  Aflatoxin M1 (AFM1) is a carcinogenic compound commonly found in milk in
excess of the WHO permissible limit, especially in developing countries.
Currently, state-of-the-art tests for detecting AFM1 in milk include
chromatographic systems and enzyme-linked-immunosorbent assays. Although these
tests provide fair accuracy and sensitivity however, they require trained
laboratory personnel, expensive infrastructure, and many hours for producing
final results. Optical sensors leveraging spectroscopy have a tremendous
potential of providing an accurate, real time, and specialists-free AFM1
detector. Despite this, AFM1 sensing demonstrations using optical spectroscopy
are still immature. Here, we demonstrate an optical sensor that employs the
principle of cavity attenuated phase shift spectroscopy in optical fiber
cavities for rapid AFM1 detection in aqueous solutions at 1550 nm. The sensor
constitutes a cavity built by two fiber Bragg gratings. We splice a tapered
fiber of $<$ 10 $\mu$m waist inside the cavity as a sensing head. For ensuring
specific binding of AFM1 in a solution, the tapered fiber is functionalized
with DNA aptamers followed by validation of the conjugation via FTIR, TGA, and
EDX analyses. We then detect AFM1 in a solution by measuring the phase shift
between a sinusoidally modulated laser input and the sensor output at resonant
frequencies of the cavity. Our results show that the sensor has the detection
limit of 20 ng/L (20 ppt) which is well below both the US and the European
safety regulations. We anticipate that the present work will lead towards a
rapid and accurate AFM1 sensor, especially for low-resource settings.
