Department Chair

Jinseok Heo

Date of Award

8-2026

Access Control

Campus-Only Access

Degree Name

Master of Science (MS)

Department

Chemistry

Advisor

Jinseok Heo, Ph.D.

First Reader

Jinseok Heo, Ph.D.

Second Reader

Sujit Suwal, Ph.D.

Abstract

Gold nanoparticle (AuNP) aggregates formed via rapid freezing in liquid nitrogen exhibit strong near-infrared (NIR) plasmonic coupling. These quick-freezing-induced AuNP aggregates (QFIAAs) form moderately sized, colloidally stable structures that remain suspended for over three months, supporting a variety of analytical applications. Mechanistically, QFIAA formation arises from the confinement of nanoparticles and ions within advancing ice grain boundaries during freeze-concentration. This mechanical crowding promotes extreme physical proximity while simultaneously elevating local ionic strength, which screens electrostatic barriers to drive the assembly of dense, tightly coupled plasmonic networks.

This study investigates the plasmonic and surface-enhanced Raman scattering (SERS) behavior of bidisperse QFIAA networks combining 15 nm and 50 nm citrate-capped AuNPs at varying volumetric fractions (15 nm:50 nm; 3:1, 1:1, and 1:3). We hypothesized that the smaller 15 nm AuNPs would intercalate within the nanogaps of the larger 50 nm host aggregates, creating additional sharp-junction hot spots through the lightning-rod effect and thereby enhancing near-field coupling. Spectroscopic evaluation against a mathematically weighted additive model revealed a complex, competing physical interplay rather than a simple synergistic enhancement. While the inclusion of 15 nm AuNPs bridged the longitudinal plasmon pathways to elevate bulk NIR extinction above the additive model, it concurrently drove an interstitial crowding effect. The smaller particles physically occupied the unperturbed junctions of the 50 nm AuNP hosts, redistributing electromagnetic energy away from extreme near-field zones and quenching localized molecular SERS intensity relative to pure 50 nm AuNP baselines.

Furthermore, comparison against the additive model—constructed by summing the spectra of independently prepared 15 nm and 50 nm QFIAA solutions weighted by their volume fractions—demonstrated that direct co-assembly under rapid freezing forces a distinct, disordered packing pathway that cannot be replicated by the linear sum of pre-formed clusters. Ultimately, these results establish critical boundary conditions for multi-scale plasmonic architectures, demonstrating that bulk NIR absorbance characteristics can decouple from local near-field enhancements when unconstrained co-assembly leads to hot-spot occlusion.

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