Engineering the synthesis of silica–gold nano-urchin particles using continuous synthesis

Abstract

Compared to freestanding nanoparticles, supported nanostructures typically show better mechanical stability as well as ease of handling. Unique shapes such as core-shells, raspberries and crescents have been developed on supported materials to gain improved chemical and optical properties along with versatility and tunability. We report the formation of hyperbranched gold structures on silica particles, silica-gold nano-urchin (SGNU) particles. Kinetic control of crystallization, fast mass transfer as well as a bumped surface morphology of the silica particles are important factors for the growth of gold branches on the silica support. Using a microfluidic platform, continuous synthesis of SGNUs is achieved with increased reaction rate (less than 12 min. of residence time), better controllability and reproducibility than that obtained in batch synthesis. The hyper-branched gold structures display surfaceenhanced Raman scattering (SERS). Introduction.Microfluidic systems offer excellent conditions for synthesis of nanomaterials by control of mixing, temperature, and pressure combined with fast heat and mass transfer. 1, 2 Moreover, they enable controlled synthesis under higher temperature and pressure conditions than typically feasible in batch with resulting faster synthesis. 1, 3 Herein, we report batch and microfluidic synthesis of hyper-branched gold structures on silica particles, silica-gold nano-urchin (SGNU) particles. The formed particles consist of a dielectric silica core decorated with densely packed gold nanobranches stretching outward. The morphology serves as an active substrate for tip-enhanced Raman scattering, 4 and could additional have applications in catalysis and bio-imaging. 5,6 Gold nanoparticles have been synthesized by a variety of approaches for diverse applications in catalysis, bio-imaging, drug delivery and photovoltaics. Their surface plasmon resonance (SPR) and surface enhanced Raman scattering (SERS) properties have been of particular interest for photothermal therapy and molecular sensing, respectively [7][8][9][10][11][12][13][14][15] . SERS active structures have been fabricated by complex with top-down methods, 4,12,16,17 but wet-chemical synthesis of gold nanoparticles offer potential advantages in simple, high yield, and limited scalable synthesis. 12,18,19,20,21,22 Gold nanostructures, such as nanorods, nanowires, tetrapods, nanoplates and star-shaped particles are particularly attractive for many applications because of their strong confinement of the electromagnetic field and high enhancement that can be tuned over a wide range of optical wave lengths in comparison with ordinary spherical structures. 11,[23][24][25][26][27] Combination of the metal nanostructure with a support material, such as in core-shells, raspberries, and crescents, affords additional advantages in tuning optical properties, improved chemical/mechanical stability, and ease of handling compared to free-standing nanoparticles.Batch methods, 20 specialized ligands 28 and additional electrochemical treat...

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