Disclosed is a composition comprising: a plurality of water-soluble nanoclusters ranging from 2 nm to 500 nm in diameter, each of the nanoclusters containing one or more metal cations, one or more anions, and one or more non-polymeric water-soluble ligands, wherein the one or more non-polymeric water-soluble ligands are selected from the group consisting of a monosaccharide, a hydrogenated monosaccharide, a disaccharide, a hydrogenated disaccharide, an oligosaccharide, and an oligosaccharide derivative, polyether, polyol, and the molar ratio among the one or more metal cations, the one or more anions, and the one or more non-polymeric water-soluble ligands is 1:0.1-9:0.1-10, wherein the water-soluble nanoclusters are prepared by a method including the following steps: - providing a first transparent aqueous solution containing one or more water-soluble salts and the one or more water-soluble ligands, in which the one or more water-soluble salts include the one or more metal cations； and the molar ratio between the one or more metal cations and the one or more water-soluble ligands is 1:0.1-10； and - mixing the first transparent aqueous solution with the one or more anions at 80-135 degrees Celsius to obtain a second transparent aqueous solution having a pH value ranging between 3.5 and 11.5, such that a plurality of water-soluble nanoclusters are formed, each ranging from 2 nm to 500 nm in diameter and containing the one or more metal cations, the one or more anions, and the one or more water-soluble ligands.
