Reflection Microspectroscopic Study of Laser Trapping Assembling of

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Reflection Microspectroscopic Study of Laser Trapping Assembling of Polystyrene Nanoparticles at Air/Solution Interface Shun-Fa Wang,† Ken-ichi Yuyama,*,† Teruki Sugiyama,*,‡ and Hiroshi Masuhara*,† †

Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu 30076, Taiwan

‡

S Supporting Information *

ABSTRACT: We present the formation of a single nanoparticle assembly with periodic array structure induced by laser trapping of 200 nm polystyrene nanoparticles at air/ solution interface of the colloidal heavy water solution. Their trapping and assembling behavior is observed by monitoring transmission and backscattering images and measuring reflection spectra under a microscope. Upon the laser irradiation into the solution surface layer, nanoparticles are gathered at and around the focal spot, and eventually a nanoparticle assembly with the size much larger than the focal volume is formed. The assembly gives structural color in visible range under halogen lamp illumination, indicating that constituent nanoparticles are periodically arrayed. Reflection spectra of the assembly show a reflection band, and its peak position is gradually shifted to short wavelength and the bandwidth becomes narrow with time, depending on the distance from the focal spot. After the laser is switched off, red-shift is observed in the reflection band. These results indicate that nanoparticles are rearranged into a densely packed periodic array during laser irradiation and diffused out to the surrounding solution after turning off the laser. These dynamics are discussed from the viewpoints of the attractive optical trapping force and the electrostatic repulsive force among nanoparticles.

1. INTRODUCTION Association of nanoparticles is important not only in fundamental science but also in industrial application, and their assemblies show unique properties of light scattering and reflection depending on their association structure. For instance, Rayleigh scattering property of plasmonic nanoparticles is drastically changed due to plasmon coupling when they are assembled.1,2 Silica and polymeric nanoparticles that are arrayed with the periodicity of a few hundred nanometers can reflect visible light of specific wavelength depending on distance between nanoparticles.3,4 One of the techniques applicable to control these assembly formation in solution is the laser trapping technique using a tightly focused single laser beam.5 The nanometer-sized trapping targets are simultaneously confined in the focal volume of about 1 μm3, and eventually one spherelike assembly is formed. This optical assembly formation was demonstrated not only for above nanoparticles1,2,6 but also for the quantum dot,7,8 polymer chain,9 micelle,10 amino acid,11 and protein.12,13 Most experiments of assembly formation by laser trapping are carried out inside the solution. This is of course because laser trapping is generally used as a three-dimensional manipulation method in solution. Meanwhile, we have explored the optical assembly phenomena by irradiating trapping laser into solution surface. In 2007, we succeeded in inducing crystallization of glycine by laser trapping at the air/solution interface of its supersaturated solution, whereas the laser trapping inside the solution gave a particle-like molecular assembly at the focal spot.11,14 At the surface layer, glycine liquidlike clusters were gathered at and around the focal spot, © XXXX American Chemical Society

and then the formed molecular assembly evolved to a crystal nucleus through molecular rearrangement induced by the gradient force. The laser trapping-induced crystallization has been confirmed for 8 amino acids, and one of typical examples was recently demonstrated for L-phenylalanine (L-Phe) in the unsaturated H2O solution.15 One notable finding in this crystallization is that laser irradiation into a central position of the resultant crystal causes crystal growth in the unsaturated solution. We assumed that the crystal growth is made possible through the trapping of the liquidlike clusters at the crystal edge. In order to directly observe trapping behavior at the crystal edge, we put 1 μm polystyrene (PS) particles into the sample solution after the crystallization of L-Phe and followed their motion during the laser irradiation into the crystal central part. Upon the irradiation, PS particles were gathered at the crystal edge that was not illuminated directly by the laser. Eventually, the particles were closely trapped at the crystal edge. On the basis of this finding, we have conceived an idea that laser trapping of nanoparticles at a surface layer of colloidal solution will lead to the formation of a colloidal crystal. Actually, we carried out preliminary experiments for this idea, succeeded in the formation of a PS nanoparticle assembly at the air/solution interface, and pointed out its possible structural Special Issue: Kohei Uosaki Festschrift Received: April 24, 2015 Revised: June 4, 2015

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DOI: 10.1021/acs.jpcc.5b03917 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C color.16 As intensively studied on colloidal crystals,3,4,17,18 this structural color should give critical information on the array structure of constituent nanoparticles. In the case of trapping in solution, the nanoparticle assembly with a size of a few micrometers is formed, but structural color is not observed. In this paper, we have applied reflection microspectroscopy to examine the structural color of a colloidal crystal-like nanoparticle assembly formed by laser trapping at a solution surface layer and elucidated its formation dynamics on the basis of the time evolution of reflection spectra. The structural color from the nanoparticle assembly was observed more clearly in backscattering image than in transmission image. In the position-dependent reflection microspectra of the backscattered light from the assembly, one reflection band was observed anywhere over the assembly. The peak position of the reflection band was shifted to short wavelength, accompanied by the decrease in the bandwidth. We explain these results from the viewpoint that nanoparticles are gathered and gradually rearranged into a periodic array due to dynamic balance between the attractive optical trapping force and the electrostatic repulsive force among nanoparticles.

Figure 1. Optical setup for the laser trapping experiment and reflection microspectroscopy.

2. EXPERIMENTS The PS nanoparticles of 208 nm in diameter were used as a target for our laser trapping experiment. A commercially available colloidal aqueous solution (Polyscience, Inc., nominal diameter; 200 nm) was diluted ten times by D2O (Aldrich, 99%) in order to suppress local temperature elevation by laser heating. H2O absorbs the 1064 nm trapping laser through the overtone vibration band of OH, and the light absorption elevates temperature in the focal volume. Actually, it is reported that the local temperature elevation is 22−24 K/W in H2O and 2.6 K/W in D2O when using a high numerical aperture (N.A.) objective lens.19 Nanoparticle concentration in the diluted solution was estimated to be 0.2 nanoparticles/μm3. The diluted colloidal solution of 15 μL was poured into a glass bottle with a highly hydrophilic surface at the inside wall and the bottom, and then the bottle was covered with another glass plate in order to suppress solvent evaporation. After adding the portion of the sample solution, it spontaneously spread over the bottom surface of the bottle, and a thin film of the solution with a thickness of 120−150 μm was formed. The glass bottle was set on the stage of an inverted microscope (Olympus, IX71) for further laser trapping experiments. Figure 1 shows an optical setup based on the inverted microscope. A linearly polarized near-infrared laser beam from Nd3+:YVO4 laser (Spectra Physics, J20I-BL-106C, λ = 1064 nm) was used as a trapping light source. The laser beam was introduced into the microscope and focused at an air/solution interfacial layer of the thin film through an objective lens (60 magnification, N.A. 0.90). The laser power throughout the objective lens was set at 1.4 W by rotating a half wave plate placed in front of a polarizing beam splitter. A diode-pumped solid-state laser beam (Altechna, λ = 532 nm) was also introduced to the microscope in the same optical path as that of the trapping laser in order to adjust the focal position. After the focus of the 532 nm laser was set at 5 μm below the air/ solution interface, the laser was switched off, and then the trapping laser was turned on. The trapping behavior of PS nanoparticles was observed in the following two ways. The sample was illuminated from above using a halogen lamp (Olympus, U-LH1001R) incorporated in the inverted microscope, and the transmission

image was monitored by a video camera (FLOVEL, ADT-33C) with an electron multiplying charge-coupled device (EMCCD). In another way, we used a different halogen lamp (MORITEX SCHOTT, Megalight100) placed outside the microscope. White light was introduced to the sample from the bottom through the objective lens as a scattering light source. Backscattering images were captured by collecting the backscattered light with the same EMCCD camera. For spectroscopic analysis, the backscattered light was introduced to a confocal scanning system (Olympus, FV-300) and detected with a CCD detector (Princeton Instrument, PIXIS 400) coupled with a spectrograph (Princeton Instrument, ACTON Spectra Pro 2300i). The detection area was limited to a region of 1.5 μm in diameter using a confocal pinhole, and the backscattering spectra were measured at a position 0, 4, or 8 μm away from the focal spot of the trapping laser, as shown in an inset of Figure 1. The spectral measurement was started in synchronization with a mechanical shutter (Shutter Instrument, LAMBDA SC) placed in the optical path. The backscattering spectrum is affected by four wavelength dependences of scattering efficiency of trapped nanoparticles, incident white light, transmittance of optics positioned between the sample and the detector, and sensitivity of the detector. We excluded the latter three from a backscattering spectrum of trapped nanoparticles in the following way. In advance, we measured a backscattering spectrum of an aluminum mirror that reflects the incident white light almost completely. The obtained spectrum from the mirror was used as a reference spectrum for the calibration. We divided the backscattering spectrum of trapped nanoparticles by this reference spectrum. The calculated spectrum was used as a reflection spectrum of trapped nanoparticles.

3. RESULTS AND DISCUSSION 3-1. Assembly Formation of Polystyrene Nanoparticles. The PS nanoparticles of 208 nm in solution were barely visible by transmission imaging, despite that the spatial resolution of our microscope system is in the level of several B

DOI: 10.1021/acs.jpcc.5b03917 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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this observation technique, the light passed through the assembly is monitored with the detector, and the illumination light passed through the surrounding solution is detected together, giving a bright image. On the other hand, the backscattering image mainly consists of specific light reflected from the assembly. This observation can generate an image, in which the structural color is detectable more clearly. Figure 3

hundred nanometers. Figure 2 shows a series of transmission images upon laser irradiation into an air/solution interfacial

Figure 2. A series of transmission images upon laser irradiation. Elapsed time from the beginning of the irradiation is given in each image.

layer of the solution thin film. The trapping process in the initial 1 min can be seen in video 1 supplied in the Supporting Information. Before the irradiation, Brownian motion of nanoparticles was observed everywhere in the image. Immediately after starting the irradiation, nanoparticles were gathered at the focal spot, and one circular assembly with a few micrometers in diameter was formed (see Figure 2, panels i−ii). Within a few seconds from the beginning of the irradiation, a few aggregates were elongated outward from the center of the small assembly. The formation of these aggregates was stochastic, and their formation behavior was different among experiments. A representative example of the aggregate formation and growth is shown in Figure 2 (panels iii−iv). There, three elongate aggregates were generated and swung randomly around the focal spot where optical potential is formed. The one side of all the aggregates seemed to be pinned at the focal spot. The aggregates were sometimes dispersed and again generated. Through their random behavior of dispersion, regeneration, and swing, the aggregates were gradually merged to each other. Eventually, a circular assembly was formed and became larger with the irradiation (Figure 2, panel v). It is noteworthy that the nanoparticle assembly formed by laser irradiation became colored under illumination with a halogen lamp. Visible colors were clearly observed at a central part of the assembly, whereas the peripheral part slightly looked pale green. We here defined that a size of the nanoparticle assembly corresponds to the colored area. The assembly size at 120 s was estimated to be 20 μm in diameter, as shown in Figure 2, panel vi. It is important and indispensable to elucidate the mechanism through which the coloration is realized. A constituent PS nanoparticle itself hardly absorbs visible light and also contains no fluorescent dyes. Direct photoexcitation of PS is difficult under the present irradiation condition, so that emission from nanoparticles is ignorable. It is reasonable to consider that the coloration is ascribed to selective reflection of specific light under the illumination. In other words, constituent PS nanoparticles in the assembly are periodically arrayed as in the case of a colloidal crystal, and its specific color is attributed to the periodicity of their array. Thus, the surface laser trapping in the colloidal system leads to the formation of a colloidal crystal-like nanoparticle assembly. 3-2. Assembly Formation Dynamics Studied by the Light Scattering Microspectroscopy. The structural color of the assembly was observed by the transmission imaging. In

Figure 3. A series of backscattering images upon laser irradiation. Elapsed time from the beginning of the irradiation is given in each image.

shows a series of the backscattering images captured with the EMCCD camera during the assembly formation by laser irradiation. Video 2 supplied in the Supporting Information shows the assembly formation process in the initial 1 min. Before starting the trapping of nanoparticles, white light as a scattering light source was introduced to the sample through the objective lens. The slightly pale white image in Figure 3 (panel i) is attributed to light reflection at air/solution interface. At a few seconds after the beginning of the irradiation, a yellow spot was observed in an area of about 2 μm surrounding the focal spot (Figure 3, panel ii). It is considered that the coloration is ascribed to backward scattered light from a small nanoparticle assembly. The colored region was expanded outward from the focal spot (Figure 3, panel iii). This extension of the colored region is possibly attributed to generation of the elongated aggregates in the transmission image. Subsequently, a few colored regions were elongated similarly from the focal spot. They were randomly swung around the focal spot and gradually merged together (Figure 3, panel iv). Finally, a circularly colored area was formed and became larger under the laser irradiation (Figure 3, panel v). The temporal change of the colored area in the backscattering images was well consistent with the formation and growth behavior of a nanoparticle assembly observed in the transmission images. In the circularly colored area in Figure 3 (panel vi), it became orange from yellow with the increasing distance from the central part. This result of the color distribution clearly supports that nanoparticles in the assembly are periodically arrayed not only in the central part but also in the peripheral area that is not irradiated by the laser directly. Periodicity of the nanoparticle array is different depending on position in the assembly, and the position-dependent reflection microspectroscopy enabled us to elucidate spatial distribution of the ordered array of nanoparticles. Figure 4a shows the time evolution of a reflection spectrum measured at the focal spot of the trapping laser. Before starting the trapping laser irradiation, only a flat spectrum due to light reflection at the air/solution interface was observed. After turning on the laser, a wide band appeared around 600 nm in a C

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Figure 4. (a) The time evolution of a reflection spectrum at the focal spot of the trapping laser. (b) The temporal changes in wavelength and reflectance at the peak position of the reflection band. (c) Normalized reflection spectra at irradiation time of 80, 90, and 100 s.

The lowering of n may be induced by the decrease in the volume fraction of nanoparticles (φp) and/or the elevation in temperature at and around the focal spot. The former can be excluded because the concentration increase during the assembly formation always accompanies the increase in φp. As described in the experimental section, the laser-induced local temperature elevation was already estimated in pure H2O and D2O under the experimental condition similar to ours.19 By assuming that the temperature elevation is simply proportional to the input laser power, the elevation in this experiment is estimated to be 3.6 K. On the basis of the reports about temperature dependence of refractive indices of PS and D2O, the decrease in their refractive indices caused by this temperature elevation was calculated to be less than 1%.21,22 The 1% decrease in n is too small to lead to the blue shift from 640 to 600 nm. The refractive index change due to local temperature elevation is ignorable, and the blue shift can be ascribed to the decrease in d. Although this variation in d accompanies the increase in n, it is numerically confirmed that the latter change is smaller than the change in d (see the Supporting Information). Thus, nanoparticles periodically arrayed in the assembly come close to each other under the laser irradiation while keeping the periodic packing structure. We found that the reflection band became a little narrower with the blue shift of the peak wavelength under the laser irradiation, as shown in Figure 4c. This spectral change means that the periodic packing structure of nanoparticles inside the assembly becomes gradually homogeneous while decreasing the distance between nanoparticles. Transmittance of a colloidal crystal becomes lower with the increase in its thickness,23 as a thick colloidal crystal causes efficient reflection of incident light. The increase in the peak reflectance of Figure 4b can be explained from the similar viewpoint of assembly growth in a vertical direction against the solution surface. The nanoparticle assembly becomes thick until 80 s; subsequently, the vertical growth is possibly stopped. The result on the transmission imaging revealed that the assembly growth in a horizontal direction is almost stopped around 80 s. The horizontal growth should be consistent with this consideration of the vertical growth, although the latter could not be observed directly at the present stage.

reflection spectrum. This spectral change should be ascribed to the formation of a nanoparticle assembly. The band shape at 500−700 nm was dynamically changed with the laser irradiation, whereas no obvious change was observed below 500 nm. Figure 4b shows the time evolution of the peak wavelength and reflectance at that wavelength. Initially, the peak position was located around 600 nm and shifted to 640 nm in 30 s. Subsequently, the peak wavelength was shifted gradually to shorter wavelength and returned to 600 nm at 120 s. The peak reflectance was increased with the irradiation and reached the maximum value of about 10% at 80 s, and then no change was observed until 120 s. The increase in the reflectance was stopped almost at the same time with the shift of the wavelength. As observed in the transmission imaging, the elongated aggregates are formed and swung randomly in the initial 30 s. At this time region, the behavior of assembly growth is stochastic, and the detected spectrum is different from time by time. Then we consider the spectral change after the 30 s irradiation, accompanied by continuous growth of a circular assembly. Blue shift of the peak wavelength during this time period can be discussed on the basis of the Bragg’s law. Light is selectively diffracted by a fine colloidal crystal according to the following Bragg’s equation:20 mλ = 2nd sin θ

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 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 = n pφp + nm(1 − φp)

The incident angle of white light against the solution surface was experimentally estimated to be 50−70 degrees, independent of the detection position (see the Supporting Information). Since the incident angle is kept during the spectral measurement, it is reasonable to consider that the blue shift of the peak wavelength from 640 to 600 nm is due to the decrease in d and/or n. D

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Figure 5. (a) The normalized reflection spectra measured at different positions for the nanoparticle assembly prepared by the 120 s irradiation. The temporal change in (b) wavelength and (c) reflectance at the band peak in reflection spectra measured at different positions during the assembly formation. The distance from the focal spot of the trapping laser is given in each graph.

Figure 6. Reflection spectra measured after turning off the trapping laser. The measurement points are (a) 0, (b) 4, and (c) 8 μm distant from the focal spot. Each spectrum was obtained with the accumulation time of 0.1 s.

wavelength and returned to 600 nm at 120 s. At an 8 μm distant position, a reflection band appeared around 50 s, and subsequently, the peak position was shifted to shorter wavelength. These results on the blue shift of the peak wavelength clearly indicate that nanoparticle rearrangement into densely packed structure proceeds at the whole area of the assembly. Figure 5c shows the time evolution of peak reflectance at respective positions. The reflectance at each point was gradually increased with time, and the increase was saturated within 100 s. The reflectance became lower with the increasing distance from the assembly center. This result implies that the assembly center is thicker compared to the assembly boundary. Nanoparticles in the assembly were immediately diffused out to the surrounding solution upon switching off the trapping laser. Due to this dynamic diffusion property of a nanoparticle assembly, we could trace the relaxation and destruction of the nanoparticle periodic array by measuring reflection spectra. The dynamic spectral changes after cutting off the laser are shown in Figure 6 (panels a−c), which were measured at different positions in the assembly prepared by the 120 s irradiation. After the irradiation was stopped, the reflection band was shifted to longer wavelength, accompanying the gradual decrease in reflectance. The reflection band at the assembly center could be observed for longer time than that of the

3-3. Position-Dependent Assembling Dynamics. We measured reflection spectra at different positions of the assembly outside the focal spot of the trapping laser by controlling the confocal scanning system. The detection position was set at 4 or 8 μm away from the focal spot. The spectral measurement was carried out for 120 s, while the growth of a nanoparticle assembly is almost saturated at this time. Figure 5a shows the normalized reflection spectra at different positions after the 120 s irradiation of the trapping laser. The comparison of these spectra gave us crucial information on the spatial distribution of the periodic array structure of nanoparticles. The reflection band peak in the assembly peripheral part was located at longer wavelength compared to the central part. On the other hand, the reflection bandwidth became narrower with the increasing distance from the focal spot. These results mean that the interparticle distance becomes longer and the structural homogeneity in the periodic array does higher toward the assembly peripheral area. Figure 5b shows the temporal change in the peak wavelength in a reflection band at each position. The spectral change at a 4 μm distant position was similar to that at the assembly center. Red-shift and subsequent blue-shift were observed in the initial 40 s. This complicated spectral change may be ascribed to the formation of the elongated aggregates and their random swing. After 50 s, the peak position was continuously shifted to shorter E

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Figure 7. (a) Possible formation mechanism of the colloidal crystal-like assembly of nanoparticles under laser irradiation into an air/solution interfacial layer.

surface are rearranged into ordered structure, and they will work as a template to arrange other nanoparticles from the solution. The circular assembly of nanoparticles is continuously grown outward from the focal spot. This assembly growth is induced by the outward scattered trapping laser, and nanoparticles outside the focal spot are confined in the extended optical potential (Figure 7b). In principle, optical potential is formed through interactions of the laser and nanoparticles. When the effective volume of nanoparticles confined in optical potential is increased, the potential becomes deeper, and further nanoparticles can be trapped there. In addition, the volume increase in the optical potential possibly causes the efficient scattering of the trapping laser, and the optical potential is extended outward from the focal spot while being deepened (see the Supporting Information). Through this dynamic evolution of the optical potential, nanoparticle concentration in the extended optical potential becomes gradually higher, and the distance among trapped nanoparticles is shortened during the assembly growth (Figure 7c). Since nanoparticles used here are negatively charged, the distance between nanoparticles is adjusted under dynamic balance between the attractive optical trapping force and the electrostatic repulsive force among nanoparticles. Finally, we discuss homogeneity of nanoparticle packing structure. With the decrease in nanoparticle distance, the structure is expected to be more homogeneous, because moving range of the nanoparticles becomes shorter. Contrary to this expectation, the experimental result showed that the periodic array structure of nanoparticles becomes more homogeneous toward the assembly peripheral area where the nanoparticle distance becomes long. We infer that strong trapping force is necessary to form the assembly but, at the assembly central part, it deforms to some extent the periodic array structure. The nonuniform distribution of strong trapping force at the focal spot reflecting its Gaussian form may cause the formation of some aggregates. The reflection spectrum from the central part of the assembly showed a broad reflection band, indicating that nanoparticles there are assembled with various nanoparticle distances in a less periodic manner.

peripheral area. The nanoparticle diffusion toward the surrounding solution is started from the assembly peripheral area, and then the relaxation and destruction of the assembly is possibly spread to the assembly central part. 3-4. Formation Mechanism of the PS Nanoparticle Assembly. On the basis of the spectroscopic analysis, we summarize the laser trapping behavior of PS nanoparticles at a solution surface layer. First, we consider the assembly growth and then discuss dynamics and mechanism of the laser trapping-induced formation of the nanoparticle assembly. The laser irradiation into the small spot of 1 μm led to the formation of an assembly of 20 μm in diameter. The temperature elevation by laser heating should be considered as a possible driving force for the growth. The local heating causes convection flow and/or thermophoresis,24−28 which will be ascribed to changes in surface tension and density of the solvent. During the laser irradiation, nanoparticles will be transported along the direction of the convection flow and/or temperature gradient. Although such diffusion was observed in the peripheral part of the observation area, nanoparticles in the central part were pulled to the focal spot, forming a nanoparticle assembly. Thus, we consider that laser heating is not a main reason for the assembly growth outward from the focal spot. The heating effect would rather interrupt the assembly growth. Therefore, as the most possible driving force for the assembly growth, we here infer that the scattered laser can form optical potential at the outside of the focal spot. This potential formation is ascribed to efficient lateral scattering of the trapping laser by the small nanoparticle assembly and/or at the air/solution interface. The optical potential extended from the focal spot possibly attracts nanoparticles in the area larger than the focal volume and leads to the assembly growth. Figure 7 shows a schematic illustration of the possible mechanism for the assembly formation and growth. Upon the laser irradiation, nanoparticles are continuously confined in the focal volume where optical potential is formed by the focused laser beam, and a small nanoparticle assembly is prepared (Figure 7a). Subsequently, the assembly evolves to a circular nanoparticle assembly with periodic array structure. It is reported that micrometer-sized particles at the solution surface are spontaneously rearranged into a periodic array.29−31 At the solution surface, nanoparticles have interactions with air as well as solvent, and their diffusion is suppressed two-dimensionally. The spontaneous nanoparticle rearrangement would be realized due to these characteristics of the solution surface. Similarly, in the present experiment, nanoparticles gathered at the solution

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SUMMARY In summary, we performed laser trapping of 208 nm PS nanoparticles at an air/solution interfacial layer, and its behavior was examined by measuring reflection spectra under the microscope as well as by observing transmission and backscattering images. The laser irradiation into the surface F

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(5) Ashkin, A. History of Optical Trapping and Manipulation of Small-Neutral Particle, Atoms, and Molecules. IEEE J. Quantum Electron. 2000, 6, 841−856. (6) Magome, N.; Kitahata, H.; Ichikawa, M.; Nomura, S. M.; Yoshikawa, K. Rhythmic Bursting in a Cluster of Microbeads Driven by a Continuous-Wave Laser Beam. Phys. Rev. E 2002, 65 (045202), 1−4. (7) Pan, L.; Ishikawa, A.; Tamai, N. Detection of Optical Trapping of CdTe Quantum Dots by Two-Photon-Induced Luminescence. Phys. Rev. B 2007, 75 (161305), 1−4. (8) Chiang, W.-Y.; Okuhata, T.; Usman, A.; Tamai, N.; Masuhara, H. Efficient Optical Trapping of CdTe Quantum Dots by Femtosecond Laser Pulses. J. Phys. Chem. B 2014, 118, 14010−14016. (9) Singer, W.; Nieminen, T. A.; Heckenberg, N. R.; RubinszteinDunlop, H. Collecting Single Molecules with Conventional Optical Tweezers. Phys. Rev. E 2007, 75 (011916), 1−5. (10) Hotta, J.; Sasaki, K.; Masuhara, H. A Single Droplet Formation from Swelled Micelles by Radiation Pressure of a Focused Infrared Laser Beam. J. Am. Chem. Soc. 1996, 118, 11968−11969. (11) Tsuboi, Y.; Shoji, T.; Kitamura, N. Optical Trapping of Amino Acids in Aqueous Solutions. J. Phys. Chem. C 2010, 114, 5589−5593. (12) Tsuboi, Y.; Shoji, T.; Nishino, M.; Masuda, S.; Ishimori, K.; Kitamura, N. Optical Manipulation of Proteins in Aqueous Solution. Appl. Surf. Sci. 2009, 255, 9906−9908. (13) Shoji, T.; Kitamura, N.; Tsuboi, Y. Resonant Excitation Effect on Optical Trapping of Myoglobin: The Important Role of a Heme Cofactor. J. Phys. Chem. C 2013, 117, 10691−10697. (14) Sugiyama, T.; Adachi, T.; Masuhara, H. Crystallization of Glycine by Photon Pressure of a Focused CW Laser Beam. Chem. Lett. 2007, 36, 1480−1481. (15) Yuyama, K.; Sugiyama, T.; Masuhara, H. Laser Trapping and Crystallization Dynamics of L-Phenylalanine at Solution Surface. J. Phys. Chem. Lett. 2013, 4, 2436−2440. (16) Yuyama, K.; Sugiyama, T.; Masuhara, H. Laser Trapping Dynamics of 200 nm-Polystyrene Particles at a Solution Surface. Proc. SPIE 2013, 8810 (88101V), 1−7. (17) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. SingleCrystal Colloidal Multilayers of Controlled Thickness. Chem. Mater. 1999, 11, 2132−2140. (18) Fudouzi, H.; Xia, Y. Colloidal Crystals with Tunable Colors and Their Use as Photonic Papers. Langmuir 2003, 19, 9653−9660. (19) Ito, S.; Sugiyama, T.; Toitani, N.; Katayama, G.; Miyasaka, H. Application of Fluorescence Correlation Spectroscopy to the Measurement of Local Temperature in Solutions under Optical Trapping Condition. J. Phys. Chem. B 2007, 111, 2365−2371. (20) Bazin, G.; Zhu, X. X. Formation of Crystalline Colloidal Arrays by Anionic and Cationic Polstyrene Particles. Soft Matter 2010, 6, 4189−4196. (21) Odhner, H.; Jacobs, D. T. Refractive Index of Liquid D2O for Visible Wavelengths. J. Chem. Eng. Data 2012, 57, 166−168. (22) Michel, P.; Dugas, J.; Cariou, J. M.; Martin, L. Thermal Variations of Refractive Index of PMMA, Polystyrene, and Poly(4methyl-1-pentene). J. Macromol. Sci., Part B 1986, 25, 379−394. (23) Gates, B.; Lu, Y.; Li, Z. Y.; Xia, Y. Fabrication and Characterization of Photonic Crystals with Well-Controlled Thickness and Stop-Band Attenuation. Appl. Phys. A: Mater. Sci. Process. 2003, 76, 509−513. (24) Braun, D.; Libchaber, A. Trapping of DNA by Thermophoretic Depletion and Convection. Phys. Rev. Lett. 2002, 89 (188103), 1−4. (25) Duhr, S.; Braun, D. Two-Dimensional Colloidal Crystals Formed by Thermophoresis and Convection. Appl. Phys. Lett. 2005, 86 (131921), 1−3. (26) Dasgupta, R.; Ahlawat, S.; Gupta, P. K. Trapping of MicronSized Objects at a Liquid−Air Interface. J. Opt. A: Pure Appl. Opt. 2007, 9, S189−S195. (27) Louchev, O. A.; Juodkazis, S.; Murazawa, N.; Wada, S.; Misawa, H. Coupled Laser Molecular Trapping, Cluster Assembly, and Deposition Fed by Laser-Induced Marangoni Convection. Opt. Express 2008, 16, 5673−5680.

layer of the colloidal solution caused the formation of a nanoparticle assembly that gives structural color in the visible region under the halogen lamp illumination. Through the spectroscopic analysis, we found that the resultant nanoparticle assembly shows a reflection peak, which indicates that constituent nanoparticles are periodically arrayed in the assembly. The time evolution of the reflection spectrum at the focus made clear that the array structure of nanoparticles gradually becomes dense and uniform under the laser irradiation. The distance between nanoparticles was shorter in the assembly center compared to the assembly boundary. On the other hand, homogeneity of the structure became higher with the increasing distance from the central part. This ordered array of nanoparticles is temporarily formed under dynamic balance between the attractive optical trapping force and the electrostatic repulsive force among nanoparticles. These findings will enable us to understand the unique behaviors of laser trapping of molecules and molecular clusters at air/ solution interface and investigate the mechanism of the laser trapping crystallization. In addition, this laser trapping technique will provide an opportunity to develop a potential preparation method of a colloidal crystal.

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ASSOCIATED CONTENT

S Supporting Information *

Video movies of the nanoparticle assembly formation, estimation of incident angle of white light, calculation of the changes in the distance between nanoparticle centers and in the refractive index of the colloidal crystal during assembly formation, and images of the trapping laser scattered from the solution surface and the nanoparticle assembly. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03917.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work is partly supported by the MOE-ATU Project (National Chiao Tung University) of the Ministry of Education, Taiwan, to H.M., the National Science Council of Taiwan to K.Y. (MOST 103-2113-M-009-022-MY2), to T.S. (NSC 102-2113-M-492-001-MY2), and to H.M. (MOST 1032113-M-009-003).

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