Refractive index based detection of acetaminophen using a simple singlemode–multimode–singlemode fiber-optic sensor

Abstract

This study presents the design, fabrication, and experimental validation of a low-cost optical sensor based on refractive index variation for the detection of acetaminophen (commonly called paracetamol) in liquid solutions. The sensor uses a multimode interference effect under an architecture built with a singlemode–no-core multimode–singlemode fibers (SMF–NC-MMF–SMF or frequently known as SMS). Taking advantage of the self-imaging effect inherent to multimodal interference and the evanescent field interaction with the surrounding media, the sensor exhibits high sensitivity to refractive index changes induced by variations in analyte concentration. Test solutions were prepared from ground commercial tablets containing 85% of active pharmaceutical ingredients. Acetaminophen concentrations in hydroalcoholic solutions, ranging from 17.54 to 128.03 g/kg, were quantified. A progressive shift of the transmission peak toward longer wavelengths, correlated with increasing acetaminophen concentration, was observed when the NC-MMF segment was immersed in each solution. The device demonstrated a spectral sensitivity of 0.024 92 nm/(g/kg), with a linearity (R2 = 0.988), and an estimated limit of detection ∼8.2 g/kg. Additionally, a negative shift attributed to the thermo-optic coefficient of the medium was observed in thermal tests conducted from 20 to 40 °C. However, crystallization effects of acetaminophen are observed above 40 °C, which limits the useful monitoring range. Compared with other methods, the proposed SMS sensor achieves comparable performance through simple construction and without specialized coatings, positioning it as a promising alternative for real-time optical monitoring of pharmaceutical compounds in liquid media.