Hg(II)-Selective Probing of Flavin Mononucleotide Helix on Single-walled Carbon Nanotubes via Multimodal NIR Detection

Abstract

The development of novel sensing methods for mercury is of paramount importance owing to the impact of this metal on human health and the environment. In this presentation, we observed that flavin mononucleotide (FMN) and its helical motif on single-walled carbon nanotube (SWNT) selectively binds to Hg2+ rather than 8 metal ions. Absorption spectroscopy-based stoichiometric studies show that FMN preferentially forms a 2:1 rather than a 1:1 complex with Hg2+ at high FMN concentrations. On the basis of the similarity to the thymine-Hg-thymine complex, it is proposed that 2:1 complex between opposing pair of FMN groups and Hg2+ comprises a Hg-bridged pair of FMN groups. Upon addition of as little as a few hundred nanomoles of Hg2+, binding of FMN−SWNT with HgCl2 induces changes in absorption and photoluminescence (PL) spectra. Moreover, FMN-SWNT displays simultaneous multiple sigmoidal changes in NIR PL of SWNT having different chiralities with Hg2+ addition. Computational calculations indicate that those optical changes originate from Hg-mediated conformational changes occurring on the helical array of FMN on SWNT. HRTEM revealed that the presence of Hg2+ in complexes with FMN-SWNT enables visualization of helical periodic undulation of FMN groups without the need for staining. Moreover, circular dichroism (CD) study revealed that FMN-SWNT decreases its dichroic bands upon the addition of Hg2+ due to the formation of a centrosymmetric FMN-Hg-FMN triad on SWNT. The use of FMN-SWNT is applicated to organomercurials detection to confirm the extensibility. The binding mode specificity and multimodal changes observed in response to Hg2+ ions suggest that systems based on FMN−SWNT can serve as in vivo NIR beacons for the detection of various mercury derivatives.