The present invention provides a nitric-oxide containing composite in the form of microparticles, wherein said microparticles comprise : (i) a core which comprises silica; (ii) a layer on said core which comprises a metal-organic framework; and (iii) nitric oxide; wherein said metal-organic framework comprises organic ligands comprising at least one amine group, said metal-organic framework is uniformly distributed on the surface of said silica core and said nitric oxide is chemisorbed within said metal-organic framework.