Direct detection of fluoride ions in aquatic samples by surface-enhanced Raman scattering

Fluorenone-naphthyl encapsulated dual sensor for recognition of F- and Hg2+: Syngenetic effect with drug sobisis and molecular docking studies

A novel fluorenone-naphthyl pendant sensor (FTU) possessing thiourea functionality has been synthesized via a simple condensation method and utilized for the recognition of F- and Hg2+ ions in the solution of CH3CN. The addition of F- and Hg2+ ions to the FTU solution led to the appearance of red-shifted absorption bands at 340 and 315 nm, respectively. On the other hand, in the fluorescence spectrum, the two-fold decrease in fluorescence intensity of probe FTU was observed with F- ions; while complete quenching of the fluorescence intensity was noticed with Hg2+ ions at 423 nm. The limit of detection values of F- and Hg2+ ions were found to be 1.02 & 29.1 nM, respectively, measured by UV-vis studies and 0.0185 & 0.81 nM, respectively, measured by fluorescence studies, which are less than recommended by WHO. DFT computational assessments and 1H NMR titration experiments pointed to F- induced deprotonation of thiourea NH signals. However, the chelation-enhanced quenching effect (CHEQ) was held responsible for fluorescence quenching with Hg2+ addition. Moreover, the in-situ formed FTU + F- complex was utilized for secondary sensing of drug sobisis. Furthermore, the real-world applicability of sensor FTU has been successfully scrutinized for the recognition of F- ions in the toothpaste samples. In addition, molecular docking studies revealed that FTU exhibited excellent antibacterial potency towards different gram-positive as well as negative strains.

Application of Fenton chemistry in electrochemical determination of pyrophosphatase activity and fluoride

Fenton chemistry has aroused widespread concern due to its application in the green oxidation and mineralization of organic wastes. Inorganic pyrophosphatase (PPase) catalyzes the hydrolysis of pyrophosphate ions (PPi) and provides a thermodynamic driving force for many biosynthetic reactions. Fluoride (F-) is widely applied to fight against tooth decay and reduce cavities. The electrochemical determination of PPase activity and F- was realized based on Fenton chemistry in this work. Glassy carbon electrode modified with poly (azure A) and acetylene black (GCE/PAA-AB) was fabricated. Hydroxyl radicals (∙OH) that were generated from a Cu2+-catalyzed Fenton-type reaction could oxidize PAA in the near-neutral medium, leading to a great increase of the cathodic peak current (Ipc). A coordination reaction between PPi and Cu2+ exerted a negative effect on Fenton reaction and hindered the Ipc enhancement. Cu2+-PPi complex was decomposed due to the hydrolysis of PPi induced by PPase, which caused the reappearance of the notably increased current response. F- could effectively inhibit PPase activity. As a result, the stable Cu2+-PPi complex remained and the high Ipc suffered from the decline again. The Ipc difference was used for the highly sensitive determination of PPase activity in the content range of 0.001-20 mU mL-1 with a detection of limit (LOD) at 0.6 μU mL-1 and that of F- in the concentration range of 0.01-100 μM with a LOD at 7 nM. The proposed PPase and F- sensor displayed a good selectivity, stability and reproducibility, and a high accuracy.

Plasmonic surface-enhanced Raman scattering nano-substrates for detection of anionic environmental contaminants: Current progress and future perspectives

Surface-enhanced Raman scattering spectroscopy (SERS) is a powerful technique of vibrational spectroscopy based on the inelastic scattering of incident photons by molecular species. It has unique properties such as ultra-sensitivity, selectivity, non-destructivity, speed, and fingerprinting properties for analytical and sensing applications. This enables SERS to be widely used in real-world sample analysis and basic plasmonic mechanistic studies. However, the desirable properties of SERS are compromised by the high cost and low reproducibility of the signals. The development of multifunctional, stable and reusable nano-engineered SERS substrates is a viable solution to circumvent these drawbacks. Recently, plasmonic SERS active nano-substrates with various morphologies have attracted the attention of researchers due to promising properties such as the formation of dense hot spots, additional stability, tunable and controlled morphology, and surface functionalization. This comprehensive review focused on the current advances in the field of SERS active nanosubstrates suitable for the detection and quantification of anionic environmental pollutants. The common fabrication methods, including the techniques for morphological adjustments and surface modification, substrate categories, and the design of nanotechnologically fabricated plasmonic SERS substrates for anion detection are systematically presented. Here, the need for the design, synthesis, and functionalization of SERS nano-substrates for anions of great environmental importance is explained in detail. In addition, the broad categories of SERS nano-substrates, namely colloid-based SERS substrates and solid-support SERS substrates are discussed. Moreover, a brief discussion of SERS detection of certain anionic pollutants in the environment is presented. Finally, the prospects in the fabrication and commercialization of pilot-scale handheld SERS sensors and the construction of smart nanosubstrates integrated with novel amplifying materials for the detection of anions of environmental and health concern are proposed.

Dual-emitting metal-organic frameworks for ratiometric fluorescence detection of fluoride and Al3+ in sequence

Fluoride (F^-) and Al³⁺ are two common ions existing in drinking water and natural water bodies. Excessive intake of F^- can lead to serious health issues such as fluorosis and bone diseases while accumulated consumption of Al³⁺ may cause neurotoxicity-based diseases. Developing a fast, reliable, and sensitive sensor for visually detecting both F^- and Al³⁺ is of great significance. In the present work, a ratiometric fluorescence sensor was constructed by incorporating rhodamine B (RhB) in situ into a zirconium-based metal-organic framework, UiO-66-NH₂. The obtained nanocomposite UiO-66-NH₂@RhB exhibited similar octahedral structure to UiO-66-NH₂ with high BET surface area, and showed two emission peaks at 450 nm and 585 nm. The blue fluorescence from UiO-66-NH₂ was enhanced by the addition of F^- while subsequent Al³⁺ addition diminished the increased fluorescence intensity, and the red emission from RhB as the reference remained unchangeable to improve the detection precision. Under optimal conditions, detection of limits as low as 1.55 μM for F^- and 0.54 μM for Al³⁺ in aqueous solution were achieved with good selectivity. High recoveries in drinking water samples were also acquired, showing potential applications of this ratiometric fluorescence sensor for practical evaluation of F^- and Al³⁺.

Coumarin based thiosemicarbazones as effective chemosensors for fluoride ion detection

Anion sensing have attained immense importance as these charged ions are prevailing in agriculture industry and in heavy industry and therefore in the environment around us, chemosensors are commencing to claim several applications as their role is being better perceived day by day. In the current study, coumarin based thiosemicarbazone R-1 (phenyl moiety) and R-2 (benzyl moiety) were synthesized. It was observed that there were variations in the sensing patterns of compound bearing benzyl group, as compared to the simple phenyl group bearing receptor. Different techniques were used to confirm the interaction of coumarin based receptors with anions. These techniques included naked-eye test, UV-visible, 1H NMR, and fluorescence spectroscopic techniques. The synthesized receptors showed selectivity for fluoride ions. Benesi-Hildebrand equation was employed for determining the detection limits and binding constants values. The synthesized receptors were employed as efficient chemosensors in real life samples and satisfactory results were obtained.

A spore-based portable kit for on-site detection of fluoride ions

The excess residues of fluoride ions cause serious human health problems, making their detection highly valuable. In this work, a whole-cell-based biosensor was presented for the detection of fluoride ions, which can inhibit the color reaction of 3,3',5,5',-tetramethylbenzidine (TMB) catalyzed by the CotA-laccase of spore surface. This reaction for the detection of fluoride ions could be read out through UV-vis spectrophotometer, smartphone, or standard colorimetric card within 10 min. Under optimum conditions, a linear range of 1-600 μmol L^-1 with a detection limit of 0.12 μmol L^-1 (3σ/k) was achieved for fluoride ions detection by using UV-vis spectrophotometer. The biosensor coupling with smartphone had a good linear response to fluoride ions concentration in the range of 5-600 μmol L^-1 with LOD of 0.90 μmol L^-1 (3σ/k). The standard colorimetric card can be directly used for recognizing the fluoride ions level via naked-eyes. A portable kit based on a colorimetric card and smartphone was developed and has been successfully applied for fluoride ions monitoring in surface waters and groundwater. This developed method has several advantages such as rapid, outstanding selectivity and anti-interference, low-cost, ease of operation and storage, and eco-friendliness, meeting the demands of point-of-care testing of fluoride ions and disease prevention.

Visual detection of fluoride based on supramolecular aggregates of perylene diimide in 100% aqueous media

A water-soluble perylene imide derivative (PDI-Glu) was synthesized and their supramolecular aggregates composed of PDI-Glu and Al³⁺ were prepared as a "turn on" fluorometric probe to monitor F^- in a purely aqueous system. Based on an "indicator displacement assay" (IDA) approach, the sensing performance and mechanism of PDI-Glu/Al³⁺ complex toward F^- were investigated by absorption and emission spectra. It was suggested that disassembly of PDI-Glu/Al³⁺ aggregates was promoted by addition of F^- through the competitive binding between Al³⁺ and F^-. The detection limit is 240 nmol/L. This method featured simple preparation, excellent water solubility, adjustable self-assembly performance, ease of observation and operation, and high selectivity and sensitivity. It was used for monitoring F^- in toothpaste and tap water samples with excellent accuracy and recovery. To the best of our knowledge, this is the first water-soluble perylene diimide-based probe for F^- detection in 100% aqueous media. We believe this work could not only extend the sensing scope of water-soluble perylene diimide, but also bring some useful information for the rapid detection of anionic analytes  in aqueous media. The disassembly of supramolecular aggregates of PDI-Glu/Al³⁺ along with significant fluorescence recovery enable a rapid and visual detection of F^- based on an "indicator displacement assay" strategy.

Synthesis and application of a new chemosensor based on the thiazolylazo-quinazolinone hybrid for detection of F- and S2- in aqueous solutions

A new chemosensor based on the thiazolylazo-quinazolinone hybrid (TAQH) was designed and synthesized for naked-eye sensitive detection of F^- and S^2-in aqueous acetonitrile solution. Spectral characterization of TAQH using FT-IR, ¹H NMR, and ¹³C NMR analysis revealed that the probe TAQH was successfully synthesized using a two steps reaction, including the diazotization-coupling and condensation reactions, respectively. The ion sensing ability of TAQH toward a wide range of anions and metal ions was evaluated by naked-eye detection method and UV-Vis absorption spectroscopy. The chemosensor TAQH displayed a fast and clear color change from yellow to red in the presence of F^- and S^2- ions, enabling easily detect with the naked eye. This clear color change is due to the effective interaction of the basic F^- and S^2- anions with hydroxyl group of chemosensor as a binding site. The experimental data also revealed that the F^- and S^2- ions were sensed by the probe TAQH over a wide pH range from 3 to 8. The results also confirmed that the TAQH has a wide linear detection range for F^- and S^2- ions. From UV-vis titration experiment, the limit of detection (LOD) for F^- and S^2- ions was found to be 3.1 μM and 5.7 μM, respectively. For quantitative measurements, the paper test strips containing TAQH were successfully fabricated and applied to detect F^- and S^2- ions in aqueous solutions. Furthermore, Job's plot based on spectroscopic data showed one-to-one stoichiometry for the interaction of anions with probe TAQH. Therefore, the proposed chemosensor with excellent features like the cost-effective, high sensitively and selectively and short response times can be utilized in any physical and biological conditions.

Luminescent metal organic frameworks-based chemiluminescence resonance energy transfer platform for turn-on detection of fluoride ion

A luminescent metal organic frameworks (MOFs)-based chemiluminescence resonance energy transfer (CRET) platform was constructed for turn-on detection of fluoride ion. A hybrid MOFs was prepared by encapsulating strong fluorescence 2',7'-dichlorofluorescein (DCF) into the frames of NH₂-MIL-101(Al) MOFs, which led to a significant suppression of fluorescence signal of DCF. In the presence of fluoride ion, it destroyed the structure of the hybrid MOFs and released DCF molecules from the frames due to the formation of more stable aluminum hexafluoride complex ions [AlF₆^3-] between fluoride ion and aluminum ion. The released DCF molecules accepted the energy originating from the chemical reaction of bis(2,4,6-trichlorophenyl)oxalate (TCPO) with hydrogen peroxide (H₂O₂), producing a strong chemiluminescence (CL) emission. The CL signal was strong dependent on the concentration of fluoride ion presented and showed a linear response in the range of 0.5-80.0 μmol L^-1 (9.5 μg L^-1-1.52 mg L^-1). The detection limit was 0.05 μmol L^-1 (about 0.95 μg L^-1) fluoride ion and the relative standard deviations was 2.3% for 40.0 μmol L^-1 fluoride ion solution (n = 11). This MOFs-based CRET method was successfully applied to the determination of fluoride ion in drinking water samples, demonstrating its potential application in analysis of real samples.

Reusable Silicon-Based SERS Chip for Ratiometric Analysis of Fluoride Ion in Aqueous Solutions

An innovative ratiometric surface-enhanced Raman scattering (SERS) sensor using a 4-mercaptoboric acid (4-MPBA)-modified silver nanoparticle-decorated silicon wafer (Si@Ag NPs chip) was proposed for the ultrasensitive determination of F^- ions in aqueous solutions. The principle of sensing strategy is based on fluoride-induced structural symmetry breaking and charge redistribution of phenylboronic acid, leading to a band shift of the C-C stretching mode of 4-MPBA from 1589 to 1576 cm^-1. Accordingly, a ratiometric signal of the area ratio (A₁₅₇₆/A₁₅₈₉) between the fluoride-bond MPBA molecules and unoccupied MPBA molecules can be used for the quantitative response of F^- ions. In comparison with other SERS-based sensing methods, this ratiometric method can avoid a large error resulting from the inhomogeneity of substrates. Under the optimized analytical conditions, the proposed SERS sensor possesses a quick response to F^- ions within 2 min and exhibits high selectivity for F^- ions with the determination limit of 10^-8 M, which is over 3 orders of magnitude lower than the World Health Organization (WHO) guideline value for F^- ions in drinking water. Of particular significance, the present sensor features favorable recyclability, which preserves suitable reproducibility during 6-time cyclic determination of F^- ions. The practical utility of this sensing system for the determination of F^- ions was tested with real water and toothpaste samples, and the results demonstrate that this sensor shows high recoveries (90-110%). Given its simple principle and easy operation, the present silicon-based SERS sensor could serve as a promising sensor for various practical applications.

Fluoride capped V6O13–reduced graphene oxide nanocomposites: high activity oxidase mimetics and mechanism investigation

A novel high activity of the oxidase-like nanozyme of V₆O₁₃–rGO NCs, whose activity can be significantly enhanced by fluoride capping through surface chemistry.

Indirect glyphosate detection based on ninhydrin reaction and surface-enhanced Raman scattering spectroscopy

Glyphosate is one of the most commonly-used and non-selective herbicides in agriculture, which may directly pollute the environment and threaten human health. A simple and effective approach to assessment of its damage to the natural environment is thus quite necessary. However, traditional chromatography-based detection methods usually suffer from complex pretreatment procedures. Herein, we propose a simple and sensitive method for the determination of glyphosate by combining ninhydrin reaction and surface-enhanced Raman scattering (SERS) spectroscopy. The product (purple color dye, PD) of the ninhydrin reaction is found to SERS-active and directly correlate with the glyphosate concentration. The limit of detection of the proposed method for glyphosate is as low as 1.43×10^-8mol·L^-1 with a relatively wider linear concentration range (1.0×10^-7-1.0×10^-4mol·L^-1), which demonstrates its great potential in rapid, highly sensitive concentration determination of glyphosate in practical applications for safety assessment of food and environment.