Dieter Neuschäfer

Novel bioaffinity sensors for trace analysis based on luminescence excitation by planar waveguides

Abstract We have developed a novel generation of optical bioaffinity sensors for ultra trace analysis. These sensors are based on luminescence generation in the evanescent field of high-refractive-index single-mode planar waveguides, With the waveguiding layers and the grating parameters chosen, very sharp discrimination of bulk against surface-confined excitation, in combination with high excitation intensities in the evanescent field, can be achieved, leading to unprecedented sensitivity. Experimental data of the optimization of the transducer parameters will be presented. Incoupling of excitation light is performed using diffractive gratings. Different methods for the detection of both transmitted and luminescence light will be presented. The transmitted excitation light can be detected either at the distal waveguide chip end or using a second outcoupling grating. Both isotropically emitted luminescence, collected by a lens located below the transducer substrate (‘volume detection’), and emission coupled back into the waveguiding layer can be monitored, the latter via a second outcoupling grating. First experimental results obtained in model bioaffinity assays will be presented, demonstrating the feasibility of the different detection methods mentioned above, as well as the superior sensitivity of our novel sensor configuration. In still preliminary experiments, 100 attomoles of fluorescently labelled DNA (16-mer oligonucleotide), applied at 100 femtomolar concentration, can be detected.

Evanescent resonator chips: a universal platform with superior sensitivity for fluorescence-based microarrays

In the present paper, we introduce for the first time a novel generation of a universal fluorescence transducer, the so-called evanescent resonator (ER) platform. The device comprises a transparent substrate and a thin dielectric surface layer containing sub-micron corrugated structures. The ER chip exhibits an inherent physical signal amplification due to confinement of excitation energy in the thin surface layer. Energy confinement is based on interference effects created by the abnormal reflection geometry and leads to efficient excitation of surface-bound fluorophores in the evanescent field of the chip. The evanescent resonator platform has the potential to increase the fluorescence yield of labelled biomolecules to more than 100-fold when compared with conventional microarray chips. The new ER device has been developed for analysis of nucleic acids from different species. However, it can be used with all kinds of biomolecular affinity systems. The platform combines superior sensitivity with exceptional reproducibility and ease of use. The chips are compatible with commercially available laser scanners, confocal microscopes, and portable or miniaturised CCD read-out equipment.

Sub-micron grating formation in Ta2O5-waveguides by femtosecond UV-laser ablation

Abstract Sub-micron-period surface gratings on Ta2O5 waveguide layers were produced by ablation with a sub-ps-UV-laser. The structure is generated by projection imaging of a primary transmission grating mask. A grating of 500 nm period with a surface modulation depth of 10 nm on a sample area of about 300 μm×300 μm can be produced with a single laser pulse of about 100 mJ/cm2 at 248 nm. Adjustment of the modulation depth to specific requirements can be accomplished by varying laser fluence or pulse number. The structures can be used as grating couplers.

Waveguide grating coupling of 2D focused beam under normal incidence: a phenomenological approach

The coupling of a 2D focused beam under normal incidence into a grating waveguide was solved by means of a coupled wave formalism where the phenomenological parameters are given by a simple plane wave diffraction analysis [1]. It is shown that the guided modes which must be considered in the situation of normal incidence are waves exhibiting a standing wave as well as a propagating character. The plane wave diffraction analysis of the poles corresponding to these modes reveals their interesting properties and helps establish the suitable phenomenological representation of the coupling event. The aim of this approach was to find out the conditions for maximum light confinement in a grating waveguide in the perspective of the high efficiency excitation of a high density pixellated array of biosensor sites.