Differential modulation system with potential for portable field detection: An ultrasensitive gentamicin optical sensor for label-free detection

In this study, we present a differential intensity-modulated system that comprises a tapered microfiber interferometer functionalized with polydopamine /AuNPs/cysteamine hydrochloride and two fiber Bragg gratings (FBGs) for detecting gentamicin in deionized water and milk. As external refractive index (RI) increases, the interference troughs of microfiber are red-shifted, while Bragg wavelengths remain unchanged, leading to a change in the difference in the reflected intensities of two FBGs, thus resulting in differential intensity modulation of external RI. The microfiber interferometer alone in this system demonstrates a wavelength response of 1314.57 nm/RIU to external RI, while the system exhibits an outstanding response of 2713.74 dB/RIU, highlighting its superior performance. The system demonstrates excellent results and selectivity when detecting gentamicin in deionized water and milk, and the wavelength response and differential intensity response reach 27 nm/mu M, 80 dB/mu M and 31 nm/mu M, 70 dB/mu M, respectively, with limit of detection of 1.117 ppb, which is lower than the maximum residue limit of 30 ppb in milk. Furthermore, a novel all-fiber demodulation scheme is presented, which holds great promise for the development of portable field measurement systems. These findings demonstrate the efficiency of differential modulation systems with implications for food safety.