pubs.acs.org/Langmuir © 2010 American Chemical Society
Tuning Size and Sensing Properties in Colloidal Gold Nanostars Silvia Barbosa,† Amit Agrawal,† Laura Rodrı´ guez-Lorenzo,† Isabel Pastoriza-Santos,*,† Ramon A. Alvarez-Puebla,† Andreas Kornowski,‡ Horst Weller,‡ and Luis M. Liz-Marzan*,†,‡ †
Departamento de Quı´mica Fı´sica and Unidad Asociada CSIC-Universidade de Vigo, 36310 Vigo, Spain, and ‡ Institute of Physical Chemistry, University of Hamburg, 20146 Hamburg, Germany Received June 24, 2010. Revised Manuscript Received August 4, 2010
Gold nanostars are multibranched nanoparticles with sharp tips, which display extremely interesting plasmonic properties but require optimization. We present a systematic investigation of the influence of different parameters on the size, morphology, and monodispersity of Au nanostars obtained via seeded growth in concentrated solutions of poly(vinylpyrrolidone) in N,N-dimethylformamide. Controlled prereduction of Au3þ to Auþ was found to influence monodispersity (narrower plasmon bands), while the [HAuCl4]/[ seed] molar ratio significantly affects the morphology and tip plasmon resonance frequency. We also varied the size of the seeds (2-30 nm) and found a clear influence on the final nanostar dimensions as well as on the number of spikes, while synthesis temperature notably affects the morphology of the particles, with more rounded morphologies formed above 60 °C. This rounding effect allowed us to confirm the importance of sharp tips on the optical enhancing behavior of these nanoparticles in surface-enhanced raman scattering (SERS). Additionally, the sensitivity toward changes in the local refractive index was found to increase for larger nanostars, though lower figure of merit (FOM) values were obtained because of the larger polydispersity.
Introduction Noble metals such as silver, gold, or copper display strong localized surface plasmon resonances (LSPRs),1 which are related to collective oscillations of conduction electrons produced during interaction of the metal with visible or near-IR light. These resonances are strongly governed by particle shape, size, and composition as well as the dielectric properties of the metal itself and the surrounding medium.1-5 Such tunability and sensitivity have motivated the development of new synthetic strategies for controlling particle shape,6-9 which in turn allow us to foresee the realization of a number of potential applications in various fields such as electronics,10,11 photonics,12-14 or sensing.15-17 In several of such applications, in particular the so-called LSPR biosensing,18 the sensitivity of the nanoparticles toward changes in the *To whom correspondence should be addressed. E-mail: pastoriza@ uvigo.es (I.P.-S.);
[email protected] (L.M.L.-M.). (1) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025 and references therein. . (2) Mulvaney, P. Langmuir 1996, 12, 788. (3) Liz-Marzan, L. M. Langmuir 2006, 22, 32. (4) Noguez, C. J. Phys. Chem. C 2007, 111, 3806. (5) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (6) Grzelczak, M.; Perez-Juste, J.; Mulvaney, P.; Liz-Marzan, L. M. Chem. Soc. Rev. 2008, 37, 1783. (7) Pastoriza-Santos, I.; Liz-Marzan, L. M. Adv. Funct. Mater. 2009, 19, 679. (8) Perez-Juste, J.; Pastoriza-Santos, I.; Liz-Marzan, L.; Mulvaney, P. Coord. Chem. Rev. 2005, 249, 1807. (9) Wiley, B.; Sun, Y.; Xia, Y. Acc. Chem. Res. 2007, 40, 1067. (10) Huang, Y.; Duan, X.; Lieber, C. M. Science 2001, 291, 630. (11) Mallouk, T. E.; Kowtyukhova, N. I. Chem.;Eur. J. 2002, 8, 4354. (12) Law, M.; Sibuly, D. J.; Johnson, J. C.; Goldberger, J.; Saykally, R. J.; Yang, P. Science 2004, 305, 1269. (13) Maier, S. A.; Brongersman, M. L.; Kik, P. G.; Meltzer, S.; Tequichia, A. A. G.; Atwater, H. A. Adv. Mater. 2001, 13, 1501. (14) Maier, S. A.; Kik, P. G.; Atwater, H. A.; Meltzer, S.; Harel, E.; Koel, B. E.; Requicha, A. A. G. Nat. Mater. 2003, 2, 229. (15) Katz, E.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 6042. (16) Schultz, D. A. Curr. Opin. Biotechnol. 2003, 14, 13. (17) Aroca, R. Surface enhanced Vibrational spectroscopy; Wiley: Weinheim, 2006. (18) Sepulveda, B.; Angelome, P. C.; Lechuga, L. M.; Liz-Marzan, L. M. Nano Today 2009, 4, 244.
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local dielectric environment is the relevant parameter to consider. Since the sensitivity is determined by the degree of confinement of the plasmon oscillations, which are responsible for the near electric field enhancement at the particle surface, anisotropic metal nanoparticles, such as rods, bipyramids, or stars, have been identified as interesting systems. Au nanostars are particularly interesting because their intrinsic properties result from hybridization of plasmons focalized at the core and the tips of the nanoparticles. The core acts as an antenna producing electromagnetic field enhancements of the tip plasmons,19-21 and the morphology of the spikes (length or aperture angle) and their number also have a strong effect on plasmon frequency and intensity. Several methods have been recently reported regarding the synthesis of branched gold nanoparticles with different sizes and number of tips.19,21-27 Most of these methods are based on the chemical reduction of a gold salt in the presence of poly(vinylpyrrolidone) (PVP) or cetyltrimethylammonium bromide (CTAB). Additionally, it has been demonstrated that these kinds of nanostructures exhibit strong surface-enhanced raman scattering (SERS) enhancing activity.26-29 We recently reported19 the synthesis of gold nanostars using a seed-mediated method in N,N-dimethylformamide (DMF), with (19) Kumar, P. S.; Pastoriza-Santos, I.; Rodrı´ guez-Gonzalez, B.; Garcı´ a de Abajo, F. J.; Liz-Marzan, L. M. Nanotechnology 2008, 19, 1–5 (015606) . (20) Hao, E.; Bailey, R. C.; Schatz, G. C.; Hupp, J. T.; Li, S. Nano Lett. 2004, 4, 327. (21) Hao, F.; Nehl, C. L.; Hafner, J. H.; Nordlander, P. Nano Lett. 2007, 7, 729. (22) Xie, J.; Lee, J. Y.; Wang, D, I. C. Chem. Mater. 2007, 19, 2823. (23) Wei, Q.; Song, H.-M.; Leonov, A. P.; Hale, J. A.; Oh, D.; Ong, Q. K.; Ritchie, K.; Wei, A. J. Am. Chem. Soc. 2009, 131, 9728. (24) Chen, S. H.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. J. Am. Chem. Soc. 2003, 125, 16186. (25) Kou, X.; Sun, Z.; Yang, Z.; Chen, H.; Wang, J. Langmuir 2009, 25, 1692. (26) Xie, J.; Zhang, Z. Q.; Lee, J. Y.; Wang, D. I. C. ACS Nano 2008, 2, 2473. (27) Khoury, C. G.; Vo-Dinh, T. J. Phys. Chem. C 2008, 112, 18849. (28) Rodrı´ guez-Lorenzo, L.; Alvarez-Puebla, R. A.; Pastoriza-Santos, I.; Mazzucco, S.; Stephan, O.; Kociak, M.; Liz-Marzan, L. M.; Garcı´ a de Abajo, F. J. J. Am. Chem. Soc. 2009, 131, 4616. (29) Pazos-Perez, N.; Barbosa, S.; Rodrı´ guez-Lorenzo, L.; Aldeanueva-Potel, P.; Perez-Juste, J.; Pastoriza-Santos, I.; Alvarez-Puebla, R. A.; Liz-Marzan, L. M. J. Phys. Chem. Lett. 2010, 1, 24.
Published on Web 08/30/2010
DOI: 10.1021/la102559e
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an excellent yield (100%) and with well-defined optical response. However, a systematic study was required to optimize the degree of control over the specific morphology and more importantly over the size of the nanostars, since, for example, the size of the core plays a critical role in the focalization of electric field at the tips. It has been demonstrated through finite-difference timedomain (FDTD) calculations21 that the Au nanostar cores act as nanoantennae, enhancing the extinction cross section (a factor of 4) and the electric near field originating from the tip plasmon mode. Thus, we systematically studied the effect of core size on nanostar formation, by using Au nanoparticles with diameters of 3, 15, and 30 nm as seeds. Additionally, the influence of other parameters such as using single crystal Pt rather than polycrystalline Au seeds, prereduction of Au3þ to Auþ before seed addition, and reaction temperature were also investigated. We evaluated the refractive index sensing capability of these nanoparticles through two parameters, the absolute plasmon shift upon changes in the dielectric environment and the figure of merit (FOM). Finally, we also compared the SERS efficiencies for both average measurements in suspension and for single particles upon deposition on active films, for nanostars with different tip sharpness.
Experimental Section Chemicals. Tetrachloroauric acid (HAuCl4 3 3H2O) and trisodium citrate dihydrate were supplied by Sigma. 1-Naphthalenethiol (1NAT) was supplied by Acros Organics, and 1,5-naphthalenedithiol (15NAT) by TCI Europe. Sodium borohydride (NaBH4, 99%), ascorbic acid, hexachloroplatinic acid hexahydrate (H2PtCl6 3 6H2O), and 1-octanethiol (1OT) were supplied by Aldrich; poly(vinylpyrrolidone) (PVP, MW=10 000 and 40 000) and N,N-dimethylformamide (DMF) were from Fluka, and glycerol was from Analema. All glassware was washed with aqua regia and HF 5% (v/v) and extensively rinsed with water. Pure grade ethanol and Milli-Q grade water were used in all preparations. All the chemicals were used without further purification.
above solution, and the mixture was sonicated until the reaction was finished (no further changes registered in the UV-vis-NIR spectra). 2-3 nm Platinum Nanoparticles. Platinum nanoparticles of 2-3 nm were prepared according to ref 33. Briefly, 5 mL of an aqueous solution of 6 mM H2PtCl6 was added to 45 mL of an ethanol-water mixture (9:1 v/v) containing 1.2 g of PVP (MW 40 000). The mixture was allowed to react for 3 h under reflux. Synthesis of Gold Nanostars. In a typical synthesis, 82 μL of an aqueous solution of 50 mM HAuCl4 was mixed with 15 mL of 10 mM PVP (MW 10 000) solution in DMF. After the complete disappearance of the Au3þ CTTS absorption band at 325 nm, a certain amount of preformed-seed dispersion was added under continuous stirring and allowed to react until completion of the reaction (no further changes in the UV-vis-NIR spectra). The ratio of [HAuCl4] to [seed] was varied within different ranges for each particular type of seed. Temperature dependence experiments were performed with the 15 nm Au seeds following the same procedures as explained above, but varying the temperature of the PVP solution between 25 and 120 °C. Refractive Index Sensitivity Measurements. Ethanolglycerol mixtures with a percentage of glycerol ranging from 0 to 80% v/v were used to change the refractive index of the gold nanoparticle colloids. The refractive index of the different ethanolglycerol mixtures was calculated according to Lorentz-Lorentz equation:34 n12 2 - 1 n1 2 - 1 n2 2 - 1 ¼ j1 2 þ j2 2 2 n12 þ 2 n1 þ 2 n2 þ 2
Gold nanoparticles of 2-3 nm were prepared as previously reported.30 Briefly, 22 μL of an aqueous solution of 0.1136 M HAuCl4 was added to 47.5 mL of a PVP (MW 10 000) solution in DMF/H2O mixture (18:1; v/v) containing 0.017 g of PVP. Next, 2.5 mL of a freshly prepared 10 mM NaBH4 solution was quickly injected under vigorous stirring. The gold sol was stirred for 2 h at room temperature. The seed was not used until 24 h after preparation to allow for complete NaBH4 decomposition and avoid further nucleation. 15 nm Gold Nanoparticles. Gold nanoparticles of 15 nm were prepared by standard citrate reduction.31 Briefly, 5 mL of 1 wt % sodium citrate aqueous solution was added under continuous stirring to a boiling aqueous solution of HAuCl4 (100 mL, 0.5 mM) and allowed to react for 15 min. Then Au particles were transferred into ethanol through PVP modification. Thus, 5 mL of a PVP (MW 10 000) aqueous solution, containing a sufficient amount to provide 60 molecules of PVP per nm2 of gold, was added dropwise to the Au colloid and allowed to react overnight. Finally, it was centrifuged at 4000 rpm for 90 min, the supernatant removed, and the particles redispersed in ethanol. 30 nm Gold Nanoparticles. Gold nanoparticles of 30 nm were prepared by growth of 15 nm seeds in DMF.32 Briefly, 58.21 μL of 128.85 mM HAuCl4 was added to 15 mL of 2.5 mM PVP (MW 10 000) solution in DMF and then sonicated until complete reduction of Au3þ to Auþ. Then 94.5 μL of a 5.64 mM colloid containing 15 nm PVP coated Au nanoparticles was added to the
where n12 is the refractive index of the mixture, n1 (1.36) and n2 (1.47) are the refractive indices of ethanol and glycerol, respectively, and j1 and j2 are their volume fractions. Figure 5S (Supporting Information) shows the linear fit of the calculated refractive index values as a function of glycerol percentage. For analyzing the effect of nanostar size on the refractive index sensitivity, gold nanostars with two different average sizes were used: core diameter 11.4 ( 2.2 nm, spike length 11.6 ( 2.4 nm (sample 1); core diameter 43.3 ( 5.2 nm, spike length 21.0 ( 3.8 nm (sample 2). Upon redispersion of the particles in different ethanol-glycerol mixtures, the corresponding vis-NIR spectra were recorded and the figure of merit calculated as the ratio between wavelength shift (at the maximum wavelength) and full width at half-maximum (fwhm). Surface-Enhanced Raman Scattering. Samples for “average SERS” were prepared by adding 10 μL aliquots of analyte solution (1-naphthalenethiol, 1NAT, 10-5 M) per milliliter of colloidal suspension. After 2 h, allowing for thermodynamic equilibrium to be reached, “average SERS” was directly recorded from these suspensions. For the fabrication of non-interacting single-particle/analyte films, smooth gold substrates (120 nm thick) were prepared by sputtering onto piranha cleaned glass slides. The sputtered substrates were then immersed in ethanol solutions containing different 1OT/15NAT ratios (100:1), with an overall concentration of 10-3 M, and maintained for 24 h, for a compact, self-assembled monolayer to be deposited.35 The slides were then extensively washed with ethanol and water, and immersed in the aqueous gold nanostars colloids for 24 h. Non-covalently attached nanoparticles were removed by extensive washing and sonication. The inelastic scattered radiation was collected with a Renishaw Invia Reflex system, equipped with a two-dimensional Peltier charge-coupled device (CCD) detector and a confocal Leica microscope. The spectrograph has 1200 g/mm grating with additional band-pass filter optics. Samples were excited with a 785 nm
(30) Teranishi, T.; Kiyokawa, I.; Miyake, M. Adv. Mater. 1998, 10, 596. (31) Graf, C.; Vossen, L. J.; Imhof, A.; van Blaaderen, A. Langmuir 2003, 19, 6693. (32) Pastoriza-Santos, I.; Liz-Marzan, L. M. Adv. Funct. Mater. 2009, 19, 679.
(33) Teranishi, T.; Hosoe, M.; Tanaka, T.; Miyake, M. J. Phys. Chem. B 1999, 103, 3818. (34) Mehra, R. Proc. - Indian Acad. Sci., Chem. Sci. 2003, 115, 147. (35) Ulman, A. Chem. Rev. 1996, 96, 1533.
Synthesis of Metal Seeds.
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2-3 nm Gold Nanoparticles.
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Figure 1. (A) Vis-NIR spectra of Au nanostars synthesized under the same experimental conditions but adding 15 nm Au seeds to the growth solution (HAuCl4 in PVP/DMF solution) after aging for different periods of time. (B) UV-visible-NIR absorption spectra of the growth solution (HAuCl4 in PVP/DMF solution) at different times after mixing. (diode) laser line. For “average SERS”, a macrosampling 90° objective adaptor was used. Spectra were collected in Renishaw extended mode with accumulation times of 10 s. Power at the sample was 63 mW. For films, the laser line was focused onto the sample in backscattering geometry using a 50 objective (NA 0.75) providing scattering areas of 1 μm2. Mapping was carried out using the Renishaw StreamLine accessory with power at the sample of 1 μW. Characterization. Optical characterization was carried out by UV-vis-NIR spectroscopy with a Hewlett-Packard HP 8453 spectrophotometer. A JEOL JEM 1010 transmission electron microscope (TEM) operating at an acceleration voltage of 100 kV was used for particle size analysis and low magnification imaging, while high resolution transmission electron microscopy (HRTEM) images were obtained using a field emission JEOL JEM-2200FS microscope operated at an accelerating voltage of 200 kV. For TEM images, samples were centrifuged at 4500 rpm at least 5-fold to remove excess PVP and redispersed in ethanol.
Results and Discussion It has been recently reported that DMF can act as a suitable reducing agent for Au3þ ions, in the presence of either PVP or gold metal seeds.32 Indeed, using relatively high concentrations of PVP, gold ions can be reduced in the absence of seeds and with no need for an external energy source. Furthermore, the kinetically controlled reaction induced by the fast reduction rate leads to preferential growth along certain crystalline faces of the initial nuclei and therefore to the formation of branched, star shaped nanoparticles.19 However, as previously reported, the presence of preformed Au seeds is highly advantageous since the particle size distribution becomes significantly narrower. So far, the fabrication of Au nanostars19,27 involved the addition of a certain amount of gold seeds (ca. 15 nm in diameter) to a freshly prepared solution of PVP in DMF, containing AuCl4- ions (growth solution). Interestingly, a more detailed investigation revealed that when the Au seeds are added to growth solutions that have been aged for different times, the final dispersions of Au nanostars display different optical spectra (see Figure 1A). In fact, we identified an optimum addition time for which the vis-NIR spectra had a narrower plasmon band, indicative of higher monodispersity. Control experiments following the damping of the Au3þ CTTS absorption band at 325 nm (see Figure 1B) revealed that this optimum time coincides with the complete reduction of Au3þ (in AuCl4-) into Auþ. Whereas the addition of Au seeds prior to complete reduction results in oxidation of the seeds themselves36 (the degree of oxidation depends on the ratio of HAuCl4 to Au (36) Rodrı´ guez-Fernandez, J.; Perez-Juste, J.; Mulvaney, P.; Liz-Marzan, L. M. J. Phys. Chem. B 2005, 109, 14257.
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seeds and occasionally the process could be followed by the naked eye since the solution turned from pink to colorless), nucleation was observed in the Au salt solution (increased plasmon absorption at 500-600 nm) when the addition was delayed for too long (beyond the complete damping of the Au3þ CTTS absorption band). Both effects can be readily related to the higher polydispersity of the particles in these cases. Therefore, the reduction of Au3þ into Auþ was followed by UV-vis spectroscopy to adjust the appropriate addition time for each particular gold salt concentration. Influence of Seed Diameter and Concentration. Although seed oxidation by Au3þ ions was not considered by Khoury and Vo-Dinh,27 they were able, following the same strategy based on PVP/DMF reduction, to tune the overall size of the obtained Au nanostars between 45 and 100 nm by adjusting the concentration of preformed Au seeds (ca. 15 nm in diameter) with respect to Au salt concentration. This brought about significant changes in the position and width of the LSPR bands. Interestingly, when similar systematic experiments were carried out (though expanding the seed concentration range) taking care of the prior reduction of Au3þ to Auþ (Figure S1 in the Supporting Information), similar trends were observed, but the plasmon bands were consistently narrower. The main observation is thus that, as the [HAuCl4]/[Au seed] ratio, R, decreases, the main LSPR band (corresponding to the tip localized plasmon mode)19 blue shifts and gets narrower while the band/shoulder corresponding to the core mode becomes more intense. Additionally, we observe that for smaller R values the tip plasmon mode becomes a shoulder while the core LSPR becomes the main band. Such changes in the optical response can be easily justified in terms of the morphology of the particles. TEM characterization revealed that smaller R values lead not only to smaller particles but also to less and shorter spikes (see Figure S1 and Table S1 in the Supporting Information). It has been shown that the LSPR band position and intensity are mainly determined by the morphology and number of spikes.19,21 Below R = 1.2, the contribution of the spikes is so small (most of the particles present just one spike or even none, see Figure S1F in the Supporting Information) that the signal is negligible and a single plasmon band is observed. The electric near field enhancement due to tip plasmon modes is additionally influenced by the size of the core,19,21 as a result of the hybridization of tip and core modes. Thus, aiming at tuning the optical response, Au nanostars were synthesized for two additional sizes of the Au nanoparticles used as seeds, 30 and 2.5 nm. As shown in Figure 2A, the tunability obtained using the 30 nm seeds was basically the same as that for 15 nm seeds; that is, the intensity of the tip LSPR band decreases with R and blue-shifts, while the intensity of the core plasmon band gradually increases. DOI: 10.1021/la102559e
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Figure 2. (A) Vis-NIR spectra of Au colloids obtained using 30 nm Au nanoparticles as seeds. In the graph, the ratio of Au salt concentration to Au seed concentration (R) is displayed. (B-D) Corresponding TEM images of samples prepared at R values of 1.67, 11.25, and 20, respectively.
Figure 3. (A) Vis-NIR spectra of Au nanostars obtained for different R values, using 2.5 nm Au nanoparticles as seeds. (B-D) Corresponding TEM images of samples with R values of 67.5 (B), 270 (C), and 808 (D).
In fact, using larger seeds seems to limit the range of R values than can be used, since above R = 11.25 a second population of smaller nanostars was obtained due to secondary nucleation (see Figure 2D) and below R = 5 the core contribution dominates over that from the spikes (likely due to their small number). On the other hand, using 2.5 nm seeds, the spectral evolution was still similar (Figure 3), but narrower plasmon bands were registered. Two LSPR bands (core and spike) can be clearly distinguished at high R, arising from the starlike morphology (Figure 3C and D), with comparable intensity since only few spikes can grow from the small cores (see discussion and Figure 5 below). The smaller number of spikes is also likely to be responsible for the narrow band of the tip plasmon mode, since 14946 DOI: 10.1021/la102559e
polydispersity is also limited when less spikes contribute to it. Even at R values as high as 270, the core plasmon mode is more intense than the tip mode, which is not registered for R
