NOVELTY - Nanophosphor comprises a first compound that is sodium gadolinium fluoride (I), where the first compound is fluoride-based one which is co-doped with cerium3+ and terbium3+.    USE - The nanophosphor is useful in a fluorescent or MRI contrast agent, infrared sensor, counterfeit prevention code and solar cell (all claimed), and invivo applications, where the prevention code is forgery prevention code.    ADVANTAGE - The nanophosphor: exhibits down-conversion light emission properties and up-conversion light emission properties (claimed) at a time, and good light emission intensity and magnetic property; provides stronger and more accurate image signal, and images from deeper region of biological tissue; prevents decline of luminance from unevenness of particle size; increases the efficiency of solar cell; provides sensor with excellent sensitivity; and is prepared simply and utilized as higher grade security code.    DETAILED DESCRIPTION - Nanophosphor comprises a first compound that is sodium-gadolinium-fluoride of formula (NaGd1-p-q-rMrF4:Ce3+p,Tb3+q) (I), where the first compound is fluoride-based one which is co-doped with cerium3+ and terbium3+.    p = 0.01-0.5;    q = 0.001-0.35;    r = greater than or equal to 0 to less than 1;    p+q+r = greater than or equal to 0.011 to less than 1; and    M = Y, La, Pr, Nd, Pm, Sm, Eu, Dy, Ho, Er, Tm, Yb and/or Lu.    An INDEPENDENT CLAIM is also included for preparing the nanophosphor, comprising: (a) preparing a first solution comprising gadolinium precursors, cerium precursors, terbium precursors and oleic acid salts; (b) preparing a second solution comprising lanthanide complexes by heat treatment of the first solution; (c) preparing a first reaction solution by mixing a forth solution with sodium precursors and fluorine precursors, where the forth solution is prepared by blending a third solution comprising oleic acid and 1-octadececen and mixture comprising the second solution; and (d) preparing nanophosphors comprising the by heat treatment of the first reaction solution.