Syoji Ito

Laser manipulation and fixation of single gold nanoparticles in solution at room temperature

A method to fix single gold nanoparticles on glass substrate was demonstrated in solution at room temperature by utilizing focused intense laser beams. A single gold nanoparticle of 80 nm was optically trapped and manipulated to a surface of a glass substrate, and then fixed on it by irradiation with ultraviolet (UV) laser light. Suitable laser fluence range for the fixation was determined to be 32–60 mJ/cm2, above which the individual nanoparticles were fragmented to several smaller fragments of 10 to 40 nm. The fixation mechanism is discussed in view of pulsed-laser-induced transient temperature elevation.

One-color reversible control of photochromic reactions in a diarylethene derivative: three-photon cyclization and two-photon cycloreversion by a near-infrared femtosecond laser pulse at 1.28 μm.

One-color control of colorization/decolorization reactions of diarylethene molecules was attained by using nonresonant high-order multiphoton absorption processes with a near-infrared (NIR) femtosecond laser pulse at 1.28 μm with 35 fs full width at half-maximum (fwhm). The intensity of a rather weak laser pulse (<1 nJ/pulse) can induce the simultaneous three-photon absorption leading to the colorization, while much weaker intensity induces two-photon absorption resulting in the decolorization. The spatial patterning concomitant with higher-order multiphoton absorption processes was also demonstrated.

Nondestructive micropatterning of living animal cells using focused femtosecond laser-induced impulsive force

Micropatterning of mouse NIH3T3 fibroblast cells was performed using focused femtosecond laser-induced impulsive force in a culture medium. The cells were detached from an upper substrate by the force and transferred to an underlying substrate with less than spatial resolution of 80μm full width at half maximum. About 80% of the cells were confirmed to be alive at 3h after the patterning. The force exerted to the cell was investigated by high-speed imaging and estimated to be an order of micronewtons. The force origin was not only due to cavitation bubble but also due to shockwave and jet flow.

Optical patterning and photochemical fixation of polymer nanoparticles on glass substrates

A method for fixing patterned nanoparticles onto a substrate was developed by combining photopolymerization with a laser manipulation technique. Nanoparticles were dispersed in ethylene glycol containing monomer, crosslinker, and photoinitiator, and gathered at the focal point of a trapping laser beam (1064 nm) just on a glass substrate. Local photopolymerization within and around the nanoparticles assembly was induced by additional irradiation of a pulsed-laser beam (355 nm), resulting in generation of polyacrylamide containing nanoparticles. The polymerized assembly was evaluated by atomic force microscope observation. By scanning both trapping and excitation laser beams, patterned nanoparticles could also be fixed on a glass substrate.

Ultrafast Photodissociation Dynamics of a Hexaarylbiimidazole Derivative with Pyrenyl Groups: Dispersive Reaction from Femtosecond to 10 ns Time Regions

The photodissociation dynamics of a hexaarylbiimidazole (HABI) derivative with two pyrenyl groups was investigated by time-resolved transient absorption spectroscopy and fluorescence measurements. Transient absorption spectroscopy revealed that photodissociation took place in the wide time region of <100 fs to 10 ns. On the other hand, fluorescence time profiles showed the dynamic red shift in the time region <100 ps. The apparent dispersive photodissociation process was attributed to the increase in the interaction between the pyrenyl moiety in the excited state and the other moiety in the ground state, resulting in the gradual increase of the activation energy for the crossing between the attractive potential surface of an excited pyrenyl unit and the repulsive potential surface.

Permanent fixing or reversible trapping and release of DNA micropatterns on a gold nanostructure using continuous-wave or femtosecond-pulsed near-infrared laser light.

The use of localized surface plasmons (LSPs) for highly sensitive biosensors has already been investigated, and they are currently being applied for the optical manipulation of small nanoparticles. The objective of this work was the optical trapping of λ-DNA on a metallic nanostructure with femtosecond-pulsed (fs) laser irradiation. Continuous-wave laser irradiation, which is generally used for plasmon excitation, not only increased the electromagnetic field intensity but also generated heat around the nanostructure, causing the DNA to become permanently fixed on the plasmonic substrate. Using fs laser irradiation, on the other hand, the reversible trapping and release of the DNA was achieved by switching the fs laser irradiation on and off. This trap-and-release behavior was clearly observed using a fluorescence microscope. This technique can also be used to manipulate other biomolecules such as nucleic acids, proteins, and polysaccharides and will prove to be a useful tool in the fabrication of biosensors.

Nanosecond to submillisecond dynamics in dye-labeled single-stranded DNA, as revealed by ensemble measurements and photon statistics at single-molecule level.

Single-molecule and ensemble time-resolved fluorescence measurements were applied for the investigation of the conformational dynamics of single-stranded DNA, ssDNA, connected with a fluorescein dye by a C6 linker, where the motions both of DNA and the C6 linker affect the geometry of the system. From the ensemble measurement of the fluorescence quenching via photoinduced electron transfer with a guanine base in the DNA sequence, three main conformations were found in aqueous solution: a conformation unaffected by the guanine base in the excited state lifetime of fluorescein, a conformation in which the fluorescence is dynamically quenched in the excited-state lifetime, and a conformation leading to rapid quenching via nonfluorescent complex. The analysis by using the parameters acquired from the ensemble measurements for interphoton time distribution histograms and FCS autocorrelations by the single-molecule measurement revealed that interconversion in these three conformations took place with two characteristic time constants of several hundreds of nanoseconds and tens of microseconds. The advantage of the combination use of the ensemble measurements with the single-molecule detections for rather complex dynamic motions is discussed by integrating the experimental results with those obtained by molecular dynamics simulation.

Fluorescent Photochromic Diarylethene That Turns on with Visible Light.

A new fluorescent photochromic diarylethene that can be activated by irradiation with 405 nm light was synthesized. The turn-on mode switching of fluorescence with visible light is favorable for application to biological systems. The fluorescence quantum yield of the photogenerated closed-ring isomer was as high as 0.8 in less or medium polar solvents, and even in polar acetonitrile the yield was higher than 0.6.

Stepwise Two-Photon-Induced Fast Photoswitching via Electron Transfer in Higher Excited States of Photochromic Imidazole Dimer

Stepwise two-photon excitations have been attracting much interest because of their much lower power thresholds compared with simultaneous two-photon processes and because some stepwise two-photon processes can be initiated by a weak incoherent excitation light source. Here we apply stepwise two-photon optical processes to the photochromic bridged imidazole dimer, whose solution instantly changes color upon UV irradiation and quickly reverts to the initial color thermally at room temperature. We synthesized a zinc tetraphenylporphyrin (ZnTPP)-substituted bridged imidazole dimer, and wide ranges of time-resolved spectroscopic studies revealed that a ZnTPP-linked bridged imidazole dimer shows efficient visible stepwise two-photon-induced photochromic reactions upon excitation at the porphyrin moiety. The fast photoswitching property combined with stepwise two-photon processes is important not only for the potential for novel photochromic materials that are sensitive to the incident light intensity but also for fundamental photochemistry using higher excited states.

Picosecond–nanosecond laser photolysis studies of a photoacid generator in solutions: Transient absorption spectroscopy and transient grating measurements

Photochemical reaction profiles of a photoacid generator, N-trifluoromethylsulfonyloxy-1,8-naphthalimide (NI-Sf), in solution phase were investigated by means of picosecond and nanosecond transient absorption spectroscopy as well as transient grating measurements. Picosecond transient absorption spectroscopy directly revealed that the lifetime of the S1 state and the intersystem-crossing yield decreased with increasing solvent polarity. On the other hand, photochemical reaction yield increased with an increase in the solvent polarity. These results indicated that the photochemical reaction started in the S1 state. Transient grating measurement directly detected the diffusion process of the proton and its diffusion coefficient was obtained to be 3.9 x 10(-9) m2 s(-1), which was several times larger than those of the usual solute molecules.