The typical high-temperature, organic phase synthesis of quantum dots (QDs) leads to high energy and solvent costs as well as toxic chemical usage, limiting process scalability requirements for commercial QD applications. Biomineralization-based routes to size-controlled QDs offer an alternative, ‘green’, and potentially scalable synthesis strategy, characterized by low temperature aqueous phase processing. Biomineralization of size-tunable CdS QDs, for example, is possible by the low-temperature, aqueous-phase enzymatic (cystathionine gamma lyase; CSE) turnover of the amino acid L-Cysteine to H2S in buffered solutions of Cd-acetate. Leveraging this and similar processes directly for scalable QD synthesis, however, is limited by the lack of cost-effective or feasible strategies for enzyme recovery and reuse. Enzyme immobilization on easily recoverable substrates offers a strategy for their facile separation from product solutions and, thereby, enzyme reuse. In this study, we demonstrate the efficacy of Pyridoxal phosphate (PLP)-dependent CSE, immobilized on nanoparticulate TiO2 substrates, for the cyclic synthesis of CdS QDs via a single-enzyme biomineralization approach. Specifically, we exploit the strong physical adsorption of CSE on nanoparticulate TiO2 aggregates for facile CSE immobilization. This allows for cyclic enzyme recovery from reaction solutions and resuspension in fresh reactant. The immobilized CSE remains active for L-cysteine turnover to H2S in buffered solutions of Cd-acetate. Following a 2 hr induction period, related to finite L-cysteine adsorption on TiO2, the immobilized CSE retains ca. 55% of its activity over 6 cycles. We also demonstrate how the activity of the immobilized CSE can be sustained or regenerated by simply dosing PLP into the fresh reaction solution. For example, PLP addition at the start of each cycle increases nominal specific activity of the immobilized CSE, and enables retention of ca. 86% of the post-induction activity over 6 cycles by mitigating substrate inhibition. While the higher enzyme activity in the case of PLP dosing leads to slightly larger CdS quantum dots relative to cycling in the absence of added PLP, CdS QDs in each case display a remarkably consistent particle size and degree of monodispersity across all post-induction cycles. This facile strategy for immobilized enzyme-mediated QD synthesis opens exciting possibilities for the scalable, ‘green’ biosynthesis of consistently sized QDs.

Scalable biomineralization of CdS quantum dots by immobilized cystathionine γ-lyase

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.