Single Enzyme Biomineralization of Semiconductor Nanocrystals

Abstract Biomineralization, the process by which living organisms generate minerals, has recently gained interest as a pathway towards the green synthesis of crystalline materials under ambient conditions for energy and catalytic applications. Semiconductor quantum dots are desirable for their size-tunable optical and electronic properties but their commercial use is currently cost-limited due to the high temperatures and stringent reaction conditions employed during synthesis. Biomineralization offers a low cost, greener approach to synthesis as quantum dots are synthesized in the aqueous phase at ambient temperature and pressure. These nanocrystals are well suited for biological applications as they are capped with DNA, amino acids, or short-chain peptides. This work seeks to better understand the biomineralization of quantum dots by Stenotrophomonas maltophilia through the study of proteins excreted in response to high concentrations of heavy metals. One enzyme, cystathionine ?-lyase (CSE), has been identified as playing the main role in both catalyzing mineralization and controlling growth by continuously generating a reactive sulfur species, H2S, from the amino acid L-cysteine. CSE was then isolated and utilized for the direct, single enzyme synthesis of many types of metal chalcogenide nanocrystals. Specifically, we have prepared and characterized CdS, PbS, and CuInS2 nanocrystals using CSE. Additionally, we demonstrate the biomineralization of core/shell quantum dots (e.g. PbS/CdS and CuInS2/ZnS) using a sequential growth method. Tunable optical properties are confirmed by absorbance and photoluminescence measurements. HRTEM and HAADF are utilized to determine the size distribution and crystal phase of the resulting nanocrystals, while single particle XEDS confirms the composition. The functional properties of these materials are demonstrated by their incorporation into quantum dot sensitized solar cells, as fluorescent markers for the bio-imaging of cancer cells, and as a stable photocatalyst for H2 generation. Lastly, nanocrystal biomineralization by CSE is studied in the context of classical theories for colloidal particle nucleation and growth to better understand the effect of synthesis parameters on the resultant quantum dot populations.