6766
Langmuir 2007, 23, 6766-6770
Free Silver Nanoparticles Synthesized by Laser Ablation in Organic Solvents and Their Easy Functionalization Vincenzo Amendola,† Stefano Polizzi,‡ and Moreno Meneghetti*,† Department of Chemical Sciences, UniVersity of PadoVa, Via Marzolo 1, I-35131 PadoVa, and Department of Physical Chemistry, UniVersity of Venezia, Via Torino 155/b, I-30172 Venezia, Italy
Downloaded via INDIAN INST OF TECH BANARAS HINDU UNIV VARANASI on June 30, 2018 at 13:05:15 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
ReceiVed December 21, 2006 Stable colloidal solutions of free silver nanoparticles (AgNPs) have been synthesized without reducing and stabilizing agents in pure acetonitrile and N,N-dimethylformamide by laser ablation of the bulk metal. Synthesis in tetrahydrofuran and dimethyl sulfoxide gave nanoparticles surrounded by a carbon shell or included in a carbon matrix. Mie theory for free and core@shell spheres accounts for the UV-vis spectra of the nanoparticles and allows their structural characterization. Transmission electron microscopy confirms the structure of the synthesized AgNPs. It is shown that free nanoparticles can be immediately functionalized, without further treatments, in the organic solvent used for the synthesis with molecules which are soluble in the same solvent.
Introduction The interest in silver nanoparticles (AgNPs) has continuously grown in recent years, due to the peculiarity of their physical and chemical properties. Fields of application of AgNPs range from nanobiotechnology1 to electronics,2 from plasmonics3 to sensoristic materials,4 from medicine5 to lithography,6 from photochromic7 to self-assembling materials,8 and from surface-enhanced Raman spectroscopy9 (SERS) to artistical manufacturing.10 The control of the size, shape, and surface functionalization is a very important issue in AgNP synthesis. The most diffused methods are based on the chemical reduction of Ag ions in solutions and give good results in terms of the final size and shape of the AgNPs. However, this type of synthesis always needs surfactants, and other chemical byproducts are present at * To whom correspondence
[email protected]. † University of Padova. ‡ University of Venice.
should
be
addressed.
E-mail:
(1) (a) Nanobiotechnology; Niemeyer, C. M., Mirkin, C. A., Eds.; Wiley-VCH Verlag GmbH & Co. KgaA: Weinheim, Germany, 2004. (b) Taxton, C. S.; Rosi, N. L.; Mirkin, C. A. MRS Bull. 2005, 30, 376-380. (c) Sonnichsen, C.; Reinhard, B. M.; Liphardt, J.; Alivisatos, A. P. Nat. Biotechnol. 2005, 23, 741-745. (d) Tom, R. T.; Samal, A. K.; Sreeprasad, T. S.; Pradeep, T. Langmuir, in press. (2) (a) Dadosh, T.; Gordin, Y.; Krahne, R.; Khivrich, I.; Mahalu, D.; Frydman, V.; Sperling, J.; Yacoby, A.; Bar-Joseph, I. Nature 2005, 436, 677-680. (b) Li, Y.; Wu, Y.; Ong, B. S. J. Am. Chem. Soc. 2005, 127, 3266-3267. (3) (a) Ozbay, E. Science 2006, 311, 189-193. (b) Krenn, J. R. Nat. Mater. 2003, 2, 210-211. (c) Taxton, C. S.; Mirkin, C. A. Nat. Biotechnol. 2005, 23, 681-682. (4) (a) Moore, B. D.; Stevenson, L.; Watt, A.; Flitsch, S.; Turner, N. J.; Cassidy, C.; Graham, D. Nat. Biotechnol. 2004, 22, 1133-1138. (b) McFarland, A. D.; Van Duyne, R. P. Nano Lett. 2003, 3, 1057-1062. (c) Haes, A. J.; Van Duyne, R. P. Anal. Bioanal. Chem. 2004, 379, 920-930. (d) Schofield, C. L.; Haines, A. H.; Field, R. A.; Russell D. A. Langmuir 2006, 22, 6707-6711. (5) (a) Choi, W. S.; Koo, H. Y.; Park, J. H.; Kim, D. Y. J. Am. Chem. Soc. 2005, 127, 16136-16142. (b) O’Neill, M. A. A.; Vine, G. J.; Beezer, A. E.; Bishop, A. H.; Hadgraft, J.; Labetoulle, C.; Walker, M.; Bowler, P. G. Int. J. Pharm. 2003, 263, 61-68. (c) Gogoi, S. K.; Gopinath, P.; Paul, A.; Ramesh, A.; Ghosh, S. S.; Chattopadhyay, A. Langmuir 2006, 22, 9322-9328. (6) (a) Huang, J.; Kim, F.; Tao, A. R.; Connor, S.; Yang, P. Nat. Mater. 2005, 4, 896-900. (b) Huang, J.; Tao, A. R.; Connor, S.; He, R.; Yang, P. Nano Lett. 2006, 6, 524-529. (7) Ohko, Y.; Tatsuma, T.; Fujii, T.; Naoi, K.; Niwa, C.; Kubota, Y.; Fujishima, A. Nat. Mater. 2003, 2, 29-31. (8) (a) Jackson, A. M.; Myerson, J. W.; Stellacci, F. Nat. Mater. 2004, 3, 330-336. (b) Malynych, S.; Robuck, H.; Chumanov, G. Nano Lett. 2001, 1, 647-649. (9) (a) Zhao, L.; Jensen, L.; Schatz, G. C. J. Am. Chem. Soc. 2006, 128, 2911-2919. (b) Kim, K.; Kim, H. S.; Park H. K. Langmuir 2006, 22, 3421-3427. (c) Yamamoto, S.; Watarai H. Langmuir 2006, 22, 6562-6569. (10) Erhardt, D. Nat. Mater. 2003, 2, 509-510.
the end of the preparation.11 Usually, one needs purified AgNPs, and all these compounds are difficult and expensive to remove. On the other hand, some important applications of AgNPs are incompatible with the presence of the reaction byproducts. For instance, SERS can be depressed by the presence of borate and citrate ions on the silver nanoparticle surface.12 Moreover, the AgNP functionalization is restricted to ligands compatible with solvents and reagents used for the reduction synthesis, which, usually, is water. Obtaining chemical-free AgNPs in a simple way originated several works on the synthesis of silver nanoparticles by laser ablation of a bulk silver plate in pure water, ethanol, and chloroform, but the final products were poorly characterized as well as their functionalization.13 Some effects due to the laser wavelength and pulse duration14 and to solutes, such as sodium dodecyl sulfate, cetyltrimethylammonium bromide, or NaCl,15 have also been reported. In this paper we present the results of the laser ablation synthesis in solution (LASiS) of stable AgNPs in pure organic solvents such as acetonitrile (AN), N,N-dimethylformamide (DMF), tetrahydrofuran (THF), and dimethyl sulfoxide (DMSO) without using reducing agents and stabilizing molecules. Free AgNPs can be obtained in particular with AN and DMF and can be easily functionalized, without further treatments, in the same solutions in which they are obtained. This gives great opportunities (11) (a) Xia, Y.; Halas, N. J. MRS Bull. 2005, 30, 330-361. (b) Sun, Y.; Xia, Y. Science 2002, 298, 2176-2179. (b) Pileni, M. P. Nat. Mater. 2003, 2, 145150. (c) Wiley, B. J.; Xiong, Y.; Li, Z. Y.; Yin, Y.; Xia, Y. Nano Lett. 2006, 6, 765-768. (d) Yu, D.; Yam, V. W. W. J. Am. Chem. Soc. 2004, 126, 1320013201. (e) Kumbhar, A. S.; Kinnan, M. K.; Chumanov, G. J. Am. Chem. Soc. 2005, 36, 12444-12445. (f) Hoppe, C. E.; Lazzari, M.; Blanco, I. P.; Lo`pezQuintela, M. A. Langmuir 2006, 22, 7027-7034. (g) Yamamoto, M.; Kashiwagi, Y.; Nakamoto, M. Langmuir 2006, 22, 8581-8586. (12) Prochazka, M.; Stepanek, J.; Vickova, B.; Srnova, I.; Maly, P. J. Mol. Struct. 1997, 410-411, 213-216. (13) (a) Compagnini, G.; Scalisi, A. A.; Puglisi, O. J. Appl. Phys. 2003, 94, 7874-7877. (b) Pyatenco, A.; Shimokawa, K.; Yamagichi, M.; Nishimiura, O.; Suzuki, M. Appl. Phys. A 2004, 79, 803-809. (c) Simakin, A. V.; Voronov, V. V.; Kirichenko, N. A.; Shafeev, G. A. Appl. Phys. A 2004, 79, 1127-1132. (d) Ganeev, R. A.; Baba, M.; Ryasnyansky, A. I.; Suzuki, M.; Kuroda; H. Opt. Commun. 2004, 240, 437-448. (14) (a) Tsuji, T.; Iryo, K.; Watanabe, N.; Tsuji, M. Appl. Surf. Sci. 2002, 202, 80-85. (b) Tsuji, T.; Kakita, T.; Tsuji, M. Appl. Surf. Sci. 2003, 206, 314-320. (c) Shafeev, G. A.; Freysz, E.; Bozon - Verduraz, F. Appl. Phys. A 2004, 78, 307-309. (15) (a) Mafune`, F.; Kohno, J.; Takeda, Y.; Kondow, T. J. Phys. Chem. B 2000, 104, 9111-9117. (b) Chen, Y.; Yeh, C. Colloids Surf. 2002, 197, 133-139. (c) Bae, C. H.; Nam, S. H.; Park, S. M. Appl. Surf. Sci. 2002, 197-198, 628-634.
10.1021/la0637061 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/10/2007
LASiS Synthesis of AgNP in Organic SolVents
Figure 1. UV-vis spectra of AgNPs synthesized in DMF (a, black), acetonitrile (b, red), THF (c, blue), and DMSO (d, green).
of easily obtaining a large number of new silver particle conjugates. Similar direct synthesis of free gold nanoparticles by LASiS in an organic solvent16 allowed, for example, their coupling with a fullerene derivative soluble in DMSO, which showed enhanced multiphoton absorption, or their embedding in a graphitic carbon matrix, which allowed control of the presence of the surface plasmon resonance of the nanoparticles.17 Here we show that AgNPs are obtained as free nanoparticles when synthesized in AN and DMF and embedded in a carbon matrix when DMSO is used. An intermediate structure with a metal core surrounded by a thin shell of carbon was obtained using THF. Fitting the AgNP surface plasmon absorption (SPA) at about 400 nm with the Mie-Gans theory allows an easy characterization of the structure of the nanoparticles, which is confirmed by high-resolution transmission electron microscopy (HRTEM) images. Experimental Section The laser ablation was carried out with Nd:YAG (Quantel YG981E) laser pulses at 1064 nm (9 ns) focused with a 10 cm focus lens on a silver plate placed at the bottom of a cell containing the solvent. Solutions were obtained with pulses of about 10 J/cm2 at a 10 Hz repetition rate for 10 min and were stable for up to several weeks. We used spectroscopic grade solvents, and the target was a plate of 99.9% silver. UV-vis spectra were recorded with a Varian Cary 5 spectrometer in 2 mm optical path quartz cells, and micro Raman spectra were obtained with a Renishaw InVia Raman microscope using the 488 nm line of an Ar laser. Samples for TEM analysis were prepared by depositing some silver colloid drops on a copper grid covered with a holey carbon film and drying them at room temperature. TEM images were collected at 300 kV with a JEOL JEM 3010 microscope equipped with a Gatan multiscan CCD camera, model 794.
Results and Discussion UV-vis spectra of the solutions obtained by laser ablation of a silver plate immersed in AN, DMF, THF, and DMSO are reported in Figure 1. One can see that AgNPs obtained in AN and DMF have a sharp SPA near 400 nm, as usual for spherical silver particles of nanometric size.18 On the contrary the AgNP (16) (a) Amendola, V.; Polizzi, S.; Meneghetti, M. J. Phys. Chem. B 2006, 110, 7232-7237. (b) Amendola, V.; Mattei, G.; Cusan, C.; Prato, M.; Meneghetti, M. Synth. Met. 2005, 155, 283-286. (17) Amendola, V.; Rizzi, G. A.; Polizzi, S.; Meneghetti, M. J. Phys. Chem. B 2005, 109, 23125-23128. (18) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; SpringerVerlag: Berlin, 1995.
Langmuir, Vol. 23, No. 12, 2007 6767
solution obtained in THF is red to the naked eye, instead of yellow as usual, its SPA being red-shifted to 455 nm and very broad. The AgNP solution obtained in DMSO does not show, on the other hand, any SPA. HRTEM images of the same samples used for Figure 1 are reported in Figure 2. AgNPs synthesized in AN (Figure 2a) and DMF (Figure 2b) show a clear crystalline structure, and they have a prevalently spherical shape and small sizes. The insets of Figure 2a,b give the size histograms obtained with more than 350 particles in both cases, which can be well fitted with lognormal curves. Average AgNP radii are found to be R ) 1.9 nm in AN, with a standard deviation of 1.5 nm, and R ) 2.2 nm in DMF, with a standard deviation of 2.5 nm. HRTEM images of the THF solution show that also in this case we produced nanoparticles of crystalline silver, but that they are always surrounded by an amorphous shell a few nanometers thick (Figure 2c). The usual log-normal distribution fits the size histogram for the AgNP core (inset of Figure 2c, obtained with more than 490 particles). The average size for the crystalline core is R ) 2.4 nm with a standard deviation of 1.1 nm, while the shell thickness ranges from subnanometric size to several nanometers for the biggest nanoparticles. HRTEM images indicate that also the DMSO solution contains crystalline AgNPs (Figure 2d), although the SPA is not observable in the UV-vis spectrum. In this case the AgNPs are found to be completely embedded in a thick amorphous matrix. We calculated, on more than 300 particles, an average radius of R ) 3.9 nm with a standard deviation of 1.9 nm (the inset of Figure 2d shows the size histogram with the usual log-normal structure). The absence of any SPA in the UVvis spectrum and the HRTEM images of AgNPs obtained in DMSO recall the case of AuNPs included in a carbon matrix obtained when the synthesis was operated in toluene.17 We confirmed the presence of the carbon matrix, for AgNPs synthesized in DMSO, observing in the Raman spectrum the presence of two bands with comparable intensities at about 1360 (D-band) and 1580 (G-band) cm-1. The strong differences observed in the UV-vis spectra of AgNPs synthesized in AN, DMF, THF, and DMSO can be understood by using the Mie theory for the calculation of the optical extinction cross section of metal nanoparticles. Colloids of spherical AgNPs have a characteristic yellow color due to the SPA located near 400 nm,11a,b,18 but in Figure 1, one can also observe an asymmetric broadening of the SPA toward longer wavelengths, which are due to a fraction of particles with spheroidal shape. The origin of these spheroids can be ascribed to aggregation processes that are very probable in a colloidal solution of free metal nanoparticles. Confirmation of this origin comes from the red-shifted broadening of the SPA of aged solutions where aggregation takes place. The SPA of spheroidal metal particles can be well reproduced by the Gans model, an extension of the Mie model for nonspherical particles.18 Although the samples contain nanoparticles with different radii and show different levels of aggregation, we demonstrated16a that fitting the UV-vis spectra of AuNPs can be obtained using the Mie and Gans models with only three fitting parameters: (i) the average radius of the nanoparticles (R); (ii) the standard deviation (σG) of a Gaussian function used to describe the statistical distribution of spheroid aspect ratios; (iii) the fraction of spherical to spheroidal gold nanoparticles.16a The same fitting procedure can be applied to the spectra of AgNPs synthesized in AN and DMF.19 (19) For the bulk silver dielectric constant we used values tabulated by Palik, E. D. Handbook of Optical Constants of Solids; Academic Press: New York, 1985.
6768 Langmuir, Vol. 23, No. 12, 2007
Amendola et al.
Figure 2. HRTEM images of AgNPs obtained in acetonitrile (a), DMF (b), THF (c), and DMSO (d). The insets show the size histograms.
Figure 3. Fittings of UV-vis spectra using the Mie theory. Fittings of AgNPs in DMF, acetonitrile, and THF are indicated by green circles, blue squares, and black tilted squares, respectively. Experimental UV-vis spectra of AgNPs in DMF, acetonitrile, and THF are indicated by the black line, red line, and blue line, respectively.
For the AN solution the fitting reported in Figure 3 indicates that the nanoparticles have an average radius of R ) 3.5 nm and that a spheroid contribution of only 2% with σG ) 1.5 is present, (20) For the graphite dielectric constant we used values tabulated by Palik, E. D. Handbook of Optical Constants of Solids; Academic Press: New York, 1985.
while for DMF R ) 5.0 nm with 100% spherical nanoparticles. The negligible amount of spheroidal particles points out the low aggregation of AgNPs obtained in these solvents. This is also confirmed by aging experiments since AN and DMF solutions are stable for weeks to months, depending on the concentration. This can be explained considering that the synthesized nanoparticles are charged and their aggregation is contrasted by Coulomb interactions. The calculated radii are slightly larger than the average radii measured by HRTEM (R )1.9 and 2.2 nm for AgNPs obtained in AN and DMF, respectively). This is understood on the basis of Mie theory, which shows that the UV-vis spectra depend on R3, namely, on the volume of the nanoparticles. In fact, averaging the volumes of the particles, observed in HRTEM images, we found a corresponding average value R ) 3.5 nm for particles synthesized in AN and R ) 5.2 nm for those synthesized in DMF, which are remarkably close to the values calculated with the fitting of the UV-vis spectra. This result confirms that the UV-vis spectra depend more on large particles than on small ones18 and that a correct comparison with TEM-based size histograms has to be considered. In the case of AgNPs synthesized in THF, fitting of the UVvis spectrum with the Mie-Gans model failed. However, we have shown above that, in this case, AgNPs are surrounded by a shell of amorphous carbon. Then, we used the Mie theory extension for a core@shell sphere to describe a AgNP@graphite structure. Fitting reaches the best agreement for a silver core of R ) 2.9 nm and a graphite shell of d ) 0.85 nm (see Figure 3). Both R and d values are compatible with HRTEM analysis, which
LASiS Synthesis of AgNP in Organic SolVents
Figure 4. UV-vis spectra of AgNPs in THF (a) and DMSO (b) deposited on soda lime slides before (black lines) and after (red lines) the heat treatment in air at 550 °C.
gives an indication of subnanometric thin carbon layers around the core particles of average radius R ) 2.4 nm. Also in this case a better agreement is obtained considering the average core volume experimentally observed, equivalent to R ) 3.0 nm. The same AgNP@graphite model can account for the quenching of the SPA for silver particles synthesized in DMSO. In fact, the model shows that the SPA of a AgNP of radius R ) 3.9 nm is completely quenched when the graphite layer reaches d ) 6.0 nm. These values are again compatible with those measured by HRTEM images. We were able to remove the amorphous carbon around the AgNPs synthesized in THF or DMSO by a heat treatment at 550 °C for 1 h in air. Parts a and b of Figure 4 show the UV-vis spectra of the samples, deposited on soda lime slides, before and after the heat treatment. For AgNPs synthesized in THF the blue shift and the shrinking of the SPA are clearly visible, which indicates the effective removal of the carbon shell. Analogous results are shown in Figure 4b for AgNPs synthesized in DMSO. The same heat treatment carried out in a nitrogen atmosphere on the same samples does not produce any difference in the UV-vis spectra, confirming the importance of removing the matrix surrounding the particles with an oxidative treatment. This result also shows that other possible reactions, for example, with the sulfur of DMSO, did not occur. Functionalization of Free AgNP. Free nanoparticles can be easily functionalized within the same solvent in which they are synthesized and without any preliminary treatment of the nanoparticles. We report in Figure 5a the spectra of free AgNPs in AN and of AgNPs functionalized with R-lypoic acid and dodecanethiol, which are soluble in AN but not, for example, in
Langmuir, Vol. 23, No. 12, 2007 6769
Figure 5. Solution spectra of free AgNPs (black line) and functionalized AgNPs with R-lipoic acid (red line) and dodecanethiol (blue line). Spectra without (a) and with (b) NaCl show the evolution of the spectra in the presence of an aggregating agent.
water. The functionalization was simply obtained by adding the molecules to the nanoparticle solution. One can see a red shift of the nanoparticle SPA with both molecules. This clearly indicates that the nanoparticles were functionalized since the shift can be understood as a consequence of a change of the dielectric constant induced in the surroundings of the nanoparticles by the new molecules linked to the surface. Further confirmation of the functionalization is found in Figure 5b where the spectra of the free and functionalized AgNPs after addition of NaCl to the solutions are given. The salt induces aggregation of the nanoparticles as can be clearly seen by the presence of the pronounced shoulder at about 520 nm in the spectrum of free AgNPs. On the other hand, spectra of the functionalized nanoparticles are very similar to those obtained without addition of the salt (see Figure 5a), showing that the functionalization was present and is able to prevent the aggregation of the nanoparticles. One should note that the solutions of the functionalized nanoparticles do not contain other molecules (reducing agents, other surfactant molecules, byproducts) which could interfere in applications such as biological or SERS experiments.
Conclusions In summary, we reported results on the laser ablation synthesis of AgNPs in organic solvents such as AN, DMF, THF, and DMSO. The optical properties of the synthesized AgNPs are strongly dependent on the solvent used, since we obtained free AgNPs using AN and DMF, particles covered with an amorphous carbon shell using THF, and particles completely embedded in
6770 Langmuir, Vol. 23, No. 12, 2007
a carbon matrix using DMSO. The different optical properties of the various type of nanoparticles have been understood within the Mie-Gans theory and supported by HRTEM analysis. It was shown how the functionalization of the free nanoparticles can be realized in an easy and immediate way by obtaining solutions which are free of other interfering molecules.
Amendola et al.
Acknowledgment. We thank G. Marcolongo for technical help and useful discussions. MIUR (Grants PRIN2004/ 200035502, FIRB/RBNE033KMA, and FIRB/RBNE01P4JF) is gratefully acknowledged for financial support. LA0637061
