Comparison of PAMAM-Au and PPI-Au nanocomposites and their catalytic activity for reduction of 4-nitrophenol

Dendrimer-Au nanocomposites are prepared in aqueous solutions using poly(amidoammine)dendrimers (PAMAM) (generation 2, 3, and 5) and poly(propyleneimine)dendrimers (PPI)(generation 2, 3, and 4) by wet chemical NaBH(4) method. The Au nanoparticles thus obtained are 2-4 nm in diameter for both dendrimers and no generation dependence on the particle size is observed, whereas the generations of the dendrimers are increased as stabilization of Au-nanoparticles is achieved with lower dendrimer concentrations. Studies of the reduction reaction of 4-nitrophenol using these nanocomposites show that the rate constants for the PAMAM dendrimers (generations 2 and 3) are higher than those for the PPI dendrimers (generations 2 and 3), while a distinct difference in the rate constants is not seen for the PAMAM dendrimer (generation 5) or the PPI dendrimer (generation 4). In addition, the rate constants for the reduction of 4-nitrophenol involving all the dendrimers decrease with increases in dendrimer concentrations.     

Preparation, characterization, and electrocatalytic activity of surface anchored, Prussian Blue containing starburst PAMAM dendrimers on gold electrodes

Gold bead electrodes were modified with submonolayers of 3-mercaptopropionic acid or 2-aminoethanethiol and further reacted with poly(amidoamine) (PAMAM) dendrimers (generation 4.0 and 3.5, respectively) to obtain films on which Prussian Blue (PB) was later absorbed to afford mixed and stable electrocatalytic layers. Experiments carried out with these novel materials not only showed an improved surface coverage of PB on the dendrimer modified electrodes as compared to PB modified gold electrodes prepared under acidic conditions, but also showed an increased stability at neutral pH values for one of the dendrimer containing substrates where the PB film on a bare gold electrode is simply not formed. The dendrimer modified electrodes were also tested as electrocatalytic substrates for the electroxidation of L(+)-ascorbic acid (AA), and it was found that their sensitivity as well as the corresponding detection limits were improved as compared to the voltammetric response of a Au-PB modified electrode. On the basis of UV-visible (UV-vis) spectroscopy and electrochemical experiments, it is suggested that the PB molecules are located within the dendritic structure of the surface attached PAMAM dendrimers.     

A simple polyacrylamide gel electrophoresis procedure for separation of polyamidoamine dendrimers

A simple, inexpensive, and rapid electrophoresis technique was developed for use as a routine tool for evaluating purity of polyamidoamine (PAMAM) dendrimers. A variety of factors influencing migration of generations 0-7 dendrimers on nongradient polyacrylamide gels were evaluated. The low generation dendrimers were found to be very sensitive to diffusion during or after electrophoresis. The proposed method incorporates steps that minimize diffusion, in order to obtain improved resolution and sensitivity, especially for the lower-molecular-weight dendrimers. This was accomplished by inclusion of a dendrimer fixation step with glutaraldehyde and performing the electrophoresis separation, fixation, staining, and destaining at 4 degrees C. PAMAM dendrimer separation was studied under basic and acidic conditions. Electrophoresis under acidic conditions gave increased resolution and sensitivity over separation at alkaline pH. Oligomers and trailing generations could be clearly separated and visualized under these conditions. The smallest PAMAM dendrimer, generation 0, was visible at 1.5 microg under the optimized acidic conditions. With slight modifications, this technique should be applicable to separation of other water-soluble dendrimers.     

Preparation of PAMAM- and PPI-metal (silver, platinum, and palladium) nanocomposites and their catalytic activities for reduction of 4-nitrophenol

Dendrimer-metal (silver, platinum, and palladium) nanocomposites are prepared in aqueous solutions containing poly(amidoamine) (PAMAM) dendrimers with surface amino groups (generations 3, 4, and 5) or poly(propyleneimine) (PPI) dendrimers with surface amino groups (generations 2, 3, and 4). The particle sizes of the metal nanoparticles obtained are almost independent of the generation as well as the concentration of the dendrimer for both the PAMAM and the PPI dendrimers; the average sizes of silver, platinum, and palladium nanoparticles are 5.6-7.5, 1.2-1.6, and 1.6-2.0 nm, respectively. It is suggested that the dendrimer-metal nanocomposites are formed by adsorbing the dendrimers on the metal nanoparticles. Studies of the reduction reaction of 4-nitrophenol by these nanocomposites show that the rate constants are very similar between PAMAM and PPI dendrimer-silver nanocomposites, whereas the rate constants for the PPI dendrimer-platinum and -palladium nanocomposites are greater than those for the corresponding PAMAM dendrimer nanocomposites. In addition, it is found that the rate constants for the reduction of 4-nitrophenol involving all the dendrimer-metal nanocomposites decrease with an increase in the dendrimer concentrations, and the catalytic activity of dendrimer-palladium nanocomposites is highest.     

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.     

Preparation, characterization, resistance to protein adsorption, and specific avidin-biotin binding of poly(amidoamine) dendrimers functionalized with oligo(ethylene glycol) on gold

Protein-resistant films derived from the fifth-generation poly(amidoamine) dendrimers (PAMAM G5) functionalized with oligo(ethylene glycol) (OEG) derivatives consisting of various ethylene glycol units (EG(n), n = 3, 4, and 6) were prepared on the self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid (MUA) on gold substrates. The resulting films were characterized by ellipsometry, contact angle goniometry, and X-ray photoelectron spectroscopy (XPS). About 35% of the peripheral amines of the dendrimers were reacted with N-hydroxysuccinimide-terminated EG(n) derivatives (NHS-EG(n)). The dendrimer films showed improved stability over octadecanethiolate SAMs on gold in hot solvents, attributed to the formation of multiple amide bonds per PAMAM unit with underlying NHS-activated MUA monolayer. The EG(n)-attached PAMAM surfaces with n = 3 reduced the adsorption of fibrinogen to approximately 20% monolayer, whereas 2-3% for n = 4 or 6. The dendrimer films with various densities of EG(n) molecules on PAMAM surfaces were prepared by immersion of the NHS-terminated MUA-functionalized gold substrates in ethanolic solutions containing PAMAM and NHS-EG(n) of various mole ratios. The density (r) of the EG(n) molecules on the PAMAM surfaces is consistent with the mole ratio (r') of NHS-EG(n)/free amine of PAMAM in solutions. The resistance to protein adsorption of the resulting surfaces is correlated with the surface density and the length of the EG chains. At their respective r, the EG(n)-modified dendrimer films resisted approximately 95% adsorption of fibrinogen on gold surfaces. Finally, the specific binding of avidin to the approximately 5% and approximately 40% biotinylated EG3 dendrimers (surface density of biotin with respect to the total number of terminal amino groups on PAMAM G5) gave rise to about 50% and 100% surface coverage by avidin, respectively.     

CE of poly(amidoamine) succinamic acid dendrimers using a poly(vinyl alcohol)-coated capillary

Various generations (G1-G8) of negatively charged poly(amidoamine) (PAMAM) succinamic acid dendrimers (PAMAM-SAH) were analyzed by CE using a poly(vinyl alcohol)-coated capillary. Due to its excellent stability and osmotic flow-shielding effect, highly reproducible migration times were achieved for all generations of dendrimer (e.g., RSD for the migration times of G5 dendrimer was 0.6%). We also observed a reverse trend in migration times for the PAMAM-SAH dendrimers (i.e., higher generations migrated faster than lower generation dendrimers) compared to amine-terminated PAMAM dendrimers reported in the literature. This reversal in migration times was attributed to the difference in counterion binding around these negatively charged dendrimers. This reverse trend allowed a generational separation for lower generation (G1-G3) dendrimers. However, a sufficient resolution for the migration peaks of higher generations (G4-G5) in a mixture could not be achieved. This could be due to their nearly identical charge/mass ratio and dense molecular conformations. In addition, we show that dye-functionalized PAMAM-SAH dendrimers can also be analyzed with high reproducibility using this method.     

Morphological change of gold-dendrimer nanocomposites by laser irradiation

Gold-dendrimer nanocomposites are prepared in aqueous solutions in the presence of poly(amidoamine)dendrimers (PAMAM) (generation 3 and 5) or poly(propyleneimine)dendrimers (PPI) (generation 3 and 4) by wet chemical NaBH(4) method. Thus prepared gold-dendrimer nanocomposites are irradiated by laser at 532 nm. UV-vis absorption spectroscopy and transmission electron microscopy reveal that the gold nanoparticles grow with the laser irradiation time as well as the fluence of the laser; in particular, the gold nanoparticles prepared at lower concentrations of PAMAM dendrimer as well as lower generations of PAMAM grow significantly. On the other hand, in the case of PPI dendrimers, the gold nanoparticles hardly grow by irradiation. In addition, dynamic light-scattering measurements show that the laser irradiation markedly promotes the association of the gold-PAMAM G3 dendrimer nanocomposites compared to that of the gold-PAMAM G5 dendrimer nanocomposites, while the sizes of association for the gold-PPI G3, G4 dendrimer nanocomposites hardly change by laser irradiation.     

Electrochemistry of non-redox-active poly(propylenimine) and poly(amidoamine) dendrimers at liquid-liquid interfaces

The electrochemistry of a series of dendrimers was examined at the interface between two immiscible electrolyte solutions (ITIES), enabling study of non-redox-active dendrimers. Different generations of poly(propylenimine) (DAB-AM-n) and poly(amidoamine) (PAMAM) dendrimers were studied. In their protonated states, the dendrimers were transferred across the ITIES, with the electrochemical behavior observed depending on the dendrimer family, the generation number, and the experimental pH. The electrochemistry of the lower generations studied was characterized by well-defined peaks for both dendrimer families and with small peak-peak separations in the case of the PAMAM family. The voltammetry of the higher generations was more complex, showing distorted voltammograms and instability of the interface. The charges of the transferring dendrimers were calculated by convolution of the voltammetric data and were similar to the theoretical charges for DAB-AM-n. For PAMAM, only the lowest generation exhibited reversible behavior, with higher generations having irreversible behavior. Using cyclic voltammetry, low micromolar concentrations of the dendrimers were detected. The results show that electrochemistry at the ITIES can be a useful method for characterization of ionizable dendrimers and that voltammetry can be a simple method for detection of low concentrations of these multicharged species.     

Analysis of poly(amidoamine)-succinamic acid dendrimers by slab-gel electrophoresis and capillary zone electrophoresis

Ethylenediamine (EDA)-core poly(amidoamine) (PAMAM) succinamic acid dendrimers (Ex.SAH, where x refers to the generation) were synthesized and analyzed by polyacrylamide gel electrophoresis (PAGE), size-exclusion chromatography (SEC), potentiometric acid-base titration, and capillary zone electrophoresis (CZE). Various generations (E1.SAH-E7.SAH) PAMAMs and a succinamic acid terminated core-shell tecto(dendrimer) (E5(E3.SAH)(n)) were first analyzed by PAGE. PAGE results show that the relative mobilities of generation 2 to generation 7 dendrimers decreased with the increasing number of generations. The molecular mass of a generation 5 core generation 3 shell tecto(dendrimer) (denoted as E5(E3.SAH)(n)) was determined to be between the Mw of E6.SAH and E7.SAH. CZE analysis allowed the evaluation of electrophoretic properties of given-generation dendrimers. The electrophoretic mobilities of individual generations PAMAM polyanions are similar, indicating that the separation mainly depends on their approximately identical charge/mass ratio. The E5(E3.SAH)(n) tectodendrimer had a lower electrophoretic mobility, which was consistent with its lower charge/mass ratio. The combination of PAGE and CZE analysis provides an alternative and effective way to characterize this group of PAMAM-succinamic acid dendrimers.     

Interaction forces between poly(amidoamine) (PAMAM) dendrimers adsorbed on gold surfaces

Interaction forces between two gold surfaces with adsorbed poly(amidoamine) (PAMAM) dendrimers (generations G3.0 and G5.0) have been investigated using colloidal probe atomic force microscopy (AFM). In the absence of dendrimers or at their low concentrations, an attractive force derived from the van der Waals interaction was observed. On the other hand, this attractive interaction changed to repulsion with increasing dendrimer concentration. The origin of the repulsion can be attributed to either an electric double layer interaction or a steric effect of the adsorbed dendrimers, depending on the concentration of dendrimer. The steric hindrance was also influenced by the generation of the dendrimer; the force-detectable distance in the presence of PAMAM G5.0 dendrimer was slightly longer than that in the presence of G3.0 dendrimer. In order to estimate the occupied area of each dendrimer adsorbed on gold, quartz crystal microbalance (QCM) measurement was also carried out.     

Aggregation‐Controlled Excimer Emission from Anthracene‐Containing Polyamidoamine Dendrimers

Lower generations of polyamidoamine (PAMAM) dendrimers were peripherally modified with anthracene moieties, and excimer emission from anthracene chromophores was investigated in an acetonitrile–water mixture at acidic and basic pH values. Results from fluorescence spectroscopic experiments suggest that 1) the propensity of anthracene‐modified PAMAM dendrimers to aggregate in acetonitrile is substantial in the presence of 15–20 vol % of water, and 2) aggregate formation in anthracene‐modified PAMAM dendrimers leads to unique morphologies in the ground state, where the anthracene units are pre‐arranged to form stable excimers upon photoexcitation. Three types of anthracene excimers are generated in the system, with face‐to‐face, angular, and T‐shaped geometry. The formation of different types of anthracene excimers was confirmed by steady‐state and time‐resolved fluorescence spectroscopic experiments. Experimental results further suggest that it is feasible to alter the type of excimer formed by anthracene units attached to the PAMAM dendrimers through altering the propensity for ground‐state aggregation. Most excitingly, increased π conjugation in the molecular framework of anthracene‐substituted PAMAM dendrimers leads to intense and exclusive excimer emission from anthracene at room temperature.     

Amperometric Detection of H2O2 Using Gold Electrodes Modified with Starburst PAMAM Dendrimers and Prussian Blue

Gold‐bead electrodes were modified by covalent bonding or physical adsorption of several Starburst PAMAM dendrimers (generations 2.0, 3.0 and 4.0) followed by absorption of Prussian Blue (PB). The covalent dendrimer‐PB‐modified electrodes can be used as amperometric sensors of H₂O₂ in aqueous solution. They offer enhanced sensitivity with correspondingly lower detection and quantification limits compared to similar amperometric detectors.     

Study on elution behavior of poly(amidoamine) dendrimers and their interaction with bovine serum albumin in asymmetrical flow field-flow fractionation

Polyamidoamine (PAMAM) dendrimers have an amine surface and an ethylenediamine core and are of great interest in various applications such as in drug delivery. Physiochemical properties of PAMAM dendrimers vary with pH. At neutral to basic pH, PAMAM dendrimers are either weakly charged or uncharged and tend to adsorb on to the neutral packing material, making chromatographic separation of the dendrimers difficult. Asymmetrical flow field-flow fractionation (AsFlFFF) was tested as an alternative to the chromatographic techniques for separation of the PAMAM dendrimers. AsFlFFF provided generation-based separation of the dendrimers even at neutral and basic pH. The elution time increased gradually as the generation number (and thus the size) increased. Separation of impurities such as generational or missing-arm impurities and aggregates from the main population was also achieved. Electrostatic and hydrophobic interactions (e.g., repulsive elecrostatic interaction among the dendrimer molecules or attractive hydrophobic interaction between the dendrimer molecules and the membrane) may result in an inaccurate size measurement. Careful optimization of experimental conditions such as the flow rate, pH, and the salt concentration may be required to minimize the interactions with the membrane. AsFlFFF was also tested for a study on the interaction between the PAMAM dendrimers and proteins. AsFlFFF was able to show the growth in the size of bovine serum albumin (BSA) when BSA is mixed with increasing amounts of PAMAM dendrimers. Results suggest that, with proper optimization, AsFlFFF could become a useful tool for separation and characterization of large charged molecules such as PAMAM dendrimers.     

Self‐assembly of a new class of amphiphilic poly(amidoamine) dendrimers and their electrochemical properties

Amphiphilic poly(amidoamine) (PAMAM) dendrimers consisting of a hydrophilic dendrimer core and hydrophobic aromatic dansyl or 1‐(naphthalenyl)‐2‐phenyldiazene (NPD) shells have been synthesized. These amphiphilic dendrimers from the zero generation to the third generation self‐assemble into vesicular aggregates in water. The self‐assembly behavior of these dendrimers strongly depends on their generations. The generation dependence has been further investigated by an exploration of their electrochemical properties. For the PAMAM–NPD aggregates, the photoisomerization process leads to a change in the aggregate size. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5512–5519, 2005     

Dendritic chelating agents. 1. Cu(II) binding to ethylene diamine core poly(amidoamine) dendrimers in aqueous solutions

This paper describes an investigation of the uptake of Cu(II) by poly(amidoamine) (PAMAM) dendrimers with an ethylenediamine (EDA) core in aqueous solutions. We use bench scale measurements of proton and metal ion binding to assess the effects of (i) metal ion-dendrimer loading, (ii) dendrimer generation/terminal group chemistry, and (iii) solution pH on the extent of binding of Cu(II) in aqueous solutions of EDA core PAMAM dendrimers with primary amine, succinamic acid, glycidol, and acetamide terminal groups. We employ extended X-ray absorption fine structure (EXAFS) spectroscopy to probe the structures of Cu(II) complexes with Gx-NH2 EDA core PAMAM dendrimers in aqueous solutions at pH 7.0. The overall results of the proton and metal ion binding measurements suggest that the uptake of Cu(II) by EDA core PAMAM dendrimers involves both the dendrimer tertiary amine and terminal groups. However, the extents of protonation of these groups control the ability of the dendrimers to bind Cu(II). Analysis of the EXAFS spectra suggests that Cu(II) forms octahedral complexes involving the tertiary amine groups of Gx-NH2 EDA core PAMAM dendrimers at pH 7.0. The central Cu(II) metal ion of each of these complexes appears to be coordinated to 2-4 dendrimer tertiary amine groups located in the equatorial plane and 2 axial water molecules. Finally, we combine the results of our experiments with literature data to formulate and evaluate a phenomenological model of Cu(II) uptake by Gx-NH2 PAMAM dendrimers in aqueous solutions. At low metal ion-dendrimer loadings, the model provides a good fit of the measured extent of binding of Cu(II) in aqueous solutions of G4-NH2 and G5-NH2 PAMAM dendrimers at pH 7.0.     

PAMAM树形分子与Cd2＋的配位作用研究及CdS/PAMAM纳米复合材料的制备与表征

CdS半导体纳米簇具有独特的光、电性能, 如何制备均匀分散的、能够稳定存在的CdS纳米簇是目前的研究热点之一. 以聚酰胺-胺(PAMAM)树形分子为模板, 原位合成了CdS纳米簇. 首先用UV-Vis分光光度法研究了与树形分子的配位机理, 得出G4.5和G5.0的平均饱和配位数分别为16和34, 并发现在G4.5PAMAM树形分子中Cd^2＋主要与最外层叔胺基配位, 在G5.0PAMAM树形分子中Cd^2＋主要与最外层伯胺基配位. 酯端基的G4.5的模板作用要明显优于胺端基的G5.0. 通过改变Cd^2＋与G4.5树形分子的摩尔比可以得到不同粒径的CdS纳米簇. 溶液的pH值对CdS纳米簇影响很大, pH在7.0左右制备的CdS纳米簇粒径小而均匀, 且溶液稳定性高. 用UV-Vis分光光度计和TEM对CdS纳米簇的大小和形貌进行了表征. 结果表明TEM观测CdS纳米簇的粒径要大于用Brus公式的估算值.     

A structural study of amphiphilic PAMAM (poly(amido amine)) dendrimers in Langmuir and Langmuir-Blodgett films

Two amphiphilic PAMAM dendrimers are synthesized by attaching 12-hydroxydodecanoic acid (HA) chains to a poly(amido amine) (PAMAM) dendrimer core (including generation I and generation II). The limiting molecular area obtained from the surface pressure-area isotherm at the air/water interface suggests the edge-on configuration for both dendrimers in Langmuir films. The edge-on arrangement is also supported by the atomic force microscopic (AFM) studies of the Langmuir-Blodgett films.     

环糊精修饰聚酰胺-胺树状高分子的合成及其与乳酸左氧氟沙星的作用研究

合成了一系列以环糊精修饰的树状高分子化合物PAMAM(G2,G4)-β-CD,用IR,1H-NMR等手段表征了其结构,并采用荧光光谱法对其在缓冲溶液中与乳酸左氧氟沙星(LFL)的相互作用进行了研究.结果表明,经环糊精修饰树状高分子的增敏率远大于未修饰的和天然环糊精,且随代数和环糊精含量的增加而增大,表明其具有强于相同代数PAMAM的分子键合能力,这些强的键合能力源于环糊精修饰树状高分子化合物中两种结构单元的疏水作用、静电作用和氢键作用的协同效应.     

Controlled interparticle spacing through self-assembly of Au nanoparticles and poly(amidoamine) dendrimers

Control of particle-particle spacing is a key determinant of optical, electronic, and magnetic properties of nanocomposite materials. We have used poly(amidoamine) (PAMAM) dendrimers to assemble carboxylic acid-functionalized mixed monolayer protected clusters (MMPCs) through acid/base chemistry between the particle and dendrimer. Small angle X-ray scattering was then used to establish average inter-MMPC distances. Five generations of PAMAM dendrimer (0, 1, 2, 4, 6) were investigated, with a monotonic increase in interparticle spacing from 4.1 to 6.1 nm observed with increasing generation.