Comparison of nanotube structures constructed from alpha-, beta-, and gamma-cyclodextrins by potential-controlled adsorption

2-O-Methylated β-Cyclodextrin as an Effective Building Block to Construct Supramolecular Assemblies with Various Morphologies and Molecular Arrangements

The preparation of supramolecular cyclodextrin (CD) assemblies and control of their assembly mode through guest inclusion in CD cavities have been actively studied. Contrarily, there are limited reports on the control of the assembly mode of guest-free CD molecules by external stimuli. Herein, we report the use of 2-O-methylated β-cyclodextrin (2-Me-β-CD) as an effective building block in fabricating supramolecular assemblies with diverse morphologies and molecular arrangements through assembly mode control by various stimuli, such as temperature and solvent. When methanol and diethyl carbonate were used as good and poor solvents, respectively, 2-Me-β-CD formed an amorphous assembly through solvent evaporation on a polyethylene terephthalate (PET) substrate. Increasing the drying temperature and using crystalline substrates, such as highly oriented pyrolytic graphite (HOPG) and sapphire, changed the assembly mode of 2-Me-β-CD to a head-to-tail channel assembly. However, when a 2-Me-β-CD/1-propanol solution was mixed with linear alkanes as a poor solvent, 2-Me-β-CD with head-to-head channel assembly was formed as a precipitate. Additionally, when the corresponding cyclic alkane was used as an alternative poor solvent, an organogel composed of 2-Me-β-CD with head-to-head channel assemblies was obtained. The organogel obtained became a precipitate composed of 2-Me-β-CD with cage-type assembly upon heating at 50 °C. Among the supramolecular assemblies fabricated in this study, the head-to-tail channel assembly is a rare molecular assembly of β-CD and its derivatives. It possesses a modified columnar cavity that has potential applications in molecular recognition and sensing.

Gold-Nanoparticle-Mediated Assembly of High-Order DNA Nano-Architectures

Constructing high-order DNA nano-architectures in large sizes is of critical significance for the application of DNA nanotechnology. Robust and flexible design strategies together with easy protocols to construct high-order large-size DNA nano-architectures remain highly desirable. In this work, the authors report a simple and versatile one-pot strategy to fabricate DNA architectures with the assistance of spherical gold nanoparticles modified with thiolated oligonucleotide strands (SH-DNA-AuNPs), which serve as "power strips" to connect various DNA nanostructures carrying complementary ssDNA strands as "plugs". By modulating the plug numbers and positions on each DNA nanostructure and the ratios between DNA nanostructures and AuNPs, the desired architectures are formed via the stochastic co-assembly of different modules. This SH-DNA-AuNP-mediated plug-in assembly (SAMPA) strategy offers new opportunities to drive macroscopic self-assembly to meet the demand of the fabrication of well-defined nanomaterials and nanodevices.

Structural Insights into the Host-Guest Complexation between β-Cyclodextrin and Bio-Conjugatable Adamantane Derivatives

Understanding the host–guest chemistry of α-/β-/γ- cyclodextrins (CDs) and a wide range of organic species are fundamentally attractive, and are finding broad contemporary applications toward developing efficient drug delivery systems. With the widely used β-CD as the host, we herein demonstrate that its inclusion behaviors toward an array of six simple and bio-conjugatable adamantane derivatives, namely, 1-adamantanol (adm-1-OH), 2-adamantanol (adm-2-OH), adamantan-1-amine (adm-1-NH₂), 1-adamantanecarboxylic acid (adm-1-COOH), 1,3-adamantanedicarboxylic acid (adm-1,3-diCOOH), and 2-[3-(carboxymethyl)-1-adamantyl]acetic acid (adm-1,3-diCH₂COOH), offer inclusion adducts with diverse adamantane-to-CD ratios and spatial guest locations. In all six cases, β-CD crystallizes as a pair supported by face-to-face hydrogen bonding between hydroxyl groups on C2 and C3 and their adjacent equivalents, giving rise to a truncated-cone-shaped cavity to accommodate one, two, or three adamantane derivatives. These inclusion complexes can be terminated as (adm-1-OH)₂⊂CD₂ (1, 2:2), (adm-2-OH)₃⊂CD₂ (2, 3:2), (adm-1-NH₂)₃⊂CD₂ (3, 3:2), (adm-1-COOH)₂⊂CD₂ (4, 2:2), (adm-1,3-diCOOH)⊂CD₂ (5, 1:2), and (adm-1,3-diCH₂COOH)⊂CD₂ (6, 1:2). This work may shed light on the design of nanomedicine with hierarchical structures, mediated by delicate cyclodextrin-based hosts and adamantane-appended drugs as the guests.

Monomolecular covalent honeycomb nanosheets produced by surface-mediated polycondensation between 1,3,5-triamino benzene and benzene-1,3,5-tricarbox aldehyde on Au(111)

Fabrication of a two-dimensional covalent network of honeycomb nanosheets comprising small 1,3,5-triamino benzene and benzene-1,3,5-tricarboxaldehyde aromatic building blocks was conducted on Au(111) in a pH-controlled aqueous solution. In situ scanning tunneling microscopy revealed a large defect-free and homogeneous honeycomb π-conjugated nanosheet at the Au(111)/liquid interface. An electrochemical potential dependence indicated that the nanosheets were the result of thermodynamic self-assembly based not only on the reaction equilibrium but also on the adsorption (partition) equilibrium, which was controlled by the building block surface coverage as a function of electrode potential.

Theoretical study of an anti-Markovnikov addition reaction catalyzed by β-cyclodextrin

β-Cyclodextrin (β-CD) has a hydrophilic exterior and a hydrophobic internal cavity, which allows it to form host-guest complexes with a wide range of guests, such as organics, inorganics, and biomolecules. The aforementioned features lead to an extensive range of applications of β-CD, as the properties of β-CD mean that it is environmentally friendly and can be recovered and reused without mass loss. Here, the β-CD-catalyzed anti-Markovnikov addition of styrene to thiophenol in the presence of aerial oxygen and in aqueous solution to give 1-phenyl-2-(phenylsulfanyl)-1-ethanol was studied using density functional theory (DFT) and the Hartree-Fock (HF) method. The optimal configuration of the inclusion complex of styrene and thiophenol within β-CD was obtained, which indicated that styrene and thiophenol enter from the secondary and primary hydroxyl ends of β-CD, respectively. Moreover, hydrogen bonding of β-CD with styrene and thiophenol contributes to the stability of the inclusion complex. An investigation of the charges from electrostatic potentials using a grid-based method (CHELPG) highlighted the distribution of atomic charges upon complexation. The reaction sites of styrene and thiophenol were determined based on electrostatic potentials (ESPs) and condensed dual descriptors. The calculated ¹H nuclear magnetic resonance (¹H NMR) spectrum of β-CD implied that the chemical shifts of its protons change and H3 and H5 move to higher fields upon complexation, while the calculated ¹³C nuclear magnetic resonance (¹³C NMR) spectrum of styrene suggested that this molecule is electrophilic. Graphical abstract ᅟ.

Lipid-bilayer-assisted two-dimensional self-assembly of DNA origami nanostructures

Self-assembly is a ubiquitous approach to the design and fabrication of novel supermolecular architectures. Here we report a strategy termed ‘lipid-bilayer-assisted self-assembly' that is used to assemble DNA origami nanostructures into two-dimensional lattices. DNA origami structures are electrostatically adsorbed onto a mica-supported zwitterionic lipid bilayer in the presence of divalent cations. We demonstrate that the bilayer-adsorbed origami units are mobile on the surface and self-assembled into large micrometre-sized lattices in their lateral dimensions. Using high-speed atomic force microscopy imaging, a variety of dynamic processes involved in the formation of the lattice, such as fusion, reorganization and defect filling, are successfully visualized. The surface modifiability of the assembled lattice is also demonstrated by in situ decoration with streptavidin molecules. Our approach provides a new strategy for preparing versatile scaffolds for nanofabrication and paves the way for organizing functional nanodevices in a micrometer space.

Self-assembly is a useful method to fabricate novel supramolecular architectures. Here, the authors use lipid-bilayer-assisted self-assembly to obtain two-dimensional crystalline DNA origami lattices, imaging dynamic assembly phenomena using high-speed atomic force microscopy.

In situ scanning tunneling microscopy investigation of subphthalocyanine and subnaphthalocyanine adlayers on a Au(111) electrode

The adsorption behaviors of subphthalocyanine (SubPc) and subnaphthalocyanine (SubNc) on the Au(111) surface were investigated by electrochemical scanning tunneling microscopy (ECSTM). Two types of ordered adlayer structures of SubPc were observed at 550 mV versus the reversible hydrogen electrode (RHE). All of the SubPc molecules take the Cl-down adsorption configuration on Au(111) in both structures. The ordered adlayers exist in the potential range between 350 and 650 mV. The SubNc molecules adsorb on Au(111) in a less-ordered pattern than the SubPc molecules. The present work provides direct evidence for understanding the potential-controlled adsorption behaviors of SubPc and SubNc on the Au(111) surface.

Cyclic voltammetry of water-soluble manganese porphyrins in the presence of cyclodextrins

The addition of cyclodextrins (α-CD, β-CD, 2-hydroxopropyl-β-CD and γ-CD) to aqueous solutions of representative cationic ( MnTMPyP ) and anionic ( MnTPPS 4) water-soluble Mn porphyrins only substantially affect the behavior of the Mn (III)/ Mn (II) couple when inclusion complexes (i.e. for MnTPPS 4) are formed. The voltammetric responses were interpreted under the assumption that the inclusion complexes are electroactive, which occurs after their adsorption on the electrode surface. In addition, the results point to a higher affinity of the cyclodextrins for Mn II TPPS 4 than for Mn III TPPS 4.

Thermodynamically controlled self-assembly of covalent nanoarchitectures in aqueous solution

The pursuit of methods for design and preparation of robust nanoarchitectonic systems with integrated functionality through bottom-up methodologies remains a driving force in molecular nanotechnology. Through the use of π-conjugated covalent bonds, we demonstrate a general substrate-mediated, soft solution methodology for the preparation of extended π-conjugated polymeric nanoarchitectures in low-dimensions. Based on thermodynamic control over equilibrium polymerization at the solid-liquid interface whereby aromatic building blocks spontaneously and selectively link, close-packed arrays composed of one-dimensional (1-D) aromatic polymers and two-dimensional (2-D) macromolecular frameworks have been prepared and characterized by in situ scanning tunneling microscopy. This methodology eliminates the necessity for severe conditions and sophisticated equipment common to most current fabrication techniques and imparts almost infinite possibilities for the preparation of robust materials with designer molecular architectures.

Two-dimensional self-assembled structures of melamine and melem at the aqueous solution-Au(111) interface

Self-assembled structures of melamine and the condensed melamine derivative melem were investigated at aqueous solution-Au(111) interfaces by cyclic voltammetry and in situ scanning tunneling microscopy (STM) observation. The adsorption/desorption behaviors of both molecules on Au(111) surfaces could be controlled by varying the electrochemical potential and solution concentration. In the negative potential region, self-assembled structures of melem and melamine were constructed by double hydrogen bonding systems between nitrogen atoms of triazine rings and amine groups. In addition, melem formed a closely packed structure at potentials of between -0.3 and -0.15 V or in solutions at higher concentrations.

Molecular Dynamics in Two-Dimensional Supramolecular Systems Observed by STM

Since the invention of scanning tunneling microscopy (STM), 2D supramolecular architectures have been observed under various experimental conditions. The construction of these architectures arises from the balance between interactions at the medium-solid interface. This review summarizes molecular motion observed in 2D-supramolecular structures on surfaces using nanospace resolution STM. The observation of molecular motion on surfaces provides a visual understanding of intermolecular interactions, which are the major driving force behind supramolecular arrangement.

Bromine-passivated Au(111) as a platform for the formation of organic self-assembled monolayers under electrochemical conditions

Self-assembled monolayers of 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP) were formed by equilibrium adsorption from a perchloric acid solution onto Au(111) using an interposed adlayer of bromide. The passivating bromide adlayer was generated by addition of 150 microM KBr to the electrochemical cell and allowed monolayer ordering at positive potentials where a disordered TPyP monolayer would be found on a bare Au(111) surface. The TPyP monolayers were characterized in situ with electrochemical scanning tunneling microscopy (EC-STM) and cyclic voltammetry. They were successfully observed at working electrode potentials between 0.0 and +1.3 V vs Ag/AgCl. This wide potential window of usability for the bromide adlayer extends to potentials more positive than what has been achieved for similar observations using iodide-modified Au(111). Within the TPyP monolayers, isolated domains with differing geometries could be distinguished, suggesting dynamic monolayer rearrangements. These results demonstrate that the presence of a passivating bromide adlayer is conducive to the formation of highly ordered organic monolayers. Indeed, bromide is not only one of the few anions that are suitable for this purpose, but it may be superior to the more frequently used iodide.

Iron-induced cyclodextrin self-assembly into size-controllable nanospheres

Iron-induced self-assembly of beta-cyclodextrin, beta-CD, into size controllable nanospheres with a well-defined spherical morphology and a relatively narrow size distribution was formed when acetone was added to a solution of beta-CD with iron(II) acetate, Fe(OAc)(2), in DMF. Thermogravimetric analysis, inductively coupled plasma atomic emission spectroscopy, and high resolution transmission electron microscopy showed that the irons were present as a well-dispersed state in the beta-CD nanospheres. In the (1)H NMR spectrum of the beta-CD/Fe(OAc)(2), beta/Fe, solution before adding acetone, the peaks corresponding to beta-CD were broadened and their spin-spin splitting had disappeared. In particular, the beta/Fe solutions were found to remain in a clean solution state at 1 week after solution preparation. These findings indicate the isolation of individual iron ions caused by the surrounding of each ion with the beta-CD molecules in the solution before the addition of acetone. X-ray crystal structure analysis, morphological observations, and N(2) adsorption and desorption experiments showed that the beta-CD nanospheres were generated by the formation of iron-embedded beta-CD primary particles with disordered cage type structure and simultaneous spherical assembly of the primary particles during the addition of acetone to the beta/Fe solution with appropriate mole ratio between beta-CD and Fe(OAc)(2). Interestingly, the size of the beta-CD nanospheres could be simply controlled by changing the speed at which acetone was added to the solution, with higher acetone addition speeds yielding smaller particles.

Self-assembly of beta-cyclodextrin in water. 2. Electron spin resonance

The interaction of amphipilic spin labels with beta-cyclodextrin was investigated using conventional electron spin resonance (ESR) spectroscopy to explore the aggregation of cyclodextrins in water. Methyl 5-doxylstearate (5-DMS) and stearic acid spin probes (n-DSA), which contain a cyclic nitroxide moiety with unpaired electrons covalently linked to the aliphatic chain carbon in positions 5, 7, 12, and 16, show that different dynamic domains coexist in beta-CD water solutions above 3 mM. The results are consistent with the formation of beta-CD self-assembled structures in water above a critical aggregation concentration and confirm the previous findings that were reported in the part 1 article of this series.

Adlayer structures of aza- and/or oxo-bridged calix[2]arene[2]triazines on Au(111) investigated by scanning tunneling microscopy (STM)

The adlayers formed by a series of aza- and/or oxo-bridged calix[2]arene[2]triazines on Au(111) surfaces were investigated by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. 1,3-Alternate configurations of these molecules are preserved on gold surfaces as in their three-dimensional crystals. STM images show that the cavity sizes of these molecules are finely tuned by substituting the bridging nitrogen atom with oxygen atoms, which change the strengths and densities of the intermolecular hydrogen bonds. Hydrogen bond interaction influences the molecular orientation and conformation in the adlayers, and it plays a key role in the formation of these two-dimensional supramolecular architectures. Coadsorption of calix[2]arene[2]triazine with 1,3,5-tris(5-carboxyamyloxy)benzene (TCAB) intervenes with the intermolecular hydrogen bond formations among the calix[2]arene[2]triazine molecules and consequently causes a conformational transition of the calixarene molecules from rhombic to square. These results demonstrate the role of hydrogen bonds in molecular assembly formations.

Cyclodextrin-based aggregates and characterization by microscopy

Cyclodextrin-based aggregates have been widely investigated with microscopies such as STM, AFM, SEM, TEM, and fluorescent microscopy to obtain the direct morphology and structure of samples. In the present review, we discuss various types of cyclodextrin aggregates, that is, native and modified cyclodextrins, inclusion complexes and their aggregates of cyclodextrins, cyclodextrin rotaxanes and polyrotaxanes, cyclodextrin nanotubes and their secondary assembly, and other high-order aggregates of cyclodextrins. Especially, we focus on the use of microscopy to characterize above aggregates. The application of modern microscopy tools promotes the investigation on cyclodextrins.

Synthesis of Oligo(ethylenediamino)-β-Cyclodextrin Modified Gold Nanoparticle as a DNA Concentrator

A novel oligo(ethylenediamino)-beta-cyclodextrin-modified gold nanoparticle (OEA-CD-NP) was synthesized as a vector for DNA binding and comprehensively investigated by means of absorption and circular dichroism spectroscopies as well as transmission electron microscopy, and its plasmid transfection efficiency as a carrier into cultivated cells in vitro was also evaluated. Possessing many hydrophobic cavities at the outer space, OEA-CD-NP may have a capability of carrying biological and/or medicinal substrates into cells, which will make it potentially applicable in many fields of material science and biological technology. In contrast with OEA-CD-NP, the oligo(ethylenediamino)-lipoic amido-modified gold nanoparticle (OEA-L-NP) without CD was synthesized to investigate the interaction with DNA. The results showed that OEA-L-NPs could only weakly bind DNA.

Micrometer-sized rodlike structure formed by the secondary assembly of cyclodextrin nanotube

In this work, we report the observation on the self-assembly of beta-CD nanotube induced by 2-phenyl-5-(4-diphenylyl)1,3,4-oxadiazole (PBD) molecule with fluorescence microscopy and transmission electron microscopy (TEM). Micrometer-sized rodlike structure is formed by the secondary assembly of cyclodextrin nanotube driven by the inter-nanotubular hydrogen bonding. The effects of pH value, urea, DMF and NaCl on the formation of the rodlike structure are investigated. Dynamic light scattering (DLS) is applied to further characterize the formation of the PBD-beta-CD nanotube. The effect of light scattering on the measurement of fluorescence anisotropy of PBD in the aqueous solutions of beta-CD is corrected. A new mechanism of cyclodextrin aggregation is proposed for this system.

Formation of supramolecular permethrin/β-cyclodextrin nanorods

Molecular dynamics simulations along with scanning tunneling microscopy (STM) imaging demonstrate the formation of a novel type of nanorods, with a stable [permethrin(beta-CD)(2)](n) structure in which the host:guest ratio is 2:1 and n=21-27 giving a length of approximately 30.0-38.5 nm and an average diameter of approximately 1.5 nm, self-assembled from an inclusion complex composed of permethrin and biocompatible beta-cyclodextrin.

Two-dimensional supramolecular arrangements of enantiomers and racemic modification of 1,1'-binaphthyl-2,2'-dicarboxylic acid

Formation of adlayers of the optically active compound 1,1'-binaphthyl-2, 2'-dicarboxylic acid (BINAC) on iodine-modified Au (111) surfaces in perchloric acid was investigated by in situ scanning tunneling microscopy (STM). Highly ordered arrays formed on the surfaces via simple spontaneous adsorption from a solution of enantiomers or the racemic BINAC, in spite of the fact that BINAC has a three-dimensionally complex stereochemical structure. Adlayers of both enantiomers essentially shared the same enantiomorphous structure. Observed parameters of the rectangular unit cell lattice for arrays of both enantiomers of BINAC were a = 2.3 +/- 0.2 nm and b = 0.7 +/- 0.2 nm. On the other hand, racemic modification formed an entirely different adlayer, which consisted of an alternate alignment of the two enantiomers, with an oblique unit cell lattice with parameters of a = 1.2 +/- 0.2 nm, b = 0.8 +/- 0.1 nm, and 74 +/- 3 degrees . No domain composed of a single enantiomer was observed. The stronger hetero-intermolecular interactions of enantiomer couples led to the formation of an alternate arrangement in the array prepared by racemic modification.