COMMUNICATION
One-pot synthesis of biopolymeric hollow nanospheres
by photocrosslinkingw
Yihua Yin,* Sha Xu, Dan Chang, Hua Zheng,* Junli Li, Xiaoping Liu,
Peihu Xu and Fuliang Xiong
Received 9th August 2010, Accepted 6th September 2010
DOI: 10.1039/c0cc03129a
Biopolymeric hollow nanospheres with stabilized structure have
been prepared by self-assembly of amphiphatic photocrosslinkable
carboxymethyl chitosans in a water medium into hollow nano-
capsules and subsequent in situ photo-crosslinking of the photo-
reactive functional groups in the shells of the nanocapsules.
of biopolymeric shell cross-linked hollow spheres. This
approach is novel in many aspects. First, the polymer which
self-assembles into the hollow nanospheres is an amphiphatic
photocrosslinkable carboxymethyl chitosan instead of a block
copolymer or another synthetic polymer and its preparation is
very simple. Second, the polymeric preparation and the self-
assembly process can be completed simultaneously in a water
medium without the aid of surfactants, toxic organic solvents,
emulsion phases, or template cores. Third, the hollow nano-
spheres were obtained directly by cross-linking of nanocapsule
precursors, the cross-linking was carried out by UV-induced
in situ cross-linking of the hydrophobic groups in the shells
without the addition of cross-linkers. Fourth, carboxylic
groups at their surface could be used for different purposes.
The one-pot synthesis of hollow nanospheres is based on
an amphiphatic photocrosslinkable carboxymethyl chitosan
(Az-CMCS, Scheme 1B). The compound can be synthesized
by mixing azidobenzaldehyde (Az) and carboxymethyl
chitosan in water solution at room temperature. The structure
of Az-CMCS was characterized by FT-IR spectrum and
1H NMR spectroscopy (see Fig. S1 and S2 in the ESIw).
Az-CMCS samples with various degrees of substitution of
Az were named. Their degrees of substitution to the C-2 amino
groups estimated by an elemental analysis are shown in
Table 1.
In recent years great efforts have been made in the develop-
ment of preparation of hollow nanoparticles with controlled
dimensions and stabilized structure. To sum up, these methods
can be divided into three kinds: (1) Wooley et al.1,2 produced
such hollow spheres via self-assembly of block copolymers into
core-shell micelles followed by cross-linking of the shell and
degradation of the core. (2) Jiang Ming et al.3,4 developed a
block-copolymer-free strategy that first obtained core-shell
micelles via self-assembly of two polymers, which are able to
connect to each other via hydrogen bonding. Subsequent
cross-linking of the shell and removing the core by dissolution
leading to shell cross-linked hollow spheres. (3) Meier5 have
prepared hydrophilic hollow nanospheres by cross-linking
polymerization of hydrophobic monomers in the template of
a lipid bilayer followed by hydrolysis of a hydrophobic ester.
The main challenges of these methods are that the synthesis
of polymer materials and/or the preparation processes of
the hollow nanospheres are relatively complicated, and the
materials which assemble into spheres also do not have
biodegradability thus leading to a great restriction on their
application. Recently, only one report describing the one-pot
synthesis of hollow nanospheres was given by Sugihara.6
However, the polymer used in the method is also a triblock
copolymer whose synthesis requires very delicate reaction
conditions.
Az-CMCS can self-assemble into monodisperse hollow
nanocapsules, and subsequently the nanocapsule solution
was exposed to UV light for 15 min using an ultraviolet lamp
(20 W, 253.7 nm) to induce in situ cross-linking of the
hydrophobic photoreactive functional groups (phenyl azide
groups) in the shells, and forming a shell cross-linked hollow
nanosphere. Fig. 1 shows the TEM photographs of the nano-
capsules before photocrosslinking (a) and the photocrosslinked
hollow nanospheres (b). The hollow nanospheres retain their
original void configuration, but their average diameters are
decreased and the contrasts between the centers and the shells
are reduced. This is attributed to the photocrosslinking leading
to a more compact structure. The photocrosslinking of
Az-CMCS in the hollow nanospheres was demonstrated by
the IR spectra (see Fig. S3 in the ESIw). Compared with FT-IR
spectrum (see curve a) of Az-CMCS in the nanocapsules
before photocrosslinking, the characteristic absorption band
at 2130 cmÀ1 is disappeared in curve b,12 which indicates the
formation of cross-linking between azide groups after UV
irradiation. Similar absorption bands have also been observed
in the literature.7–11
Chitosan is a biocompatible and biodegradable biopolymer
and is often used as a biomaterial. Recently, a photocrosslinkable
chitosan or one of its derivatives7–11 has been of particular
interest in the biomedical material field due to the following
reasons: the cross-linking reaction kinetics are rapid, the
photoreactive functional groups eliminate the use of photo-
initiators, the cross-linking reaction does not leave toxic
by-products (the only by-product is N2) and the cross-linked
chitosan or its derivative is biocompatible and biodegradable.8
In this paper, we report a new approach to one-pot synthesis
Department of Pharmaceutical Engineering,
School of Chemical Engineering, Wuhan University of Technology,
Wuhan University of Technology, Wuhan, 430070, China.
E-mail: yihuayin@yahoo.com.cn, zhenghua.whut@126.com
w Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/c0cc03129a
The formation mechanism of hollow nanospheres may be
explained on the basis of the poly-core model proposed by
ꢀc
This journal is The Royal Society of Chemistry 2010
8222 | Chem. Commun., 2010, 46, 8222–8224