QD-Biopolymer-TSPP Assembly as Efficient BiFRET Sensor for Ratiometric and Visual Detection of Zinc Ion.

In this work, we report a new type of quantum dot (QD)-based fluorescence resonance energy transfer (FRET) assembly and its utility for sensing Zn in different media. The assembly on the QD scaffold is via first coating of poly(dA) homopolymer/double-stranded DNA, followed by loading of meso-tetra(4-sulfonatophenyl)porphine dihydrochloride (TSPP), both of which are electrostatic, offering the advantages of cost-efficiency and simplicity. More importantly, the biopolymer coating minimizes the interfacial thickness to be ≤2 nm for QD-TSPP FRET, which results in improvements of up to 60-fold for single FRET efficiency and nearly 4-fold for total FRET efficiency of the QD-biopolymer-TSPP assemblies in comparison with silica-coating-based QD-TSPP assemblies. On the basis of Zn-chelation-induced spectral modulation, dual-emission QD-poly(dA)-TSPP assemblies are developed as a ratiometric Zn sensor with increased sensitivity and specificity. The sensor either in solution or on a paper substrate displays continuous color changes from yellow to bright green toward Zn, exhibiting excellent visualization capability. By utilizing the competitive displacement of Zn, the sensor is also demonstrated to have good reversibility. Furthermore, the sensor is successfully used to visualize exogenous Zn in living cells. Together the QD-biopolymer-TSPP assembly provides an inexpensive, sensitive, and reliable sensing platform not only for on-site analytical applications but also for high-resolution cellular imaging.