In situ formation of silver nanoparticles in linear and branched polyelectrolyte matrices using various reducing agents
© Chumachenko et al.; licensee Springer. 2014
Received: 23 December 2013
Accepted: 18 March 2014
Published: 4 April 2014
Silver nanoparticles were synthesized in linear and branched polyelectrolyte matrices using different reductants and distinct synthesis conditions. The effect of the host hydrolyzed linear polyacrylamide and star-like copolymers dextran-graft-polyacrylamide of various compactness, the nature of the reductant, and temperature were studied on in situ synthesis of silver sols. The related nanosystems were analyzed by high-resolution transmission electron microscopy and UV-vis absorption spectrophotometry. It was established that the internal structure of the polymer matrix as well as the nature of the reductant determines the process of the silver nanoparticle formation. Specifically, the branched polymer matrices were much more efficient than the linear ones for stable nanosystem preparation.
KeywordsBranched polymers Polyelectrolytes Dextran Polyacrylamide Grafted copolymers Hydrolysis Silver nanoparticles
During the last decade, silver nanoparticles (Ag NPs) attract significant attention due to their unique optical, thermal, and electrical properties as well as their use as antibiotic materials, photocatalysts, and conductive nano-inks [1–7]. The methods to obtain Ag NPs of well-defined morphology, size, orientation, and complex pattern are the subject of numerous researches. In principle, physical and chemical techniques for nanometer-sized metal particle preparation can be used [7–12]. Such methods as chemical vapor deposition, chemical reduction, photolytic reduction, and radiolytic reduction are among them. Reduction of metal ions into neutral clusters is a commonly used treatment in chemical synthesis.
The high reactivity of Ag NPs raises difficulties in developing stable colloidal dispersions, since Ag NPs rapidly undergo agglomeration. Therefore, it is urgent to search the methods allowing the acquisition of nanosystems with high storage stability. Silver colloids stabilized by polymers in various solvents are extensively investigated by considering the linear and star-shaped polymers, polymer brushes, block copolymers, and even dendrimers [13–19]. However, the advantages of branched polymer matrices in comparison with their linear polymer analogs for in situ nanoparticles formation are still not clear. Yet, this knowledge is needed to prove or disprove the necessity of using expensive materials. The chemical nature of the polymer matrices, the nature of the reductant, and temperature affect the shape and the size of the particles [20–25]. The internal structure of the polymers could also influence the process of nanoparticle formation. The branched polymer architecture demonstrates an improvement in the ordering phenomenon. That is why such systems can differ in functionalities from their linear analogs. In the present paper, we have focused on the study of Ag sols synthesized in situ in linear and branched polyelectrolyte polymer matrices. The effect of reductant and temperature was discussed too.
Dextran with M w = 7 × 104 g mol−1 (referred as D70 throughout) was purchased from Sigma Aldrich, St Quentin Fallavier, France. Cerium (IV) ammonium nitrate (Sigma Aldrich, St Quentin Fallavier, France) was used as initiator of radical graft polymerization. Dextran samples and the cerium salt were used without further purification. Acrylamide (Sigma Aldrich, St Quentin Fallavier, France) was twice re-crystallized from chloroform and dried under vacuum at room temperature for 24 h. NaOH from Aldrich was used for alkaline hydrolysis of polymer samples. Sodium borohydride and hydrazine hydrate (Sigma Aldrich, St. Quentin Fallavier, France) were used for chemical reduction of silver nitrate in polymer solutions in order to synthesize Ag NPs.
Branched copolymers were obtained by grafting polyacrylamide (PAA) chains onto dextran (D70) backbone . The synthesis was carried out using a ‘grafting from’ method. The theoretical number of grafting sites per polysaccharide backbone depends on the ratio of Ce (IV) concentration to dextran one . Thus, n was equal to 5 or 20, and the related dextran-graft-polyacrylamide copolymers were referred as D70-g-PAA5 and D70-g-PAA20. The linear PAA (M w = 1.40 × 106 g mol−1) was synthesized by radical polymerization. All polymers were characterized by size-exclusion chromatography (SEC).
The D70-g-PAA copolymers and linear PAA were saponified by alkaline hydrolysis using NaOH to obtain polyelectrolyte samples. The hydrolysis for all samples was carried out as follows: 2 g of D70-g-PAA (or PAA) was dissolved in 200 mL of water and then 10 mL of a 5-M NaOH aqueous solution was added. The mixture was placed in a water bath at 50°С. The probes were taken in 30 min and precipitated by acetone. All samples were freeze-dried after precipitation and kept under vacuum.
In situ synthesis of Ag NPs in linear and branched polyelectrolytes matrices
Sodium borohydride and hydrazine hydrate were used for the chemical reduction of silver nitrate dissolved in polymer solutions. This reaction led to Ag NP formation. The ratio of Ag+ ions to acrylamide monomers was 1:3.
A 0.1-M silver nitrate solution was added to a polymer solution under active stirring and was kept at such conditions during 20 min for equilibrium achievement. Then, 0.1 M of sodium borohydride or 3.8 g L−1 hydrazine hydrate aqueous solutions were added and stirred for 20 min. The chemical reduction was conducted at 20°C, 40°C, 60°C, and 80°C. The solution turned dark reddish brown immediately after adding the reductant, which indicated the Ag NP formation.
SEC analysis was carried out by using a multi-detection device consisting of a LC-10 AD Shimadzu pump (throughput 0.5 mL min−1; Nakagyo-ku, Kyoto, Japan), an automatic injector WISP 717+ from Waters (Milford, MA, USA), three coupled 30-cm Shodex OH-pak columns (803HQ, 804HQ, and 806HQ; Munich, Germany), a multi-angle light scattering detector DAWN F from Wyatt Technology (Dernbach, Germany), and a differential refractometer R410 from Waters. Distilled water containing 0.1 M NaNO3 was used as eluent. Dilute polymer solutions (c = 3 g L−1 < c* = 1 / [η]) were prepared, allowing for neglect of intermolecular correlations in the analysis of light scattering measurements.
Potentiometric titration of polyelectrolyte samples was performed using a pH meter pH-340 (Econix Express, St. Petersburg, Russia). HСl (0.2 N) and NaOH (0.2 N) were used as titrants. Polymer concentration was 2 g L−1. The polymer solutions were titrated with HCl up to pH 2 and then with NaOH up to pH 12. Previously, a fine blank titration (titration of non-hydrolyzed polymer) was made. The absorption of OH− anions was calculated through the analysis of the titration curves and then the limits of these values were used to determine the conversion degree (А) of amide groups into carboxylate ones. All measurements were performed at T = 25.0°C under nitrogen.
Viscosity measurements were performed at 25.0°C ± 0.1°C using an Ostwald-type viscometer. All polymers were dissolved in distilled water without added salt. The pH of the polyelectrolyte solutions were in the range 7.8 < pH < 8.2.
Transmission electron microscopy
The identification of Ag NPs and their size analysis were carried out using high-resolution transmission electron microscopy (TEM). A Philips CM 12 (Amsterdam, Netherlands) microscope with an acceleration voltage of 120 kV was used. The samples were prepared by spraying silver sols onto carbon-coated copper grids and then analyzed.
UV-vis spectra of silver sols were recorded by Varian Cary 50 scan UV-visible spectrophotometer (Palo Alto, CA, USA) in the range from 190 to 1,100 nm (in 2-nm intervals). Original silver sols were diluted 50 times before spectral measurements.
Results and discussion
Molecular parameters of the D70- g -PAA copolymers and the linear PAA
M w (×10−6 g mol−1)
R z (nm)
R z 2/M w (×103)
Dextran content (weight%)
The compactness becomes higher as the grafting ratio of the D70-g-PAA samples increases. However, for D70-g-PAA5 copolymers, this characteristic is close to that of linear PAA macromolecules (Table 1).
Alkaline hydrolysis of D70-g-PAA were not attended by irrelevant processes (breaking or cross-linking of macromolecules) as it was confirmed by SEC analysis of source and saponified samples.
Conversion degree of polymers (hydrolysis time 30 min)
NaBH4 as reducing agent
Hydrazine as reducing agent
The present study presents a study of Ag sols obtained in linear and branched polyelectrolyte matrices. It was revealed the effect of the internal structure of host polymer matrices depended on silver nanoparticle size, morphology, and stability. The polyelectrolyte linear polymer matrices were less efficient for silver sol manufacturing in comparison with branched ones for all reductants used. Something already contemplated and demonstrated for silver sol, synthesized in situ in the same polymer matrices using ascorbic acid as the reducing agent . It was established that the temperature of synthesis and the reductant choice drastically affect the size and shape of silver nanoparticles obtained. Stable Ag sols could not be synthesized in linear PAA matrix at 80°C, while colloids synthesized in branched matrices remained stable.
VC is a Ph.D. student in the Macromolecular Department of Kiev Taras Shevchenko National University. NK is the principal researcher and is a Ph.D. and Dr. Chemical Science degree holder. MR is and Ph.D. and Dr. of Research degree holder and the head of team ‘Polyelectrolytes Complexes and Materials’. MS is a research engineer. CB is an engineer assistant.
The synthesis of silver colloids using hydrazine hydrate as reductant has been made by O. Korychenska, the student of Kiev National Taras Shevchenko University.
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