A highly sensitive, rhodamine phenylglyoxal-based fluorophore that specifically detects protein citrullination via a chemoselective reaction between glyoxal and citrulline; reacts with any citrulline-containing protein and can be analyzed with fluorescent imaging (Ex. 532 nm; Em. 580 nm); limit of detection of ~10 ng for citrullinated histone H3 and ~1 ng for autodeiminated PAD4 when applied at 100 µM for 30 minutes at acidic pH.