High-Yield Extracellular Biosynthesis of ZnS Quantum Dots through a Unique Molecular Mediation Mechanism by the Peculiar Extracellular Proteins Secreted by a Mixed Sulfate Reducing Bacteria

Abstract

This work describes a high-yield extracellular biosynthesis of ZnS QDs via a unique molecular mediation mechanism driven by the mixed sulfate reducing bacteria (SRB). The mixed SRB have obtained the highest ever ZnS QD biosynthesis rate of 35.0–45.0 g/(L·month). The biogenic ZnS QDs with an average crystallite size (ACS) of 6.5 nm have greater PL activity and better uniformity than that of a chemical route. Peculiar extracellular proteins (EPs) with molecular weights of approximately 65 and 14 kDa specially adhere to the ZnS QDs, which cover extraordinarily high contents of acidic amino acids (14.0 mol % Glu and 13.0 mol % Asp) and of nonpolar amino acids (12.0 mol % Ala, 11.0 mol % Gly, and 7.0 mol % Phe), for novel molecular mediation. The vast amount of negative charges in Glu and Asp guides the strong absorption between the EPs and Zn2+ via electrostatic attraction to reach a maximum absorption capacity of 745.9 mg/g within 2.0 h, motivating large and rapid nucleation as the first step of biosynthesis. Meanwhile, bridging and interlinkage occur inside the EPs or between the EPs via hydrophobic interactions dominated by the nonpolar amino acids, resulting in the formation of massive microcavities to control and restrict the growth of ZnS QDs as a template. The novel molecular mediation mechanism triggered by the peculiar EPs with an extraordinary amino acid composition and structure accounts for the high-yield biosynthesis of ZnS QDs. The mixed SRB have also successfully fabricated other metal sulfide QDs, including PbS, CuS, and CdS, through the novel molecular mediation.

Controlled Biosynthesis of ZnCdS Quantum Dots with Visible-Light-Driven Photocatalytic Hydrogen Production Activity

“…As the first step in the rapid biosynthesis of ZnCdS QDs, negatively charged acidic amino acids provide a large number of adsorption sites for metal ions. A large number of non-polar amino acids make EPs form hydrophobic bridging and smaller cavities, thus controlling the growth of ZnCdS crystals [ 44 ].…”

“…In previous studies, we domesticated, screened, and isolated mixed SRB from sulphate-rich soil consisting of 25% Desulfovibrio sp., 25% Clostridiaceae sp., 25% Proteiniphilum sp., 12.5% Geotoga sp., and 12.5% Sphaerochaeta sp. The SRB, which had harvested the highest-ever extracellular biosynthesis of binary metal sulfide QDs [ 44 ], were applied to fabricate the ternary ZnCdS QDs in a modified, easier, and faster manner in the current work. The growth medium contained 0.1 g/L CaCl 2 , 1.0 g/L NH 4 Cl, 0.5 g/L K 2 HPO 4 , 0.5 g/L MgSO 4 , 0.1 mol/L sodium lactate, and 0.1 mol/L Na 2 SO 4 , at pH 7.2.…”

“…When the SRB reached logarithmic phase after about 4 days of culture, the turbidity of SRB (OD 600 ) increased from 0.1 to 0.6. At that time, the cells were removed by centrifuging at 8000 rpm for 10 min to obtain the supernatant, which contained a massive amount of biogenic H 2 S (S 2− ) from sulfate bio-reduction as well as specific EPs secreted by the SRB [ 44 ]. Subsequently, 200 mL of the SRB-derived active supernatant was mixed with 200 mL of mixed solutions of ZnSO 4 and CdCl 2 with different molar ratios (Zn:Cd = 0:1, 1:1, 3:1, 5:1, 9:1, 15:1, 1:0; total concentration of Zn 2+ and Cd 2+ = 0.1 mol/L).…”

“…Photoluminescence (PL) spectra were afforded at the excitation wavelength of 350 nm performed on a Hitachi F-4500 fluorescence spectrometer (Tokyo, Japan). According to standard procedures (ISO 13903-2005), the amino acid types and concentrations in the proteins that mediated the synthesis of ZnCdS QDs were analyzed using an amino acid analyzer (L-8900, Hitachi High-Tech, Toyko, Japan) [ 44 ].…”

“…There are many research works on the biosynthesis of ZnS and CdS QDs as binary semiconductors, but the biosynthesis of ternary ZnCdS QDs has not yet been reported, due to the difficulty in the stable synthesis of ternary materials in complex biological systems. In previous work, the highest extracellular synthesis rate of ZnS QDS was obtained by mixed sulfate-reducing bacteria (SRB), which secreted extracellular proteins (EPs) containing amino acids to strongly mediate the biosynthesis process of binary metal sulfides QDs [ 44 ]. This study provides a simple and efficient method for the extracellular biosynthesis of ZnCdS QDs by SRB for the first time.…”