Optical integrated sensors, such as Mach-Zehnder Interferometers (MZI), offer remarkable advantages in terms of sensitivity and compactness for label-free bio-sensing applications [1-2], although they need from additional structures and long arms to perform the sensing. In this work, we report a new interferometric sensing method encompassing the benefits of MZIs while reducing the footprint of the integrated device. This new approach is based on the interference between two Bloch modes propagating through a single-channel and one-dimensional photonic crystal, where the slow-wave effect takes place.
The principle of operation is the following: a single-mode waveguide working in TE polarization excites two modes, which interfere with each other by an abrupt discontinuity into the second single-mode waveguide at the output. For a given variation of the cladding refractive index unit (RIU), the propagation constant of the guided modes changes, producing an increase in the phase shift. Due to the slow-wave phenomenon whereby light travels slower than in other structures, good values of sensitivity are obtained for short device lengths in comparison to other interferometers and without the necessity of long modal sections to achieve enough phase shift (dimensions around 35 microns) . In summary, this approach could result in novel interferometric sensing structures for lab-on-a-chip integrated devices and other bio-sensing applications.
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 K. E. Zinoviev, A. B. González-Guerrero, C. Domínguez, and L. M. Lechuga, J. Light. Technol. 29, 1926 (2011).
The development and demand of label-free biosensors devices for a direct, rapid and cost effective analysis has rapidly increased during the last years. In this context, we report here our work towards the development of an integrated label-free biosensor based on an innovative nanophotonic technology for the detection of low protein concentrations. Photonic bandgap (PBG) sensing structures based on a silicon on insulator (SOI) substrate were used, as they demonstrates high sensitivities with an extremely small footprint due to the slow-wave effect .
Regarding the biofunctionalization, the thiol-ene coupling (TEC) reaction was used to covalently immobilize the bioreceptors onto the surface. TEC reaction was selected because it provides more compact functionalized surface, what is translated into a higher interaction with the photonic evanescent wave, and a spatially-specific immobilization of the bioreceptors upon UV light photo-catalysis, what can be used to biofunctionalize each sensing area with a different bioreceptor in order to perform a multiplexed sensing device . Half antibodies (hIgGs), which were immobilized using the SH groups from their hinge region, were used as bioreceptors for the specific recognition of the target protein.
To implement this biofunctionalization strategy, first the SOI surface silanization was carried out using triethoxyvinylsilane at 1% in water. The use of water as carrier for the organosilane provides several advantages such as vertical polymerization prevention, compactness and sustainability. Once the sensing surface was silanized, the TEC-based immobilization of the hIgGs onto the photonic sensors was monitored for the ``live´´ hIgG immobilization performance. BSA hIgGs obtained by the TCEP protocol were used in these experiments . The real-time monitoring of the sensing structures allowed demonstrating that the immobilization of the hIgGs only took place when the system was photo-catalysed with UV light. Finally, the recognition of the target protein (BSA) at 1 µg/mL by the PBG sensors was successfully measured.
 X. Fan, I. M. White, S. I. Shopova, H. Zhu, J. D. Suter, and Y. Sun. Anal. Chim. Acta. 2008, 620, 8-26.
 R. Alonso, P. Jiménez-Meneses, J. García-Rupérez, M.J Bañuls, Á. Maquieira. Chem.Comm. 2018, , 54, 6144--6147.