ISSN 1061-933X, Colloid Journal, 2017, Vol. 79, No. 6, pp. 735–739. © Pleiades Publishing, Ltd., 2017. Original Russian Text © E.V. Abkhalimov, V.A. Ershov, B.G. Ershov, 2017, published in Kolloidnyi Zhurnal, 2017, Vol. 79, No. 6, pp. 700–704.
An Aqueous Colloidal Silver Solution Stabilized with Carbonate Ions E. V. Abkhalimov*, V. A. Ershov, and B. G. Ershov Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, 119071 Russia *e-mail:
[email protected] Received May 31, 2017
Abstract—A method has been developed for the preparation of a stable “pure” silver hydrosol containing nanoparticles with an average size of 10 nm and stabilizing carbonate ions. The method consists in the photochemical reduction of silver ions with oxalate ions, which simultaneously generate carbonate ions. Optical spectroscopy, electron microscopy, and dynamic light scattering are used to study the hydrosol. DOI: 10.1134/S1061933X17060023
INTRODUCTION The ability of silver to inhibit the growth of dangerous microflora was revealed several thousand years ago; since then, this property of the metal has been successfully used to prolog the shelf life of water and foodstuffs. At present, silver nanoparticles are used as efficient antibacterial and antiviral remedies. The broad spectrum of the antimicrobial activity of silver and the absence of resistance of the overwhelming majority of pathogenic microorganisms to it, together with the absence of allergic reactions, make this nanomaterial the focus of much interest. Nanosized silver is used in the production of food-packaging materials, clothing, paints, enamels, everyday items, perfumes, and medicinal preparations [1–5]. The large specific surface area of silver nanoparticles greatly enhances the antimicrobial activity, while their small sizes (≤10 nm) make it possible to affect intracellular processes. Therefore, the useful properties of silver nanoparticles have attracted much attention from the point of view of their application in medicine. Chemical reduction of silver ions in aqueous solutions is one of the methods widely used for the production of silver nanoparticles. Diverse compounds, such as sodium borohydride, hydrazine, aluminum hydrides, hypophosphites, formaldehyde, oxalates, tartrates, and many other reagents, are used as reductants [6–10]. Arising metal atoms form hydrophobic nanoparticles. Various organic or inorganic stabilizers are added to solutions to prevent nanoparticles from aggregating. Polymer compounds or polar organic molecules are predominantly used, because they “envelop” metal cores to form shells around them. A stabilizer layer weakens mutual attraction between metal particles and creates a structural mechanical barrier, which prevents them from approaching each other. The surface modification of silver nanoparticles with a stabilizer, on the one hand,
provides their dispersions with stability (the particles become hydrophilic) and, on the other hand, substantially changes their physicochemical and physiological characteristics. As a rule, the used stabilizers, reductants of silver ions, and products of their degradation are not harmless. Therefore, the disinfecting action of silver is de facto complicated by possible toxic effects of the additives and the products of their degradation, which are present in solutions. Moreover, the existence of a stabilizing shell hinders the direct contact between silver particles and a biological object. This is the reason for the interest in the feasibility of stabilizing dispersions of metal nanoparticles by the electrostatic mechanism using small ions that are harmless for health and environment. In this case, an electrical double layer is formed on metal particle surface; hence, the stability of a hydrosol is achieved due to the repulsion of nanoparticles upon overlapping of their ionic atmospheres. In contrast to the stabilization with polymers, the size of a particle coated with a shell of small anions appears to be comparable with the size of its metal core, and its surface is much more accessible. The method developed by Turkevich et al. for the synthesis of gold hydrosols using citrate ions as a reductant [11–13] is one of the procedures most widely used for the preparation of silver nanoparticles. Under the action of heating or another activating factor, citrate ions reduce silver ions in aqueous solutions and, at the same time, stabilize the metal nanoparticles being formed. However, citric acid itself and the products of its degradation (acetone dicarboxylic and itaconic acids and other compounds) present in a resulting hydrosol can cause a harmful and uncontrollable effect on living organisms. Therefore, it is an urgent problem to develop a method for preparing hydrophilic silver-nanoparticle dispersions with a composition free of toxic impurities and correspond-
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ing to natural water, which primarily contains carbonate ions. Moreover, silver particles are desirable to have an average size of 5–20 nm and a rather narrow size distribution. Previously, it was shown that an efficient electrostatic mechanism of stabilizing silver hydrosols is indeed realized in the presence of carbonate ions [14, 15]. Silver nanoparticles with an average size of 10.0 ± 2.5 nm were synthesized by reducing Ag+ ions with sodium borohydride in the presence of 5 × 10–5−1 × 10–2 M Na2CO3. The obtained hydrosol had a rather high stability to aggregation. However, the silver hydrosol thus formed also contained toxic borate ions, which resulted from the degradation of the reducing agent, which was borohydride. Carbonate-ion-stabilized silver nanoparticles were also obtained with the use of tannin as a reductant [16]. The goal of this work was to obtain a “pure” silver hydrosol, which would contain metal nanoparticles and stabilizing carbonate ions. For this purpose, it was necessary to realize conditions under which, in the process of Ag+ reduction, a reductant would be transformed into a compound efficiently stabilizing nanoparticles being formed. Oxalate ions, which, being irradiated with UV light, are decomposed to yield CO 2− radical ions, were used as the reductants of Ag+ ions: (1) C 2O 42 − /\/\/\ → 2CO 2−. These radicals possess high reducing potential E0(CO2/CO 2− ) = −1.9 V [17] and efficiently reduce silver ions [18]. It should be emphasized that the reaction occurs in the bulk of a solution with the formation of silver atoms, for which E 0(Ag+/Ag0) = −1.8 V [17].
Ag + + CO 2− → Ag 0 + CO 2 (k = 4 × 10 9 L mol −1 s −1 [18]) .
(2)
The aggregation of the atoms into metal nanoparticles proceeds through several stages with the formation of intermediate silver clusters ( Ag 2+ , Ag 32 +, Ag 24 +,
Ag 82 +, etc.) [19–21]. Let us conditionally express this complex process of colloidal silver formation by the following scheme: nAg0 → Agn.
(3)
In an aqueous medium, carbon dioxide resulting from reaction (2) is transformed into carbonic acid to yield bicarbonate and carbonate ions, which stabilize colloidal silver particles that are formed: CO2 + H2O ⇆ H+ + HCO3− ⇆ 2H+ + CO 32 −.
(4)
The data obtained have confirmed the feasibility of realizing the approach proposed for the synthesis of a stable silver hydrosol containing metal nanoparticles and carbonate ions stabilizing them.
EXPERIMENTAL Silver perchlorate monohydrate (AgClO4 · H2O, 99%, Aldrich Chemical) and potassium oxalate (K2C2O4, extrapure grade, Reakhim) were used in the experiments. Pure silver hydrosol containing only nanoparticles and carbonate ions was obtained by UV irradiation of a mixed solution of silver perchlorate (source of silver ions) and potassium oxalate (reductant and source of carbonate ions). The irradiation was performed in a special unit equipped with a 2-mL quartz cell having a light path of 5 mm. The concentrations of silver perchlorate and potassium oxalate in the irradiated solutions were (1−3) × 10–4 and (3−10) × 10–4 mol/L, respectively. Before the irradiation, a solution was degassed during 5 min using a vacuum pump. In addition to carbonate ions, the “pure” silver hydrosol obtained with the used these compounds contained non-toxic K+ and ClO4 ions. The solutions were illuminated by UV light of a pulsed low-pressure xenon lamp with total radiation flux intensity I = 6.0 × 1020 quantum/s = 1.0 × 10‒3 E/s. Optical spectra were measured using a Cary 100 Scan spectrophotometer (Varian Inc., Netherlands) equipped with a thermostatted Peltier cell at temperature T = 20°C. The hydrodynamic sizes and ζ potentials of the resultant colloidal particles were measured using dynamic light scattering (DLS) with a Delisa Nano C instrument (Beckman Coulter, Inc., United States) operating at a wavelength of laser radiation λ = 658 nm. The sizes and polydispersity of the nanoparticles were determined with a JEM-2100 transmission electron microscope (TEM) (JEOL, Japan) operating at an accelerating voltage of 200 kV.
RESULTS AND DISCUSSION It has been found that UV irradiation of a deaerated aqueous solution of silver ions ((1−3) × 10−4 mol/L) containing oxalate ions ((3−10) × 10−4 mol/L) induces the reduction of the metal ions with the formation of nanoparticles. This is confirmed by the appearance and growth of an optical absorption in a range of 380−410 nm, which is typical of silver nanoparticles and caused by surface plasmon resonance (SPR) (Fig. 1). In the course of reduction, the SPR peak gradually shifts from 410 to 380 nm, while its width decreases. This effect is caused by the reduction of silver ions on the surface of particles and the “pumping” of the latter with additional electrons due to the discharge of CO 2− ion-radicals on them. An increase in the concentration of electrons in a particle induces in the growth of the SPR-band intensity ((ISPR)) and its short-wavelength shift [22, 23]. The process is ended by the complete reduction of Ag+ ions in the solution, which is evident from the achievement of a stationary optical absorption of the metal particles. COLLOID JOURNAL
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Fig. 1. Optical absorption spectra of deaerated solution containing Ag+ ions (3 × 10−4 mol/L) and C 2O 42 − ions (1 × 10−3 mol/L) after UV irradiation for (1) 0, (2) 30, (3) 60, (4), 69 (5) 78, (6) 87, (7) 96 and (8) 106 s. The insert shows variations in the SPR peak intensity (ISPR) as depending on the irradiation time.
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The TEM data indicate the formation of silver nanoparticles with a predominantly spherical shape and a narrow size distribution (Fig. 2). For example, nanoparticles formed in a deaerated solution containing Ag+ ions (3 × 10−4 mol/L) and C 2O 42 − ions (1 × 10−3 mol/L) have an average size of 10.3 ± 3.2 nm. Therewith, the average hydrodynamic diameter of the particles measured by DLS (with allowance for the shell composed of carbonate ions) is 12.2 ± 1.7 nm (Fig. 3), which is close to the size of the metal core determined by TEM. The ζ potential of the formed colloidal particles is equal to −68.3 mV. The high absolute value of the ζ potential must provide the hydrosol with a high stability to aggregation. It is worth noting that silver nanoparticles with almost the same sizes were obtained in a solution with a lower concentration of Ag+ ions equal to 1 × 10−4 mol/L. During 21-day storage, the light absorption of the deaerated colloidal silver solution undergoes some transformations (compare spectra 1 and 2 in Fig. 4). For example, the intensity of the SPR band at 380−420 nm, which is sensitive to the surface state of nanoparticles, decreases by nearly 15%. In addition, the absorption in the range of 250−300 nm attributed
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Fraction of particles, % 30
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to the electronic interband transitions in the metal (d−d-transitions) slightly decreases. In our opinion, the aforementioned changes are caused by the fact that the excess charge of a nanoparticle, which is accumulated as a result of the reductant discharge on the particle surface, disappears, and the particle “relaxes” to Extinction 3.5 3.0
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Fig. 4. Time variations in the absorption spectrum of a silver hydrosol: (1) immediately after the preparation; (2) after 21-day storage in dark; and (3−6) 30 min, 22 h, 28 h, and 4 days after the contact with air. The synthesis conditions are presented in the caption for Fig. 2.
the equilibrium with the environment. Immediately after the colloidal solution is brought in contact with air, a slight red shift (by nearly 3 nm) is observed in the SPR peak, and, in 24 h, the peak position reaches 396 nm. A further exposure of the hydrosol in air leads to more pronounced changes in its optical spectrum. The intensity of the SPR band at 400 nm decreases with a concomitant appearance of a band having a maximum within a range of 500−600 nm (see Fig. 4). This may be attributed to the formation of some amount of silver nanoparticle aggregates. The TEM examination of the hydrosol exposed in air has shown that particles coalesce into chains (Fig. 5). However, it is seen that the absorption at 250−300 nm attributed to the interband transitions remains almost the same (Fig. 4). This fact leads us to conclude that the concentration of the metal itself also remains actually unchanged. Further (for 1 or 2 months), the optical absorption of the hydrosol remains almost constant, thereby indicating its high stability. The ζ potential of silver particles in a deaerated colloidal solution remains at nearly the same level (‒60…−70 mV) for first several weeks after its preparation. The TEM data have shown that, in a colloidal solution stored under anaerobic conditions, the size (as well as polydispersity) of the particles does not change significantly and amounts to 13.4 ± 3.4 nm (Fig. 2b). The hydrodynamic diameter of silver nanoparticles varies also insignificantly: it is equal to 12.2 ± 1.8 (Fig. 3a) and 13.3 ± 4.7 nm (Fig. 3b) immediately after the synthesis and after 21-day storage, COLLOID JOURNAL
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10 nm Fig. 5. TEM image of silver nanoparticles taken after 4-day exposure of a hydrosol in air. The synthesis conditions are presented in the caption for Fig. 2.
respectively. The absolute value of the ζ potential higher than 60 mV is indicative of a high stability of the silver hydrosol stabilized with carbonate ions. The aggregation of nanoparticles is hindered by a decrease in the interfacial tension due to the formation of the electrical double layer on their surface. The appearance of the electric potential at the interface is caused by the adsorption of carbonate ions. The selection of the optimal potassium oxalate concentration for the hydrosol synthesis is determined by two factors: the insurance the complete reduction of Ag+ ions and providing the non-aggregating sol. Systematic experiments have indicated that, for a (1−3) × 10–4 mol/L Ag0 colloidal solution, this is achieved at the concentration of oxalate ions equal to (5−10) × 10–4 mol/L. Their photochemical decomposition is accompanied by the formation of (1−2) × 10– 3 mol/L carbonate ions. The same concentration was optimal for the synthesis of a carbonate-ion-stabilized silver hydrosol via the reduction of Ag+ ions with borohydride [14, 15]. Thus, the results obtained have shown that the UV irradiation of deaerated solutions containing (1−3) × 10–4 mol/L Ag+ ions and (3−10) × 10–4 mol/L oxalate ions leads to the formation of a silver hydrosol with nanoparticle sizes of 4–16 nm (the average size is 10.3 ± 3.2 nm). ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research, project nos. 15-03-02068-a and 15-03-04854-a.
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Translated by E. Khozina
