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

Abstract

Abstract 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.

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