Engineering of hybrid upconversion nanoparticles for biodetection and cancer imaging

Upconversion nanoparticles (UCNPs), which utilize the sequential absorption and energy transfer steps to emit a higher energy photon after absorbing two or more low-energy excitation photons, have evoked great attention for bio- applications in areas such as biodetection and cancer imaging (Chen et al. 2014a; Zhou et al. 2015). This conversion from long-wavelength near-infrared (NIR) stimulation to short-wavelength outcome (e.g., ultraviolet, visible and NIR) endows UCNPs with attractive optical features, such as large anti- Stokes shifts (Haase and Schafer 2011), high detection sensitivity (Cheng et al. 2010), non-blinking and non-bleaching properties (Park et al. 2009; Wu et al. 2009), low autofluorescence background (Kobayashi et al. 2009; Xiong et al. 2009a), minimal photodamage (Nam et al. 2011) and extremely high tissue penetration depth (Liu et al. 2011f). In general, UCNPs comprise an inorganic host, a sensitizer and an activator. Fluorides including NaYF4 (Ni et al. 2014b; Wang et al. 2010), NaYbF4 (Liu et al. 2012b; Xing et al. 2012a), NaGdF4 (Qiao et al. 2015) and NaLuF4 (Liu et al. 2011f), which exhibit low phonon energies 193(~350 cm^-1) and high chemical stability, have become the most popular host materials. The doped lanthanide (Ln³⁺) ions such as Er³⁺, Tm³⁺ and Ho³⁺ are frequently used as activators to generate upconversion luminescence (UCL) emission under NIR excitation. To enhance the UCL efficiency, Yb³⁺, which has a larger NIR absorption strength than other Ln³⁺ ions, is often co-doped as a sensitizer (Gai et al. 2014).