Noble-metal nanoparticles exhibit a strong interaction with light due to collective oscillations of conduction electrons in the nanoparticles, known as surface plasmon resonance (SPR) [1], which have fascinated scientists for a long time. SPR not only gives the nanoparticle a specific color but it also produces the intense local electric field with the same frequency as the excitation light source near the nanoparticle surface. This phenomenon enables spectroscopies of surface-enhanced fluorescence [2], surface-enhanced Raman scattering [3], and surface-enhanced hyper-Raman scattering [4]. Particularly junctions of nanoparticles formed in aggregates shows a giant local field enhancement due to particle plasmon coupling, that strongly depends on the structure and orientation of the aggregates [5–8]. Consequently, recent studies have been focused on the fabrication and structural control of the aggregate of noble-metal nanoparticles promising the giant local-field enhancements.

Recently we found that optical trapping potential produced in a laser focus assists and controls the self-assembly of dissolved molecules or suspended nanoparticles in solution [9–12]. The focused laser beam makes a strong gradient of the electromagnetic field. A nanoparticle, which can be regarded as an electric dipole, experiences an attractive force into the center of the laser focus where the intensity of the electromagnetic field is at a maximum (i.e., the potential energy is at a minimum). This is called the optical gradient force, which has been used for trapping and manipulation of small objects. When the particle size is much smaller than the spot size of the laser beam, a number of nanoparticles can be confined in the focal spot and form a nanoassembly due to the optical trapping potential produced by the gradient force. Some groups including ours have studied optical trapping of gold and silver nanoparticles [13–20]. In the paper reported by Svedberg et al, to form a SERS active dimer, a diffusing silver nanoparticle was captured and moved to another one stuck on a surface by using optical trapping technique [20]. Theoretical calculation had shown two optically trapped metal nanoparticles strongly interact each other, that promotes their aggregation [20–24]. It was confirmed in our previous work by means of fluorescence spectroscopy that this “laser-induced self-assembly” directs the formation and orientation of J-aggregates of cyanine dye molecules to possessing a higher polarizability [11,12].

In this paper, we report the laser-induced self-assembly of silver nanoparticles via plasmonic interactions investigated by Rayleigh scattering spectroscopy. The shape of Rayleigh scattering spectra of the silver nanoassembly produced in the laser focus strongly depends on the polarization of the laser beam. Particularly, a linearly polarized laser beam induces the formation of arrayed structure along the laser polarization, that shows a sharp SPR band and harnesses excellent plasmonic properties applicable for nonlinear surface enhanced spectroscopy.

The colloidal silver was prepared by a slight modification of a well-known citrate reduction method [25]. Electron micrograph in Fig. 1 (a) showed that the colloidal solution consists of nearly spherical nanoparticles, whose average diameter is 27 nm. Absorption spectra of these colloidal samples (Fig. 1 (b)) were measured by using a commercial uv-visible absorption spectrometer. They showed a single peak attributed to the SPR, and a broadening or an additional band indicating aggregation or polydispersion was not observed. This spectral feature proves a monodispersion of colloidal particle. The size distribution of the sample confirmed by means of dynamic light scattering had 7% standard deviation. The final silver particle concentration was about 2 × 10⁻⁹ M. As depicted in Fig. 1 (c), a near-infrared laser beam (wavelength: 1064 nm) from a continuous-wave (CW) Nd³⁺:YAG laser was introduced into an inverted microscope and focused into the colloidal solution via an oil immersion microscope objective lens (Olympus UPlanApo 100 × , N.A. = 0.5-1.3). Since this objective lens corrects for both chromatic and spherical aberration at high level, the aberration was not a significant issue for this study. The polarization of the linearly polarized laser beam was rotated by a λ/2 plate. Circular polarization was produced using a λ/4 plate. Since the height of the laser focus in solution was set to be 20 μm apart from the top of a coverslip, the surface effects were negligibly small. For Rayleigh scattering measurements, a collimated unpolarized beam of white light from a tungsten halogen lamp was illuminated onto the silver colloidal solution through an oil immersion dark-field condenser lens (Olympus U-DCW NA = 1.4). Rayleigh scattering light from the colloidal silver was collected with a microscope objective and introduced to a CCD spectrometer. An absorptive sheet polarizer was placed in front of the detector to analyze the polarization of the Rayleigh scattered light.

 

Fig. 1 (a) SEM image of and silver nanoparticles and (b) absorption spectrum of these colloidal samples. (c) Schematic of experimental setup.

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A thin layer of the silver colloidal solution was prepared by sandwiching it between two glass plates. A 300-μm pinhole was set at the conjugate spot of the laser focus to limit the detection area of the Rayleigh scattering light to a spot 1 μm in diameter centered on the focal point. Rayleigh scattering spectra of silver nanoparticles optically trapped in the laser focus were obtained by calculating I _t(λ) − I _b(λ), where I _t(λ) and I _b(λ) are Rayleigh scattering spectra from the sample without and with laser irradiation, respectively.

As linearly polarized laser beam was focused into the silver colloidal solution, Rayleigh scattering light from trapped silver nanoparticles gradually increased. A surface plasmon resonance (SPR) band could be measured after several sec and then its shape changed during the laser irradiation. In every measurement, single plasmon band shifted to low energy, as shown in Fig. 2 (a) . The splitting of this band and the sudden appearance of another band were not observed in this case. Figure 3 (a) shows four typical Rayleigh scattering spectra measured at 20 sec after the start of laser irradiation. Since the photon energy range of the tungsten halogen lamp was from 1.57 to 3.1 eV, SPR band of isolated silver nanoparticles at 3.2 eV could not be clearly detected. On the other hand, since assemblies of silver nanoparticles generally show a SPR band in energetically lower region than monomer nanoparticles [1], all of SPR bands shown in Fig. 3 (a) are assigned to those of assemblies. Moreover, Rayleigh scattering light of the trapped silver nanoparticles showed polarization anisotropy, as shown in Fig. 4 . When the analyzer is parallel (θ = 0, 180°) and perpendicular (θ = 90°) to the polarization of the laser beam, the intensity of the plasmon resonance was maximized and minimized, respectively. Furthermore, this angular dependence fits to a cos² θ curve, thereby showing that the bands shown in Fig. 3 (a) are due to a dipole mode oscillation of plasmon resonance. This polarization dependence of the Rayleigh scattering spectra clearly suggests the formation of the anisotropic assembly of silver nanoparticles in the laser focus, which is supported by Ref [17,18]. that the long axis of an anisotropic nanoparticle aligns with the trapping laser polarization. Thus we estimated the general aspects of Rayleigh scattering spectra of 27 nm-sized silver nanoparticles according to the extended Mie theory [26,27]. It was confirmed that their linearly arrayed structures have longitudinal plasmon bands in the wavelength range between1.57 and 3.1 eV and the peak positions shift to lower energy with increasing array length, while the transverse plasmon band around 3.2 eV hardly changes with array length. That is, scattering spectra shown in Fig. 3 (a) can be ascribed to the linearly arrayed structures of silver nanoparticles whose long axis aligns along the laser polarization direction and Fig. 2 (a) show their growing process in the laser focus.

 

Fig. 2 Consecutive Rayleigh scattering spectra (from bottom to top) of silver nanoparticles optically trapped in the laser focus with (a) linear and (b) circular polarization. These spectra were recorded every 5 sec from the start of laser irradiation.

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Fig. 3 Rayleigh scattering spectra of assemblies of the silver nanoparticles formed in the laser focus with (a) linear and (b) circular polarization. These spectra were recorded at 20 sec after the start of laser irradiation

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Fig. 4 Normalized intensity of the plasmon band maxima vs. rotation angles of an analyzer. The dashed line indicates a fitted cos² θ curve. The arrows show the laser polarization direction and analyzer direction. The analyzer was placed in front of the detector as shown in Fig. 1 (c).

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Actually, the same measurement by using the laser beam with circular polarization gives a contrastive result. Figure 2 (b) and 3 (b) show Rayleigh scattering spectra obtained by circular polarization. Distributions of the peak energy E _res and bandwidth Γ of the SPR (Fig. 5 )demonstrates that the plasmon band in the case of circular polarization shows a broader Γ　and a higher E _res than that in the case of linear polarization (average E _res and Γ: 1.85 and 0.128 eV for linear, 2.38 and 0.534 eV for circular polarization). This result suggests that the aggregate structure produced in the laser focus with circular polarization is relatively isotropic as compared to that in the case of linear polarization, because less anisotropy of aggregates with the same number of nanoparticles increases a peak energy and broadens a bandwidth of longitudinal plasmon modes.

 

Fig. 5 Histograms of (a, c) peak wavelength and (b, d) bandwidth of the plasmon resonance bands in the spectral measurements repeated under the same conditions as for Fig. 1. (a, b) linear and (c, d) circular polarization of the laser beam.

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We consider the mechanism responsible for the anisotropic alignment of silver nanoparticles. The interaction between colloidal nanoparticles is described by the so-called Derjaguin–Landau–Verwey–Overbeek (DLVO) potential, consisting of an attractive van der Waals potential and a repulsive Coulomb potential [28]. Figure 6 shows a DLVO potential described as a function of the center-to-center interparticle distance calculated by the assumption of some suitable parameters (see figure caption). This DLVO potential barrierprevents aggregation and keeps colloidal stabilization. When the thermal energy of the nanoparticles is beyond the potential barrier height through their collision process, their aggregation proceeds immediately. The lower the potential barrier height, the higher the possibility of aggregation. We noticed that this interparticle interaction is modified under the intense laser field because of a laser induced dipole interaction, so called optical binding. We calculated this laser induced interaction by using a simple dipole approximation [23]. The black and gray solid lines in Fig. 5 show the laser induced interaction potentials for particle alignments parallel and perpendicular to the laser polarization, respectively. Thus, total interparticle interaction potential is split into two levels, as shown in Fig. 6 (b). The split of the interaction energy should be larger than the present estimation because of the plasmon coupling between each particle [5,22,24]. When nanoparticles approach each other along the laser polarization direction, they can easily contact because of the low potential barrier. This means that a formation of the linear array structure along the laser polarization is enhanced in the intense laser field. In the case of dielectric nanoparticles of the same size at the same conditions, the split of interparticle potential is estimated to be much smaller than silver nanoparticles (not shown data). It should be noted that the total interparticle potential in Fig. 6 (b) is still possessing potential barrier even in the direction along the laser polarization and thus the linear array of nanoparticles cannot be produced only by the laser induced interaction. An increase of the nanoparticle concentration that occurs in the laser focus due to the optical trapping accelerates the aggregation, as demonstrated in the previous work by using polystylene nanoparticles [9,10]. Therefore, it is essentially relevant to the linear array formation that the anisotropic interparticle potential is working on the particle aggregation process simultaneously in the laser focus. In other words, this is laser induced self-assembly of silver nanoparticles which is defined to be the aggregation under the anisotropic perturbation of laser induced interaction.

 

Fig. 6 (a) DLVO potential U _DLVO (dotted line), attractive potential U _att (black solid line), and repulsive potential U _rep (gray solid line). U _att and U _rep correspond to the laser induced interparticle interactions in cases of the particle alignment parallel and perpendicular to laser polarization, respectively. (b) shows gray and black lines representing U _DLVO + U _rep and U _DLVO + U _att, respectively. They are plotted in units of kT against the center-to-center distance r. The parameter values for calculations were set as follows: effective Hamaker constant, A _H = 2.5 eV [28]; particle surface potential (zeta potential), Ψ ₀ = −46.5 mV; and Debye length, 1/κ = 5.5 nm. Debye length was calculated from the electrolyte concentration (3.05 mM) in aqueous solution [28].

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Attractive interaction between laser induced dipoles can be explained to be originated from a gradient force by an intense optical field produced between a gap of two nanoparticles. Especially, silver and gold nanoparticles produces an extremely enhanced optical field in their nanogap, as well-known as hot spot, because the interactive dipole is coupled with SPR [29]. That is, if the alignment of nanoparticles is attributed to the laser induced interaction, hot spots must be produced in the laser induced self-assembly. This is confirmed as follows. Turning off the white light used for Rayleigh scattering measurement, the emission was observed from the laser focus, as shown in Fig. 7 . A sharp band at 2.34 eV (532 nm) is attributed to the hyper-Rayleigh scattering (HRS) of the laser beam at 1.17 eV (1064 nm). Particularly in the case of a linearly polarized laser, the other two sharp bands were observed at 2.17 and 2.14 eV, and they are not shown in the case of circular polarization. These spectral peaks correspond to 1394 and 1583 cm⁻¹ hyper-Raman bands of symmetric and asymmetric stretching vibrations (inset of Fig. 7 (a)) that could be originated from the carboxylate group of the citrate ion adsorbed on the colloidal silver during citrate reduction synthetic process [30,31]. Occurrence of surface enhanced hyper-Raman scattering (SEHRS) due to SPR of silver nanoparticles has been confirmed by using a pulse laser excitation. We previously demonstrated SEHRS by means of CW laser excitation, but the target molecule was rhodamine 6G dye, which is excited via resonance Raman process [25]. Even in the case of resonance SEHRS of rhodamine 6G, considering the cross section of hyper Raman scattering (the order of typically 10⁻⁶⁵ cm⁴·s) and obtained signal intensity, it is estimated that the enhancement factor reaches 10²⁰. The present SEHRS from citrate ions is not caused by resonance Raman process, meaning that a giant enhancement over 10²⁰ occurs accompanied by the laser induced self-assembly of silver nanoparticles. In addition, this remarkable plasmonic property of the present array structure is proved by the especially sharp SPR band shown in Fig. 2 (a).

 

Fig. 7 Emission spectra of assemblies of the silver nanoparticles formed in the laser focus. (a) linear and (b) circular polarization of the laser beam. The inset in (a) displays an enlarged spectrum plotted as a function of Raman shift from double frequency of the irradiated laser beam.

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In conclusion, we have demonstrated that laser induced self-assembly of silver nanoparticles via plasmonic interactions results in the formation of linear array structures possessing exciting plasmonic properties. Our experimental results will constitute the novel strategy to utilize SPR in a small volume of solution, that is applicable to biomolecular sensing in a micro chamber, a microfluid, or single cell directly.