Aggregation affects optical properties and photothermal heating of gold nanospheres

Yiru Wang,Yilin Liu,Huan Yang,Zhe Gao,Zonghu Han,Christopher J Hogan,John C Bischof,Taner Akkin

doi:10.1038/s41598-020-79393-w

Abstract

Laser heating of gold nanospheres (GNS) is increasingly prevalent in biomedical applications due to tunable optical properties that determine heating efficiency. Although many geometric parameters (i.e. size, morphology) can affect optical properties of individual GNS and their heating, no specific studies of how GNS aggregation affects heating have been carried out. We posit here that aggregation, which can occur within some biological systems, will significantly impact the optical and therefore heating properties of GNS. To address this, we employed discrete dipole approximation (DDA) simulations, Ultraviolet–Visible spectroscopy (UV–Vis) and laser calorimetry on GNS primary particles with diameters (5, 16, 30 nm) and their aggregates that contain 2 to 30 GNS particles. DDA shows that aggregation can reduce the extinction cross-section on a per particle basis by 17–28%. Experimental measurement by UV–Vis and laser calorimetry on aggregates also show up to a 25% reduction in extinction coefficient and significantly lower heating (~ 10%) compared to dispersed GNS. In addition, comparison of select aggregates shows even larger extinction cross section drops in sparse vs. dense aggregates. This work shows that GNS aggregation can change optical properties and reduce heating and provides a new framework for exploring this effect during laser heating of nanomaterial solutions.

Highlights

2–5 nm gold nanoparticles (GNS) 25 nm gold nanorods (GNR) 44–60 nm nanostars Cardiac, 200 nm GNS Neuron, 750–810 nm GNR Skin, 8–10 nm GNS 10–15, 40 nm GNS 785 nm resonance GNR 805 nm resonance GNR 15, 30 nm GNS
We expected that the aggregation effect will be more prominent for dense, compact geometries due to strong overlapping and coupling of the electro-magnetic fields from adjacent particles, while GNS will behave as isolated particles when separated by sufficient distance
GNS diameter plays a role as well; the resonance peak location for 5 nm GNS is relatively insensitive to aggregate size, for peak location for 16 nm GNS increases slightly with increasing aggregate size, and the peak for 30 nm GNS decreases with increasing aggregate size

Summary

Introduction

2–5 nm GNS 25 nm GNR 44–60 nm nanostars Cardiac, 200 nm GNS Neuron, 750–810 nm GNR Skin, 8–10 nm GNS 10–15, 40 nm GNS 785 nm resonance GNR 805 nm resonance GNR 15, 30 nm GNS. There are computational studies showing GNS optical extinction spectrum shift under aggregation[21–23]. In an effort to begin to understand this, we study here the influence of GNS aggregation on laser GNS heating both computationally and experimentally. We chose a typical size range of GNS (5, 16 and 30 nm) for photothermal conversion and early-stage aggregate sizes (2 to 30 primary spheres particles) for these studies. The discrete dipole approximation (DDA) is utilized to examine how aggregation changes GNS optical properties. These calculations are compared to cuvette “laser GNS calorimetry” and extinction cross section measurements by UV–Vis

Methods

Results

Conclusion