Plasmonic Magnesium Nanoparticles Are Efficient Nanoheaters

Abstract

Understanding and guiding light at the nanoscale can significantly impact society, for instance, by facilitating the development of efficient, sustainable, and/or cost-effective technologies. One emergent branch of nanotechnology exploits the conversion of light into heat, where heat is subsequently harnessed for various applications including therapeutics, heat-driven chemistries, and solar heating. Gold nanoparticles are overwhelmingly the most common material for plasmon-assisted photothermal applications; yet magnesium nanoparticles present a compelling alternative due to their low cost and superior biocompatibility. Herein, we measured the heat generated and quantified the photothermal efficiency of the gold and magnesium nanoparticle suspensions. Photothermal transduction experiments and optical and thermal simulations of different sizes and shapes of gold and magnesium nanoparticles showed that magnesium is more efficient at converting light into heat compared to gold at near-infrared wavelengths, thus demonstrating that magnesium nanoparticles are a promising new class of inexpensive, biodegradable photothermal platforms.

Keywords: magnesium; photothermal therapy; photothermal transduction; plasmonics.

Figure 1

Impact of the differences in permittivity between Mg and Au. (A) Real and (B) imaginary complex permittivity of Au (gold trace) and Mg (gray trace). Extinction (solid) and absorption (dashed) cross sections of Mg and Au (C, D, G) nanospheres and (D, F, H) nanorods of indicated diameter (d) and length (). The rods are 40 nm in width.

Figure 2

Effects of NP shape on the plasmonic response of faceted spheroids (blue), single crystal hexagonal platelets (pink), and elongated twinned platelets (purple). (top) Model of the NP shapes. (A–C) Absorption cross sections (C. S.) of each NP shape, calculated for four NP diameters: 50 nm (solid), 100 nm (dashed), 150 nm (dash dot), and 200 nm (dotted), where diameter is defined as the longest tip-to-tip distance indicated by the dashed line in panels D–F. (D–F) Steady-state temperature line sections through the center of each NP calculated with excitation at the dipole resonance in a background of isopropanol.

Figure 3

Optical characterization of (A–C) Au and (D–G) Mg NP suspensions. The first row contains (A, B) representative scanning transmission electron microscopy (STEM) and (C–G) scanning electron microscopy (SEM) images. The second row shows the length distribution of NPs in each suspension with the averages and standard deviations indicated. The last row presents the experimentally measured normalized extinction of each NP suspension.

Figure 4

Photothermal (PT) efficiency measurements for the Mg and Au NP suspensions. Photothermal efficiency of each NP suspension (left to right: 11, 48, and 150 nm Au spheres, 38, 150, and 202 nm Mg spheroids, and 200 nm Mg platelets) at (A) 532 nm and (C) 785 nm. The height of each bar corresponds to the average photothermal efficiency, and the error bars correspond to the standard deviation, evaluated across multiple measurements. Efficiencies replot (dots) with simulation of absorption/extinction overlaid (line traces) at (B) 532 nm and (D) 785 nm. The x-axis error bars on the experimental data represent the standard deviation of the size distribution of the NP suspensions.

Figure 5

Stability of 38 nm diameter Mg faceted spheroids after five cycles of laser heating at 785 nm. Gray regions indicate laser on, and white regions indicate laser off.