recent advances in assemblies of cyclodextrins and

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Colloid & Interface Science

Recent advances in assemblies of cyclodextrins andamphiphiles: construction and regulationKaerdun Liu, Cheng Ma, Tongyue Wu, Weilin Qi, Yun Yan andJianbin Huang

AbstractCyclodextrins (CDs) had been regarded as destructors inmolecular assembly systems for a long time until CD/surfac-tants were found to assemble into high order structure drivenby hydrogen bonding between CDs. Thereafter, intensive re-searches have been conducted on construction and regulationof CD–amphiphile systems. Here, we summarized the recentprogress on construction and regulation of CDs and amphi-philes assembly. The scope of amphiphiles have beenextended from surfactants (ionic surfactants, zwitterion sur-factants, nonionic surfactants, gemini surfactant, and so on), tonontypical amphiphiles (amines, aromatic molecules, alkanes,and so on). Owing to the abundant choices of guest amphi-philes and dynamic nature of host–guest inclusive interaction,numerous regulation methods (such as enzyme, light, pH,concentration, temperature, and so on) have been used inCD–amphiphile systems. Moreover, remarks and future per-spectives are also discussed at the end of this review, which isexpected to stimulate progress on both mechanism andapplication level.

AddressesBeijing National Laboratory for Molecular Sciences (BNLMS), StateKey Laboratory for Structural Chemistry of Unstable and Stable Spe-cies, College of Chemistry and Molecular Engineering, Peking Uni-versity, Beijing, 100871, PR China

Corresponding author: Huang, Jianbin ([email protected])

Current Opinion in Colloid & Interface Science 2020, 45:44–56

This reviews comes from a themed issue on Surfactants

Edited by Romain Bordes and Yilin Wang

For a complete overview see the Issue and the Editorial

https://doi.org/10.1016/j.cocis.2019.11.007

1359-0294/© 2019 Elsevier Ltd. All rights reserved.

KeywordsCyclodextrins, Amphiphiles, Non-typical amphiphiles, Regulation, Light,Enzyme.

IntroductionCyclodextrins (CDs) are cyclic oligosaccharides pro-duced by enzymatic degradation of one of the mostessential polysaccharides, starch. Since the first discov-ery in 1891, CDs have drawn intensive attention. And


applications of CDs have been found in almost all sec-tors of industry, especially pharmacy, food, chemistry,chromatography, catalysis, biotechnology, agriculture,cosmetics, hygiene, medicine, textiles, and environment[1].

The uniqueness of CDs comes from its cage structure.Most commonly used CDs include natural a, b, and g-CDs with six, seven, and eight a-D-glucopyranose units,respectively, which are linked by a-1,4 glycosidic bondsto form a ring (Figure 1 (a)). CD has a hollow, truncatedcone appearance, and a donut, toroidal-like shape. Theouter side of the toroid is hydrophilic because of thehydroxyl groups of the glucopyranose units, while thecentral cavity of CDs is lined by skeletal carbons andethereal oxygens of glucose residues, which makes it

much less hydrophilic than the aqueous environment.Thus, the hydrophobicity of cavity enables the favorableinclusion of hydrophobic parts of guest molecules(mainly amphiphiles) in a process known as hosteguestassembly to form inclusive complex [2].

The assemblies from CDeamphiphileeinclusive com-plexes received intensive attention because of severaladvantages. Firstly, CDs, as natural products from starch,endow CDeamphiphile assemblies with high biocom-patibility. Moreover, the CDseamphiphileseinclusive

complexes have crystalline structure, representing clearmechanism during assembly. Therefore, it is appealingto fabricate CDeamphiphile assemblies with highbiocompatibility and clear mechanism. However, from ahistorical view, CDs were firstly regarded as destructorfor assemblies because the hydroxyl groups (-OH) onthe exterior of CDeamphiphileeinclusive complexeswould improve the solubility of complex, meaning nohigh order structure will appear.

To obtain high-order assembly structure from CDeamphiphile complexes, two strategies were used. Firststrategy is to chemically modify CDs by introducinghydrophobic groups, which may reduce the biocompat-ibility of natural CDs. Second one is to thread CDs onhigh molecular weight polymers [4], where the crystal-line property of CDeamphiphile complexes is usuallyabsent. It was still a challenge to fabricate high-orderassembly structures from natural CDs and small

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Figure 1

Structure and aggregation of CDs. (a) Molecular structures and dimensions of various CDs: (A) a-CD; (B) b-CD; and (C) g-CD. (b) Average size ofnative CDs aggregates versus CD concentration obtained by light scattering. (n is the number of molecules) [3]. CD, cyclodextrin.

Recent advances in assemblies of cyclodextrins Liu et al. 45

amphiphiles until Jiang et al. [5,6] reported a pioneeringresearch on assembly of b-CDs and surfactant sodiumdodecyl sulfate (SDS) in 2010.

The key conceptual advances are hydrogen bonds be-tween CDs, which can be the driving force to fabricatehigh-order assembly structure from inclusive complex.It has been known for some time that natural CDs areable to self-assemble into aggregates because ofhydrogen bonds between CDs [3,7]. As shown inFigure 1 (b), a/b/g-CD can assemble into aggregates,while the largest aggregates are observed from b-CDassemblies [8]. The driving force for native CDs toaggregate is CDeCD hydrogen bond. However, native

CD aggregates do not have well-ordered structure andtunable properties of regulation, which can be providedby assemblies with amphiphiles. Therefore, the generalhierarchical assembly process leading to high orderstructure from CDs and amphiphiles can be described asfollowing: firstly, the formation of inclusive complexoccurs driven by hosteguest interactions, then the in-clusive complex can further assemble into high-orderassembly structure driven by hydrogen bonding on theexterior of inclusive complex.

Since the report of SDSeb-CDs assembly in 2010 [5,6],numerous research studies have been conducted toexplore the construction of assemblies from CDs and


amphiphiles, to extend the scope of amphiphiles fromtypical surfactants to nontypical amphiphiles, and toregulate CDeamphiphile assemblies upon stimuli

including light, pH, enzyme, and so on.

There were already some reviews covering themodulator role of CDs in amphiphile assembly [9,10].In this review, we want to summarize the recent ad-vances on construction and regulation in assemblies ofCDs and amphiphiles. Construction of CDs andamphiphiles assembly will mainly cover the con-struction of CDeamphiphile assemblies. Regulationof CD and amphiphile assembly will emphasize theregulation of assemblies of CDs and amphiphiles.

Remarks and perspectives is the remarks andperspective.

Construction of CDs and amphiphilesassemblyAssembly of CDs and polymersWe first briefly review the assembly of CDs and

amphiphilic polymers, since researchers used polymer tothread CDs before CDeCD hydrogen bonding wasused. Native CDs can directly assemble onto polymerchains to form so-called poly (pseudo)rotaxane [11e13].The resulting assembly is reminiscent of beads on astring. Early work on poly (pseudo)rotaxane started withthreading multiple CDs onto poly (ethylene glycol)



46 Surfactants

(PEG) polymer chains to create crystalline inclusioncomplex [4]. Besides PEG polymer, alkyl chain can alsoserve as linear line to thread a-CDs [14]. The chemicalstructure of polymers can be varied to alter materialproperties of poly (pseudo)rotaxane, especially thestructure and sequence of the threading polymer [15e17]. Apart from synthetic polymers, biological poly-mers can also assemble with CDs. For example, CD can

serve as an important and promising method to controlthe assembly behavior of DNA [18,19].

Shortly thereafter, polymereCD polyrotaxanes wereused to create hydrogels. It should be noted that poly-mereCD hydrogels did not form initially upon threadingand were dependent on self-association of a-CDsthrough hydrogen bonding to form physical cross-linksbetween the linear poly (pseudo)rotaxane chains [20e22]. These research studies illustrate the importanceof CDeCD hydrogen bonds in formation of high-order

assembly structures.

In brief, polymers can form inclusive complex with CDsand thread CDs on polymer chain. Although CDs onpolymer with high molecular weight can be obtained,the assembly mechanism is usually uncertain, (forexample, the diversity of polymer mass and the numberof threaded CDs are not accurate during assembly) andCDs are not driving components during assembly. Still,some researches have shown importance and possibilityto directly use hydrogen bonding between CDs to

fabricate high order structures.

Assembly of CDs and surfactantsLearning the importance of hydrogen bonding betweenCDs, Jiang et al. [5,6] reported breaking through as-sembly of b-CDs and surfactant SDS in 2010 driving bystrong hydrogen bonding between b-CDeSDSeinclu-sive complexes. Thereafter, plenty assembly exampleswith detailed mechanism between CDs and surfactantswere reported.

Surfactants are classic amphiphiles containing both hy-drophobic groups and hydrophilic groups. Usually, thehydrophobic groups of surfactants can be incorporated

into CDs, leading to CDesurfactant inclusion complex[2]. Then CDesurfactant complex as a building blockcan further assemble into various high order structuresdriven by hydrogen bond between CDs. This two-stephierarchical assembly process is universal for manytypes of surfactants. In this assembly process, thestructure of surfactants will determine the associationstrength and ratio with CDs, and hydrogen bonding andother interactions (such as electrostatic interaction be-tween charged group on anionic or cationic surfactants)will determine the crystalline assembly of CDesurfactant complexes.


In 2010, Jiang et al. [5,6] reported that anionic surfac-tant, SDS and b-CDs can form 1:2 complex (denoted asSDS@2b-CD), which can further assemble into well-defined lamellae, tube, vesicles in concentrated orsemiconcentrated aqueous solution (Figure 2 (a)). SDSand b-CDs are able to form hosteguest complexes in 2:1stoichiometry with high binding constants by includingSDS’s hydrophobic tails into CD cavities. Then

SDS@2b-CDs, as building blocks, can assemble intochannel-type crystalline bilayer membrane. The mem-brane can laterally expand into infinite two-dimensional lamellar structures at high concentrations,disconnect and scroll up along one axis into onedimensional multilamellar microtubes upon dilution,and further close along another axis into dispersed ves-icles upon further dilution.

Detailed analysis confirms the SDS@2b-CD as buildingblocks and structure of SDS@2b-CD channel-type

crystalline bilayer membrane. Morphologies of SDSeb-CD assemblies are observed by confocal laser scanningmicroscope (CLSM), transmission electron microscopy(TEM), and atomic force microscope (AFM). Figure 2(b) (A) and (B) are full of numerous parallel lines witha uniform interval, typical for cross section of lamellarstructure. Figure 2 (b) (C) and (D) are prevailed bypairs of parallel lines, indicating the one-dimensionaltubular structure. Figure 2 (b) (E) and (F) are domi-nant by spherical structures, corresponding to vesicles.Hydrogen bonds between CDs and electrostatic in-

teractions of charged head group of SDS were proposedas driving forces.

Besides anionic surfactant SDS, zwitterion surfactantscan also form inclusion complex with b-CDs and furtherassemble into high-ordered structure in a similarmechanism [23]. Tetradecyl dimethylammonium pro-pane sulfonate (TDPS, Figure 2 (c)) and b-CDs canform 2TDPS@3b-CD building block. The2TDPS@3b-CD complex laterally expands intolamellae, the lamellae stack together in the perpendic-ular direction to form a 3D array, and the array eventually

folds up into the microtubes.

The TDPSeb-CD microtubes are long, flexible,bundling, and entangling (in great contrast to the pre-viously described SDS@2b-CD microtubes), and theycan thus form an extensive 3D network and eventually astrong hydrogel. Compared with SDS@2b-CD nano-tubes, TDPSeb-CD microtubes are more flexible,resulted from the surfactant headgroups: zwitterionicfor TDPS (electrostatic repulsion minimized) andanionic for SDS (highly electrostatically repulsive).

Similarly, hydrogen bonds between CD and CDs andelectrostatic interactions of charged head group ofTDPS are driving forces of TDPSeb-CD assembly.



Figure 2

Assembly of b-CD with SDS, TDPS and tween20. (a) Schematic self-assembly behavior of SDS@2b-CD. (A) SDS and b-CD monomers, (B) SDS@2b-CD complex, (C) and SDS@2b-CD bilayer membrane with a channel-type crystalline structure. (D), (E), and (F) the aggregates transform upon dilutionfrom lamellae via microtubes to vesicles [5,6]. (b) Morphology of SDS@2b-CD aggregates. (A) (B), FF-TEM images of lamellae. c) d), CLSM and TEMgraphics of microtubes, respectively. (E) (F), FF-TEM and AFM images of vesicles, respectively [5,6]. (c) (A), Schematic illustrations of different TDPS–b-CD complexes. (B) An array of the 2TDPS@3b-CD complex. (C) a microtube of the 2TDPS@3b-CD complex [23]. (d) Schematic illustrations of the self-assembly behavior of tween 20–b-CD complexes [24]. CD, cyclodextrin, SDS, sodium dodecyl sulfate, TDPS, tetradecyl dimethylammonium propanesulfonate; TEM, transmission electron microscopy; FF-TEM, freeze fracture transmission electron microscopy; CLSM, confocal laser scanning micro-scope; AFM, atomic force microscope.


Apart from ionic surfactants, nonionic surfactant can alsoassemble with CD and form various ordered structures[24,25]. Assembly of Tween 20 with b-CD was investi-gated in detail [24]. Tween 20 and b-CD can form 1:2inclusion complex (Tween 20@2b-CD) as building unit.

Tween 20@2b-CD will tend to assemble in a parallelway to form infinite two-dimensional bilayer driven byhydrogen bonding (Figure 2 (d)). In dilute concentra-tion range (c = 0.03e20 mM), the bilayer membranesbend into vesicles. In concentrated range (c = 6e25 wt% or 20e100 mM), the bilayer membranes stack closelyinto lamellar flakes. Moreover, many other nonionicsurfactants, including AEO3, AEO9, Tween 40, Tween


60, Tween 80, and Triton �100, can also assemble withb-CD and obtain vesicle structure in a similar way.

Surfactants with special topology (such as gemini/bolatype surfactants) can also assemble with CDs. b-CD can

interact with hydrophobic carbon chains and withpolyethylene oxide chain of a nonionic gemini surfactantto form inclusion complex [26,27]. Micelles or rod-likestructure can be obtained at different concentrationregions. b-CD can also interact with hydrophobic carbonchains of cationic gemini surfactants. Bis (dodecyldimethylammonium)diethyl ether dibromide (12-EO1-12) and b-CD assembly was investigated by NMR



Figure 3

Assembly of b-CD with geimini and trimeric surfactants. (a) (A), Molecular structure of 12-EO1-12 gemini surfactant. Schematic illustration of as-sembly structure of gemini@b-CD (B) and gemini@2b-CD (C) [28]. (b) (A) molecular structure of C12NDDA gemini surfactant. (B) Phase diagram of theC12NDDA–b-CD–H2O system at 25 C. (C) Schematic summary of the aggregation transitions of the b-CD–C12NDDA complexes with varyingC12NDDA concentrations and molar ratios of C12NDDA with b-CD [29]. (c) Aggregates transitions of the CD–DTAD complexes with the increase ofconcentration. b-CD and g-CD will form 1:1 complex with DTAD, while a-CD can form 1:1 and 2:1 complex. DTAD–CD complex will assemble into largesolid spherical aggregates, vesicles and micelles upon increasing concentration [30]. C12NDDA, novel cationic gemini surfactant with a spacer containingnaphthalene and amides. CD, cyclodextrin. DTAD, tri(dodecyldimethylammonioacetoxy)diethyltriamine trichloride, a novel cationic ammoniumtrimericsurfactant.

48 Surfactants

techniques (Figure 3 (a)) [28]. Analysis of the 1H NMRspectra and self-diffusion coefficients reveal the forma-tion of gemini@b-CD and gemini@2b-CD (1:1 and 1:2

complex) with a calculated stability constant for thesecond binding step higher than that of the first.Detailed assembly structure was illustrated in Figure 3(a) (B) (C). At low concentrations of b-CD, one of thehydrocarbon tails of the surfactant enters the cavitythrough the wide rim, with the other chain interacting


with the external surface of the b-CD. By contrast, athigher concentrations of b-CD, the two hydrocarbontails are included in two b-CD cavities.

Inclusion complex of gemini surfactant and b-CD canalso further assemble into various structures [29,31].Figure 3 (b) shows a novel cationic gemini surfactant(C12NDDA) with a spacer containing naphthalene andamides. Tuning the C12NDDA concentration and the




C12NDDAeb-CDmolar ratio allowed the production ofdifferent assembled aggregate morphologies such asmicelles, vesicles, nanowires, nanorods, and hydrogels(Figure 3 (b) (C)). Detailed investigation revealedthat C12NDDA could form inclusion complex with b-CD as C12NDDA@b-CD, C12NDDA@2b-CD,C12NDDA@3b-CD, and [email protected]@b-CD has an ‘exposed’ hydrocarbon chain,

which lead to precipitate because of strong hydrophobicinteractions. As for C12NDDA@2b-CD andC12NDDA@3b-CD, the amount of b-CD was insuffi-cient to interact with all the hydrophobic arms ofC12NDDA; thus, the complex tended to restack intospherical micelles with the hydrophobic moietiesaggregated in the core and the hydrophilic moietiesstretched into water. Moreover, the inclusion complexesassembled into micelles because of hydrophobic in-teractions, hydrogen bonding, and van der Waals forces.In C12NDDA@4b-CD complex, two hydrophobic

chains of a C12NDDAmolecule was combined with fourb-CD molecules, leading to channel-type bilayer mem-branes. As the concentration of C12NDDA increased,the bilayer membranes closed laterally along two in-plane axes to generate vesicles. When the concentra-tion of C12NDDA increased to 38 mM, the vesiclesexpanded into nanorod structures that could furthercross-link in a disordered manner to generate an opaqueand thermoreversible hydrogel through hydrogen bondinteractions, p-p stacking, and electrostatic forces.

Moreover, trimeric surfactants with three hydrophilicgroups and three hydrophobic groups can also assemblewith CDs. Tri(dodecyldimethylammonioacetoxy)dieth-yltriamine trichloride (DTAD) is a novel cationicammonium trimeric surfactant with a star-shapedasymmetric spacer (chemical structure shown inFigure 3 (c)) [30]. Upon adding CDs with differentcavity size, the a-CD@DTAD, 2a-CD@DTAD, b-CD@DTAD, and g-CD@DTAD complexes are fabri-cated. Compared with DTAD itself, these CDeDTADcomplexes show much lower critical aggregation con-centration and form more diverse aggregate structures

with varying the concentration, larger vesicles, orspherical solid aggregates of w50 nm first and thensmaller micelles of w10 nm. The aggregate transitionsare controlled by the discrepancy in the intensity ofhydrogen bonds and hydrophobic interaction, and thechanges of molecular configuration as illustrated inFigure 3 (c). These different assembly structures canalter antibacterial activity, and the formation of vesiclesis approved to be in favor of the improvement of themildness.

In brief, many types of surfactants (ionic surfactants,zwitterion surfactants, nonionic surfactants, geminisurfactant, and so on.) can form inclusion complexes


with CDs driven by hosteguest interactions. Theseinclusion complexes can further assemble into variousstructures including vesicles, nanotubes, fiber, lamellae,and so on, driven by hydrogen bonding between CDs.The broad spectrum of surfactants and final assemblystructures will provide us abundant choices and prom-ising applications.

Assembly of CDs and nontypical amphiphilesAs shown before, CDs can assemble with amphiphilesincluding polymers and surfactants driven by inclusion

of hydrophobic groups into cavity of CDs and hydrogenbonding between inclusive complexes. Besides surfac-tants, recent studies also demonstrate the assemblybetween CDs and nontypical amphiphiles (such asamines [32], aromatic molecules [33], and so on.), evennonamphiphiles (such as alkene [34,35], and so on).Similarly, these guest molecules can form inclusioncomplexes with CDs, which further assemble into or-dered structures.

Alkyl amines are polar molecules consisting hydrophobic

alkyl chains and hydrophilic NH2. After introducingalkyl amines CH3(CH2)n-1NH2 (n = 8, 9, 10) into b-CDsolution, the inclusion complex was formed and whiteprecipitates were observed (Figure 4 (a)) [32]. Detailedinvestigation revealed that CH3(CH2)n-1NH2 (n = 8, 9,10) and b-CD formed 1:1 inclusion complex, which canfurther assemble into channel-type crystalline layer.Interestingly, CH3(CH2)n-1NH2 (n = 8, 9) are orientedpredominantly toward the wider rim of b-CD, whileCH3(CH2)n-1NH2 (n = 10) threaded through both thenarrower and wider rim of b-CD. The orientation of

alkyl amine is determined by hydrogen interaction be-tween NH2 group of alkyl amine and OH group of CDs.Interestingly, the selective orientation of CH3(CH2)n-1NH2 (n = 8, 9) in b-CD leads to helices, whereas dualorientation of CH3(CH2)n-1NH2 (n = 10) results inplanar bricks observed by scanning electron microscopy(SEM).

Beyond nontypical amphiphiles alkyl amines, re-searchers even extend the scope of guest molecules thatcan assemble with CDs to fully hydrophobic molecules.

Dodecane and b-CD were found to form inclusivecomplex dodecane@2b-CD (Figure 4 (b)) [34].Because the extending length of dodecane is muchshorter than the sum of two b-CDs, the dodecanemolecule was completely buried in the channel formedby two b-CD molecules. Thus, dodecane was notdirectly involved in the interactions between thedodecane@2b-CD supramolecular complexes, so thatthe self-assembly of dodecane@2b-CD is simply that ofthe channel type b-CD dimers which is dominated byhydrogen bonds. The dodecane@2b-CD complex will

form vesicles at low concentration but bricks at high

6

Figure 4

Assembly of b-CD with alkyl amines and alkanes. (a) CH3(CH2)7,8NH2 is oriented toward to the wider rim of b-CD, and CH3(CH2)9NH2 is orientedtoward to both the wider and narrower rims of b-CD. The selective orientation of the CH3(CH2)7,8NH2 leads to helices, whereas the dual orientation ofCH3(CH2)9NH2 results in planar bricks [32]. (b) Schematic illustration for the self-assembling behavior of the dimers of 2b-CD threaded by one dodecane.The dimer is a supramolecular building block of dodecane@2b-CD [34]. (c) Schematic illustration of the self-assembly behavior of alkane/b-CD buildingblocks [35]. CD, cyclodextrin

50 Surfactants

concentration related with water-mediated hydrogenbond. At low concentrations, more water molecules areexpected to bridge the dodecane@2b-CD supramolec-

ular building blocks. The uneven distribution of watermolecules in the outer and inner side of the membraneleads to the curvature for vesicles. At high concentra-tions, dodecane@2b-CD complexes are adequate forparallel packing leading to bricks.

More systematically, influence of chain length of alkaneson assemblies with CDs are studied (Figure 4 (c)) [35].In the case of short-chained alkanes with 5e10 carbons,the building block is channel type 2alkane@2b-CD,which can be looked on as the dimer of alkane@b-CD.

Upon increasing the chain length to 12e14 carbons, thebuilding blocks take the form of [email protected], upon further increasing the chain lengthto 16 and 18 carbons, the building block changes back tothe form of 2alkane@2b-CD but with nonchannel typearrangement of b-CD. All these inclusion complexes asbuilding blocks can self-assemble into unilamellar andbilamellar vesicles in dilute solutions. Similar tododecane@2b-CD assembly, water was found to act as astructural mediator of hydrogen bonds in vesicleformation.

In summary, nontypical amphiphiles (including amines,aromatic molecules, alkanes) can assemble with CDs toform inclusion complex and further assemble into


ordered structures (helices, vesicles, and so on) drivenby CDeCD hydrogen bonds. In this process, guestmolecules can influence the inclusion formation by polar

head NH2 or chain length, leading to controllable as-sembly structures. Combining with previous 2.1e2.3sections, we overviewed the assembly of CDs and manytypes of guest amphiphiles, including polymers, surfac-tants (ionic surfactants, zwitterion surfactants, nonionicsurfactants, gemini surfactant, and so on), and nontyp-ical amphiphiles (amines, aromatic molecules, alkanes,and so on). Guest amphiphiles can form inclusioncomplex with CDs, and CDeCD hydrogen bond caninduce further assembly of these inclusion complex toform various types of ordered structures. Because of the

broad spectrum of guest amphiphiles and dynamicnature of hosteguest inclusive interaction, researchersalso illustrate plenty of ways to regulate CDeamphiphile assemblies to fabricate smart responsivesystems (covered in Regulation of CD and amphiphileassembly).

Regulation of CD and amphiphile assemblyIn general, assemblies of CDs and amphiphiles occur insequence: first CDs and amphiphiles form inclusioncomplexes, then these complexes further assemble intoordered structures. The ways to regulate the afore-mentioned assemblies can be roughly divided intoseveral categories: (1) to regulate the structure of CDsby enzyme and so on; (2) to regulate guest amphiphiles,




for example, by introducing light-, pH-, coordination-responsive moieties into guest amphiphiles; (3) toregulate hosteguest interactions and hydrogen bondingbetween CDs and amphiphiles by concentration, tem-perature, and so on. In this section, we will give anoverview of the progress in this aspect. Although moreexciting developments are still being made, the exam-ples presented herein clearly demonstrate the

numerous opportunities and practical applications thatcan stem from the diversity and complexity of CD-basedstimuli-responsive assembly systems.

EnzymesHistorically, CDs were firstly found from the enzymaticproduction of starch [1] and closely related to severalbiological pathways. On the other hand, the hydro-phobic cavity of CDs has long been investigated asartificial enzymes mimicking the substrate-bindingpockets of natural enzymes [36]. Recently, consider-able research studies have focused on introducingenzyme-responsive sites into CDeamphiphileassemblies.

The direct and general approach to enzymaticallyregulate CDeamphiphile assemblies is biodegradationof CD glucose bonds by a-amylase. a-Amylase cancleave a-1,4 linkages between glucose units of starchmolecules including CDs, which will degrade CDs intwo steps (ring opening and chain scission) givingglucose in the end. Biodegradation of CDs would lead todisassembly of CDeamphiphile assemblies. However, insome cases where CDs play destructive roles, degrada-tion of CDs can trigger assembly of amphiphiles [9]. For

example, surfactants (including TDPS, CTAB, SDS,

Figure 5

Enzyme and light methods to regulate CD-amphiphiles assembly. (a) Scenzyme-triggered monolayer formation, (C) enzyme-triggered micellization, (Dphoto-triggered release from the bacteria-like C4AG@2b-CD vesicles [38]. (cassembly using Azo@CD in (B) micelle, (C) vesicle, and (D) helix systems [3


and Triton�100) can be released from the cavity of CDsafter treatment of a-amylase. Consequently, releasedsurfactants can form monolayers, micelles, and vesicles(Figure 5 (a)) [37].

This strategy of enzyme regulation by a-amylase has alsobeen used in other CDeamphiphile systems, such asCDs/bola amphiphiles [40], to tune physical properties

of assembly. The bola-form covalent amphiphile couldself-assemble in solution, forming sheet-like aggregatesand displaying weak fluorescence because of aggregationcaused quenching. The addition of b-CD led to theformation of hosteguest complex, prohibiting the ag-gregation of naphthalene and accompanying with thesignificant recovery of fluorescence. Upon the additionof a-amylase, with the degradation b-CD, the fluores-cence would quench gradually and significantly. Quan-titatively, the quenching rate linearly correlated to theconcentration of a-amylase, which makes it promising to

diagnose the a-amylaseeassociated diseases.

Besides enzymatic degradation of CDs, a variety ofenzyme-responsive guest amphiphiles have also beenextensively used in construction of enzyme-responsiveCDeamphiphile assembly systems, such as disruptionof protamine by trypsin [41], rupture of myristoylcho-line chloride by butyrylcholinesterase (BChE) [42], andso on. BChE-responsive supramolecular nanoparticlesare fabricated by hepa-carboxylemodified CDs(carboxyl-CDs) as the macrocyclic host and cationic

enzyme-cleavable guest myristoylcholine as enzymati-cally responsive guest. After the treatment of BChE,myristoylcholine can be cleaved into myristic acid andcholine, leading disassembly of nanoparticles.

hematic illustrations of the degradation of (A) b-CD by a-amylase, (B)) and enzyme-triggered vesicle formation [37]. (b) Schematic illustration of) Schematic illustrations of the construction (A) a photoresponsive self-9]. CD, cyclodextrin



52 Surfactants

As described previously, it is clear that enzyme-responsive CDeamphiphile assembly offers hugespace of regulation and applications. Taking advantageof the facile degradation of CDs by enzymes, we antic-ipate that almost all CDeamphiphile assemblies canrespond to enzymes, which brings about limitless pos-sibility to regulate CDeamphiphile assemblies. Inaddition, since numerous enzymeesubstrate pairs have

been found in biological research, many substrates aresuitable to be introduced into amphiphiles to fabricatespecific enzyme-responsive assemblies. Moreover,enzyme-responsive CDeamphiphile assemblies mayalso be important in disease diagnosis because manydiseases are associated with abnormal enzymaticexpression and activity.

LightLight-regulated CDeamphiphile systems usually relatewith light-responsive amphiphiles. Light-responsiveassemblies attract much attention because of conve-nience, rapid response, remote control, and no chemicalcontamination. Typically, guest amphiphiles contain

photoresponsive moieties including azobezene, aryla-zopyrazole, 0-nitrobezylester, pyrenylmethyl ester,coumarin, and anthracene [43].

Among the aforementioned photo-responsive moieties,azobezene groups are a family of most widely used light-responsive moieties because of their unique properties ofisomerization in transecis structures. Trans azobenzeneisomerized from cis-azobenzene under 365 nm UV lightirradiation can be included by a-CD and b-CD. However,cis azobenzene isomerized from trans-azobenzene under

approximately 450 nm UV light irradiation is relativelyhard to include in the cavity of a-CD and b-CD. Theisomerization is reversible and controllable throughalternating the light. The huge difference of bindingability between trans/cis azobenzene and CDs providesresearchers with unlimited opportunities for remote andreversible control over assembly structures [39,44,45]and related properties such as phase transition [46], drugrelease [38], optical properties [47], and so on.

Photo-controlled inclusion and exclusion of

azobenzene-containing amphiphiles with CDs canundergo reversible assembly and disassembly, leadingto reversible transformation of assembly structure.For example, cationic surfactant 1-[10-(4-phenylazophenoxy) decyl]pyridinium bromide (termedAzoC10) can form vesicle-like aggregates in aqueoussolution [44]. After addition of a-CD, AzoC10 and a-CDwould form inclusion complex, leading to disassembly ofvesicle. Trans-AzoC10 will undergo photo-inducedisomerization under UV light and resultant cis-Az0C10has a much smaller binding constant with a-CDcompared with trans-AzoC10, leading to reassembly tovesicle structure.


Besides the vesicle morphology, other morphologiesmanipulated by light can also be achieved in CDeAzosystem. Amphiphiles containing axially chiral 1,10-binaphthyl and photoresponsive azobenzene moietiescan form inclusion complex with a-CD and bis-SC4Ahost molecules [45]. The inclusion complex canextend to polymer-like structure (so-called supramo-lecular polymer) P because of unique structure of bis-

SC4A. Trans-Azoebased P has more flexible and short-er length aggregates under cryo-TEM, while after UVirradiation cis-Azo based P can assemble into linearsingle-helical supramolecular polymer molecules withdiameters of ca. 2 nm and lengths of hundreds ofnanometers to micrometers. This could be ascribed tothe fact that UV-irradiated P containing cis-azobenzenehas a more straight structure compared with the Pcontaining trans-azobenzene. Interestingly, the single-helical structure of irradiated supramolecular polymerP can be observed, resulted from the introduction of the

axially chiral 1,10-binaphthyl units in amphiphiles.

Beyond assembly structure transformation upon light inCDeAzo system, the related properties of the systemcan also be manipulated, which resulted from assemblytransformation. For example, some researchers focus onlight-controlled phase transition, especially gelesoltransition [46]. Azobenzene units functionalized with aguanidinium group were used as the photoswitches andincorporated through a hosteguest inclusion methodinvolving a-CD functionalized with 2,6-

pyridinedicarboxylic acid groups. The guanidiniumgroups could associate with the negatively charged sur-face of sodium polyacrylate exfoliated laponite nano-sheets (SPLNs) through electrostatic interactions, thusconnecting the organic and inorganic components of thehydrogel. Owing to the conformation-dependent bindingbehavior between Azo moieties with a-CD, light couldcontrol the isomerization of azobenzene, which inducesassembly or disassembly of the inclusion complexes andfurther leads to solegel phase transition of hydrogels.

Apart from phase transition, other functions were

investigated in CDeAzo system. Light-controlled drugrelease is an attracting topic for precise delivery andrelease of drug. This can be achieved in amphiphilic [4-butyl-40-(oxy-2, 3-epoxypropyl)azobenzene] (C4AG)and b-CD aqueous system (Figure 5 (b)) [38]. C4AGand b-CD can form inclusion complex at molar ratio of1:2 as C4AG@2b-CD, and C4AG @2b-CD complex canfurther self-assemble into vesicles in water. Upon UVirradiation, many hairs may develop from the surface ofthe vesicle, just similar to cilia of bacteria. Concomi-tantly, this triggers the release of the loaded drug. The

transformation process is reversible and repeatable withalternative UVevisible light irradiation. In this way,vesicles with photo-controlled on and off membrane canbe obtained, which can be used in photo-controlledsmart drug carrier.




In general, CDeAzo system can serve as a universalcontrol unit in molecule assembly system [39]. b-CDand 4-(phenylazo)benzoic acid sodium salt form anAzo@CD photoresponsive factor to couple with light-inert amphiphilic assemblies that successfully make ageneral approach for controlling the morphology ofamphiphilic aggregates by UV or visible light irradiation(Figure 5 (c)). The key point of this strategy is the

different capabilities of forming host�guest inclusionswith CD, that is, trans-azobenzene > alkyl chain ofamphiphilic molecules > cis-azobenzene. The inclusionof trans-Azo@CD can stably coexist with light-inertamphiphilic aggregates under visible light. Upon UVlight irradiation, with the transformation from trans-Azoto cis-Azo, CDs are released into the solution and formnew inclusions with amphiphilic molecules, which leadto the disassembly of the light-inert aggregations. Themorphology of amphiphilic assemblies, such as micelles,vesicles, and helixes, and phase behavior, such as gel

formation, can be modulated upon light irradiation usingthis method.

OthersBesides enzymes and light, other stimulus (such aspH, concentration, temperature, and so on) can alsoregulate CDeamphiphile assemblies. Among thesestimuli, pH regulates guest amphiphiles of acidebaseform. Concentration and temperature mainly affectthe hosteguest interaction between CDs andamphiphiles.

Figure 6

pH, concentrationand temperature methods to regulate CD-amphiphilespseudorotaxane by a-CD and poly (ε-lysine). (D) Schematic illustration for pHaggregate growth in mixed cationic–anionic surfactant systems induced by bcomplexes with the variation of the temperature [53]. CD, cyclodextrin.


Similar to biomolecules, pH has a great impact on CDeamphiphile assemblies. Usually, molecules containingpH-responsive groups (such as peptides, carbonategroups, amine group, and so on) have been used in theconstruction of pH-responsive CDeamphiphile systems.For example, cationic poly (ε-lysine) can thread a-CDsto obtain inclusive polypseudorotaxane at condition of8.5 < pH < 12 (Figure 6 (a)) [48]. Although the inclu-

sion complexation could be observed in the pH 8.5e12,no inclusive polypseudorotaxane was found in acidic andhighly alkaline conditions. This pH-dependentcomplexation is closely related to the protonation ofamino side groups (pKa = 9) and the dissociation ofhydroxyl groups (pKa = 11.3) of a-CD. a-CD cannotinclude any polymer chains containing the protonizedamino side groups in acidic conditions, and also theionized form of a-CD in highly alkaline conditions morethan pH> 11.5 cannot work as a host for any guests. Thisillustrates pH is a feasible and powerful method to

regulate CDeamphiphile assemblies and further appli-cations have been intensively studied [49e51].

Concentration is a common factor affecting assemblybehavior in aqueous assembly system. As illustrated inprevious examples [5,24,29,32,34,35], concentrationand molar ratio of CDs and amphiphiles could lead tovarious assembly structures. Moreover, CDs can serve asmodulators to anionicecationic surfactant assemblysystem because anionicecationic surfactants can beincluded into hollow of CDs. In nonstoichiometrical

assembly. (a) (A) Poly (ε-lysine), (B) a-CD, and (C) the inclusion poly--dependent inclusion complexation [47]. (b) Schematic illustration of-CD [52]. (c) Schematic aggregate morphology transition of SDS@2b-CD



54 Surfactants

mixed cationiceanionic surfactants systems, uponaddition of b-CD, the aggregate undergoes a micellarelongation and a following micelle-to-vesicle transition,which in turn greatly influences the viscosity andabsorbance of the solutions (Figure 6 (b)) [52]. It wasfound that the selective binding of b-CD toward themajor component of a cationiceanionic surfactantmixture is thought to be responsible. This selectivity

removes the excess part of the major component fromthe aggregates, shifts the surfactant compositions in theaggregates toward an electroneutral mixing stoichiom-etry, and thus gives rise to the observed aggregategrowth and concomitant variations in solution proper-ties. This is an excellent example of using CDeamphiphile assemblies to regulate local concentration ofother assembly system, which gives us inspirations forprecise control of assembly system.

Temperature has long been used as a convenient tool to

control molecular self-assembly because noncovalentinteraction can be largely affected by temperature. InCDeamphiphile system, temperature will influence thehydrogen bonding and finally influence the assemblybehavior because CDeCD hydrogen bond is the maindriving force for CDeamphiphile complexes toassemble. In SDS and b-CD (b-CD) system, SDS and b-CD form a channel-type SDS@2b-CD supramolecularunit, which further self-assembles into nonamphiphilicvesicles and microtubes driven by hydrogen bonding[5]. Vesicle and tubular structures existed at the con-

centration range of 4e6% and 6e25% at 25 C, respec-tively. Upon decrease in temperature, the vesicles in theconcentration range of 4e6% will transform intomicrotubes, whereas with increase in temperature, themicrotubes that are formed in the concentration rangeof 6e25% transform into vesicles. The achievement ofthe transformation between the microtubes and vesiclesdepends upon the variation of the strength of hydrogenbonds (Figure 6 (c) [53]). At low temperatures and highconcentrations, the hydrogen bonding between CDs isvery strong, so that microtubes are the favorite struc-tures. Under opposite conditions, the hydrogen bonding

between water and CDs becomes dominant. Thisunique behavior may shed light on importance ofhydrogen bonding in CD-based inclusion complexes andoffer a reversible regulation method.

To summarize, various amphiphiles can be chosen tofabricate CDeamphiphile systems, which can responseto stimuli including light, pH, enzyme, and so on.Moreover, because the hosteguest interaction andhydrogen binding in CDeamphiphile assemblies arenoncovalent and dynamic interaction, various tech-

niques (including temperature, concentration, and soon) can be applied to control this hosteguest interac-tion. These studies concerning regulation of CDeamphiphile assemblies will not only help us


understand the principle rules of assembly but alsobenefit applications of CDeamphiphile assemblies.

Remarks and perspectivesIn this review, we summarized the recent progress onassemblies of CDs and amphiphiles, focusing on aspectsof construction and regulation. The scope of amphi-philes used to fabricate CDeamphiphile assemblies hasbeen widely extended from polymers (PEGs, blockpolymers, and so on), surfactants (ionic surfactants,zwitterion surfactants, nonionic surfactants, geminisurfactant, and so on), to nontypical amphiphiles(amines, aromatic molecules, alkanes, and so on). In

general, these guest amphiphiles can form inclusioncomplexes with CDs, and CDeCD hydrogen bond caninduce further assembly into various high order struc-tures. Owing to the abundant choices of guest amphi-philes and dynamic nature of hosteguesteinclusiveinteraction, elegant and convenient regulation methods(such as enzyme, light, pH, concentration, temperature,and so on) have been used to regulate CDeamphiphileassemblies.

Although remarkable advances have been made in this

field, a great deal of effort is needed:

(1) Mechanism of assembly and relationship betweenmolecular structure and assembly behavior.Although recent study revealed guest orientationinside CDs using 2D-NMR [32], detailed mecha-nism is still worth careful investigations. Forexample, the orientation and alignment of inclusioncomplex in channel-type bilayers, factors affectingcurvature of assembly structure, interactions be-tween polar heads on surfactants, and so on. Thesedetails not only enrich the scientific knowledge but

also give us hints for understanding mysterious ofassemblies in nature. Biological systems are usuallyassembled by various noncovalent interactions,among which hydrogen bonding is a vital one.Therefore, assembly of CDs and amphiphiles drivenby hydrogen bonding will offer understanding forbiological assembly process. Recently, it was foundthat crystalline bilayer constructed from channel-type CDs might resemble structure of a capsid innature [54].

(2) Applications based on CDeamphiphile assemblies.

Construction and regulation of CDeamphiphileassemblies offer great opportunities to constructresponsive functional materials for colloidal science,biomedical application, and so on. Besides, orderedstructure of CDeamphiphile assemblies also pro-vides model systems for further applications. Forexample, CDeSDS microtubes were used as aconfined space to investigate basic assembly ofcolloidal particles inside the assemblies [55]. Thisshows us a promising example to use CDe




amphiphile assemblies to promote advances incolloidal and many other fields. Moreover, CDeamphiphile assemblies are also promising forbiomedicine because CDs are highly biocompatibleand are widely used in the pharmaceutical industry.

In conclusion, we have witnessed substantial advance-ment in assembly between CDs and amphiphiles fromconstruction to regulation. Moreover, CDeamphiphileassemblies with ordered structure and multipleresponsiveness can be promising in both fundamentaland application level. Indeed, more discoveries can bestimulated to illustrate the assembly mechanism andessential applications, beneficial for us to understanddynamic nature of hosteguest interactions and masterassembly system, which may be helpful for problemssuch as food shortage, disease, and pollution.

Conflict of interest statementNothing declared.

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