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ISSN 0306-0012
Chemical Society Reviews
0306-0012(2010)39:6;1-S
TUTORIAL REVIEW
Xuehai Yan, Pengli Zhu and Junbai Li
Self-assembly and application of
diphenylalanine-based nanostructures
TUTORIAL REVIEW
Tadashi Nakata
SmI2-induced reductive cyclizations
for the synthesis of cyclic ethers
and applications in natural product
synthesis
www.rsc.org/chemsocrev Volume 39 | Number 6 | June 2010 | Pages 1861–2336
Self-assembly and application of diphenylalanine-based nanostructures
Xuehai Yan,
ab
Pengli Zhu
a
and Junbai Li*
a
Received 24th November 2009
First published as an Advance Article on the web 9th March 2010
DOI: 10.1039/b915765b
Micro- and nanostructures fabricated from biological building blocks have attracted tremendous
attention owing to their potential for application in biology and in nanotechnology. Many
biomolecules, including peptides and proteins, can interact and self-assemble into highly ordered
supramolecular architectures with functionality. By imitating the processes where biological
peptides or proteins are assembled in nature, one can delicately design and synthesize various
peptide building blocks composed of several to dozens of amino acids for the creation of
biomimetic or bioinspired nanostructured materials. This tutorial review aims to introduce a new
kind of peptide building block, the diphenylalanine motif, extracted with inspiration of a
pathogenic process towards molecular self-assembly. We highlight recent and current advances in
fabrication and application of diphenylalanine-based peptide nanomaterials. We also highlight
the preparation of such peptide-based nanostructures as nanotubes, spherical vesicles, nanofibrils,
nanowires and hybrids through self-assembly, the improvement of their properties and the
extension of their applications.
1. Introduction
Molecular self-assembly is a spontaneous process of formation of
ordered structures under thermodynamic and kinetic conditions
as a consequence of specific and local interactions of molecules
themselves.
1,2
These molecules undergo self-association usually
forming hierarchical structures at the nanoscale or at the macro-
scale. Self-assembly plays a vital role in many biological systems,
either to achieve its biological function or as part of a pathogenic
process, e.g. the formation of biological membranes upon
self-assembly of phospholipids, DNA double helix formation
through specific hydrogen bonding interactions, protein
microtubules and microfilaments as functional units for intra-
cellular interplay, as well as the formation of amyloid fibrils
relevant to a variety of neurological disorders or diseases. With
inspiration from biology, a variety of biological and biomimetic
materials have been constructed via such a ‘‘bottom-up’’
approach, molecular self-assembly.
3–9
The ordered organization
of building blocks into defined nanostructures relies on specific
molecular recognition which is facilitated by a combination of
non-covalent interactions, including hydrogen bonds, electro-
static interactions, p–pstacking, hydrophobic forces, non-specific
Van der Waals forces, and chiral dipole–dipole interactions.
Although individually these forces are relatively weak, when
a
Beijing National Laboratory for Molecular Sciences (BNLMS),
International Joint Lab, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, China. E-mail: jbli@iccas.ac.cn;
Fax: +86 10 8261 2629; Tel: +86 10 8261 4087
b
Max Planck Institute of Colloids and Interfaces, Am Mu
¨hlenberg 1,
D-14476 Golm/Potsdam, Germany
Xuehai Yan
Xuehai Yan received his BE
degree in chemical engineering
in 2002 and MS degree in
applied chemistry in 2005
from China University of
Mining and Technology. Then
he joined the Institute of
Chemistry, the Chinese
Academy of Sciences, where
he obtained his PhD in physical
chemistry in 2008. Currently
he is working as a research
fellow of the Alexander von
Humboldt Foundation at the
Max Planck Institute of
Colloids and Interfaces in
Germany. His research interests are focused on self-assembly
of biomimetic and bioinspired nanomaterials, especially peptide-
based hierarchical assembly.
Pengli Zhu
Pengli Zhu received her BS
degree in chemistry from
Henan University in 2004 and
is now a graduate student
pursuing her PhD at the
Institute of Chemistry, the
Chinese Academy of Science.
Her research interests include
designing peptide-based nano-
structures and organogels,
biomacromolecule self-assembly,
synthesis and functionalization
of nanoparticles and
nanotubes.
This journal is cThe Royal Society of Chemistry 2010 Chem.Soc.Rev., 2010, 39, 1877–1890 |1877
TUTORIAL REVIEW www.rsc.org/csr |Chemical Society Reviews
combined as a whole, they can govern the self-assembly of
molecular building blocks into superior and ordered structures.
The individual forces are also not large compared to thermal
forces—they are of a similar magnitude—thus enabling
variations of structures and properties by a small variation of
parameters.
The nanostructures fabricated from biomolecules are
attracting increasing attention owing to their biocompatibility,
their ability for specific molecular recognition, simple
chemical and biological modification and easy availability
for bottom-up fabrication. Many biomolecules such as
lipids, nucleic acids, proteins and peptides, can interact
and self-assemble into highly ordered supramolecular
architectures.
9–13
Among them, peptides composed of
several to dozens of amino acids are a class of versatile
building blocks aiming at this target.
13–15
The inherent
biological origin of peptides make them rather favorable for
medical and biological applications. In some cases, they
can also imitate the behavior and function of proteins,
offering an alternative model for gaining insight into
self-assembly and protein function. Furthermore, the self-
assembled capability of designed or extracted peptide building
blocks enables them to be readily manipulated into
well-defined nanostructures with various functions. Over the
past few decades, researchers have made significant progress in
this field. A number of peptide-based building blocks,
including cyclic peptides, dendritic peptides, amphiphile
peptides, surfactant-like oligopeptides, copolypeptides, and
aromatic dipeptides, have been designed and developed for
the creation of functional supramolecular architectures and
the exploration of their possible applications in biology and
nanotechnology.
Getting inspiration from nature is of particular importance
for designing peptide building blocks towards self-assembly.
For instance, based on the discovery of a repetitive 16-residue
peptide motif n-AEAEAKAKAEAEAKAK-c (EAK16-II), a
fragment of a left-handed Z-DNA binding protein in
yeast, Shuguang Zhang et al. designed a class of ionic
self-complementary peptides such as RDA16-I, RAD16-II,
EAK-I.
13
This class of peptide building blocks shares a
common architectural feature that both positively and
negatively charged side chains are on one side of the b-sheet
and hydrophobic side chains on the other. They have been
shown to undergo self-assembly into nanofiber scaffolds,
having potential application in 3D tissue cultures. Inspired
by the self-assembly of lipids, a class of essential components
in cell membranes, Zhang et al. designed surfactant-like
oligopeptides with hydrophobic tails and hydrophilic heads.
16
They can undergo self-assembly by the aggregation of
hydrophobic tails in water similar to a lipid molecule.
However, a possible difference for driving self-assembly still
exists compared to natural lipids because the surfactant-like
peptides also likely interact through the hydrogen bonds along
peptide backbones.
16
In addition, certain peptide building blocks for self-
assembly are derived with inspiration from a pathogenic
process. A known example is the diphenylalanine peptide
which is extracted from Alzheimer’s b-amyloid poly-
peptide as the core recognition motif for molecular self-
assembly.
17
Since the emergence of FF as a self-assembling
building block, many studies have been made to organize
the FF-based building blocks into various functional nano-
structures such as nanotubes, spherical vesicles, nanofibrils,
nanowires and ordered molecular chains (Fig. 1). Potential
applications of self-assembled FF-based nanomaterials have
also been demonstrated. It is believed that this field is
growing at an accelerating pace with the aim of improved
properties and enhanced functions of FF building blocks.
However, there exist no reports to comprehensively
summarize the development and the application of FF-based
building blocks for molecular self-assembly. Therefore,
in this review, we first focus on the self-assembly of FF-based
building blocks into various nanostructures, and the
fabrication of functional inorganic hybrids. Then we
discuss the potential applications of such assembled functional
materials in biological and non-biological areas, including
3D cell culture, drug delivery, bioimaging, biosensors
and guest encapsulation as well as templates for
nanofabrication.
Fig. 1 Schematic representation of various nanostructures formed by
self-assembly of FF-based building blocks and their potential
applications.
Junbai Li
Junbai Li obtained his PhD in
polymer science from Jilin
University in 1992, China.
He then spent several years
carrying out postdoctoral
research in colloid and inter-
face science at the Max
Planck Institute of Colloids
and Interfaces in Germany,
where he had a long term of
collaboration with the inter-
face department. He is currently
a Professor at the Institute of
Chemistry, the Chinese
Academy of Sciences and
heads the CAS key lab of
colloid & interfaces science. His main research interests are on
molecular assemblies of biomimetic systems; self assembly and
biointerfaces of molecular patterns; design and synthesis of
bioinspired materials with various nanostructures.
1878 |Chem. Soc. Rev., 2010, 39, 1877–1890 This journal is cThe Royal Society of Chemistry 2010
2. Molecular self-assembly into various
nanostructures
2.1 Nanotubes and their transition into vesicle-like structures
Biological proteins and peptides are extremely attractive as
potential building blocks for the fabrication of tubular nano-
structures. There has been rapid progress in the development
and potential application of peptide-based nanotubes.
18
The simplest peptide building block for self-assembly is
generally considered as diphenylalanine peptide (L-Phe-L-Phe,
FF, Fig. 2-1). It has been demonstrated that FF can
self-assemble into well-ordered tubular structures with a long
persistence length (B100 mm) by a combination of hydrogen
bonding and p–pstacking of aromatic residues (Fig. 3a).
17
A recent systematic X-ray diffraction (XRD) study has further
unraveled the molecular organization in the nanotubes formed
from hydrophobic dipeptides.
19
The ordered molecular
organization of FF nanotubes (FNTs) stems from a striking
three-dimensional aromatic-stacking alignment that serves as
a glue between the hydrogen-bonded cylinders of the peptide
main chain.
20
Interestingly, the introduction of a thiol group
into FF peptide can alter the self-assembly properties of
the resultant building blocks, e.g. cysteine-diphenylalanine
tripeptide (CFF, Fig. 2-2) self-assembles into spherical vesicles
rather than into nanotubes. This is ascribed to the energetic
contribution provided by the disulfide cross-links, which
makes it possible to bend and to close the stacking layer along
two axes.
21
An understanding of physical and chemical
properties of peptide nanotubes is important for future
technological application. In an atomic force microscope
(AFM) measurement upon dry-heating, the dried FNTs
display remarkable thermal stability up to 100 1C, but lose
their structural integrity with further increase in temperature
and undergo an apparent degradation at temperatures at or
above 150 1C.
22
The tubular nanostructures are considerably
rigid entities with a high Young’s modulus of around 19 GPa
23
(27 GPa in another study, where a bending-beam model to
AFM images of FNTs was applied to obtain the Young’s
modulus
24
). This makes them among the stiffest biological
materials presently known. These unique properties enable the
application of such peptide nanotubes for the fabrication of
biocompatible nanodevices.
In some cases, macroscopic organization or alignment of
self-assembled peptide nanotubes is needed to meet the
demand for the bionanotechnological application. A vertically
aligned array of FNTs is achieved by axial unidirectional
growth of nanotubes.
25
This growth process is due to the
evaporation-initiated self-assembly process, in which an FF
solution in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) is spread
over a silanized glass substrate and a thin layer composed of
nanotube arrays can be formed upon rapid evaporation of
HFIP. It has been proposed that the well-ordered organization
of nanotubes is most likely controlled by a nucleation-growth
mechanism in the vapour–liquid–solid system that exists
during the rapid evaporation of solvent. Furthermore, a
horizontal alignment of FNTs has also been demonstrated
by simply coating magnetite nanoparticles on the surface of
nanotubes and then applying an external magnetic field.
Intriguingly, individual FNTs can also be well-aligned by
exposing them to a strong magnetic field.
26
Quite different to
the magnetic alignment of proteins where ordered organization
of peptide bonds gives the major contribution to the net
diamagnetic anisotropy, the aromatic moieties of FF play a
key role in the alignment of FNTs. The ordered orientation of
aromatic rings is mainly responsible for the net anisotropy in
this structure. In turn, the alignment direction of the nano-
tubes in the magnetic field can give us information on the
orientation of aromatic rings that can be instructive for the
validation of a model structure. The ordered array or
the controlled alignment of FNTs may make it possible to
integrate them into multi-array sensitive sensors, and nano-
devices for future use.
The FNTs have been manipulated into well-organized films
on various substrates (SiO
2
, Au, Pd, alumina, mica, quartz,
InP) by self-assembly of FF in a suitable solvent, such as
N-methyl-2-pyrrolidone (NMP), which can either act as the
disassembly/disbundling agent or allow FNTs to reorganize in
a film-growth form during the solvent evaporation.
27
In
addition to the formation of FNT films, the authors successfully
fabricated silver-incorporated FNT composite films by using a
similar method combined with inclusion chemistry. It is
believed that the creation of such FNT films offers opportunities
for the construction of new complex nanostructures.
Recently, we reported that a cationic dipeptide (H-Phe-Phe-
NH
2
HCl, Fig. 2-3) derived from FF can self-assemble into
nanotubes (hereafter referred to as CDPNTs) at physiological
pH.
28,29
Morphological features of the nanotubes were
revealed by scanning electron microscopy (SEM), transmission
electron microscopy (TEM) and AFM studies. For example, a
TEM image exhibited a typical tubular structure with enough
contrast to distinguish the inner part and the periphery of the
nanotube (Fig. 3b). The SEM and AFM studies likewise
supported the morphological feature observed in the TEM
image. The circular dichroism (CD) signature obtained on the
CDPNTs has some similarities with that of a-helical poly-
peptides, and the observed extrema may correspond to a-helical
p-p* and n -p* transitions, respectively. The pathway to
form tubular structures is proposed in Fig. 3c. Peptide
Fig. 2 Molecular structures of several representative FF-based
building blocks.
This journal is cThe Royal Society of Chemistry 2010 Chem.Soc.Rev., 2010, 39, 1877–1890 |1879
monomers first stack to form a 2D layer and then allow the
closure of the 2D layer, thus leading to the tubular structures.
Interestingly, a spontaneous transformation of nanotubes
into spherical vesicle-like structures takes place by diluting a
solution containing CDPNTs at pH 7.2. The structural
transition of self-assembled materials is commonly observed
and achieved on demand. Parameters to control the morphology
mainly involve the molecular and solution parameters. For a
given building molecule, its molecular parameters such as
hydrophobic/hydrophilic properties and the architecture are
constant. Nevertheless, a different morphology can still be
obtained by tuning solution parameters such as the type of
solvents and solvent quality, the building molecule concentration,
the pH value and temperature, etc.
30
The concentration-
dependent structural transition in the peptide-based building
blocks, such as linear surfactant-like oligopeptides and
D-Phe-D-Phe dipeptides, has been confirmed.
31,32
As shown
in Fig. 4a,b, the transition between CDPNTs and vesicle-like
structures is reversibly dependent on the concentration of the
peptide building blocks, indicating that the concentration
plays a critical role in determining the final nanostructure
morphology. In other words, the conversion between tubular
and spherical structures is capable of being readily modulated
by varying the concentration of peptide building blocks. The
joined spheres in a necklace-like structure, an intermediate
state of the transition of CDPNTs into vesicle-like structures,
were observed directly by using TEM and fluorescence
microscopy (Fig. 4c,d).
Analogous to the self-assembly of FNTs, the driving forces
for the formation of CDPNTs have been proposed to be
hydrogen bonding and p–pstacking. However, the XRD
pattern of CDPNTs is somewhat different from that of FNTs,
indicating possible differences in the organization of the
cationic dipeptides.
29
This is attributed to the charged dipeptide
state and the nature of the counterions which to some extent
affect the molecular arrangement of the cationic dipeptide. At
higher concentrations of dipeptide (10 mg mL
1
), sufficient
free energy of association can be gained by the intermolecular
interactions, resulting in the tubular nanostructure. The
molecular rearrangement of dipeptide probably occurs with
the decrease in the concentration and thus leads to the formation
of vesicle-like structures, which minimizes the free-energy of
the system. Similar to the result from Monte Carlo simulations
carried out by Song et al.,
32
the dipeptide behaves somewhat
like a surfactant; the polar groups are isolated from the
hydrophobic aromatic groups to form bilayers. With the
increasing dipeptide concentration, the molecular stacking
may lead to spherical bilayer vesicles, unilamellar nanotubes,
multilamellar structures and continuous phases, respectively.
Fig. 3 TEM image of the FNTs (a) and the CDPNTs (b); reproduced in part with permission from ref. 17 and 28. Copyright 2003, American
Association for the Advancement of Science and Copyright 2007, Wiley. (c) A proposed schematic illustration of formation of FF-based
nanotubes.
Fig. 4 The reversible transition between peptide nanotubes and
vesicle-like structures: AFM height image of nanotubes (a) and
vesicle-like structures (b). (c) Fluorescent optical image of the joined
necklace-like structures composed of spherical vesicles bound with
fluorescently-labeled ss-DNA and (d) TEM image of the joined
necklace-like structures. Reproduced with permission from ref. 28
and 29. Copyright 2007 and 2008, Wiley.
1880 |Chem. Soc. Rev., 2010, 39, 1877–1890 This journal is cThe Royal Society of Chemistry 2010
On the basis of the multilamellar model of dipeptide
arrangement, a general model has been set up to gain deeper
insight into the morphological transition between CDPNTs
and vesicle-like structures.
29
To describe the formation of
different shapes, the following two-stage mechanism is taken
into account.
33,34
Firstly, monomer molecules are transferred
from the isotropic phase (I-phase) to the outermost face of the
aggregate phase (A-phase) to form a layer of thickness h,
where the layer must gain additional internal cohesion energy
to compensate for the obvious entropy decrease of the
transferred molecules. Additionally, the obtained cohesion
energy is mechanically balanced through forming a defined
shape which minimizes the total volume and surface free
energy. For our system involving a dipeptide structural transition,
the aggregation at the first stage is readily tuned by changing
the peptide concentration. Hence the gained internal stress of the
formed layer also varies with peptide concentration,
leading to the shape transition. Following this mechanism,
we derive a theoretical equation to define the critical tube-
vesicle concentration (CTVC):
29
CTVC = C
A
e
3g/C
A
dk
B
T
where C
A
is the molecule concentration of the A-phase which
is a constant for a given monomer, gis the tension of the
solution/aggregate interface, dis the molecular size, k
B
is the
Boltzmann constant, and Tis the temperature. The CTVC
value has a clear physical significance: when C
S
oCTVC,
nanotubes must transform rapidly into spherical vesicle-like
structures, in accordance with the experimental observation in
which dipeptide nanotubes spontaneously change into
metastable necklace-like structures and finally into spherical
vesicle-like structures. To intuitively comprehend the shape
transition, a rotationally symmetric hypersurface can be
constructed by Delaunay’s method: by rolling a given conical
section on a straight line in a plane, and then rotating the trace
of a focus about the line, one obtains such a surface. In our
case, the conic section is assumed to be an ellipse. A beaded
structure is obtained by the Delaunay construction with a thin
ellipse cross section (Fig. 5a). If the ellipse is very flat, then the
beaded structure becomes vesicles in a necklace-like structure
(Fig. 5b). At the other limit, if the ellipse becomes a circle the
resulting surface is a cylindrical tube (Fig. 5c). All in all, the
presented theoretical model proposes a way to engineer
assembling molecules in order to devise other systems whose
morphology could be tuned on demand. Three kinds of
free energy, internal cohesion energy, interface energy and
curvature elastic energy are mainly responsible for this class of
shape transition related to monomer concentration. It is
believed that the theoretical model on CTVC has a critical
significance in providing insight into certain kinds of shape
transitions in molecular self-assembly.
2.2 Nanofibrils and ribbons
Fibril formation upon self-assembly of peptides and proteins is
ubiquitous in biology. In particular, peptide fibrillization is
relevant to a number of diseases, including Alzheimer’s,
Parkinson’s, Huntington disease, type II diabetes, and prion
disorders with the characteristic of deposition of amyloid
fibrils in various tissues and organs.
35,36
On the other hand,
nanofibrils self-assembled from natural or de novo designed
peptides display remarkable potential for application in
bionanotechnology. Therefore, peptide nanofibrils constitute
one of the most abundant and important naturally occurring
self-assembled materials.
As the shortest structural recognition motif for the
Alzheimer’s b-amyloid polypeptide, FF plays a decisive role
in fibril formation, which is attributed to the p-stacking
interaction of FF aromatic residues.
35
Basically, FF affords
the structural communication for molecular self-association.
As highlighted above, indeed, FF can undergo self-assembly
into long and stiff nanotubes through an association of
hydrogen bonding and p–pstacking between phenyl rings.
Remarkably, it has been observed that a 9-fluorenylmethoxy-
carbonyl (Fmoc) protected diphenylalanine (Fmoc-FF, Fig. 2-4)
self-assembles into nanofibrils in water and thus results in a
hydrogel held together in a network by hydrogen bonding and
p–pinteraction.
37,38
Such a class of hydrogel is self-supporting
and has a rheological behavior with a storage modulus (G0)
that is approximately an order of magnitude larger than the
loss modulus (G00), which is a characteristic of solid-like gel
materials. Compared with other peptide or protein hydrogels,
Fmoc-FF hydrogel is considerably stronger and stiffer, and
can be stable at a wide range of temperatures and pH values
including extreme acidic conditions. Thus Fmoc-FF hydrogels
can be more advantageous for certain applications such as
controlled drug release and 3D cell culture. A molecular model
of Fmoc-FF peptide has been recently proposed to shed light
on the molecular stacking mode in the self-assembling fibrous
structures. In accordance with this model, the peptides are
aligned in an antiparallel b-sheet fashion and adjacent sheets
are interlocked through lateral p–pinteractions, thus leading
to the formation of a cylindrical structure (Fig. 6).
39
FF-based peptides can be designed to be responsive to
enzymes, thereby enzymes can be used to regulate or control
molecular self-assembly for hydrogelation, which generally
takes place in vitro or in vivo. There are two ways to achieve
the enzyme-tuned formation of hydrogels: (1) catalyzing the
bond formation of the hydrogelator; or (2) removing a blocking
group from a precursor molecule to form the hydrogelator and
Fig. 5 Illustration of Delaunay’s constructing method: (a) a sphere,
(b) neck-like structure and (c) tube with constant mean curvature. f is
indicative of the focus of an ellipse. Reprinted with permission from
ref. 29. Copyright 2008, Wiley.
This journal is cThe Royal Society of Chemistry 2010 Chem.Soc.Rev., 2010, 39, 1877–1890 |1881
the hydrogelator then self-assembles, usually forming nano-
fibers and further entangling to serve as a matrix for the
hydrogel.
15,40
A number of natural enzymes, such as thermolysin,
b-lactamase, phosphatase, and phosphatase/kinase, have been
applied to the enzyme-tuned self-assembly of FF-based
molecules. For example, Ulijn et al. reported that thermolysin
can catalyze the bond formation between Fmoc-amino acids
and FF to synthesize a hydrogelator, which self-assembles
into three-dimensional fibril networks.
41,42
This self-assembly
process is achieved by the thermolysin-catalyzed reverse
hydrolysis that is thermodynamically unfavorable, but
involves relatively small free energy changes. The production
of building blocks and subsequent self-assembly is proposed to
be a key avenue to form thermodynamically stable supra-
molecular structures, therefore paving the way to directed-
assembly of nanostructures with enhanced complexities and
fewer defects. In addition, Xu et al. presented a series of
enzyme-regulated self-assembly of nanofibrils dealing with
the FF motif by converting the enzyme-cleavable precursors
into self-assembling building blocks.
43–46
For instance,
b-lactamases, an important family of enzymes that catalyze
the hydrolysis of b-lactam antibiotics,
47
were utilized to trigger
nanofibril formation.
43
Initially, a water-soluble precursor
was synthesized by connecting a hydrophilic group to a
hydrogelator with the FF motif by using the cephem nucleus
as the linker. Through the action of b-lactamases the hydro-
gelator (C
10
H
7
CH
2
CO-phe-phe-NHCH
2
CH
2
SH) was released
to self-assemble into the nanofibrils in water and then form the
hydrogels. This facile process could detect b-lactamase in the
lysates of bacteria. Design of the enzyme-responsive molecule
is concerned with the dipeptide (FF) segment, which acts as a
major building block to control the molecular self-assembly.
Such enzyme-tuned self-assembly involving the FF motif may
be triggered inside a cell. The hydrogelator is designed to
C
10
H
7
CH
2
CO-phe-phe-NHCH
2
CH
2
OH and a cleavable
butyric acid is introduced to yield the water-soluble precursor
which does not self-assemble extracellularly.
44
The intra-
cellular self-assembly takes place upon the action of an
endogenous enzyme such as esterase which converts the
precursor molecule to the hydrogelator. The use of enzymes
in molecular self-assembly enables us to arrive at the control of
the self-assembly process. Therefore, new function and
improved property of self-assembled nanomaterials from
FF-based building blocks can be achieved upon introduction
of the enzymatic reaction either in vitro or in vivo.
Low molecular-mass organogelators (LMOGs) are known
to gelate a number of organic liquids for the formation of
organogels having unique properties and potential application
as distinct soft materials. Recently, we found that the single
dipeptide, FF, acted as a LMOG to gelate some organic
solvents such as chloroform and aromatic solvents.
48
Such
FF organogels are composed of predominantly long and
entangled fibrils (Fig. 7a) and are thermoreversible, having
an apparent sol–gel transition temperature. It is known that a
kinetically trapped state of a gel can be changed into a more
thermodynamically stable crystal upon ageing or external
induction. Through the introduction of ethanol as a co-solvent
into toluene, the structural transition of mesostable FF
organogels into microcrystals is readily realized.
49
The SEM
image (Fig. 7b) shows that thermodynamically stable
structures in the crystal phase are the flower-like microcrystals
consisting of the packing ribbons. The AFM observation
indicates that the ribbons have obvious lamellar layers
(Fig. 7c), implying that microcrystals are formed through the
hierarchical self-assembly of FF molecules. Intriguingly, in the
25% ethanol–toluene solution the dynamic morphological
transition from the kinetically trapped state of the gel into
the thermodynamically more stable crystal can be directly
observed. That provides an excellent example to help us
understand the gel–crystal transition process and to allow us
to obtain structural information in different stages of phase
transition.
The intermolecular interactions governing the self-assembly
of FF in the gels and microcrystals are determined by using
multiple spectroscopy techniques such as Fourier transform
infrared (FTIR), CD and fluorescence spectroscopy as well as
XRD and thermogravimetric analysis (TGA). The FTIR
spectrum of organogels in toluene shows the predominant
b-sheet character based on the position of the amide I band
at 1620 and 1683 cm
1
, and possibly an antiparallel
configuration.
48,49
The CD spectrum of gels gives a signature
with b-sheet arrangements of FF molecules, in agreement with
the FTIR analysis. In the crystal phase, FF molecules are
possibly more organized in a parallel b-sheet mode based on
the amide I band absorption in the vicinity of 1615 cm
1
.
49
To get information on the arranged mode of FF aromatic
residues, fluorescence spectroscopy was used to determine the
emission spectra of gels and microcrystals in toluene. In the
FF solution, the phenyl groups have an emission peak at
306 nm, which shifts to 339 nm for the gel and 285 nm for the
microcrystal.
49
The red shift in the gel phase suggests an
effective p–pstacking between the aromatic residues, and
probably in a J-aggregate fashion. By contrast, the blue shift
in the crystal phase indicates a possible extended H-aggregate
between the phenyl rings. The XRD pattern of a dried gel
Fig. 6 A model structure is created to describe the arrangement of
Fmoc-FF peptides in a hydrogel. The peptide is arranged in an
antiparallel b-sheet pattern. Reproduced with permission from
ref. 39. Copyright 2008, Wiley.
1882 |Chem. Soc. Rev., 2010, 39, 1877–1890 This journal is cThe Royal Society of Chemistry 2010
showed a sharp peak at 2yof 5.21with dspacing of 1.7 nm,
corresponding to the thickness of a b-sheet monolayer.
48
The
microcrystals formed in pure ethanol have a similar XRD
pattern as FF single crystals and nanotubes, yielding a
hexagonal packing structure. Nevertheless, the diffraction
patterns of microcrystals obtained in mixed solvents are
somewhat different from that in ethanol alone, indicating that
toluene molecules possibly interfere or take part in the self-
assembly of FF through aromatic stacking. Such a conjecture
is confirmed by the TGA of microcrystals in different
ethanol–toluene mixed solvents. The weight loss at about
100 1C in the TGA curves is assigned to the toluene molecules
embedded in the crystal lattice. The higher the toluene ratio in
the mixed solvents, the higher weight loss that is observed at
this temperature.
49
On the basis of the results above, it has
been suggested that in the gel phase FF molecules may adopt
antiparallel b-sheet secondary structures with the J-aggregate
nature of aromatic residues and in the crystal state supposed
to organize in the parallel b-sheet form with the creation of
H-aggregates of the phenyl groups (Fig. 8). Solvent properties,
such as polarity and the ability to form hydrogen bonding,
play a vital role in regulating the formation of organogels and
controlling the ultimately self-assembled structures, nano-
fibrils or microcrystals. The discussion of solvent effects on
the gelation provides new insights into the solvent–gelator
interaction and the molecular arrangement mode in the gel
phase as well as in the crystal phase.
2.3 Nanowires
Fabrication of 1D nanowires from biological building blocks
is important for the development of new materials or devices.
The FF dipeptide was recently found to be capable of self-
assembling into individually-dispersed and rigid nanowires in
carbon disulfide (CS
2
).
50
XRD measurements show that the
peptide nanowires have a hexagonal columnar arrangement,
which is in good agreement with that of a single crystal or
nanotube despite the morphological discrepancy. Such peptide
nanowires display novel liquid crystalline behavior over a wide
concentration range. When liquid nanowires are viewed under
cross polarized light, a Schlieren texture, indicative of the
characteristic morphology of nematic liquid crystalline
phase, can be observed. Furthermore, the nematic dispersed
nanowires can be well aligned upon exposure to external
electric field. It is anticipated that the liquid nanowires from
FF specific self-assembly may find potential application in
nanopatterning, reinforcing materials into nanocomposites.
50
FF nanowires vertically aligned on a solid substrate can also
be achieved through solid-phase self-assembly by using an
amorphous peptide film as a precursor. Ryi and Park grew
vertically aligned FF nanowires from a film by changing the
water activity in the vapor phase or by applying high thermal
energy.
51
It is proposed that surface nucleation and mass-
transport limitation may be the main factors for controlling
the formation of nanowires on solid surfaces in the water
vapor-mediated self-assembly, whereas the nanowires induced
by thermal ageing are likely a result of a phase transition of FF
molecular arrays. Subsequently, they improved the method for
the preparation of vertically aligned peptide nanowires from
an FF amorphous film. With the aid of aniline vapor the
uniform and vertically well-aligned nanowires are grown by
ageing the film at temperatures above 100 1C (Fig. 9).
52
SEM
images confirmed the occurrence of the vertically well-aligned
and rigid nanowires. Based on the analysis of time evolution of
the film through aniline vapor ageing, it is suggested that a
surface-initiated nucleation in the initial stage is a possible
mechanism for the growth of vertically well-aligned nano-
wires. In addition, a micro-pattern of vertically aligned FF
nanowires can be fabricated by the combination of high-
temperature aniline vapor ageing and a soft-lithographic
technique.
2.4 Ordered molecular chains on Cu surfaces
The fabrication of ordered peptide chains at the molecular
level has attracted increasing attention due to the potential
application in bionanotechnology. By using scanning
tunneling microscopy (STM), Kern et al. directly observed
the stereoselective assembly of diphenylalanine enantiomers,
L-Phe-L-Phe and D-Phe-D-Phe, into molecular pairs and chains
in a chiral recognition fashion.
53
The FF dipeptide contains
two chiral carbon centers connected through a central amide
bond, which is a key motif in molecular recognition. STM
imaging shows that the co-deposition in vacuum self-organizes
L-Phe-L-Phe and D-Phe-D-Phe on Cu surfaces into homochiral
molecular chains through mutually induced conformational
changes. The chiral recognition of FF enantiomers at the
single-molecule level provides a pronounced evidence for the
prediction of Pauling on the mechanism of dynamically
induced fit.
Fig. 7 SEM image of the dried FF gels (a); SEM image (b) and enlarged AFM 3D height image (c) of the flower-like structure in the microcrystal.
Reproduced with permission from ref. 48. Copyright 2008, American Chemical Society. Also adapted from ref. 49.
This journal is cThe Royal Society of Chemistry 2010 Chem.Soc.Rev., 2010, 39, 1877–1890 |1883
2D extended periodic arrangements of FF molecules can be
reached upon co-crystallization using terephthalic acid (TPA)
as a linker.
54
STM measurements of FF molecules deposited
on Cu surfaces reveal that individual FF molecules have a
tendency to self-assemble into 1D chains, but 2D ordered and
extended FF chains emerge on Cu surfaces when introducing
TPA as a molecular ‘‘glue’’ to bridge the isolated dipeptide
chains (Fig. 10). The formation of ordered supramolecular
structures is independent of the stoichiometry of initial FF to
TPA, in which molecular organization is self-recognizing at a
Fig. 8 Schematic illustration of the structural transition induced by varying the ethanol content in the mixed solvents and the proposed molecular
stacking in the gel and in the microcrystal. Adapted from ref. 49.
Fig. 9 A schematic illustration and cross-sectional SEM images of growing vertically well-aligned nanowires from an amorphous peptide thin film
by high-temperature aniline vapor ageing. Reproduced with permission from ref. 52. Copyright 2008, Wiley.
1884 |Chem. Soc. Rev., 2010, 39, 1877–1890 This journal is cThe Royal Society of Chemistry 2010
fixed ratio of FF to TPA (1 : 1). This facilitates production of
such ordered nanostructures on solid surfaces despite the
imprecise control of the amount of deposited components. It
has been suggested that intermolecular hydrogen bonding
might be the major driving force for the formation of the final
ordered structures.
54
3. Peptide–inorganic hybrids
A combination of inorganic functional materials (such as
nanocrystals, nanoparticles and polyoxometalates (POMs)),
which exhibit unique electronic, photonic, and catalytic
properties, with biological or biomimetic building blocks
provides a strategy towards improved properties and novel
functions of nanobiomaterials. There are many efforts exerted
on the exploration of suitable organic components to
achieve the integration with functional inorganic materials,
particularly nano-objects. Peptides, as versatile self-assembling
building blocks with unique biological functionality, possess a
remarkable potential for the fabrication of such multi-
functional hybrids. Considerable progress has been made in
the combination of functional inorganic materials with FF
dipeptides, one of the simplest peptide building blocks.
Such functional FF-based composite structures include 3D
peptide–inorganic cross-linked fibrous architectures with
optical properties,
48
biocompatible 3D colloidal spheres for
bioimaging,
55
peptide–POM hybrid spheres with an adaptive
encapsulation property
56
and photoluminescent nanotubes
with the incorporation of lanthanide ions.
57
The 2D or 3D architectures with the incorporation of
inorganic nanocrystals are of fundamental interest for
application in nanotechnology or in biology. Such spatial
network scaffolds can be readily created by using a gelator
to gelate the corresponding well-dispersed solutions containing
nanocrystals. It has been found that FF is a suitable candidate
to achieve the fabrication of 3D fibrous scaffolds.
48
Upon
simply gelating a solution of quantum dots (QDs), organogels
with the entrapment of QDs are easily obtained. Such gels
display photoluminescence (PL) from the embedded QDs
(Fig. 11a). TEM images show that the gels with encapsulated
QDs consist of cross-linking 3D fibrous networks with attachment
of QDs to the fibrils (Fig. 11b,c). The comparison between
emission spectra of QDs in gels and those of free QDs reveals
that the emission maxima of the QDs in fibrous networks are
slightly blue-shifted, indicating the attachment of QDs on
fibrils, but remaining the original PL colors (Fig. 11d). In
addition, lipophilic gold nanoparticles are manipulated into
the fibrous scaffolds by using the same method, indicating the
general suitability of this approach. It can thus be imagined
that gel materials with various optical, electronic, and
magnetic properties may be achieved through gelating the
corresponding functional nanocrystals.
Recently we initiated a new strategy to prepare water-
dispersible 3D colloidal spheres by using the FF-based
organogels with encapsulated nanocrystals as the starting
point.
55
This method involves the transfer of a nanocrytal
organogel phase into a water phase (TNOW). The detailed
processes are illustrated in Fig. 12a. The FF derivative,
cationic dipeptide (H-Phe-Phe-NH
2
HCl, Fig. 2-3), serves as
the gelator to initially prepare fibrous aggregates of QDs
(Fig. 12b) that are then dried to get the xerogel with the
attached QDs under vacuum condition. By the addition of
water and subsequent ultrasonic treatment, the resultant 3D
colloidal spheres can be obtained. The TEM image shows that
the colloidal spheres comprise the individual QDs (Fig. 12c),
which ensures the original optical property of QDs after
Fig. 10 STM images of 2D extended FF–TPA ordered molecular
chains on Cu(110). The quadrangle in the inset marks one unit of the
molecular superlattice. The ellipse and dumbbell indicate the positions
of TPA and the FF molecules, respectively. Reproduced with permission
from ref. 54. Copyright 2007, American Chemical Society.
Fig. 11 Incorporation of quantum dots in gels: (a) PL photograph of
four different encapulated QDs gels; (b) TEM image of the encapsulated
QD523 nanocrystals in the fibril network; (c) Magnified TEM image
showing the attachment of QDs to the fibril; (d) Emission spectra
(l
excitation
= 365 nm) of the free QDs in toluene (solid line) and the
encapsulated QDs in gel (dash dot). Reproduced with permission from
ref. 48. Copyright 2008, American Chemical Society.
This journal is cThe Royal Society of Chemistry 2010 Chem.Soc.Rev., 2010, 39, 1877–1890 |1885
assembly into 3D colloidal spheres. The local energy-dispersive
X-ray (EDX) spectrum further confirms that the colloidal
spheres are composed of QDs and cationic dipeptide.
Dynamic light scattering (DLS) indicates that the colloidal
spheres are stable in aqueous solution, having hydrodynamic
diameters of about 150 100 nm. Furthermore, we can also
obtain differently sized colloidal spheres relying on the
concentration of cationic dipeptide and ultrasonic time. It
has been proposed that the electrostatic interactions upon
protonation of the cationic dipeptide and hydrophobic
attraction are possibly the predominant driving forces for
the assembly process.
Apart from the nano-objects used as inorganic components
for the fabrication of peptide-based functional hybrids, POMs,
a known class of anionic oxide nanoclusters of transition
metals are possible candidates for such a fabrication owing
to their potential application in catalysis, electronics, optics,
magnetics, medicine, and biology.
58
A Keggin-type POM
(phosphotungstic acid, PTA) is selected as a polyoxoanion
model molecule, and combined with the cationic dipeptide to
assemble the expected hybrids.
56
The morphology of hybrids is
completely investigated by using SEM, TEM and DLS.
The results show well-defined spherical nanostructures with
diameters of around 150 60 nm (Fig. 13b). EDX spectroscopy
combined with SEM and FTIR spectroscopy indicate that the
hybrids consist of PTA and cationic dipeptide. Based on the
XRD and HRTEM characterization, it has been suggested
that the hybrid spheres are formed upon initial self-assembly
of peptide encapsulated clusters (PECs) by strong electrostatic
interaction, and further stacking of such PECs through
multiple non-covalent interactions (Fig. 13a). Such hybrid
spheres are responsive to external stimuli such as pH and
temperature, which is a desirable feature for extensive application
of self-assembled nanostructures in controlled release of drugs.
Intriguingly, the hybrid supramolecular network displays an
adaptive inclusion property for guests in the self-assembly
process, which is utilized to encapsulate various guest
materials.
56
Water-soluble small molecules including neutral
fluorescein isothiocyanate (FITC), positively charged
Rodamine 6G (R6G) and negatively charged Congo Red
(CR) as well as macromolecular polymers such as dextran
labeled with FITC (FITC-dextran) have been demonstrated to
enable effective incorporation into the hybrid spheres
(Fig. 13c). Such hybrid supramolecular networks also exhibit
interfacial adaptability for nanoscale guests, regardless of
hydrophobic or hydrophilic surfaces. The hybrid supra-
molecular network is demonstrated to self-assemble adaptively
on the surface of hydrophobic drug particles and form
core–shell nanostructures with drug particles as the core
(Fig. 13d). Additionally, hydrophilic gold nanoparticles
(AuNPs) are incorporated into the hybrid supramolecular
network and result in Au@hybrid core–shell spheres
(Fig. 13e). The hybrid supramolecular network self-assembled
from the association of POM and cationic dipeptide exhibits
flexibility and multifunctionality, offering new insight into
hybrid supramolecular structures and may be a starting point
for further development of bio-inorganic hybrid materials with
novel functions.
Lanthanide ions possess fascinating optical properties and
have extensive applications in lighting devices, optical fibers,
lasers, medical diagnosis, and bioimaging etc.
59
The integration
of such functional inorganic components into self-assembled
peptide-based materials is feasible. Park et al. recently
fabricated PL peptide nanotubes by local incorporation of
Fig. 12 (a) A schematic illustration for the preparation of water-dispersible 3D colloidal spheres; TEM images of the fibrous aggregates of QDs
(b) and an as-prepared colloidal sphere (c). Reproduced with permission from ref. 55. Copyright 2008, Wiley.
1886 |Chem. Soc. Rev., 2010, 39, 1877–1890 This journal is cThe Royal Society of Chemistry 2010
photosensitizers and/or lanthanides during the self-assembly
of FF nanotubes.
57
The authors found that FF nanotubes
acted not only as a matrix for lanthanide ion inclusion, but
also as an antenna (or photosensitizer). Multiple analysis
techniques, such as fluorescence quenching of FF, electron
and optical microscopy, and EDX spectroscopy, are used to
confirm the effective incorporation of lanthanide ions and
photosensitizers into FF nanotubes. It was found that the
PL intensity of lanthanide ions can be significantly enhanced
when both lanthanide ions and photosensitizers are
simultaneously incorporated into FF nanotubes. In addition,
FF nanotubes, having various PL colors covering almost
the entire visible range, can be obtained by varying the
composition of lanthanide ions and photosensitizers incorporated
into the nanotubes. It has been proposed that the striking PL
enhancement of lanthanide ions in the FF nanotubes is a result
of a cascaded energy transfer from the nanotubes to lanthanide
ions through photosensitizer molecules.
57
The introduction of
lanthanide ions into FF nanotubes adds new functions for
such self-assembled biological nanomaterials, which may
extend their application in optics and electronics.
4. Applications of FF-based nanomaterials
The self-assembling nature of FF-based peptide building
blocks and their capacity to associate with functional
inorganic components led to the production of various
functional nanostructures, thus providing broad prospects
for the potential application in biological and non-biological
fields. The biological applications involve 3D cell culture, drug
delivery, bioimaging and biosensors, etc. Self-assembled FF
nanotubes, nanoribbons and nanowires can also act as
suitable templates for the fabrication of metal nanowires,
metal oxide nanoribbons and functional polymer nanotubes.
Therefore, we focus on these points to totally summarize the
applications involving the FF-based nanostructures.
4.1 Applications in biology
For FF-based nanomaterials various potential applications in
biology have been demonstrated. For example, the fibrous
hydrogel networks have tentatively been used as an extra-
cellular matrix (ECM) for the 3D culture of cells. Chondrocyte
cells were used as model cells and incorporated into hydrogels
by mixing with an appropriate Fmoc-FF gel. Two-photon
fluorescence microscopy and environmental scanning electron
microscopy (ESEM) studies revealed the features of a number
of chondrocytes which indicates the growth and proliferation
of cells in the 3D fibrous networks.
37
Therefore, FF-based
fibrous scaffolds are promising for further development for
application in tissue engineering, which enable them to
ultimately function analogously to the natural ECM.
The FF-based nanostructures possess potential in the
delivery of drugs or genes in vitro. A recent example revealed
that oligonucleotides can be delivered to the interior of cells
by endocytosis following the spontaneous transition of self-
assembled CDPNTs into vesicles varying the concentration of
building blocks (Fig. 14).
28
CLSM and gel electrophoresis
studies confirm the stable immobilization of negatively
charged oligonucleotides on the CDPNTs and subsequent
intracellular internalization of reassembled vesicles. Furthermore,
such peptide-based delivery vehicles are biocompatible, bio-
absorbable and recyclable. It is thus possible to exploit the
FF-based nanomaterials as a new class of vectors for the
delivery of foreign substances.
Bioimaging with FF-based nanomaterials can be achieved
through the integration of functional inorganic nano-objects
into the peptide system. Upon association of cationic
dipeptide and QDs 3D colloidal spheres exhibit the capability
of bioimaging (Fig. 15).
55
As illustrated in section 3, such 3D
colloidal spheres are fabricated by a new strategy called the
TNOW method. They can be stably dispersed in a serum-
containing cell medium. A standard cytotoxicity assay, known
as 3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide
(MTT) cell-survival assay, shows that the 3D colloidal spheres
Fig. 13 (a) A schematic illustration of co-assembly of cationic dipeptide and POM into hybrid spheres; (b) SEM image of hybrid spheres;
Incorporation of guests into hybrid spheres: (c) CLSM image of FITC-doped colloids, (d) TEM image of colloids encapsulated with HB
nanoparticles, (e) TEM image of colloids with the inclusion of AuNPs. Adapted from ref. 56.
This journal is cThe Royal Society of Chemistry 2010 Chem.Soc.Rev., 2010, 39, 1877–1890 |1887
are highly biocompatible, offering perspectives for further
application in cell labeling. CLSM indicates that the cells after
incubation with 3D colloidal spheres can radiate punctual
spots of luminescence. A red FM 4-64 marker to stain cell
membranes and endosomes was used to show the location of
3D colloidal spheres inside cells (Fig. 15). The result indicates
that 3D colloidal spheres are internalized into cells and
possibly accumulated in the cytoplasm. Therefore, this
investigation presents an excellent example to attain the
functionality of biomaterials upon their integration with
functional inorganic materials, which leads to new functions
and thus enlarges their applications.
Peptide nanotubes self-assembled from FF building blocks
have been shown to have application in biosensors.
60,61
A new
electrochemical biosensing platform is fabricated by
depositing FNTs on the surface of a graphite electrode.
60
Cyclic voltammetric and time-dependent amperometric
studies demonstrate that the presence of FNTs can significantly
improve the sensitivity of electrodes. In a similar study, thiol-
modified FNTs are applied to a gold electrode and render it
sensitive to enzymes such as glucose oxidase (GOx) and
ethanol dehydrogenase (ADH).
61
Such modified electrodes
show improved sensitivity and reproducibility for detection
of glucose and ethanol based on the enzyme-related electro-
catalytic reaction. In addition, FNT modified electrodes also
have some other merits such as a nonmediated electron
transfer, a short detection time, a large current density, and
a comparatively high stability. Therefore, these findings show
that FNTs are an attractive alternative biomaterial for the
fabrication of sensors and biosensors having promising
electrochemical performance.
4.2 Applications in nanofabrication
Peptide nanostructures have exciting potential applications in
nanofabrication, in which nanoscale tubes, wires or ribbons
serve as templates for the formation of functional nano-
materials such as metal nanowires, polymers (e.g. polyaniline,
PANI) nanotubes and hollow TiO
2
nanoribbons. For
instance, water-filled FF nanotubes might be a favorable
scaffold for creating metal nanowires and composites with
embedded metal nanoparticles (Fig. 16). Reches and Gazit
have confirmed that FNTs can be used as template for
metallization.
17
A silver nanowire 20 nm in diameter is
obtained upon reduction of silver ions inside the nanotubes
followed by enzymatic degradation of peptide scaffolds.
Through the introduction of thiol-containing peptide linkers,
20 nm gold nanoparticles can be coated onto the surface of the
FNTs filled with silver nanowires, resulting in the attainment
of metal–peptide–metal trilayer coaxial nanocables with
unique electromagnetic properties.
62
In light of a similar
method, a nanotube fabricated from the D-Phe-D-Phe dipeptide
is used to template Pt-nanoparticles.
32
HRTEM and EDX
studies demonstrate the attachment of 2–3 nm Pt nanoparticles
in the walls of peptide nanotubes and the following formation
of Pt nanoparticle peptide–nanotube composites. Recently,
the vertically aligned FF nanowires on a solid substrate have
shown the ability to act as a template for the fabrication of
FF/PANI core–shell conducting nanowires.
63
The thickness of
Fig. 14 Schematic illustration of oligonucleotide delivery into cells by using the peptide carrier. Reprinted with permission from ref. 28.
Copyright 2007, Wiley.
Fig. 15 Bioimaging application of water-dispersible 3D colloidal
spheres prepared by the combination of cationic dipeptide and QDs.
Lipophilic QDs (top left) can be transformed into QD-containing
fibrils (bottom left) via addition of cationic dipeptide. By adding water
and subsequent sonication these can be converted into 3D colloidal
spheres (bottom right) which then can be internalized to cells for
bioimaging (top right). Note that top right is a fluorescence micro-
graph, the others are TEM images.
1888 |Chem. Soc. Rev., 2010, 39, 1877–1890 This journal is cThe Royal Society of Chemistry 2010
the PANI shell can be readily adjusted by either varying
the reaction time or the number of PANI layers coated.
Electrochemical analyses demonstrate that the FF/PANI
nanowires are active both chemically and electrochemically.
Individual PANI nanotubes can also be obtained by selectively
removing the FF template. This investigation indicates that
FF-based assemblies are versatile and practical for the
fabrication of functional polymer nanostructures. Most
recently, Kim et al. prepared hollow TiO
2
ribbons via templating
FF ribbon framework, which was firstly self-assembled in
chloroform from FF peptide with ultrasonic assistance.
64
The FF xerogels exhibit remarkably high thermal stability,
enabling them to undergo a functional process of atomic layer
deposition (ALD) at high temperature (140 to 160 1C). A thin
TiO
2
layer was deposited over the peptide ribbon scaffolds by
such an ALD method. After removal of the template by
calcination, the highly entangled hollow TiO
2
ribbons that
replicate the FF xerogel framework may be easily produced.
Another interesting application is concerned with the
controlled patterning of FF-based nanotubes and nanospheres
by using inkjet printing.
65
FF-based nanostructures self-
assembled in solution were used as an ‘‘ink’’, and their
patterning on indium-tin oxide (ITO) coated plastic and
transparency foil surfaces were achieved by simply using a
commercial inkjet printer. Such patterns exhibit remarkable
durability on the above surfaces. The printed area is still
visible and similar to a fresh pattern 8 months after printing.
The use of the inkjet printing technique opens an alternative
avenue for the integration of peptide nanostructures into
functional electro-organic hybrid devices, even for the creation
of electronic or medical devices.
5. Conclusions and perspectives
Molecular self-assembly, as a powerful ‘‘bottom-up’’ technique,
has been widely utilized in the fabrication of biomaterials and
nanomaterials of which many applications have been explored
and developed in fields such as material science, biology,
medicine and nanotechnology. Biomolecules, especially peptides
provide a versatile approach for creating such biological
nanomaterials, mostly by using synthetic peptide building
blocks with inspiration from natural motifs and their self-
assembling behavior. Among various peptide building blocks,
diphenylalanine (FF)-based peptides have been most popular
due to their structural simplicity, functional versatility, cost
effectiveness and widespread applications. In this review, we
have concentrated on the development and the progress of
FF-based building blocks towards self-assembly. A variety of
different supramolecular structures including nanotubes,
vesicles, nanofibrils and ribbons, nanowires, and ordered
molecular chains can be constructed by self-assembly of short
FF-based peptides. The ordered organization of these unique
superstructures has been demonstrated as a cooperation of
hydrogen bonding and p–pstacking. In addition, to improve
inherent properties of peptide nanomaterials and introduce
new functions, the integration with functional inorganic
components has been achieved, e.g. the fabrication of 3D
peptide-inorganic cross-linked fibrous architectures with
optical properties, biocompatible 3D colloidal spheres for
bioimaging, peptide-POM hybrid spheres with adaptive
encapsulation function and photoluminescent nanotubes.
A number of applications are available to these FF-based
nanomaterials not only in biology such as 3D cell cultures,
drug delivery, bioimaging and biosensors, etc. but also in
nanotechnology as templates for the production of metal
nanowires, metal oxide nanoribbons and functional polymer
nanotubes.
Although significant advances have been made in the
creation of FF-based peptide materials, this field is still in its
infancy. So far, there seems to be a notable lack of theoretical
insight into peptide nanostructure formation, e.g. into control
of the morphological diversity, into rational modification of
the peptide motifs for self-assembly and functional improvement,
and into an effective prediction for the resulting self-assembled
structures. Fabrication of materials through the incorporation
of multiple responsive elements or components is yet a
challenge. The biological assessment of peptide-based complex
materials seems also to be a less studied problem. Thus, there
still remain formidable challenges for future development and
application of peptide nanomaterials. As the future research
focus of FF-based building blocks, one may expect that the
introduction of functional units into peptide entities will offer
an effective avenue for improving the intrinsic properties,
tuning the self-assembly, and ultimately achieving multi-
functional nanomaterials for potential applications in biology
and in nanotechnology. In fact, some helpful attempts have
been made for enhancing the function of FF-based materials.
As an example on this issue, our group for the first time
achieved the hybridization of functional inorganic components
with FF-based building blocks, which thus introduced new
properties in the self-assembled peptide materials. The Ulijn
group
66
developed a FF-based hydrogel with the incorporation
of bio-recognition function by coassembling the Fmoc-FF and
Fmoc-RGD (arginine-glycine-aspartate) peptides. Such
hydrogels may be used as biomimetic fibrous scaffolds for
the 3D-culture of anchorage-dependent cells.
Acknowledgements
The authors are greatly indebted to Prof. H. Mo
¨hwald for
valuable discussion and consistent support. X. Yan acknowl-
edges support for a research fellowship from the Alexander
von Humboldt Foundation. Dr Kewei Wang is thanked for
assistance with the drawing of 3D cartoons. This research was
Fig. 16 Illustration of FF nanotubes as functional templates for the
fabrication of metal nanowires and composites embedded with metal
nanoparticles.
This journal is cThe Royal Society of Chemistry 2010 Chem.Soc.Rev., 2010, 39, 1877–1890 |1889
financially supported by the National Basic Research Program
of China (973 program) 2009CB930101, the National Nature
Science Foundation of China (No. 20833010) as well as the
German Max-Planck Society.
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1890 |Chem. Soc. Rev., 2010, 39, 1877–1890 This journal is cThe Royal Society of Chemistry 2010
