Tunable 3D Plasmonic Hotspots in AAO-Based SERS Substrates : Effect of Pore Depth and Nanoparticle Composition = AAO 기반 SERS 기판 내 제어 가능한 3차원 플라즈모닉 핫스팟 : 기공 깊이 및 나노입자 조성의 영향

다국어 초록 (Multilingual Abstract)

The development of highly sensitive and selective mercury-ion (Hg²⁺) detection systems remains a critical challenge due to the ultralow permissible exposure levels and the need for reliable monitoring in complex environments. In this work, we present a plasmonic SERS platform based on gold nanoparticle (AuNP) dimers confined within anodic aluminum oxide (AAO) nanochannels and functionalized with an imidazole-bearing perylene diimide derivative (HPH). By finely tuning the AAO pore geometry and controlling the protonation state of HPH, efficient adsorption was achieved at the optimized pH of 8, where both the carboxylate (COO⁻) and imidazole N-donor groups exhibit maximal availability for Au binding and Hg²⁺ chelation. This condition enables the formation of a uniform monolayer on the AuNP surface and ensures stable immobilization of HPH within the plasmonic nanogaps. Extensive Raman mapping conducted over 108 distinct positions across multiple substrates confirmed the exceptional spatial uniformity and reproducibility of the confined AuNP dimer hotspots. Furthermore, a semi-in situ measurement strategy—where incremental Hg²⁺ aliquots were introduced directly onto the fixed plasmonic substrate—enabled real-time tracking of coordination-driven spectral changes. Across a broad dynamic range from 1 pM to 1 μM, the Raman intensity increased monotonically with Hg²⁺ concentration, governed by the synergistic interplay between electromagnetic (EM) field enhancement in the nanogap and chemical enhancement (CM) arising from Hg²⁺–HPH coordination and associated charge-transfer interactions. The sensor also exhibited outstanding selectivity, with Hg²⁺ producing a markedly higher Raman response than various competing metal ions, attributable to its strong affinity for the deprotonated functional groups of HPH. Taken together, these results demonstrate that AAO-confined AuNP dimers functionalized with HPH offer a highly robust and ultrasensitive SERS sensing architecture, capable of reproducible hotspot generation, real-time molecular interrogation, and reliable Hg²⁺ quantification at extremely low concentrations. This platform provides a promising foundation for next-generation heavy metal sensing technologies and broader applications in environmental and biochemical monitoring.

목차 (Table of Contents)

• CHAPTER 1. Introduction 8
• CHAPTER 2. Experimental section 12
• 2.1 Materials and instruments 12
• 2.2 Synthesis of spherical gold nanoparticles (AuNPs) 12
• 2.3 Synthesis of Histidine-functionalized perylene Diimide 13
• 2.4 Ligand exchange of oleylamine with HPH on Au nanoparticles 16
• 2.5 SERS measurements 17
• CHAPTER 3. Results and discussion 18
• 3.1 Preparation of AAO 18
• 3.2 Synthesis of sphere shape Au Nanoparticles 19
• 3.3 Insertion Au nanoparticles into the AAO pores by ultrasonic process 21
• 3.4 pH-Dependent Adsorption Behavior of HPH on Au Nanoparticle Dimers 22
• 3.5 Raman Spectroscopy measurement 26
• CHAPTER 4. Conclusion and Future Perspectives 32
• CHAPTER 5. Reference 35
• Part 2. 46
• CHAPTER 1. Introduction 55
• CHAPTER 2. Experimental section 64
• 2.1 Materials and instruments 64
• 2.2 Fabrication of depth-tunable AAO templates 65
• 2.3 Synthesis of Ag nanocubes 66
• 2.4 Synthesis of Au nanoparticles 68
• 2.5 Loading of nanoparticles into AAO pores 70
• 2.6 Ligand exchange with 4-mercaptobenzoic acid (4-MBA) 75
• 2.6 SERS measurements 76
• CHAPTER 3. Results and discussion 77
• 3.1 Depth-dependent SERS performance of Au-NPs / AAO substrates and Ag-NCs / AAO substrates 77
• 3.2 SERS-based quantitative detection of 4-MBA using the optimized Au-NP/AAO substrate 86
• CHAPTER 4. Conclusion and future work 89
• CHAPTER 5. Reference 92