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Hybrid (Nano)Materials Meet Supramolecular Chemistry: A Brief Introduction to Basic Terms and Concepts
Chapter 1 Hybrid (Nano)Materials Meet Supramolecular Chemistry
system often involves a phase in which the different researchers participating in such a
project have to settle terms and deﬁnitions and have to agree on a common “language,”
because similar terms or even identical abbreviations can have different meanings in
different scientiﬁc (sub)communities. For illustrative examples, one does not have to
embark on lengthy searches but can simply take a look at the List of Abbreviations of
this book. Attempting the unequivocal use of abbreviations book-wide, we were rather
soon confronted with the impossibility of doing so. Whereas for some abbreviations
used in various chapters with different meanings the uniﬁcation was simple (e.g.,
MAA stood formerly for methacrylic acid and mercaptoacetic acid, the latter being
now abbreviated as TA for its synonym thioglycolic acid),Ã for others, it was not possible. In several cases, both abbreviations have become so common in different areas of
the chemical sciences, the natural sciences, or even society that we had to accept their
“schizophrenic nature” and kept them with two different meanings in the book.
ferrocene (or ferrocenyl)
fragment, crystallizable (of an antibody)
personal digital assistant
Therefore, the key features of supramolecular chemistry and nanomaterials important in the present context are introduced in the next two sections.
1.2 TERMS AND CONCEPTS IN SUPRAMOLECULAR
Historically, the term supramolecular chemistry has been put onto the chemical map
by J.-M. Lehn in the middle of the 1980s,1 and received broad coverage from 1987 on,
the year Lehn, D. J. Cram and C. J. Pederson won the Nobel prize in chemistry.2–4 In
short, Lehn deﬁned the term as the “chemistry of molecular assemblies and of the
molecular bond,” which is often used in its condensed form of the “chemistry
beyond the molecule.” According to Lehn, the main characteristics of a supramolecular ensemble are a certain degree of order and/or symmetry of the packing as well as an
interaction between the subunits and/or speciﬁc types of intermolecular interactions.
Since Lehn distinguishes between the supramolecular complex and the covalent molecule, these interactions are supposed to be noncovalent in nature. With regard to function or properties, supramolecular systems show certain abilities for recognition,
catalysis, and/or transport. In contrast, the covalent molecule has a unique chemical
nature, shape, and polarity (and chirality) as well as redox, optical, and/or magnetic
properties. In this sense, the covalent grafting of, for instance, a receptor unit onto a
silica nanoparticle does not create a supramolecular ensemble, but a hybrid material.
The latter nonetheless may exert a supramolecular function upon binding of a guest
(see, e.g., Chapter 11) or may provide a suitable platform to build a supramolecular
This is actually a more complicated case since the second most obvious abbreviation, TGA, also leads to a
conﬂict with a term widely used in the materials sciences, thermogravimetric analysis.
1.2 Terms and Concepts in Supramolecular Chemistry
ensemble (see, e.g., Chapter 13). The same holds for molecules attached to a metal
surface through bonds that have a pronounced covalent character, such as the gold –
thiol bond (see, e.g., Chapters 4 and 8). These hybrids are not supramolecular in
nature but composite or hybrid and can be used in a supramolecular context as discussed for instance in Chapters 15 and 23. In addition, complex molecules comprising
various active and/or addressable subunits in a covalent fashion such as for example
10 in Figure 17.9 in Chapter 17 are also not supramolecular ensembles, but the supramolecular function is only generated by using, in this case, the ferrocenyl-appended
cyclodextrin that interacts with different parts of switchable 10 through hydrophobic
forces, depending on the state the molecular machine is in.
Closely connected to the deﬁnition of Lehn is the term host – guest chemistry as
laid out by Cram.3 The same forces are at play and the main constraint with respect
to the more general concept of supramolecular chemistry pertains to the binding site
features that the two partners bring into the complex: whereas the guest can possess
divergent binding sites (extreme cases: spherical guests such as halide anions or metal
cations), the host has to be equipped with convergent binding sites (simple cases: a
crown ether as host for sodium ions5 or an oligopyrrole for the binding of chloride
ions).6 Very much related to host– guest complex formation is the term molecular
recognition, which implies that a certain receptor can discriminate between a
number of potential guests, that is, recognizes the designated guest.7 Historically,
all the concepts that are based on molecular speciﬁcity in binding are rooted in the biochemical background of the biological receptor8 and the principle of lock-and-key9
(initially coined for enzyme – substrate interactions; however, later modiﬁed for
these speciﬁc interactions to the principle of the induced ﬁt).10 P. Ehrlich and J. N.
Langley developed the concept of the biological receptor from the observations
they made during their studies of how toxins and drugs can inﬂuence certain cellular
or synaptic functions (cited in Reference 8) and their dose-effect experiments foreshadowed analytical trends such as competitive assays or displacement assays, the
latter for instance being discussed here in Chapter 19. Regarding host – guest chemistry and molecular recognition, at the opposite end of the scale of complexity of
hosts such as crown ethers and oligopyrroles thus lie antibodies11,12 or their synthetic
analogs, molecularly imprinted polymers.13,14 The latter are featured in this book in
their organic – inorganic hybrid form in Chapter 20.
A typical feature of larger and more complex hosts is that they usually bind the
guest through multiple interactions in which the binding sites act in a concerted
fashion, that is, the overall binding constant between host and designated guest is
often the sum or even larger than the sum of the binding constants of the individual
binding sites with the guest. Such cooperative effects are also found in simple systems,
for example, for crown ethers or ethylenediamine tetraacetic acid (EDTA) and metal
ions, and are termed here macrocyclic effect15 or chelate effect.†16 To be able to
Note that in analogy to cofactors and substrates in enzyme chemistry, the general deﬁnition of cooperativity
usually concerns two different binding sites for two different guests and includes positive as well as
negative cooperativity: Cooperativity results when occupation of a given binding site leads to a change on
the binding features of the other site(s), making binding either easier or more difﬁcult [J. M. Lehn,
Supramolecular Chemistry (Weinheim: VCH, 1995), 141 ff].
Chapter 1 Hybrid (Nano)Materials Meet Supramolecular Chemistry
bind a certain guest selectively, not only does the host have to possess the adequate
binding sites, but the latter have to be arranged in an optimal fashion. This means
that in supramolecular systems, preorganization often plays a critical role. Within
the context of this book, preorganization plays an outstanding role because the use
of inorganic, nanoscopically sized, structured or textured supports to preorganize
(bio)organic functionalities in such a way that they can be used in supramolecular recognition or signalling is one of the main driving forces in the ﬁeld of hybrid materials
design (see, e.g., Chapter 14). Ultimately, the preorganization of the binding sites of
the host leads to a complementarity of host and guest that entails the unique response.
Having introduced the major principles of complexation, association, and organization, it is important to review the physicochemical forces that lead to supramolecular ensemble formation. As mentioned above, supramolecular interactions are by
deﬁnition noncovalent. In the order of the polarity of the partners involved, they comprise ionic or electrostatic interactions,17,18 ion – dipole interactions,19 dipole – dipole
interactions,20 (ionic) hydrogen bonding,21–23 cation-p 24 and anion-p interactions,25
p-p stacking,26 interactions based on van der Waals forces27 and hydrophobic effects,28
that is, the speciﬁc exclusion of polar solvents, in particular water, with speciﬁc packing effects in the solid state taking a special position.29 Whereas polar and electrostatic
forces are the key players to hold together tightly the porous coordination polymers
discussed in Chapter 7, van der Waals forces and solvent exclusion effects are decisive
forces in porous hybrid materials that mimic biological receptors like binding pockets
of proteins (see, e.g., Chapter 19). Because of the special physical parameters
involved, a delicate balance of (very) polar and (very) apolar interactions usually governs the possibilities of self-assembly at interfaces and the behavior of objects such as
self-assembled monolayers (SAMs)30 or Langmuir – Blodgett ﬁlms (LB ﬁlms).31
In terms of the physicochemical control of the selectivity in supramolecular systems, both thermodynamic and kinetic gain are important. Thermodynamic aspects
most of all govern a steady-state discrimination between different guests, the best
known examples being perhaps the binding of molecular oxygen to hemoglobin in
the presence of a number of potentially competing species such as molecular nitrogen,
water, and carbon dioxide, partly being present in rather high excess.32 Kinetic driving
forces are more important in catalytically acting systems such as enzymes since here
often the binding of the designated target is comparatively weak, yet the kinetics of
turnover for the guest of choice are much faster than for other potential guests as
well as for the same reaction in liquid solution.33 The loose binding is important
because the educt and the product of the catalytic reaction commonly have different
shapes, topologies, and chemical structures and the host has to rearrange during the
reaction. If enzymes would be preorganized in a rigid way to achieve stronger binding
of the (educt) guest, the reaction would be unfavorable.
Having mentioned self-assembled objects as examples already above in the context of supramolecular forces, this paragraph brieﬂy introduces self-assembly34 and
templating or the template effect,35 both concepts being deeply rooted in supramolecular chemistry. While the term self-assembly relates to a system that performs spontaneously several steps in a single operation to create a supramolecular ensemble, the
DNA double helix being perhaps the most prominent biochemical example, templating
1.3 Terms and Concepts Relating to Nanomaterials
or template-directed synthesis requires a certain, temporary or permanent, species that
aids the conversion of reactants into a product through the formation of a supramolecular complex, for example, an alkali metal ion that promotes oxacrown synthesis.36
Besides objects such as molecular rectangles,37 capsules38 or rosettes,39 self-assembly
also includes mechanically interlocked architectures such as catenanes and rotaxanes,40 extremely relevant in the framework of hybrid molecular machines (see
Chapter 17), as well as coordination polymers (see Chapter 7). On the other hand,
recent advancements in the supramolecular chemistry of interfaces have propelled
the typical design of functional monolayers to another level of sophistication, allowing
the construction of programmed 3D architectures through layer-by-layer (LbL) assembly techniques.41 Several chapters in this book discuss the many facets related to
layered assembly in detail, including Chapter 2. In addition, micelles, liposomes,
and vesicles can be considered as dynamic supramolecular assemblies and gained
paramount importance in the biological context of the protocell.42,43 Finally, besides
the role of templates in synthesis, templating of course is intrinsically connected to
molecular imprinting and its use in chemical sensing.13,14
1.3 TERMS AND CONCEPTS RELATING TO
As fuzzy and buzzwordy as the words may sound, nanomaterials, nanochemistry, and
nanotechnology, including their nanobiological variations, form the cornerstones of
the research map that spans the size range between 1 nm and 1 mm.44,45 In view of
traditional chemistry, 1 nm is considered as being large, whereas the traditional engineer or physicist who was educated in the area of microtechnology regards 1 mm as
small;46,47 the interface is the main playground of the nanosciences. In particular, the
term nanomaterials encompasses inorganic, (bio)organic, or hybrid objects of nanometric size and bulk material that is structured or patterned at nanometric dimensions.
Typical terms of classiﬁcation include dimensionally rather constrained objects such
as nanoparticles, nanocrystals, and nanotubes,48 dimensionally less deﬁned objects
such as nanolayers and nanoﬁbers, and nanosized voids like nanopores or nanocavities. Some classes can be subdivided into certain categories such as, for instance,
nanoparticles into nanospheres, nanorods,49 or nanocubes.50 It is interesting to note
that the term nanoparticle,51–53 even in a classic supramolecular sense, that is, as colloidal drug delivery system, was established in the pharmaceutical community even a
decade before the term nanotechnology saw a ﬁrst signiﬁcant increase in published
papers in the late 1980s, after the release of Drexler’s visionary book in 1986.‡45
Moreover, the functionalization of nanoparticles with complex hosts such as antibodies was also already realized before all the inﬂuential treatises on nanotechnology
appeared.54 Returning once more to the triangle of nanomaterials, nanochemistry, and
Colloidal metal nanoparticles, however, have been known for centuries since gold and silver nanoparticles
are responsible for the brilliant reds and yellows seen in stained glass windows. Scientiﬁc research on
metal nanoparticles dates back at least to M. Faraday; see, for example, M. Faraday, Philos. Trans. 1857,
147, 145– 181.
Chapter 1 Hybrid (Nano)Materials Meet Supramolecular Chemistry
nanotechnology, today, nanomedicine most likely constitutes the fourth cornerstone
and is perhaps the fastest developing one of the nano-areas, which was foreseen in
the late 1980s55 and closes the loop to the early development of nanoparticles in
the pharmaceutical sciences.
Among the nanoscopic objects, terms such as tubes, spheres, rods, and cubes are
self-explanatory and, as described in many chapters of the book, have today been
obtained from many different materials, including carbon, silica, or metals; see, for
example, Chapters 3, 4, and 6. These objects are commonly characterized by sizes
in the lower nanometer regime. Nanoﬁbers, which can easily cross the border to
micrometer-sized objects, can have very diverse features, not only in size but also
in composition, ranging from pure carbon materials (e.g., graphitic ﬁbers)56 via polymer assemblies57 to metal oxide ﬁbers.58 Regarding topology, the term nanowire is
almost synonymous, yet it is basically used in connection with either metallic or semiconductor materials,59,60 which can, however, be also assembled by supramolecular
templating,61 or for molecular wires. Traditionally, molecular wires are conjugated
polymers designed for molecular electronics,62 yet they can be equipped with adequate groups to perform tasks of molecular recognition and act as sensory devices63
or they can show certain features that can only be obtained through the help of supramolecular forces, for example, in the case of insulated wires.64 The term nanocrystal,
on the other hand, is commonly used for typically small (mostly ,10 nm) particles
of inorganic origin, containing two different types of atoms such as CdSe, PbS,
or TiO2.65 They possess special properties arising from conﬁnement effects, that is,
in these nanoscale objects the movement of an electron is conﬁned in all three dimensions. The electrons can thus only populate discrete energy levels. Accordingly,
energy bands (e.g., of the bulk semiconductor) converge to discrete, atom-like states,
with the oscillator strength being compressed into few transitions, leading to the typically observed high absorption coefﬁcients.66 Since in semiconductors the electronic
properties are strongly governed by the transitions between the edges of valence and
conduction band, conﬁnement effects are more dramatic for these materials than for
metals (see, e.g., Chapters 5 and 10). When metallic materials are concerned which
are typically obtained by the reduction of a metal salt dissolved in an organic liquid,
producing also dimensionally small particles (mostly ,10 nm), these are mainly
referred to as (atomic) clusters.67 Inherent to both types of particles is the fact that
they are not very stable or soluble in neat form and usually carry a protective organic
shell, tightly adsorbed to the inorganic core. In the case of metal particles, particles
stabilized with an organic layer are frequently referred to as monolayer-protected
clusters.68 Because surface defects (dangling orbitals or dangling bonds) govern
especially the luminescence properties of semiconductor nanocrystals, such a capping
layer also determines the performance of these particles in desired applications (see,
e.g., Chapter 12). Besides organic ligands, the passivation procedure for semiconductor nanocrystals often comprises the growth of a second semiconductor layer onto the
core in a core-shell type of fashion (e.g., ZnS@CdSe) with subsequent capping with
organic ligands.69 In contrast to nanocrystals, the term nanocrystalline is often used
with a different connotation. These materials do not have to have nanoscale size,
but are usually composed of nanosized crystals or crystallites in a consolidated
1.3 Terms and Concepts Relating to Nanomaterials
state, the single crystallites being held together by interatomic forces acting between
the atoms in the crystals on either side of the defect core or defect site.70 In other
words, these materials consist of numerous defect cores that are embedded into (an
elastically distorted) crystal lattice(s). Since atoms at these defect sites possess unique
properties, the bulk material retains these properties in its entirety and not only at its
surface. Such materials are commonly produced by vapor deposition techniques,
the most prominent example today being perhaps nanocrystalline silicon, which consists of crystalline grains of approximately 10 to 1000 nm size. Nanolayer ﬁnally is a
very general term and is today used mainly in conjunction with composite materials
prepared by deposition techniques. The preﬁx nano relates here basically to the thickness or the structured pattern of the layer.
Talking about fabrication techniques, they fall basically in one of the two concepts
of nanotechnology, top-down and bottom-up.44 Top-down is the classic engineering
approach,71 while bottom-up relates to (bio)chemical synthesis.72 With respect to
discrete nanomaterial objects, carbon nanotubes isolated from soot on one hand and
silica materials synthesized through the sol – gel route or metal clusters through redox
reactions on the other hand can be seen as the two antipoles. The chemical bottom-up
strategies are intrinsically connected to supramolecular synthetic concepts, discussed
above. The top-down approaches important within the present context include various
deposition techniques for layered objects such as physical73 or chemical vapor deposition74 and, to a lesser extent, epitaxy techniques such as molecular beam epitaxy;75
the latter are much more frequently used in device construction, for instance in optoelectronics. Particles can be generated either by synthetic reduction (e.g., metal
particles), precipitation (e.g., semiconductor nanocrystals), and condensation techniques (e.g., using the sol – gel route), constituting the major routes, as well as by electrochemical deposition (e.g., for modiﬁed electrodes, see Chapter 9) or by physical
methods such as grinding or ball-milling (e.g., for polymer-nanoparticle hybrids),76
laser ablation (e.g., for Raman-active hybrid nanoparticles)77 and thermal decomposition.78 In addition, nanostructuring techniques comprise ﬁrst of all lithographic
methods. For inorganic or rigid organic (e.g., polymer) supports, photolithography
is mostly used,79 other lithographic techniques such as electron beam lithography80
playing a minor role. Major advances in the ﬁeld discussed in this book, however,
rely on the development of soft lithography for the attachment and patterning of
organic entities onto substrates,81 rendering the preparation of nanoarrays possible.82
Relatively straightforward is the deﬁnition of nanoscopic voids. Nanopores and
nanocavities are elongated voids or voids of any shape, and nanomaterials can incorporate especially nanopores in an ordered or disordered way. The former is of crucial
importance for many of the hybrid materials discussed in the book (e.g., in Chapters
16 or 18). Nanochannel is also frequently used instead of nanopore, often in biological or biochemical contexts. Besides nanoporous, the term mesoporous is often found
in hybrid materials research. Interestingly, the IUPAC has deﬁned the terms mesoporous (pores with diameters between 2 and 50 nm), microporous (pores with diameters ,2 nm) and macroporous (pores with diameters .50 nm), yet has not given
a deﬁnition of nanoporous in the IUPAC Recommendations on the “Nomenclature
of Structural and Compositional Characteristics of Ordered Microporous and
Chapter 1 Hybrid (Nano)Materials Meet Supramolecular Chemistry
Mesoporous Materials with Inorganic Hosts.”83 Nanoscopic voids surrounded only by
a comparatively thin layer of hybrid material are often referred to as nanocontainers.
Intensive research on these hybrid analogs of micelles or vesicles has already accomplished rather sophisticated functions as presented in Chapter 22.
The last paragraph in the nano-section lists some terms that are frequently used in
connection with “nanoscience,” that are, however, rather ill- or very broadly deﬁned
only. Among these, nanocomposite, nanodevice, nanosystem, and nanodomain are
perhaps the least speciﬁc ones because they concern any kind of composite material,
device, system, or domain that is related to nanometric dimensions. Nanoﬂuidics,
nanoelectronics, nanolithography, and other such linguistic “nanocomposites”
encompass broad deﬁnitions, yet are self-explanatory, because they are rooted in the
parent, macroscopic technique. With respect to alluding research accomplishments
in nanochemistry or nanophysics to applications of the macroscopic world, a whole
new list of terms can easily be compiled from browsing (scientiﬁc) journals. People
publishing in or writing about nanomaterials and nanotechnology often tend to
illustrate their ﬁndings with more tangible terms such as nanoshells, nanotweezers,
nanoreactors, nanorobots, or nanofabrication.84
Finally, the term hybrid is not intrinsically connected to supramolecular chemistry or
nanomaterials but is commonly used for any combination of materials that can be
classiﬁed into the various subdisciplines of the chemical sciences. As the title of
the book suggests, most important here are organic – inorganic hybrids, stretching
into the ﬁelds of biomolecular – inorganic85 and polymeric – inorganic86 materials
such as those discussed, for example, in Chapter 21. Other combinations such as
biomolecular –polymeric, organic – biomolecular, or inorganic– inorganic hybrids
(e.g., alloy nanoparticles or AuNPs on supports consisting of other metals) are of
course also possible but have less relevance in the present context. The noun or
adverb composite is often used synonymously. In the scientiﬁc literature, the
term organic –inorganic hybrid material was ﬁrst used by G. L. Wilkes’ group in
the context of organically modiﬁed silica materials prepared by the sol – gel route.87
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Supramolecular Chemistry at
KATSUHIKO ARIGA, GARY J. RICHARDS,
JONATHAN P. HILL, AJAYAN VINU, AND
2.2 SUPRAMOLECULAR CHEMISTRY IN MESOSCOPIC MEDIA
2.3 SUPRAMOLECULAR ASSEMBLY AT THE MESOSCALE
2.4 SUPRAMOLECULAR MATERIALS AT THE MESOSCALE
2.5 FUTURE PERSPECTIVES
Dimensions between the atomic/molecular and the bulk macroscopic scales are
sometimes called “mesoscopic.” Because the mesoscopic scale corresponds roughly
to the electron free path, unusual phenomena such as quantum effects can be observed,
some of which could be used in the development of single-electron devices or
quantum computers. Top-down-type nanofabrication techniques are now capable of
producing structures in this size range, and research on this subject has received
signiﬁcant attention, especially in the ﬁeld of semiconductor science and technology.
The mesoscopic size regime is also relevant to organic chemistry and biochemistry since many biological mechanisms, including information transmission across cell
membranes, ribosomal protein synthesis, mitochondrial energy conversion, and enzymatic reactions, occur within complexes at the mesoscopic level. For construction of
functional mesoscopic structures from organic and biological components, bottom-up
The Supramolecular Chemistry of Organic–Inorganic Hybrid Materials. Edited by Knut Rurack and
Copyright # 2010 John Wiley & Sons, Inc.
Chapter 2 Supramolecular Chemistry at the Mesoscale
Figure 2.1 Schematic illustration of the bottom-up approach from molecular to bulk scales.
approaches based on supramolecular self-assembly techniques have to be utilized,
because such structures require sophisticated arrangement of their components in order
to express ﬁne functions. However, although formation of self-assembling structures
at the molecular- and nano-scales is well developed, fabrication of well-organized
structures in the mesoscale range remains a challenging target.1
In Figure 2.1, schematic representations illustrate strategies for creation of bulk
materials using molecular scale interactions through bottom-up supramolecular
approaches. Control of molecular arrangement,2–6 self-assembly or designed assembly processes,7–9 and structural transcription to materials10,11 all play important roles.
Additionally, construction of mesoscopic structures is one of the most important steps.
Assembly processes involving only organic materials are sometimes not suitable for
obtaining well-structured materials at the mesoscopic level. Combination of organic
or biological moieties with inorganic materials is one of the practical solutions to this
problem because inorganic materials can improve stability of the resulting hybrids.
Fabrication of organic– inorganic hybrid materials is thus an excellent route for the
construction of mechanically stable mesoscopic structures. Immobilization of organic
and biological components on inorganic supports leads to sophisticated arrangements
of these functional components, while transcription of supramolecular self-assembled
structures into inorganic materials results in novel functional inorganic structures.
These ambitious challenges can now be achieved at the mesoscopic scale.
In this chapter, supramolecular chemistry related to developments in materials
fabrication and functionalization at the mesoscale are discussed, with an emphasis on
those systems based on organic – inorganic hybrid structures. The contents of this
chapter are classiﬁed into (1) supramolecular chemistry within mesoscopic media,
(2) supramolecular assembly at the mesoscale, and (3) supramolecular materials at
the mesoscale. Despite this classiﬁcation these topics have considerable similarities.
2.2 SUPRAMOLECULAR CHEMISTRY IN
Molecular recognition is a key event in supramolecular chemistry since it determines
how molecules assemble in a predesignated way. Most molecular recognition events in