Design of Bismuth-based luminescent materials for multi-modal imaging and optical thermometry

Upconversion phosphor materials, usually consisting of crystals doped with lanthanide ions, are attracting increasing attention and several possible applications in various fields have been proposed so far: e.g. solar cells with improved efficiency, nanomaterials for bio-imaging, microtags in anti-counterfeiting inks and lasers and novel display technologies. In recent years, lanthanide (Ln3+)-doped upconverting nanoparticles (UCNPs) have proved to be photostable and basically nontoxic, thus have been presented as efficient and versatile bioimaging probes. This type of nanoparticles can be excited with near-infrared (NIR) light, while emitting higher-energy photons in a wide range of the electromagnetic spectrum, from the ultraviolet (UV), to visible (VIS) and near infrared (NIR) regions, via a multiphoton process. In particular, operating within the biological window leads to several advantages, such as drastically reduced photodamage and autofluorescence background, and remarkable tissue penetration. On the other hand, bismuth-based luminescent materials have proved to be excellent candidates for the design of bulk and nanosized phosphors, thanks to peculiar optical characteristics and appealing properties such as low cost of production and almost non-toxicity. Driven by these factors our work is mainly focused on the development of novel nanostructures, i.e. lanthanide-doped bismuth silicate-silica core-shell nanoparticles, to be employed as biological probes. A new synthetic procedure is here developed to obtain NPs composed of a crystalline Bi2SiO5 core embedded in a glassy shell of dense SiO2. Uniform, monodispersed, crystalline and non-toxic nanoparticles are obtained. The tunability of the UC emission is investigated by co-doping the system with different combinations and relative concentrations of lanthanide ions (Yb, Er, Ho, Tm). Lanthanide-doped Bi2SiO5@SiO2 NPs are thoroughly characterized, allowing to assess their potential as bioimaging and temperature sensing nanoprobes. In fact, the strongly temperature-dependent behaviour of the upconversion photoluminescence (UCPL) in lanthanide ions, allows to develop ratiometric luminescent thermal sensors, emitting in the VIS or NIR regions, with promising properties in the biological field. The Nd3+ singly-doped Bi2SiO5@SiO2 system is also investigated as thermal bio-probe and its optical properties are compared with that of two others bismuth silicate phases of the Bi2O3-SiO2 phase diagram, namely Bi4Si3O12 and Bi12SiO20. Moreover, the synthesized NPs are multifunctional, potentially being multi-modal probes for combined optical imaging and X-ray computed tomography (CT)/single-photon emission CT (SPECT)/photoacoustic tomography (PAT), thanks to the X-ray attenuating properties of the bismuth-based matrix.