.45-Br macro-initiator), via a
PEO.sub.45-PDMA.sub.29 diblock copolymer.
[0061] FIG. 17 shows the .sup.1H NMR spectrum of the
PEO.sub.45-PDMA.sub.29-PDPA.sub.76 triblock copolymer recorded in
d.sub.5-pyridine.
[0062] FIG. 18 presents TEM images for silica rods wherein silica
deposition was performed at 1.0% copolymer concentration, the
resulting silica rods being easily (re)dispersed by
ultrasonication.
[0063] FIG. 19 illustrates comparative zeta potential vs. pH curves
obtained for the original copolymer rods prepared from the
PEO.sub.45-PDMA.sub.29-PDPA.sub.76 triblock copolymer (shown as
squares), and the final silica rods synthesised using a mixture of
1.0 ml of a 1.0 wt. % copolymer micelle solution and 0.20 g TMOS
for 20 min (shown as triangles); for comparative purposes, the zeta
potential curve obtained for an ultrafine commercial 20 nm silica
sol (Nyacol 2040) is also shown (as circles).
[0064] Particularly favourable results have been achieved with
compositions based on selectively quaternised non-crosslinked and
shell crosslinked micelles derived from tertiary amine
methacrylate-based block copolymers, a specific example being
poly[2-(diisopropylamino)ethyl
methacrylate)-block-2-(dimethylamino)ethyl methacrylate]
(PDPA-PDMA), and such materials have proved to be particularly
successful when used as templates for the biomimetic formation of
well-defined copolymer-silica nanoparticles of less than 50 nm
diameter. Diblock copolymer micelles comprising either partially or
fully quaternised poly(2-(dimethylamino)ethyl methacrylate) (PDMA)
coronas and hydrophobic poly(2-(diisopropylamino)ethyl
methacrylate) (PDPA) cores in particular have been used as
nano-sized templates for the deposition of silica from aqueous
solution under mild conditions, i.e. at pH 7.2 and 20.degree.
C.
[0065] PDPA-PDMA diblock copolymers of this type are relatively
easy to synthesise over a range of block compositions and copolymer
molecular weights using any suitable method such as group transfer
polymerisation or controlled radical polymerisation. Such diblock
copolymers dissolve molecularly in acidic solution due to
protonation of both polyamine blocks. On adjustment of the solution
pH with aqueous base, micellar self-assembly occurs at around
neutral pH; the deprotonated hydrophobic PDPA chains form the
micelle cores and the cationic (protonated) PDMA chains form the
micelle coronas. Alternatively, and depending upon the precise
block composition under investigation and the degree of
quaternisation, selected diblock copolymers can be dissolved
directly in water at around neutral pH to form well defined
micelles.
[0066] Both non-crosslinked and SCL micelles of this type can be
coated with silica without loss of colloid stability. Silica
deposition on the SCL micelles is primarily confined to the
cationic PDMA shell, leading to core-shell copolymer-silica
nanoparticles with pH-responsive PDPA cores. Moreover, in situ
silica deposition effectively stabilises the uncrosslinked
PDPA-PDMA micelles, which remain intact on lowering the solution
pH, whereas the original PDPA-PDMA micelles are found to dissociate
to give individual copolymer chains in acidic solution.
[0067] In a further embodiment of the invention, it has been shown
that a poly(ethylene oxide)-PDMA-PDEA triblock copolymer
facilitates the preparation of highly anisotropic rod-like silica
particles.
[0068] Shell crosslinking of these micelles can be readily achieved
at high dilution using 1,2-bis-(2-iodoethoxy)ethane (BIEE) as a
bifunctional quaternising reagent under mild conditions. BIEE
quaternises the PDMA chains selectively, leaving the much less
reactive PDPA chains untouched.
[0069] The general approach to the preparation of the compositions
according to the first aspect of the invention is shown in FIG. 1,
from which it will be gleaned that the thickness of the deposited
silica shell differs according to whether or not the copolymer
micelle incorporates crosslinking. The degree of quaternisation of
the PDMA block can also be an important factor. The PDMA shell has
significant cationic character due to either protonation and/or
quaternisation, so it can act both as a polymeric catalyst and also
as a physical scaffold for silica formation. Tetramethyl
orthosilicate (TMOS) was employed as a silica precursor and
biomineralization was conducted in aqueous solution at 20.degree.
C. at around neutral pH.
[0070] Thus, in the first approach, a PDPA.sub.23-PDMA.sub.68 block
copolymer is either partially or fully quaternised by treatment
with iodomethane in tetrahydrofuran at 20.degree. C. for 24 hours,
and non-crosslinked micelles are formed by dissolution at pH 2 and
adjustment of the pH to 7.2; finally, silica deposition occurs on
treatment of the micelles with tetramethyl orthosilicate for 10-40
minutes at room temperature and pH 7.2, resulting in the formation
of silica crosslinked nanoparticles having a relatively thick
silica shell when using a relatively large excess of TMOS.
[0071] Alternatively, micelles are formed by dissolution of the
PDPA.sub.23-PDMA.sub.68 block copolymer at pH 2 and adjustment of
the pH to 7.2, and the micelles are then shell crosslinked by
quaternisation by treatment with 1,2-bis-(2-iodoethoxy)ethane
(BIEE) at 20.degree. C. for 72 hours; silica deposition is then
carried out by treatment of the crosslinked micelles with
tetramethyl orthosilicate for 10-40 minutes at room temperature and
pH 7.2, resulting in the formation of silica nanoparticles having a
relatively thin silica shell when using a relatively large excess
of TMOS.
[0072] Initially, the present inventors carried out silica
deposition using non-crosslinked micelles prepared directly from
the PDPA.sub.23-PDMA.sub.68 copolymer precursor as templates.
Dynamic light scattering (DLS) studies indicated an
intensity-average diameter of 37 nm at 25.degree. C. for these
micelle templates. At pH 7.2 the PDMA chains in the micelle shell
are approximately 50% protonated, and therefore have appreciable
cationic character..sup.24
[0073] Silicification of the said micelles was achieved by mixing
2.0 ml of an aqueous micelle solution (0.25 w/v % at pH 7.2) with
1.0 ml tetramethyl orthosilicate, and then stirring the initially
heterogeneous solution under ambient conditions for 20 minutes. The
silica-coated nanoparticles thus obtained were washed with ethanol,
then subjected to three centrifugation/redispersion cycles at
16,000 rpm for 5 minutes. Redispersal of the sedimented
nanoparticles was subsequently achieved with the aid of an
ultrasonic bath.
[0074] Thermogravimetric analyses of the product indicated that the
mean diblock copolymer content of the silica nanoparticles was
about 15% by mass. A typical Transmission Electron Micrograph (TEM)
image obtained for these TMOS-treated micelles is shown in FIG. 2A.
The formation of templated silica nanoparticles with core-shell
structures is clearly observed, since the silica/PDMA hybrid shell
is more electron-dense than the PDPA chains within the micelle
cores. These nanoparticles have a number-average diameter of around
35 nm, which is in reasonably good agreement with the dimensions of
the precursor micelles. However, in addition to the formation of
templated silica nanoparticles, some ill-defined, non-templated
silica structures are also observed in FIG. 2A, indicating that the
silica formation is not particularly well controlled in this case.
Ideally, silica formation should occur exclusively on the cationic
copolymer micelles, rather than in bulk solution.
[0075] Improved control over silica deposition was, however,
achieved when employing quaternised polymers. Initial trial
experiments were conducted using PDMA homopolymer, and it was found
that on mixing 1.0 ml tetramethyl orthosilicate and 1.0 ml aqueous
PDMA homopolymer solution (concentration of DMA repeat units,
[DMA]=0.064 M) at pH 7.2 and 20.degree. C., the initially
heterogeneous solution became homogeneous after continuous stirring
for 15 minutes (hydrolysis of TMOS, which produces silicic acid,
allows the system to become homogeneous). By way of contrast, for
50% and 100% quaternised PDMA homopolymers under identical
conditions, the corresponding times required for the reaction
solutions to become homogeneous were 25 minutes and 50 minutes,
respectively. This suggests that quaternised PDMA chains catalyse
slower, and therefore perhaps more controlled, hydrolysis of the
TMOS precursor.
[0076] These experiments with PDMA homopolymer suggested that
well-controlled silica deposition might be achieved using partially
or fully quaternised PDPA.sub.23-PDMA.sub.68 copolymer micelles as
templates. Thus, experiments were conducted wherein selective
quaternisation of DMA residues was achieved using iodomethane under
mild conditions. A 0.25 wt. % aqueous solution of
PDPA.sub.23-PDMA.sub.68 copolymer micelles in which the PDMA chains
were 50% quaternised had an intensity-average diameter of 29 nm at
pH 7.2, as indicated by Dynamic Light Scattering (DLS). Tetramethyl
orthosilicate (1.0 ml) was added to 2.0 ml of the aqueous micelle
solution at 20.degree. C., and silica deposition was allowed to
continue for 20 minutes, with continuous stirring, prior to
isolation via centrifugation.
[0077] TEM images of the purified core-shell copolymer-silica
nanoparticles obtained are shown in FIG. 2B. Core-shell
nanostructures were clearly observed, with a number-average
diameter of 28.+-.3 nm. DLS studies indicated an intensity-average
diameter of 34 nm and a relatively narrow size distribution, as
illustrated in FIG. 4. In contrast to the results obtained for the
non-quaternised diblock precursor, there was no evidence for
non-templated silica structures in this case, suggesting that
secondary nucleation had been minimised.
[0078] TEM results obtained using micelles with 100% quaternised
PDMA blocks are shown in FIG. 5, from which it is apparent that
there is little or no evidence of the formation of a copolymer
core, thus confirming that partially quaternised copolymers are a
particularly preferred embodiment of the present invention.
Thermogravimetric analyses, however, indicated that the mean
diblock copolymer contents of the silica nanoparticles derived from
micelles with 50% and 100% quaternised PDMA blocks were about 18%
and 16% by mass, respectively. Thus, quaternisation of the PDMA
chains does appear to be beneficial for well-controlled silica
deposition. Moreover, these quaternised micelles produced hybrid
nanoparticles with much thicker, more well defined silica shells
relative to those obtained using non-quaternised copolymer micelles
(see FIG. 2A) under the same biomineralisation conditions.
[0079] The present inventors have also established that the
nanostructure of these copolymer-silica core-shell particles can be
simply controlled by tuning the amount of TMOS used for silica
deposition. Thus, for example, silica particles with thin shells
and large copolymer cores were obtained when using lower levels of
TMOS. Well-defined silica particles with a number-average diameter
of around 26 nm (see FIGS. 9A/9B) were formed by stirring a mixture
of 58 mg TMOS with 2 ml of a 0.25 w/v % solution of 50% quaternised
copolymer micelles for 20 minutes. As shown in Table 1,
thermogravimetric analysis of the product indicated that the mean
copolymer content of these core-shell copolymer-silica particles
was about 28% by mass, indicating a silica conversion of about 58%.
Such particles have much thinner silica shells and larger copolymer
cores. Moreover, colloidal stability was maintained even when the
reaction time was increased from 20 minutes to 8 hours when using
this reduced amount of TMOS (see FIGS. 12A/12B). The results
obtained when increasing the quantity of TMOS in the above
synthesis to 116 mg are shown in FIGS. 9C/9D. Again, there is no
evidence for non-templated silica (such as that observed in FIG.
2B), indicating efficient templating of these silica
nanostructures. Further thermogravimetric analyses indicated that
these core-shell copolymer-silica nanoparticles had lower copolymer
contents (23%) compared to the core-shell copolymer-silica
nanoparticles shown in FIGS. 9A/9C (28% copolymer content). This
indicates that higher levels of TMOS lead to more silica deposition
under otherwise identical conditions.
[0080] TEM studies provided further evidence of efficient micelle
crosslinking via biomineralisation. As shown in FIG. 2B (see inset)
and FIG. 6, the silica crosslinked micelles retain their spherical
core/shell structures after direct dispersion and drying at pH 2.
.sup.1H NMR studies of the core-shell copolymer-silica
nanoparticles at pH 2 produced a signal at .delta. 1.3-1.4 due to
the protonated PDPA chains (see FIG. 7). When the solution pH was
increased to pH 7, however, this signal disappeared as the PDPA
chains became deprotonated and hence hydrophobic. Thus, these
spectroscopic studies confirmed that the PDPA chains in the micelle
cores are pH-responsive (i.e. they can become hydrophilic at low pH
and hydrophobic at high pH), and this further illustrates the
potential use of these new core-shell copolymer-silica
nanoparticles in encapsulation/controlled release applications.
[0081] Typically, shell crosslinking is conducted at high dilution
(normally less than 0.5 wt. % copolymer micelles) in order to avoid
inter-micelle fusion. However, micelle crosslinking by biomimetic
silica deposition can be successfully performed at somewhat higher
concentrations. Thus, as shown in FIGS. 11A/11B, the mixing of 1 ml
of a solution of 1.0 w/v % copolymer micelles (50% quaternised with
respect to the PDMA shell) with 116 mg TMOS for 20 minutes produced
well-defined hybrid copolymer-silica core-shell particles with a
number average diameter of about 26 nm. Similar-sized particles
were also obtained using 2.0 w/v % copolymer micelles (see FIGS.
11C/11D). Thermogravimetric analyses (Table 1) indicated that the
mean copolymer contents of the copolymer-silica core-shell
particles shown in FIGS. 11A/11B and 11C/11D were about 20 and 22%
by mass, respectively, indicating silica conversions of 87 and 78%,
respectively. Thus this biomimetic approach to SCL micelles by
silica deposition appears to be notably efficient, and to offer
particular advantages in terms of mild reaction conditions, fast
reaction times and relatively inexpensive reagents when compared
with the methods of the prior art.
[0082] The present inventors also prepared SCL micelles by
selective quaternisation and crosslinking of the PDMA chains using
1,2-bis-(2-iodoethoxy)ethane, and evaluated the resulting cationic
micelles as templates for silica deposition. The target degree of
crosslinking for the PDMA coronal chains was 30 mol %. DLS studies
conducted at 25.degree. C. indicated an intensity-average micelle
diameter of 37 nm for the precursor SCL micelles.
[0083] Biomineralisation was performed using tetramethyl
orthosilicate under the same conditions as those employed for
non-crosslinked micelles. FIG. 3A shows a typical TEM image of the
resulting silica nanoparticles. Their intensity-average and
number-average diameters from DLS and TEM are 35 nm and 32.+-.5 nm
(see FIG. 8), respectively, which are in reasonably good agreement
with the values obtained for the SCL micelle precursor.
Furthermore, their core-shell structure is also clearly evident.
For example, the silica nanoparticle indicated by the lower white
square in FIG. 3A has a PDPA core of approximately 14 nm and a
silica/PDMA hybrid shell thickness of around 11 nm.
Biomineralisation studies with SCL micelles prepared at a target
degree of crosslinking of 50% produced similar results, as shown in
FIG. 3B. Compared to the silica nanoparticles prepared using
non-crosslinked micelles (FIG. 2A), the silica particles obtained
from SCL micelle precursors have larger cores and thinner shells.
In addition, there is no evidence for non-templated silica within
the dispersion, indicating that silica deposition is again
well-controlled.
[0084] Silica deposition was also performed at lower levels of
TMOS. Thus, on mixing a 2 ml aliquot of a 0.25 w/v % copolymer
micelle solution (50% target degree of crosslinking using BIEE)
with 58 mg TMOS for 20 minutes, silica deposition led to
aggregation, rather than a colloidally stable dispersion. TEM
studies indicated the formation of core-shell silica particles of
about 17 nm, as well as interconnected, fused primary particles
(see FIGS. 10A/10B). Thermogravimetric analyses (see Table 1)
indicated a mean copolymer content of around 30% by mass,
indicating a silica conversion of approximately 50%. The formation
of silica nanoparticles was much improved by using a slight excess
of TMOS under the same conditions. Hence, mixing 2 ml of a 0.25 w/v
% copolymer micelle solution (50% target degree of crosslinking
using BIEE) with 116 mg TMOS for 20 minutes produced a colloidally
stable dispersion, as judged by visual inspection. As shown in
FIGS. 10C/10D, hybrid copolymer-silica particles with a
number-average diameter of about 20 nm were obtained.
Thermogravimetric analyses indicated a mean copolymer content of
about 24% by mass, indicating a silica conversion of around
35%.
[0085] Silica deposition can be also controlled using SCL micelles
under initially homogeneous conditions. Thus, a 2.0 ml aliquot of a
0.25 wt. % SCL micelle solution was added to a mixture of 2.0 ml
methanol and 2.0 ml tetramethyl orthosilicate, wherein the methanol
acted as a co-solvent and ensured that the TMOS was miscible with
the aqueous phase from the beginning of the reaction. After
continuing silica deposition for 40 minutes, TEM studies of the
obtained product, as illustrated in FIG. 3C, confirmed the expected
formation of well-defined core-shell copolymer-silica
nanoparticles. Even after continuing the treatment for 120 minutes,
however, no evidence for non-templated silica nanostructures was
observed, as shown in FIG. 3D.
[0086] The SCL micelle-derived core-shell copolymer-silica
nanoparticles shown in FIG. 3A were further characterised using
thermogravimetric analyses, FT-IR spectroscopy and aqueous
electrophoresis. Thermogravimetric analyses indicated that the mean
copolymer content of the copolymer-silica particles was about 19%
by mass, whilst the FT-IR studies, illustrated in FIG. 13,
confirmed silica formation, since bands were observed at 1080, 950,
800 and 470 cm.sup.-1 for these particles, due to the presence of
the inorganic component; these bands were found to be absent in the
spectra obtained for the copolymer micelles prior to
biomineralisation. After calcination at 800.degree. C., the
characteristic bands at 1726 cm.sup.-1, associated with the
pyrolysed copolymer, completely disappeared, whilst those bands
assigned to the thermally-stable silica were still observed.
[0087] TEM studies indicated that the calcined copolymer-silica
particles became hollow silica particles after pyrolysis of the
organic component. Zeta potential measurements also supported the
deposition of silica within the coronal layer of the copolymer
micelles, as shown in FIG. 14. The precursor SCL micelles (having a
target degree of crosslinking for the PDMA chains of 30%) had
positive zeta potentials over the whole pH range investigated, due
to their cationic PDMA shells. However, the silica-coated micelles
exhibited negative zeta potentials over a wide pH range, with an
isoelectric point at around pH 3.3. This latter behaviour is
similar to that found for aqueous colloidal silica sols (see FIG.
14) and is, therefore, consistent with the SCL micelles becoming
coated with a silica overlayer.
[0088] The inventors also attempted the deposition of gold
nanoparticles within these hybrid copolymer-silica particles. In
order to achieve this, HAuCl.sub.4 was initially used to protonate
the weakly basic PDPA chains within the cores of the nanoparticles.
Then, the AuCl.sub.4.sup.- counter-ions associated with the
protonated PDPA chains were reduced in situ to produce zero-valent
gold nanoparticles, using NaBH.sub.4 as a reducing agent. The
colour of the copolymer-silica hybrid nanoparticles changed from
white to wine red after the reduction step, indicating the
formation of nano-sized gold sols. TEM observations, as illustrated
in FIG. 15, provided evidence for the generation of gold sols
within the cores of the copolymer-silica nanoparticles, although
some disruption of the silica shells was also apparent. The
experiment also provided direct evidence for the presence of the
PDPA chains within the cores of the hybrid copolymer-silica
particles.
[0089] Thus, the potential for encapsulation of other species, such
as quantum dots or biologically-active molecules, is clearly
illustrated. Indeed, as a consequence of their well-defined
nanostructures, these hybrid copolymer-silica nanoparticles have
potential applications in biolabeling, biodiagnostics, targeted
drug delivery, solubilization, catalysis and imaging, and as
fillers and coatings.
[0090] The fact that mild conditions, fast reaction times, and
accessible reagents can be utilised herein may offer clear
advantages when preparing commercially applicable processes. In
addition, the ability to control the size and/or properties of the
particles offers benefits.
[0091] The use of silica also offers particular advantages in terms
of the potential applications of the materials of the invention.
Thus, since silica is usually considered to be a `food-grade`
material, these new particles have potential applications in food
manufacturing.
[0092] It is clear from the work of the inventors that the effect
of varying the degree of quaternisation and shell crosslinking of
the diblock copolymer templates under investigation has a
significant effect on the nature of the silica nanoparticles that
are produced during in situ silica biomineralisation, since either
solid spheres (with no cavities), or structured core-shell spheres
with thin shells, or structured core-shell spheres with thick
shells can be obtained, depending on the precise nature of the
copolymer micelles.
[0093] The core-shell copolymer-silica nanoparticles of the present
invention are somewhat larger than those of the prior art (30 nm
vs. 10 nm), and this should allow higher loading capacities. The
core-shell nature of the hybrid copolymer-silica particles has been
clearly illustrated by TEM studies, and these results have been
corroborated by small angle x-ray scattering studies (SAXS). The
mean wall thicknesses obtained by TEM and SAXS are in good
agreement.
[0094] Perhaps the most significant advantage of the present
invention, however, lies in the fact that the core-forming PDPA
block in the claimed compositions is pH-responsive, and this offers
the possibility of pH-triggered release of hydrophobic actives from
the cores of the hybrid copolymer-silica nanoparticles.
[0095] The use of ABC triblock copolymers has found particular
success in the preparation of predominantly anisotropic rod-like
copolymer-silica particles, and the said nanorods should allow
zero-order diffusional release to be achieved. The synthesis of
said nanorods is illustrated in FIG. 16 wherein a poly(ethylene
oxide)-based macroinitiator (PEO.sub.45-Br) is firstly reacted with
2-(dimethylamino)ethyl methacrylate (DMA) in the presence of
copper(I) chloride, then the product is further reacted with
2-(diisopropylamino)ethyl methacrylate (DPA). The obtained
copolymer was characterised by GPC and .sup.1H NMR, and the results
are summarised in Table 2 and FIG. 17, which shows the .sup.1H NMR
spectrum of the triblock copolymer recorded in
d.sub.5-pyridine.
[0096] This copolymer was designed to self-assemble into colloidal
micellar aggregates with PDPA cores, PEO coronas and PDMA inner
shells. Since the PDMA block has a pK.sub.a of around 7.0, these
residues are approximately 50% protonated at pH 7.2. Thus, silica
deposition was expected to occur exclusively within the cationic
PDMA inner shells, with the coronal PEO blocks imparting steric
stabilization. Thus, it is believed that silica deposition can be
performed at relatively high copolymer concentrations without
inducing particle fusion.
[0097] Silica deposition was performed at 1.0% copolymer
concentration to produce the anisotropic rod-like copolymer-silica
particles, which were easily (re)dispersed by ultrasonication. The
resulting silica rods were characterized using TEM,
thermogravimetric analyses, FT-IR spectroscopy and zeta potential
measurements. FIG. 18 shows a representative TEM image of the
silica rods. FT-IR studies confirmed silica formation and polymer
encapsulation, since bands were observed at 1080, 950, 800 and 470
cm.sup.-1 due to the inorganic component, and at 1726 cm.sup.-1 due
to the carbonyl ester stretch of polymer for these silica rods.
Thermogravimetric analyses indicated that the mean copolymer
content of these hollow silica rods was about 26% by mass and, as
shown in FIG. 19, zeta potential measurements indicated the
successful coating of silica onto the copolymer micelles.
[0098] The invention will now be further illustrated, though
without in any way limiting the scope of the disclosure, by
reference to the following examples.
EXAMPLES
Example 1
[0099] PDPA.sub.23-PDMA.sub.68 diblock copolymer was synthesised by
sequential monomer addition using group transfer polymerisation
according to Chem. Commun. 1997, 671-672. Gel permeation
chromatography analysis indicated an M.sub.n of 18,000 and an
M.sub.w/M.sub.n of 1.08 using a series of near-monodisperse
poly(methyl methacrylate) calibration standards. The mean degrees
of polymerisation of the PDPA and PDMA blocks were estimated to be
23 and 68, respectively, using .sup.1H NMR spectroscopy.
[0100] Non-crosslinked micelles of the PDPA.sub.23-PDMA.sub.68
diblock copolymer (degree of quaternisation=0%) were prepared by
molecular dissolution at pH 2, followed by adjusting the solution
pH to pH 7.2 using NaOH. Dynamic light scattering (DLS) studies at
25.degree. C. indicated an intensity-average micelle diameter of 37
nm for a 0.25 wt. % copolymer micelle solution at pH 7.2.
[0101] Silicification of the said micelles was achieved by mixing
2.0 ml of an aqueous micelle solution (0.25 w/v % at pH 7.2) with
1.0 ml tetramethyl orthosilicate, and then stirring the initially
heterogeneous solution under ambient conditions for 20 minutes. The
hybrid core-shell copolymer-silica nanoparticles thus obtained were
washed with ethanol, then subjected to three
centrifugation/redispersion cycles at 16,000 rpm for 5 minutes.
Redispersal of the sedimented core-shell copolymer-silica
nanoparticles was subsequently achieved with the aid of an
ultrasonic bath.
Example 2
[0102] PDPA.sub.23-PDMA.sub.68 diblock copolymer was synthesised by
sequential monomer addition using group transfer polymerisation as
in Example 1.
[0103] Partial quaternisation of the PDMA block (targeting a degree
of quaternisation of either 50% or 100%) using iodomethane was
conducted in THF for 24 hours, as described in Macromolecules 2001,
34, 1148-1159.
[0104] Non-crosslinked micelles prepared using either 50% or 100%
quaternised PDPA.sub.23-PDMA.sub.68 diblock copolymers were also
prepared by pH adjustment, as described in Example 1. DLS studies
conducted at pH 7.2 indicated intensity-average diameters of 29 nm
and 26 nm for 0.25 wt. % aqueous solutions of 50% and 100%
quaternised copolymer micelles, respectively.
[0105] Tetramethyl orthosilicate (1.0 ml) was added at 20.degree.
C. to 2.0 ml of a 0.25 wt. % aqueous solution of
PDPA.sub.23-PDMA.sub.68 copolymer micelles in which the PDMA chains
were 50% quaternised, and silica deposition was allowed to continue
for 20 minutes, with continuous stirring, prior to isolation via
centrifugation.
[0106] DLS studies on the hybrid core-shell copolymer-silica
nanoparticles obtained using the 50% quaternised copolymer
precursor indicated an intensity-average micelle diameter of 34 nm
at around pH 7.
Example 3
[0107] PDPA.sub.23-PDMA.sub.68 diblock copolymer was synthesised by
sequential monomer addition using group transfer polymerisation,
and non-crosslinked micelles of the PDPA.sub.23-PDMA.sub.68 diblock
copolymer were prepared as described in Example 1.
[0108] Shell crosslinking of the coronal PDMA chains was achieved
by adding a bifunctional quaternising agent,
1,2-bis-(2-iodoethoxy)ethane (BIEE, 0.15 moles per DMA residue for
a target degree of cross-linking of 30%) to a 0.25%
PDPA.sub.23-PDMA.sub.68 copolymer micelle solution at pH 7.2. Shell
crosslinking was carried out at 25.degree. C. for at least 72
hours. After shell crosslinking, DLS studies indicated an
intensity-average diameter of 32 nm and TEM studies suggested a
number-average diameter of 26 nm for the dried SCL micelles. On
adjusting the aqueous SCL micelle solution to pH 2, DLS studies
indicated an intensity-average diameter of 45 nm due to swelling of
the SCL micelles.
[0109] This DLS experiment also confirmed successful shell
crosslinking, since the non-crosslinked micelles simply dissociate
at low pH to form a molecular solution, because the PDPA chains are
highly protonated, and hence no longer hydrophobic, at low pH. In
addition, SCL micelles prepared using the 50% quaternised copolymer
had an intensity-average diameter of 37 nm at pH 7.2 as indicated
by DLS.
[0110] Silica deposition was achieved by adding a 2.0 ml aliquot of
a 0.25 wt. % SCL micelle solution to a mixture of 2.0 ml methanol
and 2.0 ml tetramethyl orthosilicate, wherein the methanol acted as
a co-solvent and ensured that the TMOS was miscible with the
aqueous phase. After continuing silica deposition for 40 minutes,
TEM studies of the obtained product confirmed the formation of
well-defined core-shell copolymer-silica nanoparticles, as
illustrated in FIG. 3C. Even after continuing the treatment for 120
minutes, however, no evidence for non-templated silica
nanostructures was observed, as shown in FIG. 3D.
Example 4
[0111] PEO.sub.45-PDMA.sub.29-PDPA.sub.76 triblock copolymer was
synthesized by Atom Transfer Radical Polymerisation using a
PEO-based macro-initiator by firstly adding the macro-initiator
(1.00 g, 0.463 mmol) to a 25 ml one-neck flask, then degassing by
three vacuum/nitrogen cycles, followed by the addition of DMA (2.18
g, 13.88 mmol, target DP 30), 2,2'-bipyridine (144.5 mg, 0.925
mmol) and then 3.2 ml of a degassed 95/5 v/v IPA/water mixture. The
solution was placed in a 40.degree. C. oil bath and stirred until
homogeneous. Copper(I) chloride (45.8 mg, 0.463 mmol) was then
added and the reaction was carried out at 40.degree. C. for 3.5
hours under nitrogen with continual stirring. After this time, the
DMA monomer conversion reached 96%, as determined by .sup.1H NMR
spectroscopy.
[0112] Thereafter, a mixture of DPA (4.94 g, 23.13 mmol, target
DP=50) and 5.0 ml of a 95/5 v/v IPA/water mixture was added. The
second-stage polymerization was carried out at 40.degree. C. for
18.5 hours, before being terminated by exposure to air. .sup.1H NMR
analysis showed that the DPA monomer conversion reached 99%. The
copolymer solution was diluted with THF (200 ml) and passed through
a silica column to remove the spent catalyst. The copolymer
solution was then concentrated under vacuum and the solid copolymer
was precipitated into deionized water (100 ml) to remove residual
monomer and any unreacted PEO-DMA diblock copolymer. The purified
white copolymer was isolated by freeze-drying under vacuum
overnight to give an overall yield of 6.1 g (76%).
[0113] The micellar rods formed by the
PEO.sub.45-PDMA.sub.29-PDPA.sub.76 triblock copolymer were prepared
by molecular dissolution at pH 2, followed by adjusting the
solution pH to 7.2 using NaOH. The final copolymer concentration
was 1.0 wt. %.
[0114] Silica deposition was achieved by adding excess TMOS (0.20
g; i.e. a TMOS:copolymer mass ratio of 20:1) to 1.0 ml of copolymer
solution and silicification was then conducted for 20 minutes at
20.degree. C. and pH 7.2. Silica rods were obtained by washing with
ethanol, followed by three centrifugation/redispersion cycles at
13,000 rpm for 15 minutes.
TABLE-US-00001 TABLE 1 TGA results of silica synthesized using the
PDPA.sub.23-PDMA.sub.68 diblock copolymer micelles under various
quaternisation conditions at 20.degree. C. and pH 7.2. Precursor
micelles Mel(50) Mel(50) Mel(50) BIEE(50) BIEE(50) BIEE(50) Mel(50)
Mel(50) Concentrations/wt. % 0.25 0.25 0.25 0.25 0.25 0.25 1.0 2.0
Copolymer/mg 5 5 5 5 5 5 10 20 TMOS/mg 58 116 1000 58 116 1000 116
232 Target polymer content from reaction 18 36 1.3 18 36 1.3 18 18
feeding/wt. % Actual polymer content from TGA/wt. % 28 23 18 30 24
19 20 22 Silica Conversion/% 56 36 6 51 34 5 87 78 Diameters from
TEM (nm) 33 33 28 20 23 26 33 35
TABLE-US-00002 TABLE 2 Summary of molecular weight data obtained
for the PEO.sub.45-Br macro-initiator, PEO.sub.45- PDMA.sub.29
diblock precursor and the final PEO.sub.45-PDMA.sub.29-PDPA.sub.76
triblock copolymer. AB diblock ABC triblock ABC Triblock
composition A block Conversion Conversion Targeted Morphologies
Calculation M.sub.n M.sub.w/M.sub.n of DMA M.sub.n M.sub.w/M.sub.n
of DPA M.sub.n M.sub.w/M.sub.n Rods
PEO.sub.45-DMA.sub.29-DPA.sub.76 3,100 1.08 96 8,400 1.18 99 19,500
1.20
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