Near-Infrared Photothermal Efficient Ge Nanoparticles Synthesized via Nanosecond Laser Ablation in Liquid

We have explored germanium nanoparticles (Ge NPs) as a potential material for photothermal therapy in cancer treatment. Given the high intrinsic optical absorption of bulk germanium in the near-infrared (NIR-I) biological transparency window, pulsed laser ablation in liquid (PLAL) was employed to produce a colloid of Ge NPs with a Mie-resonant size in the range of 100–500 nm. The heating efficiency of individual Ge NPs was evaluated using 785 nm laser irradiation, while the temperature-dependent shift in the Ge–Ge Raman band was monitored simultaneously. The maximum estimated temperature increase of 480 K at a laser power density of 3 mW/μm2 for Ge NPs with a diameter of 300 nm was confirmed with no signs of oxidation or structural degradation. This value is more than four times higher than that of pure silicon NPs of a similar size. Laser heating (808 nm, 4.5 W) of an isopropanol suspension containing Ge NPs demonstrated that their resonant size enables grounds for mild photothermal therapy with a linear light-to-heat conversion efficiency response to NPs concentration reaching 17%, and the potential to heat the suspension by ∆T = 5–50°C within an NPs concentration range of 1.25–10 μg/mL.