Abstract: The invention provides modified oligonucleotides of formula (I), comprising at least one metal chelator which provides a powerful tool for study of the pharmacokinetics of siRNA and its correlation with in vivo activity. The chelated metals provide luminescent properties enable detection of the oligonucleotides through the use of time-resolved fluorescent quenching based on energy transfer from the metal ion to a nonfluorescent quencher which can be used as non-isotopic labels of oligonucleotides for diagnostics and evaluation of cellular uptake.