Geometry-Dependent
Plasmonic Tunability and Photothermal
Characteristics of Multibranched Gold Nanoantennas
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Abstract
Plasmon resonances of anisotropic
multibranched nanostructures
are governed by their geometry, allowing morphology-directed selective
manipulation of the optical properties. In this work, we have synthesized
multibranched gold nanoantennas (MGNs) of variable geometry by a one-step
seedless approach using 4-(2-hydroxyethyl)-1-piperazineethanesulfonic
acid (HEPES) as a capping and reducing agent. This approach enables
us to modulate the MGNs’ geometry by controlling three different
parameters: concentration of HEPES, concentration of Au<sup>3+</sup>, and pH of HEPES buffer. By altering the MGNs morphology with minimal
increase in the overall dimensions, the plasmon resonances were tuned
from the visible to the near-infrared. The MGNs plasmon resonances
demonstrated a nonintuitive blue-shift when pH > p<i>K</i><sub>a</sub> of HEPES which we attributed to emergence of charge
transfer oscillations formed when MGNs cluster to dimers and trimers.
Further, due to the presence of multiple sharp protrusions, the MGNs
demonstrated a refractive index sensitivity of 373 nm/RIU, which is
relatively high for this class of branched nanostructures of similar
size. Finally, the sharp protrusions of MGNs also give rise to intense
photothermal efficiencies; ∼53 °C was achieved within
5 min of laser illumination, demonstrating the efficacy of MGNs in
therapeutic applications. By modulating the mass density of MGNs,
the laser flux, and time of illumination, we provide a detailed analysis
of the photothermal characteristics of MGNs