Abstract
Surface-plasmons of metals have been utilized to enhance the Raman spectra of various adsorbed moieties for over decades. While amplification of the spectral intensity takes place on most of the metals, due to their superb properties, Au, Ag and Cu surfaces represent the benchmark in surface-enhanced Raman spectroscopy. In this paper, we show that Cu-Pd bimetal and CuPt alloy nanotubes derived from Cu nanowires by simple galvanic exchange reactions are suitable for the efficient enhancement of Raman spectra when dispersed on Si surfaces. Amplification factors of $120\times $ on Cu nanowires, $150\times $ on Cu-Pd bimetal nanotubes and $250\times $ on CuPt alloy nanotubes in reference to the substrate are measured for rhodamine 6G and methyl violet model compounds. We also show that the nanotubes dispersed on Au surfaces can contribute to a further intensity enhancement of the substrate and detect analytes adsorbed from 10−6 M analyte concentrations. Our results obtained using bimetallic and alloy nanomaterials shed light on a new strategy to synthetize and apply new types of metal nanostructures and compositions for surface-enhanced Raman spectroscopy in the future.
Highlights
SURFACE-ENHANCED Raman spectroscopy (SERS) is a powerful analytical tool to detect and quantify a broad range of chemicals even in trace quantities, and it has been widely employed in analytical chemistry [1], environmental monitoring [2], [3], biomedical diagnostics [4]-[7], food security [8], and even in forensic investigations [9], [10].Enhancement of the Raman spectra are due to two primary reasons
Cu nanowires were synthesized by a hydrothermal route, [30], [31], [33] whereas their Pd and Pt modified derivatives were obtained by galvanic replacement reactions [34] similar to those as we reported earlier
The crystal structure of the nanotubes is different as suggested by high-resolution transmission electron microscopy (TEM) (Fig. 1d-f), energy dispersive X-ray spectroscopy (EDX) (Fig. 1g,h) and X-ray diffraction (XRD) (Fig. 1i)
Summary
SURFACE-ENHANCED Raman spectroscopy (SERS) is a powerful analytical tool to detect and quantify a broad range of chemicals even in trace quantities, and it has been widely employed in analytical chemistry [1], environmental monitoring [2], [3], biomedical diagnostics [4]-[7], food security [8], and even in forensic investigations [9], [10].Enhancement of the Raman spectra are due to two primary reasons. The electric field in the proximity of resonant surface plasmons is amplified, which enhances polarization of the adsorbate and increase Raman intensity (electromagnetic effect). On the other, (partial) charge transfer between adsorbed molecules on metal surfaces can result in the change of polarizability contributing to an enhanced polarization in the electric field (chemical or charge-transfer effect) [11]. One is based on the extended range of increased electric field on the surface of a metal or semiconductor near a highly SERS active particle or film (borrowed SERS activity) [23]. The other effect is associated with plasmonic hot electron injection from the metal to the semiconductor [24], which inherently results in different local fields and can alter the metal-molecule complex influencing both electromagnetic and chemical sensing mechanisms. In addition to the above processes, the geometry, morphology, and size of particles influence SERS, as the local field is a function of surface curvature [25]-[29]
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