Quantum Dots-Ligand Complex as Ratiometric Fluorescent Nanoprobe for Visual and Specific Detection of G-Quadruplex

By complexing a nonionic G-quadruplex ligand with hybrid dual-emission quantum dots (QDs), a ratiometric fluorescent nanoprobe is developed for G-quadruplex detection in a sensitive and specific manner. The QDs nanohybrid comprised of a green-emission QD (gQD) and multiple red-emission QDs (rQDs) inside and outside of a silica shell, respectively, is utilized as the signal displaying unit. Only the presence of G-quadruplex can displace the ligand from QDs, breaking up the QDs-ligand complexation, and inducing the restoration of the rQDs fluorescence. Since the fluorescence of embedded gQD stays constant, variations of the dual-emission intensity ratios display continuous color changes from green to bright orange, which can be clearly observed by the naked eye. Furthermore, by utilizing competitive binding of a cationic ligand versus the nonionic ligand toward G-quadruplex, the nanoprobe is demonstrated to be applicable for assessing the affinity of a G-quadruplex-targeted anticancer drug candidate, exhibiting ratiometric fluorescence signals (reverse of that for G-quadruplex detection). By making use of the specificity of the ligand binding with G-quadruplex against a double helix, this nanoprobe is also demonstrated to be capable of sensitive detection of one-base mutation, exhibiting sequence-specific ratiometric fluorescence signals. By functionalizing with a nuclear localization peptide, the nanoprobe can be used for visualization of G-quadruplex in the nucleus of human cells.