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.
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The authors thank Ying Lin and Dr. Jaona Randrianalisoa for their insights in data visualization. We thank Dr. Chao Liu for his help in setting up the laser for photothermal conversion experiment. We thank Yuanchang Du from Xi’an Jiaotong University for his insights on DDSCAT software. We thank Dr. Christy Haynes for the usage of DLS and UV–Vis instruments. The authors acknowledge the Minnesota Supercomputing Institute (MSI) at the University of Minnesota for providing resources for the DDA simulation results reported within this paper. URL: http://www.msi.umn.edu. This study is partially funded by Carl and Janet Kuhrmeyer Chair and Medtronic-Bakken Endowed Chair grant of UMN.