Surface-enhanced Raman scattering (SERS) is a vibrational spectroscopy technique that amplifies molecular Raman signals using precious metal nanostructures. Recently, SERS has emerged as a powerful fingerprint identification tool for rapid, non-destructive, and ultra-sensitive detection of various chemical and biological targets, with broad applications in analysis and sensing. To further improve the sensitivity, integration, and practicality of the Raman detection system, we propose a multi-channel microfluidic D-shaped fiber SERS probe based on magnetic enrichment.

First, uniformly shaped silver nanoparticles (AgNPs) are prepared using a one-step reduction method and then adsorbed onto Fe₃O₄ microbeads through electrostatic interactions to form a Fe₃O₄@AgNPs composite structure. Next, microfluidic channels are created, using a custom-designed polydimethylsiloxane (PDMS) template. The D-shaped fiber, microfluidic channels, and a glass slide are bonded together, and the prepared Fe₃O₄@AgNPs are injected into the microchannels. Under the influence of a magnetic field, the nanoparticles are enriched in the planar region of the D-shaped fiber, forming a microfluidic D-shaped fiber SERS probe. To further analyze the enhancement mechanism of the D-shaped fiber SERS probe, we utilize COMSOL Multiphysics software for simulation. The model parameters include a cladding radius (R) of 62.5 μm, a core radius (r) of 31.25 μm, a core refractive index (n₁) of 1.46, and a cladding refractive index (n₂) of 1.44. Simulation results indicate a theoretical maximum enhancement factor (EF) of approximately 6.2×10⁴.

To experimentally verify the magnetic enrichment effect, we conduct a series of comparative experiments. First, we prepare R6G solutions with concentrations of 10^-7, 10^-8, and 10^-9 mol/L, and sequentially introduce the prepared composite structures into 3 mL of these R6G solutions. After thoroughly mixing the composites with the test solutions, 50 μL of the mixture is pipetted onto a silicon wafer. Magnetic aggregation is then induced using a magnet, followed by natural drying prior to testing. For comparison, an equal volume of the mixed solution is pipetted onto a silicon wafer without magnetic aggregation. The results clearly show that the Fe₃O₄@AgNPs composite structures significantly enhance the Raman signal of the R6G probe molecules, enabling the detection of lower concentrations of R6G. Furthermore, the signal intensity increases significantly under magnetic enrichment. At all three concentrations, the signal intensity at the 611 cm^-1 peak with magnetic aggregation is approximately twice as strong as that without magnetic aggregation (Fig. 3). To evaluate the detection performance of the microfluidic D-shaped fiber SERS probe under magnetic enrichment, we further characterize it with different concentrations of R6G (ranging from 10^-5 to 10^-8 mol/L). The Raman spectra show clear peaks corresponding to R6G (611, 772, 1182, 1310, 1363, 1506, 1570, and 1650 cm^-1) [Fig. 4(a)]. In addition, to explore its ability to detect multiple molecules in complex environments, we sequentially introduce R6G (10^-5 mol/L), MG (10^-4 mol/L), and CV (10^-3 mol/L) through different input ports for mixed detection. The resulting Raman spectra indicate that the peaks of R6G (611 cm^-1), CV (912 cm^-1), and MG (1216 cm^-1) can still be clearly distinguished in mixed conditions, demonstrating the probe’s ability to detect multiple molecules simultaneously [Fig. 4(d)]. To verify the reproducibility of the microfluidic D-shaped fiber SERS probe and its ability to detect real-world molecules, we also conduct relevant Raman tests (Fig. 5).

In this study, we successfully develop a microfluidic optical fiber SERS probe by combining D-shaped fibers with microfluidic channels via plasma surface bonding and incorporating magnetic enrichment. The performance of the SERS probe is evaluated through tests on detection limits, simultaneous detection of multiple molecules, real-world sample analysis, and reproducibility. The experimental results demonstrate that the microfluidic fiber SERS probe has high sensitivity. Rapid enrichment of the SERS substrate is achieved using an external magnetic field, resulting in the formation of additional “hot spot regions.” The detection limit for R6G reaches 10^-8 mol/L, with a maximum enhancement factor of approximately 10⁶. The probe can detect the Raman characteristic peaks of target molecules even in complex environments. In addition, the SERS probe shows excellent reproducibility. The Fe₃O₄@AgNPs composite can quickly disperse after removing the magnetic field, allowing for easy recovery and reuse, thus reducing experimental costs. This multi-channel, highly integrated microfluidic fiber SERS platform provides a practical and efficient solution for the on-site detection of biological and chemical molecules.