Studies on the annealing and antibacterial properties of the silver-embedded aluminum/silica nanospheres
© Pan et al.; licensee Springer. 2014
Received: 13 May 2014
Accepted: 9 June 2014
Published: 17 June 2014
Substantial silver-embedded aluminum/silica nanospheres with uniform diameter and morphology were successfully synthesized by sol-gel technique. After various annealing temperatures, the surface mechanisms of each sample were analyzed using scanning electron microscope, transmission electron microscope, and X-ray photoelectron spectroscopy. The chemical durability examinations and antibacterial tests of each sample were also carried out for the confirmation of its practical usage. Based on the result of the above analyses, the silver-embedded aluminum/silica nanospheres are eligible for fabricating antibacterial utensils.
During the last decades, owing to the greenhouse effect, the climatic anomaly has happened around our planet. The change in temperature is dramatic all over the world, the disease-causing germs threat humankind seriously, and consequently, various antibacterial materials, such as zeolite-based [1, 2] and phosphate-based [3, 4] compositions, ZnO nanoparticles [5, 6], TiO2 nanoparticles [7–9], Ag nanoparticles [10–13], and core-shell silica-metal nanocompositions [14–16] are developed by researchers. Generally, the core-shell silica-silver nanocomposition is the most promising item among antibacterial materials due to its unique physical and chemical properties [17, 18]. Nanosilver owns a large specific surface area and a high fraction of surface atoms; silica nanospheres, a typical dielectric core material for immobilized nanoparticles, has a good chemical and thermal stability, chemical inertness, large surface area, and outstanding compatibilities with several kinds of metal nanoshells. Moreover, sundry publication studies on core-shell silica-silver nanostructures focused on various manufacturing, optoelectronics, and catalytic and antibacterial features. It is common knowledge that the size, shape, and density of metal nanoshells and the combination of shells and cores mainly affect the presentations of the core-shell silica-silver nanocomposition, so many research groups have devoted their energies to discovering diverse ways for a very efficient sample. Therefore, throughout the previous literatures referring to the syntheses of the core-shell silica-silver nanocompositions, two approaches are majorly adopted: seed-mediated method and layer-by-layer (LbL) self-assembly technique [19, 20].
The seed-mediated method , similar with electroless plating , includes two steps. In the first step, the surfaces of core materials are activated by chemicals or metals; in the second step, handling the redox reaction leads Ag+ ions into metallic Ag, which coats the SiO2 nanocores with tiny Ag nanoshells. In order to obtain the accurate thickness of shells, these two steps must be carried out again for several times, which means that this method is a great consumption and its products as impurities. The idea of LbL self-assembly technique is based on the alternate depositions derived from the electrostatic interactions between the charged components for multilayer growth on colloid particles. According the earlier published papers [22, 23], LbL self-assembly technique is smart but time-consuming and unsuitable for production line. No doubt dealing with the drawbacks of above two major approaches would be the priority for researchers.
In this task, the authors synthesized the silver-embedded aluminum/silica nanospheres by sol-gel technique, a one-step method modified from Stöber method. The ideal silver-embedded aluminum/silica nanospheres for commercialization have to be manufactured under a high working temperature and keeping the color pure and white after work. For example, combinations of ceramics and antibacterial powders to make human-friendly utensils need to be heated and maintained at a high working temperature for calcination or sintering. However, there are few literatures putting emphasis on the annealing-induced properties of the silver-embedded aluminum/silica nanospheres, especially at high working temperatures above 600°C. Herein, authors did the annealing separately at 250°C, 400°C, 600°C, 800°C, and 1,000°C and investigate the annealing-induced properties of the silver-embedded aluminum/silica nanospheres via material and UV-visible (UV-vis) analyses. To follow the capabilities of these products realistically, the chemical durability test and antibacterial examination were handled as well.
Five reagents are used in this task as follows: tetraethyl orthosilicate (TEOS, 98%, Alfa Aesar, Ward Hill, MA, USA), absolute ethanol (99.99%, Sigma-Aldrich Corporation, St. Louis, MO, USA), silver nitrate (99.9%, Mallinckrodt Pharmaceuticals, Dublin, Ireland), aqueous ammonia solution (30% to 33%, Sigma-Aldrich), and aluminum nitrate enneahydrate (Al(NO3)3 · 9H2O, Alfa Aesar). All reagents without additional purification and deionized (DI) water (Ω >1018) were used in all processes.
Synthesis of silver embedded aluminum/silica nanospheres and annealing conditions
After obtaining as-prepared powders, silver-embedded aluminum/silica nanospheres, the annealing processes were performed in the furnace. The as-prepared powders in an aluminum boat were placed in the heating area of the quartz tube, and the pressure inside the quartz tube was kept up at 5 × 10−2 Torr by a rotary pump. The working conditions of this process were as follows: (1) The temperature was raised separately to five working parameters (250°C, 400°C, 600°C, 800°C, and 1,000°C) with the heating rate of 1.7°C/min. (2) The temperature was maintained separately at five working parameters (250°C, 400°C, 600°C, 800°C, and 1,000°C) for 1 h. (3) Each sample was cooled separately to room temperature. In the above steps, (1), (2), and (3), O2 gas was continuously injected into the quartz tube at a constant flow rate of 4 sccm. The five annealing temperatures in this experiment were 250°C, 400°C, 600°C, 800°C, and 1,000°C, and these five samples obtained were termed as SAS-250, SAS-400, SAS-600, SAS-800, and SAS-1000, respectively.
Characterizations of silver-embedded aluminum/silica nanospheres
The morphologies of each sample were characterized by scanning electron microscope (SEM, JSM-6500 F; JEOL Ltd., Tokyo, Japan) and high-resolution transmission electron microscope (HRTEM, JEM-2010). X-ray photoelectron spectroscopy (XPS) spectra were analyzed at an angle of 0° using PerkinElmer model PHI 1600 system (PerkinElmer Inc., Waltham, MA, USA) with Mg Kα line as an X-ray source and the energy resolution was 1.6 eV; all the deconvolutions of XPS curves were performed with the XPS Peak Fitting Program (XPSPEAK41, Chemistry, CUHK; Informer Technologies Inc., Copthall Roseau Valley, Dominica). UV-visible absorption spectra were recorded using a Hitachi U-3010 spectrophotometer (Hitachi Technologies, Shanghai, China).
Chemical durability test of silver-embedded aluminum/silica nanospheres in water
In this test, the focus was on the products after annealing at high temperature, so SAS-250 was ignored. First, each of the five samples of 0.25 g powder was dispersed in 10 ml water, and the Si, Al, and Ag ions released were determined after 2, 4, 6, 8, and 10 days of immersion. Second, 2 ml HNO3 (0.5 mM) was added into the retrieved solution. The concentrations of Si, Al, and Ag ions released from the sample into the water were measured using inductively couple plasma atomic emission spectroscopy (ICP-AES, ICAP 9000; Thermo Jarrell-Ash, Franklin, MA, USA) with a detectable limitation of 1 ppb.
Antibacterial examination of silver-embedded aluminum/silica nanospheres
Owing to the tracing of the antibacterial activity of products accepting annealing at a high temperature, SAS-250 was neglected in the above test. This antibacterial examination was based on Japanese Industrial Standards (JIS) Z 2801 method designed to test the antimicrobial activity or efficacy of plastics. Two bacterial types, Escherichia coli (E. coli) 8739 and Staphylococcus aureus (S. aureus) 6538P, were used for the antibacterial assays; E. coli and S. aureus belong to Gram-negative and Gram-positive bacteria, respectively. Thus, our products were verified as available for practical application in various antibacterial activities by this examination. As to the sample preparations of this test, first, a bulk using rolling depression was made by mixing 0.01 g sample powders and 0.99 g polyethylene (PE) powders at 180°C for 5 min. Next, transforming the bulk to plenty of crushed grains was done by a cutting machine. Then, forming the moderately crushed grains into an antibacterial film was carried out by depression machine at 180°C, and the average thickness of each film was less than 0.05 mm. To investigate the comparisons objectively, the placebo film was made of 100% PE as well.
The inoculum was dispersed and diluted with 1/500 NB as appropriate so that the amount of bacteria of the test inoculum was in the range of 2.5 × 105 to 10 × 105 cells/ml. After each test, the film was placed in a sterilized petri dish, using a pipette, and we took exactly 0.4 ml of the test inoculum and then instilled it onto each test film. Next, in order to incubate bacteria, the authors set the temperature of the petri dish containing the test film inculcated with the test inoculum at 35°C and a relative humidity of not less than 90% for 24 h. After incubation, the number of colonies was counted in a serially diluted petri dish. The results of this antibacterial examination will be discussed in ‘Antibacterial analysis of silver-embedded on aluminum/silica nanospheres’ section.
Results and discussion
Morphologies of silver-embedded aluminum/silica nanospheres
Al ions and Ag ions are released by the hydrolysis of silver nitrate and aluminum nitrate enneahydrate in solution B. After adding moderate ammonia drops, the combination of hydroxyl ions and a lot of cations is reacted in solution B, thus producing AgOH and Al(OH)3.
Therefore, due to this redox, the silver particles are deposited on the silica surface. Additionally, De et al.  indicate that the Ag characteristic peaks were exhibited in X-ray diffraction (XRD) pattern, since the sample accepted the annealing at and above 200°C. As for the sample which accepted the annealing under 200°C, the XRD pattern without Ag signal was also verified.
XPS analysis of silver-embedded aluminum/silica nanospheres
XPS results of Ag 0 and Ag + by deconvolution of Ag (3 d 5/2 ) curve
Percentage of Ag0
Percentage of Ag+
In terms of silver oxides, no doubt they are easily decomposed under elevated temperature. However, there are no silver oxides in each sample of our experiments. The root cause is that the nanosized Ag would be produced during annealing and further vaporized at higher temperature with constant air flow.
UV-visible spectrum analysis of silver-embedded aluminum/silica nanospheres
Chemical durability analysis of silver-embedded aluminum/silica nanospheres
Antibacterial analysis of silver-embedded aluminum/silica nanospheres
Results of the antibacterial examinations
Staphylococcus aureus ATCC 6538P
2.8 × 105
2.8 × 105
2.8 × 105
2.8 × 105
2.8 × 105
2.8 × 105
2.9 × 104
Escherichia coli ATCC 8739
9.3 × 106
9.3 × 106
9.3 × 106
9.3 × 106
9.3 × 106
9.3 × 106
Inoculation time 24 h
7.2 × 106
In summary, sol-gel technique has been successfully expanded to synthesize the silver-embedded aluminum/silica nanospheres, and the diameter of the nanospheres is about 500 nm. The ratios of Ag+ ions to Ag atoms on the surface of each sample deeply affect the optical and chemical features and are proportional to the annealing temperatures. Judging by the chemical durability and antibacterial determinations for antibacterial usages in practice, all samples are eligible but SAS-1000, which indicates that silver-embedded aluminum/silica nanospheres would be the promising candidate for manufacturing antibacterial utensils.
KYP is a PhD student at the National Tsing Hua University in Taiwan, R.O.C. and have devoted much attention to the research of nanomaterials. CHC got a master's degree from the National Tsing Hua University and now is an engineer in TSMC. YCP got a PhD degree from National Chiao Tung University and is currently a postdoctoral fellow at UC Santa Cruz in USA. CML is currently a senior supervisor in the Nano Technology Research Center. YJH is currently a vice professor at National Chiao Tung University. JWY is currently a professor at National Tsing Hua University. HCS is currently a professor at National Tsing Hua University and Chinese Culture University.
This task was sponsored by the National Science Council of the Republic of China (Taiwan) under grants NSC-102-2221-E-034-003.
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