From Nano to Micro: Synthesis and Optical Properties of

Article pubs.acs.org/Langmuir

From Nano to Micro: Synthesis and Optical Properties of Homogeneous Spheroidal Gold Particles and Their Superlattices Nicolas Pazos-Perez,† F. Javier Garcia de Abajo,‡ Andreas Fery,*,† and Ramon A. Alvarez-Puebla*,§ †

Department of Physical Chemistry II, University of Bayreuth, Universitatstr. 30, 95440 Bayreuth, Germany Instituto de OpticaCSIC, Serrano 121, 28006 Madrid, Spain § Departamento de Química Física, Universidade de Vigo, 36310, Vigo, Spain ‡

S Supporting Information *

ABSTRACT: Iodide ions have been used as an additive to fabricate homogeneous gold spheres with a la carte dimensions, ranging from the nano- (50 nm) to the microscale (ca. 1 μm). Due to the high uniformity and surface functionalization of the produced materials, they undergo spontaneous assembly into organized superlattices upon solvent drying. Thus, optical properties of the particles including localized surface plasmon resonances and surface enhanced Raman scattering (SERS) response, both in solution and organized into superlattices, are also reported.

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standard CTAB-assisted seeding growth process, where iodide ions were used to avoid the formation of anisotropic shapes. This is based on previous findings where the presence of traces of iodide, in the rage of ppm, may inhibit the growth of gold nanorods due to the blockage of determinate crystallographic facets.9 In our case, pentatwined gold seeds were used in the growth seeding procedure. When these particular seeds are used for the formation of Au nanorods, the {111} surfaces are exposed at the tips of the rods. In order to favor the elongated growth of the particles, the Au must deposit onto these surfaces. However, since iodide adsorbs strongly to Au {111} surfaces, the Au deposition is significantly slowed on these facets when this anion is present. Thereby, the addition of iodide leads to the fabrication of spheroidal particles with dimensions ranging from the nano to the microscale. The high uniformity of the particles in combination with the surfactant used to functionalize the surface of the produced materials (i.e., CTAB) additionally allows their spontaneous assembly into organized superlattices upon drying of the solvent (water). The optical properties of the particles including localized surface plasmon resonances (LSPRs) and surface enhanced Raman scattering (SERS) enhancement, both in solution and organized into superlattices, were characterized, showing a good potential as highly efficient SERS substrates.

he fascinating chemical and physical properties of metallic nanoparticles have attracted the interest of many research groups during the last decades, due to the diversity of observed phenomena,1 which offer new perspectives toward the development of innovative applications in the fields of optics, electronics, catalysis, sensing, biomedicine or environmental monitoring, among others.2 The optical response of plasmonic particles upon light irradiation is mainly determined by particle size and shape, dielectric environment and organization.3 Synthetic protocols have been devised for the preparation of particles with almost any desired geometry,4 but the preparation of monodisperse particles with larger dimensions is still restricted to around 200 nm in the case of spheres5 and several hundred in the case of anisotropic particles such as rods or plates.2b,6 On the other hand, self-assembly of colloidal nanoparticles in concentrated dispersions or upon solvent evaporation from colloidal dispersions to form large scale superlattices (or supercrystals) has recently attracted a growing interest.7 From a fundamental perspective, organization of colloidal particles provides new ways to exploit the collective plasmonic response of the individual particles and generate, for example, materials supporting extremely high electric near fields, with key applications as optical sensors.8 The formation of such supercrystals from the simple packing of spheres typically requires monodisperse spherical particles coated with an appropriate organic ligand and dispersed in a suitable solvent. We present in this paper a multistep seeding growth approach to fabricate homogeneous gold spheres with tailored dimensions ranging from the nano (50 nm) to the microscale (ca. 1 μm). The method is a simple modification of the © 2012 American Chemical Society

Special Issue: Colloidal Nanoplasmonics Received: January 19, 2012 Revised: February 25, 2012 Published: March 27, 2012 8909

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Scheme 1. Schematic Representation of the Synthetic Protocol for the Preparation of Gold Spheroidal Nanoparticles with Tunable Dimensions from 50 to 900 nma

a

(A) Production of 4−5 nm pentatwined Au nanoparticles by the fast reduction of Au3+ in to Au0 with NaBH4 in the presence of citrate ions (seeds A). (B) Seed growth process: a growth solution containing CTAB, HAuCl4 is prepared followed by the addition of ascorbic acid in order to reduce Au3+ into Au1+. Next, the seeds A are added and the Au1+ is selectively reduced to Au0 into the preformed Au nanoparticles. (C) The obtained particles (seeds B) are regrowth in order to obtain larger particles.

Figure 1. SEM micrographs of particles from 50 to 900 nm: (A) 50; (B) 113; (C) 130; (D) 154; (E) 170; (F) 214; (G) 330; (H) 425; (I) 525; (J) 585; (K) 670; and (L) 885 nm. (M) Optical image of a single particle of 525 nm.

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RESULTS AND DISCUSSION

monodisperse multiply twinned particles of 4−5 nm (seeds A, Scheme 1A). Different volumes of the initial seed solution were mixed with growth solutions containing CTAB, HAuCl4, ascorbic acid, and KI, resulting in the formation of particles of 50, 60, 70, and 80 nm with narrow size distributions (seeds B, Scheme 1B, Figure 1A). In order to obtain larger particle sizes, a second seeding growth step was carried out by addition of the growth solution to different volumes of 50 nm Au particles as seeds (Scheme 1C). This last step gives rise to spheres with average diameters of 113, 130, 150, 175, 214, 330, 425, 525,

Monodisperse spheroidal gold particles with average diameters ranging from ∼50 nm to ∼900 nm were prepared using a multistep seed mediated approach similar to those previously reported for the production of gold nanorods.10 However, the formation of nanorods was inhibited through addition of minute amounts of potassium iodide to the growth solution. Initial seeds were prepared by NaBH4 reduction of HAuCl4 using citrate as stabilizer. This method yields highly 8910

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Figure 2. Experimental and theoretical UV−vis-NIR spectra of spherical particles from 50 to 885 nm.

565, 585, 650, 670, 865, and 885 nm (Figure 1B−L). The obtained particles are remarkably homogeneous and characterized, all of them, by a positive zeta potential around 50 mV. SEM, TEM, and dynamic light scattering show that, for samples smaller than 500 nm, polydispersity never exceeds 3%; however, as size increases also does polydispersity though it never goes beyond 11% (for 885 nm; see the Supporting Information). It is worth noting that one can skip the second growth step, using the initial seeds for the preparation of all sizes; however, better monodispersity is achieved when larger particles are used as seeds. Is interesting to note as well the increase in the faceting of the particles as size increases. Optical properties of the obtained colloidal dispersions were characterized by means of UV−vis-NIR and SERS spectroscopies. Figure 2 shows the experimental and theoretical LSPR. The observed trend agrees very well with the expected changes in the optical behavior for increasing particle size11 and is supported by the theory. Particles of 50 nm present a maximum at 532 nm characteristic of their dipolar plasmon mode. However, as particle size increases above 50 nm, scattering effects become more relevant, and the band is noticeably redshifted and broadened. In fact, the dipolar mode is clearly displaced to 655, 757, 870, 896, and 1048 nm for particle sizes of 113, 130, 150, 175, and 214 nm, respectively. Interestingly, above 113 nm, the single LSPR band is accompanied by a shoulder at lower wavelengths (550 nm). This dephasing effect, due to growth of the nanoparticle which permits the accommodation of new plasmon modes within the particle surface, corresponds to the quadrupolar resonance and can be as well observed as an independent band in larger particles with the characteristic red-shift (554, 558, 560, and 566 nm for particles sizes of 130, 150, 175, and 214 nm, respectively). From particles sizes above 330 nm, scattering effects dominate the spectra. Nevertheless, a plasmonic shoulder between 700 and 800 can be still observed until diameter sized over 585 nm. The average SERS response from the colloidal dispersions of spherical particles with different sizes was tested with 2naphthalenethiol (2NAT), a well-known Raman probe (Figure

3). All the prepared colloids were found to display the same vibrational pattern, characterized by ring stretchings (1621,

Figure 3. SERS spectra of 2-naphthalenethiol (λex: 785 nm) on colloidal dispersions of spherical particles from 50 to 885 nm. Intensities compared correspond to that of C−H bending mode at 1080 cm−1.

1378 cm−1), C−H bending (1080 cm−1), ring deformation, and C−S stretching (364 cm−1) modes. However, for the same amount of gold, the intensity strongly varied as a function of size. It has been previously reported12 that the SERS intensity notably increases with size, reaching a maximum at 113 nm and then consistently decreasing as radiative damping becomes increasingly significant with size. Notably, we registered SERS signals up to particle sizes of 525 nm, in contrast with the former literature, where it has never been shown SERS enhancement for spheroidal particles larger than ∼200 nm.5b On the basis of simple packing rules for spheres, a monodisperse collection of spherical, organic ligand-coated nanocrystals is expected to form a face-centered cubic (fcc) lattice.13 When dispersed in a good solvent, the nanocrystals experience a short-ranged steric repulsion, similar to that of hard spheres.14 When the nanocrystals are compressed together and the density of the collection exceeds a critical value (e.g., 8911

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Figure 4. (A, B) Dark field images; (C) SEM images at different magnifications; (D) localized surface plasmon resonance; (E) SERS response after exposure to benzenethiol in gas phase; and (F) optical and SERS images (1073 cm−1 breathing mode) of a region of 21 × 21 μm2 (1225 spectra; 600 nm step size, see Figure 5 for the intensity scale) for a superlattice prepared with particles of 113 nm in size.

Figure 5. (A) SEM images; (B) localized surface plasmon resonance; (C) SERS response after exposure to benzenethiol in gas phase; and (D) SERS images of a region of 21 × 21 μm2 (1225 spectra; 600 nm step size) for a superlattices prepared with particles of 113 to 585 nm in size.

nanoparticles in three dimensions, with interparticle separations around 5 nm, ultimately yielding an electric field accumulation on the top layer of the superlattice.8 In fact, this incremented electric field is central for the generation of homogeneous (in terms of SERS intensity spot-to-spot) highly active optical sensors. To proof this concept, benzenethiol (BT) from the gas phase was retained on the superlattices surfaces. BT is another well-studied analyte, characterized by a strong vibrational pattern characterized by a strong ring stretching mode at 1573 cm−1, C−H bending (1022 cm−1), ring breathing (1073 and 999 cm−1), and low frequency mode with contribution of

when the solvent is evaporated from the dispersion), the nanocrystals spontaneously order into a superlattice. This ordering transition is driven by entropy. With negligible energetic interactions between nanocrystals, only the excluded volume of each particle matters and the structure with the highest entropy is favored.7c The nanocrystals in a “dry” superlattice are held together by strong cohesive interactions between neighboring ligands and nanocrystals. Figure 4A−C shows how particles of 113 nm in diameter spontaneously selfassembled into a stable superlattice.7d This interaction leads to strong plasmon coupling (Figure 4D) between neighboring 8912

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C−S stretching (417 cm−1) (Figure 4E). Signal homogeneity is demonstrated in Figure 4F. All the colloidal dispersions with particle sizes up to 585 nm were found to form superlattices upon solvent drying (Figure 5A) with crystalline domains of about several tens of micrometers, similar to those of the 113 nm particles (Figure 4A−C). Notably, larger particles produce small domains probably due to the observed increase in polydispersity. Dark-field spectroscopy shows strongly coupled plasmonic interactions, characterized by bands that vary in position and relative intensity with the particle size (Figure 5B). Figure 5C and D illustrates the SERS intensity and homogeneity of the crystals. Although it is clear that the intensities obtained for the colloidal dispersions and the superlattices are not directly comparable, it is obvious that, considering the different acquisition times and power at the sample, 10 s and 80 mW for the dispersions and 360 ms and 0.2 mW for the lattices, the films are much more efficient for optical enhancement. Further, optical enhancing tendency as a function of particle size is also very different in the crystal as compared with that of the solutions. Although when particles were in solution a maximum was identified at 113 nm, when studying solid crystals, the continuously increasing intensities were recorded up to a maximum at 214 nm diameter. From this point onward, the intensity decreases abruptly until it can be considered null at 585 nm. Remarkably, this last diameter fits with the findings reported for the corresponding colloidal dispersions, so that in both cases the largest particles showing SERS activity are those particles with diameter of 525 nm. Regarding the uniformity of the intensity within the samples, quite homogeneous SERS maps were obtained for all the samples between 50 and 330 nm, but especially between 130 and 214 nm. In summary, we have introduced a simple seeding-growth method for the preparation of spherical gold particles with tunable sizes ranging from 50 to 900 nm. The obtained particles are highly monodisperse and they can easily selfassemble into superlattices when the solvent (water) is evaporated. The optical properties of both discrete particles and superlattices were characterized. In the case of colloids, both LSPR and SERS enhancement behaved as expected, with the particles accommodating quadrupolar and higher order plasmon modes together with the dipolar ones, as particle size increases. Both types of plasmonic responses were found to red-shift with size, but for particles larger than 330 nm scattering effects dominate the optical response. SERS intensity increases as well with size until 113 nm and decreases abruptly from this point as the radiative damping becomes more significant. The last sample showing SERS activity was that of 525 nm. In the case of superlattices, LSPR shows significant particle intercoupling. This generates an extremely high electric field on the top layer of the crystals that significantly enhances the Raman signal as compared with the colloidal particles. Notably, in this case, maximum enhancement is obtained for particle sizes of 214 nm. However, the last sample showing enhancement is as well 525 nm. We anticipate these results to become central for the development of new quantitative all optical sensors with applications in a variety of fields including biomedicine, homeland security, and environmental monitoring.

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Aldrich and used without further purification. Water was purified using a Milli-Q system (Millipore). Seed Preparation and Particle Growth. Seed solution was done by preparing an aqueous solution (20 mL) containing HAuCl4 (2.5 × 10−4 M) and sodium citrate (2.5 × 10−4 M). While the mixture was vigorously stirred, NaBH4 (600 μL, 0.1 M) solution was added, observing a fast color change into red which indicates the formation of the gold particles. The seeds were left under stirring at open atmosphere for 1 h to allow the NaBH4 to decompose. Next, a growth solution was prepared by dissolving CTAB (from Mercks, 40 mL, 0.1 M) and potassium iodide (0.3 mg/gram of CTAB) in Milli-Q water followed by the addition of HAuCl4 (204 μL, 0.103 M) and ascorbic acid (294 μL, 0.1 M). After each addition, the bottles were vigorously shaken. Different volumes of seeds were added, and the solution was again vigorously shaken. The flasks were left undisturbed at 28 °C during 48 h. After this time, a small amount of gold particles is observed as sediment in the bottom of the flask. Since particles below 100 nm are stable in solution, this precipitate must be composed of larger Au structures coming from seeds with a different crystallographic structure. Carefully, the supernatant is collected and the precipitate discarded in order to ensure the monodispersity of the particles. Highly monodisperse spheroidal gold particles of ∼50, 60, 70, and 80 nm were obtained for seed volumes of 100, 75, 50, and 25 μL. In order to obtain larger particles, a second seeding-growth step was carried out using the preformed 50 nm Au particles as seeds. For the second seeding process, a new growth solution was prepared using aqueous solutions containing CTAB (0.1 M, 10 mL), HAuCl4 (42.8 μL, 0.121 M), and ascorbic acid (73.5 μL, 0.1 M). After vigorously shaking, various seed (50 nm particles) volumes were added (400, 300, 200, 150, 75, 20, 10, and 5 μL) so different particle sizes (∼115, 130, 150, 175, 215, 330, 425, and 525 nm, respectively) were obtained after reaction for 12 h. Furthermore, for the preparation of even larger particles, the 50 nm seed solution was diluted (1:7) in water. After adding different volumes of seeds (30, 25, 20, 15, 10, and 5 μL) to the growth solution, particles with sizes 565, 585, 650, 670, 865, and 885 nm were obtained. Supercrystal Assembly. Because of their narrow size distribution, the gold particles self-crystallize into 3D colloidal crystals when a concentrated drop of their aqueous suspension is cast on a substrate and allowed to dry very slowly (24 h) in a saturated moist atmosphere. In order to do that, the gold nanoparticle solutions (2.5 × 10−4 M, 10 mL) were centrifuged between 8000 and 1000 rpm depending on the nanoparticle sizes during 20 min. The supernatant was discarded, and the precipitate redispersed in 0.5 mL of H2O. The final concentration of the gold dispersions before inducing the supercrystals assembly was 5 × 10−3 M. To produce the supercrystals, 20 μL of each concentrated gold suspension was deposited on a glass slide and allowed to dry during 24 h in a saturated moist atmosphere. Samples on glass were cleaned with plasma previously to SEM characterization and SERS analysis. Plasma was generated in a Solarus (model 950) advanced plasma cleaning system under the following conditions: 27.5 sccm (standard cubic centimeters per minute) O2, 6.4 sccm H2, 70 mTorr, and exposition of 2 min. Characterization. UV−vis-NIR spectra were recorded using an Agilent 8453 diode array spectrophotometer. Zeta potential and dynamic ligth scatering were determined using a Malver Zetasizer 2000 instrument. Scanning electron microscopy (SEM) images were obtained with a JEOL JSM 6700F field-emission microscope, using either lower secondary electron image (LEI) or secondary electron image (SEI) detectors. Transmissio electron microscopy (TEM) was conducted with a LEO 922 microscope operating at an acceleration voltage of 200 kV. Dark-field imaging and spectroscopy of the Au superlattices were carried out on an inverted optical microscope (Nikon Eclipse TE-2000) equipped with an Acton SpectraPro 2150i monochromator and a Princeton Instruments Pixis 1024 chargecoupled device (CCD), which was 80 thermoelectrically cooled to −50 °C. Sample on glass slides were illuminated by white light from a 100 W tungsten lamp through a dark-field condenser (NA = 0.8). The scattered light was collected with a 40× objective (LD Plan-

EXPERIMENTAL MATERIALS AND METHODS

Chemicals. Cetyltrimethylammonium bromide, CTAB, was purchased from Merk. All other chemicals were purchased from 8913

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NEOFLUAR, NA = 0.6) and reflected to the entrance slit of 85 the monochromator for imaging and spectroscopy. Scattering spectra from individual Au nanoparticle arrays were corrected by subtracting background spectra taken from the adjacent regions containing no Au nanoparticles. Surface-Enhanced Raman Scattering Spectroscopy. SERS experiments were conducted in a micro-Renishaw InVia Reflex system. The spectrograph uses high a resolution grating (1200 grooves cm−1 for the NIR) with additional band-pass filter optics, a confocal microscope, a 2D-CCD camera, and an atomatized stage of 100 nm of spatial resolution. Excitation was carried out using a laser line at 785 nm. Average SERS of colloidal dispersions was carried out in 1 mL of particle solutions with a fixed gold concentration of 10−4 M and an analyte (2-naphthalenethiol) concentration of 10−6 M. Measurements were made by using a macrosampler accessory with integration times of 10 s and a power at the sample of 80 mW. In order to characterize the gold superlattices, benzenethiol was adsorbed in gas phase over the whole surface of the films by casting a drop of BT (0.1 M in ethanol) in a Petri box where the film was also contained. Surfaces were then mapped using the Renishaw StreamLine accessory, taking mapping areas of 21 × 21 μm2, with a step size of 0.6 nm (1225 spectra each). Acquisition times were set to 360 ms with power at the sample of 0.2 mW.

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ASSOCIATED CONTENT

S Supporting Information *

TEM images and histograms of Au colloids of different sizes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: (+49) 0921552059. (+34) 986812556. E-mail: andreas. [email protected] (A.F.); [email protected] (R.A.A.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Spanish Ministerio de Ciencia e Innovación (CTQ2011−23167) and by the German science foundation within SFB 840, TP B5.

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REFERENCES

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