Multifunctional bionanocomposites for trace detection of water contaminants

We report here research on magneto-plasmonic bionanocomposites (Fe₃O₄@SiO₂/SiκCRG/Ag) designed for the detection of water contaminants using surface enhanced Raman scattering (SERS) methods. These new substrates comprise a ferrimagnetic core (Fe₃O₄) coated with hybrid shells composed of amorphous silica and κ-carrageenan, an anionic polysaccharide. Silver nanoparticles (Ag NPs) were generated in situ via the reduction of aqueous Ag(I) in the presence of the magnetic bionanocomposites (Fe₃O₄@SiO₂/SiκCRG), resulting in SERS-active substrates with magnetic separation capabilities. The SERS performance of these substrates was evaluated for the detection of two organic dyes, methylene blue (MB) and malachite green (MG), as well as the antibiotic ciprofloxacin (CIP). We mapped the spatial distribution of both the model contaminants and Ag NPs on the magneto-plasmonic bionanocomposites using combined SERS analysis and Raman imaging studies. This approach enabled the detection of MB, MG, and CIP in spiked aqueous solutions, down to concentrations of 1 × 10^-9 M for MB and MG, and 3 × 10^-7 M for CIP. Furthermore, the suitability of these substrates for on-site analysis was demonstrated using a portable Raman device to detect single- and multi-component dye mixtures. We also explored their practical applicability in a more complex matrix by detecting CIP (3 × 10^-5 M) in spiked water sample from the Aveiro lagoon. Noteworthy, we associate the improved performance of these substrates to the role of κ-carrageenan, which facilitates the assembly of Ag NPs, creating localized regions of high electromagnetic field intensity, preserving the SERS activity even after eighteen months of storage. The impact of temperature on the SERS signal was evaluated, revealing that the bionanocomposites maintain their activity after experiencing a temperature cycle. These findings indicate that these new SERS substrates are promising for practical sensing applications, particularly for on-site detection of water contaminants in remote regions.