A graphene-based sandwich immunoassay for detecting whether a target biological substance is present in a sample, generally comprising contacting said sample with a plurality of particles coated with graphene nanosheets, each particle having at least one targeting receptor, such that the target biological substance, if present, associates with the targeting receptor, and detecting the presence of the target biological substance in the sample by subsequently contacting the sample with a detection antibody, wherein the detection antibody is capable of targeting and binding with the target biological substance if bound to the targeting receptor to yield a detectable complex. The targeting receptor can be an antibody or fragment thereof. The target biological substance can be an exosome.