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3UDC 546.5, 543.4
SYNTHESIS OF SILVER NANOPARTICLES BASED ON THE SOL-GEL PROCESS
AND THEIR CHARACTERISTICS
ABBASOVA RENA FRIDUNOVNA
Associate professor of the Department of General and Inorganic Chemistry, Baku State University,
Baku, Azerbaijan
VEYSOVA SITARA MALIK GIZI Senior Lecture of the Department of General and Inorganic Chemistry, Baku State University,
Baku, Azerbaijan
GAHRAMANOVA GUNEL HAJI GIZI Research assistant of the Department of General and Inorganic Chemistry, Baku State
University, Baku, Azerbaijan
ILYASLY TEYMUR MAMMAD OGHLU
Professor of the Department of General and Inorganic Chemistry, Baku State University,
Baku, Azerbaijan
Annotation: The paper presents an analysis of the results of synthesis of silver nanoparticles by sol-gel method using different reducing agents. The main parameters of the synthesis of nanoparticles, such as temperature, concentration and ratio of solutions of silver nitrate and reducing agent, and their influence on the size of nanoparticles and the stability of their solutions are analyzed. The stability and optical properties of the obtained solutions and nanoparticles were studied.
Key words: silver nanoparticles, synthesis, sol-gel method, optical absorption spectra
Introduction
Nanomaterials have unique chemical and physical properties, and their use in many fields stimulates the search for new synthesis methods and continuous improvement of existing ones. Silver nanoparticles have unique optical, electrical and catalytic properties [1 ;2 pp 3-13] One of the most interesting properties of silver nanoparticles is the phenomenon arising from the interaction of photons with the surface of AgNPs, in which external free electrons in the conduction band form localized plasmons. Surface plasmon resonance is the collective excitation of free electrons near the surface of nanoparticles. The electrons are restricted to certain modes of oscillation due to the size and shape of the particle, and only certain wavelengths of light cause the outer electrons to oscillate. When these resonances occur, the intensity of absorption and scattering is much higher than the same particles without plasmonic properties. [3, pp 246-249, 4, pp. 327-330] The intrinsic properties of metallic nanostructures can be modified by controlling their size, shape, composition, crystallinity and structure depending on the experimental conditions [3, pp 233-237 5, pp. 36-38]
Due to their unique properties, silver nanoparticles are already in demand in many fields, and the need for their even wider application stimulates the search for new synthesis methods and continuous improvement of existing ones [6, pp. 65-69].
There are many synthesis methods to produce silver nanoparticles (AgNPs). In our work we used one of the most popular methods - sol-gel method, due to the fact that its advantages outweigh its disadvantages [ 7, pp. 7-8, 13-24).]. The main feature of the sol-gel method is the control of the rate of sol and gel formation and, consequently, the size and microstructure of the obtained particles. An undoubted advantage of the method is the fact that the production of nanoscale crystals occurs at relatively low temperatures.
The method allowed to obtain silver NPs of different sizes and with different distributions. Based on known techniques we used different reducing agents and varied some synthesis parameters in order to evaluate their efficiency.
The synthesized AgNPs were characterized by means of UV Vis spectroscopy. The chemical activity of the particles and their antibacterial effect on the shelf life of milk were also investigated.
Experimental
For the experiments, we used silver nitrate as Ag precursor, sodium citrate, ferrous sulfate, sodium borohydride and glucose as reducing agents, water-soluble starch was used as reducing agent and stabilizer, respectively. Because, the stabilization of silver nanoparticles is important, since the latter undergo rapid oxidation and easily aggregate in solutions.
The reduction reaction is carried out under various conditions: temperature, concentration of initial reagents and storage time of colloidal solutions.
In all experiments, 0.001 M silver nitrate solution was used as a precursor, the concentrations of reducing agents were calculated according to the chemical equations of known methods. Freshly prepared solutions of dilute silver nitrate and reducing agents were mixed at different temperatures in ratios corresponding to the reactions of the processes.
Each experiment was performed both without and with the addition of starch 0.02 g of water-soluble starch as stabilizer
Experiments using citrate as a reducing agent (based on the method developed by Turkevich) were carried out by gradual addition of its 1% solution to a boiling solution of silver nitrate in a 1:1 ratio with continuous stirring on a magnetic stirrer until the color changed to pale yellow. The reaction of the process can be written as Ag+ + 2C6H5O7 3- = Ag + 2CO2 + C5H2O52- +3H2O + C5H2O42-The solution was then cooled to room temperature with the stirrer on. In this experiment, sodium citrate plays the role of both reducing agent and stabilizer, which complicates the calculation of its concentration. To find the most optimal ratio of precursor and reducing agent, we conducted a series of experiments, changing the concentration and ratio of solutions.
To carry out the synthesis, a solution of AgNO3 in an amount of 25 ml of 0.001 M was used as a precursor. We heated it to boiling (using a magnetic stirrer) in a glass (200 ml). Separately, 100 ml of 0.001 M Na3C6H5O7 solution was prepared and added dropwise to the boiling AgNO3 solution with constant stirring.
After mixing, the color of the solution changed from colorless to cloudy yellowish, indicating the reduction of silver ions. Heating was continued for 15 minutes, then the solution was cooled to room temperature.
The resulting solution without the addition of starch was stable for two weeks, then a small amount of precipitate was observed and the solution became discolored. The unstabilized solution remained visually unchanged for 7 days. The resulting solution with the addition of starch was stored in the dark and retained its color for a month.
The absorption spectra of the resulting solutions were recorded on the device. For subsequent study of sediments, the solutions were centrifuged, the sediments were washed and dried.
When obtaining silver nanoparticles by citrate-sulfate (Carey Lee method), the experiment was carried out in a similar way, only in this case prepared solutions of sodium citrate (stabilizer) and ferrous sulfate (reducing agent) separately, then mixed and immediately added to the precursor solution with vigorous stirring. The reduction of silver nitrate can be described by the following equation:
3AgNO3+3FeSO4 ^Fe2(SO4>+Fe(NO3)3+3Ag
In this case, the color of the solution changed to a darker, yellow-brown color. Stirring was carried out for an hour.
To reduce silver nitrate with sodium tetraborate, the latter is cooled to 00C and gradually added to the silver nitrate solution preheated on a magnetic stirrer.
When sodium tetrahydroborate is added to silver nitrate silver nanoparticles are formed according to the following scheme:
2AgNO3+2NaBH4+6H2O=2Ag+7H2+2NaNO3+2H3BO3 The resulting solution, even without the addition of starch, was visually stable for one month. The obtained solution even without the addition of starch was visually stable during one month, i.e. there was no significant change in the color of the solution, however, the taken spectrum showed the presence of additional new peaks characteristic for nonspherical particles with large size. The resulting solution with the addition of starch was stored in the dark and retained its color for 2 months.
When glucose was used as a reducing agent, a 0.05 M glucose solution was prepared separately. The solutions were mixed in the ratio of 1 AgNO3 : 1 C6H12O6. To obtain pH = 8-9, NH4OH was added to the solution. The process is described by following reaction:
2AgNO3+C6H12O6+H2O ^C6H12O7+2Ag+2HNO3
The experiment was carried out at 96-98 0C for 2 hours. The color of the solution slowly changed from yellow to yellowish brown, from gray to dark gray.
The obtained solution did not show good stability, which may be due to the quality of glucose or the high concentration of precursor. During the first week the solution became discolored due to rapid aggregation and precipitation.
The absorption spectra of the resulting solutions were recorded on the device spectrophotometry.
For subsequent study of sediments, the solutions were centrifuged, the sediments were washed and dried. Their chemical and antibacterial properties were then studied
Results and discussion
The aim of the work was to synthesize solutions containing silver nanoparticles, to study their physicochemical and antibacterial properties, to compare the efficiency of different reducing agents for the production of silver nanoparticles, as well as the stability and optical properties of the obtained solutions of silver nanoparticles.
To achieve the goal, the following tasks were solved: synthesis of silver nanoparticles; study of the composition and properties of sols containing silver nanoparticles; study of the antibacterial properties of sols containing silver nanoparticles.
The synthesized solutions were characterised by ultraviolet-visible (UV-Vis) spectroscopy techniques. The UV-Vis was used to study optical and structural properties of the synthesized Ag nanoparticles respectively.
Absorption spectrophotometry is one of the most informative methods for the study of nanoparticle solutions. We used Specord 210 plus Analytik Jena, Germany to register AgNPs in solution and control the dynamics of their formation. Absorption spectra were measured in 1 nm steps at wavelength 200-1000 nm in quartz cuvettes with a fourth optical path of 1 cm.
Optical absorption study in the wavelength range between 200-1000 nm reveals that strong absorbance peak was found in UV-Vis region.
It is known that the shape and size of silver nanoparticles obtained by this method depend on the temperature regime of synthesis. The nature and concentration of the reducing agent, reaction temperature and pH of the medium have a great influence on the size distribution of nanoparticles.
Silver nanoparticles of spherical shape with a diameter of 10-25 nm have an absorption peak near 400-420 nm, the size and degree of monodispersity of the particles increase with increasing temperature, rod-shaped particles with two absorption peaks in the spectra can be formed.
In our experiments at low temperature, the reaction did not proceed or proceeded very slowly. Therefore, all reactions were carried out with preheating of the precursor and on a heated magnetic stirrer. In our experiments, at nanoparticle sizes larger than 50 nm, additional peaks were observed in some solutions, which is associated with the aggregation process.
A characteristic feature of nanoparticles is their strong and specific interaction with electromagnetic radiation. A feature of the absorption spectra of nanoparticles larger than 2 nm is the
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presence of a broad band of surface plasmon resonance (SPR) in the visible region or in the near ultraviolet region adjacent to it. The spectral maximum near 400 nm corresponds to the SPR of isolated and weakly interacting silver nanoparticles. The absorption spectrum of our solutions show a pronounced maximum at 430-450 nm. After exposure for 7, 14 days and 1 month, the absorption spectrum of the solutions with starch practically does not change, which indicates the absence of active aggregation of particles. The maximum at a wavelength of 430-450 nm corresponds to spherical silver particles up to 50 nm in size.
Figure 1 is the absorption spectrum of the solution obtained using sodium tetrahydroborate. The electromagnetic absorption spectrum in the UV/Visible region shows a peak at 441 nm. The nanoparticles consist of crystalline silver, have a spherical shape with a diameter slightly larger than 50 nm. In the unstabilized solution obtained by the Carey Lee method there are maxima characteristic of bulk silver and larger aggregates of nanoparticles. (Fig. 2) Figure 2 shows the absorption spectrum of the unstabilized solution reduced by sodium citrate, the unstabilized rasters are characterized by the characteristic broadening observed.
0,4
250 350 450 550 650
W»v*len|th (nm|
Wavelength (nm]
Fig.1.Optical absorption spectrum of colloidal solution of Ag nanoparticles reduced by tetraboron stabilized with starch after 14 days of storage in a dark place
Fig.2. Optical absorption spectrum of unstabilised colloidal solution of Ag nanoparticles reduced by citrate after 14 days of storage in a dark place
Fig. 3 shows the absorption spectrum of colloidal silver solution obtained by reduction with glucose, taken after one week of preparation. On the absorption spectrum there are 2 peaks, one of which is more gentle and shifted to the long-wave region of the spectrum at 450 nm, corresponds to silver nanoparticles, nonspherical shape and larger size as a result of particle aggregation. The presence of the second peak at 350 nm corresponds to the presence of bulk silver particles in solution. Due to the high reducing ability of glucose, a large number of small clusters are formed at the initial stage, and further irreversible aggregation of these clusters results in larger agglomerates. Glucose is both a reducing agent and a stabilizer. In the unstabilized solution obtained by the Carey Lee method there are maxima characteristic of bulk silver and larger aggregates of nonspherical nanoparticles (Fig.4). Nanorods have anisotropic symmetry, and therefore two peaks are observed in the absorption spectrum, corresponding to transverse and longitudinal plasmons Transverse plasmon gives an absorption peak at 400 nm, and longitudinal can be manifested in the range from 500-1000 nm, i.e., in the near-infrared region. Its position is determined by the dimensional factors of the nanorod, namely the ratio of length to width. The absorption spectra in the longer wavelength range characterize the nanorods. The absorption maximum in the spectra of nonspherical particles is shifted to the long-wavelength region, and the larger the particles, the stronger the absorption shift.
Fig.3. Optical absorption spectrum of colloidal solution of rod-shaped Ag nanoparticles reduced by glucose, stabilized by starch, after 14 days of storage in a dark place
Fig.4. Optical absorption spectrum of unstabilized colloidal solution of Ag nanoparticles obtained by the CareyLee method stabilized with starch after 14 days of storage in a dark place.
Analyzing the results of the experiments we can say that the most stable colloidal solution was obtained using sodium tetrohydroborate as a reducing agent.
The physical properties of many substances depend on the size of the sample.
Sodium tetrahydridoborate serves as both a reducing agent and a stabilizer of the formed nanoparticles. The increase in particle size occurs due to the aggregation of clusters during the decomposition of borohydride, when the stabilizing effect of sodium tetrahydridoborate decreases.
Nanoparticles of a substance often have properties that are generally not found in samples of these substances of normal size.
After cooling to room temperature, the solution was added to a 5 mL centrifuge tube and processed at 10,000 rpm for 10 min in the centrifuge. The resulting supernatant was decanted. After three washes, the product is dried at 70 °C in a drying oven. After drying, the properties of silver nanoparticles. After three washes, the product is dried at a temperature of 70°C in a drying cabinet. After drying, the properties of silver nanoparticles were studied.
It is known that silver does not participate in most chemical reactions. However, silver nanoparticles not only become very good catalysts for chemical reactions (accelerate their occurrence), but also directly participate in chemical reactions. For example, ordinary silver samples do not react with hydrochloric acid, but silver nanoparticles react with hydrochloric acid.
To 5 ml of a solution of the obtained silver nanoparticles, 5 ml of diluted HCl was added dropwise. Then acetic acid CH3COOH was added. At the same time, a gradual dissolution of silver nanoparticles and the formation of a white precipitate when adding hydrochloric acid and discoloration of the solution when adding acetic acid were first observed.
Hydrochloric acid does not react with ordinary silver. However, silver nanoparticles react with hydrochloric acid to release hydrogen. The reason for this behavior of nanoparticles is related to the so-called surface effects. The point is that in a small particle the proportion of atoms located on the surface increases significantly. These atoms have dangling bonds, and as a result, they have higher energy and activity. The high reactivity of silver nanoparticles explains the fact that they have a strong bactericidal effect - they kill some types of pathogenic bacteria. Silver ions make it impossible for many chemical reactions to occur inside bacteria, and therefore, in the presence of silver nanoparticles, many bacteria do not reproduce.
We investigated the high reactivity of silver nanoparticles, conventional silver samples do not interact with hydrochloric acid, while silver nanoparticles react with hydrochloric acid to form a characteristic white precipitate which dissolves on addition of acetic acid.
To test the antibacterial properties, silver nanoparticle sol was preliminarily applied to fresh milk in dishes. The freshness of milk was determined by using indicators.
The pH of milk in untreated dishes decreased to 3.5 after one week, indicating souring of milk, while in treated dishes it remained 6.5 for one month, which corresponds to the pH of fresh milk. Comparison of acidity of milk in silver sol-treated and untreated dishes showed the presence of antibacterial properties of silver.
Conclusion
Silver nanoparticles were synthesized from previously prepared colloidal solutions using various reducing agents: Na3C6H5O7, FeSO4, NaBH4, C6H12O6. In our studies, the most stable solutions of nanoparticles were obtained when sodium tetrahydroborate was used as a reducing agent. The main parameters of the synthesis of nanoparticles, such as temperature, concentration and ratio of solutions of silver nitrate and reducing agent, and their effect on the size of nanoparticles and the stability of their solutions, were analyzed. It was revealed that an increase in the concentration of a silver nitrate solution violates the stability of the resulting colloidal solution, and an increase in temperature has a positive effect on the formation and growth of silver nanoparticles. The recorded absorption spectra of the resulting colloidal solutions in the range of 250 - 800 nm were analyzed. Comparison of the obtained spectra with literature data allowed us to draw a conclusion about the size and shape of the obtained nanoparticles. The high reactivity of silver nanoparticles, and, consequently, their strong bactericidal effect, was verified by reactions with acids and by measuring the pH of milk.
Acknowledgement
We would like to express our sincere gratitude to Professors of the Department of Chemistry of High Molecular Weight Compounds Rasim Alosmanov and Irada Buniyat-zade for their invaluable assistance in conducting this research.
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