Effect of aging on optical properties of bimetallic sensor chips

Dielectric-Loaded Waveguides as Advanced Platforms for Diagnostics and Application of Transparent Thin Films

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⁺β-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 → 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.

Enhanced Biosensing Activity of Bimetallic Surface Plasmon Resonance Sensor

Surface plasmon resonance (SPR) sensors present a challenge when high sensitivity and small FWHM (full width at half maximum) are required to be achieved simultaneously. FWHM is defined by the difference between the two extreme values of the independent variable at which the value of the dependent variable is equal to half of its maximum. A smaller value of FWHM indicates better accuracy of SPR measurements. Theoretically, many authors have claimed the possibility of simultaneously achieving high sensitivity and small FWHM, which in most of the cases has been limited by experimental validation. In this report, an experimental study on the improved surface plasmon resonance (SPR) characteristics of gold over silver bimetallic sensor chips of different film thicknesses is presented. A comparative study of antigen–antibody interaction of the bimetallic chip using a custom-made, low-cost, and portable SPR device based on an angular interrogation scheme of Kretschmann configuration is performed. Pulsed direct current (DC) magnetron-sputtered bimetallic films of gold over silver were used in the construction of the SPR chip. The FWHM and sensitivity of the bimetallic sensors were firstly characterized using standard solutions of known refractive index which were later immobilized with monoclonal anti-immunoglobulin G (IgG) in the construction of the SPR biochip. Spectroscopic measurements such as ultraviolet–visible light spectroscopy (UV–Vis) and Fourier-transform infrared spectroscopy (FTIR) were used for the confirmation of the immobilization of the antibody. The performance of the bimetallic SPR biochip was investigated by exposing the sensor to various concentrations of the target protein. The results indicated that the bimetallic sensors of silver/gold had a 3.5-fold reduced FWHM compared to pure gold-based sensors, indicating a higher detection accuracy. In addition, they exhibited a significant shift in resonance angle as high as 8.5 ± 0.2 due to antigen–antibody interaction, which was ~1.42-fold higher than observed for pure silver-based sensors.

Plain silver surface plasmon resonance for microarray application

The application scope of surface plasmon resonance (SPR) and SPR imaging (SPRi) is rapidly growing, and tools such as high-performance and low-cost slides could enable more rapid growth of the field. We describe herein a novel silver slide, addressing the inherent instability of plain silver structure by improving adhesion between the glass substrate and the silver layer with a thin buffer layer of gold. Covered by a self-assembled monolayer (SAM) only, SPR characteristics of the slide remain steady for more than 3 months under regular storage. In a bioassay, the slide substantiates the predicted nearly 100% sensitivity improvement over gold slides and exhibits exceptional performance stability as determined by sensitivity and resolution measurements during the extended 40,000 s multicycle experiment. We demonstrate the suitability of this new slide for large-area SPRi, describing analysis results from a 1 296-ligand protein microarray. We predict this slide structure will provide a stable, high-sensitivity solution for high-throughput SPRi applications and other surface analysis platforms.

Dual-mode surface-plasmon sensor based on bimetallic film

We propose a plasmonic structure, based on a silver-gold two-layered metallic film, where two surface plasmons (SPs) with equal propagation constants are excited simultaneously at different wavelengths. We show theoretically that the bimetallic film provides unique opportunities for manipulation of plasmons and optimization of the accuracy and cross-sensitivity. The structure can be used as an effective self-reference SP sensor in wavelength-interrogated design.