Measurements of Spontaneous and External Stimuli Molecular Release Processes from a Single Optically Trapped Poly(lactic-co-glycolic) Acid Microparticle and a Liposome Containing Gold Nanospheres.

We investigated the single particle kinetics of the molecular release processes from two types of microcapsules used as drug delivery systems (DDS): biodegradable poly(lactic-co-glycolic) acid (PLGA) and a light-triggered-degradable liposome encapsulating gold nanospheres (liposome-GNP). To optimize the design of DDS capsules, it is highly desirable to develop a method for real-time monitoring of the release process. Using a combination of optical tweezers and confocal fluorescence microspectroscopy we successfully analyzed a single optically trapped PLGA particle and liposome-GNPs in solution. From temporal decay profiles of the fluorescence intensity, we determined the time constant τ of the release processes. We demonstrated that the release rate of spontaneously degradable microcapsules (PLGA) decreased with increasing size, while conversely, the release rate of external stimuli-degradable microcapsules (liposome-GNPs) increased in proportion to their size. This result is explained by the differences in the disruption mechanisms of the capsules, with PLGA undergoing hydrolysis and the GNPs in the liposome-GNP undergoing a photoacoustic effect under nanosecond pulsed laser irradiation. The present approach offers a way forward to an alternative microanalysis system for single drug delivery nanocarriers.

Optical trapping of nanoclusters formed in a temperature-responsive ionic liquid aqueous solution under focused near-infrared laser irradiation.

Confining molecules and ions at a specific position in a solution enables the control of chemical reactions and analysis of tiny amounts of substances. Here, we demonstrate local condensation of a temperature responsive ionic liquid using optical tweezers. Two kinds of microdroplets are prepared through phase separation or nanocluster formation under irradiation of a near-infrared laser beam. The droplet formation mechanism is discussed in view of the evolution of an optical potential well and the local temperature distribution.

Optical Force-Induced Chemistry at Solution Surfaces.

When an intense 1,064-nm continuous-wave laser is tightly focused at solution surfaces, it exerts an optical force on molecules, polymers, and nanoparticles (NPs). Initially, molecules and NPs are gathered into a single assembly inside the focus, and the laser is scattered and propagated through the assembly. The expanded laser further traps them at the edge of the assembly, producing a single assembly much larger than the focus along the surface. Amino acids and inorganic ionic compounds undergo crystallization and crystal growth, polystyrene NPs form periodic arrays and disklike structures with concentric circles or hexagonal packing, and Au NPs demonstrate assembling and swarming, in which the NPs fluctuate like a group of bees. These phenomena that depend on laser polarization are called optically evolved assembling at solution surfaces, and their dynamics and mechanisms are elucidated in this review. As a promising application in materials science, the optical trapping assembly of lead halide perovskites, supramolecules, and aggregation-induced emission enhancement-active molecules is demonstrated and future directions for fundamental study are discussed.

Fluorescence Colour Control in Perylene-Labeled Polymer Chains Trapped by Nanotextured Silicon.

The ability to modulate, tune, and control fluorescence colour has attracted much attention in photonics-related research fields. Thus far, it has been impossible to achieve fluorescence colour control (FCC) for material with a fixed structure, size, surrounding medium, and concentration. Here, we propose a novel approach to FCC using optical tweezers. We demonstrate an optical trapping technique using nanotextured Si (black-Si) that can efficiently trap polymer chains. By increasing the laser intensity, the local concentration of perylene-labelled water-soluble polymer chains increased inside the trapping potential. Accordingly, the excimer fluorescence of perylene increased while the monomer fluorescence decreased, evidenced by a fluorescence colour change from blue to orange. Using nanostructure-assisted optical tweezing, we demonstrate control of the relative intensity ratio of fluorescence of the two fluorophores, thus showing remote and reversible FCC of the polymer assembly.

Nonradiative Energy Transfer through Distributed Bands in Piezochemically Synthesized Cesium and Formamidinium Lead Halide Perovskites.

Repeated absorption of emitted photons, also called photon recycling, in large crystals and thick films of perovskites leads to delayed photoluminescence (PL) and decrease of PL intensity. The role of distinct band gaps, which act as donors and acceptors of energy, and nonradiative energy transfer on such delayed, low intensity emission is yet to be rationalized. Here we report delayed emission by nonradiative energy transfer across a distribution of energy states in close-packed crystallites of cesium lead bromide CsPbBr3 , formamidinium lead bromide FAPbBr3 , or the mixed halide FAPb(BrI)3 perovskite synthesized in the form of thick pellets by the piezochemical method. The PL lifetime of the bromide-rich domain in the mixed halide pellet is considerably decreased when compared with a pure FAPbBr3 pellet. Here the domains with higher bromide composition act as the energy donor, whereas the iodide-rich domains are the acceptors. Time-resolved PL measurements of CsPbBr3 , FAPbBr3 , and the mixed halide FAPb(BrI)3 perovskite pellets help us to clarify the role of nonradiative energy transfer on photon recycling.

Anomalously Large Assembly Formation of Polystyrene Nanoparticles by Optical Trapping at the Solution Surface.

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.

Rapid localized crystallization of lysozyme by laser trapping.

Confining protein crystallization to a millimetre size was achieved within 0.5 h after stopping 1 h intense trapping laser irradiation, which shows excellent performance in spatial and temporal controllability compared to spontaneous nucleation. A continuous-wave near-infrared laser beam is tightly focused into a glass/solution interfacial layer of a supersaturated buffer solution of hen egg-white lysozyme (HEWL). The crystallization is not observed during laser trapping, but initiated by stopping the laser irradiation. The generated crystals are localized densely in a circular area with a diameter of a few millimetres around the focal spot and show specific directions of the optical axes of the HEWL crystals. To interpret this unique crystallization, we propose a mechanism that nucleation and the subsequent growth take place in a highly concentrated domain consisting of HEWL liquid-like clusters after turning off laser trapping.

Crystallization of Methylammonium Lead Halide Perovskites by Optical Trapping.

Single crystals of organolead halide perovskites attract much attention to electrooptical and photovoltaic applications. They are usually prepared in precursor solutions incubated at controlled temperatures or under optimized vapor atmosphere conditions, and thus, multiple perovskite crystals are nucleated all over the solution. Multiple nucleation of crystals prevents efficient use of precursors in the preferential growth of large single crystals. An innovative approach is presented for spatiotemporally controlled, selective nucleation and growth of single crystals of lead halide perovskites by optical trapping with a focused laser beam. Upon such trapping in unsaturated precursor solutions, nucleation of MAPbX3 (MA=CH3 NH3 + ; X=Cl- , Br- , or I- ) is induced at the focal spot through increase in the concentration of perovskite precursors in the focal volume. The rate at which the nucleated crystal grows depends upon whether the perovskite absorbs the trapping laser or not. These findings suggest that optical trapping would be useful to prepare various perovskite single crystals and modify their optical and electronic properties; thereby, offering new methods for engineering of perovskite crystals.

Photocontrolled Supramolecular Assembling of Azobenzene-Based Biscalix[4]arenes upon Starting and Stopping Laser Trapping.

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.

Femtosecond-Laser-Enhanced Amyloid Fibril Formation of Insulin.

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.

Optical Trapping-Formed Colloidal Assembly with Horns Extended to the Outside of a Focus through Light Propagation.

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.

A Single Spherical Assembly of Protein Amyloid Fibrils Formed by Laser Trapping.

Protein amyloids have received much attention owing to their correlation with serious diseases and to their promising mechanical and optical properties as future materials. Amyloid formation has been conducted by tuning temperature and chemical conditions, so that its nucleation and the following growth are analyzed as ensemble dynamics. A single spherical assembly of amyloid fibrils of cytochrome c domain-swapped dimer was successfully generated upon laser trapping. The amyloid fibrillar structure was confirmed by fluorescence characterization and electron microscopy. The prepared spheres were further manipulated individually in solution to fabricate a three-dimensional microstructure and a line pattern. Amyloid formation dynamics and amyloid-based microstructure fabrication are demonstrated based on direct observation of a single spherical assembly, which foresees a new approach in amyloid studies.

Optically Evolved Assembly Formation in Laser Trapping of Polystyrene Nanoparticles at Solution Surface.

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.

Laser trapping-induced crystallization of L-phenylalanine through its high-concentration domain formation.

We present the laser trapping-induced crystallization of L-phenylalanine through high-concentration domain formation in H2O and D2O solutions which is achieved by focusing a continuous-wave (CW) near-infrared laser beam at the solution surface. Upon laser irradiation into the H2O solution, laser trapping of the liquid-like clusters increases the local concentration, accompanying laser heating, and a single plate-like crystal is eventually prepared at the focal spot. On the other hand, in the D2O solution, a lot of the monohydrate needle-like crystals are observed, not at the focal spot where the concentration is high enough to trigger crystal nucleation, but in the 0.5-1.5 mm range from the focal spot. The dynamics and mechanism of the amazing crystallization behaviour induced by laser trapping are discussed from the viewpoints of the concentration increase due to laser heating depending on solvent, the large high-concentration domain formation by laser trapping of liquid-like clusters, and the orientational disorder of molecules/clusters at the domain edge.

Laser trapping chemistry: from polymer assembly to amino acid crystallization.

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.

Formation of a core-shell droplet in a thermo-responsive ionic liquid/water mixture by using optical tweezers.

Many chemical and biological processes involve phase separation; however, controlling this is challenging. Here, we demonstrate local phase separation using optical tweezers in a thermo-responsive ionic liquid/water solution. Upon near-infrared laser irradiation, a single droplet is formed at the focal spot. The droplet has a core consisting of highly concentrated ionic liquid. The mechanism of the core-shell droplet formation is discussed in view of the spatial distribution of optical and thermal potentials.

Generation of Ultralong Liposome Tubes by Membrane Fusion beneath a Laser-Induced Microbubble on Gold Surfaces.

Membrane fusion (MF) is one of the most important and ubiquitous processes in living organisms. In this study, we developed a novel method for MF of liposomes. Our method is based on laser-induced bubble generation on gold surfaces (a plasmonic nanostructure or a flat film). It is a simple and quick process that takes about 1 min. Upon bubble generation, liposomes not only collect and become trapped but also fuse to form long tubes beneath the bubble. Moreover, during laser irradiation, these long tubes remain stable and move with a waving motion while continuing to grow, resulting in the creation of ultralong tubes with lengths of about 50 µm. It should be noted that the morphology of these ultralong tubes is analogous to that of a sea anemone. The behavior of the tubes was also monitored by fluorescence microscopy. The generation of these ultralong tubes is discussed on the basis of Marangoni convection and thermophoresis.

Incoherent Optical Tweezers on Black Titanium.

Optical tweezers enable the manipulation of micro- and nanodielectric particles through entrapment using a tightly focused laser. Generally, optical trapping of submicron size particles requires high-intensity light in the order of MW/cm2. Here, we demonstrate a technique of stable optical trapping of submicron polymeric beads on nanostructured titanium surfaces (black-Ti) without the use of lasers. Fluorescent polystyrene beads with a diameter d = 20-500 nm were successfully trapped on black-Ti by low-intensity focused illumination of incoherent light at λ = 370 m from a Hg lamp. Light intensity was 5.5 W/cm2, corresponding to a reduced light intensity of 6 orders of magnitude. Upon switching off illumination, trapped particles were released from the illuminated area, indicating that trapping was optically driven and reversible. Such trapping behavior was not observed on nonstructured Ti surfaces or on nanostructured silicon surfaces. Thus, the Ti nanostructures were demonstrated to play a key role.

Photoinduced photoluminescence enhancement in self-assembled clusters of formamidinium lead bromide perovskite nanocrystals.

Nanocrystals of formamidinium lead bromide perovskite (FAPbBr3) self-assemble into clusters in powder and film samples and provide a prolonged photoluminescence lifetime, which is attributed to the diffusion of charge carriers through interparticle states formed among nanocrystals. Interestingly, the photoluminescence lifetime decreases and the emission intensity increases for the clusters, which is with the increase in the intensity of excitation light. By doping the nanocrystal clusters with C60, we successfully harvested the photogenerated charge carriers. Nonetheless, at high intensities of excitation, the rate of radiative recombination becomes comparable to that of the electron transfer to C60. Thus, the optimum rate of electron transfer to C60 is accomplished by minimally exciting the self-assembled nanocrystals.

Quantum dot-polymer conjugates for stable luminescent displays.

The broad absorption of light in the UV-Vis-NIR region and the size-based tunable photoluminescence color of semiconductor quantum dots make these tiny crystals one of the most attractive antennae in solar cells and phosphors in electrooptical devices. One of the primary requirements for such real-world applications of quantum dots is their stable and uniform distribution in optically transparent matrices. In this work, we prepare transparent thin films of polymer-quantum dot conjugates, where CdSe/ZnS quantum dots are uniformly distributed at high densities in a chitosan-polystyrene copolymer (CS-g-PS) matrix. Here, quantum dots in an aqueous solution are conjugated to the copolymer by a phase transfer reaction. With the stable conjugation of quantum dots to the copolymer, we prevent undesired phase separation between the two and aggregation of quantum dots. Furthermore, the conjugate allows us to prepare transparent thin films in which quantum dots are uniformly distributed at high densities. The CS-g-PS copolymer helps us in not only preserving the photoluminescence properties of quantum dots in the film but also rendering excellent photostability to quantum dots at the ensemble and single particle levels, making the conjugate a promising material for photoluminescence-based devices.