Colloids and Surfaces A: Physicochemical and Engineering Aspects
Preparation of high concentration of silver colloidal nanoparticles in layered laponite sol
Introduction
Colloidal nanometals and semiconducting nanoparticles with controllable morphology and size are of great interest in recent years due to their potential applications in microelectronics, optical, electronic, magnetic devices and their high catalytic activity [1], [2], [3], [4], [5]. To develop effective preparation methods of nanoparticles with well-controlled shapes and sizes, surfactants or protective colloids have been used to prevent the growth and aggregation of nanoparticles [6], [7], [8], [9], [10], [11]. Silver nanoparticles are also stabilized by kaolinite/DMSO complexes and are positioned within the interlayer space and on the external surfaces and the edges depending on particle sizes [12]. The interlamellar space in kaolinite limits particle growth to give small silver nanoparticles (3.8–4.2 nm) and the average size of silver particles formed increases with increasing the initial Ag+ concentration [13]. Similarly silver nanoparticles in limited concentration ranges have been chemically synthesized in the presence of laponite which acts as effective inorganic protective colloids [14]. Laponite is a synthetic polycrystalline similar in structure and composition to natural hectorite of the smectic group. The basic layered structures are builted up from two outer tetrahedral silica sheets and a central octahedral magnesia sheet. Some magnesium ions in the central sheet are substituted by lithium ions, resulting in a net negative charge of the layer, which is balanced by sodium ions located between adjacent layers in a stack. The empirical composition of laponite is Na0.7[(Si8Mg5.5Li0.3)O20(OH)4]. The sodium ions in laponite are exchangeable and in aqueous dispersions these ions diffuse into the water and plate-like particles with negatively charged faces are formed [15], [16]. Silver nanoparticles in laponite sol could be suitable to use antibacterial applications in human use, since laponite is viewed as nontoxic and environmentally inert material and used for biological fluids, cosmetics, pharmaceuticals and food. As the material to be used in wide ranges of applications of antibacterial devices, stable and high concentrations of silver nanoparticles are required. In this paper, the optimum experimental condition for preparing silver colloidal particles is described in terms of initial concentrations of AgNO3 and NaBH4.
Section snippets
Experimental
Laponite RDS (Rockwood) was used without further purification and was considered as an anionic material with negative charge (cationic exchange capacity) of about 50 mmol/100 g. Laponite RDS (4.60 g, 2.30 mmol of negative charge) in 500 ml distilled water was vigorously stirred for half an hour. When laponite solution turned to be transparent, excess NaBH4 (Aldrich, 98%, 0.444 g, 11.5 mmol) was added into laponite solution and stirred for five more minutes. Separately, analytically pure AgNO3 (0.390 g,
Colloidal silver sols in laponite with various initial concentrations of AgNO3
The order of reactant addition, dropping silver nitrate solution containing ammonia water into laponite solution with NaBH4 is important to get high concentration of silver nanoparticles and this is the standard procedure in a preparation of silver hydrosols [17]. The reverse order of reactant addition gave immediate precipitation of silver metals. The color of the solution was depended on concentrations of added AgNO3. As the concentration of AgNO3 was increased, the color of solution was
Dependence of nanosilver aggregation on the concentrations of NaBH4
Varying molar ratios of sodium borohydride to silver nitrate (NaBH4:AgNO3) were attempted at 0.928 mM of silver nitrate solution in the presence of laponite in which equimolar negative charge was contained. UV–vis spectra of different molar ratio of sodium borohydride to silver nitrate are shown in Fig. 3. N is used as molar ratio of sodium borohydride to silver nitrate (NaBH4:AgNO3) for convenience. When small amount of sodium borohydride (N = 1/2) was used, broad absorption peaks at about 390
Conclusions
High concentrations of silver colloidal sol have been obtained by using laponite as protective colloids. Literatures usually shows low concentrations of silver nanoparticle (e.g. <10−4 M) [14], [17]. Aggregation of silver nanoparticles are closely related to initial concentration of AgNO3 and amounts of NaBH4. Small silver nanoparticles adsorbed to laponite aggregates in high concentration of AgNO3 and low concentration of NaBH4. These results are proved by UV–vis absorption and transmission
Acknowledgement
The present research was conducted by the research fund of Dankook University in 2006.
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Present address: Institute of Polymer Science, Sun Yat-sen University, Guangzhou 510275, PR China.