Scalable Biomineralization of CdS Quantum Dots by Immobilized Enzyme

Significance 

Quantum dots (QDs) are man-made nanoscale crystals that can transport electrons. When UV/Visible light hits these semiconducting nanocrystals, they can emit light of various colors. These artificial semiconductor nanocrystals have as such found numerous applications in composites, solar cells and fluorescent biological imaging. These properties are due to their unique quantum confinement effects, high extinction coefficients, tunable band gap, high brightness, and widely controllable emission wavelength along with their photostability and lower photobleaching rates relative to organic dyes. Efforts over the past couple of decades to optimize lab-scale synthesis of high-quality QDs have succeeded in improving quantum efficiency, increasing quantum yield, tailoring nanoparticle size, and narrowing the size distribution. Yet, challenges associated with process scaleup have still yet to be adequately addressed. Specifically, the scale-up of laboratory processes requires industrially relevant production methods that sufficiently preserve product quality while meeting stringent demands on process complexity, economics, and environmental impact or process “greenness”. As a result, these factors present a particularly daunting challenge for common QD synthesis routes.

In general, the development of scalable, green, aqueous phase, nanomaterial synthesis routes that do not sacrifice the functional performance of the materials remains a persistent challenge. At present, bioinspired methodologies have proven effective for batchwise synthesis of highly functional nanomaterials but require scalable strategies for the recovery and reuse of costly enzymes. Bearing this in mind, Lehigh University researchers: Nur Koncuy Ozdemir, Joseph Cline, Prof. Christopher Kiely, Prof. Steven McIntosh, and Prof. Mark Snyder, demonstrated that facile immobilization of pyridoxal phosphate (PLP)-dependent cystathionine γ-lyase (CSE) on inexpensive TiO2 supports enables the cyclic biomineralization of size-controlled nanocrystalline cadmium sulfide (CdS) quantum dots (QDs). Their work is currently published in the research journal, ACS Sustainable Chemistry & Engineering.

In their innovative approach, they leveraged the unique irreversibility of CSE physisorption onto TiO2 under nominal synthesis conditions, and used that to demonstrate that the retained activity of the immobilized CSE for turnover of L-cysteine is sufficient for the slow, continuous endogenous production of H2S in buffered solutions of Cd acetate that is required to keep the CdS biomineralization system in the size-focusing regime. On the basis of the established functionality and applicability of biomineralized CdS QDs, the scientists tackled the persistent fundamental to process-scale challenges that have so far limited the commercial viability and sustainability of such QD biomineralization approaches.

The authors reported that, when supplied with pyridoxal phosphate to regenerate the active center, the immobilized enzyme demonstrated no more than ca. 5% activity loss over six cycles while maintaining a tight size distribution of the nanocrystals (1.72 ± 0.44 nm). This confirmed the promise for this approach as a simple, scalable strategy for continuous single-enzyme-based biomineralization of functional nanocrystals.

In summary, the study demonstrated the feasibility of the cyclic synthesis of consistently sized and narrowly distributed CdS QDs by an immobilized single-enzyme, aqueous phase, and low-temperature biomineralization approach. Taken together with the intrinsic multi-purposing of reagents, the simplification of QD purification by enzyme immobilization highlights the promise of the cyclic biomineralization approach for targeted reduction of some of the most common prohibitive cost drivers for QD commercialization. In a statement to Advances in Engineering, Professor Mark Snyder emphasized that the outlined new process offers a potential platform for scalable green biomineralization of CdS QDs as well the possibility for synthesizing QDs, derived from similar enzymatic processes, from across an even broader compositional spectrum.

Reference

Nur Koncuy Ozdemir, Joseph P. Cline, Christopher J. Kiely, Steven McIntosh, Mark A. Snyder. Scalable Biomineralization of CdS Quantum Dots by Immobilized Cystathionine γ‑Lyase. ACS Sustainable Chemistry & Engineering 2020; volume 8, page 15189−15198.

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