(536c) Scalable Biomineralization of CdS Quantum Dots By Immobilized Cystathionine ?-Lyase

The typical high-temperature, organic phase synthesis of quantum dots (QDs) leads to respective high energy demand and expensive and toxic chemical usage that limit the process scalability required to meet the production needs for promising commercial QD applications.^1–3 Biomineralization-based routes to size-controlled QDs offer an alternative, ‘green’, and potentially scalable synthesis strategy, characterized by low temperatures and aqueous phase processing. Biomineralization of size-tunable CdS QDs, for example, is possible by the aqueous-phase, low-temperature enzymatic (cystathionine gamma lyase) turnover of the amino acid L-Cysteine to H₂S in buffered solutions of cadmium acetate.⁴ 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 cystathionine γ-lyase (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, albeit at an expectedly reduced level relative to free enzyme. Following a 2 hr induction, 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, the 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.

References

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2. Lim, H. et al. Continuous Purification of Colloidal Quantum Dots in Large-Scale Using Porous Electrodes in Flow Channel. Sci. Rep. 7, 43581 (2017).
3. Yang, H.-J. & Tuan, H.-Y. Efficient and scalable synthesis of quantum dots using hexane as the solvent in a non-microfluidic flow reactor system. RSC Adv. 4, 51926–51934 (2014).
4. Spangler, L. C., Cline, J. P., Kiely, C. J. & McIntosh, S. Low temperature aqueous synthesis of size-controlled nanocrystals through size focusing: a quantum dot biomineralization case study. Nanoscale 10, 20785–20795 (2018).