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.

Observing the Kinetic Pathway of Nanotube Formation from Bolaamphiphiles by Time-Resolved Small-Angle X-ray Scattering.

We investigated the formation kinetics of a single monolayer nanotube from bolaamphiphiles (consisting of a sugar residue, an alkyl chain, and an amino group) in solution. In this bolaamphiphile, a transition from a monomerically dispersed state to the nanotube takes place by changing the solvent condition. This transition was induced by fast mixing with a stopped-flow apparatus. From just after the mixing, this transition process was monitored in situ by time-resolved small-angle X-ray scattering. In this manner, we were able to derive the direct structural information as a function of time during the nanotube formation. The results revealed that disklike aggregates initially formed, which then grew and closed to produce a tubular structure.

Encapsulation of Albumin in Organic Nanotube Channel: Structural Investigation by Small-Angle X-ray Scattering.

Rationally designed bolaamphiphiles are known to self-assemble into nanotube structures. Herein, we report the formation of an inclusion complex between a nanotube and bovine serum albumin (BSA). This complex formation was confirmed by ultraviolet (UV) absorbance and electrophoretic light scattering (ELS). The structure for different mixing ratios of BSA to the nanotube was also investigated by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). The structural analysis with our proposed model has revealed that the BSA molecules were contained within the internal space of the nanotube with maintenance of its tubular structure.

Cross-sectional structures of a molecular monolayer nanotube explored with SAXS: evidence for the parallel orientation of the headgroups in asymmetric bolaamphiphiles.

We investigated the cross-sectional structure of a molecular monolayer nanotube self-assembled from asymmetric bolaamphiphiles having two different hydrophilic headgroups. Small-angle X-ray scattering measurements clarified that the glucose and amino headgroups form the exterior and interior surfaces of the nanotube, respectively.

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.

Molecular-Level Understanding of the Encapsulation and Dissolution of Poorly Water-Soluble Ibuprofen by Functionalized Organic Nanotubes Using Solid-State NMR Spectroscopy.

A comprehensive study of the encapsulation and dissolution of the poorly water-soluble drug ibuprofen (IBU) using two types of organic nanotubes (ONT-1 and ONT-2) was conducted. ONT-1 and ONT-2 had similar inner and outer diameters, but these surfaces were functionalized with different groups. IBU was encapsulated by each ONT via solvent evaporation. The amount of IBU in the ONTs was 9.1 and 29.2 wt % for ONT-1 and ONT-2, respectively. Dissolution of IBU from ONT-1 was very rapid, while from ONT-2 it was slower after the initial burst release. One-dimensional (1D) (1)H, (13)C, and two-dimensional (2D) (1)H-(13)C solid-state NMR measurements using fast magic-angle spinning (MAS) at a rate of 40 kHz revealed the molecular state of the encapsulated IBU in each ONT. Extremely mobile IBU was observed inside the hollow nanosapce of both ONT-1 and ONT-2 using (13)C MAS NMR with a single pulse (SP) method. Interestingly, (13)C cross-polarization (CP) MAS NMR demonstrated that IBU also existed on the outer surface of both ONTs. The encapsulation ratios of IBU inside the hollow nanospaces versus on the outer surfaces were calculated by waveform separation to be approximately 1:1 for ONT-1 and 2:1 for ONT-2. Changes in (13)C chemical shifts showed the intermolecular interactions between the carboxyl group of IBU and the amino group on the ONT-2 inner surface. The cationic ONT-2 could form the stronger electrostatic interactions with IBU in the hollow nanosapce than anionic ONT-1. On the other hand, 2D (1)H-(13)C NMR indicated that the hydroxyl groups of the glucose unit on the outer surface of the ONTs interacted with the carboxyl group of IBU in both ONT-1 and ONT-2. The changes in peak shape and chemical shift of the ONT glucose group after IBU encapsulation were larger in ONT-2 than in ONT-1, indicating a stronger interaction between IBU and the outer surface of ONT-2. The smaller amount of IBU encapsulation and rapid IBU dissolution from ONT-1 could be due to the weak interactions both at the outer and inner surfaces. Meanwhile, the stronger interaction between IBU and the inner surface of ONT-2 could suppress IBU dissolution, although the IBU on the outer surface of ONT-2 was released soon after dispersal in water. This study demonstrates that the encapsulation amount and the dissolution rates of poorly water-soluble drugs, a class which makes up the majority of new drug candidates, can be controlled using the functional groups on the surfaces of ONTs by considering the host-guest interactions.

Effect of Photoinduced Size Changes on Protein Refolding and Transport Abilities of Soft Nanotubes.

Self-assembly of azobenzene-modified amphiphiles (Glyn Azo, n=1-3) in water at room temperature in the presence of a protein produced nanotubes with the protein encapsulated in the channels. The Gly2 Azo nanotubes (7 nm internal diameter [i.d.]) promoted refolding of some encapsulated proteins, whereas the Gly3 Azo nanotubes (13 nm i.d.) promoted protein aggregation. Although the 20 nm i.d. channels of the Gly1 Azo nanotubes were too large to influence the encapsulated proteins, narrowing of the i.d. to 1 nm by trans-to-cis photoisomerization of the azobenzene units of the Gly1 Azo monomers packed in the solid bilayer membranes led to a squeezing out of the proteins into the bulk solution and simultaneously enhanced their refolding ratios. In contrast, photoinduced transformation of the Gly2 Azo nanotubes to short nanorings (<40 nm) with a large i.d. (28 nm) provided no further refolding assistance. We thus demonstrate that pertubation by the solid bilayer membrane wall of the nanotubes is important to accelerate refolding of the denatured proteins during their transport in the narrow nanotube channels.

Lipid Nanotube Tailored Fabrication of Uniquely Shaped Polydopamine Nanofibers as Photothermal Converters.

Helically coiled and linear polydopamine (PDA) nanofibers were selectively fabricated with two different types of lipid nanotubes (LNTs) that acted as templates. The obtained coiled PDA-LNT hybrid showed morphological advantages such as higher light absorbance and photothermal conversion effect compared to a linear counterpart. Laser irradiation of the coiled PDA-LNT hybrid induced a morphological change and subsequent release of the encapsulated guest molecule. In cellular experiments, the coiled PDA-LNT efficiently eliminated HeLa cells because of its strong affinity with the tumor cells. This work illustrates the first approach to construct characteristic morphologies of PDA nanofibers using LNTs as simple templates, and the coiled PDA-LNT hybrid exhibits attractive photothermal features derived from its unique coiled shape.

Soft nanotubes acting as confinement effecters and chirality inducers for achiral polythiophenes.

Depending on their nanochannel sizes, soft nanotubes were able to not only control the conformation and aggregation state of encapsulated achiral polythiophene boronic acids but also induce chirality in the polythiophene chains that exhibit chiral recognition abilities for D, L-sugars.

Qualitative/chiral sensing of amino acids by naked-eye fluorescence change based on morphological transformation and hierarchizing in supramolecular assemblies of pyrene-conjugated glycolipids.

Supramolecular assemblies of fluorescent glycolipids exhibited molecular packing rearrangement as well as morphological transformation, in response to amino acid analytes. Naked-eye detectable fluorescence color changes and hydrogel formation as the result of the amplification of the molecular- and nanometer-scaled changes enabled not only qualitative analysis but also chiral sensing of a specific amino acid among 20 amino acids.

Photoinduced morphological transformations of soft nanotubes.

In water, synthetic amphiphiles composed of a photoresponsive azobenzene moiety and an oligoglycine hydrogen-bonding moiety selectively self-assembled into nanotubes with solid bilayer membranes. The nanotubes underwent morphological transformations induced by photoisomerization of the azobenzene moiety within the membranes, and the nature of the transformation depended on the number of glycine residues in the oligoglycine moiety (i.e., on the strength of intermolecular hydrogen bonding). Upon UV-light irradiation of nanotubes prepared from amphiphiles with the diglycine residue, trans-to-cis isomerization induced a transformation from nanotubes (inner diameter (i.d.) 7 nm), several hundreds of nanometers to several tens of micrometers in length, to imperfect nanorings (i.d. 21-38 nm). The cis-to-trans isomerization induced by continuous visible-light irradiation resulted in the stacking of the imperfect nanorings to form nanotubes with an i.d. of 25 nm and an average length of 310 nm, which were never formed by a self-assembly process. Time-lapse fluorescence microscopy enabled us to visualize the transformation of nanotubes with an i.d. of 20 nm (self-assembled from amphiphiles with the monoglycine residue) to cylindrical nanofibers with an i.d. of 1 nm; shrinkage of the hollow cylinders started at the two open ends with simultaneous elongation in the direction of the long axis.

Two-step naked-eye detection of lectin by hierarchical organization of soft nanotubes into liquid crystal and gel phases.

Depending on the concentration of a lectin analyte, supramolecular soft nanotubes, bearing recognition sites immobilized on the outer surface through ethylene glycol chains, hierarchically organized into naked-eye-detectable liquid crystals and hydrogels.

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.

Effects of PEGylation on the physicochemical properties and in vivo distribution of organic nanotubes.

Application of organic nanotubes (ONTs) into drug nanocarriers ultimately requires validation in live animals. For improving the dispersibility in biological media and in vivo distribution, the outer surface of an ONT was functionalized with polyethylene glycol (PEG) via the coassembly of an ONT-forming lipid with 5-20 mol% of a PEG-tethered lipid analogue (PEG-lipid). Firstly, the effect of PEGylation on the psysicochemical properties of ONTs, such as morphology and dispersibility, was investigated. PEGylation of ONTs slightly reduced the average length and effectively prevented the aggregation in phosphate-buffered saline (PBS). The PEGylated ONTs even showed high thermal stability in aqueous dispersion at least up to 95°C. Secondly, differential scanning calorimetry and powder X-ray diffraction indicated that ~10 mol% of PEG-lipid was completely incorporated into the ONTs, while 20 mol% of PEG-lipid encountered a partial phase separation during coassembly. In the heating differential scanning calorimetry runs, the resultant PEGylated ONTs with 5 mol% PEG-lipid showed no sign of phase separation up to 180°C under lyophilized condition, while those with 10 mol% and 20 mol% PEG-lipid showed some phase separation of the PEG-lipid above 120°C. Finally, PEGylation significantly affected the tissue distribution and prolonged the persistence time in the blood in mice. Non-PEGylated ONTs was quickly cleared from the circulation after intravenous infusion and preferentially accumulated in the lung, while PEGylated ONTs was mainly trapped in the liver and could circulate in the blood up to 24 hours. This study provided valuable information of physicochemical properties and the in vivo distribution behavior of PEGylated ONTs for their potential application into drug nanocarriers.

Encapsulation of poorly water-soluble drugs into organic nanotubes for improving drug dissolution.

Hydrocortisone (HC), a poorly water-soluble drug, was encapsulated within organic nanotubes (ONTs), which were formed via the self-assembly of N-{12-[(2-α,ß-d-glucopyranosyl) carbamoyl]dodecanyl}-glycylglycylglycine acid. The stability of the ONTs was evaluated in ten organic solvents, of differing polarities, by field emission transmission electron microscopy. The ONTs maintained their stable tubular structure in the highly polar solvents, such as ethanol and acetone. Furthermore, solution-state (1)H-NMR spectroscopy confirmed that they were practically insoluble in acetone at 25°C (0.015 mg/mL). HC-loaded ONTs were prepared by solvent evaporation using acetone. A sample with a 3/7 weight ratio of HC/ONT was analyzed by powder X-ray diffraction, which confirmed the presence of a halo pattern and the absence of any crystalline HC peak. HC peak broadening, observed by solid-state (13)C-NMR measurements of the evaporated sample, indicated the absence of HC crystals. These results indicated that HC was successfully encapsulated in ONT as an amorphous state. Improvements of the HC dissolution rate were clearly observed in aqueous media at both pH 1.2 and 6.8, probably due to HC amorphization in the ONTs. Phenytoin, another poorly water-soluble drug, also showed significant dissolution improvement upon ONT encapsulation. Therefore, ONTs can serve as an alternative pharmaceutical excipient to enhance the bioavailability of poorly water-soluble drugs.

Control of self-assembled morphology and molecular packing of asymmetric glycolipids by association/dissociation with poly(thiopheneboronic acid).

The molecular packing and self-assembled morphologies of asymmetric bolaamphiphiles, N-(2-aminoethyl)-N'-(ß-d-glucopyranosyl)alkanediamide [1(n), n = 12, 14, 16, 17, 18, and 20], were precisely controlled by association/dissociation with poly(thiopheneboronic acid) (PTB). Differential scanning calorimetry, X-ray diffraction, and infrared spectroscopy revealed that the starting film of 1(n) associated with 1 equiv of the boronic acid moiety of PTB, (Film-1(n)PTB), had antiparallel molecular packing of 1(n) moiety within the monolayer membranes. However, the molecular packing of the starting film that contained 0.5 equiv of the boronic acid moiety of PTB (Film-2eq1(n)PTB) was parallel. The dispersion of Film-1(n)PTB in water gave only nanotapes, whereas that of Film-2eq1(n)PTB in water selectively formed nanotubes, through a dissociation reaction of PTB based on the hydrolysis of the boronate esters in the complexes. The nanotapes and nanotubes memorized the antiparallel and parallel molecular packing of the starting films, respectively. Changes in the length of the oligomethylene spacer of 1(n) never affected the molecular packing or self-assembled morphologies. However, the inner diameters of the nanotubes increased irregularly in the range of 67.9-79.6 nm as the length of the oligomethylene spacer of 1(n) increased from n = 12 to n = 18.

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.

A hydro/organo/hybrid gelator: a peptide lipid with turning aspartame head groups.

This work presents a novel bola-type peptide lipid which can gelate water, organic solvents, and water/organic-solvent mixtures. In its molecular structure, an amphiphilic dipeptide aspartame (L-α-aspartyl-L-phenylalanine methyl ester) is connected at both ends of an alkylene linker. The different morphologies in the hydrogel (helical nanotapes) and the organogel (tape-like nanostructures) were visualized by energy-filtering transmission electron microscopy (EF-TEM) and energy-filtering scanning electron microscopy (FE-SEM), and the molecular arrangement was examined using X-ray diffraction (XRD), infrared (IR) spectroscopy, and circular dichroism (CD) spectroscopy. Possessing a hydrophilic aspartic acid group and a (relatively) hydrophobic phenylalanine methyl ester group, the dipeptide head group can turn about in response to solvent polarity. As a consequence, the solvent condition changed the molecular packing of the gelator and affected the overall supramolecular structure of the gel. It is noted that the peptide lipid gelated mixed solvents of water and organic solvents such as dichloromethane, liquid-paraffin, olive-oil, silicone-oils, and so on. The present hybrid gel can simultaneously hold hydrophilic and hydrophobic functional materials.

Higher lung accumulation of intravenously injected organic nanotubes.

The size and shape of intravenously injected particles can affect their biodistribution and is of importance for the development of particulated drug carrier systems. In this study, organic nanotubes (ONTs) with a carboxyl group at the surface, a length of approximately 2 µm and outer diameter of 70-90 nm, were injected intravenously into tumor-bearing mice. To use ONTs as drug carriers, the biodistribution in selected organs of ONTs postinjection was examined using irinotecan, as an entrapped water-soluble marker inside ONTs, and gadolinium-chelated ONT, as an ONT marker, and compared with that of a 3 µm fluorescently labeled spherical microparticle which was similar size to the length of ONTs. It was found that for irinotecan, its active metabolite and gadolinium-chelated ONTs were highly accumulated in the lung, but to a lower level in the liver and spleen. On the other hand, microparticles deposited less in the lung and more highly in the liver. Moreover, histologic examination showed ONTs distributed more in lung tissues in part, whereas microparticles were present in blood vessels postinjection. These preliminary results support the notion of using negatively charged ONTs as intravascular carriers to maximize accumulation in the lung whilst reducing sequestration by the liver and spleen. This finding suggested that ONTs are potential carriers for lung-targeting drug delivery.

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.