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 K_d 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. Non-lanthanide metal ions did not induce a fluorescence enhancement. Therefore, it is a general lanthanide binding aptamer. Another aptamer Tb-4 (K_d 290 nM) has some sequence similarity to a previously reported aptamer selected using Gd³⁺ (K_d 1.5 μM), as determined by a thioflavin T fluorescence assay. Compared to a previously reported aptamer named Sc-1, Tb-1 has faster exchange with EDTA for lanthanide binding, suggesting that Tb-1 is an outer-sphere ligand. By comparing different aptamers, we have gained fundamental insights into aptamer binding to lanthanide ions. Finally, using the strand displacement reaction, a detection limit of 0.5 nM Tb³⁺ was achieved in Lake Ontario water.