Organized Assemblies in Bolaamphiphile/Oppositely Charged Conventional Surfactant Mixed Systems

Self-assembly solid-state enhanced fluorescence emission of GFP chromophore analogues: Formation of microsheets and microtubes oriented by molecular skeleton

The p-, m- and o-N,N-dimethylamino analogs of the green fluorescent protein (GFP) chromophore (denoted as p-DBHI, m-DBHI and o-DBHI) were synthesized by 2,3-cycloaddition. These three compounds were structurally characterized by NMR, HRMS and single crystal X-ray diffraction and were shown to be in the Z-form in both the solid phase and solution. Their fluorescence properties and self-assembly behaviors were investigated by UV-Vis, photoluminescence spectroscopy, fluorescence microscopy and scanning electron microscopy. They exhibited low fluorescence quantum yields in both protic and aprotic solvents, which was consistent with the reported results, and strong emissions in the solid state, thus exhibiting aggregation-induced emission (AIE) behaviors. By a solvent exchange method, the p-DBHI and o-DBHI were assembled into microsheets, while the m-DBHI was assembled into microtubule-like structures. The photoluminescence properties of the assemblies were compared with those of the pristine microcrystalline powders obtained by evaporation from organic solvents. The fluorescence quantum yields of the microcrystals obtained by self-assembly were recorded to 9.86 %, 3.37 % and 31.65 %, respectively, which were much higher than those of the corresponding pristine powders (4.71 %, 2.51 % and 17.03 %). This indicated that the fluorescence properties in the solid state depended on the morphologies of the particles.

Effect of the Molecular Weight of Polyelectrolyte and Surfactant Chain Length on the Solid-Phase Molecular Self-Assembly

Solid-phase molecular self-assembly (SPMSA) is emerging as an efficient approach, leading to scale-span self-assembled supramolecular films. With SPMSA, freestanding macroscopic supramolecular films can be formed upon mechanically pressing the precipitates formed with polyelectrolytes and oppositely charged surfactants. Herein, we report that the film formation ability and the mechanical strength of the resultant film depend highly on the surfactant chain lengths and the molecular weight of polyelectrolytes. A coarse-grained molecular dynamics study revealed that the longer surfactant chains are beneficial for the ordered assembly of surfactant bilayers in the film, whereas the larger molecular weight of PE favors the enhanced mechanical strength of the film by promoting the long-range order of the surfactant bilayers. The current results disclosed the physical insight of the surfactant chain length and the molecular weight of polyelectrolytes into the film structure and mechanical strength, which is of practical importance in guiding the creation of SPMSA materials.

Self-Assembly of Aggregation-Induced-Emission Molecules

The last decade has witnessed rapid developments in aggregation-induced emission (AIE). In contrast to traditional aggregation, which causes luminescence quenching (ACQ), AIE is a reverse phenomenon that allows robust luminescence to be retained in aggregated and solid states. This makes it possible to fabricate various highly efficient luminescent materials, which opens new paradigms in a number of fields, such as imaging, sensing, medical therapy, light harvesting, light-emitting devices, and organic electronic devices. Of the various important features of AIE molecules, their self-assembly behavior is very attractive because the formation of a well-defined emissive nanostructure may lead to advanced applications in diverse fields. However, due to the nonplanar topology of AIEgens, it is not easy for them to self-assemble into well-defined structures. To date, some strategies have been proposed to achieve the self-assembly of AIEgens. Herein, we summarize the most recent approaches for the self-assembly of AIE molecules. These approaches can be sorted into two classes: 1) covalent molecular design and 2) noncovalent supramolecular interactions. We hope this will inspire more excellent work in the field of AIE.

Photo-Switchable Self-Assemblies Based on Thymine-Containing Bolaamphiphiles

The photoswitching of photosensitive bolaamphiphiles based on thymine was investigated. Topochemical principles were applied to create light-responsive supra-amphiphiles by the utilisation of dynamic covalent bonds created by the photo-dimerisation of a DNA base, thymine. In order to induce the photo-dimerisation of thymine, two bolaamphiphilic molecules were designed and synthesised to meet the required [2 π+2 π] photo-cycloaddition conditions. The amphiphiles were synthesised with different spacers and their photo-reversibility and morphologies were studied by using UV/Vis, NMR and infrared spectroscopy, rheometry, dynamic light scattering and transmission electron microscopy.

Fabrication of Propeller-Shaped Supra-amphiphile for Construction of Enzyme-Responsive Fluorescent Vesicles

Propeller-shaped molecules have been recognized to display fantastic AIE (aggregation induced emission), but they can hardly self-assemble into nanostructures. Herein, we for the first time report that ionic complexation between a water-soluble tetrapheneyl derivative and an enzyme substrate in aqueous media produces a propeller-shaped supra-amphiphile that self-assembles into enzyme responsive fluorescent vesicles. The supra-amphiphile was fabricated upon complexation between a water-soluble propeller-shaped AIE luminogen TPE-BPA and myristoylcholine chloride (MChCl) in aqueous media. MChCl filled in the intramolecular voids of propeller-shaped TPE-BPA upon supra-amphiphile formation, which endows the supra-amphiphile superior self-assembling ability to the component molecules thus leading to the formation of fluorescent vesicles. Because MChCl is the substrate of cholinesterases, the vesicles dissemble in the presence of cholinesterases, and the fluorescent intensity can be correlated to the level of enzymes. The resulting fluorescent vesicles may be used to recognize the site of Alzheimer's disease, to encapsulate the enzyme inhibitor, and to release the inhibitor at the disease site.

Controlled self-assembly of a pyrene-based bolaamphiphile by acetate ions: from nanodisks to nanofibers by fluorescence enhancement

In this paper, a pyrene moiety is incorporated into a bolaamphiphile to form a novel molecule denoted PRB. Above the critical micelle concentration, PRB forms nanodisks in the aqueous solution. The addition of acetate ions induces a morphological change in self-assembled aggregates, which convert into nanofibers with a diameter of several nanometers. More interestingly, along with the morphological change, the fluorescence of the assemblies was enhanced concomitantly, which can be attributed to the binding effect of acetate ions on pyridinium head groups of PRB.

Photo-responsive self-assemblies based on bio-inspired DNA-base containing bolaamphiphiles

Photo-responsive thymine incorporated bolaamphiphiles/superamphiphiles that can transfrom the aggregation morphology into spherical structures from ribbon-like structures by UV irradiation were investigated.

The Micelle-to-Vesicle Phase Transition in Dilute Aqueous Solution from Undecylamine Induced by Metal(II) ion (Cu2+)

A metal(II) ion (Cu2+)-induced vesicle phase was prepared from a mixture of n-undecylamine and CuCl2 in aqueous solution. Cu2+-ligand coordination with n-undecylamine results in the formation of molecular bilayers because Cu2+ can firmly bind to the amine groups of n-undecylamine which reduced the area of the head-group. In this system, no counterions in aqueous solution exist because of the Cu2+-ligand coordination, and the bilayer membranes are not shielded by salts. The vesicles were determined by transmission electron microscopy (TEM) images and dynamic light scattering (DLS) measurements.

Realization of the reversible vesicle-micelle transition of vitamin-derived bolaamphiphiles by heat change monitoring

The real-time energetics involved in the structural change of a zwitterionic vitamin-derived bolaamphiphiles (DDO) vesicles, which were induced by conventional surfactants, such as hexadecyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and Triton X-100 (TX100), was characterized by isothermal titration calorimetry (ITC). Interactions of both CTAB and SDS with DDO were accompanied with considerable heat release whereas the interaction energetics between TX-100 and the vesicles were small. However, the transition of DDO vesicles to micelles did occur upon the addition of all of the three surfactants. Fine inflection points were observed in heat flow enthalpograms, which indicated systematically the change of vesicle structure. By monitoring the interaction of CTAB with DDO, we found that heat release kept constant over a certain concentration range at higher temperatures. The repairing effect of heating was revealed and a reversible transition from micelles to vesicles of DDO was thus realized. Further encapsulation of fluorescein in DDO vesicles proved that the reversible vesicle-micelle transition was controllable. This research demonstrates that ITC combined with complementary analytical methods such as dynamic light scattering (DLS) and transmission electron microscopy (TEM) helps to get the real-time information of the structural changes of vesicles. It also shows that these synthetic novel bolaamphiphiles offer great promise for designing controllable release system.

Spontaneous Vesicle Formation from Bulky Tetrasiloxane-tailed Dicephalic Surfactant

The spontaneous vesicle formation from a novel bulky tetrasiloxane-tailed dicephalic surfactant composed of a carbohydrate and ethylene glycol ethyl ether was reported. The surface tension measurements revealed that aggregates are formed, and dynamic light scattering (DLS) measurements suggested the aggregates are vesicles. The size and morphology were observed by transmission electron microscopy (TEM). The vesicle formation was further confirmed by dye entrapment studies.

New surfactant phosphine ligands and platinum(II) metallosurfactants. Influence of metal coordination on the critical micelle concentration and aggregation properties

We have prepared the first platinum(II) metallosurfactants from a new family of linear surfactant phosphines Ph(2)P(CH(2))(n)SO(3)Na {1 (n = 2), 2 (n = 6), and 3 (n = 10)}, which were synthesized by reaction between the halosulfonates X(CH(2))(n)SO(3)Na and sodium diphenylphosphide. The metallosurfactants cis-[PtCl(2)L(2)] (L = 1-3) were obtained after reaction between the phosphines and PtCl(2) in dimethylsulfoxide. All compounds were fully characterized by the usual methods {NMR ((1)H, (13)C, (31)P, (195)Pt), IR, MS-ESI and HRMS}. By exploring the surfactant properties of phosphines 1-3 and their respective platinum metallosurfactants cis-[PtCl(2)L(2)] (L = 1-3) through surface tension measurements, dynamic light scattering spectroscopy, and cryo-TEM microscopy, we were able to analyze the influence of the metal coordination on the critical micelle concentration (cmc) and the aggregation properties. The cmc values of platinum metallosurfactants were considerably lower than those obtained for the free phosphines 1-3. This behavior could be understood by an analogy between the structure of cis-[PtCl(2)L(2)] complexes and bolaform surfactants. The calculated values of area per molecule also showed different tendencies between 1-3 and cis-[PtCl(2)L(2)] complexes, which could be explained on the basis of the possible conformations of these compounds in the air-water interface. The study of aggregates by dynamic light scattering spectroscopy and cryo-TEM microscopy showed the formation of spherical disperse medium size vesicles in all cases. However, substantial differences were observed between the three free phosphines (the population of micellar aggregates increased with long chain length) and also between phosphines and their respective metallosurfactants.

Comparative studies on the self-assembling behaviors of cationic and catanionic surfactant-like peptides

In order to introduce ionic bonds into traditional surfactant-like peptide to develop self-assembling materials with better properties, a catanionic surfactant-like peptide A(6)K(+/-) was designed by removing the protecting -NH(2) at the C-terminus of the cationic surfactant-like peptide A(6)K. Both TEM and AFM observations revealed that A(6)K(+/-) could form longer nanofibers than A(6)K did. On mica surface, A(6)K could form membrane-like structures, which were likely formed by unassembled peptide monomers, while A(6)K(+/-) only formed regular nanofibers, suggesting the absence of unassembled monomers. Pyrene probe spectrofluorometry showed that A(6)K(+/-) had a much lower critical micelle concentration (CMC) than A(6)K. The self-assembling structures of A(6)K(+/-) were also more thermostable than those of A(6)K and could endure temperature as high as 80 degrees C. Furthermore, changing the pH value to extreme acid or basic has more complicated effects on A(6)K(+/-) than A(6)K. These results indicated that the coexistence of opposite charges in the head group of A(6)K(+/-) may help it to form nanostructures with better stability and pH-sensitivity, suggesting a novel approach to fabricate nanomaterials by catanionic surfactant-like peptides.

Chiral cones and vesicles from Gemini-type fatty acid-heteroditopic amine mixtures

A twin-chiral and twin-tailed Gemini-type fatty acid with a rigid spacer, 2,3-bis(decyloxy) succinic acid, formed chiral self-assembled structures in aqueous solution with heteroditopic amines, such as 6-aminohexanoic acid and 4-aminobutyric acid. The morphology of such structures was governed by the stoichiometry between the fatty acid and the amine. Mixtures with 1 : 2 molar ratio formed exclusively spherical vesicles, while the 1 : 1 mixtures resulted in entropically driven chiral cones in addition to spherical vesicles. Surface tension measurements, light scattering, and TEM studies proved the existence of vesicles in aqueous solution. Cones were formed through disclinations introduced in the hexatic chiral membrane with specific angles, in accordance with θ = arcsin (1 - (n/6)), [0 < θ < π/2, 1 < n < 5]. Mixtures prepared with 4-aminobutyric acid formed only spherical vesicles and no conical self-assemblies, emphasizing the geometry of the Gemini-heteroditopic amine pair, which directs the morphology of the final self-assembled structures.

Temperature-dependent self-assembly and mixing behavior of symmetrical single-chain bolaamphiphiles

The temperature-dependent self-assembly and the mixing behavior of symmetrical single-chain bolaamphiphiles with different polymethylene chain lengths and different headgroup structures were investigated in water by differential scanning calorimetry (DSC), cryo transmission electron microscopy (cryo-TEM), and small angle neutron scattering (SANS). The even-numbered polymethylene-1,omega-bis(phosphocholines) (PC-C n-PC) are known to form nanofibers composed of stretched molecules with an all- trans alkyl chain conformation (Drescher, S.; Meister, A.; Blume, A.; Karlsson, G.; Almgren, M.; Dobner, B. Chem.Eur. J. 2007, 13, 5300-5307). The odd-numbered analogues were synthesized to study a possible even-odd effect of these bolaamphiphiles during their aggregation in water. In addition to these bolaamphiphiles with phosphocholine headgroups, a new series of polymethylene-1,omega-bis(phosphodimethylethanolamines) (Me2PE-Cn-Me2PE) with smaller headgroup sizes was synthesized. These bolaamphiphiles show an additional fiber-fiber transition when the alkyl chain length exceeds 26 carbon atoms. The mixing behavior of both types of bolaamphiphiles indicates that differences in the alkyl chain length up to six carbon atoms are tolerated within the fiber structure. The mixing of two Me2PE-Cn-Me2PE or PC-Cn-PC type bolaamphiphiles with different alkyl chain lengths offers the possibility to adjust the temperature, where the cross-linking of the fibers is disrupted and where the fibers break apart. As a consequence, temperature switchable hydrogels are obtained that can be fine-tuned for drug delivery applications. The comparison with dotriacontane-1,32-diyl-bis[2-(methylammonio)-ethylphosphate] (MePE-C32-MePE), a new bolaamphiphile with even smaller phosphomonomethylammonio headgroups, illustrates the importance of the headgroup size for the aggregation behavior. This bolaamphiphile self-assembles exclusively into lamellar structures, and this aggregate type persists in mixtures with the fiber forming Me2PE-C32-Me2PE.

Mixing behaviour of a symmetrical single-chain bolaamphiphile with phospholipids

The mixing behavior of the symmetric long-chain bolaamphiphile dotriacontane-1,1'-diyl-bis-[2-(trimethylammonio)ethylphosphate] (PC-C32-PC) with conventional phospholipids (DPPC, DMPC, and POPC) was investigated by differential scanning calorimetry and transmission electron microscopy. PC-C32-PC by itself forms nanofibers at room temperature, which convert to micellar aggregates at higher temperatures. The length of the bolaamphiphile corresponds roughly to the thickness of a lipid bilayer. However, a significant insertion of the bolaamphiphile into the phospholipid bilayer vesicles was not observed in any of the mixed systems. This is apparently due to packing problems caused by the larger space requirement of the phosphocholine headgroups of PC-C32-PC compared to the small cross-sectional area of the alkyl chain. Obviously, the insertion of these bolaamphiphiles into membrane structures with parallel oriented molecules leads to large void volumes because of this packing problem, so that the bolaamphiphiles prefer to self-assemble into fibers, where the chains can get in closer contact. On the other hand, up to a certain molar ratio, double-chain phospholipids can easily be inserted into the hydrophobic pockets of the bolaamphiphile fibers, where they stabilize the fiber structure.

Effect of hydrocarbon parts of the polar headgroup on surfactant aggregates in gemini and bola surfactant solutions

Cationic gemini surfactant homologues alkanediyl-alpha,omega-bis(dodecyldiethylammonium) bromide, [C12H25(CH3CH2)2N(CH2)SN(CH2CH3)2C12H25]Br2, where S = 4, 6, 8, 10, or 12, referred to as C12CSC12(Et), and cationic bolaamphiphiles BPHEAB (biphenyl-4,4'-bis(oxyhexamethylenetriethylammonium) bromide), PHEAB (phenyl-4,4'- bis(oxyhexamethylenetriethylammonium) bromide) were synthesized, and their aggregation behaviors in aqueous solution were studied and compared by means of dynamic light scattering, fluorescence entrapment, and transmission electron microscopy. Spherical vesicles were found in the aqueous solutions of these gemini and bola surfactants, which can be attributed to the increase of the hydrocarbon parts of the polar headgroup of the surfactants. In combination with the result of the other gemini with headgroup of propyl group, the increase of the hydrophobic parts of the surfactant polar headgroup will be beneficial to enhance the aggregation capability of the gemini and bola surfactants. Both of the vesicles formed in the gemini and bola systems showed good stabilities with time and temperature, but different stability with salt due to the different membrane conformations of surfactant molecules in the vesicles.

Molecular Packing Parameter in Bolaamphiphile Solutions: Adjustment of Aggregate Morphology by Modifying the Solution Conditions

The geometrical rule of molecular packing parameters in a bolaamphiphile solution was tested with experimental results. By modifying the solution conditions to change the molecular packing parameters, the morphology of the aggregate was successfully manipulated in a single-chain bolaamphiphile, disodium phenyl-1,4-bis (oxyhexanoate) (i.e., C6PhC6Na2 solution). Micelle-vesicle-tube transformation was observed by changing the pH and the addition of NaBr or octanol. In the mixed systems of oppositely charged bola/surfactants, the molecular packing parameter's role is related to the mixing ratio.

Vesicle formation and stability in aqueous mixtures of the hydrolyzed copolymer of styrene-maleic anhydride and conventional single-tailed cationic surfactants

Vesicle formation in aqueous mixtures of the hydrolyzed copolymer of styrene-maleic anhydride (HSMA) and a series of single-tailed cationic surfactants (C(n)H(2n+1)N(C(m)H(2m+1))3Br, n = 8, 10, 12, 16, m = 1, 2, 3, 4) was studied by fluorescence measurement, zeta potential measurement, and transmission electron microscopy. The driving forces of vesicle formation in this kind of system are attributed to the combination of electrostatic attraction and the hydrophobic interaction. Variation of the surfactant structure had a great influence on vesicle formation. A model for the conformation of the molecular packing in the vesicle membrane was suggested on the basis of XRD measurement and Chem3D simulation. Moreover, these vesicles showed superstability to aging time, to NaBr, and to ethanol.

Spontaneous formation of vesicles

his review highlights the relevant issues of spontaneous formation of vesicles. Both the common characteristics and the differences between liposomes and vesicles are given. The basic concept of the molecular packing parameter as a precondition of vesicles formation is discussed in terms of geometrical factors, including the volume and critical length of the amphiphile hydrocarbon chain. According to theoretical considerations, the formation of vesicles occurs in the systems with packing parameters between 1/2 and 1. Using common as well as new methods of vesicle preparation, a variety of structures is described, and their nomenclature is given. With respect to sizes, shapes and inner structures, vesicles structures can be formed as a result of self-organisation of curved bilayers into unilamellar and multilamellar closed soft particles. Small, large and giant uni-, oligo-, or multilamellar vesicles can be distinguished. Techniques for determination of the structure and properties of vesicles are described as visual observations by optical and electron microscopy as well as the scattering techniques, notably dynamic light scattering, small angle X-ray and neutron scattering. Some theoretical aspects are described in short, viz., the scattering and the inverse scattering problem, angular and time dependence of the scattering intensity, the principles of indirect Fourier transformation, and the determination of electron density of the system by deconvolution of p(r) function. Spontaneous formation of vesicles was mainly investigated in catanionic mixtures. A number of references are given in the review.

Transitions of organized assemblies in mixed systems of cationic bolaamphiphile and anionic conventional surfactants

The transition from vesicles to tubelike structures has been studied in mixed systems of cationic bolaamphiphile BPHTAB [biphenyl-4,4'-bis(oxyhexamethylenetrimethylammonium bromide)] and its oppositely charged conventional surfactants with transmission electron microscopy (TEM) and dynamic light scattering (DLS). This transition can be attributed to the fact that tube-like structures are more stable aggregates than vesicles because of the special molecular packing in the aggregates of the mixed systems. The effects of temperature and salt addition on this transition have also been investigated, and the rate of the transition was found to be strongly dependent on temperature. Addition of the appropriate amount of NaBr will accelerate the transition from vesicles to tube-like structures, but the vesicles will transform into micelles at higher salt concentration. Moreover, the micelle-vesicle transition can be realized by addition of n-octanol in the mixed system of BPHTAB/sodium caprate (SC) at higher salt concentrations.

Solvent-Driven Formation of Bolaamphiphilic Vesicles

We show that a spontaneous bending of single-layer bolaamphiphiles results from the frustration due to the competition between core-core and tail-solvent interactions. We find that spherical vesicles are stable under rather general assumptions on these interactions described within the Flory-Huggins theory. We consider also the deformation of the vesicles in an external magnetic field that has been recently experimentally observed.