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Pharmaceutics 2020, 12, 1102 2 of 17 1. Introduction Photodynamic therapy (PDT) has attracted great interest as an effective means of treating cancers [1–3]. PDT utilizes photosensitizers (PS) that absorb visible light and convert molecular oxygen to reactive oxygen species (ROS) such as singlet oxygen (1O2) or other free radicals [2,4]. The generated ROS oxidizes cellular proteins, leading to tumor ablation via such mechanisms as direct tumor cell killing, damage to tumor vasculature, and tumor suppression [5,6]. Cancer cells can be killed by the photodynamic process through several mechanisms of apoptosis, necrosis, or autophagy [5]. Necrosis is triggered by a high dose of PS/light and involves cytoplasm swelling, organelle devastation, or membrane disruption, whereas apoptosis is characterized by cell shrinkage, chromosomal DNA fragmentation, or chromatin condensation [5,7]. In the context of an intratumoral injection or tumor targeting approach, PDT has the clear advantage of localizing tumor therapy by irradiating light selectively on the tumor location, with less invasiveness and fewer side effects [5]. In addition, the treatment is of short duration, is inexpensive, and induces an immune response that can produce synergistic effects on anticancer activity [8,9]. However, the clinical use of PDT is often restricted because most photosensitizers are activated around a wavelength of ~660 nm, which cannot penetrate the skin deeply when compared to near-infrared (NIR) light (~780–940 nm) [1,10–12]. Upconverting nanoparticles (UCNPs) have unique optical properties that convert NIR light to UV or visible light [13], which enables many biomedical applications, such as bioimaging, photodynamic therapy, and combined cancer therapy [14–17]. The concept of UCNPs is based on the anti-Stokes phenomenon, in which two or more photons are absorbed and a single quantum of energy is emitted, generating short wavelength light from long wavelength light [18–20]. The structure of UCNPs is a matrix of alkali metal compounds in which the respective lanthanide elements are embedded [21,22]. In UCNPs, lanthanide elements consist of a sensitizer that absorbs long wavelengths of light and an activator that emits short wavelengths of light. In particular, the core@shell structure that separates the activator and the sensitizer is widely used to maximize the fluorescence efficiency of UCNPs [23–26]. The clinical significance of UCNPs requires the strong permeability of NIR light because the absorption of biological window I (700–980 nm) light is low in the skin and body [27]. Consequently, UCNPs have been utilized as a light-generating source (~660 nm) subsequent to their absorption of well-permeating NIR light (>800 nm), which overcomes the limitation of triggering PS in the body [28–31]. Herein, we sought to design and prepare highly red fluorescence light-emitting core-shell upconverting nanoparticles surface-modified with silica, followed by PEG-folic acid (FA/PEG) and TCPP (UCNPs@SiO2-NH2@FA/PEG/TCPP), as an efficient photodynamic agent for killing tumor cells. The UCNPs@SiO2-NH2@FA/PEG/TCPP was fabricated in a core-shell structure by doping with erbium/lutetium to mainly emit ~660 nm to activate the adjoining TCPP, and the folic acid ligand was conjugated as a targeting moiety of HeLa cells. Analytical/morphological investigations to identify the UCNPs@SiO2-NH2@FA/PEG/TCPP in vitro cytotoxicity were conducted to evaluate the antitumor potential, and an evaluation of the UCNP penetration in HeLa cell spheroids was conducted to achieve this aim (Figure 1).PDF Image | Red LED Erbium Upconverting Nanoparticles v Cervical Cancer
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