Martinez Maestro, L, Dong, N, Pedroni, M, Piccinell, F, Conti, G, Sbarbati, A, Ramı´rez-Herna´ ndez, JE, Iglesias-de la Cruz, MC, Sanz-Rodriguez, F, Juarranz, A, Chen, F, Vetrone, F, Capobianco, JA, Garcia Sole, J, Jaque, D, Bettinelli, M, Speghini, A, "NIR-to-NIR two-photon excited CaF2:Tm3+,Yb3+ nanoparticles: multifunctional nanoprobes for highly penetrating fluorescence bio-imaging", ACS Nano, 2011