Powder Technology 215-216 (2012) 219–222
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Silver-doped silica powder with antibacterial properties Askwar Hilonga a, b, Jong-Kil Kim a, b, c, Pradip B. Sarawade a, c, Dang Viet Quang a, Godlisten Shao a, Gideon Elineema a, Hee Taik Kim a,⁎ a b c
Department of Chemical Engineering, Hanyang University 1271 Sa 3-dong, Sangnok-gu, Ansan-si, Gyeonggi-do 426-791, Republic of Korea Nelson Mandela African Institute of Science and Technology in Arusha, Tanzania E&B Nanotech. Co., Ltd, Republic of Korea
a r t i c l e
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Article history: Received 8 March 2011 Received in revised form 30 June 2011 Accepted 30 September 2011 Available online 8 October 2011 Keywords: Silver-doped Sodium silicate Sol–gel method E. coli Antibacterial
a b s t r a c t In this study a simple and reproducible method was used to develop silver-doped silica powder with antibacterial properties. Silica matrices were synthesized via a sol–gel route which allows one to easily tailor textural and chemical properties. A wide range of silica-materials/products was obtained via the present route. These are: pure silver nanoparticles (Ag0), silver in ionic state (Ag+), AgCl nanoparticles, and the mixture of Ag0 and AgCl. The efficacy of these products were tested against Escherichia coli and the results demonstrate that materials that are suitable for antibacterial applications were obtained by this newly developed technique while utilizing sodium silicate, which is relatively inexpensive, as a silica precursor. This may significantly boost the industrial production of the inexpensive silver-doped silica products for various applications. A project on other innovative industrial applications of our products is in progress. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Silver embedded silica has been intensively studied in recent years due to its potential application in various fields [1–3 and references therein]. Several studies are currently reported, implying that this subject still attract attention of many researchers [4–6]. There are several methods for preparing silver-doped silica including ion exchange, multi-target sputtering, meltdown, flame spray pyrolysis, sol–gel process, and nanoporous template [7–9]. Among these methods, sol–gel has many advantages such as high purity, low processing temperatures, and homogeneity. The sol–gel method, however, has mostly been carried out on the laboratory level and has not yet been widely used in industrial processes because of the high cost of traditional precursors. The most commercially available silicon alkoxyde precursors are expensive, soluble in alcohol (additional cost), and require special safety precautions. This has led to increasing interest on the development of inexpensive water-based silica sol solutions (such as sodium silicate) suitable for large economic industrial applications [1,2,4]. Silver and silver-based compounds are well-investigated antimicrobial agents being biocompatible and non-toxic to human cells at concentrations effective against microorganisms when in the form of non-agglomerated and well dispersed nanoparticles [4,10–12]. These products have also emerged as friendly antibacterial materials
⁎ Corresponding author. Tel.: + 82 31 400 5493. E-mail address:
[email protected] (H.T. Kim). 0032-5910/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2011.09.051
for water treatment. It proved to be effective for bacterial removal from the supplied water at the point-of-use [13]. The use of silver nanoparticles is particularly potential to treat the nosocomial water which is frequently infected with antibiotic resistant bacteria [13]. Treatment of this water usually requires using high concentrated chlorine compounds which may cause a high risk of human cancer [14]. Generally, the silver-doped materials have several advantages such as high performance, low price (compared to pure silver), high chemically durability, and release silver ions for a long period of time [3]. However, few disadvantages of this material can also be pointed out. For instance, silver nanoparticles which are used for various applications (as highlighted earlier) might be released in water and can prove detrimental to the purity of water. Silver nanoparticles are bacteriostatic, which means they limit the growth of bacteria. This may result in the destruction of bacteria that help in breaking down the organic matter in water treatment plants. Due to the unknown health risks of metal nanoparticles there is also a clear need for the formation of films (containing nanoparticles powders) in which nanoparticles are tightly attached and their diffusion to the environment is inhibited [15]. Moreover, aluminum ions were also used to reinforce the silver-doped nanoparticles in order to improve the properties of the final product. It was previously reported that the aluminum ions promote chemical durability of silver-doped silica gel [1,2,16]. They dramatically decrease the elusion of silica ions into the aqueous solution and induce slow release of silver ion over a long period of time. In this work we are reporting versatile approach to synthesize materials/products that are in powdery form and are very effective as
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Scheme 1. Proposed scheme for the changes in silver state for the sample which was washed with ethanol and annealed in air condition [1].
antibacterial agents. We have conducted investigations on the efficacy of silver-doped products, namely, pure silver nanoparticles (Ag0), silver in ionic state (Ag+), AgCl nanoparticles, and the mixture of Ag0 and AgCl. The un-embedded product was also prepared and subjected to the same treatment to serve as reference. A project on further innovative industrial applications of these products is in progress. 2. Experimental 2.1. Materials Sodium silicate (24% SiO2, 7.4% Na2O) was from Shinwoo Materials Co. Ltd., South Korea. Ammonia solution (28% NH3), hydrochloric acid (36% HCl), silver nitrate (AgNO3) was purchased from Duksan Chemical. Aluminum isopropoxide (Al(OC3H7)3) was from ALDRICH. 2.2. General preparation method The method of preparation of silver-doped silica is reported by Hilonga et.al [1,2]. Generally, it involves polycondensation of silica precursor with various amounts of AgNO3 previously mixed with ammonia solution. In this study, fixed amounts of 4 g of sodium silicate (24% SiO2, 7.4% Na2O) and 0.204 g of aluminum isopropoxide (Al (OC3H7)3) were dissolved in 30 ml of 2 N HCl solution at room temperature and stirred for 10 min. Then 10 ml of 28% NH3 solution containing 1 × 10−3 M of AgNO3 was slowly added and stirred for 2 h. The as-synthesized samples obtained were treated in various ways to develop products of different kinds. The washing procedure and the annealing condition were found to be the key factors for the development of the diverse products reported in this study. Also the un-embedded samples were prepared and subjected to the same treatment to serve as reference. The final products, as briefly described in the subsequent sections, were then examined for their antibacterial efficacy.
3. Results and discussion The colors of the products obtained in this work were similar to our previous reports [1,2]. In short, the un-embedded product (before and after calcination at any temperature) as well as the Ag + appear colorless. On the other hand, Ag 0 and AgCl are yellow or brown/ gray depending on the concentration of silver/size of silver nanoparticles in the silica matrix. The appearance and disappearance of colors is claimed be associated with the change in the state of silver (Ag 0 or Ag +) at various calcination temperatures as demonstrated using XRD characterization technique. Mechanisms for the change in silver state with the increase in calcination temperature are proposed in Schemes 1 and 2 [1,2]. 3.1. AgCl nanoparticles This product was prepared by washing the as-synthesized sample with ethanol and drying it at 110 °C for 2 h; without further treatment. The characteristic five major peaks at 27.8, 32.2, 46.3, 54.8 and 57.5° correspond to the (111), (200), (220), (311) and (222) crystal planes of AgCl, as demonstrated in Fig. 1. The results indicate that AgCl is contained in this product and there is no mixture of silver nanoparticles, as evidence by the absence of Ag0 peaks. The average grain size of AgCl nanoparticles is 35 nm as calculated using Scherrer's equation: D = Kλ/(βcosθ), where K is Scherrer's constant, 0.9; λ the X-ray wavelength, 1.5406 Å; β is the full width at half-maximum (FWHM) of the diffraction peaks, rad; and θ is the Bragg diffraction angle [17]. 3.2. Mixture of Ag 0 and AgCl The mixture of Ag0 and AgCl was obtained when the as-synthesized sample was washed with ethanol and calcined at 600 °C for 2 h under the air (oxygen) annealing condition. This product revealed the presence of both Ag0 and AgCl peaks. The peaks at 38.2 and 44.4° confirmed
Scheme 2. Proposed scheme for the changes in silver state for the sample which was washed with water and annealed in inert atmosphere (argon) [2].
A. Hilonga et al. / Powder Technology 215-216 (2012) 219–222
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Fig. 3. The XRD patterns of pure silver nanoparticles (Ag0).
Fig. 1. The XRD pattern of AgCl nanoparticles.
the presence of silver nanoparticles (Ag0) with an average grain size of 43 nm, Fig. 2. 3.3. Pure silver nanoparticles (Ag 0) Pure silver nanoparticles (Ag 0) were obtained when the assynthesized sample was washed with water and calcined at 600 °C for 2 h under the inert (argon) annealing condition. The only peaks appearing at 38.2 and 44.4° in Fig. 3 confirmed the presence of pure silver nanoparticles (Ag 0). Silver nanoparticles appear as black spots spreading throughout the silica matrix, as confirmed by TEM micrograph in Fig. 4.
annealing process whereby its environment determines the final oxidation state [18]. In the pure silica matrix, Ag + ions are found to be unstable and tend to reduce to Ag 0 at higher temperatures (≥ 800 °C) but when aluminum is used as a reinforcing agent the [AlO4] − tetrahedral is formed [2,16]. The negative charge of [AlO4] − is compensated by the silver ions and, as a result, the Ag+ is localized to the [AlO4] − and does not exist as silver nanoparticles. The existence of silver in Ag+ state in silica matrix is more beneficial because it was reported that in this state the Ag ions were slowly release in water [16]. Moreover, the silver ions could only be released from the powders via ion exchange between Ag+ and hydronium (H3O+) ion from the surrounding water. The release of Ag ion is controlled by the rate of interdiffusion of these ions within the solid powders. 3.5. Antibacterial test
The silver in ionic state (Ag +) was obtained when the assynthesized sample was calcined at 1000 °C for 2 h regardless of the annealing condition. Fig. 5 shows that the samples calcined at high temperature are generally amorphous. XRD pattern for the un-embedded product is similar to that of the Ag+. It reveals amorphous character; no crystalline peak was observed before and even after calcination. Although the XRD pattern of Ag+ appears like that of un-embedded sample, it does not justify the absence of silver in this product. It was previously reported that Ag+ ions incorporated in different silicabased matrices tend to segregate towards the interface during the
In this study, Escherichia coli (E. coli), a gram negative bacterium, was selected as an indicator to test the antibacterial activity of the synthesized products. Nutrient agar was poured onto the Petri dishes and allowed to solidify. Bacteria were spread on the plate uniformly. Each of the synthesized products was gently placed on the solidified agar gel. Plates were incubated at 37 °C for 24 h to check the zone of inhibition. Fig. 6 shows the results of inhibitory zone of each of the products against E. coli. Generally, all silver-doped products showed good antibacterial activity, while un-embedded product has no inhibitory effect. In particular, the zone of inhibition is almost similar for all silver-doped products as demonstrated by arrows affixed in Fig. 6. These results indicate that the silver-doped products have an excellent antibacterial performance against E. coli. This might also apply to other microorganisms. It was previously reported that silver binds with protein molecules, causing inhibition of cellular metabolism and final
Fig. 2. The XRD pattern of the mixture of Ag0 and AgCl.
Fig. 4. TEM micrograph of silver nanoparticles (Ag0) doped in silica matrix.
3.4. Silver in ionic state (Ag +)
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4. Conclusions
Fig. 5. The XRD patterns of Ag+; similar to un-doped product (pure silica (SiO2)).
A wide range of silica-materials/products with antibacterial properties that were synthesized using inexpensive silica precursor were obtained and tested against E. coli bacteria to examine its efficacy. The products obtained are: pure silver nanoparticles (Ag 0), silver in ionic state (Ag +), AgCl nanoparticles, and the mixture of Ag 0 and AgCl. In the antibacterial activity tests, the powders containing Ag 0, Ag + and AgCl nanoparticles were compared with the un-embedded product (pure silica, SiO2). All silver-doped products showed large zone of inhibition than un-embedded product. A detailed investigation is required on the releasing mechanism of silver from each of the products reported in this work and antibacterial performance against other microorganisms.
Acknowledgments This work was supported by the research fund of Hanyang University (HY-2010-N).
References
Fig. 6. (a) AgCl nanoparticles, (b) pure silver nanoparticles (Ag0)/Ag+, (c) mixture of Ag0 and AgCl, (d) un-doped (pure silica (SiO2)).
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