MIP-Based Sensitive Lens on Silica Optical Fibers for Selective Detection of Analytes

In this work, two silica optical fiber (SOF) sensors are developed exploiting a novel sensing principle based on two SOFs coupled via a lens made on the tip of the launching SOF and covered by molecularly imprinted polymers (MIPs). Another SOF collects the focused light through a sensitive lens. In particular, the lens is formed by a fusion splicing machine, while the MIP layer is deposited on the lens by dip coating. As a proof of concept, an MIP for the detection of 4-chloro-2-methylphenoxyacetic acid (MCPA) is used. Two sensor configurations are exploited by using, as the launching SOF, a narrow-core (NC) optical fiber or a large-core (LC) optical fiber, in order to tune the detection range. When the analyte (MCPA) interacts with the MIP on the lens, the MIP's refractive index changes and, therefore, changes the focal length of the lens on the launching SOF, altering the intensity of the collected light. To demonstrate the feasibility of the sensing principle, experimental results for the MCPA detection in water were obtained by exploiting both sensor configurations based on NC and LC SOFs. The best sensor configuration based on the LC SOF presents a limit of detection (LOD) of 8 pM and a detection range from 8 pM to 1 nM. In contrast, the NC SOF sensor configuration achieves an LOD of about 89 nM and a detection range from 89 nM to 54 µM . Therefore, by replacing the launching SOF, complementary MCPA detection ranges are achieved.