WO2023172232A1

WO2023172232A1 - Synthesis method of ultra-small silver nanoparticles and silver nanoparticles obtained by using this method

Info

Publication number: WO2023172232A1
Application number: PCT/TR2023/050233
Authority: WO
Inventors: Gurkan Yesiloz; Iqra MUNIR
Original assignee: Bilkent Universitesi Ulusal Nanoteknoloji Arastirma Merkezi
Priority date: 2022-03-11
Filing date: 2023-03-09
Publication date: 2023-09-14

Abstract

The present invention relates to a method (100) of obtaining monodisperse silver nanoparticles (NPs) which are size tunable, ultra-small, offer a linearity range, at room temperature and silver nanoparticles which are obtained by using this method (100).

Description

DESCRIPTION

SYNTHESIS METHOD OF ULTRA-SMALL SILVER NANOPARTICLES AND SILVER NANOPARTICLES OBTAINED BY USING THIS

METHOD

Technical Field

The present invention relates to a method of obtaining monodisperse silver nanoparticles (NPs) which are size tunable, ultra-small, offer a linearity range, at room temperature and silver nanoparticles which are obtained by using this method.

Background of the Invention

Today, it is known that silver nanoparticles are used in commercial applications in medical and pharmaceutical sciences. However, it is seen in the said applications that silver nanoparticles synthesized naturally by biosynthesis (green synthesis) have greater biocompatibility than the ones synthesized chemically. Therefore, it is currently required to synthesize nanoparticles used in biological experiments, by biosynthesis; to obtain ultra-small (<10nm) nanoparticles; and nanoparticles need to have a certain linearity range and be monodisperse.

The Korean patent document no. KR20150071423, an application in the state of the art, discloses a method of preparing monodisperse metal nanowires with high capacity, high efficiency and controllable aspect ratio in an effective and reproducible way.

Summary of the Invention

An objective of the present invention is to realize a method of obtaining monodisperse silver nanoparticles which can be simply produced as biocompatible, is size tunable and has a linearity range, and silver nanoparticles which are obtained by using this method.

Another objective of the present invention is to realize a method of obtaining ultrasmall (<10nm) silver nanoparticles at room temperature, neutral pH conditions and without irradiance of ultraviolet light, and ultra-small nanoparticles which are obtained by means of this method.

Detailed Description of the Invention

The “Synthesis Method of Ultra-Small Silver Nanoparticles and Silver Nanoparticles Obtained by Using this Method” realized to fulfil the objectives of the present invention is shown in the figures attached, in which:

Figure l is a flowchart of the inventive method.

Figure 2 illustrates UV-Vis absorption spectrum of synthesized silver nanoparticles, and graphical analysis thereof at absorption maxima to observe linearity in varying concentrations.

Figure 3 illustrates (a) size distribution analysis of silver nanoparticles (10- 100%) with DLS, and (b) linearity curve obtained from particle size data.

Figure 4 illustrates (a) TEM / SEM micrograph images of silver nanoparticles in glycerol (10-100%) + PVP for the corresponding size measurements (Scale bars are 5nm-20nm); (i) 10% glycerol (Avg. 1.5nm); (ii) 20% glycerol (avg. 37.22 nm); (iii) 40% glycerol (avg. 37.24 nm); (iv) 60% glycerol (avg. 36.44 nm); (v) 70% glycerol (avg. 36.32 nm); (vi) 80% glycerol (avg. 37.24 nm); (vii) 90% glycerol (avg. 36.44 nm) and (b) TEM- derived size distribution histogram of synthesized silver nanoparticles.

Figure 5 illustrates FTIR absorption spectrum of silver nanoparticles in glycerol (10-100%) + PVP.

Figure 6 illustrates XRD pattern of the synthesized silver nanoparticles in glycerol (10-100%) + PVP. Figure 7 illustrates AFM analysis of silver nanoparticles (a) silver nanoparticles in 100% glycerol + PVP (b) length and height measurements of the observed particles.

100. Method

The inventive method (100) of obtaining monodisperse silver nanoparticles which are size tunable, ultra-small, offer a linearity range comprises steps of

- washing and then drying laboratory equipment to be used in synthesis of silver nanoparticles (101); obtaining glycerol solutions in varying ratios/concentrations by mixing glycerol and water in a plurality of containers, so as to have different concentrations in each container (102); obtaining PVP solutions by adding polyvinylpyrrolidone (PVP) into glycerol solutions (103); and synthesizing silver nanoparticles by adding silver nitrate (AgNOs) into PVP solutions (104).

At the step of washing and then drying laboratory equipment to be used in synthesis of silver nanoparticles (101) included in the inventive method (100), glass containers used for preparing solution and magnetic beads used for mixing solutions are washed with aqua regia -which is an acidic mixture of concentrated hydrochloric acid (HC1) and nitric acid (HNO3)- in a 3: 1 ratio (v/v) for 5-15min and then air dried.

At the step of obtaining glycerol solutions in varying ratios/concentrations by mixing glycerol and water in a plurality of containers, so as to have different concentrations in each container (102) included in the inventive method (100), 5- 10 ml deionized water is added into a container and glycerol solutions are prepared in different concentrations by adding glycerol (CxHxOx) in the range of 10-100% by volume into deionized water. Magnetic beads are put into the container wherein the prepared glycerol solutions are included and the containers are placed on a magnetic stirrer. The magnetic stirring process is carried out at room temperature until the water and the glycerol included in the containers are mixed homogeneously.

At the step of obtaining PVP solutions by adding polyvinylpyrrolidone (PVP) into glycerol solutions (103) included in the inventive method (100), 0.4% (approximately 0.02 g) PVP -which has high water solubility and exists in powder form- is added into glycerol solutions becoming homogeneous after magnetic stirring and existing in different concentrations. Magnetic stirring is performed by means of a magnetic stirrer and magnetic bead for 15-60min at room conditions, in order that 10-50% glycerol solutions wherein PVP is added become homogeneous. Magnetic stirring is performed by means of a magnetic stirrer and magnetic bead for 2-3h at room conditions, in order that 60-100 % glycerol solutions wherein PVP is added become homogeneous. Since PVP has high water solubility, it takes comparatively more time for solutions with increasing glycerol concentration to obtain a transparent homogenous PVP solution compared to solutions with low glycerol concentration at room temperature.

At the step of synthesizing silver nanoparticles by adding silver nitrate (AgNCh) into PVP solutions (104) included in the inventive method (100), 1.65% silver nitrate (approximately 0.085g) is added into PVP solutions homogenized in different concentrations. Silver nanoparticles are synthesized by leaving the solution containers, wherein silver nitrate is added, for mixing with the magnetic bead on the magnetic stirrer at room temperature of 20-25°C until the transparency of the solution starts turning into a noticeable light yellow color. Then, the magnetic bead is removed from the solution container and the solutions are retained on the laboratory bench overnight without mixing to observe the stability of the synthesis reaction. Following the retention, color of all solutions prepared in different concentrations turned from light yellow into a brown image and silver nanoparticles are obtained at room temperature. The silver nanoparticles obtained by means of the inventive method (100) are synthesized by using silver nitrate as a metal ion source, glycerol (polyol) as a reductant for both solvent and silver nitrate, polyvinylpyrrolidone (PVP) and deionized water as a stabilizer. The silver nanoparticles obtained are produced at room temperature, neutral pH conditions, without irradiance of ultraviolet light, without needing long procedures, by means of a quick, simple, reproducible and partly green synthesis method, by controlling the dispersion of particles and lowering the particle agglomeration, so as to be in ultra-small (<10nm) sizes, as size tunable in the range of 2-200nm, offer a linearity range and as monodisperse.

With the inventive method (100), one-step synthesis of silver nanoparticles in varying concentrations of glycerol (10-100%) as a reducing agent was conducted at room temperature. During the synthesis step, a sustained reaction kinetics started upon mixing the silver nitrate (metal source) with glycerol (as reductant)-PVP solution and it resulted in direct reduction of Ag+ to produce metallic Ag atoms. As the process initiated, the freshly reduced Ag (silver) atoms served as the nuclei of the nanoparticles, kept on growing continuously as the processing time exceeded, and catalyzed the reduction of the Ag metal ions remaining in the solution. Typically, such coalescence of atoms resulted in the metal clusters formation requiring stabilization by specific surfactants, polymers or ligands (for instance PVP). The synthesis continued during the course of the investigated period, as shown by the appearance and the progressive enhancement in the color intensity at each percentage. The appearance in color intensity for the sample at each percentage indicated progressive enhancement. This color change indicates the formation of nanoparticles in a solution.

The distinctive colors of metal nanoparticles are principally due to a Plasmon absorption phenomenon. It is observed when a conduction happens in electrons due to incident light on the nanoparticle surface, resulting in the absorption of electromagnetic radiation. Such spectrum of varying colors has been observed during silver nanoparticles synthesis in glycerol. A prominent increase in the color intensity was noticed in regard to increased glycerol content of the solution; thus, the variation in the color was confirmed due to difference of reduction potential provided by glycerol in each composition. This effected the size as well as absorption capacity at each percentage (i.e. 10-100%). Another correlation of the metal nanoparticles color has been defined w.r.t. shape/size of the synthesized particles along with refractive index (RI) of the solvent medium surrounding the nanoparticle.

During the studies, it was observed that the color appearance of the silver nanoparticles -which are obtained at increasing glycerol concentrations- changes from light yellow to sharp yellow. However, the presence of PVP resulted in increased stability and/or reduced dispersity of the obtained nanoparticles in the solution. This is confirmed with the presence of highly specific sized particles in a linear range, at each percentage. The color change also relates with the increase in particle size at higher percentages (i.e. 60-100%), that is also evidenced and complimented by the data of UV-Vis spectroscopy, DLS and TEM images.

The effect of glycerol as a reducing agent on the synthesis of silver nanoparticles was followed by UV-Vis spectroscopy by using absorbance measurements in the presence of PVP. Fig. 2 illustrates the UV-Vis spectra of the synthesized particles, and peak maxima at each concentration is stated in Table 1. The spectrum indicates the appearance of optical absorbance positions between 410nm and 450nm, which correlates with the surface plasmon of silver nanoparticles. Here, the peak positions of each concentration of glycerol (10-100%) undergoes a sharp blue shift at low concentration of glycerol (i.e. 10-30%), with a trivial switch to red shift at increased glycerol proportions i.e. from 50% to 100%. A static peak occurs for the middle range of glycerol (50-70%), while a significant red shift occurs at high glycerol range (80-100%). The results are in agreement with the theoretical data that a blue shift in the UV spectrum is related to the small size particles, whereas bigger nanoparticles will have a peak with red shift. However, the particular position of the absorption due to Plasmon may also rely on multiple factors like size and shape of the particle, type of solvent, and capping agent.

Table 1. Silver nanoparticle sizes obtained as a function of glycerol percentage

With the intention to highlight the size specificity in each composition (1-100%) of glycerol, a linearity curve was generated (Fig. 2) by using the absorbance values of silver nanoparticles at wavelength maxima, to find out a range where size variation appears. A consistent absorption pattern was observed when up to 50% glycerol was used as a solvent medium. Thus, a linear range was provided in the synthesis of nanoparticles; while above 70%, the absorption started increasing. And this indicates the improved absorption relevant to increased size of nanoparticles. The results herein endorse the protocol offering a linearity range to successfully generate ultra-small nanoparticles (<10nm) in single step. Furthermore, the size of the silver nanoparticles synthesized at various glycerol compositions was acquired by using DLS and TEM measurements. DLS (Dynamic Light Scattering), an extensive measuring tool to calculate the hydrodynamic size (Rh) of nanoparticles, was used to evaluate silver nanoparticles in glycerol (10-100%) through poly dispersity index (PDI) and average diameter (nm). Comprehensive analysis of the silver nanoparticles synthesized at ambient temperature was performed. Figure 2a demonstrates an overlay plot of the hydrodynamic diameter in all composition of silver nanoparticles. Amongst the glycerol range used for the synthesis of silver nanoparticles, it was detected that size distribution in the particles at up to 60% glycerol is highly stable. Here, a significant proportion of nanoparticles presented particle diameter of <10nm (i.e. 1.70nm, 2.45nm, 3.68nm, 5.27nm, 6.97nm, 7.23nm for 10-60% glycerol, respectively) and a PDI range of 0.3-0.6%. However, the size started increasing from lOnm up to 160nm (i.e. 10.09nm, 15.87nm, 67.95nm, 159.6nm) with comparatively low PDI range of 0.1-0.3%, at higher glycerol concentrations (>80- 100%). This variation in the particle size may be accredited to the varying concentration of the precursor in the reaction. And this makes a sense that increasing the concentration of glycerol increased the availability of reducing agent and hence reduced the silver nitrate to its value, at high concentrations. This results indicates that the particle size was effected from low glycerol concentration to high concentration of glycerol.

During the studies, a linearity graph was plotted from particle size acquired from DLS at each percentage. As shown in Fig. 3, the size remains <10nm up to 60% glycerol, with a stable peaks in DLS (Fig. 3 a), and starts increasing at 70% and above percentage of glycerol. Since DLS is an extremely sensitive technique to the existence of agglomerates, all the experiments were performed in triplicates in order to remove any uncertainty of the measurements with very low particle size (<10nm). Thus, this protocol is one of a kind to offer small-size particle synthesis over a range of solvent percentage, with high reproducibility (Fig. 3b).

In order to compliment the size observed on DLS, the samples were subject to TEM analysis to authenticate the obtained data for the current synthesis. Figure 4 shows TEM images of silver nanoparticles synthesized in glycerol (10-100%) + PVP, which predominates the small sized particles having spherical morphologies, with an overall size range of 2nm to 7nm for <60% glycerol preparations, whereas the overall size range is 60-160nm for glycerol solutions of 70% and above.

For the particle size distribution, histograms were generated by using Image J software and by denoting the results in Fig. 4, an increasing trend has been observed for average silver NP size from low (10%) to high (100%) glycerol concentration as 1.70nm, 2.45nm, 5.27nm, 7.32nm. 10.09nm, 15.87nm, 67.95nm, respectively. Moreover, along with the higher percentage of glycerol, the histograms of particle size also show broadening of the particles through overall wider size distribution from 0.5-20nm to 0.5-160nm. On the other hand, particles as small as 1.8nm and as large as 159nm were being found in the samples synthesized at lowest-to-highest glycerol concentrations. Change in shape from spherical to quasi-spherical was also be observed, especially at higher concentrations (80-100%) of glycerol. Since, with increasing glycerol content, increased availability of reducing agent was noticed, it is believed that the remaining Ag+ ions at higher values continue to grow along with existing silver NPs and thus alter the morphology of the particles in later stages.

Additionally, at 100% glycerol, the size appeared in DLS was 159nm (Fig. 3). SEM analysis were conducted to monitor the size and appearances of silver nanoparticles. The analysis results emphasize that PVP is a strong stabilizing agent, by showing much stabile particles with even size distribution. However, results point towards the fact that although there is higher glycerol in the medium (i.e. more reducing agent available to conduct the particle synthesis), but due to high viscosity of the surrounding medium, the reaction rate decreases at high glycerol concentration and indicates sequential big sized particle synthesis w.r.t. glycerol percentage (i.e. 10- 100%) in the medium. On the contrary, since a very slow reaction rate can result in particles agglomeration, PVP is used during synthesis to stabilize the synthesized particles at all percentages in order to prevent the related aggregation. Thus, amongst many other limitations during the synthesis of nanoparticles, appropriate reducing agent selection is one of the key factors as it has a direct dependency on the shape, size as well as particle size distribution obtained at the end of reaction.

In the current study, measurement was carried out by FTIR spectrum to categorize the potential molecules in glycerol responsible for reducing silver nitrate in order to synthesize silver nanoparticles. Figure 5 displays the FTIR absorption band patterns of silver nanoparticles synthesized in glycerol (10%-100%) with PVP. As depicted in Fig. 5, results for pure glycerol and silver nanoparticles (with peak variations) highly correlates with the literature. For instance, absorption bands at 3100-3400cm-l indicates as O-H stretch of glycerol and 2695cm- 1 as aldehyde (C- H) group produced from glycerol. Expectedly, the peaks at 2933cm-l, 2879cm-l and 2695cm-l of bare glycerol decreased significantly with the passage of silver NPs formation. Thus, it relates to the theory that it has a role in the reduction of glyceraldehyde (coming from glycerol). Other than that, two main peaks at 3307 cm'¹ and 1638 cm'¹ confirms the formation of silver nanoparticles, with the later peak indicating characteristic of C-0 stretching. However, a similar peak appeared at 1641 cm'¹ after addition of PVP which allocates as an important characteristic absorbance peak of PVP for carbonyl (C=O) stretching. Additionally, the band at 1049 cm'¹ corresponds to several functional groups like alcohol, carboxylic acid, ester, ether, and anhydrides and thus appear in all sample spectra’s with varying intensities.

The XRD pattern was obtained to confirm the crystalline nature of the silver nanoparticles. Fig. 6 shows the diffraction peaks of the silver nanoparticles which indicate the polycrystalline nature. The patterns disclose the peak diffraction corresponding to fee (face-centered cubic) crystalline silver phase, with a characteristic split in the peaks exhibited at 29 (i.e. 38.2°, 44.3° and 64.7°, as 111, 200, 220) planes, respectively. Diffraction patterns obtained by XRD explain whether the material of sample is pure or include impurities. Scherrer’s formula was used to determine the mean particle size through XRD patterns and calculate the peak position, peak intensity, and full width at half maximum (FWHM) values. The formula is as follows: d=9.9X/13Cos9, where d is the mean diameter of the nanoparticles, X is the wavelength of X-ray radiation source, 13 is the angular FWHM of the XRD peak at the diffraction angle 9(70). By using this equation, the estimated mean particle size from the major diffraction peaks was found to be 17nm, correlating with TEM and DLS data for 80% glycerol sample of silver nanoparticles. Therefore, the XRD study confirmed that the resultant particles are of silver source in the prepared samples.

For the validation of particle structure, size, and aggregation status, AFM (Atomic Force Microscope) was used further to measure silver NPs at 100% glycerol concentration. Figure 7 illustrates the well dispersed appearance of silver nanoparticles, providing a high density coverage on the silica substrate (Fig. 7a). Silver nanoparticles appear spherical in shape on AFM, which correlates with the observations from SEM images for the same sample percentage. However, SEM cannot offer any metrological information regarding the height of the nanoparticles, which was measured through AFM observations (Fig. 7b, right), along with the average length of the particles (Fig. 7b, left) that appears in the similar range as observed in the related DLS (1 9%).

For instance, an increase in hydrodynamic diameter (Z.avg), without effecting the polydispersity in DLS results matches well with the slight increase in height of the silver NPs when detected by AFM cross-section analysis with no observation of aggregates. Thus, these measurements show complementarity and coherency in the results obtained from multiple techniques in our study, for the same sample. A study conducted by Ning et al., depicted the chemical synthesis of AgNPs as silver island films, using citrate and NaBH4 reduction of silver nitrate in water and obtained silver nanoparticles of vast size range i.e. 8-599nm(71). Interestingly, their AFM images of 1 9-299nm give a good reference for our obtained results in the similar size range. For instance, an increase in hydrodynamic diameter, without effecting the polydispersity in DLS results matches well with the slight increase in height of the silver nanoparticles when detected by AFM cross-section analysis with no observation of aggregates. Thus, these measurements show complementarity and coherency in the results obtained from multiple techniques in our study, for the same sample. Within these basic concepts; it is possible to develop various embodiments of the inventive “Synthesis Method of Ultra-Small Silver Nanoparticles and Silver Nanoparticles Obtained by Using this Method”; the present invention cannot be limited to examples disclosed herein and it is essentially according to claims.

Claims

CLAIMS A method (100) of obtaining silver nanoparticles; characterized by steps of

- washing and then drying laboratory equipment to be used in synthesis of silver nanoparticles (101); obtaining glycerol solutions in varying ratios/concentrations by mixing glycerol and water in a plurality of containers, so as to have different concentrations in each container (102); obtaining PVP solutions by adding polyvinylpyrrolidone (PVP) into glycerol solutions (103); and synthesizing silver nanoparticles by adding silver nitrate (AgNOs) into PVP solutions (104). A method (100) according to Claim 1; characterized in that at the step of washing and then drying laboratory equipment to be used in synthesis of silver nanoparticles (101), glass containers used for preparing solution and magnetic beads used for mixing solutions are washed with aqua regia -which is an acidic mixture of concentrated hydrochloric acid (HC1) and nitric acid (HNO3)- at a 3:1 ratio (v/v) for 5-15min and then air dried. A method (100) according to Claim 1 or 2; characterized in that at the step of obtaining glycerol solutions in varying ratios/concentrations by mixing glycerol and water in a plurality of containers, so as to have different concentrations in each container (102), 5-10 ml deionized water is added into a container and glycerol solutions are prepared in different concentrations by adding glycerol (C3H8O3) in the range of 10-100% by volume into deionized water. A method (100) according to any of the preceding claims; characterized in that magnetic beads are put into the container wherein the prepared glycerol solutions are included and the containers are placed on a magnetic stirrer. A method (100) according to any of the preceding claims; characterized in that the magnetic stirring process is carried out at room temperature until the water and the glycerol included in the containers are mixed homogeneously. A method (100) according to any of the preceding claims; characterized in that at the step of obtaining PVP solutions by adding polyvinylpyrrolidone (PVP) into glycerol solutions (103), 0.4% (approximately 0.02 g) PVP - which has high water solubility and exists in powder form- is added into glycerol solutions becoming homogeneous after magnetic stirring and existing in different concentrations. A method (100) according to Claim 6; characterized in that magnetic stirring is performed by means of a magnetic stirrer and magnetic bead for 15-60min at room conditions, in order that 10-50% glycerol solutions wherein PVP is added become homogeneous. A method (100) according to Claim 6; characterized in that magnetic stirring is performed by means of a magnetic stirrer and magnetic bead for 2-3h at room conditions, in order that 60-100 % glycerol solutions wherein PVP is added become homogeneous. A method (100) according to any of the preceding claims; characterized in that at the step of synthesizing silver nanoparticles by adding silver nitrate (AgNCh) into PVP solutions (104), 1.65% silver nitrate (approximately 0.085g) is added into PVP solutions homogenized in different concentrations. A method (100) according to Claim 9; characterized in that silver nanoparticles are synthesized by leaving the solution containers, wherein silver nitrate is added, for mixing with the magnetic bead on the magnetic stirrer at room temperature of 20-25 °C until the transparency of the solution starts turning into a noticeable light yellow color. A method (100) according to Claim 9 or 10; characterized in that the magnetic bead is removed from the solution container and the solutions are retained on the laboratory bench overnight without mixing to observe the stability of the synthesis reaction. A method (100) according to Claim 11; characterized in that following the retention, color of all solutions prepared in different concentrations turned from light yellow into a brown image and silver nanoparticles are obtained at room temperature Silver nanoparticles which are obtained by following any of the method (100) steps in the preceding claims and using silver nitrate as a metal ion source, glycerol (polyol) as a reductant for both solvent and silver nitrate, polyvinylpyrrolidone (PVP) and deionized water as a stabilizer and at room temperature, neutral pH conditions, without irradiance of ultraviolet light, without needing long procedures, by means of a quick, simple, reproducible and partly green synthesis method, by controlling the dispersion of particles and lowering the particle agglomeration, so as to be in ultra-small (<10nm) sizes, as size tunable in the range of 2-200nm, offer a linearity range and as monodisperse.