A metal ion nanocluster having a particle size of 2 to 500 nm, a molecular weight of 3500 to 1000000 Dalton, and a formula of X(OH) a Y b Z c M d (H 2 O) e , in which X is a metal cation, Y is a water-soluble salt-forming anion, Z is a water-soluble ligand, M is a turbidity point modifier, a is 0.1 to 20, b is 0 to 9, c is 0.1 to 10, d is 0.1 to 9, and e is 0 to 25, wherein the metal cation is Mg(II), Al(III), Ca(II), Cr(III), Cu(II), Zn(II), Mn (II), Ti(IV), Fe(II/III), Co(II), Ni(II), Bi(III), V(V), La(III), Mo(VI), Sr(II), Zr(IV), or a combination thereof; the nanocluster has a turbidity point of 8 or lower; and the turbidity point modifier is a complex-forming polyvalence anion or a carboxylated polyol, wherein the water-soluble ligand is xylitol, mannitol, ribose, mannose, xylose, galactose, fructose, lactose, glucose, isomaltose, isomalt, sucrose, maltitol, trehalose, arabinose, sorbitol, polyisomaltose, isomalto-oligosaccharide, dextrin, dextran, fructooligosaccharide, or a combination thereof; and the complex-forming polyvalence anion is citrate, malate, fumarate, tartrate, lactate, oxalate, succinate, ascorbate, phosphate, pyrophosphate, glycerophosphate, or a combination thereof; wherein the metal ion nanocluster is prepared by a process including the following steps: providing an aqueous dispersion containing X, Y, Z, and OH; adjusting the pH of the aqueous dispersion to a predetermined value to obtain a pH-adjusted aqueous dispersion; heating the pH-adjusted aqueous dispersion to 50-180 °C so that a nanocluster precursor is formed; and adding M after the above steps, in which the ratio of X : Y : Z : M is 1 : 0-9 : 0.1-10 : 0.1-9. wherein the metal cation is Mg(II), Al(III), Ca(II), Cr(III), Cu(II), Zn(II), Mn (II), Ti(IV), Fe(II/III), Co(II), Ni(II), Bi(III), V(V), La(III), Mo(VI), Sr(II), Zr(IV), or a combination thereof; the nanocluster has a turbidity point of 8 or lower; and the turbidity point modifier is a complex-forming polyvalence anion or a carboxylated polyol, wherein the water-soluble ligand is xylitol, mannitol, ribose, mannose, xylose, galactose, fructose, lactose, glucose, isomaltose, isomalt, sucrose, maltitol, trehalose, arabinose, sorbitol, polyisomaltose, isomalto-oligosaccharide, dextrin, dextran, fructooligosaccharide, or a combination thereof; and the complex-forming polyvalence anion is citrate, malate, fumarate, tartrate, lactate, oxalate, succinate, ascorbate, phosphate, pyrophosphate, glycerophosphate, or a combination thereof; wherein the metal ion nanocluster is prepared by a process including the following steps: providing an aqueous dispersion containing X, Y, Z, and OH; adjusting the pH of the aqueous dispersion to a predetermined value to obtain a pH-adjusted aqueous dispersion; heating the pH-adjusted aqueous dispersion to 50-180 °C so that a nanocluster precursor is formed; and adding M after the above steps, providing an aqueous dispersion containing X, Y, Z, and OH; adjusting the pH of the aqueous dispersion to a predetermined value to obtain a pH-adjusted aqueous dispersion; heating the pH-adjusted aqueous dispersion to 50-180 °C so that a nanocluster precursor is formed; and adding M after the above steps, in which the ratio of X : Y : Z : M is 1 : 0-9 : 0.1-10 : 0.1-9. <EOS>