Shell-isolated nanoparticle-enhanced Raman spectroscopy

Abstract

Surface-enhanced Raman scattering (SERS) spectroscopy is a powerful analytical technique able to detect substances down to single molecule level. Its applications are limited, however, because to realize a substantial Raman signal requires metal substrates that either have roughened surfaces or take the form of nanoparticles. An innovative approach is now demonstrated, where the substance under investigation, on a generic substrate, is covered by a layer of 'smart dust' consisting of gold nanoparticles coated by an ultrathin insulating shell of silica or alumina. The nanoparticles provide Raman signal amplification, and the coating keeps them separate from each other and from the probed substance. The new technique, termed SHINERS (shell-isolated nanoparticle-enhanced Raman spectroscopy), is demonstrated by probing pesticide residues on the surfaces of yeast cells and citrus fruits. It could be useful in materials science and the life sciences, as well as for the inspection of food safety, drugs, explosives and environmental pollutants. Surface-enhanced Raman scattering is a powerful spectroscopy technique that can be used to study substances down to the level of single molecules. But the practical applications have been limited by the need for metal substrates with roughened surfaces or in the form of nanoparticles. Here a new approach — shell-insulated nanoparticle-enhanced Raman spectroscopy — is described, and its versatility demonstrated with numerous test substances. Surface-enhanced Raman scattering (SERS) is a powerful spectroscopy technique that can provide non-destructive and ultra-sensitive characterization down to single molecular level, comparable to single-molecule fluorescence spectroscopy1,2,3,4,5,6,7,8,9,10,11,12,13,14,15. However, generally substrates based on metals such as Ag, Au and Cu, either with roughened surfaces or in the form of nanoparticles, are required to realise a substantial SERS effect, and this has severely limited the breadth of practical applications of SERS. A number of approaches have extended the technique to non-traditional substrates14,16,17, most notably tip-enhanced Raman spectroscopy (TERS)18,19,20 where the probed substance (molecule or material surface) can be on a generic substrate and where a nanoscale gold tip above the substrate acts as the Raman signal amplifier. The drawback is that the total Raman scattering signal from the tip area is rather weak, thus limiting TERS studies to molecules with large Raman cross-sections. Here, we report an approach, which we name shell-isolated nanoparticle-enhanced Raman spectroscopy, in which the Raman signal amplification is provided by gold nanoparticles with an ultrathin silica or alumina shell. A monolayer of such nanoparticles is spread as ‘smart dust’ over the surface that is to be probed. The ultrathin coating keeps the nanoparticles from agglomerating, separates them from direct contact with the probed material and allows the nanoparticles to conform to different contours of substrates. High-quality Raman spectra were obtained on various molecules adsorbed at Pt and Au single-crystal surfaces and from Si surfaces with hydrogen monolayers. These measurements and our studies on yeast cells and citrus fruits with pesticide residues illustrate that our method significantly expands the flexibility of SERS for useful applications in the materials and life sciences, as well as for the inspection of food safety, drugs, explosives and environment pollutants.