Radiation-Chemical Synthesis of Silver Nanoparticles ... - Springer Link

ISSN 2075-1133, Inorganic Materials: Applied Research, 2016, Vol. 7, No. 5, pp. 730–736. © Pleiades Publishing, Ltd., 2016. Original Russian Text © L.N. Shirokova, A.A. Revina, V.A. Aleksandrova, A.A. Fenin, 2016, published in Perspektivnye Materialy, 2016, No. 1, pp. 40–48.

MATERIALS FOR ENSURING HUMAN VITAL ACTIVITY AND ENVIRONMENTAL PROTECTION

Radiation-Chemical Synthesis of Silver Nanoparticles in Aqueous Solution of Chitin Derivative L. N. Shirokovaa,*, A. A. Revinab, V. A. Aleksandrovaa, and A. A. Feninc aTopchiev

Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninskii pr. 29, Moscow, 119991 Russia bFrumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119071 Russia cMendeleyev University of Chemical Technology of Russia, Miusskaya pl. 9, Moscow, 125047 Russia *e-mail: [email protected] Received May 18, 2015

Abstract—It has been demonstrated for the first time that, in the process of gamma-induced reduction of Ag+ in aqueous solution of biodegradable carboxymethyl chitin (CMC) polyelectrolyte, a metal-polymer colloidal solution is formed, wherein silver nanoparticles have spherical shape and a size of about 1–5 nm. By using UV–Vis spectroscopy and transmission electron microscopy (TEM), the influence of CMC concentration and radiation doses on the formation of clusters and silver nanoparticles in the metal-polymer colloidal solution is observed. Colloidal solutions of silver nanoparticles in CMC exhibit a clear concentration-dependent bactericidal activity toward the strains of Gram-positive Staphilococcus aureus, as well as Gram-negative Salmonella tythimurium bacteria. Keywords: carboxymethyl chitin (CMC), radiation-chemical synthesis, silver nanoparticles, antibacterial activity DOI: 10.1134/S2075113316050245

INTRODUCTION Silver nanoparticles are of great scientific interest owing to a wide range of applications of such particles in the area of creation of effective antibacterial agents, electromagnetic coatings, biosensors, etc. [1, 2]. Polysaccharides are widespread compounds of complex carbohydrates, which, along with proteins and nucleic acids, are necessary for normal vital activity of organisms [2]. The choice of chitin derivatives as a polymer matrix for obtaining nanosized silver particles is due to well-known film- and fiber-forming properties of chitin and some of the derivatives thereof [3–5]. Natural origin, biocompatibility, capability of biodegradation, low toxicity, and high sorption properties of chitin derivatives with respect to water, metal ions, and organic substances determine the prospect of their use, mainly, for biology and medicine [6–8]. Chitin insolubility in common solvents limits the scope of its practical application. Chitosan, which is a N-deacetylated derivative of chitin, is soluble in aqueous solutions only in acidic environments; therefore, for further use thereof in medicine and cosmetics, removal of excess acid is required, which results in the change in shape and size of the material obtained. Unlike chitin, its carboxymethylated derivative (CMC), along with low toxicity, has good solubility in water, confirming the prospect of the use thereof as stabilizer of nanoparticles of metals.

A method for synthesis of nanoparticles in reverse micelles in two-phase aqueous-organic systems is widely known and makes it possible to obtain metal nanoparticles uniform both in size and in shape [9, 10], which may then be introduced in polymer carrier matrices [11, 12]. In addition, currently, the methods for the synthesis of nanoparticles during the reduction of metal ions directly in solution (or melt) of a high molecular weight compound are widely used and developed. If such method for the formation of metal nanoparticles in solution is used, both chemical reagents and reducing particles appearing under the influence of various types of high energy radiation, in particular, γ radiation, may be used as a reducing agent. Among chemical reducing agents, the most frequently used ones are hydrazine, hydrogen, and borohydrides [13, 14]. However, metal nanoparticles obtained by chemical reduction may be contaminated with impurities of both initial reducing agents and toxic reaction byproducts. An important advantage of the radiation-chemical reduction is the possibility of synthesis of nanosized metal particles in the absence of impurities with good reproducibility in polymer matrices [15]. In addition, radiation-chemical reduction in polymers, as was shown by example of polyelectrolytes and interpolyelectrolyte complexes, makes it possible to

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RADIATION-CHEMICAL SYNTHESIS OF SILVER NANOPARTICLES

HOH H O HO

H NH H H O C CH3

HOCH2COONa H O HO O H NH H H O C CH3

NaOH

O

ClCH2COOH

n

731

m

Fig. 1. Synthesis of carboxymethyl chitin.

obtain nanosized metal particles with a narrow particle size distribution in the range of 1–5 nm [16]. The aim of this study is the investigation of the processes of the formation of clusters and silver nanoparticles at radiation-chemical reduction of silver ions in a carboxymethyl matrix at low irradiation doses. EXPERIMENTAL Materials and Methods Chitin ((1→4)-2-β-acetamido-2-deoxy-D-glucan) with an average molecular weight of (40–45) × 10 4 derived from carapaces and pincers of crab (All-Russian Scientific Institute for Fisheries Research and Oceanography, Shchelkovo, Russia) was used in the study. Sodium hydroxide of reagent grade, 2-propanol of extrapure grade, monochloroacetic acid of reagent grade, and silver nitrate of reagent grade were used without additional treatment. Aqueous solutions of the reagents were prepared using distilled water. Water-soluble derivative 6-O-carboxymethyl-chitin (CMC) with a molecular weight of 8 × 10 4 and degree of carboxymethylation of 1.0 was obtained from chitin [17]. Monochloroacetic acid was used as esterifying agent, by which preactivated chitin was treated in the presence of an excess of sodium hydroxide in a wateralcohol medium at elevated temperature (Fig. 1). Preparation of Silver Nanoparticles Stabilized by Carboxymethyl Chitin In order to obtain a colloidal solution of silver nanoparticles, the easily water-soluble salt silver nitrate AgNO3 was used and water-soluble biodegradable polyelectrolyte CMC was used as a stabilizer of silver nanoparticles. The concentration of polymer solution was varied in the range of 0.05–2.0 wt %. Oxygen present in the reaction medium initiates a side oxidation reaction. Therefore, the resulting solution containing CMC and AgNO3 (at a concentration of AgNO3 in polymer solution of 7.5 mM) was purged with argon for 1.5–3.0 h in a reaction vessel and thoroughly sealed. Then deaerated solutions were irradiated with 60Co γ rays with use of the RHM-γ-20 isotope radiation-chemical facility at the Mendeleyev University of Chemical Technology of Russia at a dose INORGANIC MATERIALS: APPLIED RESEARCH

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rate of 0.14 ± 0.01 Gy/s within the range of absorbed doses from 1.0 to 4.0 kGy. Quantitative determination of silver nanoparticles and estimation of the stability thereof were carried out by the measurement of absorption intensity of silver nanoparticles at 420 nm using a SPECORD M40 UV-Vis spectrophotometer (Carl Zeiss, Germany). The measurement error of wavelength (±0.05 nm) and the photometric error corresponded to the technical characteristics of the spectrophotometer. Thus, the concentration of the solution of silver nanoparticles (cAgNP, mol/L) was calculated by the equation

c AgNP =

A420,AgNP , ε 420,AgNPl

where A420,AgNP is the intensity of the absorption of silver nanoparticles at 420 nm, ε420,AgNP is the molar extinction coefficient of silver nanoparticles at 420 nm equal to (1.03 ± 0.08) × 10 4 L/(mol cm) [18], and l is optical path length (cell width) in centimeters. Morphological Analysis of Silver Nanoparticles in Carboxymethyl Chitin Matrix The presence of silver nanoparticles in the CMC matrix and the shape and size of the nanoparticles were characterized by TEM and local electron diffraction on a Tecnai microscope (FEI, USA). Study of Bactericidal Properties of Metal-Polymer Colloidal Solutions The bactericidal properties of the investigated samples of colloidal solutions of silver nanoparticles obtained as a result of radiochemical reduction of Ag+ ions in CMC matrix were estimated by the zone of growth inhibition of pathogenic microbes clearly notable against a background of confluent microbial growth on nutrient media. The size of the inhibition zone determines the degree of sensitivity of microbe to antimicrobial substance. The measured diameter of the zone should pass through the center of the disk. The diameter of the zone of growth inhibition of microorganisms was measured with an accuracy of 0.1 mm. Strains of Gram-negative bacteria Salmonella typhimurium 79 and Gram-positive bacteria StaphyloNo. 5

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coccus aureus ATCC 6538P obtained from the collection of the Tarasevich State Institute of Standardization and Control of Biomedical Preparations (Russia) were used as test cultures. In order to cultivate microorganisms with concentration of 104 and 106 CFU/mL, nutrient media xylose lysine deoxycholate agar or XLD-agar (Merck, USA) and Baird–Parker-type agar (OOO Biokompas-S, Russia) for Salmonella typhimurium and Staphylococcus aureus, respectively, were taken. In each experiment two glass Petri dishes with a size of 100 × 20 mm were used, in each of which after agar was set three holes with a diameter of 9 mm were cut by a sterile drill. Then 50 μL of test colloidal solution of silver nanoparticles was put into the holes, and the dishes with cultures were incubated for 24–48 h at 37°C. Bactericidal activity data of the obtained colloidal solutions of silver nanoparticles were presented as the arithmetic mean of six parallel experiments and standard deviation. Statistical processing of the data was performed using the BioStat program for Windows XP.

The output of nanosized metal particles under the impact of high energies on the chemical system is due to generation of highly active strong reducing agents, − such as solvated electrons eaq , forming during radiolysis of water. In a number of studies, it was shown that the output of metal nanoparticles at the initial stage of radiation-chemical reduction of atoms and metal ions is preceded by a series of intermediate stages of formation of small clusters of varying stability [19–23]. In the course of their successive fusion, larger particles are formed up to the appearance of quasi-metallic (subcolloidal) and then nanosized particles. In this case, it was shown by example of silver [24, 25], (1)

Ag 2+ → Ag 32 + → Ag 42 + → Ag 82 + → … → Ag mn + , n > m,

2.0 1.5 2 1.0

3 4

0.5 1 0 200

300

400

500

600

700 λ, nm

Fig. 2. Electronic spectra of aqueous solutions containing CMC with concentration of (1) 0.05, (2) 0.5, (3) 1.0, and (4) 2.0 wt %, 7.8 mM AgNO3, and 0.05 mL of isopropanol obtained on the first day after γ radiolysis at absorbed dose of 2.0 kGy.

ions and, correspondingly, in the additional formation of nanoparticles [15], i.e., they may serve as secondary reducing agents.

RESULTS AND DISCUSSION

− Ag + + eaq → Ag 0,

2.5 Absorbance, arb. units

732

(2)

that Ag0 atoms forming from Ag+ as a result of the reaction in water (1) by passing through a series of successive transformations give a family of “magic” clusters (2). Upon radiation exposure in water, not only sol− vated electrons eaq and radicals with reducing properties but also oxidant radicals, such as hydroxyl radicals •OH, may be formed. In order to inactivate them, an acceptor of •OH radicals—isopropanol—was used [24]. It is known that alcohols serve not only as traps of hydroxyl radicals •OH forming at γ irradiation of aqueous solutions. It is essential that, at the interaction of isopropanol with •OH radicals, hydroxy isopropyl radicals are formed, which have high reduction potential and take part in reduction reaction of silver

Effect of CMC Concentration on Processes of Formation of Clusters and Silver Nanoparticles Figure 2 shows electronic spectra of the products of radiolysis of aqueous solutions of CMC in various concentrations at constant AgNO3 concentration. The increase in CMC concentration, in addition to the strengthening of the role of diffusion factor, was accompanied by an increase in the proportion of silver ions fixed by the polymer. This significantly slowed down the processes of formation of clusters. When lower concentrations less than 0.1 wt % were used, the stabilizing effect of the polymer decreased considerably; thus, in particular, at the concentration of 0.05 wt %, the formation of silver nanoparticles was not observed (Fig. 2, curve 1). At radiation-chemical reduction of silver ions in aqueous solution of 2.0 wt % of CMC and 15.6 mM AgNO3 (in CMC solution), a sharp increase in solution viscosity and gel formation took place, which was probably due to the formation of specific coordination cross-linkages between linear CMC macromolecules, where silver ions serve as coordination cross-linking agent. Upon γ irradiation of silver ions in 0.5 wt % of CMC, an increase in the intensity of optical absorption of the solution and a noticeable change in the structure of the spectrum took place (Fig. 2, curve 2). Under the action of γ irradiation, the appearance of a broad absorption band in the range of 300–500 nm is observed in the electron spectrum of the system. The shape of this absorption band changed at a further increase in the irradiation dose (Fig. 3). This indicates that, in this region of absorption, superposition of optical bands characteristic of intermediate silver clus-

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(a)

10 nm

733

10 nm

(b)

Fig. 3. Electron micrographs of films of aqueous solutions containing (a) 0.5 wt % CMC, 7.8 mM AgNO3, and 0.05 mL of isopropanol and (b) 1.0 wt % CMC, 7.8 mM AgNO3, and 0.05 mL of isopropanol after γ radiolysis at absorbed dose of 2.0 kGy.

ters is presented. In this case, a gradual disappearance of the band with a maximum at 275 nm ( Ag 24 + clusters) and the appearance of a distinct band with a maximum at 290 nm ( Ag 82 + clusters) took place. In addition, at this stage of radiation-chemical reduction of silver ions, the bands with maxima at 360–370 nm and 480–510 nm (Agn oligomeric clusters) belonging to quasi-metallic silver particles became clearly apparent. Further, the increase in the absorption intensity of quasi-metallic particles was accompanied by gradual disappearance of the absorption band with a maximum at 290 nm. In this case, a gradual shift of the absorption band of quasi-particles to 390–420 nm and a further increase in the intensity of the band, which is due to the absorption of silver nanoparticles Ag mn + appearing at the final stage of aggregation of intermediate clusters, were also observed. The shift of the optical band of quasi-metallic particles is due to transformation thereof in nanoparticles with metallic properties. As one can see from TEM data shown in Fig. 3, in the nanosystem from CMC solution with a concentration of 1 wt %, larger silver nanoparticles with a particle size of 5–10 nm are formed as compared with nanoparticles obtained from CMC solution with a concentration of 0.5 wt % (particle size of 1–5 nm). The images on electron micrographs (Fig. 3) confirm spectral data shown in Fig. 2, curves 2 and 3. Thus, the concentration of 0.5 wt % of CMC is optimal for the formation of small-sized silver nanoparticles. In this regard, it was interesting to study the influence of the dose of γ irradiation on the processes of formation of clusters and silver nanoparticles. Effect of Dose of Radiation Exposure on the Processes of Formation of Clusters and Silver Nanoparticles With the increase in the dose of γ irradiation, an increase in the intensity of optical absorption in the range of λmax (420 nm) and a notable change in the INORGANIC MATERIALS: APPLIED RESEARCH

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spectrum structure (Fig. 4a) took place. It was found (Fig. 4) that, even at small irradiation doses in the nanosystem at CMC concentration of 0.5 wt % at storage for 1 month in the dark after γ irradiation, an increase in the intensity of the absorption band with a maximum at 420 nm, which may be attributed to nanosized silver particles, took place.

It was shown that, in the nanosystem at CMC concentration of 1.0 wt % and absorbed dose of 2.0 kGy, formation of the absorption band with a maximum at 420 nm was not observed (Fig. 2, curve 3). Nevertheless, when the nanosystem was stored for 1 month in the dark after the irradiation with γ rays, an increase in the intensity of the characteristic absorption band with λmax of 420 nm took place (Fig. 5). Such phenomenon may be due to the formation of a colloidal metallic phase catalyzing subsequent reduction of metal ions and additional formation of nanostructures, which was noted by other authors [15, 24, 25]. Similar regularities were observed in nanosystems at absorbed doses from 1.0 to 3.0 kGy (Figs. 4b–4d). It should be noted that, with the increase in irradiation dose, the formation of nanoparticles from intermediate clusters occurred more intensely. In this case, the formation of narrow plasmon resonance band, which characterizes small-sized silver nanoparticles, was recorded (Fig. 3a). Thus, variation of the reaction conditions of radiation-chemical reduction of ions made it possible to obtain metal-polymer colloid solutions with various contents of silver nanoparticles. The stability of the clusters increased with the number of atoms in them and, when the size of 1–5 nm was achieved, metal particles were stable in the reaction medium. In this case, CMC serves not only as a stabilizer of nanosized silver particles but also as a matrix having influence on the processes of formation and growth of silver nanoparticles by controlling the size thereof. No. 5

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(a)

1.5 4 kGy 3 kGy 2 kGy 1 kGy

0.5 0 200

300

400

2.5 Absorbance, arb. units

Absorbance, arb. units

2.0

1.0

500

600

42 days 15 days 9 days 7 days 3 days 1 days

0.5 0 200

300

400

500

42 days 15 days 9 days 7 days 3 days 1 days

1.5 1.0 0.5 300

400

2.5

2.0

1.0

2.0

0 200

700 λ, nm

(c)

1.5

(b)

2.5

600

700 λ, nm



2.5

500

600

700 λ, nm

(d)

2.0 42 days 15 days 9 days 7 days 3 days 1 days

1.5 1.0 0.5 0 200

300

400

500

600

700 λ, nm

Fig. 4. Electronic spectra of: (a) aqueous solutions containing 0.5 wt % of CMC, 7.8 mM AgNO3, and 0.05 mL of isopropanol obtained on the first day after γ radiolysis at absorbed doses of 1.0, 2.0, 3.0, and 4.0 kGy. Time evolution of the spectra of aqueous solutions containing 0.5 wt % CMC, 7.8 mM AgNO3, and 0.05 mL of isopropanol after γ radiolysis at absorbed doses of (b) 1.0, (c) 2.0, and (d) 3.0 kGy.

Antimicrobial properties of the investigated samples of metal-polymer colloidal solutions were estimated by growth inhibition zone of pathogenic microbes standing out clearly against a background of continuous microbial growth on nutrient media. The value of the inhibition zone determines the degree of sensitivity of the microbe to the antimicrobial substance. The measuring zone diameter should pass through the center of the disk. Strains of Gram-negative bacteria Salmonella typhimurium 79 and Gram-positive bacteria Staphylococcus aureus ATCC 6538P obtained from the collection of the Tarasevich State Institute of Standardization and Control of Biomedical Preparations (Russia) were used as test cultures. Cultivation of Salmonella typhimurium strains was carried out on XLD-agar nutrient medium for 24 h at 37°C, and cultivation of Staphylococcus aureus was performed on Baird–Parker-type agar for 48 h at the same temperature; the concentrations of the bacteria were 10 4 and 106 CFU/mL, respectively. In order to

identify Salmonella and Staphylococcus, biochemical tests and special nutrient media were used. There are definite correlations between the degree of sensitivity of the microbe to the test sample and the size of the diameter of the inhibition zone. A zone with a diameter up to 10 mm or complete absence of stasis 2.5 Absorbance, arb. units

Bactericidal Activity of Colloidal Solution of Silver Nanoparticles in Carboxymethyl Chitin Matrix

2.0 42 days 15 days 9 days 7 days 3 days 1 days

1.5 1.0 0.5 0 200

300

400

500

600

700 λ, nm

Fig. 5. Time evolution of spectra of aqueous solutions containing 1.0 wt % CMC, 7.8 mM AgNO3, and 0.05 mL of isopropanol after γ radiolysis at absorbed dose of 2.0 kGy.


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Size of growth inhibition zone, mm


735

REFERENCES I II III

60 50 40 30

I II III

I II III

I II III

20 10 0

106 CFU/mL104 CFU/mL 106 CFU/mL104 CFU/mL Staphylococcus aureus Salmonella typhimurium

Fig. 6. Antimicrobial activity of colloidal solution with various concentrations with regard to investigated test cultures of microorganism: (I) 0.5, (II) 0.25, and (III) 0.17 wt %. For the initial culture (control), a growth inhibition zone was not observed.

indicates the resistance of the microbe to the substance. A zone with a diameter over 15 mm indicates that the microorganism is sensitive to the antimicrobial agent, and a zone larger that 25 mm indicates high sensitivity to the antimicrobial agent [26, 27]. The results of the investigation of antimicrobial activity of colloidal solutions diluted with distilled water to a concentration of 0.5, 0.25, and 0.17 wt % are shown in Fig. 6. From the presented data, one can see that the colloidal solution exhibits a strong bactericidal effect with respect to both Gram-positive and Gram-negative strains of microorganisms, the effect being constant at 2- and 3-fold dilution of the colloidal solution. CONCLUSIONS Carboxymethyl chitin shows the ability to stabilize intermediate clusters of silver of different nuclearity forming during radiation-chemical reduction from ions which are precursors of metal nanoparticles. Carboxymethyl chitin macromolecules influence the processes of formation and particle growth, controlling the size and shape thereof. Metal-polymer colloidal solutions with bactericidal properties may be used in medicine, in biotechnology, and also in the food and cosmetic industries. ACKNOWLEDGMENTS We are grateful to V.T. Dubinchuk (All-Russian Research Institute of Mineral Raw Materials) for carrying out TEM experiments and G.I. Timofeeva (Nesmeyanov Institute of Organoelement Compounds) for the determination of carboxymethyl chitin molecular weight by ultracentrifugation. INORGANIC MATERIALS: APPLIED RESEARCH

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Translated by V. Kudrinskaya


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