SnO_<i>x</i> Nanoflakes as Enhanced Near-Infrared Photothermal Therapy Agents Synthesized from Electrochemically Oxidized SnS₂ Powders

Near-infrared (NIR) photothermal therapy (PTT) using nanomaterials is a promising strategy for selective cancer treatment. We report two tin-based two-dimensional (2D) nanoflakes─defective SnS₂ (SnS_2-<i>x</i>) and mixed-phase SnO_<i>x</i>─synthesized via top-down ultrasonication and electrochemical exfoliation with oxidation, respectively. Both nanoflakes have thicknesses below 20 nm, and their lateral sizes (<400 nm) were confirmed by AFM, DLS, atomic force microscopy, dynamic light scattering, and transmission electron microscopy (TEM). Despite a similar optical band gap (∼1.89 eV), SnO₂ nanoflakes display a significantly enhanced NIR photothermal performance under 810 nm light emitting diode (LED) irradiation. A 3 mg/mL SnO_<i>x</i> dispersion increases in temperature by ∼19 °C after 30 min, and a 0.25 mg/mL sample achieves a photothermal conversion efficiency of 93%. X-ray photoelectron spectroscopy and TEM analyses show that SnO_<i>x</i> consists of interconnected SnO and SnO₂ nanocrystals (<5 nm), which promote nonradiative energy release due to exciton confinement effects, unlike the planar SnS_2-<i>x</i> nanoflakes that show negligible heating. <i>In vitro</i> studies demonstrate selective cytotoxicity: SnO_<i>x</i> combined with NIR light (100-200 μg/mL, 30 min, 115.2 mW/cm²) reduces viability in SW837 colorectal (-50%) and A431 skin carcinoma cells (-92%), with no cytotoxicity toward human skin fibroblasts. Importantly, the SnO_<i>x</i> nanoflakes retain both their photothermal efficiency and structural integrity after four cycles of NIR irradiation, demonstrating stability for repeated therapeutic applications. This work presents a green and scalable method to convert NIR-inactive SnS₂ into photothermally active SnO_<i>x</i> nanoflakes using only aqueous media and validates SnO_<i>x</i> as an efficient, biocompatible PTT agent using low-cost LED sources.