Fig. 1. Experimental flowchart. (a) Preparation process of Fe₃O₄@AgNPs composite structure; (b) preparation process of D-shaped fiber SERS probe integrated with microfluidic-based magnetic enrichment

Fig. 2. Simulation of enhancement principle of D-shaped fiber SERS probe. (a) Model diagram; (b) electric field distribution; (c) SEM image of Fe₃O₄@AgNPs composite structure; (d) x-y plane regional electric field distribution; (e) effect of removing different fiber core sizes on electric field intensity

Fig. 3. Comparison of Raman performance with and without magnetic enrichment. (a) Fe₃O₄@AgNPs composite structure without magnetic enrichment; (b) Fe₃O₄@AgNPs composite structure with magnetic enrichment; (c) Raman intensity gradient with magnetic enrichment; (d) Raman intensity gradient without magnetic enrichment; (e) comparison of intensity values of peak at 611 cm⁻¹ at three different concentrations

Fig. 4. Raman performance test of microfluidic D-shaped fiber SERS probe. (a) Concentration gradient test of R6G molecules ranging from 10⁻⁵ to 10⁻⁸ mol/L; (b) detection limit of 10⁻⁸ mol/L; (c) physical image of multi-molecule detection; (d) results of multi-molecule detection

Fig. 5. Reproducibility verification and real-world molecular test results. (a) Raman spectra of microfluidic D-shaped fiber SERS probe during 5 cycles of testing at 10^-6 mol/L R6G; (b) Raman intensity at 611 cm^-1 during 5 cycles of testing; (c) detection results of thiram