Optical sensing of chlorophyll(in) with dual-spectrum Si LEDs in SOI CMOS technology
preprintposted on 04.06.2021, 02:54 by Satadal Dutta, Peter Steeneken, Gerard J. Verbiest
Small and low-cost chlorophyll sensors are popular in agricultural sector and food-quality control. Combining such sensors with silicon CMOS electronics is challenged by the absence of silicon-integrated light-sources. We experimentally achieve optical absorption sensing of chlorophyll based pigments with silicon (Si) micro light-emitting diodes (LED) as light-source, fabricated in a standard SOI-CMOS technology. By driving a Si LED in both forward and avalanche modes of operation, we steer its electroluminescent spectrum between visible (400–900 nm) and near-infrared (~1120 nm). For detection of chlorophyll in solution phase, the dual-spectrum light from the LED propagates vertically through glycerol micro-droplets containing sodium copper chlorophyllin at varying relative concentrations. The transmitted light is detected via an off-chip Si photodiode. The visible to near-infrared color ratio (COR) of the photocurrent yields the effective absorption coefficient. We introduce the LED-specific molar absorption coefficient as a metric to compute the absolute pigment concentration (?~0.019 ?M) and validate the results by measurements with a hybrid spectrophotometer. With the same sensor, we also show non-invasive monitoring of chlorophyll in plant leaves. COR sensitivities of ? 3.9? x 104 M-1 and ? 5.3? x 104 M-1 are obtained for two leaf species, where light from the LED propagates diffusely through the thickness of the leaf prior to detection by the photodiode. Our work demonstrates the feasibility of realizing fully CMOS-integrated optical sensors for biochemical analyses in food sector and plant/human health.