Photoluminescence properties of quinary Ag–(In,Ga)–(S,Se) quantum dots with a gradient alloy structure for in vivo bioimaging

Multinary semiconductor quantum dots (QDs) composed of group I–III–VI elements have received much attention for wide-ranging applications due to their low level of toxicity and tunable optical properties. The chemical composition and particle size are important parameters for controlling the physicochemical properties of these QDs. In this study, we prepared quinary QDs of an Ag–(In,Ga)–(S,Se) semiconductor (AIGSSe) by thermolysis of precursors in an organic solution with two-step heat treatment and evaluated their physicochemical properties as a function of Se fraction. The energy gap (Eg) of QDs decreased from 1.9 to 1.5 eV with an increase in the Se/(S + Se) ratio from 0 to 1, accompanied by a shift of the valence band maximum to a higher energy level. The obtained AIGSSe QDs exhibited a sharp band-edge PL peak, the wavelength of which was red-shifted from 580 to 790 nm with a decrease in their Eg. The PL intensity was remarkably enlarged by the surface coating with a GaSx shell, the highest PL quantum yield being 50% for the PL peak at 580 nm. AIGSSe QDs with an Se/(S + Se) ratio of 0.50 exhibited a near-IR PL peak at 790 nm, which is suitable for in vivo bioimaging in the wavelength range of the first biological window. By incorporating GaSx-coated AIGSSe QDs (Se/(S + Se) = 0.50) into liposomes, the QDs were transferred into aqueous solutions without significant deterioration of both the PL peak outline and quantum yield, though a broad PL peak originating from defect sites appeared with a relatively large PL intensity when GaSx-coated Ag–(In,Ga)–S QDs that did not contain Se were used. In vivo imaging of a mouse was successfully performed by detecting the band-edge emission at 790 nm of AIGSSe QDs injected subcutaneously in the back of the mouse.