Silver nanoparticles in combination with acetic acid and zinc oxide quantum dots for antibacterial activities improvement—A comparative study

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Silver nanoparticles in combination with acetic acid and zinc oxide quantum dots for antibacterial activities improvement—A comparative study Sofiane Sedira a,∗ , Ahmed Abdelhakim Ayachi a , Sihem Lakehal a , Merouane Fateh b , Slimane Achour a a b

Ceramic Laboratory, University of Constantine1, Constantine, Algeria Microbiological Laboratory Engineering and Application, University of Constantine1, Constantine, Algeria

a r t i c l e

i n f o

Article history: Received 26 January 2014 Received in revised form 18 May 2014 Accepted 19 May 2014 Available online xxx Keywords: Ag NPs ZnO Qds Acetic acid Released Ag+ Bactericidal effect

a b s t r a c t Due to their remarkable antibacterial/antivirus properties, silver nanoparticles (Ag NPs) and zinc oxide quantum dots (ZnO Qds) have been widely used in the antimicrobial field. The mechanism of action of Ag NPs on bacteria was recently studied and it has been proven that Ag NPs exerts their antibacterial activities mainly by the released Ag+ . In this work, Ag NPs and ZnO Qds were synthesized using polyol and hydrothermal method, respectively. It was demonstrated that Ag NPs can be oxidized easily in aqueous solution and the addition of acetic acid can increase the Ag+ release which improves the antibacterial activity of Ag NPs. A comparative study between bactericidal effect of Ag NPs/acetic acid and Ag NPs/ZnO Qds on Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumonia and Staphylococcus aureus was undertaken using agar diffusion method. The obtained colloids were characterized using UV–vis spectroscopy, Raman spectrometry, X-ray diffraction (XRD), transmission electron microscopy (TEM) and atomic force microscopy (AFM). © 2014 Elsevier B.V. All rights reserved.

1. Introduction Zinc oxide (ZnO) is a versatile and important semiconductor which found applications not only in the optoelectronic field [1,2] but also in the bio applications because of its environmentally friendly properties [3,4]. ZnO nanoparticles are recently investigated and seem to have significantly higher antibacterial effect without reaction with human cells [5]. Liu et al. [6] have addressed an important topic about the mechanism of action of ZnO NPs on bacteria. They proved that under light irradiation, ZnO can produce electron–hole (e− , h+ ) pairs with high energies. At the surface of ZnO, the created (e− , h+ ) pairs influence the redox reaction which generates hydroxyl and perhydroxil radicals (OH− , HO2 − ) in addition to the superoxide anions (O2 − ). The microorganism cells were immediately destructed in the presence of these radicals. Another mechanism has been proposed by Sharma et al. [7]: the bactericidal effect of ZnO NPs is mainly explained by the rupture of the

lipid bilayer of bacterium which results in leakage of cytoplasmic contents. Several studies have been, also, devoted to silver nanoparticles as a broad spectrum antimicrobial agent [8]. The toxicity of these nanoparticles may be explained by: (i) degradation of the cell membrane caused by the generation of reactive oxygen species [9–11], (ii) the release of silver ions from the crystalline core of silver nanoparticles may contribute to the toxicity causing a decrease in the pH of the cell cytoplasm [12,13]. In this work, Ag NPs and ZnO Qds were synthesized using polyol and hydrothermal method, respectively. The bactericidal effect of Ag NPs on Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumonia and Staphylococcus aureus was examined, after ZnO Qds addition firstly, and acetic acid in a second step, using agar diffusion method. 2. Materials and methods 2.1. Ag NPs and ZnO QDs preparation

∗ Corresponding author. Tel.: +213 552805063; fax: +213 31818881. E-mail addresses: sofi[email protected], sofi[email protected] (S. Sedira), [email protected] (A.A. Ayachi), [email protected] (S. Lakehal), [email protected] (M. Fateh), [email protected] (S. Achour).

Ag NPs were synthesized using polyol method. As a precursor, silver nitrate (AgNO3 ) was chosen for the synthesis of Ag NPs. Ethanol 96% was adopted as reducing agent [14] and polyvinylpyrrolidone (PVP) was used as surfactant. A flask

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containing a solution of 0.1 g of AgNO3 , 1 g of PVP and 20 ml of ethanol 96% was immersed in an oil bath. The solution was then heated under reflux at 160 ◦ C with a constant stirring for 30 min. During the process, the color of the solution becomes reddish after it was transparent. After cooling, the obtained solution remains stable without any precipitation for many months. For ZnO Qds preparation, hydrothermal method was used. In this method, 20 mg of multi walled carbon nanotubes (MWCNTs) was attacked by nitric acid (HNO3 ) as reported by [15]. This mixture was then dispersed in 20 ml of dimethylformamide (DMF) by ultra-sonication for 30 min. 6 ml aliquot of the MWCNTs-HNO3 /DMF solution was then mixed with 36 ml of DMF in which 0.39 g of zinc nitrate tetrahydrate (Zn(NO3 )·4H2 O) was dissolved, and the resulting mixture was vigorously stirred for 30 min at room temperature. The mixture was then heated to 105 ◦ C at a heating rate of 2 ◦ C/min, and maintained at this temperature for 3 h. The transparent upper part of the container is composed of suspended ZnO Qds (dispersed in DMF) and the dark bottom layer contained the isolated MWCNTs-HNO3 .

3. Theory Based on the principle that Ag+ is the definitive toxicant agent responsible for killing bacteria [18], we have added acetic acid (CH3 COOH) to the Ag NPs colloid which would increase H+ concentration according to Eq. (3) and give rise to the increase of Ag+ content according to Eq. (2). Eq. (1) shows that Ag NPs can be oxidized in aqueous solution [18] and rapidly ionized in the presence of H+ to give Ag+ according to Eq. (2). 4Ag + O2 → 2Ag2 O +

(1) +

2Ag2 O + 4H → 4Ag + 2H2 O −

CH3 COOH → CH3 COO + H

(2)

+

(3)

This phenomenon is well confirmed subsequently in the Raman spectroscopy analysis section. 4. Results and discussion 4.1. XRD analysis

For XRD characterization, ZnO Qds were deposited on glass substrates using spin coating (100 t/min) followed by drying at 160 ◦ C to eliminate DMF (boiling point: 153 ◦ C). These films were then annealed at 400 ◦ C for 1 h and Ag NPs were deposited on titanium substrates using spin coating (100 t/min) followed by drying at 430 ◦ C to decompose PVP (decomposition temperature: 420 ◦ C). The XRD analysis was conducted on a D8 Siemens Advance diffractometer with CuK␣ radiation ( = 0.15418 nm). The 2 range was from 10◦ to 100◦ . The formation of Ag NPs and ZnO Qds, also, was further confirmed using UV–vis Jasco V-670 spectrometer. Raman spectra were carried out, using Bruker SENTERRA R200L spectrometer, to control the reactions which occurred after acetic acid addition in Ag NPs colloid. Furthermore, the surface enhanced Raman spectroscopy of ZnO Qds on Ag NPs films with different thicknesses was studied. The Raman spectra were recorded using the 532 nm line of an Argon ion laser. AFM images of ZnO Qds were obtained using Horiba Jobin Yvon (AIST-NT Smart SPM/AFM) in non-contact mode. To examine the size and the morphology of the Ag NPs, transmission electron microscopy (TEM; Tecnai TF20, BFTEM at 200 kV) was used.

XRD spectra of the synthesized ZnO Qds (dried at 160 ◦ C and annealed at 400 ◦ C) recorded in the 2 range 10–75◦ are presented in Fig. 1a. The diffraction pattern of the annealed ZnO Qds films exhibits seven peaks at 2 = 31.86, 34.59, 36.36, 47.78, 56.60, 62.98, 68.28 degree which were assigned to the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) (JCPDS No. 36-1451) planes of hexagonal zinc oxide. No characteristic peaks of any impurities were detected; suggesting that high quality of ZnO was obtained. However, the related ZnO peak intensities appear lower in the dried samples (Fig. 1a) which can be due to the presence of the organic matter that appears as amorphous phase in the range 10–30◦ . Fig. 1b shows the diffraction pattern of Ag NPs deposited on titanium substrates using spin coating and drying at 430 ◦ C. The peaks

1- ZnO Qds dried at 160°C

2- ZnO Qds annealed at 400°C for 1h

120 100 80

(103) (112)

(102)

140

(110)

(100)

160

intensity (arb. unit)

The agar diffusion method (Kirby-Bauer) is a relatively quick and easy to execute as semi-quantitative test to determine antibacterial activity of diffusible antimicrobial agents on treated textile material [16,17]. Bacterial inactivation tests were carried out using P. aeruginosa (ATCC: 27853), E. coli (ATCC: 25922), K. pneumonia (ATCC: 70603) and Staphylococcus aureus (ATCC: 25923) as test organisms. A spectrophotometer (JENWAY 7305) was used to ensure an identical bacterial concentration in each test by measuring the optical density (108 cells/cm3 ). One colony of each bacteria was taken out in a petri dish and grown in nutrient broth medium (Müller-Hinton agar: beef infusion 300.0 ml, caseinhydrolysate 17.5 g, starch 1.5 g, agar: 17.0 g, pH adjusted to neutral at 25 ◦ C). 10 mm diameter discs (3 M perti film) immersed in ethanol 96% and acetic acid were used as control fabrics. As test fabrics, discs immersed in Ag NPs (100 ␮g/cm3 ) with different concentration of added acetic acid and ZnO Qds were used. These discs were gently pressed onto the surface of the petri dishes and were incubated in darkness at 37 ◦ C for 18–24 h (INB 200-Memmert incubator). The antibacterial activity of fabrics was demonstrated by the diameter of the zone of inhibition in comparison to the control fabric.

(a)

180

2

60 40

1

20 0 0

10

20

30

2

40

50

60

70

80

deg)

b

60

intensity (arb. unit)

2.3. Antibacterial tests

200

(002) (101)

2.2. Characterization method

50 40

Ti (100) Ag (111)

TiO2 (110)

30 20

Ti (102) Ag (200)

Ti (110)

10 0 20

30

40

2

50

60

deg)

Fig. 1. XRD patterns of (a) dried and annealed ZnO Qds, (b) Ag NPs deposited on Ti.substrate and heat treated at 430 ◦ C.

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Fig. 2. (a) TEM and HRTEM images of the as-synthesized Ag NPs. (b) AFM image of ZnO Qds deposited on glass at 160 ◦ C with the height for selected area.

at 2 = 38.48, 53.00 and 63.07 degrees were assigned to the (1 0 0), (1 0 2) and (1 1 0) planes of titanium substrate (JCPDS No. 44-1294). The presence of a face centered cubic structure of crystalline Ag NPs is confirmed by the diffraction peaks at 2 = 38.11 and 44.27◦ assigned to the (1 1 1) and (2 0 0) planes (JCPDS No. 04-0783). The diffraction peak observed at 25.33◦ is attributed to the (1 2 0) planes of the anatase phase of TiO2 (JCPDS No. 29-1360) which can be formed during films preparation (heating at 430 ◦ C).

shows AFM image acquired in a 5 × 5 ␮m area of ZnO QDs film deposited on glass substrate and dried at 160 ◦ C. It can be inferred from this figure that ZnO QDs were more or less agglomerated to form nanoparticles as a result of heating at 160◦ C and subsequent decomposition of DMF which acts as an inhibitor against ZnO QDs agglomeration. However, a mean particle size of about 30 nm can be estimated from this figure.

4.3. UV–vis spectroscopy analysis 4.2. Ag NPs and ZnO Qds size and morphology To get information about the silver particle size and shape, TEM measurements were performed. As can be seen from the TEM image shown in Fig. 2a, the as-prepared Ag NPs are nearly spherical in shape and quite mono dispersed. The mean particle size observed varies between 20 and 100 nm. High-resolution transmission electron microscopy (HRTEM) measurement was also used in order to further investigate the crystal structure of the Ag NPs which gives a fringe spacing (0.24 nm) corresponding to the (1 1 1) lattice planes of metallic silver. The selected area electron diffraction (SAED) pattern reveals that the diffraction rings of the synthesized Ag NPs exhibit Debye–Scherrer rings assigned to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) lattice planes of the face centred cubic (fcc) Ag. Furthermore, the TEM images indicate that the polyol method, using silver nitrate as precursor, ethanol as reducer and PVP as surfactant, is an appropriate way to obtain small and well-stabilized nanoparticles. Fig. 2b

Ag NPs and ZnO Qds formation was further confirmed by the UV–vis spectra. Fig. 3 presents the absorbance spectra of ZnO Qds, Ag NPs colloids and their mixture. Ag NPs colloid shows a characteristic peak of absorbance at around 438 nm which corresponds to the plasmon resonance of Ag NPs [19]. It is well known that color transformation from transparent to yellowish brown during the preparation is the indication of Ag NP formation [20]. The absorption edge at about 358 nm corresponds to transitions across the ZnO Qds gap. The inset image shows pictures of sample vials (ZnO Qds, Ag NPs colloids and the mixture of them). No precipitation in the mixture was observed but the color became light brown which indicates absence of any undesirable reaction after mixing. In addition, the UV–vis spectrum of the mixture showed two absorptions bands, one at 358 nm attributed to the presence of ZnO Qds and the other at 438 nm related to Ag NPs without any shift in the peak wavelengths. This means that there is no substantial interaction

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a Glass

Ag

Ag

AgO

2

Intensity (arb. unit)

100000 90000 80000 70000 60000 50000 40000 30000 20000 10000

(1) Ag NPs colloid (2) 5ml AgNPs+3ml ac acid (3) 5ml AgNPs+5ml ac acid (1) (2) (3)

0

100

200

300

400

500

Raman shift (cm-1)

b

between Ag and ZnO nanoparticles in the mixture [21]. Moreover, the UV–vis analysis showed no formation of organic complexes in the mixture that can destroy the antibacterial activities. 4.4. Fluorescence emission spectrum characterization Fig. 4 shows UV–vis absorption and fluorescence emission spectra. The characteristic absorption peak of ZnO QDs is located at about 358 nm. Upon excitation with a UV–vis beam, the fluorescence spectrum of the ZnO QDs showed a maximum emission at about 559 nm. According to the literatures, this emission peak in the visible region originates from defect energy levels located in the band gap such as oxygen vacancies, surface traps and interstitial defects [22,23].

Raman spectra were measured on samples deposited on glass substrates and illuminated with 10 mw of laser power. Spectral data was collected in the continuous scan mode over the region of 50–2000 cm−1 with two 10 s integrations. Fig. 5a shows Raman spectra of Ag NPs colloids with different additions of acetic acid (Ag NPs, 5 ml of Ag NPs + 3 ml acetic acid, 5 ml of Ag NPs + 5 ml acetic acid) in the 50–500 cm−1 range. The spectrum of Ag NPs colloid without acetic acid addition shows two shoulders at about 94 and 147 cm−1 related to silver lattice vibrations [24].The peak around 240 cm−1 which is characteristic of silver oxide (Ag2 O) [24] considerably decreases in intensity after adding 3 ml of acetic acid and completely disappears after 5 ml addition of acetic acid. This phenomenon is accompanied by an increase in the relative intensity 559 nm

358 nm

a- absorption b- emission

Absorbance (arb. unit)

Fluorescence (arb. unit)

a

b

0

300

350

400

450

500

550

600

Wavelength (nm)

650

700

Fig. 4. UV–vis absorption (a) and fluorescence emission (b) spectra of ZnO QDs.

90000

D

80000

G

70000 60000

(1)

50000 40000

(2)

30000 20000

(3)

10000 1000

1200

1400

1600

1800

2000

Raman shift (cm-1)

c

D

G

11000

Intensity (arb. unit)

4.5. Raman spectroscopy analysis

(1) AgNPs colloid (2) 5ml AgNPs+3ml ac acid (3) 5ml AgNPs+5ml ac acid

100000

Intensity (arb. unit)

Fig. 3. UV–vis absorption spectra of ZnO Qds (1), Ag NPs (2), and their mixture (3) taken in colloid state.

1

10000

2

9000

1 - graphite powder 2 - graphite powder + acetic acid 8000 1300

1400

1500

1600

1700

1800

1900

Raman shift (cm -1) Fig. 5. Raman spectra of Ag NPs with acetic acid in the range 50–500 cm−1 (a), 1000–2000 cm−1 (b) and graphite powder with acetic acid (c).

of the peaks at 93 and 147 cm−1 (attributed to Ag), in accordance with the reactions shown by Eqs. (2) and (3). In the 900–2000 cm−1 range (Fig. 5b), the Raman spectra present two bands located at about 1332 and 1602 cm−1 that are related to carbon and labeled D and G, respectively. The D band is ascribed to the local defects or disorder, while the G band arises from the sp2 hybridized carbon [25,26]. The carbon may arise from the added organics or be formed during Ag NPs synthesis. A decreasing in D and G is observed as a result of acetic acid addition which can be explained by the reaction of carbon or carbon compounds. This can be seen from Fig. 5c which shows Raman spectra of graphite powder and graphite powder + CH3 COOH where both graphite G and D bands vanish after acetic acid addition. Fig. 6 presents Raman spectra of a ZnO Qds on bare glass, a second ZnO Qd film deposited on glass on which two drops of Ag NPs solution were previously spin coated and a third ZnO Qd film on glass where only one drop of Ag NPs was beforehand deposited. All spectra show two peaks, one at about

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Table 1 Variation of the diameter (in mm) of the inhibition zone against bacteria. Samples DMF Ethanol Ag NPs ZnO Qds AgNPs + ZnO Qds Ac acid 28.7 5 ml Ag + 3 ml Aca cid 5 ml Ag + 5 ml Aca cid

AgNPs + ZnO Qds

Ag NPs + Ac acid Ag NPs

Intensity (arb. unit)

2500

E. coli

S. aureus

K. pneumoniae

P. aeruginosa

0 0 28.8 15.2 29.4 0 27.2 30.1 35.4

0 0 27.2 0 27.6 0 13.0 29.4 30.2

0 0 13.0 0 14.0 0 29.6 17.2 23.2

0 0 29.8 11.0 30.1 0

4.6. Antibacterial activity of Ag NPs/ZnO Qds and Ag NPs/acetic acid

1-ZnO Qds on glass 2-ZnO Qds on one drop of Ag NPs 3-ZnO Qds on two drops of Ag NPs

2000

E2(high) 1500 E2(low)

3

1000

2

500

1 0 100

150

200

250

300

350

31.3 36.2

400

450

500

-1

Raman shift (cm ) Fig. 6. Raman spectra of (1) ZnO Qds on glass, (2) ZnO Qds on one drop of Ag NPs on glass and (3) ZnO Qds on two drops of Ag NPs on glass.

97.3 cm−1 labeled (E2 low) and the second at about 437.5 cm−1 (E2 high) which are the Raman active optical mode of the ZnO hexagonal phase [27]. The relative intensities of ZnO Qds deposited on Ag NPs are stronger than that deposited only on glass. This enhancement in Raman intensity can be explained by the surface-enhanced Raman scattering (SERS) as a result of introducing Ag NPs.

Recently, it has been demonstrated that Ag NPs exerts their antibacterial activities mainly by the released Ag+ [18]. In this Context, acetic acid was added in different quantities to increase the released Ag+ according to the reactions in (1)–(3). Moreover, several studies have shown that the bactericidal effect of Ag NPs was improved by the addition of ZnO nanoparticles [28]. A comparative study between the antibacterial activities of Ag NPs combined with ZnO Qds and Ag NPs with acetic acid tested on P. aeruginosa, E. coli, K. pneumonia and S. aureus using agar diffusion method, is presented in Figs. 7 and 8. It was found that Ag NPs are able to inhibit the bacterial growth. In addition, the antibacterial activity was improved less effectively by ZnO Qds addition. The measured inhibition zones formed after antibacterial tests on the culture plates are given in Table 1. It can be seen that ethanol, DMF and acetic acid alone have no activity on bacteria. The smallest inhibition zones were obtained for ZnO Qds. However, the inhibition zone increased significantly when acetic acid was added and the content of acetic acid increased. This is shown for the sample containing 5 ml Ag + 5 ml Ac acid where a significant improvement inhibition resulting in zones of 35.4 mm for E. coli and 36.2 mm for P. aeruginosa, respectively, can be seen. Furthermore, it was observed that E. coli and P. aeruginosa were more sensitive than K. pneumonia and S. aureus to the nanoparticles. It can be concluded that samples containing Ag NPs and acetic acid

Fig. 7. Antibacterial activity of Ag NPs combined with ZnO Qds and tested on (A) Klebsiella pneumonia, (B) Escherichia coli, (C) Staphylococcus aureus and (D) Pseudomonas aeruginosa.

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Fig. 8. Antibacterial activity of Ag NPs combined with acetic acid and tested on (A) Klebsiella pneumonia, (B) Escherichia coli, (C) Staphylococcus aureus and (D) Pseudomonas aeruginosa.

Acknowledgments The authors would like to thank FEI (Holland) for TEM analysis and Horiba Jobin Yvon (France) for AFM observation. References

Fig. 9. Schematic of Ag NPs/acetic acid mechanism of action against bacteria.

showed the highly improved antibacterial activity toward E. coli and P. aeruginosa. The acid acetic action can be explained as follows: The acid acetic delivers H+ which reacts with Ag2 O to give Ag+ ions. Then, the Ag+ reacts with the bacteria cytoplasm (increasing H+ in the cytoplasm) and leads to a decrease of the local pH of the cell which becomes more acidic (Fig. 9).

5. Conclusion Highly stable colloid Ag NPs and ZnO Qds were successfully fabricated (more than 14 months) by polyol and hydrothermal technique, respectively. Raman spectroscopy revealed that the acetic acid addition can increase the Ag+ rate in the Ag NPs colloid which is the toxicant molecular responsible for killing bacteria. Agar diffusion method showed that the antibacterial activity of Ag NPs against P. aeruginosa, E. coli, K. pneumonia and S. aureus is more enhanced with acetic acid than Ag NPs combined with ZnO Qds. Moreover, the antimicrobial activity of the colloids is found to be in the order of P. aeruginosa > E. coli > S. aureus > K. pneumonia.

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Please cite this article in press as: S. Sedira, et al., Silver nanoparticles in combination with acetic acid and zinc oxide quantum dots for antibacterial activities improvement—A comparative study, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.132