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Synthesis of PMA stabilized silver nanoparticles by chemical reduction process under a two-step UV irradiation
Applied Surface Science 256 (2010) 3812–3816
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis of PMA stabilized silver nanoparticles by chemical reduction process under a two-step UV irradiation D. Spadaro a,*, E. Barletta a, F. Barreca a, G. Curro` a, F. Neri b a b
Advanced and Nano Materials Research s.r.l., Salita Sperone, 31, 98166 Messina, Italy Dipartimento di Fisica della Materia e Ingegneria Elettronica, Universita` di Messina, Salita Sperone 31, 98166 Messina, Italy
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 August 2009 Received in revised form 29 September 2009 Accepted 12 January 2010 Available online 20 January 2010
Poly(methacrylic acid) (PMA) stabilized silver nanoparticles (Ag NPs), also used in the surface modification of clothing fibers, were fabricated via chemical reduction processes under UV irradiation. To obtain an uniform size distribution it has been designed a new ‘‘two-step’’ process which employs two different UV radiation densities in order to control the kinetics of NPs nucleation. The as produced nanoparticles were characterized by UV–vis absorption spectroscopy and TEM microscopy. The results show the reduction of the Ag ions and the nanoparticles nucleation in the first step. In the second step, the final Ag NPs size distribution is controlled through a quick cross-linking of the PMA that freezes out any further modification. A narrow size distribution with more than 80% NPs smaller than 10 nm and none larger than 25 nm was obtained and the long-term stability (one month) of the colloidal solution was verified. ß 2010 Elsevier B.V. All rights reserved.
Keywords: Silver nanoparticles Polymethacrylic acid Chemical reduction UV irradiation Bio-packaging
1. Introduction Silver nanoparticles (Ag NPs) show unique optical, electrical, magnetic and chemical properties as well as a strong biological activity mainly due to their strong plasmonic absorption in the visible range and to the very large surface to volume ratio. They are extensively applied in a number of different fields, for example as antistatic materials, cryogenic superconducting materials [1,2] and in colloidal form they can also be used for antibacterial purposes [3,4]. As for many other nanoparticles systems, their specific properties depend on structural parameters such as average size, shape and crystal structure. In the last years, the largest interest has been placed on the control of these characteristics [5,6]. In particular, the dependence of the position of the surface plasmon resonance in relation to all the above parameters has been carefully studied [7]. The fabrication method surely plays the major role in selecting the structure of the AgNPs and their tunability. Several kinds of growth methods have been studied, such as chemical reduction, gas condensation, laser irradiation and sonochemical deposition [8]. In the case of chemical bath processes, the most used fabrication way, parameters such as precursor and stabilizer concentrations, pH optimization, reducing agent type and the same solvent type have been examined and a number of synthesis methods for the preparation of size* Corresponding author. Tel.: +39 090 676 5452. E-mail address: [email protected] (D. Spadaro). 0169-4332/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.01.031
controlled Ag NPs has been defined [9]. Polymers in solution have been frequently employed to minimize or to avoid the aggregation of Ag NPs due to their strong mutual interactions after nucleation [10–19]. In fact, it was demonstrated that nanocomposites can be rapidly produced at ambient temperature by photoinitiated crosslinking polymerization of multifunctional monomers and oligomers [20,21]. For this reason a special attention has recently been devoted to the use of polyelectrolytes since they present the advantage to act both as reducing agents and stabilizing agents at the same time. With this purpose, it has been employed the polymethacrylic acid sodium salt (PMA) to obtain silver nanoparticles starting from silver nitrate by means of an UV photoreduction [3,20]. The use of PMA as polyelectrolyte offers numerous advantages due to its physical and chemical properties such as the solubility in water and the coordination ability for nanoparticles [3]. The latter capacity is due to the intrinsic polymer structure and, in particular, to the steric effect of the methyl groups that lead to metal clusters linked to the carboxylic parts of the macromolecule [22]. Furthermore, PMA does not show any toxic chemical effects [3] and for this reason it may be used in many applications where a biocompatible material is required. The choice of PMA as a capping agent is also motivated by the fact that PMA behaves as an anionic polyelectrolyte that can be used in the surface modification of clothing fibers through layer-by-layer impregnation [3]. Moreover, it fulfils the ever increasing demand for safe, eco-friendly and lowcost processing of materials.
D. Spadaro et al. / Applied Surface Science 256 (2010) 3812–3816
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To specifically promote the application of PMA-coated Ag NPs in the textile industry it is required an effort to improve their size population which strongly depends on the best coating performance of Ag nanoparticles. In this direction, we report on the study and definition of a class of PMA stabilized Ag NPs fabricated via chemical reduction processes under UV irradiation. We introduce a two-step process which employs different UV radiation densities in order to control the kinetics of NPs nucleation to obtain a narrow size distribution. 2. Experimental details Ag NPs colloids were prepared by a two-step photo-induced reduction process under UV irradiation of silver nitrate in a dilute solution of PMA used as reducing and capping agent. In particular, we used a 30% water solution of sodium stabilized PMA (Mw = 9500) and a 98% pure powder of silver nitrate (AgNO3) (Sigma–Aldrich). Reagents have been mixed in double-distilled water, with a molar ratio of 10:1 (AgNO3/PMA). In the first step, the mixture has been exposed to a radiation density of 470 nW/cm2 from a 6 W UV lamp up to a maximum time of 1 h. However, already after 5 min of exposure, the reduction of silver ions by PMA led to a change of the colour of the solution from colourless to yellow. In a second step, the mixture has been exposed to a 378 mW/cm2 irradiation from a 25 W UV lamp for 5 h. The solution colour turned to dark orange. The colloidal stability has been checked by examining the NPs characteristics after one month storage of colloids in air and dark. The process results have been monitored with UV–vis absorption spectroscopy and TEM microscopy. Absorption measurements were performed in the UV–VIS–NIR region by means of a conventional Perkin-Elmer Lambda 2 spectrometer operating in the 190–800 nm range, while TEM imaging was performed using a Zeiss Leo 912AB microscope working at 80 kV and using carbon coated copper grids as substrates. 3. Results and discussion Ag NPs were obtained by the method of UV promoted reduction using only two reagents, the Ag precursor salt (AgNO3) and the reducing-capping polymer PMA. We adopted a two-step process with the aim of improving the control of the NPs size distribution by simply allowing the Ag+ reduction reaction as the main one during the first step and favouring the PMA reticulation in the second one. During the first step, the solution exposure to a low power radiation promoted the silver reduction due to the polymer which coordinates the Ag+ ions through the COO groups. In this way, the particle nucleation begins and proceeds without any significant modification of the polymer structure. The UV power density applied during the second step, about 1000 times larger then in the first step, strongly increases the polymer reticulation kinetics, so causing a freezing of the NPs distribution already produced during the first step. During the experiment, the reduction of silver ions under UV light is evidenced by the change of the solution colour from transparent to yellow and then to dark orange [7]. The particles formation is confirmed by the UV–vis spectra due to the presence of a surface plasmon absorption band with a maximum at 430 nm which is typical for particles with diameter less than 30 nm [3]. This solution is very stable over one month storage in air and dark. In fact, in these conditions no variations have been detected in the fundamental characteristics, e.g. colour, plasmonic peak and pH. The latter parameter remains unchanged at about 8.50. The Ag/ PMA solution has been irradiated with a radiation density of 470 nW/cm2 and checked for different exposure times to individuate the most effective duration of the first step. The
Fig. 1. Absorbance spectra of the PMA-Ag NPs in solution after reduction under UV lamp at different exposure times up to 1 h irradiation.
UV–vis absorbance spectra relative to the first step are shown in Fig. 1a–c. After a 5 min irradiation, the plasmon resonance band at 426 nm has still a low intensity but the colour of the solution has changed from transparent to weak yellow. When the first step irradiation is prolonged to 10 min, the plasmon intensity increases slightly and the colour of the solution remains unchanged. A shoulder under 360 nm appears and we assign it to the Ag-PMA coordination, which is in competition with nanoparticles nucleation [23]. A stronger plasmonic peak has been obtained after an irradiation of 1 h, while the solution colour turns to a more intense yellow. A detailed analysis of the first step UV spectra shows that, varying the exposure time from 5 to 60 min, the peak width shrinks progressively of some nanometers during the irradiation process, while its intensity increases. Indeed, if the exposure time is set to 2, 5 and 24 h using the same lamp, the plasmonic peak becomes wider and is characterized by more features because the aggregation begins and the size of nanoparticles increases. This evidence is demonstrated by the UV spectra in Fig. 2 where it is shown that a weak but wide shoulder appears extending beyond 500 nm. As reported in the literature [5–7], this is due to aggregation phenomena occurrence. Therefore, we choice 1 h exposure at 470 nW/cm2 as the first step process conditions. The TEM analysis performed after the end of first step (one hour irradiation) shows that Ag NPs with a diameter of some nanometers have been formed (Fig. 3a) but these are not yet stable so they tend to agglomerate. Concerning the polymer, the main phase is constituted by a compact network which represents the typical structure of the polymer coordinated with the Ag+ ions in agreement with the UV–vis results (Fig. 3b). To get some insight about the kinetics of the processes involved in this first step, we compared the UV–vis spectra of all the irradiated samples (from 5 min up to 24 h at 470 nW/cm2). Fig. 4 shows both the plasmon peak (426 nm) and the aggregation one (500 nm) intensities as a function of the exposure times. The plasmonic peak intensity reaches the maximum after 1 h irradiation and then decreases. At the same time, the shoulder positioned around 500 nm starts to grow after (2 h) the first process has reached a maximum and then remains almost constant. Assuming that the plasmonic peak is representative of the nanoparticle nucleation while the 500 nm shoulder represents the
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D. Spadaro et al. / Applied Surface Science 256 (2010) 3812–3816
Fig. 4. Kinetics study of processes involved in the fist step: comparison between samples from 5 min to 24 h of irradiation at 470 nW/cm2.
Fig. 2. UV–Vis absorbance spectra of the mixture after reduction over 1 h irradiation up to 24 h in dark.
formation of nanoparticles aggregates, the above results indicate that the NP growth and aggregation process starts after the nucleation process has been almost completed. Moreover, it must be noted that the plasmon intensity curve, which relates to the nucleation mechanism, grows up faster than the 500 nm shoulder intensity curve. This suggests that the NP nucleation mechanism is by itself faster than the successive NP growth. Once fixed at 1 h the duration of the first step, the second step with a radiation density of 378 mW/cm2 has been carried out for 5 h. Different trials were made to select this irradiation time [24].
The UV–vis absorbance spectra of the as obtained colloidal solution is very similar to that obtained after the first step, with a band whose maximum is still located at 426 nm (Fig. 5a and b). This means that during the second step the polymer reticulates, strongly limiting any further Ag NPs nucleation, growth and aggregation. The stability of the colloidal solution has been verified after one month storage in air and dark and, as showed in Fig. 5c, the position and the intensity of the plasmon peak are unchanged. A representative TEM micrograph of the final colloidal solution shows the visual evidence of the nanoparticles stabilization (Fig. 6). The nanoparticles distribution in the polymer appears to be homogeneous and the particles are almost always isolated with no evidence of any substantial aggregation. The typical polymer coordination structures observed after the first step completion (Fig. 3b) have disappeared. The silver particles size distribution histogram shown in Fig. 7a has been obtained by the analysis of a set of TEM micrographs (Fig. 7b) of the same sample. To achieve a reliable result from the size distribution analysis we considered only nanoparticles with a diameter equal to or larger
Fig. 3. TEM micrographs of PMA-Ag NPs solution after 1 h irradiation with a radiation density of 470 nW/cm2, (a) silver nanoparticles agglomerated; (b) typical structure of the polymer coordinated with the Ag+ ions.
D. Spadaro et al. / Applied Surface Science 256 (2010) 3812–3816
Fig. 5. A comparison between the UV–vis absorbance spectra of the mixture after the first step irradiation and after the second step. The colloidal solution is stable after one month storage in air and dark.
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Fig. 6. TEM micrograph of colloidal solution after the ‘‘two-step’’ process irradiation.
Fig. 7. (a) Silver particles size distribution histogram obtained from (b) TEM micrographs of colloidal solution after the ‘‘two-step’’ process irradiation.
than 4 nm. Smaller Ag NPs are also identified but our image analysis does not allow to extract reliable results. The overall results show that more than 80% of the Ag NPs have a size smaller than 10 nm and that no particles larger than about 25 nm were detected. 4. Conclusion A two-step process for fabricating colloidal solutions of small Ag NPs capped with PMA was defined. The wet process we have developed is carried out in water and involves only the silver salt as source of Ag ions and the PMA as reducing and capping agent. The size distribution of the Ag nanoparticles is controlled during the first step by promoting a quick reduction of Ag ions with consequent NP nucleation through a relatively low energy UV irradiation. During the second step an UV irradiation at an higher energy density allows a quick cross-linking of the PMA that freezes out any further and concurrent modification of the Ag NPs distribution. The resulting NPs population has a narrow size distribution with at least 80% NPs smaller than 10 nm none larger than 25 nm.
We have verified a long-term stability of the as-obtained colloids, up to one month storage in normal conditions. The twostep process can be further developed to allow a fine selection of the Ag NP structure and size, while assuring a final capping with PMA electrolyte. This simple method offers numerous advantages because it allows to limit the number of process reagents, to employ materials that does not show any toxic chemical effects and to obtain a narrow size distribution of Ag NPs. The produced colloidal solution is bio- and eco-compatible, hence it is suited to be used as a surface treatment for the functionalization of textile fibers and to many other applications such as food packaging and bio-diagnostic probes. References [1] H.Q. Jiang, S. Manolache, A.C.L. Wong, F.S. Denes, Plasma-enhanced deposition of silver nanoparticles onto polymer and metal surfaces for the generation of antimicrobial characteristics, J. Appl. Polym. Sci. 93 (2004) 1411–1422. [2] S. Hirano, Y. Wakasa, A. Saka, S. Yoshizawa, Y. Oya-Seimiya, Y. Hishinuma, A. Nishimura, A. Matsumoto, H. Kumakura, Preparation of Bi-2223 bulk composed with silver-alloy wire, Phys. C: Supercond. 392–396 (2003) 458–462.
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[3] S.T. Dubas, P. Kumlangdudsana, P. Potiyaraj, Layer-by-layer deposition of antimicrobial silver nanoparticles on textile fibers, Colloids Surf. A 289 (2006) 105– 109. [4] J. Wang, J. Li, L. Ren, A. Zhao, P. Li, Y. Leng, H. Sun, N. Huang, Antibacterial activity of silver surface modified polyethylene terephthalate by filtered cathodic vacuum arc method, Surf. Coat. Technol. 201 (2007) 6893–6896. [5] Y. Sun, Y. Xia, Gold and silver nanoparticles: a class of chromophores with colors tunable in the range from 400 to 750 nm, Analyst 128 (2003) 686–691. [6] B. Wiley, Y. Sun, B. Mayers, Y. Xia, Shape-controlled synthesis of metal nanostructures; the case of silver, Chem. Eur. J. 11 (2005) 454–463. [7] J.J. Mock, M. Barbic, D.R. Smith, D.A. Schultz, S. Schultz, Shape effects in plasmon resonance of individual colloidal silver nanoparticles, J. Chem. Phys. 116 (15) (2002) 6755–6759. [8] I. Capek, Preparation of metal nanoparticles in water-in-oil microemulsions, Adv. Colloid Interf. Sci. 110 (2004) 49–74. [9] B. He, J.J. Tan, K.Y. Liew, H. Liu, Synthesis of size controlled Ag nanoparticles, J. Mol. Catal. A 221 (2004) 121–126. [10] R. Patakfalvi, S. Papp, I. Dekany, The kinetics of homogeneous nucleation of silver nanoparticles stabilized by polymers, J. Nanoparticles Res. 9 (2007) 353–364. [11] C. Luo, Y. Zhang, X. Zeng, Y. Zeng, Y. Wang, The role of poly(ethylene glycol) in the formation of silver nanoparticles, J. Colloid Interf. Sci. 288 (2005) 444–448. [12] Z.S. Pillai, P.V. Kamat, What factors control the size and shape of silver nanoparticles in the citrate ion reaction method? J. Phys. Chem. B 108 (2004) 945–951. [13] M.M. Oliveira, D. Ugarte, D. Zanchet, A. Zarbin, Influence of synthetic parameters on the size, structure and stability of dodecanthiol-stabilized silver nanoparticles, J. Colloid Interf. Sci. 292 (2005) 429–435.
[14] W Zhang, X. Qiao, J. Chen, Synthesis and characterization of silver nanoparticles in AOT microemulsion system, Chem. Phys. 330 (2006) 495–500. [15] W. Zhang, X. Qiao, J. Chen, Synthesis of nanosilver colloidal particles in water/oil microemulsion, Colloids Surf. A: Physicochem. Eng. Asp. 299 (2007) 22–28. [16] I. Pastoriza-Santos, L.M. Liz-Marzan, Synthesis of silver nanoprisms in DMF, Nano Lett. 2 (8) (2002) 903–905. [17] H. Zeng, J. Li, Z.L. Wang, J.P. Liu, S. Sun, Bimagnetic core/shell Fe Pt/Fe3O4 nanoparticles, Nano Lett. 4 (2004) 187–190. [18] A. Pal, S. Shah, S. Devi, Synthesis of Au, Ag, Au–Ag alloy nanoparticles in aqueous polymer solution, Colloids Surf. A 302 (2007) 51–57. [19] Y. Xiong, A.R. Siekkinen, J. Wang, Y. Yin, M.J. Kim, Y. Xia, Synthesis of silver nanoplates at high yields by slowing down the polyol reduction of silver nitrate with polyacrylamide, J. Mater. Chem. 17 (2007) 2600–2602. [20] K. Zou, X.H. Zhang, X.F. Duan, X.M. Meng, S.K. Wu, Seed-mediated synthesis of silver nanostructures and polymer/silver nanocables by UV irradiation, J. Crystal Growth 273 (2004) 285–291. [21] L. Balan, D. Burget, Synthesis of metal/polymer nanocomposite by UV-radiation curing, Eur. Polym. J. 42 (2006) 3180–3189. [22] G. Cardenas, C. Munoz, H. Carbacho, Thermal properties and TGA-FTIR studies of polyacrylic and polymethacrylic acid doped with metal clusters, Eur. Polym. J. 36 (2000) 1091–1099. [23] J.E. Huheey, E.A. Keiter, R.L. Keiter, Chimica Inorganica: Principi, Strutture, Reattivita`, Piccin (Padova), 1999. [24] D. Spadaro, E. Barletta, F. Barreca, G. Curro`, F. Neri, PMA capped silver nanoparticles produced by UV-enhanced chemical process, Appl. Surf. Sci. 255 (2009) 8403–8408.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis of PMA stabilized silver nanoparticles by chemical reduction process under a two-step UV irradiation D. Spadaro a,*, E. Barletta a, F. Barreca a, G. Curro` a, F. Neri b a b
Advanced and Nano Materials Research s.r.l., Salita Sperone, 31, 98166 Messina, Italy Dipartimento di Fisica della Materia e Ingegneria Elettronica, Universita` di Messina, Salita Sperone 31, 98166 Messina, Italy
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 August 2009 Received in revised form 29 September 2009 Accepted 12 January 2010 Available online 20 January 2010
Poly(methacrylic acid) (PMA) stabilized silver nanoparticles (Ag NPs), also used in the surface modification of clothing fibers, were fabricated via chemical reduction processes under UV irradiation. To obtain an uniform size distribution it has been designed a new ‘‘two-step’’ process which employs two different UV radiation densities in order to control the kinetics of NPs nucleation. The as produced nanoparticles were characterized by UV–vis absorption spectroscopy and TEM microscopy. The results show the reduction of the Ag ions and the nanoparticles nucleation in the first step. In the second step, the final Ag NPs size distribution is controlled through a quick cross-linking of the PMA that freezes out any further modification. A narrow size distribution with more than 80% NPs smaller than 10 nm and none larger than 25 nm was obtained and the long-term stability (one month) of the colloidal solution was verified. ß 2010 Elsevier B.V. All rights reserved.
Keywords: Silver nanoparticles Polymethacrylic acid Chemical reduction UV irradiation Bio-packaging
1. Introduction Silver nanoparticles (Ag NPs) show unique optical, electrical, magnetic and chemical properties as well as a strong biological activity mainly due to their strong plasmonic absorption in the visible range and to the very large surface to volume ratio. They are extensively applied in a number of different fields, for example as antistatic materials, cryogenic superconducting materials [1,2] and in colloidal form they can also be used for antibacterial purposes [3,4]. As for many other nanoparticles systems, their specific properties depend on structural parameters such as average size, shape and crystal structure. In the last years, the largest interest has been placed on the control of these characteristics [5,6]. In particular, the dependence of the position of the surface plasmon resonance in relation to all the above parameters has been carefully studied [7]. The fabrication method surely plays the major role in selecting the structure of the AgNPs and their tunability. Several kinds of growth methods have been studied, such as chemical reduction, gas condensation, laser irradiation and sonochemical deposition [8]. In the case of chemical bath processes, the most used fabrication way, parameters such as precursor and stabilizer concentrations, pH optimization, reducing agent type and the same solvent type have been examined and a number of synthesis methods for the preparation of size* Corresponding author. Tel.: +39 090 676 5452. E-mail address: [email protected] (D. Spadaro). 0169-4332/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.01.031
controlled Ag NPs has been defined [9]. Polymers in solution have been frequently employed to minimize or to avoid the aggregation of Ag NPs due to their strong mutual interactions after nucleation [10–19]. In fact, it was demonstrated that nanocomposites can be rapidly produced at ambient temperature by photoinitiated crosslinking polymerization of multifunctional monomers and oligomers [20,21]. For this reason a special attention has recently been devoted to the use of polyelectrolytes since they present the advantage to act both as reducing agents and stabilizing agents at the same time. With this purpose, it has been employed the polymethacrylic acid sodium salt (PMA) to obtain silver nanoparticles starting from silver nitrate by means of an UV photoreduction [3,20]. The use of PMA as polyelectrolyte offers numerous advantages due to its physical and chemical properties such as the solubility in water and the coordination ability for nanoparticles [3]. The latter capacity is due to the intrinsic polymer structure and, in particular, to the steric effect of the methyl groups that lead to metal clusters linked to the carboxylic parts of the macromolecule [22]. Furthermore, PMA does not show any toxic chemical effects [3] and for this reason it may be used in many applications where a biocompatible material is required. The choice of PMA as a capping agent is also motivated by the fact that PMA behaves as an anionic polyelectrolyte that can be used in the surface modification of clothing fibers through layer-by-layer impregnation [3]. Moreover, it fulfils the ever increasing demand for safe, eco-friendly and lowcost processing of materials.
D. Spadaro et al. / Applied Surface Science 256 (2010) 3812–3816
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To specifically promote the application of PMA-coated Ag NPs in the textile industry it is required an effort to improve their size population which strongly depends on the best coating performance of Ag nanoparticles. In this direction, we report on the study and definition of a class of PMA stabilized Ag NPs fabricated via chemical reduction processes under UV irradiation. We introduce a two-step process which employs different UV radiation densities in order to control the kinetics of NPs nucleation to obtain a narrow size distribution. 2. Experimental details Ag NPs colloids were prepared by a two-step photo-induced reduction process under UV irradiation of silver nitrate in a dilute solution of PMA used as reducing and capping agent. In particular, we used a 30% water solution of sodium stabilized PMA (Mw = 9500) and a 98% pure powder of silver nitrate (AgNO3) (Sigma–Aldrich). Reagents have been mixed in double-distilled water, with a molar ratio of 10:1 (AgNO3/PMA). In the first step, the mixture has been exposed to a radiation density of 470 nW/cm2 from a 6 W UV lamp up to a maximum time of 1 h. However, already after 5 min of exposure, the reduction of silver ions by PMA led to a change of the colour of the solution from colourless to yellow. In a second step, the mixture has been exposed to a 378 mW/cm2 irradiation from a 25 W UV lamp for 5 h. The solution colour turned to dark orange. The colloidal stability has been checked by examining the NPs characteristics after one month storage of colloids in air and dark. The process results have been monitored with UV–vis absorption spectroscopy and TEM microscopy. Absorption measurements were performed in the UV–VIS–NIR region by means of a conventional Perkin-Elmer Lambda 2 spectrometer operating in the 190–800 nm range, while TEM imaging was performed using a Zeiss Leo 912AB microscope working at 80 kV and using carbon coated copper grids as substrates. 3. Results and discussion Ag NPs were obtained by the method of UV promoted reduction using only two reagents, the Ag precursor salt (AgNO3) and the reducing-capping polymer PMA. We adopted a two-step process with the aim of improving the control of the NPs size distribution by simply allowing the Ag+ reduction reaction as the main one during the first step and favouring the PMA reticulation in the second one. During the first step, the solution exposure to a low power radiation promoted the silver reduction due to the polymer which coordinates the Ag+ ions through the COO groups. In this way, the particle nucleation begins and proceeds without any significant modification of the polymer structure. The UV power density applied during the second step, about 1000 times larger then in the first step, strongly increases the polymer reticulation kinetics, so causing a freezing of the NPs distribution already produced during the first step. During the experiment, the reduction of silver ions under UV light is evidenced by the change of the solution colour from transparent to yellow and then to dark orange [7]. The particles formation is confirmed by the UV–vis spectra due to the presence of a surface plasmon absorption band with a maximum at 430 nm which is typical for particles with diameter less than 30 nm [3]. This solution is very stable over one month storage in air and dark. In fact, in these conditions no variations have been detected in the fundamental characteristics, e.g. colour, plasmonic peak and pH. The latter parameter remains unchanged at about 8.50. The Ag/ PMA solution has been irradiated with a radiation density of 470 nW/cm2 and checked for different exposure times to individuate the most effective duration of the first step. The
Fig. 1. Absorbance spectra of the PMA-Ag NPs in solution after reduction under UV lamp at different exposure times up to 1 h irradiation.
UV–vis absorbance spectra relative to the first step are shown in Fig. 1a–c. After a 5 min irradiation, the plasmon resonance band at 426 nm has still a low intensity but the colour of the solution has changed from transparent to weak yellow. When the first step irradiation is prolonged to 10 min, the plasmon intensity increases slightly and the colour of the solution remains unchanged. A shoulder under 360 nm appears and we assign it to the Ag-PMA coordination, which is in competition with nanoparticles nucleation [23]. A stronger plasmonic peak has been obtained after an irradiation of 1 h, while the solution colour turns to a more intense yellow. A detailed analysis of the first step UV spectra shows that, varying the exposure time from 5 to 60 min, the peak width shrinks progressively of some nanometers during the irradiation process, while its intensity increases. Indeed, if the exposure time is set to 2, 5 and 24 h using the same lamp, the plasmonic peak becomes wider and is characterized by more features because the aggregation begins and the size of nanoparticles increases. This evidence is demonstrated by the UV spectra in Fig. 2 where it is shown that a weak but wide shoulder appears extending beyond 500 nm. As reported in the literature [5–7], this is due to aggregation phenomena occurrence. Therefore, we choice 1 h exposure at 470 nW/cm2 as the first step process conditions. The TEM analysis performed after the end of first step (one hour irradiation) shows that Ag NPs with a diameter of some nanometers have been formed (Fig. 3a) but these are not yet stable so they tend to agglomerate. Concerning the polymer, the main phase is constituted by a compact network which represents the typical structure of the polymer coordinated with the Ag+ ions in agreement with the UV–vis results (Fig. 3b). To get some insight about the kinetics of the processes involved in this first step, we compared the UV–vis spectra of all the irradiated samples (from 5 min up to 24 h at 470 nW/cm2). Fig. 4 shows both the plasmon peak (426 nm) and the aggregation one (500 nm) intensities as a function of the exposure times. The plasmonic peak intensity reaches the maximum after 1 h irradiation and then decreases. At the same time, the shoulder positioned around 500 nm starts to grow after (2 h) the first process has reached a maximum and then remains almost constant. Assuming that the plasmonic peak is representative of the nanoparticle nucleation while the 500 nm shoulder represents the
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Fig. 4. Kinetics study of processes involved in the fist step: comparison between samples from 5 min to 24 h of irradiation at 470 nW/cm2.
Fig. 2. UV–Vis absorbance spectra of the mixture after reduction over 1 h irradiation up to 24 h in dark.
formation of nanoparticles aggregates, the above results indicate that the NP growth and aggregation process starts after the nucleation process has been almost completed. Moreover, it must be noted that the plasmon intensity curve, which relates to the nucleation mechanism, grows up faster than the 500 nm shoulder intensity curve. This suggests that the NP nucleation mechanism is by itself faster than the successive NP growth. Once fixed at 1 h the duration of the first step, the second step with a radiation density of 378 mW/cm2 has been carried out for 5 h. Different trials were made to select this irradiation time [24].
The UV–vis absorbance spectra of the as obtained colloidal solution is very similar to that obtained after the first step, with a band whose maximum is still located at 426 nm (Fig. 5a and b). This means that during the second step the polymer reticulates, strongly limiting any further Ag NPs nucleation, growth and aggregation. The stability of the colloidal solution has been verified after one month storage in air and dark and, as showed in Fig. 5c, the position and the intensity of the plasmon peak are unchanged. A representative TEM micrograph of the final colloidal solution shows the visual evidence of the nanoparticles stabilization (Fig. 6). The nanoparticles distribution in the polymer appears to be homogeneous and the particles are almost always isolated with no evidence of any substantial aggregation. The typical polymer coordination structures observed after the first step completion (Fig. 3b) have disappeared. The silver particles size distribution histogram shown in Fig. 7a has been obtained by the analysis of a set of TEM micrographs (Fig. 7b) of the same sample. To achieve a reliable result from the size distribution analysis we considered only nanoparticles with a diameter equal to or larger
Fig. 3. TEM micrographs of PMA-Ag NPs solution after 1 h irradiation with a radiation density of 470 nW/cm2, (a) silver nanoparticles agglomerated; (b) typical structure of the polymer coordinated with the Ag+ ions.
D. Spadaro et al. / Applied Surface Science 256 (2010) 3812–3816
Fig. 5. A comparison between the UV–vis absorbance spectra of the mixture after the first step irradiation and after the second step. The colloidal solution is stable after one month storage in air and dark.
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Fig. 6. TEM micrograph of colloidal solution after the ‘‘two-step’’ process irradiation.
Fig. 7. (a) Silver particles size distribution histogram obtained from (b) TEM micrographs of colloidal solution after the ‘‘two-step’’ process irradiation.
than 4 nm. Smaller Ag NPs are also identified but our image analysis does not allow to extract reliable results. The overall results show that more than 80% of the Ag NPs have a size smaller than 10 nm and that no particles larger than about 25 nm were detected. 4. Conclusion A two-step process for fabricating colloidal solutions of small Ag NPs capped with PMA was defined. The wet process we have developed is carried out in water and involves only the silver salt as source of Ag ions and the PMA as reducing and capping agent. The size distribution of the Ag nanoparticles is controlled during the first step by promoting a quick reduction of Ag ions with consequent NP nucleation through a relatively low energy UV irradiation. During the second step an UV irradiation at an higher energy density allows a quick cross-linking of the PMA that freezes out any further and concurrent modification of the Ag NPs distribution. The resulting NPs population has a narrow size distribution with at least 80% NPs smaller than 10 nm none larger than 25 nm.
We have verified a long-term stability of the as-obtained colloids, up to one month storage in normal conditions. The twostep process can be further developed to allow a fine selection of the Ag NP structure and size, while assuring a final capping with PMA electrolyte. This simple method offers numerous advantages because it allows to limit the number of process reagents, to employ materials that does not show any toxic chemical effects and to obtain a narrow size distribution of Ag NPs. The produced colloidal solution is bio- and eco-compatible, hence it is suited to be used as a surface treatment for the functionalization of textile fibers and to many other applications such as food packaging and bio-diagnostic probes. References [1] H.Q. Jiang, S. Manolache, A.C.L. Wong, F.S. Denes, Plasma-enhanced deposition of silver nanoparticles onto polymer and metal surfaces for the generation of antimicrobial characteristics, J. Appl. Polym. Sci. 93 (2004) 1411–1422. [2] S. Hirano, Y. Wakasa, A. Saka, S. Yoshizawa, Y. Oya-Seimiya, Y. Hishinuma, A. Nishimura, A. Matsumoto, H. Kumakura, Preparation of Bi-2223 bulk composed with silver-alloy wire, Phys. C: Supercond. 392–396 (2003) 458–462.
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