From Nano to Micro: Synthesis and Optical Properties ofHomogeneous Spheroidal Gold Particles and Their SuperlatticesNicolas 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 OpticaCSIC, 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 fabricatehomogeneous gold spheres with a la carte dimensions, ranging fromthe nano- (50 nm) to the microscale (ca. 1 μm). Due to the highuniformity and surface functionalization of the produced materials,they undergo spontaneous assembly into organized superlatticesupon solvent drying. Thus, optical properties of the particlesincluding localized surface plasmon resonances and surface enhancedRaman scattering (SERS) response, both in solution and organizedinto superlattices, are also reported.

The fascinating chemical and physical properties of metallicnanoparticles have attracted the interest of many research

groups during the last decades, due to the diversity of observedphenomena,1 which offer new perspectives toward thedevelopment of innovative applications in the fields of optics,electronics, catalysis, sensing, biomedicine or environmentalmonitoring, among others.2

The optical response of plasmonic particles upon lightirradiation is mainly determined by particle size and shape,dielectric environment and organization.3 Synthetic protocolshave been devised for the preparation of particles with almostany desired geometry,4 but the preparation of monodisperseparticles with larger dimensions is still restricted to around 200nm in the case of spheres5 and several hundred in the case ofanisotropic particles such as rods or plates.2b,6 On the otherhand, self-assembly of colloidal nanoparticles in concentrateddispersions or upon solvent evaporation from colloidaldispersions to form large scale superlattices (or supercrystals)has recently attracted a growing interest.7 From a fundamentalperspective, organization of colloidal particles provides newways to exploit the collective plasmonic response of theindividual particles and generate, for example, materialssupporting extremely high electric near fields, with keyapplications as optical sensors.8 The formation of suchsupercrystals from the simple packing of spheres typicallyrequires monodisperse spherical particles coated with anappropriate organic ligand and dispersed in a suitable solvent.We present in this paper a multistep seeding growth

approach to fabricate homogeneous gold spheres with tailoreddimensions ranging from the nano (50 nm) to the microscale(ca. 1 μm). The method is a simple modification of the

standard CTAB-assisted seeding growth process, where iodideions were used to avoid the formation of anisotropic shapes.This is based on previous findings where the presence of tracesof iodide, in the rage of ppm, may inhibit the growth of goldnanorods due to the blockage of determinate crystallographicfacets.9 In our case, pentatwined gold seeds were used in thegrowth seeding procedure. When these particular seeds areused for the formation of Au nanorods, the {111} surfaces areexposed at the tips of the rods. In order to favor the elongatedgrowth of the particles, the Au must deposit onto thesesurfaces. However, since iodide adsorbs strongly to Au {111}surfaces, the Au deposition is significantly slowed on thesefacets when this anion is present. Thereby, the addition ofiodide leads to the fabrication of spheroidal particles withdimensions ranging from the nano to the microscale. The highuniformity of the particles in combination with the surfactantused to functionalize the surface of the produced materials (i.e.,CTAB) additionally allows their spontaneous assembly intoorganized superlattices upon drying of the solvent (water). Theoptical properties of the particles including localized surfaceplasmon resonances (LSPRs) and surface enhanced Ramanscattering (SERS) enhancement, both in solution and organ-ized into superlattices, were characterized, showing a goodpotential as highly efficient SERS substrates.

Special Issue: Colloidal Nanoplasmonics

Received: January 19, 2012Revised: February 25, 2012Published: March 27, 2012

Article

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■ RESULTS AND DISCUSSION

Monodisperse spheroidal gold particles with average diametersranging from ∼50 nm to ∼900 nm were prepared using amultistep seed mediated approach similar to those previouslyreported for the production of gold nanorods.10 However, theformation of nanorods was inhibited through addition ofminute amounts of potassium iodide to the growth solution.Initial seeds were prepared by NaBH4 reduction of HAuCl4using citrate as stabilizer. This method yields highly

monodisperse multiply twinned particles of 4−5 nm (seedsA, Scheme 1A). Different volumes of the initial seed solutionwere mixed with growth solutions containing CTAB, HAuCl4,ascorbic acid, and KI, resulting in the formation of particles of50, 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 thegrowth solution to different volumes of 50 nm Au particles asseeds (Scheme 1C). This last step gives rise to spheres withaverage diameters of 113, 130, 150, 175, 214, 330, 425, 525,

Scheme 1. Schematic Representation of the Synthetic Protocol for the Preparation of Gold Spheroidal Nanoparticles withTunable 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 (seedsA). (B) Seed growth process: a growth solution containing CTAB, HAuCl4 is prepared followed by the addition of ascorbic acid in order to reduceAu3+ into Au1+. Next, the seeds A are added and the Au1+ is selectively reduced to Au0 into the preformed Au nanoparticles. (C) The obtainedparticles (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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565, 585, 650, 670, 865, and 885 nm (Figure 1B−L). Theobtained particles are remarkably homogeneous and charac-terized, all of them, by a positive zeta potential around 50 mV.SEM, TEM, and dynamic light scattering show that, for samplessmaller than 500 nm, polydispersity never exceeds 3%;however, as size increases also does polydispersity though itnever goes beyond 11% (for 885 nm; see the SupportingInformation). It is worth noting that one can skip the secondgrowth step, using the initial seeds for the preparation of allsizes; however, better monodispersity is achieved when largerparticles are used as seeds. Is interesting to note as well theincrease 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 spectros-copies. Figure 2 shows the experimental and theoretical LSPR.The observed trend agrees very well with the expected changesin the optical behavior for increasing particle size11 and issupported by the theory. Particles of 50 nm present a maximumat 532 nm characteristic of their dipolar plasmon mode.However, as particle size increases above 50 nm, scatteringeffects become more relevant, and the band is noticeably red-shifted and broadened. In fact, the dipolar mode is clearlydisplaced to 655, 757, 870, 896, and 1048 nm for particle sizesof 113, 130, 150, 175, and 214 nm, respectively. Interestingly,above 113 nm, the single LSPR band is accompanied by ashoulder at lower wavelengths (550 nm). This dephasing effect,due to growth of the nanoparticle which permits theaccommodation of new plasmon modes within the particlesurface, corresponds to the quadrupolar resonance and can beas well observed as an independent band in larger particles withthe characteristic red-shift (554, 558, 560, and 566 nm forparticles sizes of 130, 150, 175, and 214 nm, respectively).From particles sizes above 330 nm, scattering effects dominatethe spectra. Nevertheless, a plasmonic shoulder between 700and 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 2-naphthalenethiol (2NAT), a well-known Raman probe (Figure

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

1378 cm−1), C−H bending (1080 cm−1), ring deformation, andC−S stretching (364 cm−1) modes. However, for the sameamount of gold, the intensity strongly varied as a function ofsize. It has been previously reported12 that the SERS intensitynotably increases with size, reaching a maximum at 113 nm andthen consistently decreasing as radiative damping becomesincreasingly significant with size. Notably, we registered SERSsignals up to particle sizes of 525 nm, in contrast with theformer literature, where it has never been shown SERSenhancement for spheroidal particles larger than ∼200 nm.5b

On the basis of simple packing rules for spheres, amonodisperse collection of spherical, organic ligand-coatednanocrystals is expected to form a face-centered cubic (fcc)lattice.13 When dispersed in a good solvent, the nanocrystalsexperience a short-ranged steric repulsion, similar to that ofhard spheres.14 When the nanocrystals are compressed togetherand the density of the collection exceeds a critical value (e.g.,

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

Figure 3. SERS spectra of 2-naphthalenethiol (λex: 785 nm) oncolloidal dispersions of spherical particles from 50 to 885 nm.Intensities compared correspond to that of C−H bending mode at1080 cm−1.

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when the solvent is evaporated from the dispersion), thenanocrystals spontaneously order into a superlattice. Thisordering transition is driven by entropy. With negligibleenergetic interactions between nanocrystals, only the excludedvolume of each particle matters and the structure with thehighest entropy is favored.7c The nanocrystals in a “dry”superlattice are held together by strong cohesive interactionsbetween neighboring ligands and nanocrystals. Figure 4A−Cshows how particles of 113 nm in diameter spontaneously self-assembled into a stable superlattice.7d This interaction leads tostrong plasmon coupling (Figure 4D) between neighboring

nanoparticles in three dimensions, with interparticle separationsaround 5 nm, ultimately yielding an electric field accumulationon the top layer of the superlattice.8 In fact, this incrementedelectric field is central for the generation of homogeneous (interms of SERS intensity spot-to-spot) highly active opticalsensors. To proof this concept, benzenethiol (BT) from the gasphase was retained on the superlattices surfaces. BT is anotherwell-studied analyte, characterized by a strong vibrationalpattern characterized by a strong ring stretching mode at1573 cm−1, C−H bending (1022 cm−1), ring breathing (1073and 999 cm−1), and low frequency mode with contribution of

Figure 4. (A, B) Dark field images; (C) SEM images at different magnifications; (D) localized surface plasmon resonance; (E) SERS response afterexposure 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; 600nm 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) SERSimages 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.

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C−S stretching (417 cm−1) (Figure 4E). Signal homogeneity isdemonstrated in Figure 4F.All the colloidal dispersions with particle sizes up to 585 nm

were found to form superlattices upon solvent drying (Figure5A) with crystalline domains of about several tens ofmicrometers, similar to those of the 113 nm particles (Figure4A−C). Notably, larger particles produce small domainsprobably due to the observed increase in polydispersity.Dark-field spectroscopy shows strongly coupled plasmonicinteractions, characterized by bands that vary in position andrelative intensity with the particle size (Figure 5B). Figure 5Cand D illustrates the SERS intensity and homogeneity of thecrystals. Although it is clear that the intensities obtained for thecolloidal dispersions and the superlattices are not directlycomparable, it is obvious that, considering the differentacquisition times and power at the sample, 10 s and 80 mWfor the dispersions and 360 ms and 0.2 mW for the lattices, thefilms are much more efficient for optical enhancement. Further,optical enhancing tendency as a function of particle size is alsovery different in the crystal as compared with that of thesolutions. Although when particles were in solution a maximumwas identified at 113 nm, when studying solid crystals, thecontinuously increasing intensities were recorded up to amaximum at 214 nm diameter. From this point onward, theintensity decreases abruptly until it can be considered null at585 nm. Remarkably, this last diameter fits with the findingsreported for the corresponding colloidal dispersions, so that inboth cases the largest particles showing SERS activity are thoseparticles with diameter of 525 nm. Regarding the uniformity ofthe intensity within the samples, quite homogeneous SERSmaps were obtained for all the samples between 50 and 330nm, but especially between 130 and 214 nm.In summary, we have introduced a simple seeding-growth

method for the preparation of spherical gold particles withtunable sizes ranging from 50 to 900 nm. The obtainedparticles are highly monodisperse and they can easily self-assemble into superlattices when the solvent (water) isevaporated. The optical properties of both discrete particlesand superlattices were characterized. In the case of colloids,both LSPR and SERS enhancement behaved as expected, withthe particles accommodating quadrupolar and higher orderplasmon modes together with the dipolar ones, as particle sizeincreases. Both types of plasmonic responses were found tored-shift with size, but for particles larger than 330 nmscattering effects dominate the optical response. SERS intensityincreases as well with size until 113 nm and decreases abruptlyfrom this point as the radiative damping becomes moresignificant. The last sample showing SERS activity was that of525 nm. In the case of superlattices, LSPR shows significantparticle intercoupling. This generates an extremely high electricfield on the top layer of the crystals that significantly enhancesthe Raman signal as compared with the colloidal particles.Notably, in this case, maximum enhancement is obtained forparticle sizes of 214 nm. However, the last sample showingenhancement is as well 525 nm. We anticipate these results tobecome central for the development of new quantitative alloptical sensors with applications in a variety of fields includingbiomedicine, homeland security, and environmental monitor-ing.

■ EXPERIMENTAL MATERIALS AND METHODSChemicals. Cetyltrimethylammonium bromide, CTAB, was

purchased from Merk. All other chemicals were purchased from

Aldrich and used without further purification. Water was purified usinga Milli-Q system (Millipore).

Seed Preparation and Particle Growth. Seed solution was doneby preparing an aqueous solution (20 mL) containing HAuCl4 (2.5 ×10−4 M) and sodium citrate (2.5 × 10−4 M). While the mixture wasvigorously stirred, NaBH4 (600 μL, 0.1 M) solution was added,observing a fast color change into red which indicates the formation ofthe gold particles. The seeds were left under stirring at openatmosphere for 1 h to allow the NaBH4 to decompose. Next, a growthsolution was prepared by dissolving CTAB (from Mercks, 40 mL, 0.1M) and potassium iodide (0.3 mg/gram of CTAB) in Milli-Q waterfollowed by the addition of HAuCl4 (204 μL, 0.103 M) and ascorbicacid (294 μL, 0.1 M). After each addition, the bottles were vigorouslyshaken. Different volumes of seeds were added, and the solution wasagain vigorously shaken. The flasks were left undisturbed at 28 °Cduring 48 h. After this time, a small amount of gold particles isobserved as sediment in the bottom of the flask. Since particles below100 nm are stable in solution, this precipitate must be composed oflarger Au structures coming from seeds with a different crystallo-graphic structure. Carefully, the supernatant is collected and theprecipitate discarded in order to ensure the monodispersity of theparticles. Highly monodisperse spheroidal gold particles of ∼50, 60,70, and 80 nm were obtained for seed volumes of 100, 75, 50, and 25μL.

In order to obtain larger particles, a second seeding-growth step wascarried out using the preformed 50 nm Au particles as seeds. For thesecond seeding process, a new growth solution was prepared usingaqueous solutions containing CTAB (0.1 M, 10 mL), HAuCl4 (42.8μL, 0.121 M), and ascorbic acid (73.5 μL, 0.1 M). After vigorouslyshaking, 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 afterreaction for 12 h.

Furthermore, for the preparation of even larger particles, the 50 nmseed solution was diluted (1:7) in water. After adding differentvolumes 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 aconcentrated drop of their aqueous suspension is cast on a substrateand 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, 10mL) were centrifuged between 8000 and 1000 rpm depending on thenanoparticle sizes during 20 min. The supernatant was discarded, andthe precipitate redispersed in 0.5 mL of H2O. The final concentrationof the gold dispersions before inducing the supercrystals assembly was5 × 10−3 M. To produce the supercrystals, 20 μL of each concentratedgold suspension was deposited on a glass slide and allowed to dryduring 24 h in a saturated moist atmosphere. Samples on glass werecleaned with plasma previously to SEM characterization and SERSanalysis. Plasma was generated in a Solarus (model 950) advancedplasma 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 anAgilent 8453 diode array spectrophotometer. Zeta potential anddynamic ligth scatering were determined using a Malver Zetasizer2000 instrument. Scanning electron microscopy (SEM) images wereobtained with a JEOL JSM 6700F field-emission microscope, usingeither lower secondary electron image (LEI) or secondary electronimage (SEI) detectors. Transmissio electron microscopy (TEM) wasconducted with a LEO 922 microscope operating at an accelerationvoltage of 200 kV. Dark-field imaging and spectroscopy of the Ausuperlattices were carried out on an inverted optical microscope(Nikon Eclipse TE-2000) equipped with an Acton SpectraPro 2150imonochromator and a Princeton Instruments Pixis 1024 charge-coupled device (CCD), which was 80 thermoelectrically cooled to−50 °C. Sample on glass slides were illuminated by white light from a100 W tungsten lamp through a dark-field condenser (NA = 0.8). Thescattered light was collected with a 40× objective (LD Plan-

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NEOFLUAR, NA = 0.6) and reflected to the entrance slit of 85 themonochromator for imaging and spectroscopy. Scattering spectra fromindividual Au nanoparticle arrays were corrected by subtractingbackground spectra taken from the adjacent regions containing no Aunanoparticles.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 confocalmicroscope, a 2D-CCD camera, and an atomatized stage of 100 nm ofspatial resolution. Excitation was carried out using a laser line at 785nm.Average SERS of colloidal dispersions was carried out in 1 mL of

particle solutions with a fixed gold concentration of 10−4 M and ananalyte (2-naphthalenethiol) concentration of 10−6 M. Measurementswere made by using a macrosampler accessory with integration timesof 10 s and a power at the sample of 80 mW. In order to characterizethe gold superlattices, benzenethiol was adsorbed in gas phase over thewhole 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 thenmapped using the Renishaw StreamLine accessory, taking mappingareas 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 0.2mW.

■ ASSOCIATED CONTENT*S Supporting InformationTEM images and histograms of Au colloids of different sizes.This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Fax: (+49) 0921552059. (+34) 986812556. E-mail: andreas.fery@uni-bayreuth.de (A.F.); ramon.alvarez@uvigo.es (R.A.A.-P.).

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was funded by the Spanish Ministerio de Ciencia eInnovacion (CTQ2011−23167) and by the German sciencefoundation within SFB 840, TP B5.

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