An improved chemical probe for the detection of the amino acid citrulline combines: 1) a reactive head formed of 1,3-dicarbonyl moiety that reacts with a citrulline side chain in an improved manner compared to currently used 1,2-dicarbonyl moieties; and 2) a modular action of the probe where citrulline side chains are labeled first using reactive heads described above, and attachment of a read-out subunit or tag, be it a fluorophore, a nanoparticle, or an antigen is performed separately. The modular nature of the chemical probe increases the sensitivity of the probes due to their smaller size. Additionally, the chemical probes of the present disclosure allow the same sample to be analyzed using a variety of read-out methods.