Optical sensing of chlorophyll(in) with dual-spectrum Si LEDs in SOI
CMOS technology
Abstract
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.