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.

Green and Low-temperature Synthesis of Carbon Dots for Simple Detection of Kaempferol

As a common flavonols, kaempferol (Kae) has a broad market as health food and medicine for its anti-inflammatory, anti-oxidation, and anti-cancer properties. In this study, a novel convenient and simple fluorescent sensor based on carbon dots (CDs) for the detection of Kae was developed. The fluorescent CDs, with excellent photo-luminescence (PL) and up-conversion luminescence (UCPL) properties, were successfully prepared by low-temperature oil bath reaction based on ascorbic acid as carbon source at 90 °C in one pot. Under the optimal conditions, the fluorescence (FL) intensity of CDs was gradually quenched by the increasing addition of Kae with a linear relationship between F0/F and Kae concentration in a wide range from 5 µM to 100 µM with a detection limit of 0.38 µM. And this designed sensor was favourably applied for the detection of Kae in actual sample (xin-da-kang tablets). Moreover, the proposed CDs has great application prospects as a drug-sensor for detecting Kae due to its simple operation, economical and green materials, low equipment requirement, and rapid detection.

Recent Advances in Silicon Quantum Dot-Based Fluorescent Biosensors

With the development of nanotechnology, fluorescent silicon nanomaterials have been synthesized and applied in various areas. Among them, silicon quantum dots (SiQDs) are a new class of zero-dimensional nanomaterials with outstanding optical properties, benign biocompatibility, and ultra-small size. In recent years, SiQDs have been gradually utilized for constructing high-performance fluorescent sensors for chemical or biological analytes. Herein, we focus on reviewing recent advances in SiQD-based fluorescent biosensors from a broad perspective and discussing possible future trends. First, the representative progress for synthesizing water-soluble SiQDs in the past decade is systematically summarized. Then, the latest achievement of the design and fabrication of SiQD-based fluorescent biosensors is introduced, with a particular focus on analyte-induced photoluminescence (fluorescence) changes, hybrids of SiQDs with other materials or molecules, and biological ligand-modification methods. Finally, the current challenges and prospects of this field are highlighted.

Synthesis of sulfhydryl functionalized silicon quantum dots with high quantum yield for imaging of hypochlorite in cells and zebrafish

Sulfhydryl functionalized silicon quantum dots (S-SiQDs) with a fluorescence quantum yield of 38.5% were synthesized using 3-mercaptopropyltrimethoxysilane (MPTMS) and m-phenylenediamine by a simple one-pot method. It is worth noting that by oxidizing the surface sulfhydryl groups and statically quenching, the fluorescence of S-SiQDs at 492 nm (excitation at 383 nm) can be selectively quenched by hypochlorite (ClO^-) in a linear range of 0.05 to 1.8 μM with a low detection limit of 13 nM. The reaction was completed in 10 s with no interference from other ROS, metal ions, anions and reducing species. The silicon source containing sulfhydryl groups was used to synthesize silicon quantum dots for the first time, and the surface of the S-SiQDs was provided with sulfhydryl groups and reacted rapidly and sensitively with ClO^-. The S-SiQDs have good photostability and biocompatibility, and can be further used for ClO^- imaging in MCF-7 cells and zebrafish, showing great promise in biological imaging. The proposed assay demonstrates that 3-mercaptopropyltrimethoxysilane is a good choice to obtain a functionalized fluorescent nanoprobe for redox species.

Nitrogen-rich silicon quantum dots: facile synthesis and application as a fluorescent "on-off-on" probe for sensitive detection of Hg2+ and cyanide ions

The sensitive and reliable detection of Hg²⁺ and CN^- as harsh environmental contaminants are of great importance. In view of this, a novel "on-off-on" fluorescent probe based on nitrogen-rich silicon quantum dots (NR-SiQDs) has been designed for sensitive detecting Hg²⁺ and CN^- ions in aqueous media. NR-SiQDs were synthesized by a facile, one-step, and environment friendly procedure in the presence of 3-aminopropyl trimethoxysilane (APTMS) and ascorbic acid (AA) as precursors, with L-asparagine as a nitrogen source for surface modification. The NR-SiQDs exhibited strong fluorescence emission at 450 nm with 42.34% quantum yield, satisfactory salt tolerance, and superior photo- and pH-stability. The fluorescence emission was effectively quenched by Hg²⁺ (turn off) due to the formation of a non-fluorescent stable NR-SiQDs/Hg²⁺ complex while after the addition of cyanide ions (CN^- ), Hg²⁺ ions can be leached from the surface of the NR-SiQDs and the fluorescence emission intensity of the quenched NR-SiQDs fully recovered (turn on) due to the formation of highly stable [Hg (CN)₄ ]^2- species. After optimizing the response conditions, the obtained limits of detection were found to be 53 nM and 0.46 μM for Hg²⁺ and CN^- , respectively. Finally, the NR-SiQDs based fluorescence probe was utilized to detect Hg²⁺ and CN^- ions in water samples and satisfactory results were obtained, suggesting its potential application for environmental monitoring.

Polymer surface ligand and silica coating induced highly stable perovskite nanocrystals with enhanced aqueous fluorescence for efficient Hg2+ and glutathione detection

The poor stability and aqueous-quenching of fluorescence of perovskite nanocrystals (NCs) hinder their application in bio-detection and bio-imaging. Herein, through the synergistic effects of polymer surface ligand and silica encapsulation, highly stable and enhanced aqueous fluorescent CsPbBr₃-mPEG@SiO₂ NCs were synthesized and used as a novel "on-off-on" fluorescent probe for highly sensitive and selective detection of mercury ions (Hg²⁺) and glutathione (GSH) in aqueous solutions. The effects of the methoxypolyethylene glycol amine (mPEG-NH₂) ligand and silica encapsulation on the stability and aqueous fluorescence of the CsPbBr₃ NCs were studied. It indicated that the aqueous fluorescence of perovskite NCs was increased by 2.59 times. The water stability was also greatly improved, with the NCs maintaining 73% of their original fluorescence after storage for 30 days in water. X-ray diffraction (XRD), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FT-IR) analyses further demonstrated that the NCs were successfully passivated by mPEG-NH₂ and silica. The fluorescence of the CsPbBr₃-mPEG@SiO₂ nanocrystals was effectively quenched by Hg²⁺ which is attributed to the electron transfer process between NCs and Hg²⁺. Then, through the interaction between Hg²⁺ and GSH, the restoration of fluorescence for CsPbBr₃-mPEG@SiO₂ was realized. The "on-off-on" fluorescent probe can be used for the detection of Hg²⁺ and GSH with a low detection limit of 0.08 nM and 0.19 μM, respectively. It also shows a fast response time and high accuracy for practical sample detection. The simple and sensitive fluorescent probe of CsPbBr₃-mPEG@SiO₂ shows great potential in environmental and biological sensing.

"ON-OFF-ON" fluorescence switches based on N,S-doped carbon dots: Facile hydrothermal growth, selective detection of Hg2+, and as a reversive probe for guanine

Water contamination due to heavy metal ions has become a major environmental problem worldwide. In this work, "on-off-on" fluorescence switches comprising N,S-doped carbon dots (N,S-CDs) have been developed for selective recognition of Hg²⁺ and as reversive probes for guanine. N,S-CDs were synthesized in a facile one-step hydrothermal approach using citric acid and methionine as precursors. The synthesized N,S-CDs display fluorescence with excitation/emission maxima of 370/440 nm and a quantum yield of 32.5%. Under the variable pH (2-12), the fluorescent N,S-CDs with a linear range from 0.05 to 100 μM displayed selective discrimination for Hg²⁺ with the limit of 6.24 nM over several other cations, anions, and neutral analytes resulting in the quenching of fluorescence response. Furthermore, the addition of guanine at the LOD of 6.4 nM can restore N,S-CDs' fluorescence in a reversible manner. For this kind of fluorescence switch, its purposed applications on environmental samples are employed successfully to detect Hg²⁺ in tap water and river water.

Sulfur quantum dot-based "ON-OFF-ON" fluorescence platform for detection and bioimaging of Cr(vi) and ascorbic acid in complex environmental matrices and biological tissues

Based on an "assembling-fission" principle, stable sulfur quantum dots (SQDs) were synthesized using sublimed sulfur as a precursor and PEG-400 as a passivator. The fluorescence intensities (FIs) of SQDs were efficiently quenched by Cr(vi) due to formation of SQD/Cr(vi) complexes through the inner-filter effect. When ascorbic acid (AA) was introduced into the SQD/Cr(vi) system, SQD fluorescence was restored due to AA-induced reduction of Cr(vi) to Cr(iii). Consequently, a SQD-based "ON-OFF-ON" platform was constructed for sequential detection of Cr(vi) and AA. Under optimized conditions, the FIs of SQDs were linearly dependent on the concentrations of Cr(vi) and AA, yielding linear ranges of 0.005-1.5 and 0.01-5.5 mM with detection limits of 1.5 and 3 μM, respectively, in waters, serum and tablet samples. After a 24 h incubation, the SQDs displayed strong, quenched and recovered blue fluorescence, respectively, in the SQD, SQD/AAO/Cr(vi) and SQD/Cr(vi) systems in live HeLa cells and zebrafish embryos/larvae. A blue fluorescence was displayed in the yolk of zebrafish embryos, and yolk and head of larvae. This study demonstrates the efficacy of SQD systems for environmental and biological applications in complex matrices, and for direct observation of Cr bioaccumulation in organisms by bioimaging.

Fluorescent and Colorimetric Dual-signal Enantiomers Recognition via Enzyme Catalysis: The Case of Glucose Enantiomers Using Nitrogen-doped Silicon Quantum Dots/Silver Probe Coupled with β-D-Glucose Oxidase

Chiral enantiomer recognition is important but facing tough challenges in the direct quantitative determination for complex samples. In this work, via chosing nitrogen-doped silicon quantum dots (N-SiQD) as optical nanoprobe and constructing N-SiQD/silver (N-SiQD/Ag NPs) complex, β-D-GOx as model enzyme and glucose enantiomers as analytes, a fluorescent and colorimetric dual-signal chiral sensing strategy was proposed herein for chiral recognition based on specific enzyme-catalyzed reaction. N-SiQD can exhibit intense fluorescence, while it can be quenched by Ag NPs owing to the formation of N-SiQD/Ag NPs. In the presence of glucose isomer, D-glucose is catalytically hydrolyzed by β-D-GOx to form H₂O₂ owing to the specific enzyme catalyzed reaction between D-glucose and β-D-GOx, and H₂O₂ can etch Ag NPs from the N-SiQD/Ag NPs probe to change the solution color from brown to colorless and restore the N-SiQD fluorescence; while these phenomena cannot be caused by L-glucose, a dual-signal sensing method was thus constructed for recognizing glucose enantiomers. It is believed that the chiral enantiomers recognition strategy via enzyme catalysis has great application for selective and quantificational detection of enantiomers in the complex sample system.

Ultrasensitive simultaneous detection of ascorbic acid, dopamine, uric acid and acetaminophen on a graphitized porous carbon-modified electrode

NHCS-1000/GCE produces a noticeable electrocatalytic response towards the anodic oxidation for the multiple sensing of AA, DA, UA and AC.

Silicon-hybrid carbon dots derived from rice husk: promising fluorescent probes for trivalent rare earth element ions in aqueous media

As a novel fluorescence probe, Si–CDs could not only be applied to distinguish four groups of different rare earth element ions (REEs) but also exhibit a rapid and sensitive response towards individual Tb3+,Eu3+ and Dy3+.

Quantum dot (QD)-based probes for multiplexed determination of heavy metal ions

Heavy metal contamination is a major global concern and additive toxicity resulting from the exposure to multiple heavy metal ions is more pronounced than that induced by a single metal species. Quantum dots (QDs) have demonstrated unique properties as sensing materials for heavy metal ions over the past two decades. With the rapid development and deep understanding on determination of single heavy metal ion using QD probes, this technology has been employed for sensing multiple metal ions. This review (with 97 refs.) summarizes the progress made in recent years in methods for multiplexed determination of heavy metal ions using QDs. Following an introduction into the importance of simultaneous quantitation of multiple heavy metal ions in environmentally relevant settings, the review discusses the applications of different types of QDs, i.e. chalcogenide, carbon, polymer and graphene in this field. Determination strategies based on fluorometric, colorimetric and electrochemical responses were reviewed including the testing mechanisms and differentiation between various metal ions. In addition, current state of the art sensor constructions, i.e. immobilization of QDs on solid substrate and sensor arrays have been highlighted. A concluding section describes the limitations, opportunities and future challenges of the QD probes. We also compiled a comprehensive table of currently available literature. The listed papers provided information in the following categories, i.e. type of QDs used, ligands or other components in the probe, metal ions tested, medium/substrate of the probe, transduction methods, discrimination mechanism, limit of detection (LOD) and concentration range. Graphic abstract.