We present a theoretical and experimental study on the biosensing sensitivity of Au/Co/Au multilayers as transducers of the magneto-optic surface-plasmon-resonance (MOSPR) sensor. We demonstrate that the sensing response of these magneto-plasmonic (MP) transducers is a trade-off between the optical absorption and the magneto-optical activity, observing that the MP multilayer with larger MO effect does not provide the best sensing response. We show that it is possible to design highly-sensitive MP transducers able to largely surpass the limit of detection of the conventional surface-plasmon-resonance (SPR) sensor. This was proved comparing the biosensing performance of both sensors for the label-free detection of short DNA chains hybridization. For this purpose, we used and tested a novel label-free biofunctionalization protocol based on polyelectrolytes, which increases the resistance of MP transducers in aqueous environments.

In this paper, we analyze the magnetoplasmonic ͑MP͒ features and sensing capabilities of Au/Fe/Au trilayer structures, as transducers of the magneto-optic surface plasmon resonance ͑MOSPR͒ biosensor. This biosensor, which can surpass the sensitivity of the standard SPR sensor, is based on a MP modulation technique generated by the simultaneous stimulation of the surface plasmon polaritons ͑SPP͒ and the transversal magneto-optical Kerr effect ͑TMOKE͒. We study the magneto-optical activity of the trilayers as a function of the thickness and position of the Fe layer. We first demonstrate that this kind of structure allows modulating the SPP through an external magnetic field and moreover, induce a strong enhancement of the TMOKE effect. The modulation of the SPP is linearly proportional to the thickness of Fe layer and inversely proportional to the distance between the Fe layer and the external dielectric medium. Finally, we experimentally confirm a twofold increase in the MOSPR sensitivity with respect to the intensity-interrogated SPR biosensor in bulk refractive-index changes, keeping a similar chemical resistance and stability, unprecedented in other MP transducers, and biofunctionalization protocols.

Micro-RNA signatures have emerged as advantageous biomarkers for disease prediction opening the route for the development of more direct and accurate therapies. There is an urgent need for reliable tools which can offer a fast, highly sensitive and selective detection of multiple miRNAs in complex matrices as opposed to the conventional techniques. Here, we demonstrate a nanophotonic biosensor with potential multiplexing capabilities based on interferometric bimodal nanowaveguides (BiMW) for ultra-sensitive detection of microRNAs in complex media. Concretely, the BiMW biosensor has been employed for the detection and quantification of miR-181a at attomolar concentrations (LOD = 23 aM) directly, and for the first time, in human urine samples of bladder cancer patients with no need for prior sample purification or amplification steps. We demonstrate the extremely selectivity of our methodology for miR-181a detection showing the discrimination of homologous sequences at single nucleotide mismatch and its pre-miRNA. A significant overexpression of miR-181a in bladder cancer patients was appreciated when compared with healthy controls, suggesting the implication of this miRNA in bladder cancer. Our results show that the BiMW biosensor can be used as an ultrasensitive and specific diagnostic tools by the early and fast detection and quantification of microRNAs for the prediction of diseases (as cancer) with well-defined microRNA signatures.

An all-optical phase modulation method for the linear readout of integrated interferometric biosensors is demonstrated, merging simple intensity detection with the advantages offered by spectral interrogation. The phase modulation is introduced in a simple and cost-effective way by tuning a few nanometers the emission wavelength of commercial laser diodes, taking advantage of their well-known drawback of power-wavelength dependence. The method is applied to the case of a bimodal waveguide (BiMW) interferometric biosensor, fabricated with standard silicon technology and operated at visible wavelengths, rendering a detection limit of 4 × 10 −7 refractive index units for bulk sensing. The biosensing capabilities of the phase-linearized BiMW device are assessed through the quantitative immunoassay of C-reactive protein, a key protein in inflammatory processes. This method can be applied to any modal interferometer.