Optically Evolved Assembly Formation in Laser Trapping of

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Optically evolved assembly formation in laser trapping of polystyrene nanoparticles at solution surface Shun-Fa Wang, Tetsuhiro Kudo, Ken-ichi Yuyama, Teruki Sugiyama, and Hiroshi Masuhara Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02433 • Publication Date (Web): 08 Sep 2016 Downloaded from http://pubs.acs.org on September 14, 2016

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Optically evolved assembly formation in laser trapping of polystyrene nanoparticles at solution surface Shun-Fa Wang,1 Tetsuhiro Kudo,1 Ken-ichi Yuyama,1,* Teruki Sugiyama,1,2,* and Hiroshi Masuhara1* 1

Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan

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Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara 630-0192, Japan

ACS Paragon Plus Environment

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KEYWORDS Polystyrene nanoparticle, Colloidal assembly, Solution surface, Optical trapping, Light scattering, Reflection mircospectroscopy

ABSTRACT

Assembling dynamics of polystyrene nanoparticles by optical trapping is studied with utilizing transmission/reflection microscopy and reflection microspectroscopy. A single nanoparticle assembly with periodic structure is formed upon the focused laser irradiation at solution surface layer and continuously grows up to a steady state within few minutes. By controlling nanoparticle and salt concentrations in the colloidal solution, the assembling behavior is obviously changed. In the high concentration of nanoparticles, the assembly formation exhibits fast growth, gives large saturation size, and leads to dense packing structure. In the presence of salt, one assembly with the elongated aggregates was generated from the focal spot and 1064-nm trapping light was scattered outwardly with directions, while a small circularly assembly and symmetrical expansion of the 1064-nm light were found without salt. The present nanoparticle assembling in optical trapping is driven through multiple scattering in gathered nanoparticles and directional scattering along the elongated aggregates derived from optical association of nanoparticles, which dynamic phenomenon is called optically evolved assembling. Repetitive trapping and release processes of nanoparticles between the assembly and the surrounding solution always proceed, and the steady state at the circular assembly formed by laser trapping is determined under optical and chemical equilibrium.


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1. Introduction Optical trapping with a tightly focused laser beam has played an innovative role for the non-contact manipulation of colloidal particles in solution.1–8 Upon the laser irradiation into a colloidal solution, gradient force toward the focal spot is exerted on nano- and micro-particles. Eventually, the particles are stably trapped in the focal volume where the deep optical potential is formed. Various methods have been developed in order to arrange plural particles at arbitrary positions. For examples, the utilization of scanning mirrors and spatial light modulators made it possible to create large numbers of optical potential wells and to form a specific two- or threedimensional pattern by confining particles in the spatially designed potential.9–12 Interestingly, optical trapping near a solution surface or a glass/solution interface generates an ordered structure of colloidal particles.13–17 When a tightly or loosely focused laser beam is irradiated into an interfacial layer in a colloidal solution, nano- and micro-particles are gathered around the focal spot, and finally an ordered structure of particles is formed. The trapping experiments using plasmonic metal nanostructures and photonic-crystal slabs also form an ordered array of colloidal particles on the substrate.18–21 Similar phenomenon of the ordered structure formation is observed in molecular system. Optical trapping at a solution surface of amino acid solutions leads to crystallization, whereas that inside the solution gives a particle-like assembly without crystal nucleation.6,22,23 The formation of these ordered structures is ascribed to characteristics of interfaces, namely, particles and molecules at interfaces show suppressed diffusion and have interactions not only with solvent but also with the substrate or air. Recently, we demonstrated the formation of a colloidal crystal-like nanoparticle assembly of 200-nm polystyrene (PS) nanoparticles by irradiating a continuous-wave near-infrared laser beam (1064 nm) into the air/solution interfacial layer.16,17 Upon the irradiation, nanoparticles


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were gathered around the focal volume (about 1 µm3), and eventually one circular assembly was grown up to the size of 20 µm in diameter. The large assembly showed structural color, meaning that constituent nanoparticles are arrayed in a periodic manner. We applied reflection microspectroscopy to this assembly formation and examined its dynamics by measuring the time evolution of reflection spectra relating to particle distance. The observed spectral change clearly indicated that nanoparticles were gradually accumulated during laser irradiation and their packing structure became tight and homogeneous. We considered that the above nanoparticle assembly formation proceeds under dynamic balance between the attractive optical trapping force and the electrostatic repulsive force among nanoparticles. In order to understand its formation mechanism deeply, it is important and indispensable to examine the assembly formation with various concentrations of particle and salt. In this paper, we have studied the assembly formation behavior in the solution surface layer by transmission/reflection microscopy and reflection microspectroscopy. Initial nanoparticle and salt concentration dependences of the colloidal crystal-like assembly formation induced by optical trapping were examined and considered. With the increase in the nanoparticle and salt concentrations, the assembly at the steady state was formed with the larger size, and the distance among constituent nanoparticles became shorter. Besides, the optical trapping initially led to the formation of the small aggregates, and subsequently a circular assembly was generated and continuously grown. On the other hand, in the absence of salt, the circular assembly was symmetrically formed without appearance of elongate aggregates. We discuss the dynamics and mechanism by considering the dynamic evolution of optical potential through homogeneous and directional scattering, which are made possible by nanoparticle association.


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2. Experiments 2-1. Optical setup The trapping experiments were carried out using an optical system almost as the same as that in our previous paper.16,17 A linearly polarized laser (1064 nm) of 1.4 W was focused at a sample solution surface layer and the trapping behavior was observed by transmission/reflection microscopy, and reflection microspectroscopy. The details of this microspectroscopy are described in Supporting Information (SI-3).

2-2. Sample preparation The trapping target was polystyrene (PS) nanoparticles with a diameter of 208 nm, same as our previous work.16,17 A colloidal aqueous solution (Polyscience, Inc., nominal diameter; 200 nm, concentration; 2 particles/µm3) was diluted by D2O (Aldrich, 99%) in order to suppress local temperature elevation by laser heating. In the experiment reported by Ito et al., local temperature elevation measured in H2O and D2O at the focal spot of a tightly focused 1064-nm laser beam are 22–24 K/W and 2.6 K/W, respectively.24 The local temperature elevation in H2O is ascribed to absorption of the 1064-nm laser through overtone of OH stretching. However, the solvent evaporation of D2O is still not prevented after several minutes irradiation, and the laser focal position is possibly defocused gradually. So the irradiation time of each sample is limited to 3 minutes at most. For experiments on the particle concentration dependence, we prepared colloidal solutions having four different concentrations (0.2, 0.1, 0.05, and 0.025 particles/µm3). We prepared these solutions in the following procedure in order to suppress the concentration change of additives in the solvent through dilution. Firstly, the purchased mother colloidal solution was


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diluted by D2O to give the concentration of 0.2 particles/µm3. The nanoparticles of the diluted colloidal solution were removed by a centrifugal filter (Amicon, 100 kDa) with centrifugation at 13000 rpm for 2 min. The separated filtrate solution was used as a solvent for dilution of the colloidal solution of 0.2 particles/µm3. The solvent is the same among the prepared colloidal solutions with four different particle concentrations. The samples for the experiments on salting effects were prepared in the following procedure in order to suppress the effects of additives that originally exist in the mother solution. The original colloidal solution was diluted by 25 times with H2O, and the diluted colloidal solution of 1 mL was centrifuged for 45 min at 13000 rpm. The supernatant (990 µL) of the centrifuged solution was removed, and a pure D2O or a sodium chloride (NaCl) D2O solution was added into the remaining solution. The nanoparticle concentration of the colloidal solution was fixed at 0.2 particles/µm3, whereas NaCl concentration was adjusted in a range from 0 to 250 µg/mL. Although the original solvent of mother colloidal solution was nearly removed, the stability of the solution tentatively remained, and no aggregates or precipitates were observed under the microscope. We expect that the nanoparticles are easily attracted with each other due to the thin diffuse electronic double layer of the surface by adding NaCl into solution, since the electric repulsive force among nanoparticles becomes weaker with the increase in NaCl concentration.

2-3. Optical trapping experiments In all experiments, the colloidal solution of 15 µL was poured into a glass bottle with a highly hydrophilic surface at the inside wall and the bottom. After adding the sample solution, it spontaneously spread over the surface of the bottom glass, and a thin film of the solution with


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thickness of 120–150 µm was formed. The bottle was covered with another glass plate in order to suppress solvent evaporation. The sample bottle was set on the stage of an inverted microscope, and the trapping laser was focused into the position 5 µm below the air/solution interface. This focal position allows more particles to be accumulated in the solution surface layer, and the assembly becomes thick enough to obtain the strong signals of the backwardly scattered light. The laser power after objective lens fixed at 1.4 W is sufficient to form the assembly whose reflection band is fully located in visible region and get its reflected light with high signal/noise ratio.


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3. Results and Discussion 3-1. Particle concentration dependence examined by transmission/reflection microscopy Here, we demonstrate particle concentration dependence of the assembly formation. Figure 1a shows one example of the temporal change in transmission images around the focal spot during laser irradiation into the air/solution interfacial layer of the colloidal solution of 0.1 particles/µm3. Upon the irradiation, nanoparticles were confined in the focal volume, and a small nanoparticle assembly was initially formed (panels (i) & (ii) in Figure 1a). Subsequently, a few aggregates were elongated from the small assembly and randomly swung around the focal spot (panel (iii) in Figure 1a). The elongate aggregates were merged to each other, and one circular assembly was formed and continuously grown in a symmetric manner (panel (iv) in Figure 1a). This trapping behavior giving a few elongate aggregates and a single circular assembly was stochastic and changed from observation to observation; however, its spatial and temporal characteristics were always same. The behavior was also observed similarly at the other concentrations. We estimated the assembly diameter from the obtained images by assuming that the assembly corresponds to a dark part within the brightest ring as shown in Figure 1a (iv). The time evolution of the assembly diameter at each concentration is shown in Figure 1b. We measured the size of the circular assembly at the irradiation stage when all elongate aggregates were merged. At the highest concentration (0.2 particles/µm3), a nanoparticle assembly of the circular shape was formed rapidly at 40 sec, and its diameter was increased monotonously till 140 sec. In the lowest concentration (0.025 particles/µm3), the assembly of the circular shape was observed from 120 sec, but its growth was saturated soon. At all concentrations, the assemblies are deemed to be in their steady states at about 180 sec. Apparently, the assembly growth rate


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became higher with the increase in the initial concentration, and the assembly size at 180 sec was increased with the concentration (Figure 1c). The size of a nanoparticle assembly at the steady state appeared to be determined by the initial nanoparticle concentration. In addition, nanoparticles at the assembly boundary showed vigorous fluctuation and random diffusion, while the assembly reached around 20 µm in diameter at the higher concentration. We suggest that nanoparticles are trapped and diffused out at the edge of the assembly and the boundary becomes unclear. The bright spot and rings in the transmission images are interpreted as an optical artifact due to the disc-like structure of the assembly. Namely, a bright spot at the assembly center in the transmission images is regarded as Arago spot which is induced by optical interference.25 This optical effect also results in bright rings around the assembly. Structural color of nanoparticle assemblies was clearly observed by reflection microscopy, where white light was introduced from the bottom of the sample through the objective lens and reflected light was detected. Figure 2 shows reflection images of the nanoparticle assemblies prepared by the 180-sec irradiation at respective concentrations. The colored area was observed around the focal spot in a dark background. The area size in the reflection image at each concentration corresponded well to the assembly size estimated in the transmission image. The fluctuation of nanoparticles at the edge was observed in the transmission images but not in reflection images. Because the reflection from the edge and the diffusing nanoparticles was weak due to the limitation of nanoparticle number, the fluctuation was not clear in reflection images. The color was generated from the whole area of the assembly, while it changed from the center to the outside. It is reported that structural color of a colloidal crystal depends on particle distance;26 therefore observation of structural color change of the present nanoparticle assembly enables us to consider the nanoparticle association structure under


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the optical trapping conditions from the viewpoint of inter-particle distance. Namely, the constituent nanoparticles are arrayed in a periodic manner over all areas of the assembly. The gradient of the color in the assembly implies that the packing structure of nanoparticles varies, depending on the positions in the assembly.


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3-2. Particle concentration dependence examined by reflection microspectroscopy Reflection spectra were sequentially measured at the focus of the trapping laser during optical trapping at each nanoparticle concentration. Figure 3a shows the time evolution of reflection spectra at intervals of 20 sec. As a representative example, we explain the result on the solution of 0.2 particles/µm3. The 20-sec laser irradiation gave a wide reflection band with peak at 610 nm. During the irradiation, the reflection band gradually shifted to 580 nm with narrowing of the band width as well as with increasing of the peak reflectance. When the concentration reduced to 0.1 particles/µm3, the reflection was generated initially at around 610 nm and eventually shifted to 590 nm. After the peak wavelength reached 590 nm at 140 sec, the spectral change was saturated, indicating that the nanoparticle packing structure becomes dense and homogeneous during optical trapping and eventually reaches a steady state. At the higher concentrations (0.2 and 0.1 particles/µm3), a reflection band shows its peak at shorter wavelength, a narrower bandwidth, and higher intensity compared to that at the lower concentrations (0.05 and 0.025 particles/µm3). We normalized reflection spectra of the assemblies formed by the 180-sec irradiation at respective concentrations (Figure 3b). These assemblies were at their steady states, where the apparent assembly sizes are kept constant and the spectral changes are saturated. The peak wavelength gives us the information on the inter-particle distance according to the following Bragg’s equation;26 mλ = 2ndsinθ, where m is diffraction order, λ is wavelength of diffracted light, n is a refractive index of the colloidal crystal, d is distance between the nanoparticle centers, and θ is an incident angle of


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light. The n can be approximately described with the volume fraction of nanoparticles (φp) and refractive indices of a constituent nanoparticle (np) and medium (nm), as follows; n = npφp + nm(1 – φp). The incident angle of white light against the solution surface is kept during the spectral measurement, so that λ depends on nd. When nanoparticles are gathered, the increase in n and the decrease in d are simultaneously induced. However, the latter is dominant compared to the change in n, as numerically confirmed in our previous paper.16 Thus, the peak position of a reflection band approximately depends on the particle distance. The results of normalized reflection spectra indicate that the assembly structure at the steady state shows the initial particle concentration dependence. At the lower concentration, nanoparticles are packed more loosely, and their packing structure becomes inhomogeneous. This is possibly ascribed to wider free space of particle motion for the lower concentration. It is noted that the peak reflectance of the reflection band also depends on the particle concentration. At the steady state, the peak reflectance became higher with the increase in the particle concentration, meaning that the higher concentration solution gives a thicker assembly. Actually it is reported that, in a colloidal crystal, its reflectance becomes high with the increase in the number of the layer.27 Thus, optical trapping of nanoparticles in a solution surface layer generates the larger, thicker, and denser assembly at the higher particle concentration.


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3-3. Salting effect examined by transmission/1064-nm scattering microscopy In the salting effect experiment, the original solvent of mother colloidal solution was nearly removed. Addition of NaCl into the colloidal solution leads to the assembly formation different from the experiment on concentration dependence, as the solvent condition was adjusted differently and the solution components are different. Figure 4 shows a series of transmission images during optical trapping at 0 and 100 µg/mL of NaCl. The comparable video obtained under these two conditions is included in Supporting Information. In the absence of NaCl (Figure 4a), a circular assembly was formed and continuously grew in a symmetric manner during the irradiation. The assembly was kept in a circular shape during its growth, and no elongate aggregate was observed until the assembly reached a steady state. At 100 µg/mL (Figure 4b), one circular small assembly was formed at 2 sec, and three aggregates were elongated outwardly from the small assembly after 3 sec. Spatial and temporal characteristics of aggregate formation are reproducible, although the detail changes from measurement to measurement. The elongate aggregates, which are possibly pinned at the focal spot, underwent the continuous growth to a large assembly. Eventually, the aggregates were merged into a circular symmetric assembly. All the assemblies were dispersed to the solution upon switching off the trapping laser. Furthermore, with the increase in NaCl concentration in the range of 0 and 100 µg/mL, the aggregate generation and its behavior were observed much obviously. It is of great interest to point out that the elongate aggregate generation within the initial 30-sec irradiation was observed only for high NaCl concentration. The formation of elongate aggregates was always observed at higher NaCl concentration, while a circular assembly was formed after 30 sec at all concentrations. It is experimentally confirmed (Figure S2) that the transmission images of nanoparticle assemblies consisting of


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different numbers of elongate aggregates, which was observed at 100 µg/mL. Usually, the number of elongate aggregates was increased with time. With the increase in the number of elongate aggregates, they became unstable and started swinging around the focal spot. Sometimes elongate aggregates dissociated into constituent nanoparticles and disappeared. One end of elongate aggregates was always pinned at the irradiation focus, and once an aggregate is put out of the focus, its morphology cannot be kept anymore. The stably pinned aggregates expanded their size, and the areas between these aggregates were gradually occupied by nanoparticles. The assembly morphology was changed dynamically and stochastically with accompanying the formation, expansion and dissolution of elongate aggregates, and finally a circular assembly was always formed. It seems that the formation of elongate aggregates and a circular assembly always take place competitively in the early irradiation stage. The aggregates elongated from the focal spot inspire us to consider that the trapping laser gathers nanoparticles, propagates through the formed nanoparticle assembly, expands the trapping potential further, and thus grows the aggregates outwardly. In the case of circular expansion, for which one typical example is given in Figure 4a, nanoparticles confined in the focal volume induce scattering of the trapping laser, and the outside particles propagate the scattered laser, forming potential just outside the circular assembly. Nanoparticles are trapped in the potential, increasing the circular assembly size. This optically evolved assembly continues till the optical potential formed by light scattering becomes comparable to thermal motional energy of nanoparticles and cannot trap nanoparticles anymore. In order to confirm this idea, we removed a short pass filter blocking the trapping laser and monitored its backscattering pattern from the assembly. Figure 5 shows the time evolution of the backscattering images of 1064-nm trapping laser at 0 and 100 µg/mL of NaCl. In the absence


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of NaCl (Figure 5a), the concentric circle pattern was observed and continuously extended to the surrounding solution in a symmetric manner. Its circular shape evolution corresponds well to Figure 4a, and therefore we consider that the scattered laser pattern is extended outwardly due to multiple scattering by the growing assembly. At the high NaCl concentration (Figures 5b), the scattered laser pattern was modified after a few seconds, and some bright spots could be seen at the edge of the laser pattern as shown in the panels (ii) & (iii). This is consistent with morphologies of nanoparticle aggregates observed in Figure 4b, so that bright spots should be responsible to the formation of elongate aggregates. It is noteworthy that, in the panels (iv) & (v) in Figure 5b, the directional scattering was clearly observed in the homogeneously extended laser pattern. Finally, the laser pattern would become a concentric circle again, and its size was larger than that in the absence of salt. The comparable video obtained under these two conditions is included in Supporting Information.


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3-4. Salting effect examined by reflection microspectroscopy Addition of NaCl also results in the formation of the larger and denser nanoparticle assembly under optical trapping conditions, which was studied by reflection microspectroscopy. Here, we prepared the colloidal solutions of 0.2 particles/µm3 containing NaCl of 0–250 µg/mL. Figure 6a shows the temporal change in the assembly size at three different NaCl concentrations (0, 50, and 100 µg/mL) determined from the transmission images. The assemblies gradually increased in their sizes under the irradiation and reached respective steady states at 120 sec. The assembly size at the steady state became large with the increase in NaCl concentration. Figure 6b shows the time evolution of reflection spectra at the focal spot at each NaCl concentration. The reflection band initially appeared at long wavelength and continuously shifted to short wavelength with increasing the reflectance. The spectral change was saturated within 120 sec. The solution of higher NaCl concentration gave a wider reflection band at shorter wavelength (Figure 6c). This result indicates that, at a high NaCl concentration, inter-particle distance in the assembly becomes short in average but has wide distribution. Thus, addition of NaCl can form the larger and denser nanoparticle assembly similarly to the case of the high nanoparticle concentration. Also the effective volume of trapped nanoparticles in the focal volume is increased through their association, resulting in efficient light scattering as well as elongate aggregate formation out of the focus. Incidentally, the reflection spectrum at the steady state of the colloidal solution without salt (a blue graph in Figure 6c) is much different, and its band is located in the long wavelength compared to that in Figure 3b. There are two peaks in the reflection band, implying that two different structures are prepared at the assembly center. It is noted that original components of the mother solution of commercial samples were removed, so that gathered nanoparticles at the focus could not form homogeneous aggregate structure.


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It is reasonable to consider that a single larger and denser nanoparticle assembly formed by adding NaCl is ascribed to weakening of electrostatic repulsive force among nanoparticles. In Figure 6d, the peak wavelength of a reflection band at a steady state is plotted as the function of NaCl concentration. The peak wavelength shifted to short wavelength with the increase in NaCl concentration in the range of 0–100 µg/mL, whereas its shift was saturated at the concentration higher than 100 µg/mL. Below a certain concentration, the electrostatic repulsive force due to surface charge on nanoparticles possibly becomes weak with the increase in the salt concentration.26 As the result, nanoparticles come closer to each other and can be packed more tightly. Above 100 µg/mL, the electrical double layer of nanoparticles is screened, and nanoparticles are gathered as close as possible, independent of the salt concentration. Thus, in the assembly formation by optical trapping, addition of NaCl that can suppress inter-particle repulsion is able to decrease the inter-particle distance similarly as in the nanoparticle concentration effect.


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3-5. Dynamics and mechanism of optically evolved assembly formation In this work, we have experimentally confirmed that a single circular assembly of nanoparticles is formed at the air/solution interface upon optical trapping and its lateral size becomes much larger than the focus size. Here, we consider dynamics and mechanism of our unique optical trapping phenomena of nanoparticles from the following viewpoints; (1) optical potential, (2) electrostatic repulsion among particles, (3) solution surface, and (4) thermodynamic equilibrium. Optical potential is initially prepared according to Gaussian form, and nanoparticles are trapped in that focal volume. The trapping light is scattered from the nanoparticles and leaked out from the outside layer of nanoparticles. Optical potential formed just outside of the assembly still attracts nanoparticles, since we introduce high intensity of 1.4W at the focus. The assembly size becomes large as the scattered light is emitted further through the additionally arranged outside layers of nanoparticles. This scattering expansion and additional gathering of nanoparticles take place in a symmetrical manner at the air/solution interface. A circular assembly is grown up to 20 µm in diameter. In case that scattering with further association develops only toward some directions, a few elongate aggregates are prepared. There, propagation of trapping laser along the elongate aggregates may have important roles. The assembly growth behavior through competitive formation of a circular assembly and elongate aggregates, giving the size much larger than the focal volume, depends on particle and salt concentrations. Nanoparticles are gathered and trapped and eventually forms an expanded assembly, while they are in principle repulsive with each other. Upon switching off the trapping laser, the assembly disappears by dispersing the constituent nanoparticles. The electrostatic repulsion among nanoparticles is critical and determines the packing structure of the assembly. The


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addition of salt into the present colloidal solution reduces this electrostatic repulsion and modifies the internal structure of the assembly. As a result, the morphology of the assembly formed at the early irradiation stage is controllable by the salt concentration. The circular assembly is always formed with low salt concentration while the elongate aggregates are often found with high concentration. The suppression of electrostatic repulsion leads to fast association at some places in the potential and results in the formation of some elongate aggregates at initial stages. At the steady states, inter-particle distances of the resultant circular assembly are shortened with higher salt concentration, giving the compact assembly. It should be pointed again that the expanded nanoparticle assembly with periodic structure and the elongate aggregates are formed only at the solution surface layer. The present phenomenon is never realized inside the solution. Solution surface must play one important role for the assembly formation. Nanoparticles at the surface are partly exposed to air, which may be energetically preferred in view of hydrophobicity. In addition, diffusion motion of nanoparticles and their convection flow at the surface and surface layers are suppressed compared to those inside the solution. In the case that the laser is focused just at a solution surface, a circular assembly possibly extends along the surface, but its thickness will not reach a certain value giving enough reflection intensity in microspectroscopy. When we focused the trapping laser at 5 µm below the solution surface, some thickness of the assembly is attained. The surface effects still operate at 5 µm, while the present phenomenon was not observed at the position tens µm below surface. Probably inside solution, three dimensional vigorous diffusion and free convection flow overcomes association with multiple scattering and directional propagation of the trapping laser.


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The nanoparticle assembly and its surrounding colloidal solution are of course in a thermodynamically equilibrated state. The size and packing structure can be explained just in terms of not only optical potential and electrostatic repulsion but also particle concentration. The higher concentration gives larger assembly size at the steady state, faster assembly growth rate, and more packed assembly structure. The salting effect can be explained in similar way. Indeed, the assembly is optically and chemically equilibrated with the surroundings as we pointed out in 1998.28 That was reported just on apparent morphology change of polymers inside solution; however, the internal structure of the assembly was confirmed to depend on concentration in this work, which is the first time to be realized by our reflection microspectroscopic measurement. Dynamics and mechanism of the present assembly formation are schematically summarized in Figure 7, which we designed as top-view. Under the focused laser irradiation, nanoparticles in the beam path are trapped to the focal spot and stayed stably there (panel (i) of Figure 7a). Nanoparticles are continuously confined around the center of optical potential (panel (ii) of Figure 7a), forming a small nanoparticle assembly with the size smaller than the focal volume. When the effective volume of nanoparticles confined in optical potential is increased through their sequential association, the potential becomes deeper, and further nanoparticles are trapped there (panel (iii) of Figure 7a). When the volume is increased to the size of the focus, the efficient scattering of the trapping laser among the associate nanoparticles leaks outward further, forming attractive potential outside the assembly. The assembly size becomes larger than the focus volume, being kept with expanded potential (panel (iv) of Figure 7a). Eventually, more nanoparticles are trapped in the extended potential out of the focus, and then a steady-state assembly becomes much larger than focal spot (panel (v) of Figure 7a).


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On the other hand, in the case of weak electrostatic repulsion in the presence of salt, elongate aggregates stuck out from the small circular assembly (panel (ii) of Figure 7b). With further laser irradiation, the number of elongate aggregates increases, simultaneously accompanying their size expansion (panels (iii) and (iv) of Figure 7b). It is worth noting that the end of the elongate aggregates is always pinned at the focal spot. Finally, all the aggregates are merged with each other to one circular assembly with large size and tight packing structure (panel (v) of Figure 7b). Sometimes, a few aggregates (three at least) are sufficient to form the circular assembly, which does not go to the path of increasing aggregate more. Although the most aggregates pinned at the focal spot stably, occasionally in the early growth stage, it detaches to the surrounding solution and immediately decomposes to constituent nanoparticles (panels (i) and (ii) of Figure 7c). According to the findings on elongate aggregate and its decomposition after detachment, we suppose that enough strong attractive interactions are generated among nanoparticles. Namely, the aggregates pinned at the focus are elongated from the small assembly, the incident laser is efficiently guided along the specific outward directions, and gives further extension of the elongate aggregates. Light propagation of trapping laser enables to gather nanoparticles out of the focal spot through the generation of elongate aggregates. As a representative example, we recently demonstrated that optical trapping of bigger 500-nm particles at a glass/solution interface leads to the formation of a nanoparticle assembly which sticks out rows of linearly aligned particles like horns through light propagation.29 The formation of a circular assembly due to multiple scattering and elongate aggregates due to light propagation possibly takes place competitively, while the circular structure is considered to be dominant at the steady state under the present conditions because the elongate aggregates are always merged to a circular assembly.


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When the initial colloidal solution is dilute, the probability for nanoparticle trapping in the optical focus is low, and the potential slowly evolves. The release of nanoparticles from the shallower and narrower optical potential is relatively preferable compared to the higher concentration solution. As the result, the lower concentration solution gives a smaller nanoparticle assembly where nanoparticles are packed in the longer particle distance. The assembly structure at the steady state is thermodynamically determined by optical properties of trapping target material, chemical characteristics of the solution concentration, and laser parameters. Indeed, optical trapping assembling is realized under “optical and chemical equilibrium”.28 The present viewpoint of optically expanded potential out of focal spot will be useful to not only to form colloidal assembly but also understand generally optical trappinginduced crystallization and phase transition in molecular system.


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Summary We demonstrated the assembly formation of 208-nm polystyrene nanoparticles by optical trapping in a solution surface layer. The assembly formation behavior at different nanoparticle and salt concentrations was examined by transmission/reflection microscopy and reflection microspectroscopy. Upon the irradiation, nanoparticles were gathered at and around the focal spot, and a steady-state nanoparticle assembly where nanoparticles are periodically arrayed was formed in a few minutes. The nanoparticle assembly growth by optical trapping out of focus underwent two processes; the generation of elongate aggregates with weak electrostatic repulsion, and the formation of a circular assembly with strong electrostatic repulsion. As the driving force for evolution of optical potential, we considered directional scattering by elongate aggregates as well as homogeneous scattering by the gathered nanoparticles. Therefore, the nanoparticle assembly was formed even out of the focal spot through the lateral and vertical potential expansion. With the increase in the nanoparticle and salt concentrations, the apparent assembly size got larger, and the nanoparticle packing structure became tighter. We explained these results by considering the dynamic evolution of optical potential under optical and chemical equilibrium, which proceeds through the supply of nanoparticles from the surrounding solution and the association of nanoparticles. These findings will be helpful to design the novel experiments of optical assembly formation of colloidal particles and molecules at the solution surface.


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SUPPORTING INFORMATION The video movies of the transmission and the 1064-nm scattering imaging during the single nanoparticle assembly formation under NaCl concentrations of 0 and 100 µg/mL. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS The present work is partly supported by MOE-ATU Project (National Chiao Tung University) of the Ministry of Education, Taiwan to H.M., and by grants from Ministry of Science and Technology, Taiwan to K.Y. (MOST 105-2113-M-009-019-), to T.S. (MOST 105-2113-M-492001-), and to H.M. (MOST 105-2811-M-009-022), and JSPS KAKENHI Grant Number JP16H06507 in Scientific Research on Innovative Areas “Nano-Material Optical-Manipulation” to T.S.


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Figure 1. (a) A series of transmission images during laser irradiation for the solution of 0.1 particles/µm3. Elapsed time from the beginning of the irradiation is given in each image. (b) The temporal change in the assembly diameter at each concentration. (c) The transmission images of the assemblies formed by the 180-sec irradiation at respective concentrations. The initial particle concentration is shown in each image.


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Figure 2. The reflection images of the nanoparticle assemblies prepared by the 180-sec irradiation for the solution of respective concentrations.


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Figure 3. (a) The time evolution of reflection spectra of the nanoparticle assemblies measured at the focal spot of the trapping laser. (b) The normalized reflection spectra of assemblies formed by the 180-sec irradiation for the colloidal solutions with respective concentrations.


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Figure 4. The time evolution of transmission images during optical trapping of the colloidal solutions by adding (a) 0 and (b) 100 µg/mL of NaCl.


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Figure 5. The time evolution of backscattering images of the 1064-nm trapping laser for the colloidal solutions of (a) 0 and (b) 100 µg/mL of NaCl.


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Figure 6. (a) The temporal change in the assembly diameter at each NaCl concentration. (b) The time evolution of reflection spectra measured at the assembly center under different salt concentrations; (i) 0, (ii) 50, and (iii) 100 NaCl µg/mL. (c) The normalized reflection spectra of assemblies formed by the 180-sec irradiation for the colloidal solutions with respective NaCl concentrations. (d) The peak wavelength of the assemblies formed by the 120-sec irradiation of the solution of various salt concentrations.


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Figure 7. Dynamics and mechanism of nanoparticle assembly formation by optical trapping at a solution surface layer.


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Optically Evolved Assembly Formation in Laser Trapping of

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