Optical Trapping of Plasmonic Mesocapsules: Enhanced Optical

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Optical Trapping of Plasmonic Mesocapsules: Enhanced Optical Forces and SERS Donatella Spadaro, Maria Antonia Iatì, Javier Perez-Pineiro, Carmen Vázquez-Vázquez, Miguel A. Correa-Duarte, Maria Grazia Donato, Pietro Giuseppe Gucciardi, Rosalba Saija, Giuseppe Strangi, and Onofrio M Marago J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10213 • Publication Date (Web): 10 Dec 2016 Downloaded from http://pubs.acs.org on December 11, 2016

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The Journal of Physical Chemistry

Optical Trapping of Plasmonic Mesocapsules: Enhanced Optical Forces and SERS D. Spadaro,†,# M. A. Iat´ı,†,# J. Pérez-Piñeiro,‡ C. Vázquez-Vázquez,‡ M. A. Correa-Duarte,‡ M. G. Donato,∗,† P. G. Gucciardi,† R. Saija,¶ G. Strangi,∗,§,k,⊥ and O. M. Maragò∗,† †CNR-IPCF, Istituto per i Processi Chimico-Fisici, I-98158, Messina, Italy ‡Department of Physical Chemistry, Biomedical Research Center (CINBIO), and Southern Galicia Institute of Health Research (IISGS), Universidade de Vigo, 36310 Vigo, Spain ¶Dipartimento di Scienze Matematiche e Informatiche, Scienze Fisiche e Scienze della Terra, Universitá di Messina, I-98166, Messina, Italy §Department of Physics, Case Western Reserve University, 44106-7079 Cleveland, USA kCNR Nanotec and University of Calabria, Rende, Italy ⊥IIT Istitituto Italiano di Tecnologia, Genova, Italy #Contributed equally to the work. E-mail: [email protected]; [email protected]; [email protected] Phone: +39 090 39762249 Abstract We demonstrate optical trapping of plasmonic silica-gold mesocapsules and their use as local SERS probes in Raman tweezers. These novel hybrid dielectric-metal particles, designed for optoplasmonic applications, are mesoscopic porous silica shells embedding gold nanospheres in their inner wall. We observe a high trapping efficiency due to plasmon-enhanced optical trapping of the gold component. Furthermore, we

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develop an accurate model of optical trapping of this hybrid system in the T-matrix framework studying how the plasmon-enhanced optical forces scale with gold nanoparticle number in the mesocapsule. The relevance of effective optical trapping in hollow plasmonic mesocapsules is twofold for detection and delivery technologies: Positioning and activation processes. In fact, the presented system allows for the opportunity to drag and locate cargo mesocapsules embedded with specific molecules that can be activated and released in-situ when a precise localization is required.


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Introduction Optical tweezers 1,2 (OT) is a key technique for the contactless manipulation of micro 3,4 and nano-sized particles. 5,6 It is based on the conservation of the electromagnetic momentum in the light-matter interaction 2 and it allows the detection and transduction of small (femtonewton) forces 7–12 and (tens of femtonewton nanometer) torques 13–15 enabling investigations in a large variety of research fields from biology, 4 physical chemistry, 16–20 and nanotechnology, 5,6 to the limits of quantum mechanics. 21 In particular, in physical chemistry OT have been used for trapping and analysis of aerosols, 16,17 optical trapping of aminoacids 18 and myoglobin, 19 as well as for applications such as single-molecule fluorescence spectroscopy, 20 imaging and absorption spectroscopy of individual gold nanoparticles, 22,23 microrheology of ionic liquids, 24,25 self-assembly of pseudoisocyanine J-aggregates, 26 optical printing 27–30 and controlled aggregation 31 of metal nanoparticles. In OT, the particles are spatially confined by optical forces in proximity of the high intensity spot of a focused laser beam if their refractive index is greater than that of the surrounding medium. 2 On the contrary, low-refractive index particles (e.g., microbubbles) are repelled from a standard Gaussian beam OT and different strategies need to be employed based on doughnut-shaped beams 32,33 or time-averaged potentials. 34 More generally the optomechanical strength is roughly related with the interaction volume, so that a thin shell or capsule has a very small optomechanical interaction compared to a solid particle. Hence, strategies to increase optical trapping forces and manipulation of capsules are key to advance in the development of multifunctional single-particle observations, where particles not only can be trapped and directed by light, but also can be individually activated through an additional stimulus for sensing, 35 catalytic 36 or drug-delivery 37,38 purposes. Additionally nano/micro-swimmers that could be guided and activated by light, 39 including drug-delivery functions will benefit from these advances. In recent years, great attention has been devoted to the trapping and manipulation of colloidal plasmonic nanoparticles. 40–47 Increased mechanical effects of light on metal nanopar3 ACS Paragon Plus Environment


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ticles with respect to dielectric particles of equivalent size are related to the occurrence of their optoplasmonics response typically in the visible or near-infrared (NIR) range. 40,41,47,48 Indeed, at the nanoscale optical trapping (gradient) forces are typically very weak as they scale with the real part of the dipolar polarizability and ultimately with particle volume. 5,6 This volume scaling is detrimental for optical trapping of typical dielectric nanomaterials against the destabilizing effects of thermal fluctuations. 2 However, the optoplasmonic response of metal nanoparticles has been exploited to increase optical trapping forces so that their stable confinement is achieved in the NIR at lower power with respect to dielectric ones. 40,41,48 Far from plasmon resonances peaks, the optical response of metal nanoparticles is (mainly) the one of the free-electron plasma, yielding a large NIR polarizability, 2 and hence a large (generally ten times larger) optical trapping force when compared to dielectric particles of comparable size. 40,41,47,48 The occurrence of plasmon resonances in metal nanoparticles is also responsible for an increase in the extinction cross-section resulting in an increase in radiation pressure. 2,5 Indeed, when light is tuned in proximity of a plasmon resonance peak, optical trapping is unstable because radiation pressure is much larger than the optical trapping (gradient) force and particles are pushed away from the trap. Thus, for stable trapping to occur one must tune the laser wavelength on the red side (generally in the NIR) of the plasmon resonance peaks, so that we can exploit the dispersive nature of the optical trapping force. 5,6,47 Gold nanoparticles as small as 10 nm in size 49 have been stably trapped in OT. However, as the particle size increases, scattering forces overwhelm the confining forces and particles are expelled from the trap limiting the size range for stable optical trapping. 41,45,46,48 Furthermore, the resonant plasmonic response of metal nanoparticles is responsible for their wavelength dependent optomechanical behaviour, so that for light nearly-resonant with the nanoparticle plasmonic response (typically in the visible spectral range) optical forces can be used to push them for controlled patterning, 27,28,30 sorting, 50 or surface-enhanced Raman spectroscopy 31 (SERS). Nearly-resonant or high power illumination of single plasmonic nanoparticles may yield significant heating effects in optical traps


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due to light absorption. 43,51–53 While on one side heating effects could be detrimental to force spectroscopy in biological systems, 51 they can be exploited to trigger the thermal release of encapsulated molecules of interest 52 or the controlled formation of micro-bubbles. 43 Several hybrid structures with two or more components have been proposed as novel colloidal materials due to their additional functionalities with respect to simple plasmonic nanoparticles. 54–60 The possibility to change the shape and the composition of the hybrid particles allows the control of their plasmonic response as well as their aggregation or compatibility with a solvent, their strength to corrosion or their magnetic or fluorescent properties. 61,62 Particularly interesting is the case of multifunctional inorganic hybrid capsules because of the positioning of plasmonic nanoparticles in the inner wall allowing to create a plasmonic environment within the hollow capsule by enhancing the potential of these functional materials. 63,64 The spatial distribution of noble metal nano-spheres gives rise to localized and collective plasmonic modes within the capsules that results in broadband plasmon fields that are harnessed for both enhanced optical trapping and metal enhanced spectroscopies. The field arising from the coupling and interplay of metal particles can be resembled to molecular hybridization processes, where the resulting field retains properties of all the interacting elements. 65,66 In addition, these convoluted plasmonic modes occupy the hollow section of the capsules producing plasmonic hot-spots in close proximity to the nanoparticle constellation. Moving away from the inner walls a fast gradient field take places leading to a much less intense plasmon field at the center of the cavity. This intense plasmon field gradient is at the basis of catalytic reaction control, 64,67,68 metal enhancement effects 55,60 and scattering field distributions 62,63 for effective optical trapping. Here, we study the optical trapping of a novel type of hybrid dielectric-metal particle designed for optoplasmonic applications, i. e., reverse bumpy-ball architectures composed of mesoporous silica shells embedding gold nanospheres. 69 We measure the optical trapping efficiency for the plasmonic mesocapsules and compare it with that of a solid latex particle of similar size and of an individual gold nanoparticle of size close to the gold nanospheres



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embedded in the mesocapsule inner shell. The hybrid mesocapsule shows a trapping efficiency much higher than that of the individual gold nanoparticle due to gold clusterization inside the mesoporous shell. Moreover, we find comparable trapping efficiency to that of the solid microparticle despite the much (about 20 times) smaller interaction volume of the hollow plasmonic mesocapsule. This large trapping efficiency for the mesocapsule is due to the plasmon-enhanced optical forces acting on its hybridized shell. Experimental results are supported by accurate theoretical light scattering calculations in the T-matrix framework that show how optical forces on a gold shell-cluster increase with the square of the gold nanoparticle number on the shell. Additionally, as a proof of concept that further extends the multifunctionality of these hybrid materials, we show how the plasmonic characteristics of such particles enable their use as single-particle probes for surface enhanced Raman spectroscopy (SERS) of molecules dispersed in the liquid environment.

Materials and Methods Plasmonic mesocapsules are synthesized using polystyrene (PS) beads ∼1400 nm as templates. These PS beads are firstly functionalized by a sequent deposition of a positive poly (allylamine hydrochloride, PAH) and negative poly (sodium styrenesulfonate, PSS) polyelectrolytes, following the layer-by-layer assembly technique. The deposition of a last layer of PAH provides the surface of the PS beads with a homogenous positive charge, which ensures the later adsorption of the gold seeds. Gold seeds (1-3 nm [Au]=10−4 M) are produced as described by Duff et al. 70 Thus, 6 mL of Au seeds solution are added to 50 mL of previously functionalized PS beads (0.25 mg/mL) under sonication. Then, the solution is centrifuged (4000 rpm, 20 min) and redispersed with milli-Q water three times, to remove the excess of Au seeds not adsorbed on the PS surface. The concentration of PS beads is adjusted to 1.25 mg mL−1 . Mesoporous silica coating is obtained following the method described by Vázquez-Vázquez et al. 71 Briefly, the previous solution of PS@Au seeds is added dropwise


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and under sonication to a mixed solution of cetyl trimethylammonium bromide (CTAB) (100 mg), deionized water (40 mL), ethanol (30 mL), ammonia aqueous solution (28 wt %,365 µL). The resulting solution is homogenized by sonication for 20 min. Then, 1 mL of a 5% (v/v) solution tetraethoxysilanesoution (TEOS) in ethanol is added drop by drop under sonication. The mixture is stirred for 2 days in order to have a homogenous silica growth. Then it is centrifuged and washed with ethanol three times. Finally, the PS templates are removed by calcination at 500 ◦ C for 10 h. Gold seeds in the inner of the capsule are grown using a solution of Au+ and formaldehyde as reducing agent. The Au+ solution is obtained mixing 459.3 µL of HAuCl4 0.1138 M and 120 mL of K2 CO3 1.8 mM for 1 h. Then, 5 mL of this Au+ solution and 30 µL of formaldehyde solution (37 wt %) are added to 2 mL Au-seeds@SiO2 mesoporous (1.25 mg mL−1 ) under vigorous stirring. After 10 min of reaction, the color changed from pink to purple-blue. Finally, the sample is centrifuged (4500 rpm, 15 min) and washed with ethanol. The experimental setups for optical trapping in the NIR (830 nm and 785 nm) are based on inverted microscopes 43,72 (see Supporting Information). In a first setup the light from a NIR laser diode at 830 nm is focused in a sample chamber (80 µL) by a high numerical aperture (NA =1.3) oil immersion objective. The available maximum power at the sample is about 26 mW. Note that at our near-infrared trapping wavelengths the absorption of ethanol or water is negligible, 2,73 and thus, related heating effects of the surrounding medium can be ignored. We use a CCD camera to image the trapped particles. Particle tracking and force sensing are obtained through back focal plane interferometry, where the interference pattern from the unscattered and scattered light by the trapped particle is collected onto a quadrant photodiode (QPD). After a suitable calibration, QPD voltage signals give direct information on the trapped particle position. 74 Additionally, Raman tweezers experiments at 785 nm are carried out in a different homebuilt setup (see Supporting Information). The beam of a diode laser, delivering about 7 mW at the sample, serves both the purposes of optical trapping and Raman excitation. It



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is expanded by a telescope to overfill the microscope objective (100x oil, NA=1.3), then it is reflected by a mirror toward a notch filter (Semrock NF03-785E-25) which, again, reflects the laser beam toward the objective lens. The sample chamber is mounted on a piezostage (Physics Instruments, P-517.3 CL) with 1 nm resolution. The backscattered light passes through the notch filter, used for Rayleigh scattering removal, and is subsequently focused by a 50 mm lens onto a Horiba Jobin-Yvon Triax 190 spectrometer (190 mm focal length) equipped with a 1200 gg/mm grating blazed at 650 nm. A high sensitivity CCD (Horiba Jobin-Yvon Synapse) is used for the signal detection. A beam splitter is inserted in the optical path to reflect 50% of the scattered light toward a CCD camera (Thorlabs USB 2.0, DCU223M), allowing for visual inspection of the trapped particle. Optical forces on particles stem from the conservation of linear momentum upon light scattering. 2 We calculate them by solving the light scattering problem in the T-matrix formalism for core-shell or cluster model particles illuminated by the tightly focused fields creating the optical tweezers (see Supporting Information). First, the optical fields in the focus of the high NA objective lens are calculated by means of the angular spectrum representation 75,76 in the absence of any particle. The resulting field is the field incident on the particles, and the radiation force exerted on any particle within the focal region is calculated by integrating the time-averaged Maxwell stress tensor in the far field: 76

Frad = r

2

I TM · ˆr dΩ ,

(1)

Ω

where the integration is over the full solid angle, r is the radius of a large sphere surrounding the particle, and TM is the time-averaged Maxwell stress tensor in the Minkowski form in a homogeneous, linear and non-dispersive medium. 2 Thus, the radiation force is expressed in terms of incident and scattered fields that can be expanded in vector spherical harmonics regular at the origin (Bessel multipoles) or at infinity (Hankel multipoles), respectively. 2 The linear relation between the scattered expansion amplitudes and the incident ones defines the


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T-matrix for the scattering process. 77 Optical force components are calculated by considering the projections of the radiation force vector on the different coordinate axes, e. g., Fx = ˆ , and calculating the corresponding numerical integral. 76 Optical forces on shell model Frad · x particles (silica or silica-gold shells) are obtained using the Wyatt generalization of Mie theory for radially symmetric spheres 58,78,79 (see Supporting Information). While for the plasmonic shell-cluster model we use the T-matrix formulation of light scattering by a cluster of spheres 43,77,80 (see Supporting Information) where the fields scattered by the spherical subunits composing the aggregate are combined by using the addition theorem of multipole fields through a transfer matrix. 2

Results and Discussion Plasmonic mesocapsules. The plasmonic mesocapsules studied in our experiments have a hybrid structure that extends itself on two different length scales, that is, they are constituted by a microscopic porous hollow silica shell with 1.4 µm diameter and 30 nm thickness embedding plasmonic Au nanospheres. These mesocapsules are hollow and permeable and Au nanoparticles are grafted on their inner walls. Figures 1a,c show TEM images at different magnifications of the as-prepared plasmonic capsules after calcination, where individual Au nanoparticles are clearly distinguished in their inner cavity. According to the statistical distribution analysis (Fig. 1d) the size of the Au nanoparticles is around 11 nm. The increase of the particles size, starting from the initial 2-3 nm Au particles, is achieved via a thermal process used also to remove the polymeric template. Hence, the large amount of small nanoparticles deposited onto the polystyrene template during the first step, ensures the close vicinity of the nanoparticles necessary for the occurrence of collective plasmonic properties as a consequence of the thermal growth. Dispersions of 2-3 nm Au nanoparticles do not show localized surface plasmon resonances (LSPR), whereas plasmonic features appear only for larger particles (Fig. 1b) as shown in our final structure.



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Figure 1: (a) TEM image of a 1.4 µm plasmonic mesocapsule. (b) Extinction spectrum of plasmonic mesocapsules dissolved in solution and sketch of the mesocapsule structure (inset). (c) TEM image close-up of a mesocapsule evidencing the Au nanoparticle size distribution, whose histogram is shown in (d), on the inner side of the mesocapsules. The averaged size of the Au nanoparticle is about 11 nm. (e,f) CCD images (see also Supporting Information Video) of a mesocapsule when trapped in the optical tweezers (e) and when released from the trap (f). Optical trapping. Mesocapsules dispersed in ethanol have been used for optical trapping experiments in the NIR at 830 nm with 26 mW trapping power (see also Supporting Information). The starting point for the measurement of optical forces is thermal noise analysis. 2,74,81 The three-dimensional motion of the optically trapped mesocapsule is well described by the Langevin equation in the overdamped regime: 82 √ ki dxi (t) = − xi + 2DWi (t), dt γ

i = x, y, z

(2)

where xi (t) is the mesocapsule displacement in the i-th direction, ki is the trap stiffness, γ = 6πηR is the friction coefficient for a spherical particle of radius R with η being the medium (ethanol) dynamical viscosity, D = kB T /γ is the Stokes-Einstein diffusion coefficient, and Wi (t) is a white noise having hWi (t)i = 0, hWi2 (t)i = 1 for each value t, and Wi (t) independent of Wi (t + τ ) for τ 6= 0. 2 Equation 2 gives rise to first-order differential equations for the autocorrelation functions (ACFs) of the particle’s displace-


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ments, Cii (τ ) = hxi (t)xi (t + τ )i, that lead to simple exponential decays for the ACFs, Cii (τ ) = Cii (0) exp(−ki τ /γ), i. e., the autocorrelation function of the particle fluctuations decays in time with the relaxation rate ωi = ki /γ. Thus, for an optically trapped spherical mesocapsule a single-exponential fit to the displacement ACFs allows to measure the trap spring constants ki , since η = 1.144 mPa·s (for ethanol at 20 ◦ C) and R = 700 nm are known.

Figure 2: Optical force calibration for optoplasmonic mesocapsules. (a) Measured tracking signals in the radial (magenta) and axial (blue) directions of the trap, representing the positional thermal fluctuations of a trapped mesocapsule. (b) Corresponding histograms of the mesocapsule position related to the probability distribution in the confining potential. (c) Autocorrelation function analysis obtained from the positional fluctuations in (a). Their decay rates are related to the spring constants of the trap and serve for the full force calibration. (d) Reconstruction of the effective optical confining potential in the radial (magenta) and axial (blue) directions. In Figure 2, we show the statistical analysis of the tracking signal (Fig. 2a) that points out, as usually observed with ordinary Gaussian beams, 74 how the trapped particles are more confined in the radial (x or y) than in the axial (z) direction (Fig. 2b-d), i.e., the width of the particle position probability distribution ρ(xi ) (solid line in Figure 2b) in the radial direction is smaller than in the axial direction (Fig. 2b). The single-exponential fit of the 11 ACS Paragon Plus Environment


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autocorrelation functions of the particle position (Fig. 2c) gives higher relaxation rates in the radial than in the axial directions. We trapped ten different mesocapsules at a depth of about 20 µm for which we collected three different QPD tracking signals. The corresponding averaged force constants from all the measurements are normalized to the power at the sample. Thus, we obtain the values kx /P = 0.9 ± 0.4 pN/(nm W), ky /P = 1.0 ± 0.5 pN/(nm W), and kz /P = 0.5±0.2 pN/(nm W). Finally, by exploiting the particle position probability distribution (fig. 2b), the optical potential U (xi ) = −kb T log[ρ(xi )] + U0 can be calculated (fig. 2d). The optical trap is well approximated by a harmonic potential and it is steeper in the radial than in the axial direction.

Figure 3: Reconstruction of Brownian motion for different type of spherical particles in the optical trap for similar experimental conditions: (a) optoplasmonic mesocapsule trapped in ethanol, R = 0.7 µm, (b) solid latex particle trapped in water, R = 1 µm, (c) individual gold nanoparticle trapped in water, R = 10 nm. Despite the hollow optoplasmonic mesocapsule has an interaction volume much smaller than the solid latex bead, it is optically confined with a similar strength due to the hybridization of its silica shell with plasmonic nanoparticles.

The calibration of the positional fluctuations and of the effective optical trapping potential enables the accurate reconstruction of the Brownian dynamics in the OT and their use for force sensing with femtonewton resolution. 7,11,74 In Fig. 3 we compare the Brownian motion of an optically trapped plasmonic mesocapsule (Fig. 3a) to that of a latex bead (Fig. 3b) with radius (R = 1 µm) close to that of the mesocapsule and to that of an individual spherical AuNP (Fig. 3c) with a radius (R = 10 nm) close to that of the AuNPs embedded in the plasmonic mesocapsule. This comparison shows visually how plasmonic 12 ACS Paragon Plus Environment

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mesocapsules are optically confined with a strength close to that of solid latex beads despite their interaction volume is reduced to their hybridized shell. On the contrary, individual bare AuNPs are much less confined and difficult to trap showing how hybridization of the silica shell with AuNPs plays the key role in increasing optical trapping efficiency in the plasmonic mesocapsule. Table 1: Optical trapping efficiencies for different types of particles: plasmonic mesocapsules, latex beads, and gold nanoparticles. The efficiency values are averaged over the spring constant measurements from ten different particles. Uncertainties represent the standard deviation from the mean values. efficiency (µm−1 ) qx qy qz

mesocapsules 0.7 µm 0.20 ± 0.08 0.2 ± 0.1 0.10 ± 0.05

latex beads 1 µm 0.55 ± 0.03 0.56 ± 0.04 0.11 ± 0.01

AuNP 10 nm (1.0 ± 0.3) ×10−2 (1.1 ± 0.3) ×10−2 (0.9 ± 0.5) ×10−2

For a direct quantitative comparison of optical forces on different particles it is often useful to divide optical forces by nm P/c, with nm being the refractive index of the surrounding medium (ethanol or water), c the velocity of light in vacuum, and P the laser power at the sample. This enables to normalize for the effects of different trapping power and surrounding medium refractive index. 2 Thus, from our trapping measurements we calculate the optical trapping efficiencies in terms of reduced spring constants, qi = cki /nm P . Table 1 compares the measured reduced spring constants, qi , calculated for the three samples shown in figure 3. The best trapping is obtained with latex beads, but also silica-Au mesocapsules have quite good efficiencies, considering that they are smaller and also hollow, and they are almost two orders of magnitude better confined than individual AuNPs, that have very small trapping efficiencies due to the volumetric scaling of optical forces at the nanoscale. 5,41,48 In our hybrid mesocapsule, the presence and aggregation of the metal nanoparticles in the inner (30 nm) shell yield an effective increase of the optical trapping force of the whole structure. Moreover, we find comparable trapping efficiency to that of a solid microparticle despite the much smaller (about 20 times) interaction volume of the hollow plasmonic meso-



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capsule. This observed large trapping efficiency for the hollow plasmonic mesocapsule is due to the plasmon-enhanced optical forces acting on the hybridized mesocapsule shell. Finally, we note that heating effects of the plasmonic samples in the trap are negligible as they generally occur at higher power 51,52 (above 50 mW) or for wavelengths resonant with the particle plasmonic response so that enough absorption is converted into heat. 43,53 We are far from both conditions. However, these are intriguing aspects as plasmonic mesocapsules can act as thermally activated cargos to release molecules chemically attached to the gold component. Further studies in a double-wavelength configuration, 20 where a NIR beam traps the mesocapsule and a visible resonant beam thermally excite the structure, can gain insights on this perspective.

Theory. To understand the experimental results, we calculate the optical trapping forces by solving the electromagnetic scattering problem in the T-matrix approach 48,76,77 for tightly focused fields 2,75,76 (see Supporting Information). While hollow spheres, with a inner index of refraction lower than the surrounding medium, cannot be trapped 33,34 in an ordinary Gaussian beam, our silica shells are permeable to ethanol and, thus, internal and external refractive indexes are the same (nm = 1.36). Moreover, the silica shells embed small Au nanoparticles on the inner silica shell wall with a random spatial distribution. To understand the role played by both the silica shell and the distribution of AuNPs in the optical trapping mechanism of the hybrid mesocapsule, we calculate the scattering process and optical forces for several model particles and compare them with those obtained for a solid latex bead and gold nanoparticles with 6 nm radius equal to the average radius of the AuNPs of the hybrid mesocapsule. In Figure 4 the optical force efficiencies, Qi = cFi /nm P (i = x, y, z), calculated for several models are presented. Optical trapping occurs when all Qi vanish with a negative derivative. 48,76 As expected, both a 1 µm latex bead (Fig. 4a) and an individual AuNPs (4b) can be trapped with different efficiencies dictated by size and material properties. Interestingly, also a 30 nm thick permeable silica shell (4c) without AuNPs inclusions can


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be trapped in two equilibrium positions, even if the trapping efficiency is about two order of magnitude lower than that of a solid latex bead (Fig. 4a). However, if we use a model (Fig. 4d) consisting in an outer silica shell and an inner homogeneous Au shell with 12 nm thickness (corresponding to the hybridizing AuNPs diameter), no trapping is obtained as scattering forces are much stronger than trapping ones because of the gold shell large extinction at our trapping wavelength (830 nm). Thus, we calculate optical forces for a spherical shell cluster (Fig. 4e) of gold particles with 6 nm radius containing an increasing number, N , of AuNPs in a regular configuration, shell-cluster model (see also Supporting Information). By increasing the number of AuNPs we observe a cross-over in the optical trapping behaviour for about 50 particles in the shell-cluster (Fig. 4f). When the particle number is too small (low number density) the optical trapping strength is related to the individual AuNP trapping that is the distance between AuNPs is so large that the interaction between the high intensity region of the OT occurs only with one particle. However, when the particle number is above 50 the increase in optical trapping follows a N 2 scaling law (dashed line in Fig. 4f). In fact, in our shell-cluster calculations the interparticle distance considered (> 80 nm) is always much larger than the particle diameter (12 nm) and a weak plasmonic coupling and small red shift occur. Thus, the increase of optical trapping is consistent with the particle number density increase on the shell within the focal interaction region. Clustering at large number density can make the plasmon band to red shift when plasmonic coupling occurs for short (roughly the particle size) interparticle distance. This can also contribute to enhance optical trapping forces in the NIR. 43

Spectroscopic applications. Finally, as a proof-of-concept we explored the use of optically trapped plasmonic mesocapsules for SERS applications. OT are tools that works intrinsically in liquid with the capability to explore soft surroundings in three-dimensions with nanoscopic precision. 83 They offer the opportunity to spectroscopically explore region of interest on the surface or even inside biological samples. 84,85 Thus, having the opportu-



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Figure 4: Optical trapping efficiencies, Qx (blue lines), Qy (green lines), Qz (red lines), calculated through the T-matrix approach for different type of model particles and cluster: (a) a latex bead (R = 1 µm), (b) an individual AuNP (R = 6 nm) similar to the particles hybridizing the mesocapsules, (c) a 30 nm thick permeable silica shell, (d) an hybrid silicagold double shell, and (e) a shell-cluster composed of 186 AuNPs distributed over a spherical surface equal to the mesocapsule one. Note how the model shell particles in (c) and (e) have two stable axial trapping positions (red dots) corresponding to their maximum overlap with the high intensity spot of the OT. Instead when a full silica-gold double shell is considered, there is no trapping point as scattering forces increase because of the large extinction of the full gold shell. (f) Optical force constant for the shell-cluster model in the axial (light propagation) direction as a function of AuNPs number, N . A quadratic power law scaling (dashed line) is superposed to the numerical data points.


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nity to combine OT with SERS (SERS tweezers) is key to probe the physical chemistry of soft environments and to observe the Raman response of molecules directly in their natural state, 31,72,86 with particular relevance for the case of in situ monitoring the evolution of analytes at very specific locations when dealing with biological systems at cellular scale. However, devising the most efficient strategy for SERS tweezers is still a challenge. This is due, on one side, to the difficulty in manipulating nanoparticles with an efficient plasmonic response, and on the other side to excite SERS without affecting the trapping behaviour. The route to accomplishing this is to design special SERS-active probes consisting of metal colloids bound to optically trappable microscopic particles. 87,88 In this context, the plasmonic mesocapsules described here represent a novel strategy for SERS tweezers. We have successfully used the plasmonic mesocapsules for SERS by means of a singlebeam Raman tweezers setup 72,86 operating at a single wavelength, 785 nm, used both for trapping and Raman excitation (see Supporting Information). In Figure 5 the spectra obtained in 2 × 10−5 M (fig. 5a) and 5 × 10−5 M (fig. 5b), methylene blue (MB) water-ethanol solutions are shown, with (red lines) and without (purple lines) a mesocapsule in the trap. In (a) with respect to (b) we increased by four times the water content to the solution to reduce the Raman contribution of ethanol (asterisks in fig. 5b) to the spectrum that would make the identification of MB Raman peaks more difficult. Even at the low concentration of 2×10−5 M (fig. 5a), and with only a mesocapsule in the trap, the presence of methylene blue in the solution is recognized, as the most representative peaks at approximately 450, 503 (δ(C − N − C) doublet) and 1440 cm−1 (α(C − H)) are observed. 89 On the contrary, no MB peaks are observed in the spectrum obtained without the mesocapsule in the trap (purple line, fig. 5a), where, only a small band at 450 cm−1 due to ethanol background is observed. Thus, the SERS enhancement of MB peaks is clearly due to the AuNPs in the inner walls of the mesocapsule itself, that can be reached by methylene blue thanks to the porosity of the silica shell. For reference and comparison we performed SERS on a drop cast dry (without ethanol)



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mesocapsule sample with a commercial Jobin-Yvon HR800 micro-Raman setup at 785 nm excitation wavelength. 31,89,90 Figure 5c shows the spectrum of a 1 × 10−4 M aqueous solution of MB molecules drop cast and dried on a glass slide (black line) and the SERS spectrum (blue line) obtained when mesocapsules are dispersed in MB solution at the same concentration, drop cast, and dried on the glass substrate. On dry samples the MB Raman signal is increased about 40 times in presence of mesocapsules (estimated from the peak at 450 cm−1 ). This amplification factor or SERS gain, 89,90 ISERS /IRaman ∼ 40, measures the absolute signal enhancement that a SERS substrate (the mesocapsule) can provide for SERS-active molecules (MB). This factor is much smaller than the so-called SERS enhancement factor, EF, defined as the scattering enhancement provided by each gold nanoparticle on a single molecule. The latter can be calculated upon normalizing the SERS and the Raman signals to the number of probed molecules in each experiment, EF = ISERS nRaman /IRaman nSERS , where nRaman and nSERS are the number of probed molecules in the Raman and SERS experiments, respectively. An estimate of the number of probed molecules, nRaman , can be obtained assuming that all molecules within the 785 nm laser spot area, Alas = πd2las /4 ∼ 2 µm2 , do contribute to the Raman signal (IRaman ), where dlas ∼ 1.5 µm is the laser spot diameter used in the micro-Raman spectrometer. On the other hand, only MB molecules adsorbed on the gold nanoparticles surface within the mesocapsule, Ameso = N πd2np ∼ 0.7 µm2 , will contribute to the SERS signal (ISERS ), where N ∼ 2 · 103 and dnp ∼ 11 nm are, respectively, the average number and average diameter of gold nanoparticles in the mesocapsule (estimated from TEM images, fig. 1). Thus, assuming that in both cases we are probing the same number of MB layers, we can estimate an enhancement factor of about EF = ISERS Alas /IRaman Ameso ∼ 102 that is a reasonable number for weakly coupled nanoparticles excited out of resonance. 89 Finally, we note that while using 785 nm wavelength ensures stable optical trapping of the mesocapsules in the Raman tweezers, this is not the most efficient wavelength for SERS detection on these structures. However, while a wavelength more resonant with the plasmon band is expected to yield a larger SERS signal this would be less suitable for trapping as the


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increased radiation pressure would destabilize the optical trap. In order to improve SERS detection with trapped mesocapsules, a double wavelength setup 20 with independent trapping (in the NIR) and resonant SERS excitation could be used. Another possible strategy could be to modify the gold nanoparticle shape, e.g., using nanorods instead of spheres, or increasing their aggregation and coupling so that plasmon resonance peaks in the red would appear, enhancing SERS with 785 nm excitation. 91

Conclusion In conclusion, we investigated the optical trapping forces on hybrid silica-gold optoplasmonic mesocapsules. Their increased trapping efficiency is governed by the plasmonic enhancement of optical forces due to the AuNPs distribution inside the permeable silica shell. By accurately solving the light scattering problem in the tightly focused spot of the OT, we calculated and compared optical forces for several model particles, finding a quadratic number scaling law for a shell-cluster model mimicking the optoplasmonic hybridized mesocapsule. Finally, by means of a Raman tweezers we used the trapped mesocapsules as local SERS molecular probes at 10−5 M concentration in a liquid environment. Hybridization of optical forces at the mesoscale hold perspectives to combine plasmonic response with dielectric scaffolding for smart cargos driven by light with an unprecedented ability for their individual localization. The opportunity to trap and direct hollow and porous capsules in biological fluids is of paramount importance for theranostics, high resolution imaging and controlled drug release.

Associated Content The Supporting Information is available free of charge on the ACS Publications website. Experimental setups, Optical forces in the T-matrix formalism for shell and cluster particles (Shell particles, Cluster particles, Convergence), Supplementary Video.



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Figure 5: (a-b) SERS spectra obtained by trapping a plasmonic mesocapsule in a Raman tweezers (785 nm trapping and excitation wavelength). Spectra in red are obtained by trapping a mesocapsule in a 2 × 10−5 M, in (a), and 5 × 10−5 M, in (b), methylene bluewater-ethanol solution. Few of the MB peaks are evident even at the lowest concentration (the δ(C − N − C) doublet and the α(C − H)). Spectra in purple are obtained in the same solution without trapped particle showing only the ethanol background. For reference and comparison, in (c) we show the spectrum of a 1 × 10−4 M aqueous solution of methylene blue drop cast and dried on a glass slide (black line) and the SERS spectrum (blue line) obtained when mesocapsules are dispersed in the MB solution, drop cast, and dried on the glass substrate. These spectra were obtained with a commercial Jobin-Yvon HR800 micro-Raman spectrometer at 785 nm excitation wavelength. The assignment of the most representative MB Raman peaks are indicated. On dry samples the MB Raman signal is increased about 40 times in presence of mesocapsules. Data are displaced for clarity.


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Acknowledgement We acknowledge support from “Programma Operativo Nazionale Ricerca e Competitività” 2007-2013, Project PAC02L3-00087 SOCIAL-NANO, MPNS COST Action 1205 “Advances in Optofluidics: Integration of Optical Control and Photonics with Microfluidics” and MP1302 “Nanospectroscopy”, and from the Ohio Third Frontier Project ”Research Cluster on Surfaces in Advanced Materials (RC-SAM) at Case Western Reserve University”. This work was also funded by Xunta de Galicia (EM2014/035), MINECO-Spain (CTM2014-58481-R), and Fundación Tatiana Perez de Guzman el Bueno.

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Optical Trapping of Plasmonic Mesocapsules: Enhanced Optical

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