Label-free optical biosensing using a horizontal air-slot SiNx microdisk resonator

We demonstrate label-free optical biosensing using a horizontal air-slotted silicon-rich silicon nitride (SiNx) microdisk resonator. Due to the strong confinement of light in the air-slot, a large resonance shift of 6.2 nm is observed upon reaction of the biotin-functionalized disk with a streptavidin solution with concentration of 2.5 mu g/ml. Assuming a linear relationship between resonance shift and streptavidin concentration, we estimate the sensitivity to be 2.5 +/- 0.2 nm/(mu g/ml). Comparing this value with surface sensitivity of 5.7 nm/ nm calculated using FDTD simulations, a detection limit of 30 +/- 2 ng/ml, is extrapolated. (C) 2010 Optical Society of America