A DNA aptamer for trivalent lanthanide ions with low nanomolar affinity

Lanthanides are extremely important for a variety of technological applications. In this work, DNA aptamers were selected using the library-immobilization method (capture-SELEX) with Tb3+ and Ce3+ as target metal ions. The Tb3+ selection yielded a new sequence named Tb-1 that has a Kd of 26.9 nM for La3+, 3.9 nM for Tb3+, and 2.3 nM for Lu3+ as determined by a DNA strand displacement assay. Binding to other metal ions cannot be detected using a fluorescence DNA strand displacement assay. Therefore, it is a general lanthanide binding aptamer. Another aptamer Tb-4 has some sequence similarity to a previously reported Gd-1 aptamer and it exhibited a Kd of 290 nM, while Gd-1 had a Kd of 1.5 µM, as determined by a thioflavin T fluorescence assay. They showed little selectivity for different lanthanides. Compared to a previously reported aptamer named Sc-1, Tb-1 is an outersphere binder and fast release of bound lanthanides upon the addition of EDTA. By comparing different aptamers, we have gained fundamental insights into aptamer binding to lanthanides. Finally, using the strand displacement reaction, a detection limit of 0.5 nM Tb3+ was achieved in Lake Ontario water.