On-chip bulk-index concentration and direct, label-free protein sensing utilizing an optical grated-waveguide cavity

A theoretical and experimental evaluation is given of a principle of direct, label-free opto-chemical sensing. According to this principle, which is applicable in compact planar optical sensors, measurand-induced wavelength shifts of the sharp fringes in the transmission spectra near the stop band edges of a resonant grating-based cavity are monitored. Such fringes are the results of Fabry–Perot resonances of the Bloch modes propagating in the cavity. Two sensor configurations have been considered, a first one for measuring the concentration of a single compound dissolved in water in the vicinity of the grating (bulk sensing), and a second one for determining the concentration of a specific compound adsorbed at the grating surface from a watery mixture of many compounds (surface sensing). In the latter, a thin interface layer which contains receptors specific to the targeted analyte, the PepN enzyme, is applied on top of the grated waveguide section. Filling of the receptors can be effectively seen as growth of an adlayer. Experimentally resolutions of 6 × 10 − 6 refractive index unit and ∼4 pm adlayer growth have been obtained for bulk and surface sensing, respectively. With a statistical analysis the limitations to obtain lower resolutions with the current set-up are identified. The compact devices (footprint ∼200 μm × 15 μm) are well suited for multi-sensing in lab-on-a-chip systems and can easily be fabricated with standard micro-fluidic and CMOS technologies.