Gold nanoparticles (NPs) have attracted much interest because of their tunable Surface Plasmon Resonance (SPR) peaks, leading to important applications in the fields of chemistry, biology and materials sciences. Especially, besides the effects of size of gold NPs in cellular and medical applications, a particle shape is another important influence for delivering it into the cell. Some research showed that gold NPs with spherical shapes took shorter wrapping time to wrap the entire bulk in comparison to the nanorod because of the decrease in the surface area.[1,2] Therefore it is urgent to obtain the homogeneous gold nanospheres without any facets. However, the crystalline gold nanoparticles (NPs) prepared in a solution phase always tend to grow anisotropic and accompany with a high tendency to form distinct facets naturally, driven by the surface free energy minimization. Therefore, it still keeps a challenge to achieve highly monodispersed, spherical Au nanoparticles with low cost and well control.
We develop a facile and effective strategy to prepare monodispersed Au spherical nanoparticles by two steps.  Large-scale monocrystalline Au nanooctahedra with uniform size were synthesized by a polyol-route and subsequently Au nanoparticles were transformed from octahedron to spherical shape in a liquid under ambient atmosphere by non-focused laser irradiation in very short time. High monodispersed, ultra-smooth gold nanospheres can be obtained by simply optimizing the laser fluence and irradiation time. Photothermal melting-evaporation model was employed to get a better understanding of the morphology transformation for the system of nanosecond pulsed-laser excitation. These Au nanoparticles were fabricated into periodic monolayer arrays or colloidosomes by interfacial self-assembly utilizing their high monodispersity and perfect spherical shape.  Importantly, such Au nanospheres arrays demonstrated very good SERS enhancement related to their periodic structure due to existence of many SERS hot spots between neighboring Au nanospheres caused by the electromagnetic coupling in an array. They also could be used as visualized bio-sensor after composite with functionalized hydrogel.  It will make them as an excellent and promising candidate for applying in sensing and spectroscopic enhancement, catalysis, energy, and biology.
Keywords: gold nanospheres; self-assembly; SERS; biosensors
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