Highly sensitive surface plasmon resonance-based sensor using green synthesized Fe₃O₄/rGO interface layer utilizing plant leaf extracts for alcohol compound detection

In this study, a fast response, real time, accurate, and non destructive alcohol detection method using surface plasmon resonance (SPR) technique was purposed. The SPR measurement was performed using 5-layers Krestchmann configuration with a layer structure of prism/Au thin film/Fe₃O₄/rGO nanocomposite/alcohol compounds/air. The Fe₃O₄/rGO nanocomposite was successfully synthesized using the green route utilizing Moringa oleifera and Amaranthus viridis leaf extract. X-ray diffraction analysis showed the nanocomposite has a face-centered cubic with an inverse spinel structure with a crystallite size of 5.6-5.8 nm. The size of Fe₃O₄ NPs in the Fe₃O₄/rGO nanocomposite was variated from 10.6-13.0 nm and showed that there is no impurities in the sample. Fourier transform infra-red analysis also validates the existence of Fe₃O₄ and rGO indicated by the FeO and CC bond, respectively. The interaction between Fe₃O₄ and rGO can also be observed through the coordinational bonding FeOC, which is validated by the presence of FeO and CO bonds. The optical properties were studied using ultraviolet-visible spectroscopy, which shows an energy gap of 2.36 eV. Magnetic properties of Fe₃O₄/rGO nanocomposite show a superparamagnetic characteristic with the saturation magnetization of 40.53 emu/g, magnetic susceptibility of 3.62 × 10^-2, and the domain size is 6.22 nm. The SPR angle shifts when applied with Fe₃O₄/rGO nanocomposite. The addition of alcohol compound further shifted the SPR angle by 0.22°, 0.61°, and 1.19° for methanol, ethanol, and IPA, respectively. This noticeable shift shows a possibility for early detection to differentiate these 3 compounds. The presence of a magnetic field further shifts the SPR angle by 0.08°, 0.08°, and 0.10° for 40, 60, and 80 Oe, which indicates an increase in sensitivity. Therefore, the combination of applied magnetic field and green synthesized Fe₃O₄/rGO nanocomposite as an eco-friendly interface layer are potential to enhance the sensitivity of SPR to detect the alcohol compounds.