Dendrimer Molecules Research Articles

Conformational properties and Rouse dynamics of different dendrimer molecules

A coarse-grained model previously proposed to perform Monte Carlo simulations for several dendrimer molecules with different topologies and chemical compositions in solution is employed now to obtain structural properties, such as the bead density profile, the asphericity and the molecular scattering factor, or form factor. It is also used to study the Rouse dynamics, including Rouse spring forces consistent with the equilibrium averages of distances between connected frictional beads and hydrodynamic interactions (Rouse–Zimm scheme). With this approach, the Rouse relaxation times and the frequency-dependent viscoelastic modulus are calculated. Since hydrodynamic interactions are included in their preaveraged form, the effect of the preaveraging approximation is explicitly discussed. The influence of the different structural and topological dependence on the dendrimer static and dynamic properties is analysed and discussed.

Label-free detection of DNA molecules on the dendron based self-assembled monolayer by electrochemical impedance spectroscopy

We report preparation of a novel platform for effective DNA hybridization and its application to the detection of single mismatched DNA. Cone-shaped dendrimer molecules have been immobilized on the gold surface at equidistance, 3.1 nm, from each other with a probe DNA molecule attached to the top of each dendrimer so that enough space would be secured for effective hybridization. This arrangement allows each probe DNA molecule to form a natural DNA double helix upon hybridization with a target DNA molecule. The single nucleotide polymorphism at either the central or end position of the 25-mer target DNA has been shown to be effectively discriminated against on this platform from each other as well as from a complementary DNA by electrochemical impedance measurements. We also report adverse effects exerted by probe ions, Fe(CN) 6 3−/4−, on DNA hybridization reactions. The significance of the results for the use in DNA analysis is discussed.

Dendrimer adsorption on charged particulate surfaces

AbstractThe adsorption of a negatively charged poly‐L‐lysine dendrimer SPL‐7013 on alumina particle surfaces was investigated as a function of solution ionic strength and pH. The Langmuir model satisfactorily describes experimental adsorption isotherms; the free energies of adsorption ΔGads, surface affinity constants, maximum adsorbed amounts, and areas per adsorbed dendrimer molecule were calculated. ΔGads for various solution conditions were in the range of − 46 to − 59 kJ mol−1; these are significantly greater in magnitude than for small molecules and reflect the multivalent nature and strong adsorption affinity of dendrimer to oppositely charged surfaces, thus confirming electrostatic attraction as a major driving force. The adsorption affinity and interfacial packing of the dendrimer increase with increased solution salt concentration due to the screening of the electrostatic forces and increased surface charge density of the adsorbent. Similarly, increased adsorption at low pH is ascribed to a greater adsorption site density on alumina. Copyright © 2008 Curtin University of Technology and John Wiley & Sons, Ltd.

Effect of molecular topology on the transport properties of dendrimers in dilute solution at Θ temperature: A Brownian dynamics study

Structure and transport properties of dendrimers in dilute solution are studied with the aid of Brownian dynamics simulations. To investigate the effect of molecular topology on the properties, linear chain, star, and dendrimer molecules of comparable molecular weights are studied. A bead-spring chain model with finitely extensible springs and fluctuating hydrodynamic interactions is used to represent polymer molecules under Theta conditions. Structural properties as well as the diffusivity and zero-shear-rate intrinsic viscosity of polymers with varied degrees of branching are analyzed. Results for the free-draining case are compared to and found in very good agreement with the Rouse model predictions. Translational diffusivity is evaluated and the difference between the short-time and long-time behavior due to dynamic correlations is observed. Incorporation of hydrodynamic interactions is found to be sufficient to reproduce the maximum in the intrinsic viscosity versus molecular weight observed experimentally for dendrimers. Results of the nonequilibrium Brownian dynamics simulations of dendrimers and linear chain polymers subjected to a planar shear flow in a wide range of strain rates are also reported. The flow-induced molecular deformation of molecules is found to decrease hydrodynamic interactions and lead to the appearance of shear thickening. Further, branching is found to suppress flow-induced molecular alignment and deformation.

Self-Organization in Dendrimer Polyelectrolytes

Results from molecular dynamics simulations are reported, describing self-organization in solutions of charged dendrimer molecules upon variation of the strength of the electrostatic interactions, in the presence of explicit solvent and counterions. Systems of two sizes bearing different surface charge densities are studied at constant temperature and volume conditions. It is found that a systematic variation of Bjerrum length triggers a mechanism associated with counterion spatial correlations, which drives the systems from an amorphous liquidlike arrangement to an ordered state bearing the symmetry of a cubic phase. The role of dendrimer interpenetration, trapping of counterions, and solvent depletion in the ordering process is also explored. As this study employs a description which takes into account all the principal internal degrees of freedom of dendrimer molecules, the conclusions drawn are expected to offer a fair description of the behavior of realistic systems bearing similar dendritic topology.

Realistic numerical simulations of dendrimer molecules

Dendrimer molecules have great potential in different applied fields because of their peculiar conformational properties. Many insights pertinent to the understanding of these properties can be provided by numerical simulation methods. Detailed models, based on atomistic representations of the molecular structures are usually employed for molecular dynamics simulations, which require an important amount of computational resources. More efficient Brownian dynamics or Monte Carlo methods can be more easily designed for coarse-grained models. However, the simplest models cannot distinguish between the different behaviours of particular dendrimer structures. Specific coarse-grained models that incorporate local details of the molecules structures are more adequate for these purposes. We describe some of the results provided by the simulation methods for particular dendrimer models and analyse their performance in the prediction of existing experimental data, discussing the current achievements and needs in this field.

Optical Energy Transport and Interactions between the Excitations in a Coumarin−Perylene Bisimide Dendrimer

Energy transfer properties of novel coumarin-perylene bisimide dendrimer are studied by means of steady state and time-resolved UV/vis spectroscopy. At low donor excitation density fast (transfer rate approximately 10 ps(-1)) and efficient (quantum yield approximately 99.5%) donor-acceptor energy transfer is observed. The random distributions of donor-acceptor orientations and distances result in nonexponential energy transfer kinetics. The energy transfer remains independent of excitation density up to densities corresponding to one absorbed photon per 10 dendrimer molecules. At higher excitation densities the transfer rate is found to increase due to excitation of multiple donors per dendrimer. Control of the donor-acceptor energy transfer rate is achieved by pre-excitation of the acceptor and monitored by prepump-pump-probe experiments, which show that the energy transfer rate can be decreased by a factor of 2. The relative orientations of transition dipole moments in the donor and acceptor molecules are found to be one of the key factors determining the energy transfer dynamics at high excitation densities.

ENFET glucose biosensor produced with dendrimer encapsulated Pt nanoparticles

The biosensors based on ENFETs for direct glucose concentration analysis have been fabricated by introducing dendrimer encapsulated Pt nanoparticles and glucose oxidase (GOx) via a layer-by-layer self-assembly method. The free amine groups located on each poly(amidoamine) dendrimer molecule were exploited to immobilize enzyme through covalent attachment. Depending on metal nanoparticles within dendrimers and biocompatibility of dendrimers, the fabricated glucose sensitive ENFET shows obviously enhanced sensitivity and extended lifetime compared with the conventional ones. The fabricated sensor has a linear range of 0.25–2.0 mM, and a detection limit of ca. 0.15 mM. The influence of buffer concentration, ionic strength and pH was discussed. The biosensor also has good stability, which response could be used for detecting of glucose samples at intervals for at least 1 month when it stored in dry state at 4 °C.

Determination of glucose by affinity adsorption solid substrate-room temperature phosphorimetry based on concanavalin agglutinin labeled with fluorescein using 1.5-generation dendrimers as sensitizer

In the presence of the heavy atom perturber Pb(Ac)2, fluorescein (HFin) can emit strong and stable room temperature phosphorescence (RTP) on the surface of a nitrocellulose membrane (NCM) at λexmax/λemmax = 480/648 nm. It can be spiked with the 1.5-generation polyamidoamine dendrimers (abbreviated: D-1.5) to emit stronger RTP. It was found that a quantitative specific affinity adsorption (AA) reaction between concanavalin agglutinin (abbreviated: Con A) labeled with D-1.5-HFin and N-acetylglucosamine (G) could be carried out on the surface of NCM. The product of the reaction (D-1.5-HFin- Con A-G) could emit strong and stable RTP, and the ΔIp was proportional to the content of G. According to the above facts, a new method for determination of G by affinity adsorption solid substrate-room temperature phosphorimetry (AA-SS-RTP) was established, based on Con A labeled with fluorescein using D-1.5 dendrimers molecules as sensitizer. The linear range of the sandwich method was 0.040–60 pg G spot−1 (corresponding concentration range: 0.10–150 ng mL−1; sample volume: 0.40 µL spot−1). The regression equation of the working curve was ΔIp = 6.880 + 5.610 mG pg spot−1, r = 0.9987. The working solutions containing 0.10 and 150 ng mL−1 G were determined repeatedly 11 times, respectively. The corresponding RSDs were 2.9 and 3.8%. The detection limit of this method calculated by 3Sb/k was 0.021 pg spot−1 (5.2 × 10−11 g mL−1). Compared with the direct method (detection limit: 0.078 pg spot−1, linear range: 0.40–40 pg G spot−1), the sensitivity was obviously improved and the linear range was wider. This method has been successfully applied to the determination of G in human plasma, as well as to the supervision and forecast of human diseases, for it is of good sensitivity, accuracy and precision.

Crystallization in concentrated dendrimer solutions

We investigate the structural formation of concentrated dendrimer solution by coarse-grained molecular dynamics simulations. With larger electrostatic screening lengths, dendrimer molecules form a crystalline structure. Dendrimer crystals melt when the screening length is decreased, ultimately showing the incipient phenomena of dendrimer aggregation. We study the profiles of the radial distribution function and the mean square displacement of dendrimer molecules in concentrated solutions, and discuss the melting transition of dendrimer crystal in terms of the Lindemann criterion.

Dendrimer based non-competitive fluoroimmunoassay for analysis of cortisol

The use of immunoassays in clinical diagnostics has stimulated the development of many sensitive and highly specific techniques to measure the presence of specific antigens in samples. This research proposes a non-competitive fluoroimmunoassay method for analysis of cortisol based on the blocking of unbound sites of the capture antibody by means of a reagent that consists of a dendrimer molecule bound to cortisol. Dendrimers are ordered branched polymers. The dendrimer–cortisol complex binds to more than one antibody at the same time, so it is more strongly bound than the analyte to the immobilized antibody. In this way, when the fluorescent-labeled antigen is added, competition for the antibody occupancy allows the removal of the analyte molecules, but not the dendrimer molecule. The measured signal is directly related to the analyte concentration (i.e. cortisol). Size exclusion chromatography (SEC) shows the presence of a high molecular weight component after dendrimer–cortisol synthesis. Results show cortisol covalently attached to PAMAM (Polyamidoamine) dendrimers can be used successfully as a blocking reagent in a non-competitive assay. The non-competitive assay standard curve shows increased fluorescence as the concentration of cortisol increases, which demonstrates that cortisol is being replaced by FITC-cortisol.

Spectral investigation of coordination of cuprum cations and protons at PAMAM dendrimer peripherally modified with 1,8-naphthalimide units

The investigations aim at revealing the ability of a 1,8-napthalimide-modified poly(amidoamine) dendrimer from second generation to respond to the presence of cuprum cations and protons in the environment. It has been established that a single Cu 2+ cation present in the dendrimer molecule is capable of quenching more than 78% of its fluorescence what is an indication of high sensitiveness. An enhancement of the fluorescence emission of the dendrimer has been observed in acidic medium. It has been established that the processes of coordinating the ions in different sites of the dendrimer are reversible.

Nanoscale Protein Pores Modified with PAMAM Dendrimers

We describe nanoscale protein pores modified with a single hyperbranched dendrimer molecule inside the channel lumen. Sulfhydryl-reactive polyamido amine (PAMAM) dendrimers of generations 2, 3 and 5 were synthesized, chemically characterized, and reacted with engineered cysteine residues in the transmembrane pore alpha-hemolysin. Successful coupling was monitored using an electrophoretic mobility shift assay. The results indicate that G2 and G3 but not G5 dendrimers permeated through the 2.9 nm cis entrance to couple inside the pore. The defined molecular weight cutoff for the passage of hyperbranched PAMAM polymers is in contrast to the less restricted accessibility of flexible linear poly(ethylene glycol) polymers of comparable hydrodynamic volume. Their higher compactness makes sulfhydryl-reactive PAMAM dendrimers promising research reagents to probe the structure of porous membrane proteins with wide internal diameters. The conductance properties of PAMAM-modified proteins pores were characterized with single-channel current recordings. A G3 dendrimer molecule in the channel lumen reduced the ionic current by 45%, indicating that the hyperbranched and positively charged polymer blocked the passage of ions through the pore. In line with expectations, a smaller and less dense G2 dendrimer led to a less pronounced current reduction of 25%. Comparisons to recordings of PEG-modified pores revealed striking dissimilarities, suggesting that differences in the structural dynamics of flexible linear polymers vs compact dendrimers can be observed at the single-molecule level. Current recordings also revealed that dendrimers functioned as ion-selectivity filters and molecular sieves for the controlled passage of molecules. The alteration of pore properties with charged and hyperbranched dendrimers is a new approach and might be extended to inorganic nanopores with applications in sensing and separation technology.

Free Energy Balance Predicates Dendrimer Binding Multivalency at Molecular Printboards

Self-assembled monolayers (SAMs) terminating in beta-cyclodextrin (beta-CD) cavities can be used to bind ink molecules and so provide a molecular printboard for nanopatterning applications. Multivalent, or multisite, binding strengthens the attachment of large inks to the printboard, yielding more robust patterns. We performed fully atomistic molecular dynamics (MD) simulations in bulk explicit solvent to probe the conformational space available to dendrimer and dendrite ink molecules, in both free and bound environments. We show that accurate treatment of both pH effects and binding conformations gives calculated binding modes in line with known binding multivalencies. We identify and quantify the steric frustration causing small, low-generation dendrimer inks to bind to the printboard using just a subset of the available anchor groups. Furthermore, we show that the enhanced binding energy of multisite attachment offsets the steric strain, the feasibility of a given binding mode thus determined by the relative magnitudes of the unfavorable steric strain and favorable multisite binding free energies. We use our experimentally validated model of dendrimer binding to predict the binding mode of novel fluorophoric dendrites and find divalent binding, consistent with confocal microscopy imaging of pattern formation at molecular printboards.

Specifics of structural organization and properties of lysine dendrimers of different generations and related supramolecular structures

Lysine dendrimers of different generations containing covalently attached anthracene-based luminescent labels (one label per six dendrimer molecules) and fifth-generation heterodendrimers containing various amino acid units located between lysine fragments were synthesized. Homodendrimers were studied by means of the polarized luminescence technique. It was found that the packing density of lysine fragments in macromolecules increases with an increase in the generation number. Their capability of forming supramolecular structures changes in this case; the number of polymacromolecular associates produced in the solution and on the polymer matrix in interpolymer complexes decreases and their complexing power in the reaction with low-molecular-mass compounds increases. It was found that dendrite molecules acquire a looser structure on passing to heterodendrimers and their complexing ability varies in the corresponding manner; the number of polymacromolecular associates in the solution and on the polymer matrix in the interpolymer complexes increases and binding of low-molecular-mass compounds is reduced.

Combined in Situ and Time-Resolved SANS and SAXS Studies of Chemical Reactions at Specific Sites and Self-Assembling Processes of Reaction Products: Reduction of Palladium Ions in Self-Assembled Polyamidoamine Dendrimers as a Template

We reported the reduction process of palladium(II) acetate, Pd(OAc)2, and the self-assembling process of the resulting palladium atoms, Pd(0), into nanoparticles in the presence of a first-generation polyamidoamine dendrimer (G1-NH2) in nonaqueous solution. The process was induced by mixing the two solutions (A) Pd(OAc)2 dissolved in N,N-dimethylformamide (DMF) and (B) G1-NH2 dissolved in methanol, under the specific condition of r ≡ [Pd(OAc)2]/[−NH2] = 3.3, where [−NH2] is the molar concentration of the primary amine groups in the dendrimer in the reaction solution. The reaction and reaction-induced self-assembling process were explored in situ by means of a combined time-resolved method of small-angle neutron scattering and small-angle X-ray scattering. The method revealed the time evolution of a series of self-assembling processes occurring in the system. (a) The dendrimer molecules and Pd(OAc)2 first self-assemble themselves rapidly into spherical aggregates with an average radius of ∼30 nm after onset of the reaction, and thereafter their size was kept almost constant. (b) Inside the aggregates, which serve as a template for the reaction, the reduction of Pd(OAc)2 to Pd(0) and their self-assembly into Pd nanoparticles with an average radius of 0.4−1.7 nm proceeds gradually with time over 12 h. The following factors are proposed to be important for understanding the series of self-assembling processes: (i) the relative strength of interactions among the constituents Pd(OAc)2, G1-NH2, DMF, and methanol and (ii) the reduction of Pd(OAc)2 with methanol as a weak reducing agent and the self-assembly of the reduction products of Pd(0) in the templates.

Conformation and distribution of groups on the surface of amphiphilic polyether-co-polyester dendrimers: Effect of molecular architecture

Amphiphilic polyester-co-polyether (PEPE) dendrimers synthesized from poly(ethylene glycol) (PEG) were examined to understand the influence of alterations in the architecture of dendrimers on their conformation at interfaces and distribution of various groups on their surface. Effect of changes in the number of branching points, type of terminal functional groups and generation of dendrimer was primarily evaluated. Dendrimers were deposited on mica by spin coating at 0.1 mg/mL. Tapping mode atomic force microscopy (AFM) was employed for the visualization of dendrimer topographies while, X-ray photoelectron spectroscopy (XPS), AFM phase and force imaging were used as the tools for characterization of their surfaces. Individual dendrimer molecules could be imaged by AFM, which showed that they are round or oval in topography. Dendrimers were also flattened on mica but the extent of flattening differed with the chemical structure; for instance, third generation dendrimers were more flattened than second generation dendrimers whereas, dendrimers with higher number of branches had greater height above the mica surface. Hydrophilic and hydrophobic groups present towards the aerial interface existed in distinct zones rather than being distributed randomly, except in dendrimer with higher number of branches. The percentage of various hydrophobic groups on the surface of dendrimer was enhanced by increase in the number of branches but, was lowered by the presence of hydroxyl groups as the pendant terminal groups. Furthermore, the core of dendrimers was not always located towards the centre, its position was found to be altered by the number of branching points, type of terminal functional groups and the generation of dendrimer.

Sensitive Biosensors Based on (Dendrimer Encapsulated Pt Nanoparticles)/Enzyme Multilayers

AbstractA sensitive enzymed‐based biosensor for glucose has been obtained by introducing dendrimer encapsulated Pt nanoparticles via a layer‐by‐layer assembling method. The free amine groups located on each poly(amidoamine) dendrimer molecule were exploited to covalently attach enzyme to the dendrimer chains using carbodiimide coupling. The resultant enzyme electrodes are shown to have excellent sensitivity (as high as 30.33 μA mM−1 cm−2) and a limit of detection (about 0.1 μmol L−1), depending on metal nanoparticles within dendrimers and the biocompatibility of dendrimers, the linear response range to glucose (from 5 μM to 1.0 mM), a fast response time (within 5 s), and good reproducibility (<8% relative standard deviation between electrodes at low substrate concentration). The sensitivities, and stabilities determined experimentally have demonstrated the potential of dendrimer encapsulated Pt nanoparticles as a novel candidate for enzymatic glucose biosensors.

Synthesis and Electrochemical Properties of Fullerene‐Rich Nanoclusters Synthesized by Cobalt‐Catalyzed Cyclotrimerization of Bis(aryl)alkyne Fullerodendrimers

A lot of balls: Up to 24 C60 units were assembled around a hexaphenylbenzene core by cobalt-catalyzed cyclotrimerization of bis(aryl)alkyne fullerodendrimers (see scheme). Electrochemical investigations reveal that interactions between C60 units within a single dendrimer molecule are extremely efficient.

Improved simulation method for the calculation of the intrinsic viscosity of some dendrimer molecules

A method previously proposed for calculating the radius of gyration and the intrinsic viscosity of dendrimers is modified to give a more accurate description of existing experimental data. The new method includes some features that were not previously considered, namely: (a) a correction term to take into account the contribution of individual friction beads, whose volumes are not negligible in comparison with the molecule size, (b) a realistic distribution of internal angles between successive beads that define branching points in the molecule, (c) a distribution of distances between branching points computed from molecular dynamics simulations of a small dendrimer with explicit solvent. Modification (a) alone is able to give a good description of the experimental results obtained for polypropylene-imide with a diaminobutane core in water, while the simultaneous use of the three modifications is needed to adequately describe the experimental data of monodendrons and tridendrons of polybenzylether in THF.