Microcone Array‐Mediated Distance Tuning for Photonic Nanojet‐Amplified SERS

ABSTRACT This study optimizes photonic nanojet (PNJ)‐amplified surface‐enhanced Raman spectroscopy (SERS) by precisely controlling the gap distance between a dielectric microsphere and the SERS‐active surface, ensuring alignment of the PNJ focal point with plasmonic hotspots. To achieve this, we developed a tunable sensing platform using poly(ethylene glycol) diacrylate (PEGDA) microcone arrays decorated with silver nanoparticles (AgNPs). These microcone arrays mechanically support high refractive index microspheres at specific heights above the SERS substrate, allowing the microsphere‐substrate gap to be systematically tuned by varying the microcone pitch. Structural parameters were validated using 3D optical profilometry, and the PNJ focal distances were modeled using finite‐difference time‐domain (FDTD) simulations to confirm optimal alignment. Our findings demonstrate that an optimal gap distance exists for each microsphere size, corresponding to the precise alignment of the PNJ's focal point with the SERS‐active surface. This optimized configuration yields a PNJ‐induced SERS enhancement of up to 15‐fold, significantly improving the limit of detection for adenine (to 1 nM) and melamine (to 0.5 µ M ). Overall, this work establishes microcone‐mediated gap control as an effective, reproducible strategy for the rational design and optimization of PNJ‐amplified SERS biosensors, with potential applications in food safety, environmental monitoring, and biomedical diagnostics.