Effect of pH on complexation in mixed dilute aqueous solutions of poly(acrylic acid), poly(ethylene glycol), and Cu2+ ions

The phase states of mixed dilute solutions of PAA, PEG, and Cu²⁺ ions largely determines the mechanism governing the growth of metal nanoparticles during the subsequent reduction of copper ions. Mixtures with PAA: PEG > 1 base-mol/base-mol and PAA: Cu²⁺ ≥ 5 base-mol/mol are studied. It is shown that the simultaneous complexation of PAA with PEG and Cu²⁺ ions in these mixtures at pH values below the intrinsic pH of a solution is accompanied by phase separation related to insolubility of PAA-PEG interpolymer complexes. A decrease in the pH of the ternary mixture is caused by the release of a strong low-molecular-mass acid due to complexation with Cu²⁺ ions. The minimum pH value, above which the PAA-PEG-Cu²⁺ system becomes single-phase (a transparent solution), depends on the concentration ratio between PAA and PEG chains (the mean degree of polymerization). This value is either 6.8–7.0 (if all macromolecules are incorporated in the insoluble interpolymer complex with PEG) or 4.0 (if chains occur in excess). Methods of preparing single-phase systems in the pH range 4.0–7.0 via exchange reactions of the PAA-Cu²⁺ complex with PEG or the nonstoichiometric soluble interpolymer complex PAA-PEG are developed. Viscometry, electron microscopy, and dynamic light scattering are used to investigate the compositions and structures of soluble complexes, in which either each chain (if the chain is long) may be linked with both PEG and Cu²⁺ ions or PAA chains are redistributed between two complexes (at comparable lengths of PAA and PEG chains).     

Formation of copper nanoparticles during the reduction of Cu2+ ions in solutions and dispersions of polycation-poly(acrylic acid) interpolymer complexes in acidic media

The phase behavior of mixed acidic solutions of the high-molecular-mass cationic polyelectrolyte poly(1,2-dimethyl-5-vinylpyridinium methyl sulfate) and PAA at a PAA-to-polycation ratio of <1 base-mol/base-mol in narrow pH ranges at 6–30°C is studied. The reduction of Cu²⁺ ions in solutions and dispersions of interpolyelectrolyte complexes yields copper nanoparticles. It is shown that the mean-square diameters of nanoparticles formed in solutions of nonstoichiometric polyelectrolyte complexes are much smaller than those obtained in solutions of each component of the polyelectrolyte complex under the same conditions. These values decrease as the thermodynamic quality of the solvent with respect to the polymer of the pseudomatrix nonstoichiometric polyelectrolyte complex worsens. The experimental data may be explained in terms of the theoretical concept of pseudomatrix formation of a new phase in polymer solutions.     

Mutual enhancement of the complexing properties of components in tertiary systems containing copper nanoparticles, poly(acrylic acid), and poly(ethylene glycol)

It is shown that nonstoichiometric interpolymer complexes composed of high-molecular-mass poly( acrylic acid) and PEG of various molecular masses are more efficient stabilizers of copper sols than each component of the complex taken separately. This conclusion is based on comparison of dimensions of copper nanoparticles in sols formed via reduction of copper(II) ions in solutions of poly (acrylic acid), PEG, and their blends and on the enhanced stability of sols protected by the interpolymer complex against aggregation and oxidation of metal particles. Much shorter PEG chains than those necessary for formation of corresponding interpolymer complexes in the absence of nanoparticles can be involved in formation of tertiary complexes including copper nanoparticles, poly(acrylic acid), and PEG. On the basis of the experimental data, it is inferred that the mutual enhancement of the complexing behavior of components occurs in tertiary complexes containing copper nanoparticles and both polymers.     

Effect of complexation of poly(acrylic acid) with Cu2+ ions on the size of copper nanoparticles prepared via reduction in aqueous solutions

Sols of zero-valence copper are prepared via the chemical reduction of Cu(II) ions by hydrazine borane in aqueous solutions of high-molecular-mass poly(acrylic acid), which forms stable complexes with copper ions at 20°C in a wide pH range. The study of the composition of coordination centers, the ligand surrounding of metal ions, and the character of distribution of copper ions over poly(acrylic acid) coils in a wide range of solution compositions and pH values shows that the size of copper nanoparticles in the sols can be controlled by varying the ratio between ligand groups (carboxylate anions in poly(acrylic acid)) and copper ions in the reaction system during the synthesis of sols. This effect can be accomplished either by variation in the initial composition of solution or change in pH (the degree of ionization of the initial poly(acrylic acid) in the presence of copper ions).     

Copper sols stabilized by poly(ethylene glycol-600-monolaurate) and its complexes with poly(acrylic acid)

Copper sols stabilized by a polymer-colloid complex are studied via dynamic light scattering and transmission electron microscopy. It is shown that the polymer-colloid complex including poly(acrylic acid) and the nonionogenic polymeric surfactant poly(ethylene glycol-600-monolaurate) is an effective protector of copper nanoparticles formed via the reduction of Cu²⁺ ions in an aqueous medium. The sizes of sol particles of the nanocomposite consisting of the polymer-colloid complex and copper nanoparticles depend on the method of preparation of the nanocomposite. The incorporation of the copper nanoparticles being formed (an average diameter of 5 nm) into particles of the polymer-colloid complex leads to an insignificant change in the sizes of the complex particles. The same sizes are typical for particles of the nanocomposite formed during the introduction of surfactant micelles in the copper sol formed in the solution of poly(acrylic acid). The interaction of copper nanoparticles formed in an aqueous medium with surfactant micelles entails their aggregation; as a result, these nanoparticles turn out to be incorporated into large aggregates with equivalent radii of up to 100 nm. When poly(acrylic acid) is incorporated into this sol, the sizes of its particles insignificantly change apparently because of the low rate of structural rearrangements accompanying the formation of the polymer-colloid complex.     

Formation of copper sols via reduction of Cu2+ ions in solutions of cationic and anionic polyelectrolytes

Zero-valence copper sols are prepared at 20°C via the chemical reduction of Cu(II) ions in aqueous solutions of high-molecular-mass cationic and anionic polyelectrolytes [(poly(1,2-dimethyl-5-vinylpyridium methyl sulfate) and poly(sodium styrenesulfonate), respectively]. In both sols, metal nanoparticles are characterized by narrow size distribution, indicating the pseudomatrix mechanism of their formation; however, the diameter of spherical copper particles formed in the polycation solution (3–14 nm) is much smaller than that of particles formed in the solution of polyanion (10–30 nm). Causes of different sizes of metal nano-particles formed in solutions of polyelectrolytes with different chain charges are discussed in terms of the pseudomatrix mechanism of new phase synthesis in polymer solutions and classical electrocapillary theory.     

Why the size of copper nanoparticles depends on the nature of the reducing agent in the preparation of sols in a cationic polyelectrolyte solution

The influence of different borane reducing agents (hydrazine borane and sodium borohydride) on the size of copper nanoparticles in sols prepared at 20°C through the reduction of Cu(II) ions in diluted aqueous solutions of high-molecular-mass poly(1,2-dimethyl-5-vinylpyridinium methyl sulfate) is studied. When hydrazine borane is used as a reducing agent, the nanoparticle size (3–14 nm in diameter) is independent of the solution pH. In the case of sodium borohydride, the transition from a highly acidic (pH 2.0) solution to solution with pH 4.5–8.5 during sol synthesis leads to a shift in the size distribution of nanoparticles toward larger diameters (from 3–14 to 10–20 nm). During the reduction of Cu(II) ions with sodium borohydride, the polymer-analogous reaction, which includes the reduction of pyridine rings of the polymer, proceeds simultaneously with the main process. The as-modified polymer is a weak polybase and, therefore, is protonated to a high extent via nitrogen atoms only in a strongly acidic solution. As pH is increased in the range 4.5–8.5, the positive charge of chains is decreased. The relationship between the nanoparticle size and variation in the positive charge of macromolecules is discussed in terms of the concept of the pseudomatrix synthesis of polymer-metal nanocomposite sols.     

Specific features of nonstoichiometric interpolymer complexes of poly(acrylic acid) and poly(ethylene glycol) as protectors of copper nanoparticles in aqueous sols

Advantages of interpolymer complexes for use as amphiphilic protectors of nanoparticles during the formation and stabilization of sols are considered. The effects of the ratio of poly(acrylic acid) and poly(ethylene glycol) and the molecular mass of poly(ethylene glycol) on the mean size and size distribution of copper nanoparticles in sols formed via the reduction of divalent copper ions in mixed aqueous solutions of these polymers are investigated. It is shown that sols of metal nanoparticles with small sizes and narrow size distributions are formed even when poly(ethylene glycols) with chain lengths below the “critical” chain length and a small PEG-to-PAA base-molar ratio are used. This is evidence for efficient protection of the formed copper nanoparticles by the interpolymer complex PEG-PAA under conditions of its instability and for self-organization of oligomeric PEG chains in complex macromolecular shields of nanoparticles.     

Limiting stability temperatures for copper sols stabilized by poly(<Emphasis Type="Italic">N</Emphasis>-vinyllactams)

It has been shown that stable copper sols cannot be prepared through the reduction of copper ions in aqueous solutions of poly(N-vinyllactams) in the presence of poly(N-vinylpyrrolidone) in the temperature range 50–70°C, while in the presence of poly(N-vinylcaprolactam), the said sols cannot be prepared at temperatures below ～12°C. This tendency is related to the thermodynamic instability of complexes formed via noncovalent interactions of copper nanoparticles with polymer macromolecules in corresponding temperature ranges. Stable sols synthesized in the presence of the above polymers at other temperatures rapidly lose stability in the said temperature ranges, and this effect is accompanied by the aggregation of metal particles. The experimental results provide evidence that there is a relationship between the stability of complexes macromolecules-metal nanoparticles and the thermodynamic quality of the solvent.     

Influence of the Molecular Mass of Poly(<Emphasis Type="Italic">N</Emphasis>-vinylpyrrolidone) on Formation of Cu<Subscript>2</Subscript>O Nanoparticles During Reduction of Divalent Copper Ions with <Emphasis Type="Italic">tert</Emphasis>-Butylamine Borane in Polymer Solution

The process of reduction of divalent copper ions with tert-butylamine borane in dilute aqueous solutions of poly(N-vinylpyrrolidone) is investigated. The influence of polymer molecular mass on properties of the resultant Cu₂O sols is studied. It is shown that Cu₂O nanoparticles with an average diameter of 6–8 nm independent of polymer molecular mass and a relatively narrow size distribution of particles are formed in the systems under study. The contour length of macromolecules and the hydrodynamic diameter of a poly(N-vinylpyrrolidone) macromolecular coil are compared with the diameter of Cu₂O particles. Poly(N-vinylpyrrolidone) with M ≥ 1 × 10⁴ can be used to produce Cu₂O nanoparticles. Poly(N-vinylpyrrolidone) with M > 4 × 10⁴ should be used for the formation of long-living Cu₂O sols.     

Cu(II)-catalyzed oscillatory chemical reactions

The role of copper ions in the copper-catalyzed chemical reactions is discussed. It is pointed out that copper ions can induce oscillatory behavior in many systems for the following reasons: (1) Copper cations can exist in three oxidation states (+1, +2 and +3); (2) Copper cations can form precipitates and stable complexes with a large number of reactants and intermediates; (3) Copper ions can participate in both oxidation and reduction processes, due to the surprisingly large range of redox potentials exhibited by the Cu²⁺/Cu⁺ and Cu³⁺/Cu²⁺ couples (known redox potentials span from 0.1 to 1.8 V, depending on the counter-ion or ligand present).     

Mediated electrochemical synthesis of copper nanoparticles in solution

Environmentally friendly mediated electrochemical synthesis of copper nanoparticles in the solution using a copper anode as a source of copper ions has been realized for the first time. It is shown that at the potential of the redox pair MV²⁺/MV?⁺ methylviologen MV²⁺ is able to mediate a reduction of Cu²⁺ ions in 60% aqueous DMF/0.1 M Bu₄NBF₄. Copper nanoparticles build large aggregates (200—250 nm) in the absence of a stabilizer. The use of polyvinylpyrrolidone as a stabilizer makes it possible to obtain smaller copper nanoparticles (20—50 nm) of spherical and oval shape and to characterize them by physicochemical methods.     

Colloidal copper in aqueous solutions: radiation-chemical reduction,mechanism of formation,and properties

Colloidal copper has been obtained by -irradiation of aqueous solutions of copper (II) perchlorate in the presence of alcohol and polyethyleneimine (PEI). The sols are spherical particles about 4 nm in diameter, which are quickly oxidized by oxygen or other oxidants. When Cu^II is not entirely incorporated into the complex with PEI, disproportionation of Cu^I aqua complexes formed affords the metal, along with Cu₂O. Reduction of the PEI complex of Cu^I by hydrated electrons gives only colloidal copper. The copper ions can be reduced on the surface of silver sols. Optical parameters of the resulting bimetallic particles have been studied. The presence of copper ions leads to broadening of the absorption band associated with the silver sols and shifts it to the UV region, which is due to the transfer of electrons from copper to silver. Three copper monolayers are enough to cause plasmon absorption of colloidal copper.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 25–30, January, 1994.     

Reduction of copper(II) ions in polyacrylic acid-polyethyleneimine complexes using X-ray radiation

The specifics of X-ray-induced reduction of copper(II) ions in the polyacrylic acid-polyethyleneimine-copper(II) (PAA-PEI-Cu²⁺) complexes were studied. It was found that the action of radiation led to the effective reduction of Cu²⁺ in the PAA-PEI-Cu²⁺ films swollen in an aqueous-alcohol medium. The formation of metal nanoparticles in irradiated samples was revealed by electron microscopy and X-ray diffraction techniques. Analysis of ESR data has demonstrated that the Cu²⁺ reduction rate increases with an increase in the initial concentration of copper ions in the samples. Formal yields of the radiation-chemical reduction of Cu²⁺ in the PAA-PEI complex films were found to be quite high (>100 ions per100 eV of energy absorbed by the swollen film), which can be explained by involvement of the radiolysis products of aqueous alcoholic solutions (outer medium) in the reduction processes.     

Interactions between macromolecules and metal nanoparticles in aqueous-saline media

The influence of the concentration of low-molecular-mass salt additives in the reaction medium on the size characteristics of copper nanoparticles in sols formed through the reduction of Cu²⁺ ions in the presence of a cationic polyelectrolyte and nonionogenic polymers with hydrophilic (poly(ethylene oxide) and hydrophobic (poly(N-vinylpyrrolidone)) main chains has been studied. Formation of sols with a narrow size (diameter) distribution of metal nanoparticle indicates the pseudomatrix character of formation of the metal phase under the studied conditions. Effects of the neutral salt and its concentration in the reaction medium on the synthesis of copper sols and on the mean size of metal nanoparticles are related to a change in the nature or character (when oppositely charged polyelectrolyte macromolecules and copper nanoparticles are involved in interaction) of noncovalent interactions stabilizing the macromolecule-nanoparticle complex on passage from the salt-free aqueous medium to the aqueous-saline medium with a sufficiently high concentration of the neutral salt.     

Removal and recovery of copper (II) ions by bacterial biosorption

Studies were conducted toinvestigate the removal and recovery of copper (II) ions from aqueous solutions by Micrococcus sp., which was isolated from a local activated sludge process. The equilibrium of copper biosorption followed the Langmuir isotherm model very well with a maximum biosorption capacity (q_max) of 36.5 mg of Cu²⁺/gofdry cell at pH 5.0 and 52.1 mg of Cu²⁺/g of dry cell at pH 6.0. Cells harvested at exponential growth phase and stationary phase showed similar biosorption characteristics for copper, Copper uptake by cells was negligible at pH 2.0 and then increased rapidly with increasing pH un til 6.0. In multim etal systems, Micrococcus sp. exhibited a preferential biosorption order: Cu−Pb>Ni−Zn. There is virtually no interference with copper uptake by Micrococcus sp. from solutions bearing high concentrations of Cl⁻, SO ₄ ²⁻ , and NO3/− (0–500 mg/L). Sulfuric acid (0.05 M) was the most efficient desorption medium, recovering >90% of the initial copper sorbed. The copper capacity of Micrococcus sp. remained unchanged after five successive sorption and desorption cycles. Immobilization of Micrococcus sp. in 2% calcium alginate and 10% polyacrylamide gel beads increased copper uptake by 61%. Biomass of Micrococcus sp. may be applicable to the development of potentially cost-effective biosorbent for removing and recovering copper from effluents.     

Preparation of Fe3O4/chitosan/poly(acrylic acid) composite particles and its application in adsorbing copper ion (II)

Fe₃O₄/chitosan/poly(acrylic acid) (Fe₃O₄/CS/PAA) composite particles, which are reusable, biodegradable and of high adsorption capacity, have been prepared through polymerizing acrylic acid in chitosan and Fe₃O₄ nanoparticles aqueous solution. By varying in-feed mole ratio of carboxyl to amino group (n^c/n^a) and reactant concentration, the average diameter of Fe₃O₄/CS/PAA composite particles can be controlled to vary from 100 to 300 nm. FT-IR, XRD and TEM were used to characterize Fe₃O₄/CS/PAA composite particles. Results showed that Fe₃O₄ was indeed incorporated into CS/PAA particles. The composite particles showed high efficient to remove copper ions (II) in aqueous solution. Adsorption kinetic studies showed that the adsorption process followed a pseudo-second-order kinetic model and the equilibrium data agreed well with the Langmuir model. The saturated adsorption capacity obtained from the experimental was 193 mg/g in close to proximity to the data 200 mg/g calculated from Langmuir model. The saturated adsorption capacity still retained 100 mg/g after three cycles of adsorption–desorption of copper ions (II).     

Synthesis and characterization of poly(acrylic acid) stabilized cadmium sulfide quantum dots

Cadmium sulfide (CdS) nanoparticles (NPs) capped with poly(acrylic acid) (PAA) were prepared in aqueous solutions from Cd(NO3)2 and Na2S. Influence of the COOH/Cd ratio (0.8-12.5), reaction pH (5.5 and 7.5), and PAA molecular weight (2100 and 5100 g/mol) on the particle size, colloidal stability, and photoluminescence were investigated. A Cd/S ratio of <1 causes ineffective passivization of the surface with the carboxylate and therefore results in a red shift of the absorption band and a significant drop in photoluminescence. Therefore, the Cd/S ratio was fixed at 1.1 for all experiments studying the mentioned variables. PAA coating provided excellent colloidal stability at a COOH/Cd ratio above 1. Absorption edges of PAA-coated CdS NPs are in the range of 460-508 nm. The size of the NPs increases slightly with increasing PAA molecular weight and COOH/Cd ratio at pH 7.5. It is demonstrated that there is a critical COOH/Cd ratio (1.5-2) that maximizes the photoluminescence intensity and quantum yield (QY, 17%). Above this critical ratio, which corresponds to smaller crystal sizes (3.7-4.1 nm) for each reaction set, the quantum yield decreases and the crystal size increases. Moreover, CdS NPs prepared at pH 7.5 have significantly higher QY and absorb at lower wavelengths in comparison with those prepared at pH 5.5. Luminescence quenching has not been observed over 8 months.     

Facile design of a metal-imprinted surface from a poly(vinyl chloride-co-acrylic acid) / poly(propylene glycol) blend

Porous thin films with memory for imprinted metals have been prepared by casting water-THF mixed solutions of a poly(vinyl chloride-co-acylic acid) / poly(propylene glycol) blend. When Cu²⁺ and Pb²⁺ ions are imprinted in the cast process, the films retain a memory for the original Cu²⁺ and Pb²⁺ ions. The imprinting processes can be thoroughly monitored by FT-IR spectroscopy.     

Komplexbildung des Kupfer(II) mit Opioidpeptiden

Complex Formation of Copper(II) with Opioid Peptides Complex compounds of copper(II) with the neuropeptide enkephaline were detected as CuH_?(n?1)L + nLi⁺ ions by means of FAB mass spectrometry. Formation constants of proton and copper(II) complexes of four opioid peptides (L-I ? L-IV) were determined potentiometrically. Complexes with methionine-containing ligands show the highest stability. Under physiological pH values all brain-existent enkephaline may be bound by endogeneous copper provided that [Cu²⁺] > 10^?7 M.