Template-free parallel one-dimensional assembly of gold nanoparticles

In this work, we have identified key process parameters to generate parallel unidirectional 1D assemblies of gold nanoparticles with the assistance of organic surfactants. By controlling the surfactant population, metal particle size, and amount of solvent for dispersion, the length of nanoparticle chains and their interchain space can be further tailored. In principle, the general findings of this work can also be extended to large-scale 1D organization of other transition/noble metal nanoparticles using simple organic surfactants.     

Preparation and characterization of poly(acrylic acid)-based nanoparticles

The present investigation describes the synthesis and characterization of nanoparticles based on poly(acrylic acid) (PAA) intramolecularly cross-linked with diamine, 2,2′-(ethylenedioxy)bis(ethylamine), using water-soluble carbodiimide. The aqueous colloid dispersions of nanoparticles were clear or mildly opalescent depending on the ratio of cross-linking, pH of the solution, and the molecular weight of PAA, finding consistent with values of transmittance between 3% and 99%. The structure was determined by nuclear magnetic resonance spectroscopy, and the particle size was identified by dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements. It was found that particle size depends on the pH, and at a given pH, it was caused by the ratio of cross-linking and the molecular weight of PAA. Particle size measured by TEM varied in the range of 20 and 80 nm. In the swollen state, the average size of the particles measured by DLS was in the range of 35–160 nm.     

Self-assembly of gold nanoparticles induced by poly(oxypropylene)diamines

A series of poly(oxypropylene)diamines D230, D400, D2000, and D4000, having molecular weights of hydrophobic segments of 230, 400, 2000, and 4000, were used as ligands to synthesize self-organized gold nanocrystals. Ligand exchange significantly reduced the average particle size and the polydispersity of nanocrystals, and this effect was more remarkable as the molar ratio of amine groups to Au3+ ions ([N]/[Au3+] ratio) was increased. Under the same [N]/[Au3+] ratio of 100, D2000 generated an ordered 2D-monolayer; however, D230 and D400 colloids formed mainly a densely packed 3D structure with minor 2-D layers, and D4000 presented disordered 3D and 2D structures. The gap among the nanoparticles was found to be increased with the increasing molecular weight of the hydrophobic segment of ligands, accompanied by the decreasing wavelength of UV-vis absorption bands. This increased gap can be interpreted as the ligand thickness calculated from the equation of steric force increasing with increasing molecular weight of the hydrophobic segment. The potential energies obtained from the calculated ligand thickness according to the soft sphere model show more steep potential wells for D230 and D400 colloids than that for the D2000 colloid. This explains why the aggregation hardly occurred for the gold nanoparticles obtained under D2000, where the nanoparticles are single crystals having face center crystal structure with a lattice constant of 2.36 A and have grain sizes close to the average particle sizes, evidenced from the results of transmission electron microscopy and X-ray diffraction spectroscopy.     

Polyelectrolyte complex nanoparticles from chitosan and poly(acrylic acid) and Polystyrene‐block‐poly(acrylic acid)

Interpolymer polyelectrolyte complexes of chitosan (CS) with poly(acrylic acid) homopolymers and polystyrene‐block‐poly(acrylic acid) diblock copolymers were prepared and characterized. The influence of the positive/negative charge balance (charge ratio), pH, and ionic strength were thoroughly studied by dynamic light scattering. The existence of a strong polyelectrolyte effect was also highlighted in this study. Domains of stability, in which nanoparticle sizes are smaller than 100 and 200 nm for complexes of CS with the homopolymer and copolymer, respectively, were identified and confirmed by scanning electron microscopy and atomic force microscopy. The charged nature of the surface of the nanoparticles was evidenced by Zeta potential measurements. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Hybridization chain reaction triggered controllable one-dimensional assembly of gold nanoparticles

Assembling and ordering nanomaterials into desirable patterns are considerably significant, since the properties of nanomaterials depend not only on the size and shape, but also on the spatial arrangement among the collective building blocks. In this work, the DNA self-assembly technology of hybridization chain reaction (HCR) provided a convenient method to yield long double-strand DNA (dsDNA) to install gold nanoparticles (AuNPs) into one dimensional assembly along the skeleton of dsDNA. Interestingly, the tunable length of AuNPs assemblies along dsDNA chain could be achieved by adjusting the reaction time of HCR, which is based on the formation of covalent bond between Au and the -SH group of DNA. Compared with weak light scattering of single AuNP, these AuNPs assemblies could be clearly imaged under the dark field microscopy, indicating that the light scattering was greatly improved after assembling.     

Folding induced assembly of polypeptide decorated gold nanoparticles

Reversible assembly of gold nanoparticles controlled by the homodimerization and folding of an immobilized de novo designed synthetic polypeptide is described. In solution at neutral pH, the polypeptide folds into a helix-loop-helix four-helix bundle in the presence of zinc ions. When immobilized on gold nanoparticles, the addition of zinc ions induces dimerization and folding between peptide monomers located on separate particles, resulting in rapid particle aggregation. The particles can be completely redispersed by removal of the zinc ions from the peptide upon addition of EDTA. Calcium ions, which do not induce folding in solution, have no effect on the stability of the peptide decorated particles. The contribution from folding on particle assembly was further determined utilizing a reference peptide with the same primary sequence but containing both D and L amino acids. Particles functionalized with the reference peptide do not aggregate, as the peptides are unable to fold. The two peptides, linked to the nanoparticle surface via a cysteine residue located in the loop region, form submonolayers on planar gold with comparable properties regarding surface density, orientation, and ability to interact with zinc ions. These results demonstrate that nanoparticle assembly can be induced, controlled, and to some extent tuned, by exploiting specific molecular interactions involved in polypeptide folding.     

Pseudotemplate synthesis of copper nanoparticles in solutions of poly(acrylic acid)-pluronic blends

It was found that the reduction of copper(II) ions in solutions of poly(acrylic acid)-pluronic blends results in a stable sol of metallic copper with a particle size below 10 nm, whereas a less stable sol with coarse aggregates of particles is formed in the presence of poly(acrylic acid) alone and an insoluble complex of this polymer with copper nanoparticles is produced in the presence of pluronic alone. The addition of poly(acrylic acid) to the complex causes the transfer of a portion of nanoparticles from the precipitate into the sol. In mixed poly(acrylic acid) and pluronic solutions, no formation of a polymeric complex with reasonable stability was detected. It was assumed that such a polycomplex is stabilized in the presence of copper nanoparticles. Owing to its amphiphilic nature, the complex forms stable protective shields on the surface of nanoparticles, and the stability of the sol is determined by free fragments of poly(acrylic acid).     

The preparation of poly(N-acetyl-α-amino acrylic acid) [poly(N-acetyl dehydroalanine)] and poly(α-amino acrylic acid) (polydehydroalanine)

Poly(N-acetyl-α-amino acrylic acid) was prepared by a free radical polymerization reaction. Mild alkaline hydrolysis of the polymer product yielded a second polymer poly(α-amino acrylic acid) (polydehydroalanine). Both polymers exhibited certain polyelectrolyte behavior, although the latter did not behave as expected for an amphoteric polyelectrolyte.     

Precipitation-redispersion of cerium oxide nanoparticles with poly(acrylic acid): toward stable dispersions

We exploit a precipitation-redispersion mechanism for complexation of short chain polyelectrolytes with cerium oxide nanoparticles to extend their stability ranges. As synthesized, cerium oxide sols at pH 1.4 consist of monodisperse cationic nanocrystalline particles having a hydrodynamic diameter of 10 nm and a molecular weight of 400 000 g mol(-1). We show that short chain uncharged poly(acrylic acid) at low pH when added to a cerium oxide sols leads to macroscopic precipitation. As the pH is increased, the solution spontaneously redisperses into a clear solution of single particles with an anionic poly(acrylic acid) corona. The structure and dynamics of cerium oxide nanosols and their hybrid polymer-inorganic complexes in solution are investigated by static and dynamic light scattering, X-ray scattering, and chemical analysis. Quantitative analysis of the redispersed sol gives rise to an estimate of 40-50 polymer chains per particle for stable suspension. This amount represents 20% of the mass of the polymer-nanoparticle complexes. This complexation adds utility to the otherwise unstable cerium oxide dispersions by extending the range of stability of the sols in terms of pH, ionic strength, and concentration.     

Immobilization of urease in poly(1-vinyl imidazole)/poly(acrylic acid) network

In this study, urease was immobilized in a polymer network obtained by complexation of poly(1-vinyl imidazole) (PVI) with poly(acrylic acid) (PAA). Preparation of the polymer network was monitored by FT-IR spectroscopy. Scanning electron microscopy (SEM) revealed that enzyme immobilization had a strong effect on film morphology. Proton conductivity of the PVI/PAA network was measured via impedance spectroscopy under humidified conditions. Values of the Michaelis-Menten constant (K _M) for immobilized urease were higher than for the free enzyme, indicating a decreased affinity of the enzyme to its substrate. The basic characteristics (pH_opt, pH_stability, T _opt, T _stability, reusability, and storage stability) of immobilized urease were determined. The results show that the PAA/PVI polymer network is suitable for enzyme immobilization.     

Swelling of ferrogels of poly(acrylic acid) with ferric oxide nanoparticles stabilized by chitosan

The processes of swelling of poly(acrylic acid) ferrogels prepared via radical polymerization in an aqueous suspension of ferric oxide nanoparticles with the weighted average size of 23.5 nm obtained by laser evaporation method and stabilized by chitosan (М = 5.3 × 10⁵ and degree of deacetylation of 62%) are studied. The swelling of washed ferrogels depends on the content of chitosan and decreases abruptly at a polymer concentration exceeding 1 g/L. At a chitosan concentration above 1 g/L, the chemical network of poly(acrylic acid) is formed on the fluctuation network of chitosan in solution. As pH increases, these ferrogels are contracted owing to formation of an interpolymer complex of chitosan with poly(acrylic acid) subchains.     

Synthesis of dense poly(acrylic acid) brushes and their interaction with amine-functional silsesquioxane nanoparticles

Poly(acrylic acid) polyelectrolyte brushes were synthesized by surface-initiated atom transfer radical polymerization (SI-ATRP) of tert-butyl acrylate on planar gold surfaces and subsequent hydrolysis. Three types of monolayers with different numbers of thiol binding sites per initiating unit were used. The binding strength to the gold surface turned out to be of crucial importance for the formation of uniform brush layers after acidic hydrolysis. The monolayers and polymer brushes were characterized by ellipsometry, infrared spectroscopy, water contact angle measurements, atomic force microscopy, and X-ray photoelectron spectroscopy. Their interaction with [(diglycidylamino)propyl]silsesquioxane nanoparticles at various pH values was studied by surface plasmon resonance.     

Calorimetric investigation of poly(methacrylic acid) and poly(acrylic acid) in aqueous solution

The enthalpy of dissociation of poly(acrylic acid) and of poly(methacrylic acid) in water and in 0.5N NaCl at 25°C has been measured over a wide range of degrees of neutralization of the polyelectrolytes. In the case of poly(methacrylic acid) the calorimetric data permit the direct evaluation of the enthalpy of conformational transition of the polymer. For this transition, with the aid of standard free energy data derived from potentiometric titrations, the change in entropy was also estimated. The relative accuracy of the thermodynamic data, and the possibility of deriving therefrom information on the mechanism of transitions of the type, globular coils → expanded coils for partially hydrophobic synthetic polyelectrolytes in aqueous solution are discussed.     

The synthesis and characterization of monodisperse poly(acrylic acid) and poly(methacrylic acid)

A number of polyacrylic (PAA) and polymethacrylic (PMAA) acids have been synthesized by living anionic polymerization of the monomeric tert-butyl esters followed by subsequent hydrolysis of the corresponding polyesters. The necessary precautions were taken in order to assure good molecular weight control, as well as high yields in the polymerization reactions. The intermediate and final polymers were characterized by gel permeation chromatography and NMR-H¹ spectrometry.     

Effect of urea on poly(methacrylic acid) and poly(acrylic acid) aqueous solutions

The effects of urea on aqueous solutions of both poly(methacrylic acid (PMA) and poly(acrylic acid) (PAA) have been investigated by using potentiometry, viscometry and study of the fluorescence of Auramine O, a cationic dye. The viscosity behaviour of unionized PMA obtained from direct dissolution of solid powder shows that the unneutralized macromolecules can be associated in water. The stability of such “aggregates” seems weak as indicated by their disappearance as soon as the charge density is very low. For PMA salt solution percolated through a cation (H⁺) exchange resin column, no association is observed. The pH-dependent conformational behaviour of PMA which, contrary to PAA, presents compact conformations in water at low charge density is discussed in terms of solvophobic/solvophilic interactions. It is shown that, even for urea concentration up to 8 M, the compact conformations of PMA are not completely destroyed. The formation of H⁺/urea complex is taken into account.     

Binding of a surfactant counterion to low-charge-density poly(acrylic acid) and poly(methacrylic acid)

The binding of a cationic surfactant, dodecylpyridinium (C12Py) chloride, with a low-charge-density poly (methacrylic acid) (PMA) was investigated in buffer solutions under the condition of constant pH. The binding isotherms with PMA consisted of two and three steps at a pH lower and higher than 3.2, respectively. Bindings in the first step were independent of pH and this step was considered to correspond to the solubilization of the hydrocarbon chains of C12Py into the nonpolar region of the compact form of PMA. This is the indication of the compact form from the binding isotherm. At pH higher than 3.2, the second step was discriminated and it depended on the pH. In the third step, a sharp rise in the degree of binding (β) was observed accompanying the solubilization of the precipitates of the PMA–C12Py complex. The binding with poly(acrylic acid) (PAA) and PMA in conventional unbuffered NaCl solutions was also examined and the pH profile of the solution during the binding process was determined. In the case of unbuffered NaCl solutions, the binding with PAA took place cooperatively at the critical association concentration (cac). The binding isotherm consisted of two steps and the pH decreased with the increase in β. The binding isotherm of PMA, on the other hand, consisted of three steps: the pH decreased slightly in the first step and considerably in the second step with the increase in β but it increased with β in the third step, exhibiting a pH minimum around 3.2. The binding in the first step coincided with that obtained in the buffered solutions. Linear relationships between β and the pH were found for both polymers. In the case of PMA, no cac was observed in both buffered and unbuffered NaCl solutions. Received: 24 January 2001 Accepted: 23 May 2001     

Adhesion of polyether-modified poly(acrylic acid) to mucin

Pluronic-PAA, a thermogelling copolymer composed of side chains of poly(acrylic acid) (PAA) grafted onto a backbone of Pluronic copolymer, is of interest as a vehicle for the controlled release of compounds. An important feature of such a vehicle is its bioadhesive/mucoadhesive properties, which in the case of Pluronic-PAA are significant due to the presence of the PAA side chains. An atomic force microscopy (AFM) method has been developed and utilized to investigate the interactions between a Pluronic-PAA-modified microsphere and mucous substrates. The bioadhesive force was successfully measured, and trends were observed under conditions of varying pH and ionic strength. Pluronic-PAA exhibits significant mucoadhesion over a range of pH values, with mucoadhesion being optimal at pH 4-5 (adhesive force approximately 80 mN/cm(2)) and dropping sharply at higher pH, to a value of approximately 20 mN/cm(2) at pH 8. The mucoadhesive force decreased with increasing ionic strength, from a value of approximately 80 mN/cm(2) in 0.025 M NaCl to approximately 25 mN/cm(2) in 1.0 M NaCl. These results have been interpreted in terms of the effect of changing pH and ionic strength on electrostatic interactions and swelling of the polymer and mucin layers. Tensiometric force measurements indicated that hydrophobic interactions, as well as hydrogen bonding and electrostatic interactions, were significant in the mucoadhesion of Pluronic-PAA copolymers. Experiments with a range of Pluronic-PAA copolymers with varying PPO contents in the Pluronic segments showed that increasing the overall PPO content increased the hydrophobicity of the polymer solutions. This was reflected in the increases in the advancing contact angles with the mucin layer, indicating that hydrophobic interactions play a role in the adhesion of Pluronic-PAA to mucin.     

Oxidative and photooxidative degradation of poly(acrylic acid)

The oxidative degradation of poly(acrylic acid) (PAA), a water soluble polymer, was studied at various temperatures with different concentrations of persulfates, potassium persulfate (KPS), ammonium persulfate (APS) and sodium persulfate (SPS). The photodegradation of PAA was also examined with APS as oxidizer. The degraded samples were analyzed for the time evolution of molecular weight distribution by gel permeation chromatography. A theoretical model based on the continuous distribution kinetics was developed that accounted for the polymer degradation and the dissociation of persulfate. The rate coefficients for the oxidative and photooxidative degradation of PAA were determined from the parametric fit of the model with experimental data. The rate of degradation increased with increasing amount of persulfate in both oxidative and photooxidative degradation. The rate of degradation also increased with increasing temperature in the case of oxidative degradation.     

Thermodynamics of temperature-sensitive polyether-modified poly(acrylic acid) microgels

The temperature-induced structural changes and thermodynamics of ionic microgels based on poly(acrylic acid) (PAA) networks bonded with poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) (Pluronic) copolymers have been studied by small-angle neutron scattering (SANS), ultra-small-angle neutron scattering (USANS), differential scanning calorimetry (DSC), and equilibrium swelling techniques. Aggregation within microgels based on PAA and either the hydrophobic Pluronic L92 (average composition, EO8PO52EO8; PPO content, 80%) or the hydrophilic Pluronic F127 (average composition, EO99PO67EO99; PPO content, 30%) was studied and compared to that in the solutions of the parent Pluronic. The neutron scattering results indicate the formation of micelle-like aggregates within the F127-based microgel particles, while the L92-based microgels formed fractal structures of dense nanoparticles. The microgels exhibit thermodynamically favorable volume phase transitions within certain temperature ranges due to reversible aggregation of the PPO chains, which occurs because of hydrophobic associations. The values of the apparent standard enthalpy of aggregation in the microgel suspensions indicate aggregation of hydrophobic clusters that are more hydrophobic than the un-cross-linked PPO chains in the Pluronic. Differences in the PPO content in Pluronics L92 and F127 result in a higher hydrophobicity of the resulting L92-PAA-EGDMAmicrogels and a larger presence of hydrophobic, densely cross-linked clusters that aggregate into supramolecular structures rather than micelle-like aggregates such as those formed in the F127-PAA-EGDMA microgels.