Biogenic ZnS Research Articles

Photocatalytic degradation of Congo Red by zinc sulfide quantum dots produced by anaerobic granular sludge

ABSTRACT Sulfate reducing bacteria present in anaerobic granular sludge mediate the metabolic conversion of sulfate to sulfide. In the presence of heavy metals, sulfides precipitate as metal sulfides. In this study, dissimilatory sulfate reduction was coupled to the precipitation of zinc as ZnS quantum dots (QDs) at ambient conditions. The biogenic ZnS QDs had average sizes of 5–7 nm and were formed within 2–4 days of incubation. X-ray diffraction analysis indicated that the biosynthesised ZnS QDs possessed a crystalline cubic lattice structure. The organics present during ZnS biosynthesis were characterized using 3D-fluorescence excitation–emission measurements (FEEM) and the presence of an organic coating on the biogenic ZnS QDs was affirmed using FTIR analysis. The UV–visible absorption spectra of the samples exhibited a prominent absorption peak below 325 nm, which is the characteristic of the surface plasmon resonance of ZnS QDs. The band gap energy of the biogenic ZnS QDs was estimated to be 3.84 eV, comparable to the values reported for chemically synthesised ZnS QDs. The direct band gap energy indicates a large redox potential and carrier mobility, which capacitate the application of these QDs as effective photocatalysts for the photo-assisted decolourization of dyes, as illustrated for the dye congo red.

Biogenic design of ZnS quantum dots - Insights into their in-vitro cytotoxicity, photocatalysis and biosensing properties

Zinc sulphide Quantum Dots (QDs) are semiconductor nanoparticles that have profound applications in medical imaging, photocatalysis and biosensing. Of late, green synthesis of ZnS QDs has emerged as an useful substitute for the routine chemical synthesis. The present study reports the biosynthesis of ZnS QDs using the fungus Aspergillus sp. under ambient conditions. In the present study, the mechanism behind the antibacterial action of the biosynthesized ZnS Quantum Dots has been explored. It was found that biogenesis using fungus has resulted in the synthesis of spherical crystalline ZnS particles of mean diameter 6.3 nm. FTIR analysis indicated the presence of active organic functional groups on the surface of the QDs. The optical and structural features of the biogenic ZnS were found to be comparable with that of the chemically synthesized quantum dots. Thereafter, the antibacterial activities of the biosynthesized ZnS QDs were observed against pathogenic bacteria. The green synthesized ZnS exhibited excellent antibacterial activity and were quantified using filter paper bioassay. Experimental evidences indicated that the antibacterial strategy relied on bacterial cell membrane disruption, leakage of the cytoplasmic contents and associated ROS mediated oxidative damages leading to bacterial cell lysis. Further, the prospects of biogenic ZnS for photodegradation of dyes and sensing of heavy metals in aqueous phase were investigated.

Solar assisted photocatalytic degradation of organic pollutants in the presence of biogenic fluorescent ZnS nanocolloids

The main aim of this study was to ascertain the photocatalytic degradation of organic pollutants present in aqueous phase using fluorescent biogenic ZnS nanocolloids produced from an endophytic fungus Aspergillus flavus. The degradation studies were carried out using different organic pollutants such as methyl violet (MV), 2,4-dichlorophenoxyacetic acid (2,4-D) and paracetamol (PARA) for 120 min, 270 min and 240 min, respectively, at pH varying from 3.0 to 11.0. The results from this study indicate that the degradation efficiency of ZnS nanocolloids for MV, 2,4-D and PARA were 87%, 33% and 51%, respectively, at the optimum concentration of 100 mg/L of the tested organic pollutants. At different time intervals, the samples were analyzed for their chemical oxygen demand (COD) and total organic carbon (TOC) contents. The reduction of COD and TOC were 78% and 74% for MV at 120 min; 55.5% and 57.2% for 2,4-D at 270 min and 47.6% and 44.5% for PARA at 240 min, respectively. The degradation pathway was determined based on the mass spectrum and the intermediates formed; in addition, the interaction between organic pollutants and nanocolloids was also elucidated based on atomic force microscopy (AFM) and fluorescence spectrum.

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.

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.

Bioinspired ZnS:Gd Nanoparticles Synthesized from an Endophytic Fungi Aspergillus flavus for Fluorescence-Based Metal Detection.

Recently, several nonconventional sources have emerged as strong hotspots for the biosynthesis of chalcogenide quantum dots. However, studies that have ascertained the biomimetic methodologies that initiate biosynthesis are rather limited. The present investigation portrays a few perspectives of rare-earth(Gd)-doped ZnS biosynthesis using the endophytic fungi Aspergillus flavus for sensing metals based on their fluorescence. Analysis of ZnS:Gd nanoparticles was performed by elemental analysis, energy-dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), photoluminescence (PL), and transmission electron microscopy (TEM). The results of TEM demonstrated that the particles were polycrystalline in nature, with a mean size of 10–18 nm. The fluorescence amenability of the biogenic ZnS nanoparticles was further used for the development of a simple and efficient sensing array. The results showed sensitive and detectable quenching/enhancement in the fluorescence of biogenic colloidal ZnS nanoparticles, in the presence of Pb (II), Cd (II), Hg (II), Cu (II) and Ni (II), respectively. The fluorescence intensity of the biogenic ZnS:Gd nanoparticles was found to increase compared to that of the ZnS nanoparticles that capacitate these systems as a reliable fluorescence sensing platform with selective environmental applications.

Bio-inspired ZnS quantum dots as efficient photo catalysts for the degradation of methylene blue in aqueous phase

ZnS quantum dots are semiconductor nanoparticles characterized by superior optoelectronic properties. The amenability of these nanoparticles in initiating efficient photo-redox reactions enable their use as photocatalysts for environmental remediation. In the present study, ZnS quantum dots were biosynthesized from Zn tolerant Penicillium sp. under ambient reaction conditions. The biogenic ZnS quantum dots were characterized using TEM, SEM, XRD, FTIR and UV–Vis absorption studies that revealed the formation of spherical particles of average diameter 11.08 nm with a zinc blend crystal structure and the optical properties were in par with chemically synthesized ZnS. The biogenic ZnS were then used for the photodegradation of methylene blue (MB) dye. It was found that the green synthesized ZnS quantum dots exhibited good photocatalytic activity with a half-life of 4 h. Further, the dye degradation efficiency was enhanced as the ZnS nanocatalyst/dye ratio increased and reached equilibrium within 6 h. The present study reports an inexpensive and scalable method to fabricate ZnS nano hybrids with practical applicability in the remediation of pollutants in textile, paper and dyeing industry.

Influence of Structural Defects on Biomineralized ZnS Nanoparticle Dissolution: An in-Situ Electron Microscopy Study.

The dissolution of metal sulfides, such as ZnS, is an important biogeochemical process affecting fate and transport of trace metals in the environment. However, current studies of in situ dissolution of metal sulfides and the effects of structural defects on dissolution are lacking. Here we have examined the dissolution behavior of ZnS nanoparticles synthesized via several abiotic and biological pathways. Specifically, we have examined biogenic ZnS nanoparticles produced by an anaerobic, metal-reducing bacterium Thermoanaerobacter sp. X513 in a Zn-amended, thiosulfate-containing growth medium in the presence or absence of silver (Ag), and abiogenic ZnS nanoparticles were produced by mixing an aqueous Zn solution with either H2S-rich gas or Na2S solution. The size distribution, crystal structure, aggregation behavior, and internal defects of the synthesized ZnS nanoparticles were examined using high-resolution transmission electron microscopy (TEM) coupled with X-ray energy dispersive spectroscopy. The characterization results show that both the biogenic and abiogenic samples were dominantly composed of sphalerite. In the absence of Ag, the biogenic ZnS nanoparticles were significantly larger (i.e., ∼10 nm) than the abiogenic ones (i.e., ∼3-5 nm) and contained structural defects (e.g., twins and stacking faults). The presence of trace Ag showed a restraining effect on the particle size of the biogenic ZnS, resulting in quantum-dot-sized nanoparticles (i.e., ∼3 nm). In situ dissolution experiments for the synthesized ZnS were conducted with a liquid-cell TEM (LCTEM), and the primary factors (i.e., the presence or absence structural defects) were evaluated for their effects on the dissolution behavior using the biogenic and abiogenic ZnS nanoparticle samples with the largest average particle size. Analysis of the dissolution results (i.e., change in particle radius with time) using the Kelvin equation shows that the defect-bearing biogenic ZnS nanoparticles (γ = 0.799 J/m2) have a significantly higher surface energy than the abiogenic ZnS nanoparticles (γ = 0.277 J/m2). Larger defect-bearing biogenic ZnS nanoparticles were thus more reactive than the smaller quantum-dot-sized ZnS nanoparticles. These findings provide new insight into the factors that affect the dissolution of metal sulfide nanoparticles in relevant natural and engineered scenarios, and have important implications for tracking the fate and transport of sulfide nanoparticles and associated metal ions in the environment. Moreover, our study exemplified the use of an in situ method (i.e., LCTEM) to investigate nanoparticle behavior (e.g., dissolution) in aqueous solutions.

Photocatalytic properties of zinc sulfide nanocrystals biofabricated by metal-reducing bacterium Shewanella oneidensis MR-1

Accumulation and utilization of heavy metals from wastewater by biological treatment system has aroused great interest. In the present study, a metal-reducing bacterium Shewanella oneidensis MR-1 was used to explore the biofabrication of ZnS nanocrystals from the artificial wastewater. The biogenic H2S produced via the reduction of thiosulfate precipitated the Zn(II) as sulfide extracellularly. Characterization by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and field emission scanning electron microscope (FESEM) confirmed the precipitates as ZnS nanocrystals. The biogenic ZnS nanocrystals appeared spherical in shape with an average diameter of 5nm and mainly aggregated in the medium and cell surface of S. oneidensis MR-1. UV–vis DRS spectra showed ZnS nanoparticles appeared a strong absorption below 360nm. Thus, the photocatalytic activity of ZnS was evaluated by the photodegradation of rhodamine B (RhB) under UV irradiation. The biogenic ZnS nanocrystals showed a high level of photodegradation efficiency to RhB coupled with a significant blue-shift of maximum adsorption peak. A detailed analysis indicated the photogenerated holes, rather than hydroxyl radicals, contributed to the photocatalytic decolorization of RhB. This approach of coupling biosynthesis of nanoparticles with heavy metal removal may offer a potential avenue for efficient bioremediation of heavy metal wastewater.

Pseudomorphs of barite and biogenic ZnS after phyto-crystals of calcium oxalate (whewellite) in the peat layer of a poor fen.

Pseudomorphs of barite (BaSO4) and Cd-rich ZnS after whewellite (CaC2O4·H2O) occur within remnants of Scots pine bark tissues in the peat layer of a poor fen located near a zinc smelter in south Poland. A two-step formation of the pseudomorphs is postulated based on SEM observations: (1) complete dissolution of whewellite, possibly caused by oxalotrophic bacteria, and (2) subsequent bacterially induced precipitation of barite and spheroidal aggregates of ZnS together with galena (PbS) in voids left by the dissolved whewellite crystals. Local increase in pH due to microbial degradation of whewellite, elevated concentrations of Zn(II) and Ba(II) in pore water due to the decomposition of atmospheric particles of sphalerite and barite in the acidic (pH 3.5–3.8) environment, oxidation of S species during drying and rewetting of the peat layer, and subsequent partial reduction of sulfate anions by sulfur-reducing bacteria were all factors likely involved in the crystallization of ZnS and barite in the microenvironment of the post-whewellite voids.

Structure and reactivity of zinc sulfide precipitates formed in the presence of sulfate-reducing bacteria

The biologically mediated formation of metal sulfide precipitates in anoxic sediments represents a potentially important mechanism for the sequestration of toxic metals. Current knowledge of the structure and reactivity of these biogenic metal sulfides is scarce, limiting the ability to effectively assess contaminant sequestration in, and remobilization from, these solids. In this study, SO4-reducing bacteria (Desulfovibrio sp.) were grown for 5 days in a high-SO4, minimal metal media amended with Zn at either 30 or 300 micromolar. Zinc speciation in the reactor solids was determined using X-ray absorption spectroscopy, and the results compared to spectra of known metal sulfide mineral phases and freshly formed metal sulfides synthesized through purely chemical processes. Biogenically mediated Zn sulfides showed significantly more short range crystallographic order than the abiotically prepared amorphous precipitates. The presence of dissolved Fe2+ at similar concentrations did not affect the nature of the Zn precipitates formed. The biogenic ZnS solids were also more resistant to re-oxidation than the chemical precipitates but more soluble than sphalerite mineral samples. These results suggest that Zn sulfides formed in anaerobic sediments are likely to be more resistant to re-oxidation than would be expected based on dissolution of Fe sulfides and/or sediment acid volatile sulfides.

Submicron-scale isotopic variations within biogenic ZnS record the mechanism and kinetics of extracellular metal-sulfide biomineralization

Submicron-scale isotopic variations within biogenic ZnS record the mechanism and kinetics of extracellular metal-sulfide biomineralization