Presenter: Anke Schütz-Trilling – Surfix (Netherlands)
In this contribution, we present the first results of an innovative integrated optical biosensing platform with an extremely high sensitivity. The platform uses the TriPleX (Si3N4/SiO2) waveguide technology and is based on an asymmetric Mach-Zehnder Interferometer (aMZI), which is intrinsically more sensitive (LoD 10-8 – 10-7 RIU) than other waveguide-based sensors (e.g. optical ring resonators) and sensors based on Surface Plasmon Resonance. The performance of the aMZI is enhanced by material-selective functionalization of Si3N4 vs. SiO2. This allows the bioreceptor to be immobilized only on the waveguide. The rest of the surface is modified with an antifouling layer, thus reducing non-specific binding. This confines analyte binding to the waveguide surface, which results in improved sensitivity and limit of detection (LoD).
The aMZI structures (Figure 1a) consist of planar waveguides with a 100 nm thick Si3N4 core embedded in a SiO2 cladding, which is locally removed in the sensing window. The aMZI biosensor platform exploits the wavelength dependence of the interference of light between the two interferometer arms. A change in the effective refractive index at the sensor surface will result in a phase shift of the interference signal and can thus be used for detecting an analyte and/or measuring changes in the bulk refractive index of the sample solution (Figure 1b).
Material-selective functionalization of the biosensor is based on surface modification of oxide-free Si3N4 using terminal alkenes. A carboxylic acid-terminated monolayer is prepared on the Si3N4 waveguide for biofunctionalization by coupling to amine groups in the bioreceptor. Next, the oxide is modified with an antifouling poly(ethylene glycol) layer using silane chemistry (figure 2a). Figure 2b shows a fluorescence image of a modified Si3N4/SiO2 surface after adsorption of a fluorescent protein.
To show the effect of material-selective surface functionalization, a direct comparison was made between sensors with material-selective and ‘regular’ uniform functionalization. In Figure 3, the adsorption of mouse IgG on both types of surfaces is shown. This experiment shows that immobilization is more efficient on the material-selectively functionalized sensor, consistent with site-selective adsorption onto the waveguide surface.
In conclusion, the results presented here show the great potential of our biosensor platform. The combination of the high intrinsic sensitivity of the aMZI and material-selective (bio)functionalization enables new applications in diagnostics or drug screening, where low concentrations of small molecules need to be detected.
This project has received funding from the European Union’s Horizon 2020 programme under grant agreement No 732309 (BioCDx).