Highly efficient multiple exciton generation in colloidal PbSe and PbS quantum dots

We report ultra-efficient multiple exciton generation (MEG) for single photon absorption in colloidal PbSe and PbS quantum dots (QDs). We employ transient absorption spectroscopy and present measurement data acquired for both intraband as well as interband probe energies. Quantum yields of 300% indicate the creation, on average, of three excitons per absorbed photon for PbSe QDs at photon energies that are four times the QD energy gap. Results indicate that the threshold photon energy for MEG in QDs is twice the lowest exciton absorption energy. We find that the biexciton effect, which shifts the transition energy for absorption of a second photon, influences the early time transient absorption data and may contribute to a modulation observed when probing near the lowest interband transition. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs, and we also introduce a new model for MEG based on the coherent superposition of multiple excitonic states.

Selenocompounds in plants and animals and their biological significance

There are several selenocompounds in tissues of plants and animals. Selenate is the major inorganic selenocompound found in both animal and plant tissues. Selenocysteine is the predominant selenoamino acid in tissues when inorganic selenium is given to animals. Selenomethionine is the major selenocompound found initially in animals given this selenoamino acid, but is converted with time afterwards to selenocysteine. Selenomethionine is the major selenocompound in cereal grains, grassland legumes and soybeans. Selenomethionine can also be the major selenocompound in selenium enriched yeast, but the amount can vary markedly depending upon the growth conditions. Se-methylselenocysteine is the major selenocompound in selenium enriched plants such as garlic, onions, broccoli florets and sprouts, and wild leeks.

Biomagnification of cadmium selenide quantum dots in a simple experimental microbial food chain

Previous studies have shown that engineered nanomaterials can be transferred from prey to predator, but the ecological impacts of this are mostly unknown. In particular, it is not known if these materials can be biomagnified-a process in which higher concentrations of materials accumulate in organisms higher up in the food chain. Here, we show that bare CdSe quantum dots that have accumulated in Pseudomonas aeruginosa bacteria can be transferred to and biomagnified in the Tetrahymena thermophila protozoa that prey on the bacteria. Cadmium concentrations in the protozoa predator were approximately five times higher than their bacterial prey. Quantum-dot-treated bacteria were differentially toxic to the protozoa, in that they inhibited their own digestion in the protozoan food vacuoles. Because the protozoa did not lyse, largely intact quantum dots remain available to higher trophic levels. The observed biomagnification from bacterial prey is significant because bacteria are at the base of environmental food webs. Our findings illustrate the potential for biomagnification as an ecological impact of nanomaterials.

Selenium contaminated waters: An overview of analytical methods, treatment options and recent advances in sorption methods

Selenium is an essential trace element for many organisms, including humans, but it is bioaccumulative and toxic at higher than homeostatic levels. Both selenium deficiency and toxicity are problems around the world. Mines, coal-fired power plants, oil refineries and agriculture are important examples of anthropogenic sources, generating contaminated waters and wastewaters. For reasons of human health and ecotoxicity, selenium concentration has to be controlled in drinking-water and in wastewater, as it is a potential pollutant of water bodies. This review article provides firstly a general overview about selenium distribution, sources, chemistry, toxicity and environmental impact. Analytical techniques used for Se determination and speciation and water and wastewater treatment options are reviewed. In particular, published works on adsorption as a treatment method for Se removal from aqueous solutions are critically analyzed. Recent published literature has given particular attention to the development and search for effective adsorbents, including low-cost alternative materials. Published works mostly consist in exploratory findings and laboratory-scale experiments. Binary metal oxides and LDHs (layered double hydroxides) have presented excellent adsorption capacities for selenium species. Unconventional sorbents (algae, agricultural wastes and other biomaterials), in raw or modified forms, have also led to very interesting results with the advantage of their availability and low-cost. Some directions to be considered in future works are also suggested.

Selenium characterization in the global rice supply chain

For up to 1 billion people worldwide, insufficient dietary intake of selenium (Se) is a serious health constraint. Cereals are the dominant Se source for those on low protein diets, as typified by the global malnourished population. With crop Se content constrained largely by underlying geology, regional soil Se variations are often mirrored by their locally grown staples. Despite this, the Se concentrations of much of the world's rice, the mainstay of so many, is poorly characterized, for both total Se content and Se speciation. In this study, 1092 samples of market sourced polished rice were obtained. The sampled rice encompassed dominant rice producing and exporting countries. Rice from the U.S. and India were found to be the most enriched, while mean average levels were lowest in Egyptian rice: approximately 32-fold less than their North American equivalents. By weighting country averages by contribution to either global production or export, modeled baseline values for both were produced. Based on a daily rice consumption of 300 g day(-1), around 75% of the grains from the production and export pools would fail to provide 70% of daily recommended Se intakes. Furthermore, Se localization and speciation characterization using X-ray fluorescence (micro-XRF) and X-ray absorption near edge structure (micro-XANES) techniques were investigated in a Se-rich sample. The results revealed that the large majority of Se in the endosperm was present in organic forms.

Selective growth of PbSe on one or both tips of colloidal semiconductor nanorods

PbSe nanocrystals with rock-salt structure are grown on the tips of colloidal CdS and CdSe nanorods. The facets of wurtzite rods provide a substrate with various degrees of reactivity for the growth of PbSe. The presence of dangling Cd bonds may explain subtle differences between nonequivalent facets resulting in the selective nucleation of PbSe only on one of the two tips of each CdS rod. This approach has the potential to facilitate the fabrication of heterostructures with tailored optical and electronic properties.

Metabolic pathway for selenium in the body: speciation by HPLC-ICP MS with enriched Se

Selenium (Se) is an ultramicro essential nutrient and both inorganic (selenite and selenate) and organic (selenocysteine and selenomethionine) forms of Se can be used as nutritional sources. Metabolic pathways for Se in the body were studied for selenite and selenate, with the use of enriched 82Se, by speciation with separation by gel filtration HPLC and detection by element-specific mass spectrometry with ionization with inductively coupled argon plasma (HPLC-ICP MS). The concentrations of 82Se in organs and body fluids and the distributions of their constituents depending on the dose and time after the intravenous administration of 82Se-selenite and -selenate to rats were determined. Selenite was taken up by red blood cells within several minutes, reduced to selenide by glutathione, and then transported to the plasma, bound selectively to albumin and transferred to the liver. Contrary to selenite, intact selenate was either taken up directly by the liver or excreted into the urine. The 82Se of selenite origin and that of selenate origin were detected in the forms of the two Se peak materials in the liver, A and B. The former one was methylated to the latter in vivo and in vitro. The latter one was identical with the major urinary metabolite and it was identified as Se-methyl-N-acetyl-selenohexosamine (selenosugar). The chemical species-specific metabolic pathway for Se was explained by the metabolic regulation through selenide as the assumed common intermediate for the inorganic and organic Se sources and as the checkpoint metabolite between utilization for the selenoprotein synthesis and methylation for the excretion of Se.

Ultimate selenium(IV) monitoring and removal from water using a new class of organic ligand based composite adsorbent

This work reports the selenium (Se(IV)) detection and removal from water by ligand functionalized organic-inorganic based novel composite adsorbent. The composite adsorbent was prepared by direct immobilization of N,N'-di(3-carboxysalicylidene)-3,4-diamino-5-hydroxypyrazole onto the mesoporous silica monolith. The adsorbent exhibited distinct color change in the presence of various concentrations of Se(IV). This was characterized by UV-vis spectroscopy, and the color change was observed by naked-eye observation. The detection limit was determined to be 1.14 μg/L. The effect of solution pH, interferential metal ions, contact time, initial Se(IV) concentration, and adsorbent regeneration were evaluated. The maximum sorption capacity was determined based on the initial concentration. The data fitted well to the Langmuir isotherm model, and the maximum Se(IV) sorption capacity was 111.12 mg/g. The presence of diverse competing ions did not affect the Se(IV) sorption capacity, and the adsorbent had almost no sorption capacity for these coexisting ions, which suggests the high selectivity to Se(IV) ions. The adsorbed Se(IV) was eluted with suitable eluent (0.10 M NaOH) and simultaneously regenerated into the initial form for the next operation. The excellent reusability of the adsorbent was justified after eight consecutive sorption-elution-regeneration cycles. The proposed adsorbent is cost-effective and environmentally friendly and a potential candidate for treatment of water containing Se(IV).

High-performance liquid chromatography of selenium compounds utilizing perfluorinated carboxylic acid ion-pairing agents and inductively coupled plasma and electrospray ionization mass spectrometric detection

Increasing speciation demands in clinical chemistry, toxicology and nutrition have made the determination of the total elements in a sample inadequate; the amount of an element and the chemical forms in which it is present need to be known. Inductively coupled plasma mass spectrometry (ICP-MS) was used after high-performance liquid chromatographic (HPLC) separation, as was electrospray ionization mass spectrometry (ESI-MS). The effect of variation of the number of carbon atoms in perfluorinated carboxylic acids used as ion-pairing agents for the separation of selenium compounds was examined. Trifluoroacetic acid (0.1%), pentafluoropropanoic acid (0.1%) or heptafluorobutanoic acid (0.1%; HFBA) were alternatively used as additives to methanol-water (1:99, v/v) solutions as mobile phases. Reversed-phase HPLC-ICP-MS with 0.1% HFBA in the mobile phase allowed more than 20 selenium compounds to be separated in 70 min in an isocratic elution mode; the separation of natural selenium-enriched sample extracts was examined and explained. The pH of the 0.1% HFBA solution was modified with hydrochloric acid or ammonia and the pH of the sample extracts before injection was modified in order to overcome unwanted double peak formation in the chromatograms of sample extracts. Oxidations of standard gamma-glutamyl-Se-methylselenocysteine and Se-methylselenocysteine were carried out using 30% H2O2 solution and identifications of selenium-containing oxidation products were made using HPLC-ICP-MS and HPLC-ESI-MS. The principal organic oxidation product in both cases was methaneseleninic acid (MeSeO2H).

Colloidal CdSe quantum wires by oriented attachment

We report here a relatively low temperature (100-180 degrees C) synthetic route to high-quality and single-crystalline CdSe nanowires using air-stable and generic chemicals. The diameter of nanowires was controlled and varied in an exceptionally small size regime, between 1.5 and 6 nm. This was achieved by using alkylamines, a single type or a mixture of two different types of amines, with different chain lengths and varying the reaction temperature. The experimental results suggest the coexistence of two types of fragments in the prewire aggregates, known as "pearl-necklace" or "string-of-pearls" in the literature, which are loosely associated and chemically fused sections.

Selenium speciation in soil and rice: influence of water management and Se fertilization

Rice (Oryza sativa) is the staple food for half of the world's population, but the selenium (Se) concentrations in rice grain are low in many rice-growing regions. This study investigated the effects of water management on the Se speciation dynamics in the soil solution and Se uptake and speciation in rice in a pot experiment. A control containing no Se or 0.5 mg kg(-1) of soil of selenite or selenate was added to the soil, and plants were grown under aerobic or flooded conditions. Flooding soil increased soluble Se concentration when no Se or selenite was added to the soil, but decreased it markedly when selenate was added. Selenate was the main species in the +selenate treatment, whereas selenite and selenomethionine selenium oxide were detected in the flooded soil solutions of the control and +selenite treatments. Grain Se concentration was 49% higher in the flooded than in the aerobic treatments without Se addition. In contrast, when selenate or selenite was added, the aerobically grown rice contained 25- and 2-fold, respectively, more Se in grain than the anaerobically grown rice. Analysis of Se in rice grain using enzymatic hydrolysis followed by HPLC-ICP-MS and in situ X-ray absorption near-edge structure (XANES) showed selenomethionine to be the predominant Se species. The study showed that selenate addition to aerobic soil was the most effective way to increase Se concentration in rice grain.

Unequal functional redundancy between the two Arabidopsis thaliana high-affinity sulphate transporters SULTR1;1 and SULTR1;2

* In Arabidopsis, SULTR1;1 and SULTR1;2 are two genes proposed to be involved in high-affinity sulphate uptake from the soil solution. We address here the specific issue of their functional redundancy for the uptake of sulphate and for the accumulation of its toxic analogue selenate with regard to plant growth and selenate tolerance. * Using the complete set of genotypes, including the wild-type, each one of the single sultr1;1 and sultr1;2 mutants and the resulting double sultr1;1-sultr1;2 mutant, we performed a detailed phenotypic analysis of root length, shoot biomass, sulphate uptake, sulphate and selenate accumulation and selenate tolerance. * The results all ordered the four different genotypes according to the same functional hierarchy. Wild-type and sultr1;1 mutant plants displayed similar phenotypes. By contrast, sultr1;1-sultr1;2 double-mutant plants showed the most extreme phenotype and the sultr1;2 mutant displayed intermediate performances. Additionally, the degree of selenate tolerance was directly related to the seedling selenate content according to a single sigmoid regression curve common to all the genotypes. * The SULTR1;1 and SULTR1;2 genes display unequal functional redundancy, which leaves open for SULTR1;1 the possibility of displaying an additional function besides its role in sulphate membrane transport.

Simultaneous monitoring of volatile selenium and sulfur species from se accumulating plants (wild type and genetically modified) by GC/MS and GC/ICPMS using solid-phase microextraction for sample introduction

A sensitive method for determining ultratrace volatile Se species produced from Brassica juncea seedlings is described. The use of a new commercially available GC/ ICPMS interface in conjunction with solid-phase micro-extraction is a promising way to perform these studies. The addition of optional gases (O2 and N2) to the argon discharge proved to increase the sensitivity for Se and S as well as for Xe, which as a trace contaminant gas, was used for ICPMS optimization studies. However, the optimization parameters differ when an optional gas is added. In the best conditions, limits of detection ranging from 1 to 10 ppt can be obtained depending on the Se compound and 30 to 300 ppt for the volatile S species. The use of GC/MS with similar sample introduction permits the characterization of several unknown species produced as artifacts from the standards. The method allows the virtually simultaneous monitoring of S and Se species from the headspace of several plants (e.g., onions, garlic, etc.) although the present work is focused on the B. juncea seedlings grown in closed vials and treated with Se. Dimethyl selenide and dimethyl diselenide were detected as the primary volatile Se components in the headspace. Sulfur species also were present as allyl (2-propenyl) isothiocyanate and 3-butenyl isothiocyanate as characterized by GC/MS.

Toxicity of quantum dots and cadmium salt to Caenorhabditis elegans after multigenerational exposure

To fully understand the biological and environmental impacts of nanomaterials requires studies that address both sublethal end points and multigenerational effects. Here, we use a nematode to examine these issues as they relate to exposure to two different types of quantum dots, core (CdSe) and core-shell (CdSe/ZnS), and to compare the effect to those observed after cadmium salt exposures. The strong fluorescence of the core-shell QDs allowed for the direct visualization of the materials in the digestive track within a few hours of exposure. Multiple end points, including both developmental and locomotive, were examined at QD exposures of low (10 mg/L Cd), medium (50 mg/L Cd), and high concentrations (100 mg/L Cd). While the core-shell QDs showed no effect on fitness (lifespan, fertility, growth, and three parameters of motility behavior), the core QDs caused acute effects similar to those found for cadmium salts, suggesting that biological effects may be attributed to cadmium leaching from the more soluble QDs. Over multiple generations, we commonly found that for lower life-cycle exposures to core QDs the parents response was generally a poor predictor of the effects on progeny. At the highest concentrations, however, biological effects found for the first generation were commonly similar in magnitude to those found in future generations.

On-Line HPLC-UV/Nano-TiO2-ICPMS System for the Determination of Inorganic Selenium Species

We have developed an UV/nano-TiO2 vapor generation (VG) device that when coupled between a chromatographic column and an ICP mass spectrometer provides a simple and sensitive hyphenated method for the determination of Se(IV) and Se(VI) without the need to use conventional chemical VG techniques. Because our proposed VG device allows both Se(IV) and Se(VI) species in the column effluent to be converted on-line into volatile Se products, which are then measured directly by the ICPMS, the safety risks and the probability of contamination arising from the use of additional chemicals are both low. To achieve the maximum signal intensity, we optimized a number of the operating parameters of the UV/nano-TiO2 VG device, including the acidity, the amounts of TiO2 and formic acid, and the length of the reaction coil, with respect to their effects on the reduction efficiency of the analyte species. This hyphenated method achieves excellent detection limits-0.06 and 0.03 ng mL(-1) for Se(IV) and Se(VI), respectively-because of the high efficiencies of the conversions of Se(IV) and Se(VI) to their respective volatile products and the lower blank level achieved, relative to those of other traditional systems. In addition, because the conversion efficiency of the analyte selenium species reached its maximum level within 36 s of irradiation, the working time (approximately 12 min) was limited primarily by time required for the chromatographic separation. A series of validation experiments-analysis of the 1643e Standard Reference Material and natural water samples-indicated that our proposed methods can be applied satisfactorily to the determination of inorganic selenium species in water samples.

Speciation of seleno compounds in yeast aqueous extracts by three-dimensional liquid chromatography with inductively coupled plasma mass spectrometric and electrospray mass spectrometric detection

A three-dimensional liquid chromatographic purification protocol based on sequential size-exclusion, anion-exchange and cation-exchange separation mechanisms was developed for the mapping of seleno compounds in aqueous yeast extracts. The method allowed the demonstration of the presence of more than 30 different seleno compounds. Semi-preparative size-exclusion and anion-exchange chromatography were optimized for maximum resolution using electrospray-compatible buffers in order to purify the compounds for mass spectrometric analysis. Molecular masses were attributed to many of the compounds on the basis of the selenium isotopic pattern in the electrospray mass spectra and of the collision-induced fragmentation patterns. Limitations preventing the ultimate identification of the selenium species detected are discussed.

Large enhancement of fluorescence efficiency from CdSe/ZnS quantum dots induced by resonant coupling to spatially controlled surface plasmons

Nanoengineered fluorescent response is reported from semiconductor core-shell (CdSe/ZnS) quantum dots in proximity to the surface plasmon polariton field of periodic Ag nanoparticle arrays. Tuning the surface plasmon polariton resonance to the quantum dot exciton emission band results in an enhancement of up to approximately 50-fold in the overall fluorescence efficiency, in a design where each Ag nanoparticle is interconnected by a continuous Ag thin film. Propagating modes of surface plasmon resonances have a direct impact on the fluorescence enhancement.

High-performance liquid chromatography of selenoamino acids and organo selenium compounds. Speciation by inductively coupled plasma mass spectrometry

As part of an ongoing study to identify selenium compounds with cancer chemopreventive activity, extracts of selenium-enriched samples were analyzed by HPLC-inductively coupled plasma (ICP)-MS. Ion-exchange, ion pair and derivatization methods for reversed-phase HPLC were considered and advantages and disadvantages for each compared. Anion exchange allows separation of selenite and selenate, but otherwise provides poor separation. Pre-column derivatization and reversed-phase chromatography provides separation of compounds with terminal amine functionalities, but many other species elute in the void volume. The ion pair method gave optimal separation and was compatible with standard ICP-MS operating conditions.

Improving selenium extraction by sequential enzymatic processes for Se-speciation of selenium-enriched Agaricus bisporus

Sample preparation methods based on the use of proteolytic and cell wall digesting enzymes for the speciation analysis of selenized mushroom were investigated. The sample (Agaricus bisporus; 160 microg total Se per g sample) was grown on compost supplemented with selenized yeast. Experiments were carried out to elucidate the possible role of the cell wall digesting enzymes--Lysing enzyme and Driselase--in the improvement of extraction efficiency with and without inhibiting proteolysis during cell wall digestion. A 3-step procedure applying Lysing enzyme and pronase gave the highest extraction efficiency (89%); however, the best species recovery was achieved by a one-step proteolytic procedure. All the procedures of selenium speciation were controlled by independent ICP-AES analysis measuring the total amount of selenium.

Selenium distribution and speciation in plant parts of wheat (Triticum aestivum) and Indian mustard (Brassica juncea) from a seleniferous area of Punjab, India

The concentration, distribution, and speciation of selenium in different parts of wheat and Indian mustard, grown in a seleniferous area in Punjab, were investigated using synchrotron based (XAS) and classical acid digestion and extraction methods. The analyses revealed a high Se enrichment in all investigated plant parts, with Se levels in the range of 133-931 mg/kg (dry weight, dw). Such high Se enrichment is mainly due to the considerable amounts of easily available Se detected in the soil, which are renewed on a yearly basis to some extent via irrigation. Speciation analysis in soil and plants indicated selenate and organic Se as major Se species taken up by plants, with a minor presence of selenite. The analyses also revealed that the highest Se enrichment occurs in the upper plant parts, in agreement with the high uptake rate and mobility of selenate within plants. In both wheat and mustard, highest Se enrichments were found in leaves (387 mg/kg·dw in wheat and 931 mg/kg·dw in mustard). Organic species (dimethylselenide and methylselenocysteine) were found in different parts of both plants, indicating that an active detoxification response to the high Se uptake is taking place through methylation and/or volatilization. The high proportion of selenate in wheat and mustard leaves (47% and 70%, respectively) is the result of the inability of the plant metabolism to completely transform selenate to non-toxic organic forms, if oversupplied. Methylselenocysteine, a common Se species in accumulating plants, was detected in wheat, suggesting that, in the presence of high Se concentration, this plant develops similar response mechanisms to accumulator plants.

Uptake and speciation of selenium in garlic cultivated in soil amended with symbiotic fungi (mycorrhiza) and selenate

The scope of the work was to investigate the influence of selenate fertilisation and the addition of symbiotic fungi (mycorrhiza) to soil on selenium and selenium species concentrations in garlic. The selenium species were extracted from garlic cultivated in experimental plots by proteolytic enzymes, which ensured liberation of selenium species contained in peptides or proteins. Separate extractions using an aqueous solution of enzyme-deactivating hydroxylamine hydrochloride counteracted the possible degradation of labile selenium species by enzymes (such as alliinase) that occur naturally in garlic. The selenium content in garlic, which was analysed by ICP-MS, showed that addition of mycorrhiza to the natural soil increased the selenium uptake by garlic tenfold to 15 microg g(-1) (dry mass). Fertilisation with selenate and addition of mycorrhiza strongly increased the selenium content in garlic to around one part per thousand. The parallel analysis of the sample extracts by cation exchange and reversed-phase HPLC with ICP-MS detection showed that gamma-glutamyl-Se-methyl-selenocysteine amounted to 2/3, whereas methylselenocysteine, selenomethionine and selenate each amounted to a few percent of the total chromatographed selenium in all garlic samples. Se-allyl-selenocysteine and Se-propyl-selenocysteine, which are selenium analogues of biologically active sulfur-containing amino acids known to occur in garlic, were searched for but not detected in any of the extracts. The amendment of soil by mycorrhiza and/or by selenate increased the content of selenium but not the distribution of detected selenium species in garlic. Finally, the use of two-dimensional HPLC (size exclusion followed by reversed-phase) allowed the structural characterisation of gamma-glutamyl-Se-methyl-selenocysteine and gamma-glutamyl-Se-methyl-selenomethionine in isolated chromatographic fractions by quadrupole time-of-flight mass spectrometry.

Selective detection and identification of Se containing compounds—review and recent developments

The complexity of selenium (Se) chemistry in the environment and in living organisms presents broad analytical challenges. The selective qualitative and quantitative determination of particular species of this element is vital in order to understand selenium's metabolism and significance in biology, toxicology, clinical chemistry and nutrition. This calls for state-of-the-art analytical techniques such as hyphenated methods that are reviewed with particular emphasis on interfaced separation with element-selective detection and identification of the detected selenium compounds. Atomic spectral element specific detection for monitoring chromatographic eluent enabled quantitative determination of selenium species in selenized yeast and qualitative measurement for breath samples. Gas chromatography with atomic emission detection (AED) of ethylated species and fluoroacid ion pair HPLC applied to the analysis of currently produced or archived selenized yeast and Brassica juncea have revealed the presence of a previously unrecognised Se-S amino acid, S-(methylseleno)cysteine.

Surveying selenium speciation from soil to cell--forms and transformations

The aim of this review is to present and evaluate the present knowledge of which selenium species are available to the general population in the form of food and common supplements and how these species are metabolized in mammals. The overview of the selenium sources takes a horizontal approach, which encompasses identification of new metabolites in yeast and food of plant and animal origin, whereas the survey of the mammalian metabolism takes a horizontal as well as a vertical approach. The vertical approach encompasses studies on dynamic conversions of selenium compounds within cells, tissues or whole organisms. New and improved sample preparation, separation and detection methods are evaluated from an analytical chemical perspective to cover the progress in horizontal speciation, whereas the analytical methods for the vertical speciation and the interpretations of the results are evaluated from a biological angle as well.

Aerobic and anaerobic biosynthesis of nano-selenium for remediation of mercury contaminated soil

Selenium (Se) nanoparticles are often synthesized by anaerobes. However, anaerobic bacteria cannot be directly applied for bioremediation of contaminated top soil which is generally aerobic. In this study, a selenite-reducing bacterium, Citrobacter freundii Y9, demonstrated high selenite reducing power and produced elemental nano-selenium nanoparticles (nano-Se⁰) under both aerobic and anaerobic conditions. The biogenic nano-Se⁰ converted 45.8-57.1% and 39.1-48.6% of elemental mercury (Hg⁰) in the contaminated soil to insoluble mercuric selenide (HgSe) under anaerobic and aerobic conditions, respectively. Addition of sodium dodecyl sulfonate enhanced Hg⁰ remediation, probably owing to the release of intracellular nano-Se⁰ from the bacterial cells for Hg fixation. The reaction product after remediation was identified as non-reactive HgSe that was formed by amalgamation of nano-Se⁰ and Hg⁰. Biosynthesis of nano-Se⁰ both aerobically and anaerobically therefore provides a versatile and cost-effective remediation approach for Hg⁰-contaminated surface and subsurface soils, where the redox potential often changes dramatically.