The present invention provides a particle aggregate comprising particles linked to other particles via linkers that are capable of being cleaved by an enzyme. The particles may be linked directly to other particles by the linkers or may be linked indirectly by means of a binding moiety. The invention also provides a particle coupled to a binding moiety via a linker that can be cleaved by an enzyme. The present invention may be used in diagnosing a disease or condition associated with the enzyme, or may be used to treat such a disease or condition whereby the drug is retained in the particle aggregate until the linkers are cleaved by the enzyme.
