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PH-controlled two dimensional gold nanoparticle aggregates for systematic study of local surface plasmon coupling
Xinheng Li1, Kaoru Tamada, Akira Baba
1Interdisciplinary Graduate School of Science and Engineering, Department of Electronic Chemistry, Tokyo Institute of Technology, 4259 Nagatsuta-cho Midori-ku, Yokohama, Kanagawa 226-8502, Japan.
Journal of Nanoscience and Nanotechnology
|May 16, 2009
Summary
Citrate-capped gold nanoparticles
Area of Science:
- Nanotechnology
- Surface Chemistry
- Plasmonics
Background:
- Gold nanoparticles (AuNPs) are widely used in various applications due to their unique optical properties.
- Controlling nanoparticle assembly and interparticle interactions is crucial for tuning their plasmonic behavior.
- Surface charge density plays a significant role in nanoparticle aggregation and optical responses.
Purpose of the Study:
- To investigate the effect of pH-controlled citrate capping on gold nanoparticle surface charge density.
- To explore the relationship between nanoparticle surface density and optical properties, particularly local plasmon coupling.
- To analyze the impact of nanoparticle aggregation on Surface Plasmon Resonance (SPR) spectra.
Main Methods:
- Synthesis of gold nanoparticles with a mixed citrate and biotin capping layer.
- Controlled immobilization of nanoparticles on a substrate via biotin-avidin interactions.
- Systematic variation of nanoparticle surface density.
- Characterization of optical properties using UV-vis-Near-IR reflection absorption spectroscopy.
- Analysis of Surface Plasmon Resonance (SPR) using Fresnel's equations and Kramers-Kronig transformations.
Main Results:
- Citrate capping allows for pH-dependent tuning of nanoparticle surface charge density.
- Increased nanoparticle surface density leads to the formation of 2D aggregates.
- Aggregates exhibit red-shifted and broadened longitudinal resonance compared to dispersed particles.
- SPR curves show significant shifts and increased intensity dips for aggregates, deviating from Maxwell-Garnett theory predictions.
- Effective dielectric constants of particle layers increase with nanoparticle density, indicating aggregate formation.
Conclusions:
- Citrate molecules on gold nanoparticles act as pH-tunable capping agents, controlling interparticle distances.
- Local plasmon coupling in high-density nanoparticle aggregates significantly influences optical responses.
- The study provides a method to control nanoparticle assembly for tailored plasmonic applications.
