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Effect of citrate on poly(vinyl pyrrolidone)-stabilized gold nanoparticles formed by PVP reduction in microwave (MW) synthesis
Materials Chemistry and Physics 137 (2012) 135e139
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Effect of citrate on poly(vinyl pyrrolidone)-stabilized gold nanoparticles formed by PVP reduction in microwave (MW) synthesis Seung Kwon Seol a, b, *, Daeho Kim a, b, Sunshin Jung a, b, Won Suk Chang a, Young Min Bae a, b, Kyeong Hee Lee a, b, Yeukuang Hwu c a b c
Advanced Medical Device Research Center, Korea Electrotechnology Research Institute (KERI), Ansan 426-910, Republic of Korea University of Science and Technology (UST), Ansan 426-910, Republic of Korea Institute of Physics, Academia Sinica, Nankang, Taipei 115, Taiwan
h i g h l i g h t s < We report the citrate effect on PVPeAuNPs formed by PVP reduction in MW synthesis. < An increase in citrate concentration leads to slow stable reactions in synthesis. < It contributes to the size decrease and the uniformity improvement of PVPeAuNPs. < Therefore, we produced uniform PVPeAuNPs with diameter of 7.94 0.14 nm in 1 min.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 February 2012 Received in revised form 20 August 2012 Accepted 26 August 2012
Colloidal PVP (poly(vinyl pyrrolidone))estabilized gold nanoparticles (PVPeAuNPs) are synthesized in aqueous solution with PVP as a reducing and stabilizing agent using a short microwave (MW) heating duration of 1 min. The size and uniformity of the synthesized PVPeAuNPs can be varied by modifying the concentration of sodium citrate (Na3Ct), which acts predominantly as mediator of the stability of PVP eAuNP formation during the rapid synthesis. Due to the increase in the Na3Ct concentration, the number of citrate ions adsorbed on the growing surface of AuNPs increase, and less reactive gold solute complexes are formed, leading to slow stable reactions that produce small, uniform colloidal PVP eAuNPs. We therefore demonstrate that by adjusting the Na3Ct concentration used in the PVP reduction, the diameter of PVPeAuNPs was varied from 19.47 3.97 nm to 7.94 0.14 nm when using constant concentrations of chloroauric acid (HAuCl4) and PVP. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Chemical synthesis Nucleation Composite materials
1. Introduction Colloidal metallic nanoparticles have received a great deal of attention, due in part to their specific properties and potential applications. In particular, gold nanoparticles (AuNPs) have become increasingly attractive because of their unique electronic, optical and chemical properties, which have given these nanoparticles the potential to be used in advanced applications in a variety of fields, such as electronics [1], sensors [2], disease diagnosis [3,4] and therapeutics [5,6].
* Corresponding author. Advanced Medical Device Research Center, Korea Electrotechnology Research Institute (KERI), Ansan 426-910, Republic of Korea. Tel.: þ82 31 8040 4182; fax: þ82 31 8040 4189. E-mail address: [email protected] (S.K. Seol). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.08.057
The properties of AuNPs depend strongly on their characteristics, such as size, shape and size distribution. Therefore, the key goal in the synthesis of AuNPs is to obtain small particles with a uniform size distribution so that the unique properties can be realized or amplified. Since Frens and Turkevich introduced the citrate reduction route for the synthesis of colloidal AuNPs [7], various synthesis approaches, such as chemical [8], electrochemical [9], radiolysis [10,11], photochemical [12] and sonochemical [13] methods, have been suggested to prepare small, uniform AuNPs. To control the size and uniformity of synthesized particles, it is important to protect the particles to prevent undesirable aggregation. High-molecular-weight polymers [14,15], ligands [16] and surfactants [17] are typically used as stabilizing reagents to prevent aggregation. Poly(vinyl pyrrolidone) (PVP) is one potential AuNPstabilizing reagent due to its high solubility in many solvents, high stability and non-toxicity. The hydrophilic headgroup (NeC] O) of PVP is attached to the surface of the particles, resulting in
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particle repulsion due to steric interactions between polymers adsorbed on the particle’s surface. Recently, several groups have been reported that PVP also acts as a reducing reagent for metal particle syntheses in aqueous solution [18e21]. Gold nanostructures with diverse shapes have been synthesized by a PVP reduction route. The method is based on the chemical reduction of gold precursors by organic radicals formed during the degradation of PVP due to chemical or thermal decomposition [18,22]. The PVP reduction route is a simple, inexpensive and environmentally friendly method for producing colloidal AuNPs. The use of conventional convective heating with an external heat source for synthesis via the PVP reduction route is an inefficient way to achieve fast and uniform heat transfer to the reactants, resulting in a slow reaction rate and a wide size distribution of the synthesized AuNPs. Recently, microwave heating has been widely used for nanoparticle synthesis [23e30]. In our previous work, we reported the formation kinetics of the microwave synthesis of citrate-reduced colloidal AuNPs [31]. Compared with the convective heating used in the standard citrate reduction route, microwave heating resulted in rapid volumetric heating, resulting in the improvement of the reaction rate and the uniformity of the synthesized AuNPs [31]. Here, we present a rapid method for the preparation of PVPe stabilized gold nanoparticles (PVPeAuNPs) by PVP reduction with microwave (MW) heating. To obtain small, uniform PVPeAuNPs using the PVP reduction route, we investigated the effect of sodium citrate (Na3Ct) as mediator of the reduction of gold ions on the size of the synthesized PVPeAuNPs.
2. Experimental PVPestabilized gold nanoparticles (PVPeAuNPs) were synthesized at atmospheric pressure by the reduction of chloroauric acid (HAuCl4) reacted with PVP (Mw ¼ 40,000) and sodium citrate (Na3Ct) in deionized water (18.2 MU cm). All chemical reagents were purchased from Aldrich. A basic solution was prepared at room temperature by mixing 2.0 mM PVP and 0.27 mM HAuCl4. The Na3Ct was added to control the chemical condition of the basic solution. A schematic illustration of the microwave heating system (2.45 GHz, 1600 W) used for the entire synthesis process is shown in Fig. 1(a) [31,32]. In this system, the amplitude of the microwave power in the microwave cavity, which was adjusted using a 3-stub tuner module, was calculated by measuring the difference between the forward- and backward-propagating microwaves. The temperature of the reaction solution was measured using an optic fiber sensor that is not affected by microwaves. A feedback system between the applied microwave power and the measured temperature allowed the manipulation of the microwave power to obtain the desired reaction temperature. The solution was put in a 50 ml glass bottle, which was stirred with a magnetic stirrer coated with Teflon during the synthesis, and this bottle was placed in the cavity. The solution temperature reached the desired value of w95 C in less than 5 s (Ramping period). During the retention time (Rt) region of microwave heating, the temperature was maintained at 95 C by altering the applied microwave power between 0 W and 28 W (on/off switching) [31]. The UVevis absorption spectra of the synthesized colloidal PVPeAuNPs were recorded using a Thermo Evolution 300 spectrophotometer, and TEM images were taken on a JEM-2100F (JEOL) instrument. For TEM analysis, specimens were prepared by placing a drop of synthesized colloidal PVPeAuNPs on a carbon-coated copper grid, followed by evaporation of the solvent. The size distribution of the colloidal PVPeAuNPs was determined by measuring the diameters of more than 200 particles.
Fig. 1. (a) A schematic illustration of microwave heating system. (b) UVevis absorption spectra of colloidal PVPeAuNPs formed at 95 C in a basic solution with 2.0 mM PVP and 0.27 mM HAuCl4 by microwave heating for Rt w 1 min. The inset is a TEM image of the synthesized colloidal PVPeAuNPs.
3. Results and discussion The synthesis of colloidal PVPeAuNPs was performed at a reaction temperature of 95 C with a microwave retention time (Rt) of 1 min in a basic solution consisting of 0.27 mM HAuCl4 and 2.0 mM PVP. Fig. 1(b) shows the UVevis absorption spectra of the synthesized PVPeAuNPs. The absorption peak at 532 nm, which is related to the surface plasmon band (SPB), indicates that nanoparticles were formed during the short heating period. The formation of PVPeAuNPs is attributed to the reduction of gold ions by PVP because of no other reducing reagents were added. In contrast to convective heating, which results in a slow reaction rate, microwave (MW) irradiation significantly accelerates the degradation of PVP by rapid volumetric heating, resulting in the formation of colloidal PVPeAuNPs in 1 min. Gold solute complexes such as AuClx ðOHÞ4x bind to the hydrophilic headgroup (NeC] O) of PVP during the initial stage of the reaction. The headgroup is the active site for the reduction of the gold ions, which react with the generated organic radicals. After the reduction, small primary AuNPs formed by the nucleation of metallic gold act as seeds for the formation of colloidal PVPeAuNPs. The steric repulsive force generated by the PVP layer adsorbed on the seed prevents the further aggregation of the nanoparticles during growth. However, even though PVP acts as stabilizing reagent, the synthesized PVPe AuNPs still exhibit irregular features and a wide size distribution, resulting from the fast random growth of nanoparticles at the initial stage of process (see the TEM image in the inset of Fig. 1(b)).
S.K. Seol et al. / Materials Chemistry and Physics 137 (2012) 135e139
To improve the uniformity of the synthesized colloidal PVPe AuNPs, we tried to modify the chemical conditions of the basic solution by adding sodium citrate (Na3Ct), which generally plays the roles of reducing agent, stabilizing agent and pH mediator in the citrate reduction route [31]. Fig. 2 presents the UVevis absorption spectra for the PVPeAuNPs formed in basic solutions containing different Na3Ct concentrations. An increase in the Na3Ct concentration contributes to a gradual decrease in the absorption intensity (Iabs) and to a blue shift in the maximum absorption position (lmax). The Iabs decrease is indicative of a reduction in the density of synthesized PVPeAuNPs due to the deceleration of the reaction rate at a constant Rt w1 min. The blue shift of lmax e from 532 nm to 518 nm e is indicative of a decrease in the average size of the PVPeAuNPs (see Fig. 2(b)). The values of Iabs and lmax are constant above the Na3Ct concentration of 15.4 mM. These results can be explained by two factors affected by the Na3Ct concentration: i) the capping capability of the stabilizing reagents and ii) the reactivity of the gold complexes. The higher colloidal stability resulting from the higher capping capability of the stabilizing reagents induces the generally uniform formation of AuNPs. The electrostatic repulsive force due to Na3Ct ions capping
Fig. 2. UVevis absorption spectra of colloidal PVPeAuNPs formed in basic solutions with different Na3Ct concentrations. (a) As the solution pH increases due to the increase in the Na3Ct concentration, the absorption intensity decreases for a constant Rt w 1 min; (b) Normalized absorption value of (a): as the Na3Ct concentration increases, lmax exhibits a blue shift from 532 nm to 518 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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the AuNPs surfaces is responsible for the colloidal stability during the citrate reduction [33,34]. In our study, because PVP has a loosely packed structure on the particles, the added Na3Ct ions improve the capping capability at the initial stage of growth by capping the surface area of the primary AuNPs to which PVP has not adsorbed. Thus, the colloidal stability of synthesized PVPeAuNPs during growth is enhanced by cooperating steric and electrostatic repulsive forces. The second factor that is affected by adding Na3Ct to a basic solution is the reactivity of the gold solute complexes, which is dependent on the solution pH. As shown in Fig. 2(a), the solution pH increases proportionally with the Na3Ct concentration. As the solution pH increases, gold solute complexes are transformed successively from AuCl4 to less reactive and lower redox potential complexes (AuCl3(OH), AuCl2(OH) 2 , AuCl(OH)3 and AuðOHÞ4 ) by substitution of Cl ions with OH ions, leading to slow, stable reactions [35]. Consequently, the higher capping capability and lower reactivity of gold solute complexes obtained by increasing the Na3Ct concentration inhibit the fast random growth of nanoparticles, decreasing the Iabs and the size of the PVPeAuNPs. We synthesized colloidal PVPeAuNPs under the two different conditions with and without PVP. Fig. 3 shows the UVevis absorption spectra of PVPeAuNPs formed in the solution consisting of 0.27 mM HAuCl4 and 15.4 mM Na3Ct with 2.0 mM PVP (Rt w 1 min) or 0 mM PVP (Rt w 1, 3, 10, 15 min). Whereas an absorbance peak with a lmax value of 518 nm is obtained with an Rt of 1 min in the presence of 2.0 mM PVP, a relatively flat absorption profile appears at the same Rt when PVP is absent. Even for an Rt of w15 min, the absorbance peak has a lower intensity than the peak obtained with 2.0 mM PVP. These results clearly show that Na3Ct has a minor role as a reducing agent with a short heating time of Rt w1 min. Fig. 4(a) and (b) shows the TEM image and the corresponding size distribution of the PVPeAuNPs synthesized in basic solution with 15.4 mM Na3Ct and without Na3Ct. By adding 15.4 mM Na3Ct, the average size of the synthesized PVPeAuNPs is reduced from 19.47 3.97 nm to 7.94 0.14 nm. Based on the TEM image and the size distribution for the particles synthesized without Na3Ct in Fig. 4(b), it is clear that the added Na3Ct results in superior PVPe AuNPs with a small size and a narrow size distribution (see Fig. 4(a)). The stability of the colloidal PVPeAuNPs synthesized under the conditions for Fig. 4(a) is also though to extend to conditions of high
Fig. 3. Comparison of the UVevis absorption spectra of colloidal PVPeAuNPs formed in a solution (0.27 mM HAuCl4 and 15.4 mM Na3Ct) containing 0 mM PVP (Rt w 1, 3, 10, 15 min) or 2.0 mM PVP (Rt w 1 min).
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Fig. 4. (a and b) TEM images and the corresponding size distribution of synthesized colloidal PVPeAuNPs in a basic solution (0.27 mM HAuCl4 and 2.0 mM PVP) with 15.4 mM Na3Ct (a) and without Na3Ct (b). The inset images in (a) and (b) show the color of synthesized colloidal PVPeAuNPs. (c) UVevis absorption spectra of the synthesized colloidal PVPeAuNPs recorded at 72 h with and without NaCl (1.0 M). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
ionic strength. Fig. 4(c) shows the behavior of PVPeAuNPs with and without NaCl (1.0 M). No significant shifts in the UVevis absorption spectra are observed within 72 h, indicating that the synthesized PVPeAuNPs are stable at a high ionic strength. This high stability indicates that the synthesized AuNPs are well covered by PVP, resulting in steric repulsion that prevents the particles from aggregating.
Fig. 5 illustrates the variation in lmax with respect to the Na3Ct concentration in a solution with 2.0 mM PVP and different HAuCl4 concentrations (0.16, 0.27 and 0.55 mM HAuCl4). Note that the tendency of the lmax variation is consistent with the dependence on the Na3Ct concentration in Fig. 2. The results clearly demonstrate that the size and uniformity of the synthesized PVPeAuNPs can be improved by adding Na3Ct to the PVP reduction route.
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References
Fig. 5. Plot of lmax vs. the Na3Ct concentration at 2.0 mM PVP and different HAuCl4 concentrations (0.16, 0.27 and 0.55 mM). Note that the increase in the Na3Ct concentration contributes to the reduction of the value of lmax, reflecting a decrease in the average size of the synthesized PVPeAuNPs.
4. Conclusions In this article, we investigated the effect of sodium citrate (Na3Ct) on PVP (poly(vinyl pyrrolidone))estabilized AuNPs (PVPeAuNPs) formed by the PVP reduction route with microwave (MW) heating for 1 min. The sodium citrate (Na3Ct) in the PVP reduction system plays an important role in dictating the size and uniformity of the colloidal PVPeAuNPs. An increase in the Na3Ct concentration induces an increase in the number of citrate ions adsorbed on the growing surface of the primary AuNPs and the formation of less reactive gold solute complexes, leading to slow, stable reactions. It contributes to a decrease in the size and an improvement in the uniformity of the synthesized colloidal PVPeAuNPs. We demonstrate that by adjusting the Na3Ct concentration, the diameter of the colloidal PVPeAuNPs is reduced from 19.47 3.97 nm to 7.94 0.14 nm. We believe that the large-scale production of colloidal PVPeAuNPs with a small size and a high uniformity can be achieved by the appropriate control of the Na3Ct concentration in the PVP reduction system when using microwave (MW) heating. Acknowledgments This work was supported by a grant (Code No. 12-12-N0101-46) from the Primary Research Program of Korea Electrotechnology Research Institute (KERI), Republic of Korea.
[1] W.S. Chang, L.S. Slaughter, B.P. Khanal, P. Manna, E.R. Zubarev, S. Link, Nanoletters 9 (2009) 1152e1157. [2] E. Katz, I. Willner, Angew. Chem. Int. Ed. 43 (2004) 6042e6108. [3] X. Qian, X.H. Peng, D.O. Ansari, Y.G. Qiqin, G.Z. Chen, D.M. Shin, L. Yang, A.N. Young, M.D. Wang, S. Nie, Nat. Biotechnol. 26 (1) (2008) 83e90. [4] D.K. Kim, S.J. Park, J.H. Lee, Y.Y. Jeong, S.Y. Jon, J. Am. Chem. Soc. 129 (2007) 7661e7665. [5] M. Ferrari, Nat. Rev. Cancer 5 (2005) 161e171. [6] B. Kang, M.A. Mackey, M.A. El-sayed, J. Am. Chem. Soc. 132 (2010) 1517e 1519. [7] J. Turkevich, P.C. Stevenson, J. Hillier, Trans. Faraday Soc. 11 (1951) 55e75. [8] M.C. Daniel, D. Astuc, Chem. Rev. 104 (2004) 293e346. [9] C.J. Huang, P.H. Chiu, Y.H. Wang, K.L. Chen, J.J. Linn, C.F. Yang, J. Electrochem. Soc. 153 (12) (2006) D193eD198. [10] C.C. Kim, C.H. Wang, Y.C. Yang, Y. Hwu, S.K. Seol, Y.B. Kwon, C.H. Chen, H.W. Liou, H.M. Lin, G. Margaritondo, J.H. Je, Mater. Chem. Phys. 100 (2006) 292e295. [11] C.H. Wang, C.J. Liu, C.L. Wang, T.E. Hua, J.M. Obliosca, K.H. Lee, Y. Hwu, C.S. Yang, R.S. Liu, H.M. Lin, J.H. Je, G. Margaritondo, J. Phys. D: Appl. Phys. 41 (2008) 195301. [12] S. Kundu, S. Panigrahi, S. Praharaj, S. Basu, S.K. Ghosh, A. Pal, T. Pal, Nanotechnology 18 (2007) 075712e075719. [13] V.G. Pol, A. Gedanken, J. Calderro-Moreno, Chem. Mater. 15 (2003) 1111e 1118. [14] T. Ahmadi, Z.L. Wang, T.C. Green, A. Henglein, M.A. El-Sayed, Science 272 (1996) 1924e1925. [15] T. Ishii, H. Otsuka, K. Kataoka, Y. Nagasaki, Langmuir 20 (2004) 561e564. [16] A.G. Kanaras, F.S. Kamounah, K. Schaumburg, C.J. Kiely, M. Brust, Chem. Commun. 20 (2002) 2294e2295. [17] J.H. Fendler, Chem. Rev. 87 (1987) 877e899. [18] M. Zhou, S. Chen, S. Zhao, J. Phys. Chem. B 110 (2006) 4510e4513. [19] I. Washio, Y. Xiong, Y. Yin, Y. Xia, Adv. Mater. 18 (2006) 1745e1749. [20] Y. Xiong, I. Washio, J. Chen, H. Cai, Z.Y. Li, Y. Xia, Langmuir 22 (2006) 8563e 8570. [21] C.E. Hoppe, M. Lazzari, I. Pardinas-Blano, M.A. Lopez-Quintela, Langmuir 22 (2006) 7027e7034. [22] M. Zhou, B. Wang, Z. Rozynek, Z. Xie, J.O. Fossum, X. Yu, S. Raaen, Nanotechnology 20 (2009) 505606e505615. [23] M. Tauji, M. Hashimoto, Y. Nishizawa, M. Kubokawa, T. Tsuji, Chem. Eur. J. 11 (2005) 440e452. [24] J.A. Gerbec, D. Magana, A. Washington, G.F. Strouse, J. Am. Chem. Soc. 127 (2005) 15791e15800. [25] M. Chu, X. Shen, G. Liu, Nanotechnology 17 (2006) 444e449. [26] C. Fan, W. Li, S. Zhao, J. Chen, X. Li, Mater. Lett. 62 (20) (2008) 3518e 3520. [27] F.K. Liu, Y.C. Chang, F.H. Ko, T.C. Chu, Mater. Lett. 58 (2004) 373e377. [28] Q. Xiang, J. Yu, B. Cheng, H.C. Ong, Chem. Asian J. 5 (2010) 1466e1474. [29] S. Komarneni, M.C. D’Arrigo, C. Leonelli, G.C. Pellacani, H. Katsuki, J. Am. Ceram. Soc. 81 (1998) 3041e3043. [30] R. Eluri, B. Paul, Mater. Lett. 76 (2012) 36e39. [31] S.K. Seol, D. Kim, S. Jung, Y. Hwu, Mater. Chem. Phys. 131 (2011) 331e335. [32] D. Kim, J. Choi, G.J. Kim, S.K. Seol, Y.C. Ha, M. Vijayana, S. Jung, B. Kim, G. Lee, S. Park, Bioresour. Technol. 102 (2011) 3639e3641. [33] B.K. Pong, H.I. Elim, J.X. Chong, W. Ji, B.L. Tout, J.Y. Lee, J. Phys. Chem. C 111 (2007) 6281e6287. [34] X. Ji, X. Song, J. Li, Y. Bai, W. Yang, X. Peng, J. Am. Chem. Soc. 129 (2007) 13939e13948. [35] D.V. Gioa, E. Matijevic, Colloids Surf. A: Phys. Eng. Aspects 146 (1999) 139e152.
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Effect of citrate on poly(vinyl pyrrolidone)-stabilized gold nanoparticles formed by PVP reduction in microwave (MW) synthesis Seung Kwon Seol a, b, *, Daeho Kim a, b, Sunshin Jung a, b, Won Suk Chang a, Young Min Bae a, b, Kyeong Hee Lee a, b, Yeukuang Hwu c a b c
Advanced Medical Device Research Center, Korea Electrotechnology Research Institute (KERI), Ansan 426-910, Republic of Korea University of Science and Technology (UST), Ansan 426-910, Republic of Korea Institute of Physics, Academia Sinica, Nankang, Taipei 115, Taiwan
h i g h l i g h t s < We report the citrate effect on PVPeAuNPs formed by PVP reduction in MW synthesis. < An increase in citrate concentration leads to slow stable reactions in synthesis. < It contributes to the size decrease and the uniformity improvement of PVPeAuNPs. < Therefore, we produced uniform PVPeAuNPs with diameter of 7.94 0.14 nm in 1 min.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 February 2012 Received in revised form 20 August 2012 Accepted 26 August 2012
Colloidal PVP (poly(vinyl pyrrolidone))estabilized gold nanoparticles (PVPeAuNPs) are synthesized in aqueous solution with PVP as a reducing and stabilizing agent using a short microwave (MW) heating duration of 1 min. The size and uniformity of the synthesized PVPeAuNPs can be varied by modifying the concentration of sodium citrate (Na3Ct), which acts predominantly as mediator of the stability of PVP eAuNP formation during the rapid synthesis. Due to the increase in the Na3Ct concentration, the number of citrate ions adsorbed on the growing surface of AuNPs increase, and less reactive gold solute complexes are formed, leading to slow stable reactions that produce small, uniform colloidal PVP eAuNPs. We therefore demonstrate that by adjusting the Na3Ct concentration used in the PVP reduction, the diameter of PVPeAuNPs was varied from 19.47 3.97 nm to 7.94 0.14 nm when using constant concentrations of chloroauric acid (HAuCl4) and PVP. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Chemical synthesis Nucleation Composite materials
1. Introduction Colloidal metallic nanoparticles have received a great deal of attention, due in part to their specific properties and potential applications. In particular, gold nanoparticles (AuNPs) have become increasingly attractive because of their unique electronic, optical and chemical properties, which have given these nanoparticles the potential to be used in advanced applications in a variety of fields, such as electronics [1], sensors [2], disease diagnosis [3,4] and therapeutics [5,6].
* Corresponding author. Advanced Medical Device Research Center, Korea Electrotechnology Research Institute (KERI), Ansan 426-910, Republic of Korea. Tel.: þ82 31 8040 4182; fax: þ82 31 8040 4189. E-mail address: [email protected] (S.K. Seol). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.08.057
The properties of AuNPs depend strongly on their characteristics, such as size, shape and size distribution. Therefore, the key goal in the synthesis of AuNPs is to obtain small particles with a uniform size distribution so that the unique properties can be realized or amplified. Since Frens and Turkevich introduced the citrate reduction route for the synthesis of colloidal AuNPs [7], various synthesis approaches, such as chemical [8], electrochemical [9], radiolysis [10,11], photochemical [12] and sonochemical [13] methods, have been suggested to prepare small, uniform AuNPs. To control the size and uniformity of synthesized particles, it is important to protect the particles to prevent undesirable aggregation. High-molecular-weight polymers [14,15], ligands [16] and surfactants [17] are typically used as stabilizing reagents to prevent aggregation. Poly(vinyl pyrrolidone) (PVP) is one potential AuNPstabilizing reagent due to its high solubility in many solvents, high stability and non-toxicity. The hydrophilic headgroup (NeC] O) of PVP is attached to the surface of the particles, resulting in
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S.K. Seol et al. / Materials Chemistry and Physics 137 (2012) 135e139
particle repulsion due to steric interactions between polymers adsorbed on the particle’s surface. Recently, several groups have been reported that PVP also acts as a reducing reagent for metal particle syntheses in aqueous solution [18e21]. Gold nanostructures with diverse shapes have been synthesized by a PVP reduction route. The method is based on the chemical reduction of gold precursors by organic radicals formed during the degradation of PVP due to chemical or thermal decomposition [18,22]. The PVP reduction route is a simple, inexpensive and environmentally friendly method for producing colloidal AuNPs. The use of conventional convective heating with an external heat source for synthesis via the PVP reduction route is an inefficient way to achieve fast and uniform heat transfer to the reactants, resulting in a slow reaction rate and a wide size distribution of the synthesized AuNPs. Recently, microwave heating has been widely used for nanoparticle synthesis [23e30]. In our previous work, we reported the formation kinetics of the microwave synthesis of citrate-reduced colloidal AuNPs [31]. Compared with the convective heating used in the standard citrate reduction route, microwave heating resulted in rapid volumetric heating, resulting in the improvement of the reaction rate and the uniformity of the synthesized AuNPs [31]. Here, we present a rapid method for the preparation of PVPe stabilized gold nanoparticles (PVPeAuNPs) by PVP reduction with microwave (MW) heating. To obtain small, uniform PVPeAuNPs using the PVP reduction route, we investigated the effect of sodium citrate (Na3Ct) as mediator of the reduction of gold ions on the size of the synthesized PVPeAuNPs.
2. Experimental PVPestabilized gold nanoparticles (PVPeAuNPs) were synthesized at atmospheric pressure by the reduction of chloroauric acid (HAuCl4) reacted with PVP (Mw ¼ 40,000) and sodium citrate (Na3Ct) in deionized water (18.2 MU cm). All chemical reagents were purchased from Aldrich. A basic solution was prepared at room temperature by mixing 2.0 mM PVP and 0.27 mM HAuCl4. The Na3Ct was added to control the chemical condition of the basic solution. A schematic illustration of the microwave heating system (2.45 GHz, 1600 W) used for the entire synthesis process is shown in Fig. 1(a) [31,32]. In this system, the amplitude of the microwave power in the microwave cavity, which was adjusted using a 3-stub tuner module, was calculated by measuring the difference between the forward- and backward-propagating microwaves. The temperature of the reaction solution was measured using an optic fiber sensor that is not affected by microwaves. A feedback system between the applied microwave power and the measured temperature allowed the manipulation of the microwave power to obtain the desired reaction temperature. The solution was put in a 50 ml glass bottle, which was stirred with a magnetic stirrer coated with Teflon during the synthesis, and this bottle was placed in the cavity. The solution temperature reached the desired value of w95 C in less than 5 s (Ramping period). During the retention time (Rt) region of microwave heating, the temperature was maintained at 95 C by altering the applied microwave power between 0 W and 28 W (on/off switching) [31]. The UVevis absorption spectra of the synthesized colloidal PVPeAuNPs were recorded using a Thermo Evolution 300 spectrophotometer, and TEM images were taken on a JEM-2100F (JEOL) instrument. For TEM analysis, specimens were prepared by placing a drop of synthesized colloidal PVPeAuNPs on a carbon-coated copper grid, followed by evaporation of the solvent. The size distribution of the colloidal PVPeAuNPs was determined by measuring the diameters of more than 200 particles.
Fig. 1. (a) A schematic illustration of microwave heating system. (b) UVevis absorption spectra of colloidal PVPeAuNPs formed at 95 C in a basic solution with 2.0 mM PVP and 0.27 mM HAuCl4 by microwave heating for Rt w 1 min. The inset is a TEM image of the synthesized colloidal PVPeAuNPs.
3. Results and discussion The synthesis of colloidal PVPeAuNPs was performed at a reaction temperature of 95 C with a microwave retention time (Rt) of 1 min in a basic solution consisting of 0.27 mM HAuCl4 and 2.0 mM PVP. Fig. 1(b) shows the UVevis absorption spectra of the synthesized PVPeAuNPs. The absorption peak at 532 nm, which is related to the surface plasmon band (SPB), indicates that nanoparticles were formed during the short heating period. The formation of PVPeAuNPs is attributed to the reduction of gold ions by PVP because of no other reducing reagents were added. In contrast to convective heating, which results in a slow reaction rate, microwave (MW) irradiation significantly accelerates the degradation of PVP by rapid volumetric heating, resulting in the formation of colloidal PVPeAuNPs in 1 min. Gold solute complexes such as AuClx ðOHÞ4x bind to the hydrophilic headgroup (NeC] O) of PVP during the initial stage of the reaction. The headgroup is the active site for the reduction of the gold ions, which react with the generated organic radicals. After the reduction, small primary AuNPs formed by the nucleation of metallic gold act as seeds for the formation of colloidal PVPeAuNPs. The steric repulsive force generated by the PVP layer adsorbed on the seed prevents the further aggregation of the nanoparticles during growth. However, even though PVP acts as stabilizing reagent, the synthesized PVPe AuNPs still exhibit irregular features and a wide size distribution, resulting from the fast random growth of nanoparticles at the initial stage of process (see the TEM image in the inset of Fig. 1(b)).
S.K. Seol et al. / Materials Chemistry and Physics 137 (2012) 135e139
To improve the uniformity of the synthesized colloidal PVPe AuNPs, we tried to modify the chemical conditions of the basic solution by adding sodium citrate (Na3Ct), which generally plays the roles of reducing agent, stabilizing agent and pH mediator in the citrate reduction route [31]. Fig. 2 presents the UVevis absorption spectra for the PVPeAuNPs formed in basic solutions containing different Na3Ct concentrations. An increase in the Na3Ct concentration contributes to a gradual decrease in the absorption intensity (Iabs) and to a blue shift in the maximum absorption position (lmax). The Iabs decrease is indicative of a reduction in the density of synthesized PVPeAuNPs due to the deceleration of the reaction rate at a constant Rt w1 min. The blue shift of lmax e from 532 nm to 518 nm e is indicative of a decrease in the average size of the PVPeAuNPs (see Fig. 2(b)). The values of Iabs and lmax are constant above the Na3Ct concentration of 15.4 mM. These results can be explained by two factors affected by the Na3Ct concentration: i) the capping capability of the stabilizing reagents and ii) the reactivity of the gold complexes. The higher colloidal stability resulting from the higher capping capability of the stabilizing reagents induces the generally uniform formation of AuNPs. The electrostatic repulsive force due to Na3Ct ions capping
Fig. 2. UVevis absorption spectra of colloidal PVPeAuNPs formed in basic solutions with different Na3Ct concentrations. (a) As the solution pH increases due to the increase in the Na3Ct concentration, the absorption intensity decreases for a constant Rt w 1 min; (b) Normalized absorption value of (a): as the Na3Ct concentration increases, lmax exhibits a blue shift from 532 nm to 518 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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the AuNPs surfaces is responsible for the colloidal stability during the citrate reduction [33,34]. In our study, because PVP has a loosely packed structure on the particles, the added Na3Ct ions improve the capping capability at the initial stage of growth by capping the surface area of the primary AuNPs to which PVP has not adsorbed. Thus, the colloidal stability of synthesized PVPeAuNPs during growth is enhanced by cooperating steric and electrostatic repulsive forces. The second factor that is affected by adding Na3Ct to a basic solution is the reactivity of the gold solute complexes, which is dependent on the solution pH. As shown in Fig. 2(a), the solution pH increases proportionally with the Na3Ct concentration. As the solution pH increases, gold solute complexes are transformed successively from AuCl4 to less reactive and lower redox potential complexes (AuCl3(OH), AuCl2(OH) 2 , AuCl(OH)3 and AuðOHÞ4 ) by substitution of Cl ions with OH ions, leading to slow, stable reactions [35]. Consequently, the higher capping capability and lower reactivity of gold solute complexes obtained by increasing the Na3Ct concentration inhibit the fast random growth of nanoparticles, decreasing the Iabs and the size of the PVPeAuNPs. We synthesized colloidal PVPeAuNPs under the two different conditions with and without PVP. Fig. 3 shows the UVevis absorption spectra of PVPeAuNPs formed in the solution consisting of 0.27 mM HAuCl4 and 15.4 mM Na3Ct with 2.0 mM PVP (Rt w 1 min) or 0 mM PVP (Rt w 1, 3, 10, 15 min). Whereas an absorbance peak with a lmax value of 518 nm is obtained with an Rt of 1 min in the presence of 2.0 mM PVP, a relatively flat absorption profile appears at the same Rt when PVP is absent. Even for an Rt of w15 min, the absorbance peak has a lower intensity than the peak obtained with 2.0 mM PVP. These results clearly show that Na3Ct has a minor role as a reducing agent with a short heating time of Rt w1 min. Fig. 4(a) and (b) shows the TEM image and the corresponding size distribution of the PVPeAuNPs synthesized in basic solution with 15.4 mM Na3Ct and without Na3Ct. By adding 15.4 mM Na3Ct, the average size of the synthesized PVPeAuNPs is reduced from 19.47 3.97 nm to 7.94 0.14 nm. Based on the TEM image and the size distribution for the particles synthesized without Na3Ct in Fig. 4(b), it is clear that the added Na3Ct results in superior PVPe AuNPs with a small size and a narrow size distribution (see Fig. 4(a)). The stability of the colloidal PVPeAuNPs synthesized under the conditions for Fig. 4(a) is also though to extend to conditions of high
Fig. 3. Comparison of the UVevis absorption spectra of colloidal PVPeAuNPs formed in a solution (0.27 mM HAuCl4 and 15.4 mM Na3Ct) containing 0 mM PVP (Rt w 1, 3, 10, 15 min) or 2.0 mM PVP (Rt w 1 min).
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Fig. 4. (a and b) TEM images and the corresponding size distribution of synthesized colloidal PVPeAuNPs in a basic solution (0.27 mM HAuCl4 and 2.0 mM PVP) with 15.4 mM Na3Ct (a) and without Na3Ct (b). The inset images in (a) and (b) show the color of synthesized colloidal PVPeAuNPs. (c) UVevis absorption spectra of the synthesized colloidal PVPeAuNPs recorded at 72 h with and without NaCl (1.0 M). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
ionic strength. Fig. 4(c) shows the behavior of PVPeAuNPs with and without NaCl (1.0 M). No significant shifts in the UVevis absorption spectra are observed within 72 h, indicating that the synthesized PVPeAuNPs are stable at a high ionic strength. This high stability indicates that the synthesized AuNPs are well covered by PVP, resulting in steric repulsion that prevents the particles from aggregating.
Fig. 5 illustrates the variation in lmax with respect to the Na3Ct concentration in a solution with 2.0 mM PVP and different HAuCl4 concentrations (0.16, 0.27 and 0.55 mM HAuCl4). Note that the tendency of the lmax variation is consistent with the dependence on the Na3Ct concentration in Fig. 2. The results clearly demonstrate that the size and uniformity of the synthesized PVPeAuNPs can be improved by adding Na3Ct to the PVP reduction route.
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References
Fig. 5. Plot of lmax vs. the Na3Ct concentration at 2.0 mM PVP and different HAuCl4 concentrations (0.16, 0.27 and 0.55 mM). Note that the increase in the Na3Ct concentration contributes to the reduction of the value of lmax, reflecting a decrease in the average size of the synthesized PVPeAuNPs.
4. Conclusions In this article, we investigated the effect of sodium citrate (Na3Ct) on PVP (poly(vinyl pyrrolidone))estabilized AuNPs (PVPeAuNPs) formed by the PVP reduction route with microwave (MW) heating for 1 min. The sodium citrate (Na3Ct) in the PVP reduction system plays an important role in dictating the size and uniformity of the colloidal PVPeAuNPs. An increase in the Na3Ct concentration induces an increase in the number of citrate ions adsorbed on the growing surface of the primary AuNPs and the formation of less reactive gold solute complexes, leading to slow, stable reactions. It contributes to a decrease in the size and an improvement in the uniformity of the synthesized colloidal PVPeAuNPs. We demonstrate that by adjusting the Na3Ct concentration, the diameter of the colloidal PVPeAuNPs is reduced from 19.47 3.97 nm to 7.94 0.14 nm. We believe that the large-scale production of colloidal PVPeAuNPs with a small size and a high uniformity can be achieved by the appropriate control of the Na3Ct concentration in the PVP reduction system when using microwave (MW) heating. Acknowledgments This work was supported by a grant (Code No. 12-12-N0101-46) from the Primary Research Program of Korea Electrotechnology Research Institute (KERI), Republic of Korea.
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