Captivating nano sensors for mercury detection: a promising approach for monitoring of toxic mercury in environmental samples

Mercury, a widespread highly toxic environmental pollutant, poses significant risks to both human health and ecosystems. It commonly infiltrates the food chain, particularly through fish, and water resources via multiple pathways, leading to adverse impacts on human health and the environment. To monitor and keep track of mercury ion levels various methods traditionally have been employed. However, conventional detection techniques are often hindered by limitations. In response to challenges, nano-sensors, capitalizing on the distinctive properties of nanomaterials, emerge as a promising solution. This comprehensive review provides insight into the extensive spectrum of nano-sensor development for mercury detection. It encompasses various types of nanomaterials such as silver, gold, silica, magnetic, quantum dot, carbon dot, and electrochemical variants, elucidating their sensing mechanisms and fabrication. The aim of this review is to offer an in-depth exploration to researchers, technologists, and the scientific community, and understanding of the evolving landscape in nano-sensor development for mercury sensing. Ultimately, this review aims to encourage innovation in the pursuit of efficient and reliable solutions for mercury detection, thereby contributing to advancements in environmental protection and public health.

The construction of dual-emissive ratiometric fluorescent probes based on fluorescent nanoparticles for the detection of metal ions and small molecules

With the rapid development of fluorescent nanoparticles (FNPs), such as CDs, QDs, and MOFs, the construction of FNP-based probes has played a key role in improving chemical sensors. Ratiometric fluorescent probes exhibit distinct advantages, such as resistance to environmental interference and achieving visualization. Thus, FNP-based dual-emission ratiometric fluorescent probes (DRFPs) have rapidly developed in the field of metal ion and small molecule detection in the past few years. In this review, firstly we introduce the fluorescence sensing mechanisms; then, we focus on the strategies for the fabrication of DRFPs, including hybrid FNPs, single FNPs with intrinsic dual emission and target-induced new emission, and DRFPs based on auxiliary nanoparticles. In the section on hybrid FNPs, methods to assemble two types of FNPs, such as chemical bonding, electrostatic interaction, core satellite or core-shell structures, coordination, and encapsulation, are introduced. In the section on single FNPs with intrinsic dual emission, methods for the design of dual-emission CDs, QDs, and MOFs are discussed. Regarding target-induced new emission, sensitization, coordination, hydrogen bonding, and chemical reaction induced new emissions are discussed. Furthermore, in the section on DRFPs based on auxiliary nanoparticles, auxiliary nanomaterials with the inner filter effect and enzyme mimicking activity are discussed. Finally, the existing challenges and an outlook on the future of DRFP are presented. We sincerely hope that this review will contribute to the quick understanding and exploration of DRFPs by researchers.

Luminescent MOF-Based Nanofibers with Visual Monitoring and Antibacterial Properties for Diabetic Wound Healing

Diabetic wound healing remains as a serious challenge for medical circles that required continuous monitoring and effective management. Herein, the glucose oxidase/carbon dots@copper-metal-organic framework-based nanofibers (GOx/CDs@MOF NFs) were proposed as a multifunctional wound dressing, aiming to visually monitor wound pH and inhibit bacterial infection. In the diabetic wound microenvironment, the GOx/CDs@MOF NFs could convert endogenous glucose into hydroxyl radial (•OH) through the cascade catalytic reaction. In vivo and vitro experimental results confirmed that the GOx/CDs@MOF NFs could efficiently kill bacteria and promote wound healing. Additionally, CDs as a pH fluorescent indicator endowed GOx/CDs@MOF NFs with sensitive and reversible fluorescent sensing behavior to wound pH, and these visual images could also be captured by smartphones and transformed into RGB color mode (red, green, blue) values, allowing for onsite evaluation of the wound status. This multifunctional wound dressing provides a smart and effective solution for diabetic wound management and takes an immeasurable step toward the development of the next generation of digitally visualized wound dressings.

Fluorescent hydrogels based on oxidized carboxymethyl cellulose with excellent adsorption and sensing abilities for Ag. Int J Biol Macromol 2022;213:955-966. [PMID: 35690162 DOI: 10.1016/j.ijbiomac.2022.06.029]

Heavy metal contamination in water and soil are harmful and destructive to the environment, it has always been regarded as a big problem. Herein, we developed a self-healing fluorescent hydrogel based on oxidized carboxymethyl cellulose with excellent sensing and adsorption abilities for Ag⁺. The detection and adsorption effects of hydrogels on heavy metal ions were studied. It turned out that the fluorescent hydrogel has sensitive detection and high adsorption capacity for Ag⁺, the detection limit was 3.798 μM, and the maximum adsorption capacity was 407 mg/g. The adsorption isotherm fitted the Langmuir model well, and the pseudo-secondary model for adsorption kinetics fitted well. The hydrogel could heal itself without external stimulus, it could be easily regenerated 7 times without loss of adsorption performance. In short, the prepared hydrogel has capability of self-healing, detecting and adsorbing heavy metal ions at the same time, good mechanical strength, these all made it a promising long-life adsorbent and provided a new way for wastewater treatment.

Detection and Sorption of Heavy Metal Ions in Aqueous Media by a Fluorescent Zr(IV) Metal-Organic Framework Functionalized with 2-Picolylamine Receptor Groups

Increasing global environmental pollution due to heavy metal ions raises the importance of research on new multifunctional materials for simultaneous detection and removal of these contaminants from water resources. In this study, we report a microporous 8-connected Zr⁴⁺ metal-organic framework (MOF) based on a terephthalate ligand decorated with a chelating 2-picolylamine side group (dMOR-2), which shows highly efficient fluorescence sensing and sorption of heavy metal cations. We demonstrate by detailed fluorescence studies the ability of a water-dispersible composite of dMOR-2 with polyvinylpyrrolidone for real-time detection of Cu²⁺, Pb²⁺, and Hg²⁺ in aqueous media. The limits of detection were found to be below 2 ppb for these species, while the system's performance is not affected by the presence of other potentially competitive ions. In addition, sorption studies showed that a composite of dMOR-2 with calcium alginate (dMOR-2@CaA) is an excellent sorbent for Pb²⁺ and Cu²⁺ ions with capacities of 376 ± 15 and 117 ± 4 mg per gram of dMOR-2@CaA, respectively, while displaying the capability for simultaneous removal of various heavy metal ions in low initial concentrations and in the presence of large excesses of other cationic species. Structural and spectroscopic studies with model ligands analogous to our material's receptor unit showed chelation to the 2-picolylamine moiety to be the main binding mode of metal ions to dMOR-2. Overall, dMOR-2 is shown to represent a rare example of a MOF, which combines sensitive fluorescence detection and high sorption capacity for heavy metal ions.

Enhancement of the Detection Performance of Paper-Based Analytical Devices by Nanomaterials

Paper-based analytical devices (PADs), including lateral flow assays (LFAs), dipstick assays and microfluidic PADs (μPADs), have a great impact on the healthcare realm and environmental monitoring. This is especially evident in developing countries because PADs-based point-of-care testing (POCT) enables to rapidly determine various (bio)chemical analytes in a miniaturized, cost-effective and user-friendly manner. Low sensitivity and poor specificity are the main bottlenecks associated with PADs, which limit the entry of PADs into the real-life applications. The application of nanomaterials in PADs is showing great improvement in their detection performance in terms of sensitivity, selectivity and accuracy since the nanomaterials have unique physicochemical properties. In this review, the research progress on the nanomaterial-based PADs is summarized by highlighting representative recent publications. We mainly focus on the detection principles, the sensing mechanisms of how they work and applications in disease diagnosis, environmental monitoring and food safety management. In addition, the limitations and challenges associated with the development of nanomaterial-based PADs are discussed, and further directions in this research field are proposed.

Turning waste into treasure: chicken eggshell membrane derived fluorescent carbon nanodots for the rapid and sensitive detection of Hg2+ and glutathione

Herein, a green, economical, and waste-utilization approach is reported for the synthesis of water-soluble carbon nanodots (C-Dots) with a high fluorescence quantum yield of 19.5%. As a common protein-rich waste, eggshell membrane was selected as a cost-effective and ideal precursor to prepare C-Dots using the microwave method. The as-prepared C-Dots showed a maximum emission at 375 nm with an excitation wavelength at 235 nm. The fluorescent C-Dots were adopted as a sensitive probe for the rapid detection of Hg²⁺ and glutathione (GSH) based on the fluorescence off and on (turn-off-on) strategy. This was ascribed to the fact that Hg²⁺ could effectively quench the fluorescence of the C-Dots and GSH was able to prevent fluorescence quenching owing to the specific binding between Hg²⁺ and GSH. The designed method exhibited a high sensitivity and selectivity towards the detection of Hg²⁺ and GSH. Under the optimized conditions, the method showed a good linear relationship with Hg²⁺ concentration in the range from 100 nM to 50 μM with a detection limit of 32.0 nM. For GSH detection, it displayed a linear range from 50 nM to 10 μM with a detection limit of 9.8 nM. Moreover, this method was successfully applied to detect GSH in human serum samples. The eggshell derived fluorescent C-Dots pave the way for economical environmental and biological analyses.

Encapsulation of gold nanoclusters: stabilization and more

Gold nanoparticles with only a few atoms, known as gold nanoclusters (AuNCs), have dimensions below 2 nm and feature singular properties such as size dependent luminescence. AuNCs are also highly photostable and have catalytic activity, low toxicity and good biocompatibility. With these properties, they are extremely promising candidates for application in bioimaging, sensing and catalysis. However, when stabilized only with small capping ligands, their use is hindered by lack of colloidal stability. Encapsulation of the AuNCs can contribute to provide a more robust protection and even to improve their properties. Here, we review the encapsulation of AuNCs in polymers, silica and metal organic frameworks (MOFs) for applications in bioimaging, sensing and catalysis.

Engineering of an Upconversion Luminescence Sensing Platform Based on the Competition Effect for Mercury-Ion Monitoring in Green Tea

Accurately monitoring mercury ions (Hg²⁺) in food and agriculture-related matrixes (e.g., green tea) is of great significance to safeguard food safety. Here, we employed upconversion nanoparticles (UCNPs) and gold nanoparticles (AuNPs) to engineer a cysteine (Cys)-assisted anti-Stokes luminescence sensing platform (UCNPs-AuNPs) for precisely detecting residual Hg²⁺ in green tea through the competition effect. Initially, AuNPs could effectively quench the luminescence of UCNPs through the luminescence resonance energy transfer process, which was then interrupted by Cys-triggered AuNP aggregation via Au-S, thereby restoring UCNP luminescence. Interestingly, owing to the competition effect with AuNPs toward Cys, Hg²⁺ could weaken the luminescence restoring efficiency, achieving a Hg²⁺ concentration-dependent luminescence change. On this basis, a facile, reliable, and sensitive upconversion luminescence sensing platform for monitoring residual Hg²⁺ in green tea was successfully established. This study offers a novel insight into integrating the competition effect and anti-Stokes luminescence for food- and agriculture-related contaminant monitoring.

Development of carbon dot-thiochrome-based sensing system for ratiometric fluorescence detection of D-penicillamine

A simple and rapid ratiometric fluorescent sensing system for D-penicillamine (D-PA) determination is developed based on yellow carbon dots (Y-CDs) combined with thiochrome (oxVB₁) for the first time. The oxidization of thiamine (VB₁) can be catalyzed by Alkaline-hydrolyzed artemisinin (a-ART) to form oxVB₁, which leads to the occurrence of fluorescence emission peak at 466 nm. Furthermore, the oxidation reaction between a-ART and VB₁ could be inhibited by D-PA, and accompanied with the decrease of fluorescence at 466 nm. However, the fluorescence peak of Y-CDs as an internal reference at 566 nm was almost unchanged. The ratiometric signal changes contributed to a robust and sensitive D-PA sensing. Under the optimal condition, a good linear response for the D-PA detection was obtained in the ranges of 0.5-50 μM with a detection limit of 0.33 μM. In addition, Y-CDs and thiochrome-based sensing system was applied to D-PA determination in real samples and obtained acceptable results. We developed a new carbon dots/thiochrome fluorescent nanoprobe for ratiometric fluorescence sensing of D-penicillamine.

Dual-emission ratio fluorescence for selective and sensitive detection of ferric ions and ascorbic acid based on one-pot synthesis of glutathione protected gold nanoclusters

A fluorometric method was proposed for the determination of Fe³⁺ and ascorbic acid (AA) based on blue and red dual fluorescence emissions of glutathione (GSH) stabilized-gold nanoclusters (AuNCs). AuNCs were synthesized from GSH and tetrachloroauric acid. The fluorescence peaks of AuNCs were at 425 nm and 585 nm, respectively. In the presence of Fe³⁺, the fluorescence peak at 425 nm can be enhanced and that at 585 nm can be quenched. There is a good linear relationship between the fluorescence intensity ratio for the 425 and 585 nm peaks (F ₄₂₅/F ₅₈₅) and the concentration of Fe³⁺ in the range of 0.75-125 μM. However, when AA was added to the AuNCs-Fe³⁺ system, the value of F ₄₂₅/F ₅₈₅ decreased consistently with the concentration of AA in the range of 0.25-35 μM. The limit of detection for Fe³⁺ and AA was 227 and 75.8 nM, respectively. The interaction between AuNCs and Fe³⁺ can induce the ligand-metal charge transfer (LMCT) effect leading to the fluorescence increment at 425 nm, while AA can reduce Fe³⁺ to Fe²⁺. The production of Fe²⁺ can not enhance or quench the fluorescence of AuNCs. By comparison with previous literature, the AuNCs prepared here show two fluorescence peaks without additional fluorescence labels. Furthermore, the method was successfully applied in the determination of Fe³⁺ and AA in some real samples, such as water, human serum and tablets.

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.

Ingenious dual-emitting Ru@UiO-66-NH2 composite as ratiometric fluorescence sensor for detection of mercury in aqueous

The increasing deterioration of ecosystem derived from heavy metals residues brings about the environmental and food contamination, which presses the exploration of facile platform for monitoring heavy metals. Herein, a ratiometric fluorescence sensor was designed for Hg²⁺ detection based on the compound of UiO-66-NH₂ and Ru(bpy)₃²⁺ (Ru@UiO-66-NH₂) which was synthesized by situ encapsulation. The innovative composite displayed two emission peaks at 437 and 604 nm, and the addition of Hg²⁺ could only quench the blue fluorescence due to static quenching and photo-induced electron transfer mechanism, providing an internal standard to promote the precision. Under optimal conditions, the ratiometric Ru@UiO-66-NH₂ probe revealed outstanding anti-interference capability and performed with a great limit of detection (LOD) of 0.053 μM for Hg²⁺, which was 2-fold lower than that of single-color UiO-66-NH₂. By merit of Ru@UiO-66-NH₂, test hydrogels were fabricated to provide a tactics for visual, rapid and on-site detection of Hg²⁺. Additionally, the dual-emitting sensing platform presented satisfactory recoveries and reliabilities in lake water, tap water, and drink water, demonstrating the application potential of this proposed ratiometric fluorescence sensor for monitoring Hg²⁺.

A simple sensitive ratiometric fluorescent probe for the detection of mercury ions in living cells and zebrafish

Mercury, as a highly toxic heavy metal, can cause very serious harm to human health and even death in severe cases. Therefore, we synthesized a novel ratiometric fluorescent probe for detecting mercury ions, with mercaptoethanol as the recognition receptor. Probe CMER could determine mercury ions in 0-1.6 μM and the detection limit is 7.6 nM. Moreover, CMER manifested a fast response for Hg²⁺ (within 5 s) and simultaneously observed that the color changed from light yellow to orange by naked eye. In addition to these preeminent spectral properties, the probe also had satisfactory bioimaging results in RAW 264.7 macrophage cells and zebrafish.

Dual-emission copper nanoclusters-based ratiometric fluorescent probe for intracellular detection of hydroxyl and superoxide anion species

A fluorescent nanoprobe based on copper nanoclusters (CuNCs) has been developed for ratiometric detection of hydroxyl radicals (^•OH) and superoxide anion radicals (O₂^•-). Two differently luminescent CuNCs, namely cyan-emissive poly(methacrylic acid)-protected copper nanoclusters (PCuNCs) and orange-emissive bovine serum albumin-protected CuNCs (BCuNCs), were conjugated to obtain a hybrid, dual-emission nanoprobe (PCuNCs-BCuNCs) with the corresponding peaks at 445 nm and 652 nm at an excitation wavelength of 360 nm. In particular, the fluorescence peak at 445 nm gradually enhanced with the incremental addition of ^•OH and O₂^•-. However, the fluorescence emission at 652 nm was greatly quenched in the presence of ^•OH, while in case of O₂^•-, the fluorescence intensity remained constant. The differential response of the PCuNCs-BCuNCs towards ^•OH and O₂^•- formed the basis of ratiometric detection. Under optimal conditions, the PCuNCs-BCuNCs exhibited good sensitivity and linearity towards ^•OH and O₂^•- with limits of detection of 0.15 μM and 1.8 μM, respectively. Moreover, the nanoprobe exhibited high selectivity for ^•OH and O₂^•- over other potential ROS interferences. Besides, PCuNCs-BCuNCs were eventually applied for qualitative and quantitative ratiometric assessment of intracellular ^•OH and O₂^•- in L-132 cells. Therefore, this strategy unveils a new potential for copper nanocluster-based sensing of ROS.

Recent Progress on Luminescent Metal-Organic Framework-Involved Hybrid Materials for Rapid Determination of Contaminants in Environment and Food

The high speed of contaminants growth needs the burgeoning of new analytical techniques to keep up with the continuous demand for monitoring and legislation on food safety and environmental pollution control. Metal-organic frameworks (MOFs) are a kind of advanced crystal porous materials with controllable apertures, which are self-assembled by organic ligands and inorganic metal nodes. They have the merits of large specific surface areas, high porosity and the diversity of structures and functions. Latterly, the utilization of metal-organic frameworks has attracted much attention in environmental protection and the food industry. MOFs have exhibited great value as sensing materials for many targets. Among many sensing methods, fluorometric sensing is one of the widely studied methods in the detection of harmful substances in food and environmental samples. Fluorometric detection based on MOFs and its functional materials is currently one of the most key research subjects in the food and environmental fields. It has gradually become a hot research direction to construct the highly sensitive rapid sensors to detect harmful substances in the food matrix based on metal-organic frameworks. In this paper, we introduced the synthesis and detection application characteristics (absorption, fluorescence, etc.) of metal-organic frameworks. We summarized their applications in the MOFs-based fluorometric detection of harmful substances in food and water over the past few years. The harmful substances mainly include heavy metals, organic pollutants and other small molecules, etc. On this basis, the future development and possible application of the MOFs have prospected in this review paper.

Integration of fluorescent probes into metal–organic frameworks for improved performances

Recent years have witnessed a rapid development of fluorescent probes in both analytical sensing and optical imaging. Enormous efforts have been devoted to the regulation of fluorescent probes during their development, such as improving accuracy, sensitivity, selectivity, recyclability and overcoming the aggregation-caused quenching effect. Metal–organic frameworks (MOFs) as a new class of crystalline porous materials possess abundant host–guest chemistry, based on which they display a great application potential in regulating fluorescent probes. This review summarized the research works on the regulation of fluorescent probes using MOFs, with emphasis on the methods of integrating fluorescent probes into MOFs, the regulation effects of MOFs on fluorescent probes, the superiorities of MOFs in regulating fluorescent probes, and the outlook of this subject. It is desirably hoped that this review can provide a useful reference for the researchers interested in this field.

This review surveyed the research works for the regulation of fluorescent probes with metal–organic frameworks based on host–guest chemistry.

DNA three-way junction-actuated strand displacement for miRNA detection using a fluorescence light-up Ag nanocluster probe

A rapid and label-free fluorescence biosensing strategy for highly sensitive detection of microRNA-122 (miR-122) has been developed by the combination of DNA three-way junction (TWJ)-actuated strand displacement and a fluorescence light-up Ag nanocluster (AgNC) probe. In the presence of target miR-122, the attachment of miR-122 to its complementary DNA results in the unblocking of the toehold and branch migration domains in the TWJ, activating the strand displacement reaction (SDR) accompanied by the proximity between the G-rich DNA probe and DNA-AgNC probe; thus a remarkably enhanced fluorescence signal of AgNCs can be obtained owing to the G-rich fluorescence enhancement mechanism. The results reveal that this biosensor exhibits superb specificity and high sensitivity toward miR-122 with a detection limit of 0.030 nM. In addition, the practicality of the biosensor is demonstrated by analyzing miR-122 in three cell lines with satisfactory results. Furthermore, by the utilization of the toehold-mediated SDR and DNA-AgNC conjugates, this proposed strategy offers the advantages of rapidness, convenience, low cost, and simplified operation without the need for biological labeling and the addition of enzymes. Thus, the constructed biosensor might provide a valuable and practical tool for detecting miRNA and the related clinical diagnosis and fundamental biomedicine research.