Assessment of TTA-Based UCNPs 441 9. Detection Applications of Lanthanide UCNPs 441 9.1. Lanthanide UCNPs as Nanothermometers 441 9.2. Upconversion Detection Based on the Inner Filter Effect 442 9.2.1. Lanthanide UCNPs as pH Sensors 442 9.2.2. Lanthanide UCNPs as CO 2 or Ammonia Probes 442 9.2.3. Lanthanide UCNPs as a Cr 6+ Probe 442 9.2.4. Lanthanide UCNPs as Probes for Antioxidants 442 9.3. Design Strategy for Upconversion LRET Detection 442 9.4. Upconversion LRET Detection by Alteration of the Spectral Overlap between Donor and Acceptor 444 9.4.1. Lanthanide UCNPs as CN − Probe 444 9.4.2. Lanthanide UCNPs as a NO 2 − Probe 444 9.4.3. Lanthanide UCNPs as a Cu 2+ Probe 444 9.4.4. Lanthanide UCNPs as Hg 2+ and MeHg + Probes 444 9.4.5. Lanthanide UCNPs as an Oxygen Probe 445 9.4.6. Lanthanide UCNPs as a pH Probe 445 9.4.7. Lanthanide UCNPs as a GSH Probe 445 9.5. UC-LRET Detection by Alteration of the Distance between Donor and Acceptor 445 9.5.1. Lanthanide UCNPs for DNA/RNA Detection 445 9.5.2. Lanthanide UCNPs for Immunoassay 446 9.5.3. Lanthanide UCNPs as Luminescent Probes Based on Ligand−Acceptor Interaction 446 9.5.4. Lanthanide UCNPs as Enzyme-Activity Assay 447 9.5.5. Lanthanide UCNPs as an ATP Probe 447 9.5.6. Lanthanide UCNPs as Hg 2+ Probe 447 9.6. Summary of Upconversion Detection Systems 447 10. Upconversion Materials as a Lighting Source 448 10.1. Solid-State TTA-Based Upconversion Film for Lighting 448 10.1.1. Co-doping Both Sensitizer and Annihilator into a Polymer Matrix 448 10.1.2. Doping the Sensitizer in an Emissive Polymer Matrix 448 10.1.3. TTA-Based Upconversion Luminescence in Nanocrystalline ZrO 2 Films 449 10.1.4. TTA-Based Upconversion Luminescence in Nanofibers and Mats 449 10.2. Lanthanide UCNPs for Lighting 449 10.3. TTA-Based Upconversion Materials for Color-Display Devices 449 10.4. Lanthanide UCNPs for Anticounterfeiting Applications 449 10.5. Lanthanide UCNPs for Fingermark Detection 449 10.6. Lanthanide UCNPs s for 3D-Displays 449 11. Upconversion Materials as a Second Excitation Source 450 11.1. Upconversion Materials for Photocurrent Generation 450 11.1.1. Lanthanide UCNPs for Photocurrent Generation 450 11.1.2. TTA-Based Upconversion Materials for Photocurrent Generation 450 11.1.3. Lanthanide UCNPs for Solar Cells 450 11.1.4. TTA-Based Upconversion Materials for Solar Cells 451 11.2. Upconversion Materials for Photocatalysis 451 11.2.1. Lanthanide UCNPs for Photocatalysis 451 11.2.2. TTA-Based Upconversion for Photocatalysis 451 11.3. Upconversion Materials for Solar Fuels 451 11.4. Upconversion Materials for Photoisomerization 451 11.4.1. Lanthanide UCNPs for Photoisomerization of Diarylethenes 451 455 Abbreviations 455 References 456

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).

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