Photon upconversion is promising for applications such as biological imaging, data storage or solar cells. Here, we have investigated upconversion processes in a broad range of gadolinium-based nanoparticles of varying composition. We show that by rational design of a core-shell structure with a set of lanthanide ions incorporated into separated layers at precisely defined concentrations, efficient upconversion emission can be realized through gadolinium sublattice-mediated energy migration for a wide range of lanthanide activators without long-lived intermediary energy states. Furthermore, the use of the core-shell structure allows the elimination of deleterious cross-relaxation. This effect enables fine-tuning of upconversion emission through trapping of the migrating energy by the activators. Indeed, the findings described here suggest a general approach to constructing a new class of luminescent materials with tunable upconversion emissions by controlled manipulation of energy transfer within a nanoscopic region.

Lanthanide-doped inorganic nanoparticles possess superior physicochemical features such as long-lived luminescence, large antenna-generated Stokes or anti-Stokes shifts, narrow emission bands, high resistance to photobleaching and low toxicity, and thus are regarded as a new generation of luminescent bioprobes as compared to conventional molecular probes like organic dyes and lanthanide chelates. These functional nanoparticles, although most of their bulk counterparts were well studied previously, have attracted renewed interest for their biomedical applications in areas as diverse as biodetection, bioimaging, and disease diagnosis and therapeutics. In this review, we provide a comprehensive survey of the latest advances made in developing lanthanide-doped inorganic nanoparticles as potential luminescent bioprobes, which covers areas from their fundamental chemical and physical features to bioapplications including controlled synthesis methodology, surface modification chemistry, optical spectroscopy, and their promising applications in diverse fields, with an emphasis on heterogeneous and homogeneous in vitro biodetection of tumor markers and multimodal bioimaging of various tumor tissues. Some future prospects and challenges in this rapidly growing field are also summarized.

Lanthanide-doped upconversion nanoparticles (UCNPs) have attracted considerable interest due to their superior physicochemical features, such as large anti-Stokes shifts, low autofluorescence background, low toxicity and high penetration depth, which make them extremely suitable for use as alternatives to conventional downshifting luminescence bioprobes like organic dyes and quantum dots for various biological applications. A fundamental understanding of the photophysics of lanthanide-doped UCNPs is of vital importance for discovering novel optical properties and exploring their new applications. In this review, we focus on the most recent advances in the development of lanthanide-doped UCNPs as potential luminescent nano-bioprobes by means of our customized lanthanide photophysics measurement platforms specially designed for upconversion luminescence, which covers from their fundamental photophysics to bioapplications, including electronic structures (energy levels and local site symmetry of emitters), excited-state dynamics, optical property designing, and their promising applications for in vitro biodetection of tumor markers. Some future prospects and efforts towards this rapidly growing field are also envisioned.