Abstract
Anisotropic metal nanoparticles have been paid much attention because the broken symmetry of these nanoparticles often leads to novel properties. Anisotropic gold nanoparticles obtained by wet chemical methods inevitably accompany spherical ones due to the intrinsically high symmetry of face-centred cubic metal. Therefore, it is essential for the purification of anisotropic gold nanoparticles. This work presents a facile, low cost while effective solution to the challenging issue of high-purity separation of seed-mediated grown NIR-responsive gold nanorods from co-produced spherical and cubic nanoparticles in solution. The key point of our strategy lies in different shape-dependent solution stability between anisotropic nanoparticles and symmetric ones and selective self-assembly and subsequent precipitation can be induced by introducing ions to the as-made nanorod solution. As a result, gold nanorods of excellent purity (97% in number density) have been obtained within a short time, which has been confirmed by SEM observation and UV-vis-NIR spectroscopy respectively. Based on the experimental facts, a possible shape separation mechanism was also proposed.
Highlights
Metal nanoparticles with specific shape have become the focus of intensive research because the physicochemical properties of these nanoparticles are highly dependent on their shapes and exposed crystal facets [1,2]
For gold nanorods (NRs), their plasmon band corresponds to absorption and scattering of light is split in two due to their anisotropic shape: one weak band at higher energy resonates along the transverse axis of the NR, while the other strong band at lower energy resonates along the longitudinal axis of the NR
This procedure results in reproducible formation of gold NRs with aspect ratios from 1.5 to 4.5 in nearly quantitative yields and their longitudinal plasmon bands of these NRs are mainly located in visible region
Summary
Metal nanoparticles with specific shape have become the focus of intensive research because the physicochemical properties of these nanoparticles are highly dependent on their shapes and exposed crystal facets [1,2]. A small amount of AgNO3 is put into the growth solution before seed addition to direct the rod growth and adjust the aspect ratio This procedure results in reproducible formation of gold NRs with aspect ratios from 1.5 to 4.5 in nearly quantitative yields (approximately 99%) and their longitudinal plasmon bands of these NRs are mainly located in visible region. Unlike longer gold NRs with aspect ratios beyond 10 which undergo gravitational settling from solution [19], colloidal gold NRs do not precipitate spontaneously because their gravitational force is insignificant as compared to Brownian motion To solve this long-existing problem, Sharma et al adopted centrifugation-assisted sedimentation to purify these NIR-responsive gold NRs by the different shape-dependent sedimentation coefficient of the nanoparticles [20]. The UV-vis-NIR absorption spectra of the gold nanoparticle solutions were recorded by a UNICO 2802S spectrophotometer (UNICO, Shanghai, China) in a wavelength range of 300 to 1,100 nm
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