Atomic force microscopy study of the adsorption and electrostatic self-organization of poly(amidoamine) dendrimers on mica

The adsorption behavior of poly(amidoamine) dendrimers to mica surfaces was investigated as a function of ionic strength and pH. The conformation and lateral distribution of the adsorbed dendrimers of generations G8 and G10 were obtained ex situ by tapping mode atomic force microscopy (AFM). The deposition kinetics of the dendrimers was found to follow a diffusion-limited process. Fractional surface coverage and pair correlation functions of the adsorbed dendrimers were obtained from the AFM images. The data are interpreted in terms of the random sequential adsorption (RSA) model, where electrostatic repulsion due to overlapping double layers is considered. Although the general trends typical for an RSA-determined process are well-reproduced, quantitative agreement is lacking at low ionic strengths.     

New insights into the structure of PAMAM dendrimer/gold nanoparticle nanocomposites

In this work, we have employed a suite of complementary analytical techniques to shed light on the nanocomposite structures formed during gold nanoparticles (AuNPs) synthesis in the presence of poly(amidoamine) (PAMAM) dendrimers. Nanocomposites of AuNPs and either fourth or eighth generation amine-terminated PAMAM dendrimers (G4 or G8) were prepared. The size distributions of AuNPs and the nanocomposites were determined by transmission electron microscopy. Atomic force microscopy phase imaging and neutral impact collision ion scattering spectroscopy (NICISS) were utilized for the first time to investigate and compare nanocomposite structures formed from G4 and G8. Our results suggest that G4 stabilized the AuNP by capping the AuNP particle surface but that a certain fraction of the gold surface was still barely covered. In contrast, the metal nanoparticle surface was completely covered by G8. In addition, NICISS results provided evidence that nanocomposites deformed when being deposited directly onto a substrate.     

Catalysis by hydrophobically modified poly(propylenimine) dendrimers having quaternary ammonium and tertiary amine functionality

Four different quaternary ammonium chloride-modified poly(propylenimine) (PPI) dendrimers were synthesized by alkylation of a PPI dendrimer having eight dimethylamino end groups with 1-bromooctane or 1-bromododecane. By varying the mole ratio of alkyl bromide to dendrimer, averages of 4-10 quaternary ammonium groups were formed. The new amphiphilic dendrimers are surface active and are micellar catalysts in water. The dendrimers have critical aggregation concentrations between 8.5 x 10(-4) and 9.0 x 10(-5) M. Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate at 25 degrees C was 650 times faster than in water alone in the presence of a dendrimer quaternized with eight dodecyl chains at a concentration of 2.45 mM in quaternary ammonium groups. The order of the catalytic efficiency of the new dendrimers decreased with the length and number of hydrophobic alkyl groups in the order (C(12))(8) > (C(12))(4) > (C(8))(10) > (C(8))(5). The pseudo-first-order rate constants for basic hydrolysis of p-nitrophenyl hexanoate in pH 9.4 buffer at 30 degrees C using the (C(12))(8) and (C(12))(4) dendrimers were 26 and 13 times higher than those for hydrolysis with no dendrimer. The kinetic data were fit to a single-site binding model to evaluate the contributions of binding constants of reactants to the dendrimers and catalytic rate constants of the bound species to the overall catalytic activity.     

一种基于胆固醇的荧光小分子胶凝剂的合成及其胶凝行为

合成并表征了一种含7-硝基苯并-2-氧杂-1,3-二唑基(7-Nitrobenzo-2-oxa-1,3-diazol-4-yl)的胆固醇衍生物(NBD-C), 考察了其在30种溶剂中的胶凝行为. 实验结果表明, NBD-C对乙腈具有很强的胶凝作用, 且该凝胶体系具有显著的剪切触变性. 对干凝胶的显微分析发现, 在不同溶剂中, NBD-C具有不同的聚集结构. 红外光谱(FTIR)、核磁共振光谱(¹H NMR)和荧光光谱研究结果表明, 除了胆固醇的范德华堆积作用之外, 分子间氢键作用也是该化合物聚集的重要驱动力.     

Synthesis and comparative studies of carbosilane liquid crystalline dendrimers with chiral terminal mesogenic groups

A new series of carbosilane liquid crystalline (LC) dendrimers from the first to the third generations with 8, 16 and 32 chiral terminal mesogenic groups, respectively, has been synthesized. The molecular structures and purity of all new compounds were confirmed by ¹H NMR spectroscopy and GPC analysis. Data of polarization microscopy and SAX analysis demonstrated that all LC dendrimers synthesized form a chiral smectic SmC* phase at temperatures below 50 °C. It has been found that bistable electrooptical switching is observed for all dendrimers. The influence of chiral mesogenic fragment length on phase behavior and ferroelectric properties of carbosilane LC dendrimers is discussed.     

Heavyweight dendritic inks for positive microcontact printing

Poly(propylene imine) dendrimers with dialkyl sulfide end groups were prepared and developed as inks for positive microcontact printing ((+)muCP) on gold. Long (C10H21-S-C10H20-), medium (C3H7-S-C4H8-), and short (CH3-S-CH2-) dialkyl sulfide end groups were attached to second- and third-generation PPI dendrimers to create a family of dendritic sulfides. The dendritic inks flatten upon adsorption and form monolayers on gold. (+)muCP was performed on gold using commercially available poly(dimethylsiloxane) as stamp material and n-octadecanethiol as etch resist. The gold beneath the dendrimers was selectively etched away with an acidic Fe(NO3)3/thiourea solution to give the positive copy of the original master pattern. The multivalent sulfide attachment and the relatively high molecular mass of these dendrimers ensured minimal lateral ink spreading and thus optimal feature reproducibility. Contact times were varied to analyze the spreading rates of the dendritic inks. The spreading rates of the dendritic inks were found to be much lower than that of pentaerythritol tetrakis(3-mercaptopropionate). (+)muCP with the new inks was extended to submicrometer features. Optical microscopy, scanning electron microscopy, and atomic force microscopy were used to characterize the etched samples. Lines with a width of 100 nm were faithfully replicated with the third-generation dendrimers bearing medium (C3-S-C4-) end groups.     

Coarse-grained molecular dynamics studies of the concentration and size dependence of fifth- and seventh-generation PAMAM dendrimers on pore formation in DMPC bilayer

We have performed molecular dynamics (MD) simulations of multiple copies of unacetylated G5 and G7 and acetylated G5 dendrimers in dimyristoylphosphatidylcholine bilayers with explicit water using the coarse-grained model developed by Marrink et al. (J. Phys. Chem. B 2007, 111, 7812) with the inclusion of long-range electrostatics. When initially clustered together near the bilayer, neutral acetylated dendrimers aggregate, whereas cationic unacetylated dendrimers do not aggregate, but separate from each other, similar to the observations from atomic force microscopy by Mecke et al. (Chem. Phys. Lipids 2004, 132, 3). The bilayers interacting with unacetylated dendrimers of higher concentration are significantly deformed and show pore formation on the positively curved portions, while acetylated dendrimers are unable to form pores. Unacetylated G7 dendrimers bring more water molecules into the pores than do unacetylated G5 dendrimers. These results agree qualitatively with experimental results showing that significant cytoplasmic-protein leakage is produced by unacetylated G7 dendrimers at concentrations as low as 10 nM, but only at a much higher concentration of 400 nM for unacetylated G5 dendrimers (Bioconjugate Chem. 2004, 15, 774). This good qualitative agreement indicates that the effect on pore formation of the concentration and size of large nanoparticles can be studied through coarse-grained MD simulations, provided that long-range electrostatic interactions are included.     

Dendronized Hyperbranched Macromolecules: Soft Matter with a Novel Type of Segmental Distribution

Dendronization of a hyperbranched polyester with different generation dendrons leads to pseudo‐dendritic structures. The hyperbranched core is modified by the divergent coupling of protected monomer units to the functional groups. Compared to dendrimers, the synthetic effort is significantly less, but the properties are very close to those of high‐generation dendrimers. The number of functional groups, molar mass, and rheology behavior even in the early generation (G1–G4) pseudo‐dendrimers strongly resembles the behavior of dendrimers in higher generations (G5–G8). Comparison of the segmental and internal structure with perfect dendrimers is performed using SANS, dynamic light scattering and viscosity analysis, microscopy and molecular dynamics simulation. The interpretation of the results reveals unique structural characteristics arising from lower segmental density of the core, which turns into a soft nano‐sphere with a smooth surface even in the first generation.     

一种含树枝单元的高流动性尼龙6的原位聚合及性能测试

将低代数的树枝状大分子聚酰胺胺(polyamidomine,即PAMAM)分别与对苯二甲酸配制成母盐溶液,再以母盐溶液,己内酰胺,封端剂一起投入高压釜进行原位聚合,通过调节PAMAM在己内酰胺中的比例及改变不同的PAMAM代数得到新型高流动性尼龙6.FTIR显示,与线性尼龙相比较,高流动性尼龙6的γ(N—H)和2δ(N—H)两处对应峰都发生了明显的蓝移.对于树枝单元含量较低(0.3 wt%~0.5 wt%)的新型尼龙6,拉伸性能没有下降,缺口冲击强度增加,而熔体流动却显著得到改善.DSC显示,新型尼龙6的结晶曲线的结晶峰温度有不同程度的降低,晶体生长速率明显减慢.     

Discotic liquid crystalline poly(propylene imine) dendrimers based on triphenylene

The design, synthesis, and mesomorphic properties of a new series of homodendrimers consisting of the commercially available poly(propylene imine) (PPI) dendrimers (G = 1-5), PPI-(NH(2))(n)() (n = 4, 8, 16, 32, 64), functionalized with a discotic triphenylene moiety are reported. The liquid crystalline behavior was investigated by means of differential scanning calorimetry (DSC), polarizing-light optical microscopy (POM), and X-ray diffractometry (XRD). All of the homodendrimers showed mesomorphic properties, with the second to fifth generations giving a hexagonal columnar mesophase (Col(h)) and the first generation a rectangular columnar mesophase (Col(r)). The X-ray study reveals that these mesophases show a highly ordered structure with segregation of triphenylenes and dendrimers into separate columns and a regular stacking distance inside the triphenylene columns. GPC analysis showed that the dendrimers had good monodispersity and MALDI-TOF studies of the first three generations gave good evidence that all of the terminal amino groups of the dendrimers were functionalized with a discotic unit.     

Langmuir-Blodgett monolayers of long-chain NBD derivatives on silver island films: Well-organized probe layer for the metal-enhanced fluorescence studies

The metal-enhanced fluorescence (MEF) from the well-organized monolayers of two newly prepared long-chain alkylamine derivatives of nitrobenzoxadiazole (NBD-C16 and NBD-C18) deposited on silver island films (SIFs) has been investigated. The NBD derivatives were conveniently prepared by using a single step procedure in quantitative yields. The monolayers of the probes on SIFs as well as on bare-glass slides were obtained by using the Langmuir-Blodgett (LB) technique. Orientation of the NBD probe molecule in LB monolayer film was measured with polarized absorption spectroscopy. The observed tilt angle of the probe transition dipole moment with respect to the surface normal of approximately 67-68 degrees was evaluated. We observed that the NBD monolayers deposited in close proximity to silver islands show about a 16-fold increase in fluorescence intensity and shortened fluorescence lifetime compared to those on bare-glass, which are due to metal enhanced fluorescence. On the other hand, the corresponding MEF from randomly oriented film obtained by using spin coating of the probes on SIFs was only 2.5-fold. Further, we deposited mixed monolayers of NBD-C16 or NBD-C18 with various molar ratios of stearic acid to understand the polarity effect on MEF. Interestingly, we found a consistent increase in MEF efficiency with increasing molar ratio of stearic acid. Along with MEF, we also found a continuous blue-shift in emission band maxima of the probes with an increasing molar ratio of stearic acid. The observed increase in MEF efficiency is justified based on cooperative effects of (1) the modulations in electronic density of the surface plasmon absorption band of SIFs and (2) defined probe orientation that might lead to preferential excitation.     

Functional Janus dendrimers containing carbazole with liquid crystalline,optical and electrochemical properties

ABSTRACT

Novel liquid crystalline Janus dendrimers that combine a mesogenic block and an electroactive block have been synthesised. The mesogenic block is based on two third-generation Percec-type dendrons bearing six or eight terminal dodecyloxy alkyl chains, whereas the electroactive blocks are formed by one or two carbazole units. The liquid crystal behaviour was investigated by polarised-light optical microscopy, differential scanning microscopy and X-ray diffraction. The Janus dendrimers with one electroactive unit exhibited cubic or columnar liquid crystal phases, whereas the Janus dendrimers with two electroactive units did not show liquid crystalline behaviour. The UV-vis absorption and emission properties of the Janus dendrimers were investigated. The spectra suggested the existence of π-π stacking and the formation of aggregates in the solid state. Electrodeposition of the carbazole-containing dendrimers afforded semi-globular particles in which the number of electropolymerizable units and the flexible or rigid character of the linker have a decisive influence in the particle size.     

DNA-templated covalent coupling of G4 PAMAM dendrimers

Generation-4 polyamidoamine (PAMAM) dendrimers were surface-functionalized with azides or alkynes and conjugated to one DNA strand. DNA-controlled self-assembly of alternating azide and alkyne dendrimers on a DNA template enabled the coupling of the dendrimers by the azide-alkyne "click" reaction to form covalently coupled dimers, trimers, and tetramers. Polymerization of the DNA-dendrimer conjugates was also demonstrated, as well as assembly in a circular structure on DNA origami and imaging by atomic force microscopy.     

Phase transition temperature of water restrained in polysulfone hollow fibres

The melting and crystallization behaviour of water restrained in polysulfone (PSF) hollow fibre has been investigated by DSC. The PSF hollow fibre was prepared by wet spinning. The morphology of the fibre was observed by scanning electron microscopy. The amount of water was varied by successive evaporation and it was observed that melting and crystallization shifted to temperatures lower than those observed for free water. DSC and NMR studies showed that water in hollow fibres is in a restricted state.     

Synthesis of Novel Amphiphilic Poly （ester-amine） Dendrimers and Their Recognition of Hg2＋ at the Air／Water Interface

Two novel amphiphilic poly(ester-amine)dendrimers were synthesized and characterized by 1H NMR,13C NMR and IR spectra.Their properties of Langmuir film were investigated at the air/water interface.The relationship between the surface pressure and area isotherms of the dendrimers was found to be dependent on the nature of subphases,Interaction between HgCl2 and the dendrimers was observed,indicating that the amphiphilic dendrimers could act as a sensor for Hg^2 .     

Formation of columnar liquid crystals on the basis of unconventional triazine-based dendrimers by the C3-symmetric approach

Two series of unconventional triazine-based dendrimers with C(2) symmetry and C(3) symmetry were prepared. The newly prepared C(3)-symmetrical dendrimers were characterized by (1)H and (13)C NMR spectroscopy, mass spectrometry, and elemental analysis. Differential scanning calorimetry, polarizing microscopy, and powder XRD showed that the C(3)-symmetrical dendrimers display columnar liquid-crystalline phases during thermal treatment, but the C(2)-symmetrical dendrimers were not observed to behave correspondingly. The molecular conformations of C(3)- and C(2)-symmetrical dendrimers were obtained by computer simulation with the MM2 model of the CaChe program in the gas phase. The simulation results reasonably explain the different mesogenicities of C(3)- and C(2)-symmetric dendrimers. This new strategy should be applicable to other types of unconventional dendrimers with rigid frameworks for displaying columnar liquid-crystalline behavior.     

Fluorescence and aggregation behavior of poly(amidoamine) dendrimers peripherally modified with aromatic chromophores: the effect of dendritic architectures

PAMAM dendrimers of the zeroth to fifth generation (G0-5) have been peripherally modified with phenyl, naphthyl, pyrenyl, and dansyl chromophores. Their fluorescence behaviors are strongly affected by the dendritic architectures at different generations. These dendrimers modified with hydrophobic chromphores can self-organize into vesicular aggregates at the low generations G0-3 in water. The size and aggregation number of these vesicles decrease with increasing generation from G0 to G3. Critical aggregation concentration determined by fluorescence spectroscopy reveals that these aggregates can be favorably formed in the order of G3 > G2 > G1. In contrast to the vesicles made from traditional amphiphilic compounds, these dendrimer-based vesicles are very adhesive due to the H-bonding interaction and entanglement of dendritic branches located in the outer layer. A large number of multivesicle assemblies, i.e., "twins" and "quins" consisting of two and five vesicles, were clearly identifiable with transmission electron (TEM) and atomic force microscopy. For the dendrimers with peripheral pyrenyl chromophores, triangle-like vesicles were observed in water. The hydrophobic interphase thickness of the vesicular bilayer is ca. 2.0-3.2 nm determined by fluorescence resonance energy transfer methods, which agrees well with the thickness directly observed with TEM.     

以聚酰胺-胺树形分子为模板制备AgI纳米簇

本文以聚酰胺-胺(PAMAM)树形分子为模板,原位制备AgI纳米簇.系统地研究了AgI纳米簇制备过程中各种反应条件如树形分子端基、反应时间、Ag⁺与PAMAM摩尔比等对AgI纳米簇粒径的影响,分别用紫外-可见光谱、荧光光谱、透射电镜等对所制备的纳米簇进行表征.在相同的条件下,以G4.5-COOH₃为模板较以G5.0-NH₂为模板制备的AgI纳米簇粒径小、分布均匀,这主要取决于G4.5-COOCH₃PAMAM树形分子所起的“内模板”作用.G4.5-COOH₃树形分子浓度为1×10^-5mol/L,Ag⁺与树形分子摩尔比为30:1时所制备的AgI纳米簇的粒径分布均匀、稳定性好,室温避光可稳定存在两个月以上.     

Dendrimer-tuned formation of fluorescent organic microcrystals. Influence of dye hydrophobicity and dendrimer charge

The reprecipitation method is a simple and useful way to prepare microcrystals through a solvent exchange process. It was applied to three fluorescent dyes of the 4-amino-7-nitrobenz-2-oxa-1,3-diazole series. Compounds 1, 2, and 3 differ by the length of the alkyl chain, which comprises 8, 12, and 18 carbon atoms, respectively. The reprecipitation process was first studied in water, in the absence of additives. The kinetics was monitored by UV/vis absorption spectroscopy. The size and shape of the microparticles were analyzed by fluorescence microscopy and transmission electron microscopy. Dyes 1 and 2 lead to microcrystals, the whole process taking much more time for 2 than for 1. The long-chained dye 3 only gave stable aggregates. Therefore, it appears that the hydrophobicity of the organic dye markedly influenced the reprecipitation process. The latter was then studied in the presence of additives. Poly(amidoamine) dendrimers, terminated by 64 carboxylate or amino groups were placed in the reprecipitation medium. They had little effect upon the formation of aggregates for dye 3. In contrast, they drastically accelerated the reprecipitation of 1 and 2 and tuned the size and shape of the microcrystals. Platelets and spindles were thus obtained by varying the nature of the dendrimer, and their optical properties were briefly investigated.     

Cu2+对联苯甲醛修饰的树状大分子聚酰胺-胺荧光性能的影响

采用小分子联苯甲醛(BPA)分别修饰第一和第二代树状大分子聚酰胺-胺(PAMAM), 合成了2种PAMAM的修饰产物G1-BPA4和G1-BPA8. 利用IR, 1H NMR及MALDI-TOF MS等手段表征了2种产物的结构, 研究了Cu2+浓度对其荧光性能的影响. 结果表明, 在一定的浓度范围内, 作为常见荧光猝灭剂的Cu2+能使2种产物的荧光均显著增强.