Surface-enhanced Raman spectroscopy (SERS) harnesses metallic nanostructures combined with optical fields to create localized surface plasmon resonance (LSPR), yielding significant Raman scattering enhancement. However, 'top-down' manufacturing methods for SERS substrates are often costly due to complex fabrication processes. Photochemical reduction synthesis, known for its high chemical purity and good process controllability, has gained attention but typically requires a high-power laser for rapid preparation. With advancements in high-power femtosecond-pulsed lasers, laser direct-writing has become viable for single-step SERS substrate fabrication. Nevertheless, the small focal laser spot limits efficiency in large-area fabrication of patterned micro-nanostructures. Dielectric microspheres, with their ability to focus incident lasers at their bottom beyond the diffraction limit, offer a solution for parallel nanomanufacturing. This study developed a one-step photochemical reduction technique for hierarchical silver micro-nanostructures using a dielectric microsphere array, demonstrating its ultra-sensitive Raman detection capability.
A polydimethylsiloxane (PDMS) film was prepared by mixing PDMS with a curing agent and then spin-cured. Barium titanate microspheres, with high refractive indices (1.9), were pressed into a monolayer close-packed array on the PDMS film via mechanical grinding. An uncured PDMS film was placed onto this array, transferring and semi-embedding the microspheres into PDMS (PDMS/MS), followed by curing. The PDMS/MS film was then covered with a silver nitrate and trisodium citrate solution. A 532 nm line CW laser, with power ranging from 49?168 μW, focused by PDMS/MS into the solution, induced the reduction reaction. Consequently, Ag+ was reduced under focused laser irradiation, forming a hierarchical silver micro-nanostructure (AgNPs/AgMRs) at the bottom of the microspheres. The surface morphology of the Ag micro-nanostructures was examined using SEM. The influence of microsphere diameter and photoreduction parameters on the morphology was both theoretically and experimentally investigated. Raman spectra of various analytes at different concentrations were acquired to optimize the hierarchical AgNP/AgMR/MS structure, with COMSOL simulations revealing the Raman enhancement mechanisms.
The study explored how microsphere diameters affect the hierarchical Ag micro-nanostructure. With a diameter increase to 21 μm, AgMRs with three concentric circles were formed. Increasing the diameter further to 39 μm resulted in the focused light energy at the microsphere bottom falling below the photochemical reduction threshold, leading to the formation of only a small amount of AgNPs (Fig. 2). Optimal photochemical reduction parameters were experimentally determined: a 1∶4 molar concentration ratio of silver nitrate to trisodium citrate, a 98 μW laser power, and an 80 s irradiation time produced clear AgMRs with high-density AgNPs at the microsphere bottom (Fig. 3). This configuration achieved a detection limit of 10-14 mol/L for methylene blue solution and an enhancement factor (ζ) of up to 9.50×109. The SERS structure exhibited good reproducibility and compatibility for practical applications, as shown in Fig. 4. Furthermore, an enhancement factor of 9.53×109 for the hierarchical AgNP/AgMR/MS structure was obtained through numerical simulation (Fig. 6), aligning well with experimental results. The Raman enhancement channels were attributed to electromagnetic enhancement from microsphere nanofocusing, localized surface plasmon resonances in AgNPs/AgMRs, and the directional antenna effect of the AgNPs/AgMRs/MS hybrid structure.
This study proposes a new technique for fabricating hierarchical metal micro-nanostructures through optical field modulation of dielectric microspheres. Rapid photochemical reduction of the hierarchical Ag micro-nanostructure was achieved using the unique focusing properties of dielectric microspheres. The impact of precursor molar concentration ratio, microsphere diameter, laser power, and irradiation time on the morphology of the hierarchical Ag micro-nanostructures was thoroughly examined. An optimal Raman-enhancing hierarchical AgNP/AgMR/MS structure was fabricated using 21 μm diameter barium titanate microspheres, a 1∶4 silver nitrate to trisodium citrate molar concentration ratio, a 98 μW laser power, and an 80 s irradiation time. Experiments and numerical simulations indicated that the Raman enhancement channels of the hierarchical AgNP/AgMR/MS structure stemmed from microsphere nanofocusing, localized surface plasmon resonance of the hierarchical Ag micro-nanostructure, and directional emission from the hybrid structure. The hierarchical AgNP/AgMR/MS hybrid structure demonstrated an enhancement factor of up to 9.50×109 and a detection limit of 10-14 mol/L for trace detection. This study provides a new strategy for creating ultra-sensitive dielectric/metal hybrid SERS substrates with low cost and high performance for practical applications.
