Linear chains of Ag nanoparticles embedded in dielectric films for SERS applications in analytical chemistry

Abstract

In line with the approach known as shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), in which Raman signal amplification of analytes is provided by metallic nanoparticles with an ultrathin silica or alumina shell, we report here on a Surface-Enhanced Raman Spectroscopy (SERS) substrate consisting of periodic lines of Ag nanoparticles embedded in dielectric surfaces for enhancing Raman signals. This paper demonstrates the possibility to use these so-called ‘PLANEDSERS’ substrates as washable and reusable chemical sensors with a good level of repeatability. Large-area Ag nanoparticle arrays are produced by glancing-angle ion-beam sputtering deposition on nanorippled patterns and are protected from the chemical environment (atmospheric or liquid solutions) by a robust and functionalizable thin dielectric layer of alumina or silicon nitride. Our results show that linear assemblies of ellipsoidal nanoparticles (size ∼15 nm) separated by interparticle gaps of approximately 5 nm generate enough near-field intensity enhancement to give rise to significant SERS signals of non-Raman-resonant bipyridine molecules without chemical contact between molecules and Ag nanoparticles. Moreover, the optical dichroic response of these plasmonic assemblies allows for the possibility of tuning the excitation wavelength of the Raman spectra over a wide spectral range. This study is a first step towards designing a substrate-platform without chemical specificity to enhance in equal manner all the weak Raman signals of usual organic molecules and to avoid loss of balance in favour of only one species as usual in SERS experiments. The quantitative detection ranges for bipyridine used as a probe test molecule are around between 10⁻³ to 10⁻⁶ M.