Surface-enhanced infrared absorption studies towards a new optical biosensor.

Reflectometric interference spectroscopy (RIfS), which is well-established in the visual regime, measures the optical thickness change of a sensitive layer caused, e.g., by binding an analyte. When operated in the mid-infrared range the sensor provides additional information via weak absorption spectra (fingerprints). The originally poor spectra are magnified by surface-enhanced infrared absorption (SEIRA). This is demonstrated using the broad complex fluid water band at 3300 cm-1, which is caused by superposition of symmetric, antisymmetric stretching vibration, and the first overtone of the bending vibration under the influence of H-bonds and Fermi resonance effect. The results are compared with a similar experiment performed with an ATR (attenuated total reflectance) set-up.

Plasmonic vertical dimer arrays as elements for biosensing.

Localized surface plasmon resonances of metallic nanoparticles can be used for biosensing because of their sensitive dependence on the refractive index of the surrounding medium. The binding of molecules to the particles causes a change of the effective refractive index in their close vicinity, which leads to a reversible shift of the resonance. We present simulations and sensing experiments of a plasmon resonance based biosensor that makes use of the narrow antisymmetric resonance in coupled plasmonic vertical dimers. The sensitivity of the antisymmetric resonance is compared with that of a surface lattice resonance for refractive index sensing of bulk and of thin layers of molecules. The functionality of such a sensor surface is demonstrated via a testosterone immunoassay for detection of antibody from a solution by binding to surface-immobilized antigen in a fluidic channel.

Size does matter! Label-free detection of small molecule-protein interaction.

This review is focused on methods for detecting small molecules and, in particular, the characterisation of their interaction with natural proteins (e.g. receptors, ion channels). Because there are intrinsic advantages to using label-free methods over labelled methods (e.g. fluorescence, radioactivity), this review only covers label-free techniques. We briefly discuss available techniques and their advantages and disadvantages, especially as related to investigating the interaction between small molecules and proteins. The reviewed techniques include well-known and widely used standard analytical methods (e.g. HPLC-MS, NMR, calorimetry, and X-ray diffraction), newer and more specialised analytical methods (e.g. biosensors), biological systems (e.g. cell lines and animal models), and in-silico approaches.

On-chip integrated mid-infrared GaAs/AlGaAs Mach-Zehnder interferometer.

We report the design, fabrication, and first functional verification of mid-infrared (MIR; 3-12 µm) Mach-Zehnder interferometers (MZIs). The developed MIR-MZIs are entirely chip-integrated solid-state devices based on GaAs/AlGaAs technology waveguide fabricated via conventional optical lithography and reactive ion etching (RIE). Thus, fabricated MIR-MZIs were combined with a broadly tunable quantum cascade laser (tQCL) providing a wavelength coverage of 5.78-6.35 µm. MIR-MZIs have been designed with a waveguide width of 5 µm to ensure single mode behavior, avoiding optically undefined interference patterns. Several structures with different opening angles of the Y-junction were fabricated and tested for maximizing IR radiation throughput. This study demonstrates the feasibility of the very first chip-integrated mid-infrared Mach-Zehnder structures via interference patterns produced by minute amounts of water deposited at different positions of the MIR-MZI structure.