Abstract
The detection performances of noble metal-based surface enhanced Raman spectroscopy (SERS) devices are determined by the compositions and geometries of the metal nanostructures, as well as the substrates. In the current study, long spiky Au-Ag alloy nanostars were synthesized, and both core diameters and spike lengths were controlled by Lauryl sulfobetaine concentrations (as the nanostructure growth skeleton). The long spiky star geometries were confirmed by transmission electron micrograph images. Elements energy dispersive spectrometer mapping confirmed that Au and Ag elements were inhomogeneously distributed in the nanostructures and demonstrated a higher Ag content at surface for potential better SERS performance. Selected synthesized spiky nanostars were uniformly assembled on multi-mode silica fiber for probe fabrication by silanization. The SERS performance were characterized using crystal violet (CV) and rhodamine 6G (R6G) as analyte molecules. The lowest detection limit could reach as low as 10−8 M, with a 6.23 × 106 enhancement factor, and the relationship between analyte concentrations and Raman intensities was linear for both CV and R6G, which indicated the potential qualitative and quantitative molecule detection applications. Moreover, the fiber probes also showed good reproducibility and stability in the ambient atmosphere.
Highlights
Noble metal nanostructures have attracted much attention for their optical and electrical properties (originating from their local surface plasmon resonance (LSPR)) for various applications in recent years [1,2,3]
Considering small particles may provide denser LSPR hot-spot and the difficulty of fiber assembling, spiky nanostructures synthesized with 50 mM Lauryl sulfobetaine (LSB) were chosen for further fiber probe fabrication and Surface enhanced Raman scattering (SERS) performance investigation
The peaks red-shifted with LSB concentration increase, and the sphere-like plasmonic peaks weakened and disappeared, which indicated the formation of long spiky brunches on the larger core
Summary
Noble metal (such as silver, gold, platinum, copper, etc.) nanostructures have attracted much attention for their optical and electrical properties (originating from their local surface plasmon resonance (LSPR)) for various applications in recent years [1,2,3]. Surface enhanced Raman scattering (SERS) is one of these important applications for ultra-low or single molecular substance detection [4]. Special nanostructure geometries provide denser hot spots [14] and larger surface area for analyte attachment for better SERS detection behavior than regular sphere nanoparticles. Sharp tips in special structures generate a much stronger LSPR electric field than sphere nanoparticles [15], due to size and space effect, for further SERS improvement. Au nanostructures show better stability while their LSPR effect and SERS detection performance were not as good as that from Ag under similar sizes or structures [26]. Au-Ag alloy nanostructures provide both a good LSPR effect and good stabilities, for potential SERS substrates, for ultra-trace molecules detection under different circumstances [27,28,29]. The spiky stars could furtherly improve detection limit, from high LSPR induced field strength on sharp tips and large surface area for analyte attachment [30,31]
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