Synthesis of Silver Nanoparticles in a Chitosan Solution (PTChit XV 2010)

Progress on Chemistry and Application of Chitin and Its ..., Volume XV, 2010 63

SYNTHESIS OF SILVER NANOPARTICLES IN A CHITOSAN SOLUTION

Zofia Modrzejewska, Roman Zarzycki, Jan Sielski

Faculty of Process and Environmental Engineering Technical University of Łódź,

ul. Wólczańska 175, 90-924 Łódź, Poland e-mail: [email protected]

AbstractIn the present study silver nanoparticles were obtained using the ability of chitosan to form chelate compounds with metal ions and taking advantage of its reducing properties. The reduction process was conducted at the temperature 60°C or applying γ radiation from 60Co cobalt source. A preliminary characteristic which allows us to realize the size of formed silver nanoparticles was made on the basis of the measurements of UV absorption and dynamic light dispersion.

Key words: chitosan salt, nanoparticle, silver.

Progress on Chemistry and Application of Chitin and Its ..., Volume XV, 201064

Z. Modrzejewska, R. Zarzycki, J. Sielski

1. IntroductionCurrently, nanotechnologies and nanomaterials belong to research priorities. Na-

no-sized substances such as fullerens, nanotubes, nanocolloids, self-organizing particles, surface structures, nanocrystals, nanosons and nanocapsules are characterized by developed wall surfaces and higher atom availability. Surface development causes that the contribution of surface atoms increases, reactivity and properties change. Thanks to nanotechnologies, materials of very high strength, unusually light, changing color depending on the environ-ment color and self-purifying can be produced. Owing to their unique properties, nano-products find wide-ranging applications, from medicine and pharmaceutics to electronic industry, optics and environmental protection.

Nanoparticles are obtained by two methods: top-down, which consists in com-minution of the material to very fine particles, and bottom-up, which includes chemical synthesis of nanomaterials (mainly the sol-gel and precipitation methods), forced and free self-organization (carbon fullerens and nanotubes).

In order to obtain a product in the form of nanopowder, either calcination or drying of the material to very fine particles, and bottom-up, which includes chemical synthesis of nanomaterials (mainly the sol-gel and precipitation methods), forced and free self-organiza-tion (carbon fullerens and nanotubes).

In order to obtain a product in the form of nanopowder, either calcination or drying of the formed gel is carried out at elevated temperature. Nanoparticles can also be obtained in a solution. However, they form clusters very quickly and require the use of stabilizers in the form of surfactants. Nanoparticles are also stabilized by polymers (polyacrylnitrile, polyvinyl pyrrolidone, polyvinyl alcohol, starch, gelatin, heparin, carboxymethylcellulose and chitosan), and also by high molecular fatty acids.

A subject of the present study are silver nanoparticles. Researchers are interested in them mainly because of their unique bactericidal and fungicidal properties. The cidal activity of silver nanoparticles depends on particle dimensions. The mechanism of silver nanoparticle activity depends on the type of microorganisms. In fungi it covers disturbance of the natural water balance by blocking the ability of fungi to bind water molecules. In the case of viruses this mechanism is reduced to the elimination of their ability of catalytic decomposition of a lipid-protein substrate.

There are also several ways in which silver nanoparticles can act on a bacterial cell. n Silver nanoparticles, which are characterized by high electric conductivity, adhere to a

bacterial cell membrane and disturb the natural gradient of electric potential which deter-mines the flow of matter and energy indispensable for bacteria life.

n Nanosilver also penetrates protoplasm through capillaries in the cell membrane, causing disturbances in the activity of mitochondria and cell nucleus.


Synthesis of Silver Nanoparticles in a Chitosan Solution

n Silver ions in microbial cells are also bound with thiol groups (-SH) which results in protein deactivation. Bacterial growth is also inhibited by atomic oxygen adsorbed on silver surface.

This diversified action of silver on bacteria causes that so far they have not been able to form an efficient defense mechanism and practically there are no strains resistant to silver bactericidal action [1 - 7].

Silver nanoparticles are usually obtained by reducing silver nitrate by sodium borohydride (NaBH4), EDTA or γ radiation. Photochemical reduction, microwave, electron beam, micelle, reverse micelle, microemulsion, lamellar liquid crystal, aerosol spraying and laser radiation are also applied [8 - 20].

In water solutions silver hydrosols agglomerate very quickly and require a stabi-lizer in the form of a surface-active compound. To stabilize silver nanoparticles also natural and synthetic polymers are used. Kapoor (1998) stabilized the photochemically reduced sil-ver particles with polyvinyl pyrrolidone (PVP), gelatin and carboxymethylcellulose (CMC), while Liu (2001) synthesized polyacrylnitrile-silver nanocomposites using γ radiation. It is also known that chitosan can be used for this purpose. Silver nanoparticles were obtained from silver nitrate introduced to chitosan salt which additionally contained alcohol (isopro-panol) enabling the growth of free radicals induced by γ irradiation and reduction of ionic form of the metal [21 - 27].

2. Experimental resultsIn the present study silver nanoparticles were obtained using the ability of chitosan

to form chelate compounds with metal ions and taking advantage of its reducing properties. The reduction process was conducted at the temperature 60°C or applying γ radiation from 60Cd cobalt source.

Sorption abilities of chitosan and a possibility to form chelate compounds are stud-ied both in literature and in our own research projects. It follows from them clearly that the capacity and mechanism of sorption depend on the form of chitosan, which is illustrated in Figs. 1 and 2. To compare the possibility of silver ion removal by means of chitosan in dif-ferent forms, studies of Ag+ adsorption by hydrogel granules and chitosan salt were carried out simultaneously, as discussed in [28]. The chitosan salt was 4% chitosan acetate, i.e. a so-lution from which chitosan hydrogel granules were produced. The experiments were carried out according to the following procedure: to silver solutions with appropriate concentration of 10 mg Ag/l and 50 mg Ag/l chitosan acetate at pH = 4 was added. The amount of chitosan in the added salt was equal to the quantity of chitosan present in the hydrogel. Results are shown in Figures 1 and 2.

It follows from the above data that neutralized chitosan in the form of hydrogel is characterized by higher sorption capacity. Sorption ability of salt solutions is lower by half. This is probably a result of the presence of practically all protonated amino groups which are reactive towards anions, while binding of cations is possible only after deprotonation of the amino group and it proceeds most probably in two stages:



RNH3+ RNH2 + H+

++ + n)RNH(MRNHnM 22 �Ì M+ + nRNH2 M(RNH2)n+

Jang’s experiments with copper show that at pH lower than 5.3, binding of cations with glucosamine molecules proceeds according to the “hanging droplet” model – the metal ion is bound with one amino group, while at pH above 5.8, like in the ‘bridge’ model – the metal ions are bound with several amino groups of the same chain or of different chains. Assuming the same mechanism of reaction for silver, the complexing of chitosan in the form of hydrogel causes in a certain sense cross-linking of the structure, while salt complexing offers a possibility of placing silver outside the molecule and enables a reaction with the products of water decomposition induced by radiation, probably according to the following reaction patterns:

H2O ⇒ eaq-, •OH, H•, H3O+, H2, H2O2Ag+ + [C6H11O4N]n ⇒ [C6H11O4N]n Ag+

[C6H11O4N]n Ag+ + eaq - ⇒ [C6H11O4N]n Ag0

[C6H11O4N]n Ag0 + Ag+ ⇒ [C6H11O4N]n Ag2+

[C6H11O4N]n Ag2+ + Ag0 ⇒ [C6H11O4N]n Agm+

[C6H11O4N]n Agm+ + eaq- ⇒ [C6H11O4N]n Agm0

[C6H11O4N]n + •OH ⇒ [C6H11O4N]n-1 [C6H11O4N] • + H2O[C6H11O4N]n Agm+ + [C6H11O4N]n-1 [C6H11O4N] • + H2O ⇒ [C6H11O4N]n Agm0 + [C6H11O4N]n-1 [C6H11O4N] + H3O+

Silver nanoparticles were obtained taking advantage of the reducing properties of chitosan salt and carrying out the reduction process at proper temperature or using γ radiation which is a subject of patent application no. P 385153 (2008) [29]. A preliminary characteristic which allows us to realize the size of formed silver nanoparticles was made on the basis of the measurements of UV absorption (PERKIN ELMER) and dynamic light

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

0

2

4

6

8

10

12

14

16

18

20

Silv

er io

n co

ncen

trat

ion

[mg/

dm3]

Time [h]

chitosan acetate, salt chitosan hydrogel, granules

0 20 40 60 80 100

0

10

20

30

40

50

60

70

80

90

100

110

120

130

Si

lver

ion

conc

entr

atio

n [m

g/dm

3]

chitosan acetate, salt chitosan hydrogel, granules

Time [h]

Figure 1. Adsorption of Ag+ from different forms of chitosan; silver nitrate 10 mg/dm3.

Figure 2. Adsorption of Ag+ from different forms of chitosan; silver nitrate 50 mg/dm3



dispersion (Malvern Instruments Zetasizer Nano ZS). UV spectra for selected samples are shown in Figure 3.

The size of silver nanoparticles can be controlled not only by chitosan parameters and the concentration of silver nitrate, but also conditions in which the reduction process is carried out. The obtained silver nanoparticles of diameters from 2 µm to 50 nm were stable in water solutions. Size distributions of the obtained particles are given in Table 1.

400 600 8000

1

(0)

(Chitosan)

(1)

λ /nm

Abso

rban

ce

(3)

Figure 3. UV spectra of chitosan acetate solutions containing silver nanoparticles; 0 – chelating process at the temperature 60°C in 24 h, 1 – chetate compounds of chitosan with silver exposed to γ radiation of 10 kGy, 3 – chetate compounds of chitosan with silver exposed to γ radiation of 50 kGy.

Table 1. Size distribution of silver nanoparticles.

Method of nanoparticle formation Particle size, nm Peak analysis by

numberPeak analysis by

intensityPeak analysis by

volume

0 – 60 °C, 24 h

57.8252897.42493

2818.5

95.74.3---

--

1.498.6

-

----

100

1 – γ radiation of 10 kGy

284.2933.4263.7967.2323.1991.7

97.03---

--

8317-

----

5050

3 – γ radiation of 10 kGy

6.39.1

89.48.5

100---

-5

9517

---

100



An optical illustration of the obtained silver nanoparticles is shown in the photo-graphs taken under an electron scanning microscope FEI Quanta 200 F. They show distinct, evenly distributed silver particles of the size of several nanometers (Figure 5) which is in agreement with the results obtained by the method of dynamic light dispersion. Nanopar-ticles form clusters of several micrometer dimensions with increased intensity (Figure 6). It is also possible to form structures of the size of several hundred nanometers (Figure 7). Silver nanoparticles can also be formed into balls (Figure 8). These structures are particu-larly interesting. Authenticity of silver present in these spherical structures is confirmed by an elemental analysis performed using Oxford EDS equipped with detector XMAX 50.

3. Conclusions Chitosan salt can be an interesting nanosilver stabilizer. The size of silver nano-particles can be controlled not only by chitosan parameters and the concentration of silver nitrate, but also conditions in which the reduction process is carried out. The obtained silver nanoparticles of diameters from 2 µm to 50 nm were stable in water solutions.

Researches on this subject are promising and require further systematic studies.

Size distribution(s)

1 5 10 50 100 500 1000 5000 10000Diameter (nm) (x10̂ 2)

50

100

% in

cla

ssSize distribution(s)

50 100 500 1000Diameter (nm)

10

20

30

40

% in

cla

ss

Size distribution(s)

5 10 50 100 500Diameter (nm)

10

20

30

40% in

cla

ss

Figure 4. Particle size distribution; a – chelating process at the tempera-ture 60 °C in 24 h, b – γ radiation of 10 kGy, c – γ radiation of 50 kGy.



Figure 5. Silver nanoparticles with chitosan salt as a stabilizer.

Figure 6. Areas with increased nanosilver concentration.

Figure 7. Nanoparticles of hundred-nanometer size.



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Figure 8. Formation of spherical entangled structures.



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Synthesis of Silver Nanoparticles in a Chitosan Solution (PTChit XV 2010)