Recent Advancements in Ln‐Ion‐Based Upconverting Nanomaterials and Their Biological Applications

Abstract

Upconversion nanoparticles (UCNPs) convert low‐energy infrared (IR) or near‐infrared (NIR) photons into high‐energy emission radiation ranging from ultraviolet to visible through a photon upconversion process. In comparison to conventional fluorophores, such as organic dyes or semiconductor quantum dots, lanthanide‐ion‐doped UCNPs exhibit high photostability, no photoblinking, no photobleaching, low cytotoxicity, sharp emission lines, and long luminescent lifetimes. Additionally, the use of IR or NIR for excitation in such UCNPs reduces the autofluorescence background and enables deeper penetration into biological samples due to reduced light scattering with negligible damage to the samples. Because of these attributes, UCNPs have found numerous potential applications in biological and medicinal fields as novel fluorescent materials. Different upconversion mechanisms commonly observed in UCNPs, various methods that are used in their synthesis, and surface modification processes are discussed. Recent applications of Ln‐UCNPs in the biological and medicinal fields, including in vivo and in vitro biological imaging, multimodal imaging, photodynamic therapy, drug delivery, and antibacterial activity, are also presented.

Photoluminescent Nanoparticles for Chemical and Biological Analysis and Imaging

“…, visible emission from NIR excitation or NIR emission from longer-wavelength NIR excitation). To date, the “UCNP” terminology has generally referred to lanthanide-doped materials, between 10–100 nm (or larger) in diameter, that comprise a host nanocrystal and one or (usually) more dopants. − The most common host materials are ternary rare-earth fluorides, with hexagonal NaYF 4 being the most prominent example, while metal oxides and phosphates are much less frequently used. The dopants are primarily trivalent lanthanide ions that drive the upconversion PL process, and transition metals are occasionally used to tune the PL properties. − Different dopants ( e.g.…”

“…To date, the “UCNP” terminology has generally referred to lanthanide-doped materials, between 10–100 nm (or larger) in diameter, that comprise a host nanocrystal and one or (usually) more dopants. − The most common host materials are ternary rare-earth fluorides, with hexagonal NaYF 4 being the most prominent example, while metal oxides and phosphates are much less frequently used. The dopants are primarily trivalent lanthanide ions that drive the upconversion PL process, and transition metals are occasionally used to tune the PL properties. − Different dopants ( e.g. , Yb 3+ , Nd 3+ , Gd 3+ , Er 3+ , Tm 3+ , Ce 3+ , Pr 3+ , Tb 3+ , Eu 3+ , Dy 3+ , Sm 3+ , Mn 2+ , Fe 2+ ) have different roles in the upconversion PL process, and the host structure also has some influence on this process ( vide infra ).…”

“…, rare-earth chlorides in oleic acid), and coprecipitation of salts ( e.g. , rare-earth EDTA with sodium fluoride), among other less common methods. − ,− Core/shell and core/multishell structures, where the shells may be doped, undoped, or differently doped, are increasingly being synthesized for optimization of the PL properties. , Shell materials are frequently but not always the same host material as the core …”

“…For many luminescent materials, anti-Stokes PL is obtained via two-photon excitation that requires the simultaneous absorption of two photons. Tightly focused, high-power pulsed lasers (with intensities of ≥10 6 W cm –2 ) , are needed to achieve the necessary flux of excitation photons for this process to have non-negligible probability. In contrast, UCNPs are able to absorb two photons sequentially, such that relatively low power continuous-wave (CW) lasers (with intensities between 1 and 10 3 W cm –2 ) , can excite upconversion PL.…”

Critical Aspects on the Chemical Stability of NaYF4-Based Upconverting Nanoparticles for Biomedical Applications

“…Sprouting from the nanotechnology revolution at the start of the present millennium, it was a matter of time that the application of these type of NPs for the nascent nanomedicine field soon ensued. In consequence, several excellent reviews appeared focusing on UCNP-based topics: general characteristics of UCNPs, ,− synthetic procedures and UCNPs optical properties, − surface chemistry and modifications, biomedical applications, − bioimaging and in vivo imaging, − drug delivery, − photodynamic-based therapies, sensing and bioassays, , optical thermometry, , toxicity, , photochemical reactions, , energy conversion, and encoding applications . Nonetheless, there is a noticeable gap of information related to the physicochemical information regarding solubility and stability; these are critical aspects for using UCNPs in biomedical applications where quality standards must be strictly met.…”