Structure and antibacterial activity of new layered perovskite compounds

TAN Shao-zao(谭绍早)¹, ZHANG Li-ling(张力玲)¹, XIA Liao-yuan(夏燎原)¹,

LIU Ying-liang(刘应亮)¹, LI Du-xin(李笃信)²

1. Department of Chemistry, Ji’nan University, Guangzhou 510632, China;

2. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China

Received 17 December 2006; accepted 2 February 2007

Abstract: New layered perovskite compounds, Ag_xNa_2-xLa₂Ti₃O₁₀ (x=0.2, 0.3 and 0.5) were synthesized by an ion-exchange reaction of Na₂La₂Ti₃O₁₀ with AgNO₃ solution and characterized by energy dispersive X-ray analysis(EDX), X-ray diffractometry(XRD), scanning electron microscopy(SEM) and X-ray photoelectron spectroscopy(XPS). The ion-exchange processes were optimized, and the antibacterial activity, light permanency and water-resistance were evaluated. Surprisedly, no significant changes in crystal structure of Na₂La₂Ti₃O₁₀ are found by the exchange of silver ions. The Ag_0.3Na_1.7La₂Ti₃O₁₀ particles conglomerate obviously with irregular shape and size. Ag_0.3Na_1.7La₂Ti₃O₁₀, possessing the minimum inhibitory concentrations(MICs) against Escherichia coli (E. coli), Staphylococcus aureus (S. aureus) of 180 mg/L and 240 mg/L, has high antibacterial activity, good light permanency and water-resistance. The ionic state silver in Ag_xNa_2-xLa₂Ti₃O₁₀ is the antibacterial active component.

Key words: layered perovskite compound; structure; antibacterial activity; ion-exchange

1 Introduction

It is widely known that inorganic antibacterial agents carrying silver have high antibacterial activity to microorganisms[1-4] and display no or little side effects on tissue[5]. The applications of the inorganic antibacterial agents are limited because of discoloration and high cost though they are used as sterilizing and antibacterial agents in paper, plastic, paint, ceramic and so on. Therefore, it is important to develop new antibacterial agents with low cost, high antibacterial activity and light permanency.

Layered perovskite compounds with Ruddlesden- Popper phase of  where A is an exchangeable alkali metal cation, A′ is an alkali metal, alkaline earth, main group and/or rare earth, and B is a transition metal, have been investigated extensively in recent years[6-7]. These compounds possessing interesting structure and ion-exchange property have been explored as ionic conductors[8-9], photocatalysts [10-12], and super-conductors[13]. However, there have been few reports on the study of the antibacterial activity, and the influence of silver valence state on antibacterial activity is not clear so far. In this paper, new layered perovskite compounds, Ag_xNa_2-xLa₂Ti₃O₁₀ (x=0.2, 0.3 and 0.5) carrying low content silver were synthesized and characterized. And the antibacterial activity, light permanency and water-resistance of the compounds were investigated.

All reagents used were of analytical purities. The parent compound (carrier) of Na₂La₂Ti₃O₁₀ was prepared by conventional solid-state reaction[9,14]. The raw materials of sodium carbonate, lanthanum oxide, and titanium oxide were mixed at the mole ratio of 1.6?2?3. A 60% (mole fraction) stoichiometric excess of sodium carbonate was added to compensate for the loss due to the evaporation of the sodium component, which can act as an oxidizing flux also[15]. The mixture was ground into powder, and heated at 105 ℃ for 10 h in air. The obtained powder was placed into alumina crucible, calcined in air at 500 ℃ for 10 h, heated at 1 000 ℃ for another 5 h, and then cooled. At last, the product was washed with deionized water and dried at 105 ℃ for 10 h.

Ag_xNa_2-xLa₂Ti₃O₁₀ was synthesized by an ion- exchange method. 1 mol Na₂La₂Ti₃O₁₀ was added into deionized water to obtain a 10% suspension, and intermixed with 0.1-0.3 mol AgNO₃, and then maintained at 40-70 ℃ for 1-7 h in a dark vessel. The products were filtered out, washed with deionized water, and dried at 105 ℃ for 10 h. After grinding and sieving, the portion with average size of 5 μm was used in property tests.

The Ag⁺ content in reaction solution was measured by a HITACHI 180-80 atomic absorption spectro- photometer. After a certain time of an ion-exchange reaction, the Ag⁺ exchange fraction (η(Ag⁺)) was calculated according to the following equation:

η(Ag⁺)=m₁(Ag⁺)/m₂(Ag⁺)×100%                (1)

where  m₁(Ag⁺) and m₂(Ag⁺) are the mass fraction of Ag⁺ in Ag_xNa_2-xLa₂Ti₃O₁₀ and initial reaction solution, respectively.

The crystal structures of the samples were characterized by XRD on a MASL-XD2 X-ray diffractometer using Cu K_α radiation (36 kV, 24 mA, λ=0.154 nm), and a scanning rate of 0.08 (?)/s was used to record the patterns in the range of 5?-65?; the components and silver content were analyzed by EDX using an Oxford ISIS-300 energy-dispersive X-ray detector; the surface morphologies of Na₂La₂Ti₃O₁₀ and Ag_0.3Na_1.7La₂Ti₃O₁₀ were observed using a JSM-6360 scanning electron microscope; the silver valence state in Ag_0.3Na_1.7La₂Ti₃O₁₀ was determined with an ESCALab220i-XL electron spectrometer.

The antibacterial activity (minimum inhibitory concentrations, MICs) of Na₂La₂Ti₃O₁₀ and Ag_xNa_2-xLa₂- Ti₃O₁₀ was estimated by a two-fold diluting method[16], and the bacteria of E. coli ATCC25922 and S. aureus ATCC6538 were selected as indicators. The light permanency was carried out by exposing the samples under fluorescent ultraviolet lamp (351 nm) for a certain time and observing the discoloration. 0.1 g Ag_0.3Na_1.7La₂Ti₃O₁₀ was soaked in 20 mL deionized water in a polypropylene bottle at 37 ℃ and rotated with a diameter of 3 cm and a rotating speed of 120 r/min. After 1-10 d of rotation, the Ag⁺ concentration released in water was measured by a HITACHI 180-80 atomic absorption spectrophotometer.

3 Results and discussion

3.1 Ion-exchange process

The antibacterial activity of Ag_xNa_2-xLa₂Ti₃O₁₀, which is directly associated with the silver content, is affected by the ion-exchange process. The influences of reaction temperature, reaction time, AgNO₃ dosage on Ag⁺ exchange fraction (or the silver content of Ag_xNa_2-xLa₂Ti₃O₁₀) were investigated as follows.

When the mole ratio of Ag to Na in initial reaction solution was 0.2?1, Ag⁺ exchange fraction was determined as functions of reaction temperature (40-70℃) and reaction time (up to 7 h). As shown in Fig.1, Ag⁺ exchange fraction increases with the increase of reaction temperature and reaction time. Furthermore, at relatively high temperature (60-70 ℃), Ag⁺ exchange fraction levels off above 6 h. The results suggest that the ion- exchange reaction reaches a high Ag⁺ exchange fraction at 60 ℃ for 6 h.

Fig.1 Effect of reaction temperature and reaction time on Ag⁺ exchange fraction (η(Ag⁺))

Ag⁺ exchange fraction and the mass fraction of Ag⁺ in Ag_xNa_2-xLa₂Ti₃O₁₀ as functions of the mole ratio Ag to Na in initial reaction solution at 60 ℃ for 6 h are shown in Fig.2. With the increase of mole ratio of Ag to Na, Ag⁺ exchange fraction declines and the mass fraction of silver increases. Surprisedly, a further increase in mole ratio of Ag to Na (beyond 0.2) led to a sharp decline in Ag⁺ exchange fraction. It may be explained that the silver ion concentration on carrier surface tended to rise with the increase of mole ratio of Ag to Na, and then more silver ions exchanged with carrier causing the increase of mass fraction of Ag⁺. At the same time, the silver ions remaining in solution might be reduced to metallic silver, which would choke exchange sites when they deposited on carrier surface. Therefore, with the increase of mole ratio of Ag to Na, more metallic silver particles and more choked exchange sites result in the decrease of Ag⁺ exchange fraction.

Fig.2 Effect of mole ratio of Ag to Na on Ag⁺ exchange fraction and mass fraction of Ag⁺ in Ag_xNa_2-xLa₂Ti₃O₁₀

According to the aforementioned results, the optimized ion exchange process is as follows: reaction temperature 60 ℃, reaction time 6 h, mole ratio of Ag to Na 0.2.

3.3 Structure analysis

The results of EDX analysis for Na₂La₂Ti₃O₁₀ and Ag_xNa_1-xLa₂Ti₃O₁₀ are shown in Table 1. According to the mole ratio of Na, La and Ti, the molecular formula of Na₂La₂Ti₃O₁₀ (sample 1) can be confirmed. The molecular formulae of Ag_xNa_1-xLa₂Ti₃O₁₀ (samples 2, 3 and 4) can also be determined because the mole ratios of Ag, Na, La and Ti are almost in accordance with the molecular formulae of Ag_0.2Na_1.8La₂Ti₃O₁₀, Ag_0.3Na_1.7- La₂Ti₃O₁₀ and Ag_0.5Na_1.5La₂Ti₃O₁₀.

Table 1 Results of EDX analysis for Na₂La₂Ti₃O₁₀ and Ag_xNa_1-xLa₂Ti₃O₁₀ (mole fraction, %)

The XRD patterns of Na₂La₂Ti₃O₁₀ and Ag_xNa_1-xLa₂Ti₃O₁₀ (x= 0.2, 0.3 and 0.5) are shown in Fig.3. Obviously, the diffraction peaks of Ag_xNa_2-xLa₂Ti₃O₁₀ (x=0.2, 0.3 and 0.5) are almost the same as that of Na₂La₂Ti₃O₁₀, which are consistent with the earlier determined pattern of compound Na₂La₂Ti₃O₁₀ with tetragonal crystalline[17]. However, small changes are observed in the unit cell parameters of Ag_xNa_1-xLa₂Ti₃O₁₀ (x= 0.2, 0.3 and 0.5) because of the different sizes between Na⁺ (0.098 nm) and Ag⁺ (0.113 nm), as can be seen in Table 2. As a result, no significant changes in crystal structure of Na₂La₂Ti₃O₁₀ are found by the exchange of silver ions.

Fig.3 XRD patterns of Na₂La₂Ti₃O₁₀ and Ag_xNa_1-xLa₂Ti₃O₁₀

Table 2 Unit cell parameters of Na₂La₂Ti₃O₁₀ and Ag_xNa_2-x- La₂Ti₃O₁₀

The SEM images of Na₂La₂Ti₃O₁₀ and Ag_0.3Na_1.7La₂Ti₃O₁₀ are shown in Fig.4. Fig.4(a) corresponding to Na₂La₂Ti₃O₁₀ particles display some conglomeration with flocculent shape and uneven size due to the high temperature solid-state reaction. Moreover, Ag_0.3Na_1.7La₂Ti₃O₁₀ particles conglomerate obviously with irregular shape and size because of the import of silver ions, as shown in Fig.4(b).

Fig.4 SEM images of Na₂La₂Ti₃O₁₀ (a) and Ag_0.3Na_1.7La₂Ti₃O₁₀ (b)

3.3 Properties evaluation

The antibacterial activity and light permanency of Na₂La₂Ti₃O₁₀ and Ag_xNa_2-xLa₂Ti₃O₁₀ (x=0.2, 0.3 and 0.5) are listed in Table 3. The MICs of Ag_xNa_2-xLa₂Ti₃O₁₀ against E. coli and S. aureus are all less than 300 mg/L, in contrast with that of higher than 2 g/L for Na₂La₂- Ti₃O₁₀. On the other hand, Na₂La₂Ti₃O₁₀, Ag_0.2Na_1.8La₂- Ti₃O₁₀ and Ag_0.3Na_1.7La₂Ti₃O₁₀ show no discoloration while Ag_0.5Na_1.5La₂Ti₃O₁₀ displays a little discoloration after exposing to UV light for 24 h. These results indicate that the antibacterial activity of Ag_xNa_2-xLa₂- Ti₃O₁₀ improves with the increase of silver content or the value of x. But at the same time, the trend to discoloration increases. It is interesting that the MICs of Ag_0.3Na_1.7La₂Ti₃O₁₀ against E. coli and S. aureus are 180 mg/L and 240 mg/L, respectively, and its discoloration is not observed after exposing to UV light for 24 h. Therefore, Ag_0.3Na_1.7La₂Ti₃O₁₀ has the potential to be commercially used in future.

Table 3 Antibacterial activity and light permanency of Na₂La₂Ti₃O₁₀ and Ag_xNa_2-xLa₂Ti₃O₁₀ (MIC/(mg?L^-1))

Fig.5 shows the mass fraction of release silver ions for Ag_0.3Na_1.7La₂Ti₃O₁₀ as a function of soaking time at 37 ℃ in deionized water. In the first two days, a large amount of silver ions rapidly releasing into water were detected. With increasing soaking time, the rate of release silver ions slows down. Surprisedly, the mass fraction of release silver ions is only 6.5% (mass fraction) after soaking for 10 d. The results indicate that Ag_0.3Na_1.7La₂Ti₃O₁₀ shows good water-resistance.

Fig.5 Relation between mass fraction of release silver ions for Ag_0.3Na_1.7La₂Ti₃O₁₀ in deionized water and soaking time

3.4 Exploration of antibacterial active component

The silver valence state in Ag_0.3Na_1.7La₂Ti₃O₁₀ was analyzed by XPS, and the Ag 3d spectrum is shown in Fig.6. The binding energies of Ag 3d_5/2 and Ag 3d_3/2 are 367.59 eV and 373.63 eV, respectively, which are in

accordance with standard parameters of ionic silver[18]. This result suggests that the silver in Ag_0.3Na_1.7La₂Ti₃O₁₀ exists as ionic state.

Fig.6 Ag 3d spectrum of Ag_0.3Na_1.7La₂Ti₃O₁₀

The effect of silve valence state on antibacterial activity was further estimated as shown in Table 4. With the lapse of UV irradiation time, for Ag_0.3Na_1.7La₂Ti₃O₁₀, the MICs increase and the discoloration becomes serious. These results can be explained that when Ag_0.3Na_1.7La₂Ti₃O₁₀ is exposed to UV light, some ionic silver is reduced to metallic silver, and the metallic silver content in Ag_0.3Na_1.7La₂Ti₃O₁₀ increases with the increase of UV irradiation time. Unfortunately, the color becomes deep, and the antibacterial activity declines.

Table 4 MICs and discoloration of Ag_0.3Na_1.7La₂Ti₃O₁₀ at different UV irradiation time (MIC/(mg?L^-1))

Therefore, it is evident that the ionic state silver in Ag_xNa_2-xLa₂Ti₃O₁₀ is the antibacterial active component.

4 Conclusions

1) New layered perovskite compounds, Ag_xNa_2-xLa₂Ti₃O₁₀ (x=0.2, 0.3 and 0.5) are synthesized by an ion-exchange reaction of Na₂La₂Ti₃O₁₀ with AgNO₃ solution; the optimized ion exchange process is as follows: reaction temperature 60 ℃, reaction time 6 h, mole ratio of Ag to Na 0.2.

2) The molecular formulae of Ag_xNa_2-xLa₂Ti₃O₁₀ (x=0.2, 0.3 and 0.5) are confirmed; no significant changes in crystal structure of Na₂La₂Ti₃O₁₀ are found by the exchange of silver ions; Ag_0.3Na_1.7La₂Ti₃O₁₀ particles conglomerate obviously with irregular shape and size.

3) Ag_0.3Na_1.7La₂Ti₃O₁₀, possessing the MICs against E. coli, S. aureus of 180 mg/L and 240 mg/L, has high antibacterial activity, good light permanency and water-resistance.

4) The ionic state silver in Ag_xNa_2-xLa₂Ti₃O₁₀ is the antibacterial active component.

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Foundation item: Projects(20676049, 50472077) supported by the National Natural Science Foundation of China; Projects(05200555; 2004B20201026) supported by the Natural Science Foundation of Guangdong Province of China; Project(2005Z3-D212) supported by the Science and Technology Project Fund of Guangzhou City of China

Corresponding author: TAN Shao-zao; Tel: +86-20-85221813; Fax: +86-20-87692365; E-mail: tanshaozao@163.com

(Edited by CHEN Wei-ping)