[reaction: see text] Water- and acid-resistant arylplatinum(II) complexes have been covalently bonded to the N-terminus of L-valine, thus providing organometallic biomolecules with excellent stability properties. Owing to the (195)Pt nucleus (I = (1)/(2)), these building blocks are potentially versatile biomarkers (e.g., MRI). Moreover, they display efficient in vitro biosensor characteristics since they detect SO(2) gas selectively and fully reversibly by an instantaneous change of the spectroscopic properties including a diagnostic (195)Pt NMR signal.