Abstract
In this work we show that ordered freestanding titanium oxide nanotubes (TiO2 NT) may be used as substrates for the simple and efficient immobilization of anisotropic plasmonic nanoparticles. This is important because anisotropic plasmonic nanostructures usually give greater spectral enhancement than spherical nanoparticles. The size of the pores in a layer of titanium oxide nanotubes can be easily fitted to the size of many silver plasmonic nanoparticles highly active in SERS (surface-enhanced Raman scattering) spectroscopy (for example, silver nanocubes with an edge length of ca. 45 nm), and hence, the plasmonic nanoparticles deposited can be strongly anchored in such a titanium oxide substrate. The tubular morphology of the TiO2 substrate used allows a specific arrangement of the silver plasmonic nanoparticles that may create many so-called SERS hot spots. The SERS activity of a layer of cubic Ag nanoparticles (AgCNPs) deposited on a tubular TiO2 substrate (AgCNPs@TiO2 NT) is about eight times higher than that of the standard electrochemically nanostructured surface of a silver electrode (produced by oxidation reduction cycling). Furthermore, a super hydrophilic character of the TiO2 nanotubes surface allows for a uniform distribution of AgCNPs, which are deposited from an aqueous suspension. The new AgCNPs@TiO2 NT hybrid layer ensures a good reproducibility of SERS measurements and exhibits a higher temporal stability of the achievable total SERS enhancement factor—one that is far better than standard SERS silver substrates. To characterize the morphology and chemical composition of such evidently improved SERS platforms thus received, we applied microscopic techniques (SEM, and scanning transmission electron microscopy (STEM)) and surface analytical techniques (Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS)).
Highlights
Raman scattering is an optical effect with efficiency being usually very low
The nanoresonator-induced increase in the efficiency of the generation of the Raman signal is so large that the Raman scattering cross-section can even be increased up to, e.g., 2 × 10−14 cm2 per molecule [6] which makes it possible to observe the Raman spectra of even a single molecule [6,7,8]
The recorded UV-Vis spectrum was typical for silver cubic nanoparticles [40], with the main surface plasmon resonance (SPR) extinction band at 440 nm
Summary
Raman scattering is an optical effect with efficiency being usually very low. A typical cross-section for Raman scattering is ca. With the discovery of the surface-enhanced Raman scattering (SERS) phenomenon in the 1970s of the last century, it allowed for a huge enhancement of recorded Raman signals for molecules (pyridine) adsorbed on silver with a roughened surface [2,3,4]. The sensitivity of SERS measurements is strongly dependent on the activity of plasmonic material used as the SERS substrate, and so the fabrication of such plasmonic nanomaterials is an important and rapidly-developing branch of nanotechnology. The quality of the SERS spectra measured strongly depends on the activity and reproducibility of the plasmonic material used as the SERS substrate. Many groups are working on developing new methods of fabricating
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