A hydrophilic nanoparticle comprising: a hydrophobic particle comprising a center particle, and a hydrophobic ligand covering a surface of the center particle; and an amphiphilic organic dye combined with the hydrophobic ligand by a hydrophobic interaction, and wherein the hydrophilic particle has a surface zeta potential lower than a surface zeta potential of the amphiphilic organic dye, characterized in that: the center particle comprises a transition metal oxide or is an up-conversion particle; the hydrophobic ligand comprises a fatty acid; the amphiphilic organic dye is selected from the group consisting of cyanine; and the hydrophilic particle is obtainable from a process for manufacturing a hydrophilic particle comprising the steps of: preparing said hydrophobic particle dispersed in an organic phase; and mixing the hydrophobic particle in the organic phase with said amphiphilic organic dye in an aqueous phase to form said hydrophilic particle, wherein the amphiphilic organic dye is directly adsorbed on a surface of the hydrophobic particle to phase-convert the hydrophobic particle to the hydrophilic particle dispersed in the aqueous phase. a hydrophobic particle comprising a center particle, and a hydrophobic ligand covering a surface of the center particle; and an amphiphilic organic dye combined with the hydrophobic ligand by a hydrophobic interaction, and wherein the hydrophilic particle has a surface zeta potential lower than a surface zeta potential of the amphiphilic organic dye, characterized in that: the center particle comprises a transition metal oxide or is an up-conversion particle; the hydrophobic ligand comprises a fatty acid; the amphiphilic organic dye is selected from the group consisting of cyanine; and the hydrophilic particle is obtainable from a process for manufacturing a hydrophilic particle comprising the steps of: preparing said hydrophobic particle dispersed in an organic phase; and mixing the hydrophobic particle in the organic phase with said amphiphilic organic dye in an aqueous phase to form said hydrophilic particle, wherein the amphiphilic organic dye is directly adsorbed on a surface of the hydrophobic particle to phase-convert the hydrophobic particle to the hydrophilic particle dispersed in the aqueous phase. the center particle comprises a transition metal oxide or is an up-conversion particle; the hydrophobic ligand comprises a fatty acid; the amphiphilic organic dye is selected from the group consisting of cyanine; and the hydrophilic particle is obtainable from a process for manufacturing a hydrophilic particle comprising the steps of: preparing said hydrophobic particle dispersed in an organic phase; and mixing the hydrophobic particle in the organic phase with said amphiphilic organic dye in an aqueous phase to form said hydrophilic particle, wherein the amphiphilic organic dye is directly adsorbed on a surface of the hydrophobic particle to phase-convert the hydrophobic particle to the hydrophilic particle dispersed in the aqueous phase. preparing said hydrophobic particle dispersed in an organic phase; and mixing the hydrophobic particle in the organic phase with said amphiphilic organic dye in an aqueous phase to form said hydrophilic particle, wherein the amphiphilic organic dye is directly adsorbed on a surface of the hydrophobic particle to phase-convert the hydrophobic particle to the hydrophilic particle dispersed in the aqueous phase.