Analytical methods for mercury speciation, detection, and measurement in water, oil, and gas

Assessment of total mercury in urban particulate matter by filter fiber assisted matrix solid-phase dispersion coupling with microplasma assisted-cold vapor generation

BACKGROUND

The evaluation of particle-bound mercury (PBM) exposure is a crucial aspect of assessing the global cycle of mercury (Hg) and its adverse effects on human health and ecosystems. Nevertheless, the precise and reliable measurement of PBM remains a formidable task because of the costly and cumbersome equipment required, as well as the inadequate sensitivities exhibited by current analytical techniques. In this study, we provided a unique and straightforward approach utilising filter fiber-assisted matrix solid-phase dispersion (FF-MSPD) in conjunction with single-drop solution electrode discharge-induced cold vapor generation atomic fluorescence spectrometry (SD-SEGD-CVG-AFS) for the precise quantification of PBM. The PBM contained in a small filter was efficiently extracted with 200 μL of eluent (0.2 % L-cysteine and 4 % HCOOH) by FF-MSPD and subsequently converted to Hg0 using SD-SEGD-CVG, before being subjected to examination using AFS.

RESULTS

The resulted limit of detection (LOD, 3σ) was 0.17 pg m-3, obtained with a sample volume of 12 m3, which was much higher than that of the techniques published in the literatures. The aforementioned technique was effectively utilised for the detection of mercury in 19 samples of PM2.5 and PM10 which were collected over a span of several months.

SIGNIFFCANCE

Contrast to conventional methods, the proposed method offers a range of distinct advantages, including simplified operation, absence of memory effects, enhanced sensitivity, substantial reduction in reagent usage, and decreased secondary pollution. These advantages are particularly valuable for advancing research on the fate, transport, and exposure routes of environmental mercury.

Controllable detection threshold achieved through the toehold switch system in a mercury ion whole-cell biosensor

Due to the toxicity of mercury and its harmful effects on human health, it is essential to establish a low-cost, highly sensitive and highly specific monitoring method with a wide detection range, ideally with a simple visual readout. In this study, a whole-cell biosensor with adjustable detection limits was developed for the detection of mercury ions in water samples, allowing controllable threshold detection with an expanded detection range. Gene circuits were constructed by combining the toehold switch system with lactose operon, mercury-ion-specific operon, and inducible red fluorescent protein gene. Using MATLAB for design and selection, a total of eleven dual-input single-output sensing logic circuits were obtained based on the basic logic of gene circuit construction. Then, biosensor DTS-3 was selected based on its fluorescence response at different isopropyl β-D-Thiogalactoside (IPTG) concentrations, exhibiting the controllable detection threshold. At 5-20 μM IPTG, DTS-3 can achieve variable threshold detection in the range of 0.005-0.0075, 0.06-0.08, 1-2, and 4-6 μM mercury ion concentrations, respectively. Specificity experiments demonstrated that DTS-3 exhibits good specificity, not showing fluorescence response changes compared with other metal ions. Furthermore spiked sample experiments demonstrated its good resistance to interference, allowing it to distinguish mercury ion concentrations as low as 7.5 nM by the naked eye and 5 nM using a microplate reader. This study confirms the feasibility and performance of biosensor with controllable detection threshold, providing a new detection method and new ideas for expanding the detection range of biosensors while ensuring rapid and convenient measurements without compromising sensitivity.

Recent progress in nanomaterial-based aptamers as biosensors for point of care detection of Hg2+ ions and its environmental applications

Among the foremost persistent heavy metal ions in the ecosystem, mercury (Hg2+) remains intimidating to the environment by producing a catastrophic effect on the environment as well as on mankind due to the exacerbation of anthropogenic activities. Therefore, it has become necessary to develop superlative techniques for its detection even at low concentrations. The conventional approaches for Hg2+ ions are quite laborious, and expensive, and require expertise in operating sophisticated instruments. To overcome these limitations, aptamer-based biosensors emerged as a promising tool for its detection. DNA-based aptamers have evolved as a significant technique by detecting them even in ppb levels. This review outlines the progress in aptamer-based biosensors from the year 2019-2023 by inducing changes in the electrochemical signal or by fluorescent/colorimetric approaches. The electrochemical sensors used nanomaterial electrodes for increasing the sensitivity whereas fluorescent and colorimetric sensors exhibit quenching or strong fluorescence in the presence of Hg2+ ions depending upon the prevailing mechanism or visible color changes. This perturbation in the signals could be attributed to the formation of the T-Hg2+ -T complex with the aptamers in the presence of ions revealing its real-time and biological applications in living or cancerous cells. Furthermore, next-generation biosensors are suggested to bring a paradigm shift to the integration of high-end smartphones, machine learning, artificial intelligence, etc.

CRISPR-Hg: Rapid and visual detection of Hg2+ based on PCR coupled with CRISPR/Cas12a

Mercury (Hg) is a notorious toxic heavy metal, causing neurotoxicity and liver damage, posing grave threats to human health and environmental safety. There is an urgent imperative for developing novel Hg2+ detection methods. In this work, we developed a CRISPR-based method for Hg2+ detection named CRISPR-Hg. A CRISPR/Cas12a system was employed and could be activated by the PCR product, generating fluorescence signals based on the trans-cleavage activity. CRISPR-Hg exhibited remarkable selectivity and specificity, achieving a detection limit of 10 pM and minimal interference with background signals. This approach has been successfully applied to detect Hg2+ in real samples, including water, soil, and mushroom. Ulteriorly, a portable device was devised to streamline the readout of fluorescence signals by a smartphone within 30 min. We offer an affordable, highly selective and visually interpretable method for Hg2+ detection, with the potential for broad application in Hg2+ monitoring for food safety and public health.

A high-throughput, straightforward procedure for biomonitoring organomercury species in human hair

Mercury is a pervasive and concerning pollutant due to its toxicity, mobility, and tendency to biomagnify in aquatic and terrestrial ecosystems. Speciation analysis is crucial to assess exposure and risks associated with mercury, as different mercury species exhibit varying properties and toxicities. This study aimed at developing a selective detection method for organic mercury species in a non-invasive biomonitoring matrix like human hair. The method is based on frontal chromatography (FC) in combination with inductively coupled plasma mass spectrometry (ICP-MS), using a low pressure, homemade, anion exchange column inserted in a standard ICP-MS introduction system, without requiring high-performance liquid chromatography (HPLC) hyphenation. In addition to the extreme simplification and cost reduction of the chromatographic equipment, the proposed protocol involves a fast, streamlined and fully integrated sample preparation process (in contrast to existing methods): the optimized procedure features a 15-min ultrasonic assisted extraction procedure and 5 min analysis time. Consequently, up to 100 samples could be analyzed daily, making the method highly productive and suitable for large-scale screening programs in public and environmental health. Moreover, the optimized procedure enables a limit of detection (LOD) of 5.5 μg/kg for a 10 mg hair microsample. All these features undeniably demonstrate a significant advancement in routine biomonitoring practices. To provide additional evidence, the method was applied to forty-nine human hair samples from individuals with varying dietary habits successfully finding a clear correlation between methylmercury levels (ranging from 0.02 to 3.2 mg/kg) in hair and fish consumption, in line with previous literature data.

Synthesis of Os@ZIF-8 nanocomposites with enhanced peroxidase-like activity for detection of Hg2

Metal organic framework (MOF)-derived nanostructures display remarkable characteristics and have broad application potential. Os@ZIF-8 nanocomposites were prepared by a depositional method. The Os nanoparticles distributed on the surface of ZIF-8. The nanocomposites displayed enhanced peroxidase-like activity with smaller Km for both 3,3',5,5'-tetramethylbenzidine (TMB) and H2O2 compared to Os NPs due to the confinement effect and large surface area that ZIF-8 provided. From the average reaction rate constants obtained from three different temperatures, the activation energy values were determined. The kinetic data indicated that the Os@ZIF-8 NCs are catalytically more active than Os NPs. In addition, quantitative measurement of Hg2+ was performed based on the formation of Os-Hg alloy. Os@ZIF-8 NCs had a wide detection range between 0 μM and 71.43 μM for Hg2+ with a limit of detection (LOD) of 2.29 μM. Using a MOF with a large surface area to load Os nanoparticles to achieve enhanced nanozyme activity is the novelty of this work.

Achieving smartphone-based colorimetric assay for Hg2+ with a bimetallic site strategy based on Hg2+-triggered oxidase-like catalytic activity of NSC/Co6Ni3S8 nanocomposite

Modulation of the nanozyme's catalytic activity is crucial for its real applications in detecting target analytes. Herein, we fabricated the nanocomposite (NSC/Co6Ni3S8) of N, S co-doped carbon and Co6Ni3S8 by a facile sol-gel approach. Compared to NSC/Ni9S8, NSC/Co6Ni3S8 with bimetallic active sites displayed better enzyme-mimetic activity. This nanocomposite could catalyze O2 to form ·O2- and oxidize colorless 3, 3', 5, 5'-tetramethylbenzidine (TMB) into blue oxTMB. The other two free radicals (h+ and ·OH) played minor roles during the catalytic reaction. Hg2+ could integrate with S2- to form HgS and the surface charges of O2 were transferred to Hg2+ to promote O2 adsorption. DFT theoretical calculations highlight that the main reasons for the enhancing effect of Hg2+ on color development results from electron transfer and increased adsorption energy of O2 molecules onto the surface of NSC/Co6Ni3S8. By employing the oxidase-like activity of NSC/Co6Ni3S8 and Hg2+-triggered promoting effect, a colorimetric sensing platform was established for Hg2+ assay with a linear range of 10-200 μg/L and detection limit of 3 μg/L. Through integration with a smartphone-based APP "Thing Identify" software, a visual colorimetric assay for Hg2+ was constructed with a detection limit of 5 μg/L. Compared to the data detected by the mercury vapor meter, the relative recoveries of 92.4-108.1% evidenced the higher accuracy of this smartphone-based visual detection. Overall, the NSC/Co6Ni3S8-based colorimetric assay is convenient, rapid, and visual, and can be applied for routine monitoring of Hg2+ in real-world waters under outdoor conditions.

Promoting sensitive colorimetric detection of hydroquinone and Hg2+ via ZIF-8 dispersion enhanced oxidase-mimicking activity of MnO2 nanozyme

Reducing the agglomeration and improving the dispersibility in water of two-dimensional (2D) nanozymes is one of the effective ways to improve their enzyme-like activity. In this work, we propose a method by constructing zeolitic imidazolate framework-8 (ZIF-8)-dispersed 2D manganese-based nanozymes to achieve the specific regulated improvement of oxidase-mimicking activity. By in-situ growth of manganese oxides nanosheets of MnO₂(1), MnO₂(2) and Mn₃O₄ on the surface of ZIF-8, the corresponding nanocomposites of ZIF-8 @MnO₂(1), ZIF-8 @MnO₂(2), and ZIF-8 @Mn₃O₄ were prepared at room temperature. The Michaelis-Menton constant measurements indicated that ZIF-8 @MnO₂(1) exhibits best substrate affinity and fastest reaction rate for 3,3',5,5'-tetramethylbenzidine (TMB). The ZIF-8 @MnO₂(1)-TMB system was exploited to detection of trace hydroquinone (HQ) based on the reducibility of phenolic hydroxyl groups. In addition, by employing the fact that the cysteine (Cys) with the excellent antioxidant capacity can bind the Hg²⁺ based on the formation of "S-Hg²⁺" bonds, the ZIF-8 @MnO₂(1)-TMB-Cys system was applied to detection of Hg²⁺ with high sensitivity and selectivity. Our findings not only provide a better understanding of the relationship between dispersion of nanozyme and enzyme-like activity, but also provide a general method for the detection of environmental pollutants using nanozymes.

Novel Amperometric Mercury-Selective Sensor Based on Organic Chelator Ionophore

A novel amperometric sensor for the direct determination of toxic mercury ions, Hg²⁺, based on the organic chelator ionophore N, N di (2-hydroxy-5-[(4-nitrophenyl)diazenyl]benzaldehyde) benzene-1,2-diamine (NDBD), and multiwalled carbon nanotubes (MWCNT) immobilized on a glassy carbon electrode surface was developed. The parameters influencing sensor performance including the ionophore concentration, the applied potential, and electrolyte pH were optimized. The sensor response to Hg²⁺ was linear between 1-25 µM with a limit of detection of 60 nM. Interferences from other heavy metal ions were evaluated and the sensor showed excellent selectivity towards Hg²⁺. The method was successfully applied to the determination of mercury ions in milk and water samples.

A Simple and Effective Visual Fluorescent Sensing Paper-Based Chip for the Ultrasensitive Detection of Mercury Ions in Environmental Water

Traces of mercury ions in environmental water can harm humans and animals. Paper-based visual detection methods have been widely developed for the rapid detection of mercury ions; however, existing methods are not sensitive enough to be used in real environments. Here, we developed a novel, simple and effective visual fluorescent sensing paper-based chip for the ultrasensitive detection of mercury ions in environmental water. CdTe-quantum-dots-modified silica nanospheres were firmly absorbed by and anchored to the fiber interspaces on the paper's surface to effectively avoid the unevenness caused by liquid evaporation. The fluorescence of quantum dots emitted at 525 nm can be selectively and efficiently quenched with mercury ions, and the ultrasensitive visual fluorescence sensing results attained using this principle can be captured using a smartphone camera. This method has a detection limit of 2.83 µg/L and a fast response time (90 s). We successfully achieved the trace spiking detection of seawater (from three regions), lake water, river water and tap water with recoveries in the range of 96.8-105.4% using this method. This method is effective, low-cost, user-friendly and has good prospects for commercial application. Additionally, the work is expected to be utilized in the automated big data collection of large numbers of environmental samples.

Natural deep eutectic solvent-based microextraction for mercury speciation in water samples

A new natural deep eutectic solvent (NADES)-based analytical method for mercury speciation in water samples is presented. A NADES (i.e., decanoic acid:DL-menthol in a molar ratio of 1:2) is used as an environmentally friendly extractant for separation and preconcentration using dispersive liquid-liquid microextraction before LC-UV-Vis. Under optimal extraction conditions (i.e., NADES volume, 50 µL; sample pH, 12; volume of the complexing agent, 100 µL; extraction time, 3 min; centrifugation speed, 3000 rpm; and centrifugation time, 3 min), the limit of detection values were 0.9 µg L^-1 for the organomercurial species and 3 µg L^-1 for Hg²⁺, which had a slightly higher value. The relative standard deviation (RSD, n = 6) has been evaluated at two concentration levels (25 and 50 µg L^-1) obtaining values for all the mercury complexes within the range of 6-12% and 8-12%, respectively. The trueness of the methodology has been evaluated using five real water samples from four different sources (i.e., tap, river, lake, and wastewater). The recovery tests have been performed in triplicate obtaining relative recoveries between 75 and 118%, with RSD (n = 3) between 1 and 19%, for all the mercury complexes in surface water samples. However, wastewater sample showed a significant matrix effect (recoveries ranged between 45 and 110%), probably due to the high amount of organic matter. Finally, the greenness of the method has also been evaluated by the analytical greenness metric for sample preparation (i.e., AGREEprep).

Elemental mercury accumulation in Eichhornia crassipes (Mart.) Solms-Laubach

The aquatic macrophyte Eichhornia crassipes has great potential for the control of Hg pollution in the environment. The aim of this study was to investigate the capability of E. crassipes to accumulate elemental mercury (Hg⁰). The plants were exposed for 30 days to 5, 10, 20, 40, and 80 mg of Hg⁰ in a 1-L Hoagland medium with the Hg⁰ settled at the bottom of the flask. The roots of the plants did not touch the mercury during the treatment. After exposure, the total Hg (T-Hg) concentrations in the roots, leaves, and stems were measured using a direct mercury (Hg) analyzer. The highest concentrations were found at 80 mg Hg⁰ treatment in the roots, leaves, and stems, in that order. The translocation factor indicated a poor capability of Hg to translocate from the roots to the shoots. The relative growth and the root-length inhibition measurements showed that the differences between Hg⁰ treatments were not significant. In addition, the treatments negatively affected the chlorophyll concentration. The carotenoid content was found to be significantly different at 20 and 40 mg of Hg⁰ in 1 L. Regarding the carbonyl index in root proteins, significant differences compared to control were found at the highest Hg treatment. Based on these results, it was shown that E. crassipes is able to take up elemental Hg from Hoagland medium. However, the Hg⁰ treatments did not show a strong stress-response activation mechanism in the evaluated plant tissues.

Removal of lead from aqueous solution using electrospun nanofibers: preparation, characterization, adsorption isotherm, and kinetic study

Lead is one of the most hazardous toxic heavy metal ions in industrial wastewater. The removal of Pb(II) from aqueous environment is an extremely essential topic due to acquiring clean water resources and its significant impact on human health. Adsorption is an effective and the most widely used method for heavy metal removal from an aqueous medium. Nanofibers have potential advantages in the adsorption of heavy metal ions from wastewater. In this study, nanofibers based on polyvinyl alcohol (PVA) were fabricated for the removal of lead ions from aqueous samples. Nanofibers produced by electrospinning were characterized by scanning electron microscopy (SEM/EDX) and Fourier transform infrared (FT-IR) techniques. A batch system was used for the adsorption of Pb(II) ions onto cross-linked PVA (%10) and PVA (%10):MSs (%2) (Malva Sylestris L. seed biomaterial) nanofibers. The effectiveness of cross-linking was determined by the water absorbency test. The pH, adsorbent amount, adsorption kinetics, isotherms, and thermodynamic values were thoroughly investigated in the adsorption tests. Pb(II) adsorption on the polymer was confirmed by EDX analysis. The optimum values found were a pH of 6, an adsorbent dose of 5.0 mg, and a contact time of 120 min. Lead ion concentrations were determined by flame atomic absorption spectrometry (FAAS). The Freundlich models could explain the results from the adsorption data. Similar results were obtained from adsorption isotherm models, and the results were found to support each other. The adsorption capacity for PVA (10%) and PVA (10%):MSs (2%) nanofibers were found to be 444.2 mg g^-1 and 588.2 mg g^-1, respectively. The adsorption capacity increases with the addition of MSs (2%) biomaterial. As a result, nanofibers can be used as effective adsorbents in the removal of Pb(II) ions. The developed methods are environmentally friendly and promising for the separation of toxic Pb(II) ions from aqueous systems, which is a major problem for environmental pollution.

Recent Advanced in MXene Research toward Biosensor Development

MXene is a rapidly emerging group of two-dimensional (2D) multifunctional nanomaterials, drawing huge attention from researchers of a broad scientific field. Reporting the synthesis of MXene was the following breakthrough in 2D materials following the discovery of graphene. MXene is considered the most recent developments of materials, including transition metal carbonitrides, nitrides, and carbides synthesized by etching or mechanical-based exfoliation of selective MAX phases. MXene has a plethora of prodigious properties such as unique interlayer spacing, high ion and electron transport, large surface area, excellent thermal and electrical conductivity, exceptional volumetric capacitance, thermal shock, and oxidation resistance, easily machinable and inherently hydrophilic, and biocompatibility. Owing to the abundance of tailorable surface function groups, these properties can be further enhanced by surface functionalization with covalent and non-covalent modifications via numerous surface functionalization methods. Therefore, MXene finds their way to a plethora of applications in numerous fields including catalysis, membrane separation, energy storage, sensing, and biomedicine. Here, the focus is on reviewing the structure, synthesis techniques, and functionalization methods of MXene. Furthermore, MXene-based detection platforms in different sensing applications are survived. Great attention is given to reviewing the applications of MXene in the detection of biomolecules, pathogenic bacteria and viruses, cancer biomarkers food contaminants and mycotoxins, and hazardous pollutants. Lastly, the future perspective of MXene-based biosensors as a next-generation diagnostics tool is discussed. Crucial visions are introduced for materials science and sensing communities to better route while investigating the potential of MXene for creating innovative detection mechanisms.

A novel fluorescent "OFF-ON" sensing strategy for Hg (II) in water based on functionalized gold nanoparticles

Mercury ion (Hg²⁺) is a heavy metal pollutant that can affect the safety of water environment and endanger human health. A novel detection strategy (GNPs-_L-Cys-Rh6G2) for Hg²⁺ based on a fluorescence "OFF-ON" was proposed. Gold nanoparticles (GNPs) were assembled with _l-cysteine (_L-Cys), which was used as a "bridge" to link with rhodamine 6G derivatives (Rh6G2). The fluorescence state transition of GNPs-_L-Cys-Rh6G2 switching from "OFF"-"ON" was observed because Hg²⁺ opened the spirolactam ring through a catalytic hydrolysis mechanism. The fluorescence signal of the GNPs-_L-Cys-Rh6G2 system mixed with Hg²⁺ in the concentration range of 10-100 μM was analyzed and determined with a limit of detection (LOD) of 2 μM (S/N = 3). Moreover, the spiked Hg²⁺ concentration in real water samples were successfully quantified by GNPs-_L-Cys-Rh6G2, which was in line with the ideal average recovery rate and relative standard deviation. The proposed strategy exhibited high specificity, sensitivity and stability, providing a novel sensing platform for heavy metal ions detection in water environment.

An Immunochromatographic Assay for the Rapid and Qualitative Detection of Mercury in Rice

Mercury is a major pollutant in food crops. In this study, we synthesized an anti-mercury monoclonal antibody (mAb; IC₅₀ was 0.606 ng mL⁻¹) with high sensitivity and specificity and different immunogens and coating antigens and developed an immuno-chromatographic assay (ICA) for the detection of mercury in rice. The ICA strip had a visible detection limit of 20 ng g⁻¹ and a cut-off value of 500 ng g⁻¹ in rice. The performance of the ICA strip was consistent with that of ICP-MS and ic-ELISA. The recoveries of mercury in rice ranged from 94.5% to 113.7% with ic-ELISA and from 93.6% to 116.45% with ICP-MS. Qualitative analysis by ICA can be obtained with the naked eye. The ICA strip is an effective and practical method for the rapid and high-throughput determination of mercury in rice.

Resource utilization of pig hair to prepare low-cost adsorbents with high density of sulfhydryl for enhanced and trace level removal of aqueous Hg(II)

Pig hair (PH), a keratinous waste, was modified by ammonium thioglycolate in a ball milling to promote its performance of Hg(II) sequestration. The ball milling broke the hydrophobic cuticle sheath and enhanced the reduction of disulfide bond, which increased the sulfydryl content of the modified PH (BTPH) from 0.07 to 11.05 μmol/g. BTPH exhibited a significantly higher capture capacity of Hg(II) (415.4 mg/g) than PH (3.1 mg/g), as well as the commercial activated carbon (219.7 mg/g), and persisted its performance over a wide range of solution pH. Meanwhile, BTPH with a distribution coefficient of 5.703 × 10⁵ mL/g could selectively capture Hg(II) from the water with the coexisting metal ions such as Mg(II), Cd(II) and Pb(II). Moreover, the low-cost BTPH could reduce the Hg(II) from 1.0 mg/L to well below the limit of drinkable water (2 μg/L) in real-world samples. Density functional theory (DFT) calculations and state-of-the-art characterizations illustrated that the binding of Hg(II) to sulfydryl groups was the main adsorption mechanism. Notably, BTPH decreased the mercury content of water spinaches from 24.1 to 0.50 mg/kg and thereby significantly reduced the phytotoxicity of Hg(II). This work therefore provides a sustainable way to utilize keratinous wastes for the remediation of aqueous Hg(II).

DNAzyme-based biosensors for mercury (Ⅱ) detection: Rational construction, advances and perspectives

Mercury contamination is one of the most severe issues in society due to its threats to public health and the ecological system. However, traditional methods for mercury ion detection are still limited by their time-consuming procedures, requirement of expensive instruments, and low selectivity. In recent decades, tremendous progress has been made in the development of functional nucleic acid-based, especially DNAzyme sensors for mercury (Ⅱ) (Hg²⁺) determination, including RNA-cleaving DNAzymes and G-quadruplex-based DNAzymes in particular. Researchers have heavily studied the construction of Hg²⁺ sensors, mainly originating from in vitro selection-derived DNAzymes, by incorporating T-Hg²⁺-T recognition moieties in existing DNAzyme scaffolds, and interfacing Hg²⁺-sensitive sequences with nanomaterials. In the last case, the employment of materials (as quenchers, signal transducers and DNA immobilizers) enriches the application scenarios of current Hg²⁺-DNAzymes, due to a combination of their functions. We summarize a broad range of sensing approaches, including optical, electrochemical, and other sensing methods, and compare their features. This review elaborates on the rational design strategies for engineering DNAzymes to selectively sense Hg²⁺, critically discusses their properties in different application scenarios, and summarizes recent advances in this field. Additionally, current progress, challenges and future perspectives are also discussed. This minireview provides deeper insights into the chemistry of these functional nucleic acids when working with Hg²⁺, explains the design ideas of DNAzyme-sensors in each platform, and reveals potential opportunities in developing more advanced DNAzyme sensors for the highly selective and sensitive recognition of Hg²⁺. ENVIRONMENTAL IMPLICATION: Mercury is one of the most toxic metallic contaminants due to its high toxicity, non-biodegradability, and serious human health risks when accumulated in the body. In the recent decade, intensive studies have focused on exploring mercury sensors by combining DNAzymes with various sensing methods, paving a promising avenue to gain ultra-high sensitivity and selectivity. However, so far, no review has introduced the recent advances on DNAzyme-based sensors for mercury detection in a critical way. In this review, we comprehensively summarized the studies on DNAzyme-based sensors for mercury detection using various sensing techniques including optical, electrochemical and other sensing methods.

Recent Trends and Future Perspectives of Emergent Analytical Techniques for Mercury Sensing in Aquatic Environments

Environmental emissions of mercury from industrial waste and natural sources, even in trace amounts, are toxic to organisms and ecosystems. However, industrial-scale mercury detection is limited by the high cost, low sensitivity/specificity, and poor selectivity of the available analytical tools. This review summarizes the key sensors for mercury detection in aqueous environments: colorimetric-, electrochemical-, fluorescence-, and surface-enhanced Raman spectroscopy-based sensors reported between 2014-2021. It then compares the performances of these sensors in the determination of inorganic mercury (Hg²⁺ ) and methyl mercury (CH₃ Hg⁺ ) species in aqueous samples. Mercury sensors for aquatic applications still face serious challenges in terms of difficult deployment in remote areas and low robustness, reliability, and selectivity in harsh environments. We provide future perspectives on the selective detection of organomercury species, which are especially toxic and reactive in aquatic environments. This review is intended as a valuable resource for scientists in the field of mercury sensing.

Ti3C2Tx MXene sensor for rapid Hg2+ analysis in high salinity environment

Mercury is one of the leading chemicals of concern and receives much attention in environmental safety. It is of great necessity to develop advanced Hg²⁺ analysis method for rapid detection and monitoring. Field-effect transistor (FET) sensor, an emerging electronic sensor, has received great attention in environmental analysis since it has unique advantages in achieving rapid analysis of chemicals. Herein, an FET sensor is constructed with Ti₃C₂T_x MXene as the channel material to detect Hg²⁺ in water. The sensor displays rapid and selective response to Hg²⁺. Moreover, the sensor achieves satisfactory performance in Hg²⁺ detection in high salinity environment (1 M NaCl), which benefits its applications in real water analysis. Based on the investigation of sensing mechanism, the strong response of Ti₃C₂T_x MXene FET sensor to Hg²⁺ is due to the adsorption and reduction of Hg²⁺ to Hg⁺ on the Ti₃C₂T_x surface. This reported label-free Ti₃C₂T_x MXene platform can detect Hg²⁺ in high salinity environment with high specificity, which has significant application potential for on-site monitoring and risk assessment of Hg²⁺ in aqueous systems.

Functionalized Silver Nanoparticles as Colorimetric and Fluorimetric Sensor for Environmentally Toxic Mercury Ions: An Overview

Nanoscience is a multifaceted field which encompasses metal nanoparticles (MNPs) having novel and size-related optical properties significantly different from the bulk level as well as at the atomic level. Amongst noble MNPs, the silver nanoparticles (AgNPs) have unique properties for metal interaction. Presently, there have been expedite reports which are taken under the review in virtue of sensing the mercury ions in aqueous media. Mercury dissemination in various forms contaminates the ecosystem. Globally mercury is ranked as the most toxic element and an urgent threat to humans since it causes major health issues. Employing MNPs, especially AgNPs for the detection of mercury ions is the economic, handy and apt method in contrast to time-consuming methods that use expensive instrumentations. The review highlights a study of colorimetric and fluorimetric detection of the level of Hg (II) ions in aqueous media selectively with high sensitivity in different courses of conditions using AgNPs synthesized by various approaches. Graphical abstract.

Improving the Voltammetric Determination of Hg(II): A Comparison Between Ligand-Modified Glassy Carbon and Electrochemically Reduced Graphene Oxide Electrodes

A new thiosemicarbazone ligand was immobilized through a Cu(I)-catalyzed click reaction on the surface of glassy carbon (GC) and electrochemically reduced graphene oxide (GC-ERGO) electrodes grafted with phenylethynyl groups. Using the accumulation at open circuit followed by anodic stripping voltammetry, the modified electrodes showed a significant selectivity and sensibility for Hg(II) ions. A detection limit of 7 nM was achieved with the GC modified electrodes. Remarkably, GC-ERGO modified electrodes showed a significantly improved detection limit (0.8 nM), sensitivity, and linear range, which we attribute to an increased number of surface binding sites and better electron transfer properties. Both GC and GC-ERGO modified electrodes proved their applicability for the analysis of real water samples.