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Hydrophilic Doped Quantum Dots “Ink” and Their Inkjet‐Printed Patterns for Dual Mode Anticounterfeiting by Reversible Cation Exchange Mechanism

Luminescent quantum dots (QDs) patterns have attracted great attention in anticounterfeiting applications because they can emit fluorescence upon excitation. Here, different from traditional luminescent patterns for anticounterfeiting purposes, a novel strategy based on the reversible cation exchange enabled Ag doped CdS quantum dots (CdS:Ag QDs) is reported to provide a higher security for anticounterfeiting. First, the reversibility of cation exchange enabled doped QDs crystal engineering have been revealed. The switch from nonluminescence to luminescence for several cycles by such a mechanism has been achieved. Second, the hydrophilic doped QDs “ink” can be prepared by surface modification, and then flexible bulk‐sized patterns can be printed on different substrates by inkjet printing technology, such as the parchment paper, PET foil, and paper currency substrates. Third, owing to the characteristic of stable crystal structure of CdS:Ag QDs, the patterns of such doped QDs on substrates can be switched by in situ cation exchange to achieve conversion of the disappearance and recovery of luminescent signals, from which a multiple dual mode anticounterfeiting verification can be provided.