Room-temperature synthesis of silica supported silver nanoparticles in basic ethanol solution and their antibacterial activity

Abstract

A facile and environmentally friendly route was developed to synthesize silica supported silver nanoparticles (Ag NPs) through the reduction of silver ions in basic ethanol solution without adding any other reducing agents or surfactants at room temperature. The structure, morphology and composition of as-prepared samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). It was found that the molar ratio of sodium hydroxide to silver nitrate was a decisive factor for the composition of the final products. If the molar ratio was larger than 1.1, the final product was pure silver particles; otherwise, part of the product was silver oxide. Moreover, it was also found that water had a negative influence on the formation of silver particles. For a simple experimental process, this is an efficient and facile method to synthesize silica supported Ag NPs in ethanol at room temperature. Additionally, since the as-prepared Ag NPs were not encapsulated with surface modifier, the Ag NPs with more active atoms exposed consequently exhibited excellent antibacterial activity against Escherichia coli.

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1. Introduction

Silver nanoparticles (Ag NPs) have been of great interest during the past decades because of their unique chemical and physical properties, which lead to their potential applications in optics, electronics, biomedicine, sensors and catalysis.^1–7 In these frequently investigated fields, it has been experimentally demonstrated that the properties of Ag NPs strongly depend on their size and shape.^8,9 Smaller particles have larger specific surface area and expose more active atoms on the surface, hence exhibiting superior activity.¹⁰ However, smaller nanoparticles are more likely to aggregate due to their higher surface energy, thus Ag NPs were usually protected by surface modifiers (such as polymers and organic ligands) in synthesis process.^2,8,9 Unfortunately, these organic modifiers would cap around the surface and interact with Ag NPs, which consequently lead to the attenuation of their properties. Hence loading Ag NPs on inorganic supporting matrix has been taken into consideration as one of the applicable strategies.^11,12 Among the inorganic solid supports, silica was regarded as ideal carrier and frequently used due to its excellent stability and surface chemistry.^6,7,13–17 And the opposite charge makes silver cations (Ag⁺) easily attach to the surface of silica spheres by electrostatics attraction.^18,19

For the aforementioned reason, various approaches have been proposed for the synthesis of Ag NPs supported on silica.^13,20–23 Among these methods, synthesis of Ag NPs from the reduction of silver cations by alcohol is very interesting. Jiang et al. have reported the reduction of Ag⁺ ions in different water–alcohol solution in the presence of colloidal silica.²² Wang et al. found that the reduction of Ag⁺ in the adsorption layer was different from that in alcohol bulk, and Ag⁺ in bulk was reduced directly by alcohol at the temperature of 60 °C.²³ Although some particular conditions (such as special pH values, addition of water, temperature, et al.) were taken into account, and valuable discussion were made in their work, the content of base or molar ratio of NaOH to Ag⁺ has not been studied, and detailed mechanism of the reduction reaction has not been discussed. Here we adopted the analogous method to prepare Ag NPs supported on silica spheres, and batch verification experiments were conducted to elucidate the mechanism of the formation of Ag particles in basic ethanol.

In present work, silica spheres introduced as solid support were prepared by the classical Stöber method.²⁴ And silica supported Ag NPs were prepared with a facile route in basic ethanol at room temperature without addition of any other strong reductant and surfactant. And the formation process of Ag particles in basic ethanol solution was discussed. Meanwhile, the as-prepared silica supported Ag NPs were applied against Escherichia coli (E. coli) to evaluate its property.

2. Materials and methods

2.1 Materials

Silver nitrate (AgNO₃; 99.8%) was supplied by Aladdin Industrial Inc. Tetraethyl orthosilicate (TEOS), sodium hydroxide (NaOH), ammonium hydroxide (NH₃·H₂O; 28%), anhydrous ethanol were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). All the reagents were used without further purification.

2.2 Synthesis of silica supported silver nanoparticles

Silica spheres were prepared by the Stöber method.²⁴ TEOS was catalysed by ammonium hydroxide to hydrolyse and subsequently condense to form spherical silica in ethanol solution. As-prepared spherical silica powder (0.1 g) were mixed with AgNO₃ (0.0085 g) in 50 mL anhydrous ethanol under vigorous magnetic stirring. Then a certain amount of NaOH (0.12 M) ethanol solution was added to former mixed solution. After continuous stirring for 1 h, the reddish brown suspensions were treated with high-speed centrifugation to separate the silica supported Ag NPs out from ethanol solution.

2.3 Characterization of as-prepared samples

X-ray diffraction (XRD) data of the obtained samples were recorded on a Philips X′ Pert PRO diffractometer (PANalytical B.V., Holland) in the 2θ range from 10° to 90° with Cu Kα radiation (λ = 1.5406 Å). UV-visible (UV-vis) absorption spectra were recorded on a UV-2550 spectrophotometer (Shimadzu Co., Japan). The solid silica supported Ag NPs were dispersed in ethanol with sonication to form homogeneous suspension. The morphology of particles was observed with a field emission scanning electron microscope (FESEM, NOVA NANO 450, FEI, Holland) equipped with an X-ray energy dispersive spectrometer (EDS). And the Ag NPs were investigated with a transmission electron microscope (TEM, TECNAI G2 F30, FEI, Holland) at an accelerating voltage of 300 kV. The samples for TEM were prepared by dropping a drop of suspension on a Cu grid coated with ultrathin carbon film and drying at ambient condition. X-ray photoelectron spectroscopy (XPS) analysis was carried out on an X-ray photoelectron spectrometer (Kratos AXIS Ultra DLD-600W, Shimadzu Co., Japan) using monochromatic Al Kα radiation. And carbon 1s line at 284.6 eV was set as binding energy (BE) reference. The content of silver element in the composite was determined by an inductively coupled plasma-mass spectrometer (ICP-MS, ELAN DRC-e, PerkinElmer Inc., USA). A certain amount of composite was nitrated and then diluted with deionized water for measurement.

2.4 Study of antibacterial activity

The antibacterial activity of as-prepared samples was performed against Escherichia coli ATCC 25922. To activate bacteria, E. coli was inoculated in 50 mL nutrient broth and cultured for 15 hours in a constant shaker at 37 °C. After cultivation, the bacteria were centrifuged at 10 000 rpm for 1 min and then resuspended in phosphate buffer solution (PBS). It was worthy of note that all the mediums and samples were sterilized at 121 °C for 20 min at first. The antibacterial test was proceeded as follows: 2.5 mg of silica supported Ag NPs powder was added into 100 mL PBS which contained bacteria of ca. 10⁷ colony forming units per millilitre (CFU mL⁻¹). The mixture was stirred with magnetic stirring apparatus. At certain intervals, 0.5 mL suspension was pipetted out and serially diluted in 4.5 mL PBS. Then 0.1 mL of proper dilution was spread on an agar plate quickly and cultivated at 37 °C for 24 h. For each dilution, two plates were inoculated and cultured as control. The number of colonies on each plate was counted as the number of bacteria that remain activated. The average number with error bars was calculated by repeating the above process three times. The plot of the number of viable bacteria versus time describes the antibacterial capacity of as-prepared samples.

3. Results and discussion

3.1 Synthesis and characterization of silica supported Ag NPs

Here the synthesis process of silica spheres supported Ag NPs was schematic illustrated in Scheme 1. At the beginning, silver ions in ethanol coupled with silicon hydroxyl groups (Si–OH) on the surface of silica spheres via electrostatic interaction. Then hydroxide ions would preferentially react with silver ions to form silver oxide particles when sodium hydroxide was added. Thereafter either Ag₂O or Ag⁺ exposed on the surface of Ag₂O were reduced by ethanol in the presence of NaOH. As the reaction went on, all Ag⁺ ions (or Ag₂O) were reduced to form silver NPs decorated on the surface of silica spheres.

Scheme 1 Schematic illustration of synthesis of silica sphere supported Ag NPs.

Fig. 1a displayed the XRD pattern of as-prepared silica spheres supported Ag NPs. Five diffraction peaks at 2θ value of 38.1, 44.1, 64.3, 77.3 and 81.5° were corresponding to (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) crystallographic planes of face-centred cubic crystalline structure of silver, respectively (JCPDS card no. 00-004-0783).^14,25 And it was observed that there were no characteristic diffraction peaks of silver oxide. The low intensity and broadening of these peaks are due to the low content and the nanocrystal nature of Ag NPs.^19,25 While a broad peak in the range from 17° to 28° could be attributed to the amorphous silica. UV-vis spectra of silica supported Ag NPs and silica suspensions were shown in Fig. 1b. The absorption peak at 430 nm represents the characteristic surface plasmon resonance (SPR) band of spherical Ag NPs, while silica suspension shows no peak in this regime.²⁶ These results revealed the formation of Ag NPs.

Fig. 1 (a) XRD pattern and (b) UV-vis spectrum of as-prepared composites.

The morphology of the as-prepared silica supported Ag NPs was displayed in Fig. 2. As shown in Fig. 2a, the monodispersed silica spheres with an average diameter of 300 nm were decorated with nanoparticles. The further EDS elemental analysis of the samples was carried out on a component attachment of SEM instrument. EDS spectrum (Fig. 2b) exhibits the characteristic peaks of silver at ca. 3.0 keV accompanied by the peaks of Si and O, which also confirms the presence of Ag in the composites. Fig. 2c showed the representative TEM image, it is more clearly shown that well dispersed Ag NPs decorate on the surface of silica spheres. Furthermore, the HRTEM image shown in Fig. 2d suggests that the silica sphere is amorphous and the small particles have a lattice distance of 0.233 nm (as marked) corresponding to the (1 1 1) crystal plane of silver.

Fig. 2 (a) SEM, (b) EDS spectrum, (c) TEM of silica supported Ag NPs and (d): HRTEM image of silver nanoparticles.

To examine the elements and their valence states, XPS elemental survey was performed and shown in Fig. 3. The wide-scan spectrum (Fig. 3a) reveals the existence of carbon, oxygen, silicon, and silver elements on the surface of as-prepared silica supported Ag NPs, which is consistent with the EDS analysis. The high-resolution spectrum of Ag (Fig. 3b) is composed of two peaks at 368 eV (for Ag 3d_5/2) and 374 eV (for Ag 3d_3/2). And the spin energy gap of the two states is 6 eV, which is evident for Ag in the metallic form (Ag⁰).²⁷ The quantitative calculation results indicate that the mass concentration of Ag atoms in the composites is 22.17%, which is far higher than the actual content in the composites (5.93%, measured by ICP). The reason to explain this result is that only the elements located in the shallow layer of composites could be detected by XPS survey. It proves that Ag element concentrates on the surface of composites.

Fig. 3 (a) XPS survey spectrum of silica supported Ag NPs composites and (b) high resolution spectrum of Ag 3d.

3.2 Antibacterial activity of silica supported Ag NPs

Ag NPs have been considered as a superior antibacterial for a long period, due to their broad spectrum antibacterial activity without resulting in antibiotic resistance. Hence the as-prepared silica supported Ag NPs were applied in antibacterial, and it was found that the composites show superior antibacterial capacity, even very low dosage (25 mg L⁻¹, silver content: 1.5 mg L⁻¹) could make the bacteria inactivated in 80 min. According to the plot of bacterial concentration versus time (shown in Fig. 4), it was clearly demonstrated that the silica spheres (25 mg L⁻¹) do not have any influence on the activity of E. coli as same as the control. With silver particles (1.5 mg L⁻¹) added, there was almost no decay of bacterial activity as well. On the contrary, the bacterial activity was decayed very quickly and lost in 80 min when 25 mg L⁻¹ of silica supported Ag NPs composites was added into bacterial suspension. Fig. 5 shows the Petri dishes with nutrient agar inoculated with 0.1 mL E. coli suspension after interacting with antibacterial agents for 80 min and incubated for 24 h at 37 °C. It was more visualized that silica supported Ag NPs composites have superior antibacterial capacity. The reason is that Ag NPs supported on silica spheres were smaller and better dispersed than Ag particles prepared by the same method (Fig. S2†).

Fig. 4 Plots of bacterial concentration versus time with different antibacterial agents.

Fig. 5 Photographs of Petri dishes with 0.1 mL E. coli suspension after interacting with antibacterial agents for 80 min and incubated for 24 h at 37 °C: (a) control, diluted 10⁴ times; (b) 25 mg L⁻¹ silica spheres, diluted 10⁴ times; (c) 1.5 mg L⁻¹ Ag powder, diluted 10⁴ times; (d) 25 mg L⁻¹ silica supported Ag NPs composites (silver content 1.5 mg L⁻¹), undiluted.

3.3 Mechanism for the formation of Ag NPs

To well understand the reaction mechanism, we put forward batch verification experiments in which there were only silver source (AgNO₃), base (NaOH) and ethanol.

As is well known, sodium hydroxide reacts chemically with silver nitrate to produce silver oxide in aqueous solution. However, if anhydrous ethanol is substituted for water as solvent, the reaction becomes different and produces not only silver oxide but also metallic silver (shown in Fig. 6). The formation of metallic silver undoubtedly results from the alternation of solvent. The alternation of solvent makes the nature of the reaction transformed to redox. Since it is unlikely that hydroxide ions (OH⁻) serve as the reducing agent in the redox,²⁸ ethanol certainly involves in the reaction as a reductant. Additionally, the standard molar Gibbs free energies of ethanol, Ag⁺, and acetaldehyde are −174.8 (liq.), 77.1 (aq.), and −127.6 (liq.) kJ mol⁻¹, respectively. Thus the reduction of silver ions by ethanol is spontaneous under standard state according to the thermodynamic theory.

Fig. 6 XRD patterns of samples prepared with different molar ratio of NaOH to AgNO₃ in ethanol solution: (a) 0.5, (b) 1, (c) 1.1, (d) 1.2 and (e) 1.5.

As aforementioned, the reduction of Ag⁺ by ethanol to generate metallic silver is theoretically spontaneous under standard state. However, the reaction would indeed arise at an extremely low rate, or even not happen at room temperature in the absence of base. Instead, the reduction reaction would come up very fast in the presence of base. Thus, we believe that NaOH plays a crucial role in the reduction of Ag⁺ by ethanol. Wang et al.²³ and Huang et al.²⁹ have reported the formation of silver particles in basic alcohols. As described in their work, hydroxide ions preferentially react with Ag⁺ to yield Ag₂O small particles, then Ag clusters would form on the surface of Ag₂O particles and grow up to form small Ag particles via autocatalytic reactions. They did very excellent jobs, and the explanation for the reduction mechanism was interesting. However, both Ag⁺ and Ag₂O could not be reduced by base-free ethanol even in the presence of Ag particles. Hence, we do believe that hydroxide ions participate in the reduction reaction. And in our experiments, it was found that the molar ratio of NaOH to AgNO₃ was a decisive factor for the formation of silver particles (shown in Fig. 6). When the molar ratio was less than 1.1, the final product was the mixture of Ag and Ag₂O. This could attribute to that hydroxide ions were protonated during the oxidation of ethanol. The exhaustion of hydroxide ions makes the reaction cease.^28,29 And when the molar ratio increased to 1.1, the final product was pure silver powder. In this case, Ag⁺ ions could be fully transferred to the form of Ag₂O, and then these Ag₂O particles could be reduced by ethanol in the presence of residual hydroxide ions. These results are in accordance with that Ag₂O could be reduced into metallic silver by ethanol in the presence of NaOH (Fig. S3†). Besides, the standard electrode potentials of E⁰ (Ag⁺/Ag) and E⁰ (Ag₂O/Ag + OH⁻) were 0.7996 and 0.342 V, respectively. Evidently, Ag₂O particles are more easily reduced than Ag⁺. It is reasonable that hydroxide ions reacted with Ag⁺ firstly to produce Ag₂O particles, then Ag₂O particles were reduced by ethanol molecules in the presence of hydroxide ions. Concretely, the whole reduction process could be illustrated by the following reactions:

2Ag⁺ + 2OH⁻ ↔ 2AgOH ↔ Ag₂O + H₂O (1)

CH₃CHO + Ag₂O + OH⁻ → CH₃COO⁻ + 2Ag + H₂O (3)

Moreover, the volume ratio of water to ethanol also has significant influence on the reduction of Ag⁺ (as shown in Fig. 7). When the volume ratio is 1%, the final product is pure silver powder; as the percentage of water increases, the final products are mixture of silver and silver oxide; and when the percentage of water increases to 20%, the final product is pure silver oxide. It has been reported that the reduction of Ag⁺ was much slower in mixtures of water with alcohols than in pure alcohols.^22,23,29 Namely, water would substantially slow down the reduction of Ag⁺ in basic alcohol solution. As reported in their work, since both NaOH and AgNO₃ have higher solubility in water than that in absolute ethanol, more water in the solution results in the less Ag⁺ ions and hydroxide ions dissolved in ethanol. Then less Ag₂O particles contact with ethanol molecules, which consequently leads to the lower rate of the reduction. Generally speaking, water slows down the reduction by inhibiting the contact of reactants. This is another evidence for that the reduction process occur on the surface of Ag₂O particles.

Fig. 7 XRD patterns of samples prepared in different volume ratio of water to ethanol solution: (a) 1%, (b) 3%, (c) 5%, (d) 10%, (e) 20%. The molar ratio of NaOH to AgNO₃ was 1.2.

4. Conclusion

An efficient and facile method for preparation of silica supported Ag NPs was reported. Silica supported Ag NPs were generated from the reduction of Ag⁺ in basic ethanol at room temperature. All the analytic data (XRD pattern, UV-visible spectra, HRTEM and XPS survey) proved that Ag NPs was synthesized on the surface of silica spheres and elemental silver was in the metallic form (Ag⁰). And it was found that the formation of silver was determined by the molar ratio of NaOH to AgNO₃. The possible reaction mechanism for the formation of metallic silver in basic ethanol solution was expounded and proved with verification experiments. The advantage of this method was that no surfactant or other strong reducing agent was introduced into the synthesis process. Thus Ag NPs were not covered with organic ligands, and as-prepared composites exhibited superior antibacterial capacity against E. coli (ATCC 25922). 25 mg L⁻¹ Ag NPs composites (silver content: 1.5 mg L⁻¹) could make the bacteria (10⁷ CFU mL⁻¹) inactivated in 80 min.

Acknowledgements

The work was financially supported by the National Basic Research Program of China (Grant No. 2009CB939705) and the Fundamental Research Funds for the Central Universities (2014TS022). The authors gratefully thank the analytic and testing centre of Huazhong University of Science and Technology (HUST) for the experimental test (XRD, SEM, TEM, and XPS).

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Footnote

† Electronic supplementary information (ESI) available: SEM and TEM images of silica spheres, TEM image of silver particles, XRD patterns of samples resulted from Ag₂O in ethanol in the absence/presence of NaOH. See DOI: 10.1039/c5ra27053g

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