Hierarchical Gold Flower with Sharp Tips from Controlled Galvanic Replacement Reaction for High Surface Enhanced Raman Scattering Activity

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Abstract

Highly branched Au flowers (AuFs) with sharp tips have been synthesized in high yield by controlling the kinetics of nanocrystal growth via galvanic replacement reaction. Controlled galvanic replacement takes place on appropriately chosen commercially available polystyrene bead supported Cu nanoparticles with HAuCl4. Such a reaction is thermodynamically driven through the exploitation of a Cu(II)/Cu(0) redox couple during nanocrystal formation. The hierarchical morphology of the resulting naked Au flowers (AuFs) depends on the galvanic exchange rate, which increases with increasing HAuCl4 concentration but over a specific concentration range. The formation of AuF is carefully studied, and a spontaneous assembly mechanism is proposed. The time-course experimental results show that the influence of electrostatic field force (EFF) of the charged resin beads is held responsible for prickly tipped AuF formation. Then the fabrication of AuF morphology becomes a large-scale free-standing synthetic protocol for a chemically stable substrate for surface-enhanced Raman scattering (SERS) studies down to the single molecular level. The SERS results guarantee that the as-prepared naked AuF is an excellent and stable SERS substrate. The prickly tips of the gold bearing enhanced field with a large number of embedded "hot spots" hidden within the oriented petals and absence of surfactant or capping agent invites a probe molecule to show the enhancement even for a concentration of 4-mercaptopyridine (4-MPy) down to 10–12 mol dm–3. The electric field distribution around the hot spot has been estimated from 3D-FDTD simulation studies. The selective enhancements of SER bands of the 4-MPy molecule have been unveiled from the view of Herzberg–Teller (HT) charge transfer (CT) mechanism.

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