Surface-enhanced Raman scattering (SERS) is a powerful analytical tool that can provide molecular vibrational fingerprint information. Due to its unique analytical advantages, SERS has been widely applied in various fields such as biomedical research, environmental monitoring, and food analysis. During the development of SERS technology, silver nanoparticles have often been used as SERS substrates due to their excellent localized surface plasmon resonance properties. However, their susceptibility to oxidation poses a significant limitation in the construction of nano-probes for practical applications. In contrast, gold nanoparticles have better chemical stability and lower biotoxicity but relatively weaker Raman signal enhancement capability. Therefore, bimetallic SERS substrates combining gold and silver are characterized by high sensitivity and stability. Compared with traditional substrates, tapered optical fibers not only have the advantages of in-situ detection and remote signal transmission but also have great potential for enhancing laser reflection and improving light collection capability, which is beneficial for collecting weak Raman signals and achieving lower detection limits. Therefore, we proposed a simple and cost-effective bimetallic tapered optical fiber SERS probe. This probe utilized a light-induced method to deposit gold and silver nanoparticles on the surface of the tapered optical fiber, providing high sensitivity and good stability.

The bimetallic tapered optical fiber SERS probe was prepared by using a light-induced method in this study. First, the tapered fiber was prepared by using the fiber fusion tapering machine. After clamping the processed fiber onto the fiber holder, the fiber fusion tapering machine was started to initiate the tapering process. Once the machine stopped operating, two tapered optical fibers were obtained. Second, gold and silver nanoparticles with an approximate diameter of 50 nm were prepared by using a chemical reduction method. Finally, the bimetallic tapered optical fiber SERS probe was prepared through the light-induced method. The laser beam emitted by a helium-neon laser was coupled into the tapered optical fiber. Subsequently, the tapered end was immersed in a mixed solution of Ag sol and Au sol (volume ratio of 1∶1, both 0.3 mL) for 60 seconds. Then, the tapered end was moved from the solution to the air and kept in that position for 90 seconds while the laser was still operating. This process was repeated 15 times [Fig. 1(a)]. At the end of the entire procedure, the bimetallic tapered optical fiber SERS probe was obtained. The surface morphology of the tapered optical fiber was characterized by scanning electron microscopy (SEM), and the performance of the optical fiber probe was tested by a confocal Raman spectrometer.

In this study, SEM analysis [Fig. 2(a₁) and (a₂)] of the prepared samples revealed a uniform distribution of metal nanoparticles on the fiber surface, exhibiting a mostly monolayer arrangement. The relative mass percentages of Ag and Au elements were found to be 2.36% and 9.21%, respectively [Fig. 2(b) and (c)]. The average particle size of the metal particles on the sample was 49 nm, with an average gap of 6.8 nm [Fig. 2(e₁) and (e₂)]. In the paper, Rhodamine 6G (R6G) was selected as the analyte molecule to evaluate the SERS performance of the prepared samples. The bimetallic tapered optical fiber SERS probe was immersed in R6G solutions with concentrations ranging from 10^-8 to 10^-10 mol/L for three minutes respectively. After removal from the solution and natural drying, Raman signal detection was performed, and the obtained enhancement factor (AEF) for the samples reached 2.07×10⁸. To demonstrate the capability of detecting non-single molecule analytes, the samples were immersed in a mixed solution of 10^-6 mol/L R6G and 10^-4 mol/L crystal violet (CV) for three minutes. After removal from the solution and natural drying, the detection was conducted, and the results indicated that the unique Raman peaks of different probe molecules could be detected in the mixed solution [Fig. 6(b)]. To demonstrate the excellent stability of the bimetallic tapered optical fiber SERS probe, it was placed for different durations of 24 hours, 48 hours, 72 hours, and 96 hours. Afterward, a comparison was made by using Raman testing of 10^-7 mol/L R6G. The results indicated that the sample exhibited good stability (Fig. 7).

In this study, to enhance the enriched density of metal nanoparticles on the surface of optical fibers and improve the stability of the Raman-enhancing properties of the optical fiber SERS composite structure, we proposed a bimetallic (gold and silver) tapered optical fiber SERS probe structure. First, gold and silver nanoparticles with uniform morphology were prepared using the chemical reduction method. Then, the bimetallic particles were enriched on the tapered optical fiber using light-induced methods. The prepared optical fiber SERS probe exhibited excellent experimental performance. In this study, the lowest detected concentration of R6G was as low as 10^-10 mol/L, and the enhancement factor was 2.07×10⁸; compared with single-metal silver optical fiber SERS probes, the stability of the bimetallic sample was improved by seven times (after 96 hours). The bimetallic tapered optical fiber SERS probe is expected to be applied in in-situ and remote detection in the future. The next research direction is to explore the key process of controllable double metal modified optical fiber, so as to further optimize the detection sensitivity and stability of the sample.