Article pubs.acs.org/JPCC
Metallic-Nanostructure-Enhanced Optical Trapping of Flexible Polymer Chains in Aqueous Solution As Revealed by Confocal Fluorescence Microspectroscopy Mariko Toshimitsu,† Yuriko Matsumura,*,§ Tatsuya Shoji,‡ Noboru Kitamura,*,†,‡ Mai Takase,‡ Kei Murakoshi,†,‡ Hiroaki Yamauchi,∥ Syoji Ito,∥ Hiroshi Miyasaka,∥ Atsushi Nobuhiro,⊥ Yoshihiko Mizumoto,⊥ Hajime Ishihara,⊥ and Yasuyuki Tsuboi*,†,‡,# †
Department of Chemical Sciences and Engineering, Graduate School of Chemical Sciences and Engineering, and ‡Department of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan § Graduate School of Life Science and Technology, Tokyo Institute of Technology, Nagatsuda, Yokohama 224-8502, Japan ∥ Division of Frontier Materials Science, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan ⊥ Department of Physics and Electronics, Osaka Prefecture University, Gakuencho 1-1, Nakaku 599-8531, Sakai, Osaka, Japan # JST (Japan Science and Technology Cooperation), PRESTO S Supporting Information *
ABSTRACT: Optical trapping of flexible polymer chains to a metallic nanostructured surface was explored by microscopic imaging and confocal fluorescence spectroscopy. A fluorescence-labeled poly(N-isopropylacrylamide) was targeted, being a representative thermo-responsive polymer. Upon resonant plasmonic excitation, it was clearly observed that polymers were assembled into the excitation area to form molecular assemblies. Simultaneously, fluorescence from the area was obviously intensified, indicating an increase in the number of polymer chains at the area. The excitation threshold of light intensity that was required for obvious trapping was 1 kW/cm2, which was much lower by a factor of 104 than that for conventional trapping using a focused laser beam. The morphology of the assemblies was sensitive to excitation intensity. We precisely evaluated temperature rise (ΔT) around the metallic nanostructure upon plasmonic excitation: ΔT ≈ 10 K at 1 kW/cm2 excitation. This temperature rise was an origin of a repulsive force that blocked stable trapping. On the basis of experimental observations and theoretical calculations, we quantitatively evaluated the plasmon-enhanced trapping force and the thermal repulsive force (Soret effect). The overall mechanisms that were involved in such plasmon-enhanced optical trapping are discussed in detail. The smooth catch-andrelease trapping (manipulation) of polymer chains was successfully demonstrated.
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INTRODUCTION Trapping and manipulating molecules in condensed phases would be a fascinating as well as challenging issue in next generation chemistry. A powerful candidate as a tool to address such issues is an optical tweezer using a focused intense laser beam,1 in which small nano-/microparticles are optically trapped at a focal point by radiation force.2,3 When d ≪ λ, where d and λ are respectively the diameter of particle and light wavelength, Rayleigh’s theory can be used to explain optical trapping of an object. The force responsible for optical trapping is called a “dipole gradient force”,1 which can be expressed by the following equations: U = −α |E|2 /2
where U is the potential energy of trapping due to the dipole gradient force, and E is the electric field vector of the incident light. α is the polarizability of a particle to be trapped, r is the radius of the particle, and ε2 is the dielectric constant of the surrounding medium. n1 and n2 are the refractive indices of the particle and the surrounding medium, respectively. According to eqs 1 and 2, trapping potential |U| is negatively proportional to the volume (r3) of a target particle. Smaller particles (such as molecules) are harder to trap. To trap a small particle (i.e., d < 100 nm), tight focusing of an intense laser beam is required. Recently, we have shown that a focused NIR laser beam (a conventional optical tweezer) can stably trap small proteins and amino acid clusters in aqueous solutions.4−6 However, intense light irradiation with more than several tens
(1) Received: May 30, 2012 Revised: June 18, 2012 Published: June 19, 2012
2
α = 4πε2r 3
(n1/n2) − 1 2
(n1/n2) + 2
(2) © 2012 American Chemical Society
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of MW/cm2 (intensity at a focal point) is required to achieve stable trapping. To optically trap small particles of molecular size, it is essential to introduce a novel principle to enhance the radiation force. Recently, a novel approach to increasing radiation force has been under active research: It is the so-called “plasmon-based optical trapping”7 or “plasmon nano-optical tweezers”,8 where small particles have been optically trapped on a metallic (Au) nanostructure on the basis of an enhanced electromagnetic field of incident light created by a surface plasmon.9 A surface plasmon is a polariton that can couple with light, and hence it is strongly localized around surfaces of noble metallic nanoparticles (localized surface plasmon, LSP).10,11 In particular, it is generally accepted that an electromagnetic field of incident light is much enhanced at a “nano-gap” between adjacent metallic nanoparticles, that is, a plasmonic nanoantenna or a plasmonic nanojunction.12−15 Such LSP-based optical trapping in a gap-mode manner has been theoretically predicted in the past decade,16 and then experimentally demonstrated by Grigorenko et al. in 2008 for the first time.17 LSP-based optical trapping can potentially overcome the disadvantages of the conventional optical trapping. It can spatially confine nanoparticles in a subdiffraction volume by much weaker light irradiation than that required in conventional optical tweezers. On the other hand, from the viewpoint of chemistry, such metallic nanostructures possess another important character as an “active hot site” for a photochemical reaction. So far, various research groups have demonstrated that quantum yields or efficiencies of photochemical processes were elevated under conditions of LSP excitation.18−27 It is noteworthy here that even nonlinear photochemical processes, such as multiphoton absorption, can be induced by the excitation of such gap-mode LSP. For instance, we have recently shown that a 2-photon photochemical reaction (photochromic ring-opening reaction of diarylethene) proceeded upon irradiation, not of pulsed laser light, but of weak CW near-infrared (NIR) light, greatly assisted by gap-mode LSP.28,29 This means that gap-mode LSP could open a new channel to up-conversion of light energy. If we can optically trap photoactive species (molecules and catalytic particles) around plasmonic nanoantennas in such a situation, photon, plasmon (LSP), a photoactive nanoparticle (photocatalysis), and a photoactive molecule would be simultaneously encountered in the same nanospace, resulting in a strong photon−molecule coupling with great efficiency. This is the origin of plasmon-accelerated photochemistry, and such a situation never appears in normal cases due to the diffraction limit of light. A large part of the targeted particles in LSP-based optical trapping have hitherto been limited to “hard spheres” such as gold nanoparticles30 and polystyrene nanobeads.17,31−35 We have also demonstrated LSP-based optical trapping of a semiconductor nanocrystal (quantum dot), whose detailed behavior was monitored by means of confocal fluorescence microspectroscopy.36 However, furthering the intriguing application of LSP-based optical trapping to acceleration of photochemical reactions, it would be fruitful to explore LSP-based optical trapping of molecular materials or “soft (flexible) particles” such as chain polymers with a photochemical activity. However, such LSP-based trapping has been the subject of limited researches (protein37 and bacteria38), and the characteristics of such LSP-based trapping still remain unclear. In the present study, we singled out a chain polymer labeled with a fluorescence-probe. The main chain was poly(N-
isopropylacrylamide) (PNIPAM), which has been well-known as a water-soluble thermo-responsive polymer. The motivation for selecting PNIPAM as a target material is described latter. LSP-based trapping of flexible polymer systems is of great significance. In a large part of the past studies using hard nanospheres, the sizes of nanospheres were much larger than the gap width of the plasmonic nanoantenna. This situation is difficult to understand because the enhancement effect of the electromagnetic field drastically decays out of the gap. This contradiction is a crucial problem to be resolved in LSP-based trapping. By contrast, polymer chains in a liquid medium are sufficiently flexible to enter a nanogap by changing their conformation, resulting in efficient exposure to an enhanced electromagnetic field (EMF), and hence stable optical trapping should be expected. In the present study, by means of microscopic observation and fluorescence microspectroscopy, we explored the LSP-based optical trapping behavior of the polymer from the above viewpoint. Fluorescence from the LSP excited area provided various insights via its intensity and spectral constitution. Various local temperature elevations, attractive forces, and thermal repulsive forces were quantitatively evaluated, based on which we have concluded a trapping mechanism.
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EXPERIMENTAL SECTION To analyze trapping behavior in detail, we newly created a fluorescence-labeled PNIPAM, whose chemical structure is shown in Figure 1a. The fluorescence probe unit was 7-
Figure 1. (a) Chemical structure of the sample, DEDPQx-PDNIPA, used in the present study. (b) Fluorescence spectra and λmax of the aqueous sample solution as a function of temperature.
diethylamino-1,3-diphenyl-1H-pyrazolo[3,4-b]quinoxaline (DEDPQx). The monomer was copolymerized slightly with the (DEDPQx) unit. For simplicity, the sample is abbreviated to PDNIPA. It was synthesized by radical copolymerization in degassed water. Synthesis of the fluorescent probe unit is described in the Supporting Information. In all of the experiments, the sample polymer was used in aqueous solution (Milli-Q water, 1.0−3.0 wt %). The hydrodynamic diameter 14611
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evaluated to be 11 and 100 nm at the coiled structure (before phase transition) and the globular aggregates (after phase separation), respectively. This process takes place rapidly with time 3. 0 kW/cm2 would be induced, not only by an effect of LSP excitation, but also by other alternative forces such as thermal effects. In such an excitation condition (Ieff > 3. 0 kW/cm2), the temperature around gold nanodimers should be more than LCST (ΔT > 24 K), and hence the phase separation takes place. Again, these morphologies were totally different from those observed for the Cr substrate sample, suggesting that these are not a simple thermal event. In addition to the Ieff value, the concentration of the sample solution is also an important experimental parameter. We carefully investigated such light-induced morphological changes for other sample solutions with different concentrations (1.0 and 2.0 wt %). We clearly observed morphological changes whose characteristics were qualitatively similar to those of Figure 4, and such results are summarized in the Supporting Information. Fluorescence Microspectroscopy for PDNIPA System upon LSP Excitation. To obtain further insights into mechanisms underlying the morphological changes induced by LSP excitation, confocal fluorescence microspectroscopy was conducted with varying Ieff. Figure 5 displays fluorescence spectra of an aqueous PDNIPA solution (3.0 wt %) under on-
solution/Cr substrate) upon 808 nm light irradiation. As is clearly seen in the series of micrographs, a morphological change was brought about by light irradiation. This is safely ascribed to the thermo-responsive phase separation of the polymer by a photothermal effect in which the Cr layer served as an efficient energy converter from light to heat. One characteristic feature of the phenomenon was that the morphological change was induced over a relatively wider area than the light irradiation area (focal spot, which is marked by a white circle in the figure). This is due to a thermal as well as molecular diffusion effects, and partly due to thermophoresis (Soret effect), which is discussed in a later section. Even after stopping LSP excitation, such morphological changes remained over several tens of seconds. This would be due to a thermal storage effect in a sample cell. Fluorescence behavior upon light irradiation is also shown in Figure 3b. The observed fluorescence from the focal spot is assigned to the DEDPQxunit. Upon the light irradiation leading to the morphological change, the fluorescence spectrum exhibited a slight blue shift that corresponded to the phase separation. In addition, fluorescence intensity was markedly decreased from the original intensity upon light irradiation. This is consistent with the microscopic observation. A light scattering effect leads to the drop in fluorescence intensity. These are the characteristics that are peculiar to the pure-thermally driven behavior for the aqueous PDNPA solution system. Figure 4 shows optical micrographs of an aqueous PDNPA solution (3.0 wt %) in the presence of a plasmonic AR-NSL
Figure 4. Optical micrographs of the sample solution (3.0 wt % aqueous PDNIPA solution) under LSP excitation. The circles noted by dotted lines correspond to the LSP excitation area. LSP excitation intensity given in the figure: (a) 1.0 kW/cm2, (b) 3.0 kW/cm2, (c) 5.0 kW/cm2.
substrate upon LSP excitation (Ieff = 1.0, 3.0, and 5.0 kw/cm2). Note that movies corresponding to these pictures are available in the Supporting Information. As is clearly realized in the figure, morphological changes suggesting assembly formation were induced around irradiation areas for the three cases. The morphologies observed here were totally different from those observed for the Cr substrate sample, suggesting that these morphological changes are not a simple thermal event. Furthermore, aspects of the assembly like morphological 14614
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laser beam, where the threshold Ieff is ∼several tens of MW/ cm2. Fluorescence spectra of the solution under LSP excitation with a more intense light (Ieff = 3.0 kW/cm2) are also shown in Figure 5b. Concerning the fluorescence intensity change (Figure 5b, left panel), one can clearly appreciate a behavior, similar to that observed for LSP excitation with Ieff = 1.0 kW/ cm2 (Figure 5a). Again, LSP excitation increased, not only total fluorescence intensity, but also the relative intensity ratio of DEDPQx-unit fluorescence. However, in the normalized spectra (Figure 5b, right panel), we detected one characteristic that distinguished this spectral behavior from that observed for the lower excitation intensity (Figure 5a). DEDPQx-unit fluorescence clearly exhibited a blue shift upon LSP excitation. The blue shift seems to be the same as that observed for a spectral change shown in Figure 1b, responding to temperature elevation above LCST. This behavior was observed when Ieff ≥ 3 kW/cm2. That is, we consider that the thermo-responsive phase separation to form a larger globular aggregates takes place in this LSP excitation condition with Ieff = 3.0 kW/cm2, which is well consistent with the result of ΔT evaluation where solution temperature exceeds LCST under LSP excitation. These results strongly suggest that larger globular aggregates of the polymer are optically trapped with great efficiency when Ieff ≥ 3 kW/cm2, because radiation force is proportional to a size of particle (r3). Also here, the LSP-enhancement effect would make a great contribution, which is quantitatively discussed in a latter section. In these observations, local increases in polymer concentration upon LSP excitation, which strongly suggest LSP-based optical trapping, were clearly verified, although the morphological change and the fluorescence behavior were switched at Ieff ≈ 3 kW/cm2. To address the dynamics of trapping behavior, below and above Ieff ≈ 3 kW/cm2, time-profiles of fluorescence excitation were measured with modulation in a repetitive onand-off LSP excitation mode, with the results being shown in Figure 6a. The ordinate axis is expressed by an enhancement factor EF:
Figure 5. Fluorescence spectral changes in the sample solution (3.0 wt % aqueous PDNIPA solution) upon LSP excitation. The right-hand panels show normalized spectra for the spectra shown in the left-hand panels. Spectra noted in black and red were measured before LSP excitation and 30 s after LSP excitation, respectively. LSP excitation intensity: (a) Ieff = 1.0 kW/cm2; (b) Ieff = 3.0 kW/cm2.
or off- LSP excitation. Spectra at the lowest Ieff (=1.0 kW/cm2), which correspond to Figure 4a, in micrography, are shown in Figure 5a. Before LSP excitation, a broad band was observed to range from 500 to 700 nm. The fluorescence band was ascribed to a spectral superposition of the DEDPQx-unit (λmax = 600 nm) fluorescence and gold luminescence (650−700 nm) originating from an interband transition of gold. Upon LSP excitation (measured at 30 s after the start of LSP excitation), total fluorescence intensity obviously increased, and also the relative intensity of DEDPQx-unit fluorescence to gold luminescence increased (in the figure, left panel). This is opposed to the behavior observed for the Cr substrate sample, indicating an attractive force toward irradiation area by LSP excitation. Obviously, these results mean an increase of the polymer concentration in the focus area, suggesting optical trapping of the polymer chains. It should be noted that a plasmonic fluorescence enhancement effect is not for the present case because DEDPQx has no absorption at the NIR laser (808 nm). In the normalized spectra (light panel), DEDPQx-unit fluorescence exhibited no any spectral change around 500−630 nm upon LSP excitation, except for the relative intensity change of gold luminescence. This indicates that the thermo-responsive phase separation hardly occurs in this LSP excitation condition with Ieff = 1.0 kW/cm2, which is well consistent with the result of ΔT evaluation, where solution temperature under LSP excitation did not reach the LCST of the solution. These results strongly suggest that individual polymer chains in the random coil structure are optically trapped to the LSP-excitation area. LSP-enhancement effect of radiation force presumably makes a significantly large contribution to the present trapping, because threshold Ieff for trapping can be much decreased to ∼ kW/cm2, as compared to that required for conventional optical trapping using a focused
EF = Fon /Foff
(3)
where Fon and Foff are averaged intensity of DEDPQx-unit fluorescence with and without LSP excitation, respectively. For all excitation intensity values, the rises and falls in fluorescence intensity were reproduced in accordance with the on-and-off excitation. At the lowest LSP excitation intensity (Ieff = 1.0 kW/ cm2), fluorescence intensity took ca. 10 s to reach its maximum after the start of excitation. Next, fluorescence intensity showed a gradual decay, and suddenly dropped responding to the stop of LSP excitation. It should be noted that the dynamic fluorescence behavior seems to be consistent with micrograph observations of Figure 4a. The slow rise would correspond to the growth of particle-like assembly, and the gradual fluorescence decay was presumably triggered by the rearrangement from the particle-like-assembly to the film-like assembly observed in Figure 4a. Also, a photobleaching or quenching by gold (that should be promoted by the rearrangement) partly contributes to the gradual decay. The sudden fluorescence drop at the stop of LSP excitation corresponds to the rapid disappearance of the assembly. On the other hand, for the three Ieff values that are >3 kW/ cm2, the fluorescence rise exhibited many sharp responses (abrupt rise within 1 s) after the start of LSP excitation. Also here, switching above Ieff ≈ 3 kW/cm2 was observed. It should 14615
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Ei = Ei0 +
∑ Gif,jPjVj j=1
(4)
where Pj = αj Ej is satisfied in each cell. αj is the polarizability, i is the number of the cell at the observation position (i = 1,..., N), j is the number at the source position (j = 1,..., N), Ei is the total response field, E0i is the incident field, Gfi,j is the free space Green function with both the transverse and the longitudinal electromagnetic components, and Vj is the volume of the jth cell. The gold nanopyramidal dimers blocks are assumed to have a Drude-type dielectric function with the parameters of gold.51,52 Figure 7a shows a 2D profile of the enhancement of the electric field strength (|E|2/|E0|2) as a function of the x and z
Figure 6. (a) Fluorescence intensity modulated by the light irradiation in a repetitive on-and-off mode. LSP excitation intensity is given in the figure. (b) Fluorescence enhancement factor as a function of Ieff.
be noted that the fluorescence intensity showed gradual decay after its maximum during LSP excitation. This is presumably ascribable to the morphological changes (size increase of assemblies and rearrangement), as observed in Figure 4b and c, or to the bleaching and quenching. Also here, the sudden fluorescence drop at the cessation of LSP excitation corresponds to the rapid disappearance of the assembly. Figure 6b shows LSP-excitation intensity dependence on the enhancement factor. The EF increases with increasing Ieff, showing a saturation tendency up to EF ≈ 5. The observation of EF > 1 means an increase in polymer concentration that is ascribable to optical trapping of the polymer chains. The morphological changes shown in Figure 3 correspond to the formation of polymer assemblies due to LSP-enhanced radiation force. The mechanism well explains a large part of the experimental results: rapid and clear trapping with increasing Ieff, Ieff dependence of EF, and fluorescence spectra. To understand the essentials of these processes, we consider that quantitative discussion is necessary on the basis of theoretical treatments, which is described in the next section. Theoretical Treatment of the PDNIPA Trapping upon LSP-Excitation. The enhanced electromagnetic field (EMF) strength of the incident light around a gold nanopyramidal dimer is calculated using the discrete integral form of the Maxwell equations, which can be solved on the basis of a discrete dipole approximation (DDA),50 where the space including nanopyramidal dimers is divided into small cubic cells. Upon light irradiation, the local electric field at a cell is obtained as the sum of the electric fields of the incident light and the electric field of the dipole emission from the surrounding cells. Microscopic quantities such as the electric field and polarization in the each cell are averaged over the volume integral in the analytical evaluation of the selfinteraction. The local electric field Ei of the ith cell is then given by the integral equation:
Figure 7. Representative examples of the theoretical analysis. (a) Electric field distribution around the Au pyramidal nanodimers. The square of LSP-enhanced electric field is normalized to that without LSP excitation. (b) Radiation force Fx as a function of particle position whose geometry is also shown in the figure. For details, see text.
axes (the x and z coordinates are indicated in the figures). The electric field is strongly enhanced at the narrow nanogaps (width = 30 nm) between the nanopyramids and at the bottom edges of the nanopyramids in the broad nanogap (width = 90 nm between apexes of the pyramids). We call the former and the latter nanostructures the “gap” and “valley”, respectively. It is obvious that electromagnetic field of incident light is much enhanced at the bottoms both of gap and valley, where the enhancement (|E|2/|E0|2) reaches around 104. 14616
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investigated in detail by Kita et al.53 According to their research, ST for PNIPAM (dH = 80 nm) in solution was evaluated to be 0.475 K−1. This value was obtained for a single PNIPAM chain in a random coil structure. Therefore, a PNIPAM globular aggregate with dH ≈ 100 nm (our sample) would have a larger ST value than 0.475 K−1. They also revealed that ST showed positive values, irrespective of temperature. This means that PNIPAM moves from a hot area to a colder area in a temperature gradient. The thermophoresis force acts as a repulsive force in the present case. It should be noted that Fth generally increases as the size of the polymer particle increases, and it would abruptly increase upon phase separation. On the basis of eq 5 and these values, we estimate FT to be |FT| > 2 fN. This value is comparable to the LSP-enhanced trapping force F (∼5 fN). That is, the attractive force and the repulsive force can compete with each other. Such a Soret effect partly contributes to the formation of the larger molecular assembly. The thermal diffusion effect would also contribute to it. In such a power balance, we successfully optically trapped flexible polymer chains on a metallic nanostructure.
Starting with this enhancement value, let us check the validity of the LSP-based trapping mechanism for the present system. As previously described, conventional optical trapping using a focused laser beam (1064 nm) has been frequently performed for PNIPAM/D2O solution where any photothermal effect is completely negligible. Sasaki et al. and ourselves have revealed that ca. Ieff ≥ 10 MW/cm2 was necessary to optically trap PNIPAM chains with a random coil structure in D2O solution.42,43 On the assumption that we can decrease the threshold Ieff by the LSP enhancement effect by a factor of |E|2/| E0|2, Ieff could be evaluated to be of the order of kW/cm2 for the present system. This is fully consistent with the experimental results. For more details, we calculated the LSP-enhanced radiation force (F) using the combination of a well-known equation F = (1/2)Re[α]∇E2 and eq 4. Here, we assume that a random coil PNIPAM is regarded as a nanosphere (hard sphere model) with d = 11 nm and n = 1.47 (refractive index). Letting the PNIPAM sphere approach the bottom of the valley under LSP excitation at Ieff = 1.0 kW/cm2, we calculated Fz as a function of position (z and x). In Figure 7b, a representative result is shown with the geometrical arrangement used for the calculation. As the PNIPAM sphere approaches the bottom of valley, the radiation force |Fz| becomes greater, irrespective of x position. |Fz| ranges from several fN (femto Newton) to several tens of fN. At the optimum x value (82 in the figure), F reaches ≤10 fN. Because a hard sphere model is adopted in the simulation by necessity, the PNIPAM sphere cannot approach the bottom of the valley (where electromagnetic field is greatest) to less than 7 nm. As a matter of course, in the real picture, PNIPAM polymer chains can reach right into the bottoms of valleys and gaps by changing conformation. That is, PNIPAM polymer chains can be subjected to a much higher F (>10 fN). It should be noted that upon LSP excitation at Ieff > 3 KW/cm2, the sample polymer was modified from the randomcoil structure to globular, resulting in molecular assembling. Upon the phase separation, the size (dH) of polymer increases from 11 nm (single random coil) to 100 nm (globular aggregate), leading to an increment in F. On the basis of these results and arguments, we conclude the origin of the present trapping is the LSP-enhanced radiation force. On the other hand, the trapped polymer assembly became larger as Ith increased, outgrowing the LSP irradiation area (Figure 3). This implies that a repulsive force excluding the polymers from the area is generated, competing with the attractive radiation force. One possible candidate for such a repulsive force is the Ludwig−Soret effect (thermophoresis force, Fth), whose contribution has been occasionally pointed out for conventional optical trapping and for LSP-based optical trapping by Gan et al.49 According to the theory of the Ludwig−Soret effect, particles in a medium should move under a temperature gradient by Fth, resulting in a concentration gradient of the particles. The direction of particle diffusion is regulated by the sign of the Soret coefficient, ST, and Fth is expressed by the following equation: Fth = −ST*kT *∇T
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CONCLUSION − TRAPPING MECHANISMS From the above analysis, we can assume the LSP-based trapping mechanism for the present polymer system. When Ieff = 1.0 kW/cm2, phase separation would not be induced. The individual polymer chains with random coil structures are optically trapped by LSP-enhanced radiation force to the gaps and the valleys of Au nanopyramidal dimers. During LSP excitation, polymer chains remain and diffuse around the gaps and valleys, sometimes undergoing fluorescence quenching by the gold surface. The small random coil polymer chains undergo a small radiation force, resulting in a gradual rise of fluorescence after the initiation of LSP excitation (slow trapping). The Soret effect is negligible (because the size of polymer is small, dH = 10 nm), resulting in trapping of a uniform film-like-assembly whose shape corresponds well to the shape of irradiation spot. These processes are seen as the rearrangement in Figure 3a. On the other hand, when Ieff > 3.0 kW/cm2, phase separation would be induced, leading to the formation of a globular aggregate with dH = 100 nm. The larger globular aggregates then are efficiently trapped by the LSP-enhanced radiation force. The larger aggregates undergo a larger radiation force, resulting in a rapid rise in fluorescence after the start of LSP excitation (quick trapping). Simultaneously, the trapped polymer assembly is forced to spread by the Soret effect (thermophoresis force). In the LSP excitation area, the assembly would be strongly trapped to appear as a core-likemorphology in Figure 3b and c. With increasing Ieff, attractive trapping force F and repulsive force Fth increase, resulting in the gradual decay of fluorescence intensity (Figure 5) and the increase in size of polymer assembly increases (Figure 3b and c). In summary, using a thermo-responsive polymer as a target material, we explored LSP-based trapping behavior employing microscopic observation and microfluorescence spectroscopy. We quantitatively discussed temperature elevation, the attractive trapping force, and the repulsive thermophoresis force. The trapping mechanism switches with respect to LSP excitation intensity. PNIPAM polymers can favorably utilize heat generation to increase their size, resulting in efficient and stable trapping by the LSP-enhanced radiation force. The use of such PNIPAM systems, that is, chemically decorating a target
(5)
where k is the Boltzmann constant, T is temperature, and ▽T corresponds to the temperature gradient. ▽T was determined to be ca. −5 × 105 K/m around the gold surface in the z direction (∂T/∂z) and was roughly estimated to be ∼106 K/m around the boundary (outer edge) of the LSP excitation area in the x direction. The Soret effect for PNIPAM solution has been 14617
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The Journal of Physical Chemistry C
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molecule with PNIPAM, will provide a novel methodology for the investigation of molecular trapping and manipulation in plasmonics.
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ASSOCIATED CONTENT
S Supporting Information *
Synthesis of the sample and movies of LSP-based optical trapping. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (Y.T.); kitamura@sci. hokudai.ac.jp (N.K.);
[email protected] (Y.M.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan in the Priority Area “Strong Photon−Molecule Coupling Fields (470)” (No. 19049004), No. 20550002, and the Global COE Program (Project No. B01: Catalysis as the Basis for Innovation in Materials Science). Y.T. and T.S. are grateful to Laser System Ltd. for financial support.
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dx.doi.org/10.1021/jp305247a | J. Phys. Chem. C 2012, 116, 14610−14618
