Controlling Electronic Energy Structure of Near-IR-Responsive Ag(In,Ga)(S,Se)2 Quantum Dots for In Vivo Bioimaging
Authors: Rismaningsih Nurmanita , Yamauchi Hiroki , Kameyama Tatsuya , Yukawa Hiroshi , Baba Yoshinobu , Uematsu Taro , Kuwabata Susumu , Torimoto Tsukasa 
Published: July 2022

[Introduction]

Since cancer is a leading cause of death worldwide, early diagnostic and treatment observation using biosensors becomes urgent. To date, the development of the biosensors is limited to the utilization of fluorescent probes such as organic dyes or fluorescent proteins, which suffer from various weakness[1]. Therefore, new probes that are more photostable than current organic fluorophores are needed.
Semiconductor called quantum dots (QDs) of a few nanometers in size exhibit unique photochemical properties different from their bulk materials, owing to the quantum size effect. Binary QDs composed of group II−VI or IV−VI semiconductors, such as CdS, CdSe, and PbS, have been intensively investigated because of intense PL, high quantum yield, resistance to photo-bleaching, and broad excitation with narrow emission bands, which made them ideal for high contrast optical imaging of biological systems. However, notable toxicity associated with Cd and Pb causes serious limitations in practical use. Recently, mixed-cation QDs, such as ZnAgInS[2], AgInGaS[3], and AgInGaSe[4], have been an environmentally friendly alternative to conventional binary QDs. Their photochemical properties were controlled through the composition of QDs as well as their particle size. Controlling the composition of QDs containing multinary chalcogen elements, that is, mixed-anion QDs, is also expected to be another strategy to tune their physicochemical properties, but the method to prepare such QDs showing intense PL has not been developed well.
In this study, we report the strategy to prepare Ag(In, Ga)(S, Se)2 (AIGSSe) QDs, of tunable photochemical properties, such as energy gap (Eg) and photoluminescence peak, with the ratio of Se/S in the particles.

[Experimental]

AIGSSe QDs were synthesized by a solution-phase synthesis method. A mixture of AgOAc, In(acac)3, and Ga(acac)3 was used as a metal ion precursor, and that of thiourea and selenourea was used as a chalcogen precursor. These were added to a test tube with a mixture solvent of oleylamine and dodecane thiol, in which the ratio of Se/(Se+S) varied from 0 to 1.0. The solution was heated at 100~250 °C, and then formed QDs were isolated by adding methanol. The resulting wet precipitates were washed several times with ethanol, followed by dissolving in chloroform. Thus-obtained AIGSSe QDs were surface-coated by GaSx shell to improve PL property. Furthermore, these QDs were incorporated into unilamellar liposomes of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) to make them dissolved in aqueous solution before being injected into the back of a mouse, and their PL intensity was measured.

[Results and Discussion]

The absorption onset was red-shifted from 580 nm to 830 nm with an increase in the Se/(S+Se) ratio in preparation, indicating that the Eg of QDs was decreased from 1.9 to 1.5 eV with an increase in the Se/(Se+S) ratio from 0 to 1.0 in QDs. Thus-obtained AIGSSe QDs exhibited broad PL peaks with relatively weak intensities. However, the surface coating of AIGSSe QDs with GaSx shell (AIGSSe@GaSx) remarkably enhanced the intensity of PL peak. The PL quantum yield (QY) of AIGSSe QDs varied in the range of 2~43%, but QDs with each Se/(S+Se) ratio exhibited increased QY after surface coating with GaSx shell. Especially, AIGSSe@GaSx with Se/(S+Se) = 0 had the highest QY value as much as 50%. The PL peak wavelength was red-shifted from 580 nm to 785 nm with a decrease in the Eg of AIGSSe core, the FWHM of PL peak being varied in the range of 0.147 ~ 0.357 eV.
TEM measurement revealed that the AIGSSe@GaSx QDs were spherical, and their average size was almost constant at ca. 4 nm with narrow size distribution, regardless of Se/(Se+S) in preparation. AIGSSe@GaSx QDs with Se/(Se+S) = 0.50 exhibited a sharp PL peak at 785 nm in first biological window, which was suitable for in vivo imaging. These QDs were encapsulated with DSPC liposomes so that the QDs were dispersed in aqueous solution. Thus-obtained AIGSSe@GaSx QD-DSPC liposomes maintained a sharp band-edge PL peak even in an aqueous dispersion, in which the PLQY was slightly decreased from 25% in chloroform to 21% in aqueous dispersions but was high enough to be used as a near-IR PL probe. We successfully detected PL emission from the QDs through the skin of a mouse, and the PL intensity was proportional to the concentration of injected QDs.

[References]

[1] T. Jin, et al., J Mater Chem B, 2020, 8, 10686.
[2] T. Kameyama, et al., J. Phys. Chem. C, 2018, 122, 13705.
[3] T. Kameyama, et al., ACS Appl. Mater. Interfaces, 2018, 10, 42844.
[4] T. Kameyama, et al., ACS Appl. Nano Mater., 2020, 3, 3275.