View
3
Download
0
Category
Preview:
Citation preview
University of Groningen
Nucleic acid amphiphilesKwak, Minseok; Herrmann, Andreas; Clever, Guido; Mao, Chengde; Shionoya, Mitsuhiko;Stulz, EugenPublished in:Chemical Society Reviews
DOI:10.1039/c1cs15138j
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.
Document VersionPublisher's PDF, also known as Version of record
Publication date:2011
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):Kwak, M., Herrmann, A., Clever, G. (Ed.), Mao, C. (Ed.), Shionoya, M. (Ed.), & Stulz, E. (Ed.) (2011).Nucleic acid amphiphiles: synthesis and self-assembled nanostructures. Chemical Society Reviews,40(12), 5745-5755. DOI: 10.1039/c1cs15138j
CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.
Download date: 10-02-2018
http://dx.doi.org/10.1039/c1cs15138jhttps://www.rug.nl/research/portal/en/publications/nucleic-acid-amphiphiles(8e89625b-4b04-418b-988c-c5877789a0b3).html
This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 5745–5755 5745
Cite this: Chem. Soc. Rev., 2011, 40, 5745–5755
Nucleic acid amphiphiles: synthesis and self-assembled nanostructuresw
Minseok Kwak and Andreas Herrmann*
Received 24th May 2011
DOI: 10.1039/c1cs15138j
This review provides an overview of a relatively new class of bio-conjugates, DNA amphiphiles,
which consist of oligonucleotides covalently bonded to synthetic hydrophobic units. The reader
will find the basic principles for the structural design and preparation methods of the materials.
Moreover, the self-assembly into superstructures of higher order will be highlighted. Finally, some
potential applications will be described.
1 Introduction
Five decades have passed since the discovery of the structure
of deoxyribonucleic acids (DNAs), ‘‘the secret of life’’, by
Watson, Crick and coworkers, key among them Franklin. This
elegant double helix has ever since fascinated researchers
throughout the natural and applied sciences. Thanks to the
invention of chemical DNA synthesis in the 1960s, broader
studies involving nucleic acids became possible, not only
in biological and medical fields but also beyond. From a chemical
point of view, the driving interest of DNA is its programmability
and self-recognition properties. These properties have been
utilized to particularly great effect in materials science to build
programmed architectures. As such, it is now recognized that
DNA is ‘‘not merely the secret of life’’.1
DNA nanotechnology is now a common term to describe
the use of DNA as a structural material rather than as a
medium for genetic information. Since Seeman’s artistic
Department of Polymer Chemistry, Zernike Institute for AdvancedMaterials, University of Groningen, Nijenborgh 4, 9747 AGGroningen, The Netherlands. E-mail: a.herrmann@rug.nl;Fax: +31 50 363 4400; Tel: +31 50 363 6318w Part of a themed issue on the advances in DNA-based nano-technology.
Minseok Kwak
Minseok Kwak studied
chemistry at the Ajou Univer-
sity in Suwon (South Korea)
and received his Master’s
degree in molecular science
and technology in 2006. He
started his PhD studies at the
Max-Planck Institute for
Polymer Research in Mainz
(Germany) under the supervi-
sion of Professor A. Herrmann
and Professor K. Müllen. In
2007, he moved to the Zernike
Institute for Advanced Materials
with the group of Andreas
Herrmann and obtained his
PhD in 2011 from the University of Groningen. He is currently
employed as a postdoctoral researcher in the group of Professor
W. M. Shih at Harvard Medical School in Boston with a Rubicon
fellowship from the Netherlands Organisation for Scientific
Research (NWO).
Andreas Herrmann
Andreas Herrmann studied
chemistry at the University of
Mainz (Germany). From 1997
to 2000 he pursued his graduate
studies at the Max-Planck
Institute for Polymer Research
in the group of Professor
K. Müllen. Then he was
employed as a consultant for
Roland Berger Management
Consultants in Munich (2001).
In the years 2002 and 2003 he
returned to academia working
on protein engineering at the
Swiss Federal Institute of Tech-
nology, Zurich, with Professor
D. Hilvert. In 2004 he was appointed as a head of a junior
research group at the Max-Planck Institute for Polymer
Research. In 2007 he moved to the Zernike Institute
for Advanced Materials at the University of Groningen,
The Netherlands, where he holds a chair for Polymer Chemistry
and Bioengineering. Andreas Herrmann was awarded the
Reimund-Stadler Prize (2008) and the Dr Hermann-Schnell
Prize (2009) from the German Chemical Society (GDCh).
Moreover, in 2009 and 2010, he received prestigious funds, the
ERC Starting Grant from the European Commission and the
Vici grant from NWO, respectively.
Chem Soc Rev Dynamic Article Links
www.rsc.org/csr TUTORIAL REVIEW
Dow
nloa
ded
by U
nive
rsit
y of
Gro
ning
en o
n 07
Feb
ruar
y 20
12
Pub
lish
ed o
n 22
Aug
ust
2011
on
http
://p
ubs.
rsc.
org
| doi
:10.
1039
/C1C
S15
138J
5746 Chem. Soc. Rev., 2011, 40, 5745–5755 This journal is c The Royal Society of Chemistry 2011
building strategy for unusual DNA structures was reported
in 1991,2 countless beautiful DNA architectures have been
described—from intricate 2D patterns to highly precise nano-
scopic tubes, convex polyhedra, strings and 3D origami. These
new geometries, the increasingly sophisticated understanding
of assembly mechanisms they generated and the opportunities
for further manipulation which they revealed opened the door
to working with this material in multidimensional systems.3–8
The key structural property that makes these designs possible
is the persistence length of double-stranded (ds) DNA of 50 nm.
Qualifying it as a semi-rigid polymer, the double helix serves as a
rigid scaffold for short nucleic acid stretches.
At the same time, attempts have been made to develop
applications for DNA by incorporating functionalities and
utilizing its information-carrying capability. For instance,
fluorescently labeled DNAs can be used to detect particular
sequences or monitor the polymerase chain reaction (PCR).9
Today a range of modifications are commercially available for
daily laboratory use: chemical functionalities for covalent-
bond formation, optical/fluorescent labels for detection, and
others for further molecular recognition. While the above
modifications simply introduce an additional function, a
particular focus in materials science has been to chemically
couple nucleic acid strands with new moieties that alter the
molecule’s structural, morphological or self-assembly properties.
Such combinations of DNA with other classes of materials,
including chemical functional groups, (bio)polymers and
inorganic nanoobjects, are known as DNA hybrid materials.
This class of materials finds application in a wide range of
research fields, for instance diagnostics and electronics, as the
properties and advantages of each component can be tailored
to meet particular needs. This has been well demonstrated
and reviewed for hybrids incorporating gold nanoparticles10
and proteins.11
In this review, the class of amphiphilic DNA hybrids will be
covered in depth. Their amphiphilic nature arises from the
hydrophilicity of the DNA backbone containing charged
phosphodiester bonds, and chemical attachment of hydro-
phobic moieties. Here, amphiphilic structures with only a single
or very short nucleotides will be excluded. Such materials most
often lack the property of predetermined DNA hybridization
so that rarely more complex superstructures are formed. This
class of modified nucleic acids is covered in an excellent
review.12 First, the classification and techniques for the syn-
thesis of amphiphilic oligonucleotides (AONs) will be intro-
duced. Then their self-assembled structures and potential
applications will be described.
2 Structural classification and synthesis
Solid-phase synthesis (SPS) and solution coupling are two
general ways for the preparation of DNA (or RNA) modified
with functional groups.13–15 Short up to long fragments of
DNA (up to 200 mers), called oligonucleotides (ONs), can be
built up by iterative chemical synthesis on solid supports using
activated phosphoramidite building blocks. The four-step
cycle includes: (1) deblocking, (2) coupling, (3) capping and
(4) oxidation.16 The SPS technique makes it possible not only
to prepare pristine ONs but also to introduce functional
groups while the ONs are still present on the support. Solution
coupling is a more widely used method to make covalent
bonds between reaction-ready DNAs (e.g. amine- or thiol-
modified, prepared by SPS) that are available from commer-
cial suppliers and the complementary functionalities (e.g.
carboxylic acid, thiol or maleimide) of the target molecules.
However, solution coupling with hydrophobic molecules often
results in low yields due to the solvent incompatibility of the
two components. For this reason, solid-phase oligonucleotide
synthesis, which affords a wide choice of organic solvents and
restricts reactions to the solid supports, is more suitable for
preparing AONs. SPS methods can be further divided into two
classes: (1) fully automated synthesis within an ON synthesizer
and (2) modified techniques including syringe synthesis and
in-flask reactions utilizing DNA as synthesized on solid
supports. Fully automated synthesis offers precise control
and monitoring of the entire procedure, relatively large scales
and higher reproducibility despite of a few disadvantages such
as a high level of expertise and high investment costs for equip-
ment. Often the modified techniques can be used to mitigate some
of these difficulties, as well as to meet more specialized needs (i.e.
longer reaction times or solvents incompatible with a synthesizer).
This review will focus on the use of SPS methods to prepare
two structural classes of AONs. The first architecture contains
terminal hydrophobic moieties while the second type of structures
consists of a brush-configuration.
Terminally-functionalized AONs
Solution coupling has been used to prepare amphiphilic
DNAs with hydrophobic molecules at either terminus of the
DNA. While yields can often be low for the reasons discussed
above, the introduction of an oligoethyleneglycol linker
between the hydrophobic moiety and the DNA enhances
solvent compatibility, giving moderate coupling yields.15 SPS
represents the method of choice for the preparation of these
materials. Unlike solution coupling, though, the incorporation
of small hydrophobic moieties does not notably lower the
coupling yield. One classic and essential method to prepare
AONs using SPS with the hydrophobic moiety covalently
connected to the 50-end of the ONs is the use of 2-cyanoethyl-
N,N-diisopropylphosphoramidite (CEPA) groups.17 Here, a
hydrophobic molecule is functionalized with CEPA groups
and subsequently applied onto the detritylated 50-end of ONs
grown on solid supports (Fig. 1A). Introducing a hydrophobe
at the 30-end of the DNA requires different strategies. One
technique necessitates starting with custom solid supports which
already contain the desired hydrophobic moieties, connected via
a cleavable linker (e.g. carboxylic ester) (Fig. 1B).15 The other
method is reverse synthesis, in which the DNA sequence is
elongated in the opposite (i.e. from the 50- to 30-end) direction
with a final addition of the desired hydrophobe at the 30-end.
In this case, the CEPA group is located at the 50-end and the
4,40-dimethoxytrityl (DMT) group at the 30-end of each nucleo-
side, as opposed to conventional DNA synthesis (compare the
structures shown in Fig. 1B). Fig. 1C portrays a range of CEPA
compounds to introduce hydrophobicity for specific purposes.
In simple hydrophobic functionalities, linear lipophilic (or
aliphatic) chains are directly connected to CEPA (1 and 2)18,19
Dow
nloa
ded
by U
nive
rsit
y of
Gro
ning
en o
n 07
Feb
ruar
y 20
12
Pub
lish
ed o
n 22
Aug
ust
2011
on
http
://p
ubs.
rsc.
org
| doi
:10.
1039
/C1C
S15
138J
This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 5745–5755 5747
Fig. 1 Synthetic schemes and precursors of DNA amphiphiles with terminal hydrophobic moieties. General solid-phase synthesis for amphiphilic
DNAs with hydrophobes at (A) 50- and (B) 30-end of the DNA; (a) deblocking of DMT; (b) coupling of activated CEPA to 50-end; (c) standard
synthesis with nucleoside phosphoramidite. After the synthesis, cleavage from the solid supports and removal of the 2-cyanoethyl group
on phosphorus yield amphiphilic DNAs. (C) Chemical structures of small hydrophobic compounds modified as CEPA. (D) Hydrophobic
CEPA-functionalized polymers. (E) Graphical representations of AONs with a hydrophobic functionality and polymer (DNA block copolymer),
left and right, respectively, at a terminus of DNA. Note that primary amines (–NH2) at nucleobases (denoted as N) are accordingly protected and
CEPA will be the linker to a DNA in AON products.
Dow
nloa
ded
by U
nive
rsit
y of
Gro
ning
en o
n 07
Feb
ruar
y 20
12
Pub
lish
ed o
n 22
Aug
ust
2011
on
http
://p
ubs.
rsc.
org
| doi
:10.
1039
/C1C
S15
138J
5748 Chem. Soc. Rev., 2011, 40, 5745–5755 This journal is c The Royal Society of Chemistry 2011
or attached to the nucleobases of the phosphoramidite building
blocks (3 and 4).20,21 Hydrophobic moieties such as the fluorous
DMT group (FDMT) of 3 offer the additional advantage of fast,
highly efficient purification using a fluorous affinity column or
cartridge. In a similar synthetic manner, cholesterol derivatives
(5, 6 and 7)22 are introduced into DNA as terminal chain motifs.
For the synthesis of 30-end conjugated AONs, compounds 7a
and 7b are attached to the solid support via an ester bond at the
hydroxy groups. A useful means to tune the degree of hydro-
phobicity in these materials is by incorporating bis-adducts of
the hydrophobe connected to CEPA or altering the length of the
linear hydrophobic chain.23
Due to the high charge density of nucleic acids, it is often
desirable in AONs to have relatively large hydrophobic seg-
ments rather than small moieties (compare Fig. 1E, especially
when self-assembled nanostructures should be achieved.
Linear DNA block copolymers (DBCs) are good candidates
for attaining balanced amphiphilicity by changing the mole-
cular weight (Mw) of the polymer segments. In general, DBCs
consist of an ON covalently connected at either end to a
hydrophobic polymer or a block copolymer, yielding amphi-
philic diblock (Fig. 1E, right image) or multiblock structures.
Since Jeong and Park generated a polymeric AON with
poly(D,L-lactic-co-glycolic acid) (PLGA) as the hydrophobic
block using solution coupling,24 analogous routes were com-
monly used. Note that in these solution based methods, there
are always two or more reacting groups for the coupling of the
hydrophobes to DNA. Moreover, the coupling of highly
hydrophobic polymers to ONs entails very low yields due to
the incompatibility of solvents for hydrophobes and DNA.
To overcome solvent compatibility limitations on the choice of
polymers, solid-phase synthesis was also adopted to prepare
DBCs, yielding AONs with more hydrophobic polymers like
poly(styrene) (PS) (8).25 Here, the hydroxy-terminal group of
PS was converted to 2-cyanoethyl-N,N-diisopropylphosphor-
amidite (CEPA) and was subsequently applied onto the
detritylated 50-end of elongated ONs on solid supports
(Fig. 1A and D). Nowadays, fully automated SPS is optimized
and thus commonly used for large-scale preparation of DBCs
in DNA synthesizers with several different hydrophobic
polymers including poly(propyleneoxide) (PPO) (9)26 and
poly(9,9-di-n-octylfluorene) (PFO) (10) (Fig. 1D and E, right
image).27 The reader is directed to two recent reviews for more
comprehensive information about other polymer hybrids of
biomolecules, including DNA.28,29
Brush-type AONs
Brush-type is a structural term for macromolecules consisting
of a linear backbone onto which multiple oligomer or polymer
segments are grafted. Such macromolecular architectures
containing DNA can be generated through SPS. As shown
in the general scheme of DNA synthesis (Fig. 1B), the presence
of CEPA and DMT groups is critical for performing the
reaction cycle. Thus both groups need to be present in a
molecule if it is to be incorporated within the DNA sequence.
To build up brush-type AONs, monomers must contain either
protruding hydrophobic moieties or functional groups which
can be converted into hydrophobic units through additional
transformations. One such building block (11) fulfilling the
former requirement was recently reported,30 resulting in lipid-
grafted ONs (Fig. 2A, left). Here, a C10-alkyl chain was
introduced at C5 of the uracil nucleobase via an ethynyl group
Fig. 2 Brush-type AONs. (A) Chemical structures of nucleoside- and polyaza crown ether monomers (11 and 12, respectively) bearing CEPA and
DMT to yield hydrophobe(s)-grafted AONs. (B) Three-dimensional representation of modified uracil nucleotide (precursor: 11) base-pairing. The
hydrophobic chain (white) does sterically not interfere with the hydrogen bond formation between modified-U and A nucleobases. Side (left) and
top view (right). (C) Brush-type 12 mer AONs containing the hydrophobic (Uracil) building block (11) at varying positions (Middle or Termini)
and in different numbers (2 or 4). (D) Representation of DNA-grafted polymers in homopolymer (left, precursor: 14a) and block copolymer motifs
(right, precursors: 14b and 15; see Fig. 3C for the chemical structures). Each sphere represents a monomer unit and red strings are grafted-ONs.
Dow
nloa
ded
by U
nive
rsit
y of
Gro
ning
en o
n 07
Feb
ruar
y 20
12
Pub
lish
ed o
n 22
Aug
ust
2011
on
http
://p
ubs.
rsc.
org
| doi
:10.
1039
/C1C
S15
138J
This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 5745–5755 5749
Fig. 3 Post-DNA-synthesis preparation of AONs. (A) After SPS, hydrophobes in organic solvents are attached to the 50-end of the ribose (*) or
at the nucleobase (**) of nucleotides on the solid support. After the coupling, liberation from the resin and deprotection of primary amines of
nucleobases are performed in concentrated ammonia to yield AON products. (B) List of reactions that allow functionalization of the DNA with
hydrophobic units. (a) Chemical functionalities (R1 and R2) can be exchanged with each other in a particular case. (b) The CEPA group is only
introduced in the hydrophobes (R2). (c) Reaction conditions may vary. (C) Hydrophobic moieties for the attachment on the solid support after
DNA synthesis. (D) Synthetic route for the on-resin-modification of ONs with hydrophobes by Huisgen cycloaddition.
Dow
nloa
ded
by U
nive
rsit
y of
Gro
ning
en o
n 07
Feb
ruar
y 20
12
Pub
lish
ed o
n 22
Aug
ust
2011
on
http
://p
ubs.
rsc.
org
| doi
:10.
1039
/C1C
S15
138J
5750 Chem. Soc. Rev., 2011, 40, 5745–5755 This journal is c The Royal Society of Chemistry 2011
using Sonogashira–Hagihara coupling. This 5-(dodec-1-ynyl)-
uracil-containing nucleoside phosphoramidite has both DMT
and CEPA at the 50- and 30-ends, respectively. This structure
allows multiple addition of the modified nucleobases at precise
positions in the target ON.31 Moreover, the relatively small
hydrophobic functional group does sterically not hinder
duplex formation with its complementary nucleobase, in this
case adenine (Fig. 2B). In spite of the short alkyl chain, the
morphological behavior of the hybrids confirms their classifi-
cation as brush-type AONs. Due to the fact that the alkyl-
modification does not lower the coupling yield compared to
commercial reagents during SPS, virtually no additional
synthetic effort is needed to include the desired number of this
modified nucleotide in the desired positions (Fig. 2C). Using
this strategy, 12 mer DNA amphiphiles with two or four lipid
tails positioned in the middle or terminus (U2M, U2T and
U4T) could be isolated with reasonable yields of 60 to 91%,
based on analytical HPLC. As such, this method represented
a facile means to tune the hydrophobicity of AONs in a fully
controlled, automated manner. In an analogous strategy, a non-
nucleoside building block (12) is also capable of linking two
nucleotides in SPS. These azacrown derivatives possess varied
hydrophobic segments in two aza-N positions.32 The duplexes
formed from such compounds exhibited tolerable stability.
Just as hydrophobic functional groups can be introduced
along a DNA chain, one can imagine a structure in which
DNAs are grafted onto a hydrophobic polymer backbone
(Fig. 2D). The first such reported hybrid was prepared via
SPS exhibiting a norbornene polymer (PNB) backbone.33
Therein, two types of brush macromolecules were introduced:
DNA and DNA/ferrocene (14a and b respectively, see
Fig. 2D). Recently, Gianneschi et al. reported an analogous
brush block copolymer (15), in which one block contained
carboxylic acid in each repeating unit, which could be con-
jugated to amino-modified DNA, while in the other block
methylbenzene was incorporated as a hydrophobic function-
ality.34 Here, amino-modified DNA on solid-supports was
reacted with the diblock copolymers to realize an amide bond
between the DNA and the polymer backbone.
Preparation of AONs without a DNA synthesizer
This ex situ conjugation method, entailing post-DNA-synthesis
reactions while still on solid supports, particularly helps to
achieve couplings in cases of incompatible solvents or catalysts,
longer reaction times or more extreme reaction conditions than
are accessible with a DNA synthesizer. This strategy can be
used to prepare both brush-type (14 and 15) and linear (13 and
16) AONs (see Fig. 3A–C). An emerging synthetic method
along these lines is the Huisgen cycloaddition, the so-called
‘‘Click chemistry’’,35 employing either azide- or acetylene-
modified DNA that is reacted with the correspondingly
functionalized hydrophobic moieties (Fig. 3D).36 However,
the conditions for this ex situ method vary depending on the
chemical structure, catalyst and chemostability and thus need
to be optimized separately for each reactant.37–39
Regardless of the type of SPS method used for synthesis, the
resulting AONs offer additional advantages in the purification
stage compared to pristine ODNs. When hydrophobic units
are introduced, the AON materials interact strongly with
chromatographic stationary phases. Thus, high performance
liquid chromatography (HPLC) with affinity columns, such
as reverse-phase or anion exchange resins, can be used to
efficiently isolate target AONs.40
3 Self-assembled structures of AONs
DNA amphiphiles on their own generally self-assemble into
structures with simple geometries: spherical or cylindrical micelle
aggregates or vesicles. In addition, these structures can be altered
by mixing in other materials of interest, by hybridization or by
enzymatic reactions, as will be discussed below.
In aqueous media, the hydrophobic units of AONs tend to
undergo microphase separation, to minimize the exposure to
water. The linear DBC with poly(butadiene), DNA-b-PB,
formed vesicles of about 80 nm in diameter (Fig. 4A).41
Thereby, the isolation of the vesicular interior from the
surrounding medium by a DBC double-layer was demon-
strated using fluorescence probes. Although the material
stability and duplex formation capability have not yet been
studied, the novel behavior of this DNA hybrid suggests
further investigations of such AON structures.42 Another
vesicular structure involving AONs used terminal bilipid
moieties to form the hydrophobic core of the bilayer.43
Through the hybridization of fluorescently labelled comple-
mentary DNAs (cDNAs), the reversible decoration of the
surface of the self-assembled aggregates by hybridization was
demonstrated. Instead of constructing vesicle structures from
Fig. 4 Microscopic images and illustrations of self-assembled
amphiphilic DNAs. (A) A TEM image (top) of PB-b-DNA (from
precursor: 16, italicized b such as X-b-Y denotes a block copolymer
composed of polymer segments X and Y that are covalently
connected) vesicles. Reproduced from ref. 41 with permission of the
Royal Society of Chemistry. (B) An AFM image (top) of PS-b-DNA
spherical micelles (unpublished). Analogous aggregates were also
formed with single-stranded DBCs (precursors: 8–10) and lipid-DNAs
(see Fig. 2C, precursor: 11) with 4–10 nm height. Scale bars: 100 nm;
height color scale: 10 nm.
Dow
nloa
ded
by U
nive
rsit
y of
Gro
ning
en o
n 07
Feb
ruar
y 20
12
Pub
lish
ed o
n 22
Aug
ust
2011
on
http
://p
ubs.
rsc.
org
| doi
:10.
1039
/C1C
S15
138J
This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 5745–5755 5751
AONs exclusively, it is much easier to incorporate AONs into
the membrane of artificial liposomes fabricated from lipids or
polymers.42
Simpler and more extensively studied systems are DNA
micelles (Fig. 4B). Many AONs have been observed to form
spherical micelles with a diameter range of 5 to 12 nm, as
measured by dynamic light scattering (DLS) and atomic force
microscopy (AFM).18,26,31 These AFM measurements further
revealed that deformation of such micelles on the surface is
depending on the hydrophobic segments. AONs containing
hydrophobic segments with higher glass transition tempera-
tures (Tg), in particular PS and PFO, endure the mechanical
forces, during the AFM measurement, more than those with
lower-Tg segments like PPO, PLGA and lipids.
One big advantage of constructing nanostructures with
DNA is that once assembled, the architectures can be further
manipulated by enzymatic reactions. Indeed, enzymatic
manipulations were also applied to linear AONs and DNA
brushes. For instance, micelles consisting of a DNA diblock
copolymer, DNA-b-PPO, were treated with the enzyme,
terminal deoxynucleotidyl transferase (TdT), which catalyzes
nucleotidic elongation of the terminal 30-end of single-stranded
(ss) DNA in the presence of deoxyribonucleoside triphosphate
(dNTP) under isothermal conditions at 37 1C. In this experi-
ment, over the course of 16 hours, the sequence-independent
DNA polymerase added approximately 60 T nucleotides to the
termini of DNA chains in the corona of the micelles (Fig. 5A),
which resulted in a height increase from 5 to 11 nm on a mica
surface.44,45 This method offers a convenient post-synthesis size
control of nucleic acid nanoobjects. Precise post-synthesis
shape control has also been demonstrated with such materials
through the addition of ssDNA: AON micelles were trans-
formed into rod-shaped aggregates through hybridization with
a long cDNA template (Fig. 5B).46 The sequence of the long
DNA template was designed to contain five repeats of the
complementary sequence of the DNA corona of spherical
micelles. Alignment of five AONs along the template, after
disaggregation of spherical micelles, resulted in rod-like aggre-
gates, where the hydrophobic polymer units were uniformly
placed between two long dsDNAs glueing the two duplexes
together. Another example of changing supramolecular AON
structures by very mild external stimuli was realized with the
enzyme, phosphodiesterase (Fig. 5C). Starting from a spherical
micelle consisting of brush-type polymeric AONs, digestion
of the DNA segments reduced the volume occupied by the
grafted nucleotides. The resulting change in the hydrophilic/
hydrophobic volume ratio induced a transformation of the
spherical nanoobjects into long cylindrical aggregates. The
second stimulus applied on the AON tube was hybridization
of longer ONs with the remaining segments of the DNA brush.
This lengthening restored the initial structural parameters,
producing spherical aggregates again.34 Such reversible tech-
niques for structural manipulation should eventually produce
potential applications of AONs in the biomedical field with
possibly the same impact as peptide amphiphiles.47
4 Applications of AONs and their nanocomposites
Utilization of pristine AONs
Many advances of AON-micellar systems in the last five years
were presented in the fields of DNA detection,25 templated
reactions,26 and drug delivery due to their self-recognition
properties and the presence of a hydrophobic core to serve as a
carrier unit.24 In particular, an AON micelle has shown great
potential in cancer therapy.48–50 PPO-b-DNA micelles were
equipped both with a hydrophobic anticancer drug and a
targeting unit. Therby, doxorubicin accumulated in the core
of the AON aggregates by simple mixing while the folic acid
targeting units were incorporated into the carrier system by
hybridization (Fig. 6A, top). These aggregates were taken up
by CaCo-2 cancerous cells through receptor mediated endo-
cytosis (Fig. 6A, bottom) and, as anticipated, resulted in
efficient cytotoxicity and high mortality.48 In a different
approach, Tan et al. demonstrated that RNA aptamers,
ONs that can specifically bind to a target molecule, can be
conjugated to lipids, resulting in micelle formation and
efficient targeting ability to a specific cancerous cell line
Fig. 5 Engineering self-assembled structures of AONs. (A) Isothermal enzymatic growth of DNA-b-PPO (precursor: 9) micelles incubated
with TdT and dTTP at 37 1C (top). AFM images of growing nanoparticles with increasing reaction times (bottom). (B) DNA-templated shape
transition from spherical micelles to rod-like aggregates (top) and an AFM image of rod-like structures (bottom). (C) Enzyme induced reversible
transformation of a DNA-brush block copolymer (top, precursor: 15) and its TEM images (bottom). Note that the outer-segments of
DNA-brushes in both spherical aggregates (top left and right of the representation) have ethylene glycol chains to lower electrostatic repulsion
between the DNA brushes. Reproduced from ref. 44, 46 and 34, respectively, with permission of Wiley-VCH Verlag GmbH & Co KGaA.
Dow
nloa
ded
by U
nive
rsit
y of
Gro
ning
en o
n 07
Feb
ruar
y 20
12
Pub
lish
ed o
n 22
Aug
ust
2011
on
http
://p
ubs.
rsc.
org
| doi
:10.
1039
/C1C
S15
138J
5752 Chem. Soc. Rev., 2011, 40, 5745–5755 This journal is c The Royal Society of Chemistry 2011
(Ramos) (Fig. 6B).50 The multimerization of the aptamer unit
at the surface of the AON aggregates significantly increased the
binding of the nanoobjects compared to the pristine nucleic
acid sequences. Although these are powerful new tools for
cancer therapy, there is an inherent limitation to the use of
micellar carriers in vivo: upon dilution below the critical micelle
concentration (CMC), they tend to disassemble. One potential
solution to this issue involves an additional nanocontainer, a
virus capsid.30 In addition to the advantages of micellar
delivery described above, it was demonstrated that micelles
of DNA amphiphiles can act as a template for the self-
assembly of Cowpea Chlorotic Mottle Virus capsids with
T = 1 and 2 geometry at neutral pH (Fig. 7A). The resulting
structures were then stable against dilution. At the same time,
this encapsulation around micelles of DNA amphiphiles
represented a general and facile supramolecular loading strategy
for hydrophobic or hydrophilic small molecules within the protein
nanocontainer, an essential process for the application of these
nanoobjects in biomedicine. The virus encapsulation of AONs is a
good example of how the properties, in this case the micellar
stability, can be greatly improved by combining AONs with other
materials.
AONs employed together with other materials
Composites are made from two or more materials to achieve
desired synergic properties out of the individual constituents.
Although the synthesis and designed self-assembly of a single
material is one of the beauties and goals of fundamental
research, more focused applications can be realized through
combination with other materials of interest. Since the essence
of DNA nanotechnology is the precise programmed assembly
of DNAs through Watson–Crick basepairing, the other com-
ponent of AONs, the hydrophobic moieties, can play an
important role as well by interacting with other hydrophobic
materials to produce versatile composites for particular
applications.
Fig. 6 Medical applications of micellar AONs in cancer therapy.
(A) A multifunctional micellar drug-carrier consisting of a DBC
(precursor: 9). The targeting unit, folic acid (FA), to cancerous cells
is attached to the outer surface of the DNA corona through hybridiza-
tion while a drug, doxorubicin (Dox), is loaded into a hydrophobic
core of the micelle (top). Fluorescence microscopy image of CaCo-2
cells that have taken up the micellar vehicle (bottom). Reproduced
from ref. 49 with permission of Wiley-VCH Verlag GmbH & Co
KGaA. (B) Graphical representation of a targeting micelle (precursor:
11) with an RNA aptamer as the corona (top). The aptamer micelle
exhibited a high cell-specific affinity to lymphoma cells (Ramos) as
shown in the fluorescence micrograph (bottom). Reproduced from
ref. 50 with permission of the National Academy of Sciences.
Fig. 7 Composite micelles consisting of AONs and viral capsids or synthetic polymers, respectively. (A) Micelles of DNA amphiphiles
(precursors: 9 and 11) loaded with either small hydrophobic compounds (top left) or with hydrophilic compounds by hybridization (top right)
templated virus capsid formation at neutral pH. TEM images of micelles incorporated in virus capsids with T = 1 or 2 geometry (bottom) and an
empty capsid formed at pH 5.0 as control (inset). Scale bars: 40 nm (adapted from ref. 30). (B) Graphical representation of a blend micelle.
A diblock DNA copolymer, PPO-b-DNA (precursor: 9), was mixed with a triblock copolymer Pluronic (PEO-b-PPO-b-PEO) that exhibits the same
hydrophobic block, PPO. Both block copolymers form mixed micelles. The hydrophobic core was further stabilized through UV-induced semi-
interpenetrating network formation, which inhibits disaggregation or precipitation under dilution or low temperature, respectively. In a similar
manner as shown in (A), the core and corona could be equipped with desired functionalities also with distance-control (adapted from ref. 53).
Dow
nloa
ded
by U
nive
rsit
y of
Gro
ning
en o
n 07
Feb
ruar
y 20
12
Pub
lish
ed o
n 22
Aug
ust
2011
on
http
://p
ubs.
rsc.
org
| doi
:10.
1039
/C1C
S15
138J
This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 5745–5755 5753
With regard to micellar aggregates, this principle has
resulted in doping electrostatically or magnetically active
materials in the hydrophobic core.51,52 In a more elegant
example, DNA diblock copolymers (DNA-b-PPO) formed
mixed micelles with Pluronict triblock architectures (PEO-b-
PPO-b-PEO) (Fig. 7B).53 This polymer–AON composite, with
a DNA/PEO corona and a PPO core, could be loaded with
hydrophobic molecules and stabilized against dissociation by
the formation of a semi-interpenetrating network, resulting in
highly stable soft particles. Furthermore, the potential of the
DNA for further self-assembly remained unchanged—the
corona could be conveniently modified by hybridization either
with dye-modified complementary DNA, demonstrating dis-
tance controlled functionalization, or with cDNA-labelled
gold nanoparticles. This composite is an excellent candidate
for a smart drug delivery vehicle, with possibly reduced
immunotoxicity due to the presence of the PEO chains on
the surface.
Vesicles or liposomes have been intensively investigated to
understand and manipulate biological events and processes.42,54
Due to the fact that their bilayer is composed of amphiphiles,
it is no surprise that AONs can also play a role in decorating
these nanocontainers with DNA. More precisely, the hydro-
phobic components of AONs can be tailored to anchor into
liposomes, leaving the DNA strands available for subsequent
supramolecular events, whether for functionalizing the surface
(Fig. 8A), anchoring the liposome or even tethering vesicles
together through hybridization (Fig. 8B. The AONs described
above, in which the hydrophobes are conjugated to the DNA
and peptide nucleic acid (PNA)55 termini, are most commonly
combined with phospholipid bilayers. Thus AONs have been
used to achieve liposome tagging,21,56,57 intervesicular
fusion,19,58 the assembly of larger containers59 and attachment
to lipid bilayers, mimicking cellular systems.22,60 With regard
to applications of AONs in animal experiments, lipophilic
units and cholesterol-conjugated small interfering (si) RNA
have been utilized to study siRNA-AON uptake into cells and
gene silencing in vivo.61
A more integrated application of AON composites was
reported with single-walled carbon nanotubes (SWNTs). In
the study, a single polymeric AON was demonstrated to be an
all-in-one solution for SWNT technology.27 The hydrophobic
segment of the AON, PFO (11), was covalently connected to a
short 22 mer via a phosphodiester bond and interacted
strongly with SWNT sidewalls by p–p interactions. This
allowed highly diameter-selective dispersion (0.8–1.1 nm) of
SWNT species out of as-produced HiPCO SWNTs (Fig. 9A).
The dispersion selected for a relatively small subset of chiral
indices, which determine the electronic properties of the
materials. In addition, the ssDNA block provided solubility
in aqueous media and, most importantly, remained available
for straightforward duplex formation with its cDNA (Fig. 9B).
Through base-pairing between the free DNA block and cDNA
conjugated to a gold nanoparticle, alignment of the metallic
particles along the SWNT was demonstrated (Fig. 9C).
A similar procedure allowed attachment of the dispersion to
a cDNA-functionalized surface. The same surface immobiliza-
tion onto lithographically prepared electrodes (Fig. 9D)
resulted in 98% fabrication efficiency for SWNT field effect
transistors. The entire process proved highly selective for
semiconducting SWNT species and represented a solution-
based, scalable method achieved purely by self-assembly. This
composite powerfully combined all the advantageous proper-
ties of the individual polymer, SWNT and DNA constituents.
5 Conclusions and outlook
Synthetic methods, chemical variety, structural parameters
and outlined functions of AONs are summarized in Table 1
for the reader’s convenience. These AONs are indeed a new
Fig. 8 AONs incorporated into lipid bilayers of liposomes.
(A) Hydrophobic parts (orange, precursors: 1–2, 4–7, 13) of AONs
could be used as anchors (left) to bilayers of liposomes. Simple tagging
of the liposomes was realized by hybridizing fluorescently labeled
cDNA (blue string, right) to the DNA segment (red string) of the
AON. (B) AONs could induce moderate vesicular fusion via duplex
formation of AONs anchored in liposomes.
Fig. 9 Utilization of AONs in a SWNT composite. (A) A polymeric
AON (precursor: 10), PFO-b-DNA, was used to disperse HiPCO
SWNTs as produced. (B) Resulting dispersions were enriched in
semiconducting SWNT species with narrow diameter distribution
due to the strong and specific interaction between the PFO block
and the SWNT surface. The presence of the protruding DNA strand
was exploited to introduce functionalities by hybridization with
cDNA-modified (blue) Au-nanoparticles (yellow spheres) (C) and to
sequence-specifically immobilize SWNTs on defined cDNA surfaces,
such as electrodes of field-effect transistors (FETs). (D) The device
yield (98%) achieved exclusively by aqueous solution processing and
hybridization was extremely high (adapted from ref. 27).Dow
nloa
ded
by U
nive
rsit
y of
Gro
ning
en o
n 07
Feb
ruar
y 20
12
Pub
lish
ed o
n 22
Aug
ust
2011
on
http
://p
ubs.
rsc.
org
| doi
:10.
1039
/C1C
S15
138J
5754 Chem. Soc. Rev., 2011, 40, 5745–5755 This journal is c The Royal Society of Chemistry 2011
class of materials that has seen rapid growth with great future
potential. Regarding the choice of synthetic methods, in
general coupling hydrophobic units to DNA in solution is a
difficult task mostly resulting in low overall yields. In contrast,
synthetic strategies based on SPS approaches are the preferred
route to fabricate AON materials. While preparing AONs
in-line guarantees good reproducibility and allows large scale
synthesis, the modification of ONs on solid supports with
hydrophobic moieties offers more flexibility regarding experi-
mental conditions and does not necessitate large equipment
costs. Within the context of polymeric AON synthesis, as of
yet no 30-amphiphilic DBCs have been reported using a fully
automated SPS approach showing that there are still synthetic
challenges to be overcome in the field. The same holds true for
the DNA part of AONs. ONs of up to hundred nucleotides,
as shown in Table 1, could be chemically synthesized. There
are still ongoing efforts to overcome this current limit, for
instance, by enzymatic reactions employing ss- and ds
ON-hybrids in combination with polymerases, ligases or
endonucleases to extend the size of nucleic acid segments.62–64
Another future task in AON synthesis is to incorporate
hydrophobic compounds that have interesting optoelectronic
properties like conjugated polymers, thereby widening the field
for potential applications in diagnostics and (nano)electronics.
The amphiphilicity of AONs allows them to aggregate into
superstructures, mostly micelle aggregates or vesicles. Thereby,
well established strategies for structure formation of small
molecule amphiphiles and block copolymers can be adopted
for AONs and their composites with other amphiphiles. And
indeed those methods are much simpler than the sophisticated
design and annealing techniques known for pristine DNA
nanoconstruction. Moreover, self-assembled nanostructures
of AONs consist of a very limited set of building blocks
compared to complex DNA nanoobjects.65,66 The aggregation
behavior of AONs can be conveniently controlled by adjusting
the molecular parameters like the length of the DNA segment
and the molecular weight or the nature of the hydrophobic
unit. Additional opportunities for manipulating their self-
assembly properties are hybridization and enzymatic modi-
fications. From another point of view, introducing amphiphilic
properties into the already well-developed field of pristine
DNA nanostructures should greatly enhance their appli-
cability. This can range from simply depositing them on
hydrophobic surfaces or generating well defined compartments
of different polarities.
Regarding applications, great potential of AONs can be
foreseen for the biomedical field. Promising examples have
demonstrated basic functions such as targeted-drug delivery or
transfection with AON nanoobjects in vitro. However, the
cytotoxicity, immunogenicity and uptake of AON nano-
structures are far from being well understood. In the future,
elucidating these issues will bring advances with AONs in the
pharmaceutical sector especially in regard to stability and
delivery of biologicals. Those challenges can only be mastered
in a multidisciplinary effort, especially with input from medical
and clinical researchers. In this respect, one can be excited
about the future demonstrations of AON nanostructures
in vivo.
Acknowledgements
We thank the EU (ERC starting grant, ECCell), the Netherlands
Organization for Scientific Research (NWO-Vici), the German
Research Foundation (DFG), and the Zernike Institute for
Advanced Materials for financial support.
References and notes
1 This quote is Nadrian C. Seeman’s, the founder of DNA nano-technology, title of his lecture since a decade.
2 J. H. Chen and N. C. Seeman, Nature, 1991, 350, 631–633.3 B. Yurke, A. J. Turberfield, A. P. Mills, F. C. Simmel andJ. L. Neumann, Nature, 2000, 406, 605–608.
Table 1 Summary of AONs covered in this review
No. Hydrophobea Func. ONb ON-Fuct. ON lengthc Couplingd Ref.
AONs terminally functionalized with a hydrophobic unit1 Lipid CEPA D, R 50-OH 20–81 SPS 18,502 Bislipid CEPA D 30-/50-OH 24–48 SPS 193 FDMT CEPA D 50-OH 30–100 SPS 204 Bislipid CEPA D 50-OH 15 SPS 215–7 Mono-/bischol CEPA D, R, P 30-/50-OH 12–42 SPS 22,56,588 PS (10K) CEPA D 50-OH 20, 25 postSPS 25,51,529 PPO (7K) CEPA D 50-OH 11, 22 SPS 26,30,4810 PFO (6K) CEPA D 50-OH 22 SPS 2713 Bislipid COOH D 50-OH 9 postSPS 4316 PB (2K) NH2 D 5
0-COOH 12 postSPS 41N/A PLGA (10K) COOH D 50-NH2 15 Solution 24AONs with DNA chains attached to a hydrophobic polymer backbone14a–b PNB (-b-Fc) CEPA D 50-OH 18, 22 postSPS 3315 PNB COOH D 50-NH2 19 postSPS 34AONs with DNA hydrophobic units attached to a DNA backbone11 Lipid CEPA/DMT D N/A 11, 12 (2, 4) SPS 30,31AONs containing a hydrophobic unit within the nucleic acid structure12 Azacrown ether CEPA/DMT D N/A 9–17 (1–3) SPS 32,60
a Numbers in parentheses represent number-averaged molecular weight (g molÿ1) of the polymers. b D: DNA, R: RNA, P: PNA. c Unit: number
of nucleotides. Numbers in parentheses represent the number of hydrophobes included in the AON. d SPS: solid-phase synthesis, postSPS: post-
synthesis after SPS of ONs.
Dow
nloa
ded
by U
nive
rsit
y of
Gro
ning
en o
n 07
Feb
ruar
y 20
12
Pub
lish
ed o
n 22
Aug
ust
2011
on
http
://p
ubs.
rsc.
org
| doi
:10.
1039
/C1C
S15
138J
This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 5745–5755 5755
4 H. Liu, Y. Chen, Y. He, A. Ribbe and C. Mao, Angew. Chem., Int.Ed., 2006, 45, 1942–1945.
5 P. Rothemund, Nature, 2006, 440, 297–302.6 P. Yin, R. F. Hariadi, S. Sahu, H. M. T. Choi, S. H. Park,T. H. Labean and J. H. Reif, Science, 2008, 321, 824–826.
7 S. Douglas, H. Dietz, T. Liedl, B. Högberg, F. Graf and W. Shih,Nature, 2009, 459, 414–418.
8 D. Han, S. Pal, J. Nangreave, Z. Deng, Y. Liu and H. Yan,Science, 2011, 332, 342–346.
9 A. N. Kapanidis and S. Weiss, J. Chem. Phys., 2002, 117,10953–10964.
10 D. A. Giljohann, D. S. Seferos, W. L. Daniel, M. D. Massich,P. C. Patel and C. A. Mirkin, Angew. Chem., Int. Ed., 2010, 49,3280–3294.
11 C. M. Niemeyer, Angew. Chem., Int. Ed., 2010, 49, 1200–1216.12 H. Rosemeyer, Chem. Biodiversity, 2005, 2, 977–1063.13 J. Goodchild, Bioconjugate Chem., 1990, 1, 165–187.14 S. Verma and F. Eckstein, Annu. Rev. Biochem., 1998, 67, 99–134.15 E. Defrancq, Y. Singh and N. Spinelli, Curr. Org. Chem., 2008, 12,
263–290.16 M. H. Caruthers, Acc. Chem. Res., 1991, 24, 278–284.17 S. L. Beaucage and M. H. Caruthers, Tetrahedron Lett., 1981, 22,
1859–1862.18 H. Liu, Z. Zhu, H. Kang, Y. Wu, K. Sefan and W. Tan,
Chem.–Eur. J., 2010, 16, 3791–3797.19 Y.-H. M. Chan, B. van Lengerich and S. G. Boxer, Proc. Natl.
Acad. Sci. U. S. A., 2009, 106, 979–984.20 W. Pearson, D. Berry, P. Stoy, K. Jung and A. D. Sercel, J. Org.
Chem., 2005, 70, 7114–7122.21 A. Gissot, C. Di Primo, I. Bestel, G. Giannone, H. Chapuis and
P. Barthelemy, Chem. Commun., 2008, 5550–5552.22 I. Pfeiffer and F. Hook, J. Am. Chem. Soc., 2004, 126,
10224–10225.23 G. Stengel, L. Simonsson, R. A. Campbell and F. Hook, J. Phys.
Chem. B, 2008, 112, 8264–8274.24 J. Jeong and T. Park, Bioconjugate Chem., 2001, 12, 917–923.25 Z. Li, Y. Zhang, P. Fullhart and C. Mirkin, Nano Lett., 2004, 4,
1055–1058.26 F. Alemdaroglu, K. Ding, R. Berger and A. Herrmann, Angew.
Chem., Int. Ed., 2006, 45, 4206–4210.27 M. Kwak, J. Gao, D. K. Prusty, A. J. Musser, V. A. Markov,
N. Tombros, M. C. A. Stuart, W. R. Browne, E. J. Boekema,G. ten Brinke, H. T. Jonkman, B. J. van Wees, M. A. Loi andA. Herrmann, Angew. Chem., Int. Ed., 2011, 50, 3206–3210.
28 M. Kwak and A. Herrmann, Angew. Chem., Int. Ed., 2010, 49,8574–8587.
29 J.-F. Lutz and H. G. Boerner, Prog. Polym. Sci., 2008, 33,1–39.
30 M. Kwak, I. Minten, D.-M. Anaya, A. Musser, M. Brasch,R. Nolte, K. Müllen, J. Cornelissen and A. Herrmann, J. Am.Chem. Soc., 2010, 132, 7834–7835.
31 M. Anaya, M. Kwak, A. J. Musser, K. Müllen and A. Herrmann,Chem.–Eur. J., 2010, 16, 12852–12859.
32 K. Rohr and S. Vogel, ChemBioChem, 2006, 7, 463–470.33 K. Watson, S. Park, J. Im, S. Nguyen and C. Mirkin, J. Am. Chem.
Soc., 2001, 123, 5592–5593.34 M.-P. Chien, A. M. Rush, M. P. Thompson and N. C. Gianneschi,
Angew. Chem., Int. Ed., 2010, 49, 5076–5080.35 H. Kolb, M. Finn and K. Sharpless, Angew. Chem., Int. Ed., 2001,
40, 2004–2021.36 G. Godeau, H. Arnion, C. Brun, C. Staedel and P. Barthelemy,
Med. Chem. Commun., 2010, 1, 76–78.37 F. Amblard, J. H. Cho and R. F. Schinazi, Chem. Rev., 2009, 109,
4207–4220.38 A. H. El-Sagheer and T. Brown, Chem. Soc. Rev., 2010, 39, 1388.39 P. M. E. Gramlich, C. T. Wirges, A. Manetto and T. Carell,
Angew. Chem., Int. Ed., 2008, 47, 8350–8358.
40 For rapid and efficient purification, anion-exchange (AIEX)chromatography is a commonly employed technique using a low-cost Tris-based buffer system. ‘‘Tris’’ is an abbreviated name oftris(hydroxymethyl)aminomethane. During AIEX chromato-graphy, Tris-HCl is used as a low saline buffer in combinationwith other chemicals: ethylenediaminetetraacetic acid, acetic acid,boric acid. In a high saline buffer, NaCl is added in high concen-tration (i.e. 1.0 M).
41 F. Teixeira Jr, P. Rigler and C. Vebert-Nardin, Chem. Commun.,2007, 1130–1132.
42 S. F. M. van Dongen, H.-P. M. de Hoog, R. J. R. W. Peters,M. Nallani, R. J. M. Nolte and J. C. M. van Hest, Chem. Rev.,2009, 109, 6212–6274.
43 M. P. Thompson, M.-P. Chien, T.-H. Ku, A. M. Rush andN. C. Gianneschi, Nano Lett., 2010, 10, 2690–2693.
44 F. E. Alemdaroglu, J. Wang, M. Börsch, R. Berger andA. Herrmann, Angew. Chem., Int. Ed., 2008, 47, 974–976.
45 J. Wang, F. E. Alemdaroglu, D. K. Prusty, A. Herrmann andR. Berger, Macromolecules, 2008, 41, 2914–2919.
46 K. Ding, F. E. Alemdaroglu, M. Börsch, R. Berger andA. Herrmann, Angew. Chem., Int. Ed., 2007, 46, 1172–1175.
47 Y. Lim, K. Moon and M. Lee, Chem. Soc. Rev., 2009, 38,925–934.
48 F. E. Alemdaroglu, C. N. Alemdaroglu, P. Langguth andA. Herrmann, Adv. Mater., 2008, 20, 899–902.
49 F. E. Alemdaroglu, C. N. Alemdaroglu, P. Langguth andA. Herrmann, Macromol. Rapid Commun., 2008, 29, 326–329.
50 Y. Wu, K. Sefah, H. Liu, R. Wang and W. Tan, Proc. Natl. Acad.Sci. U. S. A., 2010, 107, 5–10.
51 M. Sowwan, M. Faroun, E. Mentovich, I. Ibrahim, S. Haboush,F. E. Alemdaroglu, M. Kwak, S. Richter and A. Herrmann,Macromol. Rapid Commun., 2010, 31, 1242–1246.
52 X.-J. Chen, B. L. Sanchez-Gaytan, S. E. N. Hayik, M. Fryd,B. B. Wayland and S.-J. Park, Small, 2010, 6, 2256–2260.
53 M. Kwak, A. J. Musser, J. Lee and A. Herrmann,Chem. Commun.,2010, 46, 4935–4937.
54 J. Voskuhl and B. J. Ravoo, Chem. Soc. Rev., 2009, 38,495–505.
55 M. Loew, R. Springer, S. Scolari, F. Altenbrunn, O. Seitz,J. Liebscher, D. Huster, A. Herrmann and A. Arbuzova, J. Am.Chem. Soc., 2010, 132, 16066–16072.
56 F. Bombelli, F. Betti, F. Gambinossi, G. Caminati, T. Brown,P. Baglioni and D. Berti, Soft Matter, 2009, 5, 1639–1645.
57 P. Beales and T. Vanderlick, J. Phys. Chem. B, 2009, 113,13678–13686.
58 G. Stengel, R. Zahn and F. Hook, J. Am. Chem. Soc., 2007, 129,9584–9585.
59 M. Loew, L. Kang, L. Daehne, R. Hendus-Altenburger,O. Kaczmarek, J. Liebscher, D. Huster, K. Ludwig, C. Boettcher,A. Herrmann and A. Arbuzova, Small, 2009, 5, 320–323.
60 U. Jakobsen, A. C. Simonsen and S. Vogel, J. Am. Chem. Soc.,2008, 130, 10462–10463.
61 C. Wolfrum, S. Shi, K. N. Jayaprakash, M. Jayaraman, G. Wang,R. K. Pandey, K. G. Rajeev, T. Nakayama, K. Charrise,E. M. Ndungo, T. Zimmermann, V. Koteliansky, M. Manoharanand M. Stoffel, Nat. Biotechnol., 2007, 25, 1149–1157.
62 M. Safak, F. E. Alemdaroglu, Y. Li, E. Ergen and A. Herrmann,Adv. Mater., 2007, 19, 1499–1505.
63 S. Keller, J. Wang, M. Chandra, R. Berger and A. Marx, J. Am.Chem. Soc., 2008, 130, 13188–13189.
64 M. S. Ayaz, M. Kwak, F. E. Alemdaroglu, J. Wang, R. Berger andA. Herrmann, Chem. Commun., 2011, 47, 2243.
65 C. X. Lin, Y. Liu, S. Rinker and H. Yan, ChemPhysChem, 2006, 7,1641–1647.
66 S. M. Douglas, A. H. Marblestone, S. Teerapittayanon,A. Vazquez, G. M. Church and W. M. Shih, Nucleic Acids Res.,2009, 37, 5001–5006.
Dow
nloa
ded
by U
nive
rsit
y of
Gro
ning
en o
n 07
Feb
ruar
y 20
12
Pub
lish
ed o
n 22
Aug
ust
2011
on
http
://p
ubs.
rsc.
org
| doi
:10.
1039
/C1C
S15
138J
Recommended