Abstract
We hereby report a novel synthesis method of size and shape controllable gold nanoparticles that is rapid, in situ and seedless. Unlike most currently employed size and shape controllable synthesis methods, it takes place in a single step under room temperature within ~15 minutes. While mixtures of gold nanospheres around 70 nm and gold nanoplates with width ranging from 100 nm to 1000 nm can be synthesized in about 15 minutes by standard synthesis method using N-2-hydroxyethylpiperazine-N-2-ethanesulphonic acid (HEPES) to reduce Au(III), gold nanoflowers or mixtures of smaller gold nanospheres and nanoplates can be synthesized with the addition of disodium phosphate (Na2HPO4) or monosodium phosphate (NaH2PO4), respectively. Increasing the concentration of phosphate added significantly reduces the formation time of gold nanoparticles to seconds. By increasing the molar ratio of Na2HPO4: HEPES and NaH2PO4: HEPES, the size of gold nanoflowers and gold nanoparticle mixtures can be tuned from ~60 nm down to 1 nm and from ~70 nm to ~2.5 nm, respectively. The systematic structural changes are accompanied by similarly systematic colour changes associated with shifting of the surface plasmon resonance. The proposed mechanism of the synthesis process is also presented.
Highlights
Gold nanoparticles (GNPs) have received increasing attention as nanotechnology has flourished[1,2,3,4,5]
The hydroxyethylpiperazine-N-2-ethanesulphonic acid (HEPES) reduced GNP synthesis method has been applied and studied for a decade, and it has been reported that the difference of the molar ratio of chloroauric acid and HEPES strongly affects the GNP synthesized[38]
During the synthetic progress of all colloidal gold samples, the colour of the system changed from pale yellow, which is the colour of chloroauric acid, into colourless, became pink/purple/blue, representing the formation of GNPs
Summary
Gold nanoparticles (GNPs) have received increasing attention as nanotechnology has flourished[1,2,3,4,5]. In addition to nanospheres[16], GNPs with different shapes such as nanorods[17,18], nanoplates[19,20,21,22], planar nanoparticles[23,24], and branched nanostructures like nanoflowers[25,26] and nanostars[27,28] have been synthesized Among these shapes, the nanoplates have attracted particular attention due to the high local electric field gradients under illumination caused by their sharp edges, which can be applied in SERS29, and the nanoflowers, with rough surface and dense tips, are able to enhance SERS to a greater degree, and have been considered powerful SERS probes[26,30,31,32]. By adjusting the molar ratio of phosphate to HEPES, the size of nanoflowers can be modulated from ~60 nm to less than 1 nm
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