ARTICLE pubs.acs.org/JPCC
Manipulation and Raman Spectroscopy with Optically Trapped Metal Nanoparticles Obtained by Pulsed Laser Ablation in Liquids E. Messina,*,† E. Cavallaro,‡ A. Cacciola,§ R. Saija,§ F. Borghese,§ P. Denti,§ B. Fazio,‡ C. D’Andrea,‡ P. G. Gucciardi,‡ M. A. Iati,‡ M. Meneghetti,|| G. Compagnini,† V. Amendola,|| and O. M. Marago‡ †
Dipartimento di Scienze Chimiche, Universita di Catania, I-95125 Catania, Italy CNR-IPCF, Istituto per i Processi Chimico-Fisici (Messina), I-98158 Messina, Italy § Dipartimento di Fisica della Materia e Ingegneria Elettronica, Universita di Messina, I-98166 Messina, Italy Dipartimento di Scienze Chimiche, Universita di Padova, I-35131 Padova, Italy
)
‡
ABSTRACT: We investigate experimentally and theoretically optical trapping of metal nanoparticles and aggregates. In particular, we show how light forces can be used to trap individual gold nanoaggregates of controlled size and structure obtained by laser ablation synthesis in solution. Due to their surface charge, no agglomeration of isolated nanoparticles was observed during trapping experiments and reliable optical force measurements of isolated and aggregated nanoparticles was possible through an analysis of the Brownian motion in the trap. We show how the field-enhancement properties of these nanostructures enables surface-enhanced Raman spectroscopy of molecules adsorbed on aggregates optically trapped in a Raman tweezers setup. We finally discuss calculations of extinction and optical forces based on a full electromagnetic scattering theory for aggregated gold nanostructures where the occurrence of plasmon resonances at longer wavelength play a crucial role in the enhancement of the trapping forces.
’ INTRODUCTION Metal nanoparticles draw great deal of attention because of their technological applications. They hold potential as basic components for subwavelength optical devices,1 for surfaceenhanced spectroscopy,2 for biological labeling and sensing,3,4 and for cancer therapy.5 For such applications, it is crucial to prepare metal nanoparticles with desired shape and size distribution. In this context, pulsed laser ablation in liquids (PLAL) has become a key method for synthesis of nanoparticles with controlled geometry and size.6 Indeed, not only spherical particles and aggregates but even the formation of submicrometer bipyramids and other well-shaped silver particles has been recently promoted in the PLAL process.7 The intense research in this field is also motivated by the search for new multifunctional materials that will allow designing of the modern miniature electronic and optical devices for ultrafast data communication and optical data storage.8 In this regard, the interaction of light with small particles depends strongly on the size, shape, and composition of the particles, as well as on the composition of the medium in which the particles are embedded. A large number of chemical methods have been developed for the synthesis of silver and gold nanostructures that have wellcontrolled shapes, including triangular plates, cubes, wires, and rods in the form of either colloidal dispersion9 or nanostructured films.10 Laser ablation of a solid target material in a liquid allows a direct, simple, and fast technique for nanoparticles synthesis.11 The ablation of metal targets in liquid environments is considered r 2011 American Chemical Society
as a unfailing alternative to the traditional chemical reduction methods for obtaining noble metal colloids, since such a strategy is considered environmental friendly (“green” technique) with products which frequently do not need stabilizing molecules or other chemicals.12 Laser-ablation-based synthesis can be implemented in pure deionized water12 or even in biologically compatible aqueous solutions13 and can be coupled with wellestablished protocols to enhance the sensitivity of classical vibrational spectroscopies such as in the case of surface-enhanced raman scattering (SERS).14,15 Optical tweezers16-18 (OT), instruments based on a strongly focused laser beam, have been recently used to trap, manipulate, control, and assemble metal and semiconducting nanostructures,19-23 and their latest combination with Raman spectroscopy enables a thorough investigation of trapped samples.24-26 Historically, optical trapping of 36 nm gold nanoparticles was first demonstrated by Svoboda and Block.27 More recently, the trapping range of gold spherical nanoparticles was expanded up to 250 nm,28-31 and very accurate measurements of optical forces have become possible.31 A complete theory of optical trapping for spherical metal particles has been Special Issue: Laser Ablation and Nanoparticle Generation in Liquids Received: September 30, 2010 Revised: January 18, 2011 Published: February 16, 2011 5115
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Figure 1. (a) Sketch of the PLAL setup. (b) Extinction spectra for the different samples of gold nanoparticles obtained by PLAL. The spectra have been normalized to the plasmon peak at 520 nm. (c-f) TEM images taken for nonaggregated gold nanoparticles (c) and for gold nanoaggregates from samples B (d), C (e), and D (f).
developed32 that was shown to be in very good agreement with experiments. The optical trapping of nonfunctionalized gold nanoparticles obtained by chemical methods can also originate particle agglomeration due to the observation of reversible electrical conductivity changes of the solution of the nanoparticles upon laser illumination or direct heating.33 Particle agglomeration during trapping experiments can be an obstacle for the accurate determination of the optical forces generated by the trapping beam and for the investigation of the effect of the plasmonic properties of nanoparticles on the trapping process. Here we discuss optical trapping experiments on functionalized gold nanoaggregates with radii ranging from 20 to 750 nm, obtained by laser ablation in aqueous solution. Due to the surface charge of gold nanoparticles obtained by laser ablation synthesis in solution, no agglomeration of isolated nanoparticles was observed during trapping experiments and the reliable comparison of the optical trapping behavior of isolated and aggregated nanoparticles was possible, contrary to that reported in other cases by using citrate stabilized gold nanoparticles.22,33 We show how nanoaggregates have an increased trapping efficiency that depends on the relative position between the trapping wavelength and the long-wavelength plasmon resonance arising from aggregation. Furthermore, the optical properties of metal nanoaggregates make them suitable for SERS studies, because “hot spots” created by the aggregation can enhance the Raman signal from the molecules adsorbed on the nanostructures by several orders of magnitude. In this work SERS by optically trapped metal nanoparticles aggregates is reported. Finally we discuss the theory of optical trapping on gold nanoparticles through a procedure based on the Maxwell stress tensor in the transition T-matrix formalism and extend this formalism to aggregated gold nanostructures.
’ MATERIALS AND METHODS Gold nanoparticles were obtained by pulsed laser ablation in liquids (PLAL) of a pure metal target in water by irradiating with a wavelength of 1064 nm from Q-switched Nd:YAG laser pulses. The laser fluence is varied between 10 and 20 J/cm2, and the ablation time is maintained at 30 min. The experimental scheme is shown in Figure 1. At a second time, controlled amounts of pyridine are added to the as prepared gold colloids. The pyridine is able to promote the aggregation of gold nanoparticles, linking to them through the
nitrogen lone-pair electrons.34 The concentration of pyridine ranged from 0.01 to 0.25 μL/mL and the volume of the gold nanoparticles solution was 2 nM for each experiment. The small amount of pyridine used is able to promote different aggregation levels of gold nanoparticles in water solution. Subsequently, adding bovine serum albumin (BSA) to the gold nanoparticles, we can quench the aggregation process. The prepared gold nanoparticle colloids have been found to be stable for several months without any aggregation or deterioration of the spectral properties. The extinction of each sample has been monitored using a Varian Cary 5 spectrometer in the range 190-1100 nm. Prior to the UV-vis analysis, samples C and D were sonicated in order to readily resuspend all the aggregates in solution. The concentration of gold nanoparticles was evaluated by fitting the UV-vis spectra with a Mie-Gans model.34 A few drops of every sample have also been deposited onto a copper grid covered with a holey carbon films in order to estimate their structural properties. These were measured by transmission electron microscopy (TEM) at 300 kV with a JEOL JEM 3010 microscope equipped with a Gatan Multiscan CCD camera, model 794. Optical trapping experiments were carried out in a an inverted setup22,23 (laser light propagates upward to counteract gravity). A sketch is shown in Figure 2a. In brief, the light from a near-infrared laser diode 830 nm (Sanyo DL-8032-01) is focused by a 1.3NA oil immersion microscope objective. The same objective allows the imaging of the trapped nanoparticles onto a CCD camera (Figure 2b-e). Image calibration is obtained by optical trapping size-standard latex beads. A few tens of microliters from the gold colloidal solution obtained with the PLAL are loaded in a small chamber where optical trapping experiments are perfomed. The laser power during all optical trapping experiments was limited to 15 mW to avoid heating of the trapped particles and bubbling. In fact, when using the maximum available power, about 32 mW, we observed heating effects (bubbling) while trapping microaggregates from sample D. Raman tweezing24-26 experiments have been performed using a single beam configuration, in which optical trapping and Raman spectroscopy are obtained with the same laser at 785 nm. The light of the near-infrared diode (Sanyo DL-7140201S) was focused by an oil immersion objective (NA = 1.3) mounted on a Olympus-BX41microscope. The power on the sample was 3.5 mW. Optical trapping was monitored by a CCD camera. The Raman signal was collected in a backscattering 5116
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Figure 2. (a) Sketch of the optical trapping setup. A near-IR laser source (830 nm) is focused through a microscope objective with NA = 1.3. Nanoaggregate samples are loaded in a small chamber placed in the trap zone where optical trapping occurs. The light scattered and unscattered by the particle is imaged through the microscope condenser lens onto a quadrant photodiode (QPD) yielding the particle positional tracking. (b) Image of an optically trapped gold nanoaggregates from sample B. The particle has a size below the optical resolution of the microscope; thus only a diffraction limited image is visible. (c) Image of the same aggregate when the trapping laser is switched off and the particle is free to move in the liquid environment. (d, e) A large (about 2 μm) aggregate from sample D, rotating while optically trapped. The light-driven rotation is induced by the geometrical anisotropy (“windmill effect”). The unbalanced radiation torque arises from anisotropic scattering by the gold aggregate.
configuration and analyzed by a LabRam HR800 spectrometer (Horiba-Jobin Yvon) equipped with a Peltier cooled CCD as detector (HJY-Synapse).
’ RESULTS AND DISCUSSION Optical Trapping and Manipulation. Figure 1b shows the measured optical extinction spectrum of four sets of samples produced by laser ablation of gold metal plates at 1064 nm in an aqueous solution with different aggregations promoted by a specific amount of pyridine. First we note that only sample A, the nonaggregated sample, exhibits exclusively the characteristic peak at 520 nm (black line). Further information is also obtained by TEM analysis. In Figure 1c we show that the nanoparticles before the aggregation process are almost spherical with an average diameter of about 35 nm. In all other samples (B-D) the optical properties show a double plasmon resonance structure. This spectral feature is a clear indication that aggregation of the gold nanoparticles took place.36 Indeed, drastic changes in the surface plasmon absorption are observed whenever changes in the particles shape or size occur.9,37 The size distribution of gold aggregates was measured from TEM photographs. As shown in previous works,12,13 particles prepared by laser ablation have a log-normal size distribution and the standard deviation on the average size is on the order of 50% (see also Figure 1c). Moreover nanoparticles prevalently have a spherical shape, although a minor fraction of spheroidal and other nonspherical particles is also present.12,35 The structure of these nanoparticles is highly defective and often polycrystalline, which is typical of the fast nucleation and growth process, that is the formation mechanism of nanoparticles obtained by laser ablation in liquids. TEM analysis on our sample confirms that nanoparticles as obtained by laser ablation in water are prevalently spherical with average size of 35 ( 15 nm (sample A).
In panels d-f of Figure 1 we show the fractal structure of gold nanoaggregates for sample B, C, and D, respectively. By TEM, we imaged about 60 aggregates from the samples B, C, and D and we observed in all cases a fractal structure with linear or globular morphology. From these TEM images, we measured that nanoaggregates have average diameters of 90 ( 30, 250 ( 50, and 750 ( 250 nm in samples B, C, and D respectively. We performed optical trapping experiments by trapping several nanoparticles from the four samples and the trapping strength for each case was measured. Remarkably, in the case of sample 1 (i.e., nonaggregated AuNP), we did not observed any laser beam induced agglomeration of particles. This result is in contrast with that reported by other authors about optical trapping of citrate coated gold nanoparticles.33 Indeed, laser beam induced agglomeration of citrate coated gold nanoparticles is a frequent phenomenon, usually explained with the release of the stabilizing citrate molecules from the AuNP surface induced by the heating of nanoparticles trapped by the laser beam.33 In the present case, however, we used AuNP obtained by laser ablation in water. These nanoparticles are obtained as a stable colloidal suspension in water without the need for citrate molecules or other stabilizers.12,13 The Meunier and Mafune groups showed that the stability of AuNP obtained by laser ablation in water is due to their negative surface charge and that the origin of this surface charge can be explained with the presence of a small amount (about 3-7%) of surface gold atoms that are oxidized.38,39 Therefore, these charges are chemically bound to the surface of AuNP, and it is likely that they are insensitive to the moderate heating of nanoparticles trapped in the laser beam, contrary to the citrate molecules, thus avoiding particle agglomeration during the optical trapping experiments. Panels b and c of Figure 2 show images of optically trapped (Figure 2b) and untrapped (Figure 2c) gold aggregates from sample B. The size of the nanoaggregate is smaller than the diffraction limit of the imaging system; thus only a diffraction limited spot is visible. In contrast, in Figure 2d,e a large (about 2 μm) aggregate from 5117
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The Journal of Physical Chemistry C sample D is trapped and set into rotation by light. Continuous rotation in the optical trap is observed for aggregates with a strong asymmetric morphology. Such light-driven rotations22 are a consequence of the particle shape, i.e., anisotropic light scattering causes rotation about the laser propagation direction (the so-called “windmill effect”).40 Thus we performed accurate radiation force measurements always with no rotation driven on the trapped particle. Moreover we put some care in keeping the distance from the coverslip surface much larger than the particle average size, so that stable trapping is always ensured while hydrodynamic perturbation of the bottom surface can be assumed negligible. To evaluate optical forces, we use a method based on recording an interferometric particle tracking signal and evaluating the correlations between the motions in orthogonal directions.41 The interference pattern between forward-scattered and unscattered light in the back aperture of the microscope condenser is imaged onto a quadrant photodiode (QPD) from which tracking signals are derived that reveal the particle’s Brownian motion in the trap.23,41 The frequency response of this technique is limited by the bandwidth of the electronics that records the tracking signal (approximately 100 kHz in our experiment), therefore enabling detection of the overdamped motion of the trapped. Furthermore, the same laser beam that traps the nanostructure is also used for the detection of that motion, thus considerably simplifying the experimental geometry. Thus, when the particle moves from its equilibrium position in the trap, each QPD quadrant generates different photocurrents. By combination of these signals as pairwise sums, it is possible to generate signals Sx, Sy, Sz proportional to the trapped particle’s displacements in the three directions (x, y, z) defined by the trapping potential. The analysis of these signals gives a wealth of information on the measure of optical force constants and more generally on Brownian motion spectroscopy.24,42-44 The force generated by an optical tweezers is well described by an harmonic confining potential with force constants kx, ky, and kz in three spatial directions. Hence the motion of a particle in such a potential can be understood through a Langevin analysis. In particular, for a trapped spherical particle in water the equation of motion of the positional displacements xi is d ð1Þ xi ðtÞ ¼ - ωi xi ðtÞ þ ξi ðtÞ i ¼ x, y, z dt where the relaxation rates ωi = ki/γ are related to the force constants and viscous damping γ = 6πηr (Stokes’ law), η is the dynamic viscosity of water, r is the hydrodynamic radius, ξi(t) are random uncorrelated fluctuations that account for the random walk of the diffusing particle in the fluid.24 We now consider the correlation functions of the positional displacements Z Cii ðτÞ ¼ xi ðtÞxi ðt þ τÞ dτ Their time evolution can be extracted from the Langevin equations (eq 1) yielding an exponential decay with lag time τ, kB T -ωi τ Cii ðτÞ ¼ e γωi Using this simple model, we analyzed several nanoaggregates in each of the samples (A, B, C, and D) and measured the optical trapping forces with the increase of aggregation. In Figure 3a we show the decay rates ωx, ωy, and ωz averaged on 10 different aggregates for each sample as a function of the plasmon resonance peak wavelength. The increased decay rates for the larger aggregates is an indication of a larger trapping force. By using the average size obtained from TEM
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Figure 3. (a) Logarithmic plot of the relaxation rates in the x (squares), y (circles), z (triangles) directions of the optical trap as a function of the peak wavelength of the plasmon resonance. The dotted line indicates the trapping wavelength. (b) Logarithmic plot of the relaxation rates as a function of a reduced parameter (λ - λp)/λp related to the distance of the trapping wavelength from the resonance wavelength. This is evidence of a frequency scaling of the optical trapping force. The dashed line is a guide to the eye and represents a x-1 scaling law. The uncertainty on these measurements ranges between 15 and 45% and are mainly due to the size and shape distribution in each sample.
images, we can estimate the average viscous damping γ for each sample and hence obtain a value of the optical force constants normalized to laser power in the range between 0.5 and 10 pN/nmW. In particular, for aggregates in sample D with an average size of about 750 nm, we measured force constants of 10 pN/nmW, a value 50 times larger than the maximum value reported in experiments with the largest (127 nm radius) gold spherical particles.28 The large increase in the correlation decay rates and measured force constants with the growth of the gold aggregates can be understood first of all, with the trapped particle volume increases leading to a larger force with respect to the individual spherical subunits.28,31,32 Another important effect is that the aggregation yields a long wavelength plasmon resonance that shifts closer to the trapping wavelength. This plasmon resonance drives the trapping mechanism, enhancing further the optical trapping efficiency. Evidence for the role played by this shift of the resonance can be visualized by plotting the decay rates as a function of a scaled parameter (λ - λp)/λp in Figure 3b. This represents a scaled distance between the trapping wavelength and the resonance wavelength. The inverse dependence of the decay rates with this parameter is clear evidence that the closer the resonance, the larger the optical trapping forces, provided that radiation pressure and absorption are kept sufficiently low to ensure stable three-dimensional trapping. Raman Tweezers and SERS. The field-enhancement properties of metal nanoaggregates make them suited for SERS studies. In particular, exciting at wavelengths red-shifted with respect to the plasmon resonance of the aggregates, both efficient trapping and excitation of SERS signals are simultaneously obtained. As a proof of principle experiment, we have performed Raman 5118
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Figure 4. (a) Simplified sketch of the Raman tweezing. Optical trapping and Raman spectroscopy are perfomed with the same laser source (785 nm). The Raman signal from the molecules adsorbed on the trapped aggregate is collected through the same objective used for trapping. (b) SERS spectrum on a trapped aggregate from sample C. Integration time 20 s, laser power 3.5 mW. The asterisks highlight the enhanced pyridine vibrational peaks, while the BSA Raman modes are indicated in blue.
tweezing for the sample C using a 785 nm laser source for both trapping and SERS excitation (Figure 4a). In Figure 4b is shown the SERS spectrum of the molecules adsorbed on the surface of an optically trapped metal nanoaggregates, located within their “hot spots”. We can clearly distinguish the enhanced pyridine ring breathing mode45-47 at 1010 cm-1 and its ring trigonal-deformation mode45-47 at 1038 cm-1. We can also appreciate an enhancement of the CdC stretching modes around 1600 cm-1. These can be attributed to both pyridine and the exposed aromatic residues of BSA (phenylalanine, tryptophan, and tyrosine) that are the amino acids more involved in SERS phenomenon due to the weak electrostatic mutual interaction between the benzene ring of both molecules.45-49 In the 500-550 cm-1 region we find the presence of enhanced Cβ-S-S-Cβ disulfide bridges spectral features of BSA.48,49 Moreover, we recognize the other characteristic bands of the protein,48,49 Amide I and Amide III, around 1645 cm-1 and in the range between 1240 and 1275 cm-1, respectively, even if slightly changed in Raman shifts and relative intensities, probably because of the conformational changes of BSA when it is adsorbed on the surface of gold nanoparticles.50
’ THEORY The basic principle behind optical tweezers is the momentum transfer associated with the scattering of light by a particle. Indeed, light carries momentum; hence when an object scatters light, changing the light propagation direction, momentum conservation requires that the object must undergo an equal and opposite momentum change. This gives rise to a force acting on the object. In the past decade, the theoretical models used to calculate optical forces have often been based on strong approximations that limited their applications to spherical particles only. On the other hand, electromagnetic scattering theory51 is crucial for a correct modeling of radiation forces on metal nanoparticles that have a high imaginary part of their dielectric constants. In order to calculate the radiation force on these systems, we use the full scattering theory in the framework of the transition matrix (T-matrix) approach.51 This approach applies to particles of any shape and refractive index for any choice of the trapping wavelength.21,32,52 The starting point of our procedure is the formulation by Richards and Wolf53 of the field configuration in the focal region of a high numerical aperture objective in absence of any particle.
The resulting field is the incident field on the particle, and the radiation force and torque within the focal region are calculated by resorting to linear and angular momentum conservation for the combined system of field and particle.21,32,52 Thus the optical force and torque exerted on the particle turn out to be21,32,52 Z FRad ¼ r 2 ^r 3 ÆTM æ dΩ ð2Þ Ω
Z MRad ¼ - r 3 ^r 3 ÆTM æ ^r dΩ Ω
ð3Þ
where the integration is over the full solid angle, r is the radius of a large sphere surrounding the particle center of mass, and ÆTMæ is the time averaged Maxwell stress in the form of Minkowski.54 The fields used to calculate this tensor are the superposition of the incident and scattered field by the particles. Both these fields are expanded in a series of vector spherical multipole fields, in terms of whose amplitudes all the quantities of interest are expressed.51,52 The multipole amplitudes of the scattered field are calculated from those of the incident field through the transition matrix approach that applies to particles of any shape and resorts to the only approximation of truncating the series after a number of terms sufficient to ensure fair convergence of all the quantities of interest.51,52 Each element of the T-matrix is independent both on the direction of propagation and on the incident polarization. Thus they do not change when the incident field is a superposition of plane waves with different direction of propagation, i.e., for the description of a focused laser beam in the angular spectrum representation.53 We modeled gold nanoaggregates as clusters of spherical subunits (see Figure 5), whose dimensions are chosen so that the wavelengh of their longitudinal plasmon resonance corresponds with the experimental one. The TEM images suggest that the subunits have different radii, ranging from 5 up to 30 nm; thus we adopted this range of dimensions as a guide for our models. We used the optical constants for gold measured by Johnson and Christy.55 The use of such constants in the modeling of aggregates poses several problems, due to the lack of convergency in the multipole expansion.51 For this reason, we introduced the longitudinal dielectric function, described by Lindhard,56 as reported and simplified by Pack.57 We investigated the optical 5119
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Figure 5. Modeling of the extinction properties of gold nanoaggregates with increasing size. We consider a model cluster with a fixed geometric configuration shown in (a). The long wavelength plasmonic resonance, arising from the aggregation of different subunits, red shifts with the increase of the aggregate size. In (b-d) we plot the calculated extinction cross section for such a model cluster (blue solid line) and compare it with the one of the largest subunits (dashed green), the gold sphere with equivolume (dotted red), and the smallest gold sphere that contains the cluster (dot-dash cyan). In (b) the five subunits are one sphere with a radius of 10 nm, two spheres with a radius of 7.5 nm, and two spheres with a 5 nm radius. In (c) the subunits have double size with respect to (b). In (d) the subunits have four times the size than in (b).
properties of several gold aggregates, built to satisfy the requirements described above. As an example we show in Figure 5 the extinction properties of aggregates with increasing size. We kept the overall aggregate geometrical configuration fixed (Figure 5a) and changed the dimensions of the spherical subunits to build clusters with a progressively growing overall size. Each cluster is composed of five spheres. In Figure 5b we consider a cluster made of one sphere with a radius of 10 nm, two spheres with a radius of 7.5 nm, and two spheres with a 5 nm radius. For comparison we consider also the smallest sphere containing the whole aggregate that has a radius rS = 22.5 nm, and the equivalent volume sphere that has a radius rV = 12.8 nm. In Figure 5c we double the size of the cluster (this corresponds to the sample B case), i.e., one sphere with a radius of 20 nm, two spheres with a radius of 15 nm, and two spheres with a 10 nm radius. The smallest sphere containing the cluster has a radius rS = 45 nm. The equivalent volume sphere has a radius rV = 25.6 nm. In Figure 5d the cluster is made of one sphere with a radius of 40 nm, two spheres with a radius of 30 nm, and two spheres with a 20 nm radius. The smallest sphere containing the cluster has a radius rS = 90 nm. The equivalent volume sphere has a radius rV = 51.2 nm. We calculate for each cluster the radiation force Frad(R) and torque Mrad(R), the argument R denoting the position of the center of mass of the aggregate. The trapping occurs on the optical axis where all the components of force and torque vanish with a negative derivative. In the vicinity of the trapping point R0 = (0,0,z0) the components of Frad(R) can be approximated by Fradx(x, 0, z0) = -kxx, Frady(0, y, z0) = -kyy, Fradz(0, 0, z) = -kz (z - z0). Where kx, ky, and kz are the force constants. Thus we first determine the trapping position and equilibrium orientation of the cluster and then calculate the force constants around this equilibrium state. For clusters in the size range close to the ones trapped in our experiments, we calculate force constants (normalized to power) in the range 0.1-1 pN/nmW that is consistent with what we measure experimentally.
’ CONCLUSION In conclusion, we have shown that gold nanoaggregates produced by pulsed laser ablation in liquids can be carefully controlled in size and properties and can be trapped with optical tweezers working in the near-infrared region. Light-driven rotations were observed for the larger aggregates that can be accounted by the particle shape
anisotropy. We have measured the optical trapping forces exerted on them and evidenced how they obey a frequency scaling dependence related to the detuning of the trapping laser from the long wavelength plasmon peak. The possibility to use a single laser source both to efficiently trap and excite the “hot spot” regions of the gold nanoaggregates allowed surface-enhanced Raman tweezing to be performed in a liquid environment.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT M.A.I., B.F., C.D’A., P.G.G., and O.M.M. acknowledge funding from the European Union Seventh Framework Programme (FP7/ 2007-2013) under grant agreement no 241818 (FP7-HEALTH-F52009-241818-NANOANTENNA). ’ REFERENCES (1) Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Surface plasmon subwavelength optics. Nature 2003, 424, 824–830. (2) Cao, Y. C.; Jin., R. C.; Mirkin, C. A. Nanoparticles with Raman spectroscopic fingerprints for DNA and RNA detection. Science 2002, 297, 1536–1540. (3) Lin, C. J.; Yang, T.; Lee, C.; Huang, S. H.; Sperling, R. A.; Zanella, M.; Li, J. K.; Shen, J.; Wang, H.; Yeh, H.; Parak, W. J.; Chang, W. H. Synthesis, characterization, and bioconjugation of fluorescent gold nanoclusters toward biological labeling applications. ACS Nano 2009, 3, 395–401. (4) Walter, J. G.; Petersen, S.; Stahl, F.; Scheper, T.; Barcikowski, S. Laser ablation based one-step generation and bio-functionalitation of gold nanoparticles conjugated with aptamers. J. Nanobiotechnol. 2010, 8, 21. (5) Huang, X.; Jain, P. K.; El-Sayed, I. H.; El- Sayed, M. A. Gold nanoparticles: interesting optical properties and recent applications in cancer diagnostics and therapy. Nanomedicine 2007, 2, 681–693. (6) Mafune, F.; Kohno, J.; Takeda, T.; Kondow, T. Full physical preparation of size-selected gold nanoparticles in solution: laser ablation and laser-induced size control. J. Phys. Chem. B 2002, 106, 7575–7577. (7) Messina, E.; Compagnini, G.; D’Urso, L.; Puglisi, O.; Bagiante, S.; Scalese, S. Size distribution and particle shape in silver colloids prepared by laser ablation in water. Radiat. Eff. Defects Solids 2010, 165, 579–583. 5120
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