Tunable NIR-II emitting silver chalcogenide quantum dots using thio/selenourea precursors: preparation of an MRI/NIR-II multimodal imaging agent

Abstract

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 (QDs). We report facile access to NIR-II emission windows with Ag₂X (X = S, Se) QDs using easy-to-prepare thio/selenourea precursors and their analogues. The aqueous phase transfer of these QDs with a high conservation of fluorescence quantum yield (retention up to ∼90%) and colloidal stability is demonstrated. A bimodal NIR-II/MRI contrast agent with a tunable fluorescence and high T₁ relaxivity of 408 mM⁻¹ s⁻¹ per QD (size ∼ 2.2 nm) and 990 mM⁻¹ s⁻¹ per QD (size ∼ 4.2 nm) has been prepared by grafting 50 and 120 monoaqua Gd(III) complexes respectively to two differently sized Ag₂S QDs. The size of the nanocrystals is crucial for tuning the Gd payload and the relaxivity.