We develop integrated nano-interferometric transducers for high performance biosensors based on the evanescent field detection principle. They combine high sensitivity, real-time and label-free detection with mechanical stability, miniaturization and the possibility of mass-production.
The activities of this research line can be divided in three main areas:
MZI biosensor device
In an integrated Mach-Zehnder Interferometer (MZI), a Y-junction splits the guided light into two arms, the sensing arm and the reference arm. After a certain distance, the two signals are recombined into an output optical waveguide via a second Y-junction, producing the interference of both beams. Calculations and simulations have been performed to design highly sensitive MZI devices in which the interaction of the evanescent wave with the external medium is maximized. This produced MZI guiding structure based on SiO2/Si3N4/SiO2 rib waveguides, with single mode behavior for core thickness below 250 nm, rib width of 3 µm and rib depths below 2 nm. Bulk detection limit as low as Δn = 10-7 refractive index units (RIU) have been demonstrated.
About the biosensing capabilities of the MZI, for example we have shown that the lowest DNA hybridization limit in buffer solution was 10 pM. Additionally, we have detected a DNA target with two mismatching bases.
In addition, we are currently implementing multiplexed MZI for the simultaneous detection of several biomolecular interactions.
BiMW biosensor device
We have recently introduced a new interferometric waveguide biosensor, the bimodal waveguide sensor (BiMW). In the BiMW, the interference of two guided modes (fundamental and first order modes) of the same polarization occurs in a straight waveguide, without the need of a reference arm. The guiding structure is based on SiO2/Si3N4/SiO2 rib waveguides, with a 150 nm-thick single mode section and a 350 nm-thick bimodal section. The rib is 4 µm wide with a height below 2 nm.
The main advantage of this biosensor device is the simplicity of its design while its sensitivity is comparable to the MZI one (LOD of 2,5×10-7 RIU in bulk). More relevant, we have demonstrated its ability for biosensing for example by the direct detection of human hormones at physiological levels (below 8 pg/ml). Preliminary results using the BiMW device also indicated very low detection limits for bacteria (at few cfu/ml) detection and DNA/RNA hybridization, demonstrating the wide range of bioapplications of this new interferometric biosensor.
Moreover, the small foot print of the BiMW device allows the integration of a large amount of sensing elements within a single chip which is of importance for the development of multiplexed biosensors.
We aim to integrate our interferometric biosensors into lab-on-a-chip (LOC) platforms which could be used as hand-held and portable devices for point-of-care diagnosis. This device must be easy to use, fast (seconds) and should allow multiplexed detection in real-time with very low volumes (nl) of samples and reagents. The point-of-care device will allow for the identification and quantification of biomarkers at femtomolar level without using fluorescent or radioactive labels.
The lab-on-chip platform is being assembled by integrating the following parts:
- The nanophotonic sensors in a multiplexed configuration (MZI and BiMW interferometers)
- An all-optical phase modulation system to solve the complex read-out of the interferometric output.
- A 3D network of SU-8 polymer microfluidics monolithically assembled at the wafer-level, ensuring a perfect sealing and a compact packaging (collaboration with the group GEMM-I3A-University of Zaragoza, Spain)
- Grating couplers for the simultaneous coupling of light in all the waveguides for multiplexed detection (collaboration with ICFO, Barcelona, Spain)
- Reliable immobilization and regeneration protocols for the biological receptors
- Photodetectors, electronic and software control