Bimetallic AuCu nanoclusters-based florescent chemosensor for sensitive detection of Fe3+ in environmental and biological systems

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.

A Polycaprolactone-Capped ZnO Quantum Dots-Based Fluorometric Sensor for the Detection of Fe3+ Ions in Seawater

Fe3+ ion plays a very active role in life, agriculture, and industry. Human health and the environment are seriously affected by the abnormal presence or excess of this cation. Therefore, the development of a fast, reliable, sensitive, and simple fluorescent probe to detect this cation is crucial. In the present paper, polycaprolactone-capped zinc oxide quantum dots were prepared for the determination of Fe3+ ions. The proposed fluorescent chemosensor exhibited a fluorometric and strong quenching effect toward Fe3+ ions at two wavelengths (303 and 602 nm). The limit of detection (LOD) was calculated as 0.410, and 0.333µM at the mentioned wavelengths. Also, the binding stoichiometric ratio was calculated as 1:1 by Job's plot. The findings indicated that the PCL@ZnO colorimetric chemosensor could be successfully applied with reliable, and good accuracy for the detection of Fe3+ ions in real seawater samples.

A ratiometric luminescence sensing platform based on lanthanide-based silica nanoparticles for selective and sensitive detection of Fe3+ and Cu2+ ions

Detection of Fe(III) and Cu(II) in water is highly desirable because their abnormal levels can cause serious harm to human health and environmental safety. In this work, a ratiometric luminescence sensing platform based on lanthanide-based silica nanoparticles was constructed for the detection of Fe³⁺ and Cu²⁺ ions. The terbium-silica nanoparticles (named SiO₂@Tb) with dual-emission signals were successfully prepared by grafting Tb³⁺ ions onto trimellitic anhydride (TMA) functionalized silica nanospheres. It can serve as a ratiometric fluorescent probe for the detection of Fe³⁺ and Cu²⁺ ions in water with the green emission of Tb³⁺ ions as a response signal and the blue emission of silica nanospheres as the reference signal. Significantly, an easy-to-differentiate color change for visual detection was also realized. SiO₂@Tb shows high sensitivity even in very low concentration regions towards the sensing of Fe³⁺ and Cu²⁺ with low detection limits of 0.75 μM and 0.91 μM, respectively. Moreover, the mechanism for the luminescence quenching of SiO₂@Tb was systematically investigated, and was attributed to the synergetic effect of the absorption competition quenching (ACQ) mechanism and cation exchange. This study demonstrates that SiO₂@Tb can be employed as a promising fluorescent probe for the detection of Fe³⁺ and Cu²⁺ ions, and the combination of lanthanide ions with silica nanoparticles is an effective strategy to construct a ratiometric fluorescent sensing platform for the determination of analytes in environmental detection.

Deoxynivalenol fluorescence aptasensor based on AuCu bimetallic nanoclusters and MoS2

Aptamers against deoxynivalenol (DON) were selected through capture-systematic evolution of ligands by exponential enrichment. Through isothermal titration calorimetry and fluorimetric assay, aptamer candidate DN-2 demonstrated good affinity to DON with K_d value of 40.36 ± 6.32 nM. Accordingly, a Forster resonance energy transfer aptasensor was fabricated by using the aptamer DN-2 combined with AuCu bimetallic nanoclusters as energy donor and MoS₂ nanosheets as energy acceptor. Under the optimal conditions, the fluorescence response was utilized for DON quantitative determination ranging from 5 to 100 ng/mL with a detection limit of 1.87 ng/mL. The practical application of this method was verified in maize flour samples and demonstrated a satisfied recovery of 94.6 ~ 103.1%. The obtained aptamers and their application in DON determination provide a new tool for DON monitoring in various foodstuff.

Fluorescent Oxygen-Doped g-C3N4 Quantum Dots for Selective Detection Fe3+ Ions in Cell Imaging

Herein, oxygen-doped g-C₃N₄ quantum dots (OCNQDs) were fabricated through sintering and ultrasonic-assisted liquid-phase exfoliation methods. The obtained OCNQDs with uniform size show high crystalline quality, and the average diameter is 6.7 ± 0.5 nm. Furthermore, the OCNQDs display excellent fluorescence properties, good water solubility, and excellent photo stability. The OCNQDs as fluorescence probe show high sensitivity and selectivity to Fe³⁺ ions. Furthermore, the fluorescent OCNQDs are applied for live cell imaging and Fe³⁺ ions detecting in living cells with low cytotoxicity, good biocompatibility, and high permeability. Overall, the fluorescent OCNQDs fabricated in this work can be promising candidates for a range of chemical sensors and bioimaging applications.

Bovine serum albumin functionalized blue emitting Ti3 C2 MXene Quantum Dots as a sensitive fluorescence probe for Fe3+ ions detection and its toxicity analysis

In the present work, an improved class of protein functionalized fluorescent 2D Ti₃ C₂ MXene quantum dots (MXene QDs) was prepared by hydrothermal method. Exfoliated 2D Ti₃ C₂ sheets is used as the starting precursor and transport protein bovine serum albumin (BSA) is used to functionalize the MXene QDs. BSA functionalized MXene QDs exhibited excellent photophysical property and stability at various physiological parameters. The HR-TEM analysis showed that the BSA@MXene QDs are quasi-spherical in shape with a size of ~2 nm. The fluorescence intensity of BSA@MXene QDs was selectively quenched in the presence of Fe³⁺ ions. The mechanism of fluorescence quenching was further substantiated using time resolved fluorescence and Stern-Volmer analysis. The sensing assay showed a linear response within the concentration range of 0-150 μM of Fe³⁺ ions with excellent limit of detection. BSA@MXene QDs probe showed good selectivity toward ferric ions even in the presence of other potential interferences. The practical applicability of BSA@MXene QDs was further tested in real samples for Fe³⁺ ions quantification and the sensor had good recovery rates. The cytotoxicity of the BSA@MXene QDs toward the human glioblastoma cells assay revealed that BSA@MXene QDs are biocompatible at lower doses possesses significant cytotoxicity.

A visualized ratiometric fluorescence sensing system for copper ions based on gold nanoclusters/perovskite quantum dot@SiO2 nanocomposites

Excessive copper ions (Cu²⁺) cause serious environmental pollution and even endanger the health of organisms. Fluorescence chemosensing materials are widely used in the detection of metal ions due to their simple operation and high sensitivity. In this study, SiO₂-encapsulated single perovskite quantum dot (PQD@SiO₂) core-shell nanostructures which show strong, stable, and green fluorescence are synthesized and composited with gold nanoclusters (AuNCs) which show Cu²⁺-sensitive and red light-emitting fluorescence to obtain a visualized ratiometric fluorescence sensor (AuNCs/PQD@SiO₂) for the detection of Cu²⁺. In the visualized detection of Cu²⁺, the green fluorescence emitted from the ion-insensitive PQD@SiO₂ component is used as a reference signal and the red fluorescence emitted by ion-sensitive AuNC component is adopted as a sensing signal. In the presence of Cu²⁺, the red fluorescence is quenched whereas the green fluorescence remains stable, which results in a visualized fluorescence color change from orange-red to yellow and finally to green with increasing Cu²⁺ concentration. The significant change in the fluorescence color of AuNCs/PQD@SiO₂ in response to Cu²⁺ enables a rapid, sensitive, and visualized detection of Cu²⁺. Further accurate and sensitive ratiometric fluorescence analysis of Cu²⁺ can be accomplished by measuring the ratio of fluorescence intensities at 643 and 520 nm (I₆₄₃/I₅₂₀) at a certain Cu²⁺ level. The developed AuNCs/PQD@SiO₂-based sensor has been validated by its satisfactory application in the detection of Cu²⁺ in human serum and environmental water samples.

Synthesis of Copper Nanocluster and Its Application in Pollutant Analysis

Copper nanoclusters (Cu NCs) with their inherent optical and chemical advantages have gained increasing attention as a kind of novel material that possesses great potential, primarily in the use of contaminants sensing and bio-imaging. With a focus on environmental safety, this article comprehensively reviews the recent advances of Cu NCs in the application of various contaminants, including pesticide residues, heavy metal ions, sulfide ions and nitroaromatics. The common preparation methods and sensing mechanisms are summarized. The typical high-quality sensing probes based on Cu NCs towards various target contaminants are presented; additionally, the challenges and future perspectives in the development and application of Cu NCs in monitoring and analyzing environmental pollutants are discussed.

The synthesis of novel fluorescent bimetal nanoclusters for aqueous mercury detection based on aggregation-induced quenching

In this research, new bimetal nanoclusters (DAMP-AuAg BNCs) with 4,6-diamino-2-mercaptopyrimidine (DAMP) as a reducing agent and stabilizer ligand were exploited. The nanoclusters displayed excellent fluorescent properties, very small size, good stability, and water solubility. It was found that the as-prepared DAMP-AuAg BNCs exhibited strong fluorescent emission at 640 nm under an excitation wavelength of 473 nm with a large Stokes shift of 167 nm, and the red fluorescence could be readily quenched with aqueous Hg²⁺. The DAMP-AuAg BNCs showed good specificity and sensitivity toward Hg²⁺ in aqueous solution, and the fluorescence analysis of Hg²⁺ showed a wide linear range from 0.85 μM to 246 μM and a detection limit of 20 nM. It is demonstrated that strong Hg²⁺-Au⁺ interactions led to the aggregation of nanoclusters, which caused the quenching of the fluorescence, and the affinity of Hg²⁺ for nitrogen should also be considered. Due to the relevant good performance of DAMP-AuAg BNCs, they were applied to the fluorescence analysis of Hg²⁺ in real water samples and were found to be a potential fluorescent sensor for aqueous mercury ions.

Hydrothermal synthesis of two-dimensional cadmium(II) micro-porous coordination material based on Bi-functional building block and its application in highly sensitive detection of Fe3+ and Cr2O72

Metal-organic framework (MOFs), also known as porous coordination polymers (PCPs), is a new kind of crystalline porous materials, which has received extensive attention in the past few decades. As a new type of sensing material, MOFs stand out from many other traditional fluorescence sensors because of its crystal characteristics, structural diversity, stable porosity and adjustable functional characteristics. In this work, the bi-functional building block containing aromatic carboxylic acid and triazole moieties, namely 3-(1H-1,3,4-triazol-1-yl) benzoic acid, was selected as the linker to synthesize {[Cd(µ₅-L)⋅I}_n (1, HL = 3-(1H-1,3,4-triazol-1-yl)benzoic acid) by hydrothermal method with transition Cd^II metal centers. Firstly, the preliminary characterization of 1 was carried out by means of PXRD, FT-IR, and then the UV and fluorescence tests were conducted to study the fluorescence properties of 1. The crystal structure analysis indicates that Cd^II is the center and the ligand is bridged to form a two-dimensional porous structure. In addition, 1 has good selectivity for Fe³⁺ and Cr₂O₇^2-, meanwhile, it has high detection sensitivity (K_sv quenching efficiency for Fe³⁺: 1.2 × 10⁴ M^-1 and Cr₂O₇^2- 1.85 × 10⁴ M^-1) and low detection limit (Fe³⁺: 19.21 μM and Cr₂O₇^2-: 12.46 μM). The results of photoluminescence test show that 1 can detect cations and anions with high sensitivity, resist the interference of other ions, and have good reusability. As far as we know, 1 is the first example of ultra-stable two-dimensional (2D) Cadmium (II) microporous coordination material as a fluorescence sensor for Fe³⁺ and Cr₂O₇^2-.

A Fluorescent Detection for Paraquat Based on β-CDs-Enhanced Fluorescent Gold Nanoclusters

In this report, a fluorescent sensing method for paraquat based on gold nanoclusters (AuNCs) is proposed. It was found that paraquat could quench both glutathione-capped AuNCs (GSH-AuNCs) and β-cyclodextrin-modified GSH-AuNCs (GSH/β-CDs-AuNCs). The modification of β-CDs on the surface of GSH-AuNCs obviously enhanced the fluorescence intensity of GSH-AuNCs and improved the sensitivity of paraquat sensing more than 4-fold. This sensibilization was ascribed to the obvious fluorescence intensity enhancement of GSH-AuNCs by β-CDs and the “host–guest” interaction between paraquat and β-CDs. The fluorescence quenching was mainly due to the photoinduced energy transfer (PET) between GSH/β-CDs-AuNCs and paraquat. With the optimized β-CDs modification of the GSH-AuNC surfaces and under buffer conditions, the fluorescent detection for paraquat demonstrated a linear response in the range of 5.0–350 ng/mL with a detection limit of 1.2 ng/mL. The fluorescent method also showed high selectivity toward common pesticides. The interference from metal ions could be easily masked by ethylene diamine tetraacetic acid (EDTA). This method was applied to the measurement of paraquat-spiked water samples and good recoveries (93.6–103.8%) were obtained. The above results indicate that host molecule modification of fluorescent metal NC surfaces has high potential in the development of robust fluorescent sensors.

Dopamine-Modified AuCu Bimetallic Nanoclusters as Charge Transfer-Based Biosensors for Highly Sensitive Glycine Detection

Glycine is the simplest amino acid in living organisms and plays important roles in biology and medicine. However, few biosensors for glycine sensing have been reported. Herein, we present a facile strategy to construct dopamine-modified AuCu bimetallic nanoclusters (denoted as AuCu NC-DA) as charge transfer-based biosensors for highly sensitive glycine sensing. The AuCu NCs stabilized by bovine serum albumin (BSA) exhibited a fluorescence maximum at 400 nm. Because of the high affinity of BSA for dopamine (DA), the surface of the AuCu NCs was modified with DA without any complicated chemical reactions, resulting in fluorescence quenching through a charge transfer process. Among 20 amino acids, AuCu NC-DA exhibited an off/on fluorescence switching response specifically toward glycine through the formation of hydrogen bonds with oxidized DA, which inhibited the charge transfer process, leading to the emergence of a new emission peak at 475 nm. Spectroscopic and thermodynamic results combined with molecular docking analyses provided comprehensive understanding of the sensing mechanism. Furthermore, we showed that AuCu NC-DA was able to sense glycine in cells by imaging. Finally, the practicability of AuCu NC-DA for glycine detection was validated in milk drink samples. This study presents a promising type of a charge transfer-based sensor.

A novel dual-function probe for recognition of Zn2+ and Al3+ and its application in real samples

A dual-function probe NAHH based on naphthalene was synthesized and characterized. Based on the combination effects derived from the inhabitation of photo-induced electron transfer (PET) and CN isomerization, probe NAHH achieved in the recognition of Zn2+ and Al3+ both through obvious fluorescence enhancement and color changes detected by naked eye, respectively. Probe NAHH showed high sensitivity with the limit of detection as low as 3.02 × 10-7 M for Zn2+ and 7.55 × 10-8 M for Al3+, indicated the capability of probe NAHH in trace detection for Zn2+ and Al3+. The binding ratio of NAHH with Zn2+ and Al3+ were all 1:1 determined by Job plot, and the corresponding association constant was calculated as 8.48 × 104 M-1 and 4.45 × 105 M-1, respectively. The mechanism was further confirmed by FT-IR, 1H NMR titration and ESI-MS analysis. Furthermore, probe NAHH was successfully applied in logic gate construction and the detection of Zn2+ and Al3+ in Songhua River and test stripe. Fluorescence imaging experiments confirmed that NAHH could be used to monitor Zn2+ in plant root.

A fluorescence "off-on-off" sensing platform based on bimetallic gold/silver nanoclusters for ascorbate oxidase activity monitoring

Herein, papain-protected bimetallic gold/silver nanoclusters (Au/Ag NCs) were successfully synthesized and applied for the detection of ascorbate oxidase (AAO). The doping of papain-protected Au nanoclusters with Ag enhanced the fluorescence intensity with an intense red fluorescence peak at 617 nm, and the red-emitting Au/Ag nanoclusters were further used to monitor the AAO activity. The fluorescence of Au/Ag NCs could be quenched by hydrogen peroxide (H2O2) due to the generation of hydroxyl radicals (˙OH) from the reaction of Ag/Au nanoclusters and H2O2. However, the addition of ascorbic acid (AA) effectively reacted with the free radicals and caused the fluorescence recovery of the Au/Ag NCs. Furthermore, AAO could catalyze the oxidation of AA to form dehydro-ascorbate (DHA). As a result, there was not enough AA to consume the hydroxyl radicals, which resulted in a decrease in the fluorescence of the papain-capped Au/Ag NCs. Therefore, the AAO activity can be monitored by measuring the fluorescence intensity of the red-emitting Au/Ag NCs. Moreover, the developed method for AAO detection displayed a good linear relationship from 5 to 80 mU mL-1 and the detection limit was 1.72 mU mL-1. Thus, a simple and selective method for the determination of the AAO activity was constructed and satisfactory results were obtained in real sample detection.

Selective detection of Fe3+ ions based on fluorescence MXene quantum dots via a mechanism integrating electron transfer and inner filter effect

Fluorescence quantum dots (QDs) are promising functional nanomaterials in chemical biology and environmental applications, where an analyte-induced responsive system is beneficial for detecting numerous life-related molecules and pollutants. Here, fluorescent Ti3C2 MXene quantum dots (MQDs) with the size of 1.75 nm were synthesized by a simple method of hydrofluoric acid etching and dimethyl sulfoxide exfoliation to form nanosheets followed by a one-step ultrasound method. The as-synthesized MQDs showed excitation-dependent behaviour along with a fluorescence quantum yield value of 7.7%. In addition, the fluorescence of the MQDs can be significantly suppressed by Fe3+. The mechanism for the fluorescence quenching of the MQDs was systematically investigated, which was attributed to the oxidation-reduction reaction between the MQDs and Fe3+ and the inner filter effect (IFE), different from the reported Förster resonant energy transfer (FRET) mechanism for MXene nanosheets. Based on this trait, a fluorescence method for Fe3+ detection based on MQDs was demonstrated with high sensitivity and selectivity, and the limit of detection was 310 nM. The proposed method was successfully used for the sensitive detection of Fe3+ in serum and sea water. This work will not only help to understand the selectivity mechanisms of MQDs as fluorescent probes for metal ions, but also provide a smart sensing platform in biological and environmental detection.