There is provided a core-shell particle comprising a core comprising at least one active agent and a method of forming said core-shell particle, wherein said core having ions of an additive surrounding a surface of said core; and an encapsulating shell comprising (i) a polymer and/or (ii) a metal or a semi-metal oxide, phosphate or carbonate. In a preferred embodiment, chondroitin sulphate nanocrystals is first prepared by using pH recrystallization method. Sodium citrate is then added to form citrate ions-capped chondroitin sulphate particles. Poly L-lysine (PLL) and tetramethoxysilane (TMOS) are subsequently added to form a silica shell surrounding the citrate-ions capped chondroitin sulphate particles, and thus forming the claimed core-shell particle. The particles may be used as sustained release formulations in therapy.