Phospholipid Membranes as Chemically and Functionally Tunable Materials

The sheet-like lipid bilayer is the fundamental structural component of all cell membranes. Its building blocks are phospholipids and cholesterol. Their amphiphilic structure spontaneously leads to the formation of a bilayer in aqueous environment. Lipids are not just structural elements. Individual lipid species, the lipid membrane structure, and lipid dynamics influence and regulate membrane protein function. An exciting field is emerging where the membrane-associated material properties of different bilayer systems are used in designing innovative solutions for widespread applications across various fields, such as the food industry, cosmetics, nano- and biomedicine, drug storage and delivery, biotechnology, nano- and biosensors, and computing. Here, the authors summarize what is known about how lipids determine the properties and functions of biological membranes and how this has been or can be translated into innovative applications. Based on recent progress in the understanding of membrane structure, dynamics, and physical properties, a perspective is provided on how membrane-controlled regulation of protein functions can extend current applications and even offer new applications.

Lipid nanotubes: Formation and applications

Lipids, the fundamental components of cell membrane, play important roles in the whole cycle of cell life, thus attracting worldwide attention, owing to their physicochemical property and extensive use in the applications based on lipid assemblies. Compared with liposomes, lipid nanotubes (LNTs) usually possess unique properties, such as highly ordered structure, precise molecular recognition, and the possibility of substance transport, thus providing more potential applications in different research fields. However, until now, there are still quite rare cases of LNTs successfully employed in practical applications. Bearing this in mind and based on our own experience in this field, we summarized and discussed the recent progress of the fabrication approaches and representative applications of the LNTs in the past decade, which would potentially provide basic understanding and guidance towards their future development.

Fabrication of Lipid Nanotubules by Ultrasonic Drag Force

This work reports a new method of fabricating lipid nanotubules using ultrasonic Stokes drag force in theory and experiment. Ultrasonic Stokes drag force generated using a planar piezoelectric ultrasonic transducer in a remotely controllable way is introduced. When ultrasonic Stokes drag force is applied on lipid vesicles, the lipid nanotubules attached can be dragged out from the lipid film. In order to demonstrate the formation mechanism of the lipid nanotubules produced by ultrasonic drag force clearly, a theoretical kinetic model is developed. In the experiments, the lipid nanotubules can be rapidly and efficiently fabricated using this ultrasonic transducer both in deionized water and NaCl solutions with different concentrations. The stretching speed of the lipid nanotubules can reach 33 μm/s, approximately 10 times faster than that of the existing methods. The formed lipid nanotubules have a diameter of 600 ± 100 nm (>80%). The length can reach the millimeter level. This work provided a remotely controllable, highly efficient, high-velocity, and solution environment-independent approach for fabricating lipid nanotubules.

Soft-Matter Nanotubes: A Platform for Diverse Functions and Applications

Self-assembled organic nanotubes made of single or multiple molecular components can be classified into soft-matter nanotubes (SMNTs) by contrast with hard-matter nanotubes, such as carbon and other inorganic nanotubes. To date, diverse self-assembly processes and elaborate template procedures using rationally designed organic molecules have produced suitable tubular architectures with definite dimensions, structural complexity, and hierarchy for expected functions and applications. Herein, we comprehensively discuss every functions and possible applications of a wide range of SMNTs as bulk materials or single components. This Review highlights valuable contributions mainly in the past decade. Fifteen different families of SMNTs are discussed from the viewpoints of chemical, physical, biological, and medical applications, as well as action fields (e.g., interior, wall, exterior, whole structure, and ensemble of nanotubes). Chemical applications of the SMNTs are associated with encapsulating materials and sensors. SMNTs also behave, while sometimes undergoing morphological transformation, as a catalyst, template, liquid crystal, hydro-/organogel, superhydrophobic surface, and micron size engine. Physical functions pertain to ferro-/piezoelectricity and energy migration/storage, leading to the applications to electrodes or supercapacitors, and mechanical reinforcement. Biological functions involve artificial chaperone, transmembrane transport, nanochannels, and channel reactors. Finally, medical functions range over drug delivery, nonviral gene transfer vector, and virus trap.

Two-Dimensional Alignment of Self-Assembled Organic Nanotubes through Langmuir-Blodgett Technique

A C₃-symmetric molecule was found to form organic nanotubes through supramolecular gel formation in organic solvents. These nanotubes can be dispersed in toluene without destroying the tubular nanostructures. Using the dispersions of these organic nanotubes as "spreading solutions", Langmuir-spreading films of these nanotubes were formed. Through repeated compression and expansion cycles, the nanotubes can be aligned to a certain extent. The formed Langmuir films could be subsequently transferred to a solid substrate, and the well-aligned nanotube films were constructed by Langmuir-Blodgett film deposition technique. Interestingly, many guests including polymers, water-soluble or oil-soluble organic molecules can be encapsulated into the nanotubes and further spread on a water subphase. Through elaborate control, large-scale parallel alignment of self-assembled organic nanotubes encapsulated by guests was also realized. This study implies that 2D hierarchical alignment of one-dimensional organic nanostructures can be realized using a simple method.

Supramolecular Self-Assembly into Biofunctional Soft Nanotubes: From Bilayers to Monolayers

The inner and outer surfaces of bilayer-based lipid nanotubes can be hardly modified selectively by a favorite functional group. Monolayer-based nanotubes display a definitive difference in their inner and outer functionalities if bipolar wedge-shaped amphiphiles, so-called bolaamphiphiles, as a constituent of the monolayer membrane pack in a parallel fashion with a head-to-tail interface. To exclusively form unsymmetrical monolayer lipid membranes, we focus herein on the rational molecular design of bolaamphiphiles and a variety of self-assembly processes into tubular architectures. We first describe the importance of polymorph and polytype control and then discuss diverse methodologies utilizing a polymer template, multiple hydrogen bonds, binary and ternary coassembly, and two-step self-assembly. Novel biologically important functions of the obtained soft nanotubes, brought about only by completely unsymmetrical inner and outer surfaces, are discussed in terms of protein refolding, drug nanocarriers, lectin detection, a chiral inducer for achiral polymers, the tailored fabrication of polydopamine, and spontaneous nematic alignment.

Chiral Nanoarchitectonics: Towards the Design, Self-Assembly, and Function of Nanoscale Chiral Twists and Helices

Helical structures such as double helical DNA and the α-helical proteins found in biological systems are among the most beautiful natural structures. Chiral nanoarchitectonics, which is used here to describe the hierarchical formation and fabrication of chiral nanoarchitectures that can be observed by atomic force microscopy (AFM), scanning tunneling microscopy (STM), scanning electron microscopy (SEM), or transmission electron microscopy (TEM), is one of the most effective ways to mimic those natural chiral nanostructures. This article focuses on the formation, structure, and function of the most common chiral nanoarchitectures: nanoscale chiral twists and helices. The types of molecules that can be designed and how they can form hierarchical chiral nanoarchitectures are explored. In addition, new and unique functions such as amplified chiral sensing, chiral separation, biological effects, and circularly polarized luminescence associated with the chiral nanoarchitectures are discussed.

Spontaneous nematic alignment of a lipid nanotube in aqueous solutions

The dispersibility and liquid crystal formation of a self-assembled lipid nanotube (LNT) was investigated in a variety of aqueous solutions. As the lipid component, we chose a bipolar lipid with glucose and tetraglycine headgroups, which self-assembled into an LNT with a small outer diameter of 16 to 17 nm and a high axial ratio of more than 310. The LNT gave a stable colloidal dispersion in its dilute solutions and showed spontaneous liquid crystal (LC) alignment at relatively low concentrations and in a pH region including neutral pH. The LNT samples with shorter length distributions were prepared by sonication, and the relationship between the LNT axial ratio and the minimum LC formation concentration was examined. The robustness of the LNT made the liquid crystal stable in mixed solvents of water/ethanol, water/acetone, and water/tetrahydrofuran (1:1 by volume) and at a temperature of up to 90 °C in water. The observed colloidal behavior of the LNT was compared to those of similar 1D nanostructures such as a phospholipid tubule.

Capillary force-induced glue-free printing of Ag nanoparticle arrays for highly sensitive SERS substrates

The fabrication of well-ordered metal nanoparticle structures onto a desired substrate can be effectively applied to several applications. In this work, well-ordered Ag nanoparticle line arrays were printed on the desired substrate without the use of glue materials. The success of the method relies on the assembly of Ag nanoparticles on the anisotropic buckling templates and a special transfer process where a small amount of water rather than glue materials is employed. The anisotropic buckling templates can be made to have various wavelengths by changing the degree of prestrain in the fabrication step. Ag nanoparticles assembled in the trough of the templates via dip coating were successfully transferred to a flat substrate which has hydrophilic surface due to capillary forces of water. The widths of the fabricated Ag nanoparticle line arrays were modulated according to the wavelengths of the templates. As a potential application, the Ag nanoparticle line arrays were used as SERS substrates for various probing molecules, and an excellent surface-enhanced Raman spectroscopy (SERS) performance was achieved with a detection limit of 10(-12) M for Rhodamine 6G.

Shape directed biomineralization of gold nanoparticles using self-assembled lipid structures

Lipid nanostructures including rectangular, hexagonal disks and twisted ribbons were used as soft matter templates for the biomineralization of gold.

Chiral self-assembly of designed amphiphiles: influences on aggregate morphology

A series of novel amphiphiles were designed for self-assembly into chiral morphologies, the amphiphiles consisting of a glutamic acid (Glu) headgroup connected through an 11-carbon alkoxy chain to a diphenyldiazenyl (Azo) group and terminated with a variable length alkyl chain (R-Azo-11-Glu, where R denotes the number of carbons in the distal chain). TEM imaging of amphiphile aggregates self-assembled from heated, methanolic, aqueous solution showed that chiral order, expressed as twisted ribbons, helical ribbons, and helically based nanotubes, increased progressively up to a distal chain length containing eight carbons, and then decreased with further increases in distal chain length. TEM and CD showed that the chiral aggregations of single enantiomers were influenced by the molecular chirality of the headgroup. However, the assembly of D,L-10-Azo-11-Glu into nanotubes demonstrated that chiral symmetry breaking effected by the azo group was also relevant to the chiral organization of the amphiphiles. The chiral order of aggregate morphologies was additionally affected by the temperature and solvent composition of assembly in a manner correlated to the mechanism driving assembly; i.e., D,L-10-Azo-11-Glu was sensitive to the temperature of assembly but less so to solvent composition, while L-14-Azo-11-Glu was sensitive to solvent composition and not to temperature. FTIR and UV-vis spectroscopic investigations into the organization of the head and azo groups, in chiral and achiral structures, illustrated that a balance of the influences of the hydrophilic and hydrophobic components on self-assembly was required for the optimization of the chiral organization of the self-assembled structures.

Biologically responsive, sustainable release from metallo-drug coordinated 1D nanostructures

A multistep self-assembly process produced one-dimensional nanostructures that consisted of a monolayer membrane functionalized with a ligand that acted as a coordination site for an anticancer Pt complex. Control of the mode of the networks of intermolecular hydrogen bonds within the monolayer membrane of the nanostructures completely determined the morphologies of the one-dimensional nanostructures to be nanotapes having widths of 20-40 nm and nanotubes having widths of 16 nm (8 nm inner diameter and 4 nm membrane thickness). Various spectroscopic measurements and microscopic observations revealed that the ligand in a nanotape was located on the surface, whereas the ligand in a nanotube was selectively located on the inner surface of the nanochannel. We calculated the stability constants of the nanotape and nanotube with an anticancer Pt complex to be 107.81 and 106.53, respectively. The nanotape and nanotube were able to not only stably coordinate the anticancer Pt complex in Milli-Q water but also release it in phosphate-buffered saline through a ligand exchange reaction. With respect to sustainable, slow release of the drug, the nanotube, which has a nanochannel to store the drug, was superior to the nanotape.

Electric moulding of dispersed lipid nanotubes into a nanofluidic device

Hydrophilic nanotubes formed by lipid molecules have potential applications as platforms for chemical or biological events occurring in an attolitre volume inside a hollow cylinder. Here, we have integrated the lipid nanotubes (LNTs) by applying an AC electric field via plug-in electrode needles placed above a substrate. The off-chip assembly method has the on-demand adjustability of an electrode configuration, enabling the dispersed LNT to be electrically moulded into a separate film of parallel LNT arrays in one-step. The fluorescence resonance energy transfer technique as well as the digital microscopy visualised the overall filling of gold nanoparticles up to the inner capacity of an LNT film by capillary action, thereby showing the potential of this flexible film for use as a high-throughput nanofluidic device where not only is the endo-signalling and product in each LNT multiplied but also the encapsulated objects are efficiently transported and reacted.

Hybrid organic nanotubes with dual functionalities localized on cylindrical nanochannels control the release of doxorubicin

A method to control the release of the anti-cancer drug doxorubicin (Dox) from cylindrical nanocapsules, known as organic nanotubes (ONTs), is reported. Co-assembly of a tube-forming glycolipid and its hydrophobized analogue yield novel ONTs with both -COOH and hydrophobic benzyloxycarbonyl groups localized on cylindrical nanochannels. The hydrophobicity of the ONTs nanochannels is easily tunable by adjusting the mixing ratio of the two glycolipids in the co-assembly process. The resultant biologically stable ONTs are able to capture Dox with high efficiency into the cylindrical nanochannels via ion complexation between cationic Dox and anionic -COO(-) , and the release of Dox from hybrid ONTs is effectively controlled by tuning the electrostatic interaction and the hydrophobicity. This controlled release by tuning the hydrophobicity of the ONTs' nanochannels greatly reduces the cytotoxicity of Dox@ONTs for HeLa cells.

Chiral self-assembly of designed amphiphiles: optimization for nanotube formation

Four amphiphiles with L-aspartic acid headgroups (Asp) and a diphenyldiazenyl group (Azo) contained within the hydrophobic tails were designed and synthesized for self-assembly into helically based nanotubes. The amphiphiles of the form R'-{4-[(4-alkylphenyl)diazenyl]phenoxy}alkanoyl-L-aspartic acid (where R' is 10 or 11) varied only in alkyl chain lengths either side of the azo group, having 4, 7, or 10 carbon distal chains and 10 or 11 carbon proximal chains (R-Azo-R'-Asp, where R denotes the number of carbons in the distal chain and R' denotes the number of carbons in the proximal chain). Despite the molecular similarities, distinct differences were identified in the chiral order of the structures self-assembled from hot methanolic aqueous solutions using microscopy and spectroscopic analyses. This was reflected in dominant thermodynamic aggregate morphologies that ranged from amorphous material for 10-Azo-10-Asp, through twisted ribbons (196 ± 49 nm pitch) for 7-Azo-11-Asp, to the desired helically based nanotubes for 4- and 7-Azo-10-Asp (81 ± 11 and 76 ± 6 nm diameters, respectively). Another key variable in the self-assembly of the amphiphiles was the use of a second method to precipitate aggregates from solution at room temperature. This method enabled the isolation of thermodynamically unstable and key transitional structures. Helical ribbons were precursor structures to the nanotubes formed from 4- and 7-Azo-10-Asp as well as the wide, flattened nanotube structures (587 ± 85 nm width) found for 4-Azo-10-Asp. Overall, the results highlighted the interplay of influence of the headgroup and the hydrophobic tail on self-assembly, providing a basis for future rational design of self-assembling amphiphiles.

Nanostructured self assembled lipid materials for drug delivery and tissue engineering

Every living organism comprises of lipids as basic building blocks in addition to other components. Utilizing these lipids for pharmaceutical and biomedical applications can overcome biocompatibility and biodegradability issues. A well known example is liposomes (lipids arranged in lamellar structures), but other than that there are additional unique mesophasic structures of lipids formed as a result of lipid polymorphisms, which include cubic-, hexagonal- or sponge-phase structures. These structures provide the advantages of stability and production feasibility compared with liposomes. Cubosomes, which exist in a cubic structure, have improved stability, bioadhesivity and biocompatibility. Hexagonal phases or hexosomes exhibit hexagonal arrangements and can encapsulate different drugs with high stability. Lipids also forms tube-like structures known as tubules and ribbons that are also utilized in different biomedical applications, especially in tissue engineering. Immune stimulating complexes are nanocage-like structures formed as a result of interactions of lipid, antigen and Quillaja saponin. These lipidic mesophasic structures have been utilized for gene, vaccine and drug delivery. This article addresses lipid self-assembled supramolecular nanostructures, including cubosomes, hexosomes, tubules, ribbons, cochleates, lipoplexes and immune stimulating complexes and their biomedical applications.

Soft nanotube hydrogels functioning as artificial chaperones

Self-assembly of rationally designed asymmetric amphiphilic monomers in water produced nanotube hydrogels in the presence of chemically denatured proteins (green fluorescent protein, carbonic anhydrase, and citrate synthase) at room temperature, which were able to encapsulate the proteins in the one-dimensional channel of the nanotube consisting of a monolayer membrane. Decreasing the concentrations of the denaturants induced refolding of part of the encapsulated proteins in the nanotube channel. Changing the pH dramatically reduced electrostatic attraction between the inner surface mainly covered with amino groups of the nanotube channel and the encapsulated proteins. As a result, the refolded proteins were smoothly released into the bulk solution without specific additive agents. This recovery procedure also transformed the encapsulated proteins from an intermediately refolding state to a completely refolded state. Thus, the nanotube hydrogels assisted the refolding of the denatured proteins and acted as artificial chaperones. Introduction of hydrophobic sites such as a benzyloxycarbony group and a tert-butoxycarbonyl group onto the inner surface of the nanotube channels remarkably enhanced the encapsulation and refolding efficiencies based on the hydrophobic interactions between the groups and the surface-exposed hydrophobic amino acid residues of the intermediates in the refolding process. Refolding was strongly dependent on the inner diameters of the nanotube channels. Supramolecular nanotechnology allowed us to not only precisely control the diameters of the nanotube channels but also functionalize their surfaces, enabling us to fine-tune the biocompatibility. Hence, these nanotube hydrogel systems should be widely applicable to various target proteins of different molecular weights, charges, and conformations.

Longitudinal zipping/unzipping of self-assembled organic tubes

Stimuli-responsive organic tubes are an attractive supramolecular assembly which has potential applications as a controlled release vehicle. We synthesize a smart organic tube by the coassembly of lithocholic acid (LCA) and taurolithocholic acid (TLCA) in aqueous solution. The coassembled LCA/TLCA tubes can be longitudinally unzipped into flat sheets by capillary force after being dehydrated on substrates. Consequently, the encapsulated guest molecules are released from the unzipping tubes. After the release of guest molecules, the flat sheets can be zipped back into hollow tubes upon hydration with aqueous solution. The zipping/unzipping LCA/TLCA tubes provide a new type of delivery vehicles, which may have potential for surface decontaminations.

Statistical model with two order parameters for ductile and soft fiber bundles in nanoscience and biomaterials

Traditional fiber bundles models (FBMs) have been an effective tool to understand brittle heterogeneous systems. However, fiber bundles in modern nano- and bioapplications demand a new generation of FBM capturing more complex deformation processes in addition to damage. In the context of loose bundle systems and with reference to time-independent plasticity and soft biomaterials, we formulate a generalized statistical model for ductile fracture and nonlinear elastic problems capable of handling more simultaneous deformation mechanisms by means of two order parameters (as opposed to one). As the first rational FBM for coupled damage problems, it may be the cornerstone for advanced statistical models of heterogeneous systems in nanoscience and materials design, especially to explore hierarchical and bio-inspired concepts in the arena of nanobiotechnology. Applicative examples are provided for illustrative purposes at last, discussing issues in inverse analysis (i.e., nonlinear elastic polymer fiber and ductile Cu submicron bars arrays) and direct design (i.e., strength prediction).

Director configuration of liquid-crystal droplets encapsulated by polyelectrolytes

Liquid-crystal 4-n-pentyl-4'-cyanobiphenyl (5CB) droplets dispersed in aqueous solution are prepared by the assembly of poly(styrenesulfonic acid) (PSSH) and poly(styrenesulfonate sodium) (PSSNa) at the 5CB/water interface. The micrometer sized PSSH-coated 5CB droplets in the space confinement formed by two parallel glass slides break up into submicrometer sized droplets under evaporation-induced flow. We find that the size reduction of the PSSH-coated droplets is accompanied by the bipolar-to-radial configuration transition of the 5CB in the droplets, while the PSSNa-coated 5CB droplets show no size-dependent configuration transition in the same size range. Our results suggest that the size-dependent director configuration of liquid-crystal droplets encapsulated by polyelectrolytes can be modulated by changing the interface conditions, which is important in designing liquid-crystal droplets for optical and biological applications.

Radial elasticity of self-assembled lipid tubules

Self-assembled lipid tubules with crystalline bilayer walls represent useful supramolecular architectures which hold promise as vehicles for the controlled release of preloaded drugs and templates for the synthesis of one-dimensional inorganic materials. We study the local elasticity of lipid tubules of 1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine by radial atomic force microscope indentation, coupled with finite element analysis. A reduced stiffness is found to extend a distance of approximately 600 nm from the ends of lipid tubules. The middle section of lipid tubules is homogeneous in terms of their radial elasticity with a Young's modulus of approximately 703 MPa. The inhomogeneous radial elasticity likely arises from the variation of lipid packing density near the tubule ends.

Challenges and breakthroughs in recent research on self-assembly

The controlled fabrication of nanometer-scale objects is without doubt one of the central issues in current science and technology. However, existing fabrication techniques suffer from several disadvantages including size-restrictions and a general paucity of applicable materials. Because of this, the development of alternative approaches based on supramolecular self-assembly processes is anticipated as a breakthrough methodology. This review article aims to comprehensively summarize the salient aspects of self-assembly through the introduction of the recent challenges and breakthroughs in three categories: (i) types of self-assembly in bulk media; (ii) types of components for self-assembly in bulk media; and (iii) self-assembly at interfaces.

Heterogeneous and anomalous diffusion inside lipid tubules

Self-assembled lipid tubules with crystalline bilayer walls are promising candidates for controlled drug delivery vehicles on the basis of their ability to release preloaded biological molecules in a sustained manner. While a previous study has shown that the release rate of protein molecules from lipid tubules depends on the associated molecular mass, suggesting that the pertinent diffusion follows the well-known Stokes-Einstein relationship, only a few attempts have been made toward investigating the details of molecular diffusion in the tubule interior. Herein, we have characterized the diffusion rates of several molecules encapsulated in lipid tubules formed by 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DC8,9PC) using the techniques of fluorescence recovery after photobleaching (FRAP) and fluorescence correlation spectroscopy (FCS). Our results show that the mobility of these molecules depends not only on their positions in the DC8,9PC tubules but also on their respective concentrations. While the former indicates that the interior of the DC8,9PC tubules is heterogeneous in terms of diffusion, the latter further highlights the possibility of engineering specific conditions for achieving sustained release of a "drug molecule" over a targeted period of time. In addition, our FCS results indicate that the molecular diffusions inside the crystalline bilayer walls of the DC8,9PC tubules strongly deviate from the normal, stochastic processes, with features characterizing not only anomalous subdiffusions but also motions that are superdiffusive in nature.