Laser-induced nanostructuring

Yasumaru N, Miyazaki K, Kiuchi J. Control of tribological properties of diamond-Uke carbon films with femtosecond-laser-induced nanostructuring. Appl Surf Sci 2008 254 2364-8. 

The last problem of this series concerns femtosecond laser ablation from gold nanoparticles [87]. In this process, solid material transforms into a volatile phase initiated by rapid deposition of energy. This ablation is nonthermal in nature. Material ejection is induced by the enhancement of the electric field close to the curved nanoparticle surface. This ablation is achievable for laser excitation powers far below the onset of general catastrophic material deterioration, such as plasma formation or laser-induced explosive boiling. Anisotropy in the ablation pattern was observed. It coincides with a reduction of the surface barrier from water vaporization and particle melting. This effect limits any high-power manipulation of nanostructured surfaces such as surface-enhanced Raman measurements or plasmonics with femtosecond pulses. 

The extremely small cross sections for conventional Raman scattering, typically 10 111 to 10-25 cm2/molecule has in the past precluded the use of this technique for single-molecule detection and identification. Until recently, optical trace detection with single molecule sensitivity has been achieved mainly using laser-induced fluorescence [14], The fluorescence method provides ultrahigh sensitivity, but the amount of molecular information, particularly at room temperature, is very limited. Therefore, about 50 years after the discovery of the Raman effect, the novel phenomenon of dramatic Raman signal enhancement from molecules assembled on metallic nanostructures, known as surface-enhanced Raman spectroscopy or SERS, has led to ultrasensitive single-molecule detection. 

Uwada, T., Asahi, T. and Masuahra, H. (2008) Microspectroscopic analysis of nanostructures by femtosecond laser induced supercontinuum light beam (Japanese), in Nanoimaging (ed. A. Wada), NTS, Tokyo, p. 184. 

Recent advances in ultrasensitive instrumentation have allowed the detection of individual atoms and molecules in solids [174, 175], on surfaces [176, 177], and in the condensed phase [178, 179] using laser-induced fluorescence. In particular, single molecule detection in the condensed phase enables scientists to explore new frontiers in many scientific disciplines, such as chemistry, molecular biology, molecular medicine and nanostructure materials. There are several optical methods to study single molecules, the principles and application of which have been reviewed by Nie and Zare [180]. These methods are listed in Tab. 6.12. 

For mechanical lysis, nanostructured filter-Uke contractions are employed in microfluidic channels with pressure-driven cell flow. Prinz et al. utilized rapid diffusive mixing to lyse Escherichia coli cells and trap the released chromosome via dielectrophoresis (DEP). Kim et al. developed a microfluidic compact disk platform for mechanical lysis of cells using spherical particles with an efficiency of approximately 65 % however, this method is difficult to be apphed for single-cell analysis. Lee et al. fabricated nanoscale barbs in a microfluidic chip for mechanical cell lysis by shear and frictional forces. Munce et al. reported a device to lyse individual cells by electromechanical shear force at the entrance of 10 mm separation channels. The contents of individual cells were simultaneously injected into parallel channels for electrophoretic separation, which can be recorded by laser-induced fluorescence OLIF) of the labeled cellular contents. The use of individual separation channels for each cell separation eliminated possible cross-contamination from multiple cell separations in a single channel. 

Apart from the change in optical properties, laser-induced surface features can also be used for further growth of nanostructures [181]. 

Further modification of the above nanostructures is useful for obtaining new functional materials. Thirdly, we apply the dopant-induced laser ablation technique to site-selectively doped thin diblock copolymer films with spheres (sea-island), cylinders (hole-network), and wormlike structures on the nanoscale [19, 20]. When the dye-doped component parts are ablated away by laser light, the films are modified selectively. Concerning the laser ablation of diblock copolymer films, Lengl et al. carried out the excimer laser ablation of diblock copolymer monolayer films, forming spherical micelles loaded with an Au salt to obtain metallic Au nanodots [21]. They used the laser ablation to remove the polymer matrix. In our experiment, however, the laser ablation is used to remove one component of block copolymers. Thereby, we can expect to obtain new functional materials with novel nanostmctures. 

As aforementioned, diblock copolymer films have a wide variety of nanosized microphase separation structures such as spheres, cylinders, and lamellae. As described in the above subsection, photofunctional chromophores were able to be doped site-selectively into the nanoscale microdomain structures of the diblock copolymer films, resulting in nanoscale surface morphological change of the doped films. The further modification of the nanostructures is useful for obtaining new functional materials. Hence, in order to create further surface morphological change of the nanoscale microdomain structures, dopant-induced laser ablation is applied to the site-selectively doped diblock polymer films. 

In addition, two-photon excitation can be another effective way to achieve NIR-triggered PDT/PTT synergistic therapy. For example, Li and coworkers also prepared hypocrellin-loaded AuNCs for two-photon PDT/PTT of cancer. The AuNCs were optimized to have efficient NIR absorption and further coated with lipids containing hydrophobic photosensitizer hypocrellin B (HB). The photodynamic effect of the photosensitizer was quenched in the Au nanostructure, thereby reducing side effects of the photosensitizer in unintended locations. For in vitro experiments, cancer cells were treated with the nanocomplex and irradiated with a 790 nm pulsed NIR laser. The photosensitizer HB was released from the AuNCs in the intracellular region, which induced dramatic phototoxicity due to the synergistic effect of PDT and PTT under two-photon irradiation. 

Raman enhancement by SERS is mainly attributed to an electromagnetic (EM) field enhancement via localized optical fields of the metallic nanostmctures that are related to plasmon resonance excitation. The increase of the cross-section with contact between the metal nanostructure and a molecule induces an additional enhancement. Without enhancement by the electrical resonance between incident light and molecules, the total Stokes-Raman signal P (vs) is proportional to the number of molecules in the scattering volume N, the Raman cross-section without surface enhancement and the excitation laser intensity... 

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