Gold Nanostar-Based Core–Satellite Nanotags for Surface-Enhanced Resonance Raman Spectroscopy

Surface-enhanced Raman scattering (SERS) nanotags based on plasmonic nanostructures have emerged as powerful tools for ultrasensitive bioanalytical detection. Rational synthesis of SERS nanotags with high sensitivity and efficiency remains critical for achieving reproducible quantitative detection and real-world implementation. This thesis presents the systematic development of NIR-compatible gold nanostar (AuNS)-based core--satellite nanostructures engineered for ultrasensitive SERS detection. Starting from AuNSs as the core component, we explored the effects of different parameters on AuNS morphology, particle size distribution, and optical response under up-scaled synthetic conditions, achieving precise LSPR tuning to match the laser excitation wavelength at 785 nm. 4-arm star polymers synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization served as dual-functional stabilizing agents and interparticle linkers, with shorter chain lengths providing morphological stability while maintaining colloidal dispersibility. Under NIR laser irradiation, AuNS@PNIPAM displayed good photothermal performance with excellent stability. Subsequently, a strategy was developed to maximize the SERS signal of individual nanotags through rational structural design in which satellites were utilized to create additional electromagnetic hot spots and 3D loading of reporter molecules was enabled. Using 4-arm polymers as interparticle linkers, exceptionally high satellite loading of approximately 35 satellites per core was achieved, with tunable interparticle distances controlled by varying the chain length of linear spacer polymers. A polyelectrolyte-assisted deposition strategy was employed to incorporate resonant dyes lacking anchoring points as reporter molecules, enabling 3D reporter loading that transcended traditional monolayer limitations. Finally, SERS performance was evaluated through systematic measurements. Using confocal Raman microscopy, dramatic signal enhancement under in-resonance excitation compared to off-resonance conditions was observed. Peak assignment confirmed that all major Raman signals were exclusively attributed to the reporter molecules. In liquid-phase measurements, a satellite-free reference sample was designed, and through normalization of reporter molecule content, the specific contribution of satellites to SERS enhancement was isolated. Core--satellite samples exhibiting balanced plasmonic coupling and hot spot accessibility demonstrated up to 4-fold signal amplification compared to satellite-free references. Excellent linearity with concentration was observed across all samples, with the optimized formulation achieving a detection limit of 10 femtomolar. The results demonstrate that rationally designed hierarchical nanostructures enable highly sensitive and robust SERS detection, establishing a versatile platform for advanced bioanalytical applications.