Photoluminescent AuNCs@UiO-66 for Ultrasensitive Detection of Mercury in Water Samples

Fluorescent chemosensors facilitate the visualization of plant health and their living environment in sustainable agriculture

Globally, 91% of plant production encounters diverse environmental stresses that adversely affect their growth, leading to severe yield losses of 50-60%. In this case, monitoring the connection between the environment and plant health can balance population demands with environmental protection and resource distribution. Fluorescent chemosensors have shown great progress in monitoring the health and environment of plants due to their high sensitivity and biocompatibility. However, to date, no comprehensive analysis and systematic summary of fluorescent chemosensors used in monitoring the correlation between plant health and their environment have been reported. Thus, herein, we summarize the current fluorescent chemosensors ranging from their design strategies to applications in monitoring plant-environment interaction processes. First, we highlight the types of fluorescent chemosensors with design strategies to resolve the bottlenecks encountered in monitoring the health and living environment of plants. In addition, the applications of fluorescent small-molecule, nano and supramolecular chemosensors in the visualization of the health and living environment of plants are discussed. Finally, the major challenges and perspectives in this field are presented. This work will provide guidance for the design of efficient fluorescent chemosensors to monitor plant health, and then promote sustainable agricultural development.

Quantum Dot-Based Fluorometric Sensor for Hg(II) in Water Customizable for Onsite Visual Detection

An easy naked-eye detection technique for mercuric ions in water using silanized quantum dots is demonstrated. Cadmium selenide quantum dots were synthesized and rendered water soluble by silica overcoating. The quantum dot emission was instantly turned off by the mercuric ions in the analyte, enabling visual detection. The emission quenching was associated with a concomitant bathochromic shift, both in the absorption and emission profiles. The underlying mechanism is a permanent surface modification of quantum dots by mercuric ions, altering the electronic structure and, in turn, the photophysical properties. The results confirmed the potential of this simple system to be customized for on-site visual detection of mercury contamination in water bodies, biological fluids, and soil with high selectivity and sensitivity.

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.

Electrochemiluminescence paper-based screen-printed electrode for HbA1c detection using two-dimensional zirconium metal-organic framework/Fe3O4 nanosheet composites decorated with Au nanoclusters

Glycated hemoglobin (HbA1c) is one of the most popular biomarkers which can be utilized for the diagnosis and control of diabetes in clinical practice. In this study, a sandwich paper-based electrochemiluminescence (ECL) biosensor has been developed using the zirconium metal-organic framework/Fe₃O₄(trimethyl chitosan)/gold nanocluster (Zr-MOF/Fe₃O₄(TMC)/AuNCs) nanocomposite as tracing tag to label anti-HbA1c monoclonal antibody and reduced graphene oxide (rGO) as immobilization platform of sensing element. The screen-printed electrodes (SPEs) were constructed and modified by sputtering a thick layer of gold on the paper substrate, followed by electrochemical reduction of aminophenylboronic acid (APBA)-functionalized GO to rGO/APBA, respectively. Different types of surface analysis methods were applied to characterize the Zr-MOF/Fe₃O₄(TMC)/AuNCs nanomaterials fabricated. Finally, antibody-labeled Zr-MOF/Fe₃O₄(TMC)/AuNCs nanocomposites were subjected to HbA1c in the sample and on the paper-based SPE. Quantitative measurement of HbA1c was performed using ECL and cyclic voltammetry (CV) over a potential range of - 0.2 to 1.7 V vs gold reference electrode with a sweep rate of 0.2 V.s^-1 in the presence of triethylamine as a co-reactant after sandwiching the HbA1c target between antibody and APBA on the sensing area. This immunosensor demonstrated the desirable assay performance for HbA1c with a wide response range from 2 to 18% and a low detection limit (0.072%). This new strategy provides an effective method for high-performance bioanalysis and opens avenues for the development of high-sensitive and user-friendly device. Graphical abstract.

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.

pH-Responsive metal-organic framework encapsulated gold nanoclusters with modulated release to enhance photodynamic therapy/chemotherapy in breast cancer

Gold nanoclusters (AuNCs) with an ultra-small size, as new inorganic photosensitizers, have been shown to be promising in photodynamic therapy (PDT), but their application has been restricted due to short blood circulation. It is therefore important to develop stimuli-responsive AuNC-based nanoprobes to achieve highly efficient PDT. Here, metal-organic framework (MOF, ZIF-8) encapsulated AuNCs (AuNCs@MOF) were synthesized, and then they were loaded with doxorubicin (DOX) to obtain pH-responsive nanoprobes (AuNCs@MOF-DOX) with modulated release for enhanced PDT/chemotherapy. In an acidic tumor microenvironment, the structure of ZIF-8 collapsed, accelerating the release of the AuNCs and DOX in the tumor cells, and enhancing the performance of PDT/chemotherapy. Under irradiation with a 670 nm laser, a large amount of singlet oxygen was generated, and the release rate of DOX increased to 77.1% at a pH value of 5.5. By single PDT and single chemotherapy, the tumors were only partially inhibited, but they completely disappeared using the combination of PDT and chemotherapy. The prepared pH-responsive AuNCs@MOF-DOX nanoprobes with modulated release showed excellent PDT/chemotherapy performance, and will be important bi-functional nanoprobes for synergistic therapy.

Aggregation-induced emission enhancement of gold nanoclusters in metal-organic frameworks for highly sensitive fluorescent detection of bilirubin

A fluorescence analysis method based on gold nanocluster (AuNC) and metal-organic framework (MOF) composite materials (AuNCs@ZIF-8) was established for highly sensitive detection of bilirubin (BR). First, AuNCs@ZIF-8 was successfully obtained by co-precipitation and displayed an aggregation-induced emission enhancement by the confinement effect of the MOFs (i.e., ZIF-8). The product showed approximately 7.0 times enhancement in the quantum yield and longer fluorescence lifetime from 2.29 μs to 11.51 μs compared with AuNCs. When BR combined with the metal node Zn2+ of ZIF-8, the skeleton of the composite was destroyed, leading to a great decrease in the fluorescence intensity by the transformation of the AuNCs from the aggregated state to dispersed state. The linear range for the detection of BR was 0.1-5.0 μM, with the limit of detection (LOD) of 0.07 μM (S/N = 3). The AuNCs@ZIF-8 exhibited a selective response toward BR within 5 min and detected BR in human serum. The long-wavelength emission by AuNCs avoided the interference of the complex biomatrix background fluorescence, indicating their great application prospects for clinical diagnosis.

UiO series of metal-organic frameworks composites as advanced sorbents for the removal of heavy metal ions: Synthesis, applications and adsorption mechanism

Heavy metal pollution has threatened the ecological environment and human health, therefore, effective removal of these toxic pollutants from various complex substrates is of great significance. So far, adsorption is still one of the most effective approaches. Metal-organic frameworks (MOFs), which are porous crystalline materials consisting of metal ions or metal clusters and organic ligands through coordination bonds. Due to their high surface area, porosity, as well as good chemical/thermal stability, the materials have recently attracted great attention in environmental analytical chemistry. This review mainly focused on the recent studies about the applications of UiO series MOFs and their composites as the emerging MOFs, which have been used effectively for the adsorption and removal of diverse heavy metal ions from a variety of environmental samples as novel adsorption materials. Moreover, an elaboration about UiO-MOFs and its composites including the synthetic methods and the applications of these materials in the removal of heavy metal ions were presented in detail. In addition, the adsorption characteristics and mechanism of UiO-MOFs as solid sorbents for heavy metal ions were discussed, including adsorption isotherms equation, adsorption thermodynamics, and kinetics. To this end, the developing trends of MOF-based composites for the removal of heavy metal ions had also prospected. This review will provide a new idea for the study of the adsorption mechanism of heavy metal ions on sorbents and the development of high-performance media for the efficient removal of pollutants in wastewater.

AuNCs-Catalyzed Hydrogen Selenide Oxidation: Mechanism and Application for Headspace Fluorescent Detection of Se(IV)

The excellent fluorescence property of Au nanoclusters (AuNCs) has received great attention for various chemosensing and biorelated applications, but the sample matrix is still an important problem that causes undesirable fluorescence variation. On the one hand, hydride generation (HG) is an effective strategy to separate the target analyte from the complex sample matrices, but the implementation of HG with AuNC-based fluorescent assays was not realized. On the other hand, due to the ultrasmall size of AuNCs and good catalytic performance of Au, AuNCs are also featuring intriguing catalytic applications. Herein, we proposed a new type of AuNC-based fluorescence assay for Se(IV) detection, in which hydride generation of Se(IV) was coupled with the fluorescence/catalytic dual functions of AuNCs. In a batch hydride generation mode, Se(IV) was first converted to volatile H₂Se. When it spread in the headspace to contact with AuNCs supported paper, AuNC-catalyzed oxidation of H₂Se by O₂ to yield elemental selenium occurred, which further deposited on the surface of AuNCs to induce fluorescence quenching. The catalytic effect of AuNCs was studied in depth via both experimental and theoretical (density functional theory) investigations. Three main steps for H₂Se oxidation were identified, with energy barriers in the presence of AuNCs significantly lower than those without. Benefiting from the reduced matrix interference by hydride generation and the unique catalysis/fluorescence of AuNCs, the proposed assay featured high selectivity, good sensitivity, and simplicity, with successful applications for selenium detection in real samples.

Biosynthesized Highly Stable Au/C Nanodots: Ideal Probes for the Selective and Sensitive Detection of Hg2+ Ions

The enormous ongoing industrial development has caused serious water pollution which has become a major crisis, particularly in developing countries. Among the various water pollutants, non-biodegradable heavy metal ions are the most prevalent. Thus, trace-level detection of these metal ions using a simple technique is essential. To address this issue, we have developed a fluorescent probe of Au/C nanodots (GCNDs-gold carbon nanodots) using an eco-friendly method based on an extract from waste onion leaves (Allium cepa-red onions). The leaves are rich in many flavonoids, playing a vital role in the formation of GCNDs. Transmission electron microscopy (TEM) and Scanning transmission electron microscopy-Energy-dispersive X-ray spectroscopy (STEM-EDS) elemental mapping clearly indicated that the newly synthesized materials are approximately 2 nm in size. The resulting GCNDs exhibited a strong orange fluorescence with excitation at 380 nm and emission at 610 nm. The GCNDs were applied as a fluorescent probe for the detection of Hg2+ ions. They can detect ultra-trace concentrations of Hg2+ with a detection limit of 1.3 nM. The X-ray photoelectron spectroscopy results facilitated the identification of a clear detection mechanism. We also used the new probe on a real river water sample. The newly developed sensor is highly stable with a strong fluorescent property and can be used for various applications such as in catalysis and biomedicine.