Yolk-shell structure for upconverting nanoparticles: Bioimaging, drug delivery, and photodynamic therapy

Upconverting Nanoparticle (UCNP) has recently received many attentions from theranostics and nanomedicine fields because it can be designed for multi-functional targeted nanomedicines with multi-modal imaging. One of popular UCNPs is NaLnF4 : Yb,Er/Tm, which absorbs infrared and releases visible or ultraviolet light to trigger drug release or to produce singlet oxygen for therapy. Lantanides doping enhances upconversion luminescence and enable magnetic resonance imaging. In addition IR-regulating drug release reaches deep without harm. Core-shell nanostructures have been applied for the most of UCNP applications, but the therapeutic efficacy is still far away from the desired levels in nanomedicine. First, loading space is limited by the thickness or porosity of shell, so enough loading isn't guaranteed in most of core-shell structures. Thicker shell is better for higher loading, but a bigger particle size is unavoidable. Porosity isn't a parameter to control simply. Second, only outer shell surface is offered for surface modifications to specific binding or properties, which is critical of targeted therapy. However, when UCNP is housed in a yolk-shell structure, the void, which isn't available in core-shell structures, can be a solution for loading both drugs and photo triggers in drug delivery, or photosensitizers in photodynamic therapy. In addition, both inner and outer surfaces can be modified as any desired purposes. Third, the movable yolk UCNP has more chance to contact with photo triggers and photosensitizers in the void. All the benefits with yolk-shell structure are resulted in high therapeutic efficacy. In this mini review, some of yolk-shell UCNP examples are introduced for in vivo multimodal bioimaging with high contrast, IR-regulated drug release, and high efficacy in photodynamic therapy.