Laser ablation-based nanofabrication in aqueous solutions
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Laser ablation-based nanofabrication in aqueous solutions
Andrei V. Kabashin and Michel Meunier Laser Processing Laboratory, Department of Engineering Physics, Ecole Polytechnique de Montreal, Case Postale 6079, Succ. Centre-ville, Montreal, Quebec, Canada, H3C 3A7 ABSTRACT An overview of research results related to the femtosecond laser ablation-based method for nanofabri-cation in aqueous solutions is presented. The method makes possible the production of stable biofunctionalized gold nanoparticle colloids with extremely small size (down to 2-2.5 nm) and size dispersion (down to 1-1.5 nm). The colloids are of importance for biosensing applications. INTRODUCTION When nanostructured, metals can demonstrate a drastic transformation of optical properties. In particular, ensembles of metal nanoparticles show spectacular changes of color, as well as transmitted and reflected spectra, when the size and the shape of nanoparticles, or the average distance between them, change [1,2]. These color effects are related to the effective resonant absorption of light by individual nanoparticles and the excitation of surface plasmons, which are electromagnetic waves, coupled to collective oscillations of electrons in metals. After an appropriate surface modification and linking with biological species, metal (Au, Ag, Pt etc.) nanoparticles can be used as their markers. In this case, the presence of these species can be screened by a change of optical characteristics of the nanoparticles. Colloidal metal or semiconductor nanoparticles can be fabricated by a chemical method, in which a diluted metal salt is reduced in an aqueous solution with a reducing reagent [3]. However, the particles are formed in dirty environment with different impurities that complicate further stabilization and functionalization of the gold surface for biological immobilizations [4]. Although chemically prepared particles can be used for some biosensing tasks, alternative methods of the nanofabrication in a clean, well-controlled environment are still in demand. Laser ablation in a liquid environment manifested itself as one of most promising physical methods for colloidal nanoparticle fabrication. This method takes advantage of the fact that the radiation energy can be effectively transmitted through a liquid and concentrated on a solid target to achieve the ablation of material directly in liquids and thus produce colloidal nanoparticles. Most experiments were carried out with the use of nanosecond laser pulses [5-13] and the size distribution of the nanoparticles in liquids tended to be broadened since the aggregation process of hot ablated atoms after the ablation process could not be easily overcome. To reduce the size of produced particles, aqueous solutions of surfactants such as sodium dodecyl sulfate (SDS) were used, which covered the surface of nanoparticles and thus prevented them from the coagulation. This method enabled both the reduction of the mean size and size dispersion of Au and Ag nanoparticles down to 4-8 nm and 5 nm, respectiv
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