Tunable NIR-II Emitting Silver Chalcogenide Quantum Dots using Thio/Selenourea Precursors: Preparation of MRI/NIR-II Multimodal Imaging Agent
Aqueous-stable, Cd- and Pb- free colloidal quantum dots with fluorescence properties in the second near-infrared region (NIR-II, 1000-1400) are highly desirable for non-invasive deep-tissue optical imaging and biosensing. The low band-gap semiconductor, silver chalcogenide offers a non-toxic and stable alternative to existing Pd, As, Hg and Cd-based NIR-II colloidal quantum dots (CQDs). We report facile access to NIR-II emission windows with Ag2X (X=S, Se) CQDs using easy-to-prepare thio/selenourea precursors and their analogues. Aqueous phase transfer of these CQDs with a high conservation of fluorescence quantum yield (retention up to ~90%) and colloidal stability is demonstrated. Bimodal NIR-II/MRI contrast agent with tunable fluorescence and high T1 relaxivity 408 mM−1 s−1 per QD (size~ 2.2 nm) and 990 mM−1 s−1 per QD (size~ 4.2 nm) have been prepared by grafting 50 and 120 monoaqua Gd (III) complexes respectively to two different sized Ag2S CQDs. The size of the nanocrystals is crucial for tuning Gd payload and the relaxivity.