DYNAMIC COVALENT SURFACTANTS
Dynamic Covalent Surfactants
Proefschrift
ter verkrijging van de graad van doctor
aan de Technische Universiteit Delft;
op gezag van de Rector Magnificus prof.ir. K.C.A.M. Luyben,
voorzitter van het College voor Promoties,
in het openbaar te verdedigen op
dinsdag 3 juli 2012 om 15:00 uur
door
Christopher Benjamin MINKENBERG
HBO-ingenieur in de scheikunde
doctorandus in de scheikunde
geboren te Roermond
Dit proefschrift is goedgekeurd door de promotor: Prof.dr. J.H. van Esch
Samenstelling promotiecommissie:
Rector Magnificus, voorzitter
Prof.dr. J.H. van Esch, Technische Universiteit Delft, promotor
Prof.dr. S.J. Picken, Technische Universiteit Delft
Prof.dr. H.T. Wolterbeek, Technische Universiteit Delft
Prof.dr. R.J.M. Nolte, Radboud Universiteit Nijmegen
Prof.dr. J.J.L.M. Cornelissen, Universiteit Twente
Dr. R. Eelkema, Technische Universiteit Delft
Dr. E. Mendes, Technische Universiteit Delft
The research described in this thesis was funded by the Netherlands Organization for Scientific Research (NWO).
Copyright © 2012 by Christophe B. Minkenberg
All rights reserved. The author encourages the communication of scientific contents and explicitly allows reproduction for scientific purposes, provided the proper citation of the source. Parts of the thesis have been published in scientific journals and copyright is subject to different terms and conditions.
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Contents
Chapter 1
General introduction
1.1 Self-assembling systems in Nature 1
1.2 Synthetic and natural surfactant systems 1
1.3 A new approach towards dynamic surfactant systems 2
1.4 Thesis outline 2
1.5 References 3
Chapter 2
Stimuli responsive surfactants and dynamic combinatorial libraries
2.1 Introduction 5
2.2 Surfactants and surfactant aggregates 5
2.2.1 amphiphiles 5
2.2.2 aggregate morphologies 9
2.2.3 physical techniques for studying amphiphile 10
aggregation
2.3 Surfactant dynamics and responsive surfactant systems 11
2.3.1 surfactant dynamics in natural and synthetic 11
systems
2.3.2 stimuli responsive surfactants 14
2.4 Dynamic combinatorial chemistry 19
2.4.1 introduction 19
2.4.2 dynamic combinatorial libraries; disulfides 21
2.4.3 dynamic combinatorial libraries; imines 23
2.4.4 dynamic combinatorial libraries; hydrazones 25
2.4.5 combined dynamic combinatorial libraries 28
2.5 Responsive systems and nanomaterials using 30
dynamic covalent chemistry
2.5.1 introduction 30
2.5.2 responsive functional polymers using a 30
dynamic covalent chemistry approach
2.6 Conclusions and outlook 35
Chapter 3
Triggered self-assembly of simple dynamic covalent surfactants
3.1 Introduction 40
3.2 Imine association and aggregation 41
3.3 Imine stabilization by micelle formation 44
3.4 Triggered self assembly of dynamic covalent micelles 45
3.5 Conclusion 48
3.6 Synthetic procedures and methods 48
3.7 References 50
Chapter 4
Responsive vesicles from dynamic covalent surfactants
4.1 Introduction 52
4.2 Design of dynamic covalent double tailed surfactants 52
4.3 Aggregation behavior of dynamic covalent double tailed 53
Surfactants
4.4 Concentration responsive dynamic covalent vesicles 55
4.5 pH responsive dynamic covalent vesicles 59
4.6 Conclusion 60
4.7 Synthetic procedures and methods 60
4.8 References 66
Chapter 5
Thermodynamically controlled morphology and surfactant selection driven by dynamic covalent self assembly
5.1 Introduction 68
5.2 Surfactant design, critical aggregation concentrations 69
and aggregate sizes
5.3 Competitive binding experiments between one headgroup 70
and two hydrophobic tails
5.4 Competitive binding experiments between two headgroups 73
and one hydrophobic tail
5.5 Thermodynamic modeling of the competition experiments 75
5.6 Conclusions 80
5.7 Experimental section 81
Chapter 6
Responsive worm-like micelles from dynamic covalent surfactants
6.1 Introduction 95
6.2 Surfactant design and critical aggregation concentrations 96
6.3 Aggregate size determination 97
6.4 Imine association and aggregation 98
6.5 Thermodynamic modeling 100
6.6 Rheological properties 102
6.7 Stimuli responsive wormlike micelles 102
6.8 Conclusions 104
6.9 Experimental section 105
6.10 References 107
Chapter 7
Self assembly of stimuli responsive vesicle gels
7.1 Introduction 110
7.2 Building block design and aggregation 111
7.3 Aggregate morphologies 113
7.4 Constitution of vesicle gels and building block stoichiometry 114
7.5 Stimuli responsive vesicle gels 115
7.6 Conclusion 117 7.7 Experimental section 117 7.8 References 120 Summary 121 Samenvatting 125 Dankwoord 129 Curriculum Vitae 132 Publication list 133
Chapter 1 General Introduction 1.1 Self-assembling systems in Nature
Nature provides a large diversity of functional nanosystems that are all constructed by self assembly of small molecular components. These systems play important roles in catalysis, transport, organization and hierarchically controlled materials which is directly related to the natural control over structure, shape and size of these nanosystems. Besides specific recognition between proteins and many other components, also non-compatible (orthogonal) interactions play crucial roles in the formation of natural systems, like channels, motors, cytoskeleton and (micro) phase separation in biological membranes. Most important is that nature is able to control the formation of nanostructures at both the kinetic and thermodynamic level.[1]
1.2 Synthetic and natural surfactant systems
To a certain extent, mimicking natural assemblies in synthetic systems has been successful[2], leading to the development of a large diversity of (semi)synthetic self-assembled structures like foldamers[3], hydrogels[4], micelles and vesicles[5]. The morphology of surfactant aggregates is controlled by the same molecular design of the building blocks in both natural and synthetic systems in which the molecular structure of the amphiphilic building blocks regulates the packing in aggregates and therefore the aggregate morphology. This relationship has been described by Israelachvilli as the structure-shape concept[6], in which the morphology of an aggregate is related to the ratio of the headgroup area and the volume and tail length of the aliphatic surfactant tail. Next to aggregate morphology, the function of self-assembled structures is also highly related to the dynamic exchange of surfactant assemblies within an aggregate with surfactants in bulk. Amongst others, the dynamic exchange of lipids in natural bilayer membranes is enhanced by proteins and carotenoids, and is essential for cell signaling, cell division and uptake and release of cargo.
This has inspired supramolecular chemists to investigate responsive surfactant systems, with aggregate assembly and disassembly triggered by external stimuli. Especially vesicles have received much attention, because their ability to compartmentalize cargo from bulk by bilayer formation. Although diverse responsive surfactant systems have been developed[7], the slow exchange dynamics between assembly and solution makes that synthetic bilayer surfactant systems behave almost as irreversible systems. A new approach towards surfactants is necessary to create responsive synthetic surfactant aggregates with fast exchange dynamics between aggregated state and solution, which might have potential as drug delivery vehicles.
1.3 A new approach towards dynamic surfactant systems
To develop surfactant aggregates with fast exchange dynamics, new surfactant building blocks are required in which a link to fast surfactant exchange should be implemented. For this, we employ a rapid and reversible reaction between simple structured non-amphiphilic building blocks which react to form surfactants in situ by the formation of a weak covalent bond. As building blocks we used a hydrophilic aldehyde functionalized building block and hydrophobic (but water soluble) amines as surfactant precursors, which form surfactants by the formation of a dynamic covalent imine bond (Figure 1a)
Figure 1: Dynamic covalent surfactant and aggregate formation from small building blocks. a) Surfactant formation from small building blocks b) aggregation of the formed dynamic covalent surfactants
Since the imine association equilibrium in water is highly influenced by temperature and pH, a shift in equilibrium will lead to stabilization or destabilization of the surfactant structures, which will lead to a breakdown of the surfactant aggregates (Figure 1b). Therefore it will be possible to influence the dynamic properties of surfactant aggregates in water and aggregates by a fast displacement of the imine association equilibrium towards its precursors or surfactants and vice versa. This development will lead to new responsive surfactant systems.
1.4 Thesis outline
The theoretical fundaments needed for the design of dynamic covalent surfactants are described in Chapter 2. The first dynamic covalent surfactants are described in Chapter 3, in which non-amphiphilic building blocks spontaneously form surfactants in water by the formation of a dynamic covalent imine bond. These surfactants self-assemble to form micelles, in which aggregation is switched on and off by displacement of the imine equilibrium from surfactants to surfactant building blocks and vice versa by a change of the temperature or pH. In Chapter 4 a similar approach is used for the design of responsive vesicles. These vesicles spontaneously form after mixing the surfactant building blocks in water. The dynamics of exchange from surfactants from the bilayer to bulk is controlled by the fast association and dissociation of the imine surfactants, which results in shrinking and
full dissociation of the vesicles upon imine hydrolysis. In Chapter 5 competitive binding experiments were described between vesicular and micellar building blocks. It was found that vesicles were the preferred structures which is accompanied with vesicular surfactant selection. Both the selection and aggregation of the surfactants could be described by a thermodynamic model. Dynamic covalent wormlike micelles were described in Chapter 6, in which aggregation is switched on and off by displacement of the imine equilibrium from surfactants to surfactant building blocks and vice versa, again in response to a change of the temperature or pH. In Chapter 7 vesicular hydrogels were described. These systems are composed of dynamic covalent double tailed imine surfactants, which are crosslinked by the formation of reversible imine bonds. These hydrogels consist of crosslinked vesicles and are also spontaneously formed after mixing. Moreover these systems are highly responsive towards changes of the pH and temperature.
1.5 References
1) L. Stryer, Biochemistry, 1995, 4th ed., W. H. Freeman and Company
2) a) G. M. Whitesides, J. P. Mathias, C. T. Seto, Science 1991, 254, 1312 b) J. M. Lehn,
Angew. Chem. Int. Ed. Engl., 1990, 29, 1304
3) R. Breslow, S. D. Dong, Chem. Rev., 1998, 98, 1997
4) J. A. Foster, J. W. Steed, Angew. Chem. Int. Ed. 2010, 49, 6718
5) B. Alberts, D. Bray, J. Lewis, M. Raff, K. Roberts, J. D. Watson, Molecular Biology of
the cell, 1994, 3rd ed. Garland Publishing, Inc.
6) J. Israelachvili, Intermolecular & Surface Forces, 1991, 2nd ed, Academic Press.
7) a) L. Shen; J. Du; S. P. Armes; S. Liu, Langmuir 2008, 24, 10019 b) J. Gaitzch; D. Appelhans; D. Gräfe; P. Schwille; B. Voit, Chem. Commun. 2011, 47, 3466 c) H. Koo; H. Lee; S. Lee; K. H. Min; M. S. Kim; D. S. Lee; Y. Choi; I. C. Kwon; K. Kim; S. Y. Jeong.
Chem. Commun. 2010, 46, 5668 d) G. Gao; H. Heo; J. Lee; D. Lee. J. Mater. Chem. 2010, 20, 5454 e) A. O. Moughton; J. P. Patterson; R. K. O’ Reilly, Chem. Commun. 2010 f) M.
Pelletier; J. Babin; L. Tremblay; Y. Zhao. Langmuir 2008, 24, 12664 g) I. K. Voets; P. M. Moll; A. Aqil; C. Jerome; C. Detrembleur; P. de Waard; A. de Keizer; M. A. Cohen Stuart.
J. Phys. Chem. B. 2008, 112, 10833 h) S. Il Yun; G. E. Gadd; V. Lo; M. Gauthier; A.
Chapter 2
Stimuli responsive surfactants and dynamic combinatorial libraries Abstract
In this chapter the basic principles of surfactants, responsive surfactant systems and diverse dynamic combinatorial library systems will be discussed. These concepts are relevant for the understanding of the design and approach of the diverse dynamic covalent surfactant systems presented in this thesis.
2.1 Introduction
In this chapter the underlying principles of surfactant chemistry, dynamic covalent chemistry and related research will be discussed, which form the foundation of the research described in this thesis. This chapter will start with a general introduction about amphiphiles (2.2). Secondly, the dynamic properties of surfactant aggregates are discussed together with some examples of responsive surfactant systems from the scientific literature (2.3). In the fourth paragraph the principles of dynamic covalent chemistry are described (2.4), followed by its applications in Dynamic Combinatorial Libraries (DCLs) and dynamic polymers and supramolecular assemblies (2.5). This fascinating field is currently of growing interest and formed the main inspiration of the research described in this thesis.
2.2 Surfactants and surfactant aggregates
2.2.1 Amphiphiles
Amphiphiles play an important role both in Nature and in our daily life. In Nature, phospholipids act as amphiphilic building blocks for cell membranes. These semi permeable membranes separate the intercellular region from the environment, which prevents cell disintegration. In daily life amphiphiles are used as surfactants, detergents or emulsifiers in soaps or creams. The word “amphiphile” has its origins in Greek, in which “amphis” means both and “philia” to love. In other words, an amphiphile contains a hydrophilic (water loving) and hydrophobic (water fearing) part within the same molecule. Cationic, anionic or non-ionic polar groups generally represent the hydrophilic part of the surfactant, while the hydrophobic part is typically made up of one or more water insoluble hydrocarbon chains. Typical surfactant headgroups are anionic (e.g. sodiumdodecylsulfate, SDS), cationic (e.g. tetradodecylammonium bromide, TTAB) or non-ionic (oligoethylene glycol derivatives). Phospholipids are typical double tailed surfactants and frequently have a zwitterionic headgroup (Figure 1).
Dissolving hydrophobic matter in water usually leads to phase separation at higher concentrations, as typically observed in mixtures of oil and water. In contrast, amphiphiles form small aggregates in which the hydrophobic chains form the interior of the amphiphilic aggregate, which is shielded from the bulk aqueous solution by the hydrophilic headgroups. The simplest model to describe assembly of N monomers (S) assembly into aggregates or N-mers (SN) is to assume that an equilibrium will be reached between surfactant monomer
and surfactant in aggregate[1] (Figure 2).
Figure 2: Simple representation of N monomers S, assembling into aggregates (SN).
These aggregates (SN) are formed above the critical solubilisation concentration of the
individual (monomeric) amphiphiles (S), which is better known as the Critical Aggregation Concentration (CAC). At the CAC, monomer and aggregate are in equilibrium, i.e. the chemical potential of the free monomers (μ(S)) equals the chemical potential of the aggregates (μ(SN)):
( )
S
( )
S
N
(Equation 1)Likewise, x1 is the mole fraction of free monomers and xN the mole fraction of aggregated
monomers, which are related to their thermal energy (RT) by
1
ln
1 Nln
Nx
RT
RT
x
N
N
(Equation 2)in which
1and
N represent the standard chemical potential for monomers and aggregated monomers respectively. The aggregation number (N) represents the number of assembled monomers in an aggregate. Equation 2 can accordingly be rewritten as:(Equation 3)
in which xc is the mole fraction of free monomers at the CAC, xt the total mole fraction of
surfactants in solution (
x
t
x
1x
N) and K the equilibrium constant for aggregate xNformation, which directly follows from Equation 2 as:
c N c c t
x
x
K
N
x
x
x
x
1(
1)
1 N N
x
K
x
(Equation 4)It becomes clear from Equation 3 and 4 that the amount of molecular dissolved surfactant monomers increases linearly by increasing the total amount of amphiphile in solution, up to reaching the CAC, after which the molecularly dissolved surfactant concentration remains constant, because all additional added surfactant will directly fit in an aggregate[1] (Figure 3). Consequently, the amount of aggregates increases above the CAC.
Figure 3: Total surfactant concentration as function of monomer (red) and aggregate concentration (black). The plots are normalized by xc.
The stability of a surfactant aggregate is related to the interaction energies between the individual surfactant molecules within the aggregate. This stabilization energy is the result of two opposing interactions. The repulsive part of the interaction free energy tends to increase the interfacial area per molecule (
a
) that is exposed to the aqueous phase. This effect is caused by electrostatic repulsion between the amphiphilic headgroups and destabilizes the aggregate.The repulsive part of the interaction free energy is therefore related to a as:
repulsive
a
(Equation 5)In which κ contains several terms, like a hydration force, steric term and electrostatic contribution. For simplicity, we take κ as a constant.
The attractive interactions mainly arise from hydrophobic and interfacial tension forces and are therefore described by:
attractive
a
(Equation 6)in which γ represents an interfacial energy term. The total free energy per molecule in an aggregate (
N) is then the sum of the repulsive and attractive contribution:N
K
a
a
(Equation 7)Aggregation takes place at the energy minimum, so at
N/
a
0
. This results in:0 2 0 0
2
(
)
Na
a a
a
(Equation 8)In which
a
0is the optimum headgroup area per molecule. As a consequence, the energy minimum is reached whena a
0. This means that only a limited amount of surfactants can cover the optimum headgroup area, which is expressed as the aggregation number[1] (Figure 4).Figure 4: Schematic representation of repulsive and attractive forces between surfactant molecules as a function of number of surfactant molecules present. The solid line represents the sum of both attractive and repulsive forces with stable aggregates being formed at the energy minimum.
2.2.2 Aggregate morphologies
The morphology of an amphiphilic aggregate depends on the molecular architecture of the amphiphilies. Amphiphiles with a single headgroup and tail tend to form micelles. By addition of one more hydrophobic tail, bilayers would be the most probable structure. Israelachvilli[1] created a model to predict aggregate morphologies from surfactant shapes, called the structure-shape concept. Critical parameters in this model are the headgroup area per amphiphile (a) and the volume and length of the hydrophobic tail, which are collected in the so-called packing parameter (p), which can be described as:
e
V
p
A l
(Equation 9)Here V represents the volume of the hydrophobic segment, Ae the mean cross-section of the
hydrophilic headgroup and l the length of the apolar chain. It has to be noted that p is a dimensionless unit and can have several values. When p < 1/3, cones will be formed as aggregate morphology. Truncated cones will be formed when 1/3 < p < 1/2 and both truncated cones and cylindrical structures will be formed when 1/2 < p < 1 (Figure 5). When p > 1 inverted cones will be formed. Spherical micelle is the most common morphology to be formed when the amphiphile has a cone shaped architecture, while truncated cones are typical architectures leading to elongated (wormlike) micelles. By increasing the packing parameter to the region of truncated cones and cylinders, straight bilayer and vesicular structures are most common. If p > 1, inverted micelles will be formed when the amphiphiles are dispersed in apolar solvents, while precipitation is most probable in water.
0(N)
# surfactant molecules (N)
Attractive forces Repulsive forces
Figure 5: Relation between packing parameter and aggregate morphology in aqueous solvent
Although the structure-shape concept is applicable for many amphiphilic systems, important parameters like temperature[2], ionic strength, molecular conformation and hydration[3] are not included in this model. Therefore, the exact value of p and thus the overall aggregate morphology is not that straightforward to predict.
2.2.3 Physical techniques for studying amphiphile aggregation
There are many physical techniques to study the aggregation behaviour of amphiphilic molecules. Dynamic Light Scattering (DLS) is a useful technique to obtain information regarding the size and shape of the aggregate, where the scattering of incoming light by aggregates in a solution can be related to the particle size and shape[4]. Besides this, there are several techniques to determine the CAC of a surfactant, in which surface tension is the most straightforward one. Water molecules at the air-liquid interface are stabilized at the intermolecular level by hydrogen bonding, which allows the interface to resist external forces for a certain extent. This phenomenon is called surface tension, which has the dimension of force per unit length. However, when dissolving surfactant molecules in water, the air-liquid interface of water is destabilized by disturbing the intermolecular interactions between adjacent water molecules. As a consequence, changing the amphiphile concentration in the bulk will affect the concentration at the surface, which is the reason why amphiphiles are also called surface active agents or surfactants. If the surfactant concentration in solution is high enough, they tend to orientate at the air-liquid interface with their polar head in the liquid layer while their hydrophobic tails point towards the air. This orientation will drastically decrease the surface tension of water. The surface tension will decrease more with increasing surfactant concentration, until the CAC has been reached. As mentioned before, the molecularly dissolved surfactant concentrations remains constant at this stage and so does the equilibrium between surfactants in solution and at the interface, which results in a constant surface tension[5] (Figure 6).
σ
CAC
σ σ σ
c
a b d
Csurfactant Csurfactant Csurfactant Csurfactant
σ CAC σ CAC σ σ σσ σσ c a b d
Csurfactant Csurfactant Csurfactant Csurfactant
Figure 6: Surfactant behavior at the air-water interface and its effect on surface tension (σ) as function of total surfactant concentration. a) surfactants moving in bulk b) first orientation at air-water interface, which results in a decrease in surface tension c) more orientation at air-water interface d) CAC is reached, air-water interface is full and aggregates are formed
Fluorescence spectroscopy using a hydrophobic probe is an alternative technique to determine the CAC[6]. Several hydrophobic fluorescent molecules are known, that show solvochromic behaviour. This means that the stability of its excited state is dependent on the polarity of its surroundings, which can be a solvent or an aggregate micro-domain. Nile Red is such a molecule and is therefore frequently used for studying the aggregation behaviour of surfactants[7]. Micromolar concentrations of probe are mixed with the surfactant solutions. If present, Nile Red will partition into the hydrophobic microdomains of surfactant aggregates, which in turn changes the emission properties of the Nile Red probe[7]. Nile Red exhibits fluorescence maxima at 660 nm in aqueous surfactant solutions below the CAC. Increasing the surfactant concentration above its CAC will generally lead to a blue-shift of the Nile Red emission, which is indicative for the formation of surfactant assemblies and can therefore be used to determine the CAC[7]. This blue shift also provides information about the hydrophobicity of the surfactant aggregates. In general, vesicle forming surfactants have a much larger hydrophobic part than micelle forming surfactants. As a consequence, the CACs of micellar aggregates are some orders of magnitude higher (mM) than the vesicular CACs (μM or lower).[1,8] Additionally, vesicles generally show a larger blue-shift of the Nile Red emission which is indicative for enhanced hydrophobic interactions.
2.3 Surfactant dynamics and responsive surfactant systems
2.3.1 surfactant dynamics in natural and synthetic systems
In Nature, double tailed phospholipid surfactants are the building blocks for cell membranes, shielding the cell interior from extracellular influences by the formation of a bilayer membrane. The cell membrane prevents polar molecules such as amino acids, nucleic acids, phosphorylated carbohydrates, proteins, and ions from entering the cell[9]. In living systems, bilayer membranes are highly dynamic, caused by high exchange rates
between surfactants from the bulk and the bilayer, amongst others due to embedded proteins and carotenoids. This is essential for endo- and exocytosis, cell signaling and cell division. Endo– and exocytosis are essential for the transport of cargo, like encapsulation and release of vitamins, and sugars. In exocytosis, a transport vesicle fuses with the cell membrane, as such releasing its cargo into the extracellular space (Figure 7a). The reverse occurs in endocytosis, in which a part of the cell membrane is internalized forming a transport vesicle for cargo transport within the cell (Figure 7b)[9,10].
Figure 7: a) exocytosis b) endocytosis
The bilayer dynamics in synthetic surfactant differ from the natural systems, since the bilayer dynamics in synthetic double tailed surfactant vesicles are much slower. The discrepancy in bilayer dynamics can be rationalized from the surfactant thermodynamics (see Section 2.2), which implies that there is a thermodynamic equilibrium between free surfactants and surfactants in aggregates (Figure 2). Here, surfactant dynamics is described by relating the thermodynamic surfactant equilibrium to surfactant association and dissociation rates.
The rate of aggregate association and dissociation can be described as:
1 1
association rate =
k x
N (Equation 10)dissociation rate =
N Nx
k
N
(Equation 11)in which k1 represents the rate constant of aggregate association, kN the rate constant of
aggregate dissociation, x1 the mole fraction of free monomers, xN the mole fraction of
aggregated monomers and N the aggregation number. These two formulas are related to Equation 2 and 4 as:
1 1exp
(
N) /
Nk
K
N
kT
k
(Equation 12)in which in which
1and
N represent the standard chemical potential for monomers and aggregated monomers respectively and k the Boltzmann constant (k = 1.38*10-23 J/K). The critical aggregation concentration (CAC) can be approached as:
1
1
exp (
N) /
CAC
kT
K
(Equation 13)Equation 13 implies that the free energy of aggregation and CAC are related, as such also related to the association and dissociation rate of aggregation (Equation 12). In other words, the aggregate dynamics is highly related to the CAC of the surfactant. Typical CACs of micelle forming single tailed surfactants are in the range of 10-2 to 10 mM[1]. In contrast, vesicle forming double tailed surfactants have much lower CACs because of increased hydrophobicity, typically in the order of 10-3 to 10-7 mM[1]. When we consider the surfactants within an aggregate to have a characteristic collision time
t
0and the probability of a surfactant molecule leaving the aggregate each time it hits the interface is1
exp((
N
) /
kT
)
, the mean residence time of a surfactant (t
R) within an aggregate can be defined as[1]: 1 0 055
/
NkT Rt
t
t e
CAC
(Equation 14)Typical surfactant residence times at room temperature are about
t
R~ 10-4 seconds formicelles and
t
R~ 3 hours for vesicles, which indicates that there is a very fast dynamic exchange for micellar single tailed surfactants while double tailed surfactant exchange within vesicles is extremely slow. The exchange between surfactants from the inner bilayer to the outside (flip-flop) is therefore also slow (Figure 8). Synthetic double tailed surfactant vesicles can be considered as static assemblies, because the surfactant exchange rate from bilayer to bulk is extremely low[8, 11].Figure 8: Schematic representation of surfactant exchange reactions for vesicles.
It can be concluded that surfactant dynamics are highly related to the CAC and aggregate morphology. As a consequence, surfactant dynamics in vesicles are rather slow as compared to their micellar analogues, which limits their application as synthetic encapsulation and release vehicle. Therefore, several attempts have been made towards developing synthetic responsive surfactant systems, in which aggregation can be triggered by external stimuli.
2.3.2 Stimuli responsive surfactants
Synthetic surfactant assemblies could be promising drug delivery vehicles, when assembly and disassembly can be controlled by external stimuli. Diverse attempts have been made, leading to responsive surfactant assemblies that can be switched on and off by e.g. pH[12], temperature[13], light[14], redox[15] and host-guest interactions[15,16] (Figure 9). Typical building blocks for these systems are block-copolymer surfactants with stimuli responsive functional groups on the polymer backbone. By using an external trigger, these functional groups undergo a chemical or physical change, as such changing the solubility of the surfactant polymer, which results in aggregate dissociation or morphology transitions (Figure 9).
Figure 9: Diverse block-copolymer based responsive surfactant systems. a) Thermosensitive hydrolysis of a block-copolymer surfactant leading to swelling and disruption of micelles. b) Assembly of polymers triggered by redox and host-guest interactions. c) On/off aggregation of block-copolymer micelles by light induced trans-cis isomerization. d) pH induced micelle dissociation leading to the release of cargo.
However, a major drawback of block-copolymer surfactants is their poor solubility in water, which limits their applications in aqueous stimuli responsive nanosystems. Therefore diverse attempts have been made towards responsive surfactant aggregates using non-polymeric surfactants.
Peptide amphiphiles are popular candidates in non-polymeric responsive surfactant systems, also because they are generally biodegradable which could be benificial for applications. Schmuck et al.[17] used small zwitterionic peptides in which aggregation could be switched on and off by pH. Although these systems can be switched between a non-assembled state and closely packed vesicles, DMSO is still required as co-solvent to solubilize the peptide surfactants (Figure 10a).
Figure 10: a) pH triggered vesicle formation, with non-aggregating building blocks at acidic pH and vesicles at alkaline pH. b) Enzyme catalyzed dephosphorylation reaction and a schematic representation of the micelle to fiber transformation.
Ulijn et al.[18] reported on a peptide based stimuli responsive surfactant system in pure water, consisting of micelle forming surfactants with a hydrophilic phosphate headgroup and a hydrophobic fluorenyl functionalized (Fmoc) tail. After applying the enzymatic trigger phosphatase, the polar headgroup dissociates and the aggregate morphology switched to highly anisotropic gel fibers by β-sheet formation (Figure 10b). Although this particular system consists of water soluble, environmentally friendly building blocks, the morphology transition is irreversible.
Yao et al.[19] used bipyridine-tripeptide amphiphiles to switch from vesicles to fibers and vice versa. Sonication resulted in a vesicle to fiber transition, due to β-sheet formation. The β-sheets were destabilized by heating, which resulted in vesicular morphologies (Figure 11). This system is fully reversible, however the tripeptide surfactants are insoluble in water, which requires THF as a co-solvent.
Figure 11: a) Structures of bipyridine based peptide amphiphiles. b) Bipyridine peptide amphiphiles forming hydrogen bond and β-sheet structures. c, d) Models for molecular packing and conformation of amphiphiles in side vesicles shaped I) and nanofibers (U-shaped II).
Similar drawbacks were observed with non-peptide, non-polymeric switchable surfactant systems. Feng et al.[20] developed a pH switchable system where a transition from spherical to wormlike micelles in water is driven by pH induced electrostatic interactions. Although they can reversibly switch from a viscous wormlike micelle solution to a Newtonian spherical micelle solution, they still observed precipitation at distinct pH regions. Dong et
al.[21] used a simple structured single tailed bisamine surfactant which solubilizes in water after sonication and assembles in multiple morphologies dependent on the protonation degree of the amine groups. Small spherical micelles were formed at acidic pH (pH = 1.98), going to donut-shaped micelles (pH 8), tubes (pH 9.01) and perforated spherical vesicles (pH 9.97). By using temperature as second stimulus, perforated vesicles were transformed to closed vesicles and vice versa (Figure 12).
Figure 12:pH and temperature induced micelle to vesicle transition[21]
It can be concluded from these diverse responsive surfactant examples from scientific literature, that it is not that straightforward to approach fully reversible surfactant systems in water. The poor water solubility of most stimuli responsive surfactants limits the reversibility of the current responsive surfactant systems. Therefore a new general surfactant design could provide an outcome, in which the surfactant remains soluble and is easy to synthesize. We believe that a dynamic combinatorial approach with water soluble surfactant building blocks could provide and outcome. The fundaments of dynamic combinatorial chemistry will be discussed in the next chapter.
2.4 Dynamic Combinatorial Chemistry
2.4.1 Introduction
Dynamic Combinatorial Chemistry (DCC) is defined as Combinatorial Chemistry under thermodynamic control. The concept behind DCC is to achieve structural variation and component selection by thermodynamic control. To achieve this, one or more building blocks need to be mixed together which can continuously interchange and react with each other by the formation of reversible covalent bonds, as such forming an equilibrated reaction mixture of products and building blocks. Such mixture is called a dynamic combinatorial library (DCL), since all library members (i.e. all reaction products and building blocks) are in thermodynamic equilibrium, in which the stability of each library member is determined by its Gibbs Energy (Figure 13a).
Figure 13: Gibbs Energy landscape of an equilibrium mixture a) without template and b) with template selective for A.
The reversible nature of a DCL makes the library constitution adaptive and responsive to external stimuli, like pH, temperature, solvents and ionic strength. The constitution of a DCL can be directed towards certain library members by addition of a suitable template, which ideally shifts the equilibrium towards the best binding library member by lowering its Gibbs Energy (Figure 13b). A template can be a small host molecule that is added to an equilibrated DCL and weakly interacts with a specific library member guest, forming a host-guest complex (Figure 14a). Alternatively, the template is added as a guest forming a host-guest complex with a library member host (Figure 14b). Ideally in both cases, the template can be removed from the selected library member after switching of or “freezing” the dynamic exchange[22,23].
Figure 14. a) Selection of a host by a separately introduced guest b) selection of a guest by a separately introduced host c) Self templating by intramolecular folding
Self templating is an effect which is frequently observed in supramolecular polymer chemistry, but is also observed in DCC. In self-templated DCC, the equilibrium is shifted towards the library member with the lowest Gibbs energy without adding an external template. A typical example of self-templated DCC are libraries in which foldamers are formed from small foldamer precursors. In such a library, the library members most prone to fold by energetically favorable non-covalent interactions are selected. Such a self-templated dynamic combinatorial approach has been used for studying the folding of synthetic polymers.[24]
An important consideration in DCC is the choice of the dynamic covalent reaction for setting up the DCL. An applicable dynamic covalent reaction for building DCLs should: 1) be reversible on a reasonable timescale, 2) be compatible with the used experimental conditions, 3) guarantee for good solubilization of all library members to prevent slow exchange rates and kinetic traps, 4) be able to switch off, as such “freezing” the exchange leading to possible isolation of library members, and 5) ideally provide library members that are all iso-energetic to prevent a strong bias towards certain products.
The most common reactions appropriate for DCC are imine exchange, hydrazone exchange and disulfide exchange and will be discussed in the coming paragraphs. Although less
frequently used, trans(thio)esterification, transamidation, aldol exchange, Michael / retro Michael reactions, (thio)acetal exchange, alkene methathesis, Diels Alder / retro Diels Alder reactions, metal ligand exchange and hydrogen bond exchange are also appropriate reversible reactions for DCC[23].
In the next paragraphs of this chapter, an overview will be given from the most common types of DCLs that have been investigated, and their application in responsive nanosystems.
2.4.2 Dynamic combinatorial libraries; disulfides
A disulfide based dynamic combinatorial library consists of thiol functionalized building blocks capable of forming disulfide bonds with one another. Continuous exchange of the disulfide bonds leads to the formation of the disulfide library members with the lowest Gibbs energy. This specific type of DCL is of interest because of the mild reaction conditions to initiate and terminate the thiol exchange. Thiol exchange can be initiated under neutral or mildly alkaline conditions in the presence of oxygen, and quenched by acidification.[23] The first disulfide based DCL has been reported by Sanders et al.[25] using carbohydrate, amino acid and diverse synthetic thiol functionalized building blocks (Figure 15). They showed that a large structural diversity could be created within a disulfide DCL since thiol exchange is multi-directional which resulted in detection, identification and quantification of 56 out of 66 theoretically predicted tetrameric macrocyles by LC-MS.[25]
Figure 15: a) Building blocks for library design b) Schematic representation of a disulfide based DCL
Upon addition of a template, the structural diversity in such a disulfide library could be drastically reduced by shifting the equilibrium to the best template binding library member. This is very often accompanied with amplification of the selected library member, which means that the concentration of the template selected library member is higher than in the non-templated situation. This has been shown by Otto et al.[26], where the introduction of a quaternary ammonium guest selects and amplifies the best binding macrocyclic disulfide (Figure 16).
Figure 16. The best binding macrocycle is selected from a library of four products after introduction of a quaternary ammonium template.
This principle has also been applied for selective synthesis of molecular cages[27], porphyrin macrocycles[28], catenanes[29] and enantioselective selection from racemic DCLs[30,31]. Besides synthetic effort, DCLs are also an outcome as screening tool for synthetic receptors with biologically relevant guests.[32,33]. A drawback of disulfide DCLs is the equilibration time, which can vary from days to sometimes several weeks before thermodynamic equilibrium has been reached[34], which limits its applications for fast and responsive libraries. However, self templating interactions between library members can reduce the equilibration times as shown by Ravoo et al.[35] They recently developed a disulfide DCL as screening tool for peptide-carbohydrate interactions. Within this library, the peptide building blocks form dimers by reversible disulfide formation at slightly alkaline pH, in which the best binding carbohydrate fits within the peptide cavity to form a 2:1 peptide-carbohydrate complex (Figure 17). Most striking, within 24 hours thermodynamic equilibrium (prior to addition of the carbohydrate) had been reached within their peptide libraries, which suggests that hydrogen bonding interactions between peptide building
blocks pre-organize the building blocks towards their thermodynamically favored constituent within a reasonable equilibration time.
Figure 17. Peptide library members GlnOH, HisOH and AspOH with guest NANA. It was shown that HisHis was selected and even amplified after addition of guest NANA
It can be concluded that disulfide DCLs are of potential interest because of their mild reaction conditions around neutral pH and room temperature. Component exchange can be instantly switched on and off by switching the pH to mildly alkaline or acidic without re-equilibration of the library constitution. Thiol building blocks exchange on a multi-directional level, which results in DCLs with a wide diversity of library members. However, addition of a suitable template can significantly reduce this diversity towards selection of the best binding library member. Although some exceptions exist, equilibration times of disulfide DCL are slow, with typical timescales varying from days to weeks[34]. This limits the application of disulfide based DCLs for fast responsive nanosystems.
2.4.3 Dynamic combinatorial libraries; imines
Imines are interesting compounds because of their role in diverse natural deamination processes, e.g. deamination of amino acids via the formation of imines by Vitamin B6. Imines are synthesized in situ by mixing an aldehyde with amine functionalized building blocks. As a first step, the amine attacks the carbonyl of the aldehyde, forming the hemiaminal intermediate. Second, acid catalysis results in removal of water from the hemiaminal, resulting in the imine as final product. By addition of water, the imine association equilibrium can be directed towards the amine and aldehyde building blocks (Scheme 1a). Besides hydrolysis, the equilibrium can also be shifted by addition of a second amine, in which the original imine undergoes transimination (Scheme 1b). Upon introduction of a second imine, the two imines can exchange by a methathesis reaction (Scheme 1c).
Scheme 1: Imine a) association b) transimination and c) exchange
In aqueous systems, the position of the imine association equilibrium depends on the chemical structure of the aldehyde and amine building blocks. The imine association equilibrium between aliphatic aldehyde and aliphatic amines is mainly positioned towards the starting compounds. Similar equilibrium positions were found for reactions between an aromatic aldehyde with an aromatic amine, with imine association constants typically below 10 M-1[36]. However, imine association is favored (K ~ 103 M-1) when mixing an aromatic aldehyde with an aliphatic amine. Next to this, pH and temperature also directly influence the position of the equilibrium. Extensive studies point out that the position of the imine association equilibrium can be effectively shifted by variations in pH, which is highly related to the pKa of the amine building block[37]. The imine association equilibrium is most effectively positioned towards the imine side at pH > pKa (amine), since the majority of the amine is present in its unprotonated form. At pH << pKa, most of the amine is protonated, as such positioning the equilibrium towards the aldehyde and amine building blocks[36,37]. The dynamic exchange can be quenched by addition of NaBH4, as such reducing imines and aldehydes to secondary amines and alcohols[38]. This method makes it possible to precisely measure the constitution of dynamic imine libraries, based on the constitution of the corresponding static secondary amine library[38].
The nitrogen atoms of the imine bonds are good sigma-donation ligands for metals, which makes imine DCLs ideal for metal recognition. Luning et al.[39] used this characteristic to switch between macrocycles and cages by addition of Ca(II) to an imine DCL (Figure 18). After mixing a bisaldehyde, trisamine and bisamine building block, a macrocyclic compound was formed as main constituent, by specific incorporation of the bisamine building block. However, addition of an external Ca(II) template resulted in re-equilibration of the library towards a nano-cage by specific incorporation of the trisamine with the macrocycle present in trace amounts within two days (Figure 18). Since macrocycles were again the main species after removal of the Ca(II) template it can be concluded that morphological switching between nano-cages and macrocycles is fully reversible in which
macrocycle synthesis is no longer the choice of suitable building blocks, but the selection of an appropriate template.
Figure 18: Imine macrocycle formation in situ from aldehyde and amine building blocks. After addition of a Ca(II) templates macrocycles switch to nano-cages.
Besides morphology selection, imine DCLs have also been applied for amplification of chirality. Gotor et al.[40] showed this by introducing a metal template to an imine DCL, consisting of diastereomeric macrocycles. The macrocycles were formed in situ after mixing a racemic bisamine with an achiral bisaldehyde with typical equilibration times of 18 to 24 hours. They found that a single diastereoisomer was selected from the library by specific binding to a Cd(II) template, showing the potency of DCC to synthesize components of specific chirality.
It can be concluded that imine formation is a powerful dynamic covalent reaction for DCLs when combined with metal templates, with typical equilibration times of one to two days. Also, the pH and temperature dependence of the imine association equilibrium has been used to develop stimuli-responsive nanomaterials, as will be discussed in paragraph 2.5.
2.4.4 Dynamic combinatorial libraries; hydrazones
Hydrazones are interesting compounds because of their, among other, antimicrobial, antitubercular and antitumoral properties[41]. Hydrazones are synthesized by mixing an aldehyde or ketone with hydrazide functionalized building blocks. As a first step, the hydrazide attacks the carbonyl of the aldehyde or ketone, forming the hemiaminal intermediate. Second, acid catalysis results in removal of water from the hemiaminal, resulting in a hydrazone (Scheme 2a).
Scheme 2: a) Hydrazone synthesis from aldehyde and hydrazide precursors b) A typical building block for pseudo-peptide DCLs[42]. The acetal functionality is in situ converted to an aldehyde functionality by addition of acid, yielding diversity of hydrazone functionalized components.
Hydrazones have been broadly applied in DCLs as pseudo-peptide macrocycles[42] (Scheme 2b). Relative to imines, hydrazones have a greater stability and the advantage of peptide-like hydrogen bonding. Pseudo-peptide DCLs have shown to be potential receptors for a large diversity of templates, e.g. crownethers[43] and ions[444] as recently shown by Sanders
et al.[45], who developed a DCL of hydrazone macrocycles, in which the ratio of dimeric and tetrameric macrocycles within the DCL is highly related to the ionic radius of the used alkaline earth metal ion template (Figure 19).
Figure 19: a) Schematic representation of hydrazone library building blocks b)
Component selection and amplification by adding metal salt templates
Hydrazone DCLs have also been applied for biological purposes, like screening tools for enzyme inhibitors[46] and controlled release vehicles for biomolecules[47]., Typical equilibration timescales can vary from days to months.[48] A typical example has been reported by Sanders et al.[48], who developed a pseudo-peptide DCLs in which the library
constitution adapts on the introduction of the neurotransmiter acetycholine as external template (Figure 20). They found that a mixture of diverse linear and macrocyclic pseudo-peptide hydrazones was formed after solubilizing a bifunctional hydrazide-aldehyde pseudo- peptide building block in organic solvent with thermodynamic equilibrium being reached within months. At a similar timescale, a pseudo peptide [2]-catenane was extensively amplified as major library constituent after addition of mixing neurotransmitter acetylcholine to this pseudo-peptide library, which clearly shows that the product constitution re-equilibrates towards the best neurotransmitter receptor albeit at an extremely long timescale.
Figure 20: a) Schematic representation of hydrazone DCL, forming dimers (2), trimers(3), tetramers (4), pentamers (5), hexamers (6) and [2]-catenanes (7) b) extensive amplification of [2]-catenanes by addition of neurotransmitter acetylcholine.
Hydrazone exchange takes place at acidic pH (pH 4-5) and the exchange is quenched by adjusting the solution around a neutral pH (pH 6-8). At these pH values, hydrazone exchange is kinetically trapped or slowed down, with typical exchange rates of days, weeks or months. Although this is an ideal method for analysis of the library constitution by HPLC methods, it limits the relevance of hydrazone DCLs for biological purposes.
A few attempts have been made to solve this problem. Huc et al.[49] introduced electron withdrawing groups on their hydrazide building blocks, as such increasing the electrophilicity of the formed hydrazone, which led to higher exchange rates. The addition of aniline as transimination catalyst has also shown to drastically increase the exchange kinetics of hydrazone formation even up to six orders of magnitude at pH 6.2[50]. Recently, Sanders et al.[51] found that addition of a small excess of aromatic mono-hydrazide avoids kinetically trapped states for hydrazone macrocycles that are stabilized by intra – and intermolecular self templating, which alternatively required up to 50 equivalents of aniline to accelerate the exchange kinetics. Although this approach is effective in both organic and aqueous solvents, optimization is required for each system, which makes the aniline route more general to apply.
2.4.5 Combined Dynamic Combinatorial Libraries
In the previous paragraphs diverse DCLs are discussed in which dynamic exchange is based on a single type of dynamic covalent reaction with component selection taking place by a (self) templating effect. Two or more dynamic covalent reactions can also be combined within one system, forming combined DCLs. By combining two or more types of dynamic covalent reactions in one system, the library constitution can be controlled by switching specific exchange reactions on and off.
When two different dynamic covalent reactions can take place and can be switched on and off independently without influencing each other, they are said to work orthogonally. Lehn
et al.[52] showed this by forming metal complexes within a DCL, using imine formation and
metal complexation within one system. Metal-ligand exchange could be switched on and off by oxidation and reduction of the metal center without influencing imine exchange. Secondly, by changing the pH, imine exchange was reversibly switched on and off without disturbing the metal-ligand interactions (Figure 21).
Figure 21: Schematic representation of anorthogonal DCL, in which imine association and metal coordination take place orthogonally.
Hydrazone and disulfide exchange are also two dynamic covalent reactions that can work orthogonally. At acidic pH, hydrazone exchange is switched on while disulfide exchange is switched off and vice versa as shown by Furlan et al.[53]. These examples show that structural control within a DCL can be realized by using orthogonal exchange reactions. Alternatively, structural diversity within a DCL can be generated by using two dynamic covalent reactions that exchange simultaneously. This has been shown by Otto et al.[54], using a DCL that can be orthogonal or non-orthogonal depending on the pH. By mixing diverse hydrazone and disulfide building blocks, hydrazone and disulfide can be selectively
activated or deactivated. Hydrazone exchange was selectively activated at acidic pH and disulfide formation at mildly basic pH values. At intermediate pH values both exchange reactions occur simultaneously
Only one system is described in literature in which three dynamic covalent reactions are used within one system[55]. This particular system shows that by using three orthogonal exchange reactions in one system, the statistically disfavoured library members are selected. Instead of an external template they used a self-sorting mechanism as selection tool. They showed this by combining a disulfide DCL with a second DCL consisting of metal complexes. The disulfide DCL consists of one amine terminated hetero-disulfide, an amine terminated homo-disulfide and a methoxy terminated homo-disulfide, in which the hetero-disulfide is the statistically favored constituent (Scheme 3a). The metal complexes in the second DCL are kept together by imine bonds and disulfide bonds (Scheme 3b). These complexes are stable for weeks, without any re-equilibrations showing that all three types of dynamic covalent reactions are orthogonal. However, by mixing the disulfide DCL with the metal complex DCL, the constitution of the newly formed library re-equilibrates, by selection of the statistically disfavored amine terminated homo-disulfide at expense of the amine terminated hetero-disulfide and the methoxy terminated homo-disulfide (scheme 3c). This points towards a self-sorting mechanism, in which the metal complex selects its best binding ligand to maintain its electron density on the metal centre which shifts the equilibrium towards the statistically disfavored products.
Scheme 3: Triple level orthogonal DCL containing a) disulfide exchange and b) imine association and metal coordination. c) Combination of a and b in one system
It can be concluded that two or more dynamic covalent reactions can be successfully combined within one system, leading to orthogonal or non-orthognal DCLs. In orthogonal DCLs, dynamic exchange of all reactions are independent without influencing each other, leading to structural control by switching specific exchange rates on or off. The opposite is the case of non-orthogonal DCLs, in which structural diversity within a DCL is generated by simultaneous exchange reactions. This additional control in combined DCLs can result in component selection without addition of external templates.
2.5 Reponsive systems and nanomaterials using dynamic covalent chemistry
2.5.1 Introduction
During the last years, diverse attempts have been made towards the development of self-healing and smart nanomaterials, like polymers, rotaxanes and functionalized surfaces. Dynamic covalent chemistry has been used as a tool to develop such materials, in which the self-healing properties of the nanomaterial are directly related to the formation of reversible covalent bonds. In this paragraph a variety of responsive systems and nanomaterials will be discussed.
2.5.2 Responsive functional polymers using a dynamic covalent chemistry approach
For the development of high molecular weight dynamic covalent polymers from monomeric building blocks, a strong bias towards polymeric constituents is needed. In monomeric dynamic combinatorial libraries, the product constitution can be regulated by addition of an external template, which drives the reaction towards the best binding complex (see Section 2.4.1). However, for the development of dynamic covalent polymers, self-templating is a general tool to drive the reaction towards polymeric library constituents. A typical example of self-templated dynamic covalent polymerization has been reported by Moore et al.[56]. They showed that small curved imine building blocks polymerize in high molecular weight imine foldamers self-templated by hydrophobic interactions. For this they mixed two imine functionalized foldamer precursors, forming oligomeric products by transimination. As a second step, the oligomeric products start to fold in helices (Figure 22a). Intramolecular folding shifts the imine ligation equilibrium to completion, since binding a new precursor to the growing foldamer lowers the overall Gibbs energy.From a pool of diverse oligomers, the segments most prone to fold were selected, which demonstrates that folding can be used as a self-templating effect for component selection.
Figure 22: a) Formation of foldamers by transamination from curved precursors b) Formation of helical polymers from imine precursors.
Similar attempts have been made by Lehn et al. which led to the development of helical hydrazone polymers[57] and amphiphilic foldamers[58]. These observations suggest that controlling this self-templating effect could lead to self-correcting and stimuli responsive polymers. An example of this has been reported by Lehn et al.[59], in which the morphology of helical hydrazone polymers can be reversibly switched to non-helical grid like structures upon addition of a metal template (Figure 23a). Within the same system, the polymer length could be tuned by shifting the imine equilibrium towards its building blocks by pH.
In a similar system[60], linear dynamic covalent imine polymers could be reversibly switched to macrocycles upon addition of a metal template (Figure 23b). Additionally, the physical properties of the polymers could be tuned upon varying the type of subsitutents on the bisaldehyde and bisamine building blocks, which resulted in a variety of flexible and stiff polymers after quenching the imine exchange and isolation of the polymer.
Figure 23: a) Lewis-acid catalyzed formation of helical strands, which transform in trid-like arrays upon metal coordination b) Reversible polymer to metallo-macrocycle association upon metal coordination c) Flexible and stiff polymeric materials upon mixing bishydrazides and bisaldehydes with flexible and stiff spacers respectively.
Alternatively, the material properties of dynamic covalent polymers can also be tuned by exchange reactions. Lehn et al.[61,62] showed this for a series of hydrazone polymers. They pointed out that the flexibility of the spacer of the bisaldehyde or bishydrazide building block is directly related to the mechanical properties of the polymer films[61]. Flexible films were obtained by mixing flexible building blocks, while the polymer film becomes more rigid after mixing with rigid building blocks (Figure 23c). Using a similar approach, color and fluorescent properties of polymer films can be tuned by hydrazone bond exchange and recombination using building block with different conjugation length[62].
Dynamic covalent core crosslinked star (CCS) polymers have been developed by Fulton et
al.[63]. The building blocks of these polymers consist of aldehyde and amine functionalized diblock copolymers, which core crosslink by the formation of imine bonds (Figure 24a). By this, the inner core of a CCS polymer is shielded from its external environment and can therefore be utilized as carriers for small molecules. The molecular weight and aggregate size of these polymers could be regulated by the mixing ratio of aldehyde and amine functionalized diblock copolymers. Recently, amphiphilic water soluble CCS polymers were synthesized, which were able to encapsulate and release the hydrophobic Nile Red probe in their inner polymeric core, triggered by temperature and pH[64] (Figure 24b).
Figure 24: a) Formation of dynamic covalent core cross-linked star polymer chains through dynamic covalent imine formation b) Temperature and pH reversible encapsulation of Nile Red by water soluble CCS polymers. It should be noted that the applied temperature shift only induces release of the Nile Red probe, since not all imine linkages dissociate in this temperature regime.
A striking step towards mechanosensitive nanomaterials has been made by Otto et al.[65] They showed that a dynamic combinatorial disulfide library, mainly containing of trimeric and tetrameric macrocycles with peptide side chains, spontaneously converts into hexamers and heptamers upon mechanical agitation (Figure 25a). Both hexamers and heptamers assemble in micron long fibers by β-sheet formation, while the trimers and tetramers do not form fibers. Shaking a solution with trimeric and tetrameric macrocycles selectively yields hexamer fibres, while the trimers and tetramers are almost fully consumed. Stirring selectively yields heptamer fibers. Moreover, shaking and stirring also led to self-replication of the fibers, since the fiber are broken by mechanical agitation and can serve as new fiber nuclei (Figure 25b). In later work[66], hexamer fibers were converted to mechanically stable gels upon photo-irradiation by rearranging the disulfide bonds within the fibers (Figure 25c).
Figure 25: a) Oxidation of a dithiol peptide building block gave a library of trimers and tetramers, which are consumed to form hexamers or heptamers upon shaking and stirring b) schematic representation of fiber self-reproduction upon mechanical agitation c) Peptide hexamers before and after photo-irradiation, forming fibers before irradiation and gel fibers after photo-irradiation.
It can be concluded that dynamic covalent chemistry provides a lot of opportunities towards new nanomaterials like stimuli responsive polymers and mechanosensitive gels.
2.6 Conclusions and outlook
Surfactants consist of a hydrophobic tail and a polar headgroup and self-assemble in aggregates after passing the critical aggregation concentration. In aqueous environment, self-assembly is driven by hydrophobic interactions which led to a broad diversity of surfactant morphologies. Surfactant assemblies play an essential role in living systems, because of their ability to encapsulate and release cargo in and from the aggregate interior, which is due to the fast exchange dynamics of surfactant molecules from aggregates to the bulk and vice versa.
These surfactant properties led to the development of synthetic responsive surfactant systems as drug delivery vehicles, in which association and dissociation of surfactant aggregates is triggered by external stimuli. However, the solubility of stimuli responsive surfactants in water is generally low, which limits its application as stimuli responsive surfactant assemblies to irreversible systems with slow exchange dynamics.
In dynamic combinatorial libraries, building blocks exchange with each other, by the formation of reversible covalent bonds, yielding a mixture of components in which the stability of each library member is thermodynamically controlled. After addition of a suitable template, the library constitution adapts towards the library member which forms the most stable binding complex, as such selecting the best binding library member. Additionally, external stimuli like pH and temperature can shift the equilibrium between the library constituents. This altogether resulted in stimuli responsive surfaces and a diversity of nanomaterials. High molecular weight stimuli responsive polymers were formed from small monomeric building blocks by a self-templating mechanism. Next to this, common synthetic problems in rotaxane synthesis were overcome by a self-templated dynamic combinatorial approach.
We believe that the generally slow exchange dynamics in synthetic responsive surfactant systems could be overcome by a dynamic combinatorial approach, in which the exchange dynamics of surfactants from and to the bulk is regulated by the reversible formation of dynamic covalent surfactants from small non-amphiphilic building blocks. This would result in a new type of surfactant systems, in which aggregation could be switched on and off by shifting the equilibrium of dynamic covalent bond formation towards surfactant building blocks and surfactants, triggered by external stimuli. Results of this concept will be discussed in the remainder of this Thesis.
