Two-Photon Fluorescence Spectroscopy of Individually Trapped Pseudoisocyanine J-Aggregates in Aqueous Solution

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.

Nanoparticle Assembling Dynamics Induced by Pulsed Optical Force

Femtosecond (fs) laser trapping dynamics is summarized for silica, hydrophobically modified silica, and polystyrene nanoparticles (NPs) in aqueous solution, highlighting their distinct optical trapping dynamics under CW laser. Mutually repulsive silica nanoparticles are tightly confined under fs laser compared to CW laser trapping and, upon increasing laser power, they are ejected from the focus as an assembly. Hydrophobically modified silica and polystyrene (PS) NPs are sequentially ejected just like a stream or ablated, giving bubbles. The ejection and bubbling take place with the direction perpendicular to laser polarization and its direction is randomly switched from one to the other. These characteristic features are interpreted from the viewpoint of single assembly formation of NPs at an asymmetric position in the optical potential. Temporal change in optical forces map is prepared for a single PS NP by calculating scattering, gradient, and temporal forces. The relative contribution of the forces changes with the volume increase of the assembly and, when the pushing force along the trapping pulse propagation overcome the gradient in the focal plane, the assembly undergoes the ejection. Further fs multiphoton absorption is induced for the larger assembly leading to bubble generation. The assembling, ejection, and bubbling dynamics of NPs are characteristic features of pulsed optical force and are considered as a new platform for developing new material fabrication method.

Solvent Effects in Highly Efficient Light-Induced Molecular Aggregation

It has been reported that when irradiated with laser light non-resonant with the main absorption peaks, porphyrin molecules (4-[10,15,20-tris(4-sulfophenyl)-21,24-dihydroporphyrin-5-yl]benzenesulfonic acid, TPPS) in an aqueous solution become 10,000 to 100,000 times more efficient in light-induced molecular aggregation than expected from the ratio of gradient force potential to the thermal energy of molecules at room temperature. To determine the mechanism of this phenomenon, experiments on the light-induced aggregation of TPPS in alcohol solutions (methanol, ethanol, and butanol) were performed. In these alcohol solutions, the absorbance change was orders of magnitude smaller than in the aqueous solution. Furthermore, it was found that the absorbance change in the aqueous solution tended to be saturated with the increase of the irradiation intensity, but in the ethanol solution, the absorbance change increased linearly. These results can be qualitatively explained by the model in which intermolecular light-induced interactions between molecules within a close distance among randomly distributed molecules in the laser irradiation volume are highly relevant to the signal intensity. However, conventional dipole–dipole interactions, such as the Keesom interaction, are not quantitatively consistent with the results.

Soft trapping lasts longer: Dwell time of a Brownian particle varied by potential shape

It is often regarded that the dwell time (or residence time, escape time, trapping duration) of trapped Brownian particles is described by the multiplication of two separate factors, i.e., the diffusive traveling time of the trapping domain size without taking into account the trapping force, and the stochastic event of overcoming the trapping energy by thermal one instantaneously. However, we show that the ratio of dwell time to the typical traveling time for the trapping domain size depends on the shape of the force field. The shape of the trapping potential affects this ratio even if the trapping energy gap is the same and the smooth potential has a single minimum. Our finding suggests the possible application of the potential shape to realize the desired trapping characteristics.

Plasmonic optical trapping of nanometer-sized J- /H- dye aggregates as explored by fluorescence microspectroscopy

In the present study, we explored plasmonic optical trapping (POT) of nanometer-sized organic crystals, carbocyanine dye aggregates (JC-1). JC-1 dye forms both J- and H- aggregates in aqueous solution. POT behavior was analyzed using fluorescence microspectroscopy. POT of JC-1 aggregates was realized in an increase in their fluorescence intensity from the focus area upon plasmon excitation. Repeating on-and-off plasmonic excitation resulted in POT of JC-1 aggregates in a trap-and-release mode. Such POT of nanometer-sized dye aggregates lying in a Rayleigh scattering regime (< 100 nm) is important toward molecular manipulation. Furthermore, interestingly, we found that the J-aggregates were preferentially trapped than H-aggregates. It possibly indicates semi-selective optical trapping of nanoparticles on the basis of molecular alignments.

Two-photon excited fluorescence from a pseudoisocyanine-attached gold tip via a plasmonic-photonic hybrid system

A plasmonic-photonic hybrid system with efficient coupling of light from a fiber-coupled microspherical cavity to localized surface plasmon (LSP) modes of a gold-coated tip was proposed, which was composed of a fiber-coupled microspherical cavity and a pseudoisocyanine (PIC)-attached gold tip. To prove efficient excitation of LSP at the gold-coated tip, we experimentally demonstrated two-photon excited fluorescence from the PIC-attached gold-coated tip via a fiber-coupled microspherical cavity under a weak continuous wave excitation condition. This hybrid system could focus the incident light with coupling efficiency of around 64% into a nanoscale domain of the metal tip with an effective area of a 79-nm circle.

Whirl-enhanced continuous wave laser trapping of particles

Tightly focused laser beams can trap micro- and nanoparticles suspended in liquids in their focal spots enabling different functionalities including 3D manipulations and assembling. Here, we report on remarkably strong liquid-liquid phase separation and crystallization experiments in para-nitroaniline dissolved in 1,4-dioxane. For optical trapping of para-nitroaniline we used low-power, weakly focused light beam from continuous-wave laser partially absorbed by the solute. The experiments were performed in solution deposited on glass with an upper free-surface and solution contained between two glass plates. The usual gradient force field and scattering force solely are insufficient to properly describe the observed particle gathering effects extending far beyond the optical trap potential. The concept of whirl-enhanced and temperature assisted optical trapping is postulated. The relative simplicity of the used geometry for trapping will broaden the understanding of the light-matter interaction and promises the widespread application of the observed effect in optically controlled crystallization.

Two-photon excited fluorescence from a pseudoisocyanine-attached gold-coated tip via a thin tapered fiber under a weak continuous wave excitation

A simple tapered fiber based photonic-plasmonic hybrid nanostructure composed of a thin tapered fiber and a pseudoisocyanine (PIC)-attached Au-coated tip was demonstrated. Using this simple hybrid nanostructure, we succeeded in observing two-photon excited fluorescence from the PIC dye molecules under a weak continuous wave excitation condition. From the results of the tip-fiber distance dependence and excitation polarization dependence, we found that using a thin tapered fiber and an Au-coated tip realized efficient coupling of the incident light (~95%) and LSP excitation at the Au-coated tip, suggesting the possibility of efficiently inducing two-photon excited fluorescence from the PIC dye molecules attached on the Au-coated tip. This simple photonic-plasmonic hybrid system is one of the promising tools for single photon sources, highly efficient plasmonic sensors, and integrated nonlinear plasmonic devices.

Laser Trapping and Crystallization Dynamics of l-Phenylalanine at Solution Surface

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.

Trapping and two-photon fluorescence excitation of microscopic objects using ultrafast single-fiber optical tweezers

Analysis of trapped microscopic objects using fluorescence and Raman spectroscopy is gaining considerable interest. We report on the development of single fiber ultrafast optical tweezers and its use in simultaneous two-photon fluorescence (TPF) excitation of trapped fluorescent microscopic objects. Using this method, trapping depth of a few centimeters was achieved inside a colloidal sample with TPF from the trapped particle being visible to the naked eye. Owing to the propagation distance of the Bessel-like beam emerging from the axicon-fiber tip, a relatively longer streak of fluorescence was observed along the microsphere length. The cone angle of the axicon was engineered so as to provide better trapping stability and high axial confinement of TPF. Trapping of the floating objects led to stable fluorescence emission intensity over a long period of time, suitable for spectroscopic measurements. Furthermore, the stability of the fiber optic trapping was confirmed by holding and maneuvering the fiber by hand so as to move the trapped fluorescent particle in three dimensions. Apart from miniaturization capability into lab-on-a-chip microfluidic devices, the proposed noninvasive microaxicon tipped optical fiber can be used in multifunctional mode for in-depth trapping, rotation, sorting, and ablation, as well as for two-photon fluorescence excitation of a motile sample.

Optical trapping through the localized surface-plasmon resonance of engineered gold nanoblock pairs

We have investigated the plasmonic trapping of dielectric nanoparticles by using engineered gold nanoblock pairs with ~5-nm gaps. Pairs with surface-plasmon resonance peaks at the incident wavelength allow the trapping of 350-nm-diameter nanoparticles with 200 W/cm2 laser intensities, and their plasmon resonance properties and trapping performance are drastically modified by varying the nanoblock size of ~20%. In addition, plasmon resonance properties of nanoblock pairs strongly depend on the direction of the linear polarization of the incident laser, which determines the trapping performance.

Evaluation of radiation force acting on macromolecules by combination of Brownian dynamics simulation with fluorescence correlation spectroscopy

The effect of optical gradient force from a focused laser beam on the fluorescence correlation spectroscopy (FCS) was investigated by a computing method based on Brownian dynamics simulation. A series of calculations revealed that, in relatively shallow optical force potential up to 1.0kTR (TR=298.15 K), the conventional theoretical model of FCS without consideration of the optical gradient force could evaluate the increase in the average number of molecules and the diffusion time in the potential. On the other hand, large deviation between the simulated fluorescence correlation curve and the theoretical model was observed under the potential depth >1.0kTR. In addition, by integrating the optical force potential with the temperature elevation under optical trapping condition, it was deduced that the temperature rise does not seriously affect the average number of particles in the sampling area, but the average residence time is more sensitively affected by the temperature elevation. The present study using the simulation also provides a method to experimentally estimate molecular polarizabilities from FCS measurements.

Controlling the formation of cyanine dye H- and J-aggregates with cucurbituril hosts in the presence of anionic polyelectrolytes

The presence of anionic polyelectrolytes enhances the tendency of cationic cyanine dyes to form aggregates in aqueous media. In this work we investigate the interactions between two cyanine dyes, pseudoisocyanine (PIC) and pinacyanol (PIN), with polystyrenesulfonate (PSS) as the key additive to develop J- and H-aggregates. We also take advantage of the binding properties of the cucurbit[7]uril (CB7) host to control formation of these aggregates through its host-guest interactions with the dye molecules. UV/Vis absorption spectroscopic studies clearly demonstrate the PSS-enhanced formation of J-aggregates in the case of PIC and H-aggregates in the case of PIN. Electrostatic interactions between the cyanine dye molecules and the polyelectrolyte chains assist the formation of J- or H-aggregates at very low dye concentrations (ca. 10 microM). Optimum development of dye aggregates was observed at a sulfonate/dye molar ratio of about 3:1. Departures from this stoichiometric ratio seem to perturb the optimal aggregate structure. Furthermore, the presence of CB7 was found to effectively disrupt the interactions responsible for dye aggregation. Thus, CB7 completely disrupts the J-aggregates formed by PIC and the H-aggregates (as well as lower concentrations of J-aggregates) formed by PIN. UV/Vis and emission spectroscopic studies clearly indicate that binding of CB7 to both dye molecules removes them from the aggregate structures. Our spectroscopic data clearly indicate that regulation of the relative molar ratios of dye, CB7 host, and polyelectrolyte sulfonate groups leads to a quantitative control of dye aggregation, yielding variable amounts of PIC J- and PIN H-aggregates in these solutions.

Control of H- and J-aggregate formation via host-guest complexation using cucurbituril hosts

The binding interactions between two cyanine dyes, pseudoisocyanine (PIC) and pinacyanol (PIN), and the cucurbit[n]uril hosts, cucurbit[7]uril (CB7) and cucurbit[6]uril (CB6), were investigated by electronic absorption spectroscopy and DFT computational methods. The CB7 host forms more stable complexes with both dyes than CB6 and the computational studies suggest that the cavity of the smaller host CB6 is not threaded by the dyes. The equilibrium association constants (K) for complexation by CB7 were measured and found to be 2.05 x 10(4) and 3.84 x 10(5) M(-1) for PIC and PIN, respectively, in aqueous media at 23 degrees C. CB7 complexation was found to effectively disrupt the intermolecular forces responsible for the aggregation of both dyes. Thus, CB7 completely disrupts the J-aggregates formed by PIC and the H-aggregates (as well as lower concentrations of J-aggregates) formed by PIN. In both cases a competing guest, 1-aminoadamantane (AD), could be used to adjust the extent of aggregation of the cyanine dye. AD regulates aggregate formation because it forms an extremely stable complex with CB7 (K approximately = 10(12) M(-1)) and exerts a tight control on the CB7 concentration available to interact and bind with the dye.

Effects of optical trapping and liquid surface deformation on the laser microdeposition of a polymer assembly in solution

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.