Suitable extraction of soils and sediments for mercury species and determination combined with the cold vapor generation atomic absorption spectrometry technique

N- and S-codoped carbon quantum dots for enhancing fluorescence sensing of trace Hg2

Carbon-quantum-dot-based fluorescence sensing of Hg2+ is a well-known cost-effective tactic with fast response and high sensitivity, while rationally constructing heteroatom-doped carbon quantum dots with improved fluorescence sensing performances through tuning the electronic and chemical structures of the reactive site still remains a challenging project for monitoring trace Hg2+ in aquatic ecosystems to avoid harm resulting from its high toxicity, nonbiodegradabilty and accumulative effects on human health. Herein, intriguing N,S-codoped carbon quantum dots were synthesized via a facile one-step hydrothermal procedure. As an admirable fluorescent probe with plentiful heteroatom-related functional groups, these N,S-codoped carbon quantum dots can exhibit an absolute fluorescence quantum yield as high as 11.6%, excellent solubility and stability over three months, remarkable sensitivity for Hg2+ detection with an attractive detection limit of 0.27 μg L-1 and admirable selectivity for Hg2+ against thirteen other metal ions. Density functional theory calculations reveal that electron-enriched meta-S of the unique graphitic N with homocyclic meta-thiophene sulfur structure can regulate this N site to have more electrons and preferable affinity towards Hg, hence achieving enhanced fluorescence quenching due to greater charge transfer from N to Hg after the coordination interaction. This strategy provides a promising avenue for precisely designing purpose-made quantum dots with the dedicated fluorescence sensing applications.

Covalent organic framework-loaded silver nanoparticles as robust mimetic oxidase for highly sensitive and selective colorimetric detection of mercury in blood

Covalent organic frameworks (COFs) were fabricated with a hierarchical flower-like hollow structure, possessing a large specific surface area, high porosity, and excellent environmental stability. In situ growth of noble silver nanoparticles (AgNPs) onto COFs was conducted yielding COF-Ag nanozymes. The structural advantages of COFs can ensure the uniform dispersion and effective size control of AgNPs. More interestingly, the oxidase-like catalytic activity of the obtained COF-Ag nanozymes could be enhanced in the presence of Hg²⁺ ions, which could specifically interact with AgNPs to form Ag-Hg alloys. A COF-Ag catalysis-based colorimetric platform was thereby constructed for highly selective and sensitive analysis of Hg²⁺ ions, showing a linear concentration range from 0.050 to 10.0 μM, with a limit of detection of about 3.7 nM. Besides, the developed colorimetric strategy was successfully applied for detecting Hg²⁺ ions in human blood with favorable detection recoveries, indicating its potential for applications in the biomedical analysis, environmental monitoring, and food safety fields.

A signal-on fluorescent biosensor for mercury detection based on a cleavable phosphorothioate RNA fluorescent probe and metal-organic frameworks

Mercury contamination is a major environmental concern. In this work, we used a cleavable phosphorothioate (PS) fluorescence probe quenched by UiO-66-NH₂ to develop a "signal-on" fluorescent biosensor for Hg²⁺ detection. The probe was bound to UiO-66-NH₂ through π-π stacking and hydrogen bonding, thereby extinguishing the fluorescence of the FAM-labelled probe. The PS site was cleaved in the presence of Hg²⁺, releasing the FAM group and significantly enhancing the fluorescence signal. The intensity of the fluorescence linearly rose as the Hg²⁺ concentration increased in the range of 1-100 nM (R² = 0.994), and the limit of detection was 0.118 nM (S/N = 3). This biosensor demonstrated high selectivity for Hg²⁺ and was effectively applied to quantification of Hg²⁺ in various water samples with acceptable recovery rates. These results suggest that this practical, straightforward technology is a good option for monitoring mercury ions in the environment.

Assignation of inorganic mercury and methylmercury mass fractions in a soil matrix certified reference material by two analytical methodologies based on species-specific isotope dilution mass spectrometry and chromatographic separation

Assignment of inorganic mercury and methyl mercury mass fractions at an ultra-trace level in soil certified reference material EnvCRM 03 with a complex matrix composition was undertaken. Inorganic mercury and methyl mercury contents by species-specific isotope dilution inductively coupled plasma MS with on-line HPLC or with classical off-line chromatography were established. Different extraction protocols: sequential extraction ((1) H₂ SO₄ /KBr/CuSO₄ ; (2) dichloromethane; (3) Na₂ S₂ O₃ ) or one-step extraction (diluted HCl) in solid-liquid systems were verified. Sequential extraction allowed quantification and separation of inorganic mercury and methyl mercury on HPLC column in one chromatographic run and were found to be (316 ± 10) μg kg^-1 (U = 3.2%, k = 2) and (0.53 ± 0.02) μg kg^-1 (U = 3.8%, k = 2), respectively. Extraction by diluted HCl and application of classical off-line chromatography led to the separation of methyl mercury from predominant inorganic mercury form and was found to be (0.54 ± 0.03) μg kg^-1 (U = 5.4%, k = 2). To the best-obtained literature knowledge, there was no available soil material aimed for speciation analysis of inorganic mercury and methyl mercury so far. Both developed analytical methodologies were found to be equally sensitive and could be successfully applied for mercury species determination in samples with the complex matrix. This article is protected by copyright. All rights reserved.

A portable dual-mode colorimetric platform for sensitive detection of Hg2+ based on NiSe2 with Hg2+-Activated oxidase-like activity

The nanozyme-based colorimetric strategy for heavy metal detection has broad application prospects nowadays. However, the inefficient recognition capabilities of nanozyme sensors for targets hinder its further application. Herein, the authors synthesize bare nickel selenide (NiSe₂) via a one-step hydrothermal reaction, in which the Se element possesses a strong binding ability with mercury (Hg). As expected, NiSe₂ exhibits oxidase-like activity in the presence of Hg²⁺, that is, Hg²⁺ can enhance the oxidase-like activity of NiSe₂. The enhanced mechanism is the accelerated electron transfer between NiSe₂-Hg²⁺ and substrate caused by the formation of Hg-Se bonds. Besides, the oxidase-like activity of NiSe₂ exhibits excellent selectivity, sensitivity and stability in response to Hg²⁺, which enables NiSe₂-Hg²⁺ to efficiently oxidize colorless TMB to blue TMB even in harsh environments. Based on this, a dual-mode colorimetric sensor integrating solution reaction and test paper is developed for the detection of Hg²⁺. In the Hg²⁺ concentration range of 10-700 nM, the colorimetric platform presents a liner response to Hg²⁺, which can reach a low LOD of 5.18 nM in solution reaction and 8.42 nM in the test paper. The proposed strategy can also be applied to real water samples with good recovery and excellent self-calibration capability.

Combination of Vortex Agitation and Ultrasonic Irradiation for Mercury Removal from Sediment by Acid Extraction

The removal of Hg from solid waste plays a key role in environmental protection. In this study, a fast, simple, and effective method for the removal of Hg by acid extraction, using a combination of vortex agitation and ultrasonic irradiation along with hydrobromic acid (HBr), was established using Hg-containing solid wastes. The optimal durations of vortex agitation and ultrasonic irradiation with 48% v/v of HBr were 5 and 6 min, respectively, and total Hg (T-Hg) extraction was achieved within 21 min. The proposed method was validated using the Certified Reference Material (CRM) ERM CC580 (estuarine sediment) and CRM NMIJ 7302-a (marine sediment). Under the optimized conditions, the efficiency rates of T-Hg extraction in both CRMs were 99.5% and 94.2%, with repeatabilities of 3.21% RSD and 2.31% RSD, respectively. The proposed extraction method can also be used for the remediation of Hg in other environmental matrices.

A novel colorimetric sensor for naked-eye detection of cysteine and Hg2+ based on "on-off" strategy using Co/Zn-grafted mesoporous silica nanoparticles

In an attempt to explore the significance of inorganic mimetic enzymes as sensors, this study introduces a naked-eye analytical sensing platform for the detection of L-cysteine (cys), mercury ions (Hg²⁺) based on (turn off/turn-on) catalytic activity of zinc and cobalt grafted mesoporous silica nanoparticles (MSNs). To this end, Zn-MSN and Co/Zn-MSN catalysts were synthesized and characterized using XRD, FT-IR, FESEM, TEM, and nitrogen adsorption-desorption methods. Then, using the intrinsic peroxidase-like activity of as-synthesized samples, the oxidation reactions of the chromogenic substrate (2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS)) was designed using H₂O₂, which produced green colored cation radical of ABTS. Considering the high peroxidase-like activity of Co/Zn-MSN in comparison to Zn-MSN, it was employed to detect cys and then Hg²⁺. The results indicated that the strong interaction between cys and Co/Zn-MSN was proved by a limit of detection (LOD) down to 0.24 nM and the linear relationship from 0.8-50 nM (turn off). Given the fact that Hg²⁺ has a high-affinity tendency to combine with cys, we were suggested a novel colorimetric path for sensing of Hg²⁺ in the presence of cys (turn on). Based on this method, LOD was found 0.17 nM with the linear range of 0.57-50 nM. Taken together, results showed that the as-prepared catalysts are superior to other nanoparticles as a sensor to measure the target molecules in biological monitoring and clinical diagnostics.

Simple and Equipment-Free Paper-Based Device for Determination of Mercury in Contaminated Soil

This work presents a simple and innovative protocol employing a microfluidic paper-based analytical device (µPAD) for equipment-free determination of mercury. In this method, mercury (II) forms an ionic-association complex of tetraiodomercurate (II) ion (HgI₄²⁻_(aq)) using a known excess amount of iodide. The residual iodide flows by capillary action into a second region of the paper where it is converted to iodine by pre-deposited iodate to liberate I_2(g) under acidic condition. Iodine vapor diffuses across the spacer region of the µPAD to form a purple colored of tri-iodide starch complex in a detection zone located in a separate layer of the µPAD. The digital image of the complex is analyzed using ImageJ software. The method has a linear calibration range of 50–350 mg L⁻¹ Hg with the detection limit of 20 mg L⁻¹. The method was successfully applied to the determination of mercury in contaminated soil and water samples which the results agreed well with the ICP-MS method. Three soil samples were highly contaminated with mercury above the acceptable WHO limits (0.05 mg kg⁻¹). To the best of our knowledge, this is the first colorimetric µPAD method that is applicable for soil samples including mercury contaminated soils from gold mining areas.

Reusable, facile, and rapid aptasensor capable of online determination of trace mercury

Herein, we reported a homemade waveguide-based evanescent wave aptasensor for the facile online monitoring of mercury pollution. The aptasensor exploited the high selectivity of hairpin structure-based thymidine-Hg²⁺-thymidine coordination chemistry (T-T mismatch) for Hg²⁺ recognition and the stably regenerable capability of DNA-functionalized waveguide surfaces. The presence of Hg²⁺ caused the T-T mismatch of Cy5.5-labeled T-rich single-stranded DNA sequences. The formed hairpin structures blocked the further hybridization of T-rich single-stranded DNA sequences with the complementary DNA strands that are modified on the waveguide surface; this phenomenon was accompanied by the decrease in the fluorescent signals excited by the evanescent wave. The limit of detection in real water samples was determined to be 0.2 μg/L, which was comparable with that of 0.4 μg/L in an ultrapure water under controlled conditions. And the linear range was observed from 1.4 µg/L to 240.7 µg/L. The negligible environmental matrix effect on the performance ensured the reliability of the proposed aptasensor. Moreover, the cross reactivity of this method toward other investigated metal ions was negligible. Through the delicate surface modification with DNA molecules covalently, the chip was reused at least 31 times with a relative standard deviation (RSD) of less than 19%. A Hg²⁺ pollution accident was successfully detected within 30 min, shedding new light in pollution monitoring, environment restoration, and emergency treatment.

Nanozyme-based sensing platforms for detection of toxic mercury ions: An alternative approach to conventional methods

Mercury (Hg) is known as a poisonous heavy metal which stimulates a wide range of adverse effects on the human health. Therefore, development of some feasible, practical and highly sensitive platforms would be desirable in determination of Hg²⁺ level as low as nmol L^-1 or pmol L^-1. Different approaches such as ICP-MS, AAS/AES, and nanomaterial-based nanobiosensors have been manipulated for determination of Hg²⁺ level. However, these approaches suffer from expensive instruments and complicated sample preparation. Recently, nanozymes have been assembled to address some disadvantages of conventional methods in the detection of Hg²⁺. Along with the outstanding progress in nanotechnology and computational approaches, pronounced improvement has been attained in the field of nanozymes, recently. To accentuate these progresses, this review presents an overview on the different reports of Hg²⁺-induced toxicity on the different tissues followed by various conventional approaches validated for the determination of Hg²⁺ level. Afterwards, different types of nanozymes like AuNPs, PtNPs for quantitative detection of Hg²⁺ were surveyed. Finally, the current challenges and the future directions were explored to alleviate the limitation of nanozyme-based platforms with potential engineering in detection of heavy metals, namely Hg²⁺. The current overview can provide outstanding information to develop nano-based platforms for improvement of LOD and LOQ of analytical methods in sensitive detection of Hg²⁺ and other heavy metals.

Mercury speciation, transformation, and transportation in soils, atmospheric flux, and implications for risk management: A critical review

Mercury (Hg) is a potentially harmful trace element in the environment and one of the World Health Organization's foremost chemicals of concern. The threat posed by Hg contaminated soils to humans is pervasive, with an estimated 86 Gg of anthropogenic Hg pollution accumulated in surface soils worldwide. This review critically examines both recent advances and remaining knowledge gaps with respect to cycling of mercury in the soil environment, to aid the assessment and management of risks caused by Hg contamination. Included in this review are factors affecting Hg release from soil to the atmosphere, including how rainfall events drive gaseous elemental mercury (GEM) flux from soils of low Hg content, and how ambient conditions such as atmospheric O₃ concentration play a significant role. Mercury contaminated soils constitute complex systems where many interdependent factors, including the amount and composition of soil organic matter and clays, oxidized minerals (e.g. Fe oxides), reduced elements (e.g. S^2-), as well as soil pH and redox conditions affect Hg forms and transformation. Speciation influences the extent and rate of Hg subsurface transportation, which has often been assumed insignificant. Nano-sized Hg particles as well as soluble Hg complexes play important roles in soil Hg mobility, availability, and methylation. Finally, implications for human health and suggested research directions are put forward, where there is significant potential to improve remedial actions by accounting for Hg speciation and transportation factors.

Electrogenerated chemiluminescence biosensor for detection of mercury (II) ion via target-triggered manipulation of DNA three-way junctions

A new electrogenerated chemiluminescence (ECL) biosensor is fabricated for the determination of mental ion incorporating DNA three-way junction structure (DNA-TWJ). As a model system, Hg²⁺ was chosen as an analyte. The ECL biosensor was fabricated by covalently coupling Hg²⁺ special DNA-TWJ tagged with ruthenium (II) complex (Ru) (named TW/Ru-J1) to the surface of glassy carbon electrode that had been covalently modified with 4-aminobenzoic acid via electrochemical oxidations. Upon binding of Hg²⁺ to the TW/Ru-J1, the confirmation of TW/Ru-J1 changed and induced Ru away from surface of electrode and thus led to a low ECL signal. The signal linearly decreases with the concentration of Hg²⁺ in the range from 0.1 pM to 10 pM with a detection limit of 0.04 pM This study could be easily extended to various analytical platforms for the detection of many kinds of analytes or their interactions such as DNA/RNA, DNAzyme/target, aptamer/target, and antibody/antigen.

Removal of inorganic mercury by selective extraction and coprecipitation for determination of methylmercury in mercury-contaminated soils by chemical vapor generation inductively coupled plasma mass spectrometry (CVG-ICP-MS)

A procedure is developed for selective extraction of methylmercury (CH₃Hg⁺) from heavily Hg-contaminated soils and sediments for determination by chemical vapor generation inductively coupled plasma mass spectrometry (CVG-ICP-MS). Soils artificially contaminated with 40 μg g^-1 inorganic mercury (Hg²⁺) or methylmercury chloride (CH₃HgCl) were agitated by shaking or exposing to ultrasounds in dilute hydrochloric acid (HCl) or nitric acid (HNO₃) solutions at room temperature. Extractions in HCl (5 or 10% v/v) resulted in substantial leaching of Hg²⁺ from soils, whereas 5% (v/v) HNO₃ provided selectivity for quantitative extraction of CH₃Hg⁺ with minimum Hg²⁺ leaching. Agitation with ultrasounds in 5% (v/v) HNO₃ for about 3 min was sufficient for extraction of all CH₃Hg⁺ from soils. Coprecipitations with Fe(OH)₃, Bi(OH)₃ and HgS were investigated for removal of residual Hg²⁺ in soil extracts. Hydroxide precipitations were not effective. Thiourea or l-cysteine added to soil extracts prior to hydroxide precipitation improved precipitation of Hg²⁺, but also resulted in removal of CH₃Hg⁺. HgS precipitation was made with dilute ammonium sulfide solution, (NH₄)₂S. Adding 30 μL of 0.35 mol L^-1 (NH₄)₂S to soil extracts in 5% (v/v) HNO₃ resulted in removal of all residual Hg²⁺ without impacting CH₃Hg⁺ levels. Vapor generation was carried out by reacting Hg²⁺-free soil extracts with 1% (m/v) NaBH₄. No significant interferences were observed from (NH₄)₂S on the vapor generation from CH₃Hg⁺. The slopes of the calibration curves for CH₃HgCl standard solutions in 5% (v/v) HNO₃ with and without (NH₄)₂S were similar. Limits of detection (LOD, 3s method) were around 0.08 μg L^-1 for 5% (v/v) HNO₃ blanks (n = 10) and 0.10 μg L^-1 for 5% (v/v) HNO₃ + 0.005 mol L^-1 (NH₄)₂S blanks (n = 10). Percent relative standard deviation (%RSD) for five replicate measurements varied between 3.1% and 6.4% at 1.0 CH₃HgCl level. The method is validated by analysis of two certified reference materials (CRM); purely Methylmercury sediment (SQC1238, 10.00 ± 0.291 ng g^-1 CH₃Hg⁺) and Hg-contaminated Estuarine sediment (ERM - CC580, 75 ± 4 ng g^-1 CH₃Hg⁺ and 132 ± 3 μg g^-1 total Hg). CH₃Hg⁺ values for SQC1238 were between 13.0 and 13.2 ng g^-1, and 79 and 81 ng g^-1 for ERM - CC580. Hg-contaminated soils (57-96 μg g^-1 total Hg) collected from the floodplains of Oak Ridge, TN were analyzed for CH₃Hg⁺ using the procedure by CVG-ICPMS. CH₃Hg⁺ levels ranged from 30 to 51 ng g^-1 and did not correlate with total Hg levels (R² = 0.01).

Catalase based hydrogen peroxide biosensor for mercury determination by inhibition measurements

A new amperometric hydrogen peroxide enzyme inhibition biosensor for the indirect determination of toxic mercury ions, Hg²⁺, based on catalase immobilized on a glassy carbon electrode surface by cross-linking with glutaraldehyde and bovine serum albumin, is reported. The parameters influencing biosensor performance were optimized, including enzyme loading, the amount of hydrogen peroxide, the applied potential and electrolyte pH. It was shown that the inhibition of catalase by Hg²⁺ species is irreversible, with a linear inhibition response between 5×10^-11 and 5×10^-10M. The limit of detection calculated as 10% inhibition was 1.8×10^-11M and is the lowest reported until now. Electrochemical impedance spectroscopy was successfully used as a diagnostic of inhibition. Interferences from other heavy metal ions and organic pesticides were evaluated and the inhibition showed very good selectivity towards Hg²⁺. The method was successfully applied to the determination of mercury ions in different types of water sample.

Fluorescence probe for mercury(ii) based on the aqueous synthesis of CdTe quantum dots stabilized with 2-mercaptoethanesulfonate

A new capping ligand, 2-mercaptoethanesulfonate (MES), for CdTe quantum dots passivation provide high selectivity and sensitivity in Hg(ii) determination.

A Highly Sensitive and Selective Catalytic Determination of Mercury in Environmental Samples

A simple, selective and highly sensitive spectrophotometric method has been developed for mercury determination utilizing its catalytic effect on the isoniazid-hexacyanoferrate (II) reaction. The paper presents for the first time (1) the catalytic effect of Hg (I) on the cited ligand substitution reactions and (2) the activating effect of thiourea on the behavior of mercury. The reaction was monitored spectrophotometrically at 423 nm using the initial rate method. The optimized reaction conditions were 5.0 mmol L(-1) hexacyanoferrate (II), 0.5 mmol L(-1) isoniazid, 150 mmol L(-1) citrate buffer (pH 3.30 ± 0.05), and 0.2 mmol L(-1) thiourea, at 50°C. Linear calibration graphs were obtained for 1-100 and 1-55 µg L(-1) with detection limits, based on the 3Sb-criterion, of 1.2 and 1.8 µg L(-1) of Hg (II) and Hg (I), respectively. The method was conveniently applied to samples of wastewaters, inactivated vaccines, and frozen Bass fish fillet, without any prior separation or preconcentration.

Selective detection of Hg2+ using fluorescent rhodamine-functionalized Fe3O4 nanoparticles

A rhodamine derivative-functionalized nanomaterial 1 was able to detect and remove Hg²⁺ selectively with a fluorescence turn-on signal.