[1] Auzel, F. Upconversion and anti-stokes processes with f and d ions in solids. Chem. Rev. 104, 139–174 (2004). doi: 10.1021/cr020357g
[2] Liu, G. K. Advances in the theoretical understanding of photon upconversion in rare-earth activated nanophosphors. Chem. Soc. Rev. 44, 1635–1652 (2015). doi: 10.1039/C4CS00187G
[3] Tu, L. P. et al. Excitation energy migration dynamics in upconversion nanomaterials. Chem. Soc. Rev. 44, 1331–1345 (2015). doi: 10.1039/C4CS00168K
[4] Chen, G. Y. et al. Upconversion nanoparticles: design, nanochemistry, and applications in theranostics. Chem. Rev. 114, 5161–5214 (2014). doi: 10.1021/cr400425h
[5] Zhou, B. et al. Controlling upconversion nanocrystals for emerging applications. Nat. Nanotechnol. 10, 924–936 (2015). doi: 10.1038/nnano.2015.251
[6] Wang, F. et al. Microscopic inspection and tracking of single upconversion nanoparticles in living cells. Light. Sci. Appl. 7, 18007 (2018). doi: 10.1038/lsa.2018.7
[7] Zuo, J. et al. Near infrared light sensitive ultraviolet-blue nanophotoswitch for imaging-guided "off-on" therapy. ACS Nano 12, 3217–3225 (2018). doi: 10.1021/acsnano.7b07393
[8] Yang, D. M. et al. Hollow structured upconversion luminescent NaYF4: Yb3+, Er3+ nanospheres for cell imaging and targeted anti-cancer drug delivery. Biomaterials 34, 1601–1612 (2013). doi: 10.1016/j.biomaterials.2012.11.004
[9] Hou, Z. Y. et al. Hydrogenated titanium oxide decorated upconversion nanoparticles: facile laser modified synthesis and 808 nm near-infrared light triggered phototherapy. Chem. Mater. 31, 774–784 (2019). doi: 10.1021/acs.chemmater.8b03762
[10] Teh, D. B. L. et al. A flexi-PEGDA upconversion implant for wireless brain photodynamic therapy. Adv. Mater. 32, 2001459 (2020). doi: 10.1002/adma.202001459
[11] Chen, S. et al. Near-infrared deep brain stimulation via upconversion nanoparticle – mediated optogenetics. Science 359, 679–684 (2018). doi: 10.1126/science.aaq1144
[12] Liang, L. L. et al. Upconversion amplification through dielectric superlensing modulation. Nat. Commun. 10, 1391 (2019). doi: 10.1038/s41467-019-09345-0
[13] Ji, Y. N. et al. Huge upconversion luminescence enhancement by a cascade optical field modulation strategy facilitating selective multispectral narrow-band near-infrared photodetection. Light. Sci. Appl. 9, 184 (2020). doi: 10.1038/s41377-020-00418-0
[14] Boyer, J. C. & van Veggel, F. C. J. M. Absolute quantum yield measurements of colloidal NaYF4: Er3+, Yb3+ upconverting nanoparticles. Nanoscale 2, 1417–1419 (2010). doi: 10.1039/c0nr00253d
[15] Faulkner, D. O. et al. Absolute quantum yields in NaYF4: Er, Yb upconverters - synthesis temperature and power dependence. J. Mater. Chem. 22, 24330–24334 (2012). doi: 10.1039/c2jm33457g
[16] Wang, F., Wang, J. & Liu, X. G. Direct evidence of a surface quenching effect on size-dependent luminescence of upconversion nanoparticles. Angew. Chem. Int. Ed. 49, 7456–7460 (2010). doi: 10.1002/anie.201003959
[17] Rinkel, T. et al. Synthesis of 10 nm β-NaYF4: Yb, Er/NaYF4 core/shell upconversion nanocrystals with 5 nm particle cores. Angew. Chem. Int. Ed. 55, 1164–1167 (2016). doi: 10.1002/anie.201508838
[18] Zhao, J. B. et al. Upconversion luminescence with tunable lifetime in NaYF4: Yb, Er nanocrystals: role of nanocrystal size. Nanoscale 5, 944–952 (2013). doi: 10.1039/C2NR32482B
[19] Chen, G. et al. Upconversion emission enhancement in Yb3+/Er3+-codoped Y2O3 nanocrystals by tridoping with Li+ ions. J. Phys. Chem. C 112, 12030–12036 (2008). doi: 10.1021/jp804064g
[20] He, J. J. et al. Plasmonic enhancement and polarization dependence of nonlinear upconversion emissions from single gold nanorod@SiO2@CaF2: Yb3+, Er3+ hybrid core–shell–satellite nanostructures. Light. Sci. Appl. 6, e16217 (2017). doi: 10.1038/lsa.2016.217
[21] Zhang, F. et al. Fabrication of Ag@SiO2@Y2O3: Er nanostructures for bioimaging: tuning of the upconversion fluorescence with silver nanoparticles. J. Am. Chem. Soc. 132, 2850–2851 (2010). doi: 10.1021/ja909108x
[22] Zhou, J. J. et al. Activation of the surface dark-layer to enhance upconversion in a thermal field. Nat. Photonics 12, 154–158 (2018). doi: 10.1038/s41566-018-0108-5
[23] Zou, W. Q. et al. Broadband dye-sensitized upconversion of near-infrared light. Nat. Photonics 6, 560–564 (2012). doi: 10.1038/nphoton.2012.158
[24] Garfield, D. J. et al. Enrichment of molecular antenna triplets amplifies upconverting nanoparticle emission. Nat. Photonics 12, 402–407 (2018). doi: 10.1038/s41566-018-0156-x
[25] Xu, J. T. et al. Highly emissive dye-sensitized upconversion nanostructure for dual-photosensitizer photodynamic therapy and bioimaging. ACS Nano 11, 4133–4144 (2017). doi: 10.1021/acsnano.7b00944
[26] Yi, G. S. & Chow, G. M. Water-soluble NaYF4: Yb, Er(Tm)/NaYF4/polymer core/shell/shell nanoparticles with significant enhancement of upconversion fluorescence. Chem. Mater. 19, 341–343 (2007). doi: 10.1021/cm062447y
[27] Johnson, N. J. J. et al. Direct evidence for coupled surface and concentration quenching dynamics in lanthanide-doped nanocrystals. J. Am. Chem. Soc. 139, 3275–3282 (2017). doi: 10.1021/jacs.7b00223
[28] Zuo, J. et al. Employing shells to eliminate concentration quenching in photonic upconversion nanostructure. Nanoscale 9, 7941–7946 (2017). doi: 10.1039/C7NR01403A
[29] Chen, X. et al. Confining energy migration in upconversion nanoparticles towards deep ultraviolet lasing. Nat. Commun. 7, 10304 (2016). doi: 10.1038/ncomms10304
[30] Wen, S. H. et al. Advances in highly doped upconversion nanoparticles. Nat. Commun. 9, 2415 (2018). doi: 10.1038/s41467-018-04813-5
[31] Liu, Q. et al. Single upconversion nanoparticle imaging at sub-10 W cm-2 Irradiance. Nat. Photonics 12, 548–553 (2018). doi: 10.1038/s41566-018-0217-1
[32] Rabouw, F. T. et al. Quenching pathways in NaYF4: Er3+, Yb3+ upconversion nanocrystals. ACS Nano 12, 4812–4823 (2018). doi: 10.1021/acsnano.8b01545
[33] Guo, S. H. et al. Sensitive water probing through nonlinear photon upconversion of lanthanide-doped nanoparticles. ACS Appl. Mater. Interfaces 8, 847–853 (2016). doi: 10.1021/acsami.5b10192
[34] Chen, D. Q. et al. Water detection through Nd3+-sensitized photon upconversion in core-shell nanoarchitecture. J. Mater. Chem. C 5, 5434–5443 (2017). doi: 10.1039/C7TC01373F
[35] Arppe, R. et al. Quenching of the upconversion luminescence of NaYF4: Yb3+, Er3+ and NaYF4: Yb3+, Tm3+ nanophosphors by water: the role of the sensitizer Yb3+ in non-radiative relaxation. Nanoscale 7, 11746–11757 (2015). doi: 10.1039/C5NR02100F
[36] Hyppänen, I. et al. Environmental impact on the excitation path of the red upconversion emission of nanocrystalline NaYF4: Yb3+, Er3+. J. Phys. Chem. C 121, 6924–6929 (2017). doi: 10.1021/acs.jpcc.7b01019
[37] Boyer, J. C. et al. Surface modification of upconverting NaYF4 nanoparticles with PEG-phosphate ligands for NIR (800 nm) biolabeling within the biological window. Langmuir 26, 1157–1164 (2010). doi: 10.1021/la902260j
[38] Stouwdam, J. W. et al. Lanthanide-doped nanoparticles with excellent luminescent properties in organic media. Chem. Mater. 15, 4604–4616 (2003). doi: 10.1021/cm034495d
[39] De, G. J. H. et al. Effect of OH on the upconversion luminescent efficiency of Y2O3: Yb3+, Er3+ nanostructures. Solid State Commun. 137, 483–487 (2006). doi: 10.1016/j.ssc.2005.12.034
[40] Homann, C. et al. NaYF4: Yb, Er/NaYF4 core/shell nanocrystals with high upconversion luminescence quantum yield. Angew. Chem. Int. Ed. 57, 8765–8769 (2018). doi: 10.1002/anie.201803083
[41] Baumer, A., Ganteaume, M. & Klee, W. E. Determination of OH ions in hydroxyfluorapatites by infrared spectroscopy. Bull. Minéral. 108, 145–152 (1985). doi: 10.3406/bulmi.1985.7864
[42] Zhang, J. H. et al. Observation of efficient population of the red-emitting state from the green state by non-multiphonon relaxation in the Er3+–Yb3+ system. Light. Sci. Appl. 4, e239 (2015). doi: 10.1038/lsa.2015.12
[43] Chen, Q. S. et al. Confining excitation energy in Er3+-sensitized upconversion nanocrystals through Tm3+-mediated transient energy trapping. Angew. Chem. Int. Ed. 56, 7605–7609 (2017). doi: 10.1002/anie.201703012
[44] Vetrone, F. et al. Concentration-dependent near-infrared to visible upconversion in nanocrystalline and bulk Y2O3: Er3+. Chem. Mater. 15, 2737–2743 (2003). doi: 10.1021/cm0301930
[45] Brushteǐn, A. I. Hopping mechanism of energy transfer. Sov. J. Exp. Theor. Phys. 35, 882–885 (1972).
[46] Zuo, J. et al. Precisely tailoring upconversion dynamics via energy migration in core-shell nanostructures. Angew. Chem. Int. Ed. 57, 3054–3058 (2018). doi: 10.1002/anie.201711606
[47] Meijer, M. S. et al. Absolute upconversion quantum yields of blue-emitting LiYF4: Yb3+, Tm3+ upconverting nanoparticles. Phys. Chem. Chem. Phys. 20, 22556–22562 (2018). doi: 10.1039/C8CP03935F