Development of a compact plasmo-photonic multiplexed sensor chip assembly setup with microfluidic module

An optical biosensor chip using an integrated plasmo-photonic waveguide platform with on-chip light generation and microfluidic channels is being developed within the European project GRACED which is aiming at fabricating a prototype of low-cost and portable analytical instrument for food quality monitoring. The ultra-high sensitivity detection configuration used is a bimodal Mach-Zhender interferometer waveguide coupled with low-footprint plasmonic sensing areas which will be functionalized with specific antibodies and designed to work at 1550 nm [1,2]. The sensing areas are multiplied on the chip in order to detect different analytes simultaneously. Waveguide excitation can be realized via fiber array and butt coupling but this requires precise alignment that is difficult to implement for a cheap and easy-to-use instrument. That is why we also develop the integration of the IR light sources on the chip based on the local deposition of quantum dots (QDs). The IR-emitting QDs can be excited simply thanks to a collimated UV beam without the need for high precision alignment.

We fabricate microfluidic channels in PDMS by molding with a glass and metal holder and assemble this MicroFluidic Module (MFM) to the chips in order to flow the analyte solutions onto the different sensing areas. We present a mechanical clamping system we have developed which allows to easily assemble and align the chip and the MFM, to add a printed circuit board if needed, a regulated temperature control and at the same time allow for the positioning of a fiber array or far-field optical detection on the sides of the chip and a UV pumping beam going to excite the QDs at the surface of the chip through the transparent MFM. The MFM is pressed on the chip with the help of a mechanical clamp which assures contact without leaks. This assembly is fully reversible so that the MFM can be reused many times for different sensor chips. We can handle up to 8 flowing channels in a 20x20 mm² footprint, with high flow rates without any leak. With this mechanical system, the alignment accuracy that can be achieved between the microfluidic channels and the sensing areas is typically in the order of 0.1 mm, which is enough for 8 sensing areas in a 20x20mm² chip. The experimental setup developed paves the way towards a compact transportable setup that promises to perform multiplexed high –performance analyses for different applications, like e.g. food, environment or health diagnostic.

As an alternative to clamping, we have also developed an adhesive bonding method compatible with the presence of antibodies (or any other biological recognition elements) immobilized on the surface of the sensor. This bonding is conceived to be irreversible, but this greatly simplifies the system as in that case the MFM and the chip are aligned optically and sealed, such that there is no need for a mechanical alignment system. In order to be compatible with the presence of biological materials deposited in the sensing areas, no plasma-, no silane-, no thermal- based bonding methods are allowed. With these constraints, we use unpolymerized PDMS as the adhesive. A thin PDMS film is deposited on the surface of the MFM by a transfer method, then the MFM is aligned optically with the chip and assembled. Finally, PDMS is polymerized at a temperature low enough to avoid denaturation of the antibodies (<40°C). This method allows us to flow the buffer at high rates without any leak.