Simple electrochemical sensing for mercury ions in dairy product using optimal Cu2+-based metal-organic frameworks as signal reporting

Magnetically-assisted electrochemical immunoplatform for simultaneous detection of active and total prostate-specific antigen based on proteolytic reaction and sandwich affinity analysis

Simultaneous detection of active and inactive proteases is clinically meaningful for improving diagnostic specificity. In this work, we reported an electrochemical method for simultaneous immunoassays of active and total proteases. Magnetic beads (MBs) were used as the solid supports for immobilization of capture antibodies and enrichment of targets. For the detection of active protease, the proteolytic-reaction-based analysis was carried out by the generation of Cu2+-binding peptide, in which a label-free peptide was used as the proteolytic substrate. The redox potential of the resulting peptide-Cu2+ complex was intrinsically distinguished from that of free Cu2+, thus allowing the "signal-on" detection of active protease. For the immunoassay of total protease in a sandwich-like format, electroactive metal-organic frameworks (Cu-MOFs) were used as the signal tags. The captured Cu-MOFs could directly produce a well-defined electrochemical signal from the reduction of Cu2+ ions. The analytical performances of the immunoplatform were evaluated by determining the model analytes of free and total prostate-specific antigen (fPSA and tPSA) in buffer and serum. The detection limits were found to be 0.3 pM for fPSA and 2 pM for tPSA. This work proposed a new strategy for simultaneous detection of active and total proteases, which should be evaluable for clinical diagnosis and treatment of protease-relative diseases.

Parthenium hysterophorus derived nanostructures as an efficient carbocatalyst for the electrochemical sensing of mercury(II) ions

The sustainable utilization of resources motivate us to create eco-friendly processes for synthesizing novel carbon nanomaterials from waste biomass by minimizing chemical usage and reducing energy demands. By keeping sustainability as a prime focus in the present work, we have made the effective management of Parthenium weeds by converting them into carbon-based nanomaterial through hydrothermal treatment followed by heating in a tube furnace under the nitrogen atmosphere. The XPS studies confirm the natural presence of nitrogen and oxygen-containing functional groups in the biomass-derived carbon. The nanostructure has adopted a layered two-dimensional structure, clearly indicated through HRTEM images. Further, the nanomaterials are analyzed for their ability towards the electrochemical detection of mercury, with a detection limit of 6.17 μM, while the limit of quantification and sensitivity was found to be 18.7 μM and 0.4723 μM μA-1 cm-2, respectively. The obtained two-dimensional architecture has increased the surface area, while the nitrogen and oxygen functional groups act as an active site for sensing the mercury ions. This study will open a new door for developing metal-free catalysts through a green and sustainable approach by recycling and utilization of waste biomass.

Ecofriendly ratiometric colorimetric determination of mercury(II) ion in environmental water samples using gallic acid-capped gold nanoparticles

Today, the monitoring and determination of heavy metal pollutants in the environment is an essential requirement for the environmental and research communities. Mercury ion is one of the most hazardous heavy metals, and scientists are trying to develop new methods for its detection. In this study, a new colorimetric sensor based on aggregation gallic acid-capped gold nanoparticles (GA-AuNPs) for the determination of mercury ions in environmental water samples was presented. The green synthesized GA-AuNPs exhibited a sharp surface plasmon resonance peak at 515 nm. The addition of mercury ions changed the surface properties of GA-AuNPs, resulting in the formation of a new peak near 670 nm due to the aggregation of GA-AuNPs, and an obvious color change from red to purple occurred. Thus, mercury ions were detected based on the change in the absorbance ratio (A670/A515). The developed sensor can determine the mercury ions in the concentration range of 78.0 nM to 8.3 µM with a detection limit of 5.5 nM. Based on the Environmental Protection Agency (EPA) and the World Health Organization (WHO) reports, the amount of Hg2+ ions in fresh water should be between 10.0 and 30.0 nM. The results indicate that the developed sensor can detect and determine trace amounts of Hg2+ ions in environmental water samples.

Analyte-induced laccase-mimicking activity inhibition and conductivity enhancement of electroactive nanozymes for ratiometric electrochemical detection of thiram

Most nanozyme-based electrochemical sensing strategies depend on the catalytic formation of electroactive substances, while the electrochemical properties of nanozymes have rarely been explored. In this study, magnetic nanoparticles encapsulated metal-organic framework served as precursors to prepare bioinspired nanozymes with both laccase-mimicking activity and electroactivity. Owing to the strong affinity between thiram (THR) and Cu(II) active sites in the nanozymes, the binding of THR inhibited nanozyme catalytic activity toward catechol (CT) oxidation and enhanced nanozyme conductivity. A lower oxidation current (ICT) of CT was accompanied by a higher oxidation signal (ICu) of Cu(II), allowing a ratiometric electrochemical response of the electroactive nanozymes toward the incoming THR. The signal ratio (ICu/ICT) displayed a good linear relationship over a THR concentration range of 10.0 nM-3.0 μM with a limit of detection of 0.15 nM, and the entire THR detection process was rapidly accomplished within 5 min. The high sensitivity and selectivity of the developed electrochemical strategy guaranteed the reliable detection of THR in fruit, vegetable, and river water samples. This study provides new insights into the development of nanozymes for electrochemical analysis.

Portable smartphone-assisted highly sensitive detection of mercury ions based on gold nanoparticle-modified NH2-UiO-66 metal-organic framework

A novel portable smartphone-assisted colorimetric method was reported for the determination of Hg2+ with good analytical performance. A Zr(IV)-based metal-organic framework functionalized with amino groups (NH2-UiO-66) has been adopted as a supporting platform to anchor gold nanoparticles (AuNPs), avoiding the migration and aggregation of AuNPs. With the addition of Hg2+, the formation of gold amalgam proved possible to enhance peroxidase-like activity of the composite (AuNPs/NH2-UiO-66), accelerating the oxidization of zymolyte 3,3',5,5'-tetramethylbenzidine (TMB). In the meantime, the color of the reaction solution turned a vivid blue, and the red, green, and blue (RGB) values of the solution color changed accordingly. On account of this strategy, the quantitative detection of Hg2+ could be achieved. After the optimization of the experiment conditions, the average color intensity (Ic) resulting from RGB values was linear related to the concentration of Hg2+ from 10 to 100 nM, accompanied with a detection limit (LOD) down to 5.4 nM calculated by 3σ/S. The successful application of the designed method has been promoted to detect Hg2+ in some water samples, displaying a great potential in practical application. Furthermore, the use of a smartphone made our proposed method simple and accurate, and thus puts forward a possible way for in situ and real-time monitoring.

Detection mechanism and the outlook of metal-organic frameworks for the detection of hazardous substances in milk

Milk has a high nutritional value. However, milk is easily contaminated in the production, processing, and storage processes, which harms consumers' health. Therefore, the harmful substances' detection in milk is important. Metal-organic frameworks (MOFs) have proven high potential in food safety detection due to their unique porous structure, large effective surface area, large porosity, and structural tunability. This article systematically describes the detection mechanism of fluorescence, electrochemical, colorimetric, and enzyme-linked immunosorbent assay based on MOFs. The progress of the application of MOFs in the detection of antibiotics, harmful microorganisms and their toxins, harmful ions, and other harmful substances in milk in recent years is reviewed. The structural tunability of MOFs enables them to be functionalized, giving the ability to be applied to different detection methods or substances. Therefore, MOFs can be used as an advantageous sensing material for detecting harmful substances in the complex environment of milk.

Electrochemical detection of glycoproteins using boronic acid-modified metal-organic frameworks as dual-functional signal reporters

The sensitive analysis of glycoproteins is of great importance for early diagnosis and prognosis of diseases. In this work, a sandwich-type electrochemical aptasensor was developed for the detection of glycoproteins using 4-formylphenylboric acid (FPBA)-modified Cu-based metal-organic frameworks (FPBA-Cu-MOFs) as dual-functional signal probes. The target captured by the aptamer-modified electrode allowed the attachment of FPBA-Cu-MOFs based on the interaction between boronic acid and glycan on glycoproteins. Large numbers of Cu2+ ions in FPBA-Cu-MOFs produced an amplified signal for the direct voltammetric detection of glycoproteins. The electrochemical aptasensor showed a detection limit as low as 6.5 pg mL-1 for prostate specific antigen detection. The method obviates the use of antibody and enzymes for molecular recognition and signal output. The dual-functional MOFs can be extended to the design of other biosensors for the determination of diol-containing biomolecules in clinical diagnosis.

A polystyrene-based ESIPT fluorescent polymeric probe for highly sensitive detection of chromium(vi) ions and protein staining

A "two-step" preparation method of an excited-state intermolecular proton transfer (ESIPT) fluorescent polymer (f-PP) is reported here. The synthesis of f-PP involves the acetylation of polystyrene and a "multicomponent one pot" reaction. The as-prepared polymer bears a group of ESIPT fluorescent units, enabling it to exhibit high brightness, moderate solubility and ESIPT fluorescence. F-PP gives off tautomeric bright green fluorescence under UV-tamp and the dual-emission could be specifically suppressed by Cr(vi). This phenomenon cannot be elicited by other competing species. On this basis, an ESIPT polymeric probe-based method for the determination of Cr(vi) was developed, offering high sensitivity (19.5 nM) and selectivity. The f-PP was successfully used to detect Cr(vi) in real water samples by standard adding methods, indicating its application feasibility.

Competitive electrochemical sensing for cancer cell evaluation based on thionine-interlinked signal probes

The development of effective methods for tracking cancer cells is of significant importance in the early diagnosis and treatment of tumor diseases. Compared with the developed techniques, the electrochemical assay has shown considerable potential for monitoring glycan expression on the cell surface using nondestructive means. However, the application expansion of the electrochemical strategy is strongly impeded owing to its dependence on electroactive species. In this study, a competitive electrochemical strategy was reported for monitoring cancer cells based on mannose (a typical glycan) as a clinical biomarker. Herein, functionalized carbon nanotubes were used to load the thiomannosyl dimer, and thionine-interlinking signal probes were designed for competitive recognition. After effective competition between cancer cells and the anchored mannose, a decreased current was obtained as the cell concentration increased. Under optimal conditions, the proposed biosensor exhibited attractive performance for cancer cell analysis with a detection limit as low as 20 cells per mL for QGY-7701 and 35 cells per mL for QGY-7703, facilitating great promise for the sensitive detection of cancer cells and thus showing potential applications in cancer diagnosis.

Metal-organic frameworks for food contaminant adsorption and detection

Metal-organic framework materials (MOFs) have been widely used in food contamination adsorption and detection due to their large specific surface area, specific pore structure and flexible post-modification. MOFs with specific pore size can be targeted for selective adsorption of some contaminants and can be used as pretreatment and pre-concentration steps to purify samples and enrich target analytes for food contamination detection to improve the detection efficiency. In addition, MOFs, as a new functional material, play an important role in developing new rapid detection methods that are simple, portable, inexpensive and with high sensitivity and accuracy. The aim of this paper is to summarize the latest and insightful research results on MOFs for the adsorption and detection of food contaminants. By summarizing Zn-based, Cu-based and Zr-based MOFs with low cost, easily available raw materials and convenient synthesis conditions, we describe their principles and discuss their applications in chemical and biological contaminant adsorption and sensing detection in terms of stability, adsorption capacity and sensitivity. Finally, we present the limitations and challenges of MOFs in food detection, hoping to provide some ideas for future development.

Coordination engineering strategy of iron single-atom catalysts boosts anti-Cu(II) interference detection of As(III) with a high sensitivity

Mutual interference issues between heavy metal ions tremendously affect the detection reliability and accuracy in water quality analysis, especially the serious interference of Cu(II) on the detection of As(III) is greatly hard to overcome, which needs to be solved urgently. Herein, iron single-atom catalysts with different coordination structures of FeN₂C₂ and FeN₃P are constructed to selectively catalyze the detection of As(III) in the coexistence of Cu(II). FeN₃P achieves a high sensitivity of 3.90 µA ppb^-1 toward As(III) in NH₄Cl/NH₃·H₂O electrolyte (pH 8.0), completely avoiding Cu(II)-interference. Moreover, the turnover frequency (TOF) of FeN₃P is an order of magnitude higher than that of FeN₂C₂. X-ray absorption fine structure (XAFS) spectroscopy and density functional theory (DFT) calculations demonstrate that an As-O bond of H₃AsO₃ is broken by the strong affinities between both P and O atoms and Fe and As atoms, and H₃AsO₃ are preferentially reduced by FeN₃P during adsorptive process. Meanwhile, the low reaction energy barrier of the rate-determined step for As(III) reduction over FeN₃P also accelerates the deposition of As(III) and enhances its response signals. The free-Cu(II) are difficult to adsorb on FeN₃P and do not compete with As(III) for Fe active sites, which contributes to the excellent anti-Cu(II) interference capability.

Rapid Determination of Mercury Ions in Environmental Water Based on an N-Rich Covalent Organic Framework Potential Sensor

In this article, an N-rich covalent organic framework (COFTFPB-TZT) was successfully synthesized using 4,4′,4′-(1,3,5-triazine-2,4,6-triyl) trianiline (TZT), and 4-[3,5-bis (4-formyl-phenyl) phenyl] benzaldehyde (TFPB). The as-prepared COFTFPB-TZT possesses irregular cotton wool patches with a large specific surface area. A novel selective electrode based on COFTFPB-TZT was used for the determination of Mercury ions. The abundance of N atoms in COFTFPB-TZT provides more coordination sites for Hg2+ adsorption, resulting in a change in the surface membrane potential of the electrode to selectively recognize Hg2+. Under optimal experimental conditions, the ion-selective electrode shows a good potential response to Hg2+, with a linear range of 1.0 × 10−9∼1.0 × 10−4, a Nernst response slope of 30.32 ± 0.2 mV/-PC at 25°C and a detection limit of 4.5 pM. At the same time, the mercury-ion electrode shows a fast response time of 10 s and good reproducibility and stability. The selectivity coefficients for Fe2+, Zn2+, As3+, Cr6+, Cu2+, Cr3+, Al3+, Pb2+, NH4+, Ag+, Ba2+, Mg2+, Na+, and K+ are found to be small, indicating no interference in the detection system. The proposed method can be successfully applied to the determination of Hg2+ in 3 typical environmental water samples, with a recovery rate of 98.6–101.8%. In comparison with the spectrophotometric method utilizing dithizone, the proposed method is simple and fast and holds great potential application prospects in environmental water quality monitoring and other fields.

Simple Design Concept for Dual-Channel Detection of Ochratoxin A Based on Bifunctional Metal-Organic Framework

A simple fluorescence and electrochemical dual-channel biosensor based on bifunctional Zr(IV)-based metal-organic framework (Zr-MOF) was proposed to detect Ochratoxin A (OTA). The bifunctional Zr-MOF, with photoluminescence properties and enormous electroactive ligands, was exploited to load OTA-specific aptamers for designing signal probes, greatly simplifying the probe-fabrication process and improving sensing reliability. Upon specific recognition of aptamer toward OTA, the anchored probe was released from the sensing interface into the reaction solution. In this circumstance, the increased amount of the signal probe in reaction solution led to an enhanced fluorescence response, while the decreased amount of the signal probe on the sensing interface resulted in a diminished electrochemical response. According to the dual-channel signal change with increasing OTA concentration, the visual fluorescence strategy was established for intuitive OTA detection, and meanwhile, sensitive electrochemical assay with a detection limit of 0.024 pg/mL was also achieved with the help of one-step electrodeposition as a sensing platform. Moreover, the proposed dual-channel assay has been successfully applied to determine OTA levels in corn samples with rapid response, superior accuracy, and high anti-interference capability, providing a promising method for food safety monitoring.

Ultrasensitive electrochemical sensor for mercury ion detection based on molybdenum selenide and Au nanoparticles via thymine-Hg2+-thymine coordination

An ultrasensitive and specific-selection electrochemical sensor was constructed for Hg²⁺ detection based on Au nanoparticles and molybdenum selenide (Au NPs@MoSe₂) as well as the thymine-Hg²⁺-thymine (T-Hg²⁺-T) coordination. Herein, Au NPs@MoSe₂ not only could improve the sensitivity due to the large surface area and good electrical conductivity but also offered more sites to immobilize thiol-labeled T-rich hairpin DNA probes (P-1), which has a specific recognition for Hg²⁺ and methylene blue-labeled T-rich DNA probes (MB-P). When Hg²⁺ and MB-P exist, P-1 and MB-P can form a stable T-Hg²⁺-T complex. Then, methylene blue can be close to the electrode and detectable via differential pulse voltammetry (DPV). Benefiting from the specific recognition of T-Hg²⁺-T and the merits of Au NPs and MoSe₂, the fabricated biosensor presented an ultrasensitive and highly selective performance. The DPV responses had a positive linear relationship with Hg²⁺ concentrations over ten orders of magnitude from 1.0 × 10^-16 to 1.0 × 10^-7 mol L^-1. The detection limit was down to 1.1 × 10^-17 mol L^-1. Moreover, the developed sensor exhibited a promising application for trace Hg²⁺determination in water samples.

Recyclable macromolecular thermogels for Hg(II) detection and separation via sol-gel transition in complex aqueous environments

The sensitive detection and quantitative separation of toxic heavy metal ions in aqueous media are of great importance. In this study, a thermogelling poly(ε-caprolactone)-poly(ethylene glycol)-poly(ε-caprolactone) (PCL-PEG-PCL) triblock copolymer (P1) was synthesized, and difluoroboron dipyrromethene (BODIPY) fluorophore integrated with thiosemicarbazide units was attached to the chain ends of P1 through consecutive post-polymerization modifications, leading to P4. P4 exhibited rapid and selective detection of Hg(II) in 100% aqueous media via turn-on fluorescence emission with a limit of detection (LOD) of as low as 0.461 μM. This turn-on emission behavior is attributed to the suppression of C˭N isomerization caused by the formation of a coordination complex between P4 and Hg(II) ions. The selective and quantitative removal of Hg(II) among various metal ions was achieved by trapping chelated Hg(II) ions inside the dehydrated P4 gel via thermo-controlled sol-gel-dehydrated gel transitions. Treating the Hg(II) ion-trapped dehydrated gels with sodium sulfide (Na₂S) in acetone/water at room temperature led to HgS precipitates, and P4 in solution was dried and recycled. This recyclable thermoresponsive macromolecular probe is promising for not only Hg(II) detection but also its separation and removal from complex aqueous environments.

Progress in Metal-Organic Frameworks Facilitated Mercury Detection and Removal

Metal Organic Frameworks (MOFs) are noted as exceptional candidates towards the detection and removal of specific analytes. MOFs were reported in particular for the detection/removal of environmental contaminants, such as heavy metal ions, toxic anions, hazardous gases, explosives, etc. Among heavy metal ions, mercury has been noted as a global hazard because of its high toxicity in the elemental (Hg0), divalent cationic (Hg2+), and methyl mercury (CH3Hg+) forms. To secure the environment and living organisms, many countries have imposed stringent regulations to monitor mercury at all costs. Regarding the detection/removal requirements of mercury, researchers have proposed and reported all kinds of MOFs-based luminescent/non-luminescent probes towards mercury. This review provides valuable information about the MOFs which have been engaged in detection and removal of elemental mercury and Hg2+ ions. Moreover, the involved mechanisms or adsorption isotherms related to sensors or removal studies are clarified for the readers. Finally, advantages and limitations of MOFs in mercury detection/removal are described together with future scopes.

Efficient preparation of dual-emission ratiometric fluorescence sensor system based on aptamer-composite and detection of bis(2-ethylhexyl) phthalate in pork

A ratiometric fluorescence sensor system is proposed for detecting bis(2-ethylhexyl) phthalate (DEHP) in pork, which is based on aptamer recognition with molybdenum disulfide quantum dots and cadmium telluride quantum dots (MoS₂ QDs/CdTe-Apta). Two signals exist in the system, among which the response signal is transmitted by CdTe-Apta. The amide condensation between aptamers and CdTe QDs shortens the distance between CdTe QDs and DEHP, thus quenching the fluorescence of CdTe QDs, possibly through a photoinduced electron transfer mechanism. The MoS₂ QDs deliver the self-calibration signal, and the fluorescence of MoS₂ QDs remains almost constant when co-existing with DEHP. Linearity (R² = 0.9536) was established for the DEHP concentration range 0.005-3.0 mg·L^-1, with a limit of detection of 0.21 μg·L^-1. The system was successfully applied in the determination of DEHP in pork. The system has potential for the quantitative determination of DEHP in practical applications.

A novel copper(i) metal–organic framework as a highly efficient and ultrasensitive electrochemical platform for detection of Hg(ii) ions in aqueous solution

A novel copper(i) metal–organic framework was constructed and used to modify a glassy carbon electrode, and exhibits excellent electrochemical sensing of Hg(ii) ions.

Preparation of boron nitrogen co-doped carbon quantum dots for rapid detection of Cr(VI)

A novel fluorescent probe based on the static quenching and the inner filter effect between boron nitrogen co-doped carbon quantum dots (B, N-CDs) and Cr(VI) was developed for the quantitative determination of Cr(VI) in real water samples. B, N-CDs were prepared using the hydrothermal method with ammonium citrate and bis(pinacolato) diboron as raw materials. Compared with undoped CDs, the fluorescence properties of the B, N-CDs were improved. The fluorescence quantum yield of the B, N-CDs was as high as 59.01%. After optimization of the experimental parameters, the B, N-CDs could be used as a fluorescence probe to detect Cr(VI). Strong linear correlation (R² = 0.9986) was established in the Cr(VI) concentration range 0.3-500 μM, and a detection limit of 0.24 μM was achieved. Moreover, the B, N-CDs successfully detected Cr(VI) in real water samples, indicating that they have broad application prospects in the sensitive detection of Cr(VI).

Bio- and Biomimetic Receptors for Electrochemical Sensing of Heavy Metal Ions

Heavy metals ions (HMI), if not properly handled, used and disposed, are a hazard for the ecosystem and pose serious risks for human health. They are counted among the most common environmental pollutants, mainly originating from anthropogenic sources, such as agricultural, industrial and/or domestic effluents, atmospheric emissions, etc. To face this issue, it is necessary not only to determine the origin, distribution and the concentration of HMI but also to rapidly (possibly in real-time) monitor their concentration levels in situ. Therefore, portable, low-cost and high performing analytical tools are urgently needed. Even though in the last decades many analytical tools and methodologies have been designed to this aim, there are still several open challenges. Compared with the traditional analytical techniques, such as atomic absorption/emission spectroscopy, inductively coupled plasma mass spectrometry and/or high-performance liquid chromatography coupled with electrochemical or UV-VIS detectors, bio- and biomimetic electrochemical sensors provide high sensitivity, selectivity and rapid responses within portable and user-friendly devices. In this review, the advances in HMI sensing in the last five years (2016-2020) are addressed. Key examples of bio and biomimetic electrochemical, impedimetric and electrochemiluminescence-based sensors for Hg2+, Cu2+, Pb2+, Cd2+, Cr6+, Zn2+ and Tl+ are described and discussed.