A metal ion nanocluster having a formula of X(OH) a Y b Z c M d (H 2 O) e . X, Y, Z, M, a, b, c, d, and e are defined herein. The nanocluster has a particle size of 2 to 500 nm and a molecular weight of 3500 to 1000000 Dalton. Also disclosed are a composition containing the nanocluster and a method of preparing the nanocluster.