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Molecules 2019, 24, 2692 2 of 17 follows: After the applied photosensitizer is attached to the bacteria, the laser light is introduced and the photosensitizer absorbs light energy into the singlet state from the stable triplet state. The singlet photosensitizer is extremely unstable and will instantaneously release the energy, returning to a triplet state [4]. The released energy is absorbed by tissue oxygen, forming reactive oxygen species (ROS) which have strong oxidation and high reactivity, causing the rapid lipid oxidation of bacteria, especially the destruction of vulnerable membrane lipids, and eventually bacterial death [5]. Antimicrobial PDT has been considered to be an important method in killing inflammation-related pathogens around teeth. Hass et al. firstly conducted the in vitro study on this point [6]. Three bacteria, Prevotella intermedia (P. intermedia), Aggregatibacter actimycetemcomitans (A. actimycetemcomitans), Porphyromonas gingivalis (P. gingivalis), were selected and treated with aPDT method, respectively. All above three species were completely killed, confirming the high efficacy of bacteria inhibition compared with other treatment methods [6]. Chan et al. used methylene blue (MB) as a photosensitizer to kill three kinds of bacteria, P. gingivalis, A. actimycetemcomitans and Fusobacterium nucleatum (F. nucleatum) with inhibition rate of 95% [7]. Ayano et al. investigated the aPDT effect on P. gingivalis with photosensitizer rose bengal (RB) and the results show that aPDT from RB can effectively kill P. gingivalis with blue light [8]. Goulart et al. found A. actimycetemcomitans can be effectively inhibited with MB under the irradiation of light from 400 to 500 nm, and the number of surviving bacteria was significantly reduced [9]. Eick et al. investigated the aPDT from the toluidine blue (TB) with light excitation (625–635 nm), which can significantly reduce A. actimycetemcomitans biofilm activity formed by two different strains [10]. Street et al. combined the MB and lasers (650 to 675 nm) and found more effective treatment effect on F. nucleatum planktonic bacteria than that of biofilms, indicating that the effect of PDT on bacteria is also related to the bacteria form [11]. Although photodynamic therapy has made important progress in the treatment of periodontal disease, it is still in its primary stage, and there are some serious problems that are still necessary to be solved before clinical application. The most important one of conventional photodynamic therapy is the weak tissue penetration of ultraviolet or visible light. Therefore, it is highly desirable to design and prepare a photodynamic therapy system with the infrared irradiation light, which can penetrate deep tissues. Upconversion fluorescence is an anti-stokes process that allows the long excitation wavelength to be converted to short emission wavelength. Rare earth doped upconversion nanoparticles (UCNPs) are the most outstanding representatives which can convert the infrared light to visible emission light by the continuous two-photon or multiphoton energy transfer process [12–14]. Upconversion brings many advantages: First, there is zero noise in biological background because biological tissue does not emit light under near-infrared (NIR) light, resulting in high signal-to-noise ratio in applications [15]. Second, the NIR excitation light used for up-conversion luminescence is located in the optical imaging window of the biological tissue, and has a deep tissue penetration. Red light can penetrate inside the tissue for 0.5 cm, while 980 nm light can penetrate more than 1 cm [16]. Third, some other advantages should be mentioned, such as narrow emission band, high color purity and stability, low toxicity and no photobleaching [17,18]. Therefore, if the up-conversion material were introduced in the periodontal deep tissue, the NIR light can be converted into light in different wavelengths from ultraviolet to NIR, satisfying the need to excite photosensitizers with different absorption bands, and well compensating for the low penetration of conventional photodynamic therapy light sources. Note that the PDT triggered by upconversion light was first designed and applied in tumor therapy [19]. However, until now there have been no reports about such upconversion nanomaterials-based photodynamic therapy on periodontitis treatment, which is highly expected to be achieved if non-invasive periodontitis treatment is considered. In this work, we designed a new UCNP/Ce6 composite nanomaterial with enhanced red light emission and efficient aPDT bactericidal performance. The combination of Ce6 and NaYF4:Yb,Er UCNPs was realized by using the amphiphilic silane modification technique, which involved the hydrophobic-hydrophobic interaction between hydrophobic side chain of the silane and hydrophobicPDF Image | Anti-Biofilm Property of Bioactive Upconversion Nanocomposites
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