Fabrication of surface-enhanced Raman scattering (SERS) nanostructures with both high sensitivity and spectral reproducibility, which are key requirements for routine SERS quantitative (sensing) applications, is a great challenge. Here we report on the multilayered design of SERS-active substrates formed by uniform Ag nanoislands stabilized by a magnetron-sputtered polytetrafluoroethylene (msPTFE) film of nanometre thickness. The preparation of large scale (∼20 cm²) nanostructures with long (∼months) time stability was fast (∼minutes), cheap and repeatable. SERS performance of our substrates was subsequently improved by step-by-step optimisation of the fabrication procedure and introduction of the additional silver layer, separating the supporting glass from the msPTFE film, which contributed to the total enhancement factor by another order of magnitude. Such substrates provided high SERS sensitivity with a SERS enhancement factor of about 2 orders of magnitude larger than commonly reported in the literature for Ag nanoislands grown directly on glass. The limits of detection were determined as ∼1 × 10⁻¹⁰ M, 1 × 10⁻⁷ M and 1 × 10⁻⁶ M for testing analytes methylene blue (MB), 5,10,15,20-tetrakis(1-methyl-4-pyridyl)porphyrin (H₂TMPyP) and cysteine, respectively. SERS substrates showed excellent spectral reproducibility both across a single substrate and different substrate batches. The relative standard deviation (RSD) of the SERS signal did not exceed 20% for different concentrations of MB and H₂TMPyP, respectively. Therefore, Ag nanoislands stabilized by the msPTFE film are due to high sensitivity, stability and excellent spectral reproducibility very promising substrates for SERS biomolecular quantitative (sensing) applications.

A sometimes overlooked degree of freedom in the design of many spectroscopic (mainly Raman) experiments involve the choice of experimental geometry and polarization arrangement used. Although these aspects usually play a rather minor role, their neglect may result in a misinterpretation of the experimental results. It is well known that polarization- and/or angular- resolved spectroscopic experiments allow one to classify the symmetry of the vibrations involved or the molecular orientation with respect to a smooth surface. However, very low detection limits in surface-enhancing spectroscopic techniques are often accompanied by a complete or partial loss of this detailed information. In this review, we will try to elucidate the extent to which this approach can be generalized for molecules adsorbed on plasmonic nanostructures. We will provide a detailed summary of the state-of-the-art experimental findings for a range of plasmonic platforms used in the last ~ 15 years. Possible implications on the design of plasmon-based molecular sensors for maximum signal enhancement will also be discussed.

Ag nanorod arrays prepared by oblique angle vapor deposition (OAD) represent regular, large area substrates for surface‐enhanced Raman scattering (SERS) spectroscopy. We studied uniformity and spectral reproducibility of silver OAD‐fabricated substrates (AgOADs) by spectral mapping of methylene blue. The results demonstrate good reproducibility apart from occasional “hot‐spot” sites where the intensity is higher. The number of “hot‐spots” represents 2%–6% of SERS‐active sites of mapping substrate area. We were able to obtain good SERS spectra of testing amino acid tryptophan at 1 × 10⁻⁵ M concentration and three different free‐base porphyrins down to ∼10⁻⁷ M concentration. We found out that keeping the AgOADs in a vacuum chamber overnight prevents the surface from binding any contaminants from the ambient atmosphere, without significant reduction in the SERS enhancement. Such substrates provide stable SERS enhancement even when stored for 1 year after preparation.

Silver nanorod arrays prepared by oblique angle deposition (AgOADs) represent versatile, simple and inexpensive substrates for high sensitivity surface enhanced Raman scattering (SERS) applications. Their anisotropic nature suggests that their optical responses such as the SERS signal, the depolarization ratio, reflectivity and ellipsometric parameters critically depend on the states of polarization, nanorod angular arrangement and specific illumination-observation geometry. SERS polarization and angular dependences of AgOADs were measured using methylene blue (MB) molecule. Our study constitutes, to our knowledge, the most detailed investigation of such characteristics of plasmonic nanostructures to date. This is due to the 90°-scattering geometry used in which two out of three Euler angles determining the nanorod spatial orientation and four polarization combinations can be varied simultaneously. We attributed the anisotropic optical response to anisotropic (pseudo)refractive index caused by different periodicity of our structures in different directions since the plasmonic properties were found rather isotropic. For the first time we demonstrate very good correspondence between SERS intensities and ellipsometric parameters for all measured configurations as compared on the basis of the surface selection rules. Obtained results enable quantitative analysis of MB Raman tensor elements, indicating that the molecules adsorb predominantly with the symmetry axis perpendicular to the surface.

Surface-enhanced fluorescence (SEF) requires the absorption/emission band of the fluorophore, the localized surface plasmon resonance (LSPR) of the nanostructure and the excitation wavelength to fall in the same (or very close) spectral range. In this paper, we monitor the SEF intensity and lifetime dependence of riboflavin (vitamin B2) adsorbed on a spacer-modified Ag substrate with respect to the thickness of the spacer. The substrates were formed by silver nanoislands deposited onto magnetron-sputtered polytetrafluoroethylene (ms-PTFE). The spacer was formed by the ms-PTFE layer with the thickness ranging from ~5 to 25 nm. The riboflavin dissolved in dimethylsulfoxide (DMSO) at a 10 µM concentration forms, at the ms-PTFE surface, a homogeneous layer of adsorbed molecules corresponding to a monomolecular layer. The microspectroscopic measurements of the adsorbed layer were performed through a sessile droplet; our study has shown the advantages and limitations of this approach. Time-resolved fluorescence enabled us to determine the enhanced fluorescence quantum yield due to the shortening of the radiative decay in the vicinity of the plasmonic surface. For the 5 nm ms-PTFE layer possessing the largest (estimated 4×) fluorescence enhancement, the quantum yield was increased 2.3×.