Growth of Sharp Tips on Gold Nanowires Leads to Increased Surface

Nov 5, 2009 - We report the formation of gold nanoparticles with a novel and useful morphology, comprising nanowires fully covered with sharp tips (th...
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Growth of Sharp Tips on Gold Nanowires Leads to Increased Surface-Enhanced Raman Scattering Activity

Nicol as Pazos-P erez,* Silvia Barbosa, Laura Rodríguez-Lorenzo, Paula Aldeanueva-Potel, n A. Alvarez-Puebla, and Luis M. Liz-Marzan* Jorge P erez-Juste, Isabel Pastoriza-Santos, Ramo Departamento de Química Física and Unidad Asociada CSIC, Universidade de Vigo, 36310 Vigo, Spain

ABSTRACT We report the formation of gold nanoparticles with a novel and useful morphology, comprising nanowires fully covered with sharp tips (thorned nanowires). The synthesis is based on a seeded-growth approach based the rapid overgrowth of ultrathin gold wires in N,N-dimethylformamide, in the presence of poly(vinylpyrrolidone). The process allows a fine control over the thickness of the final wires, as well as the tunability of the number and sharpness of the thorns. These new plasmonic nanostructures display extremely strong optical enhancing properties and can be readily used as platforms for SERS and for integration in ultrasensitive optical devices. SECTION Nanoparticles and Nanostructures

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he controlled synthesis of anisotropic nanoparticles, in particular those with sharp tips such as triangles, elongated nanorods, nanowires (NWs), or star-shaped colloids,1-7 is a key subject in modern nanoscience because of their potential applications in technologies such as photonics, electronics, or sensing.8-11 These particles can sustain large electromagnetic fields at their ends (in nanorods and NWs),12 at vertices (in triangles)13 or at tip apexes (in nanostars),14 upon excitation with light of appropriate energy. In fact, such field localization can be readily exploited for surface enhanced spectroscopies and thus, for the fabrication of ultrasensitive devices.14 A practical advantage can be foreseen for nanomaterials that are able to sustain such high field localization at a high density of sites within its structure, thus becoming excellent substrates for surface-enhanced Raman scattering (SERS) detection, for example. Therefore, we report in this letter a seed-mediated approach for the synthesis of thorned gold NWs (TNWs) in poly(vinylpyrrolidone) (PVP)/N,N-dimethylformamide (DMF) solution. This process involves the use of ultrathin gold NWs as elongated templates for the growth of a large number of thorns with sharp apexes. Characterization of the SERS optical enhancing properties of the materials produced was achieved by using a novel method that discriminates the enhanced Raman signal from morphologically different regions within the nanomaterial. The thorned NWs were first deposited onto a smooth gold film by using an aromatic dithiol (1,5-naphthalenedithiol, 15NAT) monolayer as a linker, so that the SERS signal arises from the enhancement produced exclusively by those tips that are in contact with the dithiol monolayer.14 Additionally, the total enhancement, produced by the tip apexes, tip sides, and central wires, was estimated through SERS analysis upon adsorption of benzenethiol (BT) from the vapor phase, which occurs all over the sample.15

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The synthetic procedure leading to the formation of thorned NWs starts with the preparation of ultrathin NWs in an organic solvent as recently reported.4,16 In a typical synthesis, HAuCl4 was dissolved in hexane, assisted by addition of oleylamine, to form a yellow-orange solution upon vortexing of the mixture. It is worth noting that oleylamine acts not only as a capping agent but also induces the unidirectional growth of the structures.4,17 Tri-isopropylsilane was then added, and the solution was stored at room temperature for 24 h to ensure complete HAuCl4 reduction and subsequently centrifuged. The NWs were redispersed in chloroform (see transmission electron microscopy (TEM) image in Figure 1A). Phase transfer into ethanol was achieved by using PVP as a phase transfer agent (see Supporting Information, SI, for details), resulting in a transparent NW dispersion.18,19 Representative images of the ultrathin NWs before and after solvent transfer (Figure S1) clearly demonstrate successful transfer of nonaggregated NWs, with no apparent changes in their thickness. However, some modifications can be observed in their morphology, with the development of some thicker nodules in their bodies that twist the structure generating sharp angles and, in some cases, fracturing the NWs, but the processes involved in such morphological changes are not understood at present. It is also evident that the presence of PVP on the surface wipes out the high degree of order observed in the assembly of oleylamine-stabilized NWs. Figure 1B-F, and SI Figures S2 and S3 show representative TEM images of the NWs at different stages during overgrowth of the PVP-stabilized NWs by using DMF as both a reducing agent and a solvent.20 Briefly, solutions with constant Received Date: September 21, 2009 Accepted Date: October 15, 2009 Published on Web Date: November 05, 2009

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Figure 2. Ultraviolet-visible (UV-vis) spectra of dispersions of ultrathin gold NWs (A) before, and (B-F) after overgrowth with increasing [HAuCl4]/[Seed] ratios from 11 to 130. Consecutive spectra were shifted upward to improve readability.

Figure 1. (A) TEM image of ultrathin gold NWs (average diameter ∼1.6 nm) after redispersion in CHCl3. (B-F) Representative TEM images of the particles resulting from seeded-growth of ultrathin gold NWs with different [HAuCl4]/[Seed] ratios: 11 (B), 22 (C), 33 (D), 43 (E), and 130 (F). The scale applies to all TEM images. (G) Representative SEM image of the particles shown in F.

formation. Ultrathin NWs exhibit a weak localized surface plasmon resonance (LSPR) band centered at 520 nm, consistent with the transverse LSPR of elongated gold nanocrystals. This band becomes slightly more intense and red-shifts for the two smaller ratios, which is very likely related to the thickness increase and formation of nodules within the NW structure. Notably, for [HAuCl4]/[Seed] ratios above 20, not only the first band is significantly red-shifted (up to 540 nm, mainly ascribed to the growth of the central wire), but remarkably a new band also arises around 680 nm, indicating the onset of thorn formation. With even higher ratios, when the thorns are clearly apparent in the TEM images (Figure 1E, F), the new band becomes clearly dominant, and red-shifts to the near-infrared (NIR) (∼710 to ∼780 nm) as has been reported for gold nanoparticles with acute tips.6,14,21 Bearing in mind the optical properties derived from the growth of thorns on the NW structure, it is very likely that these structures may focalize the electromagnetic field at their thorn apexes, thereby becoming an optimal platform for SERS ultradetection. To demonstrate this statement, we devised an initial experiment in which a SERS molecular probe was attached precisely onto the apex of the thorns (Scheme 1). To achieve this goal, a self-assembled monolayer of the analyte (15NAT) was deposited on a smooth gold substrate. Subsequently, the supported monolayer (with plenty of thiol functionalities) was immersed in a diluted TNW suspension, so that individual TNWs can stick onto the free thiol groups exclusively through their thorn apexes,

concentration of PVP in DMF (10 mM, 15 mL), and varying concentrations of HAuCl4 and corresponding ratios with respect to the concentration of Au in the NW seed solution ([HAuCl4]/[Seed]: 11-130) were prepared. HAuCl4 was added to the solutions prior to addition of the gold NWs (seeds) to ensure prereduction of Au(III) to Au(I). Finally, a constant volume of NW dispersion was added to the growth solution (see SI for experimental details). Low [HAuCl4]/[Seed] ratios result in an increased number of gold nodules within the rods, and their subsequent fracture (Figure 1B,C). As the gold precursor-to-seed ratio is increased, the central wires become thicker while the nodules continue growing (Figure 1D). From this point on, the increase in the [HAuCl4]/[Seed] ratio has no incidence on the central wire thickness but does affect the development of thorns around it (Figure 1E). Further increase in the concentration of the gold salt results in an even higher thorn density (Figure 1F). Although in the TEM images the three-dimensional morphology of the particles cannot be appreciated, in the representative scanning electron microscopy (SEM) image shown in Figure 1G it is clear that the thorns grow randomly at multiple sites of the NWs surface and in different directions. As expected for small Au nanostructures, the observed morphological changes significantly influence the resulting optical properties. Shown in Figures 2 and S4 are the UV-vis spectra of the NW colloids at various stages during TNW

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Figure 3. (A) SEM micrograph of noninteracting TNWs self-assembled onto a dithiol-modified gold film. (B) SERS maps for BT (red) and 15NAT (blue) acquired on the same region of the sample, and representative SEM image of one of the TNWs. (C) Spectral windows used for map acquisition for BT (red, 905-1190 cm-1) and 15NAT (blue, from 1670-1580 cm-1). Dashed lines correspond to reference SERS spectra of BT and 15NAT. (D) Comparison of intensities between BT and 15NAT for all samples as a function of [HAuCl4]/[Seed] ratio. The corresponding SERS maps are presented in Figure S6.

Scheme 1. Schematic Representation of the Two Retention Modes of Analyte Molecules on the TNW Surface That Were Used for the Evaluation of SERS Efficiency

because of sterical hindrance (Figure 3A).14 Additionally, a parallel study was carried out to characterize the optical enhancing properties due to all parts (i.e., NW body, ends, tip body, and apexes that are not in contact with the dithiolated monolayer) of the nanostructure, for which BT was allowed to adsorb onto the assembled TNWs from the gas phase.15 Figure 3B shows the SERS intensity maps for both analytes, 15NATand BT, adsorbed on TNWs (sample F in Figures 1 and 2). To ensure proper recognition of each probe, maps were collected with a confocal microscope attached to a Raman spectroscopy system, in the same spatial region at two different spectral windows (Figure 3C), namely, from 905 to 1190 cm-1, to identify the characteristic peaks of BT (999 and 1034 cm-1, corresponding to the ring breathing modes, and 1023 cm-1 for CH bending)22 and from 1670 to 1580 cm-1 to recognize 15NAT (1351 and 1551 cm-1 assigned to ring stretchings, and 1447 cm-1 for CH bending).23 Notably, the SERS map for 15NAT shows higher intensity than that for BT. This is important because, even though the analyte concentration cannot be accurately quantified, it is clear that the relative amount of 15NAT in contact with the particles is very low as compared to that of BT. More importantly, the SERS

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cross-section of 15NAT is 10 times lower than that of BT (see Figure S5, SI). Under these circumstances, it is clear not only that the nanothorn apexes efficiently focalize the near field, but also that they must provide an additional source of enhancement, since BT is expected to be retained at the apex of the free thorns as well. This extra optical enhancement may be ascribed to the strong quantum confinement24 generated at the sandwich created between the thorn apex and the gold smooth surface, as has been reported for nanostars.14 These results are also consistent with those obtained from the SERS characterization of all samples previously described (see Figures 3C and S6). First, the SERS intensity for 15NAT was determined to be much larger than that for BT in the two TNW samples (Figure 3D; samples E and F in Figures 1 and 2). Second, the intermediate sample, when thorn growth has only started (Figure 3D; sample D in Figures 1 and 2), displays a significantly lower intensity, and the signal arising from BT is much more intense than that from 15NAT, which implies that the focalization of the near-field at tip apexes is not efficient, as they are not long and sharp in this sample. In this case, the quantum confinement generated at the sandwich created between the thorn apex and the gold smooth surface is not

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efficient either, as previously demonstrated for spheres, and therefore the signal for BT is higher than that for 15NAT. Finally, samples with no thorns yielded very weak SERS intensities, and no signal could be acquired from the starting, ultrathin NWs. In summary, we have demonstrated the preparation of elongated, thorn-shaped particles by using a seeding growth method starting from ultrathin gold NWs. The resulting thorned Au NWs display extremely strong optical enhancing properties and can be readily used as platforms for SERS and for integration in ultrasensitive optical devices.

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ACKNOWLEDGMENT S.B. and R.A.A.-P. acknowledge the JdC

(MiCInn, Spain) and RyC (MEC, Spain) programs, respectively. This work has been funded by the Spanish Ministerio de Ciencia e Innovaci on (Grants MAT2007-62696, MAT2008-05755, and Consolider Ingenio 2010-CSD2006-12) and Xunta de Galicia (Grants PGIDIT06TMT31402PR, 08TMT008314PR).

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SUPPORTING INFORMATION AVAILABLE Experimental

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details, TEM/SEM images, and SERS spectra and maps. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

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Corresponding Author: *Author to whom correspondence should be addressed: E-mail: [email protected] (N.P.-P.); [email protected] (L.M.L.-M.).

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