Immobilization of gold nanorods onto electrospun polycaprolactone fibers via polyelectrolyte decoration--a 3D SERS substrate.

We report the fabrication of a homogeneous and highly dense gold nanorod (AuNR) assembly on electrospun polycaprolactone (PCL) fibers using electrostatic interaction as the driving force. Specifically, decoration of a poly(sodium 4-styrenesulfonate) (PSS) layer onto the AuNRs imposed negative charges on the nanorod surface, and the interactions between PSS and the AuNRs were investigated using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). Positive charges on the PCL fibrous substrate were established via polyelectrolyte layer-by-layer deposition, which was investigated using multiple characterization techniques. Driven by the attractive electrostatic interaction, immobilization of AuNRs on the PCL fibers was initiated upon substrate immersion, and the kinetics of the immobilization process were studied using UV-vis spectroscopy. Electron microscopy characterization of the AuNR/PCL nanocomposite fibers reveals a uniform AuNR coating on the fiber surface with the immobilized AuNR density being high enough to provide full surface coverage. By using both 4-mercaptopyridine and Rhodamine 6G as probe molecules, the performance of the AuNR/PCL fibrous mesh as a three-dimensional (3D) surface-enhanced Raman scattering (SERS) substrate was investigated. The nanocomposite fibers allowed detection at concentrations as low as 10(-7) M of the probe molecule in solution and exhibited excellent reproducibility in the SERS measurements. In addition, a comparison between the 3D AuNR/PCL fibrous mesh and a 2D AuNR/PCL film reveals that the enhanced surface area in the 3D substrate effectively improved the SERS performance with a 6-fold increase in the Raman intensity.