Quantum dot–layered double hydroxide composites for near-infrared emitting codes†
Near-infrared (NIR)-emitting PbS-based quantum dots (QDs) and exfoliated layered double hydroxide (LDH) nanosheets were self-assembled to form photostable composites for NIR-emitting codes. PbS QDs prepared by a pyrolysis synthetic method or PbS–CdS (core–shell) QDs prepared by a cation exchange using PbS QDs were transferred to aqueous solutions by surface ligand exchange to dihydrolipoic acid (DHLA). The DHLA-decorated QDs and exfoliated LDH nanosheets were assembled by electrostatic interactions during vortexing. The QD–LDH composite films prepared by drop-casting exhibit more than 500% higher NIR PL intensities than the counterpart QD films (without LDH matrices) under identical conditions (i.e., QD concentration). Photostability tests under continuous light irradiation reveal that the PbS–CdS QD films are more photostable than the PbS QD films. Formation of the QD–LDH composites gives rise to further enhancement in the photostability of the film. The photostable PbS–CdS QD–LDH composite films were used to create novel NIR-emitting codes. NIR fluorescence images of a pattern of two kinds of PbS–CdS QD–LDH composite films were recorded with an InGaAs CCD camera equipped with optical filters.