(708b) Biomineralization of Functional AIS/ZnS Quantum Dots

Semiconductor nanocrystals or quantum dots (QDs) are used for a wide range of applications spanning bioimaging, photocatalysis, LEDs, and solar cells due to their unique optical and electronic properties.¹ While II-VI QDs are popular because of their luminescent properties such as high quantum yield, tunable emission wavelength, and narrow emission spectra, their intrinsic toxicities limit their direct usage in optoelectronics and biological systems.² Alternative, heavy metal free (i.e., green) I-III-VI chalcopyrite multicomponent semiconductors such as AgInS₂ and CuInS₂ have attracted considerable interest as a result of their long PL lifetime and tunable composition, the latter enabling tailored spectroscopic properties spanning the UV to the NIR region.^2,3 Among I-III-VI materials, Ag-In-S (AIS) QDs gain considerable attention due to their non-toxic nature, attractive bandgap (1.87 eV) and Bohr diameter (5.5 nm), and the fact that their intrinsic PL quantum efficiencies (~10%) can be tuned to the levels commensurate with II-VI type QDs by the introduction of wide band gap materials (ZnS) as well as size/composition control.^2,3 The synthesis of such multicomponent QDs generally employs hydrothermal, solvothermal and single precursor thermal decomposition processes.³ The associated high temperatures and/or toxic solvents, however, undercut the overall sustainability of these ‘green’ materials. Low temperature, aqueous phase biomineralization-based routes to size controlled QDs offer an alternative, ‘green’, and potentially scalable⁴ synthesis strategy.⁵

In this study, we have investigated the sequential synthesis of functional AIS/ZnS QDs with tunable optical properties from biomineralized In-S nanocrystals. Specifically, we demonstrate how aqueous-phase, low-temperature enzymatic (cystathionine gamma lyase) turnover of the amino acid L-cysteine yields sufficient endogenous H₂S in buffered solutions of indium chloride for the nucleation of narrowly distributed In-S nanocrystals. Cysteine simultaneously serves as a source of H₂S and stabilizer of the biomineralized In-S throughout the subsequent Ag nitrate-based cation exchange process leading to AIS QDs. We demonstrate how the solution-phase metal precursor ratio (In:Ag) offers a facile knob for directly tuning the composition of the AIS product, and, thereby, the photoluminescence quantum yield (PL QY). We further elucidate and exploit the kinetics of Ag exchange, through controlled incubation, for improving PL lifetime and PL QY, likely by structural annealing. The addition of ZnS more than doubles this PL QY while retaining the optical properties of the AIS core. The resulting AIS/ZnS QDs offer promise for a range of applications. Specifically, we demonstrate their efficacy for visible light photocatalytic H₂ generation. In addition, we confirm the non-toxic nature of the AIS/ZnS QDs through cell viability studies and, upon decoration with key antibodies, show their suitability for targeted bioimaging of THP-1 macrophages. Ultimately, this study advances the versatility of single enzyme biomineralization as a means for the more broadly sustainable and potentially scalable synthesis of various ‘green’, non-toxic metal sulfide semiconductors for applications spanning photocatalysis to bioimaging.