Rare-earth upconversion nanophosphors (UCNPs), when excited by continuous-wave near-infrared light, exhibit a unique narrow photoluminescence with higher energy. Such special upconversion luminescence makes UCNPs promising as bioimaging probes with attractive features, such as no auto-fluorescence from biological samples and a large penetration depth. As a result, UCNPs have emerged as novel imaging agents for small animals. In this critical review, recent reports regarding the synthesis of water-soluble UCNPs and their surface modification and bioconjugation chemistry are summarized. The applications of UCNPs for small-animal imaging, including tumor-targeted imaging, lymphatic imaging, vascular imaging and cell tracking are reviewed in detail. The exploration of UCNPs as multifunctional nanoscale carriers for integrated imaging and therapy is also presented. The biodistribution and toxicology of UCNPs are further described. Finally, we discuss the challenges and opportunities in the development of UCNP-based nanoplatforms for small-animal imaging (276 references).

The application of phosphorescent heavy-metal complexes with d⁶ , d⁸ and d¹⁰ electron configurations for bioimaging is a new and promising research field and has been attracting increasing interest. In this critical review, we systematically evaluate the advantages of phosphorescent heavy-metal complexes as bioimaging probes, including their photophysical properties, cytotoxicity and cellular uptake mechanisms. The progress of research into the use of phosphorescent heavy-metal complexes for staining different compartments of cells, monitoring intracellular functional species, providing targeted bioimaging, two-photon bioimaging, small-animal bioimaging, multimodal bioimaging and time-resolved bioimaging is summarized. In addition, several possible future directions in this field are also discussed (133 references).

Recently, the use of phosphorescent heavy-metal complexes as chemosensors has attracted increasing interest due to their advantageous photophysical properties. This critical review focuses on the design principles and the recent development of phosphorescent chemosensors for metal cations, anions, pH, oxygen, volatile organic compounds and biomolecules based on some heavy-metal complexes (such as Pt(II)-, Ru(II)-, Re(I)-, Ir(III)-, Cu(I)-, Au(I)- and Os(II)-based complexes), in which the variation in phosphorescence signals induced by the interaction between heavy-metal complexes and analytes is utilized (217 references).

Photothermal therapy (PTT) at present, following the temperature definition for conventional thermal therapy, usually keeps the temperature of lesions at 42–45 °C or even higher. Such high temperature kills cancer cells but also increases the damage of normal tissues near lesions through heat conduction and thus brings about more side effects and inhibits therapeutic accuracy. Here we use temperature-feedback upconversion nanoparticle combined with photothermal material for real-time monitoring of microscopic temperature in PTT. We observe that microscopic temperature of photothermal material upon illumination is high enough to kill cancer cells when the temperature of lesions is still low enough to prevent damage to normal tissue. On the basis of the above phenomenon, we further realize high spatial resolution photothermal ablation of labelled tumour with minimal damage to normal tissues in vivo. Our work points to a method for investigating photothermal properties at nanoscale, and for the development of new generation of PTT strategy.

Anti-Stokes shift luminescence is a special optical process, which converts long-wavelength excitation to short-wavelength emission. This unique ability is especially helpful for bio-applications, because the longer-wavelength light source, usually referring to near infrared light, has a larger penetration depth offering a longer working distance for in vivo applications. The anti-Stokes shift luminescence signal can also be distinguished from the auto-fluorescence of biological tissues, thus reducing background interference during bioimaging. Herein, we summarize recent advances in anti-Stokes shift luminescent materials, including lanthanide and triplet–triplet-annihilation-based upconversion nanomaterials, and newly improved hot-band absorption-based luminescent materials. We focus on the synthetic strategies, optical optimization and biological applications as well as present comparative discussions on the luminescence mechanisms and characteristics of these three types of luminescent materials.