ABSTRACT
An alternative approach to classical surface plasmon resonance spectroscopy is dielectric-loaded waveguide (DLWG) spectroscopy, widely used in the past decades to investigate bio-interaction kinetics. Despite their wide application, a successful and clear approach to use the DLWGs for the one-step simultaneous determination of both the thickness and refractive index of organic thin films is absent in the literature. We propose here, for the first time, an experimental protocol based on the multimodal nature of DLWGs to be followed in order to evaluate the optical constants and thickness of transparent thin films with a unique measurement. The proposed method is general and can be applied to every class of transparent organic materials, with a resolution and accuracy which depend on the nature of the external medium (gaseous or liquid), the geometrical characteristics of the DLWG, and the values of both the thickness and dielectric constant of the thin film. From the experimental point of view, the method is demonstrated in a nitrogen environment with an accuracy of about 3%, for the special case of electroluminescent thin films of Eu3+ß-diketonate complexes, with an average thickness of about 20 nm. The high value of the refractive index measured for the thin film with the Eu(btfa)3(t-bpete) complex was confirmed by the use of a spectroscopic model based on the Judd-Ofelt theory, in which the magnetic dipole transition 5D0 â 7F1 (Eu3+) for similar films containing Eu3+ complexes is taken as a reference. The DLWGs are finally applied to control the refractive index changes of the organic thin films under UVA irradiation, with potential applications in dosimetry and monitoring light-induced transformation in organic thin films.
ABSTRACT
The resonant wave modes in monomodal and multimodal planar Surface Plasmon Resonance (SPR) sensors and their response to a bidimensional array of gold nanoparticles (AuNPs) are analyzed both theoretically and experimentally, to investigate the parameters that rule the correct nanoparticle counting in the emerging metal nanoparticle-amplified surface plasmon resonance (PA-SPR) spectroscopy. With numerical simulations based on the Finite Element Method (FEM), we evaluate the error performed in the determination of the surface density of nanoparticles σ when the Maxwell-Garnett effective medium theory is used for fast data processing of the SPR reflectivity curves upon nanoparticle detection. The deviation increases directly with the manifestations of non-negligible scattering cross-section of the single nanoparticle, dipole-dipole interactions between adjacent AuNPs and dipolar interactions with the metal substrate. Near field simulations show clearly the set-up of dipolar interactions when the dielectric thickness is smaller than 10 nm and confirm that the anomalous dispersion usually observed experimentally is due to the failure of the effective medium theories. Using citrate stabilized AuNPs with a nominal diameter of about 15 nm, we demonstrate experimentally that Dielectric Loaded Waveguides (DLWGs) can be used as accurate nanocounters in the range of surface density between 20 and 200 NP/µm², opening the way to the use of PA-SPR spectroscopy on systems mimicking the physiological cell membranes on SiO2 supports.
