Search

The major photoproducts from benzophenone (BP) adsorbed on the Ti---Al binary oxides were benzhydrol and benzpinacol where benzhydrol yield depended on Ti---Al ratio. The conclusions drawn were that more yield of benzhydrol was observed when BP protonated species were more predominant on the surfaces.

The solvent dependence of the spectral position of the charge-transfer fluorescence maxima, quantum yield, excited state depopulation kinetics and resulting radiative rate constant are investigated. Single photon timing and picosecond transient absorption experiments in a polar and an apolar solvent are performed to get more information about the fluorescent excited states. Results indicate that emission occurs from a highly polar excited state and that the high quantum yield increases as solvent polarity increases. Moreover, solvents of intermediate polarity show a maximum radiative rate constant.

Fluorescence measurements under a confocal microscope were done to define the role of radiation force in molecular assembling in a polymer solution using pyrene-labeled poly(N-isopropylacrylamide) as test case. The results confirmed that micropolarity around the pyrene chromophore in water is high and that the association of the chromophore is inefficient. It was also confirmed that irradiation results to a decrease in micropolarity and an enhancement of excimer formation.

Optical matter, a transient arrangement formed by the interaction of light with micro/nanoscale objects, provides responsive and highly tunable materials that allow for controlling and manipulating light and/or matter. A combined experimental and theoretical exploration of optical matter is essential to advance our understanding of the phenomenon and potentially design applications. Most studies have focused on nanoparticles composed of a single material (either metallic or dielectric), representing two extreme regimes, one where the gradient force (dielectric) and one where the scattering force (metallic) dominates. To understand their role, it is important to investigate hybrid materials with different metallic-to-dielectric ratios. Here, we combine numerical calculations and experiments on hybrid metal-dielectric core-shell particles (200 nm gold spheres coated with silica shells with thicknesses ranging from 0 to 100 nm). We reveal how silica shell thickness critically influences the essential properties of optical binding, such as interparticle distance, reducing it below the anticipated optical binding length. Notably, for silica shells thicker than 50 nm, we observed a transition from a linear arrangement perpendicular to polarization to a hexagonal arrangement accompanied by a circular motion. Further, the dynamic swarming assembly changes from the conventional dumbbell-shaped to lobe-like morphologies. These phenomena, confirmed by both experimental observations and dynamic numerical calculations, demonstrate the complex dynamics of optical matter and underscore the potential for tuning its properties for applications.

Mapping of the spatial and temporal motion of particles inside an optical field is critical for understanding and further improvement of the 3D spatio-temporal control over their optical trapping dynamics. However, it is not trivial to capture the 3D motion, and most imaging systems only capture a 2D projection of the 3D motion, in which the information about the axial movement is not directly available. In this work, we resolve the 3D incorporation trajectories of 200 nm fluorescent polystyrene particles in an optical trapping site under different optical experimental conditions using a recently developed widefield multiplane microscope (imaging volume of 50 × 50 × 4 μm 3 ). The particles are gathered at the focus following some preferential 3D channels that show a shallow cone distribution. We demonstrate that the radial and the axial flow speed components depend on the axial distance from the focus, which is directly related to the scattering/gradient optical forces. While particle velocities and trajectories are mainly determined by the trapping laser profile, they cannot be completely explained without considering collective effects resulting from hydrodynamic forces.

We demonstrated the optical trapping-induced formation of a single large disc-like assembly (∼50 μm in diameter) of polystyrene (PS) nanoparticles (NPs) (100 nm in diameter) at a solution surface. Different from the conventional trapping behavior in solution, the assembly grows from the focus to the outside along the surface and contains needle structures expanding radially in all directions. Upon switching off the trapping laser, the assembly disperses and needle structures disappear, while the highly concentrated domain of the NPs is left for a while. The single assembly is quickly restored by switching on the laser again, where the needle structures are also reproduced but in a different way. When a single 10 μm PS microparticle (MP) is trapped in the NP solution, a single disc-like assembly containing needle structures is similarly prepared outside the MP. Based on backscattering imaging and tracking analyses of the MP at the solution surface, it is proposed that scattering and propagation of the trapping laser from the central part of the NP assembly or the MP lead to this new phenomenon.

In colloidal solution, nanoparticles can be optically trapped by a tightly focused laser beam, and they are assembled in a focal spot whose diameter is typically about one micrometer. We herein report that a large submillimeter sized assembly of polystyrene microparticles with necklace-like patterns are prepared by laser trapping at a solution surface. The light propagation outside the focal spot is directly confirmed by 1064 nm backscattering images, and finite difference time domain simulation well supports the idea that an optical potential is expanded outside the focal spot based on light propagation through whispering gallery mode. This demonstration opens a new method for fabrication of a millimeter-order huge assembly by a single tightly focused laser beam.

Laser trapping has been utilized as tweezers to three-dimensionally trap nanoscale objects and has provided significant impacts in nanoscience and nanotechnology. The objects are immobilized at the position where the tightly focused laser beam is irradiated. Here, we report the swarming of gold nanoparticles in which component nanoparticles dynamically interact with each other, keeping their long interparticle distance around the trapping laser focus at a glass/solution interface. A pair of swarms are directionally extended outside the focal spot perpendicular to the linear polarization like a radiation pattern of dipole scattering, while a doughnut-shaped swarm is prepared by circularly polarized trapping laser. The light field is expanded as scattered light through trapped nanoparticles; this modified light field further traps the nanoparticles, and scattering and trapping cooperatively develop. Due to these nonlinear dynamic processes, the dynamically fluctuating swarms are evolved up to tens of micrometers. This finding will open the way to create various swarms of nanoscale objects that interact and bind through the scattered light depending on the properties of the laser beam and the nanomaterials.

Femtosecond (fs)-laser-induced crystallization as a novel crystallization technique was proposed for the first time by our group, where the crystallization time can be significantly shortened under fs laser irradiation. Similarly, we have further extended our investigation to amyloid fibril formation, also known as a nucleation-dependence process. Here we demonstrate that the necessary time for amyloid fibril formation can be significantly shortened by fs laser irradiation, leading to favorable enhancement. The enhancement was confirmed by both spectral measurements and direct observations of amyloid fibrils. The thioflavin T fluorescence intensity of laser-irradiated solution increased earlier than that of the control solution, and such a difference was simultaneously revealed by ellipticity changes. At the same time before intensity saturation in fluorescence, the number of amyloid fibrils obtained under laser irradiation was generally more than that in the control solution. Besides, such an enhancement is correlated to the laser power threshold of cavitation bubbling. Possible mechanisms are proposed by referring to fs-laser-induced crystallization and ultrasonication-induced amyloid fibril formation.

Well-packed thylakoids known as grana are one of the major functional sites for photosynthesis in algae and plants. Their highly ordered structures can be considered as a few hundred nanometer-sized particles having distinct scattering cross sections from other various macromolecular organizations inside plant cells. With this background we show that elastic light scattering imaging and microspectroscopy is an important tool for investigating structure and organization of grana inside a single chloroplast in plant cells. We have demonstrated this noninvasive method to identify the distribution of grana in intact fresh leaf of robust and rapidly growing Egaria densa, which is also known as Anachris and among the most popular aquarium plants. The scattering efficiency spectra of their individual grana fairly resemble cooperative absorption spectra of porphyrins and carotenoids. We found that the electronic structure of the stacked thylakoids shows granum size-dependence, indicating that size of grana is one of the critical parameters in the regulation of the photochemical functions in the thylakoid.

Laser trapping in chemistry covers various studies ranging from single molecules, nanoparticles, and quantum dots to crystallization and liquid-liquid phase separation of amino acids. In this work, a supramolecular assembly of azobenzene-based biscalix[4]arene is generated in ethyl acetate using laser trapping; its nucleation and growth are elucidated. No trapping behavior was observed when a 1064 nm laser beam was focused inside of the solution; however, interesting assembling phenomena were induced when it was shined at the air/solution interface. A single disk having two layers was first prepared at the focal point of ∼1 μm and then expanded to the size of a few tens of micrometers, although no optical force was exerted outside of the focal volume. Upon switching the trapping laser off, needles were generated at the outer layer of the assembly, giving a stable sea urchin-like morphology to the generated assembly. At a 30-50% dilution of the initial solution in ethyl acetate, a mushroom-like morphology was also observed. Laser trapping-induced assembly of azobenzene-based biscalix[4]arene was quite different from the sharp-ellipsoidal aggregates obtained by the spontaneous evaporation of the solution. These trapping phenomena were specifically observed for biscalix[4]arene in the trans conformation of azo-benzene moiety but not for the cis-form, suggesting that the laser trapping of this azobenzene-based biscalix[4]arene is photocontrollable. Dynamics and mechanism of the supramolecular assembling are considered, referring to laser trapping-induced nucleation and liquid-liquid phase separation of amino acids.

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 circular 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.

We report optical trapping and assembling of colloidal particles at a glass/solution interface with a tightly focused laser beam of high intensity. It is generally believed that the particles are gathered only in an irradiated area where optical force is exerted on the particles by laser beam. Here we demonstrate that, the propagation of trapping laser from the focus to the outside of the formed assembly leads to expansion of the assembly much larger than the irradiated area with sticking out rows of linearly aligned particles like horns. The shape of the assembly, its structure, and the number of horns can be controlled by laser polarization. Optical trapping study utilizing the light propagation will open a new avenue for assembling and crystallizing quantum dots, metal nanoparticles, molecular clusters, proteins, and DNA.

The development in optical trapping and manipulation has been showing rapid progress, most of it is in the small particle sizes in nanometer scales, substituting the conventional continuous-wave lasers with high-repetition-rate ultrashort laser pulse train and nonlinear optical effects. Here, we evaluate two-photon absorption in optical trapping of 2.7 nm-sized CdTe quantum dots (QDs) with high-repetition-rate femtosecond pulse train by probing laser intensity dependence of both Rayleigh scattering image and the two-photon-induced luminescence spectrum of the optically trapped QDs. The Rayleigh scattering imaging indicates that the two-photon absorption (TPA) process enhances trapping ability of the QDs. Similarly, a nonlinear increase of the two-photon-induced luminescence with the incident laser intensity fairly indicates the existence of the TPA process.

We present laser trapping behavior of l-phenylalanine (l-Phe) at a surface of its unsaturated aqueous solution by a focused continuous-wave (CW) near-infrared (NIR) laser beam. Upon the irradiation into the solution surface, laser trapping of the liquid-like clusters is induced concurrently with local laser heating, forming an anhydrous plate-like crystal at the focal spot. The following laser irradiation into a central part of the plate-like crystal leads to laser trapping at the crystal surface not only for l-Phe molecules/clusters but also for polystyrene (PS) particles. The particles are closely packed at crystal edges despite that the crystal surface is not illuminated by the laser directly. The molecules/clusters are also gathered and adsorbed to the crystal surface, leading to crystal growth. The trapping dynamics and mechanism are discussed in view of optical potential formed at the crystal surface by light propagation inside the crystal.

We experimentally demonstrate reordering throughout the inside of an individual bipolar nematic liquid-crystalline microdroplet optically trapped by a highly focused laser beam, when the laser powers are above a definite threshold. The threshold depends on the droplet size and laser polarization. A physical interpretation of the results is presented by considering the nonlocal orientations of the nematic liquid-crystal molecules in the droplets with the dimensions on the order of the focal spot diameter or larger. On the basis of the finite size approximation, we show that the dependence of threshold power on the droplet size is calculated to be in qualitative agreement with the experimental data.

Laser trapping has served as a useful tool in physics and biology, but, before our work, chemists had not paid much attention to this technique because molecules are too small to be trapped in solution at room temperature. In late 1980s, we demonstrated laser trapping of micrometer-sized particles, developed various methodologies for their manipulation, ablation, and patterning in solution, and elucidated their dynamics and mechanism. In the 1990s, we started laser trapping studies on polymers, micelles, dendrimers, and gold, as well as polymer nanoparticles. Many groups also reported laser trapping studies of nanoclusters, DNA, colloidal suspensions, etc. Following these research streams, we have explored new molecular phenomena induced by laser trapping. Gradient force leading to trapping, mass transfer by local heating, and molecular reorientation following laser polarization are intimately coupled with molecular cluster and aggregate formation due to their intermolecular interactions, which depend on whether the trapping position is at the interface/surface or in solution. In this Account, we summarize our systematic studies on laser trapping chemistry and present some new advances and our future perspectives. We describe the laser trapping of nanoparticles, polymers, and amino acid clusters in solution by focusing a continuous wave 1064 nm laser beam on the molecules of interest and consider their dynamics and mechanism. In dilute solution, nanoparticles with weak mutual interactions are individually trapped at the focal point, while laser trapping of nanoparticles in concentrated solution assembles and confines numerous particles at the focal spot. The assembly of polymers during their laser trapping extends out from the focal point because of the interpolymer interactions, heat transfer, and solvent flow. When the trapping laser is focused at an interface between a thin heavy water solution film of glycine and a glass substrate, the assembled molecules nucleate and evolve to a liquid-liquid phase separation, or they will crystallize if the trapping laser is focused on the solution surface. Laser trapping can induce spatiotemporally the liquid and solid nucleation of glycine, and the dense liquid droplet or crystal formed can grow to a bulk scale. We can control the polymorph of the formed glycine crystal selectively by tuning trapping laser polarization and power. These results provide a new approach to elucidate dynamics and mechanism of crystallization and are the fundamental basis for studying not only enantioselective crystallization but also confined polymerization, trapping dynamics by ultrashort laser pulses, and resonance effect in laser trapping.

We have developed a novel laser-induced crystallization method utilizing local heat-induced bubble/water interface. Continuous laser beam of 1064 nm is focused on a gold nanoparticles thin film surface covered with glycine supersaturated aqueous solution. Light absorption of the film due to localized plasmon resonance caused local heating at the focal position and produced a single thermal vapor microbubble, which generated thermal gradient followed by convection flow around the bubble and eventually induced glycine crystallization and growth. The crystallization mechanism is discussed by considering gathering and accumulating molecules around the bubble/water interface assisted by convection flow and temperature jump., (© 2012 American Chemical Society)

Controlling chemical reactions within a small space is a significant issue in chemistry, and methods to induce reactions within a desired position have various potential applications. Here we demonstrate localized, efficient photopolymerization by radiation pressure. We induced a one-photon UV polymerization of liquid acrylate solutions in the optical-trapping potential of a focused near-IR (NIR) laser beam, leading to the confinement of solidification to a minute space with dimensions smaller or equal to one-fifth of the wavelength of the NIR laser. Our approach can produce solidification volumes smaller than those achievable with conventional one-photon polymerization, thus enabling the production of tiny polymeric structures that are smaller than the diffraction limit of the trapping light. This is the first demonstration of a radiation pressure effect on a photochemical reaction.

We have studied the photochromic reaction of p-hydroxycinnamic-thiophenyl ester in the microcrystalline state. We attributed the fluorescence spectral evolution of the microcrystal, under UV irradiation, to the photoinduced trans-to-cis isomerization. The photocyclic behavior of the chromophore was demonstrated by cis-to-trans back reaction under a subsequent visible light irradiation. In addition, the [2 + 2] topochemical photocyclodimer was observed as another photoproduct. It is considered that the cooperative photoisomerization is initiated at the local lattice distortion and free spaces around the [2 + 2] cyclodimer near the crystal surface, and the photoisomerization induces larger lattice deformation and further photoisomerization in the interior of the crystal.

A polymer microassembly is formed by focusing a near-infrared (NIR) laser beam in a thin film of a polymer solution. We have investigated the mechanism of laser microdeposition of a polyfluorene assembly by measuring the surface deformation of the solution film and the morphology of the deposited assembly. It is clearly observed that a rupture is formed at the laser focus in the solution film by using laser interferometric imaging. The time necessary for the rupture formation and the volume of the deposited microassembly are analyzed as a function of laser power. Experimental results suggest that the solution surface deformation induced by local laser heating and optical trapping effects determined the volume of the laser microdeposition. By combining this method with multiple optical trapping, a polymer microassembly with a polygonal morphology is formed on the glass substrate.

We have demonstrated size-selective optical trapping and deposition of polymer and aromatic molecules from binary mixed solution. As a near-infrared laser beam is tightly focused in polystyrene and perylene mixed solution and dropped on a glass substrate, a molecular assembly is deposited at the laser focus and fixed on the substrate. The fluorescence spectrum of the deposited microassembly depends on the laser power; perylene monomer fluorescence is dominant in the case of high laser power, whereas excimer emission of perylene crystal is observed in the case of low laser power. This suggests that polystyrene molecules are preferentially deposited by focusing a higher laser power so that the ratio of polystyrene and perylene in the assembly can be controlled by laser power. This mechanism can be explained in view of the molecular size selectivity in optical trapping.

We have investigated a pseudoisocyanine dye aqueous solution including nanometer-sized J-aggregates by combining optical trapping and two-photon fluorescence spectroscopy. By focusing an intense near-infrared laser into an 8 x 10(-3) M solution, the intense fluorescence from J-aggregates for a few to tens of seconds is observed intermittently, indicating that individual J-aggregates are trapped in and diffuse out from a focal spot. The peak position and full width at half-maximum of the J-band are different from each other. By measuring 171 J-aggregates, it was found that J-aggregates can be classified largely into two groups. The existence of two kinds of groups of J-aggregates could be attributed to the difference in the nucleation process, which is affected by the substrate. J-aggregates possessing a J-band of a narrower bandwidth in a shorter wavelength region are trapped for a longer period of time, indicating that highly ordered J-aggregates are trapped for a longer period of time because of their high polarizability.

Femtosecond near-infrared laser microscope was developed with a home-built cavity-dumped chromium:forsterite laser as a light source centered at 1.26 microm. Optimization of the pulse duration achieved 35 fs fwhm at the sample position of the microscope after passing through a 100x objective. This system was applied to the detection of multiphoton fluorescence of some organic microcrystals. Excitation intensity dependence and the interferometric autocorrelation detection of the fluorescence clearly demonstrated that simultaneous three- and four-photon absorption processes are responsible for the production of the excited state for perylene and anthracene microcrystals, respectively. The spatial resolution along the optical axis and its dependence on the order of the multiphoton process were also discussed.

We report on a spreading behavior of phospholipid membranes that arise from a lump of phospholipid (a lipid source) on topographically patterned substrates immersed in an aqueous solution. Microgrooves with well-defined shapes were prepared on Si111 surfaces by anisotropic etching in an alkaline solution. A spreading front that consists of membrane lobes and a single lipid bilayer was observed on the patterned silicon substrates by utilizing fluorescence interference contrast (FLIC) microscopy. FLIC images indicate that the membrane lobes span the microgrooves, while the underlying single lipid bilayer spread along the surface of the microgrooves. In fact, fluorescent polystyrene nanoparticles could be encapsulated in the microgrooves that were completely covered with the membrane lobes. The groove-spanning behavior of membrane lobes is discussed in terms of a balance between adhesion and bending energies of lipid bilayers.

Organic dye nanoparticles 1-13 nm in height and 10-45 nm in width were prepared by the reprecipitation method. With single-molecule/nanoparticle spectroscopy, two distinct types of nanoparticles were found: particles with blue emission and particles with red emission. The difference in spectral characteristics is attributed to the presence of two morphological types of particles in the samples. The presence of two types of nanoparticles in the samples was further corroborated by our ability to separate blue nanoparticles from red nanoparticles by centrifugation.

Ordered fibril- and particle-like assemblies of poly(2,7-(9,9-bis(2-ethylhexyl)fluorene)) can be formed by photon force of a focused near-infrared laser beam during the drying process of its tetrahydrofuran solution on a glass substrate. These formations have been achieved controllably by combining laser irradiation with convection in the cast solution; that is, when viscous drag of the solution in the convection is stronger than the photon force, the fibril-like assemblies can be formed. Molecular orientation in the assemblies differs from that in self-assembled fibril-like structures, and maybe it can be controlled by the polarization direction of the focused laser beam. We have demonstrated that the length and width of the assemblies can be controlled by the irradiation time, the laser power, the concentration of the solution, and the convection rate in the solution. On the other hand, when the viscous drag of the solution in the convection is weak compared to the photon force, particle-like assemblies in which molecular orientation is controlled by polarization direction are formed.

We report on the growth of giant membrane lobes that is mechanically driven by wetting fronts of phospholipid membranes at water-solid interfaces and a strategy to control the two-dimensional structure of the membrane lobes on a solid surface. The growth of giant membrane lobes was observed on a single-lipid bilayer which spread from a lump of phospholipid deposited on a silica-glass substrate or an oxidized silicon wafer in aqueous solutions of NaCl, KCl, MgCl2, or CaCl2 at relatively high salt concentrations. Most of the membrane lobes were very flat unilamellar tubes elongating from the lump of phospholipid, and their length reached 1 mm in 5 h. Experimental findings clearly indicate that the membrane lobes are adherent to the surface of the single-lipid bilayer and are mechanically elongated from the lump of phospholipid by the sliding motion of the single-lipid bilayer. We could control the two-dimensional structure of the membrane lobes on the substrate by controlling the spreading direction of the single-lipid bilayer using Pt micropatterns that were deposited on the smooth surface of the oxidized silicon wafer.

The primary photochemical event of photoactive yellow protein (PYP) was studied by laser flash photolysis experiments on a subpicosecond-nanosecond time scale. PYP was excited by a 390-nm pulse, and the transient difference absorption spectra were recorded by a multichannel spectrometer for a more reliable spectral analysis than previously possible. Just after excitation, an absorbance decrease due to the stimulated emission at 500 nm and photoconversion of PYP at 450 nm were observed. The stimulated emission gradually shifted to 520 nm and was retained up to 4 ps. Then, the formation of a red-shifted intermediate with a broad absorption spectrum was observed from 20 ps to 1 ns. Another red-shifted intermediate with a narrow absorption spectrum was formed after 2 ns and was stable for at least 5 ns. The latter is therefore believed to correspond to I1 (PYP(L)), which has been detected on a nanosecond time scale or trapped at -80 degrees C. Singular value decomposition analysis demonstrated that the spectral shifts observed from 0.5 ps to 5 ns could be explained by two-component decay of excited state(s) and conversion from PYP(B) to PYP(L). The amount of PYP(L) at 5 ns was less than that of photoconverted PYP, suggesting the formation of another intermediate, PYP(H). In addition, the absorption spectra of these intermediates were calculated based on the proposed reaction scheme. Together, these results indicate that the photocycle of PYP at room temperature has a branched pathway in the early stage and is essentially similar to that observed under low-temperature spectroscopy.