One key point in the preparation of optical fiber surface-enhanced Raman spectroscopy (SERS) probes is the enrichment of metal nanoparticles on the fiber end face. When using self-assembly methods to enrich metal nanoparticles, it is necessary to soak the fiber in a solution to deposit the metal particles, which requires a relatively long time, typically 1?2 h. In addition, photolithography methods involve high equipment costs. To reduce both time and experimental costs, we propose the use of laser-induced metal deposition technology. Laser-induced metal deposition, a method that has become relatively popular in recent years, enables rapid and controllable enrichment of metal particles on the fiber end face. To address the fragility of tapered fibers, a more robust spherical-tip fiber is fabricated. The spherical-tip structure overcomes the evanescent field limitations of traditional planar optical fibers and achieves synergistic optimization of spatial light field compression and electromagnetic enhancement through curvature tuning. By controlling the gradient distribution of silver nanoparticles during the laser-induced deposition process, an Ag NP aggregation zone is formed at the apex of the sphere, effectively increasing the density of hot spots.

In this paper, spherical-tip optical fibers with controllable radii are prepared using a fusion splicer via a melting process. Fibers with radii of 70, 80, 90, and 100 μm are fabricated by adjusting the discharge time and intensity. Uniformly shaped silver nanoparticles are synthesized using a chemical reduction method. A 532 nm laser is then introduced into the prepared spherical-tip fibers, and via laser-induced deposition methods, silver nanoparticles are enriched at the fiber tip, forming the spherical-tip fiber-optic SERS probe. Scanning electron microscopy (SEM) is used to characterize the morphology and size of the probes with different tip radii. A confocal Raman microscope is employed to optimize the Raman characteristic parameters (radius R, cycle number N, and evaporation time t₂) and to perform Raman measurements. Moreover, COMSOL software is used to model and simulate the spherical fiber tip, and radius parameters are simulated and analyzed.

We demonstrate that spherical-tip fibers with controllable radii can be prepared via a simple melting method (Fig. 1). The spherical-tip fiber-optic SERS probe shows favorable Raman performance. The maximum electric field at the tip increases initially and then decreases as the radius increases. At a radius of 80 μm, the maximum electric field intensity reaches 27 V/m (Fig. 3). Through optimization of R, N, and t₂, the Raman response of the spherical-tip fiber is enhanced (Fig. 4). The optimized probe achieves a detection limit for R6G as low as 10^-10 mol/L. Tests with multiple fibers demonstrate that the prepared optical fiber SERS probe has good stability, with a relative standard deviation (dRSD) of 12.2%. In addition, the probe detects multiple molecules (MG, CV, R6G) and real-world molecules (uric acid), confirming its practical application (Fig. 5).

We present a high-performance SERS probe based on a spherical-tip optical fiber. Efficient fabrication is achieved by combining the melting method with a simplified laser-induced nanoparticle self-assembly process. The probe demonstrates excellent sensitivity, achieving a detection limit as low as 10^-10 mol/L and strong enhancement capability, with an analytical enhancement factor of 2.07×10⁸ for R6G detection. The spherical geometry enhances the coupling between the localized surface plasmon resonance (LSPR) of silver nanoparticles and the light field through total internal reflection, increasing both light field energy density and the distribution intensity of hot spots. The probe also demonstrates good stability (dRSD=12.2%) and effective detection of malachite green, crystal violet, and uric acid, validating its utility in multiplexed analysis under complex conditions. Future work will focus on two directions: 1) development of a controllable self-cleaning probe structure to improve long-term stability and reusability via surface chemical modification or photocatalytic design; 2) optimizing the laser-induced nanostructure assembly process by integrating a machine learning-driven parameter control strategy to push the limits of single-molecule detection and expand its application in in vivo biosensing.