Surface-enhanced Raman spectroscopy using a coffee-ring-type three-dimensional silver nanostructure

We demonstrated surface-enhanced Raman spectroscopy using a coffee-ring-type three-dimensional silver nanostructure (Ag3D). Ag3D has high activity for surface-enhanced Raman scattering (SERS). We fabricated Ag3D using convective self-assembly to form a ‘coffee-ring’ structure that consisted of a mixed solution of silver particles and polystyrene latex beads. The mechanism of the formation of Ag3D is described by the competition among a radial flow, a Marangoni recirculating flow, DVLO interactions and sedimentation. Using Ag3D, measurements of a trace amount of 4,4′-bipyridine (4bpy) in aqueous solution were performed, and typical spectra of 4bpy were observed within 1 min of dropping 4bpy. By optimizing the Ag3D preparation conditions, we could observe the characteristic enhanced Raman spectrum of 1 nM 4bpy solution. In addition, we succeeded in observing chloride activation, which increased the intensity of the Raman spectra by a factor of two to three on addition of sodium chloride to Ag3D. SERS measurements using Ag3D were stable even with passing time, whereas the peak intensity drastically dropped within a few minutes in the case with silver nanoparticles only.