A multispectral resonant waveguide nanopatterned chip for robust oil quality monitoring

Oil quality control is an important issue in the food industry and consumer market. Consequently, on-site control with a user-friendly device at a reasonable cost is needed. Simple optical imaging of resonant peak on a nanostructured chip by a consumer grade camera allows real-time label-free biomolecules sensing and bulk refractive index measurements. Multispectral capability on the same nanostructured chip is introduced for the first time here, both through theory and experiments. We demonstrate that with optimized nanostructuration, the whole visible spectrum can be sensed using the same chip support. By analyzing oil samples from deep frying process, we found cooking process can induce a small (∼10−3) but stable and detectable change of the refractive due to the accumulation of polar materials. This is measured accurately thanks to the high-sensitivity of our refractive index sensing chip (Δn ∼ ± 1 × 10−5). Refractive index change of a mixture of animal fat and vegetable oil is also studied. We compare refractive index change at the 2 extremities of the visible spectrum (λ  480 nm and λ  630 nm), and confirm and ∼2 times larger change for small wavelength. Possible integration of our device with powerful imaging capability in recent popular smartphones is also discussed.