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 detection of Hg²⁺ and CN⁻ ions in aqueous medium. NR-SiQDs were synthesized using 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 photostability and pH stability. The fluorescence emission was effectively quenched using Hg²⁺ (turn-off) due to the formation of a nonfluorescent stable NR-SiQDs/Hg²⁺ complex, whereas after the addition of cyanide ions (CN⁻), Hg²⁺ ions could 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-SiQD-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.     

Spectrofluorimetric determination of magnesium ions in water,ampoule, and suspension samples using a fluorescent azothiazol-benzenesulfonamide derivative

In this study, a fluorescence azothiazol-benzenesulfonamide derivative (M-sensor) was prepared for the determination of Mg²⁺ ions in different samples. The utilized M-sensor exhibited an emission fluorescence activity at 587 nm upon excitation at 537 nm. The developed method was based on the quenching effect of Mg²⁺ ions on the fluorescence intensity of the M-sensor with the above-mentioned fluorescence features. Furthermore, the utilized M-sensor was complexed with Mg²⁺ ions in the molar ratio of 1:1 (Mg²⁺ to M-sensor) and the selectivity of M-sensor toward Mg²⁺ against other metals ions, and the reversibility and reusability of the sensor were studied and verified. After optimization of the fluorometric detection, the quenching effect was directly proportional to the increase in the concentration of Mg²⁺ in the linear range 100–600 ng ml⁻¹ with a limit of detection value of 18 ng ml⁻¹. The fluorescence sensor was successfully applied with good recovery for the determination of Mg²⁺ in water samples and different pharmaceutical samples (ampoules and suspension) without any interference from aluminium.     

A fluorescent terbium-metal–organic framework material for high-sensitivity detection of vomitoxin and oxytetracycline hydrochloride in water

A unique luminescent lanthanide metal–organic framework (LnMOF)–based fluorescence detection platform was utilized to achieve sensitive detection of vomitoxin (VT) and oxytetracycline hydrochloride (OTC-HCL) without the use of antibodies or biomolecular modifications. The sensor had a fluorescence quenching constant of 9.74 × 10⁶ M⁻¹ and a low detection limit of 0.68 nM for vomitoxin. Notably, this is the first example of a Tb-MOF sensor for fluorescence detection of vomitoxin. We further investigated its response to two mycotoxins, aflatoxin B1 and ochratoxin A, and found that their Stern–Volmer fluorescence quenching constants were lower than those of VT. In addition, the fluorescence sensor realized sensitive detection of OTC-HCL with a detection limit of 0.039 μM. In conclusion, the method has great potential as a sensitive and simple technique to detect VT and OTC-HCL in water.     

Visualizing stimulus-responsive dual-ligand fluorescent probes for hypochlorite: A novel strategy for real application in tap water

As an effective ingredient of disinfectants, ClO⁻ inevitably remains in water, which induces potential health hazards such as lung damage and kidney disease. In this study, we synthesized stimulus-responsive dual-ligand luminol-Tb-GMP coordination polymer nanoparticles (luminol-Tb-GMP CPNPs) as highly selective fluorescent probes for the real-time and visual detection of ClO⁻. CPNPs consist of Tb³⁺, a nuclear metal, that coordinates with GMP and luminol, an auxiliary ligand. GMP can be oxidized by ClO⁻ and damage its structure, resulting in fluorescence quenching of CPNPs. The two-ligand CPNPs sensor has a rapid fluorescent response, significant fluorescent color change, and high sensitivity, with a linear range of 2–18 μM and a detection limit of 0.14 μM. It has been successfully used to detect ClO⁻ in tap water, fountain water, and drinking water. Simultaneously, the portable filter paper strip was prepared to expand the range of applications outside the laboratory, which will provide a promising application for the real-time and semiquantitative analysis of ClO⁻.     

A deep-red xanthene-based highly sensitive fluorescent probe for detection of hypochlorite

As an oxidant, deodorant and bleaching agent, the hypochlorous acid (HClO) and hypochlorite (ClO⁻) are widely used in corrosion inhibitors, textile dyes, pharmaceutical intermediates and in our daily lives. However, excess usage or aberrant accumulation of ClO⁻ leads to tissue damage or some diseases and even cancer. Therefore, it is necessary to develop a fluorescent probe that specifically identifies ClO⁻. In this article, we synthesized a deep-red xanthene-based fluorescent probe (XA-CN). The strong electron deficient group dicyano endows the probe XA-CN deep-red fluorescent emission with high solubility, selectivity and sensitivity for ClO⁻ detection. Studies showed that the probe demonstrated turn-off fluorescence (643 nm) at the presence of ClO⁻ in dimethylsulfoxide/phosphate-buffered saline 1:1 (pH 9) solution with a limit of detection of 1.64 μM. Detection mechanism investigation revealed that the electron deficient group -CN and the hydroxyl group was oxidized into aldehyde or carbonyl groups at the presence of ClO⁻, resulting ultraviolet-visible absorption of the probe blue shifted and turned-off fluorescence. Furthermore, XA-CN was successfully used for the detection of ClO⁻ in tap water samples.     

Turn‐off–on chemiluminescence determination of cyanide

A flow injection chemiluminescence (FI–CL) method was developed for the determination of cyanide (CN⁻) based on the recovered CL signal by Cu²⁺ inhibiting a glutathione (GSH)‐capped CdTe quantum dot (QD) and hydrogen peroxide system. In an alkaline medium, strong CL signals were observed from the reaction of CdTe QDs and H₂O₂, and addition of Cu²⁺ could cause significant CL inhibition of the CdTe QDs–H₂O₂ system. In the presence of CN⁻, Cu²⁺ can be removed from the surface of CdTe QDs via the formation of particularly stable [Cu(CN)_n]^(n‐1)– species, and the CL signal of the CdTe QDs–H₂O₂ system was efficiently recovered. Thus, the CL signals of CdTe QDs–H₂O₂ system were turned off and turned on by the addition of Cu²⁺ and CN⁻, respectively. Further, the results showed that among the tested ions, only CN⁻ could recover the CL signal, which suggested that the CdTe QDs–H₂O₂–Cu²⁺ CL system had highly selectivity for CN⁻. Under optimum conditions, the CL intensity and the concentration of CN⁻ show a good linear relationship in the range 0.0–650.0 ng/mL (R² = 0.9996). The limit of detection for CN⁻ was 6.0 ng/mL (3σ). This method has been applied to detect CN⁻ in river water and industrial wastewater with satisfactory results. Copyright © 2014 John Wiley & Sons, Ltd.     

Fluorescence detection of carbofuran in aqueous extracts based on dual-emission SiO2@Y2O3:(Eu3+,Tb3+)@MIP core-shell structural nanoparticles

A novel double-windows fluorescence sensor for carbofuran (CF) detection was successfully developed based on rare-earth Eu,Tb-doped Y₂O₃@SiO₂-based molecularly imprinted nanoparticles (MINs) with a multilayer core-shell structure. The recognition process of the MINs for CF was fairly fast and needed only ~8 min to reach a dynamic equilibrium. Interestingly, one fluorescence attenuation window was found with an increase in CF concentration (Q) from 0.1 to 10 μg ml⁻¹ and with a limit of detection (LOD) of 0.04 μg ml⁻¹ at 544 nm belonging to the Tb³⁺ emission, as well as another fluorescence enhanced window within the CF concentration range 10–100 μg ml⁻¹ (LOD = 4 μg ml⁻¹) at 617 nm of Eu³⁺ emission in the dispersed rare-earth-doped MIN colloidal aqueous solution. Luminescence resonance energy transfer from CF to Eu³⁺ and an inner filter effect of CF towards Tb³⁺, as well from the two independent detection windows were clearly observed simultaneously. The competition experiment displayed hardly any marked interference during detection of CF following addition of its analogues (carbaryl, isoprocarb, aldicarb, methomyl, and etofenprox). Moreover, the MINs could also be applied to accurately detect CF in rhubarb and wolfberry samples with recoveries of 85.7–92.2%. This sensing system has high specific recognition and a wide detection range for CF and provides new opportunities for pesticide detection.     

Sensitive and selective detection of Cu(II) ion: A new effective 1,8‐naphthalimide‐based fluorescence ‘turn off’ sensor

The present study reports the development of a new 1,8‐naphthalimide‐based fluorescent sensor V for monitoring Cu(II) ions. The sensor exhibited pH independence over a wide pH range 2.52–9.58, and indicated its possible use for monitoring Cu(II) ions in a competitive pH medium. The sensor also showed high selectivity and sensitivity towards the Cu(II) ions over other competitive metal ions in DMSO–HEPES buffer (v/v, 1:1; pH 7.4) with a fluorescence ‘turn off’ mode of 79.79% observed. A Job plot indicated the formation of a 1:1 binding mode of the sensor with Cu(II) ions. The association constant and detection limit were 1.14 × 10⁶ M^–1 and 4.67 × 10^–8 M, respectively. The fluorescence spectrum of the sensor was quenched due to the powerful paramagnetic nature of the Cu(II) ions. Potential application of this sensor was also demonstrated when determining Cu(II) ion levels in two different water samples.     

A ‘turn-off’ fluorescence sensor for selective Hg(II) based on phenothiazine derivative

Given how crucial it is to preserve a human-safe and sustainable environment, the rapid discovery of possibly lethal heavy metals such as Hg(II) has drawn much attention in recent years. A novel sensor, known as (E)-2-((10-octyl-10H-phenothiazin-3-yl)methylene)hydrazine-1-carbothioamide (PTZHC), was synthesized as a fluorescence ‘on–off’ sensor for Hg²⁺ ions. Coordination alters organic molecule electron densities, quenching the fluorescence intensity. PTZHC was described completely with the help of FTIR and ¹H-NMR spectrum studies. The Hg²⁺ ion was successfully detected using the PTZHC sensor even when there were other metal ions present. The limit of the detection was estimated to be 2.5 × 10⁻⁸ M and the Job's plot examination implied that PTZHC was bound to Hg²⁺ with a simple 1:1 stoichiometry in s CH₃CN/H₂O (9:1, v/v) suspension. To further cast light on the bridged effect on geometric and optoelectronic characteristics, time-dependent density functional theory (TD-DFT) at the B3LYP/6-31G(d) level and DFT were both examined.     

A ratiometric fluorescence sensor based on carbon quantum dots realized the quantitative and visual detection of Hg2+

In this paper, based on the fluorescence of carbon quantum dots (CQDs) quenched by mercury ions (Hg²⁺) and the nonresponse of Hg²⁺ to rhodamine B fluorescence, a dual emission ratio fluorescence sensor was constructed to realize the quantitative detection of Hg²⁺. Under excitation at 365 nm, the fluorescence spectrum showed double emission peaks at 437 nm and 590 nm, corresponding to the fluorescence emissions of CQDs and rhodamine B, respectively. This method quantitatively detected Hg²⁺ based on the linear relationship between the ratio of the intensities of the two emission peaks F₄₃₇/F₅₉₀ and the concentration of Hg²⁺. The detection range was 10–70 nM, and the limit of detection (S/N = 3) was 3.3 nM. In addition, this method could also realize the qualitative and semiquantitative detection of Hg²⁺ according to the fluorescence colour change of the probe under ultraviolet light. After various evaluations, the method could be successfully applied to the quantitative and visual detection of Hg²⁺ in tap water, and demonstrated excellent selectivity, anti-interference performance, and repeatability of the method.     

A ratiometric fluorescent probe for rapid and specific detection of hypochlorite

Hypochlorite (ClO⁻), as a kind of essential reactive oxygen species, plays a crucial role in vitro and in vivo. Here, a ratiometric fluorescent probe ( TPAM ) was designed and constructed for sensing ClO⁻ based on substituted triphenylamine and malononitrile, which exhibited obvious colour transfer from orange to colourless under daylight accompanied by noticeable fluorescence change from red to green in response to ClO⁻. TPAM could effectively monitor ClO⁻ with the merits of fast response, excellent selectivity, high sensitivity and a low detection limit of 0.1014 μM. ¹H NMR, mass spectra and theoretical calculations proved that ClO⁻ caused the oxidation of the carbon–carbon double bond in TPAM , resulting in compound 1 and marked changes in colour and fluorescence. In addition, TPAM was utilized for imaging ClO⁻ in living cells successfully with good photostability and biocompatibility.     

Determination of lead(II) in food samples using a functionalized paper-based fluorescent sensor modified by carbon dots

This work discusses surface modification of cellulose paper specimens for compatibility with nitrogen and sulfur co-doped carbon dots (NSCDs) for lead ion sensing. The interaction of carbon dots (CDs) and cellulose fibers was investigated using silane or chitosan-modified cellulose papers. It was found that modified papers could reduce undesirable redistribution of CDs, during paper drying. Also, only chitosan-modified filter paper was suitable for the successful immobilization of NSCDs. The effect of paper type, chitosan amount, pH, and NSCDs concentration was also studied, and a Whatman No. 42 filter paper modified with chitosan (1% w/v), pH 8.0, and an NSCD concentration of 2.5 g L⁻¹ being selected for further studies. The sensor exhibited high selectivity for lead(II) compared with other metal ions because lead(II) resulted in the most significant changes in the emitted light intensity. Variations in NSCDs fluorescence were measured using a fluorescence imaging system. The NSCDs-paper sensor showed a linear relationship between mean fluorescence intensity and lead(II) in the concentration range of 5.00–1.25 × 10² μmol L⁻¹ with a correlation coefficient (R²) of 0.9988 and a detection limit of 4.50 μmol L⁻¹. The suggested method showed satisfying results for lead(II) determination in different samples as a fast and low-cost approach with on-site application.     

Signal‐on fluorescence assay for pyrophosphate ions based on DNA‐stabilized silver nanoclusters

Pyrophosphate anion (P₂O₇^4?, PPi) is considered as a potential biomarker for arthritic diseases because high levels of PPi may result in calcium pyrophosphate dehydrate crystal deposition diseases. In this study, a simple fluorescence method for PPi was demonstrated by organic integration of the efficient fluorescence quenching ability of copper ions to DNA‐scaffolded silver nanoclusters and the strong affinity of PPi towards copper ions. This simple fluorescence sensor showed a low detection limit (0.28 μM based on signal/noise = 3) towards the detection of PPi. Practical application of this method was also validated by detection of PPi in the synovial fluid.     

Gold nanoparticles‐based fluorescence enhancement of the terbium–levofloxacin system and its application in pharmaceutical preparations

A sensitive fluorescence (FL) technique is proposed for the determination of levofloxacin (LVX). The method is based on the fact that the weak FL signal of the Tb(III)–LVX system is strongly enhanced in the presence of gold nanoparticles. Gold nanoparticles were prepared by the citrate reduction of HAuCl₄ and characterized by transmission electron microscopy (TEM). Levofloxacin and Tb(III) ion form a fluorescence complex in aqueous solution, and its maximum emission wavelength was found at 545 nm. Optimal conditions for the formation of the levofloxacin–Tb(III) complexes were studied. Levofloxacin was detected by measuring the FL intensity, which increases linearly with the concentration of LVX in the range 6.2 × 10⁻¹⁰–2.6 × 10⁻⁸ mol/L. Recovery of the target analytes was > 96% with good quality parameters: linearity (r² > 0.996), limit of detection (LOD) and limit of quantification (LOQ) values 2.1 × 10⁻¹⁰ mol/L and 7.2 × 10⁻¹⁰ mol/L, and run‐to‐run and day‐to‐day precisions with relative standard deviations (RSDs) around 3%. Thus, the proposed method can be successfully applied to the routine determination of levofloxacin in pharmaceutical preparations. Copyright © 2011 John Wiley & Sons, Ltd.     

A ratiometric fluorescence strategy based on copper nanoclusters/carbon dots for sensitive detection of doxorubicin

Sensitive detection of doxorubicin (DOX) is critical for clinical theranostics. A novel ratiometric fluorescence strategy based on the inner filter effect (IFE) has been established for the sensitive detection of DOX by designing a ratiometric fluorescence probe. In the presence of DOX, the fluorescence intensity of copper nanoclusters (CuNCs) at 485 nm decreases, and the fluorescence intensity of carbon dots at 560 nm increases. Therefore, DOX can be quantitatively detected by measuring the ratio of the fluorescence intensities at 560 and 485 nm (F₅₆₀/F₄₈₅). The F₅₆₀/F₄₈₅ ratio exhibits a linear correlation with the DOX concentration in the range from 1.0 × 10⁻⁸ M to 1.0 × 10⁻⁴ M with the detection limit of 3.7 nM. Furthermore, this method was also successfully applied to the analysis of DOX in human plasma samples, affording an effective platform for drug safety management.     

A novel chromone Schiff base as Zn2+ turn-on fluorescent chemosensor in a mixed solution

In this study, a novel fluorescent chemosensor 1 based on chromone-3-carboxaldehyde Schiff base was synthesized and featured through nuclear magnetic resonance (NMR) and mass spectra. Spectroscopic investigation indicated that the fluorescent sensor showed high selectivity toward Zn²⁺ over other metal ions and that the detection limit of 1 could reach 10⁻⁷ M. These indicated that 1 acted as a highly selective and sensitive fluorescence chemosensor for Zn²⁺.     

Synthesis,characterization, and evaluation of (E)-methyl 2-((2-oxonaphthalen-1(2H)-ylidene)methylamino)acetate as a biological agent and an anion sensor

An amino acid based and bidentate Schiff base, (E)-methyl 2-((2-oxonaphthalen-1(2H)-ylidene)methylamino)acetate (ligand), was synthesized from the reaction of glycine-methyl ester hydrochloride with 2-hydroxy-1-naphthaldehyde. Characterization of the ligand was carried out using theoretical quantum–mechanical calculations and experimental spectroscopic methods. The molecular structure of the compound was confirmed using X-ray single-crystal data, NMR, FTIR and UV–Visible spectroscopy, which were in good agreement with the structure predicted by the theoretical calculations using density functional theory (DFT). Antimicrobial activity of the ligand was investigated for its minimum inhibitory concentration (MIC) to several bacteria and yeast cultures. UV–Visible spectroscopy studies also shown that the ligand can bind calf thymus DNA (CT-DNA) electrostatic binding. In addition, DNA cleavage study showed that the ligand cleaved DNA without the need for external agents. Energetically most favorable docked structures were obtained from the rigid molecular docking of the compound with DNA. The compound binds at the active site of the DNA proteins by weak non-covalent interactions. The colorimetric response of the ligand in DMSO to the addition of equivalent amount of anions (F⁻, Br⁻, I⁻, CN⁻, SCN⁻, ClO₄⁻, HSO₄⁻, AcO⁻, H₂PO₄⁻, N₃⁻ and OH⁻) was investigated and the ligand was shown to be sensitive to CN⁻ anion.     

Yb-TCPP metal–organic framework as fluorescence sensor for detecting tetracycline in milk

Developing effective means for detecting contamination in milk during production, processing, and storage is both important and challenging. Tetracycline (TC), due to its use in treating animal infections, is among the most prevalent organic pollutants in milk, posing potential and significant threats to human health. However, efficient and in situ monitoring of TC remains lacking. Nevertheless, we have successfully developed a highly sensitive and selective fluorescence method for detecting TC in milk using a metal–organic framework material made from Yb-TCPP (ytterbium-tetra(4-carboxyphenyl)porphyrin). The calculated Stern–Volmer constant (K_SV) was 12,310.88 M⁻¹, and the detection limit was 2.44 × 10⁻⁶ M, surpassing previous reports. Crucially, Yb-TCPP fluoresces in the near-infrared region, promising its development into a specific fluorescence detection product for practical TC detection in milk, offering potential application value.     

A highly selective and sensitive boronic acid-based sensor for detecting Pd2+ ion under mild conditions

Herein, a boronic acid-based sensor was reported selectively to recognize Pd²⁺ ion. The fluorescence intensity increased 36-fold after sensor binding with 2.47 × 10⁻⁵ M of Pd²⁺ ion. It was carried out in the 99% aqueous solution for binding tests, indicating sensor having good water solubility. In addition, it is discernible that Pd²⁺ ion turned on the blue fluorescence of sensor under a UV–lamp (365 nm), while other ions (Ag⁺, Al³⁺, Ba²⁺, Ca²⁺, Cr²⁺, Cd²⁺, Co²⁺, Cs²⁺, Cu²⁺, Fe²⁺, Fe³⁺, K⁺, Li⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺ and Zn²⁺) did not show the similar change. Furthermore, sensor has a low limit of detection (38 nM) and high selectivity, which exhibits the potential for the development of Pd²⁺ recognition in practical environments.     

Design and synthesis of a new fluorescent probe for cascade detection of Zn2+ and H2PO4− in water and targeted imaging of living cells

Design and synthesis of new fluorescence probes with good water‐solubility is of great importance to better understanding the significant role of ions which are related to biology and the environment. As important ions, zinc ion (Zn²⁺) and dihydrogen phosphate ion (H₂PO₄^?) display essential roles in living systems, and quantitative detection of these ions in water is still a challenge. In order to consider the significant role of the galactose moiety in the design of a water‐soluble fluorescence sensor, herein, we have developed a novel probe, Gal‐AQTF, for the cascade detection of Zn²⁺ and H₂PO₄^? with excellent selectivity in water. Through the introduction of the galactose moiety onto the sensor AQTF, which has been reported earlier by us, the water‐solubility, cell compatibility and targeting ability were enhanced. Gal‐AQTF has been successfully applied in the imaging of the living cells of HepG2 and A549, and illustrated good selectivity for the HepG2 cells which overly express the asialoglycoprotein (ASGP) receptor.