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

By thermal decomposition in the presence only of oleylamine, sub-10 nm hexagonal NaLuF(4)-based nanocrystals codoped with Gd(3+), Yb(3+), and Er(3+) (or Tm(3+)) have been successfully synthesized. Sub-10 nm β-NaLuF(4): 24 mol % Gd(3+), 20 mol % Yb(3+), 1 mol % Tm(3+) nanocrystals display bright upconversion luminescence (UCL) with a quantum yield of 0.47 ± 0.06% under continuous-wave excitation at 980 nm. Furthermore, through the use of β-NaLuF(4):Gd(3+),Yb(3+),Tm(3+) nanocrystals as a luminescent label, the detection limit of <50 nanocrystal-labeled cells was achieved for whole-body photoluminescent imaging of a small animal (mouse), and high-contrast UCL imaging of a whole-body black mouse with a penetration depth of ~2 cm was achieved.

Water-soluble upconversion luminescent (UCL) nanoparticles based on triplet-triplet annihilation (TTA) were successfully prepared by coloading sensitizer (octaethylporphyrin Pd complex) and annihilator (9,10-diphenylanthracene) into silica nanoparticles. The upconversion luminescence quantum yield of the nanoparticles can be as high as 4.5% in aqueous solution. As determined by continuous kinetic scan, the nanoparticles have excellent photostability. Such TTA-based upconversion nanoparticles show low cytotoxicity and were successfully used to label living cells with very high signal-to-noise ratio. UCL imaging with the nanoparticles as probe is capable of completely eliminating background fluorescence from either endogenous fluorophores of biological sample or the colabeled fluorescent probe. In particular, such blue-emissive upconversion nanoparticles were successfully applied in lymph node imaging in vivo of living mouse with excellent signal-to-noise ratio (>25), upon low-power density excitation of continuous-wave 532 laser (8.5 mW cm(-2)). Such high-contrast and low-power excited bioimaging in vivo with a blue-emissive upconversion nanoparticle as probe may extend the arsenal of currently available luminescent bioimaging in vitro and in vivo.

A general strategy for constructing high-effective upconversion nanocapsules based on triplet-triplet annihilation (TTA) was developed by loading both sensitizer and annihilator into BSA-dextran stabilized oil droplets. This strategy can maintain high translational mobility of the chromophores, avoid luminescence quenching of chromophore by aggregation, and decrease the O2-induced quenching of TTA-based upconversion emission. Pt(II)-tetraphenyl-tetrabenzoporphyrin (PtTPBP) and BODIPY dyes (BDP-G and BDP-Y with the maximal fluorescence emission at 528 and 546 nm, respectively) were chosen as sensitizer/annihilator couples to fabricate green and yellow upconversion luminescent emissive nanocapsules, named UCNC-G and UCNC-Y, respectively. In water under the atmospheric environment, interestingly, UCNC-G and UCNC-Y exhibit intense upconversion luminescence (UCL) emission (λex = 635 nm) with the quantum efficiencies (ΦUCL) of 1.7% and 4.8%, respectively, whereas very weak UCL emission (ΦUCL < 0.1%) was observed for the corresponding previous reported SiO2-coating nanosystems because of aggregation-induced fluorescence quenching of annihilators. Furthermore, application of theses upconversion nanocapsules for high-contrast UCL bioimaging in vivo of living mice without removing the skin was demonstrated under 635-nm excitation with low power density of 12.5 mW cm(-2).