Detecting aflatoxin M1 using wavelength-scanned fiber cavity attenuated phase shift spectroscopy

Aflatoxin M1 (AFM1) is a carcinogenic compound usually found in milk, especially in developing countries. Significantly, AFM1 remains stable in milk even during pasteurization and heat treatments and thus poses a health hazard to humans, particularly children. Currently, well-established methods for detecting AFM1 include ELISA and chromatography. Although these approaches are reasonably accurate, they require a skilled workforce, costly setups, and several hours to generate results. We demonstrate the first application of wavelength-scanned cavity attenuated phase shift spectroscopy (WS-CAPS) in fiber cavities for AFM1 sensing to overcome the problems mentioned above. We build the sensor by forming a cavity with two fiber Bragg gratings. An SMF28 tapered fiber is spliced into the cavity as a sensing head. We bioconjugate the tapered fiber with DNA aptamers and validate the functionalization with EDX analysis. We use the coupled-mode theory to arrive at mathematical equations for conducting the WS-CAPS measurements. The WS-CAPS measurements primarily include detecting the phase of an output sinusoid with respect to an input sinusoid coming from a wavelength and amplitude modulated laser. The phase changes are directly related to AFM1 binding events at the functionalized tapered fiber. Our demonstrated sensor can detect AFM1 as low as 20 ppt (20 ng/L) in an aqueous solution, which is better than the safety limits imposed by European and USA regulatory bodies. In contrast to traditional CAPS systems with free space cavities, our WS-CAPS sensing modality allows us to use a source wavelength independent of an analyte's absorption. We strongly believe that the current work will lead towards developing accurate, rapid, and specialist-free sensors for a wide variety of applications in food security and point-of-care settings, especially for low-resource settings.