Reversibly controlled morphology transformation of an amphiphilic DNA-dendron hybrid

Article abstract of DOI:10.1039/c2cc30776f

Spherical micelles and nanofibers assembled from an amphiphilic DNA-dendron hybrid can be reversibly achieved under optimum assembly conditions. The possible morphology transformation mechanism is proposed and further confirmed by X-ray diffraction, TEM a

Full text of DOI:10.1039/c2cc30776f

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COMMUNICATION
Reversibly controlled morphology transformation of an amphiphilic
DNA–dendron hybridw
Liying Wang,a Yu Feng,a Zhongqiang Yang,b Yan-Mei He,a Qing-Hua Fan*a and
Dongsheng Liu*b
Received 3rd February 2012, Accepted 22nd February 2012
DOI: 10.1039/c2cc30776f
Spherical micelles and nanofibers assembled from an amphiphilic
DNA–dendron hybrid can be reversibly achieved under optimum
assembly conditions. The possible morphology transformation
mechanism is proposed and further confirmed by X-ray diffraction,
TEM and DLS experiments.
The self-assembly of block copolymers has become an effective
tool in achieving diverse nanostructures, which have great
potential applications in materials science and drug delivery.1
Controlling the aggregate morphology is of great value, because
Fig. 1 Chemical structure of G2Cl-18 and schematic illustration of
different morphologies are related to different functions.2 There-
morphology transformation of G2Cl-18 assemblies at different assembly
fore, significant efforts have been made to control the assembly
conditions.
morphology of block copolymers.3
DNA block copolymers (DBC)4 as a newly developed hybrid
we purified the target product by the reverse-phase HPLC with a
C4 column (Fig. S1, ESIw). Though in a previous study we found
that this hybrid could assemble into long nanofibers in aqueous
phase, here we sought to determine the conditions for forming
spherical micelles. We utilized a typical dialysis method for
formation of block copolymer micelles.9 First, we need to seek
a common solvent for both the dendron and the DNA part. As
characterized by 1H NMR in the 1 : 1 v/v mixed deuterated
tetrahydrofuran (THF) and water solution, the hybrid showed
sharp peaks at about 6.8 and 5.0 ppm which correspond to the
hydrogens of the dendron part (Fig. S2, ESIw). We also did not
detect any aggregates under this condition by transmission
electron microscopy (TEM), indicating that the mixture of
1 : 1 v/v THF : water is an ideal solvent for G2Cl-18. While in
pure deuterated water, both peaks disappeared, as the dendron is
locked in the hydrophobic core. In a typical experiment for
preparing spherical micelles, G2Cl-18 was dissolved in a 1 : 1 v/v
mixture of THF and water to get a concentration of 100 mM.
Next, water was added slowly under vigorous stirring until the
volume ratio of THF decreased to 10 vol%. The residual THF
was removed by dialysis against water for 24 h. The aggregates
were observed by TEM as shown in Fig. 2A. Spherical micelles
with an average diameter of 10 nm were observed, which agreed
well with the dynamic light scattering (DLS) result of about
11.4 nm (Fig. S3, ESIw). These results indicated that spherical
micelles of the DNA–dendron hybrid could be successfully achieved
by controlling the assembly conditions via a dialysis method.
Next, we aimed to explore the assembly conditions to switch
phases between spherical micelles and nanofibers of the DNA–
dendron hybrid. In an attempt to transform the spherical micelles
material composed of DNA and synthetic polymers are receiving
increasing attention due to their excellent programmability and
structural diversity inherited from DNA.5 Several amphiphilic
DBC have been developed and assembled into a variety of
nanostructures.6 It is reported that the morphology of the DBC
assemblies can be tuned by utilizing the sequence-selectivity of
DNA via hybridization and enzymatic cleavage.6c,7 However,
controlling the morphology of the DBC aggregate only through
the sample preparation method, without any change of the DBC
structure, has not been exploited. Herein, we have investigated the
assembling conditions of an amphiphilic DNA–dendron hybrid8
to form spherical micelles and long nanofibers in aqueous solution,
examined the reversible phase transitions and discussed the
possible transformation mechanism.
As illustrated in Fig. 1, we focused on the DNA–dendron
hybrid G2Cl-18 which has been recently reported by our group,
see details in ESI.w8a The dendron part is a Freche
´
t type
poly(benzyl ether) dendron and the DNA contains 18 bases with
a random sequence. In order to improve the separation efficiency,
a Beijing National Laboratory for Molecular Sciences, CAS Key
Laboratory of Molecular Recognition and Function, Institute of
Chemistry and Graduate School, The Chinese Academy of Sciences,
Beijing 100190, China. E-mail: fanqh@iccas.ac.cn
b Optoelectronics & Molecular Engineering of the Ministry of
Education, Department of Chemistry, Tsinghua University, Beijing
100084, China. E-mail: liudongsheng@tsinghua.edu.cn
w Electronic supplementary information (ESI) available: The detailed
synthetic routes, HPLC, 1H NMR, DLS, DSC and TEM data. See
DOI: 10.1039/c2cc30776f
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3715–3717 3715

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solution to 90 1C for 30 min and then allowed it to slowly cool
to room temperature with a rate of 1.5 1C hÀ1, nanofibers were
observed (Fig. S5, ESIw). We suppose nanofibers to be the
thermodynamically stable morphology in aqueous solution. It
is noted that if samples were prepared by rapid cooling in an
ice bath after the heating procedure, flower-like aggregates
were formed (Fig. S6, ESIw). Careful examination of these
flower-like structures revealed that they are in fact composed
of individual nanofibers. This kind of flower-like morphology
was also reported by the group of Winnik and Manners,11
however, so far the reason for the aggregate behavior is still
unclear. We infer that these flower-like assemblies are kinetic
aggregations of the individual nanofibers. On the other hand,
the nanofibers could be transformed into spherical micelles
when 10 vol% THF was added, suggesting that the spherical
micelles are a thermodynamically stable morphology in the
mixed solvent of 1 : 10 v/v THF : water.
Then why could spherical micelles not transform into the
more thermostable nanofibers after removing THF by dialysis?
We found the melting point of the dendron part to be 114 1C as
measured by DSC (Fig. S9, ESIw), indicating that the dendron
is in solid phase at room temperature. Although the spherical
micelles prefer to transform into nanofibers as the content
of THF decreased in the system, the locked dendron part
kinetically hinders this transformation. On the other hand,
Zhang and Eisenberg have reported that the possible mechanism
of the spherical to cylindrical nanofiber transformation involves
adhesive collisions of small spherical micelles.12 However, DNA
is a highly charged macromolecule, and the static repulsion
increases the energy barrier for adhesive collisions between
micelles. Therefore, the kinetics via the collision mechanism can
be very slow. We also tried to anneal G2Cl-18 (20 mM) dissolved
in 1 : 10 v/v THF : water at 90 1C for 30 min, and observed that
both spherical micelles and nanofibers coexisted in the resulting
solution (Fig. S7, ESIw), with spherical micelles as the major
species. It is possible that the free energies of the nanofibers
and the spherical micelles are in fact close to each other; the
kinetic factor determines the predominant morphology and
the assembly conditions play an important role in morphology
transformation.
Fig. 2 TEM images of G2Cl-18 spherical micelles prepared by dialysis
method (A) and morphological changes after stepwise annealing (B),
THF addition (C) and re-annealing (D); DLS result of the reversible
transformation between spherical micelles and nanofibers after step by
step annealing and THF addition (E). The scale bar is 200 nm.
into nanofibers, we applied a similar procedure for prepara-
tion of nanofibers as previously reported. The spherical micelle
solution prepared above was first annealed at 90 1C for 30 min
in the presence of 5 vol% dichloromethane (DCM), to destroy
any undesired aggregates, and then cooled to room tempera-
ture naturally. As expected, long nanofibers with the average
diameter of about 20 nm were clearly observed by TEM
(Fig. 2B). In order to switch nanofibers back to micelles, we
added 10 vol% THF and allowed the solution to stay under
room temperature for 2 days. After that, THF was removed
by dialysis against water for 24 h. Spherical micelles were
observed again as confirmed by TEM (Fig. 2C). Certainly, we
could regain long nanofibers by annealing the micelle solution
(Fig. 2D). The whole reversible morphology transformation
was also examined by DLS, Fig. 2E, the average diameter of the
micelles was 11.4 nm at the initial state, and then changed to
1.1 mm nanofibers by annealing. Similarly, cycling the assembly
conditions resulted in assemblies size switching between around
10 nm and 1 mm in DLS (Fig. S3, ESIw). Moreover, we observed
the intermediate state, i.e., necklace like structure, when the
nanofibers transformed into spherical micelles (Fig. S4, ESIw),
this result has a good agreement with previous studies.10
We also sought to elucidate if the two morphologies of
spherical micelles and nanofibers were under thermodynamic
or kinetic control. We investigated the influence of different
cooling rates upon the morphology. If we heated the G2Cl-18
We further explored the driving force for nanofiber formation
because this morphology of G2Cl-18 is unusual for an amphiphilic
block copolymer with a block ratio of 1 : 5.6. Spherical micelles
should have been the stable structure in the G2Cl-18 system
according to the current theories for the formation of block
copolymer micelles.1b,13 Several groups have also reported similar
phenomena.14 We believe that besides the hydrophobic effect,
other interactions may exist in our system. In an attempt to
gain more details of the micellar structure, X-ray diffraction
(XRD) experiments were undertaken. Nanofibers of G2Cl-18
were deposited on aluminium substrates, two obvious peaks
with d spacing of 3.57 and 7.08 A were observed (Fig. 3A),
corresponding to one and two fold of the distance of p–p
stacking in the dendron core. The distance is relatively big
compared to the typical p–p stacking distance of 3.4 A,
suggesting that the p–p stacking interaction was not that strong.
However, once 10 vol% THF was added to the formed nanofibers
and the solution was kept under room temperature for 2 days,
the XRD peak disappeared, indicating that the p–p stacking
c
3716 Chem. Commun., 2012, 48, 3715–3717
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as templates to direct the arrangement of AuNPs. In short, this
work is the first example to explore the reversible morphology
transformation of a DNA–dendron hybrid without changing
the DBC structure, providing a unique approach to tune the
morphology of amphiphilic assemblies and in the end would
benefit the development of general and applicable methods
in designing functional and smart nanomaterials. Taking
advances of biotechnology and materials science into account,
we believe that our hybrids have potential application in
biosensor,17 drug delivery1 and other related fields.
Fig. 3 XRD profiles of the G2Cl-18 nanofibers and MOMG2Cl dendron
(A) and samples prepared on the substrate at 90 1C, spherical micelles and
nanofibers with 10 vol% THF addition in the control experiment (B).
The authors thank Prof. Monika Schoenhoff and Dr Jan
Gauczinski of University of Munster (Germany) for the NMR
¨
interaction was destroyed. Neither the sample prepared at
90 1C nor the spherical micelles showed observable XRD signals
(Fig. 3B), suggesting little p–p stacking effect in these two
samples. In fact, p–p stacking was also proven to be the driving
force in the assembly of the sole dendron part.15 The XRD
pattern of MOMG2Cl is shown in Fig. 3A for comparison.
Finally, to demonstrate the functionalization potential of
the system, we used complementary strand modified 5 nm
gold nanoparticles (AuNPs) to hybridize with G2Cl-18.
The DNA–AuNP conjugates were prepared according to the
published method.16 When the G2Cl-18 nanofibers were mixed
with the DNA–AuNP conjugates, long AuNP chains were
achieved as shown in TEM, Fig. 4A, compared to the randomly
dispersed AuNPs in the control experiment (Fig. S8, ESIw). We
also mixed the G2Cl-18 spherical micelles with the DNA–AuNP
conjugates. As expected, clusters were observed, as DNA–AuNP
conjugates hybridized with G2Cl-18 and accumulated around
spherical micelles. It is known that without particular modification,
AuNPs randomly disperse in solution instead of forming clusters,
therefore, we conclude that the clusters in Fig. 4B were indeed
templated by the spherical micelles. The average diameter of
the clusters was about 25 nm, which is reasonable considering
the diameter of the AuNPs.
test. We also thank National Basic Research Program of China
(973 program, No. 2011CB935701), the National Natural
Science Foundation of China (No. 21121004 & 91027046),
and NSFC-DFG joint project TRR61 for financial support.
Notes and references
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In conclusion, we have investigated the self-assembly behavior
of the DNA–dendron hybrid G2Cl-18 in detail. The morphology
of the resulting assemblies can be well controlled by tuning the
assembly environment. The two aggregates, spherical micelles and
nanofibers, could be reversibly transformed by only alternatively
changing the assembly conditions, and the process was confirmed
by TEM and DLS. Reasons for this transformation were
proposed according to the achievable experiment results. p–p
stacking of the G2Cl-18 is considered to be the driving force of
the nanofiber formation as characterized by XRD. We also
showed that spherical micelles and nanofibers could function
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Fig. 4 TEM images of 5 nm AuNP chains and clusters templated
by G2Cl-18 nanofibers (A) and spherical micelles (B) respectively.
The scale bar is 100 nm.
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Chem. Commun., 2012, 48, 3715–3717 3717