The invention provides a nano-sized particle comprising a cross-linked polymer, wherein the polymer is selected from the group consisting of a polyacrylic acid homopolymer; polymethacrylic acid homopolymer; poly(alkylcyanoacrylate) polymer; a copolymer comprising at least two monomers selected from acrylic acid, methacrylic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate, and alkyl cyanoacrylate/cyanoacrylic acid monomers; carboxymethyl cellulose; alginic acid polymer, polylactic-polyglycolic acid (PLGA), and xanthan gum; and wherein said polymer is cross-linked with a metal ion. A process for preparing such particles is also provided.