Platform- and label-free detection of lead ions in environmental and laboratory samples using G-quadraplex probes by circular dichroism spectroscopy

Guanine-rich quadruplex (G-QD) are formed by conversion of nucleotides with specific sequences by stabilization of positively charged K(+) or Na(+). These G-QD structures differentially absorb two-directional (right- and left-handed) circularly polarized light, which can discriminate the parallel or anti-parallel structures of G-QDs. In this study, G-QDs stabilized by Pb(2+) were analyzed by a circular dichroism (CD) spectroscopy to determine Pb(2+) concentration in water samples. Thrombin aptamer (TBA), PS2.M, human telomeric DNA (HTG), AGRO 100, and telomeric related sequence (T2) were studied to verify their applicability as probes for platform- and label-free detection of Pb(2+) in environmental as well as laboratory samples. Among these nucleotides, TBA and PS2.M exhibited higher binding constants for Pb(2+), 1.20–2.04 × 10(6)/M at and 4.58 × 10(4)–1.09 × 10(5)/M at 100 micromolar and 100 mM K(+) concentration, respectively. They also exhibited excellent selectivity for Pb(2+) than for Al(3+), Cu(2+), Ni(2+), Fe(3+), Co(2+), and Cr(2+). When Pb(2+) was spiked into an effluent sample from a wastewater treatment plant (WWTP), its existence was detected by CD spectroscopy following a simple addition of TBA or PS2.M. By the addition of TBA and PS2.M, the Pb(2+) signals were observed in effluent samples over 0.5 micromolar (100 ppb) concentration. Furthermore, PS2.M caused a Pb(2+)-specific absorption band in the effluent sample without spiking of Pb(2+), and could be induced to G-QD structure by the background Pb(2+) concentration in the effluent, 0.159 micromolar concentration (3.30 ppb). Taken together, we propose that TBA and PS2.M are applicable as platform- and label-free detection probes for monitoring Pb(2+) in environmental samples such as discharged effluent from local WWTPs, using CD spectroscopy.