Local functionalization of CMOS-compatible Si3N4 Mach-Zehnder interferometers with printable functional polymers

Abstract A CMOS-compatible integrated four-channel silicon nitride (Si 3 N 4 ) waveguide based Mach-Zehnder interferometric (MZI) sensor platform was realized with the goal to take maximum advantage of the inherent self-referencing property of MZI-sensors with both sensing and reference waveguides exposed to the sample liquid. To study the self-referencing characteristics of the MZI-sensor system, we compared two sensor configurations—an asymmetric MZI configuration, where the reference waveguide was covered by a cladding, and a symmetric MZI configuration, where the reference waveguide was also in contact with the measurement liquid. To enable biomolecular measurements, we developed an inkjet printing procedure for functional polymers (biotin-modified polyethyleneimine (PEI-B) macromolecules), which allows for local functionalization of sensing waveguides in a cost effective mass-fabrication compatible manner. We could show that for high refractive index changes of the measurement liquid (Δn = 0.007) the background signal can be suppressed by a factor of ∼42 for the symmetric MZI configuration compared to the asymmetric MZI configuration. For the characterization of the functional polymer printing procedure we compared locally functionalized asymmetric and symmetric MZIs with non-locally functionalized asymmetric MZIs, which were prepared with silanization/biotinylation based approach. These locally functionalized MZIs where used for the detection of single stranded DNA (150 bp) with concentrations ranging from 2.5 to 100 nM. The limits of detection were 1.9 nM and 1.6 nM for the non-locally and locally functionalized asymmetric MZI-sensors, respectively. The locally functionalized symmetric MZI-sensors had a four times better LOD of 0.4 nM. These results proof the superior performance of symmetric biosensor arrays with locally deposited functional polymers by means of inkjet printing.