Scalable Biomineralization of CdS Quantum Dots by Immobilized Cystathionine γ-Lyase

Abstract

The development of scalable, green, aqueous phase, nanomaterial synthesis routes that do not sacrifice the functional performance of the materials remains a persistent challenge. Bioinspired methodologies have proven effective for batchwise synthesis of highly functional nanomaterials but require scalable strategies for the recovery and reuse of costly enzymes. Herein, we demonstrate that facile immobilization of pyridoxal phosphate (PLP)-dependent cystathionine γ-lyase (CSE) on inexpensive TiO2 supports enables the cyclic biomineralization of size-controlled nanocrystalline CdS quantum dots (QDs). When supplied with PLP to regenerate the active center, the immobilized enzyme demonstrates 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 demonstrates the promise for this approach as a simple, scalable strategy for continuous single-enzyme-based biomineralization of functional nanocrystals.

Sequential, low-temperature aqueous synthesis of Ag–In–S/Zn quantum dots via staged cation exchange under biomineralization conditions

“…The evolution of the spectra over only the initial 5 h incubation period, namely the increase in intensity and red shift, is consistent with the estimated time for CSE-based turnover of approximately all free cysteine according to our earlier investigation of nominal CSE kinetics. 43 Tauc plot analysis estimates a corresponding band gap energy of the resulting indium sulfide particles of 3.8 eV, consistent with the formation of quantum confined In 2 S 3 particles 71,72 given the 2.1–2.3 eV band gap reported for bulk In 2 S 3 . 25,73…”

“…The biomineralization of binary QDs can be accomplished by low-temperature enzymatic (CSE) turnover of l -cysteine to endogenous H 2 S in buffered aqueous solutions of metal salts. 43–47 Direct extension of this scalable, 43 ‘green’, one-pot aqueous technique to the synthesis of ternary QDs requires compatible stability and reactivity of the various metal salt precursors under optimal enzymatic conditions ( i.e. , pH ∼ 9, 37 °C).…”

“…This must be done in order to offset the loss of cysteine occurring during Ag + exchange as a result of its turnover by any persistently active CSE and its dimerization to form cystine. 43 Screening of cysteine dosing concentrations (4–32 mM) revealed the need to dose cysteine at a concentration of 32 mM prior to introduction of the zinc acetate precursor (Scheme 1a ( t 2 )) in order for the biomineralization solutions of In/Ag ∼ 5 AIS QDs to remain stable over the course of the 30 min incubation employed for Zn 2+ addition. Quantitative interpretation of the stabilization mechanism resulting from the concomitant molar ratio of Cys/Zn ∼ 5 is complicated by the numerous complexes that Zn 2+ forms under biomineralization conditions.…”

“…42 Alternatively, extracellular single enzyme-based biomineralization of size-tunable metal sulfide QDs (e.g., CdS, ZnS, PbS, CuInS 2 and CuZnSnS 4 ) has been demonstrated through the continuous turnover of L-cysteine to reactive HS À by an engineered strain of cystathionine g-lyase (CSE) in the presence of buffered solutions of metal salts. [43][44][45][46][47][48][49] Such enzymatic systems offer the benefits of biological processing without facing the purification challenges of cell-based systems.…”

“…The intrinsic biocompatibility over relevant timescales and ease of functionalizing the AIS/Zn QDs with targeting antibodies consequently offer the possibility for their broader customization based on unique receptor-antibody combinations characteristic of other cellular systems. More generally, the critical insight into cation exchange under biomineralization conditions advances the versatility of single enzyme biomineralization as a basis for the broadly sustainable and potentially scalable 43 synthesis of various 'green', non-toxic multi-component metal sulfide semiconductor QDs for a wide range of applications.…”