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Plasmon-Exciton Interactions on Single Thermoresponsive Platforms Demonstrated by Optical Tweezers Silvia Horme~no,†,‡,§ Neus G. Bastus,||,^ Andrea Pietsch,|| Horst Weller,|| J. R. Arias-Gonzalez,*,†,‡ and Beatriz H. Juarez*,† †
IMDEA Nanoscience, Campus de Cantoblanco 28049, Madrid, Spain Department of Macromolecular Structure, Centro Nacional de Biotecnología (CNB-CSIC), Campus de Cantoblanco 28049, Madrid, Spain § Universidad Autonoma de Madrid, Campus de Cantoblanco 28049, Madrid, Spain Institute of Physical Chemistry and Center for Applied Nanotechnology, Grindelallee, 117, 20146, Hamburg, Germany
)
‡
bS Supporting Information ABSTRACT: Optical and hydrodynamic-size studies on single bare thermo-responsive microspheres, and microspheres covered either with Au nanoparticles, CdSe/CdS quantum dots, or a combination of both have been performed by optical tweezers. The photothermal heating of water in the focal region boosts the shrinkage of the microspheres, an effect that is intensified in the presence of Au nanoparticles. In contrast, bigger microspheres are measured when they are covered with quantum dots. Plasmon-exciton interactions are observable in the trap in the combined Au and quantum dots hybrid systems. KEYWORDS: Optical tweezers, quantum dots, thermo-responsive pNIPAM microspheres, Au nanoparticles, plasmon-exciton interactions, hydrodynamic size
L
aser trapping of individual micro and nano-objects has been shown to be beneficial for many applications, especially in biology.1 Indeed, it completely removes ensemble averaging, so that the heterogeneity of populations and the dynamical fluctuations of individual trapped items come to light. Recent work by Oddershede's group shows the relevance of optical tweezers to study nanoparticles (NPs) for biophysical and biomedical purposes. In particular, it has been demonstrated that colloidal quantum dots (QDs) can be optically trapped and manipulated in 3D by an infrared (IR) continuous wave laser.2 Besides, it has been recently reported that a single IR laser beam can simultaneously trap and excite individual QDs through a two-photon absorption mechanism.3 Direct measurements of the heating associated with IR optical trapping of spherical gold NPs have also been performed.4,5 In addition, optical trapping has been proposed as a tool to produce patterns6,7 and controlled imprinting.8 Elongated objects like gold nanorods9,10 and semiconductor nanowires11 have been the focus of optical trapping experiments. The combination of Au NPs and QDs has been profusely studied either to evaluate the quenching mechanism described by the energy transfer theory1214 or the possible amplified emission.15,16 Both mechanisms, especially the former, have been employed for sensitive DNA detection and for developing specific biosensors based on immunofluorescence.17 On the other hand, poly-(N-isopropylacrylamide) (pNIPAM) is one of the most extensively used polymers to produce thermo-responsive r 2011 American Chemical Society
gels. Linear pNIPAM has a lower critical solution temperature (LCST) of 32 C in aqueous solution.18 pNIPAM hydrogels undergo a volume phase transition at ca. 32 C and can be designed to respond to different stimuli such as temperature, pH, ionic strength, or magnetic fields.19,20 The combination of pNIPAM beads and QDs represents very interesting platforms for temperature responsive emission due to energy transfer21 and multicolor optical coding in biological assays.22 pNIPAM beads have also been studied in combination with Au NPs as platforms to control plasmon resonances23,24 and sensing.25 Up to our knowledge, there is no information regarding the manipulation of QDs and Au NPs immobilized on single pNIPAM microspheres by optical tweezers. In this work bare pNIPAM thermoresponsive microspheres, and microspheres covered either with Au NPs (pNIPAM/Au), CdSe/CdS QDs (pNIPAM/QDs) or a combination of both (pNIPAM/Au/QDs) have been studied by optical tweezers. The size and optical behavior of these systems in the trap have been characterized and related to the effects that the presence of NPs produces upon trapping. The optical trapping setup is a force sensor calibrated from the light momentum conservation principle and thus, it provides force measurements that are independent of the size, shape, or refractive index of the Received: July 26, 2011 Revised: October 7, 2011 Published: October 17, 2011 4742
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Figure 1. (A) Transmission electron microscopy image of a hybrid pNIPAM/Au/QDs microsphere. (B) Absorption spectrum of Au NPs (black curve) and emission spectrum of QDs (red curve).
trapped object.26 This calibration method has been successfully used in molecular and cell biophysics27,28 and we show here the advantages that it also offers for materials and chemical sciences. Specifically, the unique calibration procedure of our system brings the possibility to characterize single specimens whose physical and optical properties change with time. The herein used optical tweezers system consists of two equal counterpropagating laser beams (two 835 nm diode lasers) brought to the same focus.26 This design offers improved trapping efficiency for metallic particles since the opposite absorption and scattering forces, which point along the propagating direction of the beams,29 provide axial stability to the particle, thus leading to a net 3D gradientforce trap. In addition to the optics, the setup includes an automated hydrodynamic flow system consisting of computercontrolled solenoid valves, glass microdispensers, and pressure sensors.30 This system provides a precise method to inject low volumes of suspended specimens in the proximity of the laser’s common focal region and ensures reproducible results. Specifically, low concentrations of particles are gently pumped from the reservoir through a very short fluid path, thus reducing the exposure of the sample to changes in environmental conditions before each assay. The fluid system produces laminar flow fields that exert forces on the pNIPAM microspheres as low as a few picoNewtons, thus facilitating particle sorting and maximizing individual manipulation throughput. In this work, CdSe/CdS QDs have been produced in organic medium, Au NPs in aqueous medium and both have been further ligand exchanged with biocompatible ligands based on amine-poly-(ethylene oxide)31 (Supporting Information). The combination of biocompatible elements (pNIPAM, and NPs), along with the capability of optical trapping for visualization, heating, and manipulation, may lead to a better understanding of the dynamic behavior of pNIPAM-based NP platforms for potential biological studies such as interactions with cells and internalization processes,32 including force measurement of membrane compliance.33 Results. The synthesis of pNIPAM beads and further immobilization of NPs on their surface were carried out following a method described elsewhere,34 where amino-capped-PEO nanocrystals showed strong interaction with pNIPAM beads containing maleic acid moieties (Supporting Information). Figure 1A shows a transmission electron micrograph of a pNIPAM bead with immobilized Au NPs and QDs on the surface. The darker spots correspond to metallic NPs of ∼10 nm diameter while the regions in between them are fully covered by ∼4 nm QDs (a magnified image can be found in Figure S1 in the Supporting Information). Figure 1B shows the absorption and emission spectra of Au NPs and QDs dispersions respectively, prior to the immobilization on the pNIPAM spheres. The presence of
Figure 2. (A) Illustration of the dual-beam optical-tweezers setup (not to scale). Two 835 nm diode lasers are used to generate the optical trap in the interior of a fluidics chamber. A blue LED is used to illuminate the sample in K€ohler configuration. A pressure system is used to control the flow speed (typically, ∼1 μL/s) of the sample solution in the fluidics chamber. (B) Power spectrum density of pNIPAM microspheres with and without Au NPs and/or QDs. Dashed curves are fittings to the theory (see Supporting Information). Arrows demarcate the approximate fitted interval. Inset: illustrative scheme of a pNIPAM microsphere held in the optical trap.
pNIPAM red shifts the plasmon resonance of Au NPs by 5 nm (not shown) due to multiple scattering among the Au NPs and pNIPAM surfaces as has been described elsewhere for similar systems.35,36 The emission spectrum of QDs after the immobilization on pNIPAM remains unchanged, thus showing the negligible influence of the beads on the QDs emission properties. pNIPAM microspheres were flowed at a controlled rate by means of a glass capillary inserted in a fluidics chamber as shown on the right upper part of Figure 2A. Specifically, a pressure bottle was used to pump an aqueous solution with pNIPAM particles by air pressure into the glass capillary. This glass capillary injected the suspension of particles directly to the central channel of the fluidics chamber, where the optical trapping assay took place (see further description of the fluidics system in the Supporting Information). Once individual specimens were trapped in solution, scattered light was directed to position-sensitive photodetectors (PSD) by polarizing beamsplitter cubes (PBS) for the analysis of the force (see Figure 2A and the Supporting Information for further details). 4743
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Nano Letters Our apparatus gives a direct measurement of the force exerted on a trapped specimen by the change in momentum flux in the trapping light beams due to their interaction with the trapped specimen. This calibration method is valid provided that nearly all of the light exiting the trap is collected. We confirmed that the presence of NPs on the pNIPAM platforms did not affect this calibration method. In this regard, we checked that the total laser power registered by the PSD when the trapped specimen contained Au NPs and/or QDs did not appreciably differ from the case in which no specimen was present in the optical trap. Since this method is not based on trap stiffness, the force calibration is independent of the particle’s size, shape, or refractive index, the viscosity or refractive index of the solution, or variations in laser power.26 Microspheres, whose size compares with optical wavelengths, became discernible in aqueous solution in a chargecoupled device camera due to their Brownian motion. By optically maneuvering individual pNIPAM microspheres with and without Au and QDs on their surface, we investigated their hydrodynamic size and optical behavior. We inferred the size of each specimen in water from measurements of its drag coefficient by means of the thermal-fluctuation spectral analysis of the force (Supporting Information). The validity of this method was demonstrated in former work with centrosomes, biological organelles whose size is similar to the pNIPAM particles used in this work. Centrosomes also experience changes in size due to environmental stimuli (pH variations).27 The error in size determination, as checked against polystyrene microparticles of known diameter, was kept below 6% (largest percent error in mean diameter relative to vendor's measurements). Figure 2B compares the power spectral densities for individual pNIPAM, pNIPAM/Au, pNIPAM/QDs, and hybrid pNIPAM/ Au/QDs microspheres trapped inside the fluid chamber at room temperature (22 ( 2) C. Since specimens were trapped more than 80 μm away from the surfaces of the fluid microchamber, wall effects were negligible. The spectrum curves, which correspond to pNIPAM microspheres with different Au and QDs concentrations on the surface, differ both in plateau height and length in the double-logarithmic plot due to their different drag coefficient and corner frequency. These two parameters were obtained from the fitting to the equilibrium power spectral density equation of force fluctuations of an overdamped particle in a harmonic potential, as described in the Supporting Information. At both low and high frequencies, electronic noise may add to the thermal fluctuations, causing deviations from the expected spectrum.2628 Therefore, to extract the information relative to the size of the particles out of the fitting analysis, nonlinear regressions were performed for frequencies between ∼300 and ∼2000 Hz (see arrows in Figure 2B). The curve trace within this bandwidth contains a large number of Fourier-transformed data points and still addresses the effects of both the corner frequency and the plateau height in the double-logarithmic plot. As a further check, we selected slightly larger and shorter bandwidth ranges to study the behavior of the fitted parameters. The differences were negligible, thus indicating the robustness of the parameters determination. Additionally, we observed the presence of a peak at low frequencies (90 ( 30 Hz, number of studied beads, n = 37) in the power spectrum of pNIPAM/Au microspheres. This peak appeared whenever an Au/pNIPAM particle, with or without QDs, was trapped and correlated with a direct observation of slow oscillations of the particle in the trap. These observations therefore reflect the interaction of the trapping light with metal NPs, which is probably due to radiometric forces caused by
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Figure 3. Hydrodynamic size of optically trapped pNIPAM microspheres. Size distributions in water measured for (A) bare pNIPAM (n = 113), (B) pNIPAM/QDs (n = 65), (C) pNIPAM/Au (n = 71), and (D) pNIPAM/Au/QDs (n = 52). Black lines in the histograms are Lorentzian fits. (E) Comparative illustration of the average sizes at t = 0 in the trap for the various pNIPAM beads studied.
anisotropic heating of the randomly arranged Au particles, with the possible incidence of aggregates, and thermophoretic forces due to the subsequent anisotropic heating of the aqueous environment.37 The peak position may probably be dependent on the Au content (mass, NPs size, and surface distribution of the NPs on each pNIPAM platform). In the proximity of the plasmon resonance of the Au NPs, the composed microspheres are more sensitive to these phenomena due to the abrupt change of the gradient force.29 Figure 2B, inset, illustrates the experimental configuration during the analysis of optically held pNIPAM microspheres. Figure 3 shows the size distribution of the different pNIPAM microspheres obtained from the power spectrum analysis method for individually manipulated specimens. The method yielded average diameters that were remarkably different depending on the presence of Au or QDs, thus manifesting underlying light-NP interaction effects. The average bare pNIPAM diameter from the thermal-noise analysis is (596 ( 262) nm (n = 113, λ = 835 nm) (Figure 3A). Dynamic light scattering measurements of a dispersion of the same bare pNIPAM spheres lead to a mean diameter of (840 ( 40) nm (20 C, λ = 635 nm).34 As will be discussed below, the reduced size obtained by optical tweezers may be attributed to the heating effect of the optical trap. We also note that the width of the distribution of bare pNIPAM microspheres is notably large. We attribute the heterogeneity of bare pNIPAM population to the fact that their size depends on the photothermal water heating inside and around the microsphere. In such a case, solution temperature variations may affect critically the 4744
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Nano Letters temporal response of the highly thermo-sensitive polymer.38 To fully understand the heating mechanism of pNIPAM in the optical trap, the IR interaction with water molecules surrounding and within the pNIPAM network should be taken into account. We performed experiments in heavy water (D2O is nearly transparent at 835 nm) and we obtained that the hydrodynamic size of bare pNIPAM beads was (1123 ( 150) nm (n = 55), a size 47% bigger than that obtained for pNIPAM beads in water (Figure S2 in the Supporting Information). This result confirms that pNIPAM collapse transition is induced by the photothermal heating of water in the focal region, in agreement with previous work of irradiated pNIPAM solutions.38 This result also indicates that the photothermal local heating of water induced by the trapping laser28 in a single bead is strong enough to induce a phase transition of the polymer (local temperature higher than the LCST). Our result also emphasizes the importance of the local heating in determining not only the size of the trapped specimen, but also in evaluating the impact of the increased temperature for experiments performed in biological media. We found a bigger average diameter for pNIPAM/QDs (d = (906 ( 98) nm (n = 65)) (Figure 3B). This result suggests that the QDs that cover the surface of pNIPAM (as previously ascertained by AFM images)34 may reduce the heating effect of water in the interior of the microsphere and subsequent pNIPAM shrinkage. The average diameter obtained for pNIPAM/ Au microspheres (Figure 3C) is (295 ( 45) nm (n = 71). Thus, the presence of Au NPs reduces the size of pNIPAM bare spheres to one-half. This result can be explained in terms of the heating process induced by the relaxation of excited Au NPs in the trap, a profusely studied effect for diagnosis and hyperthermia treatment, especially for Au nanorods.39 In the experiments reported here, the excitation of Au NPs and subsequent relaxation induces heating of the pNIPAM network. Irradiation of metallic NPs has been shown to induce phase transitions in biological lipid bilayer systems4,5,40 and, as we observe in this work, to increase the temperature of pNIPAM well above its LCST. It is, however, worth mentioning that the excitation of Au NPs is done far from their plasmon resonance, where a maximum effect is expected. An estimation of the heating dynamics shows that a single Au NP requires a few nanoseconds to increase by ∼4 K its surface temperature in water (Figure S3 in the Supporting Information). More accurate heating values should be obtained by direct measurement of the heating profile, as previously performed for a single Au NP.4,5,40 The fact that pNIPAM microspheres experience a phase transition indicates that the temperature increase within the pNIPAM matrix is larger than a few degrees. In this regard, we believe that a collective effect due to the high concentration of Au NPs on the surface increases the temperature of the pNIPAM bead thus boosting its shrinkage by an enhancing mechanism in which heat fluxes are added within the pNIPAM microsphere.12 The particular geometry of the Au-NP ensemble on pNIPAM, a 10 nm thick spherical shell in which the distance between neighboring NPs is on average below 100 nm, gives rise to an effect similar to that of greenhouse in which laser-induced heating toward the interior of the pNIPAM microsphere is both generated and trapped by the ensemble of Au NPs. Moreover, the average diameter of pNIPAM/Au/QDs microspheres is (395 ( 36 nm) (n = 52) (Figure 3D). This value is smaller than the pNIPAM/QDs diameter due to the additional heating effect of Au NPs, but larger than the pNIPAM/Au diameter due to the presence of QDs, as mentioned above. These results emphasize the sensing capability of the system to external temperature changes
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Figure 4. Time dependence of pNIPAM microspheres size in the optical trap (minute range). Data points at each time represent the peak position of the fitted histograms. Vertical bars are the corresponding full width at half-maximum addressing population spreads within experimental error.
(see Figure 3E for a schematic comparison of average sizes). The aforementioned average diameters correspond to the size measured a few seconds (∼510 s) after each specimen was optically trapped (t = 0). To ascertain whether the values obtained by optical tweezers are significantly different, a student t test for values at t = 0 was performed. The statistical analysis supports that changes in diameter are the result of different coverage and not of chance variation (significance level α = 0.01). To investigate the size dependence with time in the optical trap, we performed serial size measurements for up to 10 min after trapping. Figure 4 shows the results for the variety of microspheres studied. Each data point represents the peak of the fitted histogram (those at t = 0 correspond to histograms in Figure 3). The initial size experienced a slight average decrease over the 10 min which the experiments lasted. In order to know whether this decrease is statistically significant a t test comparing values at t = 0 and t = 10 min was performed (α = 0.01). The result indicates that this decrease is statistically not significant for all cases but for the case of pNIPAM/Au/QDs. Thus, the collapse of the polymer takes place within the first instants upon trapping (just before the first size measurement is performed) for bare pNIPAM and spheres covered with exclusively QDs or Au NPs. However, pNIPAM beads covered with both types of NPs, Au and QDs, require more time to attain their final collapsed state. This fact may be a consequence of the opposite effects exhibited by the metallic and the semiconducting NPs, competing for shrinking the bead (in the case of Au) or preventing it (in the case of QDs). We also detected the emission of QDs by using appropriate filters to block the light from the trapping lasers and the illumination lamp in the CCD camera. Since the wavelength of the laser trap is 835 nm, and the absorption edge of the here used QDs lies beyond 600 nm, the excitation of QDs on the surface of pNIPAM microspheres held in the optical trap may take place by a two-photon absorption, in agreement with previous observations.3 We found that pNIPAM/QDs microspheres fluoresce in the optical trap for (15 ( 5) min (n = 23). This time is much longer than the average time that pNIPAM/Au/QDs microspheres fluoresce in the optical trap, which is (70 ( 57) s (n = 53) (movies available in the Supporting Information). This fact 4745
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Figure 5. Quenching of pNIPAM/Au/QDs. (A) Initially, T < LCST and d denotes the average distance between Au NPs and QDs on the surface of the pNIPAM in this state. (B) When a pNIPAM/Au/QDs bead is in the optical trap, IR light is absorbed by Au and QDs, hence emitting heat and fluorescence, respectively. (C) Heat emission causes T > LCST and subsequent pNIPAM shrinkage. d0 denotes the average distance between Au NPs and QDs which decreases (d0 < d) in these conditions giving rise to luminescence quenching. (D) Snapshots of the quenching process for a single pNIPAM/Au/QDs bead in the optical trap obtained by video-microscopy. Time of residence in the optical trap is indicated. (E) Steady-state temperature-dependent quenching of hybrid pNIPAM/Au/QDs beads as ensemble. Inset: Photoluminescence peak versus temperature.
together with the collapse of pNIPAM/Au/QDs microspheres indicates that the observed quenching of QDs fluorescence is due to their approach to Au NPs as depicted in Figure 5. Figure 5AC illustrates the shrinkage of pNIPAM/Au/QDs microspheres upon trapping along with the luminescent quenching. Figure 5D shows several snapshots corresponding to the luminescent signal at different times (0, 20, 40, and 60 s) of a single trapped pNIPAM/QDs/Au bead. In these images, the laser and the illumination signals were properly filtered except for the last frame on the right, which was recorded with unfiltered illumination light. As can be appreciated in this frame, the dark spot confirms that the microsphere remained in the trap after the luminescent intensity had fallen below the detection limit of the CCD camera (after approximately 1 min), thus indicating that luminescence vanishes due to quenching. Induced heating by Au NPs provokes an extra shrinkage of the pNIPAM bead, and a subsequent approach of QDs to Au quencher centers. The longer time required for pNIPAM/Au/QDs beads to attain their final collapsed state (as described in Figure 4) may explain the duration of the luminescence for about 1 min in the trap. Steady-state fluorescence measurements of the hybrid pNIPAM/Au/QDs
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systems are shown in Figure 5E. These measurements were done in a fluorometer equipped with a heating system and an external temperature controller. A dispersion of pNIPAM/Au/QDs beads was first measured at room temperature (∼23 C) in a cuvette and then, measurements were performed at different temperatures by heating up the cuvette. As depicted in Figure 5, the fluorescent intensity gradually decreases as the temperature increases, which is in agreement with the observed behavior of the individual pNIPAM/Au/QDs beads in the optical trap. This behavior accounts for the decrease in average distance between emitters (QDs) and quenchers (Au NPs) as the bead diameter is reduced by the effect of heating. Quenching of the QD emission by proximal Au NPs has been extensively explained by F€orster theory.41 As previously reported by several authors,42,43 quenching efficiency is directly related to both the spectral overlap of Au NPs absorption with the QDs emission and the distance between the two kinds of NPs. In this work, pNIPAM/Au/QDs are composed of a high concentration of QDs and Au NPs randomly distributed around the pNIPAM bead surface. The inset in Figure 5E plots the intensity of the signal with the temperature fitted following a F€orster approach. The result discards a strong plasmon coupling among Au NPs for the relative amount of Au/QDs present in this sample and a linear dependence of the temperature with the distance between QDs and NPs. It is known that the number of metallic NPs, their geometry, and the distance between Au NPs and QDs play a role in the emission intensity and the energy transfer rate. Our results suggest a method to study quenching times and plasmon-coupling interactions between metallic NPs by controlling the Au/QDs ratio on pNIPAM platforms based on the thermo-responsive character of this polymer. Summary. We have studied the hydrodynamic size and optical response of systems composed of semiconductor fluorescent NPs (QDs) and Au NPs immobilized on individual thermoresponsive microspheres (pNIPAM) by optical tweezers. While moderate shrinkage is observed for bare pNIPAM beads, severe shrinkage is observed for pNIPAM/Au ones due to the extra heating induced by the presence of a surface ensemble of Au NPs. The presence of QDs on the pNIPAM beads reduces the local heating of water in the polymeric network leading to higher sizes. While a constant emission (minute range) from the pNIPAM/ QDs beads is recorded, emission and subsequent quenching due to energy transfer from the QDs to the metallic NPs take place in the optical trap for pNIPAM/Au/QDs microspheres. This effect is the consequence of plasmon-exciton interactions as QDs and Au NPs get closer upon pNIPAM shrinkage. These systems combine the ability for thermal sensing (due to the properties of pNIPAM systems and Au NPs), and labeling (due to the QDs), representing very interesting platforms for the design of thermal sensors in biological studies. The ability to trap and manipulate specimens along with the possibility to immobilize cells or organelles in fixed positions open the way to carry out imaging studies between interacting pNIPAM beads and cells. These studies may contribute to shed more light on the required forces for internalization processes.32,33 In addition, the use of thermoresponsive elements as the ones presented here may also be an effective way to measure and calibrate the temperature in optical trapping set-ups.28
’ ASSOCIATED CONTENT
bS
Supporting Information. Synthesis and ligand exchange procedure for Au NPs and CdSe/CdS QDs. Protocols for the
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Nano Letters synthesis of pNIPAM microspheres and NPs immobilization. A magnified version of the TEM image shown in Figure 1A. Detailed descriptions of both the experiments with optical tweezers and the fluidics system. Distribution of hydrodynamic diameters of optically trapped pNIPAM microspheres in D2O. Estimation of the heat transferred from Au NPs to pNIPAM beads. Selected movies of individually trapped pNIPAM/QDs and pNIPAM/ Au/QDs beads. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail: (B.H.J.)
[email protected]; (J.R.A.-G.)
[email protected]. Telephone: +34 91 497 8600. Fax: +34 91 394 68 55. Present Addresses ^
Institut Catala de Nanotecnologia (ICN), Campus UAB, 08193 Bellaterra, Barcelona, Spain.
’ ACKNOWLEDGMENT This work has been partially supported by ERG FP7-PEOPLE-ERG-2008, the Spanish Ministry of Science and Innovation through RYC-2007-01709, RYC-2007-01765, and MAT 200913488 and the Comunidad de Madrid through NANOBIOMAGNET S2009-MAT-1726. N.G.B. thanks “Generalitat de Catalunya” for financial support through the “Beatriu de Pinos” program. The authors thank P. Morales and C. Klinke for experimental help and A. Blanco for fruitful discussions. ’ REFERENCES (1) Hormeno, S.; Arias-Gonzalez, J. R. Biol. Cell 2006, 98 (12), 679–95. (2) Jauffred, L.; Richardson, A. C.; Oddershede, L. B. Nano Lett. 2008, 8 (10), 3376–80. (3) Jauffred, L.; Oddershede, L. B. Nano Lett. 2010, 10 (5), 1927–30. (4) Bendix, P. M.; Reihani, S. N.; Oddershede, L. B. ACS Nano 2010, 4 (4), 2256–62. (5) Kyrsting, A.; Bendix, P. M.; Stamou, D. G.; Oddershede, L. B. Nano Lett. 2011, 11 (2), 888–892. (6) Woerdemann, M.; Glasener, S.; Horner, F.; Devaux, A.; De Cola, L.; Denz, C. Adv. Mater. 2010, 22 (37), 4176–9. (7) Vossen, D. L. J.; Fific, D.; Penninkhof, J.; van Dillen, T.; Polman, A.; van Blaaderen, A. Nano Lett. 2005, 5 (6), 1175–1179. (8) Urban, A. S.; Lutich, A. A.; Stefani, F. D.; Feldmann, J. Nano Lett. 2010, 10 (12), 4794–4798. (9) Pelton, M.; Liu, M.; Kim, H. Y.; Smith, G.; Guyot-Sionnest, P.; Scherer, N. F. Opt. Lett. 2006, 31 (13), 2075–2077. (10) Selhuber-Unkel, C.; Zins, I.; Schubert, O.; S€onnichsen, C.; Oddershede, L. B. Nano Lett. 2008, 8 (9), 2998–3003. (11) Reece, P. J.; Toe, W. J.; Wang, F.; Paiman, S.; Gao, Q.; Tan, H. H.; Jagadish, C. Nano Lett. 2011, 11 (6), 2375–2381. (12) Govorov, A. O.; Bryant, G. W.; Zhang, W.; Skeini, T.; Lee, J.; Kotov, N. A.; Slocik, J. M.; Naik, R. R. Nano Lett. 2006, 6 (5), 984–994. (13) Thomas, M.; Greffet, J.-J.; Carminati, R.; Arias-Gonzalez, J. R. Appl. Phys. Lett. 2004, 85 (17), 3863–3865. (14) Pons, T.; Medintz, I. L.; Sapsford, K. E.; Higashiya, S.; Grimes, A. F.; English, D. S.; Mattoussi, H. Nano Lett. 2007, 7 (10), 3157–64. (15) Kulakovich, O.; Strekal, N.; Yaroshevich, A.; Maskevich, S.; Gaponenko, S.; Nabiev, I.; Woggon, U.; Artemyev, M. Nano Lett. 2002, 2 (12), 1449–1452. (16) Paltiel, Y.; Aharoni, A.; Banin, U.; Neuman, O.; Naaman, R. Appl. Phys. Lett. 2006, 89, 033108.
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