Characterization of silver nanoparticles synthesized

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Characterization of silver nanoparticles synthesized on titanium dioxide fine particles

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IOP PUBLISHING

NANOTECHNOLOGY

Nanotechnology 19 (2008) 065711 (8pp)

doi:10.1088/0957-4484/19/6/065711

Characterization of silver nanoparticles synthesized on titanium dioxide fine particles ˜ ˜ on3,5, A Aragón-Pina ˜ 2, N Nino-Mart´ ınez1,2, G A Mart´ınez-Castan´ 4 1 F Mart´ınez-Gutierrez , J R Mart´ınez-Mendoza and Facundo Ruiz1 ´ Facultad de Ciencias, UASLP, Alvaro Obregón 64, CP 78000, San Luis Potos´ı, SLP, Mexico ´ Instituto de Metalurgia, Facultad de Ingenier´ıa, UASLP, Alvaro Obregón 64, CP 78000, San Luis Potos´ı, SLP, Mexico 3 Maestria en Ciencias Odontológicas, Facultad de Estomatolog´ıa, UASLP, Avenida Manuel Nava 2, Zona Universitaria, San Luis Potos´ı, SLP, Mexico 4 ´ Facultad de Ciencias Qu´ımicas, UASLP, Alvaro Obregón 64, CP 78000, San Luis Potos´ı, SLP, Mexico 1 2

E-mail: [email protected]

Received 5 September 2007, in final form 5 December 2007 Published 23 January 2008 Online at stacks.iop.org/Nano/19/065711 Abstract Silver nanoparticles with a narrow size distribution were synthesized over the surface of two different commercial TiO2 particles using a simple aqueous reduction method. The reducing agent used was NaBH4 ; different molar ratios TiO2 :Ag were also used. The nanocomposites thus prepared were characterized using transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), dynamic light scattering (DLS) and UV–visible (UV–vis) absorption spectroscopy; the antibacterial activity was assessed using the standard microdilution method, determining the minimum inhibitory concentration (MIC) according to the National Committee for Clinical Laboratory Standards. From the microscopy studies (TEM and STEM) we observed that the silver nanoparticles are homogeneously distributed over the surface of TiO2 particles and that the TiO2 :Ag molar ratio plays an important role. We used three different TiO2 Ag molar ratios and the size of the silver nanoparticles is 10, 20 and 80 nm, respectively. It was found that the antibacterial activity of the nanocomposites increases considerably comparing with separated silver nanoparticles and TiO2 particles.

megaterium using environmental light [13]. Few studies have investigated the application of TiO2 in life science [14]. It has been reported that the catalytic and bactericide properties of TiO2 can be improved by growing particles of a noble metal (Ag, Au or Cu) over its surface [13], or inside its matrix as reported by Thiel et al [15]. In this work, silver nanoparticles were synthesized on the surface of TiO2 fine particles using a simple aqueous reduction method and the composites thus obtained (TiO2 @Ag) were characterized using TEM, STEM, SEM, EDS, XRD, UV–vis spectroscopy, DLS and XPS. An antibacterial activity test (NCCLS M7A4, 1997) was conducted in order to confirm the improved bactericide properties of the composites obtained.

1. Introduction Titanium dioxide (TiO2 ) is one of the most popular semiconductor materials; it is commercially available and can be used in many catalytic applications [1–8]. TiO2 has a wide band gap (3 and 3.23 eV for anatase and rutile, respectively) which makes this material transparent to visible light, i.e., no photon absorption occurs at wavelengths beyond 380 nm and catalytic reactions using pure TiO2 must be carried out using ultraviolet photons. Titanium dioxide is a material that also presents antibacterial activity [9–12]; this antibacterial activity has been studied over E. coli and B. 5 Author to whom any correspondence should be addressed.

0957-4484/08/065711+08$30.00

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© 2008 IOP Publishing Ltd Printed in the UK

Nanotechnology 19 (2008) 065711

N Niño-Mart´ınez et al

images were obtained on a JEOL 2010F. Scanning electron microscopy (SEM) analysis was performed on a Phillips XL30 SEM equipped with an EDS spectrometer EDAX DX-4 Model. XPS analysis of the powder samples was carried out using a Kratos AXIS ULTRA XPS system fitted with a monochromated Al Kα x-ray source and a hemispherical analyzer with eight channeltrons. The source was operated at 10 mA and 15 kV. UV–vis spectroscopy, SEM, EDS, XRD and XPS analyses were made using dried powders and TEM, STEM and DLS analyses were made using aqueous dispersions of the TiO2 @Ag composites.

Table 1. Description of the samples prepared in this work. Sample label

TiO2

Molar ratio (TiO2 :Ag)

TiO2 1@Ag25 TiO2 1@Ag10 TiO2 2@Ag10

DuPontTM Ti-Pure® R-902 DuPontTM Ti-Pure® R-902 Degussa P25

25:1 10:1 10:1

2. Experimental details 2.1. Materials TiO2 particles (DuPontTM Ti-Pure® R-902 and Degussa P25), AgNO3 (Sigma Aldrich, ACS Reagent), NaBH4 (Sigma Aldrich, ACS Reagent) and NH4 OH (30% w/w aqueous solution, Sigma Aldrich, ACS Reagent) were used as received without further purification.

2.4. Antibacterial test The antimicrobial activity of the synthesized composites was tested using the standard microdilution method, which determines the minimum inhibitory concentration (MIC) leading to inhibition of bacterial growth (NCCLS M7-A4, 1997). Disposable microtitration plates were used for the tests. The composites in dispersion form were diluted 2– 128 times with 100 μl of Mueller–Hinton broth inoculated with the tested bacteria at a concentration of 105 CFU ml−1 . The minimum inhibitory concentration (MIC) was read after 24 h of incubation at 37 ◦ C as the MIC of the tested substance that inhibited the growth of the bacterial strain. The dispersions were used in the form in which they had been prepared. Therefore, control bactericidal tests of solutions were performed containing all the reaction components.

2.2. Synthesis method For a typical procedure, 0.2000 g (2.5 mmol) of commercial TiO2 particles were dispersed in 100 ml of deionized water by using an ultrasonic treatment for approximately five minutes; immediately afterwards 0.0169 g (0.1 mmol) or 0.0425 g (0.25 mmol) of AgNO3 was added. The solution was magnetically stirred for about 30 min at pH = 7. After this, 0.1 mmol or 0.25 mmol of sodium borohydride, previously dissolved in 10 ml of deionized water, was added as reducing agent. The pH of the reaction media was adjusted to 10 by adding NH4 OH, and finally the solution was magnetically stirred for 30 min. After several experiments these conditions (amount of sodium borohydride added, pH value and stirring time) were chosen because they allow us to control the size and size distribution of the silver nanoparticles. The vigorous chemical reduction yields a brownish dispersion; there is a change of color (from white to brownish), and the reaction is completed after approximately 3 min; the additional 27 min of magnetic stirring allowed the narrowing of the size dispersion (Ostwald ripening) [16]. After this, the products obtained (TiO2 @Ag) were filtered, washed and dried for further characterization. Hereafter, DuPontTM particles will be named as TiO2 1 and Degussa particles will be named as TiO2 2. Three different samples were synthesized; the samples obtained using TiO2 1 and molar ratios of 25:1 and 10:1 (TiO2 :Ag) will be named as TiO2 1@Ag25 and TiO2 1@Ag10 respectively. The sample prepared using TiO2 2 and a molar ratio of 10:1 (TiO2 :Ag) will be named as TiO2 2@Ag10 (see table 1).

3. Results and discussion 3.1. Synthesis Silver ions (Ag+ ) can be deposited over the surface of TiO2 particles by cationic adsorption. TiO2 is an amphoteric oxide with an isoelectronic point IEP = 6 [17]. When the pH value of a TiO2 dispersion is lower than 6 the main surface species is –OH+ 2 , and when the pH value of a TiO2 dispersion is bigger than 6 the main surface species is –O− ; in the latter case the surface of TiO2 particles is negatively charged and silver ions can be deposited over its surface [13]. In this work, in order to ensure a complete adsorption of the silver ions, a mixture of TiO2 particles and silver ions (added as silver nitrate) was magnetically stirred for about 30 min at pH = 7. After that, the reduction reaction proceeded on the surface of TiO2 particles. The synthesis of TiO2 @Ag composites can be summarized as follows:

(i) TiO2 + Ag+ → TiO2 @(Ag+ )

2.3. Characterization

(ii) TiO2 (Ag+ ) + BH− 4 → TiO2 @Ag

The composites produced were characterized by UV–vis spectroscopy using an S2000-UV–vis spectrometer from OceanOptics Inc. Dynamic light scattering analysis was performed in a Malvern Zetasizer Nano ZS. X-ray diffraction patterns were obtained on a GBC-Difftech MMA model, with ˚ Transmission electron Cu Kα irradiation at λ = 1.54 A. microscopy (TEM) analysis was performed on a JEOL JEM1230 at an accelerating voltage of 100 kV, and the STEM

at pH = 7 at pH = 10.

3.2. SEM and EDS analysis Figure 1 shows SEM images for TiO2 particles before and after the silver synthesis. We can see that the TiO2 1 particles are pseudospherical and their sizes range from 200 to 450 nm (figures 1(a)); TiO2 2 particles present strong agglomeration 2



a)

b)

c)

d)

Figure 1. SEM images of the starting materials and the composites synthesized in this work: (a) TiO2 1, (b) TiO2 2, (c) TiO2 1@Ag25 and (d) TiO2 2@Ag10.

when seen by SEM (figures 1(b)); the morphology and particle size of these particles cannot be resolved using this technique. The insets in figures 1(a) and (b) show EDS analysis results. The chemical elements present in TiO2 1 particles are Ti, O, Al and Si; TiO2 2 particles are composed of Ti and O. From SEM images of the composites obtained it is evident that starting particles do not change in morphology (figures 1(c) and (d)); the deposited silver nanoparticles cannot be seen but their presence is detected using EDS analysis (insets in figures 1(c) and (d)).

and a standard deviation of 2.0 nm. Using TEM we can confirm the size of the TiO2 1 particles, and the most important information extracted from figure 2(a) is the irregular thin layer observed on the surface of TiO2 particles; this could be a layer made of SiO2 and Al2 O3 (according to the results obtained in the EDS analysis). It is shown in figure 2(b) that TiO2 2 particles have a spherical morphology and a particle size ranging from 15 to 70 nm. Figures 2(c)–(e) show the images for TiO2 1@Ag25, TiO2 1@Ag10 and TiO2 2@Ag10 samples, respectively. Using TiO2 1 particles and increasing the amount of silver nitrate in reaction we cannot produce more Ag nanoparticles as we expected; instead, silver nanoparticles already formed on the surface of TiO2 1 grow (figures 2(c) and (d); figures 3(a) and (b)). The reason for this unexpected behavior could be the presence of the irregular SiO2 –Al2 O3 thin layer on the surface of TiO2 1 particles (the presence of these oxides in DuPontTM Ti-Pure® R-902 is also reported in the datasheet of the product). SiO2 and Al2 O3 have no reactivity if they are not activated with a more complicated process than just varying the pH value [18, 19]; the presence of a transition element is very important, so Ag nanoparticles are formed only

3.3. TEM and STEM Size distribution analysis was done on the Ag nanoparticles prepared over the surface of TiO2 particles; each analysis accounts for 200 particles, and the results are presented as histograms in figures 2(c)–(e). The Ag nanoparticles in the TiO2 1@Ag25 sample have a mean size of 20.6 nm and a standard deviation of 5.1 nm; the Ag nanoparticles in the TiO2 1@Ag10 sample have a mean size of 77.3 nm and a standard deviation of 18.4 nm; finally, the Ag nanoparticles in the TiO2 2@Ag10 sample have a mean size of 8.2 nm 3



a)

b)

25

TiO2_1@Ag10

TiO2_1@Ag 25 20

Frequency (%)

Frequency (%)

20 15 10

15 10 5

5

0

0 10

15

20

25

30

35

40

60

80

100

120

Particle size (nm)

Particle size (nm)

c)

d)

35

TiO2_2@Ag10

Frequency (%)

30 25 20 15 10 5 0

4

5

6

7

8

9

10 11 12 13 14

Particle size (nm)

e) Figure 2. TEM images of the starting materials and the composites synthesized in this work: (a) TiO2 1, (b) TiO2 2, (c) TiO2 1@Ag25, (d) TiO2 1@Ag10 and (e) TiO2 2@Ag10. The arrow in (a) shows the irregular thin layer on the surface of these particles.

on the spots where there is no SiO2 –Al2 O3 thin layer; in a short time, these spots are replete and the remaining silver ions are deposited over the first silver nanoparticles formed, and finally they grow. If we use TiO2 2 instead of TiO2 1 and if we maintain the concentration of Ag+ as a constant, the amount of silver nanoparticles over the surface of TiO2 particles increases considerably (figures 2(d) and (e)); the reason for this could be,

again, the presence of the SiO2 –Al2 O3 thin layer on the surface of TiO2 1 and the fact that Degussa P25 is reported as the most reactive phase of TiO2 [20]. 3.4. XRD analysis Figure 4(a) shows the diffraction pattern obtained for the TiO2 1@Ag25 composite: only peaks from rutile phase 4



b)

a)

Figure 3. STEM images of the samples (a) TiO2 1@Ag25 and (b) TiO2 1@Ag10. These images show that the silver nanoparticles are bigger in sample TiO2 1@Ag10 than in sample TiO2 1@Ag25.

Figure 4. XRD analysis of samples (a) TiO2 1@Ag25, (b) TiO2 1@Ag10 and (c) TiO2 2@Ag10.

appear; no peaks from elemental silver appear, probably due to the detection limit of the instrument. Figure 4(b) shows the diffraction pattern obtained for the TiO2 1@Ag10 sample;

it presents peaks from rutile and silver. The amount of AgNO3 used to prepare this sample is more than for sample TiO2 1@Ag25; thus the silver amount present in the composite 5



Figure 6. UV–vis spectra of the starting materials and the composites synthesized in this work.

Figure 5. XPS analysis of the sample TiO2 1@Ag25.

is larger and can be detected by XRD. The x-ray diffractogram for the TiO2 2@Ag10 sample shows reflections due to rutile and anatase phases of TiO2 and also shows reflections from elemental silver (figure 4(c)). These results confirm that the nature of the synthesized particles is elemental silver.

Table 2. Minimum inhibition concentrations of TiO2 particles, Ag nanoparticles and TiO2 @Ag composites. Material

Minimum inhibition concentrationc (μg ml−1 ) Bacteria

3.5. XPS analysis

TiO2 1 TiO2 2 TiO2 1@Ag25 TiO2 1@Ag10 TiO2 2@Ag10 Ag nanoparticlesb

Because of the detection limit of the XRD instrument we were unable to detect silver on sample TiO2 1@Ag25, so XPS analyses were done in order to confirm the presence of Ag. Figure 5 shows the spectrum obtained: the peaks found at 368.1 and 373.9 eV confirm the presence of silver in the form of Ag0 [21, 22]; these results agree with those obtained using EDS analysis.

E. coli —a —a 130.2 (0.651) 358.5 (35.94) 190.1 (19.01) 13.02

S. aureus —a —a 250 (1.25) 333.3 (33.3) 208.3 (20.67) 16.67

a

No antibacterial activity was found with the concentrations tested in this work. b 20 nm Ag nanoparticles were synthesized under the same conditions as the composites but without the presence of TiO2 particles. c Values in parentheses represent the calculated content of silver in the composites. This silver content was calculated using the results from atomic absorption spectroscopy (AAS) performed on the supernatant after filtering the composites. The Ag content in TiO2 @Ag = Ag+ added –Ag in supernatant.

3.6. UV–vis analysis The absorption spectra of the composites are presented in figure 6; they feature the band edge of TiO2 (415 nm for TiO2 1 particles and 380 nm for TiO2 2 particles). A weak band near to 450 nm corresponding to the signal of silver nanoparticles [23] is present in spectra of TiO2 1@Ag25 and TiO2 1@Ag10 composites; a well defined band at 450 nm arises in the spectrum of the TiO2 2@Ag10 sample. Thus, these composites could be used in photocatalysis without the use of UV light.

3.8. Antibacterial results Minimum inhibitory concentration values were obtained for the synthesized composites tested against E. coli (Gram negative bacteria, ATCC 25922) and S. aureus (Gram positive bacteria, ATCC 25923). The results are presented as average values ion table 2 (the Kruskal–Wallis test was applied). Control sample containing all the initial reaction components showed no antibacterial activity. The TiO2 1@Ag25 sample has higher antibacterial activity than the other composites and presents higher antibacterial activity than TiO2 particles. If we compare the MIC of TiO2 1@Ag25 with that of silver nanoparticles we can see that the MIC of the latter is lower, but the silver content in TiO2 1@Ag25 sample is much lower (almost 20 times) than the MIC of silver nanoparticles; thus, we can say that there is a real synergetic antibacterial activity in these composites. So far it is clear that bigger nanoparticles decrease the antibacterial activity in our nanocomposites: TiO2 1@Ag10 composites

3.7. DLS analysis Figure 7 shows the DLS analyses made on TiO2 1 particles before and after the synthesis of silver nanoparticles. These analyses were used to probe whether silver nanoparticles were really attached to the surface of TiO2 particles. From figure 7(a), the TiO2 1 particles, before silver synthesis, have a mean diameter of 334 nm (93.1 nm width); after silver synthesis their mean diameter slightly grows (388 nm; 105 nm width) due to the presence of silver nanoparticles (figure 7(b)). This result confirms that silver nanoparticles really are attached to TiO2 particles; if this were not the case we would see two peaks in figure 7(b), one corresponding to silver nanoparticles and the other one corresponding to TiO2 particles. 6



b)

a)

Figure 7. DLS analyses of sample TiO2 1: (a) before and (b) after the synthesis of silver nanoparticles.

present more silver content than TiO2 1@Ag25 composites but their size is bigger and the antibacterial activity decreases. The fact that TiO2 1 and TiO2 2 particles showed no antibacterial activity is due to the test conditions: the test was performed on dark. It is reported [8, 14] that the bactericide activity of TiO2 is directly related to ultraviolet light absorption and the formation of free radicals, so in dark conditions TiO2 particles present no bactericide activity, which is consistent with our results. On the other hand, all the TiO2 @Ag composites show antibacterial activity even though no light is present. The antibacterial mechanism of these composites is under investigation by our group. Comparing our results with those reported by Thiel et al (they use a different method to prepare nanocomposites but perform similar evaluations of their bactericidal effects) we found that the best results obtained in their work in liquid medium were achieved using a concentration of the composite of 76 900 μg ml−1 (10 g in 130 ml) with 3 wt% of silver content (0.72 at.%), while the best bactericide effect found in our work was achieved with a concentration of 130.2 μg ml−1 with the sample TiO2 1@Ag25 with 0.5 wt% of silver content, i.e., although our composites are bigger in size the bactericide concentration is lower. The main difference between these composites is that our composites present the silver nanoparticles on their surface, promoting contact with bacteria, while Thiel’s composites have silver in their matrix which makes the direct contact of silver with bacteria cells difficult; this could be the main reason why both nanocomposites present different antibacterial activities.

The antibacterial activity of TiO2 nanoparticles was improved and is dependent on the sort of TiO2 particles. In this work, the best results were achieved using TiO2 DupontTM particles and a TiO2 :Ag molar ratio of 1:25.

Acknowledgments This work was partially supported by Fondo de Apoyo a la Investigación (FAI) of Universidad Autónoma de San Luis Potos´ı (UASLP) and CONACYT-61257. N Niño-Mart ´ınez would like to thank CONACYT for scholarship No. 185006.

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4. Conclusions Silver nanoparticles were synthesized over the surface of two different commercial TiO2 particles. The composites thus obtained were characterized: using XRD, XPS and UV–vis analysis it was demonstrated that the nature of the nanoparticles prepared is elemental silver; these silver nanoparticles are well distributed over the surface of TiO2 particles and their average sizes are 10, 20 and 80 nm, depending on the TiO2 :Ag ratio. 7


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