[1] [1] P. N. Prasad, Introduction to Biophotonics, Wiley-Interscience (2004).

[2] [2] Y. F. Kong, J. Chen, F. Gao, R. Brydson, B. Johnson, G. Heath, Y. Zhang, L. Wu, D. J. Zhou, “Near-infrared fluorescent ribonuclease- A-encapsulated gold nanoclusters: Preparation, characterization, cancer targeting and imaging," Nanoscale 5, 1009–1017 (2013).

[3] [3] P. F. Rong, P. Huang, Z. G. Liu, J. Lin, A. Jin, Y. Ma, G. Niu, L. Yu, W. B. Zeng, W. Wang, X. Y. Chen, “Protein-based photothermal theranostics for imaging-guided cancer therapy," Nanoscale 7, 16330–16336 (2015).

[4] [4] J. Qian, Z. Zhu, A. Qin, W. Qin, L. Chu, F. Cai, H. Zhang, Q. Wu, R. Hu, B. Z. Tang, “High‐order non‐linear optical effects in organic luminogens with aggregation‐induced emission," Adv. Mater. 27, 2332–2339 (2015).

[5] [5] X. Wu, X. X. He, K. M. Wang, C. Xie, B. Zhou, Z. H. Qing, “Ultrasmall near-infrared gold nanoclusters for tumor fluorescence imaging in vivo," Nanoscale 2, 2244–2249 (2010).

[6] [6] J. V. Frangioni, “In vivo near-infrared fluorescence imaging," Curr. Opin. Chem. Biol. 7, 626–634 (2003).

[7] [7] Q. Liu, B. D. Guo, Z. Y. Rao, B. H. Zhang, J. R. Gong, “Strong two-photon-induced fluorescence from photostable, biocompatible nitrogen-doped graphene quantum dots for cellular and deep-tissue imaging," Nano Lett. 13, 2436–2441 (2013).

[8] [8] N. J. Durr, T. Larson, D. K. Smith, B. A. Korgel, K. Sokolov,A.Ben-Yakar,“Two-photon luminescence imaging of cancer cells using molecularly targeted gold nanorods," Nano Lett. 7, 941–945 (2007).

[9] [9] N. N. Dong, M. Pedroni, F. Piccinelli, G. Conti, A. Sbarbati, J. E. Ramirez-Hernandez, L. M. Maestro, M. C. Iglesias-de la Cruz, F. Sanz-Rodriguez, A. Juarranz, F. Chen, F. Vetrone, J. A. Capobianco, J. G. Sole, M. Bettinelli, D. Jaque, A. Speghini, “NIR-to-NIR two-photon excited CaF2: Tm3+, Yb3+ Nanoparticles: Multifunctional nanoprobes for highly penetrating fluorescence bio-imaging," Acs. Nano 5, 8665–8671 (2011).

[10] [10] F. Helmchen, W. Denk, “Deep tissue two-photon microscopy," Nat. Meth. 2, 932–940 (2005).

[11] [11] B. G. Wang, K. Konig, K. J. Halbhuber, “Twophoton microscopy of deep intravital tissues and its merits in clinical research," J. Microsc-Oxford 238, 1–20 (2010).

[12] [12] M. Albota, D. Beljonne, J. L. Bredas, J. E. Ehrlich, J. Y. Fu, A. A. Heikal, S. E. Hess, T. Kogej, M. D. Levin, S. R. Marder, D. McCord-Maughon, J. W. Perry, H. Rockel, M. Rumi, C. Subramaniam, W. W. Webb, X. L. Wu, C. Xu, “Design of organic molecules with large two-photon absorption cross sections," Science 281, 1653–1656 (1998).

[13] [13] Y. I. Park, K. T. Lee, Y. D. Suh, T. Hyeon, “Upconverting nanoparticles: A versatile platform for wide-field two-photon microscopy and multimodal in vivo imaging," Chem. Soc. Rev. 44, 1302–1317 (2015).

[14] [14] M. Pollnau, D. R. Gamelin, S. R. Luthi, H. U. Gudel, M. P. Hehlen, “Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems," Phys. Rev. B 61, 3337–3346 (2000).

[15] [15] J.-W. Shen, J. Wang, D. Kong, X.-P. Yan, “Sub-20 nm sandwich-structured NaGdF4:Yb/Tm@NaLuF4:Yb/Tm@NaYF4 nanocrystals for in vivo upconversion luminescence/computed tomography imaging," RSC Adv. 4, 5088 (2014).

[16] [16] Z. Deng, X. Li, Z. Xue, M. Jiang, Y. Li, S. Zeng, H. Liu, “A high performance Sc-based nanoprobe for through-skull fluorescence imaging of brain vessels beyond 1500 nm," Nanoscale 10, 9393–9400 (2018).

[17] [17] W. You, D. Tu, W. Zheng, X. Shang, X. Song, S. Zhou, Y. Liu, R. Li, X. Chen, “Large-scale synthesis of uniform lanthanide-doped NaREF4 upconversion/downshifting nanoprobes for bioapplications," Nanoscale 10, 11477–11484 (2018).

[18] [18] F. Wang, X. Liu, “Upconversion multicolor finetuning: Visible to near-infrared emission from lanthanide-doped NaYF4 nanoparticles," J. Am. Chem. Soc. 130, 5642–5643 (2008).

[19] [19] Y. Zhang, J. Qian, D. Wang, Y. Wang, S. He, “Multifunctional gold nanorods with ultrahigh stability and tunability for in vivo fluorescence imaging, SERS detection, and photodynamic therapy," Angewandte Chemie 52, 1148–1151 (2013).

[20] [20] A. Zebibula, N. Alifu, L. Xia, C. Sun, X. Yu, D. Xue, L. Liu, G. Li, J. Qian, “Ultrastable and biocompatible NIR-II quantum dots for functional bioimaging," Adv. Funct. Mater. 28, 1703451 (2018).

[21] [21] F. Wang, R. Deng, X. Liu, “Preparation of coreshell NaGdF4 nanoparticles doped with luminescent lanthanide ions to be used as upconversion-based probes," Nat. Protocols 9, 1634–1644 (2014).

[22] [22] X. Ge, L. Dong, L. Sun, Z. Song, R. Wei, L. Shi, H. Chen, “New nanoplatforms based on UCNPs linking with polyhedral oligomeric silsesquioxane (POSS) for multimodal bioimaging," Nanoscale 7, 7206–7215 (2015).

[23] [23] S. Wang, W. Xi, F. Cai, X. Zhao, Z. Xu, J. Qian, S. He, “Three-photon luminescence of gold nanorods and its applications for high contrast tissue and deep in vivo brain imaging," Theranostics 5, 251–266 (2015).

[24] [24] W. Liu, Y. Wang, X. Han, P. Lu, L. Zhu, C. Sun, J. Qian, S. He, “Fluorescence resonance energy transfer (FRET) based nanoparticles composed of AIE luminogens and NIR dyes with enhanced three-photon near-infrared emission for in vivo brain angiography," Nanoscale 10, 10025–10032 (2018).

[25] [25] Q. Li, Z. Wang, Y. Chen, G. Zhang, “Elemental bioimaging of PEGylated NaYF4:Yb/Tm/Gd upconversion nanoparticles in mice by laser ablation inductively coupled plasma mass spectrometry to study toxic side effects on the spleen, liver and kidneys," Metallomics Integr. Biometal Sci. 9, 1150–1156 (2017).

[26] [26] C. T. Xu, P. Svenmarker, H. Liu, X. Wu, M. E. Messing, L. R. Wallenberg, S. Andersson-Engels, “High-resolution fluorescence diffuse optical tomography developed with nonlinear upconverting nanoparticles," ACS Nano 6, 4788–4795 (2012).

[27] [27] H. Liu, C. T. Xu, D. Lindgren, H. Xie, D. Thomas, C. Gundlach, S. Andersson-Engels, “Balancing power density based quantum yield characterization of upconverting nanoparticles for arbitrary excitation intensities," Nanoscale 5, 4770–4775 (2013).

[28] [28] J. Zhou, Z. Liu, F. Y. Li, “Upconversion nanophosphors for small-animal imaging," Chem. Soc. Rev. 41, 1323–1349 (2012).

[29] [29] X. Q. Ge, L. Dong, L. N. Sun, Z. M. Song, R. Y. Wei, L. Y. Shi, H. G. Chen, “New nanoplatforms based on UCNPs linking with polyhedral oligomeric silsesquioxane (POSS) for multimodal bioimaging," Nanoscale 7, 7206–7215 (2015).