Design,synthesis and photocytotoxicity of upconversion nanoparticles: Potential applications for near‐infrared photodynamic and photothermal therapy

Photodynamic therapy (PDT) and photothermal therapy (PTT) are emerging modalities for the treatment of tumors and nonmalignant conditions, based on the use of photosensitizers to generate singlet oxygen or heat, respectively, upon light (laser) irradiation. They have potential advantages over conventional treatments, being minimally invasive with precise spatial‐temporal selectivity and reduced side effects. However, most clinically employed PDT agents are activated at visible (vis) wavelengths for which the tissue penetration and, hence, effective treatment depth are compromised. In addition, the lipophilicity of near‐infrared (NIR) photothermal agents limits their use and efficiency. To achieve combined PDT/PTT effects, both excitation wavelengths need to be tuned into the NIR spectral window of biological tissues. This paper reports the synthesis of neodymium‐doped upconversion nanoparticles (NaYF₄:Yb,Er,Nd@NaYF₄:Nd) that convert 800 nm light into vis wavelengths, which can then activate conventional photosensitizers on the nanoparticle surface for PDT. Covalently bonded IR‐780 dyes can readily be activated by 800 nm laser irradiation. The PEGylated nanoplatform exhibited a narrow size distribution, good stability and efficient generation of singlet oxygen under laser irradiation. The in vitro photocytotoxicity of this engineered nanoplatform as either a PDT or PTT agent in HeLa cells is demonstrated, while fluorescence microscopy in nanoplatform‐incubated cells highlights its potential for bioimaging.