A method for the detection of fibrin in a source, in particular the in vivo detection of a fibrin in a patient, the method comprising supplying to the source or patient an amount of a detectable reagent comprising a plurality of discrete particles, each of the particles comprising a plurality of layers of carbon and being capable of binding to fibrin; and detecting the presence of the particles in the source. The particles may also comprise a detectable marker encased in said plurality of layers of carbon, the presence of said marker being capable of detection in said source.