We review the most important achievements published in the last five years in the field of silicon-based optical biosensors. We focus specially on label-free optical biosensors and their implementation into lab-on-a-chip platforms, with an emphasis on developments demonstrating the capability of the devices for real bioanalytical applications. We report on novel transducers and materials, improvements of existing transducers, new and improved biofunctionalization procedures as well as the prospects for near future commercialization of these technologies.

In this review, we present the current trends in photonic biosensors, focusing on devices based on lab-on-a-chip (LOC) systems capable of simultaneously detecting multiple real-life diseases on a single platform. The first section lists the advantages and challenges of building LOC platforms based on integrated optics. Some of the most popular materials for the fabrication of microfluidic cells are also shown. Then, a review of the latest developments in biosensors using the evanescent wave detection principle is provided; this includes interferometric biosensors, ring resonators, and photonic crystals, including a brief description of commercial solutions, if available. Then, a review of the latest advances in surface plasmon resonance (SPR) biosensors is presented, including localized-SPRs (LSPRs). A brief comparison between the benefits and required improvements on each kind of biosensor is discussed at the end of each section. Finally, prospects in the field of LOC biosensors based on integrated optics are glimpsed.

A label‐free interferometric transducer showing a theoretical detection limit for homogeneous sensing of 5 × 10^–8 RIU, being equivalent to a protein mass coverage resolution of 2.8 fg mm^–2, is used to develop a high sensitive biosensor for protein detection. The extreme sensitivity of this transducer combined with a selective bioreceptor layer enables the direct evaluation of the human growth hormone (hGH) in undiluted urine matrix in the 10 pg mL^–1 range. magnified image

There is still a need of a compact and cost-effective solution for efficient light in-coupling in integrated waveguides employed in the photonic biosensors activity. Especially when these waveguides are of submicron dimensions and operate at visible wavelengths. The employment of a vertically stacked taper with a larger input area is proposed to solve this need. The design of the taper is divided in two stages: in the first stage, light is guided downwards by two vertically stacked tapers; in the second stage, an inverted taper directly confines the light inside the waveguide. The design parameters are optimized using commercial software obtaining a total theoretical coupling efficiency of 72.25%. The taper is manufactured using SU-8 polymer as main material employing standard photolithography techniques at wafer level. After characterization, the results show the practicality of the taper when coupling light from macro sources to the nanometric waveguides obtaining an experimental coupling efficiency of 55%. With this vertical taper, a compact, easy-to-couple and cost-effective solution is accomplished for waveguide based biosensors operating at visible wavelengths, opening the route for a truly portable point-of-care biosensor for low cost and label-free diagnostics.

Summary form only given. Despite the intense developments in the field of integrated optics, there is still a need of a compact, easy and cost-effective solution for light in-coupling in integrated photonic optical waveguides for biosensing, which are designed to be single use devices in clinical environments. The majority of current solutions are based on grating couplers fabricated with e-beam lithography, which increases the device cost [1]. For example, in the case of our bimodal waveguide interferometric biosensor [2], gratings require tapers of up to 2000 μm to guide the light towards the waveguide entrance.We propose instead an adiabatic vertical two-stage taper (see Figure 1a), which can be fabricated using standard lithography techniques at wafer level by slightly modifying our current fabrication process. The taper is compact enough (~1200 μm) while offering theoretical coupling efficiencies up to 72% with an input area of 10 μm x 6 μm instead of the actual 3 μm x 0.15 μm, facilitating the coupling process. Vertical adiabatic tapers have been previously proposed in the short IR range but its fabrication at wafer level and its integration into a photonic visible biosensor have not been yet reported [3,4].

We present our last achievements towards the assembly of lab-on-a-chip platforms based on ultrasensitive Mach-Zehnder interferometers. In particular, we will focus on the development of a label-free and real-time multiplexed detection scheme.