A rapid, high-yield and large-scale synthesis of uniform spherical silver nanoparticles by a microwave-assisted polyol process

Abstract

Silver nanoparticles are often employed in medical devices and consumer products due to their antibacterial action. For this, reliable syntheses with quantitative yield are required. Uniform spherical silver nanoparticles with a diameter of about 180 nm were synthesized by carrying out the polyol synthesis in a microwave. Silver nitrate was dissolved in ethylene glycol and poly(N-vinyl pyrrolidone) (PVP) was added as capping agent. The particles were characterized by SEM, HRTEM, XRD, and DLS. The results are compared with the classical method where silver nitrate is reduced by glucose in aqueous solution, heated with an oil-bath. The microwave-assisted synthesis leads to an almost quantitative yield of uniform silver nanoparticles after 20 min reaction time and gives exclusively spherical particles without other shapes like triangles, rods or prisms. Diethylene glycol as solvent gave a more homogeneous particle size distribution than ethylene glycol. For both kinds of particles, dissolution in ultrapure water was examined over a period of 29 days in the presence of oxygen. The dissolution was comparable in both cases (about 50% after 4 weeks), indicating the same antibacterial action for particles from the microwave and from the glucose synthesis.

---

Introduction

Silver nanoparticles have been known and applied for more than 100 years in the form of colloidal silver. Notably, they have become of great interest to the scientific community due to their interesting physical and antimicrobial properties and potential applications. Many products containing colloidal silver like biocides, pigments, catalysts and also consumer products and medical devices are commercially available.^1–3

Along with this growing interest, sophisticated synthetic routes were developed to obtain silver nanoparticles with defined size and shape, with more than 250 publications alone by Y. Xia et al.^4,5 Most of these methods are based on the bottom-up approach, starting with a solution of a soluble silver salt like silver nitrate which is subsequently reduced to silver nanoparticles in the presence of stabilizing and capping agents, e.g. poly(N-vinylpyrrolidone) PVP or citrate.^6–8 The polyol process uses a reducing solvent like ethylene glycol for reduction.

However, there are still many open questions. While the synthesis of well-defined particles is in principle possible, the reproducibility of these results with satisfying quality, purity and yield remains challenging and depends on a large set of specific synthetic parameters, such as batch and manufacturer of the chemicals used, shape and size of the stirring bar, or the surface area of the reaction vessel.^9–11 If one is interested in biological experiments or a large-scale application of such protocols, it is inevitable to eliminate these problems and to establish new and more reliable synthetic pathways that are capable to produce nanoparticles with constant quality in high yields.

A typical synthesis involves the reduction of AgNO₃ with glucose. However, this leads to a mixture of particles with different shape as we have shown recently.⁸ Furthermore, the yield is low as we will show below.

Microwave-assisted syntheses of inorganic nanoparticles have received growing attention in recent years due to the several advantages compared to conventional heating in an oil bath like increased reaction rates and yields as well as better selectivity and reproducibility.^12–14 It is also worth to mention that modern microwave reactors for chemical synthesis are usually equipped with various sensors and are able to control critical reaction parameters such as irradiation power, temperature and pressure very precisely.¹³

The combined method of microwave heating and polyol process was found to be a very powerful method for the highly reproducible and phase-pure synthesis of binary intermetallic phases in the systems Bi–M (M = Ni, Rh, Pd, Ir).^15–19 Even for the intermetallic compounds, the preparation of particles with uniform size and shape was possible.^15,19 Moreover, the microwave-assisted synthesis can lead to extraordinary size-dependent properties of the intermetallic phases, e.g. excellent catalytic performance,¹⁵ reversible intercalation of oxygen at room temperature,¹⁷ and coexistence of ferromagnetism and superconductivity.¹⁶

We have transposed this concept to the synthesis of uniform silver nanoparticles. Thereby, the reaction rate is considerably increased and artefacts due to the mixing process (addition of AgNO₃ into the hot ethylene glycol in the hot polyol process) are avoided. A full synthesis can be easily carried out within 30 min, followed by isolation of the particles. In contrast, a classical polyol synthesis requires pre-heating of the solvent in most cases and therefore takes between 60 min and 15 h, depending on the conditions chosen, not counting the heating and cooling time.^9,20

We have recently shown that different analytical methods for the characterization of metallic nanoparticles have different advantages and limitations and that it is therefore highly recommended to apply more than just one method.²¹ Consequently, we have carefully characterized the particles synthesized in the microwave as well as the particles from the glucose-synthesis with scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), X-ray powder diffraction (XRD) and dynamic light scattering (DLS).

Concerning possible applications, especially with respect to its antibacterial action, it is important to know that dispersed silver nanoparticles dissolve in presence of oxygen under release of Ag⁺ ions.^22,23 We have therefore analysed and compared the dissolution kinetics of microwave- and glucose-synthesized particles with dialysis.

Experimental

As we know from our own experience and from the literature, the reproducible synthesis of uniform, well-defined silver nanoparticles is highly sensitive to a large number of synthetic parameters, e.g. manufacturer and purity of the applied reagents.^5,9–11,24 Therefore we describe our methods below in as much detail as possible.

All chemicals were used as received without further purification. To avoid contaminations, particularly with traces of iron and other metals, all manual operations were carried out with a glass spatula. Glass flasks and stirring bars were either freshly purchased or cleaned with aqua regia and ultrapure water (18 MΩ cm⁻¹) several times.

Microwave-assisted synthesis of spherical silver nanoparticles

30 mg silver nitrate (≥99.9%, Carl-Roth, cat. no. 7908.1) and 20 mg poly(N-vinylpyrrolidone) (MW ≈ 55 000 g mol⁻¹, Sigma-Aldrich, cat. no. 856568) were completely dissolved in 5 mL diethylene glycol (≥99.0%, Sigma-Aldrich, cat. no. 32 160) at room temperature. The solution assumed a light yellow colour (due to plasmon resonance), indicating the formation of small Ag seeds. The vial was then sealed with a PTFE cap and transferred to the microwave reactor (CEM Discover SP; 200 W). After stirring for 1 min at room temperature, the solution was rapidly heated to 160 °C by the microwave at the maximum possible rate. This heating took about 120 s. Temperature and stirring were continued for different times, up to 60 min. During this time, the reaction was monitored with an in-built video camera, showing a colour change from light yellow over red and green to dark brown. To stop the reaction at a defined endpoint, the solution was quickly cooled to room temperature with compressed air in the microwave within about 120 s and then added to cold (25 °C) ultrapure water.

The nanoparticles were collected by ultracentrifugation (66 000g, 30 min) and redispersed in ultrapure water. To determine the size and the shape of the particles, 10 μL of the suspension were placed on a silicon wafer, dried in air and analysed by scanning electron microscopy (ESEM Quanta 400 FEG, FEI Company). High-resolution imaging was performed using an aberration-corrected FEI Titan transmission electron microscope operated at 300 kV. The particle size distribution and the zeta potential were measured by dynamic light scattering (Malvern Zetasizer Nano ZS, 633 nm laser). X-ray powder diffraction was carried out on a Bruker D8 Advance instrument in Bragg–Brentano mode with Cu Kα radiation (1.54 Å, 40 kV and 40 mA).

After ultracentrifugation, a sample of 0.5 mL was taken both from the precipitated product and from the supernatant. Concentrated nitric acid and ultrapure water were added and the concentration of silver was measured by atomic absorption spectroscopy (Thermo Electron Corporation, M-Series, detection limit 1 μg L⁻¹). Additionally, the glass flask was rinsed with 5 mL of concentrated nitric acid. A sample was taken and analysed by atomic absorption spectroscopy to determine the amount of silver in the metallic residue (“mirror”).

The synthesis was carried out with different reaction times (0, 1, 2, 5, 10, 20, and 60 min) at 160 °C to examine the influence of time on the nanoparticles' size and shape. “0” denotes a rapid heating (about 120 s) to 160 °C, followed by immediate cooling to room temperature. The influence of the solvent was analysed by replacing diethylene glycol by ethylene glycol (≥99.9%, VWR, cat. no. 24041.297).

Glucose synthesis of spherical silver nanoparticles

The synthesis of silver nanoparticles by reduction of silver nitrate with glucose was first published by Wang et al.⁷ Briefly, 2 g glucose (≥99.5%, Sigma-Aldrich, cat. no. G7528) and 1 g poly(N-vinylpyrrolidone) (PVP; MW ≈ 40 000, Sigma-Aldrich, cat. no. 81420) were dissolved in 40 mL ultrapure water and stirred at 90 °C for 45 min under ambient conditions. 1 mL of a 2.72 M solution of AgNO₃ (≥99.9%, Carl-Roth, cat. no. 7908.1) in water was quickly injected into the hot solution, and the temperature was kept for 60 min under continuous stirring. After cooling to room temperature, the product was collected by ultracentrifugation (29 400g, 30 min). The amount of silver in the residue as well as in the supernatant was determined by atomic absorption spectroscopy. After further purification, shape and size of the particles were analysed as described above.

Dissolution in ultrapure water

The dissolution kinetic for both kind of particles was examined by dialysis in triplicate for each kind of nanoparticles. After purification by ultracentrifugation, 4 mL silver nanoparticle dispersion (0.1 mg mL⁻¹ Ag) were transferred in dialysis bags (Spectra/Por Biotech; cellulose ester; MWCO 100 000) and immersed in 396 mL ultrapure water. Dialysis bags were rinsed with ultrapure water several times before they were applied. Temperature, pH-value and oxygen saturation of the medium were recorded. The dialysis was then carried out under slow stirring for 696 hours (29 days) in sealed polypropylene-tubes. The dissolved amount of Ag⁺ was determined by atomic absorption spectroscopy on aliquots of 9 mL.

Results and discussion

Synthesis and yield

We have carried out the reduction of silver ions to silver nanoparticles by the solvent diethylene glycol in the microwave at different heating times and analysed the nature of silver in the system. We have distinguished between silver particles as obtained by ultracentrifugation, metallic silver (“mirror”) on the wall of the glass vessel, and dissolved silver in the supernatant. It can be safely assumed that the last fraction consists either of unreacted AgNO₃ or of very small prenucleation clusters because ultracentrifugation at 66 000g was used to separate the particles from the supernatant (Fig. 1). Any particles, except for ultrasmall clusters (1–2 nm) will precipitate under these conditions.

Fig. 1 Silver species from the reaction in the microwave after different reaction times at 160 °C. For comparison, a glucose reduction reaction was carried out for 60 min at 90 °C.

The reaction was almost complete after 20 min, leading to a total mass of 20.0 ± 1.3 mg silver that was separated by ultracentrifugation. The yield was >98% and can be considered as quantitative, unlike in the glucose reduction reaction where we found a nanoparticle yield of only 2 to 3%. At a longer reaction time, a silver mirror at the vessel wall appeared, obviously from deposited silver nanoparticles. Therefore, it is important to stop the synthesis at about 20 min. The camera in the microwave showed that the solution became turbid after about 5 min, in accordance with the data shown in Fig. 1 where the reaction sets in after about 5 min.

To verify the purity of the synthesized particles, X-ray powder diffraction was performed. Fig. 2 shows the diffraction pattern. The peaks are corresponding to metallic silver with a face-centred cubic (fcc) crystal structure (ICDD-PDF4 #04-0783), indicating the formation of pure silver nanoparticles.

Fig. 2 X-ray powder diffractogram of silver nanoparticles from microwave synthesis (20 min at 160 °C).

Particle size and shape

The particle size and shape were very uniform as shown by dynamic light scattering (DLS; Fig. 3) and scanning electron microscopy (SEM; Fig. 4). After 20 min, the average hydrodynamic particle diameter was 182 nm with a PDI of 0.18 and a zeta potential of −30 mV. After the glucose synthesis, the average hydrodynamic particle diameter was 120 nm with a PDI of 0.13 and a zeta potential of −20 mV. Both kinds of particles were electrosterically stabilized. Notably, the particles from microwave synthesis were slightly larger than those from the glucose reduction reaction.

Fig. 3 Particle size distribution by dynamic light scattering for samples from the microwave (20 min) and glucose.

Fig. 4 Scanning electron micrograph of very uniform spherical silver nanoparticles after 20 min synthesis at 160 °C in the microwave with the solvent diethylene glycol.

Interestingly, the silver nanoparticles were inhomogeneous in size and shape in the initial stages of the reaction. Some particles with irregular shapes and diameters of up to 400 nm have formed, while others were still smaller than 50 nm and may be considered as seeds. After 5 min, the particles became very homogenous in size and shape. They were almost spherical after 20 min microwave synthesis (Fig. 4). Contrary to the glucose synthesis,⁸ there were neither triangles nor rods (Fig. 5).

Fig. 5 Scanning electron micrograph of silver nanoparticles with different shape (spheres, triangles, rods) after 60 min glucose reduction at 90 °C.

To examine the influence of the solvent, diethylene glycol was replaced with ethylene glycol which is typically used in polyol synthesis. All other parameters were kept constant, and the product was characterised after 20 min reaction time after the same purification steps as described above. The hydrodynamic particle diameter was 152 nm and therefore about 30 nm lower compared to the product obtained by reduction with diethylene glycol. PDI and zeta potential were similar with 0.17 and −26 mV, respectively.

However, scanning electron microscopy (Fig. 6) indicated a more polydisperse particle size distribution compared to diethylene glycol. With respect to these results, the use of diethylene glycol is preferable.

Fig. 6 Scanning electron micrograph of silver nanoparticles after 20 min synthesis at 160 °C in the microwave with the solvent ethylene glycol. The particles are much more polydisperse than with the solvent diethylene glycol (compare to Fig. 3).

To further analyse the size, shape and crystal structure, high resolution transmission electron microscopy (HRTEM) was performed for the particles from the microwave synthesis with the solvent diethylene glycol after 20 min reaction time (Fig. 7) as well as for the particles from the glucose synthesis (Fig. 8).

Fig. 7 High resolution transmission electron microscopy of silver nanoparticles after 20 min synthesis at 160 °C in the microwave with the solvent diethylene glycol.

Fig. 8 High resolution transmission electron microscopy of silver nanoparticles after 60 min glucose reduction at 90 °C.

The diameter of the metallic particle core was determined from scanning electron microscopy and transmission electron microscopy. Particles from the microwave synthesis (80–120 nm) are almost twice as big as particles from glucose synthesis (40–80 nm). In both cases, the particles are predominantly spherical and multiply twinned. Some rods and triangles were observed after glucose synthesis, while they were absent after microwave synthesis. Furthermore, particles from the microwave appear to contain more grains than the particles from the glucose synthesis, i.e. they grew in a less regular way, possibly by the higher nucleation and crystal growth rate in the microwave.

The regular fivefold symmetry of fivefold twinned particles from glucose synthesis has been observed earlier⁸ and points to a more regular crystal growth process in the glucose reduction process.

Dissolution in ultrapure water

Dissolution kinetics for both glucose- and microwave-synthesized silver nanoparticles in ultrapure water were determined by dialysis (Fig. 9). We have shown earlier that colloidal silver particles will dissolve in aqueous dispersion under release of Ag⁺ ions when oxygen is present and when they are not stabilized by biomolecules such as cysteine.^22,23 Ag⁺ ions are able to pass the membrane and can be analysed with atomic absorption spectroscopy in the surrounding medium, while the nanoparticles are retained inside the dialysis tube.

Fig. 9 Dissolution kinetics of silver nanoparticles from glucose- and microwave synthesis after 60 min and 20 min reaction time, respectively.

After 696 hours (29 days) stirring in ultrapure water (T = 25 ± 1 °C; pH = 4.8; oxygen saturation = 93%), 54 ± 3 wt% Ag of the glucose-synthesized and 42 ± 8% wt% Ag of the microwave-synthesized particles were dissolved as Ag⁺ ions. The amount of dissolved silver from the glucose particles matches the value of ∼53% which was determined by Kittler et al. for the same particles under the same conditions.²² Particles from the microwave synthesis dissolve at a slower rate, which may be explained by their bigger size and smaller specific surface area.

Summary

A variety of microwave-assisted methods for preparation of silver nanoparticles has been described in the literature so far. For example, spherical silver nanoparticles can be prepared by microwave irradiation of silver nitrate in ethanol²⁵ or in the presence of “green” reagents like amino acids and starch.²⁶ Saraf et al. have reported a protocol for the synthesis of silver nanocubes with edge lengths between 450 and 500 nm in the presence of Au seeds.²⁷ They used poly(styrene sulfonate) as complexing agent for Ag⁺ ions that supported the formation of cubic structures. Guittard, Rossi et al. have used a modified polyol process to produce silver nanoplates and prisms of high quality.²⁸ Zhao et al. showed that microwave irradiation enhances the particle formation; however, they preheated the solvent to 160 °C and then injected the reactants, followed by a couple of seconds of microwave irradiation. No details on the nature of the microwave were given, except for the microwave power.²⁹ Tsuji et al. used PtCl₆²⁻ as seeds for the polyol synthesis of silver nanoparticles in ethylene glycol, induced by a microwave. Depending on the chain length of the PVP used, they obtained different particle morphologies, but typically as a mixture.^30–32 Similar results were obtained by Liu et al. with gold nanoseeds.^33,34

Despite all of this work, little is known about the specific advantages of microwave-assisted methods in comparison to classical synthesis in an oil-bath. Moreover, many microwave-based methods described in the literature give only polydisperse, spherical particles or mixtures of several morphologies. Crucial experimental and analytical data such as concentration, temperature, irradiation power, yields, colloidal stability or particle size distribution in solution are often missing. The microwave-based method leads to almost quantitative yields of well-defined, colloidally stable and monodisperse spherical silver nanoparticles.

Conclusions

The application of microwave radiation is advantageous for a fast synthesis of uniform silver nanoparticles with almost quantitative yield. The particles are almost exclusively spherical in contrary to the products obtained by reduction of silver nitrate with glucose in aqueous solution. Both ethylene glycol and diethylene glycol were used as solvent, but diethylene glycol is preferable as it leads to much more homogenous particles. In comparison to the reduction by glucose, the yield is much higher. The particles show a well-defined release of silver ions and are therefore suitable as antibacterial agent.

Acknowledgements

This work was supported by the Deutsche Forschungsgemeinschaft (DFG) in the frameworks of the Priority Programs SPP 1313 and SPP 1708.

References

1. L. W. Toy and L. Macera, J. Am. Assoc. Nurse Pract., 2011, 23, 183–192.
2. B. Nowack, H. F. Krug and M. Height, Environ. Sci. Technol., 2011, 45, 1177–1183.
3. S. Chernousova and M. Epple, Angew. Chem., Int. Ed., 2013, 52, 1636–1653.
4. Y. Xia, X. Xia, Y. Wang and S. Xie, MRS Bull., 2013, 38, 335–344.
5. Y. N. Xia, Y. Xiong, B. Lim and S. E. Skrabalak, Angew. Chem., Int. Ed., 2009, 48, 60–103.
6. B. Wiley, Y. Sun and Y. Xia, Acc. Chem. Res., 2007, 40, 1067–1076.
7. H. S. Wang, X. L. Qiao, J. G. Chen and S. Y. Ding, Colloids Surf., A, 2005, 256, 111–115.
8. S. Banerjee, K. Loza, W. Meyer-Zaika, O. Prymak and M. Epple, Chem. Mater., 2014, 26, 951–957.
9. S. E. Skrabalak, L. Au, X. D. Li and Y. N. Xia, Nat. Protoc., 2007, 2, 2182–2190.
10. E. V. Panfilova, B. N. Khlebtsov, A. M. Burov and N. G. Khlebtsov, Colloid J., 2012, 74, 99–109.
11. S. Ahlberg, A. Antonopulos, J. Diendorf, R. Dringen, M. Epple, R. Flöck, W. Goedecke, C. Graf, N. Haberl, J. Helmlinger, F. Herzog, F. Heuer, S. Hirn, C. Johannes, S. Kittler, M. Köller, K. Korn, W. G. Kreyling, F. Krombach, J. Lademann, K. Loza, E. M. Luther, M. Malissek, M. C. Meinke, D. Nordmeyer, A. Pailliart, J. Raabe, F. Rancan, B. Rothen-Rutishauser, E. Rühl, C. Schleh, A. Seibel, C. Sengstock, L. Treuel, A. Vogt, K. Weber and R. Zellner, Beilstein J. Nanotechnol., 2014, 5, 1944–1965.
12. M. Baghbanzadeh, L. Carbone, P. D. Cozzoli and C. O. Kappe, Angew. Chem., Int. Ed., 2011, 50, 11312–11359.
13. I. Bilecka and M. Niederberger, Nanoscale, 2010, 2, 1358–1374.
14. M. N. Nadagouda, T. F. Speth and R. S. Varma, Acc. Chem. Res., 2011, 44, 469–478.
15. D. Köhler, M. Heise, A. I. Baranov, Y. Luo, D. Geiger, M. Ruck and M. Armbrüster, Chem. Mater., 2012, 24, 1639–1644.
16. T. Herrmannsdörfer, R. Skrotzki, J. Wosnitza, D. Köhler, R. Boldt and M. Ruck, Phys. Rev. B: Condens. Matter Mater. Phys., 2011, 83, 140501.
17. M. Heise, B. Rasche, A. Isaeva, A. I. Baranov, M. Ruck, K. Schäfer, R. Pöttgen, J. P. Eufinger and J. Janek, Angew. Chem., Int. Ed., 2014, 53, 7344–7348.
18. M. Heise, J. H. Chang, R. Schönemann, T. Herrmannsdörfer, J. Wosnitza and M. Ruck, Chem. Mater., 2014, 26, 5640–5646.
19. R. Boldt, A. Grigas, M. Heise, T. Herrmannsdörfer, A. Isaeva, S. Kaskel, D. Köhler, M. Ruck, R. Skrotzki and J. Wosnitza, Z. Anorg. Allg. Chem., 2012, 638, 2035–2043.
20. S. H. Im, Y. T. Lee, B. Wiley and Y. Xia, Angew. Chem., Int. Ed., 2005, 44, 2154–2157.
21. D. Mahl, J. Diendorf, W. Meyer-Zaika and M. Epple, Colloids Surf., A, 2011, 377, 386–392.
22. S. Kittler, C. Greulich, J. Diendorf, M. Köller and M. Epple, Chem. Mater., 2010, 22, 4548–4554.
23. K. Loza, J. Diendorf, C. Greulich, L. Ruiz-Gonzales, J. M. Gonzalez-Calbet, M. Vallet-Regi, M. Koeller and M. Epple, J. Mater. Chem. B, 2014, 2, 1634–1643.
24. J. Zeng, Y. Zheng, M. Rycenga, J. Tao, Z. Y. Li, Q. Zhang, Y. Zhu and Y. N. Xia, J. Am. Chem. Soc., 2010, 132, 8552–8553.
25. A. Pal, S. Shah and S. Devi, Mater. Chem. Phys., 2009, 114, 530–532.
26. S.-H. Yu, M. Zhang, K. Wang, S.-B. Wang and B. Hu, J. Phys. Chem. C, 2008, 112, 11169–11174.
27. S. Kundu, V. Maheshwari, S. Niu and R. F. Saraf, Nanotechnology, 2008, 19, 065604.
28. T. Darmanin, P. Nativo, D. Gilliland, G. Ceccone, C. Pascual, B. de Berardis, F. Guittard and F. Rossi, Colloids Surf., A, 2012, 395, 145–151.
29. T. Zhao, J. B. Fan, J. Cui, J. H. Liu, X. B. Xu and M. Q. Zhu, Chem. Phys. Lett., 2011, 501, 414–418.
30. M. Tsuji, K. Matsumoto, P. Jiang, R. Matsuo, X. Tang and K. S. N. Kamarudin, Colloids Surf., A, 2008, 316, 266–277.
31. M. Tsuji, K. Matsumoto, N. Miyamae, T. Tsuji and X. Zhang, Cryst. Growth Des., 2007, 7, 311–320.
32. M. Tsuji, Y. Nishizawa, K. Matsumoto, N. Miyamae, T. Tsuji and X. Zhang, Colloids Surf., A, 2007, 293, 185–194.
33. F. K. Liu, P. W. Huang, Y. C. Chang, C. J. Ko, F. H. Ko and T. C. Chu, Cryst. Growth Des., 2005, 273, 439–445.
34. F. K. Liu, P. W. Huang, T. C. Chu and F. H. Ko, Mater. Lett., 2005, 59, 940–944.

---