Quantum dots and polymer hybrid composites: new insights into fluorescence switch and turn-on anion sensing

We present a previously unexplored mechanism for a quantum dot (QD) fluorescence switch, and introduce its application to turn-on sensing of anions. A hybrid composite is formed from polyacrylic acid (PAA) and the surface of CdTe QDs through strong coordination interactions between the carboxy groups of PAA and the Cd atoms on the surface of the QDs. Such interactions cause almost complete quenching of the fluorescence of the QDs via the pull effect of PAA. Carbonate, silicate or phosphate anions are then added in the pH 8.2 solution of the QD-PAA composites. These anions are partially hydrolyzed and protonated, and then react with the coordinating oxygen atoms due to the formation of hydrogen bonds. This leads to a decrease in the pull effect of PAA, and eventually gives rise to a strong recovery of fluorescence. The detection limits are 0.29, 0.02, and 0.76 mM, respectively, for carbonate, silicate and phosphate. The surface modulation method presented here provides a novel strategy for the design of QD-based chemosensors for carbonate, silicate or phosphate which otherwise are difficult to detect.